The Effect of Terminal Bromoalkoxy Substituents on the Photochromism of Functionalized 2H-Chromenes

A series of 2,2-diphenyl-2H-chromene derivatives bearing electron-donating groups at the benzopyran ring have been synthesized. A significant effect of the bromoalkoxy chain length and the concentration of chromenes on the conversion of closed (CF) into open forms under UV irradiation and the equilibrium between TC (transoid-cis) and TT (transoid-trans) isomers in the photostationary state was found. For chromene derivatives, a sequential (CF -> TC -> TT) or parallel (CF -> TC; CF -> TT) mechanism of transformation of the closed form into an open one is possible. A high solution concentration contributes to a significant shift of the photochromic equilibrium towards the TT isomer. The proposed scheme of the ring-opening reaction was confirmed by the analysis of the NMR spectra of acetonitrile solutions of chromene derivatives obtained by gradual irradiation. The results obtained are important for a deeper understanding of the nature of photochromic reactions.

Automated summary

A series of 2,2-diphenyl-2H-chromene derivatives with electron-donating groups on the benzopyran ring have been synthesized. The study found the significant effect of bromoalkoxy chain length and chromene concentration on the conversion of closed form into open form under UV irradiation. The proposed scheme of the ring-opening reaction was confirmed by NMR analysis, providing insights into the nature of photochromic reactions.