Photochemistry and Photophysics of Coordination Compounds

8 V. Balzani et al.
(Fig. 5). For most Cr(III) complexes, e.g., for [Cr(NH3)6] 3+ ,thelowest-energy
transition is metal centered and the resulting πM(t2g) 2 σ ∗ M (eg) configuration
gives rise to 4 T2g and 4 T1g excited states (Fig. 5). Several other coordination
compounds, including the complexes of the lanthanide ions, have an openshell
ground-state configuration and, as a consequence, a ground state with
high-multiplicity and low-energy intraconfigurational metal-centered excited
states.
Fig. 5 Configurations (a) andstate(b) diagrams for an octahedral Cr(III) complex. Only
the lower-lying excited states of each configuration are shown [20]
In conclusion, metal complexes tend to have more complex and specific
Jablonski diagrams than organic molecules. Points to be noticed are: (1) spin
multiplicity other than singlet and triplet can occur, but for each electronic
configuration the state with highest multiplicity remains the lowest one;
(2) excited states can exist that belong to the same configuration of the ground
state (this implies that the ground state has the highest multiplicity); and
(3) more than one pair of states of different multiplicity can arise from a single
electron configuration. In the following, in order to discuss some general
concept of molecular photochemistry we will make use of a generic Jablonski
diagram based on singlet and triplet states.
2.3
Light Absorption and Intramolecular Excited-State Decay
Figure 6 shows a schematic energy level diagram for a generic molecule [23].
In principle, transitions between states having the same multiplicity are allowed,
whereas those between states of different multiplicity are forbidden.
Therefore, the electronic absorption bands observed in the UV–visible spec-