Highly Reversible Supramolecular Light Switch for NIR Phosphorescence Resonance Energy Transfer

Enhancing Purely Organic Room Temperature Phosphorescence via Supramolecular Self-Assembly

Long-lived and highly efficient room temperature phosphorescence (RTP) materials are in high demand for practical applications in lighting and display, security signboards, and anti-counterfeiting. Achieving RTP in aqueous solutions, near-infrared (NIR) phosphorescence emission, and NIR-excited RTP are crucial for applications in bio-imaging, but these goals pose significant challenges. Supramolecular self-assembly provides an effective strategy to address the above problems. This review focuses on the recent advances in the enhancement of RTP via supramolecular self-assembly, covering four key aspects: small molecular self-assembly, cocrystals, the self-assembly of macrocyclic hosts and guests, and multi-stage supramolecular self-assembly. This review not only highlights progress in these areas but also underscores the prominent challenges associated with developing supramolecular RTP materials. The resulting strategies for the development of high-performance supramolecular RTP materials are discussed, aiming to satisfy the practical applications of RTP materials in biomedical science.

Water/Light Multiregulated Supramolecular Polypseudorotaxane Gel with Switchable Room-Temperature Phosphorescence

Water/light regulated room-temperature phosphorescence (RTP) of polypseudorotaxane supramolecular gel is constructed by threading the poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) chain with the bromoaromatic aldehyde into mono-(6-ethylenediamine-6-deoxygenated)-β-cyclodextrin (ECD) cavities and further assembling with negatively charged Laponite XLG (CNS) and diarylethene derivative (DAE) through electrostatic interaction. This hydrogel exhibits significant blue fluorescence emission; instead, after lyophilization to xerogel, the system exhibits both blue fluorescence and yellow RTP based on the rigid network structure of the xerogel, which restricts the vibration of the phosphor and suppresses the nonradiative relaxation process. Interestingly, the addition of excess ECDs to the gel system can enhance the RTP emission. Furthermore, the reversible luminescence behavior can be adjusted by the photoresponsive isomerism of DAE and humidity. This polypseudorotaxane supramolecular gel system provides a novel strategy for constructing tunable RTP materials.

Cucurbituril-based supramolecular host-guest complexes: single-crystal structures and dual-state fluorescence enhancement

Two supramolecular complexes were prepared using cucurbiturils [CBs] as mediators and a four-armed p-xylene derivative (M1) as a guest molecule. The single crystals of these two complexes were obtained and successfully analyzed by single-crystal X-ray diffraction (SCXRD). An unexpected and intriguing 1 : 2 self-assembly arrangement between M1 and CB[8] was notably uncovered, marking its first observation. These host-guest complexes exhibit distinctive photophysical properties, especially emission behaviors. Invaluable insights can be derived from these single-crystal structures. The precious single-crystal structures provide both precise structural information regarding the supramolecular complexes and a deeper understanding of the intricate mechanisms governing their photophysical properties.

Phosphorescence resonance energy transfer from purely organic supramolecular assembly

Phosphorescence energy transfer systems have been applied in encryption, biomedical imaging and chemical sensing. These systems exhibit ultra-large Stokes shifts, high quantum yields and are colour-tuneable with long-wavelength afterglow fluorescence (particularly in the near-infrared) under ambient conditions. This review discusses triplet-to-singlet PRET or triplet-to-singlet-to-singlet cascaded PRET systems based on macrocyclic or assembly-confined purely organic phosphorescence introducing the critical toles of supramolecular noncovalent interactions in the process. These interactions promote intersystem crossing, restricting the motion of phosphors, minimizing non-radiative decay and organizing donor-acceptor pairs in close proximity. We discuss the applications of these systems and focus on the challenges ahead in facilitating their further development.

Red Room-Temperature Phosphorescence Supramolecular Assemblies Based on Cucurbit[7]uril: Reversible Temperature Stimulation Response and Cell-Specific Silver Ion Imaging

Solid-state materials with efficient room-temperature phosphorescence (RTP) emission have been widely used in materials science, and organic RTP-emitting systems with heavy-metal doping in aqueous solutions have attracted much attention in recent years. A novel supramolecular interaction was induced by host-guest assembly using cucurbit[7]uril (Q[7]) as the host and brominated naphthalimide phosphor as the guest. This interaction was further enhanced through synergistic chelation stimulated by analytical silver ion complexation. This approach facilitated the system's structural rigidity, intersystem crossing, and oxygen shielding. We achieved deep red phosphorescence emission in aqueous solution and ambient conditions along with quantitative determination of silver ions. The new complex exhibited good reversible thermoresponsive behavior and was successfully applied for the first time to target phosphorescence imaging of silver ions in the mitochondria of A549 cancer cells. These results are beneficial for constructing novel RTP systems with stimulus-responsive luminescence in aqueous solution, contributing to future research in bioimaging, detection, optical sensors, and thermometry materials.

