Systematic Study of Polarizability and Microscopic Third-Order Optical Nonlinearity in Thiophene Oligomers

摘要

A systematic study of the dependence of the band gap, the linear optical susceptibility, the polarizability alpha, and the second hyperpolarizability gamma, on the number of repeat unit is conducted for the thiophene series from monomer to hexamer. The linear optical susceptibilities for oligomers have been determined from the refractive index measurements on vacuum deposited films using the m lines technique. The orientationally averaged polarizabilities alpha have been measured from refractive index measurements of THF solutions. The orientationally averaged second hyperpolarizabilities gamma have been measured by degenerate four-wave mixing studies of THF solutions. The validity of the Lorentz-Lorenz approximation is tested and found to be satisfactory. The experimental values of alpha and gamma for thiophene and alpha for bithiophene are found to be in qualitative agreement with those obtained by a recent ab initio calculation which used the finite field method and included diffuse polarization functions. The experimentally observed dependence of alpha and gamma on the number N of the thiophene repeat unit is compared with that predicted by a free electron model, PPP methods, and the ab initio calculations. Reprints. (mjm)