Luminescent Guest Research Articles

Preparation of Zn-based MOFs photoluminescent coatings on cotton fabrics and their application in anti-counterfeiting

In this paper, a series of new photoluminescent coatings on cotton fabrics were prepared by integrating Zn-based MOFs (ZnBDC), and ZnBDC was encapsulated with luminescent guests (Eu3+ and crystal violet (CV)). The results showed that a large amount of ZnBDC nanomaterials could effectively grow on cotton fabric, and the encapsulation of luminescent guests did not damage their structural integrity and morphology. The emission spectra of different encapsulated samples exhibited different fluorescence under excitation at 321 nm. Notably, different from the single Eu3+ emission observed in single Eu3+-encapsulated ZnBDC cotton fabric, the emission intensity of Eu3+ in dual (Eu3+ and CV) encapsulated sample changed significantly at different bands, and it dominated by an unusual 5D0 → 7F4 transition (693 nm). The transformation of a specific symmetric site of Eu3+ is responsible for this unusual emission. Moreover, the fluorescence color of encapsulated samples could be coded by regulating the concentration of encapsulated CV according to the special features of Eu3+ photoluminescence. Therefore, photoluminescent cotton fabrics have a potential application in the anti-counterfeiting of textiles.

Exploring the Photophysical and Mechanical Behavior of Fluorescent Metal-Organic Framework Monoliths.

Luminescent metal-organic frameworks exhibit great potential as materials for nanophotonic applications because of their programmable properties and tunable structures. In particular, luminescent guests (LG) can be hosted by metal-organic frameworks due to their porosity and guest confinement capacity, forming LG@MOF composite systems. However, such guest-host systems are mainly produced as loose powders, preventing their widespread use in practical devices and technological applications that require implementation of a stable continuum solid. In this regard, using monolithic MOF hosts might be a workable option to solve this challenge. Herein, we reported the facile synthesis and fabrication of novel prototypical sol-gel monolithic systems, designated as LG@monoMOF. Red (rhodamine B), blue (7-methoxycoumarin), and yellow (fluorescein) emitting dyes were encapsulated in a robust UiO-66 monolithic host, resulting in the red, blue, and yellow light-emitting luminescent monoliths. The mechanical and photophysical characterization of the three LG@monoMOF systems was systematically carried out in order to unravel the role of guest-host interactions in the mechanical and optical response of the bespoke LG@monoMOF composites.

Waterproof Room‐Temperature Phosphorescence Films by Host‐Guest Inclusion and Hydrogen Bonding Network

AbstractSupramolecular room temperature phosphorescence (RTP) materials are attractive due to their excellent RTP characteristics and water resistance. By evaluating the photophysical properties of HA/BisNpI‐doped materials, it is proved that hyaluronic acid (HA) is a promising matrix and the rigid plane N,N′‐(1,4‐phenylene)‐bis(1,8‐naphthalimide) (Ph‐BisNpI) is an excellent luminescent guest molecule for constructing RTP materials. Moreover, it is found that γ‐cyclodextrin (γ‐CD) can successfully bind BisNpI guest molecule. Accordingly, a tightly arranged ternary supramolecular system with HA as matrix, Ph‐BisNpI as the guest, and γ‐CD as the host is constructed by host‐guest interaction and hydrogen bonding. As expected, the obtained ternary supramolecular system HA/Ph@CD8 displays a superior RTP performance (52.7 ms longer lifetime than HA/Ph), water‐resistance, and even aqueous RTP (τ = 207 µs), which significantly extends the application of RTP in aqueous solution. These results are possibly attributed to the inclusion of γ‐CD on Ph‐BisNpI guest molecules and the self‐assembly of γ‐CD and HA to form a rigid hydrogen bonding network, which effectively inhibits the non‐radiative decay of Ph‐BisNpI and shields the triplet excitons from water and oxygen. Furthermore, the prepared ternary supramolecular RTP materials are successfully used as anticounterfeiting ink and exhibit broader application prospects in water environments.

Unveiling MOF-808 photocycle and its interaction with luminescent guests.

