A theoretical study of second hyperpolarizabilities for donor–acceptor–donor molecules

Abstract

The linear polarizabilities α and second hyperpolarizabilities γ of molecules with donor–acceptor–donor (D–A–D) motifs were calculated by a semi-empirical molecular orbital method through the finite-field approach. The bond-length alternation (BLA) parameter was used to consider the structure–property relationship for this class of molecule. By tuning the molecular ground-state geometries with the applied electric field, the dependence of the longitudinal components, α_xx and γ_xxxx, on the BLA parameter is obtained, differing from those for push–pull chromophores. With the reduction of BLA, the value of α_xx increases monotonously, while γ_xxxx is firstly positive and increases, reaches a maximum, then decreases and crosses through zero, ultimately approaching a negative maximum. The extension of the conjugation paths connecting the acceptor and the two donors leads to a rapid increase in α_xx and γ_xxxx values for short oligomers, accompanied by a sign reversion (from positive to negative) in γ_xxxx for some oligomers with highly polarized structures. A strategy for designing and synthesizing D–A–D-like molecules with large positive or negative third-order optical non-linearities is suggested based on the calculated results.