Clustering-triggered Emission of Cellulose and Its Derivatives

Characterizing the photoluminescence of fluorescein-labeled cellulose in aqueous and alcohol solutions: influence of the cellulose backbone

Although many dyes have been introduced into cellulose, whether bound to its backbone or within a cellulose matrix, few studies have determined whether the backbone statically or dynamically quenches the photoluminescence of the dye. To advance cellulosic fluorescent films, the influence of the cellulose backbone on photoluminescence must be understood. We determined the fluorescence properties of fluorescein isothiocyanate (FITC) and fluorescein-labeled cellulose (FLC) in water and alcohol, including their quantum yields [Formula: see text], lifetimes [Formula: see text], and rates of radiative [Formula: see text] and nonradiative [Formula: see text] decay. Dissolved FLC had a ~ 30× lower [Formula: see text] than FITC, suggesting that incorporating FITC into the cellulose backbone remarkably reduces the fluorescence efficiency. The FLC solutions had a six-fold lower [Formula: see text] than their FITC counterparts but a 10-20 times higher [Formula: see text]. Presumably, this was because the cellulose backbone interacted weakly with the fluorescein moieties, suggesting a quenching mechanism that can be termed quasi-static, corresponding to static quenching between the fluorescein moieties and cellulose backbone, in addition to the fluorescence quenching caused by the intramolecular nonradiative processes of fluorescein, as observed in conventional molecules. Using the Strickler‒Berg formula, we deduced the analytical radiative decay rate constants [Formula: see text] and eventually estimated the number of very short-lived fluorescein moieties per single fluorescent fluorescein moiety, corresponding well with static quenching.

Elucidation of the photoluminescence mechanism and determination of the configuration content of arabinose isomer solution by fluorescence analysis

A fluorescence method has been successfully constructed to accurately measure the D/L-Arb configuration content in optical isomers, and its application in ion detection has been expanded, which has greater sensitivity and universality than the circular dichroism (CD) method. It also promotes the study of the emission mechanism of nonconventional luminogens.

Nonconjugated Polyurethane Derivatives with Aggregation-Induced Luminochromism for Multicolor and White Photoluminescent Films

A simple and effective strategy to obtain solid-state multicolor emitting materials is a particularly attractive topic. Nonconventional/nonconjugated polymers are receiving widespread attention because of their advantages of rich structural diversity, low cost, and good processability. However, it is difficult to control the molecular conformation or to obtain the crystal structure of amorphous molecules, which means it is a challenge to obtain nontraditional polymeric materials with multicolor emission. In this work, a polyurethane derivative (PUH) with red-shifted emission was synthesized by a simple one-pot polymerization reaction. By exploiting the aggregation-induced luminochromism of PUH, a series of plastic films with tunable emission from blue to orange, and white-light emission, was obtained by doping different amounts of PUH into poly(methyl methacrylate) (PMMA), thereby changing the aggregation degree of PUH. This work demonstrates the excellent promise of polyurethane derivatives for the simple fabrication of large-scale flexible luminescent films.

Recent Progress in Nonconventional Luminescent Macromolecules and their Applications

Traditional π-conjugated luminescent macromolecules typically suffer from aggregation-caused quenching (ACQ) and high cytotoxicity, and they require complex synthetic processes. In contrast, nonconventional luminescent macromolecules (NCLMs) with nonconjugated structures possess excellent biocompatibility, ease of preparation, unique luminescence behavior, and emerging applications in optoelectronics, biology, and medicine. NCLMs are currently believed to produce inherent luminescence due to through-space conjugation of overlapping electron orbitals in solid/aggregate states. However, as experimental facts continue to exceed expectations or even overturn some previous assumptions, there is still controversy about the detailed luminous mechanism of NCLMs, and extensive studies are needed to further explore the mechanism. This Perspective highlights recent progress in NCLMs and classifies and summarizes these advances from the viewpoint of molecular design, mechanism exploration, applications, and challenges and prospects. The aim is to provide guidance and inspiration for the huge fundamental and practical potential of NCLMs.

