Persistent Room-temperature Phosphorescence Research Articles

Photoactivated Multicolor Organic Phosphorescence in Intrinsically Stretchable, Self‐Healable and Recyclable Polymers

AbstractSmart persistent organic room‐temperature phosphorescence (RTP) materials, capable of responding to microenvironmental changes, are critically important in various optoelectronic applications. However, conventional persistent RTP materials usually have not stretchability and flexibility, limiting some practical applications. This study reports a facile and one‐pot photo‐initiated copolymerization method to prepare photoactivated persistent RTP polymers with multicolored afterglow, excellent stretchability, self‐healable and recyclable properties. Impressively, the polymers can be stretched up to 500% without significant loss in RTP intensity and lifetime, and they possess a self‐healing ability with a healing efficiency of 69.6%. Utilizing these photoactivated and stretchable persistent RTP polymers, dual encryption can be achieved under UV irradiation and stretching conditions, thereby enhancing the security levels of the stored confidential information. Overall, this study represents the first example of self‐healable and recyclable stimuli‐responsive RTP materials, demonstrating their bright future for the flexible and wearable optoelectronics.

Impurities in Arylboronic Esters Induce Persistent Afterglow.

Several recent reports suggest that arylboronic esters can exhibit room temperature phosphorescence (RTP), an optical property that is desirable for applications in security printing, oxygen sensing, and bioimaging. These findings challenged the fundamental notion that heavy elements or changes in orbital symmetry were required for intersystem crossing to occur in organic compounds. As we had not observed long afterglow in the many arylboronic esters we had synthesized over many years, we suspected that the RTP observed in these systems had a simpler explanation: the materials reported were impure. Herein, we synthesized 12 arylboronic esters that were previously reported to show RTP, and carefully purified them by column chromatography, recrystallization, and sublimation. We re-examined their photophysical properties alongside single-crystal X-ray diffraction analysis and detailed theoretical studies. While 4 of the 12 compounds showed long afterglows as crude products, none of them showed persistent RTP after careful purification. We also successfully isolated the impurity 4-amino-3,5-bis(pinacolatoboryl)benzonitrile (2), identifying it as the impurity responsible for inducing delayed fluorescence in 3,5-bis(pinacolatoboryl)benzonitrile (1). Doping 1 with 1.0 mol % 2 led to a persistent afterglow with a lifetime of 67 ms, which is mediated by a dimer charge transfer state. Our findings call for a re-examination of previous studies reporting RTP from arylboronic esters, highlight the importance of careful purification in photophysical research, and provide a practical strategy for designing organic materials with a long afterglow.

Exploration of the Photoluminescence Behavior and Emission Mechanism of Thioester Polyacrylamide Tablets During the Gradual Increase of Molecular Weight.

To enhance the photoluminescence (PL) of unconventional luminescent compounds, particularly their persistent room temperature phosphorescence (p-RTP) performance, compressing the powder into tablets has been demonstrated as a viable approach. Nevertheless, the alterations in the emission capability of PL in compacted tablets have not been comprehensively investigated. In this study, four polyacrylamide (PAM) with controllable molecular weight (MW) are fabricated from powder to tablets, and their PL emission properties are thoroughly examined and compared with corresponding powders to elucidate the emission mechanism. As MW increases, both PL and p-RTP emissions of the tablets gradually intensify, exhibiting significant enhancement compared to the corresponding powder while retaining the characteristic blue shift. Through small angle X-ray scattering (SAXS), construction of molecular models for tablets, detailed analysis of molecular interactions, and theoretical calculations are conducted to reasonably explain these emission phenomena using clustering-triggered emission (CTE) and average packing density promoted emission (PDE) mechanisms. These findings not only advance the understanding of nonconventional luminogens' emission mechanisms but also offer new insights for preparing nonconventional luminescent polymers with controllable p-RTP emission performance.

Programmable Persistent Room Temperature Phosphorescence Switches through Wavelength-Selective Photoactivation.

