Conformer-Mediated Helical Chirality in 2D Layered Hybrid Perovskites

Two-dimensional (2D) chiral hybrid perovskites A₂PbI₄ (A: chiral organic ion) enable chirality controlled optoelectronic and spin-based properties. A⁺ organic sublattice induces chirality into the semiconducting [PbI₄]^2- inorganic sublattice through non-covalent interactions at organic-inorganic interface. Often, the A⁺ cations in the lattice have different orientations, leading to asymmetry in the non-covalent interactions. In a novel approach, we use different conformers of A⁺ cations to create asymmetry in the non-covalent interactions, thereby, achieving chiral perovskites with rare helical enantiomorphic structures. We prepared (R-IdPA)₂PbI₄ and (S-IdPA)₂PbI₄ (IdPA: 1-iodopropan-2-ammonium) which crystallize in the helical enantiomorphic space groups P4₃2₁2 and P4₁2₁2, respectively. The gauche- and anti-conformers of IdPA⁺ are arranged alternatively in the hybrid structure. Importantly, the anti-conformer of IdPA⁺ ion have significantly stronger electrostatic, N-H⋅⋅⋅I hydrogen bonding, and I⋅⋅⋅I halogen bonding interactions with the [PbI₄]^2- sublattice, compared to the gauche-conformer. This periodic asymmetry in non-covalent interactions caused by the alternative arrangement of gauche- and anti-conformers induces chirality in the inorganic sublattice with four-fold screw axes (4₃ and 4₁). The enantiomers (R-/S-IdPA)₂PbI₄ show mirror-image like circular dichroism from excitonic absorption of the inorganic sublattice. This conformer-based design of chiral hybrid perovskites in helical space groups broadens material choices for advanced optoelectronic applications.