Ultralong room temperature phosphorescence from amorphous organic materials toward confidential information encryption and decryption

Achieving Stable and Switchable Ultralong Room-Temperature Phosphorescence from Polymer-Based Luminescent Materials with Three-Dimensional Covalent Networks for Light-Manipulated Anticounterfeiting

Developing polymer-based organic afterglow materials with switchable ultralong organic phosphorescence (UOP) that are insensitive to moisture remains challenging. Herein, two organic luminogens, BBCC and BBCS, were synthesized by attaching 7H-benzo[c]carbazole (BBC) to benzophenone and diphenyl sulfone. These two emitters were employed as guest molecules and doped into epoxy polymers (EPs), which were constructed by in situ polymerization to achieve polymer materials BBCC-EP and BBCS-EP. It was found that BBCC-EP and BBCS-EP films exhibited significant photoactivated UOP properties. After light irradiation, they could produce a conspicuous organic afterglow with phosphorescence quantum yields and lifetimes up to 5.35% and 1.91 s, respectively. Meanwhile, BBCS-EP also presented photochromic characteristics. Upon thermal annealing, the UOP could be turned off, and the polymer films recovered to their pristine state, showing switchable organic afterglow. In addition, BBCC-EP and BBCS-EP displayed excellent water resistance and still produced obvious UOP after soaking in water for 4 weeks. Inspired by the unique photoactivated UOP and photochromic properties, BBCC and BBCS in the mixtures of diglycidyl ether of bisphenol A (DGEBA) and 1,3-propanediamine were employed as security inks for light-controlled multilevel anticounterfeiting. This work may provide helpful guidance for developing photostimuli-responsive polymer-based organic afterglow materials, especially those with stable UOP under ambient conditions.

Activating Molecular Room-Temperature Phosphorescence by Manipulating Excited-State Energy Levels in Poly(vinyl alcohol) Matrix

Poly(vinyl alcohol) (PVA) has been found as a wonderful matrix for chromophores to boost their room-temperature phosphorescence (RTP) character by forming abundant hydrogen bonding. Despite the well-utilized protective effect, the constructive role in accelerating the intersystem crossing is less investigated. Here, we focus on its role in manipulating the excited-state energy level to facilitate multiple intersystem crossing channels. Six benzoyl carbazole derivatives do not emit RTP in their solutions, powders, or crystals but exhibit significantly persistent RTP signals when embedded into the PVA matrix. Charge-transfer excited states were trapped by cofacial stacking in crystal, which blocks the intersystem crossing channels. In the PVA matrix, the allowed broad distribution of charge-transfer states covers the locally excited states, offering multiple intersystem crossing pathways via spin-vibronic orbit coupling. Consequently, efficient and persistent heavy-atom-free phosphors have been developed with the highest quantum yields of 7.7% and the longest lifetime of 2.3 s.

Room temperature phosphorescence of 2-aminopyridine with direct triplet state excitation

Phosphorescence emission at room temperature has been observed from 2-Aminopyridyne (2APi) embedded in poly (vinyl alcohol) (PVA) films. The gated emission with UV excitation at 305 nm results in a residual delayed fluorescence at around 350 nm and a broad phosphorescence spectrum with a maximum of around 500 nm. The phosphorescence excitation spectrum of 2APi - doped PVA film differs from the absorption spectrum in the long-wavelength part, showing a band at about 400-450 nm. The phosphorescence spectrum measured with a blue (420 nm) excitation closely resembles the spectrum measured with 305 nm excitation. Whereas the phosphorescence anisotropy measured with UV excitation is low and negative, with the blue excitation, the anisotropy is high and positive. The phosphorescence lifetimes (a fraction of a millisecond) are similar for UV and blue excitations. Both phosphorescence emissions with either UV or blue excitation strongly depend on temperature.

