Photo stability recombination

The stability of the flavin radicals does not speak for, but rather against a radical mechanism of photoreduction, since only very small amounts of radicals (if at all) are observed in these reactions in spite of the stability of these well-known species. This is one of the remarkable common features of flavin photo- and biochemistry. Hence, if in a reaction FlJx + RHRFl-h H practically no free radical RFl can be observed in spite of its stability, recombination of the solvated radical pair must be faster than diffusion, and faster than its formation. This turns the question of a radical path into semantics. 

The first standalone photo water-splitting device having a practical conversion efficiency was described by Kainthla and Khan (1987). Their cell is shown in Fig. 10.26. Both electrodes were simultaneously irradiated. The calculated optimal match between materials of the two electrodes was found in theory to be p-InP as cathode and n-GaAs, the latter protected from electrochemical oxidation by a film of Mn02. The InP was duly decorated with platinum to catalyze H recombination the Mn02 allowed photo 02 evolution on its surface with a stability unchanged over prolonged periods. 

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