Atomically Thin Sheets of Lead-Free 1D Hybrid Perovskites Feature Tunable White-Light Emission from Self-Trapped Excitons.

Low-dimensional organic-inorganic perovskites synergize the virtues of two unique classes of materials featuring intriguing possibilities for next-generation optoelectronics: they offer tailorable building blocks for atomically thin, layered materials while providing the enhanced light-harvesting and emitting capabilities of hybrid perovskites. This work goes beyond the paradigm that atomically thin materials require in-plane covalent bonding and reports single layers of the 1D organic-inorganic perovskite [C7 H10 N]3 [BiCl5 ]Cl. Its unique 1D-2D structure enables single layers and the formation of self-trapped excitons, which show white-light emission. The thickness dependence of the exciton self-trapping causes an extremely strong shift of the emission energy. Thus, such 2D perovskites demonstrate that already 1D covalent interactions suffice to realize atomically thin materials and provide access to unique exciton physics. These findings enable a much more general construction principle for tailoring and identifying 2D materials that are no longer limited to covalently bonded 2D sheets.

Multinary Halogenido Bismuthates beyond the Double Perovskite Motif.

Both lead halide perovskites and bismuth-based double perovskites have generated intense research interest in the past few years. There is, however, a broader class of bismuthates that transcends the double perovskite motif. These multinary halogenido bismuthates remain severely underexplored and offer rich research opportunities with regard to new structure motifs and material properties. In this Forum Article, we want to provide both an overview of the work on this class of compounds that has been done in the last 2 decades and an example of how new compounds can be obtained and which challenges are associated with their synthesis. We present the synthesis and characterization of six new bismuthates, (PBz4)3Bi3Br12 (1) (PBz4)2(MeCN)2Cu2Bi2Br10 (2), (PBz4)Bi2I7 (3), (PBz4)2(MeCN)2Cu2Bi2I10 (4), (PBz4)2AgBi2I9 (5), and (PBz4)3Bi3I12·C4H8O (6), based on the tetrabenzylphosphonium cation PBz4+. 2, 4, and 5 represent new multinary bismuthates, featuring both a familiar anion motif with a new element combination in 2 and a previously unknown anionic chain in 5. We use this series of compounds to further elucidate the influence of the anion composition, nuclearity, and dimensionality on the compounds' onset of absorption and discover that an additional factor, the connectivity between coordination polyhedra, plays a role in copper iodido bismuthates.

Synthesis of a two-dimensional organic-inorganic bismuth iodide metalate through in situ formation of iminium cations.

(Me2C[double bond, length as m-dash]NMe2)Bi2I7 represents a new layered organic-inorganic iodido bismuthate. It displays an unprecedented anion topology, a low band gap and good stability. Advanced electronic structure analysis finds the II interactions to be decisive for the compound's structural and electronic properties.

Divergent Optical Properties in an Isomorphous Family of Multinary Iodido Pentelates.

Multinary organic-inorganic metal halide materials beyond the perovskite motif can help to address both fundamental aspects such as the electronic interactions between different metalate building units and practical issues like stability and ease of preparation in this new field of research. However, such multinary compounds have remained quite rare for the halogenido pentelates, as the formation of simpler side phases can be a significant hindrance. Here, we report a family of four new multinary iodido pentelates [PPh4]2[ECu2I7(nitrile)] (E = Sb, Bi; nitrile = acetonitile or propionitrile), including the first metalate with a Cu-I-Sb unit. The compounds can be obtained by facile solution or mechanochemical methods and display good stability up to 160 °C. A comparison with compounds containing binary anions [EI6]3- reveals that, unexpectedly, the addition of the iodido cuprate unit causes a blue-shift in the absorption of the antimonates but a red-shift in the bismuthates. Photoluminescence investigations at 10 K show that the compounds display broad luminescence bands that correspond well with the trend in their onset of absorption. Overall, the work highlights that multinary, non-perovskite halogenido metalates can be a valuable expansion of the chemistry of metal halide perovskites.

Surprising discoveries on the way to an old compound: four transient iodido antimonates.

During the synthesis of the literature-known iodido antimonate [Cu(MeCN)4]4[Sb3I11]2 (MeCN = acetonitrile), four transient compounds, [Cu(MeCN)4]4[Sb6I22]·2MeCN (1), [Cu(MeCN)4]4[Sb7I25]·MeCN (2), [Cu(MeCN)4]4[Sb10I34] (3) and [Cu(MeCN)4]4[Sb8I28] (4), were identified. The compounds appeared within hours or days and subsequently re-dissolved in the mother liquor, leading to [Cu(MeCN)4]4[Sb3I11]2 as the final product. Single crystal X-ray analysis showed that all four compounds feature unprecedented anion motifs.

Ternary Iodido Bismuthates and the Special Role of Copper.

Two new, isostructural members of the title material class, [PPh4]4[Cu2Bi2I12] (1) and [PPh4]4[Ag2Bi2I12] (2), have been prepared via a facile solution route. The crystal structure of both compounds features a tetranuclear [M2Bi2I12]4- (M = Cu, Ag) anion that displays an unprecedented face-sharing mode of connection between BiI6 octahedra and MI4 tetrahedra, enabling close Bi···M contacts. The two compounds allow for a direct experimental and quantum chemical investigation of the influence of group 11 metal cations on the optical and electronic properties of ternary iodido bismuthate anions, indicating that Cu+ is a better electronic match than Ag+, resulting in a significantly lower optical band gap of the copper compound.