A Theoretical Exploration of Supramolecular Architectures as High Performance Nonlinear Optical (NLO) Materials

Abstract

Density functional theory (DFT) calculations have been 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 -1.02 kcal/mol to -7.70 kcal/mol. 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 bond orbital (NBO) analyses were also computed. The non-covalent interaction index (NCI) and quantum theory of atoms in molecules (QTAIM) analyses have also been performed to validate the nature of inter- and intra-molecular interactions in these complexes. A substantial enhancement in the first hyperpolarizability (βₒ) values of the designed supramolecular complexes has been observed driven by the charge transfer from the pyridyl moiety of alkoxystilbazole to Azo-X. The highest first hyperpolarizability (βₒ) value of 1.3×104 au is observed for supramolecular complex of p-nitro substituted azobenzene with alkoxystilbazole (ID complex). It was confirmed on a purely theoretical basis that both the type of noncovalent interactions present and the substituent group incorporated influence the nonlinear optical response (NLO) of the systems.