Substituting effect on first hyperpolarizability of R-terminated polysilaacetylene oligomers

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/n) 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.