Long Persistent Luminescence Materials Research Articles

Full-color, time-valve controllable and Janus-type long-persistent luminescence from all-inorganic halide perovskites

Long persistent luminescence (LPL) has gained considerable attention for the applications in decoration, emergency signage, information encryption and biomedicine. However, recently developed LPL materials – encompassing inorganics, organics and inorganic-organic hybrids – often display monochromatic afterglow with limited functionality. Furthermore, triplet exciton-based phosphors are prone to thermal quenching, significantly restricting their high emission efficiency. Here, we show a straightforward wet-chemistry approach for fabricating multimode LPL materials by introducing both anion (Br−) and cation (Sn2+) doping into hexagonal CsCdCl3 all-inorganic perovskites. This process involves establishing new trapping centers from [CdCl6-nBrn]4− and/or [Sn2-nCdnCl9]5− linker units, disrupting the local symmetry in the host framework. These halide perovskites demonstrate afterglow duration time ( > 2,000 s), nearly full-color coverage, high photoluminescence quantum yield ( ~ 84.47%), and the anti-thermal quenching temperature up to 377 K. Particularly, CsCdCl3:x%Br display temperature-dependent LPL and time-valve controllable time-dependent luminescence, while CsCdCl3:x%Sn exhibit forward and reverse excitation-dependent Janus-type luminescence. Combining both experimental and computational studies, this finding not only introduces a local-symmetry breaking strategy for simultaneously enhancing afterglow lifetime and efficiency, but also provides new insights into the multimode LPL materials with dynamic tunability for applications in luminescence, photonics, high-security anti-counterfeiting and information storage.

Deciphering the luminescence mechanism: Investigating ESIPT-attributed HPI2C molecule with coupled thermally activated delayed fluorescence and phosphorescence properties

Long persistent luminescence (LPL) including thermally activated delayed fluorescence (TADF) and phosphorescence is widely applied in luminogens. Recently, Pan et al. pioneered the discovery of TADF and phosphorescence in the HPI2C molecule by manipulating temperature in line with the excited-state intramolecular proton transfer (ESIPT) dynamics (J. Am. Chem. Soc. 2022, 144, 2726). They blazed a new trail to explore the switching mechanism between TADF and phosphorescence in single molecule. Nevertheless, luminescence mechanism for HPI2C cannot be explained solely through experimental means. Theoretically, we demonstrate that the ESIPT and photocyclization channel induced conical intersection processes facilitate the fluorescence quenching of the Enol* state. Moreover, we prove the cruciality of the T2 state in the TADF phenomenon involved in Keto* state and reconsider the mechanism of the dual phosphorescence. Our investigation is favorable to provide theoretical support for the design of the LPL materials based on the ESIPT-attributed molecules.

Achieving Seconds‐to‐Hours Duration‐Tunable Organic Long Persistent Luminescence from Carbon Dots‐Based Exciplex Systems by Energy Gaps Regulation†

Comprehensive SummaryDuration‐tunable afterglow materials have garnered considerable attention in various applications. Herein, carbon dots (CDs)‐based long persistent luminescence (LPL) composites with a tunable duration in an ultrawide range of seconds‐to‐hours levels were designed and prepared for the first time. In contrast to the established CD‐based afterglow materials, we reported that CD‐based composites exhibit LPL in the form of exciplexes and long‐lived charge‐separated states, enabling the LPL to be prolonged from several seconds to over one hour, exceeding the typical regulation range (limited to 1 min). Further studies revealed that the relationship between the excited and charge‐transfer states of CDs plays a pivotal role in activating the LPL and regulating its duration. Furthermore, these composites exhibited high photoluminescence (PL) quantum yields of up to 60.63%, and their LPL was robust under ambient conditions, even in aqueous media. Their robust and superior LPL performance endows these composites with a strong competitive advantage in dynamic display systems, such as tags for time‐resolved data encryption and displays of the remaining time of takeaways. This study offers an approach to preparing CDs‐based LPL composites with tunable durations and may provide new insights for the development of rare‐earth‐free LPL materials.

