Organic Long-Persistent Luminescence from a Thermally Activated Delayed Fluorescence Compound

Dual-Mechanism Design Strategy for High-Efficiency and Long-Lived Organic Afterglow Materials

Organic room-temperature phosphorescence (RTP) and afterglow materials hold great potential for various applications, but there remain inherent trade-offs between the afterglow efficiency and the lifetime. Here, we propose a dual-mechanism design strategy, leveraging the RTP or thermally activated delayed fluorescence (TADF) mechanism for a high afterglow efficiency and the organic long-persistent luminescence (OLPL) mechanism for a prolonged afterglow duration. The intramolecular charge transfer (ICT)-type difluoroboron β-diketonate molecules with a large S1 dipole moment are doped as the luminescent component into the organic matrix with a large dipole moment, and a series of TADF-type afterglow materials can be achieved with an afterglow efficiency of up to 88.7% and an afterglow lifetime of 200 ms. To prolong the afterglow duration, an electron donor is introduced as the third component to generate traps and facilitate charge separation. The obtained materials exhibit a dual afterglow mechanism, first exhibiting a TADF/RTP afterglow with an afterglow efficiency of up to 50.9%, followed by an hours-long OLPL afterglow emission with an afterglow efficiency of up to 13.1%. Further investigations reveal that an appropriate heavy-atom effect can facilitate the intersystem crossing process, which can promote the charge separation process and thus improve the OLPL afterglow performance. Additionally, rare-earth upconversion materials are introduced into OLPL materials to enable their near-infrared excitation properties, showcasing their potential applications in bioimaging.

Highly stable color-tunable organic long-persistent luminescence from a single-component exciplex copolymer for in vitro antibacterial

Developing exciplex-based organic long-persistent luminescence (OLPL) materials with high stability is very important but remains a formidable challenge in a single-component system. Here, we report a facile strategy to achieve highly stable OLPL in an amorphous exciplex copolymer system via through-space charge transfer (TSCT). The copolymer composed of electron donor and acceptor units can not only exhibit effective TSCT for intra/intermolecular exciplex emission but also construct a rigid environment to isolate oxygen and suppress non-radiative decay, thereby enabling stable exciplex-based OLPL emission with color-tunable feature for more than 100 h under ambient conditions. These single-component OLPL copolymers demonstrate robust antibacterial activity against Escherichia coli under visible light irradiation. These results provide a solid example to exploit highly stable exciplex-based OLPL in polymers, shedding light on how the TSCT mechanism may potentially contribute to OLPL in a single-component molecular system and broadening the scope of OLPL applications.

Enabling Visible-Light-Charged Near-Infrared Persistent Luminescence in Organics by Intermolecular Charge Transfer

Visible light is a universal and user-friendly excitation source; however, its use to generate persistent luminescence (PersL) in materials remains a huge challenge. Herein, the concept of intermolecular charge transfer (xCT) is applied in typical host-guest molecular systems, which allows for a much lower energy requirement for charge separation, thus enabling efficient charging of near-infrared (NIR) PersL in organics by visible light (425-700 nm). Importantly, NIR PersL in organics occurs via the trapping of electrons from charge-transfer aggregates (CTAs) into constructed trap states with trap depths of 0.63-1.17 eV, followed by the detrapping of these electrons by thermal stimulation, resulting in a unique light-storage effect and long-lasting emission up to 4.6 h at room temperature. The xCT absorption range is modulated by changing the electron-donating ability of a series of acenaphtho[1,2-b]pyrazine-8,9-dicarbonitrile-based CTAs, and the organic PersL is tuned from 681 to 722 nm. This study on xCT interaction-induced NIR PersL in organic materials provides a major step forward in understanding the underlying luminescence mechanism of organic semiconductors and these findings are expected to promote their applications in optoelectronics, energy storage, and medical diagnosis.

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.

