Deep Insight into the Photochromism of meta- and para- Monosubstituted Tetraphenylethenes

Photocyclization of tetraphenylethenes (TPEs) was critical to reveal the underlying mechanism of photochromism of TPE derivatives. To explore the photocyclization process of TPEs, a series of monosubstituted TPE derivatives were designed as model molecules, with substituents (N(CH₃)₂, NH₂, OH, OCH₃, COOCH₃, CN, and NO₂) decorated to the TPE skeleton on meta and para positions, respectively. The photochromic property of the TPE derivatives was explored in DMSO solution and the PMMA matrix, respectively. It showed that TPE derivatives with a meta-substituted group were obviously more susceptible to the photocyclization reaction than their para-isomers. Meanwhile, electron-donating groups are more favorable for facilitating the photocyclization reaction than the electron-withdrawing group. Compared with DMSO, a rigid matrix (PMMA) can effectively elongate the lifetime of radical intermediates and photocyclization intermediates.