Organic long persistent luminescence

Enabling long-lived organic room temperature phosphorescence in polymers by subunit interlocking

Long-lived room temperature phosphorescence (LRTP) is an attractive optical phenomenon in organic electronics and photonics. Despite the rapid advance, it is still a formidable challenge to explore a universal approach to obtain LRTP in amorphous polymers. Based on the traditional polyethylene derivatives, we herein present a facile and concise chemical strategy to achieve ultralong phosphorescence in polymers by ionic bonding cross-linking. Impressively, a record LRTP lifetime of up to 2.1 s in amorphous polymers under ambient conditions is set up. Moreover, multicolor long-lived phosphorescent emission can be procured by tuning the excitation wavelength in single-component polymer materials. These results outline a fundamental principle for the construction of polymer materials with LRTP, endowing traditional polymers with fresh features for potential applications.

Long-lived room temperature phosphorescence (LRTP) is important in organic photonics but exploring a universal approach to obtain LRTP in amorphous polymers is challenging. Here the authors present a facile chemical strategy to achieve ultralong phosphorescence in polymers by ionic bonding cross-linking.

A building-block design for enhanced visible-light switching of diarylethenes

Current development of light-responsive materials and technologies imposes an urgent demand on visible-light photoswitching on account of its mild excitation with high penetration ability and low photo-toxicity. However, complicated molecular design and laborious synthesis are often required for visible-light photoswitch, especially for diarylethenes. Worse still, a dilemma is encountered as the visible-light excitation of the diarylethene is often achieved at the expense of photochromic performances. To tackle these setbacks, we introduce a building-block design strategy to achieve all-visible-light photochromism with the triplet-sensitization mechanism. The simply designed diarylethene system is constructed by employing a sensitizer building-block with narrow singlet-triplet energy gap (ΔE_ST) to a diarylethene building-block. A significant improvement on the photochromic efficiency is obtained as well as an enhanced photo-fatigue resistance over those under UV irradiation. The balance between the visible-light excitation and decent photochromism is thus realized, promoting a guiding principle for the visible-light photochromism.

Ultralong room-temperature phosphorescence of a solid-state supramolecule between phenylmethylpyridinium and cucurbit[6]uril

Long-lived organic room-temperature phosphorescence (RTP) has received great attention because of its various potential applications. Herein, we report a persistent RTP of a solid-state supramolecule between a cucurbit[6]uril (CB[6]) host and a heavy-atom-free phenylmethylpyridinium guest. Significantly, the long-lived phosphorescence completely depends on the host-guest complexation, revealing that the non-phosphorescent guest exhibits a 2.62 s ultralong lifetime after being complexed by CB[6] under ambient conditions. The ultralong RTP is because of tight encapsulation of CB[6], which boosts intersystem crossing, suppresses nonradiative relaxation and possibly shields quenchers. Moreover, several phosphorescent complexes possessing different lifetimes are prepared and successfully applied in triple lifetime-encoding for data encryption and anti-counterfeiting. This strategy provides a new insight for realizing purely organic RTP with ultralong lifetime and expands its application in the field of information protection.

Phosphorescent Material Search Using a Combination of High-Throughput Evaluation and Machine Learning

High-throughput experiments including combinatorial chemistry are useful for generating large amounts of data within a short period of time. Machine learning can be used to predict the regularity of a response variable using a statistical model of a data set. Because a combination of these methods can accelerate the material development, we applied such a combination to a search of semiconducting thin films prepared on an Eu and Dy codoped SrAl₂O₄-based phosphorescent material to improve the lifetime of its afterglow. Oxide targets MgO, GeO₂, Ga₂O₃, ZnO, Bi₂O₃, Ta₂O₅, TiO₂, and Y₂O₃ were deposited to form a thin film on a SrAl₂O₄ substrate as a combinatorial library with a systematical change in these ratios. The sample was calcined under several conditions, and a data set of 800 examples was obtained using a high-throughput evaluation. The 800 examples were then randomly divided into training and test data sets. The lifetime of the afterglow was interpolated through machine learning using the film thickness of each element and the calcined condition of the training data set as explanatory variables. The accuracy of the interpolation was evaluated using a correlation coefficient and the root mean squared error of the predicted values with respect to the experimental values of the test data set. As a result, it was found that a MgO thin film is effective at improving the lifetime of the afterglow and that its optimum condition is a film thickness of approximately 100 nm with calcination at 400-600 °C in air.

