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                                                                ARTICLE IN PRESS
                                                                Journal of Luminescence 130 (2010) 415–423



                                                           Contents lists available at ScienceDirect


                                                          Journal of Luminescence
                                                 journal homepage: www.elsevier.com/locate/jlumin




Study of solvent-conjugated polymer interactions by polarized spectroscopy:
MEH–PPV and Poly(9,90 -dioctylfluorene-2,7-diyl)
Rafael F. Cossiello a, Mariano D. Susman b, Pedro F. Aramendıa b,n, Teresa D.Z. Atvars a,n
                                                            ´
a
                                                   ´
 Universidade Estadual de Campinas, Instituto de Quımica, Caixa Postal 6154, Campinas, 13084-971 SP, Brazil
b
                                    ´            ´        ´         ´      ´                                                                               ´
 INQUIMAE and Departamento de Quımica Inorganica, Analıtica y Quımica Fısica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellon 2, Ciudad
Universitaria, C1428EHA, Buenos Aires, Argentina




a r t i c l e in fo                                  abstract

Article history:                                     Absorption, emission, and anisotropy measurements were performed on poly-[2-methoxy-5(20 -
Received 3 April 2009                                ethylhexyloxy)–1,4-phenylenevinylene] (MEH–PPV) and poly-(9,90 -dioctylfluorene-2,7-diyl) (PF) solu-
Received in revised form                             tions of various solvents, and in thin films deposited from them. The good correlation of MEH–PPV
6 October 2009
                                                     absorption and emission energy with Hildebrandt’s dispersive parameter indicate that dispersive forces
Accepted 19 October 2009
                                                     regulate the effective extent of the luminophore. The excitation and the emission spectra of a and b
Available online 27 October 2009
                                                     chains can be distinguished in PF solutions using the steady-state anisotropy. PF films show greater
Keywords:                                            memory effect from the solutions from which they were spun than MEH–PPV. Anisotropy of MEH–PPV
Fluorescence anisotropy                              is very low, both in solutions and in films reflecting efficient energy migration. Anisotropy of PF in
Solvent effects
                                                     solutions and films demonstrates great differences in energy transfer efficiency within the a and b
Solutions and films
                                                     phases, while there is no energy transfer between these chain conformations.
Conjugated polymers
Polyfluorene                                                                                                              & 2009 Published by Elsevier B.V.
MEH–PPV




1. Introduction                                                                        consequences such as the presence of defects, stress, non-uniform
                                                                                       thickness, etc, that greatly lower electrical and electrolumines-
    Conjugated polymers attracted much attention in the last                           cence performance [10–31].
thirty years since the discovery of the electron conductivity of                          There are several experimental techniques useful in studies of
poly(acetylene) and later of poly(aniline) and several other                           macromolecular chain conformations: scanning electron micro-
polymers and copolymers [1,2]. More recently, during the nine-                         scopy, atomic force microscopy, small angle X-ray scattering and
ties, the scientific and technological interests were renewed by                        wide angle X-ray diffraction [26,27], near-field optical microscopy
the discovery of the electroluminescence of conjugated polymers                        [24,25,29], confocal microscopy and single molecule spectroscopy
and copolymers [2].                                                                    [32–34], optical microscopy, and luminescence spectroscopy
    These conjugated polymers are materials with a great variety                       [32–35]. In particular, the steady-state photoluminescence aniso-
of chemical structures, synthesized by distinct procedures, with                       tropy provides information on molecular conformation and
multiple functionalities conferring different colors to operating                      reorientation during the lifetime of the excited sate [35–43].
devices, from violet to red [3–14]. Each one of these particular                          In multiple chromophoric systems, such as conjugated poly-
chemical structures may have distinct physical, chemical and                           mers, the excitation energy can be converted to fluorescence
electrical properties. In addition, because of the lower melting                       emission whose anisotropy depends strongly on the chain
point (in the case of semicrystalline materials) and lower glass                       conformational relaxation, on the energy transfer processes and
transition temperature compared to inorganic semiconductors,                           on the energy migration probability. In general, when the energy
the conjugated electroluminescent polymers can easily be                               migration is faster than the emission decay, a strong depolariza-
processed and thin films can be prepared by in situ polymeriza-                         tion of the fluorescence takes place [36–43].
tion or by deposition on a substrate using spin coating, self                             Film morphology depends on several processing parameters
assembly, dip coating and casting [1]. The facility to process                         [17–29,43,44], but it is particularly influenced by the solvent
organic polymers has, on the other hand, various morphological                         used. Polymer chains form coils in unfavorable solvents to
                                                                                       minimize their exposure to the solvent and this has a strong
                                                                                       impact on its photophysical properties [23,31,33,36,45–47]. For
    n                                                                                  example, the excimer emission can be enhanced in poor solvents
    Corresponding authors. Tel.: + 55 19 35214729.
                                                        ´
    E-mail addresses: pedro@qi.fcen.uba.ar (P.F. Aramendıa),                           while it is minimized in good solvents that induce a more
tatvars@iqm.unicamp.br (T.D.F. Atvars).                                                extended conformation [48,49]. The reason for the changes in

0022-2313/$ - see front matter & 2009 Published by Elsevier B.V.
doi:10.1016/j.jlumin.2009.10.006
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                                                          O                            under mild heating until complete dissolution. All solutions were
                                                                                       maintained in dark in a sealed flask. Samples of thin films were
                                                                                       prepared by dip coating or by spin coating of poly(ethylene
                                      N                                                terephthalate) (PET) slices (1 Â 2 Â 0.5 cm) into the polymer
                                                                          n
               C8H17        C8H17                                                      solutions. Samples were dried for 2 h at room temperature after
                                                          O                            dipping to remove residual solvent.
                       PF                                   MEH-PPV

Fig. 1. Chemical structures of poly-(9,90 -dioctylfluorenyl-2,7-diyl)-end capped        2.2. Methods
with dimethyl phenylene (PF) and poly-[2-methoxy-5-(20 -ethylhexyloxy)-p-
phenylene-vinylene] (MEH–PPV).
                                                                                          Electronic absorption spectra were recorded in a Hewlett–
                                                                                       Packard-8452A UV–vis spectrometer. When necessary, polarized
                                                                                       absorption was detected by placing film polarizers before and
                                                                                       after the sample. Each polarized spectrum is referenced to the
photo and electroluminescence properties in films prepared from
                                                                                       baseline obtained with the corresponding orientation of the
different solvents is that a part of the conformation in solution
                                                                                       polarizer. The steady-state fluorescence spectroscopy was per-
should be maintained in the solid state, which is called the
                                                                                       formed on a PC1 photon counting spectrofluorimeter from ISS Inc.
memory effect [30–34].
                                                                                       or on a PTI quantamaster spectrofluorometer. Slits were selected
    Hildebrand solubility parameter is often used to quantify
                                                                                       for a spectral resolution between 4 and 8 nm in excitation and in
polymer–solvent interactions either in the global form or through
                                                                                       emission. For fluorescence emission and excitation spectra of
its specific components that describe the hydrogen bonding
                                                                                       solutions, a square quartz cuvette, 1 cm side, was employed.
interaction (dH), the dipolar interactions (dp) and the dispersive
                                                                                       Spectra of films were recorded in a back-face configuration to
interactions (dd) [50–53]. These components account better for
                                                                                       diminish stray light reaching the detector.
specific polymer–solvent interactions. For example, although
                                                                                          Fluorescence anisotropy was measured using a couple of film
toluene (d =18.2 J1/2 cm À 3/2) and ethyl acetate (d =18.2 J1/2 cm À 3/2)
                                                                                       polarizers on the excitation and emission beams in either of the
have the same Hildebrand solubility parameters, their compo-
                                                                                       spectrofluorimeters operating in a L-format. The fluorescence
nents (dH, dd, dp) are different as well as their solvation ability
                                                                                       anisotropy is calculated according to Eq. (1), where I is the
for poly-[2-methoxy-5(20 -ethylhexyloxy)–1,4-phenylenevinylene]
                                                                                       fluorescence intensity at one specific wavelength and ‘‘v’’ and ‘‘h’’
(MEH–PPV) [46].
                                                                                       are the relative orientation of the polarizers, denoting perpendi-
    The knowledge of the solvation ability is particularly impor-
                                                                                       cular and on the plane of incidence, respectively. The first letter of
tant when multi-layer electroluminescent devices are desirable.
                                                                                       the subscript represents the orientation of the excitation polarizer
Here the morphology of one layer must be maintained during the
                                                                                       and the second that of the emission polarizer [54]:
deposition of the subsequent layers; a condition that requires a
good solvent for one material and a poor solvent for the other.                                  Ivv À GIvh
                                                                                       rðlÞ ¼                                                            ð1Þ
Therefore, in the present work we explore the applicability of the                              Ivv þ2GIvh
Hildebrand parameter to discuss the behavior of MEH–PPV and
                                                                                       where Ivv and Ivh are the spectral intensity determined using the
poly-(9,90 -dioctylfluorene-2,7-diyl) (PF) (Fig. 1) in solutions of
                                                                                       polarizer and the analyzer in vertical positions and with the
toluene, chloroform, dichloromethane, tetrahydrofuran (THF),
                                                                                       polarizer in the vertical and the analyzer in the horizontal
ethyl acetate, dimethylsulfoxide (DMSO), and acetonitrile and
                                                                                       position, respectively. G is a correction factor of instrumental
the consequence of the solvent–polymer interaction on the
                                                                                       response given by Eq. (2):
photoluminescence features of polymers in solution and in films
deposited from different solvents. To analyze the solvent quality                                  Ihv ðlemÞ
                                                                                       GðlemÞ ¼                                                          ð2Þ
we use the steady-state spectroscopy and fluorescence anisotropy                                    Ihh ðlemÞ
measurements in both dilute polymer solutions and thin films
                                                                                       where Ihv and Ihh are the spectra recorded with polarizer in
prepared by dip or spin coating. Differences in photoluminescence
                                                                                       horizontal position in both cases, and the analyzer in vertical and
spectra and in fluorescence anisotropy are discussed in terms of
                                                                                       horizontal positions, respectively [54].
the chain conformation in solution and in films, and their
implications for energy migration and transfer.

                                                                                       3. Results
2. Experimental section
                                                                                       3.1. Fluorescence spectra and anisotropy of MEH–PPV in solutions
2.1. Materials
                                                                                          The electronic absorption spectra of MEH–PV in 18 mg L À 1
   End      capped     poly-(9,90 -dioctylfluorenyl-2,7-diyl)    (PF)                   solutions (see Fig. 2) show broad absorption bands. Broad
                       1À
Mw $ 40 À 120 kg mol was purchased from ADS Inc. and poly-                             absorption spectra of conjugated polymers result of the
[2-methoxy-5-(20 -ethylhexyloxy)-p-phenylene-vinylene]        Mw ¼                     convolution of electronic transitions from macromolecular
           1À
181 kg mol     (MEH–PPV), from Sigma-Aldrich. They were used                           segments of different conjugation lengths overlapped with
as received. Dichloromethane (Merck 98% and Carlo Erba,                                several vibronic states [2–25,32–34,55–57].
Analytical grade), toluene (Merck 98% and Merck, spectroscopical                          Absorption spectra of MEH–PPV in acetonitrile, DMSO and
grade) were fractionally distilled before use, chloroform (Labsynth                    ethyl acetate are different from those in other solvents (Fig. 2a).
99.8%). THF (Merck 98.5% or Aldrich ACS—99 +%) was distilled                           The lower solubility of MEH–PPV in these solvents and, conse-
with Na at reduced pressure, ethyl acetate (Merck 99% and EM                           quently, the lower concentration in solution leads to a weak
Science ACS, for HPLC), DMSO (Merck 98%), and acetonitrile                             absorbance (less than 0.1). Concentration in these cases are
(Merck 99%) were dried before use.                                                     smaller than 18 mg L À 1. The absorption bands are centered at
   Polymer solutions in each solvent were prepared by dissolving                       425 nm for acetonitrile, 480 nm for ethyl acetate and DMSO,
the solid polymer under stirring, during several hours in the dark                     492 nm for THF and 496 nm for chloroform and toluene.
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                                                                                                                                   0.20

                                  1.0                                                                                              0.16




                                                                                                              Anisotropy / < r >
          Relative absorbance




                                  0.8                                                                                              0.12

                                  0.6                                                                                              0.08

                                                                                                                                   0.04
                                  0.4
                                                                                                                                   0.00
                                  0.2                                                                                                     580   600       620      640
                                                                                                                                                 λ / nm
                                  0.0
                                     350   400    450     500       550      600                    Fig. 3. Fluorescence anisotropy (lexc = 495 nm), /rS of MEH–PPV in dilute
                                                                                                    solutions of___ethyl acetate, -&-&-, chloroform; -n-n-, THF; -J-J-, toluene;
                                                    λ / nm
                                                                                                    - Â - Â -, DMSO; and - - -, acetonitrile. Polymer concentration as in Fig. 2.