The world of metal-organic frameworks (MOFs) has become a hot topic in recent years due to the extreme variety and tunability of their structures. There is evidence of MOFs that exhibit intrinsic luminescence properties that arise directly from their organic components or from the interaction between them and metallic counterparts. A new perspective is to exploit the porous nature of MOFs by encapsulating luminescent guests, such as organic dyes, in order to explore possible changes in the luminescence activity of the combined systems. This work is focused on the optical study of zirconium-based MOF-808 and its interaction with encapsulated rhodamine B molecules. Using a plethora of different techniques, we were able to unravel its photocycle. MOF-808 displays intrinsic luminescence activity that derives from an energy transfer process from the linker to the metal sites occurring in 300 ps. The emission is a singlet-singlet transition in aqueous solution, and it is a triplet transition in powdered form. After exploring the bare MOF, we combined it with rhodamine B molecules, following an easy post-synthetic process. Rhodamine B molecules were found to be encapsulated in MOF pores and interact with the MOF's matrix through nanosecond energy transfer. We created a totally new dual-emitting system and suggested a way, based on the time-resolved studies, to clearly unravel the photocycle of MOFs from the very first photoexcitation.

Bidirectional photomagnetism, exciplex fluorescence and dielectric anomalies in a spin crossover Hofmann-type coordination polymer.

Stepped spin crossover (SCO) complexes with three or more spin states have promising applications in high-order data storage, multi-switches and multi-sensors. Further synergy with other functionalities, such as luminescence and dielectric properties, will provide a good chance to develop novel multifunctional SCO materials. Here, a bent pillar ligand and luminescent pyrene guest are integrated into a three-dimensional (3D) Hofmann-type metal-organic framework (MOF) [Fe(dpoda){Au(CN)2}2]·pyrene (dpoda = 2,5-di-(pyridyl)-1,3,4-oxadiazole). The magnetic data show an incomplete and two-step SCO behavior with the sequence of 1 ↔ 1/2 ↔ 1/4. The rare bi-directional light-induced excited spin-state trapping (LIESST) effect and light-induced stepped thermal relaxation after LIESST are observed. The pyrene guests interact with dpoda ligands via offset face-to-face π⋯π interactions to form intermolecular exciplex emissions. The competition between thermal quenching and stepped SCO properties results in a complicated and stepped exciplex fluorescence. Moreover, the stepped dielectric property with higher dielectric permittivity at lower temperature may be related to the more frustrated octahedral distortion parameters in the intermediate spin states. Hence, a 3D Hofmann-type MOF with bent pillar ligands and fluorescent guests illustrates an effective way for the development of multifunctional switching materials.

One-pot synthesis of aggregation-induced multi-emission and solid-state room-temperature-phosphorescence carbon dots

Multi-emission carbon dots (M-CDs) have broad applications. However, reports of the synthesis of M-CDs and their emission mechanism are limited. In this study, M-CDs with three emissions are synthesized by a one-pot solvothermal method, and the luminescence mechanism is investigated in detail. The prepared M-CDs show significant concentration-dependent properties. For high-concentration M-CDs (HC CDs), the surface-defect and carbon-core luminescence states are related to emission centers at 423 and 586 nm, respectively. The emission center at 493 nm are mainly due to the self-absorption of M-CDs. Conversely, low-concentration M-CDs (LC CDs) have 423 and 586 nm emissions. The blue CDs (B-CDs) and red CDs (R-CDs) are extracted from the prepared M-CD solution. The structural and optical properties indicate that the 493 nm peak is related to aggregation and self-absorption between B-CDs and R-CDs in HC CDs. Surprisingly, solid CDs (S-CDs) with solid-state room-temperature phosphorescence (RTP) are simultaneously obtained during the preparation of the M-CDs. The mechanism investigation results show that S-CDs are composed of CDs and NaHCO3, and NaHCO3 acts as an oxygen barrier to prevent the quenching of the triplet and rigidly holds the luminescent guests to suppress vibrational dissipation. The M-CDs are successfully applied to detect Morin, while the S-CDs are demonstrated for optical anti-counterfeiting applications.

Carbon dots@zeolitic imidazolate framework nanoprobe for direct and sensitive butyrylcholinesterase assay with non-enzymatic reaction

