cyclic voltammetry discussed

1. By Suchismita Ghosh

2. Voltammetric
system.
Working
(indicator)
electrode.
Reference
electrode
Counter
(auxiliary)
electrode
Ionic
elctrolyte.
Sample
dissolved
in solvent.

3. Polarography
• Polarisation
•Diffused Current
• Residual Current
• Migration Current
• Half wave potential
Working electrode replaced by unconventional micro electrode (e.g.
DME)
Ilkövic Equation:
(id)av=607nD1/2Cm2/3t1/6 at 298K
Nernst Equation: (for oxidation)
E=E1/2+(0.059/n)log{(id-i)/i}

4. Linear sweep Voltammetry
 Initial (E1) and final (E2) potential are chosen.
 E2>E1 => +ve/anodic/oxidative scan.
 E2<E1 => -ve/cathodic/ reductive scan.
 Signal is obtained if any electron transfer occurred in
the desired window.

5. Cyclic Voltammetry
Randles Sevcik equation:
ip=-(2.96x105)ACn3/2D1/2ν1/2
Advantages:
 Complete electrochemical
information obtained.
 Idea about reversibility of the
reaction.
 Gives idea about Electron
transfer kinetics and
mechanism.

6. Recognition of perfectly reversible
reaction
 Peak current(ip)
increases linearly
with ν1/2
 ipc=ipa
 ∆Ep=Epc-Epa=0.059/n at
298K

7. A typical example
Working electrode- Platinum
Reference electrode-SCE
Electro-active species-K3Fe(CN)6
Supporting Electrolyte-KNO3
Electrode Reaction-
FeIII(CN)6
3- + e FeII(CN)6
4-

8. Electrochemistry of some mono-
porphyrins
N
N
H
N
H
N
R
R
R
R
N
N
H
N
H
N
R
R
R
R
N
N
N
N
N
N
H
N
H
N
R
R
R
R
N
N
(P)H2
(PTA)H2
(PQ)H2

9. CV observations
 Positive shift in E1/2
 Additional reduction peak
for (PQ)H2 and (PTA)H2.
 TA centered multi-
electron transfer.
 Oxidation pattern of
(PQ)H2 and are similar but
additional peak in 0.48V
in (PTA)H2 .

10. ΔE1/2 values:
ΔE1/2 trend PTA<PQ<P.

11. CV of metalated Porphyrin
N
N
N
N
R
R
R
R
M
N
N
N
N
M=Cu, Zn
N
N
N
N
R
R
R
R
M
N
N
(PTA)M
(PQ)M

12. CV observations:
 Two oxidation and three
reduction peaks.
 Both oxidations are porphyrin
centered.
 First two reduction are
porphyrin centered.
 Last reduction peak TA
centered.
 HOMO-LUMO gap (ΔE1/2)
(PTA)M<(PQ)M<(P)M.

13. CV of TA linked Bis-porphyrins
H2(P)-TA-(P)H2
N
N
H
N
H
N
R
R
R
R
N
N
N
N
H
N
N
N
H
N
R R
R

14. Cv observations:
 Four well seperated reduction peaks
obtained.
 ΔE1/2 of the first two reduction gives
measure of interaction between TA
linked macrocycles.
 First 1 electron reduction easier for
TA linked bis-pophyrin.

15. CV of metallated form:
N
N
N
N
R
R
R
R
N
N
N
N
N
N
N
N
R R
R
M M
M(P)-TA-(P)M
M=Cu(II);Zn(II)

16. CV Observations:
 ΔE1/2 between first two
reduction 500 mV higher
than 230 mV of non
metalated form.
 Greater interaction between
TA linked macrocycles.
 When M=Co ΔE1/2 between
first two reduction much
smaller(140 mV).
 CoII/CoI process involved

17. Some Porphyrins with Co as metal
N
N
N
N
R
R
R
R
Co
N
N
N
N
R
R
R
R
Co
Cl
N
N
N
N
R
R
R
R
N
N
N
N
N
N
N
N
R R
R
Co Co
(P)CoII
(P)CoIIICl
CoII(P)-TA-(P)-CoII

18. CV observations:
 Reduction peak for both the
compounds at -0.87 V for CoII/CoI
process.
 Irreversible peaks at -0.26 ,-0.04 V
and 0.64 V are for CoII/CoIII
processes.
 Two ring centered oxidation peaks
negatively shifted in chloride
bound form due to increased
anion binding constant.

19. CV observations:
 Similar oxidation pattern in TA linked Bis porphyrin as compared to mono porphyrin
form indicating equivalent non-interacting redox centres.
 Four well separated reduction peaks indicating interaction between TA linked
macrocycles.

20. Redox properties of some
corrole complexes:
N
N N
N
Co
L
X
X
X

21. CV Observations:
Electroreduction
 First reduction
(CoIII/CoII)quasi-
reversible
 First reduction more
facile in DMF
 Second
reduction(CoII/CoI)
irreversible in DCM.

22. Electrooxidation
 First two oxidations reversible in both DCM
and DMF.
 Oxidation more facile in DMF.
 Third reversible oxidation cannot be detected
in DMF due to its anodic potential limit(+1.47
to -3.90)

23. Electrochemistry of manganese
containing corroles

24. CV in Pyridine
 One e oxidation peak(MnIII to MnIV) as pyridine
strongly coordinates to MnIII center.
 Additional re-reduction peak in case of dyads.
 Reduction peak due to MnIII to MnII.
 Slightly different reduction peak in 4 due to
flexibility of spacer group(diphenylether).

25. CV Observations:
 The first oxidation peak(MnIII to
MnIV)is split into two processes due
to two different forms of axially
ligated MnIV one is
[(Cor)MnIV(PhCN)]+ and other
(Cor)MnIVX. X= unknown anion.
 Further ring centered second
oxidation peak at 1.07 to 1.10 V.
 Two reduction peaks due to EC
mechanism.
(in PhCN)

26. Conclusion
 Idea about the mechanism of redox reactions.
 Effect of solvent or external elements.
 Idea about HOMO-LUMO gap of a molecule.
 Interaction between the various moieties present in a
molecule.