Quinolinium-based organic electro-optic crystals: Crystal characteristics in solvent mixtures and optical properties in the terahertz range

We investigate crystal characteristics of ionic organic π-conjugated crystals based on 4-hydroxy-3-methoxystyryl quinolinium (HMQ), which are recently developed acentric crystals showing a large nonlinear optical response and a high THz generation efficiency. Among a few of HMQ derivatives, HMQ-T crystal consisting of the HMQ cation and the 4-methylbenzenesulfonate anion, is chosen for the investigation of solubility, morphology and crystal growth characteristics including optical properties in various single solvents and solvent mixtures. Compared to the solvent mixture of non-aromatic methanol and acetonitrile, HMQ-T crystals in a solvent mixture containing an aromatic solvent (methanol/toluene mixture) exhibits a similar solubility variation with the solvent composition ratio, but based on distinguishable intermolecular interactions with solvent molecules. In addition, solvent mixtures containing an aromatic solvent show a significantly different equilibrium between benzenoid and quinoid forms of the HMQ cation. In contrast to the strong influence of solvent characteristics on solubility behavior, the investigated solvent systems only slightly influence the crystal morphology. Large HMQ-T single crystals with size up to 37 × 6 × 2 mm3 are grown by slow cooling method. We also determine the refractive index and absorption of as-grown single HMQ-T crystals in the broad THz frequency range of 0.3-10 THz.