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

Fast electron-transfer reactions of newly synthesized (carbazole)(n) dendrimers (n = 1, 3 and 7), which are connected with C-60 with a short linkage, have been investigated in polar benzonitrile. The (carbazole)(n)-C-60 dendrimers were characterized by spectroscopic, computational and electrochemical methods. The geometric and electronic structures of the C-60-(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 (C-60(center dot-)-(carbazole)(n)(center dot+)). The redox measurements revealed that the charge separation from carbazole to the singlet-excited state of C-60 is thermodynamically feasible in polar benzonitrile. The femtosecond transient absorption measurements in the visible-NIR region revealed fast charge separation (similar to 10(11) s(-1)) from the carbazole to the singlet-excited state of C-60 producing the charge-separated states (C-60(center dot-)-(carbazole)(n)(center dot+)) 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 C-60, as well as the ground states. The higher charge separation/charge recombination ratios (similar to 800) suggested the potential of compounds 1-3 to be light harvesting systems.