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 within the realm of molecular photochemistry, one can design mimicking systems via the production of a reactive one-electron charge transfer intermediate. Reported in this review are the photochemical and photophysical properties of a group of symmetric, multinuclear complexes of the general form [L′(CN)₄M(CN)-(PtL₄)-(NC)-M(CN) ₄L′]^n- (where M is a Group 8 metal, L is an amine, and L′ is a σ-donor ligand) that provide for apparent photoinduced multielectron charge transfer. These 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 nm produces a net two-electron charge transfer with a quantum yield of ca. 0.1 in an aqueous solvent. However, multielectron charge transfer photochemistry can be observed for M = Os or Ru only by using a mixed DMSO-aqueous solvent, in which the cyanide to water hydrogen bonding found in pure aqueous solvent is destroyed, thereby, shifting the redox potential of the cyanometalates to values similar to M = Fe. The observed reaction 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 Hush, 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 products or to the starting material. Well-defined oligomers and polymers of the iron-based system have been synthesized. The photochemical reactivity and photophysics of these species are found to be a function of molecular geometry. In the case of the polymeric systems, one-dimensional, two-dimensional, and network materials can be synthesized using electrochemical techniques to control the polymer reactivity sites. Both solution and surface-confined photochemistry can be observed for these systems.