Heteroatom-facilitated blue to near-infrared emission of nonconjugated polyesters

Making nonconjugated polymers to emit visible light remains a formidable challenge, let alone near-infrared (NIR) light, although NIR luminophores have many advanced applications. Herein, we propose an electron-bridging strategy of using heteroatoms (O, N, and S) to achieve tunable emission from blue to NIR regions (440-800 nm) in nonconjugated polyesters. Especially, sulfur-containing polyester P4 exhibits NIR clusteroluminescence (CL) on changing either the concentration or excitation wavelength. Experimental characterization and theoretical calculation demonstrate that the introduction of heteroatoms significantly enhances the through-space interactions (TSIs) via the electron-bridging effect between heteroatoms and carbonyls. The strength of the electron-bridging effect follows the order of S > N > O, based on two synergistic effects: electronic structure and van der Waals radius of heteroatoms. This work provides a low-cost, scalable platform to produce new-generation nonconjugated luminophores with deeper insight into the photophysical mechanism.

Micro assembly strategies for enhancing solid-state emission of cellulose nanocrystals and application in photoluminescent inks

Crystalline cellulose exhibits photoluminescent properties, making it ideal for solid-state emission through properly assembling crystal arrays. However, assembling in water or other polar solvents poses structural integrity issues. To address this, a micro-assembly method is proposed. Cellulose nanocrystals (CNCs) are organized within a sub-micrometer-sized ZIF-8 metal-organic framework and coated with TiO2. Notably, the assembly within ZIF-8 improves the CNCs' emission quantum yield to 37.8 %. The bonding between ZIF-8 and CNCs relies on electrostatic interactions and hydrogen bonds, which are sensitive to polar solvents. Yet, the sturdy coordination bonds between TiO2 and ZIF-8 enhance resistance. Solvent-resistance tests confirm that TiO2 prevents CNC assembly breakdown, resulting in only an 8.0 % drop in photoluminescent intensity in an alkaline solution after 24 h, compared to 33 % without the coating. For anti-counterfeiting purposes, TiO2@ZIF-8@CNC is combined with a polymer matrix, allowing information to be revealed under specific wavelengths using screen-printed labels.

Facile Preparation of Full-Color Tunable Room Temperature Phosphorescence Cellulose via Click Chemistry

Sustainable long-lived room temperature phosphorescence (RTP) materials with color-tunable afterglows are attractive but rarely reported. Here, cellulose is reconstructed by directed redox to afford ample active hydroxyl groups and water-solubility; arylboronic acids with various π conjugations can be facilely anchored to reconstructed cellulose via click chemistry within 1 min in pure water, resulting in full-color tunable RTP cellulose. The rigid environment provided by the B─O covalent bonds and hydrogen bonds can stabilize the triplet excitons, thus the target cellulose displays outstanding RTP performances with the lifetime of 2.67 s, phosphorescence quantum yield of 9.37%, and absolute afterglow luminance of 348 mcd m-2 . Furthermore, due to the formation of various emissive species, the smart RTP cellulose shows excitation- and time-dependent afterglows. Taking advantages of sustainability, ultralong lifetime, and full-color tunable afterglows, et al, the environmentally friendly RTP cellulose is successfully used for nontoxic afterglow inks, delay lighting, and afterglow display.

Color-Tunable, Excitation-Dependent, and Water Stimulus-Responsive Room-Temperature Phosphorescence Cellulose for Versatile Applications

Smart-response materials with ultralong room-temperature phosphorescence (RTP) are highly desirable, but they have rarely been described, especially those originating from sustainable polymers. Herein, a variety of cellulose derivatives with 1,4-dihydropyridine (DHP) rings are synthesized through the Hantzsch reaction, giving impressive RTP with a long lifetime of up to 1251 ms. Specifically, the introduction of acetoacetyl groups and DHP rings promotes the spin-orbit coupling and intersystem crossing process; and multiple interactions between cellulose induce clustering and inhibit the nonradiative transitions, boosting long-live RTP. Furthermore, the resulting transparent and flexible cellulose films also exhibit excitation-dependent and color-tunable afterglows by introducing different extended aromatic groups. More interestingly, the RTP performance of these films is sensitive to water and can be repeated in response to wet/dry stimuli. Inspired by these advantages, the RTP cellulose demonstrates advanced applications in information encryption and anti-counterfeiting. This work not only enriches the photophysical properties of cellulose but also provides a versatile platform for the development of sustainable afterglows.

Room-temperature phosphorescent materials derived from natural resources

Room-temperature phosphorescent (RTP) materials have enormous potential in many different areas. Additionally, the conversion of natural resources to RTP materials has attracted considerable attention. Owing to their inherent luminescent properties, natural materials can be efficiently converted into sustainable RTP materials. However, to date, only a few reviews have focused on this area of endeavour. Motivated by this lack of coverage, in this Review, we address this shortcoming and introduce the types of natural resource available for the preparation of RTP materials. We mainly focus on the inherent advantages of natural resources for RTP materials, strategies for activating and enhancing the RTP properties of the natural resources as well as the potential applications of these RTP materials. In addition, we discuss future challenges and opportunities in this area of research.

