Hydro-Photo-Thermo-Responsive Multicolor Luminescence Switching of a Ternary MOF Hybrid for Advanced Information Anticounterfeiting

Developing smart materials capable of solid-state multicolor photoluminescence (PL) switching in response to multistimuli is highly desirable for advanced anticounterfeiting. Here, a ternary MOF hybrid showing hydro-photo-thermo-responsive multicolor PL switching in the solid state is presented. This hybrid is constructed by co-immobilizing Eu3+ and methyl viologen (MV) cations within an anionic MOF via the cation-exchange approach. The confined guest cations are well arranged in the framework channels, facilitating the synergistic realization of stimuli-responsive multiple PL color-switching through intermolecular coupling. The hybrid undergoes a rapid and reversible PL color-switching from red to blue upon water simulation, which is achieved by activating the blue emission of the framework linker while simultaneously quenching the Eu3+ emission. Furthermore, the hybrid displays photo-thermo-responsive PL switching from red to dark. UV-light irradiation or heating triggers the chromic conversion of MV to its colored radical form, which exhibits perfect spectral overlap with Eu3+, thus activating Förster resonance energy transfer (FRET) from Eu3+ to MV radicals and quenching the Eu3+ emission. Inspired by these results, PL morse patterns are designed and fabricated using a novel triple-level encryption strategy, showcasing the exciting potential of this hybrid in advanced anticounterfeiting applications.

Quest for Multifunctionality: Current Progress in the Characterization of Heterometallic Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are a class of porous, crystalline materials that have been systematically developed for a broad range of applications. Incorporation of two or more metals into a single crystalline phase to generate heterometallic MOFs has been shown to lead to synergistic effects, in which the whole is oftentimes greater than the sum of its parts. Because geometric proximity is typically required for metals to function cooperatively, deciphering and controlling metal distributions in heterometallic MOFs is crucial to establish structure-function relationships. However, determination of short- and long-range metal distributions is nontrivial and requires the use of specialized characterization techniques. Advancements in the characterization of metal distributions and interactions at these length scales is key to rapid advancement and rational design of functional heterometallic MOFs. This perspective summarizes the state-of-the-art in the characterization of heterometallic MOFs, with a focus on techniques that allow metal distributions to be better understood. Using complementary analyses, in conjunction with computational methods, is critical as this field moves toward increasingly complex, multifunctional systems.

Hybrid nanospheres of silica covalently containing yellow-emitting cationic iridium(III) complex: preparation and application in white light-emitting diodes

A yellow-emitting cationic iridium(III) complex [(dfppy)2Ir(TBD)]PF6 (TBD: N4,N4'-bis(3-(triethoxysilyl)propyl)-[2,2'-bipyridine]-4,4'-dicarboxamide; dfppy: 2-(2,4-difluorophenyl)pyridine) containing hydrolysable alkoxysilanes was synthesized. Then, a series of silica-based hybrid nanospheres with diameters of around 400 nm was prepared via the hydrolysis of this complex together with tetraethyl orthosilicate (TEOS, a silica source). When the amount of the complex used was 5.0 wt%, hybrid nanospheres showed the best photoluminescence (PL) properties, relative to the PL quantum yield of pure solid [(dfppy)2Ir(TBD)]PF6 (12.7%), that of hybrid nanospheres increased to 26.2%. Moreover, the thermal decomposition temperature (Td) of pure solid [(dfppy)2Ir(TBD)]PF6 was 331 °C, the Td of the complex in hybrid nanospheres increased to 447 °C. However, the yellow light emission was almost unchanged and was still located at 500-750 nm with a maximum wavelength (λem,max) of 577 nm. Under the excitation of blue-emitting chips (λem,max ≈ 455 nm), cold/neutral/warm white light-emitting diodes (WLEDs) with good luminous quality can all be fabricated using these hybrid nanospheres as phosphors in epoxy resin at different blending concentrations. Compared with two or three iridium(III) complexes being contained in silica-based particles as phosphors as described in literatures, in this study, silica-based hybrid nanospheres covalently containing only one yellow-emitting cationic iridium(III) complex as phosphors provide a more effective and simpler method for preparation high-performance WLEDs.

