Synthesis and fast electron-transfer reactions of fullerene–carbazole dendrimers with short linkages

Abstract

Fast electron-transfer reactions of newly synthesized (carbazole)_n dendrimers (n = 1, 3 and 7), which are connected with C₆₀ with a short linkage, have been investigated in polar benzonitrile. The (carbazole)_n–C₆₀ dendrimers were characterized by spectroscopic, computational and electrochemical methods. The geometric and electronic structures of the C₆₀–(carbazole)_n dendrimers were examined by using the ab initio B3LYP/6-311G method. The distribution of the highest occupied frontier molecular orbital (HOMO) was found on the carbazole (Cz) entities, whereas the lowest unoccupied molecular orbital (LUMO) was located on the fullerene entity, suggesting the formation of the charge-separated (CS) states (C₆₀˙⁻–(carbazole)_n˙⁺). The redox measurements revealed that the charge separation from carbazole to the singlet-excited state of C₆₀ is thermodynamically feasible in polar benzonitrile. The femtosecond transient absorption measurements in the visible-NIR region revealed fast charge separation (∼10¹¹ s⁻¹) from the carbazole to the singlet-excited state of C₆₀ producing the charge-separated states (C₆₀˙⁻–(carbazole)_n˙⁺) with lifetimes of 1.25–1.30 ns. The complementary nanosecond transient absorption measurements in the microsecond region revealed that the charge-separated states decayed to populate the triplet states of C₆₀, as well as the ground states. The higher charge separation/charge recombination ratios (∼800) suggested the potential of compounds 1–3 to be light harvesting systems.