Solar-Driven Conversion of CO2 to C2 Products by the 3d Transition Metal Intercalates of Layered Lead Iodides.

Photocatalytic CO2 reduction to high-value-added C2+ products presents significant challenges, which is attributed to the slow kinetics of multi-e- CO2 photoreduction and the high thermodynamic barrier for C-C coupling. Incorporating redox-active Co2+/Ni2+ cations into lead halide photocatalysts has high potentials to improve carrier transport and introduce charge polarized bimetallic sites, addressing the kinetic and thermodynamic issues, respectively. In this study, a coordination-driven synthetic strategy is developed to introduce 3d transition metals into the interlamellar region of layered organolead iodides with atomic precision. The resultant bimetallic halide hybrids exhibit selective photoreduction of CO2 to C2H5OH using H2O vapor at the evolution rates of 24.9-31.4 µmol g-1 h-1 and high selectivity of 89.5-93.6%, while pristine layered lead iodide yields only C1 products. Band structure calculations and photoluminescence studies indicate that the interlayer Co2+/Ni2+ species greatly contribute to the frontier orbitals and enhance exciton dissociation into free carriers, facilitating carrier transport between adjacent lead iodide layers. In addition, Bader charge distribution calculations and in situ experimental spectroscopic studies reveal that the asymmetric Ni-O-Pb bimetallic catalytic sites exhibit intrinsic charge polarization, promoting C-C coupling and leading to the formation of the key *OC-CHO intermediate.

Highly Stable MOF-Type Lead Halide Luminescent Ferroelectrics.

Lead halide molecular ferroelectrics represent an important class of luminescent ferroelectrics, distinguished by their high chemical and structural tunability, excellent processability and distinctive luminescent characteristics. However, their inherent instability, prone to decomposition upon exposure to moisture and light, hinders their broader ferroelectric applications. Herein, for the first time, we present a series of isoreticular metal-organic framework (MOF)-type lead halide luminescent ferroelectrics, demonstrating exceptional robustness under ambient conditions for at least 15 months and even when subjected to aqueous boiling conditions. Unlike conventional metal-oxo secondary building units (SBUs) in MOFs adopting highly centrosymmetric structure with limited structural distortion, our lead halide-based MOFs occupy structurally deformable [Pb2X]+ (X=Cl-/Br-/I-) SBUs that facilitate a c-axis-biased displacement of Pb2+ centers and substantially contribute to thermoinducible structural transformation. Importantly, this class of MOF-type lead halide ferroelectrics undergo ferroelectric-to-paraelectric phase transitions with remarkably high Curie temperature of up to 505 K, superior to most of molecular ferroelectrics. Moreover, the covalent bonding between phosphorescent organic component and the light-harvesting inorganic component achieves efficient spin-orbit coupling and intersystem crossing, resulting in long-lived afterglow emission. The compelling combination of high stability, ferroelectricity and afterglow emission exhibited by lead halide MOFs opens up many potential opportunities in energy-conversion applications.

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.

Modulating Inorganic Dimensionality of Ultrastable Lead Halide Coordination Polymers for Photocatalytic CO2 Reduction to Ethanol.

Lead halide hybrids have shown great potentials in CO2 photoreduction, but challenging to afford C2+ reduced products, especially using H2O as the reductant. This is largely due to the trade-off problem between instability of the benchmark 3D structures and low carrier mobility of quasi-2D analogues. Herein, the lead halide dimensionality of robust coordination polymers (CP) was modulated by organic ligands differing in a single-atom change (NH vs. CH2), in which the NH groups coordinate with interlamellar [PbI2] clusters to achieve the important 2D→3D transition. This first CP based on 3D cationic lead iodide sublattice possesses both high aqueous stability and a low exciton binding energy of 25 meV that is on the level of ambient thermal energy, achieving artificial photosynthesis of C2H5OH. Photophysical studies combined with theoretical calculations suggest the bridging [PbI2] clusters in the 3D structure not only results in enhanced carrier transport, but also promotes the intrinsic charge polarization to facilitate the C-C coupling. With trace loading of Rh cocatalyst, the apparent quantum efficiency of the 3D CP reaches 1.4 % at 400 nm with a high C2H5OH selectivity of 89.4 % (product basis), which presents one of the best photocatalysts for C2 products to date.

