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

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

Hybridization of short-range and long-range charge transfer boosts room-temperature phosphorescence performance

At present, mainstream room-temperature phosphorescence (RTP) emission relies on organic materials with long-range charge-transfer effects; therefore, exploring new forms of charge transfer to generate RTP is worth studying. In this work, indole-carbazole was used as the core to ensure the narrowband fluorescence emission of the material based on its characteristic short-range charge-transfer effect. In addition, halogenated carbazoles were introduced into the periphery to construct long-range charge transfer, resulting in VTCzNL-Cl and VTCzNL-Br. By encapsulating these phosphors into a robust host (TPP), two host-guest crystalline systems were further developed, achieving efficient RTP performance with phosphorescence quantum yields of 26% and phosphorescence lifetimes of 3.2 and 39.2 ms, respectively.

Room-temperature phosphorescence from organic materials in aqueous media

In recent years, organic materials with room-temperature phosphorescence (RTP) features have gained significant attention due to their wide applications in the fields of bioimaging, light-harvesting materials, encryption technology, etc. Although several examples of organic RTP materials in the crystalline state and polymer-based systems have been reported in the last decade or so, achieving organic RTP in the solution phase, particularly in the aqueous phase has remained a challenging task. Herein in this review, we summarize the progress in this direction by highlighting design strategies based on supramolecular scaffolding and host-guest complexation and the applications of such aqueous organic RTP materials in bioimaging, sensing, etc.

Stretchable phosphorescent polymers by multiphase engineering

Stretchable phosphorescence materials potentially enable applications in diverse advanced fields in wearable electronics. However, achieving room-temperature phosphorescence materials simultaneously featuring long-lived emission and good stretchability is challenging because it is hard to balance the rigidity and flexibility in the same polymer. Here we present a multiphase engineering for obtaining stretchable phosphorescent materials by combining stiffness and softness simultaneously in well-designed block copolymers. Due to the microphase separation, copolymers demonstrate an intrinsic stretchability of 712%, maintaining an ultralong phosphorescence lifetime of up to 981.11 ms. This multiphase engineering is generally applicable to a series of binary and ternary initiator systems with color-tunable phosphorescence in the visible range. Moreover, these copolymers enable multi-level volumetric data encryption and stretchable afterglow display. This work provides a fundamental understanding of the nanostructures and material properties for designing stretchable materials and extends the potential of phosphorescence polymers.

Förster Resonance Energy Transfer: Stimulus-Responsive Purely Organic Room Temperature Phosphorescence through Dynamic B-N bond

Recently, stimulus-responsive organic materials with room-temperature phosphorescence (RTP) properties have attracted significant attention owing to their potential applications in chemical sensing, anticounterfeiting, and displays. However, molecular design currently lacks systematicity and effectiveness. Herein, we report a capture-release strategy for the construction of reversible RTP via B/N Lewis pairs. Specifically, the RTP of the Lewis acid of 7-bromo-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (BrBA) can be deactivated through capturing by the Lewis base, N,N-diphenyl-4-(pyridin-4-yl)aniline (TPAPy), and reactivated by dissociation of B-N bonds to release BrBA. Reversible RTP is attributed to the exceptional self-assembly capability of BrBA, whereas the tunable RTP colors are derived from distinct Förster resonance energy transfer (FRET) processes. The potential applications of RTP materials in information storage and anti-counterfeiting were also experimentally validated. The capture-release approach proposed in this study offers an effective strategy for designing stimulus-responsive materials.

Phosphorescent acyclic cucurbituril solid supramolecular multicolour delayed fluorescence behaviour

Organic photoluminescent macrocyclic hosts have been widely advanced in many fields. Phosphorescent hosts with the ability to bind organic guests have rarely been reported. Herein, acyclic cucurbituril modified with four carboxylic acids (ACB-COOH) is mined to present uncommon purely organic room-temperature phosphorescence (RTP) at 510 nm with a lifetime of 1.86 μs. Its RTP properties are significantly promoted with an extended lifetime up to 2.12 s and considerable quantum yield of 6.29% after assembly with a polyvinyl alcohol (PVA) matrix. By virtue of the intrinsic self-crimping configuration of ACB-COOH, organic guests, including fluorescence dyes (Rhodamine B (RhB) and Pyronin Y (PyY)) and a drug molecule (morphine (Mor)), could be fully encapsulated by ACB-COOH to attain energy transfer involving phosphorescent acyclic cucurbituril. Ultimately, as-prepared systems are successfully exploited to establish multicolor afterglow materials and visible sensing of morphine. As an expansion of phosphorescent acyclic cucurbituril, the host afterglow color can be readily regulated by attaching different aromatic sidewalls. This study develops the fabrication strategies and application scope of a supramolecular phosphorescent host and opens up a new direction for the manufacture of intelligent long-lived luminescent materials.

