The important role of the bromo group in improving the properties of organic nonlinear optical materials

Several kinds of bromo substituted organic conjugation compounds including chalcones, alpha-cinnamylideneacetophenones, benzophenones, stilbenes and diphenylacetylenes have been synthesized, and their powder second harmonic generation (SHG) efficiency, cut-off wavelength lambda(cut-off) and thermal stability have been studied. Their molecular first-order hyperpolarizabilities beta were investigated by CNDO/S-CI semiempirical calculation methods. The results reveal that the bromo group can obviously improve the molecular first-order hyperpolarizabilities and can effectively reduce the dipole-dipole interactions between the molecules. Thus the bromo group favors the acentric crystal structure, and therefore the SHG effect of the material. The transparency and the thermal stability of the materials can also be improved noticeably when the compounds are substituted with a bromo group. In conclusion, the bromo group is an excellent candidate group in designing effective organic second-order nonlinear optical materials.