Direct MLCT absorption

In an entirely different experimental approach the unsymmetrical mixed-valence ion shown in equation (76) was subjected to laser flash photolysis.100 Excitation was carried out into the MLCT absorption band of the Ru11 -> 7t (pz) chromophore. Following excitation, one of the deactivation channels leads to the unstable mixed-valence isomer and its subsequent relaxation to the final, stable oxidation state distribution was observed directly using picosecond laser techniques. 

While the photoexcitation can result in usefijl NO dissociation (41), the reproducible (30) MLCT absorptions do not usually involve very intense bands (10,30). Their position in the UV—vis region and the moderate intensity may lead to obscuring by more prominent bands from charge transfer transitions directed at unoccupied molecular orbitals of other redox-active ligands. 

Reactive MMCT states cannot only be generated by direct light absorption into MMCT bands but also by other processes [60]. The absorption spectrum of (NH3)5Co -pyz-Fe (CN)5 (pyz = pyrazine) does not display a MMCT bad which seems to be obscured by an intense Fe(II) pyz MLCT absorption. Nevertheless, MLCT excitation leads to the same products which should be obtained by direct MMCT excitation. 

These bands show negative solvatochromism as revealed by band shifts to lower energy in less polar solvents [5, 7, 8, 12], The direction of the solvent dependence is associated with a reduced (and reversed) molecular dipole in their MLCT excited states. Emissions from these complexes are typically broad and structureless, and they also often exhibit a rigidochromic effect [7-12], Tables 1 and 2 summarize the luminescence characteristics and environmental effects on absorption and emission maxima for rhenium(I) tricarbonyl diimine complexes. 

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