                                                                                                    usually emit at the red-edge of the intrachain species [23,25,29–
                                  1.0
                                                                                                    31,33,46]. Therefore, in the absence of aggregates, even for good
          Relative fluorescence




                                  0.8                                                               solvents, the band profile is not expected to be the same
                                                                                                    [23,25,29–31,33,46]. This can explain slight differences between
                                  0.6                                                               the emission spectra in good solvents while in these same
                                                                                                    environments the absorption spectra remain practically the same.
                                  0.4                                                               The emission spectrum of MEH–PPV in ethyl acetate presents two
                                                                                                    bands, the higher energy attributed to the intrachain exciton
                                  0.2                                                               (isolate chains) and the lower energy attributed to the interchain
                                                                                                    exciton (aggregate involving more than one macromolecular
                                  0.0                                                               segment). In poor solvents, such as acetonitrile and DMSO, only
                                     500    550       600         650          700
                                                                                                    the emission of aggregates is observed. In other words, in good
                                                    λ / nm                                          solvents the conformation of polymeric chains is more extended
Fig. 2. (a) Relative absorption and (b) relative fluorescence spectra (lexc = 495 nm)
                                                                                                    and the interchain interactions, i.e. p–p stacking, are minimized
of MEH–PPV in solution of: ___, ethyl acetate; -&-&-, chloroform; -n-n-, THF;                       by the solvation layer, while in poor solvents interchain interac-
-J-J-, toluene; (these later three spectra almost overlap in absorption and in                      tions or coil conformation favoring intersegmental interactions
emission), - Â - Â -, DMSO; and - - -. acetonitrile. Polymer concentration was                      increase the probability of aggregation [25,32,35,48,49].
18 mg L À 1 except in ethyl acetate, acetonitrile and DMSO (see text).
                                                                                                        We also found that the steady-state emission anisotropy, /rS,
                                                                                                    of MEH–PPV in these five solutions (Fig. 3, Table 1) is independent
                                                                                                    of the emission wavelength and has distinct values for three types
    The MEH–PPV fluorescence emission spectra of 18 mg L À 1                                         of solvents (as classified above): /rS= 0.114 in chloroform,
solutions (Fig. 2b) show that in good solvents (toluene, chloro-                                    /rS= 0.116 in toluene and /rS=0.120 in THF (good solvents);
form and THF) the emission maximum is centered at lem = 555                                         /rS=0.166 in ethyl acetate (solvent with intermediate quality), and
nm, with shoulders at 600 and 650 nm, corresponding to a                                            /rS=0.093 in DMSO and /rS=0.035 in acetonitrile (poor solvents).
vibronic progression originated on the C–C vibrational stretching                                   Transient measurements have shown that immediately after
modes of aromatic rings. A very small blue shift is observed in THF                                 excitation MEH–PPV anisotropy in dilute chlorobenzene solution
(lem =552 nm) when compared to toluene solutions, which can be                                      is r0 =0.4, the theoretically maximum value, and then the value
attributed to solvatochromic effects, in agreement with previous                                    decreases to r=0.15 with a characteristic lifetime of 20 ps and to
observations in THF and chlorobenzene [31]. There is a                                              r=0.08 at longer times (limiting anisotropy, rp) [54]. Considering an
pronounced spectral red-shift of the emission spectra (lem = 592                                    excited state lifetime of 250 ps, the above values render /rS=0.11,
nm) for MEH–PPV in poor solvents (DMSO and acetonitrile) with a                                     in good agreement with our results (/rS=r0tr/(tr + tf)+rp; where tr
complete absence of higher energy emission band. This result is                                     is the characteristic time for the anisotropy decay and tf is the
explained by MEH–PPV aggregation [20,22,25,28,29,31,37,46].                                         excited state lifetime). In THF solution, the anisotropy of MEH–PPV
Distinctly from both previous solvent sets, the spectrum in ethyl                                   was measured to decrease from 0.4 to r=0.18–0.20 with a
acetate shows two prominent bands; a higher energy band                                             characteristic lifetime of 150 ps [31], considering the reported
slightly blue-shifted, lem = 552 nm, compared to good solvents                                      excited state lifetime of 300 ps, a value of /rSE0.13 can be
and a lower energy band with maximum at lem =590 nm                                                 calculated, also in good agreement with our results.
comparable to that observed in poor solvents. In all cases, it is
worth noting that the fluorescence emission is not the mirror
image of the absorption spectrum, indicating that distinct sites                                    3.2. Fluorescence spectra and anisotropy of MEH–PPV films
are absorbing and emitting [20,22,25,28,29,31,37,46].
    The steady-state fluorescence emission of MEH–PPV in dilute                                          The steady-state fluorescence spectra of MEH–PPV thin films
solutions of good solvents is attributed to the intrachain exciton                                  deposited on PET surface by dip coating from solutions of
being composed by three vibronic bands appearing at 564, 599                                        different solvents (Fig. 4) are red-shifted (lem (THF)= 587 nm,
and 632 nm [23,24,29–31,33,46]. The larger apparent Stokes shift                                    lem (chloroform)= 588 nm and lem (toluene)= 600 nm) compared
originates from conformational relaxation, energy migration or                                      to dilute solutions of good solvents and ethyl acetate. We were
energy transfer processes, as usually observed for conjugated                                       not able to prepare films from DMSO and acetonitrile solutions
polymers [38–42]. The relative intensity of these bands depends                                     due to the low MEH–PPV solubility. The emission spectra of
on the electron-phonon coupling, on the macromolecular con-                                         films are very similar to those observed in solution of poor
formation relaxation, and on the presence of aggregates that                                        solvents (Fig. 2), confirming that in solution of acetonitrile and
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Table 1
The steady-state anisotropy of MEH–PPV and PF in different solvents (subscript s) and of thin films (subscript f) spun from solutions with the same solvents.

                                              /rSs (MEH–PPV)                /rSs (PF)                                                        /rSf (MEH–PPV)         /rSf (PF)

                                                                            a band                        b band

 Ethyl acetate                                0.166 70.008                   0.057 0.01                   0.307 0.05                         0.02 70.01             nd
 Chloroform                                   0.114 70.006                   0.057 0.01a                  0.307 0.05a                        0.04 70.01             0.03 7 0.01a
 Toluene                                      0.116 70.005                  0.125 7 0.005                 0.35 70.01                         0.07 70.01             0.06 7 0.02
 THF                                          0.120 70.008                  0.130 7 0.005                 0.26 70.01                         0.01 70.01             0.04 7 0.01
 DMSO                                         0.093 70.008                  nd                            nd                                 nd                     Nd
 Acetonitrile                                 0.035 70.007                  nd                            nd                                 nd                     nd

nd: not determined.
      a
          Values in dichloromethane.



                                                                                                                                 0.125
                                     1.0
              Normalized intensity




                                                                                                                                 0.100




                                                                                                              Anisotropy / <r>
                                     0.8
                                                                                                                                 0.075
                                     0.6
                                                                                                                                 0.050
                                     0.4

                                     0.2                                                                                         0.025

                                     0.0                                                                                         0.000
                                        300   400       500       600          700                                                    550   575      600      625   650
                                                       λ / nm                                                                                       λ / nm

Fig. 4. Normalized fluorescence excitation (lem = 600 nm) and emission (lexc = 495                   Fig. 5. Fluorescence anisotropy (lexc = 495 nm), /rS, of MEH–PPV in films spun
nm) spectra of MEH–PPV films prepared by dip coating from solutions in—ethyl                         from solutions of ___,.ethyl acetate; —, chloroform; -n-n-, THF; and -J-J-,
acetate, -&-&-, chloroform; -n-n-, THF; and -o-o-, toluene.                                         toluene.




DMSO the emission is originated from interchain or                                                  processes or the highly unlikely conformational relaxation) are
intersegmental species. As already observed, the emission of                                        more efficient in solid state than in dilute solutions.
MEH–PPV films is not the mirror image of the excitation spectrum
because the system undergoes a fast energy migration from the
absorbing luminophores towards the lower emissive center in                                         3.3. Fluorescence spectra and anisotropy of PF in solutions
addition to the conformational relaxation processes, which are
expected to play a minor role in the solid state. Due to these                                          The electronic absorption spectra of PF were recorded in
processes there is a large apparent Stokes shift between emission                                   solutions (0.50 g L À 1) of toluene, chloroform, THF, acetonitrile,
and excitation spectra [17–23,29,33,34,56].                                                         ethyl acetate and DMSO (Fig. 6a). In toluene, chloroform, THF and
   There are several reasons for both red-shift and differences in                                  DMSO solutions the absorption band is broad and centered at
the intensity ratio of vibronic bands when MEH–PPV films are                                         around 390 nm. This band is attributed to the a-backbone
spun from different solutions: the spectral red-shift can be                                        conformations, which is disordered and usually observed in
produced by inner filter effect usually observed for thick or                                        solutions and in thermally stabilized films [13,14,41–43,58–60].
concentrated samples [54]. It can also be attributed to chain                                       A small red-shift is observed for THF and DMSO solutions
conformation. Because these are very thin films, the absorbance is                                   (labs = 394 nm). The electronic absorption band of PF in
low, and there is a great spectral shift between absorption and                                     acetonitrile solution is broader with a higher intensity peak
emission, chain conformation and its distribution is responsible                                    centered at 383 nm, a shoulder at the red-edge (402 nm) and a
for different emission spectra.                                                                     lower intensity sharper band at 436 nm. This band appears with
   Morphology of MEH–PPV films has been studied by several                                           low intensity in the other PF solutions. The absorption spectrum
high-resolution techniques such as near-field scanning optical                                       of PF in ethyl acetate solutions is quite different from other
microscopy, transmission electron microscopy, wide angle X-ray                                      solvents. There is a splitting of the absorption band in a higher
scattering, polarized optical microscopy [24,26,28,29,35] and                                       energy component with a lower intensity centered at 387 nm and
single molecule spectroscopy [32–34]. It is accepted that MEH–                                      a higher intensity component at its red-edge (405 nm) in addition
PPV films have a low degree of crystallinity although in the film                                     to the lower intensity band at 436 nm. This lowest energy band is
short-range chain packing domains are built in the nanometric                                       also relatively more intense than in other solvents. This spectrum
scale consistent with the emission of aggregated forms [20,24].                                     can be considered as an overlap of two absorptions; from the
   The steady-state fluorescence anisotropies of MEH–PPPV films                                       luminophores in chains with the a-conformations as in toluene,
spun from solutions of different solvents are: /rS= 0.07 for                                        chloroform, THF and DMSO and in chains with the b
toluene, /rS= 0.04 for chloroform, /rS= 0.02 for ethyl acetate and                                  conformations absorbing at the red-edge. It has been proposed
/rS= 0.01 for THF (Fig. 5, Table 1). These values are the average in                                that the 0–0 vibronic band at 436 nm corresponds to the
the 575–650 nm range where emission intensity is appreciable.                                       luminophores of polyfluorenes in b conformations with other
MEH–PPV anisotropy values are one order of magnitude smaller                                        vibronic components overlapped with the higher energy band of
than in solution indicating that fluorescence depolarization                                         a-conformers [13,14,41–43,58–60]. The b conformations have
regardless its mechanism (energy migration, energy transfer                                         more planar polymer backbone [13,14,41–43,58–60]. In
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                                                                                                   nm) indicating that although in a lower concentration, that
                                                                                                   species is present even in good solvents.
                                  1.0
          Normalized absorbance


                                                                                                       It is note worthy that while PF absorption spectrum in DMSO
                                  0.8                                                              solutions is similar to those in good solvents, the emission is
                                                                                                   similar to poor solvents. The emission spectra of PF in acetonitrile
                                  0.6                                                              show the spectral features typical of the b-chain conformation.
                                                                                                   The 0–0 phonon band (lem =438 nm) seems to be a mirror
                                  0.4                                                              image of the lower energy absorption band (the 0–0 absorption
                                  0.2                                                              band at 436 nm). This behavior has been already reported
                                                                                                   for the b-phase emission spectrum, which exhibits a sharper
                                  0.0                                                              profile, a smaller Stokes shift and a more resolved vibronic
                                    300       350             400           450                    structure [61].
                                                    λ / nm                                             The discussion of the solvent quality for PF can be done in the
                                                                                                   same terms as we did for MEH–PPV. Based on the Hildebrand
                                                                                                   solubility parameters THF, chloroform, dichloromethane and
                                  1.0                                                              toluene are good solvents for PF; DMSO has an intermediate
          Normalized intensity




                                                                                                   quality character while ethyl acetate and acetonitrile are poor
                                  0.8                                                              solvents. Therefore, we can say that good solvents for PF should
                                                                                                   have dispersive and dipolar components in the range16.8r
                                  0.6
                                                                                                   dd r18.0 J1/2 cm À 3/2 and 1.4r dp r5.7 J1/2 cm À 3/2, respectively. Be-
                                  0.4                                                              cause DMSO has the dispersive component (dd =18.4 J1/2 cm À 3/2) near
                                                                                                   the limit of the ideal range and a larger dipolar (dp =16.4 J1/2 cm À 3/2)
                                  0.2                                                              component, it behaves as a solvent with intermediate quality. On the
                                                                                                   other hand, for acetonitrile (a poor solvent) the dd =15.3 J1/2 cm À 3/2
                                  0.0                                                              present a low value, while the dipolar component is much larger
                                        350   400       450         500       550                  (dp =18.0 J1/2 cm À 3/2); for ethyl acetate (also a poor solvent) the
                                                    λ / nm                                         dd =15.8 J1/2 cm À 3/2 shows low value, while the dipolar component is
                                                                                                   within the adequate range (dp =5.3 J1/2 cm À 3/2). Taking all together
Fig. 6. (a) Normalized absoption and (b) fluorescence excitation and emission
spectra of PF in solutions of —, ethyl acetate; -&-&-, dichloromethane; -n-n-,
                                                                                                   we can say that the most important component controlling the PF
THF; -J-J-, toluene; - Â - Â -, DMSO; and - - -, acetonitrile. For emission, kexc = 395            solubility is the dispersive interactions. According to the experimental
or 400 nm. For excitation, kem = 465 nm.                                                           range of Hansen’s solubility [50–53], we can estimate the solubility
                                                                                                   parameters of PF as dd =18.5 J1/2 cm À 3/2, dp =2.0 J1/2 cm À 3/2 and dH =
                                                                                                   1.0 J1/2 cm À 3/2, with a global parameter about dd =18.6 J1/2 cm À 3/2.
                                                                                                   Again, the global Hildebrand solubility parameter cannot explain the
                                                                                                   solubility ordering of ethyl acetate since it has similar solubility
conclusion, based on the electronic absorption spectra, toluene,                                   parameter, although the photophysical behavior of the solution is
chloroform, THF, and DMSO are good solvents for PF but even for                                    completely different.
these solvents there is a low concentration of chains ordered as in                                    The steady-state fluorescence anisotropy was also determined
the b phase. In poor solvents (ethyl acetate) there is a larger                                    for PF in solutions (Fig. 7, Table 1). The steady-state anisotropy
amount of chains in the b phase, while in acetonitrile there is a                                  strongly depends on excitation and emission wavelengths.
mixture of isolated luminophores and a relative high                                               The general features of the dependence of anisotropy with
concentration of segments ordered as in the b phase.                                               wavelength are similar in all four solvents. THF and toluene are
    Fluorescence emission and excitation spectra were recorded in                                  shown in Fig. 7, as an example (similar spectra in
solutions of THF, toluene, dichloromethane and ethyl acetate at a                                  dichloromethane and in ethyl acetate are shown in Fig. 3 of the
concentration of 0.18 g L À 1 and are displayed in Fig. 6b. Excitation                             Supporting Information). The steady-state anisotropy increases
spectra are slightly wavelength-dependent in all cases, and are                                    with excitation wavelength almost in the same way irrespective
different from the absorption spectra (excitation spectra recorded                                 of whether it is detected in the second emission band (typically
at three different emission wavelengths, lem = 415, 440 and                                        around 440 nm) or in the emission shoulder around 465 nm. The
465 nm, are shown in Fig. 1 in Supporting Information).                                            maximum value of the anisotropy is reached in all cases in the
Considering that there are two emitting species, the appointed                                     excitation band corresponding to the b phase (420–440 nm).
differences can be attributed to different emission yields or                                      Values as high as 0.35 are measured in toluene and in ethyl
relative absorption of a and b phases.                                                             acetate for this band. This indicates absence of depolarization
    Fluorescence emission spectra of the PF solutions (THF,                                        effects such as torsional motions or energy transfer. In the UV
dichloromethane, toluene and ethyl acetate) were also recorded                                     edge of the excitation band, the value of the steady-state
for excitation wavelengths from lexc = 380–430 nm (Supporting                                      anisotropy is close to 0.1 in all cases. This indicates a great
Information—Fig. 2). The dependence of the emission spectra                                        depolarization.
with excitation wavelength is more pronounced. Upon excitation                                         On the other hand, the steady-state emission anisotropy shows
wavelength increase, the maximum corresponding to the a phase                                      very different values and wavelength dependence when the
decreases relative to the maximum around 438 nm, and the band                                      samples are excited at 400 nm, corresponding mainly to the
structure becomes more pronounced. In ethyl acetate, which                                         absorption of the isolated luminophores, than when they are
shows a poorly resolved band emission structure at excitation                                      excited at around 435 nm, in the maximum corresponding to the
wavelength below 400 nm, clearly resolved bands are displayed                                      b phase. In the first case, a low anisotropy is observed with slight
when excitation is performed above 410 nm, similar to other                                        maxima coinciding with the emission maxima at around 440 and
solvents [13,14,41,42,58]. In addition, the relative intensity of the                              465 nm. When excitation is performed at 435 nm, the values of
lower energy emission bands (465 and 500 nm) become higher                                         the steady-state anisotropy are higher and show an increase in
for excitation at the red-edge of the excitation band (lexc 4415                                   the tendency with wavelength.
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                       0.30                                       THF                                                           1800000                                      THF




