Polarizability and hyperpolarizability of push–pull quinoid molecules Simulation of donor–acceptor pairs

AM1 calculations are performed to study the non-linear optical (NLO) properties of push–pull quinones. The variations of molecular structure, linear (α) and NLO polarizabilities (β, γ) for a set of push–pull quinones are discussed in terms of quinoid–benzenoid character (QBC), and the strength of donor–acceptor pairs (δDA). The parameter δDA, already used for the polyenebridge, is still valid in rationalizing the donor–acceptor strength for quinoid derivatives. In addition, we examined how an external electric field created by ‘Sparkles’ in the MOPAC package drives a quinoid molecule from the quinoid limit to the benzenoid limit, and how it affects the NLO properties. The results show that the effect produced by Sparkles is qualitatively similar to that of donor–acceptor pairs.