Single crystals of 2-methylbenzimidazolium perchlorate were prepared for the first time with a slow evaporation method from an aqueous solution of a mixture of 2-methylbenzimidazole (MBI) crystals and perchloric acid HClO4. The crystal structure was determined by single crystal X-ray diffraction (XRD) and confirmed by XRD of powder. Angle-resolved polarized Raman and Fourier-transform infrared (FTIR) absorption spectra of crystals consist of lines caused by molecular vibrations in MBI molecule and ClO4- tetrahedron in the region ν = 200-3500 cm-1 and lattice vibrations in the region of 0-200 cm-1. Both XRD and Raman spectroscopy show a protonation of MBI molecule in the crystal. An analysis of ultraviolet-visible (UV-Vis) absorption spectra gives an estimation of an optical gap Eg~3.9 eV in the crystals studied. Photoluminescence spectra of MBI-perchlorate crystals consist of a number of overlapping bands with the main maximum at Ephoton ≅ 2.0 eV. Thermogravimetry-differential scanning calorimetry (TG-DSC) revealed the presence of two first-order phase transitions with different temperature hysteresis at temperatures above room temperature. The higher temperature transition corresponds to the melting temperature. Both phase transitions are accompanied by a strong increase in the permittivity and conductivity, especially during melting, which is similar to the effect of an ionic liquid.
Keywords: Raman scattering; UV-Vis absorption; dielectric properties; phase transitions; photoluminescence; semi-organic crystals; single crystal and powder XRD.