Sunlight-Activated Hour-Long Afterglow from Transparent and Flexible Polymers

Afterglow materials featuring long emission durations ranging from milliseconds to hours have garnered increasing interest owing to their potential applications in sensing, bioimaging, and anti-counterfeiting. Unfortunately, polymeric materials rarely exhibit afterglow properties under ambient conditions because of the rapid nonradiative decay rate of triplet excitons. In this study, hour-long afterglow (HLA) polymer films are fabricated using a facile molecular doping strategy. Flexible and transparent polymer films emitted a bright afterglow lasting over 11 h at room temperature in air, which is one of the best performances among the organic afterglow materials reported to date. Intriguingly, HLA polymer films can be activated by sunlight, and their cyan afterglow in air can be readily observed by the naked eye. Moreover, the HLA color of the polymer films could be tuned from cyan to red through the Förster resonance energy transfer mechanism. Their application in flexible displays and information storage has also been demonstrated. With remarkable advantages, including an hour-long and bright afterglow, tunable afterglow colors, superior flexibility and transparency, and ease of fabrication, the HLA polymer paves the way for the practical application of afterglow materials in the engineering sector.

High-Resolution Afterglow Patterning Using Cooperative Vapo- and Photo-Stimulation

Bright afterglow room-temperature phosphorescence (RTP) soon after ceasing excitation is a promising technique for greatly increasing anti-counterfeiting capabilities. The development of a process for rapid high-resolution afterglow patterning of crystalline materials can improve both high-speed fabrication of anti-counterfeiting afterglow media and stable afterglow readout compared with those achieved with amorphous materials. Here, the high-resolution afterglow patterning of crystalline materials via cooperative organic vapo- and photo-stimulation is reported. A single crystal of (S)-(-)-2,2'-bis(diphenylphosphino)-5,5',6,6',7,7'8,8'-octahydro-1,1'-binaphthyl [(S)-H8-BINAP] doped with (S)-(-)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl [(S)-BINAP] shows green afterglow RTP. Crystals of (S)-BINAP-doped (S)-H8-BINAP changed to an amorphous state with no afterglow capability on weak continuous photoirradiation under dichloromethane (DCM) vapor. Photoirradiation induced oxidation of the (S)-H8-BINAP host molecule in the crystal. The oxidized (S)-H8-BINAP forms on the crystal surface strongly interacted with DCM molecules, which induces melting of the (S)-BINAP-doped (S)-H8-BINAP crystal and trigger formation of an amorphous state without an afterglow capability. High-resolution afterglow patterning of the crystalline film is rapidly achieved by using cooperative organic vapo- and photo-stimulation. In addition to the benefit of rapid afterglow patterning, the formed afterglow images of the crystalline film can be repeatedly read out under ambient conditions without DCM vapor.

A flexible ligand and halogen engineering enable one phosphor-based full-color persistent luminescence in hybrid perovskitoids

Color-tunable room temperature phosphorescent (RTP) materials have raised wide interest due to their potential application in the fields of encryption and anti-counterfeiting. Herein, a series of CdX2-organic hybrid perovskitoids, (H-apim)CdX3 and (apim)CdX2 (denoted as CdX-apim1 and CdX-apim2, apim = 1-(3-aminopropyl)imidazole, X = Cl, Br), were synthesized using apim with both rigid and flexible groups as ligands, which exhibit naked-eye detectable RTP with different durations and colors (from cyan to red) by virtue of different halogen atoms, coordination modes and the coplanar configuration of flexible groups. Interestingly, CdCl-apim1 and CdX-apim2 both exhibit excitation wavelength-dependent RTP properties, which can be attributed to the multiple excitation of imidazole/apim, the diverse interactions with halogen atoms, and aggregated state of imidazoles. Structural analysis and theoretical calculations confirm that the aminopropyl groups in CdCl-apim1 do not participate in luminescence, while those in CdCl-apim2 are involved in luminescence including both metal/halogen to ligand charge transfer and twisted intramolecular charge transfer. Furthermore, we demonstrate that these perovskitoids can be applied in multi-step anti-counterfeiting, information encryption and smart ink fields. This work not only develops a new type of perovskitoid with full-color persistent luminescence, but also provides new insight into the effect of flexible ligands and halogen engineering on the wide-range modulation of RTP properties.

