Ion-Pair Charge Transfer Photochemistry in Rhenium(I) Borate Salts.
The photophysics and photochemistry of the salt [(bpy)Re(CO)(3)(py)(+)][BzBPh(3)(-)] (ReBo, where bpy = 2,2'-bipyridine, py = pyridine, Bz = C(6)H(5)CH(2) and Ph = C(6)H(5)) has been investigated in THF and CH(3)CN solutions. UV-visible absorption and steady-state emission spectroscopy indicates that in THF ReBo exists primairly as an ion-pair. A weak absorption band is observed for the salt in THF solution that is assigned to an optical ion-pair charge transfer transition. Stern-Volmer emission quenching studies indicate that BzBPh(3)(-) quenches the luminescent dpi (Re) --> pi (bpy) metal-to-ligand charge transfer excited state of the (bpy)Re(CO)(3)(py)(+) chromophore. The quenching is attributed to electron transfer from the benzylborate anion to the photoexcited Re(I) complex, (bpy(-)(*))Re(II)(CO)(3)(py)(+) + BzBPh(3)(-) --> (bpy(-)(*))Re(I)(CO)(3)(py) + BzBPh(3)(*). Laser flash photolysis studies reveal that electron transfer quenching leads to irreversible reduction of the Re(I) cation to (bpy(-)(*))Re(I)(CO)(3)(py). Photoinduced electron transfer is irreversible owing to rapid C-B bond fragmentation in the benzylboranyl radical, PhCH(2)BPh(3)(*) --> PhCH(2)(*) + BPh(3)(*). Quantitative laser flash photolysis experiments show that the quantum efficiency for production of the reduced complex (bpy(-)(*))Re(I)(CO)(3)(py) is unity, suggesting that C-B bond fragmentation in the benzylboranyl radical occurs more rapidly than return electron transfer within the geminate radical pair that is formed by photoinduced electron transfer.
McCosar
Bruce H.
BH
Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200.
Schanze
Kirk S.
KS