title: 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.
authors.affiliation: Área de Química Física, Universidad de Extremadura , Avda de Elvas s/n, 06071 Badajoz, Spain.
authors.forename: Jose C
authors.initials: JC
authors.lastname: Corchado
authors.forename: M Luz
authors.initials: ML
authors.lastname: Sánchez
authors.forename: Ignacio
authors.initials: I
authors.lastname: Fdez Galván
authors.forename: M Elena
authors.initials: ME
authors.lastname: Martín
authors.forename: Aurora
authors.initials: A
authors.lastname: Muñoz-Losa
authors.forename: Rute
authors.initials: R
authors.lastname: Barata-Morgado
authors.forename: Manuel A
authors.initials: MA
authors.lastname: Aguilar