Bistable Charge-Transfer Complex Formation of Redox-Active Organic Molecules Based on Intermolecular HOMO−LUMO Interaction Controlled by the Redox Reactions

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 charge-transfer (CT) complex. Initially, we examined the CT complex formation of CL2- and TCNE2- with hydrocarbons (BP, hexamethylbenzene (HMB), and anthracene (AN)). Spectroelectrochemistry evidently gave the intermolecular CT spectra in the CL2-−BP and TCNE2-−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 dm3 mol-1 at 25 °C for the CL2- and TCNE2- complexes in CH2Cl2 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 CL2-−BP and TCNE2-−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 CL2-−BP and TCNE2-−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.