Mechanism of cis-Azobenzene Photoisomerization: Role of the Rydberg 3s-Orbital of the Azo Group
Yu. A. Mikheev
Российский журнал физической химии А
https://doi.org/10.1134/S0036024422030177
An 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 λex = 435 nm. It is noted that the Rydberg 3s orbital (R3s) of the azo group of cis-AB induces reversible electron (e–) wave tautomerization between the e-neutral form of cis-AB (E,\(R_{{{\text{3s}}}}^{0}\)) and the e-polarized form E+,\(R_{{{\text{3s}}}}^{{1-}}\), as with e-tautomerization between the neutral form Z,\(R_{{{\text{3s}}}}^{0}\) and polarized form Z+,\(R_{{{\text{3s}}}}^{{1-}}\) of trans-AB. The orange color of cis-AB is explained by its e‑tautomers E+,\(R_{{{\text{3s}}}}^{{1-}}\) having chromogenic phenylaminyl-type cations (PhAT) like Z+,\(R_{{{\text{3s}}}}^{{1-}}\) in trans-AB; the stronger coloration of cis-AB relative to trans-AB reflects a higher equilibrium proportion of e-tautomers E+,\(R_{{{\text{3s}}}}^{{1-}}\) in cis-AB, relative to the proportion of Z+,\(R_{{{\text{3s}}}}^{{1-}}\) in trans-AB. A mechanism of cis-AB photoisomerization is proposed in which e-tautomers E+*,\(R_{{{\text{3s}}}}^{{1-}}\) excited by pulses with λex = 435 nm undergo transition to the E++*,\(R_{{{\text{3s}}}}^{{2-}}\) state with two PhAT cations. It is shown that the Coulomb repulsion acting in the E++*,\(R_{{{\text{3s}}}}^{{2-}}\) 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++*,\(R_{{{\text{3s}}}}^{{2-}}\) state is isomerized with constant τ = 170 fs into a planar trans-intermediate (Z++*,\(R_{{{\text{3s}}}}^{{2-}}\)) 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.
- Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, 119334, Moscow, RussiaYu. A. Mikheev
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