Designing Nonconventional Luminescent Materials with Efficient Emission in Dilute Solutions via Modulation of Dynamic Hydrogen Bonds

Nonconventional luminescent materials (NLMs) which do not contain traditional aromatic chromophores are of great interest due to their unique chemical structures, optical properties, and their potential applications in various areas, such as cellular imaging and chemical sensing. However, most reported NLMs show weak or no emission in dilute solutions, which severely limits their applications. In this work, dynamic hydrogen bonds were utilized to design NLMs with efficient emission in dilute solutions. To further validate the results, polymers P1 and P2 were successfully prepared and investigated. It was found that the luminescence quantum efficiency of P1 and P2 at a concentration of 0.1 mg/mL in water solution was 8.9 and 0.6%, respectively. The high efficiency can be attributed to the fact that polymer P1 has more intra- or intermolecular dynamic hydrogen bonds and other short interactions than P2 in dilute solutions, allowing P1 to achieve the through-space conjugation effect to increase the degree of system conjugation, restrict molecular motion, and decrease nonradiative transitions, which can effectively improve luminescence. In addition, polymer P2 exhibits the characteristics of clustering-triggered emission, excitation wavelength-dependent and concentration-dependent fluorescence properties, excellent photobleaching resistance, low cytotoxicity, and selective recognition of Fe³⁺. The present study investigates the manipulation of luminescence properties of NLMs in dilute solutions through the modulation of dynamic hydrogen bonds. This approach can serve as a semi-empirical technique for designing and building innovative NLMs in the times ahead.

Preparation and Characterization of Carbazole-Based Luminogen with Efficient Emission in Solid and Solution States

Organic luminescent materials with high luminescence efficiency in both solution and solid states, namely dual-state emission (DSE), have attracted considerable attention due to their promising applications in various fields. In order to enrich the variety of DSE materials, carbazole, similar to triphenylamine (TPA), was utilized to construct a novel DSE luminogen named 2-(4-(9H-carbazol-9-yl)phenyl)benzo[d]thiazole (CZ-BT). CZ-BT exhibited DSE characteristics with fluorescence quantum yields of 70, 38 and 75% in solution, amorphous and crystalline states, respectively. CZ-BT shows thermochromic and mechanochromic properties in solution and solids, respectively. Theoretical calculations show that there is a small conformational difference between the ground state and the lowest singly excited state of CZ-BT and that it exhibits a low non-radiative transition characteristic. The oscillator strength during the transition from the single excited state to the ground state reaches 1.0442. CZ-BT adopts a distorted molecular conformation with intramolecular hindrance effects. The excellent DSE properties of CZ-BT can be explained well using theoretical calculations and experimental results. In terms of application, the CZ-BT has a detection limit for the hazardous substance picric acid of 2.81 × 10^-7 mol/L.

D-O-A based organic phosphors for both aggregation-induced electrophosphorescence and host-free sensitization

Pure organic phosphors capable of room-temperature phosphorescence show a great potential in organic light-emitting diodes, while it is limited by the big challenge to realize efficient electroluminescence under electric excitation. Herein, we develop a class of organic phosphors based on acridine as the electron donor, triazine as the electron acceptor and oxygen as the bridge between them. Benefitting from the characteristic donor-oxygen-acceptor geometry, these compounds are found to behave an exciting aggregation-induced organic room-temperature electrophosphorescence, and achieve a record-high external quantum efficiency of 15.8% for non-doped devices. Furthermore, they can sensitize multi-resonant emitters in the absence of any additional wide bandgap host, leading to an effective narrowband emission with a peak external quantum efficiency of 26.4% and a small full-width at half maximum of 26 nm. The results clearly indicate that donor-oxygen-acceptor geometry is a promising strategy to design organic phosphors suitable for organic light-emitting diodes.