Lead Halide Hybrids Templated by Two Coordinating Ligands for Enhanced and Stable Self-Trapped Emission.

Lead halide hybrids templated by coordinating ligands are a class of ultrastable broadband self-trapped emitters that overcome the stability problems of conventional ionically bound halide hybrids. However, enhancing their photoluminescence (PL) performances by crystal engineering remains a huge challenge. Herein, for the first time, we have successfully employed the synthetic strategy of two coordinating ligands to synthesize a series of layered lead halide coordination polymers, [Pb6X10]2+(chdc2-)(2,2'-bpy)2 (X = Cl/Br, chdc = trans-1,4-cyclohexanedicarboxylate), which involves chdc as a pillaring strut and 2,2'-bpy as a chelating ligand. The introduction of a chelating ligand (2,2'-bpy) enables stronger lattice distortion of lead halide layers and enhances UV-light absorption and ligand-to-metal charge transfer (LMCT) process, thereby achieving a substantial improvement of photoluminescence quantum yields (PLQYs) over the control layered materials templated by a single chdc ligand. This class of lead halide hybrids templated by two coordinating ligands exhibit chemical "inertness" after being subjected to various chemical conditions for 48 h, maintaining stable and efficient broadband emission. Density functional theory calculations and femtosecond transient absorption spectra (fs-TA) demonstrate that the broadband emission originates from self-trapped excitons, which are more populated with the LMCT contribution from 2,2'-bpy. This study shows a rational strategy at the molecular level to modulate the photophysical properties of chemically robust lead halide coordination polymers.

Fabrication of Robust and Porous Lead Chloride-Based Metal-Organic Frameworks toward a Selective and Sensitive Smart NH3 Sensor.

Organolead halide materials have shown promising optoelectronic properties that are suitable for light-emitting diodes (e.g., strong photoluminescence, narrow emission width, and high charge carrier mobility). However, the vast majority of them have no open porosity or open metal sites for host-guest interactions and are therefore not widely applicable in intrinsic fluorescent sensing of small molecules. Herein, we report a lead chloride-based metal-organic framework (MOF) with high porosity and stability and promising photoluminescent characteristics, performing as a sensitive, selective, and long-term stable fluorescence probe for NH3. For the first time, a homemade dynamic real-time photoluminescence monitoring system was developed, which showed that our haloplumbate-based MOF has an immediate response and an extremely low limit of detection (12 ppm) toward NH3. A variety of experimental characterization and theoretical calculations evidenced that the photoluminescence quenching was ascribed to the coordination between NH3 guests and exposed Pb2+ centers in MOFs. Moreover, a portable on-site smart NH3 detector was designed based on this haloplumbate-MOF using a 3D printer, and the quantitative recovery experiment demonstrated the effective detection of NH3 in the range of 15-150 ppm. This study opens a new pathway to design organolead halide-based MOFs to perform on-site chemical sensing of small molecules and shows their high potential to monitor safety concentrations of NH3 in different industrial sites.

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.

Efficient, broadband self-trapped white-light emission from haloplumbate-based metal-organic frameworks.

Metal-organic frameworks (MOFs) with low-dimensional, deformable haloplumbate secondary building units (SBUs) are an emerging class of intrinsic white-light emitters combining advantageous properties of both MOFs and lead perovskites. Herein, we have successfully synthesized two MOFs with haloplumbate SBUs occupying an extremely high degree of structural strain with local zigzag Pb-X-Pb-X (X = Cl/Br) connectivity located in single-stranded helices. Thus, the electron-phonon coupling in the deformable SBUs affords intrinsic white-light emission and moderately high external photoluminescence quantum efficiencies of 12-15%, superior to our previously reported MOFs. Moreover, the excellent photocarrier diffusion properties of lead perovskites have been successfully incorporated into the MOFs with high chemical robustness in moisture (up to 90% relative humidity, RH).

A moisture-stable organosulfonate-based metal-organic framework with intrinsic self-trapped white-light emission.

