A comparative study of 4-nitroaniline, 1,5-diamino-2,4- dinitrobenzene and 1,3,5-triamino-2,4,6-trinitrobenzene and their molecular engineering for second-order nonlinear optics

Abstract

We report here a comparative study of the second-order nonlinear optical properties of 4-nitroaniline (p-NA), 1, 5-diamino-2, 4-di-nitrobenzene (DADB) and 1, 3, 5-triamino-2, 4, 6-trinitrobenzene (TATB). First hyperpolarizabilities (β) of p-NA, DADB, and TATB calculated using the Molecular Orbital Package (MOPAC) AM1 method were found to be 12.017×10-30, 6.793×10-30 and -0.196×10-30 esu, respectively. These results demonstrate that the number and location of donor-acceptor substituents on the benzene ring strongly influence the molecular hyperpolarizability. Both p-NA and DADB show no powder second-harmonic generation (SHG). On the other hand, the TATB has a SHG intensity three times that of standard urea as reported by Ledoux et al. [Chem. Phys. Lett. 172 (1990) 440]. The physical properties of these three molecules are discussed here in view of second-order nonlinear optics. In particular, the molecular engineering of DADB leads to novel two-dimensional charge transfer molecules. First hyperpolarizability of model compounds of DADB, i.e., N, N' -dimethy1-2, 4-dinitro-1, 5-diaminobenzene and N, N' -diethyl-2, 4-dinitro-1, 5-diaminobenzene were also calculated using the same quantum chemistry program. The first hyperpolarizability of 10.642×10-30 esu was estimated for N, N' -diethyl-2, 4-dinitro-1, 5-diaminobenzene which is almost the same as of p-NA molecule. The molecularly designed amphiphile N, N' -dioctadecyl-2, 4-dinitro-1, 5-diaminobenzene (DIODD) showed second-order nonlinear optical susceptibility χ(2)111 of 22×10-9 esu and χ(2)133 of 7.7×10-9 esu from the Langmuir- Blodgett monolayer. An analysis of the second-order nonlinear optical properties of two-dimensional charge transfer molecules is presented.