Theoretical study of fulleropyrrolidines by density functional and time-dependent density functional theory

Density functional theory (DFT) calculations have been employed in a study of various fulleropyrrolidines in order to determine their electronic structure and in particular the effect on the chemical properties of the pyrrolidine nitrogen atom of fullerene as well as of additional substituents. It is found that fullerene causes a large chemical shift in the calculated N 1s binding energy, while further substitution at N has smaller effect independent of the type of substituent. Time-dependent DFT calculations have been employed for the calculation of the energy of the lower-lying electronic states of fulleropyrrolidines, as well as on higher-lying, states, in view of the intense interest related to photoinduced charge-transfer processes in these systems. The TDDFT calculations on the lower-lying states converge on the same lowest excitation energy for most of the substituted fulleropyrrolidines, favoring excitations from occupied orbitals localized on C-60 even when the highest occupied molecular orbital (HOMO) is localized on the substituent groups. Good agreement is found with the experimental lowest excitation energy in fullerene and in the fulleropyrrolidine-porphyrin system. Finally, an excitation to a higher-lying state, related to the charge-transfer process, has been determined by TDDFT calculations on a porphyrin-fulleropyrrolidine system, where it is found that the absorption characteristics of the individual substituent are transferable to the combined system.