Long-Lived Room-Temperature Phosphorescence for Visual and Quantitative Detection of Oxygen

An unconventional organic molecule (TBBU) showing obvious long-lived room temperature phosphorescence (RTP) is reported. X-ray single crystal analysis demonstrates that TBBU molecules are packed in a unique fashion with side-by-side arranged intermolecular aromatic rings, which is entirely different from the RTP molecules reported to date. Theoretical calculations verify that the extraordinary intermolecular interaction between neighboring molecules plays an important role in RTP of TBBU crystals. More importantly, the polymer film doped with TBBU inherits its distinctive RTP property, which is highly sensitive to oxygen. The color of the doped film changes and its RTP lifetime drops abruptly through a dynamic collisional quenching mechanism with increasing oxygen fraction, enabling visual and quantitative detection of oxygen. Through analyzing the grayscale of the phosphorescence images, a facile method is developed for rapid, visual, and quantitative detection of oxygen in the air.

Tricomponent Exciplex Emitter Realizing over 20% External Quantum Efficiency in Organic Light-Emitting Diode with Multiple Reverse Intersystem Crossing Channels

With the naturally separated frontier molecular orbitals, exciplexes are capable of thermally activated delayed fluorescence emitters for organic light-emitting diodes (OLEDs). And, the current key issue for exciplex emitters is improving their exciton utilization. In this work, a strategy of building exciplex emitters with three components is proposed to realize multiple reverse intersystem crossing (RISC) channels, improving their exciton utilization by enhancing upconversion of nonradiative triplet excitons. Accordingly, a tricomponent exciplex DBT-SADF:PO-T2T:CDBP is constructed with three RISC channels respectively on DBT-SADF, DBT-SADF:PO-T2T, and CDBP:PO-T2T. Furthermore, its photoluminescence quantum yield and rate constant of the RISC process are successfully improved. In the OLED, DBT-SADF:PO-T2T:CDBP exhibits a remarkably high maximum external quantum efficiency (EQE) of 20.5%, which is the first report with an EQE over 20% for the OLEDs based on exciplex emitters to the best of our knowledge. This work not only demonstrates that introducing multiple RISC channels can effectively improve the exciton utilization of exciplex emitters, but also proves the superiority of the tricomponent exciplex strategy for further development of exciplex emitters.

Two-photon-excited ultralong organic room temperature phosphorescence by dual-channel triplet harvesting

Small energy gap boosts dual-channel triplet harvesting via TADF and UOP, which suppresses long-lived triplet concentration quenching. An infrared laser (808 nm) is able to induce persistent emission under ambient conditions.

Due to inefficient molecular design strategies, two-photon-excited ultralong organic room temperature phosphorescence (TPUOP) has not yet been reported in single-component materials. Herein, we present an innovative design method by dual-channel triplet harvesting to obtain the first bright TPUOP molecule with a lifetime of 0.84 s and a quantum efficiency of 16.6%. In compound o-Cz the donor and acceptor units are connected at the ortho position of benzophenone, showing intramolecular space charge transfer. Therefore, the two-photon absorption ability is improved due to the enhanced charge transfer character. Moreover, the small energy gap boosts dual-channel triplet harvesting via ultralong thermally activated delayed fluorescence and H-aggregation phosphorescence, which suppresses the long-lived triplet concentration quenching. Through two-photon absorption, a near-infrared laser (808 nm) is able to trigger the obvious ultralong emission under ambient conditions. This research work provides valuable guidance for designing near-infrared-excited ultralong organic room temperature phosphorescence materials.

