Crystal structure and characterization of a novel acentric imidazolium analog [C3N2H5+][Br-]

► Strong polar (R3) room temperature phase of [C3N2H5+][Br-] and nonlinear optical properties. ► Phase transitions at 200 K is due to the reorientational motion of the cations (three-site model). ► Enhanced electric permittivity in the high temperature phase comes from permanent dipole moment of cation and its mutual orientation with respect to the Br- anions.