Tuning second-order nonlinear optical properties of the two-dimensional benzene/carborane compounds with phenyl carbazoles: Substituent effect and redox switch

Density functional theory (DFT) calculations have been carried out to investigate the substituent effects and redox switching of the second-order nonlinear optical (NLO) properties of a series of the two-dimensional (2D) diaryl-substituted benzenes/carboranes. The calculated static first hyperpolarizabilities (βtot) of the neutral compounds increase with replacing the central benzene with carborane, extending the π-conjugated bridge, and introducing an electron donor into carbazole moiety. Particularly, a dramatic enhancement of the βtot values for the oxidized/reduced species is observed. The βtot values of one-electron oxidized/reduced species for compound 3 are 14 times and 32 times as large as that of its neutral counterpart, respectively. Furthermore, the electronic structure analysis shows that the carbazole performs as an oxidized center and the benzene/carborane performs as a reduced center. Interestingly, the second-order NLO responses are two-dimensional in characteristic with the large off-diagonal tensor values. This work not only provides more penetrating insight into the second-order NLO properties of 2D benzene/carborane-based compounds, but also obtains an efficient approach for devising redox-switchable NLO materials.