Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 121
antisymmetrical (Ψ ) with respect to rotation around the C2 axis retained by
each Ru(bpy) unit. A more detailed picture of the highest occupied molecular
orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) is
shown in Fig. 1b [14–16]. The HOMOs are πMa1(d) and πMe(d), which are
mainly localized on the metal; the LUMOs are π ∗ L a2(Ψ )andπ ∗ L
e(Ψ ), which
are mainly localized on the ligands. The ground state of the complex is a singlet,
derived from the πMe(d) 4 πMa1(d) 2 electronic configuration.
According to Kasha’s rule, only the lowest excited state and the upper states
that can be populated on the basis of the Boltzmann equilibrium distribution
may play a role in determining the photochemical and photophysical properties.
The MC excited states of d 6 octahedral complexes are strongly displaced
with respect to the ground-state geometry along metal–ligand vibration coordinates
[17, 18].
When the lowest excited state is MC, it undergoes fast radiationless deactivation
to the ground state and/or ligand dissociation reactions (Fig. 3). As
a consequence, at room temperature the excited-state lifetime is very short,
no luminescence emission can be observed [19], and very rarely bimolecular
(or supramolecular) reactions can take place. LC and MLCT excited states
are usually not strongly displaced compared to the ground-state geometry.
Thus, when the lowest excited state is LC or MLCT (Fig. 3) it does not undergo
fast radiationless decay to the ground state and luminescence can usually be
observed. The radiative deactivation rate constant is somewhat higher for
3 MLCT than for 3 LC because of the larger spin–orbit coupling effect. For this
reason, the 3 LC excited states are longer lived at low temperature in a rigid
matrix and the 3 MLCT excited states are more likely to exhibit luminescence
at room temperature in fluid solution.
Fig. 3 Schematic representation of two limiting cases for the relative positions of 3 MC and
3 LC (or 3 MLCT) excited states