Redox control of light-induced charge separation in a transition metal cluster: photochemistry of a methyl viologen-substituted [Os3(CO)10(alpha-diimine)] cluster.
(Sub)picosecond transient absorption (TA) and time-resolved infrared (TRIR) spectra of the cluster [Os3(CO)10-(AcPy-MV)]2+ (the dication AcPy-MV = AcPy-MV2+ = [2-pyridylacetimine-N-(2-(1'-methyl-4,4'-bipyridine-1,1'-diium-1-yl)ethyl)](PF6)2) (1(2+)) reveal that photoinduced electron transfer to the electron-accepting 4,4'-bipyridine-1,1'-diium (MV2+) moiety competes with the fast relaxation of the initially populated sigmapi excited state of the cluster to the ground state and/or cleavage of an Os-Os bond. The TA spectra of cluster 1(2+) in acetone, obtained by irradiation into its lowest-energy absorption band, show the characteristic absorptions of the one-electron-reduced MV*+ unit at 400 and 615 nm, in accordance with population of a charge-separated (CS) state in which a cluster-core electron has been transferred to the lowest pi orbital of the remote MV2+ unit. This assignment is confirmed by picosecond TRIR spectra that show a large shift of the pilot highest-frequency nu(CO) band of 1(2+) by ca. +40 cm(-1), reflecting the photooxidation of the cluster core. The CS state is populated via fast (4.2 x 10(11) s(-1)) and efficient (88%) oxidative quenching of the optically populated sigmapi excited state and decays biexponentially with lifetimes of 38 and 166 ps (1.2:1 ratio) with a complete regeneration of the parent cluster. About 12% of the cluster molecules in the sigmapi excited state form long-lived open-core biradicals. In strongly coordinating acetonitrile, however, the cluster core-to-MV2+ electron transfer in cluster 1(2+) results in the irreversible formation of secondary photoproducts with a photooxidized cluster core. The photochemical behavior of the [Os3(CO)10(alpha-diimine-MV)]2+ (donor-acceptor) dyad can be controlled by an externally applied electronic bias. Electrochemical one-electron reduction of the MV2+ moiety prior to the irradiation reduces its electron-accepting character to such an extent that the photoinduced electron transfer to MV(*+) is no longer feasible. Instead, the irradiation of reduced cluster 1(*+) results in the reversible formation of an open-core zwitterion, the ultimate photoproduct also observed upon irradiation of related nonsubstituted clusters [Os3(CO)10(alpha-diimine)] in strongly coordinating solvents such as acetonitrile.