One-Dimensional Organic-Inorganic Lead Bromide Hybrids with Excitation-Dependent White-Light Emission Templated by Pyridinium Derivatives

Organic-inorganic hybrid metal halides have attracted widespread attention due to their excellent tunability and versatility. Here, we have selected pyridinium derivatives with different substituent groups or substitution positions as the organic templating cations and obtained six 1D chain-like structures. They are divided into three types: type I (single chain), type II (double chain), and type III (triple chain), with tunable optical band gaps and emission properties. Among them, only (2,4-LD)PbBr₃ (2,4-LD = 2,4-lutidine) shows an exciton-dependent emission phenomenon, ranging from strong yellow-white to weak red-white light. By comparing its photoluminescence spectrum with that of its bromate (2,4-LD)Br, it is found that the strong yellow-white emission at 534 nm mainly came from the organic component. Furthermore, through a comparison of the fluorescence spectra and lifetimes of (2,4-LD)PbBr₃ and (2-MP)PbBr₃ (2-MP = 2-methylpyridine) with similar structures at different temperatures, we confirm that the tunable emission of (2,4-LD)PbBr₃ comes from different photoluminescent sources corresponding to organic cations and self-trapped excitons. Density functional theory calculations further reveal that (2,4-LD)PbBr₃ has a stronger interaction between organic and inorganic components compared to (2-MP)PbBr₃. This work highlights the importance of organic templating cations in hybrid metal halides and the new functionalities associated with them.

(C5H6N)Pb2X5 (X = Cl, Br): Hybrid Lead Halides Based on Seven-Coordinate Pb(II)

Hybrid lead halides are a diverse family of compounds, of interest for their optoelectronic properties, that vary in the dimensionality and connectivity of their inorganic substructures. The great majority of these compounds are based on lead-centered octahedra, with few examples featuring inorganic architectures containing higher coordination numbers. Here, we report the synthesis and characterization of a pyridinium lead bromide phase that is based on seven-coordinate Pb(II) centers. Through edge- and face-sharing, the polyhedra form a corrugated, two-dimensional inorganic substructure. Electronic structure calculations were used to examine the band structure and the role of the stereoactive lone pair in the inherently asymmetric, seven-coordinate Pb(II) geometry. For reference, we have visualized the role of the lone pair in the binary halide PbBr₂, which also has a seven-coordinate inner ligand sphere. A comparison of the new structure with the limited number of existing hybrid lead halides with similar inorganic architectures highlights the templating role of the organic cation for these compounds. We also contribute characterization and discussion of isomorphic pyridinium lead chloride, which had been deposited in the Cambridge Structural Database but never, to our knowledge, addressed in the literature. The compounds were synthesized using solution conditions and structures determined with single-crystal X-ray diffraction. The materials were also characterized via powder X-ray diffraction, combustion elemental analysis, and diffuse reflectance UV-vis spectroscopy. While the structures reported here are centrosymmetric, the seven-coordinate, capped trigonal prismatic geometry that we have identified is a source of local asymmetry that could be used as a component in designing globally noncentrosymmetric structures.

Structural chemistry of layered lead halide perovskites containing single octahedral layers

We present a comprehensive review of the structural chemistry of hybrid lead halides of stoichiometry APbX 4, A 2PbX4 or A A'PbX 4, where A and A' are organic ammonium cations and X = Cl, Br or I. These compounds may be considered as layered perovskites, containing isolated, infinite layers of corner-sharing PbX 4 octahedra separated by the organic species. First, over 250 crystal structures were extracted from the CCDC and classified in terms of unit-cell metrics and crystal symmetry. Symmetry mode analysis was then used to identify the nature of key structural distortions of the [PbX 4]∞ layers. Two generic types of distortion are prevalent in this family: tilting of the octahedral units and shifts of the inorganic layers relative to each other. Although the octahedral tilting modes are well known in the crystallography of purely inorganic perovskites, the additional layer-shift modes are shown to enormously enrich the structural options available in layered hybrid perovskites. Some examples and trends are discussed in more detail in order to show how the nature of the interlayer organic species can influence the overall structural architecture; although the main aim of the paper is to encourage workers in the field to make use of the systematic crystallographic methods used here to further understand and rationalize their own compounds, and perhaps to be able to design-in particular structural features in future work.

Using steric hindrance to manipulate and stabilize metal halide perovskites for optoelectronics

The chemical instability of metal halide perovskite materials can be ascribed to their unique properties of softness, in which the chemical bonding between metal halide octahedral frameworks and cations is the weak ionic and hydrogen bonding as in most perovskite structures. Therefore, various strategies have been developed to stabilize the cations and metal halide frameworks, which include incorporating additives, developing two-dimensional perovskites and perovskite nanocrystals, etc. Recently, the important role of utilizing steric hindrance for stabilizing and passivating perovskites has been demonstrated. In this perspective, we summarize the applications of steric hindrance in manipulating and stabilizing perovskites. We will also discuss how steric hindrance influences the fundamental kinetics of perovskite crystallization and film formation processes. The similarities and differences of the steric hindrance between perovskite solar cells and perovskite light emission diodes are also discussed. In all, utilizing steric hindrance is a promising strategy to manipulate and stabilize metal halide perovskites for optoelectronics.

