Quinoxaline-based molecular crystals with tunable supramolecular networks and nonlinear optical properties: an integrated experimental, DFT and TD-DFT study.

Abstract

Quinoxaline-based molecular crystals are promising platforms to understand the relation of molecular architecture, supramolecular organization and nonlinear optical behavior. Three functionalized quinoxaline derivatives, 6-(3,5-bis(trifluoromethyl)phenyl)quinoxaline (TFPQ), 2-(quinoxalin-6-yl)phenol (QP) and 6-(pyridin-4-yl)quinoxaline (PQ), were synthesized through Suzuki cross-coupling reactions and characterized using a combination of theoretical and experimental methods. The synthesized compounds were structurally characterized using FT-IR, UV-visible, 1H-NMR, and 13C-NMR spectroscopy. The single-crystal X-ray diffraction (SC-XRD) analysis confirmed the solid state structures of TFPQ and QP. The weak C-H⋯F interaction is the main stabilizer of the crystal packing of TFPQ while in the case of QP, a directed supramolecular network was formed via O-H⋯N and C-H⋯O interactions. Hirshfeld surface analysis further confirmed distinct packing environments, with the H⋯F contacts dominant at 37% and 37.9% for TFPQ, whereas H⋯C contacts are dominant for the crystal packing of QP with a contribution of 39.9%. The B3LYP/6-311G(d,p) level DFT calculations showed HOMO-LUMO gaps of 4.49, 4.12 and 4.49 eV for TFPQ, QP and PQ, respectively, and revealed substituent-dependent charge-transfer and molecular hyperpolarizability responses. Importantly, the non-centrosymmetric symmetry of TFPQ enables an electric-dipole second-order response, which is symmetry-forbidden for centrosymmetric QP in bulk second-harmonic generation (SHG). Since SHG was not measured experimentally, the computed β tensor is considered a molecular NLO descriptor, rather than a direct proof of bulk SHG.

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