High-efficiency blue thermally activated delayed fluorescence from donor-acceptor-donor systems via the through-space conjugation effect

The photophysical optimization of donor (D)-acceptor (A) molecules is a real challenge because of the intrinsic limitation of their charger transfer (CT) excited states. Herein, two D-A-D molecules featuring blue thermally activated delayed fluorescence (TADF) are developed, in which a homoconjugated acceptor 5,10-diphenyl-5,10-dihydrophosphanthrene oxide (DPDPO2A) is incorporated to bridge four carbazolyl or 3,6-di-t-butyl-carbazolyl groups for D-A interaction optimization without immoderate conjugation extension. It is shown that the through-space conjugation effect of DPDPO2A can efficiently enhance intramolecular CT (ICT) and simultaneously facilitate the uniform dispersion of the frontier molecular orbitals (FMO), which remarkably reduces the singlet-triplet splitting energy (ΔE ST) and increases FMO overlaps for radiation facilitation, resulting in the 4-6 fold increased rate constants of reverse intersystem crossing (RISC) and singlet radiation. The maximum external quantum efficiency beyond 20% and the state-of-the-art efficiency stability from sky-blue TADF OLEDs demonstrate the effectiveness of the "conjugation modulation" strategy for developing high-performance optoelectronic D-A systems.

Crucial Breakthrough of Functional Persistent Luminescence Materials for Biomedical and Information Technological Applications

Persistent luminescence is a phenomenon in which luminescence is maintained for minutes to hours without an excitation source. Owing to their unique optical properties, various kinds of persistent luminescence materials (PLMs) have been developed and widely employed in numerous areas, such as bioimaging, phototherapy, data-storage, and security technologies. Due to the complete separation of two processes, -excitation and emission-, minimal tissue absorption, and negligible autofluorescence can be obtained during biomedical fluorescence imaging using PLMs. Rechargeable PLMs with super long afterglow life provide novel approaches for long-term phototherapy. Moreover, owing to the exclusion of external excitation and the optical rechargeable features, multicolor PLMs, which have higher decoding signal-to-noise ratios and high storage capability, exhibited an enormous application potential in information technology. Therefore, PLMs have significantly promoted the application of optics in the fields of multimodal bioimaging, theranostics, and information technology. In this review, we focus on the recently developed PLMs, including inorganic, organic and inorganic-organic hybrid PLMs to demonstrate their superior applications potential in biomedicine and information technology.

Exciplex Organic Light-Emitting Diodes with Nearly 20% External Quantum Efficiency: Effect of Intermolecular Steric Hindrance between the Donor and Acceptor Pair

Exciplex emitters have emerged as an important class of thermally activated delayed fluorescence (TADF) materials for highly efficient OLEDs. A TADF exciplex emitter requires an intermolecular donor/acceptor pair. We have synthesized a bipolar donor-type material, DPSTPA, which was used to pair with known acceptor materials (2CzPN, 4CzIPN, or CzDBA). The OLEDs based on the exciplex emitters, DPSTPA/X, where X = 2CzPN and CzDBA, give green and orange-red colors with record-high external quantum efficiencies (EQEs) of 19.0 ± 0.6 and 14.6 ± 0.4%, respectively. In contrast, the exciplex pair DPSTPA/4CzIPN gave a very low photoluminescence quantum yield (PLQY) and a very low EQE value of the device. The DFT calculations indicate that the intermolecular distance between the donor and the acceptor plays a key factor for the PLQY and EQE. The observed low PLQY and the poor device performance for the DPSTPA/4CzIPN pair are probably because of the relatively long distance between the DPSTPA and 4CzIPN in the thin film caused by the four congested carbazole (Cz) groups of 4CzIPN, which effectively block the interaction of the nitrile acceptor with the triphenylamino donor of DPSTPA.

A Highly Efficient Red Metal-free Organic Phosphor for Time-Resolved Luminescence Imaging and Photodynamic Therapy

Developing highly efficient red metal-free organic phosphors for biological applications is a formidable challenge. Here, we report a novel molecular design principle to obtain red metal-free organic phosphors with long emission lifetime (504.6 μs) and high phosphorescence efficiency (14.6%) from the isolated molecules in the crystal. Furthermore, the well-dispersed phosphorescent nanodots (PNDs) with the particle size around 5 nm are prepared through polymer-encapsulation in an aqueous solution, which show good biocompatibility and low cytotoxicity. The metal-free PNDs are successfully applied to time-resolved luminescence imaging to eliminate background fluorescence interference both in vitro and vivo as well as effective photodynamic anticancer therapy for the first time. This work will not only pave a pathway to develop highly efficient metal-free RTP materials but also expand the scope of their applications to biomedical fields.

