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 (R/S-PCA)4Cu2PbBr8·H2O (R/S-CuPbBr, R/S-PCA = R/S-3-piperidinecarboxylic
acid) and investigate their behavior under external stimuli including
pressure and temperature. The R/S-CuPbBr compounds crystallize in a noncentrosymmetric monoclinic C2 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 (R/S-PCA)3Pb2Br7·H2O (R/S-PbBr) was synthesized, crystallizing in the noncentrosymmetric
orthorhombic P212121. These materials exhibit broadband yellow emission and circularly
polarized luminescence emission at room temperature. The glum values of R/S-CuPbBr
and R/S-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 R/S-CuPbBr and R/S-PbBr are indirect and direct bandgap semiconductors, respectively.
More importantly, R-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.