2. I. Introduction
1. In most chemical reactions molecules are in their ground electronic
states.
2. In photochemical reactions one or more of the reacting molecules are
promoted by absorption of light (visible to UV) to an electronically
excited state – Light as a chemical reagent
3. A molecule is formed which is already in an electronically excited
state. When this molecule returns to its ground state, light is
produced. – Light as a chemical product
C
O
OH
C
OH
C
OH
O
+ +
Benzophenone Isopropanol Benzopinacol Acetone
h
2
NH
NH
O
O
NH2
O
O
O
O
NH2
H2O2, -
OH
K3Fe(CN)6
+ Light
3. A. Excited States and the Ground State
1. Electrons can move from the ground state to an excited state if energy is
supplied, in a photochemical reaction this energy is in the form of light
2. The energy difference between electronic energy levels is quantized, so
only light of discrete frequencies will cause a transition to occur.
E = h (h = Planck’s constant, = frequency))
3. The frequencies of light that correspond to the energy difference
between electronic energy levels in covalent bonds fall in the visible to
ultraviolet region of the spectrum.
4. Functional groups that contain bonds that undergo a given absorption
are called chromophores
Ground State
Excited State
Many frequencies
of light bombard
the chemical bond
The energy levels of chemical bonds are
quantized! E = h
Only a photon of the exact energy will be
absorbed and cause promotion of an
electron to the excited state
4. B. Singlet and Triplet States
1. In molecules all electrons in the ground state are paired with each member
of the pair possessing an opposite spin (Pauli principle)
2. If one of the electrons is promoted to another orbital of higher energy, the
promoted electron is no longer constrained by the Pauli principle and may
posses either a parallel or opposite spin to it’s former partner
3. If a molecule contains two unpaired electrons of the same (or parallel) spin
is called a triplet
4. If a molecule contains two unpaired electrons of opposite spins it is called
a singlet
Ground State
Triplet Excited State
h
Ground State
Singlet Excited State
h
5. B. Singlet and Triplet States
5. In principle there is a triplet state for every corresponding singlet state
6. By Hund’s rules the excited triplet state should be lower in energy than the
corresponding excited singlet state (it costs more energy to pair an
electron than place it in a new orbital)
7. It would seem then, that the promotion of a given electron from the ground
state could result in either an excited triplet state or excited singlet state,
just a different amount of energy is required
8. This is not the case because these transitions may be “forbidden” by the
rules of quantum mechanics.
9. These restrictions are known as “selection rules”:
• Spin-forbidden transitions: transitions where a electron changes its
direction of spin are “forbidden”. To change spin involves a change in
angular momentum.
• Therefore singlet-triplet and triplet-singlet transitions are forbidden and
singlet-singlet and triplet-triplet transitions are allowed
• “Forbidden” in this case indicates “highly improbable”.
These transitions do occur, slowly and with low intensity.
• The difference in rate between an allowed transition and a “forbidden”
one lead to the difference between many photophysical processes.
6. Fluorescence:
S1 S0 + h
•Fluorescence – Emission of light when mole returns from excited singlet
state to ground state; does not require change in spin orientation (more
common of relaxation)
•The molecule relaxes from the lowest vibrational energy level of excited
State to a vibrational energy level of ground state.
(10-9 s)
•The energy of the emitted photon is lower than that of the incident
photons
Vibrational levels in the excited states and ground states are similar
Fluorescence emission spectrum is mirror image of absorption spectrum
7. Fluorescence is a photoluminescence process in
which atoms or molecules are excited by
absorption of electromagnetic radiation. The
excited species then relax to the ground state,
giving up their excess energy as photons. One of
the most attractive features of molecular
fluorescence is its inherent sensitivity, which is
often one to three orders of magnitude better than
absorption spectroscopy.
8. Another advantage is the large linear
concentration range of fluorescence methods,
which is significantly greater than those
encountered in absorption spectroscopy.
Fluorescence methods are, however, much less
widely applicable than absorption methods
because of the relatively limited number of
chemical systems that show appreciable
fluorescence.
9. Molecular fluorescence is measured by exciting
the sample at the absorption wavelength, also
called excitation wavelength, and measuring the
emission at a longer wavelength called the
emission or fluorescence wavelength. Usually,
fluorescence emission is measured at longer
wavelength to the incident beam so as to avoid
measuring the incident radiation. The short-lived
emission that occurs is called fluorescence.
10. Excitation Spectra and Fluorescence Spectra
Because the energy differences between
vibrational states is about the same for both
ground and excited states, the absorption, or
excitation spectrum, and the fluorescence
spectrum for a compound often appear as
approximate mirror images of one another.
There are many exceptions to this mirror-image
rule, particularly when the excited and ground
states have different molecular geometries or
when different fluorescence bands originate from
different parts of the molecule.
