Impact of Van der Waals interactions on structural and nonlinear optical properties of azobenzene switches

Azobenzene derivatives are popular photochromes in nonlinear optics (NLO) due to their ability to convert between two conformational isomers, the stable trans (E) and the metastable cis (Z), with a large contrast in their first hyperpolarizability. Recently, the chemical functionalization of azobenzene molecules with bulky substituents has been shown to significantly impact the photochemical process, due to intramolecular Van der Waals (VdW) interactions that stabilize the Z isomers [1]. In addition, VdW interactions are also expected to impact the NLO properties of azobenzenes.

In this work, we perform a theoretical study of the structure and NLO properties of a collection of azobenzene molecules symmetrically substituted in meta-position with functional groups of different bulkiness. We assess the performance of a large set of density functional approximations in reproducing the geometry, the relative energy and first hyperpolarizability of E and Z isomers in comparison with ab initio calculations. Moreover, we analyze the individual contribution of the substituents on the response of this series of compounds, giving insights on the precise role of the functional groups responsible for dispersion interactions.

[1] Schweighauser et al., Angew.Chem. Int. Ed. 54, 13436(2015).