Butyrylcholinesterase (BChE) was clinically considered an important biomarker for the diagnosis of liver diseases. Ellman's strategy (enzyme-catalyzed reaction) is normally utilized as a standard protocol for BChE activity sensing due to the different properties of substrate choline (Ch) and product thiocholine (TCh). However, there remain respective drawbacks besides the laborious operation procedure and time-consuming. We herein report the first example of using nanoscale zeolitic imidazolate framework (ZIF-8), self-assembled from 2-Methylimidazole (2-MIm) and Zn2+, to directly and target detect BChE. Encapsulation of fluorescent carbon dots (CDs) into ZIF-8 suppresses the emission of CDs, while the competitive coordination between BChE and the metal node of ZIF-8 can result in the release of CDs for BChE sensing. The sensing mechanism was studied in depth via experimental investigations, and multifarious luminescent guests, metal nodes, and organic ligands were employed to confirm that the rupture of ZIF-8 was due to the connection of BChE and Zn2+ caused by electrostatic attraction. The limit of detection (LOD) as low as 1.5 U L−1 was achieved with this sensing platform. Finally, this platform was applied to quantify BChE in 40 clinical serum samples of patients with clinical liver disease and showed great consistency with clinical diagnostic results.

Luminescent Guests Encapsulated in Metal–Organic Frameworks for Portable Fluorescence Sensor and Visual Detection Applications: A Review

Metal-organic frameworks (MOFs) have excellent applicability in several fields and have significant structural advantages, due to their open pore structure, high porosity, large specific surface area, and easily modifiable and functionalized porous surface. In addition, a variety of luminescent guest (LG) species can be encapsulated in the pores of MOFs, giving MOFs a broader luminescent capability. The applications of a variety of LG@MOF sensors, constructed by doping MOFs with LGs such as lanthanide ions, carbon quantum dots, luminescent complexes, organic dyes, and metal nanoclusters, for fluorescence detection of various target analyses such as ions, biomarkers, pesticides, and preservatives are systematically introduced in this review. The development of these sensors for portable visual fluorescence sensing applications is then covered. Finally, the challenges that these sectors currently face, as well as the potential for future growth, are briefly discussed.

Dual-emitting cellulose nanocrystal hybrid materials with circularly polarized luminescence for anti-counterfeiting labels

Cellulose nanocrystal (CNC) hybrid materials with numerous optical states have great potential as anti-counterfeiting labels and information encryption materials. However, it is challenging to construct multicolor emitting materials with tunable behaviors, which can dramatically enhance anti-counterfeiting abilities. Here, free-standing composite films with vivid multi-structural colors and dual-emitting fluorescence are successfully fabricated through a host-guest coassembly strategy. The lanthanide complex and an aggregation-induced emission molecule (tetraphenylethylene derivative, TPEC) are selected as luminescent guests, which are integrated into the chiral nematic structure of CNCs. The obtained photonic films display broadband reflection across the visible spectrum, which may be attributed to the chiral nematic domains with variations in the helical pitches and helical axis orientations. Under 254 nm excitation, the film exhibits bright red emission, while blue–green emission switching occurs under 365 nm excitation. The broad reflection band of the film covers both the green and red fluorescent emission centers, and right circularly polarized luminescence emission with different dissymmetry factors is produced due to the selective reflection of the left chiral nematic structure. A large glum value up to −0.21 at 600 nm was realized. Additionally, CNC-based materials with tailored shapes are further used in anti-counterfeit tags and decorative applications.

Recent Advances in Organic Room Temperature Phosphorescence of Triphenylamine‐Based Doping Systems

AbstractThe host–guest doping strategy has become one of the most effective approaches to realize pure organic room temperature phosphorescence (RTP). Crystalline small molecules can provide a rigid environment that restricts the molecular motion of luminescent guests, thus enhancing phosphorescence emission. Triphenylamine (TPA) is a classic crystalline small molecule matrix that, when doped with designed guest molecules, enables tunable RTP properties and diverse functionalities. These properties afford TPA‐based doping systems with numerous applications in anticounterfeiting, bio‐imaging, and multiple responsive sensing. In this concept, design strategies, luminescence mechanisms, and applications of TPA‐based host‐guest doping systems are summarized. Moreover, the future prospects of TPA‐based RTP materials are proposed.

Circularly polarized luminescence in molecular recognition systems: Recent achievements.

Circularly polarized luminescence (CPL) dyes are recognized to be new generation materials and have been actively developed. Molecular recognition systems provide nice approaches to novel CPL materials, such as stimuli-responsive switches and chemical sensing materials. CPL may be induced simply by mixing chiral or achiral, luminescent or nonluminescent host and guest; there are several combinations. Molecular recognition can potentially save time and effort to construct well-ordered chiral structures with noncovalent attractive interactions as compared with the multi-step synthesis of covalently bonded dyes. It is a challenging subject to engage molecular recognition events with CPL, and it is important and interesting to see how it is achieved. In fact, simple molecular recognition systems can even enable the fine adjustment of CPL performance and detailed conformational/configurational analysis ofthe excited state. Here we overview the recent achievements of simple host-guest complexes capable of exhibiting CPL, summarizing concisely the host/guest structures, CPL intensities, and characteristics.

