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

In the absence of oxygen, the photolytically generated C-60(.-) moiety in ZnP.+-C-60(.-) and ZnP.+-H2P-C-60(.-) 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-C-60 and ZnP-H2P-C-60 donor-acceptor ensembles act in benzonitrile as efficient photocatalysts for the uphill oxidation of NADH analogues by HV2+. In the case Of ZnP-C-60, 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-C-60 and ZnP-H2P-C-60 agree well with the quantum yields of radical ion pair formation, ZnP.+-C-60(.-) and ZnP.+-H2P-C-60(.-), 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 C-60(.-) 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-C-60) in the overall photooxidation of an NADH analogue by HV2+. By contrast, the reactivity of the triad (i.e., ZnP-H2P-C-60) shows little effects upon admitting O-2.