Direct population of triplet excited states through singlet-triplet transition for visible-light excitable organic afterglow

Invoking efficient afterglow in metal-free organic molecules represents an important material advancement. However, organic afterglow suffers from low intensity and efficiency and generally needs to be excited by UV light owing to its spin-forbidden phosphorescent nature that essentially requires facile intersystem crossing (ISC). Here, we propose a strategy to bypass the traditional ISC through facilitating singlet-triplet transition to directly populate triplet excited states from the ground state by combining synergetic effects of both heavy/hetero-atom incorporation and aromatic aggregation. Verified by systematic experimental and computational investigations, this unique singlet-to-triplet absorption results in a much improved organic afterglow quantum efficiency up to 9.5% with a prolonged lifetime of 0.25 s under visible-light irradiation. Fundamentally, this work illustrates for the first time the great potential of the direct population method to red-shift the excitation wavelength and improve the afterglow efficiency, offering important clues for the development of triplet-state involved organic optoelectronic technologies.

Enhancing the performance of pure organic room-temperature phosphorescent luminophores

Once considered the exclusive property of metal complexes, the phenomenon of room-temperature phosphorescence (RTP) has been increasingly realized in pure organic luminophores recently. Using precise molecular design and synthetic approaches to modulate their weak spin-orbit coupling, highly active triplet excitons, and ultrafast deactivation, organic luminophores can be endowed with long-lived and bright RTP characteristics. This has sparked intense explorations into organic luminophores with enhanced RTP features for different applications. This Review discusses the fundamental mechanism of RTP in pure organic luminophores, followed by design principles, enhancement strategies, and formulation methods to achieve highly phosphorescent and long-lived organic RTP luminophores even in aqueous media. The current challenges and future directions of this field are also discussed in the summary and outlook.

Recent Advances in Organic Light-Emitting Diodes Based on Pure Organic Room Temperature Phosphorescence Materials

Pure organic room temperature phosphorescence (RTP) materials have attracted extensive attention in recent years due to their unique characteristics, such as flexible design method, low toxicity, low cost, as well as the ease of production at scale. The involvement of triplet state and direct radiative transition from the triplet state show that RTP materials have great potential as a new generation emitter in organic light-emitting diodes (OLEDs). Based on the mechanism of phosphorescence, various methods have been developed to achieve RTP emissions in the crystal state. However, the observation of RTP in the thin film state is much more difficult to achieve because of the lower degree of rigidity and suppression of the non-radiative transition. In this mini-review, molecular design strategies developed to achieve RTP emissions and their application in OLEDs are summarized and discussed. The conclusion and outlook point to great potential as well as the challenges for the continued study of pure organic RTP materials-based OLEDs.

Realizing 20% External Quantum Efficiency in Electroluminescence with Efficient Thermally Activated Delayed Fluorescence from an Exciplex

The investigation of nondoped exciplex blends of 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), working as the one-electron acceptor molecule, with different electron donors is reported. The emissions of these exciplexes span from the blue to orange-red regions, showing clear contribution from thermally activated delayed fluorescence (TADF) and delayed fluorescence originated from nongeminate recombination of charge carriers created by the dissociation of optically generated exciplexes. We focus our studies on the properties of TADF in these systems, covering in particular the physical meaning of the different transient components observed in their luminescence decays. Our results unravel the intricate role of reverse intersystem crossing due to spin-orbit coupling and possibly also due to hyperfine interactions and internal conversion, which affect the efficiency of the TADF mechanism. Remarkable performances are obtained in prototype organic light-emitting diodes fabricated with some of these blends. Green exciplex blends, in particular, exhibited the current efficiency of 60 cd A^-1, power efficiency of 71 lm W^-1, and external quantum efficiency of 20%. We believe that our results will contribute significantly to highlight the potential advantages of intermolecular exciplexes in the area of organic light-emitting diodes.

