Solvation-driven excited-state dynamics of [Re(4-Et-Pyridine)(CO)3(2,2′-bipyridine)]+ in imidazolium ionic liquids.: A time-resolved infrared and phosphorescence study

l Excited-state dynamics of [Re(Etpy)(CO)(3)(bpy)](+) was studied in three imidazolium ionic liquids by time-resolved IR and emission spectroscopy on the picosecond to nanosecond time scale. Low-lying excited states were characterized by TD-DFT calculations, which also provided molecular dipole moment vectors in the relevant electronic states. TRIR spectra in ionic liquids show initial populations of two excited states: predominantly bpy-localized (IL)-I-3 and (MLCT)-M-3, characterized by v(CO) bands shifted to lower and higher frequencies, respectively, relative to the ground state. Internal conversion of (IL)-I-3 to the lowest triplet (MLCT)-M-3 occurred on a time scale commensurate with solvent relaxation. The v(CO) IR bands of the (MLCT)-M-3 state undergo a dynamic shift to higher wavenumbers during relaxation. Its three-exponential kinetics were determined and attributed to vibrational cooling (units of picoseconds), energy dissipation to the bulk solvent (tens of picoseconds), and solvent relaxation, the lifetime of which increases with increasing viscosity: [EMIM]-BF4 (330 ps) < [BMIM]BF4 (470 ps) < [BMIM]PF6 (1570 ps). Time-resolved phosphorescence spectra in [BMIM]PF6 show a similar to 2 ns drop in intensity due to the (IL)-I-3 -> (MLCT)-M-3 conversion and a dynamic Stokes shift to lower energies with a lifetime decreasing from 1.8 ns at 21 degrees C to 1.1 ns at 37 degrees C, due to decreasing viscosity of the ionic liquid. It is proposed that solvent relaxation predominantly involves collective translational motions of ions. It drives the (IL)-I-3 -> (MLCT)-M-3 conversion, increases charge reorganization in the lowest excited-state (MLCT)-M-3, and affects vibrational anharmonic coupling, which together cause the dynamic shift of excited-state IR bands. TRIR spectroscopy of carbonyl-diimine complexes emerges as a new way to investigate various aspects of solvation dynamics, while the use of slowly relaxing ionic liquids offers new insight into the photophysics of Re(I) carbonyl polypyridyls.