Polymer-Based Long-Lived Phosphorescence Materials over a Broad Temperature Based on Coumarin Derivatives as Information Anti-Counterfeiting

The development of new polymer-based room-temperature phosphorescence materials is of great significance. By a special molecule design and a set of feasible property-enhancing strategies, coumarin derivatives (CMDs, M_a-M_f) were doped into polyvinyl alcohol (PVA), polyacrylamide (PAM), corn starch, and polyacrylonitrile (PAN) as information anti-counterfeiting. CMDs-doped PVA and CMDs-doped corn starch films showed long-lived phosphorescence emissions up to 1246 ms (M_a-PVA) and 697 ms (M_a-corn starch), reaching over 10 s afterglow under naked eye observation under ambient conditions. Significantly, CMDs-doped PAM films can display long-lived phosphorescence emissions in a wide temperature range (100-430 K). For example, the M_e-PAM film has a phosphorescence lifetime of 16 ms at 430 K. The use of PAM with the strong polarity and rigidity has expanded the temperature range of long-life polymer-based phosphorescent materials. The present long-lived phosphorescent systems provide the possibility for developing new polymer-based organic afterglow materials with robust phosphorescence.

Rational molecular and doping strategies to obtain organic polymers with ultralong RTP

Organic-doped polymers and room-temperature phosphorescence (RTP) mechanisms have been widely reported. However, RTP lifetimes >3 s are rare and RTP-enhancing strategies are incompletely understood. Herein, we demonstrate a rational molecular doping strategy to obtain ultralong-lived, yet bright RTP polymers. The n-π* transitions of boron- and nitrogen-containing heterocyclic compounds can promote a triplet-state population, and the grafting of boronic acid onto polyvinyl alcohol can inhibit molecular thermal deactivation. However, excellent RTP properties were achieved by grafting 1-0.1% (N-phenylcarbazol-2-yl)-boronic acid rather than (2-/3-/4-(carbazol-9-yl)phenyl)boronic acids to afford record-breaking ultralong RTP lifetimes up to 3.517-4.444 s. These results showed that regulation of the interacting position between the dopant and matrix molecules to directly confine the triplet chromophore could more effectively stabilize triplet excitons, disclosing a rational molecular-doping strategy for achieving polymers with ultralong RTP. Based on the energy-donor function of blue RTP, an ultralong red fluorescent afterglow was demonstrated by co-doping with an organic dye.

Substrate-Responsive Pillar[5]arene-Based Organic Room-Temperature Phosphorescence

Room-temperature phosphorescence (RTP) is a photophysical phenomenon typically associated with a long-lived emission that can be detected by the unaided eye. Several natural proteins display RTP, as do certain artificial polymers. In both cases, the RTP is ascribed to effective intramolecular through-space electronic communication. However, small molecules with internal electronic communication that enable RTP are relatively rare. Herein, we describe an alkyl halide-responsive RTP system consisting of a meta-formylphenyl-bearing pillar[5]arene derivative that supports effective through-space charge transfer (TSCT) within the pillararene cavity. Treatment with bromoethane, a heavy atom-containing guest for the pillar[5]arene host, serves to enhance the emission. An isomeric para-formylphenyl-bearing pillar[5]arene system proved ineffective in producing an RTP effect. Quantum chemical calculations based on single-crystal X-ray diffraction analyses provided insights into the structural determinants governing TSCT between the 1,4-dimethoxybenzene donor units and the formylphenyl groups of the pillar[5]arene, as well as the associated energy gaps and intersystem crossing channels. We believe that the present system and the associated mechanistic analysis provide the foundation for design of new small molecule with tunable RTP features.

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.

Functional Roles of Polymers in Room-Temperature Phosphorescent Materials: Modulation of Intersystem Crossing, Air Sensitivity and Biological Activity

Organic room-temperature phosphorescent (RTP) materials routinely incorporate polymeric components, which usually act as non-functional or "inert" media to protect excited-state phosphors from thermal and collisional quenching, but are lesser explored for other influences. Here, we report some exemplary "active roles" of polymer matrices played in organic RTP materials, including: 1) color modulation of total delayed emissions via balancing the population ratio between thermally-activated delayed fluorescence (TADF) and RTP due to dielectric-dependent intersystem crossing; 2) altered air sensitivity of RTP materials by generating various surface morphologies such as nano-sized granules; 3) enhanced bacterial elimination for enhanced electrostatic interactions with negatively charged bio-membranes. These active roles demonstrated that the vast library of polymeric structures and functionalities can be married to organic phosphors to broaden new application horizons for RTP materials.

Chemometrics-assisted mechanism study of the room-temperature phosphorescence on nanoscopic boronate assemblies

Boronate assemblies, derived from benzene-1,4-diboronic acid, showed room-temperature phosphorescence (RTP) in both the solid-state and dispersions, being quite sensitive to the preparation conditions. Our chemometrics-assisted quantitative structure-property relationship (QSPR) analysis between the nanostructure and RTP behavior of boronate assemblies led us not only to understand their RTP mechanism but also to predict the RTP properties of unknown assemblies from the PXRD patterns.

