Investigation of interligand electron transfer in polypyridyl complexes of Os(II) using femtosecond polarization anisotropy methods:: Examination of Os(bpy)32+ and Os(bpy)2(mab)2+

Femtosecond pump-probe polarization anisotropy measurements are used to investigate the interligand electron transfer (ILET) dynamics in two polypyridyl Os(II) complexes following photoexcitation to a metal-to-ligand charge transfer (MLCT) state. The two complexes studied are Os(bpy)(3)(2+) (bpy = 2,2'-bipyridine) and a mixed-ligand analogue, Os(bpy)(2)(mab)(2+), where mab is a mono-amide functionalized bipyridine ligand. In acetonitrile, the bpy --> bpy ILET time is 8.7 ps in Os(bpy)(3)(2+) and the bpy --> mab ILET is 1.5 ps in Os(bpy)(2)(mab)(2+), A solvent dependence study reveals that the ligand-ligand electron-transfer time (in both complexes) scales with the reorganization time of the solvent, suggesting that ILET in these complexes occurs in the adiabatic limit. An analysis of the anisotropy amplitudes for the excited-state absorption in Os(bpy)(3)(2+) may provide evidence for the formation of a delocalized (or partially delocalized) excited state produced by optical excitation.