Hybrid multifunctional materials have great potential in a wide variety of applications due to their flexible combination of organic and inorganic components. Introducing chiral organic modules into the metal halide frameworks can effectively generate multifunctional materials, achieving new functionalities with noncentrosymmetric structures. Here, by incorporating (R)-or (S)-piperidine-3-carboxylic acid (R/S-PCA) as the templating cation, we report the synthesis and characterization of three pairs of new 2D chiral hybrid Cu(I) halides, namely, (R/S-PCA)CuBr 2 , (R/S-PCA)CuBr 2 •0.5H 2 O, and (R/S-PCA)CuI 2 . These chiral Cu(I) halides crystallize in the noncentrosymmetric space group C2 and belong to a new structural type similar to layered silicates. The optical absorption edges of these chiral materials can be tuned by changing the halide or upon the absorption of water and range from 2.70 to 3.66 eV. A dynamic conversion between (R/S-PCA)CuBr 2 and (R/S-PCA)CuBr 2 •0.5H 2 O occurs through exposure to moisture or vacuum drying along with changes in the reversible bandgap and photoluminescence. Chiroptical properties such as circular dichroism, circular polarized light emission, and second harmonic generation are investigated. Density functional theory calculations (DFT) show the indirect and direct bandgap natures of these Cu(I) halides and reveal the mechanism for the broadband self-trapped exciton emission at the excited state. The fascinating structural type, chiroptical properties, and reversible hydrochromic behavior of these Cu(I)-based halides make them viable candidates for next-generation multifunctional optoelectronic materials.

No abstract

Layered hybrid perovskites comprise modular components that are individually highly tunable, resulting in materials with a range of structure and properties. In these layered materials, the usual assumption is of two-dimensional electronic behavior, because of the relatively large separations between the inorganic layers. Here, we report two layered hybrid lead iodide perovskites that possess unusually short interlayer distances: (IPA)2(MA)Pb2I7 and (ACA)(MA)PbI4 (IPA = iso-propylammonium, MA = methylammonium, ACA = acetamidinium). These compounds are prepared from mixing small organic cations, where they crystallize in the Ruddlesden-Popper type structure, and a structure with alternating cations in the interlayer space, respectively. The crystal structures are compared in detail with related structures, and electronic structures are analyzed using density-functional theory-based calculations. Time-resolved microwave conductivity measurements are employed to provide insight into charge transport in these compounds. This work exemplifies the unusual templating role of small organic cations in the layered halide compounds.

Non‐covalent interactions play an essential role in directing the self‐assembly of hybrid organic–inorganic crystals. In hybrid halide perovskites, hydrogen bonding has been the paramount non‐covalent interaction. Here, we show another non‐covalent interaction, namely, the halogen bond interaction, that directs a symmetry‐breaking assembly in a new series of two‐dimensional (2D) perovskites (ICH2CH2NH3)2(CH3NH3)n−1PbnI3n+1 (n is the layer thickness, n=1–4). Structural analysis shows that the halogen bond strength varies with the layer thickness. For the odd number (n=1, 3) layered perovskites, stronger halogen interaction leads to centrosymmetric structures, whereas for the n=2 layered perovskites, weaker halogen bonds result in non‐centrosymmetric structures. Transient reflection spectroscopy shows a suppressed radiative recombination rate (k2≈0) and prolonged spin lifetime for n=2 structure, suggesting an enhanced Rashba band splitting effect. The structural asymmetry is further confirmed with a reversible bulk photovoltaic effect. Our work provides a new design strategy to enable hybrid perovskites with emerging properties and functionalities associated with structural asymmetry.

Hybrid two-dimensional (2D) halide perovskites have emerged as an important class of high-performance semiconductors because of their excellent physical properties and structural diversity. Here, we report a new family of hybrid 2D Ge-based perovskites that have the Dion–Jacobson structure type, with the general formula A­(MA) n–1Ge n X3n+1, where A = 2-(aminomethyl)­pyridinium (2AMPY), 3-(aminomethyl)­pyridinium (3AMPY), 4-(aminomethyl)­pyridinium (4AMPY), or 4-(aminomethyl)­piperidinium (4AMP), X = Br or I, and n = 1 or 2. Single-crystal X-ray diffraction shows that most of the bromide structures are centrosymmetric, whereas the iodide analogues are all non-centrosymmetric. The optical band gaps are effectively regulated by the level of octahedral distortion of the corresponding [GeX6], where a larger distortion corresponds to a larger band gap. These compounds exhibit relatively weak photoluminescence, which can be observed at low temperatures. All of the Ge-based iodide perovskites exhibit good second-harmonic generation (SHG) responses, with the highest reaching 0.53 × AgGaS2 for (4AMPY)­GeI4, where their particle-dependent SHG has been investigated. First-principles calculations of the SHG properties are in good agreement with the experimental results. Furthermore, layer transformation can be achieved from the (100)- to (110)-oriented configuration by mixing the halides, which is a rare example. These results showcase the versatility and diversity of employing an asymmetric dication and the Ge2+ metal to achieve SHG active non-centrosymmetric materials, providing new strategies for the design of lead-free alternatives with interesting photophysical properties.