Controlling multicolor persistent room-temperature phosphorescence (RTP) through photoirradiation holds fundamental significance but remains a significant challenge. In this study, we engineered a wavelength-selective photoresponsive system utilizing the Förster resonance energy transfer strategy. This system integrates a photoactivated long-lived luminescent material as the energy donor with a fluorescent photoswitch as the energy acceptor, facilitating programmable persistent luminescence switches. Distinct afterglow color states, such as initial nonemissive, green, yellow, and orange, were achieved through irradiation at 400 nm, 365 nm, and 254 nm, respectively. Based on this capability, we established an interacting network for multistate afterglow color switching among these four emissive states. In addition, we demonstrate the potential of this wavelength-selective photoresponsive system in the photo-controlled rewritable printing of multicolor afterglow images on a single thin film. This work represents a substantial step towards the fabrication of sophisticated wavelength-selective photoresponsive systems, potentially revolutionizing applications in optical data storage, security labeling, and smart displays by enabling precise control over photoresponsive behaviors under various photoirradiation wavelengths.

Study on the Influence of Host-Guest Structure and Polymer Introduction on the Afterglow Properties of Doped Crystals.

Due to their low cost, good biocompatibility, and ease of structural modification, organic long-persistent luminescence (LPL) materials have garnered significant attention in organic light-emitting diodes, biological imaging, information encryption, and chemical sensing. Efficient charge separation and carrier migration by the host-guest structure or using polymers and crystal to build rigid environments are effective ways of preparing high-performance materials with long-lasting afterglow. In this study, four types of crystalline materials (MODPA: DDF-O, MODPA: DDF-CHO, MODPA: DDF-Br, and MODPA: DDF-TRC) were prepared by a convenient host-guest doping method at room temperature under ambient conditions, i.e., in the presence of oxygen. The first three types exhibited long-lived charge-separated (CS) states and achieved visible LPL emissions with durations over 7, 4, and 2 s, respectively. More surprisingly, for the DDF-O material prepared with PMMA as the polymer substrate, the afterglow time of DDF-O: PMMA was longer than 10 s. The persistent room-temperature phosphorescence effect caused by different CS state generation efficiencies and rigid environment were the main reason for the difference in LPL duration. The fourth crystalline material was without charge separation and exhibited no LPL because it was not a D-A system. The research results indicate that the CS state generation efficiency and a rigid environment are the key factors affecting the LPL properties. This work provides new understandings in designing organic LPL materials.

Proton transfer induced persistent triplet charge transfer phosphorescence in molecule-doped polymer systems

The intermolecular interactions between small molecules and the matrix in a highly polar environment could easily lead to non-radiative transitions, which significantly hampers the development of high-performance persistent room-temperature phosphorescence (pRTP) materials in such condition. Herein, we introduce a novel strategy that integrates consecutive proton transfer and photo-induced charge transfer to facilitate triplet charge transfer emission in highly polar polymer matrix. In particular, doping aza-arene guests including 1,10-phenanthroline (1,10-Phen), 2,9-dimethyl-1,10-phenanthroline (DM-Phen), and 7,8-benzoquinoline (7,8-BQ) into polyacrylic acid (PAA) resulted in superior pRTP properties compared to other polymer hosts. Spectroscopic investigations and theoretical calculations elucidate that this is attributed to the proton transfer and triplet charge transfer characteristics between the host and guest. Moreover, due to the rigid environment and space confinement provided by PAA, pRTP with the lifetime up to 841 ms is achieved. Capitalizing on the excellent water solubility of the doped PAA films, the inkjet printings have been executed, and showcasing the promising the potential applications of these pRTP materials in the field of information encryption.

Ultralong room temperature phosphorescence and broad color-tunability persistent luminescence via new strategy