Switchable and Highly Robust Ultralong Room-Temperature Phosphorescence from Polymer-Based Transparent Films with Three-Dimensional Covalent Networks for Erasable Light Printing

In this work, an efficient polymer-based organic afterglow system, which shows reversible photochromism, switchable ultralong organic phosphorescence (UOP), and prominent water and chemical resistance simultaneously, has been developed for the first time. By doping phenoxazine (PXZ) and 10-ethyl-10H-phenoxazine (PXZEt) into epoxy polymers, the resulting PXZ@EP-0.25 % and PXZEt@EP-0.25 % films show unique photoactivated UOP properties, with phosphorescence quantum yields and lifetimes up to 10.8 % and 845 ms, respectively. It is found that the steady-state luminescence and UOP of PXZ@EP-0.25 % are switchable by light irradiation and thermal annealing. Moreover, the doped films can still produce conspicuous UOP after soaking in water, strong acid and base, and organic solvents for more than two weeks, exhibiting outstanding water and chemical resistance. Inspired by these exciting results, the PXZ@EP-0.25 % has been successfully exploited as an erasable transparent film for light printing.

Color-Tunable Dual-Mode Organic Afterglow from Classical Aggregation-Caused Quenching Compounds for White-Light-Manipulated Anti-Counterfeiting

Color-tunable dual-mode organic afterglow excited by ultraviolet (UV) and white light was achieved from classical aggregation-caused quenching compounds for the first time. Specifically, two luminescent systems, which could produce significant organic afterglow composed of persistent thermally activated delayed fluorescence and ultralong organic phosphorescence under ambient conditions, were constructed by doping fluorescein sodium and calcein sodium into aluminum sulfate. Their lifetimes surpassed 600 ms, and the dopant concentrations were as low as 5×10^-6  wt %. Moreover, the persistent luminescence colors of the materials could be tuned from blue to green and then to yellow by simply varying the concentrations of guest compounds or the temperature in the range of 260-340 K. Inspired by these exciting results, the afterglow materials were used for UV- and white-light-manipulated anti-counterfeiting and preparation of elastomers with different colors of persistent luminescence.

Sensitive room-temperature phosphorescence for luminometric and visual monitoring of the dynamic evolution of acrylate-vinylidene chloride copolymers

Unlike fluorescence, room-temperature phosphorescence (RTP) has never been utilized to monitor the dynamic variation of polymer. In the present study, acrylate-vinylidene chloride (VDC) copolymers were doped with a good RTP molecule, N-hydroxyethyl 4-bromo-1,8-naphthalimide (HBN). During the maturation process, marked RTP-intensity enhancement of HBN was observed due to the crystallinity increase of copolymers, verified by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). For ensuring the more efficient RTP emission of HBN, copolymers with a higher content of crystallizable VDC segments and a more polar acrylate comonomer, i.e. methyl acrylate (MA) were preferred. According to the RTP characterizations, the following deductions could be obtained: (1) Maturation for 8-9 days at room temperature was needed for the copolymers with a high VDC content to ensure the complete crystallization; (2) Raising the maturation temperature to 50 and 70 °C not only accelerated the crystallization rate, but also increased the crystallinity of copolymers; (3) RTP method was more sensitive to the slight crystallinity variation than XRD and FTIR. Moreover, the dynamic maturation processes of acrylate-VDC copolymers could be also visually monitored through contacting with certain organic solvents that led to the emission color transition from orange to blue.

The Effect of Molecular Conformations and Simulated "Self-Doping" in Phenothiazine Derivatives on Room-Temperature Phosphorescence

The research of purely organic room-temperature phosphorescence (RTP) materials has drawn great attention for their wide potential applications. Besides single-component and host-guest doping systems, the self-doping with same molecule but different conformations in one state is also a possible way to construct RTP materials, regardless of its rare investigation. In this work, twenty-four phenothiazine derivatives with two distinct molecular conformations were designed and their RTP behaviors in different states were systematically studied, with the aim to deeply understand the self-doping effect on the corresponding RTP property. While the phenothiazine derivatives with quasi-axial (ax) conformation presented better RTP performance in aggregated state, the quasi-equatorial (eq) ones were better in isolated state. Accordingly, the much promoted RTP performance was achieved in the stimulated self-doping state with ax-conformer as host and eq-one as guest, demonstrating the significant influence of self-doping on RTP effect.

