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

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 (E_int) 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 (G_H-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 (β₀) 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 β₀ 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 β₀ 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 β₀ values of the E (trans) and Z (cis) forms were compared to demonstrate the two-way photoinduced switching mechanism.