Cyano-containing tetraphenylethene isomers: similar bright mechanoluminescence, but diverse recoverable processes†
Two simple luminogens p-CN and m-CN were obtained by introducing a cyano group at the para- and meta- position of the tetraphenylethene (TPE) skeleton, respectively. The two luminogens exhibited bright blue mechanoluminescence (ML) even under subdued light both in their crystalline and as-prepared powder state. They realized in situ recovery of ML with a simple thermal treatment, but with diverse recovery processes: p-CN returned to its original blue fluorescence, while m-CN transformed into cyan fluorescence. Molecule p-CN crystallized into a single crystal p-CN-c with blue emission, while m-CN crystallized into two polymorphs, m-CN-c1 and m-CN-c2, with blue emission and cyan emission, respectively. Crystallography analysis suggested that their ML was due to the interlocked molecular packing and intact three-dimensional supramolecular networks stabilized by the multiple strong intermolecular interactions. Their diverse recovery processes upon thermal stimuli were ascribed to their different transition path between their polymorphs. p-CN returned to molecular packing similar to its single crystal, while m-CN irreversibly transformed from packing similar to crystal m-CN-c1 into that of m-CN-c2, which had lower energy and a more twisted conformation. These results provide an important insight for better deciphering the relationship between ML and molecular packing, and help to optimize the rational molecular design of ML materials.