<p>A novel organic–inorganic metal-assisted single crystal, 1,4-Diazabicyclo[2.2.2]octane-1,4-diium hexaaqua-nickel sulfate (DHNS), was synthesized using the slow evaporation solution technique. In this process, 1,4-Diazobicyclo[2.2.2]octane coordinated with nickel sulfate in the presence of nitric acid and water as solvent is mixed to form a solution. The crystallographic data obtained from single-crystal X-ray diffraction (SCXRD) shows that the crystal formed from the solution has a crystal lattice centrosymmetric monoclinic with space group P2₁/n. The vibrational modes were assigned using Fourier transform infrared spectroscopy (FTIR) which confirms the presence of functional groups in the compound, hence aligning with the crystallographic study. The optical transparency in the UV–Visible region was studied using UV–Visible spectroscopy, which supports the crystal’s potential application in the field of photonics with a cut-off wavelength of 450nm and an optical band gap of 3.54 eV. The Urbach energy of the single crystal is about 0.7568eV. Thermal behavior of the crystal was analyzed through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), showing multistep multiphase degradation showing an endothermic peak of 115ºC. Further, the nonlinear optical (NLO) properties such as nonlinear absorption, nonlinear refractive index, and third-order susceptibility χ⁽³⁾ were obtained using the Z-scan technique, and the results show a promising enhanced NLO response compared to that of potassium dihydrogen phosphate (KDP).</p>