1.4 An Overview of Photochemical and Photophysical Processes
9
excited state, its energy must be conserved, so a certain amount of electronic
energy will be converted into vibrational energy. The latter will be transferred
to the environment, within times of the order of 10 ps in condensed phase and
much more slowly in gas phase. Ultrafast decays occur in less than 100 fs, but
spin-changing radiationless transitions can be much slower, in the μs range or
even more. Radiationless transitions between states belonging to the same spin
manifold are called “internal conversion” (IC). When the initial and final state are
of different spin the process is called “intersystem crossing” (ISC).
• Quenching:
A
∗
+ B → A + B
(1.28)
The excited molecule A
∗ interacts with the quencher B and transfers to it part of the
excitation energy. When they move apart, none of them is electronically excited
and the excess energy has been transferred to the nuclear degrees of freedom
(vibrational, rotational, and translational) of both molecules, much in the same
way as in the case of unimolecular radiationless decay. In all bimolecular processes
of course the rate depends on the concentrations of both species, [A
∗ ] and [B].
However, in order to compare with the rates of unimolecular processes the common
factor [A
∗ ] is irrelevant and one can adopt the point of view of a single A
∗ molecule.
With an efficient quencher B at reasonable concentrations (or partial pressures in
gas phase), quenching can compete with unimolecular processes if the lifetime of
A
∗ is 1 ns or longer. These considerations apply also to excitation transfer, charge
transfer, and bimolecular reactions (see below).
• Excitation transfer or sensitization:
A
∗
+ B → A + B
∗
(1.29)
As in quenching, the excited sensitizer A
∗ donates energy to B, but the transferred
energy is sufficient to electronically excite the acceptor. As a consequence, a minor
fraction of the excitation energy goes into the nuclear degrees of freedom.
• Photoisomerization:
A
∗
→ B
(1.30)
The excited molecule A isomerizes to B and reverts to the ground state by radiationless decay. The photon energy is used to overcome the activation barrier of the
reaction by performing part of the structural change in the excited state. A detailed
analysis of the dynamics, performed by experimental or theoretical means, may
reveal the temporal relationship of the two events, isomerization and radiationless
transition: in fact, they may turn out to be essentially simultaneous. If the A →
B isomerization is endothermic, a usually minor fraction of the photon energy is
stored as chemical energy while the rest is dissipated in the environment.
• Photodissociation:
A
∗
→ B + C
(1.31)
9
excited state, its energy must be conserved, so a certain amount of electronic
energy will be converted into vibrational energy. The latter will be transferred
to the environment, within times of the order of 10 ps in condensed phase and
much more slowly in gas phase. Ultrafast decays occur in less than 100 fs, but
spin-changing radiationless transitions can be much slower, in the μs range or
even more. Radiationless transitions between states belonging to the same spin
manifold are called “internal conversion” (IC). When the initial and final state are
of different spin the process is called “intersystem crossing” (ISC).
• Quenching:
A
∗
+ B → A + B
(1.28)
The excited molecule A
∗ interacts with the quencher B and transfers to it part of the
excitation energy. When they move apart, none of them is electronically excited
and the excess energy has been transferred to the nuclear degrees of freedom
(vibrational, rotational, and translational) of both molecules, much in the same
way as in the case of unimolecular radiationless decay. In all bimolecular processes
of course the rate depends on the concentrations of both species, [A
∗ ] and [B].
However, in order to compare with the rates of unimolecular processes the common
factor [A
∗ ] is irrelevant and one can adopt the point of view of a single A
∗ molecule.
With an efficient quencher B at reasonable concentrations (or partial pressures in
gas phase), quenching can compete with unimolecular processes if the lifetime of
A
∗ is 1 ns or longer. These considerations apply also to excitation transfer, charge
transfer, and bimolecular reactions (see below).
• Excitation transfer or sensitization:
A
∗
+ B → A + B
∗
(1.29)
As in quenching, the excited sensitizer A
∗ donates energy to B, but the transferred
energy is sufficient to electronically excite the acceptor. As a consequence, a minor
fraction of the excitation energy goes into the nuclear degrees of freedom.
• Photoisomerization:
A
∗
→ B
(1.30)
The excited molecule A isomerizes to B and reverts to the ground state by radiationless decay. The photon energy is used to overcome the activation barrier of the
reaction by performing part of the structural change in the excited state. A detailed
analysis of the dynamics, performed by experimental or theoretical means, may
reveal the temporal relationship of the two events, isomerization and radiationless
transition: in fact, they may turn out to be essentially simultaneous. If the A →
B isomerization is endothermic, a usually minor fraction of the photon energy is
stored as chemical energy while the rest is dissipated in the environment.
• Photodissociation:
A
∗
→ B + C
(1.31)