A theoretical study on the thermal cis-trans isomerization of azoheteroarene photoswitches

Quantum chemical calculations of a novel class of five-membered azo photoswitches arylazopyrazoles have been carried out with the aim of describing the photochemical properties of their thermal cis -> trans isomerization. The effects of substituents, in particular their number, position and nature of the electron donating groups (EDGs), on activation energies have been systematically researched with the ultimate goal of illuminating the isomerization process. It is found that all parameters are highly dependent on the substitution pattern on the heteroarene, allowing a planar or twisted geometry, which in turn has a significant impact on the absorption spectra, the energy levels of molecular orbitals, the transition properties of the trans/cis isomers, the half-lives and the rate constants for the thermal cis -> trans isomerization of the compounds. The nature of the transition state(s) and its dependence on substituents and the environment are discussed. Energy barriers were determined from two-dimensional potential energy surfaces of the ground states.