Deep Insight into the Photochromism of <i>meta</i>- and <i>para</i>- 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 <i>meta</i> and <i>para</i> 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 <i>meta</i>-substituted group were obviously more susceptible to the photocyclization reaction than their <i>para</i>-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.