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Reactions of a chiral and quasi-spherical molecule [1 S ,4 S -2,5-2.2.1-H 2 dabch]I 2 ( 1 ) with alkali metal halide MX (M = Na, K, Cs; X = Cl, Br) at room temperature produced a series of organic-inorganic hybrid (OIH) materials [1 S ,4 S -2,5-2.2.1-H 2 dabch]NaBr 3 ( 2 ), [1 S ,4 S -2,5-2.2.1-H 2 dabch]CsCl 3 ·H 2 O ( 3 ) and [1 S ,4 S -2,5-2.2.1-H 2 dabch]KBr 3 ·H 2 O ( 4 ). The single-crystal X-ray diffraction analysis revealed that the organic-inorganic framework structures comprised of the templating ligand and alkali metal halides (NaBr, CsCl, KBr) displayed dimensions spanning from one-dimensional (1D) to three-dimensional (3D). Moreover, the results of both differential scanning calorimetry (DSC) and dielectric measurements demonstrated that compounds 1-4 displayed reversible, high-temperature phase transitions and noticeable dielectric anomalies. In addition, the temperature-dependent second harmonic generation (SHG) results revealed crystals 1 and 3 can switch from the SHG-ON to the SHG-OFF state, which was proved by the variable-temperature X-ray diffraction. This research aims to streamline the exploration of multifunctional second harmonic generation (SHG) and dielectric materials that have been synthesized using chiral ligands and alkali metals. This will provide researchers with enhanced opportunities to delve further into this specific research domain.

Switchable second harmonic generation (SHG) materials have potential applications in information storage, signal processing, and so on because they can switch between SHG-on and SHG-off states. In this work, we designed and synthesized three organic-inorganic hybrid Rb halide three-dimensional (3D) perovskite materials [1 S ,4 S 2,5-2.2.1-H 2 dabch]RbX 3 ·0.5H 2 O (X = Cl, 1 ; Br, 2 ; I, 3 ) based on the chiral 1 S ,4 S -2,5-diazabicyclo[2.2.1]heptane (1 S ,4 S -2,5-2.2.1-dabch). The selection of homochiral organic cations ensures that the compounds 1 ∼ 3 crystallize in the noncentrosymmetric and chiral space group P 2 1 2 1 2 1 , which further leads to reversible SHG responses of the three compounds. Through differential scanning calorimetry (DSC) and dielectric measurements, it revealed that the phase transition point of the compounds 1 ∼ 3 increased with RbCl, RbBr, and RbI. This is because the hydrogen interaction H···X between the inorganic framework [RbX 3 ] n and the organic cation [1 S ,4 S -2,5-2.2.1-H 2 dabch] 2+ is increased with the order of I > Br > Cl. This study can provide an effective molecular design strategy for the exploration and construction of temperature-tunable SHG switching materials.

Even though hybrid organic-inorganic perovskites (HOIPs) have been studied by many scholars in recent years, there are not many reports on three-dimensional (3D) alkali metal cesium halide perovskites. Here, we report an unprecedented 3D HOIP molecule (1,5-3.2.2-H 2 dabcn)CsBr 3 ( 1 ), in which the 3D anionic framework is constructed by corner-sharing CsBr 6 octahedra and organic cations [1,5-3.2.2-H 2 dabcn] 2+ are located in the cavities formed by the octahedra. Organic cations interact with an inorganic metal frame via two N-H···Br hydrogen bonds. Compound 1 undergoes a reversible order-disorder phase transition and exhibits switchable dielectric and second-harmonic generation (SHG) properties. Interestingly, product 1 crystallizes in a non-centrosymmetric space group Pmn 2 1 at the low-temperature phase (LTP) and transforms into a centrosymmetric space group P 2/ m at the high-temperature phase (HTP). The space group Pmn 2 1 in the LTP has a higher symmetry than P 2/ m in the HTP. This inverted symmetry breaking is very unusual.

An organic-inorganic hybrid molecule [3.3.0-H 2 dabco]ZnBr 4 ( 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 ZnBr 2 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 P 1̅ 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.