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 C₇₀ 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 C₇₀*^- (λ_max = 1374 nm) has been detected, accompanied by the decay of the triplet - triplet absorption band of ³C₇₀* at 960 nm in laser flash photolysis of the t-BuBNAH-C₇₀ system. The initial electron transfer from t-BuBNAH to ³C₇₀* to produce t-BuBNAH*⁺ and C₇₀*^- is followed by fast C - C bond cleavage in t-BuBNAH*⁺ to give t-Bu* which is coupled with C₇₀*^- to yield t-BuC₇₀^-. The limiting quantum yield for formation of t-BuC₇₀^- (Φ_∞ = 0.45) is significantly larger than the value (Φ_∞ = 0.21) of t-BuC₆₀^- produced when C₇₀ is replaced by C₆₀. When t-BuBNAH is replaced by 1-benzyl-1,4-dihydronicotinamide (BNAH) or the dimeric form [(BNA)₂], the selective one-electron reduction of C₇₀ 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 C₇₀*^- in the photoreduction of C₇₀ by BNAH and (BNA)₂ exceed unity; Φ_∞ = 2.0 and 1.9, both of which are also larger than the corresponding values for formation of C₆₀*^- (Φ_∞ = 1.3 and 0.80, respectively). The enhanced reactivity of C₇₀ as compared to C₆₀ is ascribed to a more localized unpaired electron and negative charge in C₇₀*^- 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-isopropy 1-1-benzyl- 1,4-dihydronicotinamide (i-PrBNAH), the photochemical reaction with C₇₀ yields not only C₇₀*^- but also the isopropylated anion (i-PrC70^-).