Inverse kinetic isotope effect in the excited-state relaxation of a Ru(II)-aquo complex: revealing the impact of hydrogen-bond dynamics on nonradiative decay.

Photophysics of the MLCT excited-state of [Ru(bpy)(tpy)(OH2)](2+) (1) and [Ru(bpy)(tpy)(OD2)](2+) (2) (bpy = 2,2'-bipyridine and tpy = 2,2':6',2″-terpyridine) have been investigated in room-temperature H2O and D2O using ultrafast transient pump-probe spectroscopy. An inverse isotope effect is observed in the ground-state recovery for the two complexes. These data indicate control of excited-state lifetime via a pre-equilibrium between the (3)MLCT state that initiates H-bond dynamics with the solvent and the (3)MC state that serves as the principal pathway for nonradiative decay.