Ionic crosslinking of alginate/carboxymethyl chitosan fluorescent hydrogel for bacterial detection and sterilization

Herein, a polysaccharide-based fluorescent hydrogel with multi-responsiveness simply implemented by concurrent effects of ionic crosslinking/rehydration processes is presented. Specifically, the alginate and carboxymethyl chitosan are chosen to prepare the interpenetrating polymer matrix while a pair of metal cations has been selectively sequentially integrated to alter hydrogel mechanical and fluorescent properties. Experimental results indicate the hydrogels show tunable fluorescent emission in response to multiple cations and pH conditions, and display a reversible "ON/OFF" fluorescent response to Mⁿ⁺/ethylenediaminetetraacetic acid. Moreover, this synergistic ionic crosslinking strategy is proved to be highly effective in preparing multifunctional metallohydrogels possessing robust/anisotropic mechanical properties, typical shape memory and cation/pH-responsive fluorescence performance, and a proof-of-application for bacterial detection and sterilization has also been demonstrated. Therefore, we believe this study would provide new insights into multifunctional luminescent hydrogels for advanced biomedical systems.

Poly-L-aspartic acid based nonconventional luminescent biomacromolecules with efficient emission in dilute solutions for Al3+ detection

Nonconventional luminescent macromolecules exhibiting bright fluorescence or phosphorescence emission at high concentrations and solid-state have attracted significant attention due to their promising application in different fields. However, most reported nonconventional luminescent macromolecules show weak or no emission in dilute solutions, limiting their large-scale applications. Herein, nonconventional luminescent biomacromolecules with hydrophobic rigid chains, hydrophilic flexibility and inter- or intra-molecular hydrogen bonding interactions were proposed to achieve effective luminescence in dilute solutions. Poly-L-aspartic acid (PASA) with a fluorescence quantum yield of 4.6 % in a dilute solution (0.8 mg/mL) was synthesized to validate this design strategy. The fluorescence intensity of PASA solution increased with the increase of the concentration, demonstrating a clustering-triggered emission (CTE) effect. Furthermore, the fluorescence intensity significantly enhanced when adding Al³⁺ into PASA aqueous solution via the Al³⁺ recognition effect. The detection limits for Al³⁺ (1.71 × 10^-6 mol/L) meet the World Health Organization (WHO) requirements for food detection. At last, PASA solid-state samples exhibit room temperature phosphorescence emission.

Water-soluble polymers with aggregation-induced emission and ultra-long room temperature phosphorescence

Achieving sky blue fluorescence emission and durable green RTP emission materials under air conditions by free radical polymerization.

Synthesis and Characterization of Cellulose Diacetate-Graft-Polylactide via Solvent-Free Melt Ring-Opening Graft Copolymerization

Cellulose diacetate (CDA) and L-lactide (L-LA) were used to prepare CDA-g-PLLA with a low glass transition temperature under different process conditions. Given the high glass transition temperature (T_g) of CDA, the thermal processing performance of CDA is poor, which greatly limits its application fields. To decrease the T_g of CDA, graft copolymerization was used in this research. A CDA-g-PLLA graft copolymer was synthesized by grafting CDA with L-LA under different reaction conditions using stannous octanoate as the catalyst and variations in the grafting rate under different reaction conditions were compared. The chemical structure and crystal structure of the CDA-g-PLLA were investigated, and thermal properties were also studied. The results showed that the grafting rate was the highest at the L-LA/CDA mass ratio of 4:1 under a reaction temperature of 150 °C for 90 min, and no poly-L-lactide (PLLA) homopolymer was found among the CDA-g-PLLA graft copolymers after purification. The T_g of CDA-g-PLLA was 54.2 °C, and the initial temperature of weightlessness of CDA-g-PLLA was 218.7 °C. The regularity of the original CDA molecular chains was destroyed after grafting PLLA molecular chains. In this research, we investigated the optimal grafting conditions for CDA-g-PLLA and the CDA-g-PLLA had a low T_g, which improves the thermal processing performance of CDA and broadens its application prospects in the industry.

Clustering-Triggered Emission of EPS-605 Nanoparticles and Their Application in Biosensing

Natural carbohydrates with intrinsic luminescent properties have drawn increasing attention thanks to their fundamental importance and promising applications. To expand the range of natural nonconventional biomacromolecule luminogens and to gain deep insights into their emission mechanism, we prepared EPS-605, a naturally occurring spherical nanoparticle based on negatively charged exopolysaccharides (EPS), and studied its emission behavior. It was found that EPS-605 was highly emissive in the aggregate state, such as powder and film. Furthermore, EPS-605 aqueous solutions exhibited concentration-enhanced emission characteristics. According to fluorescence spectra and confocal images, the fluorescence phenomenon of EPS-605 was not affected by the pH value and the carbon sources. The emission behavior of EPS-605 was attributed to the clustering-triggered emission (CTE) mechanism. Moreover, EPS-605 was successfully utilized for Fe3+ detection since its fluorescence could be selectively quenched by Fe3+. It could be used to detect Fe3+ with a low limit of detection (0.06 μM) and a wide detection range from 0.05 to 250 μM. Overall, these findings not only benefit the exploitation of EPS-based nonconventional biomacromolecule luminogens, but also reveal the potential applications of EPS-605 in biosensing/bioimaging, anticounterfeiting, and encryption owing to its excellent biocompatibility, environmental friendliness, and intrinsic photoluminescence property.

