Photochemistry and Photophysics of Coordination Compounds

136 S. Campagna et al.
However, some experimental data contrast with the scheme shown in
Fig. 11. For example, a time-resolved resonance Raman study indicates that
in [Ru(bpy)3] 2+ even the initial excitation is localized [172]. Moreover, the recently
reported femtosecond transient absorption spectrum of [Ru(bpy)3] 2+
in the UV region suggests that complete relaxation within the emitting triplet
MLCT state takes several tens of picoseconds, and that randomization of
triplet MLCT is complete in less than 500 fs [173]. If this latter point is correct,
interligand hopping should largely occur from nonrelaxed states, probably
even partly in the singlet state. In the relaxed triplet MLCT state, interligand
hopping could be slower, but it would be difficult to measure since randomization
would already have happened.
Fig. 11 Picture of the early-time dynamics of light excitation in the MLCT singlet of
[Ru(bpy)3] 2+ . A delocalizated Franck–Condon state is formed (a), which becomes localized
on a single ligand (b) and then becomes “randomized” by interligand hopping (c)
Related to the excited-state dynamics at short times after excitation, broadband
femtosecond fluorescence spectroscopy of [Ru(bpy)3] 2+ has been recently
reported, as already mentioned [171]. The authors get 15 ± 10 fs as the
lifetime for the singlet emission, which is centered at about 520 nm.
4.3
Ru(II) Complexes Based on Tridentate Polypyridine Ligands
An important family of Ru(II) polypyridine complexes is that based on
tridentate ligands, with [Ru(terpy)2] 2+ as a prototype (terpy = 2,2 ′ :6 ′ ,2 ′′ -
terpyridine). The absorption, emission, and redox properties of [Ru(terpy)2] 2+
are similar to those of [Ru(bpy)3] 2+ ,exceptthat[Ru(terpy)2] 2+ is essentially
nonluminescent at room temperature, with a lifetime of the 3 MLCT
state in degassed acetonitrile at room temperature of about 250 ps (meas-