Achieving Ultralong Room-Temperature Phosphorescence in Covalent Organic Framework System

The combination of room-temperature phosphorescence (RTP) and covalent organic frameworks (COFs) would give rise to a new class of functional materials with sensing and responsive properties. However, such organic materials have been rarely reported, especially for those with long phosphorescence lifetimes. Here we report the incorporation of RTP emitters into COFs either via chemical decoration or noncovalent doping to achieve ultralong RTP in a COF system. The RTP emitters are designed with small phosphorescence rates and consequently exhibit ultralong phosphorescence lifetimes when nonradiative decay and oxygen quenching are suppressed in COF system. The RTP-COF materials have been found to possess oxygen sensing properties with large response of phosphorescence lifetimes.

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.

Chaperone Mimetic Strategy for Achieving Organic Room-Temperature Phosphorescence based on Confined Supramolecular Assembly

The development of organic materials that deliver room-temperature phosphorescence (RTP) is highly interesting for potential applications such as anticounterfeiting, optoelectronic devices, and bioimaging. Herein, a molecular chaperone strategy for controlling isolated chromophores to achieve high-performance RTP is demonstrated. Systematic experiments coupled with theoretical evidence reveal that the host plays a similar role as a molecular chaperone that anchors the chromophores for limited nonradiative decay and directs the proper conformation of guests for enhanced intersystem crossing through noncovalent interactions. For deduction of structure-property relationships, various structure-related descriptors that correlate with the RTP performance are identified, thus offering the possibility to quantitatively design and predict the phosphorescent behaviors of these systems. Furthermore, application in thermal printing is well realized for these RTP materials. The present work discloses an effective strategy for efficient construction of organic RTP materials, delivering a modular model which is expected to help expand the diversity of desirable RTP systems.

Continuous Condensed Triplet Accumulation for Irradiance-Induced Anticounterfeit Afterglow

Afterglow room-temperature emission that is independent of autofluorescence after ceasing excitation is a promising technology for state-of-the-art bioimaging and security devices. However, the low brightness of the afterglow emission is a current limitation for using such materials in a variety of applications. Herein, the continuous formation of condensed triplet excitons for brighter afterglow room-temperature phosphorescence is reported. (S)-(-)-2,2'-Bis(diphenylphosphino)-1,1'-binaphthyl ((S)-BINAP) incorporated in a crystalline host lattice showed bright green afterglow room-temperature phosphorescence under strong excitation. The small triplet-triplet absorption cross-section of (S)-BINAP in the whole range of visible wavelengths greatly suppressed the deactivation caused by Förster resonance energy transfer from excited states of (S)-BINAP to the accumulated triplet excitons of (S)-BINAP under strong continuous excitation. The steady-state concentration of the triplet excitons for (S)-BINAP reached 2.3 × 10-2  M, producing a bright afterglow. Owing to the brighter afterglow, afterglow detection using individual particles with sizes approaching the diffraction limit in aqueous conditions and irradiance-dependent anticounterfeiting can be achieved.

Recent advances in the design of afterglow materials: mechanisms, structural regulation strategies and applications

Afterglow materials are attracting widespread attention owing to their distinctive and long-lived optical emission properties which create exciting opportunities in various fields. Recent research has led to the discovery of many new afterglow materials featuring high photoluminescence quantum yields (PLQY) and lifetimes of up to several hours under ambient conditions. Afterglow materials are typically categorized according to their luminescence mechanism, such as long-persistent luminescence (LPL), room temperature phosphorescence (RTP), or thermally activated delayed fluorescence (TADF). Through rational design and novel synthetic strategies to modulate spin-orbit coupling (SOC) and populate triplet exciton states (T1), luminophores with long lifetimes and bright afterglow characteristics can be realized. Initial research towards afterglow materials focused mainly on pure inorganic materials, many of which possessed inherent disadvantages such as metal toxicity or low energy emissions. In recent years, organic-inorganic hybrid afterglow materials (OIHAMs) have been developed with high PLQY and long lifetimes. These hybrid materials exploit the tunable structure and easy processing of organic molecules, as well as enhanced SOC and intersystem crossing (ISC) processes involving heavy atom dopants, to achieve excellent afterglow performance. In this review, we begin by briefly discussing the structure and composition of inorganic and organic-inorganic hybrid afterglow materials, including strategies for regulating their lifetime, PLQY and luminescence wavelength. The specific advantages of organic-inorganic hybrid afterglow materials, including low manufacturing costs, diverse molecular/electronic structures, tunable structures and optical properties, and compatibility with a variety of substrates, are emphasized. Subsequently, we discuss in detail the fundamental mechanisms used by afterglow materials, their classification, design principles, and end applications (including sensing, anticounterfeiting, and photoelectric devices, among others). Finally, existing challenges and promising future directions are discussed, laying a platform for the design of afterglow materials for specific applications.