Biological Synthesis and Process Monitoring of an Aggregation-Induced Emission Luminogen-Based Fluorescent Polymer

As the most abundant and renewable biopolymer on earth, cellulose can be functionalized for various advanced applications by chemical modification. In addition, fluorescent polymers with aggregation-induced emission (AIE) are generally prepared using chemical approaches, and the biosynthesis of AIE-active polymers are rarely investigated. Herein, fluorescent cellulose was successfully synthesized by bacterial fermentation, where glucosamine-modified AIE luminogen was incorporated into cellulose to achieve AIE-active biopolymers. Excitingly, real-time visualization of the synthetic process was realized, which is crucial for investigating the process of bacterial fermentation. The biosynthesized cellulose exhibited better performance with uniform fluorescence distribution and high stability, compared with that prepared by physical absorption. Additionally, fluorescent mats were fabricated by electrospinning of AIE-active cellulose, demonstrating its great potential applications in flexible display and tissue engineering.

Sustainable aggregation-induced emission material based on pectin-l-lysine: Potential antibacterial and monitoring in food spoilage

The demand of smart food detection system which in detecting food spoilage is increasing. In this work, a new type of aggregation-induced emission (AIE) compound was synthesized based on pectin (P) and l-lysine (Lys). P-Lys is an AIE active compound which has the advantages of simple synthesis, easy modification and processability, it also has good water solubility and biocompatibility. Moreover, P-Lys has potential application in detecting Fe³⁺ (oxidation from Fe²⁺) and bacterial in monitoring pork spoilage. In addition, P-Lys also has spectral antibacterial properties which can prevent pork spoilage. The research results shown that P-Lys, as a new type of food testing agent has a useful future in monitoring and protecting the freshness of food.

Aliphatic Polyesters with White-Light Clusteroluminescence

Single-molecule white-light emission (SMWLE) has many advantages in practical applications; however, the fabrication of SMWLE from nonconjugated luminescent polymers, namely, clusteroluminogens (CLgens), is still a big challenge. Herein, the first example of linear nonconjugated polyesters with SMWLE is reported. Twenty-four kinds of nonconjugated aliphatic polyesters with tunable clusteroluminescence (CL) colors and efficiency were synthesized by the copolymerization of six epoxides and four anhydrides. Experimental and calculation results prove that, at the primary structure level, the balance of structural flexibility and rigidity via adjusting the side-chain length significantly enhances the efficiency of CL without wavelength change. However, altering the chemical structures of the monomer from succinic anhydride to trans-maleic anhydride (MA), cis-MA, and citraconic anhydride (CA), secondary structures of these polyesters change from helix to straight and folding sheet accompanied by gradually red-shifted CL from 460 to 570 nm due to the increase in through-space n-π* interactions, as demonstrated by the computational and experimental results. Then, pure SMWLE with CIE coordination (0.30, 0.32) based on overlapped short-wavelength and long-wavelength CL is achieved in CA-based polyesters. This work not only provides further insights into the emission mechanism of CL but also provides a new strategy to manipulate the properties of CL by regulating the hierarchical structures of CLgens.

Tunable Photoluminescence Properties of Cotton Fiber With Gradually Changing Crystallinity

