Facile Synthesis of Ln-Doped Hydrogen-Bonded Organic Frameworks with Rare-Earth-Characteristic Long Persistent Luminescence.

Tunable luminescence-assisted information storage and encryption holds increasing significance in today's society. A promising approach to incorporating the benefits of both organic long persistent luminescent (LPL) materials and rare-earth (RE) luminescence lies in utilizing organic host materials to sensitize RE luminescence, as well as hydrogen-bonded organic framework (HOF) phosphorescence Förster resonance energy transfer to RE compound luminescence. This work introduces a one-pot, in situ pyrolytic condensation method, achieved through high-temperature melting calcination, to synthesize lanthanide ion-doped HOF materials. This method circumvents the drawback of molecular triplet energy annihilation, enabling the creation of organic LPL materials with RE characteristics. The HOF material serves as the host, exhibiting blue phosphorescence and cyan LPL. By fine-tuning the doping amount, the composite material U-Tb-100 achieves green LPL with a luminescent quantum yield of 56.4%, and an LPL duration of approximately 2-3 s, demonstrating tunable persistence. Single-crystal X-ray diffraction, spectral analysis, and theoretical calculation unveil that U-Tb-100 exhibits exceptional quantum yield and long-lived luminescence primarily due to the efficient sensitization of U monomer to RE ions and the PRET process between U and RE complexes. This ingenious strategy not only expands the repertoire of HOF materials but also facilitates the design of multifunctional LPL materials.

Oxygen-Tolerant Near-Infrared Organic Long-Persistent Luminescent Copolymers.

Organic materials exhibiting long-lasting emission in the near infrared are expected to have applications in bio-imaging and other areas. Although room temperature phosphorescence and thermally activated delayed fluorescence display long-lived emission of approximately one minute, organic long-persistent luminescence (OLPL) systems with a similar emission mechanism to inorganic persistent emitters can emit for several hours at room temperature. In particular OLPL with a hole-diffusion mechanism can function even in the presence of oxygen. However, ionic materials lack long-term stability in neutral organic host owing to aggregation and phase separation. In this study, we synthesized polymers with stable near-infrared persistent luminescence at room temperature via the copolymerization of electron donors and acceptors. The copolymers exhibit long-persistent luminescence (LPL) at temperatures below the glass transition temperature and can be excited by approximately the entire range of visible light. LPL properties and spectra can be controlled by the dopant.

Enhanced Charge Separation for Efficient Photocatalytic H2 Production by Long-Lived Trap-State-Induced Interfacial Charge Transfer.

Photogeneration of charge carriers in semiconductors provides the scientific fundamental for photocatalytic water splitting. However, an ongoing challenge is the development of a new mechanism promoting charge carrier separation. Here we propose a trap-state-induced interfacial charge-transfer transition mechanism (TSICTT), in which electrons in long-lived trap states recombine with holes on the valence band (VB) of the semiconductor, thus prolonging the electron lifetime. We demonstrate this concept in the Sr4Al14O25:Eu2+, Dy3+/CdS (SAO/CdS) heterostructure, where trapped electrons with a lifetime of up to several hours in the SAO persistent luminescence phosphor (PLP) can continuously consume holes on the VB of CdS nanoparticles (NPs). We discover that the interfacial interaction and the work function difference between SAO and CdS are crucial for the TSICTT, which finally contributes to the improved H2 production from 34.4 to 1212.9 µmol gCdS-1 h-1 under visible-light irradiation. This model introduces a new strategy to manipulate charge carrier transport for the effective utilization of solar energy.

Achieving Color-Tunable Long Persistent Luminescence in Cs2 CdCl4 Ruddlesden-Popper Phase Perovskites.

Two-dimensional (2D)-halide perovskites have been enriched over recent years to offer remarkable features from diverse chemical structures and environmental stability endowed with exciting functionalities in photoelectric detectors and phosphorescence systems. However, the low conversion efficiency of singlet to triplet in 2D hybrid halide perovskites reduces phosphorescence lifetimes. In this study, the long persistent luminescence of 2D all-inorganic perovskites with a self-assembled 2D interlayer galleries structure is investigated. The results show that the decay time of the long persistent luminescence increases from 450 s to 600 s, and the luminescence color changes from cyan to orange, and the thermal stability of photoluminescence enhances dramatically after replacing Cd2+ by appropriate Mn2+ ions in 2D Cs2 CdCl4 Ruddlesden-Popper phase perovskites. Furthermore, diversified anti-counterfeiting modes are fabricated to highlight the promising applications of Cs2 CdCl4 perovskite systems with tunable persistent luminescence in advanced anti-counterfeiting. Therefore, our studies provide a novel model for realizing tunable long persistent luminescence of perovskite with 2D self-assembled layered structure for advanced anti-counterfeiting.

