Enhanced reactivity of C70 in the photochemical reactions with NADH and NAD dimer analogues as compared to C60 via photoinduced electron transfer

Visible light irradiation of a benzonitrile solution containing C-70 and an NADH analogue, 4-tert-butyl-1-benzyl-1,4-dihydronicotinamide (t-BuBNAH), results in efficient formation of the tert-butylated anion (t-BUC70-) accompanied by the oxidation of t-BuBNAH to BNA(+). The transient formation of C-70 (lambda max = 1374 nm) has been detected, accompanied by the decay of the triplet-triplet absorption band of C-3(70)* at 960 nm in laser flash photolysis of the t-BuBNAH-C-70 system. The initial electron transfer from t-BuBNAH to C-3(70)* to produce t-BuBNAH(.+) and C-70 is followed by fast C-C bond cleavage in t-BuBNAH(.+) to give t-Bu-. which is coupled with C-70(.-) to yield t-BUC70.-. The limiting quantum yield for formation of t-BuC70(-) (Phi(infinity), = 0.45) is significantly larger than the value (Phi(infinity), = 0.21) of t-BuC60.- produced when C-70 is replaced by C-60 When t-BuBNAH is replaced by l-benzyl-1,4-dihydronicotinamide (BNAH) or the dimeric form [(BNA)(2)], the selective one-electron reduction of C-70 to C70 - is attained through photoinduced electron transfer from BNAH or (BNA)2 to the triplet excited state of C70 The limiting quantum yields for formation of C-70(.-) in the photoreduction of C-70 by BNAH and (BNA)2 exceed unity; Phi(infinity), = 2.0 and 1.9, both of which are also larger than the corresponding values for formation of C-60(.-) (Phi(infinity) = 1.3 and 0.80, respectively). The enhanced reactivity of C-70 as compared to C-60 is ascribed to a more localized unpaired electron and negative charge in C-70(.-) due to loss of symmetry which facilitates the follow-up reaction in competition with the back electron transfer to the ground-state reactant pair. In the case of 4-isopropyl-1-benzyl-1,4-dihydronicotinamide (i-PrBNAH), the photochemical reaction with C-70 yields not only C-70(.-) but also the isopropylated anion (i-PrC70-).