Influence of backbone conformation on the third-order optical nonlinearity in polyacetylene

Conformational effects in the third-order nonlinear optical susceptibilities χ(3) as well as the linear susceptibilities χ(1) of polyacetylene are investigated by the Hückel theory within the frozen lattice approximation. The third-neighbor electron transfer energies (t3) at the cis-formed C–C–C–C units are considered for cis-transoid (CT) and trans-cisoid (TC) conformers. Inclusion of t3 definitely reduces (enhances) χ(1) and χ(3) in CT (TC) conformer. As a result, χ(3) of CT conformer is 6–9 times smaller than that of trans-transoid conformer. The intrinsic nonlinearity represented by χ(3)/χ(1) is also significantly affected by the backbone conformation.