Unlock lasing in two-dimensional metal halide perovskites by tuning ns² lone pairs

Despite the proliferation of lasing reports in three-dimensional (3D) lead halide perovskites, robust lasing in their single-layer two-dimensional (2D) counterparts, (LA)₂PbX₄ (LA = spacer cation; X = halide), has remained elusive. Here, we uncover the critical role of metal ns²-lone-pair stereochemistry in modulating excitonic properties and enabling lasing in these materials. Using a library of (LA)₂BI₄ [B = Pb (lead), Sn (tin), or Ge (germanium)], we identify key structural descriptors that link lone pair activity to exciton-phonon coupling and exciton-exciton annihilation. Within a given B-cation series, rigid frameworks with shorter B─I bonds suppress lone pair activity and favor free exciton emission, while enhanced lone pair activity-tuned by the spacer and B-cation-can localize excitons, increase dielectric screening, and suppress exciton-exciton annihilation at high excitation fluences. By balancing these effects through cation selection, we demonstrate lasing in a newly synthesized Pb-based (2FBMZ)₂PbI₄ (2FBMZ = 5,6-difluoro-1H-benzimidazole cation) and achieve the most thermally stable lasing in its Sn-based analog. These insights provide design principles for high-brightness 2D perovskite photonic devices.