Developing Luminescent Ratiometric Thermometers Based on a Covalent Organic Framework (COF)

Covalent Organic Frameworks (COFs), an emerging class of crystalline porous materials, are proposed as a new type of support for grafting lanthanide ions (Ln³⁺) and employing these hybrid materials as ratiometric luminescent thermometers. A TpBpy‐COF—prepared from 1,3,5‐triformylphloroglucinol (Tp) and 2,2′‐bipyridine‐5,5′‐diamine (Bpy) grafted with Eu/Tb and Dy acetylacetone (acac) complexes can be successfully used as a luminescent thermometer in the 10–360 K (Eu) and 280–440 K (Tb) ranges with good sensing properties (thermal sensitivity up to 1.403 % K^?1, temperature uncertainty δT<1 K above 110 K). For the Eu/Tb systems, we observe an unusual and rarely reported behavior, that is, no thermal quenching of the Tb³⁺ emission, a result of the absence of ion‐to‐ligand/host energy back‐transfer. The LnCOF materials proposed here could be a new class of materials employed for temperature‐sensing applications following up on the well‐known luminescent metal–organic framework thermometers.     

A Ratiometric Luminescent Thermometer Co‐doped with Lanthanide and Transition Metals

Herein, we report the fabrication of a sensitive ratiometric and colorimetric luminescent thermometer with a wide operating‐temperature range, from cryogenic temperatures up to high temperatures, through the combination of lanthanide and transition metal complexes. Benefiting from the transition metal complex as a self‐reference, the lanthanide content in the mixed‐coordination complex, Eu_0.05(Mebip‐mim bromine)_0.15Zn_0.95(Mebip‐mim bromine)_1.9, was lowered to 5 %.     

A Highly Stable Yttrium Organic Framework as a Host for Optical Thermometry and D2O Detection

The yttrium organic framework (Y_0.89Tb_0.10Eu_0.01)₆(BDC)₇(OH)₄(H₂O)₄ (BDC=benzene-1,4-dicarboxylate) is hydrothermally stable up to at least 513 K and thermally stable in air in excess of 673 K. The relative intensities of luminescence of Tb³⁺ and Eu³⁺ are governed by Tb³⁺-to-Eu³⁺ phonon-assisted energy transfer and Tb³⁺-to-ligand back transfer and are responsible for the differing temperature-dependent luminescence of the two ions. This provides a ratiometric luminescent thermometer in the 288–573 K temperature range, not previously seen for MOF materials, with a high sensitivity, 1.69±0.04 % K⁻¹ at 523 K. In aqueous conditions, loosely bound H₂O can be replaced by D₂O in the same material, which modifies decay lifetimes to yield a quantitative luminescent D₂O sensor with a useful sensitivity for practical application.     

Real-Time In Situ Volatile Organic Compound Sensing by a Dual-Emissive Polynuclear Ln-MOF with Pronounced LnIII Luminescence Response

Lanthanide metal–organic frameworks (Ln-MOFs) are promising for luminescence detection of volatile organic compound (VOC) vapors, but usually suffer from the silent or quenched Ln³⁺ emission. Herein, we report a new dual-emissive Eu-MOF composed of the coordinatively unsaturated Eu₉ clusters that afford abundant open metal sites to form a confined “binding pocket” to facilitate the preconcentration and recognition of VOCs. Single-crystal structural analyses reveal that specific analytes can replace the OH oscillators in the first coordination sphere of Eu³⁺ and form a unique hydrogen-bonding second-sphere adduct tying adjacent Eu₉ clusters together to minimize their nonradiative vibrational decay. With the promoted Eu³⁺ luminescence, the MOF realizes real-time in situ visual sensing of THF vapor (<1 s) and shows a quantitative ratiometric response to the vapor pressure with a limit of detection down to 17.33 Pa. Also, it represents a top-performing ratiometric luminescent thermometer.     