The alkali mercerizing process of semicrystalline cotton fiber (CF) is widely used in the printing and dyeing industry. The crystallinity change in the mercerizing process has been studied and certain laws have been obtained, but there is still a certain distance between the theoretical research results and the practical applications. CF is almost composed of cellulose, combined with the photoluminescence (PL) phenomenon of cellulose; herein, the varying crystallinity is correlated with its PL behavior after being treated with different concentrations of NaOH. In line with the characteristics of nonconventional luminogens, CF enjoys excitation-dependent emission and persistent room temperature phosphorescence (p-RTP) behavior. The emission spectra of all samples under the same excitation wavelength indicate that the change of CF crystallinity has a significant impact on its fluorescence and p-RTP emission. As the concentration of NaOH increases, the varying trend of quantum efficiency (QY) is consistent with the changed crystallinity of CF. Interestingly, the lifetime of p-RTP is exactly the opposite of the crystallinity change law. Clustering-triggered emission (CTE), crystallization-Induced Phosphorescence (CIP) mechanism, and the swelling due to hydrated sodium ions can reasonably explain these interesting photophysical processes, which also can be supported by theoretical calculations. The above studies have basically clarified the inherent law between the crystalline change of CF and the PL emission behavior during the alkali treatment process, which can be used as a theoretical reference for real-time monitoring of CF crystallinity changes using the spectral method in the actual cotton mercerizing process.

Irreversible Humidity-Responsive Phosphorescence Materials from Cellulose for Advanced Anti-Counterfeiting and Environmental Monitoring

Organic phosphorescence materials have many unique advantages, such as a large Stokes shift, high signal-to-noise ratio, and no interference from background fluorescence and scattered light. But, they generally lack responsiveness. Herein, we developed a new type of biopolymer-based phosphorescence materials with excellent processability and irreversible humidity-responsiveness, via introducing the imidazolium cation to cellulose chain. In the resultant cellulose derivatives, the imidazolium cation promotes the intersystem crossing, meanwhile the cation, chloride anion, and hydroxyl group form multiple hydrogen bonding interactions and electrostatic attraction interactions, which successfully inhibit the nonradiative transitions. As a result, the ionic cellulose derivatives exhibit green phosphorescence at room temperature and can be processed into phosphorescent films, coatings, and patterns. More interestingly, their phosphorescence emission changes when the different processing solvents are used. The ionic cellulose derivatives processed with acetone have a negligible phosphorescence, while they give an irreversible humidity-responsive phosphorescence, which means that the ionic cellulose derivatives processed with acetone exhibit significantly enhanced phosphorescence once they meet water vapor. Such novel irreversible responsive phosphorescence materials have huge potential in advanced anticounterfeiting, information encryption, molecular logic gates, smart tags, and process monitoring.

Ultralong phosphorescence cellulose with excellent anti-bacterial, water-resistant and ease-to-process performance

Herein, we present a phosphorescent cationized cellulose derivative by simply introducing ionic structures, including cyanomethylimidazolium cations and chloride anions, into cellulose chains. The imidazolium cations with the cyano group and nitrogen element promote intersystem crossing. The cyano-containing cations, chloride anions and hydroxyl groups of cellulose form multiple hydrogen bonding interactions and electrostatic attraction interactions, effectively inhibiting the non-radiative transitions. The resultant cellulose-based RTP material is easily processed into phosphorescent films, fibers, coatings and patterns by using eco-friendly aqueous solution processing strategies. Furthermore, after we construct a cross-linking structure by adding a small amount of glutaraldehyde as the cross-linking agent, the as-fabricated phosphorescent patterns exhibit excellent antibacterial properties and water resistance. Therefore, considering the outstanding biodegradability and sustainability of cellulose materials, cellulose-based easy-to-process RTP materials can act as antibacterial, water-resistant, and eco-friendly phosphorescent patterns, coatings and bulk materials, which have enormous potential in advanced anti-counterfeiting, information encryption, disposable smart labels, etc.

Room Temperature Phosphorescence Emission From Multi-States

Organic room temperature phosphorescence (RTP) materials have received considerable attention due to their fascinating photophysical properties. During the past decade, various organic luminogens exhibiting RTP emission in solid states were reported. However, the phosphorescence emission of organic compounds can hardly be observed in their solutions at room temperature. Herein, we reported two fluorene derivatives that can emit RTP in degassed organic solvents, polymer doped film, and crystalline states. Furthermore, those RTP luminogens emitted different colors with different phosphorescence lifetimes in multi-states. These results indicated that the phosphorescence performance can be adjusted flexibly in different condensed states. To our knowledge, this is the first example possessing diverse organic RTP at multi-states, including solution state.