Boosting the efficiency of organic persistent room-temperature phosphorescence by intramolecular triplet-triplet energy transfer

Persistent luminescence is a fascinating phenomenon with exceptional applications. However, the development of organic materials capable of persistent luminescence, such as organic persistent room-temperature phosphorescence, lags behind for their normally low efficiency. Moreover, enhancing the phosphorescence efficiency of organic luminophores often results in short lifetime, which sets an irreconcilable obstacle. Here we report a strategy to boost the efficiency of phosphorescence by intramolecular triplet-triplet energy transfer. Incorpotation of (bromo)dibenzofuran or (bromo)dibenzothiophene to carbazole has boosted the intersystem crossing and provided an intramolecular triplet-state bridge to offer a near quantitative exothermic triplet–triplet energy transfer to repopulate the lowest triplet-state of carbazole. All these factors work together to contribute the efficient phosphorescence. The generation and transfer of triplet excitons within a single molecule is revealed by low-temperature spectra, energy level and lifetime investigations. The strategy developed here will enable the development of efficient phosphorescent materials for potential high-tech applications.

The potential of organic materials with persistent room-temperature phosphorescence for high-tech application is limited by their low efficiency. Here, the authors report a strategy to enhance persistent room-temperature phosphorescence efficiency via intramolecular triplet-triplet energy transfer.

Mechanochromic Wide-Spectrum Luminescence Based on a Monoboron Complex

A reversible mechanochromic luminescent material based on a simple tetrahedral monoboron complex (B-1) is described. Interestingly, in addition to amorphous powders (P), the compound could exist in three unique crystal states (A, B, and C), showing efficient green-to-red luminescent colors, which is a result of wane and wax of dual emissions of the compound. Surprisingly, one of the emissions increases significantly with increasing temperature, fully offsetting the quenching effect of temperature-assisted internal conversion process. The four states are fully interconvertible through grinding and heating, allowing color writing/painting with a single ink.

Binary small molecule organic nanoparticles exhibit both direct and diffusion-limited ultrafast charge transfer with NIR excitation

Here we describe a facile, one-step synthesis of a binary organic nanoparticle composed completely of NIR-absorbing small molecules, a quatterylene diimide and a vanadyl napthalocyanine, using Flash Nanoprecipitation. We show that the molecules are co-encapsulated within an amphiphilic block copolymer shell by observing distinct ultrafast dynamics in the binary nanoparticles compared to nanoparticles of their individual components, which we rationalize as a photoinduced charge transfer. We then draw similarities between the charge transfer dynamics studied in our system and the charge dissociation process in macroscale organic bulk heterojunction blends for OPV applications by assigning the ultrafast time component (∼10 ps) to direct interfacial charge transfer and the slow component (70-200 ps) to diffusion limited charge transfer. This discovery can inspire the development of mixed-composition nanoparticles with new functionality for optoelectronic and theranostic applications.

Polymorphic Pure Organic Luminogens with Through-Space Conjugation and Persistent Room-Temperature Phosphorescence

Pure organic luminogens with persistent room-temperature phosphorescence (p-RTP) have attracted increasing attention owing to their vital significance and potential applications in security inks, bioimaging, and photodynamic therapy. Previously reported p-RTP luminogens normally possessed through-bond conjugation. In this work, we report a pure organic luminogen, AN-MA, the Diels-Alder cycloaddition adduct of anthracene (AN) and maleic anhydride (MA), which possesses isolated phenyl groups and an anhydride moiety. AN-MA exhibits aggregation-enhanced emission (AEE) characteristics with efficiency of approximately 2 % and up to 8.5 % in solution and crystals, respectively. Two polymorphs of AN-MA were readily obtained that were able to generate UV emission from individual phenyl rings together with bright blue emission owing to the effective through-space conjugation. Moreover, p-RTP with a lifetime of up to approximately 1.6 s was obtained in the crystals. These results not only reveal a new system with both fluorescence and RTP dual emission but also suggest an alternative through-space conjugation strategy towards pure organic p-RTP luminogens with tunable emissions.

