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.

本研究旨在厘清折叠、扭曲与弯曲在拥挤型多环芳香烯烃（overcrowded polycyclic aromatic enes, PAEs）固态热致变色机制中的相对贡献。选取两类典型的杂聚与同聚代表物——2-(噻吨-9-亚基)茚满-1,3-二酮（1）与9,9'-双-9(10H)-蒽亚基-10,10'-二酮（双蒽酮，2），通过变温单晶X射线衍射（120–530 K）与固态紫外-可见光谱法对其结构展开研究。除化合物1的三环部分发生可忽略程度的舒展外，此项针对固态PAEs高温结构的首次直接衍射研究并未发现时空平均的分子内结构存在显著且可检测的变化，由此表明PAE型热致变色并非源于相变，亦非由折叠、扭曲和/或弯曲致使材料大部分区域产生显著永久性分子畸变所引发。与之相反，实验观测与理论建模结果显示，颜色变化可能源于一种动态过程：分子围绕中心桥键的两部分发生热振动加剧，进而导致吸收光谱改变。除可逆着色现象外，本研究还观测到热疲劳引发的不可逆过程，该过程可生成能够吸收可见光的稳定化学产物。研究表明，此类稳定产物具备导电性，并可对多种生物标志物的氧化反应起到电催化作用。