On the Relation Between Conformational Changes and Optical Properties in Oligothiophenes, 2. Linear and Nonlinear Optical Properties

The dependence of microscopic electronic non‐resonant polarizabilities (α) and second‐order hyperpolarizabilities (γ) of short oligothiophenes on the mutual torsional ring orientation was investigated by employing the time‐dependent Hartree–Fock method within the AM1 approximation. The α and γ quantities exhibit the uniform shape of the torsional dependence regardless of the mutual rings' orientations for all the systems under study (minima occur for perpendicular geometries and maxima are indicated for planar structures). The frequency dependence of γ hyperpolarizability below the first resonance frequency is fairly well evaluated in the form of a simple power series. The trends of AM1 results agree with the current theoretical treatments as well as experimental data.