Diboraanthracene-Doped Polymer Systems for Colour-Tuneable Room-Temperature Organic Afterglow

Organic ultralong room temperature phosphorescence (RTP), or organic afterglow, is a unique phenomenon, gaining widespread attention due to its far-reaching application potential and fundamental interest. Here, two laterally expanded 9,10-dimesityl-dihydro-9,10-diboraanthracene (DBA) derivatives are demonstrated as excellent afterglow materials for red and blue-green light emission, which is traced back to persistent thermally activated delayed fluorescence and RTP. The lateral substitution of polycyclic DBA scaffold, together with weak transversal electron-donating mesityl groups, ensures the optimal molecular properties for (reverse) intersystem crossing and long-lived triplet states in a rigid poly(methyl methacrylate) matrix. The achieved afterglow emission quantum yields of up to 3 % and 15 %, afterglow lifetimes up to 0.8 s and 3.2 s and afterglow durations up to 5 s and 25 s (for red and blue-green emitters, respectively) are attributed to the properties of single molecules.

Recent advances in room temperature phosphorescence materials: design strategies, internal mechanisms and intelligent optical applications

Room temperature phosphorescence (RTP) materials comprising organic-inorganic hybrid, pure organic, and polymer RTP materials have been a research focus due to their tunable molecular structures, long emission lifetimes and extensive optical applications. Many design methods including halogen bonding interactions, heavy atom effect, metal-organic frameworks, polymerization, host-guest doping, and H-aggregation have been developed by RTP researchers. Narrowing the energy gap between the S₁ and lowest T_n states, enhancing the intersystem crossing (ISC) rate, increasing the spin-orbit coupling (SOC) value and stabilizing triplet emission states are the core factors to promoting RTP performance. In this review, lots of cases of organic-inorganic hybrid, pure organic, and polymer RTP materials with advanced design strategies, excellent RTP properties and intelligent applications have been classified and sorted. Their molecule structural designability and stimulus responsiveness endow them with RTP adjustability, which makes them excellent phosphors for modern optical applications. This review provides a systematic case elaboration of typical RTP systems in recent years and identifies the future challenges to improving RTP performance and finding novel applications.

A Dynamic Supramolecular H-bonding Network with Orthogonally Tunable Clusteroluminescence

Enabling dynamically tunable emissive systems offers opportunities for constructing smart materials. Clusteroluminescence, as unconventional luminescence, has attracted increasing attention in both fundamental and applied sciences. Herein, we report a supramolecular poly(disulfides) network with tunable clusteroluminescence. The reticular H-bonds synergize the rigidity and mobility of dynamic networks, and endow the resulting materials with mechanical adaptivity and robustness, simultaneously enabling efficient clusteroluminescence and phosphorescence at 77 K. Orthogonally tunable luminescence are achieved in two manners, i.e., slow backbone disulfide exchange and fast side-chain metal coordination. Further exploration of the reprocessability and chemical closed-loop recycling of intrinsic dynamic networks for sustainable materials is feasible. We foresee that the synergistic strategy of dynamic chemistry offers a novel pathway and potential opportunities for smart emissive materials.

Crucial Role of Defect States in the Ultralong Phosphorescence of Organic Molecular Crystals

Ultralong organic phosphorescence (UOP) in pure organic molecular crystals has attracted a lot of interest recently. There is much debate on the emission mechanism of this UOP. Two recent experimental works published in Nat. Photonics 2019, 13, 406-411 and Nat. Mater. 2021, 20, 175-180 attribute UOP in the 2,4,6-trimethoxy-1,3,5-triazine (TMOT) crystals and the carbazole crystals to H-aggregation of the TMOT molecules or the formation of charge-transfer excitons between the carbazole and impurity molecules. Our first-principles many-body Green's function theory calculations show that the lowest triplet states of these two crystals are in fact the localized defect states originating from the twisted TMOT molecules and the impurities, respectively. Energies of the H-aggregation-induced exciton and the charge-transfer exciton are too high to account for UOP. UOP should be mainly due to the little orbital overlap between the localized defect state and the delocalized band edges of the crystal. Strong intermolecular interactions suppress nonradiative decay of the triplet exciton localized on the defect.

