Coupled protonic and electronic conduction in the molecular conductor [2-(2-1H-benzimidazolyl)-1H-benzimidazolium]-TCNQ

A novel molecular based proton-electron mixed conductor, (H3BBIM⁺)(TCNQ)(Cl^-)_0.5(H₂O) (1), where H3BBIM⁺ is 2-(2-1H-benzimidazolyl)-1H-benzimidazolium and TCNQ is 7,7,8,8-tetracyano-p-quinodimethane, was synthesized. The salt exhibited peculiar phase transitions as a result of proton-electron coupling phenomena within the crystal. Salt 1 is composed of a closed-shell H3BBIM⁺ cation and an open-shell TCNQ anion radical, and was obtained by electrocrystallization in a buffered CH₃CN solution. Crystal 1 was constructed from the segregated uniform stacks of H3BBIM⁺ and TCNQ. The regular stack of partially electron-transferred TCNQ^-0.5 provided a one-dimensional electron-conducting column. Between the regular H3BBIM⁺ columns, a channel-like sequence of holes was formed at the side-by-side space that is filled with disordered Cl^- ions and H₂O molecules, and which offer a proton-conducting path. The electrical conductivity at room temperature (10 S cm^-1) was greater by a magnitude of four than the protonic conductivity (1 x 10^-3 S cm^-1). Electronic conduction changed from metallic (T > 250 K) to semiconducting (250 > T > 100 K), then insulating (T < 100 K). Protonic conductivity was observed above 200 K. The continuous metal-semiconductor transition at 250 K is caused by the formation of the Cl^- superstructure, whereas the disappearance of protonic conductivity at 200 K is related to the rearrangement of the [Cl^--(H₂O)₂] sublattice within the channel. The magnetic susceptibility continuously shifted from Pauli paramagnetism (T > 250K) to the one-dimensional linear Heisenberg antiferromagnetic chain (T < 250 K). Lattice dimerization in regular TCNQ columns was confirmed by the appearance of vibrational a_g mode at low temperatures. The strong localization of conduction electrons on each TCNQ dimer caused a Mott transition at 100 K. The melting and freezing of the [Cl^--(H₂O)₂] sub-lattice within the channel was correlated to the conduction electrons on the TCNQ stack and the protonic conductivity.