Hydrogen-Bond-Directed Assembly of an Intramolecular Charge-Transfer Compound for Second-Order Nonlinear Optics

A novel benzothiadiazole-based nonlinear optical crystal (BTN-1) capable of intramolecular charge transfer was grown via solvent evaporation. Single-crystal X-ray diffraction reveals it crystallizes in the noncentrosymmetric space group, P21. Intramolecular hydrogen bonds induce two distinct molecular conformations, which weaken intermolecular π–π stacking, promote charge transfer, and facilitate a “head-to-tail” packing motif. This process has been further elucidated by Hirshfeld surface and electrostatic potential analyses. Theoretical calculations demonstrate an exceptionally high first hyperpolarizability (βtot = 112.09 × 10–30 esu), exceeding that of urea by about 300 times. Second-harmonic generation (SHG) measurements confirm strong frequency doubling across a broad wavelength range, with intensity systematically dependent on the polarization angle, validating its excellent nonlinear optical (NLO) performance. This study presents a promising NLO material and demonstrates an effective design strategy for functional crystals by the modulation of intramolecular charge transfer.