                                                                                                    Emission intensity (a.u.)
                       0.25                                                                                                     1500000
                                                                                                                                1200000
                       0.20
          Anisotropy




                                                                                                                                 900000
                       0.15
                                                                                                                                 600000
                       0.10                                                                                                      300000
                       0.05                                                                                                              0

                       0.00                                                                                                              350          400            450        500
                              360 380 400 420 440 460 480 500                                                                                               λ / nm
                                          λ / nm

                                                                                                                                                                           Toluene
                        0.4                                                                                                     1200000




                                                                                                    Emission intensity (a.u.)
                                                             Toluene
                                                                                                                                1000000
                                                                                                                                 800000
          Anisotropy




                                                                                                                                 600000
                        0.2
                                                                                                                                 400000
                                                                                                                                 200000
                                                                                                                                         0
                                                                                                                                          350     400                450       500
                        0.0                                                                                                                                 λ / nm
                           350        400            450             500
                                                                                          Fig. 8. Spectral deconvolution of excitation and emission spectra of PF in solution
                                            λ / nm                                        of THF and toluene. Dotted lines correspond to total excitation (lem = 465 nm) and
                                                                                          emission (lexc = 400 nm) spectra. Points correspond to the components of the a
Fig. 7. The steady-state anisotropy of PF solutions in THF and toluene. Dotted lines
                                                                                          phase (hollow squares, excitation, and full squares, emission, f1, see text and Eqs.
correspond to excitation (lem = 465 nm) and emission (lexc = 400 nm) spectra.
                                                                                          (3)–(5)) and the b phase (hollow circles, excitation, and full circles, emission, f2,
Scatter points represent the steady-state anisotropy in the excitation spectra,
                                                                                          see text and Eqs. (3)–(5)).
lem =464 nm (empty squares) and lem = 437 nm (empty circles), and in the
emission spectra, lexc = 400 nm (full squares) and lexc =434 nm (full circles).




                                                                                                                                1.0
   Results are compatible with homo energy transfer between
                                                                                                    Normalized intensity




isolated luminophores being the main depolarization pathway.                                                                    0.8
   Taking into account that anisotropy is additive and that there
are at least two species responsible for the emission, we can write                                                             0.6

/rS ¼ r1 f1 þr2 f2                                                              ð3Þ                                             0.4
f1 þ f2 ¼ 1                                                                     ð4Þ
                                                                                                                                0.2
   In the previous equations, fi are the fractions of the emission
intensity corresponding to each species, and ri are their respective                                                            0.0
steady-state anisotropies, which, for the following calculations,                                                                  350          400              450            500
will be considered to be wavelength-independent in the excita-                                                                                         λ / nm
tion and emission wavelength ranges measured.
   Eqs. (3) and (4) can be solved for the intensities of each                             Fig. 9. Fluorescence excitation (lem = 465 nm) and emission (lexc = 400 nm) spectra
component, thus allowing calculation of their emission and                                of PF films spun from solutions of: -&-&-, dichloromethane; -n-n-, THF -J-J-
                                                                                          toluene.
excitation spectra
             /rS À r2
I1 ¼ It f1 ¼            ;
              r1 À r2
                       r1 À /rS                                                           [45], are in line with our measurements. On the other hand, the b
I2 ¼ It À I1 ¼ It f2 ¼                                                          ð5Þ       phase displays very high anisotropy, indicating that hardly any
                         r1 À r 2
                                                                                          depolarization occurs once excitation energy reaches this center.

    Arbitrarily we identify 1 with the b phase and 2 with the a
phase. The deconvoluted spectra are shown in Fig. 8 for the case of                       3.4. Fluorescence spectra and anisotropy of PF films
THF and toluene. Values of r1 and of r2 are listed in Table 1.
    For the isolated luminophore of PF in solution the anisotropy is                         The fluorescence excitation and emission spectra of PF
low. Reported values of /rS=0.096 (lexc o380 nm) measured for                             spun from solutions of toluene, dichloromethane and THF
poly[9,9-di(ethylhexyl)fluorene] in methylcyclohexane dilute so-                           are shown in Fig. 9. Excitation spectra show differences in the
lution [44] and /rS =0.10 for PF in methylcyclohexane solution                            relative contributions of b-phase components, these decrease the
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                      0.07                                                                                                 24000

                      0.06                                                                                                 23000




                                                                                                       Wavenumber (cm-1)
                                                                                                                           22000
                      0.05
                                                                                                                           21000
         Anisotropy




                      0.04
                                                                                                                           20000
                      0.03                                                                                                 19000
                      0.02                                                                                                 18000
                                                                                                                           17000
                      0.01
                                                                                                                           16000
                      0.00                                                                                                         14    15    16    17     18   19   20
                          350             400            450            500
                                                                                                                                               δd(J1/2 cm-3/2)
                                                λ / nm
                       Cossiello et al.                                                                                     Cossiello et al.

                                                                                             Fig. 11. Energy for the absorption and emission maxima of MEH–PPV in solution
Fig. 10. The steady-state anisotropy of PF films spun from THF solution. Dotted
                                                                                             of different solvents as a function of the Hildebrandt’s dispersion parameter, dd.
lines correspond to total excitation (lem = 465 nm) and emission (lexc =400 nm)
                                                                                             Upper curves and squares: absorption data. Lower curves and circles: emission
spectra. Points represent the steady-state anisotropy in the excitation spectra,
                                                                                             data. Hollow symbols are data from Ref. [66] and filled symbols are data from this
lem =464 nm (hollow triangles) and lem = 437 nm (full triangles), and in the
                                                                                             work. Values for toluene and chloroform from both sources coincide. Linear
emission spectra, lexc = 400 nm (hollow squares) and lexc = 434 nm (full squares).
                                                                                             regression line and 95% confidence limits are shown for each correlation.