Organic persistent room temperature phosphorescence enabled by carbazole impurity

The molecular design of metal-free organic phosphors is essential for realizing persistent room-temperature phosphorescence (pRTP) despite its spin-forbidden nature. A series of halobenzonitrile-carbazoles has been prepared following a one-pot nucleophilic substitution protocol involving commercially available and laboratory-synthesized carbazoles. We demonstrate how halo- and cyano-substituents affect the molecular geometry in the crystal lattice, resulting in tilt and/or twist of the carbazole with respect to the phenyl moiety. Compounds obtained from the commercially available carbazole result in efficient pRTP of organic phosphors with a high quantum yield of up to 22% and a long excited state lifetime of up to 0.22 s. Compounds obtained from the laboratory-synthesized carbazole exhibit thermally activated delayed fluorescence with an excited state lifetime in the millisecond range. In-depth photophysical studies reveal that luminescence originates from the mixed locally excited state (³LE, nπ*)/charge transfer state.

The effect of substituents and molecular aggregation on the room temperature phosphorescence of a twisted π-system

Room temperature phosphorescence of an intrinsically apolar twisted π-system is modulated by polar substituents. Persistent phosphorescence is visible by eye in poder, induced by molecular aggregation.

Highly Efficient and Ultralong Afterglow Emission with Anti-Thermal Quenching from CsCdCl3 : Mn Perovskite Single Crystals

Triplet exciton-based long-lived phosphorescence is severely limited by the thermal quenching at high temperature. Herein, we propose a novel strategy based on the energy transfer from triplet self-trapped excitons to Mn²⁺ dopants in solution-processed perovskite CsCdCl₃ . It is found the Mn²⁺ doped hexagonal phase CsCdCl₃ could simultaneously exhibit high emission efficiency (81.5 %) and long afterglow duration time (150 s). Besides, the afterglow emission exhibits anti-thermal quenching from 300 to 400 K. In-depth charge-carrier dynamics studies and density functional theory (DFT) calculation provide unambiguous evidence that carrier detrapping from trap states (mainly induced by Cl vacancy) to localized emission centers ([MnCl₆ ]^4- ) is responsible for the afterglow emission with anti-thermal quenching. Enlightened by the present results, we demonstrate the application of the developed materials for optical storage and logic operation applications.

Color-Tunable Binary Copolymers Manipulated by Intramolecular Aggregation and Hydrogen Bonding

Construction of color-tunable luminescent polymeric materials with enhanced emission intensity and room-temperature phosphorescence (RTP) performance regulated by a single chromophore component is highly desirable in the scope of photoluminescent materials. Herein, a set of binary copolymers were facilely synthesized using free radical polymerization by selecting different types of polymer matrix and N-substituted naphthalimides (NPA) as chromophores. Surprisingly, the fluorescence emission of copolymers could be remarkably enhanced, because of the intramolecular aggregation of NPA manipulated by a single polymer chain in both solution and solid state. Moreover, RTP signals of binary copolymers were all clearly observed in the air without any processing procedure, because of the embedding of phosphors into hydrogen bonding networks after copolymerization with vinyl-based acrylamide monomers. Taking advantages of the synergistic effect of copolymerization-induced aggregation and copolymerization-induced rigidification to promote optical performance, UV stimulus-responsive luminescent polymer films with processability, flexibility, and adjustable emission wavelength were simply prepared using a drop-casting method in large scale, the setting of which is the basis for application in the fields of organic optoelectronics, information security, and bioimaging/sensing.

Hydrogen-bond organized 2D metal-organic microsheets: direct ultralong phosphorescence and color-tunable optical waveguides

Ultralong phosphorescent materials have numerous applications across biological imaging, light-emitting devices, X-ray detection and anti-counterfeiting. Triplet-state molecular phosphorescence typically accompanies the singlet-state fluorescence during photoluminescence, and it is still difficult to achieve direct triplet photoemission as ultralong room temperature phosphorescence (RTP). Here, we have designed Zn-IMDC (IMDC, 4,5-imidazoledicarboxylic acid) and Cd-IMDC, two-dimensional (2D) hydrogen-bond organized metal-organic crystalline microsheets that exhibit rarely direct ultralong RTP upon UV excitation, benefiting from the appropriate heavy-atom effect and multiple triplet energy levels. The excitation-dependent and thermally stimulated ultralong phosphorescence endow the metal-organic systems great opportunities for information safety application and temperature-gated afterglow emission. The well-defined 2D microsheets present color-tunable and anisotropic optical waveguides under different excitation and temperature conditions, providing an effective way to obtain intelligent RTP-based photonic systems at the micro- and nano-scales.