An Efficient Visible and Near‐Infrared (NIR) Emitting SmIII Metal–Organic Framework (Sm‐MOF) Sensitized by Excited‐State Intramolecular Proton Transfer (ESIPT) Ligand

We report herein an unprecedented example of a luminescent Sm^III metal–organic framework (Sm‐MOF), in which both the visible and near‐infrared (NIR) emissions of Sm³⁺ ions are able to be sensitized by an excited‐state intramolecular proton transfer (ESIPT) ligand. Due to the solvent‐mediated interchange between enol and keto excited states of the ligand and subsequent energy transfer rate to Sm³⁺ ions, the luminescent decay lifetime of the Sm‐MOF can be tuned in different solvent‐grinding systems.     

A Highly Chemically Stable Metal–Organic Framework as a Luminescent Probe for the Regenerable Ratiometric Sensing of pH

A heteroatom‐rich 3D noninterpenetrating metal–organic framework (MOF) Cd‐EDDA constructed from an ethylene glycol ether bridging tetracarboxylate ligand H₄EDDA (5,5′‐(ethane‐1,2‐diylbis(oxy))diisophthalic acid) shows good chemical resistance to both acidic and alkaline solutions with a pH ranging from 2.0 to 12.2. There is a corresponding ratiometric luminescence response to pH from 2.0 to 11.5, and the sensing mechanism is also discussed through ion chromatography and molecular force field‐based calculations. Importantly, the probe can easily be regenerated simply by modulating the pH of the solution, thus being the first example of a regenerable MOF‐based ratiometric luminescent probe for pH.     

Selective Sensing of Fe3+ and Al3+ Ions and Detection of 2,4,6‐Trinitrophenol by a Water‐Stable Terbium‐Based Metal–Organic Framework

A water‐stable luminescent terbium‐based metal–organic framework (MOF), {[Tb(L₁)_1.5(H₂O)] ? 3 H₂O}_n (Tb‐MOF), with rod‐shaped secondary building units (SBUs) and honeycomb‐type tubular channels has been synthesized and structurally characterized by single‐crystal X‐ray diffraction. The high green emission intensity and the microporous nature of the Tb‐MOF indicate that it can potentially be used as a luminescent sensor. In this work, we show that Tb‐MOF can selectively sense Fe³⁺ and Al³⁺ ions from mixed metal ions in water through different detection mechanisms. In addition, it also exhibits high sensitivity for 2,4,6‐trinitrophenol (TNP) in the presence of other nitro aromatic compounds in aqueous solution by luminescence quenching experiments.     

Well‐Defined Metal–Organic Framework Hollow Nanocages

Metal–organic frameworks (MOFs) have demonstrated great potentials in a variety of important applications. To enhance the inherent properties and endow materials with multifunctionality, the rational design and synthesis of MOFs with nanoscale porosity and hollow feature is highly desired and remains a great challenge. In this work, the formation of a series of well‐defined MOF (MOF‐5, Fe^II‐MOF‐5, Fe^III‐MOF‐5) hollow nanocages by a facile solvothermal method, without any additional supporting template is reported. A surface‐energy‐driven mechanism may be responsible for the formation of hollow nanocages. The addition of pre‐synthesized poly(vinylpyrrolidone)‐ (PVP) capped noble‐metal nanoparticles into the synthetic system of MOF hollow nanocages yields the yolk–shell noble metal@MOF nanostructures. The present strategy to fabricate hollow and yolk–shell nanostructures is expected to open up exciting opportunities for developing a novel class of inorganic–organic hybrid functional nanomaterials.     

A Luminescent Dye@MOF Platform: Emission Fingerprint Relationships of Volatile Organic Molecules

Self‐assembly of luminescent moieties into porous metal–organic frameworks (MOFs) has generated many luminescent platforms for probing volatile organic molecules (VOMs). However, most of those explored thus far have only been based on the luminescence intensity of one transition, which is not efficient for probing different VOMs. We have synthesized a luminescent MOF material containing 1D nanotube channels, and further developed a luminescent dye@MOF platform to realize the probing of different VOMs by tuning the energy transfer efficiency between two different emissions. The dye@MOF platform exhibits excellent fingerprint correlation between the VOM and the emission peak‐height ratio of ligand to dye moieties. The dye@MOF sensor is self‐calibrating, stable, and instantaneous, thus the approach should be a very promising strategy to develop luminescent materials with unprecedented practical applications.     

