Enhanced Reactivity of C₇₀ in the Photochemical Reactions with NADH and NAD Dimer Analogues As Compared to C₆₀ via Photoinduced Electron Transfer

Abstract

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 -BuC 70 - ) accompanied by the oxidation of t -BuBNAH to BNA + . The transient formation of C 70 •- ( λ max = 1374 nm) has been detected, accompanied by the decay of the triplet−triplet absorption band of 3 C 70 * at 960 nm in laser flash photolysis of the t -BuBNAH-C 70 system. The initial electron transfer from t -BuBNAH to 3 C 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 -BuC 70 - . The limiting quantum yield for formation of t -BuC 70 - ( Φ ∞ = 0.45) is significantly larger than the value ( Φ ∞ = 0.21) of t -BuC 60 - produced when C 70 is replaced by C 60 . When t -BuBNAH is replaced by 1-benzyl-1,4-dihydronicotinamide (BNAH) or the dimeric form [(BNA) 2 ], the selective one-electron reduction of C 70 to C 70 •- is attained through photoinduced electron transfer from BNAH or (BNA) 2 to the triplet excited state of C 70 . The limiting quantum yields for formation of C 70 •- in the photoreduction of C 70 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 C 60 •- ( Φ ∞ = 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 -PrC 70 - ).