Exploration of violet-to-blue thermally activated delayed fluorescence emitters based on "CH/N" and "H/CN" substitutions at diphenylsulphone acceptor. A DFT study

The violet-to-blue thermally activated delayed fluorescence (TADF) emitters were created employing several substituents based on 5,5-dimethyl-5,10-dihydropyrido [2,3-b][1,8] naphthyridine-diphenylsulphone (DMDHPN-DPS) called 1a via "CH/N" and "H/CN" substitutions at the diphenylsulphone acceptor (DPS) moiety. The parent compound 1a was selected from our former work after extensive research employing "CH/N" substitution on Dimethyl-acridine (DMAC) donor moiety. There is a little overlap amid the highest occupied molecular orbitals (HOMOs) and lowest un-occupied molecular orbitals (LUMOs) due to the distribution of HOMOs and LUMOs primarily on the DMDHPN donor and the DPS acceptor moieties, respectively. It resulted in a narrower energy gap (∆E ST) between the lowest singlet (S1) and triplet (T1) excited state. In nearly all derivatives, the steric hindrance results in a larger torsional angle (85°-98°) between the plane of the DMDHPN and the DPS moieties. The predicted ΔE ST values of the compounds with "H/CN" substitution were lower than those of the comparable "CH/N" substituents, demonstrating the superiority of the reversible inter-system crossing (RISC) from the T1 → S1 state. All derivatives have emission wavelengths (λ em) in the range of 357-449 nm. The LUMO → HOMO transition energies in the S1 states are lowered by the presence of -CN groups or -N = atoms at the ortho or meta sites of a DPS acceptor unit, causing the λ em values to red-shift. Furthermore, the λ em showed a greater red-shift as there were more-CN groups or -N = atoms. Three of the derivatives named 1b, 1g, and 1h, emit violet (394 nm, 399 nm, and 398 nm, respectively), while two others, 1f and 1i, emit blue shade (449 nm each) with reasonable emission intensity peak demonstrating that these derivatives are effective violet-to-blue TADF nominees. The lower ΔE ST value for derivative 1i (0.01 eV) with λ em values of 449 nm make this molecule the finest choice for blue TADF emitter amongst all the studied derivatives. We believe our research might lead to the development of more proficient blue TADF-OLEDs in the future.

Materials for Electrochemiluminescence: TADF, Hydrogen-Bonding, and Aggregation- and Crystallization-Induced Emission Luminophores

Electrochemiluminescence (ECL) is a rapidly growing discipline with many analytical applications from immunoassays to single-molecule detection. At the forefront of ECL research is materials chemistry, which looks at engineering new materials and compounds exhibiting enhanced ECL efficiencies compared to conventional fluorescent materials. In this review, we summarize recent molecular design strategies that lead to high efficiency ECL. In particular, we feature recent advances in the use of thermally activated delayed fluorescence (TADF) emitters to produce enhanced electrochemiluminescence. We also document how hydrogen bonding, aggregation, and crystallization can each be recruited in the design of materials showing enhanced electrochemiluminescence.

Mechanism landscape in pyrylium induced organic afterglow systems

Manipulation of excited states and their dynamics represents a central topic in luminescence systems. We report an unexpected emergence of a high-performance organic afterglow in pyrylium induced photopolymerization systems, as well as the establishment of the mechanism landscape of the afterglow systems as a function of monomer types. In the case of methyl methacrylate, after pyrylium-catalyzed photopolymerization, the obtained materials exhibit a TADF-type organic afterglow with an afterglow efficiency of 70.4%. By using heavy-atom-containing methacrylate, the external heavy atom effect speeds up phosphorescence decay and switches on room-temperature phosphorescence in pyrylium-polymer systems. When 9-vinylcarbazole is used, the resultant materials display organic long persistent luminescence with hour-long durations and emission maxima around 650 nm. The intriguing mechanism landscape reflects the delicate balance of multiple photophysical processes in the pyrylium induced organic afterglow systems, which has been rarely explored in the reported studies.

