Hydrogen-Bonded Imidazolium Hydrogen Succinate Single Crystal: Structure–Property Correlation and Nonlinear Optical Performance

Organic salts of imidazole with carboxylic acids are attractive nonlinear optical (NLO) materials because charge-assisted hydrogen bonds combine wide UV transparency with tunable polarizability. Here, imidazolium hydrogen succinate (IMSU) single crystals (up to 19 × 13 × 7 mm3) were grown by slow evaporation from aqueous ethanol and studied by single-crystal and powder X-ray diffraction, FT-IR, UV–Vis–NIR, thermal, dielectric, Kurtz–Perry and Z-scan measurements, supported by DFT (B3LYP/6-311++G(d,p)), Hirshfeld, NBO and frontier-orbital analyses. Proton transfer from succinic acid to imidazole yields a salt that crystallizes in the centrosymmetric triclinic space group P-1, in which N–H···O and O–H···O hydrogen bonds build zig-zag chains. The crystals show a 232 nm cut-off, ~80% visible–NIR transmittance, a direct band gap of 5.4 eV, thermal stability up to ~248 °C and a frequency-stable permittivity (ε′ ≈ 121–125) with low loss. Because bulk second-order susceptibility is symmetry-forbidden in P-1, the powder SHG signal (0.89 × KDP) is attributed to interfacial (surface) contributions. A Z-scan at 1064 nm reveals nonlinear absorption and self-defocusing (n2 < 0), identifying IMSU as a wide-gap third-order NLO material for optical limiting.