Theoretical Study of Solvent Effects on the Ground and Low-Lying Excited Free Energy Surfaces of a Push–Pull Substituted Azobenzene

Abstract

The ground and low-lying excited free energy surfaces of 4-amino-4′-cyano azobenzene, a molecule that has been proposed as building block for chiroptical switches, are studied in gas phase and a variety of solvents (benzene, chloroform, acetone, and water). Solvent effects on the absorption and emission spectra and on the <i>cis–trans</i> thermal and photo isomerizations are analyzed using two levels of calculation: TD-DFT and CASPT2/CASSCF. The solvent effects are introduced using a polarizable continuum model and a QM/MM method, which permits one to highlight the role played by specific interactions. We found that, in gas phase and in agreement with the results found for other azobenzenes, the thermal <i>cis–trans</i> isomerization follows a rotation-assisted inversion mechanism where the inversion angle must reach values close to 180° but where the rotation angle can take almost any value. On the contrary, in polar solvents the mechanism is controlled by the rotation of the CNNC angle. The change in the mechanism is mainly related to a better solvation of the nitrogen atoms of the azo group in the rotational transition state. The photoisomerization follows a rotational pathway both in gas phase and in polar and nonpolar solvents. The solvent introduces only small modifications in the nπ* free energy surface (<i>S</i>₁), but it has a larger effect on the ππ* surface (<i>S</i>₂) that, in polar solvents, gets closer to <i>S</i>₁. In fact, the <i>S</i>₂ band of the absorption spectrum is red-shifted 0.27 eV for the <i>trans</i> isomer and 0.17 eV for the <i>cis</i>. In the emission spectrum the trend is similar: only <i>S</i>₂ is appreciably affected by the solvent, but in this case a blue shift is found