The first polyanion-substitution-driven centrosymmetric-to-noncentrosymmetric structural transformation realizing an excellent nonlinear optical supramolecule [Cd 4 P 2 ][CdBr 4 ].

Crystallographically, noncentrosymmetricity (NCS) is an essential precondition and foundation of achieving nonlinear optical (NLO), pyroelectric, ferroelectric, and piezoelectric materials. Herein, structurally, octahedral [SmCl ₆ ] ^3- is substituted by the acentric tetrahedral polyanion [CdBr ₄ ] ^2- , which is employed as a templating agent to induce centrosymmetric (CS)-to-NCS transformation based on the new CS supramolecule [Cd ₅ P ₂ ][SmCl ₆ ]Cl (1), thereby providing the NCS supramolecule [Cd ₄ P ₂ ][CdBr ₄ ] (2). Meanwhile, this replacement further results in the host 2D _∞ ² [Cd ₅ P ₂ ] ⁴⁺ layers converting to yield the twisted 3D _∞ ³ [Cd ₄ P ₂ ] ²⁺ framework, which promotes the growth of bulk crystals. Additionally, phase 2 possesses well-balanced NLO properties, enabling considerable second-harmonic generation (SHG) responses (0.8-2.7 × AgGaS ₂ ) in broadband spectra, the thermal expansion anisotropy (2.30) together with suitable band gap (2.37 eV) primarily leading to the favorable laser-induced damage threshold (3.33 × AgGaS ₂ ), broad transparent window, and sufficient calculated birefringence (0.0433) for phase-matching ability. Furthermore, the first polyanion substitution of the supramolecule plays the role of templating agent to realize the CS-to-NCS transformation, which offers an effective method to rationally design promising NCS-based functional materials.
Competing Interests: There are no conflicts to declare.
(This journal is © The Royal Society of Chemistry.)