Reversible Acid-Base Long Persistent Luminescence Switch Based on Amino-Functionalized Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) with long persistent luminescence (LPL) have attracted extensive research attention from researchers due to their potential applications in information encryption, anticounterfeiting technology, and security logic. In contrast to short-lived fluorescent materials, LPL materials offer a visible response that can be easily distinguished by the naked eye, thereby facilitating a much clearer visualization. However, there are few reports on functional LPL MOF materials as probes. In this article, two amino-functional LPL MOFs (VB4-2D and VB4-1D) were synthesized. They both exhibited adjustable fluorescence and phosphorescence from blue to green and from cyan to green, respectively. Notably, the MOFs emitted bright and adjustable LPL upon the removal of the different radiation sources. The basic amino functional groups in the MOFs exhibited acid and ammonia sensitivity, and fluorescence and phosphorescence emission intensities can be burst and restored in two atmospheres, respectively, which can be cycled multiple times. Furthermore, LPL intensity undergoes switching between two different conditions as well, which can be visually discerned by the naked eye, enabling visual sensing of volatiles by LPL. This combination of photoluminescence and the visual LPL switching behavior of acids and bases in functional MOFs may provide an effective avenue for stimulus response, anticounterfeiting, and encryption applications.

Unveiling the Potential of Sunlight‐Driven Multifunctional Blue Long Persistent Luminescent Materials via Cutting‐Edge Trap Modulation Strategies

AbstractNumerous challenges persist in the field of long persistent luminescence (LPL) materials, including the unclear mechanism of persistent luminescence, low efficiency of sunlight excitation, and limited availability of commercial materials. To address these issues, a LPL material, SrAl2Si2O8:x% Eu2+,y% Dy3+, is synthesized for excitation by UV lamps or sunlight. Dy3+ doping remarkably extends the afterglow duration from 2 to 7 h under UV lamp excitation, exhibiting a remarkable enhancement of 350%. Notably, SrAl2Si2O8:4.0% Eu2+, 3.0% Dy3+ shows 2 h afterglow duration when exposed to sunlight, surpassing other LPL materials and expanding potential applications. XPS and XANES measurements confirm Eu2+ as the luminescent and persistent luminescent centers, while Dy3+ increases trap diversity. Through meticulous analysis of the EPR and XPS data, it is elucidated that the material encompasses two distinct types of traps, namely oxygen vacancies and may be traps. This provides a basis for subsequent trap analysis in LPL materials. Microstructural analysis examined ion coordination environment changes due to Eu2+ and Dy3+ doping. The mechanical luminescent properties are characterized, and a plausible afterglow mechanism is proposed based on the experimental results, offering insights for further research into LPL materials.

High-temperature dynamic luminescence of MgGa2O4: Tb3+, Er3+ phosphors for advanced anti-counterfeiting and information encryption

Long persistent luminescence (LPL) anti-counterfeiting technology is widely used in optical information storage, information security, and anti-counterfeiting fields due to the advantages of non-interference with background light and high imaging resolution. However, the performance of most LPL materials is greatly compromised in harsh environments such as high temperature, which seriously affects the effectiveness of anti-counterfeiting measures due to the accelerated decay rate of LPL luminescence. In this work, we synthesized a series of MgGa2O4: Tb3+ (MGO: Tb3+), which possess stable crystal structure with obvious LPL properties above room temperature, and the optimum temperature is determined to be 423 K. Moreover, the additional introduction of Er3+ ions enhances the security of anti-counterfeiting, and the combination of photo-stimulated luminescence (PSL) and upconversion (UC) phenomena enables color tuning from red to orange to green by extending the 980 nm laser irradiation time. Finally, the anti-counterfeiting strategy utilizing MGO: Tb3+ and MGO: Tb3+, Er3+ possesses an extremely high level of security, making it an attractive performance for designing advanced anti-counterfeiting technology.

