Phenolic Benzothiazolium-Based Nonlinear Organic Crystals for Terahertz Generation

We investigated a new series of phenolic benzothiazolium-based ionic organic nonlinear optical crystals with various counter anions. The newly designed benzothiazolium-based cationic chromophore showed a strong second harmonic generation (SHG) signal, confirming its nonlinear optical properties. The cationic chromophore with the methyl-substituted anion exhibited a nearly parallel molecular ordering, resulting in a large macroscopic optical nonlinearity. Interestingly, within the crystals, the methyl-substituted anions orthogonally aligned to the cationic chromophores. Orthogonal cation-anion coupling can effectively reduce the influence of entire-molecular phonon vibrations along the molecular axis of the anions on low-energy region terahertz (THz) self-absorption, compared to parallel cation-anion coupling. Note that suppressing THz self-absorption caused by the vibrations of a part of the molecule or the entire molecule is a crucial issue in the field of ionic organic nonlinear optical crystals. We attempted to maximize the suppression effect by replacing the methyl group on the anion with halogen substituents, which share similar space-filling characteristics. These halogen substituents induce strong intermolecular interactions via their partially positive σ-holes and the partially negative belts, which can suppress THz self-absorption.