Open-Shell Singlet Nature and σ-/π-Conjugation Effects on the Third-Order Nonlinear Optical Properties of Si Chains: Polysilane and Poly(disilene-1,2-diyl)

We theoretically investigate the open-shell singlet nature and sigma-/pi-conjugation effects on the longitudinal second hyperpolarizabilities (gamma) of one-dimensional chains involving silicon silicon bonds, that is, polysilane and poly(disilene-1,2-diyl), by comparison with their carbon analogues, polyethylene and polyacetylene, respectively. It is found that poly(disilene-1,2-diyl) has less bond length alternation than polyacetylene and that sigma-conjugation of polysilane is less effective on the enhancement of gamma than pi-conjugation of polyacetylene, whereas pi-conjugation of poly(disilene-1,2-diyl) indicates a more than 20 times greater enhancement of the gamma than that of polyacetylene, which is known to be a typical nonlinear optical molecule with large gamma, for one-dimensional chains involving 3-5 double bonds. Further theoretical analyses of poly(disilene-1,2-diyl) reveal the sigma- and pi-electrons contribute negatively and positively to the gamma, respectively. The latter contribution is significantly larger than the former and thus causes the remarkable enhancement of gamma amplitudes due to the emerging open-shell singlet nature in the long,pi-conjugation length.

我们从理论上研究了含硅硅键的一维链（即聚硅烷和聚（1,2 - 二硅烯基））的开壳单重态性质以及σ - /π - 共轭对其纵向二阶超极化率（γ）的影响，并分别与它们的碳类似物聚乙烯和聚乙炔进行了比较。研究发现，聚（1,2 - 二硅烯基）的键长交替比聚乙炔小，并且聚硅烷的σ - 共轭对γ增强的效果不如聚乙炔的π - 共轭，然而对于含3 - 5个双键的一维链，聚（1,2 - 二硅烯基）的π - 共轭使γ的增强幅度是聚乙炔的20多倍，而聚乙炔是一种已知的具有大γ的典型非线性光学分子。对聚（1,2 - 二硅烯基）的进一步理论分析表明，σ电子和π电子分别对γ有负向和正向贡献。后者的贡献明显大于前者，因此由于在长π - 共轭长度下出现开壳单重态性质，导致γ幅值显著增强。