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.

The electronic effect of the thioether linkage between Tyr 272 and Cys 228 (the novel organic cofactor) of galactose oxidase has been examined by using model compounds, 2-(methylthio)-p-cresol (1H), 2-(methylthio)-4,6-dimethylphenol (2H), and 2-(methylthio)-4-methyl-6-[[bis[2-(2-pyridyl)ethyl]amino]methyl]phenol (3H), the physicochemical properties of which are compared to those of 2-[[bis[2-(2-pyridyl)ethyl]amino]methyl]-4-methylphenol (4H) and p-cresol (5H). (1)H NMR and electrochemical studies indicate that the methylthio group has essentially an electron-donating nature. On the other hand, the lower pK(a) values of 1H and 2H as compared to that of 5H suggest that the methylthio group also has a 2ppi-3dpi electron conjugative effect, stabilizing the negative charge on the phenolate oxygen. Furthermore, the electron-sharing conjugative effect of the substituent in the radical state has been clearly demonstrated by ESR studies and semiempirical molecular orbital calculations. Dimer copper(II) complexes [Cu(II)(2)(3(-)())(2)](PF(6))(2) (7) and [Cu(II)(2)(4(-)())(2)](PF(6))(2) (8) were prepared, and the crystal structures were determined by the X-ray diffraction method. Electrochemical analyses of the monomeric species [Cu(II)(3(-)())(py)](PF(6)) (9) and [Cu(II)(4(-)())(py)](PF(6)) (10) generated in situ by adding an external ligand such as pyridine (py) reveal that the methylthio substituent in the copper complex shows electronic effects similar to those of the free ligand stabilizing the phenoxyl radical state of the cofactor moiety in the Cu(II) complex.

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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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