Effects of main chain conformation on third-order non-linear optical susceptibilities of polysilanes - Theoretical study on their second hyperpolarizabilities and bulk susceptibilities

Static third-order non-linear optical susceptibilities chi((3)) of polysilanes were estimated at molecular and crystalline levels in order to examine effects of main chain conformation on their chi((3)) values. A combination of an ab initio coupled perturbed Hartree-Fock (CPHF) method and the oriented-gas approximation, which was previously developed by the author (T. Hamada, J. Chem. Sec., Faraday Trans., 1996, 92, 3165), was employed for the estimation. Molecular calculation results showed cis and gauche Si-Si bonds reduce gamma of polysilanes, and trans-planar polysilanes have larger second hyperpolarizabilities gamma (molecular chi((3))) than trans-cis, alternating trans-gauche, and 7/3 helix polysilanes. On the other hand, crystal calculation results indicated chi((3)) of polysilane crystals are influenced not only by the main chain conformation of the polysilanes, but also by the molecular weight of their repeating unit (M-Si). These findings were used to propose a molecular design method for polysilane chi((3)). The method showed trans-planar poly(dimethylsilane) (PDMS) and poly(diethylsilane) (PDES) are attractive materials for experimental studies, owing to their enhanced chi((3)) values.