Density Functional Theory Study of Hydrogen Adsorption in a Ti‐Decorated Mg‐Based Metal–Organic Framework‐74

The Ti‐binding energy and hydrogen adsorption energy of a Ti‐decorated Mg‐based metal–organic framework‐74 (Mg‐MOF‐74) were evaluated by using first‐principles calculations. Our results revealed that only three Ti adsorption sites were found to be stable. The adsorption site near the metal oxide unit is the most stable. To investigate the hydrogen‐adsorption properties of Ti‐functionalized Mg‐MOF‐74, the hydrogen‐binding energy was determined. For the most stable Ti adsorption site, we found that the hydrogen adsorption energy ranged from 0.26 to 0.48 eV H₂^?1. This is within the desirable range for practical hydrogen‐storage applications. Moreover, the hydrogen capacity was determined by using ab initio molecular dynamics simulations. Our results revealed that the hydrogen uptake by Ti‐decorated Mg‐MOF‐74 at temperatures of 77, 150, and 298 K and ambient pressure were 1.81, 1.74, and 1.29 H₂ wt %, respectively.     

A Multi‐responsive Regenerable Europium–Organic Framework Luminescent Sensor for Fe3+, CrVI Anions,and Picric Acid

A novel luminescent microporous lanthanide metal–organic framework (Ln‐MOF) based on a urea‐containing ligand has been successfully assembled. Structural analysis revealed that the framework features two types of 1D channels, with urea N?H bonds projecting into the pores. Luminescence studies have revealed that the Ln‐MOF exhibits high sensitivity, good selectivity, and a fast luminescence quenching response towards Fe³⁺, Cr^VI anions, and picric acid. In particular, in the detection of Cr₂O₇^2? and picric acid, the Ln‐MOF can be simply and quickly regenerated, thus exhibiting excellent recyclability. To the best of our knowledge, this is the first example of a multi‐responsive luminescent Ln‐MOF sensor for Fe³⁺, Cr^VI anions, and picric acid based on a urea derivative. This Ln‐MOF may potentially be used as a multi‐responsive regenerable luminescent sensor for the quantitative detection of toxic and harmful substances.     

Enhancing Multicomponent Metal–Organic Frameworks for Low Pressure Liquid Organic Hydrogen Carrier Separations

The resurgence of interest in the hydrogen economy could hinge on the distribution of hydrogen in a safe and efficient manner. Whilst great progress has been made with cryogenic hydrogen storage or liquefied ammonia, liquid organic hydrogen carriers (LOHCs) remain attractive due to their lack of need for cryogenic temperatures or high pressures, most commonly a cycle between methylcyclohexane and toluene. Oxidation of methylcyclohexane to release hydrogen will be more efficient if the equilibrium limitations can be removed by separating the mixture. This report describes a family of six ternary and quaternary multicomponent metal–organic frameworks (MOFs) that contain the three‐dimensional cubane‐1,4‐dicarboxylate (cdc) ligand. Of these MOFs, the most promising is a quaternary MOF (CUB‐30), comprising cdc, 4,4′‐biphenyldicarboxylate (bpdc) and tritopic truxene linkers. Contrary to conventional wisdom that adsorptive interactions with larger, hydrocarbon guests are dominated by π–π interactions, here we report that contoured aliphatic pore environments can exhibit high selectivity and capacity for LOHC separations at low pressures. This is the first time, to the best of our knowledge, where selective adsorption for cyclohexane over benzene is witnessed, underlining the unique adsorptive behavior afforded by the unconventional cubane moiety.     

Core‐Shell Structured Layered Lanthanide‐Organic Complexes with Stilbazolium‐type Dye Encapsulation for Multifunctional Performances

