A theoretical study on the structural dependences of third-order optical nonlinearities of heterocycle-substituted polymethine cyanine chromophores

The linear and third-order nonlinear optical properties of four polymethine cyanines (PC-1–PC-4) were investigated by UV–visible absorption spectroscopy and degenerate four-wave mixing (DFWM) technique. The second-order hyperpolarizabilities γ of the four chromophores achieve 10−31 esu. The dependence of their third-order optical nonlinearities on the molecular structure was discussed based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. The calculated second-order hyperpolarizabilities γ well-reproduce the experimental trends. The results show that the third-order optical nonlinearities of the chromophores can be drastically enhanced by bulky heteroatom (such as selenium) with low electro-negativity, or extended π-conjugated terminal group.