Nonlinear properties of benzothiadiazole-based intramolecular charge-transfer compounds

Intramolecular charge-transfer (ICT) chromophores have received a lot of attention due to their wide range of applications in nonlinear optical materials. In this work, we introduced an electron-rich benzothiadiazole group into the classical organic nonlinear material dimethylaminonitrostilben (DANS), which improved the efficiency of ICT and increased the dipole moment difference between the ground state and the excited state of the chromophore. We synthesized three benzothiadiazole derivatives, and the nonlinear optical properties of these chromophores were characterized by the Z-scan method. The nonlinear response is one order of magnitude higher than that of DANS. The analysis of UV-vis absorption spectra combined with the results of DFT theoretical calculations revealed the difference in the dipole moment between the ground and excited states. The difference in the dipole moments (Delta mu) between the ground state and excited state is responsible for the different nonlinear properties. The refractive indices n(2) of the three chromophores increase in the following order: BTN-2 > BTN-1 > BTN-3.