Tuning Head-to-Tail Packing in Polyfluorinated Schiff Bases for Enhanced NLO Response and High Refractive Index toward THz Applications

Abstract Schiff bases, which feature CN backbones and extended π-conjugation, are attractive building blocks for nonlinear optical (NLO) crystals. However, high-performance Schiff-base NLO crystals remain difficult to design because strong push−pull electronic systems often favor centrosymmetric packing through dipole−dipole interactions, thereby cancelling the macroscopic second-order response. To address this challenge, we incorporated a thiomethyl electron donor together with trifluoromethyl (−CF3) or trifluoromethoxy (−OCF3) acceptors into a Schiff-base framework. Two halogenated Schiff-base derivatives, (E)-1-(4-(methylthio)phenyl)-N-(4-(trifluoromethyl)phenyl)methanimine (TFNMLS, C15H12F3NS, Pca21) and (E)-1-(4-(methylthio)phenyl)-N-(4-(trifluoromethoxy)phenyl)methanimine (TFONMLS, C15H12F3NOS, Pc), were designed, synthesized, and crystallized in non-centrosymmetric space groups. Theoretical analysis indicates that steric hindrance from the fluorinated groups directs polar crystal packing, while the thiomethyl group promotes intramolecular charge transfer. These synergistic effects afford large theoretical NLO coefficients of 17 and 10 pm/V for TFNMLS and TFONMLS, respectively. Experimentally, the two crystals exhibit powder SHG efficiencies of 12 and 5.1 times that of KDP, together with high refractive indices, broad optical transparency (400−1800 nm), and good thermal stability. Polarization-resolved SHG measurements reveal pronounced optical anisotropy dominated by the d33 component. Finally, both crystals generate terahertz (THz) time-domain signals via optical rectification and effectively fill the intrinsic 1.1 THz spectral gap commonly observed in ionic crystals such as DAST, demonstrating promising low-frequency spectral continuity.