Pure organic materials with ultralong room-temperature phosphorescence (RTP) and persistent luminescence in broad color gamut exhibit tremendous potential and broad application prospects due to their unique optical properties. This article proposes a simple strategy, polyatomic synergistic effect, to endow persistent luminescent materials with ultralong lifetime and broad color-tunability through polyatomic synergistic effect and non-traditional phosphorescence resonance energy transfer (PRET). By leveraging the polyatomic synergistic effect to enhance the intersystem crossing (ISC) in bibenzimidazole (BBI) derivatives and suppress the non-radiative transition process, ultralong persistent room-temperature phosphorescence has been successfully achieved after incorporating BBI-Cl-M into poly(methyl methacrylate) (PMMA) to form a rigid matrix(BBI-Cl-M@PMMA). Specifically, the ester functionalized bibenzimidazole with modified chlorine on molecular skeleton (BBI-Cl-M) demonstrates a remarkable phosphorescent lifetime (τp) of up to 256.4 ms. In addition, the behaviors and mechanism of RTP via polyatomic synergistic effect have been further understood by theoretical calculation and single crystal analysis. Subsequently, utilizing BBI-Cl-M as the energy donor and Rhodamine B (RB) as the energy acceptor, persistent and multicolor organic afterglow covering from green to red has been realized successfully by simply regulating the doping composition and concentration of PRET systems. These RTP materials have also been applied in underwater afterglow emission and multilevel anti-counterfeiting technology successfully.

Modulating Heavy Atom Effect in Germylene for Persistent Room Temperature Phosphorescence.

It is worth but still challenging to develop the low-valent main group compounds with persistent room temperature phosphorescence (pRTP). Herein, we presented germylene-based persistent phosphors by introduction of low-valent Ge center into chromophore. A novel phosphors CzGe and its series of derivatives, namely CzGeS, CzGeSe, CzGeAu, and CzGeCu, were synthesized. Experiments and theoretical calculations reveal that the pRTP behavior were "turn on" due to the heavy atom effect of germylene. More importantly, the low-valent of oxidation state and structural traits propelled GeCz had a balance between the intersystem crossing and the shortening of lifetime caused by the heavy atoms, resulting the ultralong lifetime of 309 ms and phosphorescent quantum efficiency of 15.84 %, which is remarkable among heavy main group phosphors. This research provides valuable insights to the design of heavy atoms in phosphors and expand the applications of germylene chemistry.

Unexpected tunable photoluminescence and emission mechanism during the gradual increase of cyclic glucose units

Excitation-dependent (λex-dependent) emission is one of the most common emission characteristics of nonconventional luminophores. Previously, we have reported D-xylose crystals has an obvious λex-dependent color tunable persistent room temperature phosphorescence (p-RTP) from blue to green, but its p-RTP emission at long wavelength is very weak. To further explore this attractive multicolor p-RTP behavior, and to enhance its tunability, the photoluminescence (PL) behavior of three common cyclodextrins (CDs), namely α-CD, β-CD and γ-CD, including solution and single crystals, were studied. Their single crystals demonstrate λex-dependent multicolor p-RTP properties in the order of β-CD>α-CD>γ-CD, rather than the strongest with the least glucose units (α-CD). Among them, the λex-dependent tunable p-RTP of β-CD and α-CD is superior to that of D-(+)-xylose single crystal. The above unexpected phenomena could be explained reasonably through detailed single crystal structure analysis and theoretical calculations. The emission behavior of solution and single crystals of CDs can be reasonably explained by the clustering-triggered emission (CTE) mechanism. Furthermore, CD-based covalent organic frameworks (COF) and supramolecules are readily fabricated, which own significantly enhanced p-RTP emission. Due to the intriguing PL property as well as hydrophilic rim and low biotoxicity of CDs, successful imaging of HCT116 cells and encryption applications were demonstrated. It is believed that the intrinsic PL properties of CDs, especially their regularly adjustable multicolor p-RTP that varies with the number of glucose units, have significant guidance for the development of a series of multicolor luminescent materials.

Excellent Persistent Near-Infrared Room Temperature Phosphorescence from Highly Efficient Host-Guest Systems.

Organic near-infrared (NIR) room temperature phosphorescence (RTP) materials become a hot topic in bioimaging and biosensing for the large penetration depth and high signal-to-background ratio (SBR). However, it is challenging to achieve persistent NIR phosphorescence for severe nonradiative transitions by energy-gap law. Herein, a universal system with persistent NIR RTP is built by visible (host) and NIR phosphorescence (guest) materials, which can efficiently suppress the nonradiative transitions by rigid environment of crystalline host materials with good matching, and further promote phosphorescence emission by the additional phosphorescence resonance energy transfer (≈100%) between them. The persistent NIR phosphorescence with ten-folds enhancement of RTP lifetimes, compared to those of guest luminogens, can be achieved by modulation of aggregated structures of host-guest systems. This work provides a convenient way to largely prolong the phosphorescence lifetimes of various NIR luminogens, promoting their application in afterglow imaging with deeper penetration and higher SBRs.

