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 experim entally and theoretically investigated, based on an intermolecular interact ion which characteristically occurs in the electrogenerated dianions Formin g a pi-pi type charge-transfer (CT) complex. Initially, we examined the CT complex formation of CL2- and TCNE2- with hydrocarbons (BP, hexamethylbenze ne (HMB), and anthracene (AN)). Spectroelectrochemistry evidently gave the intermolecular CT spectra in the CL2--BP and TCNE2--BP systems at 500 and 5 50 nm, respectively. The CT interaction between the dianions and BP was mea sured as the positive shift of the second reduction potential with increasi ng concentrations of BP. This behavior allowed the formation constants to b e estimated as 33.9 and 20.3 dm(3) mol(-1) at 25 degrees C for the CL2- and TCNE2- complexes in CH2Cl2 containing 0.5 mol dm(-3) tetrabutylammonium pe rchlorate, respectively. Temperature dependence of the formation constants yielded the formation energy as 31.6 and 39.8 kJ mol(-1) for the CL2--BP an d TCNE2--BP systems, respectively. However, the CT spectra and the marked b ehavior in the voltammograms were not observed in the dianion systems invol ving 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 c omplexes. 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 HO MO-LUMO interaction, with trichromic change arising from the neutral comple x Formation, the anion radical generation, and the dianion complex formatio n.