4‑Nitro-phenylalanine\nBromides: D−π–A\nStructural Features and Their Nonlinear and Linear Optical Properties

Nonlinear optical materials have important applications\nin optical\ncommunication, information processing, biomedicine, and other fields.\nHerein, two organic chiral 4-nitro-d(l)-phenylalanine\nbromides (4NDPAB and 4NLPAB) were successfully synthesized and fully\ncharacterized. Single-crystal X-ray diffraction data reveal the <i>P</i>2<sub>1</sub> non-centrosymmetric structure, in which the\nchiral cations and Br<sup>–</sup> anions are linked through\nfour intermolecular hydrogen bonds. Chirality is identified by circular\ndichroism spectroscopy. One enantiomer, 4NDPAB, is described as an\nexample. It exhibits an absorption edge at 435 nm (<i>E</i><sub>g</sub> = 2.85 eV) and demonstrates a high thermal stability\nup to 230 °C. Additionally, 4NDPAB shows strong second harmonic\ngeneration (SHG), 1.8 times higher than that of commercial KDP, and\nlarge birefringence (Δ<i>n</i><sub>exp</sub> = 0.178).\nDensity functional theoretical calculations and analyses confirm that\nthe strong SHG intensity and excellent anisotropy performance are\ndue to the D−π–A structural nature of 4-nitro-phenylalanine\nand its parallel crystal packing arrangement.