Triethylamine vapor-induced cyclization reaction in cocrystals leading to cocrystal-to-polycrystal transformation

The structural transformation of crystals triggered by external stimuli is a fascinating area in materials science and supramolecular chemistry. Despite the potential of organic reactions driving crystal conversions, its exploration remains limited, primarily due to challenges in maintaining crystallinity during significant structural changes. In this study, we present an intriguing example of a triethylamine vapor-induced cyclization reaction in a cocrystal, leading to its conversion into polycrystals. Initially, a charge transfer cocrystal with hydrogen-bond interactions denoted as AOTC, was prepared from 9-anthracene-substituted indolino-oxazolidine (Box) in its open form and 1,2,4,5-tetracyanobenzene (TCNB). Treatment of cocrystal AOTC with triethylamine vapor effectively induced a cyclization reaction, resulting in the formation of single-crystal AIC (closed form Box) and cocrystal ACTC (a cocrystal of AIC and TCNB), both of which were suitable for direct X-ray single crystal diffraction analysis. Experimental and theoretical analysis revealed that the cocrystal-to-polycrystal transformation was primarily driven by the cyclization reaction and the synergistic effects of intermolecular D-A and C-H···N hydrogen bond interactions. Additionally, this unique base-induced transformation was utilized for information storage applications. This research not only provides a rare instance of cocrystal-to-polycrystal transformation through a simple yet effective approach but also offers a strategy for crystal engineering.