Structure and Proton-Transfer Mechanism in One-Dimensional Chains of Benzimidazoles

Planar, 1D and hydrogen-bonded chains of benzimidazole molecules have been fabricated through surface-assisted self-assembly on Ag(111) and Au(111) and investigated with scanning tunneling microscopy. The hydrogen bond between the benzimidazoles and the coupling to the molecular π-electron system, of the type -C - N···H-N-C - , which exists in bulk crystals and gives rise to ferroelectricity at room temperature, is also observed in the supported 1D chains. Inspired by this finding, the proton-transfer mechanism in 1D chains of benzimidazoles in the gas phase and on coinage metal surfaces was investigated with density functional theory (DFT) calculations. It is demonstrated that the proton transfer, which is needed to reverse the dipole moment along a model chain, is a low-energy process in the gas phase. The substrate shapes this energy barrier and lowers it as compared with free chains. A hydrogen-transfer pathway via a tautomerized state is identified, and because of the relative instability of the tautomerized state, a concerted or cascaded proton transfer along the chains seems plausible. This study predicts that 1D organic ferroelectrics based on benzimidazoles can exist if the molecule-substrate interactions are appropriately controlled.