Near-Infrared Afterglow Luminescent Aggregation-Induced Emission Dots with Ultrahigh Tumor-to-Liver Signal Ratio for Promoted Image-Guided Cancer Surgery

Afterglow imaging through the collection of persistent luminescence after the stopping of light excitation holds enormous promise for advanced biomedical uses. However, efficient near-infrared (NIR)-emitting afterglow luminescent materials and probes (particularly the organic and polymeric ones) are still very limited, and their in-depth biomedical applications such as precise image-guided cancer surgery are rarely reported. Here, we design and synthesize a NIR afterglow luminescent nanoparticle with aggregation-induced emission (AIE) characteristics (named AGL AIE dots). It is demonstrated that the AGL AIE dots emit rather-high NIR afterglow luminescence persisting over 10 days after the stopping of a single excitation through a series of processes occurring in the AIE dots, including singlet oxygen production by AIE luminogens (AIEgens), Schaap's dioxetane formation, chemiexcitation by dioxetane decomposition, and energy transfer to NIR-emitting AIEgens. The animal studies reveal that the AGL AIE dots have the innate property of fast afterglow signal quenching in normal tissues, including the liver, spleen, and kidney. After the intravenous injection of AGL AIE dots into peritoneal carcinomatosis bearing mice, the tumor-to-liver ratio of afterglow imaging is nearly 100-fold larger than that for fluorescence imaging. The ultrahigh tumor-to-liver signal ratio, together with low afterglow background noise, enables AGL AIE dots to give excellent performance in precise image-guided cancer surgery.

Efficient and Long-Lived Room-Temperature Organic Phosphorescence: Theoretical Descriptors for Molecular Designs

Room-temperature phosphorescence (RTP) with long afterglow from pure organic materials has attracted great attention for its potential applications in biological imaging, digital encryption, optoelectronic devices, and so on. Organic materials have been long considered to be nonphosphorescent owing to their weak molecular spin-orbit coupling and high sensitivity to temperature. However, recently, some purely organic compounds have demonstrated highly efficient RTP with long afterglow upon aggregation, while others fail. Namely, it remains a challenge to expound on the underlying mechanisms. In this study, we present the molecular descriptors to characterize the phosphorescence efficiency and lifetime. For a prototypical RTP system consisting of a carbonyl group and π-conjugated segments, the excited states can be regarded as an admixture of n → π* (with portion α) and π → π* (portion β). Starting from the phosphorescent process and El-Sayed rule, we deduced that (i) the intersystem crossing (ISC) rate of S₁ → T _n is mostly governed by the modification of the product of α and β and (ii) the ISC rate of T₁ → S₀ is determined by the β value of T₁. Thus, the descriptors (γ = α × β, β) can be employed to describe the RTP character of organic molecules. From hybrid quantum mechanics and molecular mechanics (QM/MM) calculations, we illustrated the relationships among the descriptors (γ, β), phosphorescence efficiency and lifetime, and spin-orbit coupling constants. We stressed that the large γ and β values are favorable for the strong and long-lived RTP in organic materials. Experiments have reported confirmations of these molecular design rules.

Facile synthesis of a micro-scale MOF host–guest with long-lasting phosphorescence and enhanced optoelectronic performance

Micro-scale MOF host–guest with tunable phosphorescence and enhanced optoelectronic performance can be obtained by a facile and scalable precipitation process in aqueous solution.

Reversible two-channel mechanochromic luminescence for a pyridinium-based white-light emitter with room-temperature fluorescence–phosphorescence dual emission

For a pyridinium-based emitter with fluorescence–phosphorescence dual emission, mechanical grinding induces phosphorescence disappearance and mechanical pressing induces extraordinary phosphorescence enhancement.

Room temperature phosphorescence of Mn(ii) and Zn(ii) coordination polymers for photoelectron response applications

Mn(ii) and Zn(ii) based coordination polymers have been synthesized, which show different mechanism of room temperature phosphorescence and special photoelectron response performance.