Weakly-coordinating organosulfonate linkers are significantly less studied in MOFs, but promising in terms of enhancing the structural strain of SBUs owing to their versatile coordination towards metals. Herein, we have successfully synthesized a porous, moisture-resistant organosulfonate-based MOF with structurally deformable 1D zigzag [CdCl]nn+ chains. The material is one rare MOF exhibiting intrinsic broadband white-light emission with an external quantum efficiency exceeding 10%, probably owing to the strong electron-phonon coupling in the orgnosulfonate-bridged deformable inorganic units. Moreover, the porosity residing in the MOF allows for facile, controllable cation exchange to introduce a second emissive center (e.g. Mn2+), which is an excellent supplement to the deficiency of the pristine MOF and achieves a high color rendering index of 92.

Intrinsic White-Light-Emitting Metal-Organic Frameworks with Structurally Deformable Secondary Building Units.

The secondary building units in metal-organic frameworks (MOFs) are commonly well-defined metal-oxo clusters or chains with very limited structural strain. Herein, the structurally deformable haloplumbate units that are often observed in organolead halide perovskites have been successfully incorporated into MOFs. The resultant materials are a rare class of isoreticular MOFs exhibiting large Stokes-shifted broadband white-light emission, which is probably induced by self-trapped excitons from electron-phonon coupling in the deformable, zigzag [Pb2 X3 ]+ (X=Cl, Br, or I) chains. In contrast, MOFs with highly symmetric, robust haloplumbate chains only exhibit narrow UV-blue photoemission. The designed MOF-based intrinsic white-light photoemitters have a number of advantages over hybrid inorganic-organic perovskites in terms of stability and tunability, including moisture resistance, facile functionalization of photoactive moieties onto the organic linkers, introduction of luminescent guests.

An ultrastable metal-organic material emits efficient and broadband bluish white-light emission for luminescent thermometers.

We discover a rare bluish white-light-emitting Sb3+-based coordination polymer with an unusually large Stokes shift of 230 nm (2.3 eV), ascribed to the assymetric-symmetric coordination shift of the Sb3+ centers. The photoemission renders both a high photoluminescence quantum yield exceeding 30% and a large full width at half maximum of 116 nm. Moreover, the strongly light-emissive material exhibits a linear relationship between the correlated color temperature and the absolute temperature in a wide range (157-457 K), thus presenting the first solid-state luminescent thermometer based on photoemission energy.

Cationic two-dimensional inorganic networks of antimony oxide hydroxide for Lewis acid catalysis.

We have successfully synthesized a rare example of inorganic layered materials possessing a positive charge, which is well outside the isostructural set of layered double hydroxides. This layered architecture consists of two-dimensional corrugated [Sb2O2(OH)]+ layers with linear α,ω-alkanedisulfonate anions residing in the interlamellar space. This cationic material displays a chemical robustness under highly acidic aqueous conditions (pH = 1). Combining the robust nature and the high density of SbIII sites on the exposed crystal facets, our cationic layered material is an efficient, recyclable catalyst for cyanosilylation of benzaldehyde derivatives with trimethylsilyl cyanide. In addition, the Lewis acidity of the SbIII sites also catalyzes the ketalization of carbonyl groups under "green" solvent-free conditions.

Intrinsic Broadband White-Light Emission from Ultrastable, Cationic Lead Halide Layered Materials.

We report a family of cationic lead halide layered materials, formulated as [Pb2 X2 ]2+ [- O2 C(CH)2 CO2- ] (X=F, Cl, Br), exhibiting pronounced broadband white-light emission in bulk form. These well-defined PbX-based structures achieve an external quantum efficiency as high as 11.8 %, which is comparable to the highest reported value (ca.9 %) for broadband phosphors based on layered organolead halide perovskites. More importantly, our cationic materials are ultrastable lead halide materials, which overcome the air/moisture-sensitivity problems of lead perovskites. In contrast to the perovskites and other bulk emitters, the white-light emission intensity of our materials remains undiminished after continuous UV irradiation for 30 days under atmospheric conditions (ca.60 % relative humidity). Our mechanistic studies confirm that the broadband emission is ascribed to short-range electron-phonon coupling in the strongly deformable lattice and generated self-trapped carriers.