Hydrogen-Bonded Architectures and Field-Induced Polarization Switching in Bridged Bis(benzimidazole) Crystals

The imidazole ring is chemically stable and forms an excellent building block for compounds that show externally switchable polarization in their crystalline states. We examined the structural properties of nine bridged bis(benzimidazole)s, of which seven were new compounds, in an attempt to identify substances with highly polarizable dielectric properties arising from the multistability of their polar supramolecular architectures. Selected bridging o-phenylene, thien-2,5-diyl, or 1H-imidazole-4,5-diyl units improved the thermal robustness as expected. Solution evaporation or vacuum sublimation of the bridged bis(benzimidazole)s afforded single crystals, with or without solvent molecules, in which the molecules were hydrogen bonded to form either extended ribbons or cyclic oligomeric structures. 2,2'-(1,2-Phenylene)bis-1H-benzimidazole (BI2P) formed ladder-like ribbons that exist as low- or high-dipole configurations depending on the parallel or antiparallel orientation of the NH hydrogen atoms between the two benzimidazole rings. These two configurations were nearly degenerate in stability, permitting them to be electrically switched, as evidenced by their double (P-E) hysteresis curves. The switching process was ascribed to an order-disorder-type paraelectric-to-ferroelectric phase transition rather than to an antiferroelectric-to-ferroelectric phase transition. BI2P is also unique as a paraelectric crystal in which the ferroelectric state is field induced but is unexpectedly inaccessible by exclusively thermal stimulation.