direction of dichloromethane, THF and toluene. Emission                                      Emission spectra show bands corresponding to these two species.
spectra are also quite similar in the three films, with a slight                              No other emitting species is observed due to excitation at 495 nm,
bathochromic shift (1 nm) in the series toluene, dichloromethane                             where the weak luminescent interchain aggregates do not
and THF. The emission spectra show absence of the a-phase                                    appreciably absorb [56].
emission, as reported by several authors [13,14,62].                                             In most cases, preferential solvation has been analyzed in
    The emission spectra show a slight change with emission                                  terms of planarity [20,23–25] instead of solubility parameters or
wavelength; a decrease in the tendency in the contribution of the                            other representations of polymer/solvent interactions [46,48–53].
438 nm maximum with respect to the 465 nm one is noted upon                                  Here we analyze the solvent quality in terms of the components of
with increase in the excitation wavelength (see Fig. 4 in                                    the Hildebrand solubility parameters for all solvents [50–53]. By
Supporting Information). This behavior parallels that in solution                            comparing these components with profiles of the emission
and is a consequence of increased absorption of the b-phase                                  spectra, we conclude that good solvents for MEH–PPV show
component with increase in the excitation wavelength [63].                                   values of the dispersive and dipolar components in the range
    The emission spectra in the solid state highly resemble the                              16.8r dd r18.0 J1/2 cm À 3/2 and 1.4r dp r5.7 J1/2 cm À 3/2, respec-
emission obtained for the f2 component (b phase), by spectral                                tively [50–53]. Solvents such as toluene, chloroform and THF have
deconvolution of the steady-state spectra using the wavelength-                              dd and dp within these ranges while ethyl acetate has a dipolar
dependent total anisotropy (Fig. 8).                                                         component as large as dp = 15.8 J1/2 cm À 3/2. Nevertheless, these
    The steady-state excitation and emission anisotropies of PF                              four solvents have the global solubility parameter in the range
films spun from THF solution are shown in Fig. 10 (similar results                            18.7–18.2 J1/2 cm À 3/2, which evidences the failure of the global
in films spun from dichloromethane and from toluene solutions                                 Hildebrand parameters to explain the MEH–PPV solubility
are displayed in Fig. 5 of the Supporting Information). The values                           differences observed in these solvents. On the other hand,
are quite low, very similar, and they show practically no                                    matching of dispersive and dipolar components adequately
dependence with wavelength in all films, either in excitation or                              describes the solubility. In addition, both acetonitrile (dd = 15.3
in emission. Any variation tendency is here within the                                       J1/2 cm À 3/2, dp = 18.0 J1/2 cm À 3/2) and DMSO (dd =18.4 J1/2 cm À 3/2,
experimental uncertainty in /rS determination of 70.02. The                                  dp = 16.4 J1/2 cm À 3/2) have the Hildebrand solubility parameters
low values of the steady-state anisotropy in the solid phase point                           out of the range adequate for a good solubility of MEH–PPV, which
to a very fast pooling of the electronic excitation energy prior to                          was estimated in a previous work to be 18.7 J1/2 cm À 3/2[46]. From
emission. The facts that /rS values are lower in films than those                             the knowledge of the experimental range of the Hansen’s
for the a phase in solution, they are wavelength-independent and                             parameter [53–55], we estimate the partial solubility parameters
there is no emission originated in the a phase in the films, all                              for the MEH–PPV as dd =18.0 J1/2 cm À 3/2, dp =4.0 J1/2 cm À 3/2,
indicate complete energy transfer to the b phase prior to                                    dH =3.0 J1/2 cm À 3/2 and d =18.7 J1/2 cm À 3/2.
emission, contrary to what is observed in dilute solution.                                       Absorption and emission maxima show a good correlation
Evidently, the molecular proximity in the solid state is                                     with the Hildebrandt’s dispersion component, as observed in
responsible for this fact.                                                                   Fig. 11. Correlation with polarity, hydrogen bonding and the total
                                                                                             Hildebrandt solubility parameters are definitely uncorrelated
                                                                                             (see Supporting information). Data obtained in this work
4. Discussion                                                                                are integrated with literature data, with which they perfectly
                                                                                             match [66].
   The absorption spectra of MEH–PPV show a great difference in                                  Values for acetonitrile in absorption and in emission as well as
acetonitrile (the worst solvent used) in comparison with other                               data for DMSO in emission fall quite apart from the correlation
solvents. The absorption at 425 nm reflects the chain collapse in                             because the spectroscopically active species is not the same as in
acetonitrile [64]. The spectral shift in other solvents can be                               the other cases. The good correlation with dispersion forces and
attributed to different amount of isolated and aggregated                                    not with polar or hydrogen bonding components of Hildebrandt’s
intrachain chromophores and solvatochromic effects [31,65,66].                               parameters indicate that dispersion is the most determining
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interaction that regulates the spectroscopic features of MEH–PPV               of a 30 nm3 molecule (ca. 10 monomer units either of PF or of
in solution. The traditional interpretation of solvent shift                   MEH–PPV) in a 1 mPa s viscous medium at 300 K. Therefore, the
produced by dispersion interactions is that they shift the energy              rotational correlation time of both, MEH–PPV and PF surely
of electronic states involved in the transitions. In this case,                exceeds a value of various ns in solvents used and rotation is
polymer–solvent interactions also change the effective conjuga-                discarded in films.
tion length [65,67], and thus enhance the energy of the electronic                 Anisotropy depolarization by energy migration was measured
transition.                                                                    in various conjugated polymers [39,56]. It has a very fast
   Films of MEH–PPV deposited from different solvents                          component ( o50 ps), accounting for most of the depolarization
show exclusively emission from aggregates. There is a gradual                  (from the maximum value of 0.4–0.35 to a value near 0.15), and a
change in the spectra that is in contrast with sharp differences               slower component (ca. 100 ps) with a small contribution to the
observed in the molecular orientation when isolated                            depolarization and a residual value of the anisotropy in the range
polymer chains are deposited on glass from dichloromethane or                  0.05–0.1 [31,55]. This residual value is higher for stiffer polymer
toluene solutions [67]. This difference is easily explained                    backbones, accounting for the fact that almost parallel energy
by the effect of chain interactions in compact polymer films                    migration does not contribute to depolarization [68]. For example,
compared to isolated molecules. The different spectral distribu-               the energy migration in MEH–PPV is approximately three times
tion displayed in emission can be explained by heterogeneity in                faster than in polythiophenes due to the disordered chain
chain conformation in solution, which is transferred to the films               conformation in MEH–PPV and it plays an important role on the
[65,67].                                                                       anisotropy depolarization [38]. Nevertheless, in the case of PF,
   The absorption and emission spectra of PF show two distinct                 although energy migration is an important pathway for the
bands. In absorption a sharp peak around 435 nm and a broader                  anisotropy depolarization, some anisotropy remains even in the
band at ca 400 nm. The emission shows peaks at 420 and 445 nm,                 solid state, which means that the energy migration and other
and a shoulder at 465 nm. The different nature of the bands can be             energy transfer processes are not totally efficient [39,40,42,
better distinguished by the steady-state emission and excitation               44,47,59].
anisotropies. This is the basis of the spectral decomposition                      When we compare the anisotropy in solutions and in the solid
shown in Fig. 8. The electronic transitions identified by the                   state there is a remarkable increase in the depolarization of MEH–
squares in Fig. 8 are attributed to the a phase [13,14,41–                     PPV emission on moving from poor solvents (DMSO and
43,45,59,60,62], whereas the ones identified by the circles are                 acetonitrile) to polymer films, independently of the solvent (Table
attributed to the b phase [13,14,41–43,45,59,60,62]. The consis-               1). Aggregation decreases /rS for MEH–PPV either in poor
tency of the spectra derived in different media and their                      solvents or in films. Because the rotation and conformational
agreement with published data, validates the analysis. The                     relaxation do not play any important role under these conditions,
tendency of fluorescence anisotropy with excitation wavelength                  the fluorescence depolarization should be more important due to
was reported previously [45].                                                  the greater efficiency of the interchain and intersegmental energy
   We have shown here that the photoluminescence features of                   transfer. Therefore, regardless of which MEH–PPV system we
MEH–PPV in solution can be explained by solute solvent                         analyze (solution or films), fast energy migration/energy transfer
dispersion interactions. For PF in solution we have shown that                 processes are always efficient and consequently its anisotropy
the absorption and emission spectra of a and b chains can be                   depolarization is very effective.
separated using the steady-state anisotropy. For both polymers,                    The behavior of the PF anisotropy is different compared to
the spectral features in films are dependent on the chain                       MEH–PPV. In this system, the a and the b phases behave
conformation in the solvent from which they were spun, as                      quite different; while the homo energy transfer between a
already reported in the literature.                                            segments highly depolarizes its emission, the same homo
   To discuss the changes in the steady-state anisotropy in                    transfer within the b phase is quite ineffective. Furthermore, the
solutions and in thin films, we must consider the polymer                       additivity of anisotropy accounts for the observed wavelength
properties such as rotational correlation time of the polymer                  dependence of this parameter in excitation and in emission. This
chain, the fluorescence lifetime, the efficiency of the energy                   additivity means that the two chain conformations behave
migration, the efficiency of the conformational relaxation and                  independent of each other with respect to the electronic
the energy transfer processes of the electronic excited states                 energy relaxation, evidencing the lack of energy transfer between
[9,13,14,18–20,23–26,34,41–43,47,55,56,58,59]. All these pro-                  them. There is a remarkable coincidence between the excitation
cesses can contribute to the decay in the steady-state anisotropy.             and emission spectra of the high anisotropy component
Molecular relaxation is expected to play a minor role in rigid                 and the spectrum of the b phase reported in the literature [60].
luminophores with parallel absorption and emission dipoles.                    These spectroscopic evidences confirm the ordered structure of
Indeed this is the case in conjugated polymers that have a                     the b phase, and the lack of electronic energy exchange with
maximum anisotropy close to 0.4 as in the case of both MEH–PPV                 the a phase in solution. Finally, the anisotropy in films
and PF [9,13,14,18–20,23–26,34,41–43,47,55,56,58,59]. Confor-                  is almost independent of the solvent quality and there is
mational relaxation was demonstrated to be only partially                      a very effective energy transfer from the a to the b phases
responsible for depolarization of fluorene oligomers and depolar-               (Table 1).
ization after excitation in the red-edge of the absorption spectrum
[18,39,40,43,44,47,59,61]. Furthermore, conformational relaxa-
tion is expected to play a still minor role in the depolarization              Acknowledgements
of the conjugated and more rigid MEH–PPV [16,18,21,22,
25,26,28,30,32,36].                                                                TDZA and RFC thank FAPESP, MCT/CNPq/INEO, CAPES and
   Molecular rotation is the other parameter that contributes to               FAEPEX/Unicamp for financial support and fellowships. PFA is a
depolarization if it can take place with comparable rate or faster                                                      ´
                                                                               member of Carrera del Investigador Cientıfico (Research Staff)
than the decay of the excited state. Molecular rotation is excluded                                                                   ´
                                                                               from CONICET (Consejo Nacional de Investigaciones Cientıficas y
for the conjugated polymers studied here. On one hand, the                       ´
                                                                               Tecnicas, Argentina) and thanks support by grants PIP 5470
polymers have a sub-nanosecond excited state lifetime. On the                  (CONICET), PICT 10621 (ANPCyT, Argentina), X086 (UBA), and
other hand, 1 ns is approximately the rotational correlation time              CAPGBA03/02 (Brazil-Argentina).
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Appendix A. Supplementary material                                                            [33] T. Huser, M. Yan, J. Photochem. Photobiol. A: Chem 144 (1) (2001) 43.
                                                                                              [34] S.S. Sartori, S. De Feyter, J. Hofkens, M. Van der Auweraer, F. De Schryver, K.
                                                                                                   Brunner, J.W. Hofstraat, Macromolecules 36 (2) (2003) 500.
    Supplementary data associated with this article can be found                              [35] S.H. Chen, A.C. Su, Y.F. Huang, C.H. Su, G.Y. Peng, S.A. Chen, Macromolecules
in the online version at doi:10.1016/j.jlumin.2009.10.006.                                         35 (11) (2002) 4229.
                                                                                              [36] I. Soutar, C. Jones, D.M. Lucas, L. Swanson., J. Photochem. Photobiol. A: Chem.
                                                                                                   102 (1) (1996) 87.
                                                                                              [37] K.M. Gaab, C.J. Bardeen., J. Phys. Chem. A 108 (49) (2004) 10801.
References                                                                                    [38] A. Synak, G. Gondek, P. Bojarski, L. Kulak, A. Kubicki, M. Szabelski, P. Kwiek.,
                                                                                                   Chem. Phys. Lett. 399 (1–3) (2004) 114.
                                                                                                                                       ¨
                                                                                              [39] M. Hennecke, T. Damerau, K. Mullen, Macromolecules 26 (13) (1993) 3411.
 [1] S. Miyate, H.S. Nalva, in: Organic Electroluminescent Materials and Devices, ,           [40] L.M. Herz, R.T. Phillips., Phys. Rev. 61 (20) (2000) 13691.
     1998.                                                                                    [41] F.B. Dias, A.L. Mac  -anita, J.S. Melo, H.D. Burrows, R. Gunter, U. Scherf, A.P.
                                                                                                                                                                ¨
 [2] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H.                      Monkman., J. Chem. Phys. 39 (15) (2003) 7119.
     Friend, P.L. Burns, A.B. Holmes., Nature 347 (6293) (1990) 539.                                                                 -
                                                                                              [42] F.B. Dias, J. Morgado, A.L. Mac anita, P.C. Costa, H.D. Burrows, A.P. Monkman,
 [3] L. Akcelrud., Prog. Polym. Sci. 28 (6) (2003) 875.                                            Macromolecules 39 (17) (2006) 5854.
 [4] A.M. Machado, J.D.D. Neto, R.F. Cossiello, T.D.Z. Atvars, L. Ding, F.E. Karasz, L.       [43] M. Grell, D.D.C. Bradley, G. Ungar, J. Hill, K.S. Whitehead, Macromolecules 32
     Akcelrud, Polymer 46 (8) (2005) 2452.                                                         (18) (1999) 5810.
 [5] A.M. Machado, M. Munaro, T.D. Martins, L.Y.A. Davila, R. Giro, M.J. Caldas,              [44] A.J. Cadby, P.A. Lane, H. Mellor, S.J. Martin, M. Grell, C. Giebeler, D.D.C.
     T.D.Z. Atvars, L.C. Akcelrud, Macromolecules 39 (9) (2006) 3398.                              Bradley, M. Wohlgenannt, C. An, Z.V. Vardeny., Phys. Rev. B 62 (23) (2000)
 [6] S.H. Yang, C.F.J. Lee., Optoelectron. Adv. Mater. 9 (4) (2007) 2078.                          15604.
 [7] L.H. Chang, Y.D. Lee, C.T. Chen, Macromolecules 39 (9) (2006) 3262.                      [45] H.L. Vaughan, F.M.B. Dias, A.P. Monkman., J. Chem. Phys. 122 (1) (2005)
 [8] H.A. Zhang, Y. Li, Q. Jinag, M.G. Xie, J.B. Peng, Y. Cao., J. Mater. Sci. 42 (12)             014902.
     (2007) 4476.                                                                             [46] R.F. Cossiello, L. Akcelrud, T.D.Z. Atvars., J. Braz., Chem. Soc. 16 (1) (2005) 74.
 [9] M.B. Ramey, J.A. Hiller, M.F. Rubner, C.Y Tan, K.S. Schanze, J.R. Reynolds,              [47] G. Fytas, H.G. Nothofer, U. Scherf, D. Vlassopoulos, G. Meier, Macromolecules
     Macromolecules 38 (2) (2005) 234.                                                             35 (2) (2002) 481.
[10] Z. Gu, Y.-J. Bao, Y. Zhang, M. Wang, Q.-D. Shen, Macromolecules 39 (9) (2006)            [48] M.L. Winnik., Photophysical and Photochemical Tools in Polymer Science, vol.
     3125.                                                                                         182, D. Riedel, New York, 1986.
[11] E.E. Nesterov, Z.G. Zhu, T.M. Swager., J. Am. Chem. Soc. 127 (25) (2005) 10083.          [49] J.L. Halary, L. Monnerie., Photophysical and Photochemical Tools in Polymer
[12] J. Gruber, R.W.C. Li, L.H.J.M.C. Aguiar, T.L. Garcia, H.P.M. Oliveira, T.D.Z. Atvars,         Science, vol. 182, D. Riedel, New York, 1986, p. 589.
     A.F. Nogueira., Synth. Met. 156 (24) (2006) 104.                                         [50] M.H. Hansen.Ind. Eng. Chem. Prod. Res. Dev. 8 (1969) 2.
[13] U. Scherf, D. Neher (Eds.), Polyfluorenes Adv. Polym. Sci., vol. 212, Springer-           [51] A.F.M. Barton, in: Handbook of Solubility Parameters and Other Cohesion
     Verlag, Berlin, 2008.                                                                         Parameters, CRC Press, Boca Raton, Florida, 1983, p. 232.
[14] U. Scherf, E.J.W. List., Adv. Mater. 14 (7) (2002) 477.                                  [52] A.F.M. Barton., Chem. Rev. 75 (6) (1975) 731.
[15] T. Kawase, T. Shimoda, C. Newsome, H. Sirringhaus, R.H. Friend., Thin Solid              [53] J. Brandrup, E.H. Immergut, E.A. Grulke, in: Polymer Handbook, Fourth ed.,
     Films. 438 (2003) 279.                                                                        Academic Press, New York, 1993, p. 2336.
[16] E.E. Gurel, Y. Pang, F.E. Karasz., Thin Solid Films. 417 (1–2) (2002) 147.               [54] M.M.L. Grage, P.W. Wood, A. Ruseckas, T. Pullerits, W. Mitchell, P.L. Burn,
[17] S.R. Amrutha, M.J. Jayakannan., Phys. Chem. B 110 (9) (2006) 4083.                            I.D.W. Samuel, V. Sundstrom., J. Chem. Phys. 118 (16) (2003) 7644.
[18] R. Deans, J. Kim, M.R. Machacek, T.M. Swager., J. Am. Chem. Soc. 122 (35)                [55] C.J. Collison, L.J. Rothberg, V. Treemaneekarn, Y. Li, Macromolecules 34 (7)
     (2000) 8565.                                                                                  (2001) 2346.
[19] D.T. McQuade, J. Kim, T.M. Swager., J. Am. Chem. Soc. 122 (24) (2000) 5885.              [56] U. Lemmer, S. Heun, R.F. Mahrt, U. Scherf, M. Hopmeier, U. Siegner, E.O. Gobel,
[20] B.J. Schwartz., Ann. Rev. Phys. Chem. 54 (2003) 141.                                                            ¨
                                                                                                   K. Mullen, H. Bassler., Chem. Phys. Lett. 240 (4) (1995) 373.
[21] T. Zyung, S.D. Jung, D.H. Hwang., Synth. Met. 117 (1-3) (2001) 223.                      [57] J.R. Lakowicz, in: Principles of Fluorescence Spectroscopy, Academic Press,
[22] S. Lee, J.Y. Lee, H. Lee., Synth. Met. 101 (1–3) (1999) 248.                                  New York, 1999.
[23] T.Q. Nguyen, R.Y. Yee, B.J. Schwartz., J. Photochem. Photobiol. A: Chem. 144             [58] S.H. Chen, A.C. Su, C.H. Su, S.A. Chen, Macromolecules 38 (2) (2005) 379.
     (1) (2001) 21.                                                                           [59] S.H. Chen, H.L. Chou, A.C. Su, S.A. Chen, Macromolecules 37 (18) (2004) 6833.
[24] T.Q. Nguyen, B.J. Schwartz, R.D. Schaller, J.C. Johnson, L.F. Lee, L.H. Haber, R.J.      [60] M.J. Winokur, J. Slinker, D.L. Huber., Phys. Rev. B 67 (18) (2003) 184106.
     Saykally., J. Phys. Chem. B 105 (22) (2001) 5153.                                                    ¨
                                                                                              [61] J.G. Muller, E. Atas, C. Tan, K.S. Schanze, V.D. Kleiman., J. Am. Chem. Soc. 128
[25] R.D. Schaller, L.F. Lee, J.C. Johnson, L.H. Haber, R.J. Saykally, J. Vieceli, I.              (12) (2006) 4007.
     Benjamin, T.Q. Nguyen, B.J. Schwartz., J. Phys. Chem. B 106 (37) (2002) 9496.            [62] D. Marushchak, L.B.A. Johansson., J. Fluorescence 15 (5) (2005) 797.
[26] U. Jeng, C.H. Hsu, H.S. Sheu, H.Y. Lee, A.R. Inigo, H.C. Chiu, W.S. Fann, S.H.           [63] H. Azuma, K. Asada, T. Kobayashi, H. Naito., Thin Solid Films 509 (1–2) (2006)
     Chen, A.C. Su, T.L. Lin, K.Y. Peng, S.A. Chen, Macromolecules 38 (15) (2005)                  182.
     6566.                                                                                    [64] H.Z. Tang, M. Fujiki, M. Motonaga, Polymer 43 (23) (2002) 6213.
                                    ´       ´
[27] C.Y. Yang, F. Hide, M.A. Dıaz-Garcıa, A.J. Heeger, Y. Cao, Polymer 39 (11)               [65] R. Traiphol, N. Charoenthai, T. Srikhirin, T. Kerdcharoen, T. Osotchan, T.
     (1998) 2299.                                                                                  Maturos, Polymer 48 (3) (2007) 813.
[28] J. Teetsov, D.A.V. Bout., J. Phys. Chem. B 104 (40) (2000) 9378.                                                                                    ´        ¨
                                                                                              [66] O. Narwark, S.C.J. Meskers, R. Peetz, E. Thorn-Csanyi, H. Bassler., Chem. Phys.
[29] C.R. McNeill, H. Frohne, J.L. Holdsworth, P.C. Dastoor., Nano Lett. 4 (12) (2004)             294 (1) (2003) 1.
     2503.                                                                                    [67] S. Quan, F. Teng, Z. Xu, L. Qian, Y. Hou, Y. Wang, X. Xu., Eur. Polym. J. 42 (1)
[30] V. Cimrova, D. Neher., Synth. Met. 76 (1–3) (1996) 125.                                       (2006) 228.
[31] T.Q. Nguyen, V. Doan, B.J. Schwartz., J. Chem. Phys. 110 (8) (1999) 4068.                [68] R. Traiphol, P. Sanguansat, T. Srikhirin, T. Kerdcharoen, T. Osotchan,
[32] D.Y. Kim, J.K. Grey, P.F. Barbara., Synth. Met. 156 (2–4) (2006) 336.                         Macromolecules 42 (20) (2006) 1165.

