Substituting effect on first hyperpolarizability of R-terminated polysilaacetylene oligomers

摘要

Theoretical analyses of the behavior with chain length of the static first hyperpolarizabilities for a series of all-trans polysilaacetylene (PSA) oligomers with identical R-terminal groups at each end have been performed by using high-accuracy ab initio quantum mechanics method (MP2/6-31+G*). Moreover, 15 R groups have been assessed to study the substituting effect on the β values. Our results indicate that these R–(SiHCH)n–R oligomers represent the character similar to that of the conventional push–pull chromophores. Besides, for some substituted PSA chromophores, to further increase the conjugated chain length could not effectively affect the β values because the first hyperpolarizabilities reach the saturated values. The evolution of the first hyperpolarizability per unit cell (β) reveals that the R substituents strongly affect β for short oligomers but have a limit influence on β for long chains. More importantly, the chain-end asymmetry plays a key role to contribute to β for these substituted PSA oligomers. On the other hand, it also shows that the bond length alternation (Δr) can be used to roughly estimate the relative magnitudes of β among the different R-terminal substituted PSA oligomers.