Highly Sensitive Ratiometric Fluorescent Flexible Sensor Based on the RhB@ZIF-8@PVDF Mixed-Matrix Membrane for Broad-Spectrum Antibiotic Detection

Antibiotics play an essential role in the treatment of various diseases. However, the overuse of antibiotics has led to the pollution of water bodies and food safety, affecting human health. Herein, we report a dual-emission MOF-based flexible sensor for the detection of antibiotics in water, which was prepared by first encapsulating rhodamine B (RhB) by a zeolite imidazolium ester skeleton (ZIF-8) and then blending it with polyvinylidene difluoride (PVDF). The luminescent properties, structural tunability, and flexible porosity of the MOF-based composites were combined with the processability and flexibility of polymers to prepare luminescent membranes. The sensor is capable of dual-emission ratiometric fluorescence sensing of nitrofurantoin (NFT) and oxytetracycline (OTC), exhibiting sensitive detection of fluorescence burst and fluorescence enhancement, respectively, with detection limits of 0.012 μM and 8.9 nM. With the advantages of visual detection, high sensitivity, short detection time, and simplicity, the highly sensitive ratiometric fluorescent flexible sensor has great potential for detecting antibiotics in an aqueous environment. It will further stimulate interest in luminescent MOF-based mixed matrix membranes and their sensing applications.

Fluorescence detection platform of metal-organic frameworks for biomarkers

Sensitive and selective detection of biomarkers is crucial in the study and early diagnosis of diseases. With the continuous development of biosensing technologies, fluorescent biosensors based on metal-organic frameworks have attracted increasing attention in the field of biomarker detection due to the combination of the advantages of MOFs, such as high specific surface area, large porosity, and structure with tunable functionality and the technical simplicity, sensitivity and efficiency and good applicability of fluorescent detection techniques. Therefore, researchers must understand the fluorescence response mechanism of such fluorescent biosensors and their specific applications in this field. Of all biomarkers applicable to such sensors, the chemical essence of nucleic acids, proteins, amino acids, dopamine, and other small molecules account for about a quarter of the total number of studies. This review systematically elaborates on four fluorescence response mechanisms: metal-centered emission (MC), ligand-centered emission (LC), charge transfer (CT), and guest-induced luminescence change (GI), presenting their applications in the detection of nucleic acids, proteins, amino acids, dopamine, and other small molecule biomarkers. In addition, the current challenges of MOFs-based fluorescent biosensors are also discussed, and their further development prospects are concerned.

Confinement of Luminescent Guests in Metal–Organic Frameworks: Understanding Pathways from Synthesis and Multimodal Characterization to Potential Applications of LG@MOF Systems

This review gives an authoritative, critical, and accessible overview of an emergent class of fluorescent materials termed “LG@MOF”, engineered from the nanoscale confinement of luminescent guests (LG) in a metal–organic framework (MOF) host, realizing a myriad of unconventional materials with fascinating photophysical and photochemical properties. We begin by summarizing the synthetic methodologies and design guidelines for representative LG@MOF systems, where the major types of fluorescent guest encompass organic dyes, metal ions, metal complexes, metal nanoclusters, quantum dots, and hybrid perovskites. Subsequently, we discuss the methods for characterizing the resultant guest–host structures, guest loading, photophysical properties, and review local-scale techniques recently employed to elucidate guest positions. A special emphasis is paid to the pros and cons of the various methods in the context of LG@MOF. In the following section, we provide a brief tutorial on the basic guest–host phenomena, focusing on the excited state events and nanoscale confinement effects underpinning the exceptional behavior of LG@MOF systems. The review finally culminates in the most striking applications of LG@MOF materials, particularly the “turn-on” type fluorochromic chemo- and mechano-sensors, noninvasive thermometry and optical pH sensors, electroluminescence, and innovative security devices. This review offers a comprehensive coverage of general interest to the multidisciplinary materials community to stimulate frontier research in the vibrant sector of light-emitting MOF composite systems.

