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)(H(2)O) (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 CH(3)CN 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 H(2)O 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 (1x10(-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(-)-(H(2)O)(2)] sublattice within the channel. The magnetic susceptibility continuously shifted from Pauli paramagnetism (T>250 K) 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(-)-(H(2)O)(2)] sublattice within the channel was correlated to the conduction electrons on the TCNQ stack and the protonic conductivity.