Theoretical Study On Thermal Cis-to-trans Isomerization Of BF₂-coordinated Azo Compounds Of The Para-substitution With Electron Donating Groups

The molecular structure of the conventional azobenzene(AB) consists of two phenyl rings linked by azo group(N=N), and it can be reversibly switched between trans and cis isomers. In the past decades, quantum chemical calculations of various AB and its derivatives have been carried out with goal to explain photo and thermal isomerization. The substituent effects, in particular their type(electron donating group, electron withdrawing group), number, and positioning(para-, meta-, and ortho-position of the phenyl ring) have been systematically studied on the isomerization pathways. The trans → cis isomerization is induced by illumination with light, and the cis → trans isomerization in the solution can be achieved by light illumination or in the dark. Moreover, in the presence of gold, Ag, and Cu nanoparticles, the activation barriers of the thermal isomerization is found to be lowered, compared to the activation barriers in the solution. In conclusion, the isomerization of AB and its derivatives has been reported largely all over the world.However, thermal cis → trans isomerization for a series of BF2-coordinated azo compounds of the para-substitution with electron donating groups have been systematically investigated at the first time. The density-functional theory calculations exhibited good performance to provide better understanding of the effects of parasubstitution with electron donating groups. It is found that the different electron donation groups can significantly affect the absorption spectra, the energy levels of molecular orbitals, the transition properties for the trans isomers, the rate constants and the half-lives for the thermal cis → trans isomerization. Our calculated half-lives for the thermal cis → trans isomerization are in qualitative agreement with the experimental values. Specifically, we have evaluated the thermal rate constants at 294 K. The relationship between the thermal isomerization and the para-substituted electron-donating group reveals that the inversion mechanism is preferred for the substituent species while the rotation mechanism is more favorable for non-substituent molecule.