Photochemistry and Photophysics of Coordination Compounds

10 V. Balzani et al.
by the reciprocal of the summation of the deactivation rate constants. For the
molecule of Fig. 6,
τ(S1)=
1
(kic + kfl + kisc)
1
τ(T1)=
(k ′ . (3)
isc + kph)
The lifetimes of the lowest spin-allowed and spin-forbidden excited state
(τ(S1) andτ(T1) in the example of Fig. 6) are approximately 10 –9 –10 –7 sand
10 –3 –100 s, respectively, for organic molecules, but they become shorter by
several orders of magnitude for metal complexes. For example, the lifetime of
the lowest spin-forbidden excited state of naphthalene is around 2 s, whereas
that of [Ru(bpy)3] 2+ , because of the presence of the heavy Ru ion, is about
1 µs [24].
The quantum yields of fluorescence (ratio between the number of photons
emitted by the lowest spin-allowed excited state, S1 in Fig. 6, and the number
of absorbed photons) and phosphorescence (ratio between the number of
photons emitted by the lowest spin-forbidden excited state, T1 in Fig. 6, and
the number of absorbed photons) can range between 0 and 1 and are given
by the following expressions:
Φ fl =
k fl
(kic + k fl + kisc)
kph × kisc
Φph =
(k ′ isc + kph)
. (5)
× (kic + kfl + kisc)
The excited-state lifetimes and fluorescence and phosphorescence quantum
yields of a great number of organic molecules and metal complexes are
known [24].
3
Bimolecular Processes
3.1
General Features
In fluid solution, when the intramolecular deactivation processes are not too
fast, i.e., when the lifetime of the excited state is sufficiently long, an excited
molecule ∗ A may have a chance to encounter a molecule of another solute B.
In such a case, some specific interaction can occur leading to the deactivation
of the excited state by second-order kinetic processes. The two most important
types of interactions in an encounter are those leading to electron or
(2)
(4)