Picosecond Relaxation of3MLCT Excited States of [Re(Etpy)(CO)3(dmb)]+and [Re(Cl)(CO)3(bpy)] as Revealed by Time-Resolved Resonance Raman, UV−vis, and IR Absorption Spectroscopy

Abstract

Picosecond dynamics of Re → polypyridine 3MLCT excited states of [Re(Etpy)(CO)3(dmb)]+ and [Re(Cl)(CO)3(bpy)] were investigated by time-resolved UV−vis, resonance Raman, and IR spectroscopy. Raman bands due to NN•- intraligand vibrations of the excited molecules increase with time during the first 15−20 ps after excitation. The time constant of 6 ± 2 ps was estimated for the increase of areas of excited-state Raman bands of [Re(Etpy)(CO)3(dmb)]+. The growth of Raman bands is accompanied by an increase of the near-UV transient absorption band at 375 nm, which corresponds to a ππ* transition of the dmb•- ligand of the 3MLCT excited state [ReII(Etpy)(CO)3(dmb•-)]+. These effects are attributed to structural reorganization during vibrational cooling, during which the electronic dipole moment and/or vibrational overlap integrals increase. IR bands due to CO stretching vibrations and some of the Raman bands undergo dynamical upward shift and narrowing, that occur with time constants between 1 and 11 ps, manifes...