Redox Control of Light-Induced Charge Separation in a Transition Metal Cluster:  Photochemistry of a Methyl Viologen-Substituted [Os3(CO)10(α-diimine)] Cluster

• Published in: Inorganic chemistry 2005-03, Vol.44 (5), p.1319-1331
• Main Authors: Vergeer, Frank W., Kleverlaan, Cornelis J., Matousek, Pavel, Towrie, Michael, Stufkens, Derk J., Hartl, František
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic049191n

(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 σπ* 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 π* orbital of the remote MV2+ unit. This assignment is confirmed by picosecond TRIR spectra that show a large shift of the pilot highest-frequency ν(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 × 1011 s-1) and efficient (88%) oxidative quenching of the optically populated σπ* 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 σπ* 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(α-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(α-diimine)] in strongly coordinating solvents such as acetonitrile.