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)CdI4and (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 small birefringence of 0.02 @ 546 nm, weak and non-phase-matchable SHG response (0.3 × KH2PO4, KDP), and 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), sevenfold higher than that of (H2DABCO)CdI4, and achieves phase-matchability. Additionally, (PIP)·CdI3(HDABCO) exhibits a good thermal stability (180 ℃), short λcutoff edge (273 nm) and wide bandgap (4.05 eV), indicating its potential as a high-performance ultraviolet SHG crystal.