Chirality-Dependent Structural Transformation in Chiral 2D Perovskites under High Pressure

Chiral perovskites have attracted considerable attention owing to their potential applications in spintronic- and polarization-based optoelectronic devices. However, the structural chirality/asymmetry transfer mechanism between chiral organic ammoniums and achiral inorganic frameworks is still equivocal, especially under extreme conditions, as the systematic structural differences between chiral and achiral perovskites have been rarely explored. Herein, we successfully synthesized a pair of new enantiomeric chiral perovskite (S/R-3PYEA)PbI₄ (3PYEA²⁺ = C₅NH₅C₂H₄NH₃²⁺) and an achiral perovskite (rac-3PYEA)PbI₄. Hydrostatic pressure was used, for the first time, to systematically investigate the differences in the structural evolution and optical behavior between (S/R-3PYEA)PbI₄ and (rac-3PYEA)PbI₄. At approximately 7.0 GPa, (S/R-3PYEA)PbI₄ exhibits a chirality-dependent structural transformation with a bandgap "red jump" and dramatic piezochromism from translucent red to opaque black. Upon further compression, a previously unreported chirality-induced negative linear compressibility (NLC) is achieved in (S/R-3PYEA)PbI₄. High-pressure structural characterizations and first-principles calculations demonstrate that pressure-driven homodirectional tilting of homochiral ammonium cations strengthens the interactions between S/R-3PYEA²⁺ and Pb-I frameworks, inducing the formation of new asymmetric hydrogen bonds N-H···I-Pb in (S/R-3PYEA)PbI₄. The enhanced asymmetric H-bonding interactions further break the symmetry of (S/R-3PYEA)PbI₄ and trigger a greater degree of in-plane and out-of-plane distortion of [PbI₆]^4- octahedra, which are responsible for chirality-dependent structural phase transition and NLC, respectively. Nevertheless, the balanced H-bonds incurred by equal proportions of S-3PYEA²⁺ and R-3PYEA²⁺ counteract the tilting force, leading to the absence of chirality-dependent structural transition, spectral "red jump", and NLC in (rac-3PYEA)PbI₄.