On the Origin of the Solid-State Thermochromism and Thermal Fatigue of Polycyclic Overcrowded Enes

Aimed at unraveling the relative contribution of the folding, twisting and bending in the mechanism of the solid-state thermochromism of overcrowded polycyclic aromatic enes (PAEs), the structures of two typical heteromeric and homomeric representatives, 2-(thioxanthen-9-ylidene)indane-1,3-dione (1) and 9,9′-bi-9(10H)-anthracenylidene-10,10′-dione (bianthrone, 2), were studied by temperature-resolved single crystal X-ray diffraction (120–530 K) and solid-state UV–visible spectroscopy. Aside from negligibly small unfolding of the tricyclic moiety of 1, this first direct diffraction study of the high-temperature structures of solid PAEs did not unravel any significant and detectable changes in the time- and space-averaged intramolecular structures, thus, showing that the PAE-type thermochromism is not due to phase transitions or to major and permanent molecular distortions of a large portion of the material that would be caused by folding, twisting and/or bending. Instead, the experimental observations and theoretical modeling indicated that the color change is probably due to a dynamic process, where the absorption spectrum changes as a result of enhanced thermal oscillations of the two halves of the molecules around the central bridge. In addition to the reversible coloration, we also observed irreversible processes of thermal fatigue that afford stable chemical products that absorb in the visible region. We showed that the stable products are conductive and they act electrocatalytically toward oxidation of several biomarkers.