From poor-active to high performance via heteroleptic tetrahedra engineering: rational design of non-π-conjugated hybrids for phase-matchable ultraviolet nonlinear optics

Abstract

In the exploration of new second-harmonic generation (SHG) crystals, non-π-conjugated organic–inorganic metal halides (OIMHs), particularly Zn/Cd-based OIMHs, have long been overlooked. This is primarily due to the high symmetry and low polarizability of both metal-halide anions (typically regular tetrahedra or octahedra) and non-π-conjugated organic cations. Herein, we report two novel SHG-active non-π-conjugated OIMHs: (H₂DABCO)CdI₄ and (PIP)·CdI₃(HDABCO), where DABCO = C₆H₁₂N₂ (1,4-diaza[2.2.2]bicyclooctane) and PIP = C₄H₁₀N₂ (piperazine). (H₂DABCO)CdI₄ (P2₁2₁2₁), with discrete (H₂DABCO)²⁻ cations and (CdI₄)²⁻ anions, exhibits a poor SHG performance, including a small birefringence of 0.02@546 nm, weak and non-phase-matchable SHG response (0.3 × KH₂PO₄, KDP), and a narrow bandgap of 3.84 eV. However, by constructing higher-polarizability [CdI₃(HDABCO)] heteroleptic tetrahedra through organic–metal coordination bonds and incorporating PIP units, the two modules are assembled into the polar space group Cc via hydrogen bonding. Consequently, (PIP)·CdI₃(HDABCO) demonstrates a dramatically enhanced SHG effect (2.1 × KDP), which is sevenfold higher than that of (H₂DABCO)CdI₄, and achieves phase-matchability. Additionally, (PIP)·CdI₃(HDABCO) exhibits good thermal stability (180 °C), a short λ_cutoff edge (273 nm) and wide bandgap (4.05 eV), indicating its potential as a high-performance ultraviolet SHG crystal.