Water-Stable 1D Hybrid Tin(II) Iodide Emits Broad Light with 36% Photoluminescence Quantum Efficiency

Embedding Te4+ into Sn4+-Based Metal Halide To Passivate Structure Defects for High-Performance Light-Emitting Application

Low-dimensional lead-halide hybrids are an emerging class of optical functional material but suffer the problems of toxicity and poor air stability. Among lead-free metal halides, tin(IV)-based metal halides are promising optoelectronic materials due to their robust structure and environmental friendliness. However, their photoluminescence (PL) properties are poor, and the underlying mechanisms are still elusive. Herein, a stable Sn4+-based halide hybrid, (C4H7N2)2SnCl6, was developed, which however exhibits poor PL properties at room temperature (RT) due to the lattice defects and the robust crystal structure. To enhance its PL efficiency, the Te4+ ion with a stereoactive 5s2 lone pair has been introduced into the lattice. As a result, Te4+-doped (C4H7N2)2SnCl6 displays broadband orange emission (∼640 nm) with a PL efficiency of ∼46% at RT. Interestingly, Te4+-doped (C4H7N2)2SnCl6 shows triple emission bands at 80 K, which could be due to the synergistic effect of the organic cations and the self-trapped state induced by Te4+. Additionally, high-performance white light-emitting diodes were prepared using Te4+-doped (C4H7N2)2SnCl6, revealing the potential of this material for lighting applications. This study provides new insight into the PL mechanism of Sn4+-based metal-halide hybrids and thus facilitates the design and development of eco-friendly light-emitting metal halides.

Revealing the Structure-Luminescence Relationship in Robust Sn(IV)-Based Metal Halides by Sb3+ Doping

Low-dimensional hybrid metal halides are an emerging class of materials with highly efficient photoluminescence (PL), but the problems of poor stability remain challenging. Sn(IV)-based metal halides show robust structure but exhibit poor PL properties, and the structure-luminescence relationship is elusive. Herein, two Sn(IV)-based metal halides (compounds 1 and 2) with the same constituent ((C6H16N2)SnCl6) but different crystal structures have been prepared, which however show poor PL properties at room temperature due to the absence of active ns2 electrons. To improve materials' PL properties, Sb3+ with active 5s2 electrons was embedded into the lattice of Sn4+-based hosts. As a result, efficient emissions were achieved for Sb3+-doped compounds 1 and 2 with a maximum PL efficiency of 14.28 and 62%, respectively. Experimental and calculation results reveal that the smaller distorted lattice structure of the host could result in the blueshift of the emission from Sb3+. Thus, a tunable color from red to orange was realized. Benefiting from the broadband efficient emission from Sb3+-doped compound 2, an efficient white light-emitting diode with a high color rendering index of up to 92.3 was fabricated to demonstrate its lighting application potential. This work promotes the understanding of the influence of robust Sn(IV)-based host lattice on the PL properties of Sb3+, advancing the development of environmentally friendly, low-cost, and high-efficiency Sn(IV)-based metal halides.

A flexible ligand and halogen engineering enable one phosphor-based full-color persistent luminescence in hybrid perovskitoids

Color-tunable room temperature phosphorescent (RTP) materials have raised wide interest due to their potential application in the fields of encryption and anti-counterfeiting. Herein, a series of CdX2-organic hybrid perovskitoids, (H-apim)CdX3 and (apim)CdX2 (denoted as CdX-apim1 and CdX-apim2, apim = 1-(3-aminopropyl)imidazole, X = Cl, Br), were synthesized using apim with both rigid and flexible groups as ligands, which exhibit naked-eye detectable RTP with different durations and colors (from cyan to red) by virtue of different halogen atoms, coordination modes and the coplanar configuration of flexible groups. Interestingly, CdCl-apim1 and CdX-apim2 both exhibit excitation wavelength-dependent RTP properties, which can be attributed to the multiple excitation of imidazole/apim, the diverse interactions with halogen atoms, and aggregated state of imidazoles. Structural analysis and theoretical calculations confirm that the aminopropyl groups in CdCl-apim1 do not participate in luminescence, while those in CdCl-apim2 are involved in luminescence including both metal/halogen to ligand charge transfer and twisted intramolecular charge transfer. Furthermore, we demonstrate that these perovskitoids can be applied in multi-step anti-counterfeiting, information encryption and smart ink fields. This work not only develops a new type of perovskitoid with full-color persistent luminescence, but also provides new insight into the effect of flexible ligands and halogen engineering on the wide-range modulation of RTP properties.