Hydrated hydroxide complex dominates the AIE property of nonconjugated polymeric luminophores

Nontraditional intrinsic luminescence (NTIL) which always accompanied with aggregation-induced emission (AIE) features has received considerable attention due to their importance in the understanding of basic luminescence principle and potential practical applications. However, the rational modulation of the NTIL of nonconventional luminophores remains difficult, on account of the limited understanding of emission mechanisms. Herein, the emission colour of nonconjugated poly(methyl vinyl ether-alt-maleic anhydride) (PMVEMA) could be readily regulated from blue to red by controlling the alkalinity during the hydrolysis process. The nontraditional photoluminescence with AIE property was from the new formed p-band state, resulting from the strong overlapping of p orbitals of the clustered O atoms through space interactions. Hydrated hydroxide complexes embedded in the entangled polymer chain make big difference on the clustering of O atoms which dominates the AIE property of nonconjugated PMVEMA. These new insights into the photoluminescence mechanism of NTIL should stimulate additional experimental and theoretical studies and could benefit the molecular-level design of nontraditional chromophores for optoelectronics and other applications. This article is protected by copyright. All rights reserved.

Bathochromic-Shifted Emissions by Postfunctionalization of Nonconjugated Polyketones

Most nontraditional intrinsic luminescent (NTIL) polymers currently show blue fluorescence. Tuning the emission color of NTIL polymers is of fundamental importance for their applications, but it still remains a scientific challenge. Herein, we initially develop an efficient strategy for bathochromic shifting of NTIL polymers by through-space acceptor-donor charge transfer between the in chain and the side chain. A variety of functionalized polyketones (FPK-R; where R = H, Ph, Me, tBu, F, and Cl) with furan rings built into the polymer chain were prepared by the Paal-Knorr reaction. FPK-R polymers showed bright and bathochromic-shifted fluorescence compared with their counterparts. The emission color could be tuned by changing the postfunctionalization conversion and varying the styrenic monomer substituent. Experimental and theoretical investigations revealed that the color tunability originated from enhanced through-space charge transfer between the side chain phenyl and the in chain furan rings.

Through-Space Interactions in Clusteroluminescence

Conventional π-conjugated luminophores suffer from problems such as emission quenching, biotoxicity, environmental pollution, etc. The emerging nonconjugated and nonaromatic clusteroluminogens (CLgens) are expected to overcome these stubborn drawbacks, so research of CLgens shows great significance not only for practical application but also for the construction of fundamental photophysical theories. This perspective summarizes the unusual features of CLgens in comparison to traditional chromophores, such as nonconjugated molecular structures, unmatched absorption and excitation, excitation-dependent luminescence, multiple emission peaks, and room-temperature phosphorescence. Different from the theory of through-bond conjugation in π-conjugated luminophores, through-space interactions, including through-space n···n interaction and through-space n···π interaction, are regarded as the emitting sources of nonconjugated CLgens. In addition, the formation of network clusters is proposed as an efficient strategy to improve the performance of CLgens, and their potential applications of anticounterfeiting, photoelectronic devices, and bioimaging are prospected.

Nonconventional luminophores: characteristics, advancements and perspectives

Nonconventional luminophores devoid of remarkable conjugates have attracted considerable attention due to their unique luminescence behaviors, updated luminescence mechanism of organics and promising applications in optoelectronic, biological and medical fields. Unlike classic luminogens consisting of molecular segments with greatly extended electron delocalization, these unorthodox luminophores generally possess nonconjugated structures based on subgroups such as ether (-O-), hydroxyl (-OH), halogens, carbonyl (CO), carboxyl (-COOH), cyano (CN), thioether (-S-), sulfoxide (SO), sulfone (OSO), phosphate, and aliphatic amine, as well as their grouped functionalities like amide, imide, anhydride and ureido. They can exhibit intriguing intrinsic luminescence, generally featuring concentration-enhanced emission, aggregation-induced emission, excitation-dependent luminescence and prevailing phosphorescence. Herein, we review the recent progress in exploring these nonconventional luminophores and discuss the current challenges and future perspectives. Notably, different mechanisms are reviewed and the clustering-triggered emission (CTE) mechanism is highlighted, which emphasizes the clustering of the above mentioned electron rich moieties and consequent electron delocalization along with conformation rigidification. The CTE mechanism seems widely applicable for diversified natural, synthetic and supramolecular systems.

