Quantum chemistry study on nonlinear optical properties of hemicyanine dye derivatives with different electron donor groups

We have designed 9 hemicyanine dye derivatives with different electron donor groups. The geometric structures are optimized by using density functional theory approach at B3LYP/6-31G(d,p) level of theory. The static first hyperpolarizability, absorption spectra and molecular frontier orbital properties of hemicyanine dyes are studied by using second-order Møller–Plesset perturbation theory (MP2) and time-dependent density functional theory (TDDFT) methods, respectively. It is found that the first excited state transition corresponds to charge transfer from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), and is responsible for molecular second-order nonlinear optical (NLO) properties. Our results reveal that the static first hyperpolarizability of hemicyanine derivatives has a linear relationship with the HOMO–LUMO energy gap. In addition, we also find the linear effect of charge transfer magnitude Δq on the first hyperpolarizability β. Our study may be helpful to further understand the structure–property relationship of the organic NLO materials.