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

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

Potential of nonporous adaptive crystals for hydrocarbon separation

Hydrocarbon separation is an important process in the field of petrochemical industry, which provides a variety of raw materials for industrial production and a strong support for the development of national economy. However, traditional separation processes involve huge energy consumption. Adsorptive separation based on nonporous adaptive crystal (NAC) materials is considered as an attractive green alternative to traditional energy-intensive separation technologies due to its advantages of low energy consumption, high chemical and thermal stability, excellent selective adsorption and separation performance, and outstanding recyclability. Considering the exceptional potential of NAC materials for hydrocarbon separation, this review comprehensively summarizes recent advances in various supramolecular host-based NACs. Moreover, the current challenges and future directions are illustrated in detail. It is expected that this review will provide useful and timely references for researchers in this area. Based on a large number of state-of-the-art studies, the review will definitely advance the development of NAC materials for hydrocarbon separation and stimulate more interesting studies in related fields.

Composite Mesoporous Silica Nanoparticles with Dual-color Afterglow for Cross-correlation-based Living Cell Imaging

Room temperature phosphorescence (RTP) materials are characterized with emission after removing the excitation source. Such long-lived emission feature possesses great potential in biological fluorescence imaging because it enables a way regarding temporal dimension for separating the interference of autofluorescence and common noises typically encountered in conventional fluorescence imaging. Herein, we constructed a new type of mesoporous silica nanoparticles (MSNs)-based composite nanoparticles (NPs) with dual-color long-lived emission, namely millisecond-level green phosphorescence and sub-millisecond-level delayed red fluorescence by encapsulating a typical RTP dye and Rhodamine dye in the cavities of the MSNs with the former acting as energy donor (D) while the latter as acceptor (A). Benefiting from the close D-A proximity, energy match between the donor and the acceptor and the optimized D/A ratio in the composite NPs, efficient triplet-to-singlet Förster resonance energy transfer (TS-FRET) in the NPs occurred upon exciting the donor, which enabled dual-color long-lived emission. The preliminary results of dual-color correlation imaging of live cells based on such emission feature unequivocally verified the unique ability of such NPs for distinguishing the false positive generated by common emitters with single-color emission feature.

Time-Dependent Polychrome Stereoscopic Luminescence Triggered by Resonance Energy Transfer between Carbon Dots-in-Zeolite Composites and Fluorescence Quantum Dots

Luminescence multiplexing shows promising application prospects in information security, yet even outstanding time division multiplexing can only carry limited luminescence information. Time-space division multiplexing can greatly expand the information capacity by simultaneously transferring luminescence information in both time and space dimensions. Herein, time-dependent polychrome stereoscopic luminescence system has been successfully developed by designing a 3D luminescence system based on resonance energy transfer (RET), in which afterglow lifetime easily regulated carbon dots-in-zeolite composites are used as energy donors and multicolor fluorescence quantum dots (QDs) as energy acceptors. Taking perovskite QDs (PeQDs) as example, by matching the energy donors with different afterglow lifetimes and the energy acceptors with different fluorescence colors, tunable afterglow emission of PeQDs with wavelength within 463-614 nm and lifetime within 232-1500 ms can be realized, in which the maximal RET efficiency reaches 95%. As a proof of concept, such novel luminescence system that carries eight layers of luminescence information involving four dimensions (time and 3D space) is successfully applied in advanced time-space division multiplexing. This work opens a new perspective for the application of time-space integrated luminescence systems in advanced information multiplexing.

Dynamic B/N Lewis Pairs: Insights into the Structural Variations and Photochromism via Light-Induced Fluorescence to Phosphorescence Switching

Ultralong afterglow emissions due to room-temperature phosphorescence (RTP) are of paramount importance in the advancement of smart sensors, bioimaging and light-emitting devices. We herein present an efficient approach to achieve rarely accessible phosphorescence of heavy atom-free organoboranes via photochemical switching of sterically tunable fluorescent Lewis pairs (LPs). LPs are widely applied in and well-known for their outstanding performance in catalysis and supramolecular soft materials but have not thus far been exploited to develop photo-responsive RTP materials. The intramolecular LP M1BNM not only shows a dynamic response to thermal treatment due to reversible N→B coordination but crystals of M1BNM also undergo rapid photochromic switching. As a result, unusual emission switching from short-lived fluorescence to long-lived phosphorescence (rad-M1BNM, τ_RTP =232 ms) is observed. The reported discoveries in the field of Lewis pairs chemistry offer important insights into their structural dynamics, while also pointing to new opportunities for photoactive materials with implications for fast responsive detectors.

