Selective one-electron and two-electron reduction of C60 with NADH and NAD dimer analogues via photoinduced electron transfer

The selective one-electron reduction of C-60 to C-60(.-) is attained through photoinduced electron transfer from an NADH analogue, 1-benzyl-1,4-dihydronicotinamide (BNAH), and the dimer analogue [(BNA)(2)] to the triplet excited stare of C-60 The limiting quantum yield for formation of C-60(.-) in the case of (BNA)(2) exceeds unity; Phi(infinity) = 1.3. Ln this case, the initial electron transfer from (BNA)(2) to the triplet excited state (C-3(60)*) is followed by fast C-C bond cleavage in the resulting (BNA)(2)(.+) to give BNA(.) and BNA(+) and the second electron transfer from BNA(.) to C-60 yields BNA(+) and C-60(.-) when (BNA)(2) acts as a two-electron donor to produce 2 equiv of C-60(.-) When BNAH is replaced by 4-tert-butylated BNAH (t-BuBNAH), the photochemical reaction with C-60 yields not C-60(.-) but instead the tert-butylated anion (t-BuC60-) selectively. In this case, the initial electron transfer from t-BuBNAH to C-3(60)* is also followed by fast C-C bond cleavage in t-BuBNAH(.+) to give t-Bu-., which is coupled with C-60(.-) produced in the electron transfer to yield t-BuC60-. The selective two-electron reduction of C-60 to 1,2-dihydro[60]fullerene (1,2-C60H2) is also attained with the use of another NADH analogue 10-methyl-9,10-dihydroacridine (AcrH(2)), under visible light irradiation in deaerated benzonitrile solution containing trifluoroacetic acid. The studies on the quantum yields, the kinetic deuterium isotope effects, and the quenching of the triplet-triplet absorption of C-60 by AcrH(2) have revealed that the photochemical reduction proceeds via photoinduced electron transfer from 10-methyl-9,10-dihydroacridine to the triplet excited state of C-60, which is followed by proton transfer from AcrH(2)(.+) to C-60(.-) and a second electron transfer from the deprotonated acridinyl radical (AcrH(.)) to C60H. in the presence of trifluoroacetic acid to yield the final products 10-methylacridinium ion (AcrH(+)) and 1,2-C60H2. The transient spectra of the radical ion pair formed in the photoinduced electron transfer have been detected successfully in laser flash photolysis of each NADH analogue-C-60 system. The mechanistic difference between the selective one- and two-electron reductions of C-60 is discussed On the basis of the difference in the redox and acid-base propel-ties of NADH and the dimer analogues.