With the help of ab initio tools taking into account dynamic electron correlation effects,we study the longitudinal electronic first hyperpolarizability of carbon-silicon analogues to polyacetylene.It turns out that the MP2/6-31G(d)//HF/6-31G(d) scheme is suitable to obtain a semiquantitative accuracy for the first hyperpolarizability of long polysilaacetylene oligomers.The conformation of the chain has a crucial impact on its second-order nonlinear optical properties.We also show that,for some chain lengths,the frequency dispersion effects may have a huge impact,even when far away from resonance.These phenomena are rationalized in terms of delocalization and asymmetry.
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