12
1 Introduction to Photochemistry
lead to the ground state, while ISC populates T 1 . From the triplet state the decay to
S 0 can take place by phosphorescence emission or ISC. Actually in many cases the
vibrational relaxation and the electronic transitions (especially IC between excited
states) occur in the same timescale and considerably affect each other, so the slanted
arrows can be a closer representation of the physical reality.
Many of the products of primary events are reactive species that readily undergo
“secondary” chemical or photophysical transformations. For instance, starting from
neutral molecules with even numbers of electrons, neutral radicals are produced by
homolytic photodissociation and hydrogen abstraction, oppositely charged ion pairs
by proton transfer; radical cations by photoionization and electron transfer; and
radical anions too by the last process. All these species are likely to react in different
ways and times, depending on the environment. The photosensitization produces
excited species that can undergo substantially the same processes listed above as
“primary.” The ground state products of several primary events, radiationless decay
in the first place, are endowed with a large amount of energy in the nuclear degrees
of freedom and can give place to “hot ground state” reactions that would not occur
at normal temperatures.
1.5 Quantum Yields
To any well-identified photochemical or photophysical process X we can associate a
quantum yield Φ, which is by definition the ratio between the number of molecules
undergoing that process and the number of absorbed photons:
Φ X ≡
number o f molecules undergoing process X
number o f absor bed photons
.
(1.36)
If more than one light absorbing compound is present, we only count the photons
absorbed by the species originating the chain of events to which process X belongs.
The process X , however complex and not known in detail, can be univocally identified
on the basis of a chemical product it generates or a molecular state it populates: for
instance, we can talk about the triplet quantum yield of a given compound, without
knowing through which sequence of radiationless transitions its triplet states are
populated. If, as usual, one absorbed photon corresponds to one excited molecule, the
denominator of Eq. (1.36) is equal to the number of excited molecules. The quantum
yield is measured over a given interval of time, which is assumed to encompass
all possible occurrences of X : for instance, if X is an excited state process, the
measurement time must be much longer than the lifetime. Alternatively, in a steadystate situation as to the (fast) process X , the quantum yield can be evaluated as a rate
ratio:
Φ X =
rate o f process X
rate o f photon absor ption
.
(1.37)
1 Introduction to Photochemistry
lead to the ground state, while ISC populates T 1 . From the triplet state the decay to
S 0 can take place by phosphorescence emission or ISC. Actually in many cases the
vibrational relaxation and the electronic transitions (especially IC between excited
states) occur in the same timescale and considerably affect each other, so the slanted
arrows can be a closer representation of the physical reality.
Many of the products of primary events are reactive species that readily undergo
“secondary” chemical or photophysical transformations. For instance, starting from
neutral molecules with even numbers of electrons, neutral radicals are produced by
homolytic photodissociation and hydrogen abstraction, oppositely charged ion pairs
by proton transfer; radical cations by photoionization and electron transfer; and
radical anions too by the last process. All these species are likely to react in different
ways and times, depending on the environment. The photosensitization produces
excited species that can undergo substantially the same processes listed above as
“primary.” The ground state products of several primary events, radiationless decay
in the first place, are endowed with a large amount of energy in the nuclear degrees
of freedom and can give place to “hot ground state” reactions that would not occur
at normal temperatures.
1.5 Quantum Yields
To any well-identified photochemical or photophysical process X we can associate a
quantum yield Φ, which is by definition the ratio between the number of molecules
undergoing that process and the number of absorbed photons:
Φ X ≡
number o f molecules undergoing process X
number o f absor bed photons
.
(1.36)
If more than one light absorbing compound is present, we only count the photons
absorbed by the species originating the chain of events to which process X belongs.
The process X , however complex and not known in detail, can be univocally identified
on the basis of a chemical product it generates or a molecular state it populates: for
instance, we can talk about the triplet quantum yield of a given compound, without
knowing through which sequence of radiationless transitions its triplet states are
populated. If, as usual, one absorbed photon corresponds to one excited molecule, the
denominator of Eq. (1.36) is equal to the number of excited molecules. The quantum
yield is measured over a given interval of time, which is assumed to encompass
all possible occurrences of X : for instance, if X is an excited state process, the
measurement time must be much longer than the lifetime. Alternatively, in a steadystate situation as to the (fast) process X , the quantum yield can be evaluated as a rate
ratio:
Φ X =
rate o f process X
rate o f photon absor ption
.
(1.37)