(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 beam spectral bandwidth and amplitude. In this study, two hybrid metal halides, namely (C14H14N2)ZnCl4 and (C14H14N2)CdCl4, were successfully synthesised by combining planar π-conjugated (C6H10N2)2+ cationic groups, and [ZnCl4]2-/[CdCl4]2- anionic tetrahedral genes. Both of them crystallize in the non-centrosymmetric space group, with 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 strong birefringence (0.325 and 0.341 at 1064 nm), making them to be promising NLO materials. Meanwhile, both crystals show broadband green photoluminescence induced by the self-trapped excitons. 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.