Enhancing tetraphenylethene cyclization as photoswitch

Abstract

Tetraphenylethene (TPE), a star building block with promising aggregation-induced emission, has received much interest. Given that its intramolecular Woodward-Hoffmann cyclic intermediate instantaneously converts back to the original state within several picoseconds, the essentially photochromic characteristic of TPE is little investigated. Achieving a visible photocyclization of TPE is still an unsolved issue and considered as the bottleneck in the further advancement of applications. We report a strategy of attaching carbonate ester onto the TPE skeleton (TPE-4C) to enhance TPE photocyclization stability. As demonstrated, the incorporated cholesteryloxycarbonyloxy substituents in TPE-4C can increase the energy barrier for cycloreversion, thereby exhibiting extremely thermal stability of photocyclic intermediate upon UV irradiation, prolonging its lifetime from 63 picoseconds to 46 s by 7.2 × 10¹¹-fold. The photoinduced cyclization of TPE-4C could be monitored with naked eyes, and the photocyclization/cycloreversion is achieved by turning on/off UV light along with a relative fatigue resistance. Encapsulation of TPE-4C into the liquid crystal can induce a striking phase transformation (achiral↔chiral), which can be applicable to encode optical information. Employing carbonate ester into the TPE unit plays a vital role in enhancing the unprecedented TPE photocyclization stability, providing a toolbox to allow TPE-based photocyclization to be visually monitored.