Broadband terahertz (THz) wave generators based on widely used ultrafast solid‐state lasers operating near 800 nm are very attractive for diverse linear and nonlinear THz applications. Here new electro‐optic molecular salt crystals are developed for efficient THz wave generation directly pumpable by a conventional femtosecond (fs) Ti:sapphire amplifier. Electron‐rich naphthalene‐sulfonate anions (naphthalene‐2‐sulfonate and naphthalene‐1‐sulfonate) are introduced in new electro‐optic quinolinium crystals, instead of previously used benzene‐sulfonate anions with smaller size. In new electro‐optic crystals, highly nonlinear quinolinium‐based molecular cations are optimally aligned for maximizing the diagonal electro‐optic response. The quinolinium‐based crystals prepared using a simple cleaving method exhibit excellent crystal characteristics suitable for efficient THz wave generation, good optical quality with two parallel surfaces having high flatness, and desired crystal thickness. Pumped with 100 fs pulses from a Ti:sapphire amplifier at 800 nm, the quinolinium‐based crystals enable efficient THz wave generation. Compared to the standard inorganic 1.0 mm thick ZnTe crystal, 4.7 times higher field amplitude and a broader bandwidth with the upper cutoff frequency of 6.0 THz are obtained. Therefore, introducing naphthalene‐sulfonate anions in molecular salt electro‐optic crystals is a promising way for designing electro‐optic crystals for THz wave generation by pumping at the technologically important Ti:sapphire wavelength.

We investigate the origins of solubility behavior of polar π-conjugated crystals in mixed solvent systems and the influence of nonpolar aromatic solvents on solubility and crystal growth characteristics. In solvent mixtures of polar nonaromatic methanol and nonpolar aromatic toluene (or o-xylene), solubility of three analogous polar crystals, consisting of a π-conjugated phenyltriene bridge between electron donor and acceptor groups, shows a maximum at the midpoint of composition. The maximal solubility mole fraction in such mixed solvent systems can be as high as 67 times as that in a single solvent. For comparison, a mixed solvent system consisting of only polar nonaromatic solvents (methanol and acetonitrile) is also examined. Main intermolecular interactions between the solute and solvent molecules in mixed solvent systems, related to the solubility behavior, are experimentally provided by the UV−vis absorption spectroscopy and 1 H NMR measurements. In the presence of aromatic solvents, additional strong interactions between the solvent and πconjugated solute molecules, such as face-to-face and edge-to-face π−π interactions including π•••HO−Ar hydrogen bonds are clearly observed. The aromatic characteristics of solvents also importantly influence the morphology and polymorphism of single crystals grown by solution crystal-growth methods.

Three-stage pH-switchable organic second-order nonlinear optical (SO NLO) chromophores are synthesized and characterized by wavelength-dependent linear and nonlinear spectroscopy. The chromophores exhibit huge SO NLO responses in their "on" stages, and large switching contrasts between adjacent stages in both SO NLO response and fluorescence quantum yield, with moreover different "on/off" sequences for closely related compounds.

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