LIGHT-INDUCED MULTIELECTRON CHARGE-TRANSFER PROCESSES OCCURRING IN A SERIES OF GROUP-8-PLATINUM CYANOBRIDGED COMPLEXES

Although true multielectron charge transfer processes do not exist wit hin the realm of molecular photochemistry, one can design mimicking sy stems via the production of a reactive one-electron charge transfer in termediate. Reported in this review are the photochemical and photophy sical properties of a group of symmetric, multinuclear complexes of th e general form [L'(CN)(4)M (CN)-(PtL4)-(NC)-M (CN)(4)L'](n-) (where M is a Group 8 metal, L is an amine, and LI is a sigma-donor ligand) tha t provide for apparent photoinduced multielectron charge transfer. The se complexes exhibit intense intervalence charge transfer (IVCT) bands in the blue portion of the optical spectrum (350-450 nm). In the case where M = Fe, irradiation into the IVCT band centered at 425 MI produ ces a net two-electron charge transfer with a quantum yield of ca. 0.1 in an aqueous solvent. However, multielectron charge transfer photoch emistry can be observed for M=Os or Ru only by using a mixed DMSO-aque ous solvent, in which the cyanide to water hydrogen bonding found in p ure aqueous solvent is destroyed, thereby, shifting the redox potentia l of the cyanometalates to values similar to M=Fe. The observed reacti on is found to selectively yield two-electron products. The reactivity of these complexes as a function of Group 8 metal and solvent system is nicely predicted using the charge transfer theories of Marcus and H ush, with the source of the differential reactivity being the shift in the relative activation barriers for the conversion of a one-electron [Fe-III, Pt-III, Fe-II] intermediate to the observed two-electron pro ducts or to the starting material. Well-defined oligomers and polymers of the iron-based system have been synthesized. The photochemical rea ctivity and photophysics of these species are found to be a function o f molecular geometry. In the case of the polymeric systems, one-dimens ional, two-dimensional, and network materials can be synthesized using electrochemical techniques to control the polymer reactivity sites. B oth solution and surface-confined photochemistry can be observed for t hese systems. (C) 1997 Elsevier Science S.A.