Elucidation of an Aggregate Excited State in the Electrochemiluminescence and Chemiluminescence of a Thermally Activated Delayed Fluorescence (TADF) Emitter

The electrochemistry, electrochemiluminescence (ECL), and chemiluminescence (CL) properties of a thermally activated delayed fluorescence (TADF) emitter 4,4'-(1,2-dihydroacenaphthylene-5,6-diyl)bis(N,N-diphenylaniline) (TPA-ace-TRZ) and three of its analogues were investigated. TPA-ace-TRZ exhibits both (a) delayed onset of ECL and (b) long-persistent luminescence, which we have attributed to the formation of an aggregate excited state in excimer or exciplex form. The evidence of this aggregate excited state was consistent across ECL annihilation and coreactant pathways as well as in CL. The absolute ECL efficiency of TPA-ace-TRZ using benzoyl peroxide (BPO) as a coreactant was found to be 0.028%, which was 9-fold stronger than the [Ru(bpy)₃]²⁺/BPO reference coereactant system. Furthermore, the absolute CL quantum efficiency of TPA-ace-TRZ was determined to be 0.92%. The performance and flexibility of the TADF emitter TPA-ace-TRZ under these various emissive pathways are highly desirable toward applications in sensing, imaging, and light-emitting devices.

Host-Guest Doping in Flexible Organic Crystals for Room-Temperature Phosphorescence

Organic single crystals (OSCs) with excellent flexibility and unique optical properties are of great importance due to their broad applicability in optical/optoelectronic devices and sensors. Nevertheless, fabricating flexible OSCs with room-temperature phosphorescence (RTP) remains a great challenge. Herein, we propose a host-guest doping strategy to achieve both RTP and flexibility of OSCs. The single-stranded crystal is highly bendable upon external force application and can immediately return to its original straight shape after removal of the stress, impressively emitting bright deep-red phosphorescence. The theoretical and experimental results demonstrate that the bright RTP arises from Förster resonance energy transfer (FRET) from the triphenylene molecules to the dopants. This strategy is both conceptually and synthetically simple and offers a universal approach for the preparation of flexible OSCs with RTP.

Conformational isomeric thermally activated delayed fluorescence (TADF) emitters: mechanism, applications, and perspectives

Thermally activated delayed fluorescence (TADF) materials have received enormous attention and the mechanism behind them has been investigated in depth. It has been found that some donor-acceptor (D-A) type TADF emitters could obviously exhibit dual stable conformations in the ground states and their distributions significantly affect the physical properties and device performances. Therefore, professional analysis and a summary of the relationship between molecular structures and performances are very important. In this review, we first summarize the mechanism and properties of TADF emitters with conformational isomerism. We also classify their recent progress according to their different applications, and provide an outlook on their perspectives.

Effect of void-carbon on blue-shifted luminescence in TADF molecules by theoretical simulations

Thermally activated delayed fluorescence (TADF) molecules have a theoretical 100% photoluminescence quantum yield in comparison with traditional fluorescent materials, leading to broad application in organic light-emitting diode (OLED). However, the application of TADF molecules with conjugated donor-acceptor structures in blue OLED remains a challenge due to their generally narrow energy gap between frontier molecular orbitals. Recently, a strategy has been approved in the improvement of the performance in TADF, in which void-carbon atoms between donor and acceptor fragments (donor-void-acceptor (D-v-A)) could regulate blue light emission. In this study, we first select three reported isomers followed by two proposed D-v-A TADF isomers to verify the feasibility of the void-carbon strategy through evaluation of the electronic structures in the excited state and photophysical properties. We further proposed a series of TADF molecules by replacing different donor and acceptor fragments to assess the applicability of the void-carbon strategy from the aspect of simulations in electronic structures, different properties of donor and acceptor fragments, photophysical properties, and analysis in the molecular conjugation. The results indicate that void-carbon strategy has conditional feasibility and applicability. Donor-acceptor molecular properties could be tuned through void-carbon strategy on aromatic acceptor fragments during the selection of promising candidates of TADF molecules. However, the void-carbon strategy does not work for the molecules with antiaromatic acceptor fragments, where the steric hindrance of the molecules plays a dominant role. Our work provides insightful guidance for the design of the blue-emission TADF molecules.

