Theoretical study on the reaction mechanism of the thermal cis–trans isomerization of fluorine-substituted azobenzene derivatives

This research was based on the quantum chemical calculations of a set of valid photoswitches of azobenzene compounds, with the aim of describing their thermal isomerization. The influences of familiar fluorine substitution and additional electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) on the para-position were also systematically studied. The results show that the presence of fluorine in different ortho-positions has a distinct effect on the molecular orbital distribution of the E isomer, which realizes the purpose of splitting the n → π* transition between the E and Z isomers. On this basis, further para-substitution can allow tunability on the order of the energy level to the molecular orbitals through their influence on the conjugation pattern of the compound. It is the modification of the substituent on these positions that allows the photoisomerization to proceed under visible wavelength light surroundings. The thermal Z → E isomerization mechanism has also been analyzed, and a detailed comparison of these compounds has been made with respect to the thermal half-life τ(1/2), and the rate constants k(Z–E). The results reveal that isomerization is thought to be a process of globally structural change, during which the effect of the substituents is determined by the extent of their influence on the conjugated system.