Multi-Emission from Single Metal-Organic Frameworks under Single Excitation

Multi-emission materials have come to prominent attention ascribed to their extended applications other than single-emission ones. General and robust design strategies of a single matrix with multi-emission under single excitation are urgently required. Metal-organic frameworks (MOFs) are porous materials prepared with organic ligands and metal nodes. The variety of metal nodes and ligands makes MOFs with great superiority as multi-emission matrices. Guest species encapsulated into the channels or pores of MOFs are the additional emission sites for multi-emission. In this review, multi-emission MOFs according to the different excitation sites are summarized and classified. The emission mechanisms are discussed, such as antenna effect, excited-state intramolecular proton transfer (ESIPT) and tautomerism for dual-emission. The factors that affect the emissions are revealed, including ligand-metal energy transfer and host-guest interaction, etc. Multi-emission MOFs could be predictably designed and prepared, once the emissive factors are controlled rationally in combination with the different multi-emission mechanisms. Correspondingly, new and practical applications are realized, including but not limited to ratiometric/multi-target sensing and bioimaging, white light-emitting diodes, and anti-counterfeiting. The design strategies of multi-emission MOFs and their extensive applications are reviewed. The results will shed light on other multi-emission systems to develop the structure-derived functionality and applications.

Multifunctional lanthanide MOF luminescent sensor built by structural designing and energy level regulation of a ligand

In order to reduce usage cost and simplify the detection process, it is necessary to develop multifunctional and multi-emitter Ln-MOF luminescent sensors.

Luminescent MOFs (LMOFs): Recent Advancement Towards a Greener WLED Technology

The replacement of the traditional incandescent, halogen and fluorescent lamps by white light emitting diodes (WLEDs) is expected to reduce the global electricity consumption by one-third by 2030, according to...

Phase selectivity and tunable photophysical nature of rare earth metal-organic frameworks of EuxY1-x-PTC (H3PTC = 2,4,6-pyridine tricarboxylic acid; x = 0-1)

Two rare earth metal-organic frameworks (MOFs), [Y2(PTC)2(H2O)2]·3H2O (Y-PTC) and [Eu2(PTC)2(H2O)5]·H2O (Eu-PTC) together with the solid solutions [Eu2xY2(1-x)(PTC)2(H2O)5]·H2O (EuxY1-x-PTC, x = 0.013-0.82), were synthesized hydrothermally, and characterized by microanalysis, IR spectroscopy, TG, powder, and single crystal X-ray diffraction techniques. Eu-PTC and Y-PTC showed different crystal structures; however, all solid solutions were isomorphic to Eu-PTC even at x = 0.013, leading to the IR spectra and TG plots of the solid solutions to be similar to those of Eu-PTC but distinct from those of Y-PTC. DFT calculations for the crystal lattice energy demonstrated that the procedure for the crystallizing nucleation of Eu-PTC occurred prior to that of Y-PTC in the reaction solution, leading to the all solid solutions being isomorphic to Eu-PTC. The solid emission spectra at ambient condition showed that Y-PTC emitted ligand-based phosphorescence at 433 nm with a quantum yield (QY) of 27.02%, while Eu-PTC and EuxY1-x-PTC (x = 0.013-0.82) emitted the characteristic luminescence of Eu3+ ions, and most solid solutions showed higher QYs than Eu-PTC; in particular, the QY of Eu0.195Y0.805-PTC was up to 29.48%, i.e., increased by 10% regarding Eu-PTC (19.86%). Interestingly, solid solutions with x = 0.013-0.395 showed excitation-wavelength-dependent luminescence, and such type of luminescence MOFs have promising applications including the areas of precise temperature, gas sensing and information encryption or anti-counterfeiting materials.

Eu-based coordination polymer microrods for low-loss optical waveguiding application

Lanthanide-based coordination polymers (CPs) have received great attention due to their tuneable structures and excellent luminescence properties. However, limited by the stability and micro/nanoscale morphology, a very small number of lanthanide-based CPs have been used for photonic applications. Herein, we present the synthesis of Eu-based CPs (compound 2) with highly regular one-dimensional (1D) microrod morphology by in situ structure transformation from compound 1. Moreover, the Eu-based CP microrods have an excellent stability and a high photoluminescence quantum yield (PLQY), and the distance-dependent PL spectra also exhibited typical characteristics of photoactive waveguides with a low optical-loss coefficient (0.00894 dB μm^-1) during propagation. These intriguing behaviors not only extend the research field of optical waveguides through lanthanide-based CPs, but also provide an opportunity for further application in optical communication systems.

