Time-resolved IR studies of [Re(LL)(CO)4]+.

There is continuing interest in the excited-state behavior of transition metal-carbonyl complexes containing polypyridyl ligands.1-4 Their unique photophysical and photochemical properties are useful in a variety of areas, from biochemistry to materials science.5-15 Complexes of the form [M(bpy)(CO)4] (M is Cr, Mo, or W, and bpy is 2,2′-bipyridine) exhibit particularly intriguing excitedstate behavior. For these complexes, the lowest-lying excited state is typically a metal-to-ligand charge-transfer (MLCT) state, with a close-lying ligand field (LF) state to higher energy.16,17 There has been considerable interest in understanding the interplay of these two states. Following initial studies by Wrighton and Morse,15,18 Manuta and Lees19 performed extensive studies on the photosubstitution reactions of [M(bpy)(CO)4]. Irradiation into either the LF or MLCT absorption band led to substitution, but the quantum yield for photosubstitution decreased considerably for the latter. Manuta and Lees concluded that either the MLCT state is very weakly photoactive, or a low-lying 3LF state (close in energy to the MLCT state and in thermal equilibrium with it) is responsible for the longerwavelength photochemistry. Rillema and co-workers observed that the excited-state properties of [Re(bpy)(CO)4] required the inclusion of a lowlying ligand-centered (LC) state to account for the observed optical properties.20,21 High-resolution absorption and luminescence measurements by Strouse and co-workers on a series of single crystals of [Re(LL)(CO)4](PF6) (LL is 2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine, or 1,10-phenanthroline) at 298 and 15 K indicated the presence of metal-ligand vibrational sidebands and a shortened luminescence decay, consistent with a 3LC state weakly mixed with the lowest-lying 1MLCT state.22,23 The best description proposed for the excited state in these materials was a nominally LC state with an admixture of 1-3% charge-transfer character arising from a dipole exchange mechanism for wave function mixing.22,23 Time-resolved infrared (TRIR) spectroscopy has proved useful in describing the nature of convoluted excited states of carbonyl-containing complexes in solution.24-26 The ν(CO) bands are particularly helpful due to their high oscillator strengths and interaction with the metal center. In this paper, ground-state and TRIR data are presented in the ν(CO) region for [Re(LL)(CO)4]* (LL is 2,2′-bipyridine (bpy), 4,4′-dimethyl2,2′-bipyridine (dmb), or 1,10-phenanthroline (phen)) in 1,2dichloroethane following 354.7 nm excitation. Changes in the ν(CO) bands are used to characterize the photochemical behavior of these complexes. The data demonstrate formation of a unique excited state described by an admixture of CT character into an LC excited state, followed by photolysis of a single carbonyl ligand to form a fac-Re(CO)3 solvento complex.