Femtosecond pump-probe spectroscopy of trinuclear transition metal mixed-valence complexes

The dynamics of relaxation following photoinduced metal-to-metal charge-transfer (MMCT) processes of the trinuclear mixed-valence complexes [(NC)(5)Ru-II-CN-Pt-IV(NH3)(4)-NC-Ru-II(CN)(5)](4-) and [(NC)(5)Fe-II-CN-Pt-IV(NH3)(4)-NC-Fe-II(CN)(5)](4-) were investigated with femtosecond pump-probe spectroscopy. Relaxation dynamics following photoinduced M(II) --> Pt(IV) MMCT varied with solvent and the identity of the M(II) centers. The back-electron-transfer (BET) time scale of [(NC)(5)Ru-II-CN-Pt-IV(NH3)(4)-NC-Ru-II(CN)(5)](4-) ranged from 476 +/- 23 fs in water to I 110 +/- 288 fs in a water/DMSO mixture with mole fraction DMSO of 0.1. The time scale of vibrational relaxation in the ground state ranged from 1.5 to 2.7 ps. Electron-transfer and ground-state vibrational relaxation dynamics of [(NC)(5)Fe-II-CN-Pt-IV(NH3)(4)-NC-(FeCN)-C-II)(5)](4-) were convolved, with relaxation to ground and product electronic states occurring within 3.5 ps. The electron-transfer dynamics of both complexes are significantly slower than the solvation dynamics of water or DMSO. The solvent dependence of the BET time scale of [(NC)(5)Ru-II-CN-Pt-IV(NH3)(4)-NC-(RuCN)-C-II)(5)](4-) is attributed to solvent-induced movement of the ground-state potential energy surface and the resulting changes in the driving force for BET. These effects enable systematic control over both the quantum yield of photodissociation and the dynamics of relaxation following photoinduced MMCT.