Photoreduction of carbon dioxide (C02) to formate (HCOz-) can be achieved by UV-irradiation of the system involving phenazine (Phen) as a photosensitizer, a cobalt complex of cyclam (Co"'L, L = cyclam = 1,4,8,-1 1-tetraazacyclotetradecane) as an electron mediator, and triethylamine (TEA) as an electron donor. Reduction products from the system are HC02-and a small quantity of CO and H2. The quantum yield is 0.07 for the formation of HC02-at 1 = 313 nm. Preferential electron transfer from the photoformed radical anion of phenazine (Phen'-) to Co'I'L is confirmed by EPR analysis and the reaction between Phen'-and Co"'L with a second-order rate constant, k = 4.3 x lo9 M-' s-I. The resulting Co"L reacts with phenazinyl radical, giving [CoL(H)I2+ by hydrogen transfer from phenazinyl radical. The effective insertion of a C02 molecule into [CoL(H)I2+ yields HC02-selectively.
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