Noncovalent Polymerization-Activated Ultrastrong Near-Infrared Room-Temperature Phosphorescence Energy Transfer Assembly in Aqueous Solution

Noncovalent macrocycle-confined supramolecular purely organic room-temperature phosphorescence (RTP) is a current research hotspot. Herein, a high-efficiency noncovalent polymerization-activated near-infrared (NIR)-emissive RTP-harvesting system in aqueous solution based on the stepwise confinement of cucurbit[7]uril (CB[7]) and β-cyclodextrin-grafted hyaluronic acid (HACD), is reported. Compared with the dodecyl-chain-bridged 6-bromoisoquinoline derivative (G), the dumbbell-shaped assembly G⊂CB[7] presents an appeared complexation-induced RTP signal at 540 nm via the first confinement of CB[7]. Subsequently, benefitting from the stepwise confinement encapsulation of the β-cyclodextrin cavity, the subsequent noncovalent polymerization of the binary G⊂CB[7] assembly enabled by HACD can contribute to the further-enhanced RTP emission intensity approximately eight times in addition to an increased lifetime from 59.0 µs to 0.581 ms. Moreover, upon doping a small amount of two types of organic dyes, Nile blue or tetrakis(4-sulfophenyl)porphyrin as an acceptor into the supramolecular confinement assembly G⊂CB[7] @ HACD, efficient RTP energy transfer occurs accompanied by a long-lived NIR-emitting performance (680 and 710 nm) with a high donor/acceptor ratio. Intriguingly, the prepared RTP-harvesting system is successfully applied for targeted NIR imaging of living tumor cells by utilizing the targeting ability of hyaluronic acid, which provides a new strategy to create advanced water-soluble NIR phosphorescent materials.

Near-Infrared Phosphorescent Switch of Diarylethene Phenylpyridinium Derivative and Cucurbit[8]uril for Cell Imaging

Near-infrared (NIR) pure organic room-temperature phosphorescence (RTP) materials have received growing research interest due to their wide application in the fields of high-resolution bioimaging and luminescent materials. In this work, the authors report a macrocycle-confined pure organic RTP supramolecular assembly, which is constructed by diarylethene phenylpyridinium derivative (DTE-TP) and cucurbit[8]uril (CB[8]). Compared with CB[6] and CB[7], the larger cavity of CB[8] induces molecular folding and enhances the intramolecular charge transfer interactions, which leads to the obtained assembly emitting efficient NIR phosphorescence at 700 nm. Due to the photochromism of the diarylethene core, the NIR phosphorescence is reversibly regulated by light irradiation at wavelengths of 365 and >600 nm. Furthermore, cell-based experiments show that this supramolecular assembly is located in the lysosomes and displays a NIR phosphorescence at 650-750 nm. In addition, by means of phosphorescence resonance energy transfer, the obtained assembly exhibits a red-shifted NIR emission at 817 nm. This supramolecular phosphorescent switch provides a convenient path for the modular design of water-soluble pure organic room-temperature NIR phosphorescent materials.

A Highly Efficient Phosphorescence/Fluorescence Supramolecular Switch Based on a Bromoisoquinoline Cascaded Assembly in Aqueous Solution

Despite ongoing research into photocontrolled supramolecular switches, reversible photoswitching between room-temperature phosphorescence (RTP) and delayed fluorescence is rare in the aqueous phase. Herein, an efficient RTP-fluorescence switch based on a cascaded supramolecular assembly is reported, which is constructed using a 6-bromoisoquinoline derivative (G3 ), cucurbit[7]uril (CB[7]), sulfonatocalix[4]arene (SC4A4), and a photochromic spiropyran (SP) derivative. Benefiting from the confinement effect of CB[7], initial complexation with CB[7] arouses an emerging RTP signal at 540 nm for G3 . This structure subsequently coassembles with amphiphilic SC4A4 to form tight spherical nanoparticles, thereby further facilitating RTP emission (≈12 times) in addition to a prolonged lifetime (i.e., 1.80 ms c.f., 50.1 µs). Interestingly, following cascaded assembly with a photocontrolled energy acceptor (i.e., SP), the efficient light-driven RTP energy transfer occurs when SP is transformed to its fluorescent merocyanine (MC) state. Ultimately, this endows the final system with an excellent RTP-fluorescence photoswitching property accompanied by multicolor tunable long-lived emission. Moreover, this switching process can be reversibly modulated over multiple cycles under alternating UV and visible photoirradiation. Finally, the prepared switch is successfully applied to photocontrolled multicolor cell labeling to offer a new approach for the design and fabrication of novel advanced light-responsive RTP materials in aqueous environments.

Cucurbit[n]uril-based host-guest interaction enhancing organic room-temperature phosphorescence of phthalic anhydride derivatives in aqueous solution

A cyan RTP in aqueous solution was enhanced by supramolecular host–guest complexation of water-soluble halogen-substituted phthalic anhydride (PA) derivatives with cucurbit[n]urils (CB[n]s).