Recent progress in ion-regulated organic room-temperature phosphorescence

Organic room-temperature phosphorescence (RTP) materials have attracted considerable attention for their extended afterglow at ambient conditions, eco-friendliness, and wide-ranging applications in bio-imaging, data storage, security inks, and emergency illumination. Significant advancements have been achieved in recent years in developing highly efficient RTP materials by manipulating the intermolecular interactions. In this perspective, we have summarized recent advances in ion-regulated organic RTP materials based on the roles and interactions of ions, including the ion-π interactions, electrostatic interactions, and coordinate interactions. Subsequently, the current challenges and prospects of utilizing ionic interactions for inducing and modulating the phosphorescent properties are presented. It is anticipated that this perspective will provide basic guidelines for fabricating novel ionic RTP materials and further extend their application potential.

Abnormal thermally-stimulated dynamic organic phosphorescence

Dynamic luminescence behavior by external stimuli, such as light, thermal field, electricity, mechanical force, etc., endows the materials with great promise in optoelectronic applications. Upon thermal stimulus, the emission is inevitably quenched due to intensive non-radiative transition, especially for phosphorescence at high temperature. Herein, we report an abnormal thermally-stimulated phosphorescence behavior in a series of organic phosphors. As temperature changes from 198 to 343 K, the phosphorescence at around 479 nm gradually enhances for the model phosphor, of which the phosphorescent colors are tuned from yellow to cyan-blue. Furthermore, we demonstrate the potential applications of such dynamic emission for smart dyes and colorful afterglow displays. Our results would initiate the exploration of dynamic high-temperature phosphorescence for applications in smart optoelectronics. This finding not only contributes to an in-depth understanding of the thermally-stimulated phosphorescence, but also paves the way toward the development of smart materials for applications in optoelectronics.

Modulation of Deep-Red to Near-Infrared Room-Temperature Charge-Transfer Phosphorescence of Crystalline "Pyrene Box" Cages by Coupled Ion/Guest Structural Self-Assembly

Organic cocrystals obtained from multicomponent self-assembly have garnered considerable attention due to their distinct phosphorescence properties and broad applications. Yet, there have been limited reports on cocrystal systems that showcase efficient deep-red to near-infrared (NIR) charge-transfer (CT) phosphorescence. Furthermore, effective strategies to modulate the emission pathways of both fluorescence and phosphorescence remain underexplored. In this work, we dedicated our work to four distinct self-assembled cocrystals called "pyrene box" cages using 1,3,6,8-pyrenetetrasulfonate anions (PTS4-), 4-iodoaniline (1), guanidinium (G+), diaminoguanidinium (A2G+), and hydrated K+ countercations. The binding of such cations to PTS4- platforms adaptively modulates their supramolecular stacking self-assembly with guest molecules 1, allowing to steer the fluorescence and phosphorescence pathways. Notably, the confinement of guest molecule 1 within "pyrene box" PTSK{1} and PTSG{1} cages leads to an efficient deep-red to NIR CT phosphorescence emission. The addition of fuming gases like triethylamine and HCl allows reversible pH modulations of guest binding, which in turn induce a reversible transition of the "pyrene box" cage between fluorescence and phosphorescence states. This capability was further illustrated through a proof-of-concept demonstration in shrimp freshness detection. Our findings not only lay a foundation for future supramolecular designs leveraging weak intermolecular host-guest interactions to engineer excited states in interacting chromophores but also broaden the prospective applications of room-temperature phosphorescence materials in food safety detection.

Cucurbit[8]uril Confinement-Based Secondary Coassembly for High-Efficiency Phosphorescence Energy Transfer Behavior

