Chiral Two-Dimensional Cu–Pb Bromides: Circularly Polarized Luminescence and Pressure-Enhanced Optical Properties

The search for high-performance double perovskite-related materials remains constrained by the limited synthetic accessibility of bimetallic halides compared to their conventional halide double perovskite counterparts, leaving substantial unexplored territory in this domain. A promising structural modification strategy involves the incorporation of chiral organic moieties into the metal halide frameworks, enabling precise engineering of noncentrosymmetric structures toward targeted functional properties. Here, we report a pair of chiral two-dimensional (2D) Cu(I)-Pb bimetallic bromides (<i>R</i>/<i>S</i>-PCA)₄Cu₂PbBr₈·H₂O (<i>R</i>/<i>S</i>-CuPbBr, <i>R</i>/<i>S</i>-PCA = <i>R</i>/<i>S</i>-3-piperidinecarboxylic acid) and investigate their behavior under external stimuli including pressure and temperature. The <i>R</i>/<i>S</i>-CuPbBr compounds crystallize in a noncentrosymmetric monoclinic <i>C</i>2 space group, consisting of inorganic bimetal [Cu₂PbBr₈] layers and organic layers formed via hydrogen bonding interactions. For comparison, another pair of 2D Pb-based bromides (<i>R</i>/<i>S</i>-PCA)₃Pb₂Br₇·H₂O (<i>R</i>/<i>S</i>-PbBr) was synthesized, crystallizing in the noncentrosymmetric orthorhombic <i>P</i>2₁2₁2₁. These materials exhibit broadband yellow emission and circularly polarized luminescence emission at room temperature. The <i>g</i>_lum values of <i>R</i>/<i>S</i>-CuPbBr and <i>R</i>/<i>S</i>-PbBr are 8.63 × 10^-3 and -7.99 × 10^-3, 4.33 × 10^-3 and -3.52 × 10^-3, respectively. Density functional theory (DFT) calculations reveal <i>R</i>/<i>S</i>-CuPbBr and <i>R</i>/<i>S</i>-PbBr are indirect and direct bandgap semiconductors, respectively. More importantly, <i>R</i>-CuPbBr exhibits dramatic enhancements in optical properties under high pressure, with an 8-fold increase in photoluminescence and 44-fold boost in second-harmonic generation at elevated pressure.