Switchable NLO response induced by rotation of metallacarboranes [Ni(III/IV)(C2B9H11)2](-/0) and C-,B-functionalized derivatives.

The rotary motion based on metallacarboranes around a molecular axis can be controlled by simple electron transfer processes, which provides a basis for the structure-property relationship for the nonlinear optical (NLO) switching. However, this phenomenon has not been previously reported in the development of NLO properties of metallacarboranes. In this work, the metallacarboranes Ni(III/IV)(C2B9H11)2 and their C-,B-functionalized derivatives are studied by the density functional theory (DFT) method. By calculating relative energies, we obtained the stable states before and after rotation controlled by simple electron transfer. Then, the static and frequency-dependent second-order NLO properties were calculated by several DFT functionals. According to the TDDFT results, the large NLO responses of the studied compounds are mainly caused by substituent group-to-carborane cage charge transfer (L'LCT) and substituent group-to-metal charge transfer (L'MCT) processes. The order of first hyperpolarizabilities (β values) illustrates that the NLO response can be enhanced by introducing a strong electron-donating group. Significantly, the geometric interconversions resulting from the redox reaction of 1C/1T-6C/6T allow the NLO responses to be switched "ON" or "OFF". The B(9,9')-methoxyphenyl-functionalized derivative of nickelacarborane, having low energetic cost and large different NLO responses between two states (from 0 to 20 998 a.u.), can be an excellent switchable NLO material.