MULTIPLEX PROTON-TRANSFER AND ELECTRON-TRANSFER NATURES BASED ON THE 2,2'-BI-1H-IMIDAZOLE SYSTEM .1. 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 (H4BIM(2+)) with aromatic 6 pi-6 pi character. The high PT character of H3BIM(+) (2-(2-1H-imidazolyl)-1H-imidazolium) and H4BIM(2+) compared with that of the corresponding hydroquinone system was clearly observed. The onsite Coulomb repulsion energy for electron-transfer (U-1(CT)) was evaluated for various pi-electron acceptor and donor systems. It was found that the U-1(CT) values decrease linearly with increasing the length of a molecule, which can divide the molecular systems into two classifications, i.e. electron accepters and dications. The onsite Coulomb repulsion energy for two-step proton-transfer processes (U-1(PT)) was newly defined and evaluated for the H2BIM, H4BIM(2+), hydroquinone (H2Q), and bis(4-hydroxyphenyl) disulfide (HPDS) systems. The U-1(PT) values become constant above a certain molecular length (r(c)). Below r(c) the U-1(PT) values increase linearly with decreasing the distance r regardless of Wurster and Weitz structures, approximately. The H2BIM (H4BIM(2+)) system has large U-1(PT) values in comparison with the H2Q and HPDS systems.