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4437MA-CVE sample. The technical assistance of Mrs. L. Mansuy in the synthesis of dialkyl viologens is also gratefully acknowledged. Registry No. di-C4 viologen, 36437-30-6; di-C, viologen, 6 159-05-3; di-C8 viologen, 32449-1 8-6; di-C4 viologen radical cation, 87922-26-7; di-C, viologen radical cation, 631 19-42-6; di-C8 viologen radical cation, 49765-26-6; PSSH, 50851-57-5; MA-CVE, 26935-44-4. have tried to interpret the data on semiquantitative grounds after having developed a kinetic model which takes very satisfactorily into account the disappearance of the reduced viologens in the absence of stabilizing agent. Acknowledgment.We are indebted to Dr. E. Pefferkorn (Institut Charles Sadron, Strasbourg) for providing us with the Photoreduction of C02 to formic acid (HCO,) and a small quantity of carbon monoxide (CO) can be achieved in nonaqueous polar solvent by using oligob-phenylenes) (OPP-n) as a photocatalyst and triethylamine (TEA) as a sacrificial electron donor under >290-nm irradiation, where the photocatalysis in N,N-dimethylformamide leads to the most effective formation of HC0,-and CO. Among OPP-n, OPP-3 and OPP-4 show high photocatalytic activity for the formation of HC02-, in which the apparent quantum yields of HCOT formation for OPP-3 and OPP-4 are 0.072 and 0.084, respectively. Although photocatalyst OPP-3 itself concurrently undergoes photo-Birch reduction during the photocatalysis, the turnover number for the formation of HC02-based on the reacted OPP-3 is calculated to be 4, implying a cyclic activity of the reduction system. The laser flash photolysis and pulse radiolysis studies reveal that the photocatalysis initially starts from the reductive quenching of the singlet state of OPP-n by TEA followed by the formation of the radical anion of OPP-n, OPP-n'-, resulting in direct electron transfer from OPP-n'-to C 0 2 molecules.

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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Multiphoton reduction of Eu 3+ to Eu 2+ in methanol was induced by irradiation with the second harmonic (394 nm, 2 ps) of a Ti:sapphire laser. The conversion efficiencies were increased to 0.3 for short pulse excitation, as compared to on the order of 10 -5 for a nanosecond dye laser pulse. It appears that a three-photon process pumps a deep charge-transfer state from which Eu 3+ efficiently dissociates to Eu 2+ . On the basis of the excitation spectra, the first intermediate state is the 5 L 6 state, which is one of the f-electronic excited states. The Eu 2+ concentration increased shot by shot and approached a plateau. The saturation behavior will be explained in terms of the back photooxidation of Eu 2+ to Eu 3+ .