Palladium and Platinum

Photochemical properties of transition metal complexes are discussed as they relate to possible schemes for energy storage. Complexes of copper(I) with π-acceptor ligands have been used to sensitize isomerizations of organic molecules, and there is room for considerable further investigation in this area. Electron transfer reactions have been more intensively studied. These reactions fall into three broad categories: ionization, oxidative addition/reductive elimination, and excited state electron transfer. Experiments in the first two categories have generally led to rather poor storage efficiency and have required ultraviolet light. But long-lived excited states of coordination compounds can participate in chemical reactions with high efficiency, and such systems are promising for solar energy storage. Specific systems of this type that are discussed in detail involve electron transfer quenching of the lowest emissive excited states of Ru(2, 2′-bipyridine) 3 2+ , M 2 (diisocyanoalkane) 4 2+ (M=Rh, Ir), and Mo 6 Cl 14 2− .

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Solar energy storage reactions involving metal complexes

Storage of light energy by chemical systems: comment on long-term efficiency of iterative cyclic reactions

The readily available benzodihydropentalene 6 and dimethyl acetylene-dicarboxylate react to give norbornadiene diester 8, with a three-carbon bridge from C1 to C2. Inrradiation of 8 gives the corresponding C1-C2 bridged quadricyclane diester 9, a new ring system. Diester 9 is quite stable, reverting to 8 with a t1/2 of 30 min at 170°C. The corresponding diacid 11, also prepared, reverts to its norbornadiene precursor at a considerably lower temperature, possibly as a consequence of acid catalysis.

1,2-Bridged Quadricyclanes

One of the best photochemical storage systems of solar energy is the interconversion between norbornadiene and quadricyclane. A kinetic study of norbornadiene photoisornerizalion sensitized with acetophenone, using ethanol as the solvent is presented. For the temperature range studied (25–55 °C), photoisomerization leads to quadricyclane, and no byproducts of the reaction were detected. Results show that a temperature increase leads to a slight decrease in the reaction rate.

E. M. Sweet

Papers

Catalysts for the Isomerization of Quadricyclane to Norbornadiene in a Photochemical Energy Storage System