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 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.