Enhanced Red Persistent Room-Temperature Phosphorescence Induced by Orthogonal Structure Disruption during Electronic Relaxation

Bright, persistent, room-temperature phosphorescence (RTP) at long wavelengths is crucial for high-resolution imaging in the absence of in vivo autofluorescence. However, efficient long-wavelength RTP is difficult. Here, enhanced red RTP based on a unique mechanism was observed from deuterated dibenzo[g.p]chrysenes substituted with a phenoxazine. The yield was 16%, with an average lifetime of 1.8 s. An orthogonal dihedral angle between the dibenzo[g.p]chrysene and the phenoxazine in the lowest excited singlet state allowed a forbidden fluorescence to increase triplet generation. When the dihedral angle changed, disengagement of the forbidden fluorescence from the excited singlet state occurred, and the lowest triplet excited state had a facilitated phosphorescence rate without increasing its nonradiative transition rate. The facilitated phosphorescence rate as well as the large triplet yield led to the enhanced red RTP.

Efficiently increasing the radiative rate of TADF material with metal coordination

Herein, a simple and straightforward method to reduce dramatically the lifetime of a pure organic thermally activated delayed fluorescence (TADF) material VIA metal coordination is demonstrated. We designed a mononuclear silver complex [Ag(PPh₂CH₃)(TCzBN-PyPz)]BF₄ (1) with a new emissive TCzBN-PyPz ligand. Even though the ligand and the metal complex have very similar emissive efficiencies and maximal peaks, over three orders of magnitude shorter lifetime of 0.59 μs for the complex than 2074 μs for ligand were obtained. Compared to other methods, the present protocol seems to be simple and highly effective.

Boosting Organic Afterglow Performance via a Two-Component Design Strategy Extracted from Macromolecular Self-Assembly

Because intersystem crossing and phosphorescence decay are spin-forbidden in organic systems, it is challenging to obtain high-performance organic afterglow materials. Inspired by two-component design strategy from macromolecular self-assembly, here we report the utilization of synthetic polymers to control the excited state properties of difluoroboron β-diketonate (BF₂bdk) and deuterated BF₂bdk compounds for the fabrication of room-temperature organic afterglow materials. The polymer component can interact with BF₂bdk excited states by dipole-dipole interactions, lower BF₂bdk S₁ levels with insignificant effect on T₁ levels, reduce ΔE_ST, and thus enhance intersystem crossing of BF₂bdk excited states. The polymer component can also suppress intramolecular motion of BF₂bdk triplets and protect BF₂bdk triplets from oxygen quenching. The obtained BF₂bdk-polymer afterglow materials exhibit emission lifetimes up to 2.2 s and high photoluminescence quantum yields under ambient conditions, display excellent processability and flexibility, and can function as efficient donors for excited state energy transfer to construct red afterglow materials.

Halide-containing organic persistent luminescent materials for environmental sensing applications

Great progress has been made in the development of various organic persistent luminescent (OPL) materials in the past few years, and increasing attention has been paid to their interesting applications in environmental sensing due to their long emission lifetimes and high sensitivity. Especially, the introduction of different halogen elements facilitates highly efficient OPL emission with distinct lifetimes and colours. In this review, we summarize the current status of the halide-containing OPL materials for environmental sensing applications. To begin with, the photophysical processes and luminescence mechanisms of OPL materials are expounded in detail to better understand the relationship among molecular structures, OPL properties, and sensing applications. Then, representative halide-containing material systems, such as small molecules, polymers, and doping systems, are summarized with their interesting applications in sensing temperature, oxygen, H₂O, UV light and organic solvents. In addition, several challenges and future research opportunities in this field are discussed. This review aims to provide some reasonable guidance on the material design of OPL sensors and their practical applications, and tries to provide a new perspective on the application direction of organic optoelectronics.

This review presents a summary of the molecular design of halide-containing organic persistent luminescent materials, and their environmental sensing applications.