Aqueous supramolecular long-lived near-infrared (NIR) material is highly attractive but still remains great challenge. Herein, we report cucurbit[8]uril confinement-based secondary coassembly for achieving NIR phosphorescence energy transfer in water, which is fabricated from dicationic dodecyl-chain-bridged 4-(4-bromophenyl)-pyridine derivative (G), cucurbit[8]uril (CB[8]), and polyelectrolyte poly(4-styrene-sulfonic sodium) (PSS) via the hierarchical confinement strategy. As compared to the dumbbell-shaped G, the formation of unprecedented linear polypseudorotaxane G⊂CB[8] with nanofiber morphology engenders an emerging phosphorescent emission at 510 nm due to the macrocyclic confinement effect. Moreover, benefiting from the following secondary assembly confinement, such tight polypseudorotaxane G⊂CB[8] can further assemble with anionic polyelectrolyte PSS to yield uniform spherical nanoparticle, thereby significantly strengthening phosphorescence performance with an extended lifetime (i.e., 2.39 ms, c.f., 45.0 μs). Subsequently, the organic dye Rhodamine 800 serving as energy acceptor can be slightly doped into the polyelectrolyte assembly, which enables the occurrence of efficient phosphorescence energy transfer process with efficiency up to 80.1% at a high donor/acceptor ratio, and concurrently endows the final system with red-shifted and long-lived NIR emission (710 nm). Ultimately, the as-prepared assembly is successfully exploited as versatile imaging agent for NIR window labeling and detecting in living cells.

Efficient Visible-Light-Activated Ultra-Long Room-Temperature Phosphorescence Triggered by Multi-Esterification

The development of ultra-long room-temperature phosphorescence (UL-RTP) in processable amorphous organic materials is highly desirable for applications in flexible displays, anti-counterfeiting, and bio-imaging. However, achieving efficient UL-RTP from amorphous materials remains a challenging task, especially with activation by visible light and a bright afterglow. Here we report a general and rational molecular-design strategy to enable efficient visible-light-excited UL-RTP by multi-esterification of a rigid large-plane phosphorescence core. Notably, multi-esterification minimizes the aggregation-induced quenching and accomplishes a 'four birds with one stone' possibility in the generation and radiation process of UL-RTP: i) shifting the excitation from ultraviolet light to blue-light through enhancing the transition dipole moment of low-lying singlet-states, ii) facilitating the intersystem crossing process through the incorporation of lone-pair electrons, iii) boosting the decay process of long-lived triplet excitons resulting from a significantly increased transition dipole moment, and iv) reducing the intrinsic triplet nonradiative decay by substitution of high-frequency vibrating hydrogen atoms. All these factors synergistically contribute to the most efficient and stable visible-light-stimulated UL-RTP (lifetime up to 2.01 s and efficiency up to 35.4 % upon excitation at 450 nm) in flexible films using multi-esterified coronene, which allows high-tech applications in single-component time-delayed white light-emitting diodes and information technology based on flashlight-activated afterglow encryption.

Naphthyl Substituted Impurities Induce Efficient Room Temperature Phosphorescence

Accidentally, it was found that triphenylamine (TPA) from commercial sources shows ultralong yellow-green room temperature phosphorescence (RTP) like commercial carbazole, which however disappears for lab-synthesized TPA with high purity. Herein, we for the first time identify the impurity types that cause RTP of commercial TPA, which are two N, N-diphenyl-naphthylamine isomers. Due to similar molecular polarity and very trace amount (≈0.8 ‰, molar ratio), these naphthyl substituted impurities can be easily overlooked. We further show that even at an extremely low amount (1000000 : 1, mass ratio) of impurities, RTP emission is still generated, attributed to the triplet-to-triplet energy transfer mechanism. Notably, this doping strategy is also applicable to the triphenylphosphine and benzophenone host systems, of which strong RTP emission can be activated by simply doping the corresponding naphthyl substituted analogues into them. This work therefore provides a general and efficient host/guest strategy toward high performance and diverse organic RTP materials.

Revisiting organic charge-transfer cocrystals for wide-range tunable, ambient phosphorescence

Simple and efficient designs that enable a wide range of phosphorescence emission in organic materials have ignited scientific interest across diverse fields. One particularly promising approach is the cocrystallization strategy, where organic cocrystals are ingeniously formed through relatively weaker and dynamic non-covalent interactions. In our present study, we push the boundaries further by extending this cocrystal strategy to incorporate donor-acceptor components, stabilized by various halogen bonding interactions. This non-covalent complexation triggers ambient, charge-transfer phosphorescence (3CT), which can be precisely tuned across a broad spectrum by a modular selection of components with distinct electronic characteristics. At the core of our investigation lies the electron-deficient phosphor, pyromellitic diimide, which, upon complexation with different donors based on their electron-donating strength, manifests a striking array of phosphorescence emission from CT triplet states, spanning from green to yellow to reddish orange accompanied by noteworthy quantum yields. Through a systematic exploration of the electronic properties using spectroscopic studies and molecular organization through single-crystal X-ray diffraction, we decisively establish the molecular origin of the observed phosphorescence. Notably, our work presents, for the first time, an elegant demonstration of tunable 3CT phosphorescence emission in intermolecular donor-acceptor systems, highlighting their immense significance in the quest for efficient organic phosphors.

