Photoswitchable nonlinear optical properties of azobenzene-based supramolecular complexes: insights from density functional theory.

Density functional theory (DFT) calculations were performed for a series of supramolecular assemblies containing azobenzene (Azo-X where X = I, Br and H) and alkoxystilbazole subunits to evaluate their electronic, linear and nonlinear optical properties. These assemblies are derivatives of azobenzene, obtained by the substitution of electron-withdrawing and electron-donating groups onto the molecular skeleton. The interaction energies (Eint) of all the designed supramolecular complexes (IA-IF, IIA-IIF and IIIA-IIIF) range from -1.0 kcal mol-1 to -7.7 kcal mol-1. The electronic properties of these hydrogen/halogen bond driven supramolecular assemblies such as vertical ionization energies (VIE), HOMO-LUMO energy gap (GH-L), excitation energies, density of states (DOS) and natural population (NPA) analyses were also computed. The non-covalent interaction index (NCI) and quantum theory of atoms in molecules (QTAIM) analyses were also performed to validate the nature of inter- and intra-molecular interactions in these complexes. A substantial enhancement in the first hyperpolarizability (ß0) values of the designed supramolecular complexes was observed, which is driven by the charge transfer from the pyridyl moiety of alkoxystilbazole to Azo-X. The highest ß0 value of 1.3 × 104 au was observed for the supramolecular complex of p-nitro substituted azobenzene with alkoxystilbazole (ID complex). Moreover, the results show that the substitution of electron-withdrawing groups on Azo-X can also bring larger ß0 values for such complexes. It was confirmed on a purely theoretical basis that both the types of noncovalent interactions present and the substituent group incorporated influence the nonlinear optical (NLO) response of the systems. Furthermore, the ß0 values of the E (trans) and Z (cis) forms were compared to demonstrate the two-way photoinduced switching mechanism.