Visible-light-excited robust room-temperature phosphorescence of dimeric single-component luminophores in the amorphous state

Organic room temperature phosphorescence (RTP) has significant potential in various applications of information storage, anti-counterfeiting, and bio-imaging. However, achieving robust organic RTP emission of the single-component system is challenging to overcome the restriction of the crystalline state or other rigid environments with cautious treatment. Herein, we report a single-component system with robust persistent RTP emission in various aggregated forms, such as crystal, fine powder, and even amorphous states. Our experimental data reveal that the vigorous RTP emissions rely on their tight dimers based on strong and large-overlap π-π interactions between polycyclic aromatic hydrocarbon (PAH) groups. The dimer structure can offer not only excitons in low energy levels for visible-light excited red long-lived RTP but also suppression of the nonradiative decays even in an amorphous state for good resistance of RTP to heat (up to 70 °C) or water. Furthermore, we demonstrate the water-dispersible nanoparticle with persistent RTP over 600 nm and a lifetime of 0.22 s for visible-light excited cellular and in-vivo imaging, prepared through the common microemulsion approach without overcaution for nanocrystal formation.

Controllable Blue Shift and Enhancement Emission during the Gradually Increasing Molecular Weight of Polyacrylamide.

Nonconventional luminescent polymers have become research hotspots due to their advantages such as persistent room temperature phosphorescence (p-RTP) emission and strong film-forming properties. It is proven that the molecular weight (MW) of such luminescent polymers has a significant impact on their emission over a large range, generally with a red shift as the MW increases. Herein, four controllable MW polyacrylamides are prepared via reversible addition-fragmentation chain transfer polymerization (RAFT), and their photoluminescence quantum yield and p-RTP lifetimes gradually increase with the increasing MW. The emission of p-RTP gradually shifts blue with increasing MW, which is likely due to the gradually changing interactions between the electron-rich portion in RAFT reagent and the increasing acrylamide (AM) units in the molecular chain. These can be reasonably explained through small angle X-ray scattering, the clustering-triggered emission (CTE) mechanism, and supported by theoretical calculations. Powder with controllable p-RTP capability has the potential for strategic anti-counterfeiting encryption. The above results not only promote the development of the CTE mechanism toward more precise explanations but also provide new ideas for the preparation of nonconventional luminescent polymers with controllable p-RTP emission performance.

The Impact of Molecular Packing on Organic Room Temperature Phosphorescence and Corresponding Stimulus Response Effect

AbstractOrganic luminescent materials exhibiting persistent room temperature phosphorescence (RTP) have garnered considerable interest due to their versatile applications. However, the mechanism is still unclear, especially for the molecular structure‐molecular arrangement‐property relationship. Herein, six derivatives of phenothiazine 5,5‐dioxide are synthesized, revealing their distinct RTP properties and the associated underlying mechanisms. Careful analyses of their single crystal structures, combined with initial theoretical calculations, indicate that the persistent RTP effect is facilitated by effective intermolecular π–π interactions in the solid state led by folded conformation. Specifically, PTZO‐CF3‐3F exhibits distinctive photo‐activated phosphorescence both in crystal and grinding state in response to the changes of molecular arrangement. Besides, a porous structure is found to be present in PTZO‐CF3‐2F crystal, offering it great sensitivity to oxygen for both RTP intensity and lifetime. Accordingly, the visual oxygen warning device is fabricated successfully. Furthermore, PTZO‐CF3‐3F with photo‐activated RTP effect in a grinding state is successfully utilized in the applications of anti‐counterfeiting and optical logic gates, expanding the scope of stimulus‐responsive phosphorescence materials for broader applications.