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Journal of Luminescence

  • 1. This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright
  • 2. Author's personal copy ARTICLE IN PRESS Journal of Luminescence 130 (2010) 415–423 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin Study of solvent-conjugated polymer interactions by polarized spectroscopy: MEH–PPV and Poly(9,90 -dioctylfluorene-2,7-diyl) Rafael F. Cossiello a, Mariano D. Susman b, Pedro F. Aramendıa b,n, Teresa D.Z. Atvars a,n ´ a ´ Universidade Estadual de Campinas, Instituto de Quımica, Caixa Postal 6154, Campinas, 13084-971 SP, Brazil b ´ ´ ´ ´ ´ ´ INQUIMAE and Departamento de Quımica Inorganica, Analıtica y Quımica Fısica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellon 2, Ciudad Universitaria, C1428EHA, Buenos Aires, Argentina a r t i c l e in fo abstract Article history: Absorption, emission, and anisotropy measurements were performed on poly-[2-methoxy-5(20 - Received 3 April 2009 ethylhexyloxy)–1,4-phenylenevinylene] (MEH–PPV) and poly-(9,90 -dioctylfluorene-2,7-diyl) (PF) solu- Received in revised form tions of various solvents, and in thin films deposited from them. The good correlation of MEH–PPV 6 October 2009 absorption and emission energy with Hildebrandt’s dispersive parameter indicate that dispersive forces Accepted 19 October 2009 regulate the effective extent of the luminophore. The excitation and the emission spectra of a and b Available online 27 October 2009 chains can be distinguished in PF solutions using the steady-state anisotropy. PF films show greater Keywords: memory effect from the solutions from which they were spun than MEH–PPV. Anisotropy of MEH–PPV Fluorescence anisotropy is very low, both in solutions and in films reflecting efficient energy migration. Anisotropy of PF in Solvent effects solutions and films demonstrates great differences in energy transfer efficiency within the a and b Solutions and films phases, while there is no energy transfer between these chain conformations. Conjugated polymers Polyfluorene & 2009 Published by Elsevier B.V. MEH–PPV 1. Introduction consequences such as the presence of defects, stress, non-uniform thickness, etc, that greatly lower electrical and electrolumines- Conjugated polymers attracted much attention in the last cence performance [10–31]. thirty years since the discovery of the electron conductivity of There are several experimental techniques useful in studies of poly(acetylene) and later of poly(aniline) and several other macromolecular chain conformations: scanning electron micro- polymers and copolymers [1,2]. More recently, during the nine- scopy, atomic force microscopy, small angle X-ray scattering and ties, the scientific and technological interests were renewed by wide angle X-ray diffraction [26,27], near-field optical microscopy the discovery of the electroluminescence of conjugated polymers [24,25,29], confocal microscopy and single molecule spectroscopy and copolymers [2]. [32–34], optical microscopy, and luminescence spectroscopy These conjugated polymers are materials with a great variety [32–35]. In particular, the steady-state photoluminescence aniso- of chemical structures, synthesized by distinct procedures, with tropy provides information on molecular conformation and multiple functionalities conferring different colors to operating reorientation during the lifetime of the excited sate [35–43]. devices, from violet to red [3–14]. Each one of these particular In multiple chromophoric systems, such as conjugated poly- chemical structures may have distinct physical, chemical and mers, the excitation energy can be converted to fluorescence electrical properties. In addition, because of the lower melting emission whose anisotropy depends strongly on the chain point (in the case of semicrystalline materials) and lower glass conformational relaxation, on the energy transfer processes and transition temperature compared to inorganic semiconductors, on the energy migration probability. In general, when the energy the conjugated electroluminescent polymers can easily be migration is faster than the emission decay, a strong depolariza- processed and thin films can be prepared by in situ polymeriza- tion of the fluorescence takes place [36–43]. tion or by deposition on a substrate using spin coating, self Film morphology depends on several processing parameters assembly, dip coating and casting [1]. The facility to process [17–29,43,44], but it is particularly influenced by the solvent organic polymers has, on the other hand, various morphological used. Polymer chains form coils in unfavorable solvents to minimize their exposure to the solvent and this has a strong impact on its photophysical properties [23,31,33,36,45–47]. For n example, the excimer emission can be enhanced in poor solvents Corresponding authors. Tel.: + 55 19 35214729. ´ E-mail addresses: pedro@qi.fcen.uba.ar (P.F. Aramendıa), while it is minimized in good solvents that induce a more tatvars@iqm.unicamp.br (T.D.F. Atvars). extended conformation [48,49]. The reason for the changes in 0022-2313/$ - see front matter & 2009 Published by Elsevier B.V. doi:10.1016/j.jlumin.2009.10.006
  • 3. Author's personal copy ARTICLE IN PRESS 416 R.F. Cossiello et al. / Journal of Luminescence 130 (2010) 415–423 O under mild heating until complete dissolution. All solutions were maintained in dark in a sealed flask. Samples of thin films were prepared by dip coating or by spin coating of poly(ethylene N terephthalate) (PET) slices (1 Â 2 Â 0.5 cm) into the polymer n C8H17 C8H17 solutions. Samples were dried for 2 h at room temperature after O dipping to remove residual solvent. PF MEH-PPV Fig. 1. Chemical structures of poly-(9,90 -dioctylfluorenyl-2,7-diyl)-end capped 2.2. Methods with dimethyl phenylene (PF) and poly-[2-methoxy-5-(20 -ethylhexyloxy)-p- phenylene-vinylene] (MEH–PPV). Electronic absorption spectra were recorded in a Hewlett– Packard-8452A UV–vis spectrometer. When necessary, polarized absorption was detected by placing film polarizers before and after the sample. Each polarized spectrum is referenced to the photo and electroluminescence properties in films prepared from baseline obtained with the corresponding orientation of the different solvents is that a part of the conformation in solution polarizer. The steady-state fluorescence spectroscopy was per- should be maintained in the solid state, which is called the formed on a PC1 photon counting spectrofluorimeter from ISS Inc. memory effect [30–34]. or on a PTI quantamaster spectrofluorometer. Slits were selected Hildebrand solubility parameter is often used to quantify for a spectral resolution between 4 and 8 nm in excitation and in polymer–solvent interactions either in the global form or through emission. For fluorescence emission and excitation spectra of its specific components that describe the hydrogen bonding solutions, a square quartz cuvette, 1 cm side, was employed. interaction (dH), the dipolar interactions (dp) and the dispersive Spectra of films were recorded in a back-face configuration to interactions (dd) [50–53]. These components account better for diminish stray light reaching the detector. specific polymer–solvent interactions. For example, although Fluorescence anisotropy was measured using a couple of film toluene (d =18.2 J1/2 cm À 3/2) and ethyl acetate (d =18.2 J1/2 cm À 3/2) polarizers on the excitation and emission beams in either of the have the same Hildebrand solubility parameters, their compo- spectrofluorimeters operating in a L-format. The fluorescence nents (dH, dd, dp) are different as well as their solvation ability anisotropy is calculated according to Eq. (1), where I is the for poly-[2-methoxy-5(20 -ethylhexyloxy)–1,4-phenylenevinylene] fluorescence intensity at one specific wavelength and ‘‘v’’ and ‘‘h’’ (MEH–PPV) [46]. are the relative orientation of the polarizers, denoting perpendi- The knowledge of the solvation ability is particularly impor- cular and on the plane of incidence, respectively. The first letter of tant when multi-layer electroluminescent devices are desirable. the subscript represents the orientation of the excitation polarizer Here the morphology of one layer must be maintained during the and the second that of the emission polarizer [54]: deposition of the subsequent layers; a condition that requires a good solvent for one material and a poor solvent for the other. Ivv À GIvh rðlÞ ¼ ð1Þ Therefore, in the present work we explore the applicability of the Ivv þ2GIvh Hildebrand parameter to discuss the behavior of MEH–PPV and where Ivv and Ivh are the spectral intensity determined using the poly-(9,90 -dioctylfluorene-2,7-diyl) (PF) (Fig. 1) in solutions of polarizer and the analyzer in vertical positions and with the toluene, chloroform, dichloromethane, tetrahydrofuran (THF), polarizer in the vertical and the analyzer in the horizontal ethyl acetate, dimethylsulfoxide (DMSO), and acetonitrile and position, respectively. G is a correction factor of instrumental the consequence of the solvent–polymer interaction on the response given by Eq. (2): photoluminescence features of polymers in solution and in films deposited from different solvents. To analyze the solvent quality Ihv ðlemÞ GðlemÞ ¼ ð2Þ we use the steady-state spectroscopy and fluorescence anisotropy Ihh ðlemÞ measurements in both dilute polymer solutions and thin films where Ihv and Ihh are the spectra recorded with polarizer in prepared by dip or spin coating. Differences in photoluminescence horizontal position in both cases, and the analyzer in vertical and spectra and in fluorescence anisotropy are discussed in terms of horizontal positions, respectively [54]. the chain conformation in solution and in films, and their implications for energy migration and transfer. 3. Results 2. Experimental section 3.1. Fluorescence spectra and anisotropy of MEH–PPV in solutions 2.1. Materials The electronic absorption spectra of MEH–PV in 18 mg L À 1 End capped poly-(9,90 -dioctylfluorenyl-2,7-diyl) (PF) solutions (see Fig. 2) show broad absorption bands. Broad 1À Mw $ 40 À 120 kg mol was purchased from ADS Inc. and poly- absorption spectra of conjugated polymers result of the [2-methoxy-5-(20 -ethylhexyloxy)-p-phenylene-vinylene] Mw ¼ convolution of electronic transitions from macromolecular 1À 181 kg mol (MEH–PPV), from Sigma-Aldrich. They were used segments of different conjugation lengths overlapped with as received. Dichloromethane (Merck 98% and Carlo Erba, several vibronic states [2–25,32–34,55–57]. Analytical grade), toluene (Merck 98% and Merck, spectroscopical Absorption spectra of MEH–PPV in acetonitrile, DMSO and grade) were fractionally distilled before use, chloroform (Labsynth ethyl acetate are different from those in other solvents (Fig. 2a). 99.8%). THF (Merck 98.5% or Aldrich ACS—99 +%) was distilled The lower solubility of MEH–PPV in these solvents and, conse- with Na at reduced pressure, ethyl acetate (Merck 99% and EM quently, the lower concentration in solution leads to a weak Science ACS, for HPLC), DMSO (Merck 98%), and acetonitrile absorbance (less than 0.1). Concentration in these cases are (Merck 99%) were dried before use. smaller than 18 mg L À 1. The absorption bands are centered at Polymer solutions in each solvent were prepared by dissolving 425 nm for acetonitrile, 480 nm for ethyl acetate and DMSO, the solid polymer under stirring, during several hours in the dark 492 nm for THF and 496 nm for chloroform and toluene.
