Alkynyl Pyridinium Crystals for Terahertz Generation

The design and synthesis of three organic nonlinear optical crystals is presented for terahertz (THz) generation that incorporates the optimal characteristics of known organic nonlinear optical crystals 4-(4-(dimethylamino)styryl)-1-methylpyridinium 4-methylbenzenesulfonate (DAST) and 2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium 2,4,6-trimethylbenzenesulfonate (HMQ-TMS). The three crystals feature the 4-((4-(dimethylamino)phenyl)ethynyl)-1-methylpyridin-1-ium (4DEP) cation and three different anion combinations. Gas phase ab initio calculations of the cation show that 4DEP has a larger hyperpolarizability than cations from both DAST and HMQ-TMS. To obtain the molecular packing needed for large nonlinear optical susceptibility and efficient THz generation, three anions are tested that result in noncentrosymmetric crystals: 4-methylbenzenesulfonate, 3-nitrobenzenesulfonate, and napthalene-2-sulfonate. After synthesis and crystallization, the crystal structure is characterized via X-ray diffraction (XRD) analysis, and the relative THz generation efficiency for ideal crystals is predicted. 4DEP-N2S shows a roughly 30% larger relative second-order nonlinear optical susceptibility compared to HMQ-TMS and DAST and, therefore, shows promise as a new advanced THz generation crystal.