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, αxx and γxxxx, on the BLA parameter is obtained, differing from those for push–pull chromophores. With the reduction of BLA, the value of αxx increases monotonously, while γxxxx 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 αxx and γxxxx values for short oligomers, accompanied by a sign reversion (from positive to negative) in γxxxx 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.