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 = C6H5CH2 and Ph = C6H5) has been investigated in THF and CH3CN solutio ns. W-visible absorption and steady-state emission spectroscopy indica tes that in THF ReBo exists primarily as an ion-pair. A weak absorptio n band is observed for the salt in THF solution that is assigned to an optical ion-pair charge transfer transition. Stern-Volmer emission qu enching studies indicate that B(2)BPh(3)(-) quenches the luminescent d pi (Re) --> pi (bpy) metal-to-ligand charge transfer excited state o f the (bpy)Re(CO)(3)(py)(+) chromophore. The quenching is attributed t o 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 t hat electron transfer quenching leads to irreversible reduction of the Re(I) cation to (bpy(-.))Re-I(CO)(3)(py). Photoinduced electron trans fer is irreversible owing to rapid C-B bond fragmentation in the benzy lboranyl radical, PhCH(2)BPh(3)(.) --> PhCH(2)(.) + BPh(3)(.). Quantit ative laser flash photolysis experiments show that the quantum efficie ncy for production of the reduced complex (bpy(-.))Re-I(CO)(3)(py) is unity, suggesting that C-B bond fragmentation in the benzylboranyl rad ical occurs more rapidly than return electron transfer within the gemi nate radical pair that is formed by photoinduced electron transfer.