Theoretical studies on the electronic spectra and second-order nonlinear optical properties of N-methyl-2-(2′-thiophene)-pyrrolo[3,4]C60

Abstract

AM1 semiempirical calculation method was used to study the structures and electronic properties of N-methyl-2-(2′-thiophene)-pyrrolo[3,4]C60 (MTPC) and N-methyl-pyrrolo[3,4]C60 (MPC). Geometry optimization results indicate that MTPC possesses eight stable isomers (four pairs of enantiomers), and among which, MTPC-1A, MTPC-3A, MTPC-1B, MTPC-3B correspond to the lowest total energies and they have larger dipole moments. The frontier molecular orbitals of C60 moiety dictated that of MTPC and MPC. For MTPC and MPC, there exists stronger intramolecular electron transfer, with C60 moiety as the electron acceptor while thiophene and pyrrolidine part are electron donors. Compared with that of N-methyl-pyrrolo[3,4]C60 (MPC), the C60 moiety of MTPC isomers possesses less net charge. Based on the AM1 geometry optimization, the electronic spectra and second-order nonlinear optical polarizabilities (β) of MTPC and MPC were calculated by using INDO/SCI method combined with sum-over-states (SOS) expression. The calculated results reveal that there exist weak absorption peaks above 400nm for MTPC and MPC. The thiophene-containing molecule (MTPC) exhibits good optical nonlinearity with the β values slightly less than that of MPC. For MTPC, β values vary with different isomers. When electron donor such as amino connected with thiophene, β values increase by one factor.