Multicolour room temperature phosphorescence of carbon nitride nanoparticles in sodium borohydride and borax matrix

Carbon nitride based materials find applications in photocatalysis, sensing, and bioimaging. The fluorescence properties of carbon nitride based nanomaterials is widely studied. However, its phosphorescence phenomenon is relatively less explored. Herein, we report a simple and economic one step thermal synthesis of highly fluorescent pure carbon nitride nanoparticles (CNNPs) from organic solvent formamide at 220 °C, just above its boiling point without any further passivation or purification.The fluorescent CNNPs is dispersed in sodium borohydride and borax which act as suitable host matrix resulting in activation of persistent room temperature phosphorescence of CNNPs with high phosphorescence lifetimes of 1.144 s and 0.737 s respectively. Moreover, the CNNPs-Sodium borohydride composite display tricolour, blue, green and yellow-orange phosphorescence under 254 nm, 365 nm and 400 nm light respectively while CNNPs-Borax composite show dual blue and green phosphorescence under 254 nm and 365 nm UV light respectively. Hence, these CNNPs-Composite may be used as multicolour anti-counterfeiting materials in future.

Improvement in persistent room temperature phosphorescence of polymeric systems via a melt‐pressing method

AbstractPolymer‐based afterglow materials with persistent room temperature phosphorescence (pRTP) have attracted ever‐increasing attention due to their excellent mechanical property, suitability for large‐area production, and diverse processing methods. However, most pRTP polymers were solution‐processed into the films via casting. Herein, a simple processing method of melt‐pressing was proposed, which can improve the phosphorescence of various emitters when doped into polyamide‐12 (PA12) or its copolymer PA4 matrix. Notably, for Be@PA4 and Ch@PA4 (benzo[c]phenanthrene, Be and chrysene, Ch), their casted films did not show any afterglow, while the melt‐pressed ones exhibited bright afterglow of over 4.0 s, even if there were no hydrogen or ionic interaction between the emitters and the matrix. Detailed investigations demonstrate the melt‐pressing procedure can increase the non‐covalent interaction among the matrix as well as offer a rigid and dense environment. The melt‐pressed RTP improvement strategy was also applied to other polar polymers. Additionally, the melt‐pressing method would be of great potential in high‐level anti‐counterfeiting like preparing colorful camouflage patterns. Therefore, we offered a feasible method of melt‐pressing to improve the phosphorescence of polymeric materials without a change in chemical composition or feeding ratio. This work also gained a deeper understanding of the luminescence of polymer‐based systems.

Dopants Induce Persistent Room Temperature Phosphorescence in Triarylamine Boronate Esters

AbstractPurely organic materials exhibiting room temperature phosphorescence (RTP) are promising candidates for oxygen sensors and information encryption owing to their cost‐effective and environmentally friendly nature. Herein, we report a bimolecular RTP system where DTBU acts as the guest and TBBU serves as the host. In contrast to previously reported results, we find that both pure DTBU and TBBU do not exhibit RTP in the solid state even under N2 atmosphere. A DTBU/TBBU system with a low doping ratio (0.1 mol %) exhibits persistent yellowish‐green afterglow with a lifetime of 340 ms and is highly sensitive to oxygen. A DTBU/TBBU system with a higher doping ratio (10 mol %) maintains a phosphorescence lifetime of 179 ms under air. Applications of DTBU/TBBU at varied doping ratios in both oxygen sensing and information encryption are demonstrated. We propose that the T1 state of TBBU acts as an energy transfer intermediate between Tn and T1 of DTBU, ultimately leading to the generation of persistent RTP. Overall, this work demonstrates the critical importance of material purity in the design of RTP systems, and how an understanding of host–guest doping enables their photophysical properties to be precisely tuned.

Dopants Induce Persistent Room Temperature Phosphorescence in Triarylamine Boronate Esters.

Purely organic materials exhibiting room temperature phosphorescence (RTP) are promising candidates for oxygen sensors and information encryption owing to their cost-effective and environmentally friendly nature. Herein, we report a bimolecular RTP system where DTBU acts as the guest and TBBU serves as the host. In contrast to previously reported results, we find that both pure DTBU and TBBU do not exhibit RTP in the solid state even under N2 atmosphere. A DTBU/TBBU system with a low doping ratio (0.1 mol %) exhibits persistent yellowish-green afterglow with a lifetime of 340 ms and is highly sensitive to oxygen. A DTBU/TBBU system with a higher doping ratio (10 mol %) maintains a phosphorescence lifetime of 179 ms under air. Applications of DTBU/TBBU at varied doping ratios in both oxygen sensing and information encryption are demonstrated. We propose that the T1 state of TBBU acts as an energy transfer intermediate between Tn and T1 of DTBU, ultimately leading to the generation of persistent RTP. Overall, this work demonstrates the critical importance of material purity in the design of RTP systems, and how an understanding of host-guest doping enables their photophysical properties to be precisely tuned.