Long‐Persistent Circularly Polarized Luminescence from a Host‐Guest System Regulated by the Multiple Roles of a Gold(I)‐Carbene Motif

AbstractThe promotion of intersystem crossing (ISC) is critical for achieving a high‐efficiency long‐persistent luminescence (LPL) from organic materials. However, the use of a transition‐metal complex for LPL materials has not been explored because it can also shorten the emission lifetime by accelerating the phosphorescence decay. Here, we report a new class of LPL materials by doping a monovalent Au‐carbene complex into a boron‐embedded molecular host. The donor‐acceptor systems exhibit photoluminescence with both high efficiencies (>57 %) and long lifetimes (ca. 40 ms) at room temperature. It is revealed that the Au atom promotes the population of low‐lying triplet excited states of the host aggregate (T1*) which can be converted into the charge‐transfer (CT) state, thereby resulting in afterglow luminescence. Moreover, the use of a chirality unit on the guest molecule results in the LPL being circularly polarized. This work illustrates that transition‐metal complexes can be used for developing organic afterglow systems by exquisite control over the excited state mechanism.

Long-Persistent Circularly Polarized Luminescence from a Host-Guest System Regulated by the Multiple Roles of a Gold(I)-Carbene Motif.

The promotion of intersystem crossing (ISC) is critical for achieving a high-efficiency long-persistent luminescence (LPL) from organic materials. However, the use of a transition-metal complex for LPL materials has not been explored because it can also shorten the emission lifetime by accelerating the phosphorescence decay. Here, we report a new class of LPL materials by doping a monovalent Au-carbene complex into a boron-embedded molecular host. The donor-acceptor systems exhibit photoluminescence with both high efficiencies (>57 %) and long lifetimes (ca. 40 ms) at room temperature. It is revealed that the Au atom promotes the population of low-lying triplet excited states of the host aggregate (T1 *) which can be converted into the charge-transfer (CT) state, thereby resulting in afterglow luminescence. Moreover, the use of a chirality unit on the guest molecule results in the LPL being circularly polarized. This work illustrates that transition-metal complexes can be used for developing organic afterglow systems by exquisite control over the excited state mechanism.

Ultralong Room‐Temperature Phosphorescence from Organic–Inorganic Hybrid Perovskitoid Based on Metal‐Organic Complex Component

AbstractOrganic–inorganic hybrid perovskite or perovskitoid phosphorescent materials have aroused extensive research interests due to their applications in organic light emitting diodes, anti‐counterfeiting, display, and other fields. Herein, a new strategy is proposed, employing metal‐organic complex (MOC) cations as the organic components, which can increase the spin–orbit coupling (SOC) and intersystem crossing (ISC) efficiencies of the organic units, and hybrid with inorganic parts to form a novel type MOC–inorganic composed perovskitoid. In detail, a lead‐free MOC–inorganic perovskitoid material, named P‐Cl, is synthesized by self‐assembly of preassembled MOC from metallized crown ethers with inorganic cadmium halide component. In the solid state, the assembled perovskitoid exhibits a tunable fluorescence color from blue to green, during which a cold white light emission is achieved. Interestingly, after removal of the radiation source, the perovskitoid still emits a bright and color‐tunable long persistent luminescence (LPL) from cyan to green. Single‐crystal X‐ray diffraction analysis and theoretical calculations reveal that the bright LPL of P‐Cl arises from the improvement of the metal‐organic part to increase the SOC and the interlocking of metal halogenated layers to inhibit the nonradiative transition. The proposed strategy not only broadens the range of organic–inorganic perovskitoid hybrid materials but also facilitates the design of multifunctional LPL materials.

