THEORETICAL INVESTIGATION OF 2-PHOTON ABSORPTION AND 3RD-HARMONIC GENERATION IN A LONG THIOPHENE OLIGOMER - ALPHA-HEPTATHIOPHENE

Taking into account the full chemical and molecular structure, we calc ulate the two-photon absorption spectrum (TPA) and the frequency-depen dent third-harmonic generation (THG) of a long thiophene oligomer (T7, the alpha-heptathiophene) by means of a multireference-determinant-co nfiguration interaction method based on the intermediate-neglect-of-di fferential-overlap Hamiltonian. The TPA spectrum is dominated by a ver y intense peak associated with an mA excited state (m = 6). Three exci ted states (1B, mA, and m'A) contribute primarily to the static third- order nonlinear optical response; however, a more complex model includ ing both the 2A state and higher-lying B states is needed for descript ion of all the features observed m the frequency-dependent THG spectru m. Extrapolation of the results from the oligomer to the polymer is a nontrivial task because of the presence of double resonances that are characteristic of finite chains.