Effect of cis / trans isomerization on the photochromic performances of triphenylethylene

Revealing the structure–property relationships is crucial for promoting the development of photochromic materials and achieving the desired function in practical applications. Herein, a series of triphenylethylene photochromic materials (TP, TP-F, TP-Cl and TP-Br) have been synthesized and their photochromic properties were investigated. Monohalogenated TP-F, TP-Cl and TP-Br display obvious photochromic behaviors in solution, the solid state and polymer film. It is found that the halogen atoms with weak electron withdrawing ability can dramatically enhance the photocolorabilities of triphenylethylene. Moreover, TP-Cl and TP-Br exhibit different fatigue processes from TP-F. The further characterization reveals that the key factor leading to the diverse photochromic properties is cis / trans isomerization of triphenylethylene. The coloration of triphenylethylene mainly comes from the photo-triggered cyclization of cis -isomers. The initial attenuation of TP-Cl and TP-Br under UV irradiation is mostly caused by cis–trans photoisomerization, rather than the oxygen-involved photooxidation reactions. Based on the controllable “turn-on” and “turn-off” photobehaviors, TP-Cl and TP-Br were applied in the photoswitchable patterns, showing an enhanced signal-to-background ratio.