Structural materials with afterglow room temperature phosphorescence activated by lignin oxidation

Sustainable afterglow room temperature phosphorescence (RTP) materials, especially afterglow RTP structural materials, are crucial but remain difficult to achieve. Here, an oxidation strategy is developed to convert lignin to afterglow materials with a lifetime of ~ 408 ms. Specifically, lignin is oxidized to give aromatic chromophores and fatty acids using H₂O₂. The aromatic chromophores are locked by a fatty acid-based matrix by hydrogen bonds, triggering enhanced spin orbit coupling and long afterglow emission. More interestingly, motivated by this discovery, an auto fabrication line is built to convert wood (natural structural materials) to wood with afterglow RTP emission (RTP wood) via in situ oxidation of naturally-occurring lignin located in the wood cell walls to oxidized lignin (OL). The as-prepared RTP wood exhibits great potential for the construction of sustainable afterglow furniture. With this research we provide a new strategy to promote the sustainability of afterglow RTP materials and structural materials.

Sustainable afterglow room temperature phosphorescence (RTP) Structural materials are difficult to achieve. Here, the authors demonstrate a wood based RTP material by oxidation of lignin to realize an afterglow RTP material with a lifetime of ~ 408 ms.

Organic phosphorescent nanoscintillator for low-dose X-ray-induced photodynamic therapy

X-ray-induced photodynamic therapy utilizes penetrating X-rays to activate reactive oxygen species in deep tissues for cancer treatment, which combines the advantages of photodynamic therapy and radiotherapy. Conventional therapy usually requires heavy-metal-containing inorganic scintillators and organic photosensitizers to generate singlet oxygen. Here, we report a more convenient strategy for X-ray-induced photodynamic therapy based on a class of organic phosphorescence nanoscintillators, that act in a dual capacity as scintillators and photosensitizers. The resulting low dose of 0.4 Gy and negligible adverse effects demonstrate the great potential for the treatment of deep tumours. These findings provide an optional route that leverages the optical properties of purely organic scintillators for deep-tissue photodynamic therapy. Furthermore, these organic nanoscintillators offer an opportunity to expand applications in the fields of biomaterials and nanobiotechnology.

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.

Ultralong organic phosphorescence from isolated molecules with repulsive interactions for multifunctional applications

Intermolecular interactions, including attractive and repulsive interactions, play a vital role in manipulating functionalization of the materials from micro to macro dimensions. Despite great success in generation of ultralong organic phosphorescence (UOP) by suppressing non-radiative transitions through attractive interactions recently, there is still no consideration of repulsive interactions on UOP. Herein, we proposed a feasible approach by introducing carboxyl groups into organic phosphors, enabling formation of the intense repulsive interactions between the isolated molecules and the matrix in rigid environment. Our experimental results show a phosphor with a record lifetime and quantum efficiency up to 3.16 s and 50.0% simultaneously in film under ambient conditions. Considering the multiple functions of the flexible films, the potential applications in anti-counterfeiting, afterglow display and visual frequency indicators were demonstrated. This finding not only outlines a fundamental principle to achieve bright organic phosphorescence in film, but also expands the potential applications of UOP materials.

Poly(arylene piperidine) Quaternary Ammonium Salts Promoting Stable Long-Lived Room-Temperature Phosphorescence in Aqueous Environment

Room-temperature phosphorescence (RTP) materials have garnered considerable research attention owing to their excellent luminescence properties and potential application prospects in anti-counterfeiting, information storage, and optoelectronics. However, several RTP systems are extremely sensitive to humidity, and consequently, the realization of long-lived RTP in water remains a formidable challenge. Herein, a feasible and effective strategy is presented to achieve long-lived polymeric RTP systems, even in an aqueous environment, through doping of synthesized polymeric phosphor PBHDB into a poly(methyl methacrylate) (PMMA) matrix. Compared to the precursor polymer PBN and organic molecule HDBP, a more rigid polymer microenvironment and electrostatic interaction are formed between the PMMA matrix and polymer PBHDB, which effectively reduce the nonradiative decay rate of triplet excitons and dramatically increase the phosphorescence intensity. Specifically, the phosphorescence lifetime of the PBHDB@PMMA film (1258.62 ms) is much longer than those of PBN@PMMA (674.20 ms) and HDBP@PMMA (1.06 ms). Most importantly, a bright-green afterglow can be observed after soaking the PBHDB@PMMA film in water for more than a month. The excellent water resistance and reversible response properties endow these systems with promising potential for dynamic information encryption even in water.