Thermally Activated Long Persistent Luminescence of Organic Inorganic Metal Halides.

Long persistent luminescence (LPL) materials of SrAl2 O4 doped with Eu2+ or Dy3+ can maintain emission over hours after ceasing the excitation but suffer from insolubility, high cost, and harsh preparation. Recently, organic LPL of guest-host exciplex systems has been demonstrated via an intermediate charge-separated state with flexible design but poor air-stability. Here, we synthesized a nontoxic two-dimensional organic-inorganic metal hybrid halides (OIMHs), called PBA2 [ZnX4 ] with X=Br or Cl and PBA=4-phenylbenzylamine. These materials exhibit stable LPL emission over minutes at room-temperature, which is two orders of magnitude longer than those of previously reported OIMHs. The mechanism study shows that the LPL emission comes from thermally activated charge separation state rather than room-temperature phosphorescence. Moreover, the LPL of PBA2 [ZnX4 ] can be excited by low power sources, representing an effective strategy for developing low-cost and high-stability LPL systems.

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.

One/Two-Photon-Excited ESIPT-Attributed Coordination Polymers with Wide Temperature Range and Color-Tunable Long Persistent Luminescence.

The multiple metastable excited states provided by excited-state intramolecular proton transfer (ESIPT) molecules are beneficial to bring temperature-dependent and color-tunable long persistent luminescence (LPL). Meanwhile, ESIPT molecules are intrinsically suitable to be modulated as D-π-A structure to obtain both one/two-photon excitation and LPL emission simultaneously. Herein, we report the rational design of a dynamic CdII coordination polymer (LIFM-106) from ESIPT ligand to achieve the above goals. By comparing LIFM-106 with the counterparts, we established a temperature-regulated competitive relationship between singlet excimer and triplet LPL emission. The optimization of ligand aggregation mode effectively boost the competitiveness of the latter. In result, LIFM-106 shows outstanding one/two-photon excited LPL performance with wide temperature range (100-380 K) and tunable color (green to red). The multichannel radiation process was further elucidated by transient absorption and theoretical calculations, benefiting for the application in anti-counterfeiting systems.

Manipulation of Triplet Excited States in Two-Component Systems for High-Performance Organic Afterglow Materials.

The past several years have witnessed the tremendous development of novel chemical structures, new design strategies and intriguing applications in the field of room-temperature phosphorescence (RTP) and organic afterglow materials. This Review article focuses on recent advancements of high-performance organic afterglow materials obtained by two-component design strategies such as a dopant-matrix, donor-acceptor, sensitization, and energy-transfer strategies. Based on some cutting-edge studies, organic afterglow efficiency has been largely improved, exceeding 90 % in several cases. Organic afterglow durations reach tens of seconds in phosphorescence systems and hours in donor-acceptor systems. Organic afterglow brightness outcompetes some inorganic afterglow materials in the first several seconds after ceasing excitation source. Organic afterglow colors cover the whole visible regions and extend to near-infrared regions with respectful afterglow efficiency. On the basis of these achievements, researchers demonstrate promising applications of organic afterglow materials in diverse fields, which has also been reviewed.

Biocompatible Probes Based on Rare-Earth Doped Strontium Aluminates with Long-Lasting Phosphorescent Properties for In Vitro Optical IMAGING.

In recent decades, the demand for biomedical imaging tools has grown very rapidly as a key feature for biomedical research and diagnostic applications. Particularly, fluorescence imaging has gained increased attention as a non-invasive, inexpensive technique that allows real-time imaging. However, tissue auto-fluorescence under external illumination, together with a weak tissue penetration of low wavelength excitation light, largely restricts the application of the technique. Accordingly, new types of fluorescent labels are currently being investigated and, in this search, phosphorescent nanoparticles promise great potential, as they combine the interesting size-dependent properties of nanoscale materials with a long-lasting phosphorescence-type emission that allows optical imaging well after excitation (so avoiding autofluorescence). In this work, core-shell structures consisting of SrAlO:Eu,Dy luminescent cores encapsulated within a biocompatible silica shell were prepared, showing a green persistent phosphorescence with an afterglow time of more than 1000 s. A high-energy ball milling procedure was used to reduce the size of the starting phosphors to a size suitable for cellular uptake, while the silica coating was produced by a reverse micelle methodology that eventually allows the excitation and emission light to pass efficiently through the shell. Confocal fluorescence microscopy using HeLa cancer cells confirmed the potential of the all-ceramic composites produced as feasible labels for in vitro optical imaging.