Multicolor Phosphorescence Modulated by Excitation and Temperature in Zn-Based Coordination Polymers

Color-tunable room-temperature phosphorescence (RTP) with potential in many fields is of great importance but extremely challenging. It is necessary to comprehend the correlation between the molecular structure and property to design and synthesize such materials. Metal-organic coordination polymers (CPs) with good predesignability and precise structure have become a platform to construct RTP materials. Herein, three zinc-based CPs containing halogen and a flexible tetradentate ligand are synthesized. All of these CPs present two constant emission regions and an excitation-dependent emission region. Structure-property analysis shows that these emissions originate from isolated chromophores and dimerized chromophores as well as various charge transfers. The phosphorescence colors of these CPs can be modulated by excitation and temperature. This study provides a novel strategy to construct multicolor and multiresponsive RTP materials based on metal-organic coordination polymers.

Fast and Tunable Phosphorescence from Organic Ionic Crystals

Crystalline diphosphonium iodides [MeR2 P-spacer-R2 Me]I with phenylene (1, 2), naphthalene (3, 4), biphenyl (5) and anthracene (6) as aromatic spacers, are photoemissive under ambient conditions. The emission colors (λem values from 550 to 880 nm) and intensities (Φem reaching 0.75) are defined by the composition and substitution geometry of the central conjugated chromophore motif, and the anion-π interactions. Time-resolved and variable-temperature luminescence studies suggest phosphorescence for all the titled compounds, which demonstrate observed lifetimes of 0.46-92.23 μs at 297 K. Radiative rate constants kr as high as 2.8×105  s-1 deduced for salts 1-3 were assigned to strong spin-orbit coupling enhanced by an external heavy atom effect arising from the anion-π charge-transfer character of the triplet excited state. These rates of anomalously fast metal-free phosphorescence are comparable to those of transition metal complexes and organic luminophores that utilize triplet excitons via a thermally activated delayed fluorescence mechanism, making such ionic luminophores a new paradigm for the design of photofunctional and responsive molecular materials.

Anion-π Interaction in a Diketopyrrolopyrrole Derivative

In this work, an N-substituted diketopyrrolopyrrole (DPP) derivative Ph-DPP was synthesized, showing interaction toward Lewis alkaline anions such as F-. The typical electron-transfer-dominated anion-π interaction product Ph-DPP•- and unexpected isomer product i-Ph-DPP were both observed, and their formation mechanism was studied by density functional theory calculations, suggesting that a deprotonation initiation route is favored, which gives interesting insight for understanding the debatable role of F- in such non-covalent intermolecular interactions.

Conformationally confined three-armed supramolecular folding for boosting near-infrared biological imaging

Herein, a triphenylamine derivative (TP-3PY) possessing 4-(4-bromophenyl)pyridine (PY) as an electron-accepting group and tris[p-(4-pyridylvinyl)phenyl]amine (TPA) with large two-photon absorption cross-sections as an electron-donating group was obtained, and showed intense absorption in the visible light region (λmax = 509 nm) and weak near-infrared (NIR) fluorescence emission at 750 nm. After complexation with cucurbit[8]uril (CB[8]), TP-3PY showed bright NIR fluorescence emission at 727 nm and phosphorescence emission at 800 nm. When the supramolecular assembly (TP-3PY⊂CB[8]) further interacted with dodecyl-modified sulfonatocalix[4]arene (SC4AD), the fluorescence and phosphorescence emissions were further enhanced at 710 and 734 nm, respectively. However, only the fluorescence emission of TP-3PY was enhanced in the presence of cucurbit[7]uril (CB[7]) and SC4AD. More interestingly, the photoluminescence of TP-3PY⊂CB[8]@SC4AD and TP-3PY⊂CB[7]@SC4AD assemblies could be excited by both visible (510 nm) and NIR light (930 nm). Finally, these ternary supramolecular assemblies with bright NIR light emission were applied to lysosome imaging of tumor cells and real-time biological imaging of mice.