White light and long persistent luminescence from metal cluster-based metal-organic frameworks

Metal-organic frameworks (MOFs) with long persistent luminescence (LPL) have raised particular attention among researchers for their potential applications in optical recording devices, biological imaging, security systems, and so on. Compared with traditional MOFs materials constructed by the single metal as a node, MOFs materials with multinuclear cluster substructure units can effectually facilitate the fixation of molecular conformations through multiple coordination bonds, which would form a dense stacking and rigid environment to suppress the non-radiation transition of triplet excitons. At the same time, metal clusters can act as heavy atoms to increase the spin–orbit coupling to a certain extent, promote Sn-Tn energy conversion to effectively improve the ISC efficiency and enhance the LPL performance. However, MOFs-based LPL materials centred on multinuclear clusters are rarely studied. In this paper, two LPL MOFs (SUST-WJ-12 and SUST-WJ-13) based on the polydentate ligand 2-hydroxyniotinic acid (H2L) with multinuclear cluster are reported and their formulas are [CdL]n and [Zn3(L)2(HCOO)(OH)]n, respectively. In the solid state, they both exhibit excitation-dependent emission color-tuning from blue to green, in which SUST-WJ-12 obtains a white light emission under 280 nm excitation. Importantly, two MOFs both give the long-lived phosphorescence with the lifetimes of 69.92 and 63.22 ms, respectively, and emit the bright and visible LPL for 2–3 s after the removal of the excitation light source. Single-crystal X-ray diffraction analysis and theoretical calculations reveal that LPL of two MOFs arise from the ligand center which is enhanced by metal cluster. The thus obtained LPL provides potentials in lighting and security systems and so on.

High-Efficiency and Stable Long-Persistent Luminescence from Undoped Cesium Cadmium Chlorine Crystals Induced by Intrinsic Point Defects.

Application of long-persistent luminescence (LPL) materials in many technological fields is in the spotlight. However, the exploration of undoped persistent luminescent materials with high emission efficiency, robust stability, and long persistent duration remains challenging. Here, inorganic cesium cadmium chlorine (CsCdCl3 ) is developed, featuring remarkable LPL characteristics at room temperature, which is synthesized by a facile hydrothermal method. Excited by ultraviolet light, the CsCdCl3 crystals exhibit an intense yellow emission with a large photoluminescence quantum yield of ≈90%. Different from the reported systems with lanthanides or transition metals doping, the CsCdCl3 crystals without dopants perform yellow LPL with a long duration of 6000 s. Joint experiment-theory characterizations reveal the intrinsic point defects of CsCdCl3 act as the trap centers of excited electrons and the carrier de-trapping process from such trap sites to localized emission centers contributes to the LPL. Encouraged by the attractive fluorescence and persistent luminescence as well as good stability of CsCdCl3 against environment oxygen/moisture (75%), heat (100°C for 10h), and ultraviolet light irradiation, an effective dual-mode information storage-reading application is demonstrated. The results open up a new frontier for exploring LPL materials without dopants and provide an opportunity for advanced information storage compatible for practical applications.

High‐Temperature and Dynamic RGB (Red‐Green‐Blue) Long‐Persistent Luminescence in an Anti‐Kasha Organic Compound

AbstractOrganic LPL (long‐persistent luminescence) materials have sparked extensive research interest due to the ultralong‐lived triplet states. Although numerous organic LPL materials have been reported, most of the triplet emission was static and monotonous. Therefore, LPL materials with dynamic triplet emission are urgently required. A triamino‐s‐triazine derivative 1 with dynamic LPL was fabricated. The single‐crystal structure shows that the abundant intermolecular interactions and small free volume restrict the molecular motion and avoid the quenchers. Spectral and theoretical calculations upheld the existence of multiple excited states in 1, and the migration of electrons between multiple excited states is very sensitive to external stimuli. By modulating the stimulus, the residence of electrons in different triplet states can be manipulated to achieve RGB LPL. Importantly, blue LPL was achieved by manipulating the anti‐Kasha emission. And the red LPL can still be observed at high temperature.

