A pair of chiral organic–inorganic hybrid tin compounds showing reversible phase transition and enhanced SHG properties

This study synthesized and characterized two zero-dimensional chiral organic–inorganic hybrid isomers (R-APH₂)SnCl₆ (1) and (S-APH₂)SnCl₆ (2) (where AP is 3-aminopyrrolidine). Their phase transition behaviors, chiral optical properties, and crystal structures were investigated via differential scanning calorimetry (DSC), vibrational circular dichroism (VCD), second harmonic generation (SHG) measurements, and high/low-temperature single-crystal X-ray diffraction analysis. The results showed that the two compounds undergo high-temperature reversible first-order phase transitions at 422/448 K and 418/448 K, respectively, with the high/low-temperature single-crystal symmetry exhibiting the rare characteristic of inverse temperature-induced symmetry breaking (ITSB). SHG tests revealed that during the phase transition, the compounds display a unique antisymmetric nonlinear optical switching effect: the low-temperature phase (chiral space group P2₁2₁2₁) is in the “SHG-low” state, while the high-temperature phase (non-centrosymmetric space group P2₁) transitions to the “SHG-high” state. The symmetric signals of VCD spectra at specific wavenumbers confirm their enantiomeric properties. Further research reveals that the synergistic displacement of organic cations (R/S-APH₂²⁺) and the distortion synergy of inorganic metal frameworks ([SnCl₆]²⁻) constitute the core phase transition mechanism that drives changes in crystal symmetry and optical properties. This study provides a reference for the development of low-dimensional chiral materials with high phase transition temperatures, facilitating their applications in optoelectronic devices, chiral sensing, and other fields.