Theoretical study of substituent and charge effects on the thermal cis → trans isomerization of ortho-fluoroazobenzenes photoswitches

Abstract

Besides the photochromism isomerization, scientists are likely to focus more and more on the influence on the electrochemical reduction conditions of the azobenzene isomerization process. Diverse azobenzene compounds were chosen to study the behavior of their trans and cis isomers upon electrochemical reduction. Hence, thermal reaction mechanisms for the neutral, cationic and anionic azobenzene derivatives isomerization with a wide variety of electron-donating and electron-withdrawing groups in the para-position of the benzene ring in ground state have been systematically examined applying the density functional theory method. Some prominent changes in structure, energy barrier and reaction rate arose from charge changes of azobenzenes have been detected and interpreted in terms of the strengths of substituents, the polarity of solvents and charge effect. Our findings indicate that the radical anion of the cis isomer can isomerize to the corresponding trans configured counterpart rapidly with a tremendously enhanced rate and obvious decreased energy barriers with respect to the neutral species as the distribution of the electron density changed, signifying that the metastable isomer of a photoswitch is able to isomerize to the more stable one catalytically upon reduction and the charge effect has obvious influence on the reaction mechanism.