Promoting the formation of metal-carboxylate coordination to modulate the dimensionality of ultrastable lead halide hybrids

Crystal engineering of metal halide hybrids is critical to investigate their structure-property relationship and advance their photophysical applications, but there have been limited efforts to employ coordination chemistry to precisely control the dimensionality of metal halide sublattices. Herein, we present a coordination-assembly synthetic strategy developed for the rational modulation of lead halide dimensionality, realizing the transition from 2D to 3D architectures. This manipulation is achieved by utilizing three organocarboxylates featuring the identical cyclohexane backbone unit. Specifically, the 1,4-cyclohexanedicarboxylate and 1,2,4,5-cyclohexanetetracarboxylate ligands facilitate the formation of quasi-2D layered structures, characterized by weakly corrugated and strongly corrugated lead halide layers, respectively. Importantly, the introduction of the 1,2,3,4,5,6-cyclohexanehexacarboxylate ligand results in coordination architectures featuring 3D lead chloride/bromide sublattices. The formation of the 3D coordination architectures templated by the 1,2,3,4,5,6-cyclohexanehexacarboxylate ligand affords extended wavelength coverage and superior carrier transport properties compared to their quasi-2D layered analogues. Importantly, both the 2D and 3D lead halide-based coordination polymers exhibit high aqueous stability over a wide pH range, outperforming the conventional ionic-bound lead halides. Notably, the chemically stable 3D lead bromide exhibits efficient photocatalytic ethylbenzene oxidation with the conversion rate of 498 μmol g-1 h-1, substantially higher than its 2D lead bromide counterparts. This work highlights the important role of coordination chemistry in the rational design of metal halide hybrids, which is crucial for advancing their photophysical properties and applications.

One-Year Water-Stable and Porous Bi(III) Halide Semiconductor with Broad-Spectrum Antibacterial Performance

Hybrid metal halide semiconductors are a unique family of materials with immense potential for numerous applications. For this to materialize, environmental stability and toxicity deficiencies must be simultaneously addressed. We report here a porous, visible light semiconductor, namely, (DHS)Bi2I8 (DHS = [2.2.2] cryptand), which consists of nontoxic, earth-abundant elements, and is water-stable for more than a year. Gas- and vapor-sorption studies revealed that it can selectively and reversibly adsorb H2O and D2O at room temperature (RT) while remaining impervious to N2 and CO2. Solid-state NMR measurements and density functional theory (DFT) calculations verified the incorporation of H2O and D2O in the molecular cages, validating the porous nature. In addition to porosity, the material exhibits broad band-edge light emission centered at 600 nm with a full width at half-maximum (fwhm) of 99 nm, which is maintained after 6 months of immersion in H2O. Moreover, (DHS)Bi2I8 exhibits bacteriocidal action against three Gram-positive and three Gram-negative bacteria, including antibiotic-resistant strains. This performance, coupled with the recorded water stability and porous nature, renders it suitable for a plethora of applications, from solid-state batteries to water purification and disinfection.

Porous and Water Stable 2D Hybrid Metal Halide with Broad Light Emission and Selective H2 O Vapor Sorption

In this work we report a strategy for generating porosity in hybrid metal halide materials using molecular cages that serve as both structure-directing agents and counter-cations. Reaction of the [2.2.2] cryptand (DHS) linker with Pb^II in acidic media gave rise to the first porous and water-stable 2D metal halide semiconductor (DHS)₂ Pb₅ Br₁₄ . The corresponding material is stable in water for a year, while gas and vapor-sorption studies revealed that it can selectively and reversibly adsorb H₂ O and D₂ O at room temperature (RT). Solid-state NMR measurements and DFT calculations verified the incorporation of H₂ O and D₂ O in the organic linker cavities and shed light on their molecular configuration. In addition to porosity, the material exhibits broad light emission centered at 617 nm with a full width at half-maximum (FWHM) of 284 nm (0.96 eV). The recorded water stability is unparalleled for hybrid metal halide and perovskite materials, while the generation of porosity opens new pathways towards unexplored applications (e.g. solid-state batteries) for this class of hybrid semiconductors.

