Mechanism of cis -Azobenzene Photoisomerization: Role of the Rydberg 3 s -Orbital of the Azo Group

n analysis is performed of data on the femtosecond photoisomerization of cis -azobenzene ( cis ‑AB) under the action of Vis laser radiation with a wavelength of λ ex = 435 nm. It is noted that the Rydberg 3 s orbital ( R 3s ) of the azo group of cis -AB induces reversible electron ( e – ) wave tautomerization between the e -neutral form of cis -AB ( E , R_3s^0 ) and the e -polarized form E + , R_3s^1- , as with e -tautomerization between the neutral form Z , R_3s^0 and polarized form Z + , R_3s^1- of trans -AB. The orange color of cis -AB is explained by its e ‑tautomers E + , R_3s^1- having chromogenic phenylaminyl-type cations (PhAT) like Z + , R_3s^1- in trans -AB; the stronger coloration of cis -AB relative to trans -AB reflects a higher equilibrium proportion of e -tautomers E + , R_3s^1- in cis -AB, relative to the proportion of Z + , R_3s^1- in trans -AB. A mechanism of cis -AB photoisomerization is proposed in which e -tautomers E + * , R_3s^1- excited by pulses with λ ex = 435 nm undergo transition to the E ++ *, R_3s^2- state with two PhAT cations. It is shown that the Coulomb repulsion acting in the E ++ *, R_3s^2- state between PhAT cations pushes them into an intermediate planar cis -state with constant τ = 47 fs. The energy stored in cis -AB is then added to that of photoexcitation, and the E ++ *, R_3s^2- state is isomerized with constant τ = 170 fs into a planar trans -intermediate ( Z ++ * , R_3s^2- ) that relaxes mainly to trans -AB. Schemes of the relaxation channels are given along with reasons for the change in the quantum yields of the cis ↔ trans isomerization of AB when the solvent n -heptane is replaced with a water–alcohol mixture.