Theoretical study of static second-order nonlinear optical properties of push-pull heteroquinonoid dimers

The static first-hyperpolarizabilities (β) of a number of donor-acceptor substituted heteroquinonoid dimers has been calculated analytically by using the time-dependent Hartree Fock (TDHF) method and split-valence basis sets. The evolution pattern of β of mono-substituted quinoid rings has been found to mirror the pattern of variation of β of the corresponding polyenes. The charge transfer (CT) characteristics of the chosen quinonoid chromophores have been studied in terms of effective CT parameters (ηfXR) which have been found to fairly correlate with the calculated β of mono-substituted quinonoids. The relative trend in β of quinonoid dimers has been rationalized on the basis of calculated hardness parameter η and ηf(=ηfDR1+ηfAR2) which exhibit linear relationships with the ZINDO-S/CI calculated vertical transition energy corresponding to HOMO→LUMO excitation. The analytically evaluated response property, β correlates fairly with the corresponding two-state calculated β0 and relevant spectroscopic quantities. The general qualitative trend in β obtained for 3-21G and 6-31G** basis sets has been found to be same. The aromatic factor does not play any significant role in the evolution of second-order NLO responses in heteroquinonoid compounds.