The host-guest inclusion driven by host-stabilized charge transfer for construction of sequentially red-shifted mechanochromic system

Developing more extensive methods to understand the underlying structure-property relationship of mechanochromic luminescent molecules is demanding but remains challenging. Herein, the effect of host-guest interaction on the mechanochromic properties of organic molecules is illustrated. A series of pyridinium-functionalized triphenylamine derivatives show bathochromic-shifted emission upon mechanical stimulation. These derivatives bind to cucurbit[8]uril to form homoternary host-guest inclusion complexes through host-stabilized intermolecular charge transfer interactions. Remarkably, the homoternary complexes exhibit longer emission than that of free guests in the solid state (even longer than ground guests), and a further bathochromic-shifted emission is observed upon grinding. Additionally, a heteroternary complex constructed through the encapsulation of pyrene (donor) and pyridinium (acceptor) guest pair in cucurbit[8]uril also displays the mechanochromic luminescent property. This work not only discloses the effect of host-guest inclusion on the mechanochromic property of organic molecules, but also provides a principle and a facile way to design the sequentially red-shifted mechanochromic materials.

Anion-π Type Aggregation-Induced Emission Luminogens

As a type of important non-covalent interactions that can efficiently prohibit π-π interaction to avoid quenching of luminescence, anion-π interactions are receiving growing attention for the fabrication of aggregation-induced emission luminogens (AIEgens) since 2017. The obtained anion-π type AIEgens can be applied in the fields of wash-free bioimaging and long-term tracking of subcellular organelle, photodynamic anti-cancer and anti-bacterial therapy due to their good water solubility, superior photostability and excellent reactive oxygen species generation ability. Moreover, anion-π type AIEgens were also further constructed for room temperature phosphorescence by taking advantages of the heavy-atom participated anion-π interactions. This concept article provides a brief summary of this field, mainly focusing on the design strategy, photophysical properties and applications of anion-π type AIEgens.

Simple Stimulus-Responsive Organic Long Persistent Luminescence Systems Based on Methoxy-Functionalized Triphenylphosphine

Triarylphosphine-based pure organic long persistent luminescence materials are rarely investigated because of their poor stability and low photoluminescence quantum yield. Herein, we demonstrate that the introduction of a methoxy group (TPP-o-3OMe) at the ortho position of triphenylphosphine (TPP) can essentially promote the n → π* transition and promote intersystem crossing to generate more triplet excitons. Simultaneously, generating abundant intramolecular and intermolecular hydrogen bonds to stable excited triplet excitons is beneficial, thereby causing high-efficiency phosphorescence emission (τ_p = 394.1 ms; Φ_p = 9.28%). Interestingly, it shows a good acid response to protonic acids and can often be cycled many times under the heating or ammonia fumigation conditions. This research provides a new idea for enriching the types of pure organic room-temperature phosphorescent materials, widening their applications in the fields of anticounterfeiting and smart response, and promotes the further development of this field.

Room temperature charge-transfer phosphorescence from organic donor–acceptor Co-crystals

Engineering the electronic excited state manifolds of organic molecules can give rise to various functional outcomes, including ambient triplet harvesting, that has received prodigious attention in the recent past. Herein, we introduce a modular, non-covalent approach to bias the entire excited state landscape of an organic molecule using tunable ‘through-space charge-transfer’ interactions with appropriate donors. Although charge-transfer (CT) donor–acceptor complexes have been extensively explored as functional and supramolecular motifs in the realm of soft organic materials, they could not imprint their potentiality in the field of luminescent materials, and it still remains as a challenge. Thus, in the present study, we investigate the modulation of the excited state emission characteristics of a simple pyromellitic diimide derivative on complexation with appropriate donor molecules of varying electronic characteristics to demonstrate the selective harvesting of emission from its locally excited (LE) and CT singlet and triplet states. Remarkably, co-crystallization of the pyromellitic diimide with heavy-atom substituted and electron-rich aromatic donors leads to an unprecedented ambient CT phosphorescence with impressive efficiency and notable lifetime. Further, gradual minimizing of the electron-donating strength of the donors from 1,4-diiodo-2,3,5,6-tetramethylbenzene (or 1,2-diiodo-3,4,5,6-tetramethylbenzene) to 1,2-diiodo-4,5-dimethylbenzene and 1-bromo-4-iodobenzene modulates the source of ambient phosphorescence emission from the ³CT excited state to ³LE excited state. Through comprehensive spectroscopic, theoretical studies, and single-crystal analyses, we elucidate the unparalleled role of intermolecular donor–acceptor interactions to toggle between the emissive excited states and stabilize the triplet excitons. We envisage that the present study will be able to provide new and innovative dimensions to the existing molecular designs employed for triplet harvesting.

A modular, non-covalent donor–acceptor strategy is proposed to bias the excited-state manifold of organic systems and to realize unprecedented charge-transfer phosphorescence.