Development of silica-coated rare-earth doped strontium aluminate toward superhydrophobic, anti-corrosive and long-persistent photoluminescent epoxy coating

Long-persistent phosphorescent smart paints have the ability to continue glowing in the dark for a prolonged time period to function as energy-saving products. Herein, new epoxy/silica nanocomposite paints were prepared with different concentrations of lanthanide-doped aluminate nanoparticles (LAN; SrAl₂ O₄ : Eu²⁺ , Dy³⁺ ). The LAN pigment was firstly coated with SiO₂ utilizing the heterogeneous precipitation technique to provide LAN-encapsulated between SiO₂ nanoparticles (LAN@SiO₂ ). The epoxy/silica/lanthanide-doped aluminate nanoparticles (ESLAN) nanocomposite paints were coated on steel. The prepared ESLAN paints were studied by transmission electron microscope (TEM), infrared spectra (FTIR), scanning electron microscope (SEM), X-ray fluorescence analysis (XRF), and energy-dispersive X-ray spectra (EDS). The transparency and coloration properties of the nanocomposite coated films were explored by CIE Lab parameters and photoluminescence spectra. The ultraviolet-induced luminescence properties of the transparent coated films demonstrated greenish phosphorescence at 518 nm upon excitation at 368 nm. Both hardness and hydrophobic activities were investigated. The anticorrosion activity of the nanocomposite films coated onto mild steel substrates immersed in NaCl_(aq) (3.5%) was studied by the electrochemical impedance spectral (EIS) analysis. The silica-containing coatings were monitored to exhibit anticorrosion properties. Additionally, the nanocomposite films with LAN@SiO₂ (25%) exhibited the optimized long-lasting luminescence properties in the dark for 90 minutes. The nanocomposite films showed highly reversible and durable long-lived phosphorescence.

Recent Advances on Host-Guest Material Systems toward Organic Room Temperature Phosphorescence

The design and characterization of purely organic room-temperature phosphorescent (RTP) materials for optoelectronic applications is currently the focus of research in the field of organic electronics. Particularly, with the merits of preparation controllability and modulation flexibility, host-guest material systems are encouraging candidates that can prepare high-performance RTP materials. By regulating the interaction between host and guest molecules, it can effectively control the quantum efficiency, luminescent lifetime, and color of host-guest RTP materials, and even produce RTP emission with stimuli-responsive features, holding tremendous potential in diverse applications such as encryption and anti-counterfeiting, organic light-emitting diodes, sensing, optical recording, etc. Here a roundup of rapid achievement in construction strategies, molecule systems, and diversity of applications of host-guest material systems is outlined. Intrinsic correlations between the molecular properties and a survey of recent significant advances in the development of host-guest RTP materials divided into three systems including rigid matrix, exciplex, and sensitization are presented. Providing an insightful understanding of host-guest RTP materials and offering a promising platform for high throughput screening of RTP systems with inherent advantages of simple material preparation, low-cost, versatile resource, and controllably modulated properties for a wide range of applications is intended.

Highly efficient organic long persistent luminescence based on host–guest doping systems

Recently, organic long persistent luminescence (OLPL) has attracted widespread attention as a new luminescence pathway initiated by the exciplex. However, the low quantum yield, few alternative molecules and high fabrication cost seriously slow down the development of OLPL materials. Herein, a series of simple multi-guest/host OLPL materials with a high quantum yield are reported by doping four phenothiazine derivative guest molecules into 9H-xanthen-9-one host matrices. The F-substituted phenothiazine derivative doping system displays highly efficient emission with 46.3% quantum yield in air. Meanwhile, these OLPL materials provide broad opportunities for further application in the field of heat resistance due to their highly efficient luminescence at high temperatures.

A series of high quantum yield organic long persistent luminescence (OLPL) materials were obtained by doping four phenothiazine derivatives into a host molecule (9H-xanthen-9-one). Power-law decay is exhibited by OLPL systems.

Merging photoinitiated bulk polymerization and the dopant-matrix design strategy for polymer-based organic afterglow materials

The polymer produced by photoinitiated bulk polymerization promotes intersystem crossing of luminescent dopants and switches on room-temperature organic afterglow.

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 Cu^I 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 Cu^I complexes, resulting in semi-free halogen ions and in situ generated Cu^I 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.

