Photochemistry of fac-[Re(bpy)(CO)3Cl]

The photochemistry of fac-[Re(bpy)(CO)3Cl] (1?a; bpy=2,2'-bipyridine) initiated by irradiation using <330 nm light has been investigated. Isomerization proceeded in THF to give the corresponding mer-isomer 1?b. However, in the presence of a small amount of MeCN, the main product was the CO-ligand-substituted complex (OC-6-24)-[Re(bpy)(CO)2Cl(MeCN)] (2?c; bpy=2,2'-bipyridine). In MeCN, two isomers, 2?c and its (OC-6-34) form (2?a), were produced. Only 2?c thermally isomerized to produce the (OC-6-44) form 2?b. A detailed investigation led to the conclusion that both 1?b and 2?c are produced by a dissociative mechanism, whereas 2?a forms by an associative mechanism. A comparison of the ultrafast transient UV-visible absorption, emission, and IR spectra of 1?a acquired by excitation using higher-energy light (e.g., 270 nm) and lower-energy light (e.g., 400 nm) gave detailed information about the excited states, intermediates, and kinetics of the photochemical reactions and photophysical processes of 1?a. Irradiation of 1?a using the higher-energy light resulted in the generation of the higher singlet excited state with t=25 fs, from which intersystem crossing proceeded to give the higher triplet state (3HES(1)). In THF, 3HES(1) was competitively converted to both the triplet ligand field (3LF) and metal-to-ligand charge transfer (3MLCT) with lifetimes of 200 fs, in which the former is a reactive state that converts to [Re(bpy)(CO)2Cl(thf)]+ (1?c) within 10 ps by means of a dissociative mechanism. Re-coordination of CO to 1?c gives both 1?a and 1?b. In MeCN, irradiation of 1?a by using high-energy light gives the coordinatively unsaturated complex, which rapidly converted to 2?c. A seven-coordinate complex is also produced within several hundred femtoseconds, which is converted to 2?a within several hundred picoseconds.