Hemocompatibility investigation and improvement of near-infrared persistent luminescent nanoparticle ZnGa2O4:Cr3+ by surface PEGylation

ZnGa₂O₄:Cr³⁺ hemocompatibility was systematically investigated from the aspects of hemolysis, erythrocyte morphology, coagulation and complement system activation, and greatly improved by surface PEGylation.

Achieving Thermo-Mechano-Opto-Responsive Bitemporal Colorful Luminescence via Multiplexing of Dual Lanthanides in Piezoelectric Particles and its Multidimensional Anticounterfeiting

Optical characteristics of luminescent materials, including emission color (wavelength), lifetime, and excitation mode, play crucial roles in data communication and information security. Conventional luminescent materials generally display unicolor, unitemporal, and unimodal (occasionally bimodal) emission, resulting in low-level readout and decoding. The development of multicolor, multitemporal, and multimodal luminescence in a single material has long been considered to be a significant challenge. In this study, for the first time, the superior integration of colorful (red-orange-yellow-green), bitemporal (fluorescent and delayed), and four-modal (thermo-/mechano-motivated and upconverted/downshifted) emissions in a particular piezoelectric particle via optical multiplexing of dual-lanthanide dopants is demonstrated. The as-prepared versatile NaNbO₃ :Pr³⁺ ,Er³⁺ luminescent microparticles shown are particularly suitable for embedding into polymer films to achieve waterproof, flexible/wearable and highly stretchable features, and synchronously to provide multidimensional codes that can be visually read-out using simple and commonly available tools (including the LED of a smartphone, pen writing, cooling-heating stimuli, and ultraviolet/near-infrared lamps). These findings offer unique insight for designing highly integrated stimuli-responsive luminophors and smart devices toward a wide variety of applications, particularly advanced anticounterfeiting technology.

Resonance-Activated Spin-Flipping for Efficient Organic Ultralong Room-Temperature Phosphorescence

Triplet-excited-state-involved photonic and electronic properties have attracted tremendous attention for next-generation technologies. To populate triplet states, facile intersystem crossing (ISC) for efficient exciton spin-flipping is crucial, but it remains challenging in organic molecules free of heavy atoms. Here, a new strategy is proposed to enhance the ISC of purely organic optoelectronic molecules using heteroatom-mediated resonance structures capable of promoting spin-flipping at large singlet-triplet splitting energies with the aid of the fluctuation of the state energy and n-orbital component upon self-adaptive resonance variation. Combined experimental and theoretical investigations confirm the key contributions of the resonance variation to the profoundly promoted spin-flipping with ISC rate up to ≈10⁷ s^-1 in the rationally designed NPX (X = O or S) resonance molecules. Importantly, efficient organic ultralong room-temperature phosphorescence (OURTP) with simultaneously elongated lifetime and improved efficiency results overcoming the intrinsic competition between the OURTP lifetime and efficiency. With the spectacular resonance-activated OURTP molecules, time-resolved and color-coded quick response code devices with multiple information encryptions are realized, demonstrating the fundamental significance of this approach in boosting ISC dynamically for advanced optoelectronic applications.

Organic Long-Persistent Luminescence from a Flexible and Transparent Doped Polymer

Long-persistent luminescence (LPL) materials have a wide range of applications, such as in architectural decorations, safety signs, watch dials, and glow-in-the-dark toys. Present LPL materials based on inorganics must be processed into powders and blended with polymer matrices before use. However, micropowders of inorganic LPL materials show poor compatibility with common polymers, limiting the mechanical properties and transparency of the composites. Here, a polymer-based organic LPL (OLPL) system that is flexible, transparent, and solution processable is reported. Following low-power excitation at room temperature, this polymer-based OLPL system exhibits LPL after phosphorescence from the donor.

