Jiang Xiaofan, Tao Yu, Gu Jiazhen, Jin Leyang, Li Chen, Zhang Wenkai, Fu Yongping
Beijing National Laboratory for Molecular Science, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.
Department of Physics and Applied Optics Beijing Area Major Laboratory, Center for Advanced Quantum Studies, Beijing Normal University, Beijing 100875, China.
Dalton Trans. 2023 Oct 31;52(42):15489-15495. doi: 10.1039/d3dt01627g.
The stereochemical expression of the 6s lone pair on the lead atom has a significant impact on the crystal structures and physical properties of lead halide perovskites. Two-dimensional (2D) lead bromide perovskites often exhibit a broadband emission, yet the structural origin of the broadband emission has been under debate. Here, we report the synthesis and characterization of a 2D lead bromide hybrid (4-chlorophenylammonium)PbBr that consists of a combination of the octahedral unit PbBr and the rarely observed capped octahedral unit PbBr through corner-sharing and edge-sharing linkages. The seven-coordination geometry indicates a strong stereo-active lone pair on the Pb cation. By comparing this structure with two representative 2D perovskites, (benzylammonium)PbBr and (4-aminotetrahydropyran)PbBr, we establish how the lone pair expression affects the local coordination geometry of the Pb cation and the resulting optical and electronic properties. As the Pb-Br bond length increases, the lone pair expression leads to off-centering displacement of Pb within the octahedra and then the formation of seven-coordination capped octahedra. Density functional theory calculations indicate that the off-centering distorted octahedra and capped octahedra are due to the asymmetric distribution of the Pb electrons that have both s and p orbital characteristics. Spectroscopic studies show the photoluminescence spectra evolving from narrowband emission to broadband emission with increasing LPE, as well as softer and more anharmonic lattice vibrations that facilitate exciton self-trapping. Our results demonstrate that lone pairs could be a powerful design rule for developing light emitting materials.
铅原子上6s孤对电子的立体化学表达对卤化铅钙钛矿的晶体结构和物理性质有重大影响。二维（2D）溴化铅钙钛矿通常表现出宽带发射，但其宽带发射的结构起源一直存在争议。在此，我们报道了一种二维溴化铅杂化物（4-氯苯铵）PbBr₃的合成与表征，它由八面体单元PbBr₆和罕见的帽式八面体单元PbBr₅通过角共享和边共享连接组合而成。七配位几何结构表明Pb阳离子上存在强立体活性孤对电子。通过将该结构与两种代表性的二维钙钛矿（苄基铵）PbBr₃和（4-氨基四氢吡喃）PbBr₃进行比较，我们确定了孤对电子表达如何影响Pb阳离子的局部配位几何结构以及由此产生的光学和电子性质。随着Pb-Br键长增加，孤对电子表达导致Pb在八面体内发生偏心位移，进而形成七配位帽式八面体。密度泛函理论计算表明，偏心畸变八面体和帽式八面体是由于具有s和p轨道特征的Pb电子不对称分布所致。光谱研究表明，随着孤对电子效应（LPE）增加，光致发光光谱从窄带发射演变为宽带发射，同时晶格振动更柔和且更非谐，这有利于激子自陷。我们的结果表明，孤对电子可能是开发发光材料的一个有力设计规则。
