Theoretical investigation on two-dimensional molecule-based second-order nonlinear optical materials of the disubstituted o-carborane derivatives

Recent studies show that non-dipolar molecular structures, having multidimensional (nD) nonlinear optical (NLO) characteristics, can be regarded as excellent candidates for the second-order NLO applications. Then, the second-order NLO response of a series of o-carborane (C2B10H12) derivatives was investigated by density functional theory (DFT) method. The results suggest that their second-order nonlinearities are dominated by two second-order polarizability tensor components (βzzz and βzyy). Electronic transitions to crucial excited states show that y-polarized transition, contributed to the off-diagonal second-order polarizability tensor (βzyy), possesses lower transition energy compared with z-polarized transition accounted for the diagonal second-order polarizability tensor (βzzz), which led to the large in-plane nonlinear anisotropy (μ=βzyy/βzzz) value, as well as good 2D second-order NLO properties. Moreover, the βtot values gradually increase from the monosubstituted (MS) to disubstituted (DS) o-carboranes. With increasing the strength of donor groups in DS molecules, the βtot values are also enhanced. The micromechanism of the second-order NLO properties has been analyzed from the standpoint of dipole moment, transition electronic dipole moment and transition energy. Our studies show that these complexes can be used as excellent 2D second-order NLO materials from the standpoint of large β, high transparency, and μ values.