Synthesis, SC-XRD, Key Electronic Properties of Nitro Containing Schiff Base Chromophores: An Experimental and Computational Study

Two nitro-containing Schiff base crystals (MNIB, NHMN) were synthesized, and structural elucidation was accomplished via SC-XRD analysis. The optoelectronic and nonlinear optical (NLO) characteristics were investigated at the M06/6-311G(d,p) level of DFT and TD-DFT methods. The structural results illustrated that MNIB existed in keto form, whereas NHMN stabilized as an enol tautomeric form. Moreover, a comparative analysis of the geometrical parameters showed good agreement between the experimental SC-XRD data and the optimized DFT results. Hirshfeld surface and interaction energy analyses at the B3LYP/6-31G(d,p) level demonstrated that Coulombic and dispersion interactions play key roles in crystal packing stabilization. The calculated energy gaps (3.29–3.47 eV) and absorption maxima (481 and 440 nm) indicate favorable optoelectronic behavior. DOS, TDM, and FMO analyses confirmed efficient intramolecular charge transfer in both crystals. The calculated NLO parameters showed that NHMN had significantly better electronic properties than MNIB, including dipole moment (7.43 D), polarizability (5.35 × 10 −23 esu), first hyperpolarizability (5.39 × 10 −29 esu), and second hyperpolarizability (2.31 × 10 −34 esu). The high values of β total and γ total , along with efficient chargetransfer properties, indicate the high potential of the entitled crystals for further applications in optoelectronics and NLO characteristics. The calculated β is the intrinsic molecular NLO response, but due to centrosymmetric crystal structures, bulk second-order nonlinear optical effects are symmetry forbidden in the crystalline phase.