High-Temperature and Dynamic RGB (Red-Green-Blue) Long-Persistent Luminescence in an Anti-Kasha Organic Compound.

Organic LPL (long-persistent luminescence) materials have sparked extensive research interest due to the ultralong-lived triplet states. Although numerous organic LPL materials have been reported, most of the triplet emission was static and monotonous. Therefore, LPL materials with dynamic triplet emission are urgently required. A triamino-s-triazine derivative 1 with dynamic LPL was fabricated. The single-crystal structure shows that the abundant intermolecular interactions and small free volume restrict the molecular motion and avoid the quenchers. Spectral and theoretical calculations upheld the existence of multiple excited states in 1, and the migration of electrons between multiple excited states is very sensitive to external stimuli. By modulating the stimulus, the residence of electrons in different triplet states can be manipulated to achieve RGB LPL. Importantly, blue LPL was achieved by manipulating the anti-Kasha emission. And the red LPL can still be observed at high temperature.

Organic Long-Persistent Luminescence from a Single-Component Aggregate.

Long-persistent luminescence (LPL), also known as afterglow, is a phenomenon in which the material shows long-lasting luminescence after the cessation of the excitation source. The research of LPL continues to attract much interest due to its fundamental nature and its potential in the development of the next generation of functional materials. However, most of the current LPL materials are multicomponent inorganic systems obtained after harsh synthetic procedures and often use rare-earth metals. Recently, metal free organic long-persistent luminescence (OLPL) has gained much interest because it can bypass many of the disadvantages of inorganic systems. To date, the most successful method to generate OLPL systems is to access charge-separated states through binary donor-acceptor exciplex systems. However, it has been reported that the ratios of the binary systems affect OLPL properties, complicating the reproducibility and large-scale production of OLPL materials. Simpler OLPL systems can overcome these issues for the benefit of the development and adoption of OLPL systems. Here, we report on the rational design and synthesis of a single-component OLPL system with detectable afterglow for at least 12 min under ambient conditions. This work exemplifies an easy design principle for new OLPL materials. The investigation of the material provides valuable insights toward the generation of OLPL from a single-component system.

SrAl2O4 crystallite embedded inorganic medium with super-long persistent luminescence, thermoluminescence, and photostimulable luminescence for smart optical information storage

As a leader of long persistent luminescence (LPL) materials, the optical properties of aluminate phosphor have remained unsurpassed for many years. As a powder material, its practical application will always be limited to the field of security signs. In this paper, the SrAl 2 O 4 : Eu 2 + , Dy 3 + inorganic solid material with comparable LPL properties to powder materials was obtained. The crystallization mechanism and crystallite micro-morphology of inorganic glass materials have been studied, and a new opinion is put forward that the large-size SrAl 2 O 4 crystallites in the glass matrix are stacked by rod-shaped crystals arranged in a regular direction. In addition, the SrAl 2 O 4 : Eu 2 + , Dy 3 + glass obtained cannot only collect high-energy photons but also is sensitive to low-energy sunlight. The results show that the material exhibits superior performance in LPL, thermoluminescence, and photostimulable luminescence. Based on this property, a new application of this material in the field of information storage was explored. This paper has a certain reference value for the development and application of aluminate LPL materials in the field of smart optical information storage.

Temperature-dependent long persistent luminescence properties and trap distributions in Sr2MgAl22O36: Eu2+

Long persistent luminescence (LPL) materials can be released its stored energy by various temperature induced thermal disturbance for a period of time. However, the accelerated release of trapped carriers at higher temperatures inhibits the use of LPL materials in extreme environments. In this work, a series of blue-emitting LPL phosphors Sr2MgAl22O36: Eu2+ were prepared by high temperature solid phase reaction. The investigation of photoluminescence (PL) and LPL preliminary indicated that the optimal room-temperature LPL time is 80 s, and the depth of trap was about 0.84 eV and 1.078 eV. Analyses of temperature-dependent LPL decay curves and thermoluminescence (TL) spectra show that the LPL initial brightness and duration time increase first and then decrease with the increase of temperature, and the optimal temperature is 453 K. These findings ensure the application of the discovered LPL phosphors above room temperature, including optical information storage and anti-counterfeiting among others.

