First-order hyperpolarizability of pyridinium N-phenolate betaine dye: Ab initio study

In the present study, we report on the results of calculations of non-linear optical properties (NLO) of the simplest pyridinium N-phenolate betaine dye [4-(1-pyridinium-1-yl)phenolate]. The two contributions to the first-order hyperpolarizability (β), namely vibrational (βv) and electronic (βe) were calculated at the Hartree–Fock (HF) level as well as using second-order Møller–Plesset (MP2) perturbation theory. The evaluation of both contributions to β using the MP2 method brought an interesting and unusual observation: Contrary to other π-conjugated molecules, the inclusion of electron correlation diminished the values of (βe) for the investigated betaine dye. The effect upon including electron correlation through second-order in perturbation treatment leads to the change of sign of vibrational counterpart of the first-order NLO response of the molecule under study.