(C14H14N2)MCl4 (M = Zn, Cd): two hybrid metal halides exhibiting strong second harmonic generation responses, large birefringence, and broadband photoluminescence

Abstract

Nonlinear optical (NLO) crystals are vital materials in optoelectronics and laser technology, as they can expand laser applications by converting the light beam wavelength and amplitude. In this study, two hybrid metal halides, namely (C₁₄H₁₄N₂)ZnCl₄ and (C₁₄H₁₄N₂)CdCl₄, were successfully synthesised by combining planar π-conjugated (C₁₄H₁₄N₂)²⁺ cationic groups, and [ZnCl₄]²⁻/[CdCl₄]²⁻ anionic tetrahedral genes. They both crystallize in the non-centrosymmetric space groups, monoclinic Cm and orthorhombic Cmc2₁ for (C₁₄H₁₄N₂)ZnCl₄ and (C₁₄H₁₄N₂)CdCl₄, respectively. They exhibit excellent optical properties, including significant phase-matched second-harmonic generation responses (1.63 and 1.36 × KDP), wide band gaps (3.20 and 3.13 eV), and large birefringence (0.325 and 0.341 at 1064 nm), making them promising NLO materials. Meanwhile, both crystals show broadband green photoluminescence. Theoretical studies and crystal structure analyses reveal that their large SHG effects and birefringence are primarily from the synergistic interaction between π-conjugated organic cations and distorted [ZnCl₄]²⁻/[CdCl₄]²⁻ tetrahedra. This work provides a new way for designing high-performance hybrid metal halide NLO materials with broad luminescence.