Symmetry-Breaking Triplet Excited State Enhances Red Afterglow Enabling Ubiquitous Afterglow Readout.

Molecular vibrations are often factors that deactivate luminescence. However, if there are molecular motion elements that enhance luminescence, it may be possible to utilize molecular movement as a design guideline to enhance luminescence. Here, the authors report a large contribution of symmetry-breaking molecular motion that enhances red persistent room-temperature phosphorescence (RTP) in donor-π-donor conjugated chromophores. The deuterated form of the donor-π-donor chromophore exhibits efficient red persistent RTP with a yield of 21% and a lifetime of 1.6s. A dynamic calculation of the phosphorescence rate constant (kp) indicates that the symmetry-breaking movement has a crucial role in selectively facilitating kp without increasing nonradiative transition from the lowest triplet excited state. Molecules exhibiting efficient red persistent RTP enable long-wavelength excitation, indicating the suitability of observing afterglow readout in a bright indoor environment with a white-light-emitting diode flashlight, greatly expanding the range of anti-counterfeiting applications that use afterglow.

A Universal Strategy for Tuning Afterglow Color In Situ Via Radiative Energy Transfer

AbstractColorful afterglow materials have promising applications in optoelectronic displays. To effectively fine‐tune the afterglow color, an assembly structure that includes UV lamp, persistent room temperature phosphorescence (pRTP) layer, and fluorescent layer (from the bottom to the top) with superimposition of pRTP and fluorescence, is designed. The fluorescence component is emitted from a PMMA film doped with asymmetric 4‐(7‐methoxy‐5,6‐dinitrobenzo[c][1,2,5,]thiadiazol‐4‐yl)‐N,N‐diphenylaniline (TPA‐TD), which has a broad absorption that covers the entire visible region, facilitating effective radiative energy transfer between various pRTP materials and TPA‐TD. Due to its aggregation‐induced‐emission nature, the fluorescence intensity of TPA‐TD enhances with increased doping concentration in PMMA films. Meanwhile, the transmission of TPA‐TD film reduces as the doping concentration increases, which results in a reduction of pRTP intensity through the TPA‐TD film. Therefore, the afterglow color can be fine‐tuned in situ by regulating the ratio of pRTP and fluorescence.

Regioisomers containing triarylboron-based motifs as multi-functional photoluminescent materials: from dual-mode delayed emission to pH-switchable room-temperature phosphorescence.

Triarylboron compounds have been established as promising candidates in optoelectronic applications. However, realizing multi-functional properties in triaryl boron-based materials remains challenging. Herein, we present two regioisomers, 1 and 2, designed judiciously by connecting a dimethylamino donor and a dimesitylboryl acceptor at 1,4 and 2,6-positions of the naphthalene spacer, respectively. Both compounds 1 and 2 display simultaneous, delayed fluorescence and persistent room-temperature phosphorescence (580 nm, τ av = 168 ms, Φ = 76% for 1; 550 nm, τ av = 129 ms, Φ = 88% for 2 in 1 wt% PMMA), with the delayed fluorescence bands being sensitive to doping concentration (in PMMA). Notably, compound 1 in 1 wt% PMMA films demonstrates a reversibly switchable single-molecule phosphorescence from orange (580 nm) to green (λ Ph = 550 nm, τ av = 42 ms) in response to pH, which can be utilized for anti-counterfeiting applications. These results were further corroborated by studying the respective cationic salts 1-OTF and 2-OTF. Moreover, 1 and 2 exhibited blue-shifted fluorescence in response to mechanical pressure. Compound 2 also showed three-photon (σ3P) absorption properties which were better compared to those of compound 1.