18
1 Introduction to Photochemistry
We see that, once the spectrum of the exciting light is fixed, the photochemical rate
constant is proportional to the total irradiance.
Equation (1.62) can be simplified if the quantum yield is approximately constant
in the frequency interval [ν a , ν b ] and vanishes elsewhere. This is rather common: for
instance, in gas phase the photodissociation quantum yield of a small molecule with
dissociation energy E diss is almost one for ν > E diss / h and drops to zero below this
threshold (see also the last part of Sect. 3.11). In such cases, we can write
J X,K J
(ν a ,ν b )
exc,K Φ X,K .
(1.65)
When using monochromatic light of frequency ν exc these relationships simplify
without approximations:
J X,K = J
(ν exc )
exc,K Φ X,K (ν exc ) = C ε,σ ε K (ν exc ) I ph,tot Φ X,K (ν exc ) .
(1.66)
For this reason, and for a better reproducibility, many experiments are performed
with monochromatic light.
The case in which the decay of the excited state(s) of molecule K is slow enough
as to be monitored with the available techniques calls for a kinetic treatment that
takes into account their transient populations. However, it is not always possible to
define a set of rate constants for the primary processes, because the excited state
dynamics may depend on a complex interplay of structural changes, energy transfer
to the environment and radiationless electronic transitions, as will be discussed in
Chaps. 4 and 5. A typical case in which the definition of rate constants for a kinetic
treatment is viable occurs when the electronically excited molecules can become
thermally equilibrated as to their nuclear degrees of freedom by interaction with the
environment. In condensed phase, this requires lifetimes much longer than 10 ps,
which are not uncommon.
As an example, it has became clear thanks to experimental and theoretical work
that the excited state dynamics of benzophenone is a rather complicated sequence
of fast transitions of IC and ISC type, involving S 2 (the state connected with S 0
by the strongest optical transition), S 1 , T 2 , and higher triplets (see [5, 8, 9] and
refs. therein). Almost 100% of the benzophenone molecules end up in the lowest
triplet state, T 1 . Then, a slow ISC to the ground state takes place, in competition
with phosphorescence emission. Given its high triplet quantum yield and the long
lifetime of its T 1 state, benzophenone is a good triplet sensitizer. Triplet sensitization
is used to populate the triplet states of other molecules in which the ISC from singlet
to triplet states is not efficient. A further advantage is the possibility to irradiate with
longer wavelengths than those needed to directly excite the energy acceptor, when the
absorbing singlet states of the latter absorb at short wavelengths. Moreover, in certain
cases it may be convenient to bypass the singlet states of the acceptor, when they
would undergo undesired reactions. In Fig. 1.2 we show the Jablonski diagrams of
benzophenone and of naphthalene. The triplet quantum yield of the latter, in benzene
at 29
◦ C, is Φ T = 0.39 and that of phosphorescence is Φ P = 0.03. That means that
39% of the excited naphthalene molecules populate T 1 , while the others decay to
1 Introduction to Photochemistry
We see that, once the spectrum of the exciting light is fixed, the photochemical rate
constant is proportional to the total irradiance.
Equation (1.62) can be simplified if the quantum yield is approximately constant
in the frequency interval [ν a , ν b ] and vanishes elsewhere. This is rather common: for
instance, in gas phase the photodissociation quantum yield of a small molecule with
dissociation energy E diss is almost one for ν > E diss / h and drops to zero below this
threshold (see also the last part of Sect. 3.11). In such cases, we can write
J X,K J
(ν a ,ν b )
exc,K Φ X,K .
(1.65)
When using monochromatic light of frequency ν exc these relationships simplify
without approximations:
J X,K = J
(ν exc )
exc,K Φ X,K (ν exc ) = C ε,σ ε K (ν exc ) I ph,tot Φ X,K (ν exc ) .
(1.66)
For this reason, and for a better reproducibility, many experiments are performed
with monochromatic light.
The case in which the decay of the excited state(s) of molecule K is slow enough
as to be monitored with the available techniques calls for a kinetic treatment that
takes into account their transient populations. However, it is not always possible to
define a set of rate constants for the primary processes, because the excited state
dynamics may depend on a complex interplay of structural changes, energy transfer
to the environment and radiationless electronic transitions, as will be discussed in
Chaps. 4 and 5. A typical case in which the definition of rate constants for a kinetic
treatment is viable occurs when the electronically excited molecules can become
thermally equilibrated as to their nuclear degrees of freedom by interaction with the
environment. In condensed phase, this requires lifetimes much longer than 10 ps,
which are not uncommon.
As an example, it has became clear thanks to experimental and theoretical work
that the excited state dynamics of benzophenone is a rather complicated sequence
of fast transitions of IC and ISC type, involving S 2 (the state connected with S 0
by the strongest optical transition), S 1 , T 2 , and higher triplets (see [5, 8, 9] and
refs. therein). Almost 100% of the benzophenone molecules end up in the lowest
triplet state, T 1 . Then, a slow ISC to the ground state takes place, in competition
with phosphorescence emission. Given its high triplet quantum yield and the long
lifetime of its T 1 state, benzophenone is a good triplet sensitizer. Triplet sensitization
is used to populate the triplet states of other molecules in which the ISC from singlet
to triplet states is not efficient. A further advantage is the possibility to irradiate with
longer wavelengths than those needed to directly excite the energy acceptor, when the
absorbing singlet states of the latter absorb at short wavelengths. Moreover, in certain
cases it may be convenient to bypass the singlet states of the acceptor, when they
would undergo undesired reactions. In Fig. 1.2 we show the Jablonski diagrams of
benzophenone and of naphthalene. The triplet quantum yield of the latter, in benzene
at 29
◦ C, is Φ T = 0.39 and that of phosphorescence is Φ P = 0.03. That means that
39% of the excited naphthalene molecules populate T 1 , while the others decay to