High first-hyperpolarizabilities of thiobarbituric acid derivative-based donor-π-acceptor nonlinear optical-phores: Multiple theoretical investigations of substituents and conjugated bridges effect

A number of compounds including thiobarbituric acid derivative-based acceptor unit and diverse donor moieties along with two types of pi-conjugated bridges have been constructed to aid in the design of the superior nonlinear optical (NLO) materials. The effect of varying the donor and bridge parts of this donor-bridge-acceptor system was analyzed in terms of structural and opto-electronic parameters such as bond length alternation and ultraviolet-visible absorption spectra. Various functionals with aug-cc-pVDZ basis set including B3LYP, PBE0, PBE38, BMK, CAM-B3LYP, and omega B97XD were employed to calculate the static and dynamic first-hyperpolarizabilities, and also the linear polarizabilities. Furthermore, the variation of the static first-hyperpolarizabilities has been explained satisfactorily in the light of the sum-over-states method and two-level model. The comprehensive study indicates that the coplanar compound D-5 with the low electronic absorption energy, strongest oscillator strength paired with the largest ground, and excited state dipole moment difference generates a dramatical increase in its static and dynamic first-hyperpolarizabilities, which would be the most worthwhile target for development as the NLO-phores.