Photoinduced charge separation and charge recombination in [60]fullerene-ethylcarbazole and [60]fullerene-triphenylamines in polar solvents

Molecules of C₆₀ covalently connected with N-ethylcarbazole (EtCz) and triphenylamine (TPA) have been synthesized. Photoinduced electron transfer in C₆₀-EtCz and C₆₀-TPA has been studied in polar and nonpolar solvents using time-resolved transient absorption and fluorescence measurements. From the fluorescence lifetimes, the excited singlet state of the C₆₀ moiety (¹C*₆₀) of C₆₀-TPA generates predominantly C₆₀^̇- - TPA^̇+, which decays quickly to the ground state within 6 ns even in polar solvents. In the case of C₆₀-EtCz, on the other hand, about half of the ¹C*₆₀ moiety generates short-lived C ₆₀^̇--EtCz^̇_, while the other half of the ¹C*₆₀ moiety is transferred to the ³C*₆₀ moiety via intersystem crossing in dimethylformamide, in which the energy level of C₆₀ ^̇--EtCz^̇+ is lower than that of ³C*₆₀. Thus, the charge separation takes place via ³C*₆₀ generating C₆₀^̇-- EtCz^̇+, having a lifetime as long as 300 ns, probably because of the triplet spin character of C₆₀^̇--EtCz ^̇+. A special property of the EtCz moiety to stabilize the hole in the charge-separated state was revealed.