Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds 9
Fig. 6 Schematic energy level diagram for a generic molecule
trum of such a generic molecule would display bands corresponding to the
S0 → Sn transitions of the diagram. For metal complexes, which usually are
highly symmetric species, symmetry selection rules can also play a role in determining
the intensity of the absorption bands. Furthermore, the presence of
a heavy atom (namely, the metal) relaxes the spin-conservation rule.
The excited states are unstable species that decay not only by intramolecular
chemical reactions (e.g., dissociation, isomerization) but also (actually,
more often) by intramolecular radiative and nonradiative deactivations.
When a species is excited to upper spin-allowed excited states, it usually
undergoes a fast and 100% efficient radiationless deactivation (internal conversion,
ic) to the lowest spin-allowed excited (S1 in Fig. 6). Setting aside the
intramolecular photochemical processes, such an excited state undergoes deactivation
via three competing first-order processes: nonradiative decay to the
ground state (internal conversion, rate constant kic); radiative decay to the
ground state (fluorescence, kfl); and intersystem crossing (isc) to the lowest
triplet state T1 (kisc). In its turn, T1 can undergo deactivation via nonradiative
(intersystem crossing, k ′ isc ) or radiative (phosphorescence, kph) decayto
the ground state S0. When the species contains heavy atoms, as in the case
of metal complexes, the formally forbidden intersystem crossing and phosphorescence
processes become faster. The lifetime (τ) of an excited state, i.e.,
the time needed to reduce the excited-state concentration by 2.718, is given