Brian W. Pfennig

Photoinduced multielectron charge transfer processes in Group 8 — platinum cyanobridged supramolecular complexes

Abstract Mixed valence compounds have attracted considerable attention because of their capability 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) 4 M(II) CN Pt(IV)(NH 3 ) 4 NC M(II)(CN) 4 L] 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.

Photoinduced multielectron charge transfer in the polymetallic intervalent complex [(NC)5Fe-CN-Pt(NH3)4-NC-Fe(CN)5]4− and related species

Abstract The complex anion, [(NC) 5 Fe II CN-Pt IV (NH 3 ) 4 -NC-Fe II (CN) 5 ] 4− , is synthesized from the redox reaction of K 3 Fe(CN) 6 and Pt(NH 3 ) 4 (NO 3 ) 2 in aqueous solution. Irradiation of the intervalent (IT) absorption band of the complex effects a net two electron transfer, regenerating the molecules from which it was formed. The IT band energy can be modulated by changing the cyanometalate species employed or through the use of a noncoordinating solvent such as DMSO. The photochemistry of the polymeric “(-Fe-Pt-) n ” cyanide-bridged complex is also discussed.

The Design of Reagents for Photochemical and Thermal Multiple Electron Transfer Reactions

Abstract Most, if not all, electron transfer reactions involving more than one electron can be considered as a series of sequential one electron transfer steps when evaluated on an appropriate timescale. Once the critical activation barrier for the first electron transfer to occur has been surmounted, the ensuing electrons are generally transferred in rapid succession. A number of multiple electron, inorganic systems, particularly those containing platinum, are evaluated; and the underlying principles for the design of these systems are discussed. Both thermally and photochemically activated systems are considered. Additionally, new thermodynamic criteria are proposed for certain inner sphere electron transfer reactions.