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

Abstract

Static third-order non-linear optical susceptibilities χ⁽³⁾ of polysilanes were estimated at molecular and crystalline levels in order to examine effects of main chain conformation on their χ⁽³⁾ values. A combination of an abinitio coupled perturbed Hartree–Fock (CPHF) method and the oriented-gas approximation, which was previously developed by the author (T. Hamada, J. Chem. Soc., FaradayTrans., 1996, 92, 3165), was employed for the estimation. Molecular calculation results showed cis and gauche Si–Si bonds reduce γ of polysilanes, and trans-planar polysilanes have larger second hyperpolarizabilities γ (molecular χ⁽³⁾) than trans–cis, alternating trans–gauche, and 7/3 helix polysilanes. On the other hand, crystal calculation results indicated χ⁽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 χ⁽³⁾. The method showed trans-planar poly(dimethylsilane) (PDMS) and poly(diethylsilane) (PDES) are attractive materials for experimental studies, owing to their enhanced χ⁽³⁾ values.