Circularly Polarized Luminescence from Zero-Dimensional Hybrid Lead-Tin Bromide with Near-Unity Photoluminescence Quantum Yield

Zero-dimensional (0D) hybrid metal halides with perfect host-guest structures are promising candidates to construct circularly polarized luminescence (CPL)-active materials. However, it still remains challenging to obtain 0D chiral metal halides with simultaneously strong CPL and high photoluminescence quantum yield. Here, a new enantiomeric pair of 0D hybrid lead-tin bromides, (RR/SS-C₆ N₂ H₁₆ )₂ Pb_0.968 Sn_0.032 Br₆  ⋅ 2H₂ O (R/S-PbSnBr ⋅ H₂ O), is reported. The R/S-PbSnBr ⋅ H₂ O compounds not only show intriguing self-trapped exciton emissions with near-unity quantum yield, but also present intense CPL with a dissymmetry factor g_lum of ±3.0×10^-3 . Such CPL activities originate from the asymmetric [SnBr₆ ]^4- luminophores in R/S-PbSnBr ⋅ H₂ O, due to the induced structural chirality by the organic ligands via N-H⋅⋅⋅Br hydrogen bonds. Furthermore, CPL emissions with tunable colors from R/S-PbSnBr ⋅ H₂ O and dehydrated compounds are reversibly observed, which extends their chiroptical applications.

Spacer Switched Two-Dimensional Tin Bromide Perovskites Leading to Ambient-Stable Near-Unity Photoluminescence Quantum Yield

Semiconductor nanostructures with near-unity photoluminescence quantum yields (PLQYs) are imperative for light-emitting diodes and display devices. A PLQY of 99.7 ± 0.3% has been obtained by stabilizing 91% Sn²⁺ in the Dion-Jacobson (8N8)SnBr₄ (8N8-DJ) perovskite with 1,8-diaminooctane (8N8) spacer. The PLQY is favored by a longer spacer molecule and out-of-plane octahedral tilting. The PLQY shows one-month ambient stability under high relative humidity (RH) and temperature. With n-octylamine (8N) spacer, Ruddlesden-Popper (8N)₂SnBr₄ (8N-RP) also shows PLQY of 91.7 ± 0.6%, but it has poor ambient stability. The 5-300 K PL experiments decipher the self-trapped excitons (STEs) where the self-trapping depth is 25.6 ± 0.4 meV below the conduction band because of strong carrier-phonon coupling. The microsecond long-lived STE dominates over the band edge (BE) peaks at lower excitation wavelengths and higher temperatures. The higher PLQY and stability of 8N8-DJ are due to the stronger interaction between SnBr₆^4- octahedra and 8N8 spacer, leading to a rigid structure.

Inorganic Chain Mediated Excitonic Properties in One-Dimensional Lead Halide Hybrid Perovskites

Semiconductors that emit intrinsic white light are considered next-generation lighting sources. Herein, the broadband emission of one-dimensional (1D) lead halide perovskites, TMAPbBr_3-xI_x (x = 0, 1, 1.5, 2, 3; TMA⁺ = tetramethylammonium), is systematically investigated. Lattice distortion causes the conversion of dark excitons to bright self-trapped excitons. Owing to its strongly localized exciton recombination and high absorption probability, TMAPbBr₃ is the most viable in this family. A delocalized hole increases the nonradiative recombination rate of excitons in TMAPbBr_3-xI_x alloys. In 1D TMAPbBr_3-xI_x perovskites, the vibration mode of the Pb-X bond stretching of the PbX₆ octahedra contributes more to the effect on exciton-phonon coupling than the mode of the X-Pb-X angle bending. Pb-X bond stretching and spontaneous polarization can tune exciton binding energy. This systematic study of excitonic behavior in 1D compounds relates the nature of ground states to the unknown excited states and provides the rational design of materials with stable and efficient broadband emission.

New Organic-Inorganic Salt Based on Fluconazole Drug: TD-DFT Benchmark and Computational Insights into Halogen Substitution

In this study, we report the synthesis of a new organic–inorganic molecular salt of the clinically used antifungal drug fluconazole, (H2Fluconazole).SnCl₆.2H₂O. By detailed investigation and analysis of its structural properties, we show that the structure represents a 0D structure built of alternating organic and inorganic zig-zag layers along the crystallographic c-axis and the primary supramolecular synthons in this salt are hydrogen bonding, F···π and halogen bonding interactions. Magnetic measurements reveal the co-existence of weak ferromagnetic behavior at low magnetic field and large diamagnetic contributions, indicating that the synthesized material behaves mainly as a diamagnetic material, with very low magnetic susceptibility and with a band gap energy of 3.6 eV, and the salt is suitable for semiconducting applications. Extensive theoretical study is performed to explain the acceptor donor reactivity of this compound and to predict the Cl-substitution effect by F, Br and I. The energy gap, frontier molecular orbitals (FMOs) and the different chemical reactivity descriptors were evaluated at a high theoretical level. Calculations show that Cl substitution by Br and I generates compounds with more important antioxidant ability and the intramolecular charge transfer linked to the inorganic anion.

