Third-order optical nonlinearity in new pi-conjugated polymers: Polydiethynylsilane and polysilole

The longitudinal components of the static polarizability (alpha(L)) and the second hyperpolarizability (gamma(L)) are investigated for polydiethynylsilane (PDES) and polysilole (PS). Those properties are calculated for oligomers consisting of up to 12 Si-rings using the INDO finite-field method. The variation in alpha(L) and gamma(L) of PDES and PS are interpreted in terms of optical nonlinearity gamma(L)/alpha(L) intrinsic to their conjugated skeletons which are characterized as trans-cisoidal (TC) or cis-transoidal (CT) polyacetylene. Moreover, the geometrical and electronic effects of the drop SiR(2) bridging group (R = Me, H, F) on gamma(L) are clarified in comparison with the case of polythiophene and polypyrrole.