Bistable complex formation systems consisting of biphenylene (BP) and redox-active organic molecules such as chloranil (CL) and TCNE have been experimentally and theoretically investigated, based on an intermolecular interaction which characteristically occurs in the electrogenerated dianions forming a π-π type chargetransfer (CT) complex. Initially, we examined the CT complex formation of CL 2and TCNE 2with hydrocarbons (BP, hexamethylbenzene (HMB), and anthracene (AN)). Spectroelectrochemistry evidently gave the intermolecular CT spectra in the CL 2--BP and TCNE 2--BP systems at 500 and 550 nm, respectively. The CT interaction between the dianions and BP was measured as the positive shift of the second reduction potential with increasing concentrations of BP. This behavior allowed the formation constants to be estimated as 33.9 and 20.3 dm 3 mol -1 at 25 °C for the CL 2and TCNE 2complexes in CH 2 Cl 2 containing 0.5 mol dm -3 tetrabutylammonium perchlorate, respectively. Temperature dependence of the formation constants yielded the formation energy as 31.6 and 39.8 kJ mol -1 for the CL 2--BP and TCNE 2--BP systems, respectively. However, the CT spectra and the marked behavior in the voltammograms were not observed in the dianion systems involving HMB and AN. The RHF/6-31G(d) calculations reveal that the CL 2--BP and TCNE 2--BP complex formations are due to molecular recognition based on the favorable intermolecular HOMO-LUMO interaction of the dianions with BP, and the geometries of the dianion complexes differ from those of the neutral complexes. This background led to the development of redox-mediated bistable complex formation systems characterized by the geometrical alteration and the chromatic change. The interconversion of the bistable complex formation in the systems is modulated through redox control of the intermolecular HOMO-LUMO interaction, with trichromic change arising from the neutral complex formation, the anion radical generation, and the dianion complex formation.

Formal redox potentials E°' involving neutral species R and radical anions R(•-) in ionic liquids (ILs) composed of ammonium, pyridinium, and imidazolium cations are discussed from the point of view of the adiabatic electron affinity as a molecular property. The dependence of the 1,4-benzoquinone (BQ)/BQ(•-) redox process in CH2Cl2 and CH3CN is primarily investigated over a wide concentration range of ILs as the supporting electrolyte. A logarithmic relationship involving a positive shift of E°' with increasing concentration is obtained when the concentration is changed from 0.01 to 1.0 M. The relationship of E°' at IL concentrations greater than 1.0 M gradually reaches a plateau and remains there even for the neat ILs. It is found that the E°' values in the neat ILs are not influenced by the measurement conditions, and that they remain considerably dependent on the nature and concentration of the electrolyte when measured using the traditional method involving molecular solvents combined with a supporting electrolyte (0.1-0.5 M). The difference in the E°' values observed in the ammonium and pyridinium ILs is only several millivolts. In addition, ESR and self-consistent isodensity polarized continuum model calculation results reveal that the potential shift toward positive values upon the transition from molecular solvents containing ILs to neat ILs is adequately accounted for by changes in the electrostatic interaction of R(•-) taken into the cavity composed of the solvent and IL. On the other hand, the first reduction waves of quinones, electron-accepting molecules, and polynuclear aromatic hydrocarbons are reversibly or quasi-reversibly observed in the ILs. The electrochemical stability of the ILs is exploited in the facile measurement of these quasi-reversible waves at quite negative potentials, such as for the naphthalene (NP)/NP(•-) couple. Notably, the E°' values obtained in the ammonium ILs correlate well with the calculated standard redox potentials and are linearly fitted with high correlation over all classes of compounds using a single regression equation based on Koopmans' theorem.

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