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.

High‐Efficiency Luminescent Solar Concentrators Based on Antifogging and Frost‐Resisting Fluorescent Polymers: Adding Multiple Functions for Sustained Performance

AbstractLuminescent solar concentrators (LSCs) are light‐management devices able to harvest and downshift sunlight, making it available to edge‐coupled photovoltaic cells for light‐to‐electricity conversion. When operating in real‐life outdoor scenarios, LSCs can be exposed to humid/freezing environments which may yield fogging and frosting, ultimately resulting in detrimental performance decay owing to reduced photon absorption and photon trapping efficiency within the waveguide. To address this issue, the first demonstration of an antifogging/frost‐resisting highly efficient thin‐film LSC is presented in this work, based on a tailored semi‐interpenetrating polymer network serving as the host matrix material, where the antifogging behavior is imparted by the simultaneous presence of hydrophilic and hydrophobic segments in the macromolecular structure, whereas the fluorescent response is achieved via the incorporation of a coumarin‐functionalized methacrylic monomer. By judicious selection and compositional tuning of an additional luminescent guest dopant with excellent spectral match with the fluorescent host matrix, highly efficient (≈98%) energy transfer is achieved. Optimized LSC devices exhibit external and internal photon efficiencies as high as 3.96% and 34.79%, respectively. In addition, persistent and reproducible antifogging/frost‐resisting functionality is demonstrated during aggressive aging tests under UV‐light, with preserved fluorescence emission and prolonged and sustained device efficiency.

Phenylpyrimidine-Based Iridium Complex for High-Performance Green-Yellow Phosphorescent Organic Light-Emitting Diodes

In this work, a phenylpyrimidine-based green-yellow iridium complex, (bpp)2Ir(acac), is prepared. Tert-butyl group is introduced to weaken the intermolecular interaction and endow (bpp)2Ir(acac) with favorable physical properties. Phosphorescent device which utilizes (bpp)2Ir(acac) as the luminescent guest material displays high efficiencies (73.9 cd/A, 84.8 lm/W, 21.2%), low efficiency roll-off (from 100 to 10000 cd/m2: 5.1%), relatively low concentration sensitivity (from 5% to 12%) and very stable electroluminescent spectra (from 1 cd/m2 to maximum luminance). As a consequence of the results proved that (bpp)2Ir(acac) is a good phosphorescent material.

Crystalline‐State Solvent: Metal‐Organic Frameworks as a Platform for Intercepting Aggregation‐Caused Quenching

Comprehensive SummaryThe sequestration of organic luminescent molecules (OLMs) within cage‐based metal‐organic frameworks (MOFs) as a dispersion platform has been developed to impede aggregation‐caused quenching (ACQ). The homogenous encapsulation of distinct luminescent guests of different sizes and emissive behaviors in the cage structure of a MOF resulted in high fluorescent quantum yields of 44.8% for DAPI@NKU‐110 (DAPI = 4',6‐diamidino‐2‐phenylindole), 65.4% for TPPA@NKU‐110 (TPPA = tris(4‐(pyridin‐4‐ yl)phenyl)amine), 31.3% for R6G@NKU‐110 (R6G = Rhodamine 6G), and 58.3% for PY@NKU‐110 (PY = Pyronin Y), attributable to the confinement effect caused by the rigid cages of NKU‐110. More significantly, a positive correlation of the encapsulated quantity of OLMs with their concentration in the in‐situ solvothermal reaction was unveiled by spectral analysis and utilized to facilely fabricate a white‐light‐emitting crystal material TPPA+R6G@NKU‐110. This material features a large crystal size on the millimeter‐scale, broadband white emission, ideal CIE coordinates (0.33, 0.34), and a high quantum yield (49.1%) when excited at 365 nm. Moreover, such a strategy can be easily generalized to an abundance of other cage‐based MOFs and a plentiful volume of OLMs for the future development of colorful, high performance luminescent materials.

A Luminescent Guest@MOF Nanoconfined Composite System for Solid-State Lighting.