  • 4. Author's personal copy ARTICLE IN PRESS R.F. Cossiello et al. / Journal of Luminescence 130 (2010) 415–423 417 0.20 1.0 0.16 Anisotropy / < r > Relative absorbance 0.8 0.12 0.6 0.08 0.04 0.4 0.00 0.2 580 600 620 640 λ / nm 0.0 350 400 450 500 550 600 Fig. 3. Fluorescence anisotropy (lexc = 495 nm), /rS of MEH–PPV in dilute solutions of___ethyl acetate, -&-&-, chloroform; -n-n-, THF; -J-J-, toluene; λ / nm - Â - Â -, DMSO; and - - -, acetonitrile. Polymer concentration as in Fig. 2. usually emit at the red-edge of the intrachain species [23,25,29– 1.0 31,33,46]. Therefore, in the absence of aggregates, even for good Relative fluorescence 0.8 solvents, the band profile is not expected to be the same [23,25,29–31,33,46]. This can explain slight differences between 0.6 the emission spectra in good solvents while in these same environments the absorption spectra remain practically the same. 0.4 The emission spectrum of MEH–PPV in ethyl acetate presents two bands, the higher energy attributed to the intrachain exciton 0.2 (isolate chains) and the lower energy attributed to the interchain exciton (aggregate involving more than one macromolecular 0.0 segment). In poor solvents, such as acetonitrile and DMSO, only 500 550 600 650 700 the emission of aggregates is observed. In other words, in good λ / nm solvents the conformation of polymeric chains is more extended Fig. 2. (a) Relative absorption and (b) relative fluorescence spectra (lexc = 495 nm) and the interchain interactions, i.e. p–p stacking, are minimized of MEH–PPV in solution of: ___, ethyl acetate; -&-&-, chloroform; -n-n-, THF; by the solvation layer, while in poor solvents interchain interac- -J-J-, toluene; (these later three spectra almost overlap in absorption and in tions or coil conformation favoring intersegmental interactions emission), - Â - Â -, DMSO; and - - -. acetonitrile. Polymer concentration was increase the probability of aggregation [25,32,35,48,49]. 18 mg L À 1 except in ethyl acetate, acetonitrile and DMSO (see text). We also found that the steady-state emission anisotropy, /rS, of MEH–PPV in these five solutions (Fig. 3, Table 1) is independent of the emission wavelength and has distinct values for three types The MEH–PPV fluorescence emission spectra of 18 mg L À 1 of solvents (as classified above): /rS= 0.114 in chloroform, solutions (Fig. 2b) show that in good solvents (toluene, chloro- /rS= 0.116 in toluene and /rS=0.120 in THF (good solvents); form and THF) the emission maximum is centered at lem = 555 /rS=0.166 in ethyl acetate (solvent with intermediate quality), and nm, with shoulders at 600 and 650 nm, corresponding to a /rS=0.093 in DMSO and /rS=0.035 in acetonitrile (poor solvents). vibronic progression originated on the C–C vibrational stretching Transient measurements have shown that immediately after modes of aromatic rings. A very small blue shift is observed in THF excitation MEH–PPV anisotropy in dilute chlorobenzene solution (lem =552 nm) when compared to toluene solutions, which can be is r0 =0.4, the theoretically maximum value, and then the value attributed to solvatochromic effects, in agreement with previous decreases to r=0.15 with a characteristic lifetime of 20 ps and to observations in THF and chlorobenzene [31]. There is a r=0.08 at longer times (limiting anisotropy, rp) [54]. Considering an pronounced spectral red-shift of the emission spectra (lem = 592 excited state lifetime of 250 ps, the above values render /rS=0.11, nm) for MEH–PPV in poor solvents (DMSO and acetonitrile) with a in good agreement with our results (/rS=r0tr/(tr + tf)+rp; where tr complete absence of higher energy emission band. This result is is the characteristic time for the anisotropy decay and tf is the explained by MEH–PPV aggregation [20,22,25,28,29,31,37,46]. excited state lifetime). In THF solution, the anisotropy of MEH–PPV Distinctly from both previous solvent sets, the spectrum in ethyl was measured to decrease from 0.4 to r=0.18–0.20 with a acetate shows two prominent bands; a higher energy band characteristic lifetime of 150 ps [31], considering the reported slightly blue-shifted, lem = 552 nm, compared to good solvents excited state lifetime of 300 ps, a value of /rSE0.13 can be and a lower energy band with maximum at lem =590 nm calculated, also in good agreement with our results. comparable to that observed in poor solvents. In all cases, it is worth noting that the fluorescence emission is not the mirror image of the absorption spectrum, indicating that distinct sites 3.2. Fluorescence spectra and anisotropy of MEH–PPV films are absorbing and emitting [20,22,25,28,29,31,37,46]. The steady-state fluorescence emission of MEH–PPV in dilute The steady-state fluorescence spectra of MEH–PPV thin films solutions of good solvents is attributed to the intrachain exciton deposited on PET surface by dip coating from solutions of being composed by three vibronic bands appearing at 564, 599 different solvents (Fig. 4) are red-shifted (lem (THF)= 587 nm, and 632 nm [23,24,29–31,33,46]. The larger apparent Stokes shift lem (chloroform)= 588 nm and lem (toluene)= 600 nm) compared originates from conformational relaxation, energy migration or to dilute solutions of good solvents and ethyl acetate. We were energy transfer processes, as usually observed for conjugated not able to prepare films from DMSO and acetonitrile solutions polymers [38–42]. The relative intensity of these bands depends due to the low MEH–PPV solubility. The emission spectra of on the electron-phonon coupling, on the macromolecular con- films are very similar to those observed in solution of poor formation relaxation, and on the presence of aggregates that solvents (Fig. 2), confirming that in solution of acetonitrile and
  • 5. Author's personal copy ARTICLE IN PRESS 418 R.F. Cossiello et al. / Journal of Luminescence 130 (2010) 415–423 Table 1 The steady-state anisotropy of MEH–PPV and PF in different solvents (subscript s) and of thin films (subscript f) spun from solutions with the same solvents. /rSs (MEH–PPV) /rSs (PF) /rSf (MEH–PPV) /rSf (PF) a band b band Ethyl acetate 0.166 70.008 0.057 0.01 0.307 0.05 0.02 70.01 nd Chloroform 0.114 70.006 0.057 0.01a 0.307 0.05a 0.04 70.01 0.03 7 0.01a Toluene 0.116 70.005 0.125 7 0.005 0.35 70.01 0.07 70.01 0.06 7 0.02 THF 0.120 70.008 0.130 7 0.005 0.26 70.01 0.01 70.01 0.04 7 0.01 DMSO 0.093 70.008 nd nd nd Nd Acetonitrile 0.035 70.007 nd nd nd nd nd: not determined. a Values in dichloromethane. 0.125 1.0 Normalized intensity 0.100 Anisotropy / <r> 0.8 0.075 0.6 0.050 0.4 0.2 0.025 0.0 0.000 300 400 500 600 700 550 575 600 625 650 λ / nm λ / nm Fig. 4. Normalized fluorescence excitation (lem = 600 nm) and emission (lexc = 495 Fig. 5. Fluorescence anisotropy (lexc = 495 nm), /rS, of MEH–PPV in films spun nm) spectra of MEH–PPV films prepared by dip coating from solutions in—ethyl from solutions of ___,.ethyl acetate; —, chloroform; -n-n-, THF; and -J-J-, acetate, -&-&-, chloroform; -n-n-, THF; and -o-o-, toluene. toluene. DMSO the emission is originated from interchain or processes or the highly unlikely conformational relaxation) are intersegmental species. As already observed, the emission of more efficient in solid state than in dilute solutions. MEH–PPV films is not the mirror image of the excitation spectrum because the system undergoes a fast energy migration from the absorbing luminophores towards the lower emissive center in 3.3. Fluorescence spectra and anisotropy of PF in solutions addition to the conformational relaxation processes, which are expected to play a minor role in the solid state. Due to these The electronic absorption spectra of PF were recorded in processes there is a large apparent Stokes shift between emission solutions (0.50 g L À 1) of toluene, chloroform, THF, acetonitrile, and excitation spectra [17–23,29,33,34,56]. ethyl acetate and DMSO (Fig. 6a). In toluene, chloroform, THF and There are several reasons for both red-shift and differences in DMSO solutions the absorption band is broad and centered at the intensity ratio of vibronic bands when MEH–PPV films are around 390 nm. This band is attributed to the a-backbone spun from different solutions: the spectral red-shift can be conformations, which is disordered and usually observed in produced by inner filter effect usually observed for thick or solutions and in thermally stabilized films [13,14,41–43,58–60]. concentrated samples [54]. It can also be attributed to chain A small red-shift is observed for THF and DMSO solutions conformation. Because these are very thin films, the absorbance is (labs = 394 nm). The electronic absorption band of PF in low, and there is a great spectral shift between absorption and acetonitrile solution is broader with a higher intensity peak emission, chain conformation and its distribution is responsible centered at 383 nm, a shoulder at the red-edge (402 nm) and a for different emission spectra. lower intensity sharper band at 436 nm. This band appears with Morphology of MEH–PPV films has been studied by several low intensity in the other PF solutions. The absorption spectrum high-resolution techniques such as near-field scanning optical of PF in ethyl acetate solutions is quite different from other microscopy, transmission electron microscopy, wide angle X-ray solvents. There is a splitting of the absorption band in a higher scattering, polarized optical microscopy [24,26,28,29,35] and energy component with a lower intensity centered at 387 nm and single molecule spectroscopy [32–34]. It is accepted that MEH– a higher intensity component at its red-edge (405 nm) in addition PPV films have a low degree of crystallinity although in the film to the lower intensity band at 436 nm. This lowest energy band is short-range chain packing domains are built in the nanometric also relatively more intense than in other solvents. This spectrum scale consistent with the emission of aggregated forms [20,24]. can be considered as an overlap of two absorptions; from the The steady-state fluorescence anisotropies of MEH–PPPV films luminophores in chains with the a-conformations as in toluene, spun from solutions of different solvents are: /rS= 0.07 for chloroform, THF and DMSO and in chains with the b toluene, /rS= 0.04 for chloroform, /rS= 0.02 for ethyl acetate and conformations absorbing at the red-edge. It has been proposed /rS= 0.01 for THF (Fig. 5, Table 1). These values are the average in that the 0–0 vibronic band at 436 nm corresponds to the the 575–650 nm range where emission intensity is appreciable. luminophores of polyfluorenes in b conformations with other MEH–PPV anisotropy values are one order of magnitude smaller vibronic components overlapped with the higher energy band of than in solution indicating that fluorescence depolarization a-conformers [13,14,41–43,58–60]. The b conformations have regardless its mechanism (energy migration, energy transfer more planar polymer backbone [13,14,41–43,58–60]. In
  • 6. Author's personal copy ARTICLE IN PRESS R.F. Cossiello et al. / Journal of Luminescence 130 (2010) 415–423 419 nm) indicating that although in a lower concentration, that species is present even in good solvents. 1.0 Normalized absorbance It is note worthy that while PF absorption spectrum in DMSO 0.8 solutions is similar to those in good solvents, the emission is similar to poor solvents. The emission spectra of PF in acetonitrile 0.6 show the spectral features typical of the b-chain conformation. The 0–0 phonon band (lem =438 nm) seems to be a mirror 0.4 image of the lower energy absorption band (the 0–0 absorption 0.2 band at 436 nm). This behavior has been already reported for the b-phase emission spectrum, which exhibits a sharper 0.0 profile, a smaller Stokes shift and a more resolved vibronic 300 350 400 450 structure [61]. λ / nm The discussion of the solvent quality for PF can be done in the same terms as we did for MEH–PPV. Based on the Hildebrand solubility parameters THF, chloroform, dichloromethane and 1.0 toluene are good solvents for PF; DMSO has an intermediate Normalized intensity quality character while ethyl acetate and acetonitrile are poor 0.8 solvents. Therefore, we can say that good solvents for PF should have dispersive and dipolar components in the range16.8r 0.6 dd r18.0 J1/2 cm À 3/2 and 1.4r dp r5.7 J1/2 cm À 3/2, respectively. Be- 0.4 cause DMSO has the dispersive component (dd =18.4 J1/2 cm À 3/2) near the limit of the ideal range and a larger dipolar (dp =16.4 J1/2 cm À 3/2) 0.2 component, it behaves as a solvent with intermediate quality. On the other hand, for acetonitrile (a poor solvent) the dd =15.3 J1/2 cm À 3/2 0.0 present a low value, while the dipolar component is much larger 350 400 450 500 550 (dp =18.0 J1/2 cm À 3/2); for ethyl acetate (also a poor solvent) the λ / nm dd =15.8 J1/2 cm À 3/2 shows low value, while the dipolar component is within the adequate range (dp =5.3 J1/2 cm À 3/2). Taking all together Fig. 6. (a) Normalized absoption and (b) fluorescence excitation and emission spectra of PF in solutions of —, ethyl acetate; -&-&-, dichloromethane; -n-n-, we can say that the most important component controlling the PF THF; -J-J-, toluene; - Â - Â -, DMSO; and - - -, acetonitrile. For emission, kexc = 395 solubility is the dispersive interactions. According to the experimental or 400 nm. For excitation, kem = 465 nm. range of Hansen’s solubility [50–53], we can estimate the solubility parameters of PF as dd =18.5 J1/2 cm À 3/2, dp =2.0 J1/2 cm À 3/2 and dH = 1.0 J1/2 cm À 3/2, with a global parameter about dd =18.6 J1/2 cm À 3/2. Again, the global Hildebrand solubility parameter cannot explain the solubility ordering of ethyl acetate since it has similar solubility conclusion, based on the electronic absorption spectra, toluene, parameter, although the photophysical behavior of the solution is chloroform, THF, and DMSO are good solvents for PF but even for completely different. these solvents there is a low concentration of chains ordered as in The steady-state fluorescence anisotropy was also determined the b phase. In poor solvents (ethyl acetate) there is a larger for PF in solutions (Fig. 7, Table 1). The steady-state anisotropy amount of chains in the b phase, while in acetonitrile there is a strongly depends on excitation and emission wavelengths. mixture of isolated luminophores and a relative high The general features of the dependence of anisotropy with concentration of segments ordered as in the b phase. wavelength are similar in all four solvents. THF and toluene are Fluorescence emission and excitation spectra were recorded in shown in Fig. 7, as an example (similar spectra in solutions of THF, toluene, dichloromethane and ethyl acetate at a dichloromethane and in ethyl acetate are shown in Fig. 3 of the concentration of 0.18 g L À 1 and are displayed in Fig. 6b. Excitation Supporting Information). The steady-state anisotropy increases spectra are slightly wavelength-dependent