Highly Stable Metal-Free Long-Persistent Luminescent Copolymer for Low Flicker AC-LEDs.

Organic room-temperature long-persistent luminescent materials are promising light-emitting materials for encryption, architectural decoration, organic solar cells, and biomedical applications. However, their unstable structures and thermal- and humidity-induced emission quenching have greatly limited their utility and reliability. Here, we report a metal-free nonconjugated copolymer that possesses stable photoluminescence at both high temperature and humidity. The room-temperature long-persistent luminescence (LPL) of this copolymer lasts for more than 15 s and can be recovered in high humidity conditions by heating to remove moisture. Copolymer LPL can be achieved with various excitation wavelengths, ranging from ultraviolet to near-infrared, and the LPL color can be adjusted accordingly. The high initial LPL intensity and ultrafast filling time of the copolymer makes it suitable for low flicker alternating current-driven light-emitting diodes (AC-LEDs).

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.

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

Materials 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.

Organic Long Persistent Luminescence Through In Situ Generation of Cuprous(I) Ion Pairs in Ionic Solids.

Recent development of most organic long persistent luminescence (OLPL) systems employed binary or tertiary doping. However, the design strategies towards OLPL materials with hour-long afterglow duration are still quite limited. Here, we propose a novel OLPL system through melt-casting method with 0.1 mol % of CuI complexes: 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl BINAP-CuX (X=Cl, Br and I) doped into the triphenylphosphine (TPP) host. The charge separation was initiated prior to excitation through host coordination with CuI complexes, resulting in semi-free halogen ions and in situ generated CuI cations, which forms TPP + BINAP-CuX ionic pairs and subsequently ionic solids. The OLPL lifetime can be readily modulated by different halogen atoms and the afterglow can last up to more than 3 hours perceivable to human eyes. This is a rare example of OLPL initiated through host-guest coordination that could potentially expand the definition of OLPL systems and design strategies.

A Metal-Organic Supramolecular Box as a Universal Reservoir of UV, WL, and NIR Light for Long-Persistent Luminescence.

Long persistent luminescence (LPL) materials have a unique photophysical mechanism to store light radiation energy for subsequent release. However, in comparison to the common UV source, white-light (WL) and near-infrared (NIR) excited LPL is scarce. Herein we report a metal-organic supramolecular box based on a D-π-A-type ligand. Owing to the integrated one-photon absorption (OPA) and two-photon absorption (TPA) attributes of the ligand, the heavy-atom effect of the metal center, as well as π-stacking and J-aggregation states in the supramolecular assembly, LPL can be triggered by all wavebands from the UV to the NIR region. This novel designed supramolecular kit to afford LPL by both OPA and TPA pathways provides potential applications in anti-counterfeiting, camouflaging, decorating, and displaying, among others.

Optoelectronic Synapse Behaviors in Tb3+ and Al3+ Co-Doped CaSnO3 with Long-Persistent Luminescence.

Neuromorphic computation draws inspiration from the remarkable features of the human brain including low energy consumption, parallelism, adaptivity, cognitive functions, and learning ability. These qualities hold the promise of unlocking groundbreaking computational techniques that surpass the limitations of traditional computing systems. This paper reports a remarkable photo-synaptic behavior in the field of rare earth ion-doped luminescent oxides by using long-persistent luminescence (LPL). This system utilizes electron trap states to regulate the synaptic behavior, operating through a fundamentally different mechanism from that of electronic-based synaptic devices. To realize this strategy, Tb3+ doped CaSnO3, which shows a significant LPL property under UV-light excitation, is prepared. The luminescent system shows key neuromorphic characteristics such as paired-pulse facilitation, pulse-number/timing dependent potentiation, and pulse-number/timing dependent short- to long-term plasticity transition, which are required for realizing synaptic devices. This feature expands the way for advanced neuromorphic technologies employing light stimuli.

Computational, optical and feasibility studies of organic luminescence TMB-PPT blend for photovoltaic application.

