LONG-DISTANCE, BRIDGE-MEDIATED ELECTRON-TRANSFER IN A LIGAND-BRIDGED RE-I-RE-I COMPLEX

Laser flash excitation (420 nm) of CH3CN solutions containing [(py-PTZ)(CO)(3)Re(mu-bbpe)Re(CO)(3)(MQ(+))](PF6)(3), (py-PTZ is 4-(10-methylphenothiazine)-4'-methyl-2,2'-bipyridine; bbpe is 1,2-trans-bis (4'-methyl-2,2'-bipyrid-4-yl)ethene; MQ(+) is N-methyl-4,4'-bipyridinium hexafluorophosphate) results in formation of the redox-separated state [(py-PTZ(.+))(CO)(3)Re-I(mu-bbpe)Re-I(CO)(3)(MQ(.))](PF6)(3) as shown by transient resonance Raman measurements. It appears during the (similar to 5 ns) laser pulse following Re-I --> bbpe, pi --> pi*(bbpe) excitation and a series of intramolecular electron transfers. The rate constant for back electron transfer is k(CH3CN, 298 K) = 5.56 x 10(7) s(-1). Similarly, Re-I --> bbpe, pi --> pi*(bbpe) excitation of [(py-PTZ)(CO)(3)Re(mu-bbpe)Re(CO)(3)(py-PTZ)](PF6)(2) leads to [(py-PTZ(.+))(CO)(3)Re-I(mu-bbpe(.-))Re-I(CO)(3)(py-PTZ)](PF6)(2) during the laser pulse. It returns to the ground state with k(CH3CN, 298 K) = 2.33 x 10(7) s(-1). In the model [(4-Etpy)(CO)(3)Re(mu-bbpe)Re(CO)(3)(4-Etpy)](PF6)(2), there is a lowest-lying pi-pi* excited state and a close-lying MLCT state which dominates excited state decay and emission at room temperature. The results of transient absorption measurements suggests that these states coexist at room temperature and undergo relatively slow interconversion.