Organic–Inorganic Hybrid Compound [H₂‑1,5-Diazabicyclo[3.3.0]octane]ZnBr₄ with Reverse Symmetry Breaking Shows a Switchable Dielectric Anomaly and Robust Second Harmonic Generation Effect

An organic–inorganic hybrid molecule [3.3.0-H₂dabco]­ZnBr₄ (1) with switchable phase transition, dielectric anomaly, and second harmonic generation (SHG) effect was synthesized by reaction of 1,5-diazabicyclo[3.3.0]­octane (3.3.0-dabco) with ZnBr₂ in concentrated hydrobromic acid aqueous solution. Differential scanning calorimetry (DSC) and dielectric measurements revealed that 1 exhibits a reversible high-temperature phase transition, accompanied by a distinct step-like dielectric anomaly at 373 K. Exceptionally, the single crystal structure analysis at different temperatures revealed that 1 undergoes reverse symmetry breaking during the phase transition, in which the high-symmetry space group Cc in the low temperature phase (LTP) is transformed to the low-symmetry space group P1̅ in the high temperature phase (HTP). In addition, with the conversion from the non-centrosymmetric (NCS) to the centrosymmetric (CS) space group, the SHG of 1 can switch from SHG-ON to SHG-OFF for at least four cycles without obvious decay.