White Light from Blue Fluorescence and Sensitized Yellow Long-Afterglow Phosphorescence of o-Terphenyl in Its π-Acid···Base Adduct with Ag3Pz3

White-light emission with CIE (0.32, 0.34) was realized for an o-terphenyl (oTP) molecule in its Ag₃[3,5-(CF₃)₂Pz]₃ (Pz = pyrazolate) sandwiching adduct, originating from the hybrid of fluorescence and sensitized long-afterglow phosphorescence of oTP. Density functional theory (DFT) and time-dependent DFT calculations revealed that heavy-atom effects of Ag play a crucial role in sensitization of the triplet of oTP, giving off its ultralong phosphorescence.

A stable luminescent zinc–organic framework as a dual-sensor toward Cu2+ and Cr2O72−, and excellent platform-encapsulated Ln3+ for systematic color tuning and white-light emission

A stable three-fold interpenetrated framework (Zn-MOF), showing chemical sensing for Cu²⁺ and Cr₂O₇²⁻, tunable luminescence and white-light emission after encapsulating lanthanide cations, was synthesized and characterized.

Novel single component tri-rare-earth emitting MOF for warm white light LEDs

Single-phase white phosphors could overcome many drawbacks of traditional phosphors, and warm white light phosphors have been considered suitable for indoor illumination applications. Thus, synthesizing new single-phase warm white phosphors is significant for the production of LEDs. Based on the above considerations, a novel single component warm white light phosphor SmxTbyDy0.2-x-y-metal organic frameworks (MOFs) has been prepared successfully in this study. The compound Sm0.1Tb0.04Dy0.06-MOF shows ideal warm white light emission with CIE coordinates (0.333, 0.3522), a color rendering index (CRI) value of 86.7, a low correlated color temperature (CCT) value of 4444 K, and good heating/cooling circulation. In addition, the LED devices fabricated with this novel warm white light phosphor have excellent warm white light quality even at high temperatures, which is necessary for its practical application.

Controllable synthesis of isoreticular pillared-layer MOFs based on N-rich triangular prism building units: gas adsorption and luminescent properties

Isoreticular 1–4 MOFs with rare lon topologies have been rationally synthesized based on novel trigonal-prism building units.

Warm-White-Light-Emitting Diode Based on a Dye-Loaded Metal-Organic Framework for Fast White-Light Communication

A dye@metal-organic framework (MOF) hybrid was used as a fluorophore in a white-light-emitting diode (WLED) for fast visible-light communication (VLC). The white light was generated from a combination of blue emission of the 9,10-dibenzoate anthracene (DBA) linkers and yellow emission of the encapsulated Rhodamine B molecules. The MOF structure not only prevents dye molecules from aggregation-induced quenching but also efficiently transfers energy to the dye for dual emission. This light-emitting material shows emission lifetimes of 1.8 and 5.3 ns for the blue and yellow components, respectively, which are significantly shorter than the 200 ns lifetime of Y₃Al₅O₁₂:Ce³⁺ in commercial WLEDs. The MOF-WLED device exhibited a modulating frequency of 3.6 MHz for VLC, six times that of commercial WLEDs.

Introduction of Red-Green-Blue Fluorescent Dyes into a Metal-Organic Framework for Tunable White Light Emission

The unique features of the metal-organic frameworks (MOFs), including ultrahigh porosities and surface areas, tunable pores, endow the MOFs with special utilizations as host matrices. In this work, various neutral and ionic guest dye molecules, such as fluorescent brighteners, coumarin derivatives, 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM), and 4-(p-dimethylaminostyryl)-1-methylpyridinium (DSM), are encapsulated in a neutral MOF, yielding novel blue-, green-, and red-phosphors, respectively. Furthermore, this study introduces the red-, green-, and blue-emitting dyes into a MOF together for the first time, producing white-light materials with nearly ideal Commission International ed'Eclairage (CIE) coordinates, high color-rendering index values (up to 92%) and quantum yields (up to 26%), and moderate correlated color temperature values. The white light is tunable by changing the content or type of the three dye guests, or the excitation wavelength. Significantly, the introduction of blue-emitting guests in the methodology makes the available MOF host more extensive, and the final white-light output more tunable and high-quality. Such strategy can be widely adopted to design and prepare white-light-emitting materials.