Tunable Solid-State Thermochromism: Alkyl Chain Length-Dependent Conformational Isomerization of Bianthrones.

The introduction of linear alkoxy chains into bianthrone promoted the aggregation of the minor twisted conformer and made it compete with the major folded conformer in the solid-state. Rapid evaporation approach to the methoxy- and n-butoxy-substituted derivatives provided powder mixtures of folded and twisted conformers as metastable solids. Upon heating the powders, different lengths of alkyl chains converged to different conformers via solid-state isomerization reaction with an apparent color change. Monitoring by FT-IR spectroscopy and powder XRD ensured that these isomerization reactions involve only conformational isomerization without pyrolysis. In addition, single crystal X-ray structure analysis revealed that twisted conformers with longer alkyl chains are stabilized in the solid-state by the synergistic effect of their inter-chain hydrophobic interactions and π-π stacking interactions.