Mechanism of trans -Azobenzene Izomerization: Role of the Rydberg 3 s -Orbital of the Azo-Group and Phenylaminyl-Type Cations

An analysis is performed of data on the trans → cis -photoisomerization of azobenzene ( trans -AB) in light of the Rydberg 3 s orbital ( R 3 s ) of the azo-group induces the formation of electronically ( e -) polarized tautomers ( Z + ) in addition to its electrically neutral tautomer ( Z ). The Z + tautomers have a chromogenic center in the form of a phenylaminyl type cation (PhAT) with a Vis excitation band at λ m = 445–450 nm. It is found that the isomerization of trans -AB under the action of Vis light and UV radiation proceeds through the photoexcited polarized e -tautomer Z + *, which undergoes adiabatic changes in its e -configurations under the influence of the R 3 s -orbital. The Z + * state (with one PhAT cation) then transitions to the Z ++ * state (with two PhAT cations), which participates in competitive transformations: isomerization in cis -AB and a return to Z + *, which relaxes in Z + . It is shown that a rotational isomerization pathway can form when there are no steric obstacles blocking rotation around the N–N bond. When there are such obstacles, isomerization proceeds through adiabatic displacement of the ph-rings in intermediate Z ++ *, first into its rectilinear conformation, and then toward one another through the stage of intermediate e -polarized excited state cis -AB*, which has a three-electron bond in addition to the σ-bond of the azo-group between adjacent sp 2 orbitals.