Correlating Molecular Hyperpolarizability and Macroscopic Nonlinear Optical Response in Azobenzene Triazole Glycoconjugates

Abstract

Two azobenzene triazole glycoconjugates, A and B, with the same chromophore core and saturated glucopyranoside spacer, but differing in the terminal para substituent of the phenoxy group (NO2 for A and NH2 for B), were synthesized and investigated. Their linear and nonlinear optical (NLO) properties were investigated using UV–visible spectroscopy, second and third harmonic generation measurements on PMMA thin films with corona poling, and Z-scan experiments in THF solutions. Maker fringe measurements performed at 1064 nm with 30 ps pulses gave effective second-order susceptibilities of 1.91 and 1.44 pm V–1 for A and B, respectively. The THG signal recorded for the film containing A was approximately twice that for the film containing B. Z-scan measurements at 532 nm revealed positive nonlinear refractive indices for both compounds along with a stronger nonlinear absorption response for A. DFT and TD-DFT calculations showed relatively similar frontier orbital energy gaps and second hyperpolarizability, while differences were observed in the space charge distribution and first hyperpolarizability. Because the saturated glucopyranoside unit breaks the extended π-conjugation between the terminal phenoxy substituent and the azobenzene triazole core, the experimental differences cannot be attributed to a fully conjugated donor–acceptor charge transfer pathway alone. Instead, they likely arise from the combined contributions of molecular conformation, local dipole distribution, excited-state processes, intermolecular interactions, and chromophore orientation in polymer films. These results provide a more detailed understanding of the structure-property relationships in azobenzene triazole glycoconjugates exhibiting nonlinear second- and third-order optical responses.