Host‐guest encapsulation of functional organic dye into a porous metal‐organic framework can give rise to the development of new functional materials. In this work, by intercalating the stilbazolium‐type dye (DEAST)I (4′‐diethylamino‐N‐methyl stilbazolium) into four lanthanide layered metal‐organic complexes (Ln‐LMOCs), i. e. {[Ln(BTB)(H₂O)₂]?3(DMF)?2(H₂O)}_n (Ln=La (1), Nd (2), Sm (3), Er (4)), four responsive (DEAST)I@Ln‐LMOC composites have been prepared, serving as multifunctional performance platform. The core–shell structures of (DEAST)I@Ln‐LMOC composites have been fully characterized by IR, UV/Vis, PXRD, SEM, TEM, TGA and ESR. Significantly, after intercalation of dyes, the (DEAST)I@Ln‐LMOC composites exhibit enhanced luminescent sensing properties in detecting Fe³⁺ with much higher water stabilities. The luminescent sensing behavior stems from the fluorescence resonance energy transfer (FRET) from the π‐electron‐rich BTB ligands to the Fe³⁺, and their higher water stabilities are induced by electrostatic interactions and lower porosity. Specially, the characteristic emissions of Sm³⁺ will not be affected after the encapsulation guest dyes, which provide a theoretical guide for the modulation of luminescence devices. Finally, better ion conductivities and diminished photocurrents can be achieved after the embedding of the functional organic dye. In all, the formation of (DEAST)I@Ln‐LMOC composites with core–shell structures can be utilized as a multifunctional platform with good stability.     

Superionic Conduction over a Wide Temperature Range in a Metal–Organic Framework Impregnated with Ionic Liquids

Most molecules in confined spaces show markedly different behaviors from those in the bulk. Large pores are composed of two regions: an interface region in which liquids interact with the pore surface, and a core region in which liquids behave as bulk. The realization of a highly mobile ionic liquid (IL) in a mesoporous metal–organic framework (MOF) is now reported. The hybrid shows a high room‐temperature conductivity (4.4×10^?3 S cm^?1) and low activation energy (0.20 eV); both not only are among the best values reported for IL‐incorporated MOFs but also are classified as a superionic conductor. The conductivity reaches over 10^?2 S cm^?1 above 343 K and follows the Vogel–Fulcher–Tammann equation up to ca. 400 K. In particular, the hybrid is advantageous at low temperatures (<263 K), where the ionic conduction is superior to that of bulk IL, making it useful as solid‐state electrolytes for electrochemical devices operating over a wide temperature range.     

A Fluorescent Zirconium‐Based Metal‐Organic Framework for Selective Detection of Nitro Explosives and Metal Ions

In this work, a new porous Zr‐based metal‐organic framework (MOF ) with a large Brunner‐Emmet‐Teller (BET ) surface area was prepared by the solvothermal method using 4,4’‐(naphthalene‐1,4‐diyl)dibenzoic acid (NDDA ) as the organic ligand, and the luminescent detection performance was studied systematically. The experiments combing with computations indicate that the as‐synthesized material can sensitively and selectively detect nitro explosives and metal ions, especially for 2,4,6‐trinitrophenol (TNP ) and Fe³⁺, due to the possible electron transfer from inorganic moieties to organic moieties with naphthalene part. Interestingly, owing to its high porosity and large surface area, this Zr‐MOF showed quick luminescent response time (in 1 min) for TNP and Fe³⁺. The results obtained may provide useful information for the design of MOFs with the large permanent porosity in sensing applications for large molecules in the future.     

In Situ Enzyme Immobilization with Oxygen-Sensitive Luminescent Metal–Organic Frameworks to Realize “All-in-One” Multifunctions

Metal–organic frameworks (MOFs) for enzyme immobilization have already shown superior tunable and designable characteristics, however, their diverse responsive properties have rarely been exploited. In this work we integrated a responsive MOF into a MOF–enzyme composite with the purpose of designing an “all-in-one” multifunctional composite with catalytic and luminescence functions incorporated into a single particle. As a proof-of-concept, glucose oxidase (GOx) was encapsulated in situ within an oxygen (O₂)-sensitive, noble-metal-free, luminescent Cu^I triazolate framework (MAF-2), denoted as GOx@MAF-2. Owing to the rigid scaffold of MAF-2 and confinement effect, the GOx@MAF-2 composite showed significantly improved stability (shelf life of 60 days and heat resistance up to 80 °C) as well as good selectivity and recyclability. More importantly, owing to the O₂ sensitivity of MAF-2, the GOx@MAF-2 composite exhibited a rapid and reversible response towards dissolved O₂, thereby allowing direct and ratiometric sensing of glucose without the need for chromogenic substrates, cascade enzymatic reactions, or electrode systems. High sensitivity with a detection limit of 1.4 μm glucose was achieved, and the glucose levels in human sera were accurately determined. This strategy has led to a new application for MOFs that can be facilely extended to other MOF–enzyme composites due to the multifunctionality of MOFs.     