Molecular Thermal Motion Modulated Room-Temperature Phosphorescence for Multilevel Encryption

The stimulus-responsive room-temperature phosphorescence (RTP) materials have become an increasingly significant topic in the fields of bioimaging, sensing, and anticounterfeiting. However, this kind of materials is scarce to date, especially for the ones with delicate stimulus-responsive behavior. Herein, a universal strategy for multilevel thermal erasure of RTP via chromatographic separation of host-guest doping RTP systems is proposed. The tunable host-guest systems, matrix materials, heating temperature, and time are demonstrated to allow precise six-level data encryption, QR code encryption, and thermochromic phosphorescence encryption. Mechanistic study reveals that the thermal-responsive property might be attributed to molecular thermal motion and the separation effect of the silica gel, which provides expanded applications of host-guest RTP materials such as cold chain break detection. This work offers a simple yet universal way to construct advanced responsive RTP materials.

Organic phosphorescent scintillation from copolymers by X-ray irradiation

Scintillators that exhibit X-ray-excited luminescence have great potential in radiation detection, X-ray imaging, radiotherapy, and non-destructive testing. However, most reported scintillators are limited to inorganic or organic crystal materials, which have some obstacles in repeatability and processability. Here we present a facile strategy to achieve the X-ray-excited organic phosphorescent scintillation from amorphous copolymers through the copolymerization of the bromine-substituted chromophores and acrylic acid. These polymeric scintillators exhibit efficient X-ray responsibility and decent phosphorescent quantum yield up to 51.4% under ambient conditions. The universality of the design principle was further confirmed by a series of copolymers with multi-color radioluminescence ranging from green to orange-red. Moreover, we demonstrated their potential application in X-ray radiography. This finding not only outlines a feasible principle to develop X-ray responsive phosphorescent polymers, but also expands the potential applications of polymer materials with phosphorescence features.

Structurally Resemblant Dopants Enhance Organic Room-Temperature Phosphorescence

Doping has shown very promising potential in endowing room-temperature phosphorescence (RTP) properties of organic phosphors with minimal effort. Here, a new isomer design and doping strategy is reported that is applicable to dibenzothiophene (DBT) and its derivatives. Three isomers are synthesized to study the dopant effect on enhancing RTP of DBT derivatives. It is found that isomer dopants bearing close resemblance to the host with matched highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels and small energy difference between singlet- and triplet-excited states can yield efficient RTP for the doped system. Meanwhile, phosphorescence color from yellow to red is achieved by varying isomer dopants used for doping the DBT derivatives. This work represents an RTP enhancement strategy based on isomer design and doping to construct luminescent organic phosphors.

Efficient and Color-Tunable Dual-Mode Afterglow from Large-Area and Flexible Polymer-Based Transparent Films for Anti-Counterfeiting and Information Encryption

It remains a great challenge to develop polymer-based materials with efficient and color-tunable organic afterglow. Two indolocarbazole derivatives IaCzA and IbCzA have been synthesized and doped into poly(vinyl alcohol) (PVA) matrices. It is found that the resulting films can produce unique dual-mode afterglow, which is composed of persistent thermally activated delayed fluorescence and ultralong organic phosphorescence. Besides, the IbCzA-doped PVA film exhibits intense blue afterglow with Φ_afterglow and τ_afterglow up to 19.8 % and 1.81 s, respectively, representing state-of-the-art dual-mode organic afterglow performance. Moreover, our reported film has high flexibility, excellent transparency, and large-area producibility; and the afterglow color of the film can be linearly tuned by temperature. Inspired by these distinctive properties, the PVA doped with IbCzA was employed as temperature-sensitive security ink for anti-counterfeiting and information encryption.