in all cases, and are with excitation wavelength almost in the same way irrespective different from the absorption spectra (excitation spectra recorded of whether it is detected in the second emission band (typically at three different emission wavelengths, lem = 415, 440 and around 440 nm) or in the emission shoulder around 465 nm. The 465 nm, are shown in Fig. 1 in Supporting Information). maximum value of the anisotropy is reached in all cases in the Considering that there are two emitting species, the appointed excitation band corresponding to the b phase (420–440 nm). differences can be attributed to different emission yields or Values as high as 0.35 are measured in toluene and in ethyl relative absorption of a and b phases. acetate for this band. This indicates absence of depolarization Fluorescence emission spectra of the PF solutions (THF, effects such as torsional motions or energy transfer. In the UV dichloromethane, toluene and ethyl acetate) were also recorded edge of the excitation band, the value of the steady-state for excitation wavelengths from lexc = 380–430 nm (Supporting anisotropy is close to 0.1 in all cases. This indicates a great Information—Fig. 2). The dependence of the emission spectra depolarization. with excitation wavelength is more pronounced. Upon excitation On the other hand, the steady-state emission anisotropy shows wavelength increase, the maximum corresponding to the a phase very different values and wavelength dependence when the decreases relative to the maximum around 438 nm, and the band samples are excited at 400 nm, corresponding mainly to the structure becomes more pronounced. In ethyl acetate, which absorption of the isolated luminophores, than when they are shows a poorly resolved band emission structure at excitation excited at around 435 nm, in the maximum corresponding to the wavelength below 400 nm, clearly resolved bands are displayed b phase. In the first case, a low anisotropy is observed with slight when excitation is performed above 410 nm, similar to other maxima coinciding with the emission maxima at around 440 and solvents [13,14,41,42,58]. In addition, the relative intensity of the 465 nm. When excitation is performed at 435 nm, the values of lower energy emission bands (465 and 500 nm) become higher the steady-state anisotropy are higher and show an increase in for excitation at the red-edge of the excitation band (lexc 4415 the tendency with wavelength.
  • 7. Author's personal copy ARTICLE IN PRESS 420 R.F. Cossiello et al. / Journal of Luminescence 130 (2010) 415–423 0.30 THF 1800000 THF Emission intensity (a.u.) 0.25 1500000 1200000 0.20 Anisotropy 900000 0.15 600000 0.10 300000 0.05 0 0.00 350 400 450 500 360 380 400 420 440 460 480 500 λ / nm λ / nm Toluene 0.4 1200000 Emission intensity (a.u.) Toluene 1000000 800000 Anisotropy 600000 0.2 400000 200000 0 350 400 450 500 0.0 λ / nm 350 400 450 500 Fig. 8. Spectral deconvolution of excitation and emission spectra of PF in solution λ / nm of THF and toluene. Dotted lines correspond to total excitation (lem = 465 nm) and emission (lexc = 400 nm) spectra. Points correspond to the components of the a Fig. 7. The steady-state anisotropy of PF solutions in THF and toluene. Dotted lines phase (hollow squares, excitation, and full squares, emission, f1, see text and Eqs. correspond to excitation (lem = 465 nm) and emission (lexc = 400 nm) spectra. (3)–(5)) and the b phase (hollow circles, excitation, and full circles, emission, f2, Scatter points represent the steady-state anisotropy in the excitation spectra, see text and Eqs. (3)–(5)). lem =464 nm (empty squares) and lem = 437 nm (empty circles), and in the emission spectra, lexc = 400 nm (full squares) and lexc =434 nm (full circles). 1.0 Results are compatible with homo energy transfer between Normalized intensity isolated luminophores being the main depolarization pathway. 0.8 Taking into account that anisotropy is additive and that there are at least two species responsible for the emission, we can write 0.6 /rS ¼ r1 f1 þr2 f2 ð3Þ 0.4 f1 þ f2 ¼ 1 ð4Þ 0.2 In the previous equations, fi are the fractions of the emission intensity corresponding to each species, and ri are their respective 0.0 steady-state anisotropies, which, for the following calculations, 350 400 450 500 will be considered to be wavelength-independent in the excita- λ / nm tion and emission wavelength ranges measured. Eqs. (3) and (4) can be solved for the intensities of each Fig. 9. Fluorescence excitation (lem = 465 nm) and emission (lexc = 400 nm) spectra component, thus allowing calculation of their emission and of PF films spun from solutions of: -&-&-, dichloromethane; -n-n-, THF -J-J- toluene. excitation spectra /rS À r2 I1 ¼ It f1 ¼ ; r1 À r2 r1 À /rS [45], are in line with our measurements. On the other hand, the b I2 ¼ It À I1 ¼ It f2 ¼ ð5Þ phase displays very high anisotropy, indicating that hardly any r1 À r 2 depolarization occurs once excitation energy reaches this center. Arbitrarily we identify 1 with the b phase and 2 with the a phase. The deconvoluted spectra are shown in Fig. 8 for the case of 3.4. Fluorescence spectra and anisotropy of PF films THF and toluene. Values of r1 and of r2 are listed in Table 1. For the isolated luminophore of PF in solution the anisotropy is The fluorescence excitation and emission spectra of PF low. Reported values of /rS=0.096 (lexc o380 nm) measured for spun from solutions of toluene, dichloromethane and THF poly[9,9-di(ethylhexyl)fluorene] in methylcyclohexane dilute so- are shown in Fig. 9. Excitation spectra show differences in the lution [44] and /rS =0.10 for PF in methylcyclohexane solution relative contributions of b-phase components, these decrease the
  • 8. Author's personal copy ARTICLE IN PRESS R.F. Cossiello et al. / Journal of Luminescence 130 (2010) 415–423 421 0.07 24000 0.06 23000 Wavenumber (cm-1) 22000 0.05 21000 Anisotropy 0.04 20000 0.03 19000 0.02 18000 17000 0.01 16000 0.00 14 15 16 17 18 19 20 350 400 450 500 δd(J1/2 cm-3/2) λ / nm Cossiello et al. Cossiello et al. Fig. 11. Energy for the absorption and emission maxima of MEH–PPV in solution Fig. 10. The steady-state anisotropy of PF films spun from THF solution. Dotted of different solvents as a function of the Hildebrandt’s dispersion parameter, dd. lines correspond to total excitation (lem = 465 nm) and emission (lexc =400 nm) Upper curves and squares: absorption data. Lower curves and circles: emission spectra. Points represent the steady-state anisotropy in the excitation spectra, data. Hollow symbols are data from Ref. [66] and filled symbols are data from this lem =464 nm (hollow triangles) and lem = 437 nm (full triangles), and in the work. Values for toluene and chloroform from both sources coincide. Linear emission spectra, lexc = 400 nm (hollow squares) and lexc = 434 nm (full squares). regression line and 95% confidence limits are shown for each correlation. direction of dichloromethane, THF and toluene. Emission Emission spectra show bands corresponding to these two species. spectra are also quite similar in the three films, with a slight No other emitting species is observed due to excitation at 495 nm, bathochromic shift (1 nm) in the series toluene, dichloromethane where the weak luminescent interchain aggregates do not and THF. The emission spectra show absence of the a-phase appreciably absorb [56]. emission, as reported by several authors [13,14,62]. In most cases, preferential solvation has been analyzed in The emission spectra show a slight change with emission terms of planarity [20,23–25] instead of solubility parameters or wavelength; a decrease in the tendency in the contribution of the other representations of polymer/solvent interactions [46,48–53]. 438 nm maximum with respect to the 465 nm one is noted upon Here we analyze the solvent quality in terms of the components of with increase in the excitation wavelength (see Fig. 4 in the Hildebrand solubility parameters for all solvents [50–53]. By Supporting Information). This behavior parallels that in solution comparing these components with profiles of the emission and is a consequence of increased absorption of the b-phase spectra, we conclude that good solvents for MEH–PPV show component with increase in the excitation wavelength [63]. values of the dispersive and dipolar components in the range The emission spectra in the solid state highly resemble the 16.8r dd r18.0 J1/2 cm À 3/2 and 1.4r dp r5.7 J1/2 cm À 3/2, respec- emission obtained for the f2 component (b phase), by spectral tively [50–53]. Solvents such as toluene, chloroform and THF have deconvolution of the steady-state spectra using the wavelength- dd and dp within these ranges while ethyl acetate has a dipolar dependent total anisotropy (Fig. 8). component as large as dp = 15.8 J1/2 cm À 3/2. Nevertheless, these The steady-state excitation and emission anisotropies of PF four solvents have the global solubility parameter in the range films spun from THF solution are shown in Fig. 10 (similar results 18.7–18.2 J1/2 cm À 3/2, which evidences the failure of the global in films spun from dichloromethane and from toluene solutions Hildebrand parameters to explain the MEH–PPV solubility are displayed in Fig. 5 of the Supporting Information). The values differences observed in these solvents. On the other hand, are quite low, very similar, and they show practically no matching of dispersive and dipolar components adequately dependence with wavelength in all films, either in excitation or describes the solubility. In addition, both acetonitrile (dd = 15.3 in emission. Any variation tendency is here within the J1/2 cm À 3/2, dp = 18.0 J1/2 cm À 3/2) and DMSO (dd =18.4 J1/2 cm À 3/2, experimental uncertainty in /rS determination of 70.02. The dp = 16.4 J1/2 cm À 3/2) have the Hildebrand solubility parameters low values of the steady-state anisotropy in the solid phase point out of the range adequate for a good solubility of MEH–PPV, which to a very fast pooling of the electronic excitation energy prior to was estimated in a previous work to be 18.7 J1/2 cm À 3/2[46]. From emission. The facts that /rS values are lower in films than those the knowledge of the experimental range of the Hansen’s for the a phase in solution, they are wavelength-independent and parameter [53–55], we estimate the partial solubility parameters there is no emission originated in the a phase in the films, all for the MEH–PPV as dd =18.0 J1/2 cm À 3/2, dp =4.0 J1/2 cm À 3/2, indicate complete energy transfer to the b phase prior to dH =3.0 J1/2 cm À 3/2 and d =18.7 J1/2 cm À 3/2. emission, contrary to what is observed in dilute solution. Absorption and emission maxima show a good correlation Evidently, the molecular proximity in the solid state is with the Hildebrandt’s dispersion component, as observed in responsible for this fact. Fig. 11. Correlation with polarity, hydrogen bonding and the total Hildebrandt solubility parameters are definitely uncorrelated (see Supporting information). Data obtained in this work 4. Discussion are integrated with literature data, with which they perfectly match [66]. The absorption spectra of MEH–PPV show a great difference in Values for acetonitrile in absorption and in emission as well as acetonitrile (the worst solvent used) in comparison with other data for DMSO in emission fall quite apart from the correlation solvents. The absorption at 425 nm reflects the chain collapse in because the spectroscopically active species is not the same as in acetonitrile [64]. The spectral shift in other solvents can be the other cases. The good correlation with dispersion forces and attributed to different amount of isolated and aggregated not with polar or hydrogen bonding components of Hildebrandt’s intrachain chromophores and solvatochromic effects [31,65,66]. parameters indicate that dispersion is the most determining