Small Organic Molecular-Based Hybrid Halides with High Photoluminescence Quenching Temperature

Organic-inorganic metal halides (OIMHs) exhibit excellent photoelectric properties; however, their high-temperature light-emission stability requires further improvement. Here, we report three isostructural OIMHs (C₂H₈N)₄InCl₇, (C₂H₈N)₄SbCl₇, and (C₂H₈N)₄SbBr₇ (C₂H₈N⁺ = dimethylammonium). They are all crystallized in the P2₁2₁2 space group with a zero-dimensional (0D) structure, with orange-red photoluminescence (PL) under 365 nm UV excitation. Among them, (C₂H₈N)₄InCl₇ exhibits the strongest PL with a photoluminescence quantum yield (PLQY) of 13.9% at room temperature. Optical property measurements and density functional theory unveil that the luminescence of (C₂H₈N)₄InCl₇ at 405 and 620 nm is due to free exciton and self-trapped exciton emission, respectively. It is worth noting that (C₂H₈N)₄InCl₇ shows a high PL quenching temperature, maintaining 50% of its room-temperature PL intensity at 425 K, which is rare in OIHMs. This is much higher than the application temperature of phosphors in practical solid-state lighting applications (363-383 K). In this temperature range, the luminous intensity of (C₂H₈N)₄InCl₇ exceeds 60% of that at room temperature. The high PL quenching temperature observed in (C₂H₈N)₄InCl₇ indicates the potential of OIMHs for applications in phosphor-converted light-emitting diodes.

Layered 2D Halide Perovskites beyond Ruddlesden-Popper Phase: Tailored Interlayer Chemistries for High-Performance Solar Cells

Layered halide perovskites (LHPs) with crystallographically 2D structures have gained increasing interest for photovoltaic applications due to their superior chemical stability and intriguing anisotropic properties, as compared to their conventional 3D perovskite counterparts. The mostly studied LHPs are Ruddlesden-Popper (RP) phases, which suffer from a carrier-transport bottleneck due to the van der Waals gap associated with their intrinsic organic interlayer structures. To address this issue, Dion-Jacobson (DJ) and alternating-cation-interlayer (ACI) LHPs have rapidly emerged, which exhibit unique structural and (opto)electronic characteristics that may resemble the 3D counterparts owing to the eliminated or reduced van der Waals gap. Improved photophyiscal properties have been achieved in DJ and ACI LHPs, leading towards better photovoltaic performance. Here we provide a comprehensive discussion on the merits and promise of DJ and ACI LHPs from a chemistry perspective. Then, we review recent progress on synthesis and tailoring of DJ and ACI LHP crystals and thin films, as well as their optoelectronic properties and photovoltaic performance. Finally, we discuss future directions to realize the full potential of DJ and ACI LHPs for high-performance solar cells and beyond.

Room Temperature Fabrication of Stable, Strongly Luminescent Dion-Jacobson Tin Bromide Perovskite Microcrystals Achieved through Use of Primary Alcohols

Lead-free two-dimensional metal halide perovskites have recently emerged as promising light-emitting materials due to their improved stability and attractive optical properties. Herein, a facile room temperature wet milling method has been developed to make Dion-Jacobson (DJ) phase ODASnBr4 perovskite microcrystals, whose crystallization was accomplished via the aid of introduced primary alcohols: ethanol, butanol, pentanol, and hexanol. Due to the strong intermolecular hydrogen bonding, the use of ethanol promoted the formation of non-doped ODASnBr4 microcrystals, with an emission peaked at 599 nm and a high photoluminescence quantum yield (PL QY) of 81%. By introducing other primary alcohols with weaker intermolecular hydrogen bonding such as butanol, pentanol, and hexanol, [SnBr6]4- octahedral slabs of the DJ perovskite microcrystals experienced various degrees of expansion while forming O-H…Br hydrogen bonds. This resulted in the emission spectra of these alcohol-doped microcrystals to be adjusted in the range from 572 to 601 nm, while keeping the PL QY high, at around 89%. Our synthetic strategy provides a viable pathway towards strongly emitting lead-free DJ perovskite microcrystals with an improved stability.

