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 (MLCT)-M-3 state that initiates H-bond dynamics with the solvent and the (MC)-M-3 state that serves as the principal pathway for nonradiative decay.