Femtosecond Pump−Probe Spectroscopy of Trinuclear Transition Metal Mixed-Valence Complexes

Abstract

The dynamics of relaxation following photoinduced metal-to-metal charge-transfer (MMCT) processes of the trinuclear mixed-valence complexes [(NC)5RuII−CN−PtIV(NH3)4−NC−RuII(CN)5]4- and [(NC)5FeII−CN−PtIV(NH3)4−NC−FeII(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)5RuII−CN−PtIV(NH3)4−NC−RuII(CN)5]4- ranged from 476 ± 23 fs in water to 1110 ± 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)5FeII−CN−PtIV(NH3)4−NC−FeII(CN)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. Th...