Thermal Equilibrium Between Quinoid/Biradical Forms Enhancing Electrochemical Amphotericity

Upon dibenzo annulation on Thiele's hydrocarbon (tetraphenyl-p-quinodimethane), the quinoid form and the biradical form adopt quite different geometries, and thus are no longer resonance structures. When these two forms can interconvert rapidly due to the small energy barrier (ΔG^≠), the equilibrated mixture contains both forms in a ratio that is determined by the energy difference (ΔG^o) between the two forms. For a series of tetrakis[5-(4-methoxyphenyl)-2-thienyl]-substituted derivatives, the more stable quinoid form and the metastable biradical form coexist in solution as an equilibrated mixture due to small ΔG^≠ (<15 kcal mol^-1) and ΔG^o (1-4 kcal mol^-1), in which the proportion of the two forms can be regulated by temperature. Since the biradical form can undergo easy two-electron (2e) oxidation to the corresponding dications as well as easy 2e-reduction to the dianions, it exhibits very high electrochemical amphotericity. This character with a record-small span for not only the first oxidation and reduction potentials but also the second those, [E₁ ^sum≈E₂ ^sum=E₂ ^ox-E₂ ^red=ca. 1.4 V], is attained through thermally enhanced conversion to the biradical form from the corresponding quinoid form, the latter of which is less amphoteric due to higher E^ox and lower E^red values.