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 cis-trans 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 cis-trans 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 (S1), but it has a larger effect on the ππ* surface (S2) that, in polar solvents, gets closer to S1. In fact, the S2 band of the absorption spectrum is red-shifted 0.27 eV for the trans isomer and 0.17 eV for the cis. In the emission spectrum the trend is similar: only S2 is appreciably affected by the solvent, but in this case a blue shift is found.

The Kinetics of the Cis-to-Trans Thermal Isomerization of 4-Anilino-4’- Nitroazobenzene are Highly Influenced by Solvent Polarity

“…Azobenzenes are common organic dyes that have been extensively studied both experimentally and theoretically owing to their potential applications in material science, medicinal chemistry, molecular switches and other devices [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. They are photo-reactive molecules that undergo reversible photo-isomerization from the more stable trans-isomer to the less stable cis-isomer.…”

“…The isomerization mechanism of 4A4NAB can thus be defined as a rotation around the azo bond assisted by a linearization of the angle of the ring attached to the electronwithdrawing substituent. The extent of such assistance depends on the polarity of the solvent, being less important as the polarity increases [8]. Indeed, as the solvent polarity increases, a noticeable decrease in the activation energy, entropy, enthalpy and Gibbs free energy of activation for the cis-trans isomerization process is observed, a result typical of para-donor, para-acceptor type azobenzenes.…”

The Quest for Photoswitches Activated by Near‐Infrared Light: A Theoretical Study of the Photochemistry of BF2‐Coordinated Azo Derivatives

“…There has been a large number of works devoted to designing azobenzene derivatives with improved properties by using simple theoretical arguments. The leading ideas have been to push the ππ* excitation to lower energies through the incorporation of electron‐withdrawing or ‐donating groups at the ortho and para positions or by extending the π system of the molecule . This has mostly led to systems with the desired redshift in the ππ* excitation but also to a (usually undesired) increase in cis ‐to‐ trans thermal isomerization .…”

Photoswitching in nanoporous, crystalline solids: an experimental and theoretical study for azobenzene linkers incorporated in MOFs

“…Here, we focused on the rotation mechanism, which seems to be the prevailing isomerization mechanism. 28,29 In Fig. 4(a) and (b), 2D projections of the corresponding high-dimensional ground-state potential energy hypersurface (PES) are plotted for the two MOFs.…”