Synthesis, growth and characterization of 4-bromo-4′-nitrobenzylidene aniline (BNBA): a novel nonlinear optical material with a (3+1)-dimensional incommensurately modulated structure

The organic nonlinear optical material of 4-bromo-4'-nitrobenzylidene aniline (BNBA, C13H9BrN2O2), a new derivative of benzylideneaniline family, was synthesized and purified by repeated recrystallization. Single crystals of BNBA were grown in acetone solvent by slow evaporation at room temperature. Its (3+1-)dimensional incommensurately modulated structure was elucidated from single crystal X-ray diffraction data obtained at 173 K. The non-centrosymmetric structure was solved and refined in the monoclinic superspace group A2(alpha 0 gamma)0; the unit cell parameters are a = 10.5217(10) angstrom, b = 16.2535(16) angstrom, c = 7.4403(7) angstrom, beta = 110.709(7)degrees, modulation vector q = 0.0658(1)a(star)-0.2658(1)c(star). At 290 K, the structure is partially disordered, however, the local structure of domains is identical to that at 173 K. The H-1 and C-13 NMR and Fourier Transform Infrared and FT-Raman spectroscopic studies confirmed the molecular structure. The Kurtz powder second-harmonic generation (SHG) test reveals that the SHG efficiency of BNBA is about 9.4 times higher than that of potassium dihydrogen phosphate. According to the thermogravimetric, differential thermal and differential scanning calorimetric analyses, BNBA is a stable phase starting from room temperature and up to the melting point, 436 K. The optical nonlinearity of BNBA was investigated at 532 nm using 5 ns laser pulses, employing the open aperture Z-scan technique.