Uphill photooxidation of NADH analogues by hexyl viologen catalyzed by zinc porphyrin-linked fullerenes

In the absence of oxygen, the photolytically generated C₆₀^.- moiety in ZnP^.+-C₆₀^.- and ZnP^.+-H₂P-C₆₀^.- radical ion pairs undergoes one-electron oxidation by hexyl viologen (HV²⁺), whereas the ZnP^.+ moiety is reduced by NADH analogues (1-benzyl-1,4-dihydronicotinamide and 10-methyl-9,10-dihydroacridine). Thus, both ZnP-C₆₀ and ZnP-H₂P-C₆₀ donor-acceptor ensembles act in benzonitrile as efficient photocatalysts for the uphill oxidation of NADH analogues by HV²⁺. In the case of ZnP-C₆₀, the quantum yield of the photocatalytic reaction increases with increasing concentration of HV²⁺ or an NADH analogue to reach a limiting value of 0.99. The limiting quantum yields of ZnP-C₆₀ and ZnP-H₂P-C₆₀ agree well with the quantum yields of radical ion pair formation, ZnP^.+-C₆₀^.- and ZnP^.+-H₂P-C₆₀^.-, respectively. In the presence of oxygen, the lifetimes of the radical ion pairs are, however, markedly reduced because of an oxygen-catalyzed back electron transfer process between C₆₀^.- and ZnP^.+. Such an impact on the radical ion pair lifetime consequences a significant decrease in the photocatalytic reactivity of the dyad (i.e., ZnP-C₆₀) in the overall photooxidation of an NADH analogue by HV²⁺. By contrast, the reactivity of the triad (i.e., ZnP-H₂P-C₆₀) shows little effects upon admitting O₂.