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.