Selective One-Electron and Two-Electron Reduction of C60 with NADH and NAD Dimer Analogues via Photoinduced Electron Transfer

Abstract

The selective one-electron reduction of C60 to C60•- 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 state of C60. The limiting quantum yield for formation of C60•- in the case of (BNA)2 exceeds unity; Φ∞ = 1.3. In this case, the initial electron transfer from (BNA)2 to the triplet excited state (3C60*) 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 C60 yields BNA+ and C60•-, when (BNA)2 acts as a two-electron donor to produce 2 equiv of C60•-. When BNAH is replaced by 4-tert-butylated BNAH (t-BuBNAH), the photochemical reaction with C60 yields not C60•- but instead the tert-butylated anion (t-BuC60-) selectively. In this case, the initial electron transfer from t-BuBNAH to 3C60* is also followed by fast C−C bond cleavage in t-BuBNAH•+ to give t-Bu•, which is coupled with C60•- produced in the electron transfer to yield t-BuC60-. The selective two-electron reduction of C60 to 1,2-dihydro[60]fullerene (1,2-C60H2) is also attained with the use of another NADH analogue, 10-methyl-9,10-dihydroacridine (AcrH2), 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 C60 by AcrH2 have revealed that the photochemical reduction proceeds via photoinduced electron transfer from 10-methyl-9,10-dihydroacridine to the triplet excited state of C60, which is followed by proton transfer from AcrH2•+ to C60•- 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−C60 system. The mechanistic difference between the selective one- and two-electron reductions of C60 is discussed on the basis of the difference in the redox and acid−base properties of NADH and the dimer analogues.