The twisted form of bianthrone is known as a metastable state provided by a photo-induced or thermal-induced isomerization of the folded form, and thus prevents the isolation and the detailed analysis of its electronic structure. In this study, an unsymmetrical bianthrone (2), consisting of the electron-withdrawing anthrone and electron-donating acridane, have been synthesized and shown to exhibit a solvent-polarity-dependent isomerization reaction between the folded and twisted isomers. With increasing the polarity of the solvent, 2 showed an isomerization reaction from the folded form to the twisted form. The stabilization of the twisted isomer in polar solvents can be interpreted as proof of its relatively large zwitterionic character. The DMF solution of 2 displayed paramagnetically-broadened NMR signals from the thermally populated triplet state resulting from rotation of the weakened ethylenic double bond of the twisted isomer.

A dyad comprised of a derivative of anthranol with an improved electron-donating ability and a derivative of acridine having an improved electron-accepting ability was prepared using a palladium-catalyzed C–H arylation process. The dyad assembles to form two crystal polymorphs, one with a color close to that of the dyad in solution and the other with a completely different color. The polymorphs are comprised of hydrogen-bonded chain aggregates that have different hydrogen-bonding motifs. Structural and spectroscopic analyses showed that the unusually colored crystals possess a stronger intermolecular hydrogen bond. In addition, this crystal displays a charge-transfer (CT) absorption band, corresponding to one-electron transfer from the anthranol unit to the acridine unit, that is red-shifted and enhanced relative to those of the other crystal and the dyad in solution. The combined theoretical and experimental results suggest that hydrogen-bonding and charge-transfer interactions take place synergistically in the unusually colored crystals.