Low-dimensional metal halides have been the focus of intense investigations in recent years following the success of hybrid lead halide perovskites as optoelectronic materials. In particular, the light emission of the low-dimensional halides based on the 5s2 cations Sn2+ and Sb3+ have found utility in a variety of applications complementary to those of the 3D halide perovskites thanks to their unusual properties such as broadband emission and highly temperature-dependent lifetimes. These properties derive from the exceptional chemistry of the 5s2 lone pair, however the terminology and explanations given for such emission vary widely, hampering efforts to build a cohesive understanding of these materials towards efficient optoelectronic devices. In this Perspective, we provide a structural overview of these materials with a focus on the dynamics driven by the stereoactivity of the 5s2 lone pair to identify the structural features that enable strong emission. We relate the different theoretical models which have been able to explain the success of these bright 5s2 emission centers into a cohesive framework, which is then applied to the array of compounds recently developed by our group and other researchers, demonstrating its utility and generating a holistic picture of the field from the point of view of a materials chemist. We highlight the state-of-the-art materials and applications which demonstrate the unique capabilities of these versatile emissive centers and identify promising future directions in the field of low-dimensional 5s2 metal halides.

The spatial localization of charge carriers to promote the formation of bound excitons and concomitantly enhance radiative recombination has long been a goal for luminescent semiconductors. Zero‐dimensional materials structurally impose carrier localization and result in the formation of localized Frenkel excitons. Now the fully inorganic, perovskite‐derived zero‐dimensional Sn^II material Cs₄SnBr₆ is presented that exhibits room‐temperature broad‐band photoluminescence centered at 540 nm with a quantum yield (QY) of 15±5 %. A series of analogous compositions following the general formula Cs_4−xA_xSn(Br_1−yI_y)₆ (A=Rb, K; x≤1, y≤1) can be prepared. The emission of these materials ranges from 500 nm to 620 nm with the possibility to compositionally tune the Stokes shift and the self‐trapped exciton emission bands.

Luminescent organic–inorganic low‐dimensional ns² metal halides are of rising interest as thermographic phosphors. The intrinsic nature of the excitonic self‐trapping provides for reliable temperature sensing due to the existence of a temperature range, typically 50–100 K wide, in which the luminescence lifetimes (and quantum yields) are steeply temperature‐dependent. This sensitivity range can be adjusted from cryogenic temperatures to above room temperature by structural engineering, thus enabling diverse thermometric and thermographic applications ranging from protein crystallography to diagnostics in microelectronics. Owing to the stable oxidation state of Sb³⁺, Sb(III)‐based halides are far more attractive than all major non‐heavy‐metal alternatives (Sn‐, Ge‐, Bi‐based halides). In this work, the relationship between the luminescence characteristics and crystal structure and microstructure of TPP₂SbBr₅ (TPP = tetraphenylphosphonium) is established, and then its potential is showcased as environmentally stable and robust phosphor for remote thermography. The material is easily processable into thin films, which is highly beneficial for high‐spatial‐resolution remote thermography. In particular, a compelling combination of high spatial resolution (1 µm) and high thermometric precision (high specific sensitivities of 0.03–0.04 K⁻¹) is demonstrated by fluorescence‐lifetime imaging of a heated resistive pattern on a flat substrate, covered with a solution‐spun film of TPP₂SbBr₅.

Two-dimensional hybrid organic-inorganic lead halides perovskite-type compounds attracted immense scientific interest due to their remarkable optoelectronic properties and tailorable crystal structures. In this work, we present a new layered hybrid lead halide, namely [CH(NH2)2][C(NH2)3]PbI4, wherein puckered lead-iodide layers are separated by two small and stable organic cations: formamidinium, CH(NH2)2+, and guanidinium, C(NH2)3+. This perovskite is thermally stable up to 255 oC, exhibits room-temperature photoluminescence in the red region with a quantum yield of 3.5%, and is photoconductive. This study highlights a vast structural diversity that exists in the compositional space typically used in perovskite photovoltaics.

Traditional fluorescence-based tags, used for anticounterfeiting, rely on primitive pattern matching and visual identification; additional covert security features such as fluorescent lifetime or pattern masking are advantageous if fraud is to be deterred. Herein, we present an electrohydrodynamically printed unicolour multi-fluorescent-lifetime security tag system composed of lifetime-tunable lead-halide perovskite nanocrystals that can be deciphered with both existing time-correlated single-photon counting fluorescence-lifetime imaging microscopy and a novel time-of-flight prototype. We find that unicolour or matching emission wavelength materials can be prepared through cation-engineering with the partial substitution of formamidinium for ethylenediammonium to generate “hollow” formamidinium lead bromide perovskite nanocrystals; these materials can be successfully printed into fluorescence-lifetime-encoded-quick-read tags that are protected from conventional readers. Furthermore, we also demonstrate that a portable, cost-effective time-of-flight fluorescence-lifetime imaging prototype can also decipher these codes. A single comprehensive approach combining these innovations may be eventually deployed to protect both producers and consumers.