Multiplex Proton-Transfer and Electron-Transfer Natures Based on the 2,2′-Bi-1H-imidazole System. I. Acid Dissociation Constants and Redox Properties in Solution

The 2,2′-bi-1H-imidazole (H2BIM) derivatives were studied from the point of the interplay of proton-transfer (PT) and electron (charge)-transfer (CT) interactions. Seven new states among the speculated 25 independent species were identified based on the study of their redox and acid dissociation properties. Isoelectronic substitution of four sulfur atoms of the dication state of tetrathiafulvalene (TTF2+) by four imino groups was found to increase the stability of the dication state 2,2′-bi-1H-imidazolium (H4BIM2+) with aromatic 6π–6π character. The high PT character of H3BIM+ (2-(2-1H-imidazolyl)-1H-imidazolium) and H4BIM2+ compared with that of the corresponding hydroquinone system was clearly observed. The on-site Coulomb repulsion energy for electron-transfer (U1CT) was evaluated for various π-electron acceptor and donor systems. It was found that the U1CT values decrease linearly with increasing the length of a molecule, which can divide the molecular systems into two classifications, i.e. electron acceptors and dications. The on-site Coulomb repulsion energy for two-step proton-transfer processes (U1PT) was newly defined and evaluated for the H2BIM, H4BIM2+, hydroquinone (H2Q), and bis(4-hydroxyphenyl) disulfide (HPDS) systems. The U1PT values become constant above a certain molecular length (rc). Below rc the U1PT values increase linearly with decreasing the distance r regardless of Würster and Weitz structures, approximately. The H2BIM (H4BIM2+) system has large U1PT values in comparison with the H2Q and HPDS systems.