Synthesis of Crystalline Fluoro-Functionalized Imines, Single Crystal Investigation, Hirshfeld Surface Analysis, and Theoretical Exploration

This investigation is focused on the synthesis of two halo-functionalized crystalline Schiff base (imine) compounds: (<i>E</i>)-2-methoxy-6-(((3-(trifluoromethyl)phenyl)imino)methyl)phenol (<b>MFIP</b>) and (<i>E</i>)-1-(((2-fluorophenyl)imino)methyl)naphthalen-2-ol (<b>FPIN</b>) by the condensation reaction of substituted benzaldehydes and substituted aniline. The crystal structures of <b>MFIP</b> and <b>FPIN</b> were determined unambiguously by single-crystal X-ray diffraction (SC-XRD) studies. Intermolecular interactions and the role of fluorine atoms in the stabilization of the crystal packing are explored for both compounds using Hirshfeld surface analysis. Accompanied with experimental studies, quantum chemical calculations were also performed for comprehensive structure elucidation at the M06/6-311G(d,p) level of theory. A comparison of experimental and density functional theory results for geometrical parameters exhibited excellent agreement. Interestingly, Frontier molecular orbitals and natural bond orbital (NBO) findings revealed that intramolecular charge transfer and hyper-conjugation interactions had played a significant role to stabilize the molecules. Both compounds exhibited a relatively larger value of hardness with a smaller global softness, which, as proposed by the SC-XRD and NBO study, shows a higher stability. Nonlinear optical (NLO) findings showed that <b>FPIN</b> manifested a larger value of linear polarizability (<<i>a</i>> = 293.06 a.u.) and second-order hyperpolarizability (<γ> = 3.31 × 10⁵ a.u.) than <b>MFIP</b> (<<i>a</i>> = 252.42 and <γ> = 2.08 × 10⁵ a.u.) due to an extended conjugation. The above-mentioned findings of the entitled compounds may play a crucial role in NLO applications.