A series of rhodamine B (RhB) encapsulated zeolitic imidazolate framework-8 (RhB@ZIF-8) composite nanomaterials with different concentrations of guest loadings have been synthesized and characterized in order to investigate their applicability to solid-state white-light-emitting diodes (WLEDs). The nanoconfinement of the rhodamine B dye (guest) in the sodalite cages of ZIF-8 (host) is supported by fluorescence spectroscopic and photodynamic lifetime data. The quantum yield (QY) of the luminescent RhB@ZIF-8 material approaches unity when the guest loading is controlled at a low level: 1 RhB guest per ~7250 cages. We show that the hybrid (luminescent guest) LG@MOF material, obtained by mechanically mixing a suitably high-QY RhB@ZIF-8 red emitter with a green-emitting fluorescein@ZIF-8 “phosphor” with a comparably high QY, could yield a stable, intensity tunable, near-white light emission under specific test conditions described. Our results demonstrate a novel LG@MOF composite system exhibiting a good combination of photophysical properties and photostability, for potential applications in WLEDs, photoswitches, bioimaging and fluorescent sensors.

Optical Properties and Applications of Crystalline Materials

Optical Properties and Applications of Crystalline Materials

Preparation and photoluminescence of functionalized cotton fabric by double luminescent guests-encapsulated ZnBDC metal-organic framework

A series of functionalized cotton fabrics (LG-ZnBDC-CCtn) were prepared by attachment of ZnBDC metal-organic framework with LbL deposition method on carboxylated cotton fabric (CCtn) and further encapsulation of ZnBDC with different luminescent guests (LG = Tb3+, La3+, Rhodanmine B or/and acridine orange). The characterization of LG-ZnBDC-CCtn by XRD, SEM and EDS showed that the structural integrity and morphology of ZnBDC on the CCtn surface were not destroyed after encapsulation. The photoluminescence behavior of sample LG-ZnBDC-CCtn was investigated by fluorescence excitation and emission spectra. After encapsulated with different luminescent guests, samples LG-ZnBDC-CCtn emitted different fluorescence. Although sample Tb/RhB-ZnBDC-CCtn with double LGs (Tb3+ and RhB) showed almost same visible color with RhB-ZnBDC-CCtn under sunlight, the orange yellow fluorescence of sample Tb/RhB-ZnBDC-CCtn was quite different with that of the single LG encapsulated sample Tb-ZnBDC-CCtn (green) or RhB-ZnBDC-CCtn (rose). A similar phenomenon was also observed in La/AO-ZnBDC-CCtn system. The fluorescence change of sample Tb/RhB-ZnBDC-CCtn could be due to a two-step fluorescence resonance energy transfer (FRET) among ZnBDC framework and luminescent guests. The visible color-change properties and technical performance of sample LG-ZnBDC-CCtn were evaluated with LAB color space and color strength.

Fluorescence Photoswitching in a Series of Metal‐Organic Frameworks Loaded with Different Anthracenes

AbstractThe confinement of luminescent guest molecules in porous host materials can induce photophysical properties different from either component in isolation. In this work, we studied several host‐guest systems consisting of anthracene and its substituted analogs adsorbed in a series of metal‐organic frameworks (MOFs) and inorganic molecular sieves. Fluorescence photoswitching of the guest molecules through photoinduced dimerization is observed only in MOFs with a favorable pore volume and geometry.

Recent Progress in Luminous Particle‐Encapsulated Host–Guest Metal‐Organic Frameworks for Optical Applications

AbstractMetal‐organic frameworks (MOFs) as a unique class of porous crystalline materials have received comprehensive attention in the past decades. Among them, luminescent MOFs (LMOFs) have been a well‐known branch on account of their relevance as optical materials in contemporary era. The establishment of host–guest hybrid LMOF materials has triggered a hot search area owing to the extensive range of applications that can be adjusted by soliciting different functional luminescent guests. Current developments in the thought‐provoking application of host–guest LMOFs have stretched into far‐ranging fields, including light regulation, imaging, sensing, data encryption, etc. This review discusses on the hybridization of MOFs with functional luminescent particles (LPs) as guests for the construction of a series of extraordinary host–guest luminescent composite materials (LP@MOFs). The basic aspects, involving the preconcentration of MOF hosts and luminous guests as well as the comprehensive synthetic strategies of LP@MOFs are introduced. Furthermore, the applications of luminescent LP@MOF materials are highlighted, with a particular emphasis on the present and continuing challenges and the demand trends of such materials that need to be explored.