Dimensional reduction of the small-bandgap double perovskite Cs2AgTlBr6

Quantum confinement effects in lower-dimensional derivatives of the ABX3 (A = monocation, X = halide) single perovskites afford striking optical and electronic changes, enabling applications ranging from solar absorbers to phosphors and light-emitting diodes. Halide double perovskites form a larger materials family, known since the late 1800s, but lower-dimensional derivatives remain rare and prior work has revealed a minimal effect of quantum confinement on their optical properties. Here, we synthesize three new lower-dimensional derivatives of the 3D double perovskite Cs2AgTlBr6: 2D derivatives with mono- (1-Tl) and bi-layer thick (2-Tl) inorganic sheets and a quasi-1D derivative (1'-Tl). Single-crystal ellipsometry studies of these materials show the first clear demonstration that dimensional reduction can significantly alter the optical properties of 2D halide double perovskites. This large quantum confinement effect is attributed to the substantial electronic delocalization of the parent 3D Ag-Tl perovskite. Calculations track the evolution of the electronic bands with dimensional reduction and the accompanying structural distortions and show a direct-to-indirect bandgap transition as the 3D perovskite lattice is thinned to a monolayer in 1-Tl. This bandgap transition at the monolayer limit is also evident in the calculations for 1-In, an isostructural, isoelectronic analogue to 1-Tl in which In3+ replaces Tl3+, underscoring the orbital basis for the direct/indirect nature of the bandgap. Thus, in complement to the massive compositional diversity of halide double perovskites, dimensional reduction may be used as a systematic route for harnessing electronic confinement effects and obtaining new electronic structures.

Molecular Engineering of Pure 2D Lead-Iodide Perovskite Solar Absorbers Displaying Reduced Band Gaps and Dielectric Confinement

Pure 2D lead-iodide perovskites typically demonstrate poor charge transport and compromised visible light absorption, relative to their 3D congeners. This hinders their potential use as solar absorbers. Herein, the systematic tuning of pyridinium-based templating cations is reported to introduce intermolecular interactions that provide access to a series of new 2D lead-iodide perovskites with reduced inter-octahedral distortions (largest Pb-(μ-I)-Pb bond angles of 170-179°) and very short inorganic interlayer separations (shortest I⋅⋅⋅I contacts ≤4.278-4.447 Å). These features manifest in reduced band gaps (2.35-2.46 eV) and relaxed dielectric confinement (excitonic binding energies of 130-200 meV). As a consequence, they demonstrate (more than ten-fold) improved photo- and electrical conductivities relative to conventional 2D lead-iodide perovskites, such as that templated by 2-(1-naphthyl)ethylammonium. Through computational studies, the origin of this behavior was shown to derive from a combination of short iodoplumbate layer separations and the aromaticity of the organic dications.

Organic cation directed hybrid lead halides of zero-dimensional to two-dimensional structures with tunable photoluminescence properties

A series of new inorganic–organic hybrid lead halides have been solvothermally synthesized and characterized with tunable luminescence properties from yellow, orange to red emissions with the largest reported Stokes shift.

Thermochromism of bromotellurates(iv): experimental insights

Thermally induced changes in optical properties (thermochromism) in an extended series of 16 bromotellurate(iv) complexes were investigated; regularities valid for this class of compounds were established.

Structural Diversity in White-Light-Emitting Hybrid Lead Bromide Perovskites

Hybrid organic-inorganic halide perovskites are under intense investigations because of their astounding physical properties and promises for optoelectronics. Lead bromide and chloride perovskites exhibit intrinsic white-light emission believed to arise from self-trapped excitons (STEs). Here, we report a series of new structurally diverse hybrid lead bromide perovskites that have broad-band emission at room temperature. They feature Pb/Br structures which vary from 1D face-sharing structures to 3D corner- and edge-sharing structures. Through single-crystal X-ray diffraction and low-frequency Raman spectroscopy, we have identified the local distortion level of the octahedral environments of Pb²⁺ within the structures. The band gaps of these compounds range from 2.92 to 3.50 eV, following the trend of "corner-sharing < edge-sharing < face-sharing". Density functional theory calculations suggest that the electronic structure is highly dependent on the connectivity mode of the PbBr₆ octahedra, where the edge- and corner-sharing 1D structure of (2,6-dmpz)₃Pb₂Br₁₀ exhibits more disperse bands and smaller band gap (2.49 eV) than the face-sharing 1D structure of (hep)PbBr₃ (3.10 eV). Using photoemission spectroscopy, we measured the energies of the valence band of these compounds and found them to remain almost constant, while the energy of conduction bands varies. Temperature-dependent PL measurements reveal that the 2D and 3D compounds have narrower PL emission at low temperature (∼5 K), whereas the 1D compounds have both free exciton emission and STE emission. The 1D compound (2,6-dmpz)₃Pb₂Br₁₀ has the highest photoluminescence quantum yield of 12%, owing to its unique structure that allows efficient charge carrier relaxation and light emission.

Halobismuthates with halopyridinium cations: appearance or non-appearance of unusual colouring

A series of chloro- and bromobismuthate complexes with bromopyridinium cations were prepared and characterized by XRD analysis and diffuse reflectance spectroscopy.

Bromine-rich complexes of bismuth: experimental and theoretical studies

Three novel polybromide complexes of bismuth(iii) – (2,6-MePyH)₃[BiBr₅(Br₃)] (1), (H₂bpp){[BiBr₅](Br₂)} (2) and (3-ClPyH)₂{[BiBr₅](Br₂)} (3) – were synthesized.

Assembly of donor–acceptor hybrid heterostructures based on iodoplumbates and viologen coordination polymers

Two D–A hybrid heterostructures have been formed by the insertion of electron-rich iodoplumbates into viologen coordination polymers, which exhibit highly effcient photocatalytic degradation RhB under visible light irradiation.