Water-Stable Zero-Dimensional (C4H9)4NCuCl2 Single Crystal with Highly Efficient Broadband Green Emission

Here, we report (C₄H₉)₄NCuCl₂ single crystals with a luminous intensity that remains largely the same after soaking in water for 24 h. (CH₉)₄NCuCl₂ has a new type zero-dimensional framework, in which the isolated [CuCl₂]^- anions are wrapped by organic (C₄H₉)₄N⁺ cations. As expected, (C₄H₉)₄NCuCl₂ shows a broad emission band at 508 nm with a photoluminescence quantum yield of approximately 82% at room temperature, stemming from self-trapped exciton (STE) emission. Temperature-dependent photoluminescence measurement reveals that there is an energy barrier ΔE (24.0 meV) between the intrinsic state and STE state, which leads to the increase in emission intensity with an increase in temperature (98-278 K), while the emission intensity begins to decrease when the temperature is higher than 278 K due to the effects of both thermal quenching and carrier scattering. Our findings provide a new idea for the design of lead-free anti-water stability metal halide materials.

Tunable Broad Light Emission from 3D "Hollow" Bromide Perovskites through Defect Engineering

Hybrid halide perovskites consisting of corner-sharing metal halide octahedra and small cuboctahedral cages filled with counter cations have proven to be prominent candidates for many high-performance optoelectronic devices. The stability limits of their three-dimensional perovskite framework are defined by the size range of the cations present in the cages of the structure. In some cases, the stability of the perovskite-type structure can be extended even when the counterions violate the size and shape requirements, as is the case in the so-called "hollow" perovskites. In this work, we engineered a new family of 3D highly defective yet crystalline "hollow" bromide perovskites with general formula (FA)_1-x(en)_x(Pb)_1-0.7x(Br)_3-0.4x (FA = formamidinium (FA⁺), en = ethylenediammonium (en²⁺), x = 0-0.44). Pair distribution function analysis shed light on the local structural coherence, revealing a wide distribution of Pb-Pb distances in the crystal structure as a consequence of the Pb/Br-deficient nature and en inclusion in the lattice. By manipulating the number of Pb/Br vacancies, we finely tune the optical properties of the pristine FAPbBr₃ by blue shifting the band gap from 2.20 to 2.60 eV for the x = 0.42 en sample. A most unexpected outcome was that at x> 0.33 en incorporation, the material exhibits strong broad light emission (1% photoluminescence quantum yield (PLQY)) that is maintained after exposure to air for more than a year. This is the first example of strong broad light emission from a 3D hybrid halide perovskite, demonstrating that meticulous defect engineering is an excellent tool for customizing the optical properties of these semiconductors.

Intrinsic self-trapped broadband emission from zinc halide-based metal-organic frameworks

Organolead halide perovskites are an emerging class of intrinsic self-trapped broadband emitters, but suffer from lead toxicity and stability problems. Herein, we report a series of metal-organic frameworks (MOFs) based on 0-D zinc halide secondary building units (SBUs), which emit large Stokes shifted broadband bluish-white light. A variety of photophysics studies demonstrate that the broadband emission probably originates from self-trapped excitons, owing to the structurally deformable SBUs. Among the intrinsic self-trapped emitters, these MOFs are very rare examples that exhibit both long-term environmental stability and contain non-toxic elements. Moreover, the open porosity enables the MOF to serve as a host matrix for encapsulating green-emitting Alq3 molecules, exhibiting cold white-light chromatic coordinates of (0.27,0.36) and a correlated color temperature of 8321 K.

Lead-Free Halide Perovskites for Light Emission: Recent Advances and Perspectives

Lead-based halide perovskites have received great attention in light-emitting applications due to their excellent properties, including high photoluminescence quantum yield (PLQY), tunable emission wavelength, and facile solution preparation. In spite of excellent characteristics, the presence of toxic element lead directly obstructs their further commercial development. Hence, exploiting lead-free halide perovskite materials with superior properties is urgent and necessary. In this review, the deep-seated reasons that benefit light emission for halide perovskites, which help to develop lead-free halide perovskites with excellent performance, are first emphasized. Recent advances in lead-free halide perovskite materials (single crystals, thin films, and nanocrystals with different dimensionalities) from synthesis, crystal structures, optical and optoelectronic properties to applications are then systematically summarized. In particular, phosphor-converted LEDs and electroluminescent LEDs using lead-free halide perovskites are fully examined. Ultimately, based on current development of lead-free halide perovskites, the future directions of lead-free halide perovskites in terms of materials and light-emitting devices are discussed.