  • 9. Author's personal copy ARTICLE IN PRESS 422 R.F. Cossiello et al. / Journal of Luminescence 130 (2010) 415–423 interaction that regulates the spectroscopic features of MEH–PPV of a 30 nm3 molecule (ca. 10 monomer units either of PF or of in solution. The traditional interpretation of solvent shift MEH–PPV) in a 1 mPa s viscous medium at 300 K. Therefore, the produced by dispersion interactions is that they shift the energy rotational correlation time of both, MEH–PPV and PF surely of electronic states involved in the transitions. In this case, exceeds a value of various ns in solvents used and rotation is polymer–solvent interactions also change the effective conjuga- discarded in films. tion length [65,67], and thus enhance the energy of the electronic Anisotropy depolarization by energy migration was measured transition. in various conjugated polymers [39,56]. It has a very fast Films of MEH–PPV deposited from different solvents component ( o50 ps), accounting for most of the depolarization show exclusively emission from aggregates. There is a gradual (from the maximum value of 0.4–0.35 to a value near 0.15), and a change in the spectra that is in contrast with sharp differences slower component (ca. 100 ps) with a small contribution to the observed in the molecular orientation when isolated depolarization and a residual value of the anisotropy in the range polymer chains are deposited on glass from dichloromethane or 0.05–0.1 [31,55]. This residual value is higher for stiffer polymer toluene solutions [67]. This difference is easily explained backbones, accounting for the fact that almost parallel energy by the effect of chain interactions in compact polymer films migration does not contribute to depolarization [68]. For example, compared to isolated molecules. The different spectral distribu- the energy migration in MEH–PPV is approximately three times tion displayed in emission can be explained by heterogeneity in faster than in polythiophenes due to the disordered chain chain conformation in solution, which is transferred to the films conformation in MEH–PPV and it plays an important role on the [65,67]. anisotropy depolarization [38]. Nevertheless, in the case of PF, The absorption and emission spectra of PF show two distinct although energy migration is an important pathway for the bands. In absorption a sharp peak around 435 nm and a broader anisotropy depolarization, some anisotropy remains even in the band at ca 400 nm. The emission shows peaks at 420 and 445 nm, solid state, which means that the energy migration and other and a shoulder at 465 nm. The different nature of the bands can be energy transfer processes are not totally efficient [39,40,42, better distinguished by the steady-state emission and excitation 44,47,59]. anisotropies. This is the basis of the spectral decomposition When we compare the anisotropy in solutions and in the solid shown in Fig. 8. The electronic transitions identified by the state there is a remarkable increase in the depolarization of MEH– squares in Fig. 8 are attributed to the a phase [13,14,41– PPV emission on moving from poor solvents (DMSO and 43,45,59,60,62], whereas the ones identified by the circles are acetonitrile) to polymer films, independently of the solvent (Table attributed to the b phase [13,14,41–43,45,59,60,62]. The consis- 1). Aggregation decreases /rS for MEH–PPV either in poor tency of the spectra derived in different media and their solvents or in films. Because the rotation and conformational agreement with published data, validates the analysis. The relaxation do not play any important role under these conditions, tendency of fluorescence anisotropy with excitation wavelength the fluorescence depolarization should be more important due to was reported previously [45]. the greater efficiency of the interchain and intersegmental energy We have shown here that the photoluminescence features of transfer. Therefore, regardless of which MEH–PPV system we MEH–PPV in solution can be explained by solute solvent analyze (solution or films), fast energy migration/energy transfer dispersion interactions. For PF in solution we have shown that processes are always efficient and consequently its anisotropy the absorption and emission spectra of a and b chains can be depolarization is very effective. separated using the steady-state anisotropy. For both polymers, The behavior of the PF anisotropy is different compared to the spectral features in films are dependent on the chain MEH–PPV. In this system, the a and the b phases behave conformation in the solvent from which they were spun, as quite different; while the homo energy transfer between a already reported in the literature. segments highly depolarizes its emission, the same homo To discuss the changes in the steady-state anisotropy in transfer within the b phase is quite ineffective. Furthermore, the solutions and in thin films, we must consider the polymer additivity of anisotropy accounts for the observed wavelength properties such as rotational correlation time of the polymer dependence of this parameter in excitation and in emission. This chain, the fluorescence lifetime, the efficiency of the energy additivity means that the two chain conformations behave migration, the efficiency of the conformational relaxation and independent of each other with respect to the electronic the energy transfer processes of the electronic excited states energy relaxation, evidencing the lack of energy transfer between [9,13,14,18–20,23–26,34,41–43,47,55,56,58,59]. All these pro- them. There is a remarkable coincidence between the excitation cesses can contribute to the decay in the steady-state anisotropy. and emission spectra of the high anisotropy component Molecular relaxation is expected to play a minor role in rigid and the spectrum of the b phase reported in the literature [60]. luminophores with parallel absorption and emission dipoles. These spectroscopic evidences confirm the ordered structure of Indeed this is the case in conjugated polymers that have a the b phase, and the lack of electronic energy exchange with maximum anisotropy close to 0.4 as in the case of both MEH–PPV the a phase in solution. Finally, the anisotropy in films and PF [9,13,14,18–20,23–26,34,41–43,47,55,56,58,59]. Confor- is almost independent of the solvent quality and there is mational relaxation was demonstrated to be only partially a very effective energy transfer from the a to the b phases responsible for depolarization of fluorene oligomers and depolar- (Table 1). ization after excitation in the red-edge of the absorption spectrum [18,39,40,43,44,47,59,61]. Furthermore, conformational relaxa- tion is expected to play a still minor role in the depolarization Acknowledgements of the conjugated and more rigid MEH–PPV [16,18,21,22, 25,26,28,30,32,36]. TDZA and RFC thank FAPESP, MCT/CNPq/INEO, CAPES and Molecular rotation is the other parameter that contributes to FAEPEX/Unicamp for financial support and fellowships. PFA is a depolarization if it can take place with comparable rate or faster ´ member of Carrera del Investigador Cientıfico (Research Staff) than the decay of the excited state. Molecular rotation is excluded ´ from CONICET (Consejo Nacional de Investigaciones Cientıficas y for the conjugated polymers studied here. On one hand, the ´ Tecnicas, Argentina) and thanks support by grants PIP 5470 polymers have a sub-nanosecond excited state lifetime. On the (CONICET), PICT 10621 (ANPCyT, Argentina), X086 (UBA), and other hand, 1 ns is approximately the rotational correlation time CAPGBA03/02 (Brazil-Argentina).
  • 10. Author's personal copy ARTICLE IN PRESS R.F. Cossiello et al. / Journal of Luminescence 130 (2010) 415–423 423 Appendix A. Supplementary material [33] T. Huser, M. Yan, J. Photochem. Photobiol. A: Chem 144 (1) (2001) 43. [34] S.S. Sartori, S. De Feyter, J. Hofkens, M. Van der Auweraer, F. De Schryver, K. Brunner, J.W. Hofstraat, Macromolecules 36 (2) (2003) 500. Supplementary data associated with this article can be found [35] S.H. Chen, A.C. Su, Y.F. Huang, C.H. Su, G.Y. Peng, S.A. Chen, Macromolecules in the online version at doi:10.1016/j.jlumin.2009.10.006. 35 (11) (2002) 4229. [36] I. Soutar, C. Jones, D.M. Lucas, L. Swanson., J. Photochem. Photobiol. A: Chem. 102 (1) (1996) 87. [37] K.M. Gaab, C.J. Bardeen., J. Phys. Chem. A 108 (49) (2004) 10801. References [38] A. Synak, G. Gondek, P. Bojarski, L. Kulak, A. Kubicki, M. Szabelski, P. Kwiek., Chem. Phys. Lett. 399 (1–3) (2004) 114. ¨ [39] M. Hennecke, T. Damerau, K. Mullen, Macromolecules 26 (13) (1993) 3411. [1] S. Miyate, H.S. Nalva, in: Organic Electroluminescent Materials and Devices, , [40] L.M. Herz, R.T. Phillips., Phys. Rev. 61 (20) (2000) 13691. 1998. [41] F.B. Dias, A.L. Mac -anita, J.S. Melo, H.D. Burrows, R. Gunter, U. Scherf, A.P. ¨ [2] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Monkman., J. Chem. Phys. 39 (15) (2003) 7119. Friend, P.L. Burns, A.B. Holmes., Nature 347 (6293) (1990) 539. - [42] F.B. Dias, J. Morgado, A.L. Mac anita, P.C. Costa, H.D. Burrows, A.P. Monkman, [3] L. Akcelrud., Prog. Polym. Sci. 28 (6) (2003) 875. Macromolecules 39 (17) (2006) 5854. [4] A.M. Machado, J.D.D. Neto, R.F. Cossiello, T.D.Z. Atvars, L. Ding, F.E. Karasz, L. [43] M. Grell, D.D.C. Bradley, G. Ungar, J. Hill, K.S. Whitehead, Macromolecules 32 Akcelrud, Polymer 46 (8) (2005) 2452. (18) (1999) 5810. [5] A.M. Machado, M. Munaro, T.D. Martins, L.Y.A. Davila, R. Giro, M.J. Caldas, [44] A.J. Cadby, P.A. Lane, H. Mellor, S.J. Martin, M. Grell, C. Giebeler, D.D.C. T.D.Z. Atvars, L.C. Akcelrud, Macromolecules 39 (9) (2006) 3398. Bradley, M. Wohlgenannt, C. An, Z.V. Vardeny., Phys. Rev. B 62 (23) (2000) [6] S.H. Yang, C.F.J. Lee., Optoelectron. Adv. Mater. 9 (4) (2007) 2078. 15604. [7] L.H. Chang, Y.D. Lee, C.T. Chen, Macromolecules 39 (9) (2006) 3262. [45] H.L. Vaughan, F.M.B. Dias, A.P. Monkman., J. Chem. Phys. 122 (1) (2005) [8] H.A. Zhang, Y. Li, Q. Jinag, M.G. Xie, J.B. Peng, Y. Cao., J. Mater. Sci. 42 (12) 014902. (2007) 4476. [46] R.F. Cossiello, L. Akcelrud, T.D.Z. Atvars., J. Braz., Chem. Soc. 16 (1) (2005) 74. [9] M.B. Ramey, J.A. Hiller, M.F. Rubner, C.Y Tan, K.S. Schanze, J.R. Reynolds, [47] G. Fytas, H.G. Nothofer, U. Scherf, D. Vlassopoulos, G. Meier, Macromolecules Macromolecules 38 (2) (2005) 234. 35 (2) (2002) 481. [10] Z. Gu, Y.-J. Bao, Y. Zhang, M. Wang, Q.-D. Shen, Macromolecules 39 (9) (2006) [48] M.L. Winnik., Photophysical and Photochemical Tools in Polymer Science, vol. 3125. 182, D. Riedel, New York, 1986. [11] E.E. Nesterov, Z.G. Zhu, T.M. Swager., J. Am. Chem. Soc. 127 (25) (2005) 10083. [49] J.L. Halary, L. Monnerie., Photophysical and Photochemical Tools in Polymer [12] J. Gruber, R.W.C. Li, L.H.J.M.C. Aguiar, T.L. Garcia, H.P.M. Oliveira, T.D.Z. Atvars, Science, vol. 182, D. Riedel, New York, 1986, p. 589. A.F. Nogueira., Synth. Met. 156 (24) (2006) 104. [50] M.H. Hansen.Ind. Eng. Chem. Prod. Res. Dev. 8 (1969) 2. [13] U. Scherf, D. Neher (Eds.), Polyfluorenes Adv. Polym. Sci., vol. 212, Springer- [51] A.F.M. Barton, in: Handbook of Solubility Parameters and Other Cohesion Verlag, Berlin, 2008. Parameters, CRC Press, Boca Raton, Florida, 1983, p. 232. [14] U. Scherf, E.J.W. List., Adv. Mater. 14 (7) (2002) 477. [52] A.F.M. Barton., Chem. Rev. 75 (6) (1975) 731. [15] T. Kawase, T. Shimoda, C. Newsome, H. Sirringhaus, R.H. Friend., Thin Solid [53] J. Brandrup, E.H. Immergut, E.A. Grulke, in: Polymer Handbook, Fourth ed., Films. 438 (2003) 279. Academic Press, New York, 1993, p. 2336. [16] E.E. Gurel, Y. Pang, F.E. Karasz., Thin Solid Films. 417 (1–2) (2002) 147. [54] M.M.L. Grage, P.W. Wood, A. Ruseckas, T. Pullerits, W. Mitchell, P.L. Burn, [17] S.R. Amrutha, M.J. Jayakannan., Phys. Chem. B 110 (9) (2006) 4083. I.D.W. Samuel, V. Sundstrom., J. Chem. Phys. 118 (16) (2003) 7644. [18] R. Deans, J. Kim, M.R. Machacek, T.M. Swager., J. Am. Chem. Soc. 122 (35) [55] C.J. Collison, L.J. Rothberg, V. Treemaneekarn, Y. Li, Macromolecules 34 (7) (2000) 8565. (2001) 2346. [19] D.T. McQuade, J. Kim, T.M. Swager., J. Am. Chem. Soc. 122 (24) (2000) 5885. [56] U. Lemmer, S. Heun, R.F. Mahrt, U. Scherf, M. Hopmeier, U. Siegner, E.O. Gobel, [20] B.J. Schwartz., Ann. Rev. Phys. Chem. 54 (2003) 141. ¨ K. Mullen, H. Bassler., Chem. Phys. Lett. 240 (4) (1995) 373. [21] T. Zyung, S.D. Jung, D.H. Hwang., Synth. Met. 117 (1-3) (2001) 223. [57] J.R. Lakowicz, in: Principles of Fluorescence Spectroscopy, Academic Press, [22] S. Lee, J.Y. Lee, H. Lee., Synth. Met. 101 (1–3) (1999) 248. New York, 1999. [23] T.Q. Nguyen, R.Y. Yee, B.J. Schwartz., J. Photochem. Photobiol. A: Chem. 144 [58] S.H. Chen, A.C. Su, C.H. Su, S.A. Chen, Macromolecules 38 (2) (2005) 379. (1) (2001) 21. [59] S.H. Chen, H.L. Chou, A.C. Su, S.A. Chen, Macromolecules 37 (18) (2004) 6833. [24] T.Q. Nguyen, B.J. Schwartz, R.D. Schaller, J.C. Johnson, L.F. Lee, L.H. Haber, R.J. [60] M.J. Winokur, J. Slinker, D.L. Huber., Phys. Rev. B 67 (18) (2003) 184106. Saykally., J. Phys. Chem. B 105 (22) (2001) 5153. ¨ [61] J.G. Muller, E. Atas, C. Tan, K.S. Schanze, V.D. Kleiman., J. Am. Chem. Soc. 128 [25] R.D. Schaller, L.F. Lee, J.C. Johnson, L.H. Haber, R.J. Saykally, J. Vieceli, I. (12) (2006) 4007. Benjamin, T.Q. Nguyen, B.J. Schwartz., J. Phys. Chem. B 106 (37) (2002) 9496. [62] D. Marushchak, L.B.A. Johansson., J. Fluorescence 15 (5) (2005) 797. [26] U. Jeng, C.H. Hsu, H.S. Sheu, H.Y. Lee, A.R. Inigo, H.C. Chiu, W.S. Fann, S.H. [63] H. Azuma, K. Asada, T. Kobayashi, H. Naito., Thin Solid Films 509 (1–2) (2006) Chen, A.C. Su, T.L. Lin, K.Y. Peng, S.A. Chen, Macromolecules 38 (15) (2005) 182. 6566. [64] H.Z. Tang, M. Fujiki, M. Motonaga, Polymer 43 (23) (2002) 6213. ´ ´ [27] C.Y. Yang, F. Hide, M.A. Dıaz-Garcıa, A.J. Heeger, Y. Cao, Polymer 39 (11) [65] R. Traiphol, N. Charoenthai, T. Srikhirin, T. Kerdcharoen, T. Osotchan, T. (1998) 2299. Maturos, Polymer 48 (3) (2007) 813. [28] J. Teetsov, D.A.V. Bout., J. Phys. Chem. B 104 (40) (2000) 9378. ´ ¨ [66] O. Narwark, S.C.J. Meskers, R. Peetz, E. Thorn-Csanyi, H. Bassler., Chem. Phys. [29] C.R. McNeill, H. Frohne, J.L. Holdsworth, P.C. Dastoor., Nano Lett. 4 (12) (2004) 294 (1) (2003) 1. 2503. [67] S. Quan, F. Teng, Z. Xu, L. Qian, Y. Hou, Y. Wang, X. Xu., Eur. Polym. J. 42 (1) [30] V. Cimrova, D. Neher., Synth. Met. 76 (1–3) (1996) 125. (2006) 228. [31] T.Q. Nguyen, V. Doan, B.J. Schwartz., J. Chem. Phys. 110 (8) (1999) 4068. [68] R. Traiphol, P. Sanguansat, T. Srikhirin, T. Kerdcharoen, T. Osotchan, [32] D.Y. Kim, J.K. Grey, P.F. Barbara., Synth. Met. 156 (2–4) (2006) 336. Macromolecules 42 (20) (2006) 1165.