Cluster-Based Metal-Organic Frameworks: Modulated Singlet-Triplet Excited States and Temperature-Responsive Phosphorescent Switch

Luminescent metal-organic frameworks (MOFs) have received much attention due to their applications in color displays, sensors, and smart materials. However, how to balance the energy distribution between singlet and triplet excited states for a new generation of persistent luminescent MOFs is still a challenging goal. In this work, we report that the construction of cluster-based MOFs can supply an effective way to modulate the fluorescence and room-temperature phosphorescence (RTP) emission based on adjustable π-π stacking, halogen-bonding interaction, and metal-cluster units. Compared to the pristine ligand (5-bromoisophthalic acid) with obvious spin-orbit coupling, Zn₅ and Zn₃ cluster-based MOFs exhibit tunable photoluminescence (such as fluorescence and RTP wavelength, lifetime, and quantum yield). The ultralong-lived RTP visualization and temperature-dependent luminescence also provide the Zn₅ cluster-based MOF as a new type of anticounterfeiting and temperature-responsive phosphorescent switch material. Therefore, this work highlights the first example of cluster-based MOFs as ultralong-lived persistent luminescent materials for tuning singlet and triplet excited states, which may be extended to other similar systems for developing ultralong RTP and delayed fluorescence materials.

The methylation effect in prolonging the pure organic room temperature phosphorescence lifetime

Prolonging the phosphorescence lifetime of pure organic phosphorescent materials by a methyl-substitution strategy is described. We present a chemical strategy for improving the phosphorescence lifetime of triplet excitons under ambient conditions by incorporating methyl groups into the chemical structures. This is observed by a long-lived phosphorescence lifetime of up to 0.83 s detected for methylated 9-(4-(mesitylsulfonyl)phenyl)-9H-carbazole (3M), compared to 0.36 s for 9-(4-(phenylsulfonyl)phenyl)-9H-carbazole (0M) without any methyl groups. Additionally, enhanced phosphorescence efficiency can be obtained at an appropriate methylation degree, because of the smaller ΔE _ST (singlet and triplet energy gap) and ΔE _TT* (normal phosphorescence and long-lived phosphorescence energy gap). A comprehensive investigation on the packing mode in the crystalline state reveals that the methyl groups occupy the free volume and result in a suppression of non-radiative decay, accounting for the enhanced phosphorescence lifetime.

Time-gated triplet-state optical spectroscopy to decipher organic luminophores embedded in rigid matrices

Wet-chemically synthesized inorganic materials often exhibit luminescence behavior. We have recently shown that the amorphous yttrium-aluminium-borate (a-YAB) powders obtained by sol-gel and modified Pechini methods exhibit organic impurities, responsible for their intense visible photoluminescence and phosphorescence afterglow. However, the heterogeneity of impurity organic compounds and difficulties in their intact extraction from the solid inorganic host matrix limit the extraction-based chemical analysis for luminophore identification. Here, we propose a photo-physical route based on time-gated triplet-state optical spectroscopy (TGTSS) to construct the electronic structures of the trapped unknown luminophores, which successfully illustrates the luminescence properties of a-YAB powders in more detail and also provides important insights intrinsic to the nature of the luminophores. The experimental results accompanied with TD-DFT calculations of the theoretical electronic structures thus help us to propose the probable luminophore compounds trapped in rigid a-YAB matrices. We anticipate that the present approach will open new opportunities for analyzing similar complex luminescent materials, including carbon dots, graphene oxides, etc., which is vital for their improvement.

Alkyl Chain Introduction: In Situ Solar-Renewable Colorful Organic Mechanoluminescence Materials

Mechanoluminescence (ML) materials are environmentally friendly and emit light by utilizing mechanical energy. This has been utilized in light sources, displays, bioimaging, and advanced sensors. Organic ML materials are strongly limited to application by in situ unrepeatable ML. Now, in situ solar-renewable organic ML materials can be formed by introducing a soft alkyl chain into an ML unit. For the first time, the ML from these polycrystalline thin films can be iteratively produced by simply recrystallizing the fractured crystal in situ after a contactless exposure to sunlight within a short time (≤60 s). Additionally, their ML color and lifetime can be also easily tuned by doping with organic luminescent dyes. Therefore, large-area sandwich-type organic ML devices can be fabricated, which can be repeatedly used in a colorful piezo-display, visual handwriting monitor, and sensitive optical sensor, showing a lowest pressure threshold for ML of about 5 kPa.