Air-stable organic glow-in-the-dark materials

The future of organic glow-in-the-dark materials is shining brightly now thanks to a study reporting a way to make the easily processed, low-cost materials stable in air ( Nat. Mater. 2021, DOI: 10.1038/s41563-021-01150-9 ). Commercial materials that exhibit the glowing effect known as long-persistent luminescence (LPL) typically include metal oxides and rare earth metals. These materials, which put the glow in glow-in-the-dark watch faces, tend to require high-temperature processing and must be ground into fine particles and dispersed in solvents or polymers for use in applications. Organic LPL materials are metal-free, readily soluble, and easily processed in liquid solvents. Those properties make the materials easy to put into paints, polymers, and fabrics for use in emergency signs, safety apparel, and other products . Compared with their inorganic counterparts, however, organic LPL materials have tended to glow weakly and become dim quickly, especially upon exposure to air. Now a team including

Photophysical tuning of small-molecule-doped organic crystals with long-persistent luminescence by variation of dopants

Design of host-guest structure has been an important direction in the development of organic long-persistent luminescence (LPL) materials, which have challenged traditional inorganic materials in some applications, such as anti-counterfeiting and bioimaging. The charge separation of donor-acceptor between host and guest is an effective way to induce LPL emission. Here, three crystalline materials were prepared by a convenient host-guest doping method at room temperature under ambient surrounding, i.e. in the presence of oxygen. Two kinds of crystals with long-lived charge separated states exhibited visible LPL emission with over 8 s and 6 s of duration. The persistent room-temperature phosphorescence effect caused by different generation efficiency of charge separated states was the main reason for the difference of LPL duration time. The crystal without charge separation had no LPL phenomenon because it was not a donor-acceptor system. This work indicates that the generation of charge separated states and their generation efficiency are the two key factors affecting the LPL properties and provides a new understanding for the design of organic LPL materials.

Research Advances on Human-Eye-Sensitive Long Persistent Luminescence Materials.

Based on the actual application requirements of multicolor long persistent luminescence (LPL) materials, we highlight the recent developments in the last decade on human-eye-sensitive LPL materials and try to make a full list of known LPL compounds possessing wavelengths of 400–600 nm and a duration time longer than 10 h (>0.32 mcd/m2); these are more sensitive to the human eye's night vision and can be used throughout the night. We further emphasize our group research of novel LPL materials and the regulation of LPL color to enable a full palette. In the end, we try to summarize the challenges and perspectives of LPL materials for potential research directions based on our limited understandings. This review could offer new enlightenment for further exploration of new LPL materials in the visible light range and related applications.

Multi‐Mode Color‐Tunable Long Persistent Luminescence in Single‐Component Coordination Polymers

AbstractMaterials with tunable long persistent luminescence (LPL) properties have wide applications in security signs, anti‐counterfeiting, data encrypting, and other fields. However, the majority of reported tunable LPL materials are pure organic molecules or polymers. Herein, a series of metal‐organic coordination polymers displaying color‐tunable LPL were synthesized by the self‐assembly of HTzPTpy ligand with different cadmium halides (X=Cl, Br, and I). In the solid state, their LPL emission colors can be tuned by the time‐evolution, as well as excitation and temperature variation, realizing multi‐mode dynamic color tuning from green to yellow or green to red, and are the first such examples in single‐component coordination polymer materials. Single‐crystal X‐ray diffraction analysis and theoretical calculations reveal that the modification of LPL is due to the balanced action from single molecule and aggregate triplet excited states caused by an external heavy‐atom effect. The results show that the rational introduction of different halide anions into coordination polymers can realize multi‐color LPL.