Color-Tunable Supramolecular Luminescent Materials

Constructing multicolor photoluminescent materials with tunable properties is an attractive research objective on account of their abundant applications in materials science and biomedical engineering. By comparison with covalent synthesis, supramolecular chemistry has provided a more competitive and promising strategy for the production of organic materials and the regulation of their photophysical properties. By taking advantage of dynamic and reversible noncovalent bonding interactions, supramolecular strategies can, not only simplify the design and fabrication of organic materials, but can also endow them with dynamic reversibility and stimuli responsiveness, making it much easier to adjust the superstructures and properties of the materials. Occasionally, it is possible to introduce emergent properties into these materials, which are absent in their precursor compounds, broadening their potential applications. In an attempt to highlight the state-of-the-art noncovalent strategies available for the construction of smart luminescent materials, an overview of color-tunable materials is presented in this Review, with the emphasis being placed on the examples drawn from host-guest complexes, supramolecular assemblies and crystalline materials. The noncovalent synthesis of room-temperature phosphorescent materials and the modulation of their luminescent properties are also described. Finally, future directions and scientific challenges in the emergent field of color-tunable supramolecular emissive materials are discussed.

Brighten Triplet Excitons of Carbon Nanodots for Multicolor Phosphorescence Films

Triplet excitons usually do not emit light under ambient conditions due to the spin-forbidden transition rule, thus they are called dark excitons. Herein, triplet excitons in carbon nanodots (CNDs) are brightened by embedding the CNDs into poly(vinyl alcohol) (PVA) films; flexible multicolor phosphorescence films are thus demonstrated. PVA chains can isolate the CNDs, and excited state electron or energy transfer induced triplet exciton quenching is thus reduced; while the formed hydrogen bonds between the CNDs and PVA can restrict vibration/rotation of the CNDs, thus further protecting the triplet excitons from nonradiative recombination. The lifetimes of the flexible multicolor phosphorescence films can reach 567, 1387, 726, and 311 ms, and the longest-lasting phosphorescence film can be observed by naked eyes for nearly 15 s even after bending 5000 times. The phosphorescence films can be processed into various patterns, and a dynamic optical signature concept has been proposed and demonstrated based on the phosphorescence films.

Clustering and halogen effects enabled red/near-infrared room temperature phosphorescence from aliphatic cyclic imides

Pure organic room temperature phosphorescence (RTP) materials become increasingly important in advanced optoelectronic and bioelectronic applications. Current phosphors based on small aromatic molecules show emission characteristics generally limited to short wavelengths. It remains an enormous challenge to achieve red and near-infrared (NIR) RTP, particularly for those from nonaromatics. Here we demonstrate that succinimide derived cyclic imides can emit RTP in the red (665, 690 nm) and NIR (745 nm) spectral range with high efficiencies of up to 9.2%. Despite their rather limited molecular conjugations, their unique emission stems from the presence of the imide unit and heavy atoms, effective molecular clustering, and the electron delocalization of halogens. We further demonstrate that the presence of heavy atoms like halogen or chalcogen atoms in these systems is important to facilitate intersystem crossing as well as to extend through-space conjugation and to enable rigidified conformations. This universal strategy paves the way to the design of nonconventional luminophores with long wavelength emission and for emerging applications.

Pure organic room temperature phosphorescence (RTP) materials become increasingly important but achieving red and near-infrared (NIR) RTP remains challenging. Here, the authors demonstrate that succinimide derived cyclic imides can emit RTP in the red and NIR spectral range with outstanding efficiencies of up to 9.2%.

Background-free room temperature phosphorescence and digital image colorimetry detection of melamine by carbon nitride quantum dots in cellulose matrix with smartphone-based portable device

Carbon nitride quantum dots (CNQDs) were embedded in the sodium carboxymethyl cellulose (CMC) matrix to form CNQDs-CMC film to explore the room temperature phosphorescence (RTP) of CNQDs, which suppress the non-radiative relaxation process due to the internal hydrogen bonding interactions between CMC and CNQDs. Then, a simple, inexpensive, background-free miniature device integrating with CNQDs-CMC film and smartphone was fabricated for rapid and quantitative detection of melamine (MEL). In the present of MEL, the yellow RTP color of the CNQDs-CMC film was quenched and photographed by the smartphone. The Color Recognizer APP in the smartphone recognized the red (R) value for quantitative detection of MEL. Thus, digital image colorimetry (DIC) determination of MEL was achieved due to the visible RTP color change of CNQDs-CMC film. The smartphone-based miniature device provided a promising platform for the on-site monitoring analytes in the complex matrix including food safety, environmental screening, health monitoring, and disease prevention.