Photoinduced multielectron charge transfer processes in Group 8 - Platinum cyanobridged supramolecular complexes

Mixed valence compounds have attracted considerable attention because of their capabil-ity for photoinduced electron transfer, which has potential applications in energy conversion and photocatalysis. In such applications, the ability to transfer multiple electrons with a single photon is desirable. Symmetric, multinuclear complexes of the form [L(NC)₄M(II)-CN-Pt(IV)(NH₃)₄-NC-M(II)(CN) ₄L]^4- (where M is a Group 8 metal and L is a CN^- or a σ-donor ligand) provide for such photoinduced multielectron charge transfer processes. These complexes exhibit intense metal-metal charge transfer (MMCT) bands in the blue portion of the spectrum (350-450 nm). In the case where M = Fe, irradiation into the MMCT band centered at 425 nm produces a net two electron charge transfer with a quantum yield of 0.01. The observed reaction is found to yield only two electron products. Well defined oligomers and polymers of the iron based system can be synthesized either as soluble materials or adherent films on electrode surfaces. 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. Polymer modified electrodes exhibit a photocurrent response which is diagnostic for the photochemistry occurring within the film. The redox potential of the primary photoproducts are found to be very sensitive to the number of bridging cyanide ligands per iron center and thus, to the degree of branching of the polymer. Correctly selected polymer morphologies lead to primary photoproducts on the electrode surface which are capable of oxidizing chloride to chlorine. This chemistry can be used to produce a photochemical energy conversion cycle in which visible light induces the oxidation of halides to energy rich halogens.