The red light emission in 2D (C4SH3CH2NH3)2SnI4 and (C4OH7CH2NH3)2SnI4 perovskites

Two-dimensional (2D) perovskites have a large exciton binding energy due to the structure of the quantum confinement, which produces a faster radiative recombination, and so are promising potential materials for light-emitting diodes. However, most of the highly efficient hybrid halide perovskites are based on the toxic Pb-based materials, so the replacement of Pb with less toxic and suitable substitute elements has been investigated for environmental efficient materials. Herein, we report the Sn-based 2D perovskites, which include (TPM)2SnI4 (TPM = C4SH3CH2NH3) and (TFF)2SnI4 (TFF = C4OH7CH2NH3), as red emission materials. Structural characterization by single crystal X-ray diffraction reveals that (TPM)2SnI4 undergoes a structural evolution from the orthorhombic space group Cmc21 (100 K) to Pbca (298 K), while the (TFF)2SnI4 perovskite exhibits the monoclinic space group P21/c at 100 K and 298 K. The inorganic framework of (TFF)2SnI4 was separated by the bilayer TFF chains with an empty space, which is an effective structure to increase the quantum confinement effect. The band gaps of the (TPM)2SnI4 (1.80 eV) and (TFF)2SnI4 (1.73 eV) compounds indicate the direct band gap semiconductor materials. From the time-resolved photoluminescence results, it can be seen that (TPM)2SnI4 produces uniform short emission (0.73 ns) throughout the entire powder crystals, whereas (TFF)2SnI4 has a uniform and long emission life time (47 ns). Temperature-dependent photoluminescence (PL) studies indicate that the (TPM)2SnI4 and (TFF)2SnI4 perovskites have a strong split red emission at low temperature due to the vibration of the inorganic framework. As the temperature increases, the PL spectra shift to the high energy region and the emission intensity decreases. The PL spectra of (TPM)2SnI4 and (TFF)2SnI4 perovskites have maximum peak wavelengths at 622 nm and 640 nm, and show the photoluminescence quantum yields of 0.30% and 1.71%, respectively.

Water-Stable DMASnBr3 Lead-Free Perovskite for Effective Solar-Driven Photocatalysis

Water-stable metal halide perovskites could foster tremendous progresses in several research fields where their superior optical properties can make differences. In this work we report clear evidence of water stability in a lead-free metal halide perovskite, namely DMASnBr₃ , obtained by means of diffraction, optical and X-ray photoelectron spectroscopy. Such unprecedented water-stability has been applied to promote photocatalysis in aqueous medium, in particular by devising a novel composite material by coupling DMASnBr₃ to g-C₃ N₄ , taking advantage from the combination of their optimal photophysical properties. The prepared composites provide an impressive hydrogen evolution rate >1700 μmol g^-1  h^-1 generated by the synergistic activity of the two composite costituents. DFT calculations provide insight into this enhancement deriving it from the favorable alignment of interfacial energy levels of DMASnBr₃ and g-C₃ N₄ . The demonstration of an efficient photocatalytic activity for a composite based on lead-free metal halide perovskite in water paves the way to a new class of light-driven catalysts working in aqueous environments.

Efficient Lone-Pair-Driven Luminescence: Structure-Property Relationships in Emissive 5s2 Metal Halides

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 low-dimensional halides based on the 5s2 cations Sn2+ and Sb3+ has found utility in a variety of applications complementary to those of the three-dimensional halide perovskites because of its unusual properties such as broadband character and highly temperature-dependent lifetime. These properties derive from the exceptional chemistry of the 5s2 lone pair, but the terminology and explanations given for such emission vary widely, hampering efforts to build a cohesive understanding of these materials that would lead to the development of 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 unite the different theoretical models that 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 those state-of-the-art materials and applications that demonstrate the unique capabilities of these versatile emissive centers and identify promising future directions in the field of low-dimensional 5s2 metal halides.