Thermally activated delayed fluorescence materials as organic photosensitizers

Photosensitizer molecules play a crucial role in materials and life sciences. Efforts to improve their performance and reduce the associated costs are therefore vital for advancing environmentally friendly light-driven technologies. In this Feature Article, we describe the use of photosensitizers that make use of thermally activated delayed fluorescence (TADF), their benefits compared to conventional fluorescent and phosphorescent sensitizers, and the efforts of our group and others to develop emitters with application-tailored properties. The key feature is the diversity of accessible excited state pathways, which may be tuned by molecular and supramolecular approaches to suit a particular problem. This unique property has allowed TADF emitters to become competitive for applications including TADF-sensitized fluorescence in light emitting diodes and chemical sensing, organic long persistent luminescence, photodynamic therapy, and non-coherent photon upconversion.

TADF-Type Organic Afterglow

We report a highly efficient dopant-matrix afterglow system enabled by TADF mechanism to realize afterglow quantum yields of 60-70 %, which features a moderate rate constant for reverse intersystem crossing (k_RISC ) to simultaneously improve afterglow quantum yields and maintain afterglow emission lifetime. Difluoroboron β-diketonate (BF₂ bdk) compounds are designed with multiple electron-donating groups to possess moderate k_RISC values and are selected as luminescent dopants. The matrices with carbonyl functional groups such as phenyl benzoate (PhB) have been found to interact with and perturb BF₂ bdk excited states by dipole-dipole interactions and thus enhance the intersystem crossing of BF₂ bdk excited states. Through dopant-matrix collaboration, the efficient TADF-type afterglow materials have been achieved to exhibit excellent processability into desired shapes and large-area films by melt casting, as well as aqueous afterglow dispersions for potential bioimaging applications.

Two-Photon Ionization Induced Stable White Organic Long Persistent Luminescence

Organic long persistent luminescence (OLPL) materials with afterglow duration in the scale of minutes or even hours are still rare. Most OLPL systems are based on exciplexes, which require complicated multi-component system in order to realize white afterglow but with slightly compromised duration and color stability. In this work, OLPLs lasting from 20 to 40 minutes are realized in a simple binary system based on two-photon ionization mechanism, which can simultaneously harvest excitons from both singlet and triplet excited states, making it potentially one of the most promising candidates to achieve stable white OLPL. Through modulation and optimization of dopant molecules in dibenzo[b,d]thiophen-2-yldiphenyl phosphine oxide host, the emission profiles of afterglow can be readily tuned from cyan (0.19, 0.22), cold white (0.31, 0.35), standard white (0.33, 0.33) to warm white (0.31, 0.46), with excellent color consistency.

Aromatic Electrophilic Directing for Fluorescence and Room-Temperature Phosphorescence Modulation

The ability to modulate luminescence is crucial for organic light-emitting molecules. However, the correlation between molecular structure and emission is not always obvious and systematic. Here, using a well-established empirical rule on electrophilic substitution involving directing groups in organic chemistry, we present a model system, where two luminophores are covalently linked to benzene ortho, meta, and para to each other, to demonstrate that the rule can also be useful in predicting the fluorescence and phosphorescence behaviors of these disubstituted benzene molecules. The benzene ring works as a "molecular wire" that transduces electron density when the two luminophores form ortho- and para-isomers, while little to no transduction can be noted for the meta-isomer, based on well-established organic chemistry. We anticipate that many more "textbook examples" of electronic directing in organic chemistry can be used for systematic modulation of molecular fluorescence and room-temperature phosphorescence.

Harvesting triplet excitons for near-infrared electroluminescence via thermally activated delayed fluorescence channel

Near-infrared (NIR) emission is useful for numerous practical applications, such as communication, biomedical sensors, night vision, etc., which encourages researchers to develop materials and devices for the realization of efficient NIR organic light-emitting devices. Recently, the emerging organic thermally activated delayed fluorescence (TADF) emitters have attracted wide attention because of the full utilization of electron-generated excitons, which is crucial for achieving high device efficiency. Up to now, the TADF emitters have shown their potential in the deep red/NIR region. Considering the color purity and efficiency, however, the development of NIR TADF emitters still lags behind RGB TADF emitters, indicating that there is still much room to improve their performance. In this regard, this perspective mainly summarizes the past progress of molecular design on constructing TADF NIR emitters. We hope this perspective could provide a new vista in developing NIR materials and enlighten breakthroughs in both fundamental research and applications.