Solvatochromism and cis-trans isomerism in azobenzene-4-sulfonyl chloride

Solvatochromism exhibited by azobenzene-4-sulfonyl chloride (here abbreviated as Azo-SCl) has been investigated in a series of non-polar, polar-aprotic and polar-protic solvents. The UV-vis spectra of Azo-SCl exhibit two long-wavelength bands, observed at 321-330 nm (band-I) and 435-461 nm (band-II), which are ascribed to the π*-π (S₂ ← S₀) and π*-n (S₁ ← S₀) transitions, respectively. The shorter wavelength band indicates a reversal in solvatochromism, from negative to positive solvatochromism, for a solvent with a dielectric constant of 32.66 (which is characteristic of methanol), while the longer wavelength band signposts negative solvatochromism in all range of solvent's dielectric constant investigated, demonstrating different interactions with the solvents in the S₂ and S₁ excited states. Using Catalán and Kamlet-Taft solvation energy models, we found that the shift in the solvatochromic behavior of band-I (S₂ ← S₀) happens because solvent dipolarity/polarizability and hydrogen bonding affect the S₂ state in opposite ways. Dipolarity/polarizability stabilizes the S₂ state compared to the ground state, while hydrogen bonding destabilizes it. In contrast, for S₁, both effects work together to destabilize the excited state. For all studied solvents, UV irradiation (λ ≥ 311 nm; room temperature) was found to lead to fast trans-cis azo photoisomerization. In the absence of light, the photogenerated cis form quickly converts back to the trans form. Interpretation of the experimental data is supported by quantum chemical calculations undertaken within the Density Functional Theory (DFT) framework, including Time Dependent DFT calculations for excited states.