Layered and\nLow-Dimensional Lead, Silver, and Bismuth\nHalide Perovskites Directed by Halogen-Substituted Spacer Cations

Hybrid\norganic–inorganic metal halides provide\na diverse\nparameter space in which the optoelectronic properties can be tuned\nthrough the composition. The compositional tunability extends to the\nmetal site, which can be expanded from single valent metals (<i>e.g</i>., Pb<sup>2+</sup>) to multivalent metals (<i>e.g</i>., Ag<sup>+</sup> and Bi<sup>3+</sup>), and the dimension (2D, 1D,\nor 0D). However, a deeper understanding of how the organic cations\ntemplate these metal halide structures is needed. Here, we synthesize\nand study the structures of a series of new layered and low-dimensional\nmetal (Pb, Ag, and Bi) halides templated by the halogenated aryl spacer\ncations 2-chlorobenzylammonium (2ClBZ) and 3-chloro-2-fluorobenzylammonium\n(3Cl2FBZ). We report new lead perovskites, (3Cl2FBZ)<sub>2</sub>PbBr<sub>4</sub>, (2ClBZ)<sub>3</sub>PbI<sub>5</sub>, and (3Cl2FBZ)<sub>2</sub>PbI<sub>4</sub>, and compare them to their silver and/or bismuth\nanalogs (2ClBZ)<sub>4</sub>AgBiBr<sub>8</sub>, (3Cl2FBZ)<sub>4</sub>AgBiBr<sub>8</sub>, (2ClBZ)<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub>, and (3Cl2FBZ)<sub>4</sub>Bi<sub>2</sub>I<sub>10</sub>. In all structures,\nthe halogen-substituted cations result in 2D or “pseudo-2D”\nlayering, but the different halogen substituents introduce different\ndistortions (tilting, octahedral distortion) and dimensional reduction\nto 1D or 0D depending on the metal and halide compositions. Optical\nabsorption measurements reveal the bandgaps are tunable through metal\nsites, dimension, and cations to different extents. Furthermore, the\n1D (3Cl2FBZ)<sub>4</sub>Bi<sub>2</sub>I<sub>10</sub> crystallizes\nin the noncentrosymmetric space group <i>Cmc</i>2<sub>1</sub> and exhibits second-harmonic generation (SHG). The organic–inorganic\ninteractions and resultant structural distortions examined here provide\ninsights toward the engineering of noncentrosymmetry and dimensional\ncontrol in hybrid metal halide perovskites.