Two cocrystals of 18-crown-6 with isophthalic acid derivatives, 5-hydroxyisophthalic acid and trimesic acid, have been successfully grown by the slow evaporation solution growth technique. Crystal structures of (18-crown-6)·6(5-hydroxyisophthalic acid)·10(H2O) (I) and (18-crown-6)·2(trimesic acid)·2(H2O) (II) elucidated by single crystal X-ray diffraction reveal that both cocrystals pack the centrosymmetric triclinic space group . The molecules are associated by strong/weak hydrogen bonds, π⋯π and H⋯H stacking interactions. Powder X-ray diffraction analyses, experimental and simulated from single-crystal diffractogram data have been matched. The vibrational patterns in FT-IR spectra are used to identify the functional groups. The band gap energy is estimated by the application of the Kubelka-Munk algorithm. Hirshfeld surfaces derived from X-ray diffraction analysis reveal the type of molecular interactions and their relative contributions. The constructed supramolecular assembly of crown ether cocrystal is thoroughly described. Both cocrystals exhibit a significant third-order nonlinear optical response and it is observed that (I) possesses a significant first-order molecular hyperpolarizability whereas it is negligible for (II). © 2020 International Union of Crystallography.