For enhanced applications of solar cells, organic luminescence materials like long persistent luminescence (LPL) present one of the promising avenues for light enhancement. Currently, most existing luminescent materials are based on an inorganic system that requires rare elements such as europium and dysprosium, with a very high processing temperature. Adopting organic luminescence materials that are free from rare elements is necessary, considering the low-temperature fabrication and low material cost. In this work, we investigate the optical properties of an organic luminescence blend consisting of 2,8-bis(diphenylphosphoryl)dibenzo [b,d]thiophene (PPT) and N,N,N',N'-tetramethylbenzidine (TMB) through computational studies and experimental validations. Optical characteristics of the luminescence materials like optical absorption, photoluminescence, and time-resolved photoluminescence spectroscopy are characterized. To validate the functionality of the organic luminescence blend, the material is incorporated into the perovskite solar cell structure. Unfortunately, the blend is unable to emit sufficient illumination over extended periods due to its low intersystem crossing efficiency and weak spin-orbit coupling. Although the power conversion efficiency of the Luminescence/FTO/TiO2/Perovskite/Carbon structure is observed to be small under dark conditions, the application of organic luminescence materials can be further enhanced and explored.

Stepwise Charge/Energy Transfer in MR-TADF Molecule-Doped Exciplex for Ultralong Persistent Luminescence Activated with Visible Light.

Organic long-persistent luminescence (OLPL), which relies on energy storage for delayed light emission by the charge separation state, has attracted intense attention in various optical applications. However, charge separation (CS) is efficient only under ultraviolet excitation in most OLPL systems because it requires a driving force from the large energy difference between the local excited (LE) and charge transfer (CT) states. In this study, a multiresonance thermally activated delayed fluorescence (MR-TADF) molecule is incorporated into an exciplex system to achieve efficient OLPL in a composite material activated by visible light via a stepwise charge/energy transfer process. The enhanced absorption of the composite material facilitated a tenfold increase in the duration of the OLPL, which can last for several hours under visible light excitation. The excited state of the MR-TADF molecule tends to charge transfer to the acceptor, followed by energy transfer to the exciplex, which benefits from the small difference between the LE and CT states owing to the inherent CS characteristics of the opposing resonance effect. Afterglow displays of these composite materials are fabricated to demonstrate their considerable potential in encryption patterns and emergency lights, which take advantage of their excellent processability, visible light activation, and tunable luminescence properties.

Multicolor-emitting Er3+ and Er3+/Yb3+ doped Zn2GeO4 phosphors combining static and dynamic identifications for advanced anti-counterfeiting application.

Anti-counterfeiting labels based on luminescence materials are a newly emerging technique for protecting legal goods and intellectual property. In the anti-counterfeiting field to prevent forgery and cloning, luminescence materials with properties different from the commercialized and traditional ones are in urgent need. In this work, multicolor-emitting Er3+ single-doped and Er3+/Yb3+ co-doped Zn2GeO4 phosphors combining static and dynamic identifications were developed in order to achieve advanced anti-counterfeiting application. The variation of trap content with increasing the doping content of rare earth ions was analyzed through X - ray photoelectron spectroscopy, thermoluminescence analysis. It was found that there are two types of traps with different depth in Zn2GeO4 phosphors. The depths of the traps were experimentally confirmed to be 0.68 and 0.79 eV, respectively. The transient photocurrent response measurement confirmed the existence of charge carriers, and the mechanism for long persistent luminescence was deduced. The multicolor upconversion mechanisms under 980 and 1550 nm excitation were also discovered. Based on the multicolor steady and transient emission features, an anti-counterfeiting pattern was designed using the phosphors. Static and dynamic identification was demonstrated and presented in detail. Finally, it is indicated that the studied phosphors are excellent candidates for potential applications in luminescence anti-counterfeiting labels.

Effective Long Afterglow Amplification Induced by Surface Coordination Interaction.

Long-persistent luminescent (LPL) materials have attracted considerable research interest due to their extensive applications and outstanding afterglow performance. However, the performance of red LPL materials lags behind that of green and blue materials. Therefore, it is crucial to explore novel red LPL materials. This study introduces a straightforward and viable strategy for organic-inorganic hybrids, wherein the organic ligand 1,3,6,8-Tetrakis(4-carboxyphenyl)pyrene (TCPP) is coordinated to the surface of a red persistent phosphor Sr0.75 Ca0.25 S:Eu2+ (R) through a one-step method. TCPP serves as an antenna, facilitating the transfer of absorbed light energy to R via triplet energy transfer (TET). Notably, the initial afterglow intensity and luminance of R increase by twofold and onefold, respectively, and the afterglow duration extends from 9 to 17 min. Furthermore, this study involves the preparation of a highly flexible film by mixing R@TCPP with high-density polyethylene (HDPE) to create a sound-controlled afterglow lamp. This innovative approach holds promising application prospects in flexible large-area luminescence, flexible wearables, and low-vision lighting.

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 10 h), 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.