Cation-dominated second-harmonic generation in chiral tetrahydro-1-naphthylammonium halides

Nonlinear optical (NLO) materials have shown promising application prospects in fields such as optoelectronic devices and medical imaging. Traditional NLO materials are primarily constructed around anionic groups, while the role of cations is often overlooked. Herein, we report two NLO materials whose second-order nonlinear optical responses are predominantly dominated by chiral organic cations. Using enantiopure R-/S-1,2,3,4-tetrahydro-1-naphthylamine (R-/S-THNA) as the chiral building block, we synthesized the enantiomeric pairs of R-/S-THNA·HCl and R-/S-THNA·HBr by reacting with HCl and HBr, respectively. High-quality centimeter-sized single crystals with exceptional thermal stability and excellent ultraviolet transparency were obtained. These compounds crystallize in non-centrosymmetric space groups, thereby endowing them with second-harmonic generation (SHG) effects. Contrasting with the traditional anionic group theory model, their SHG effects originate from the non-spherical THNA⁺ cations, which exhibit similar first hyperpolarizabilities for both the R- and S-THNA⁺ cations, whereas the contribution from the spherical halide anions is negligible. This strategy of directly utilizing chiral organic cations as the SHG-active units provides a new direction for expanding the library of nonlinear optical materials.