Eye‐Catching Dual‐Fluorescent Dynamic Metal–Organic Framework Senses Traces of Water: Experimental Findings and Theoretical Correlation

A guest‐dependent dynamic fivefold interpenetrated 3D porous metal–organic framework (MOF) of Zn^II ions has been synthesized that exhibits selective carbon dioxide adsorption. Furthermore, the MOF shows excellent luminescence behavior, which is supported by a systematic study on the guest‐responsive multicolor emission of a suspension of the MOF. The dual‐emission behavior arises from the excited‐state intramolecular proton transfer (ESIPT), and the compound also shows remarkable potential to detect traces of water in various organic solvents. The experimental observations were also painstakingly authenticated by using time‐dependent density‐functional‐theory (DFT) calculations.     

A Metal–Organic Framework Containing Unusual Eight‐Connected Zr–Oxo Secondary Building Units and Orthogonal Carboxylic Acids for Ultra‐sensitive Metal Detection

Two metal–organic frameworks (MOFs) with Zr–oxo secondary building units (SBUs) were prepared by using p,p′‐terphenyldicarboxylate (TPDC) bridging ligands pre‐functionalized with orthogonal succinic acid (MOF‐ 1 ) and maleic acid groups (MOF‐ 2 ). Single‐crystal X‐ray structure analysis of MOF‐ 1 provides the first direct evidence for eight‐connected SBUs in UiO‐type MOFs. In contrast, MOF‐ 2 contains twelve‐connected SBUs as seen in the traditional UiO MOF topology. These structural assignments were confirmed by extended X‐ray absorption fine structure (EXAFS) analysis. The highly porous MOF‐ 1 is an excellent fluorescence sensor for metal ions with the detection limit of <0.5 ppb for Mn²⁺and three to four orders of magnitude greater sensitivity for metal ions than previously reported luminescent MOFs.     

A Redox‐Active 2D Metal–Organic Framework for Efficient Lithium Storage with Extraordinary High Capacity

Metal–organic framework cathodes usually exhibit low capacity and poor electrochemical performance for Li‐ion storage owing to intrinsic low conductivity and inferior redox activity. Now a redox‐active 2D copper–benzoquinoid (Cu‐THQ) MOF has been synthesized by a simple solvothermal method. The abundant porosity and intrinsic redox character endow the 2D Cu‐THQ MOF with promising electrochemical activity. Superior performance is achieved as a Li‐ion battery cathode with a high reversible capacity (387 mA h g^?1), large specific energy density (775 Wh kg^?1), and good cycling stability. The reaction mechanism is unveiled by comprehensive spectroscopic techniques: a three‐electron redox reaction per coordination unit and one‐electron redox reaction per copper ion mechanism is demonstrated. This elucidatory understanding sheds new light on future rational design of high‐performance MOF‐based cathode materials for efficient energy storage and conversion.     

Dimerization of Organic Dyes on Luminescent Gold Nanoparticles for Ratiometric pH Sensing

Synergistic effects arising from the conjugation of organic dyes onto non‐luminescent metal nanoparticles (NPs) have greatly broadened their applications in both imaging and sensing. Herein, we report that conjugation of a well‐known pH‐insensitive dye, tetramethyl‐rhodamine (TAMRA), to pH‐insensitive luminescent gold nanoparticles (AuNPs) can lead to an ultrasmall nanoindicator that can fluorescently report local pH in a ratiometric way. Such synergy originated from the dimerization of TAMRA on AuNPs, of which geometry was very sensitive to surface charges of the AuNPs and can be reversely modulated through protonation of surrounding glutathione ligands. Not limited to pH‐insensitive dyes, this pH‐dependent dimerization can also enhance the pH sensitivity of fluorescein, a well‐known pH‐sensitive dye, within a larger pH range, opening up a new pathway to design ultrasmall fluorescent ratiometric nanoindicators with tunable wavelengths and pH response ranges.