Boron-Catalyzed Polymerization of Phenyl-Substituted Allylic Arsonium Ylides toward Nonconjugated Emissive Materials from C3/C1 Monomeric Units

Two novel allylic arsonium ylide monomers with a phenyl (steric and electronic effect) group at different positions were synthesized and used in boron-catalyzed polymerization to produce a series of well-defined polymers, poly(2-phenyl-propenylene-co-2-phenyl-propenylidene) (P2-PhAY) and poly(3-phenyl-propenylene-co-3-phenyl-propenylidene) (P3-PhAY), with unusual structures but a controllable molecular weight and relatively low polydispersity. The backbone of these polymers consists of a mixture of C1 (chain grows by one carbon atom at a time) and C3 (chain grows by three carbon atoms at a time) monomeric units, as determined by ¹H, ¹³C, and ¹H-¹³C HSQC 2D NMR. Based on the experimental results and density functional theoretical (DFT) calculations, we were able to propose a mechanism that takes into account not only the steric hindrance, but also the electron effect of the phenyl group. In addition, a nontraditional intrinsic luminescence was observed from the nonconjugated P2-PhAY and P3-PhAY; such unexpected emission is attributed to the formation of C3-unit clusters, as evidenced by ultraviolet-visible and fluorescence spectroscopy.

Generating circularly polarized luminescence from clusterization-triggered emission using solid phase molecular self-assembly

Purely-organic clusterization-triggered emission (CTE) has displayed promising abilities in bioimaging, chemical sensing, and multicolor luminescence. However, it remains absent in the field of circularly polarized luminescence (CPL) due to the difficulties in well-aligning the nonconventional luminogens. We report a case of CPL generated with CTE using the solid phase molecular self-assembly (SPMSA) of poly-L-lysine (PLL) and oleate ion (OL), that is, the macroscopic CPL supramolecular film self-assembled by the electrostatic complex of PLL/OL under mechanical pressure. Well-defined interface charge distribution, given by lamellar mesophases of OL ions, forces the PLL chains to fold regularly as a requirement of optimal electrostatic interactions. Further facilitated by hydrogen bonding, the through-space conjugation (TSC) of orderly aligned electron-rich O and N atoms leads to CTE-based CPL, which is capable of transferring energy to an acceptor via a Förster resonance energy transfer (FRET) process, making it possible to develop environmentally friendly and economic CPL from sustainable and renewable materials.

Tunable Photoluminescence Properties of Microcrystalline Cellulose with Gradually Changing Crystallinity and Crystal Form

Nonconventional luminogens with persistent room temperature phosphoresce (p-RTP) are attracting increasing attention owing to their momentous significance and diverse technical applications in optoelectronic and biomedical. So far, the p-RTP emission of some amorphous powders or single crystals has been studied in depth. The p-RTP emission of amorphous and fully crystalline states and their emission properties are widely divergent, while the difference of their p-RTP emission mechanism is still controversial. The relevance between crystallinity change and p-RTP properties is rarely studied. Furthermore, there is almost no research on the photoluminescence (PL) property change and emission mechanism under the crystal form transformation of semi-crystalline polymer. Herein, microcrystalline cellulose (MCC) is chosen as a model compound to explore its crystallinity and the change in luminescence during the crystal form transformation to make up for this gap. By precisely adjusting the crystallinity and crystal cellulose conversion of MCC, the changing trend of quantum efficiency, and p-RTP lifetime is consistent with the change of crystallinity, and the cellulose I may be more beneficial to PL emission than cellulose II. Clustering-triggered emission mechanism can reasonably explain these interesting photophysical processes, which also can be supported by single-crystal analysis and theoretical calculations.

Seeking Aggregation-Induced Emission Materials in Food: Oat β-Glucan and Its Diverse Applications

With the basic understanding and broad application prospects of luminescent materials, the emission mechanism of unconventional luminescent agents has been revealed gradually. Here, we report a non-conjugated biomass material, oat β-glucan (oat-β-Glu), which actually does not emit light in a dilute solution but emits significantly when forming aggregates. Inherently visible emission of oat-β-Glu from the concentrated solutions and solid state could be observed. In addition, we have observed room temperature phosphorescence in oat-β-Glu powders, which is also unusual in pure organic materials. It can be proposed that the luminescence property of oat-β-Glu originates from the spatial conjugation of the oxygen atoms of oat-β-Glu. This clustering-triggered emission mechanism may well be expanded to other unconventional biomacromolecules, inspiring the rational design of luminescent agents. Due to its good biocompatibility and intrinsic emission characteristics, oat-β-Glu has shown great potential application prospects in bioimaging and biosensors.

Seeking brightness from nature: Sustainable AIE macromolecule with clustering-triggered emission of xanthan gum and its multiple applications

Unconventional biomacromolecule luminescent agents have attracted widespread attention due to the potential applications in diverse fields. In order to explore new luminescent agents and gain a comprehensive understanding of their emission mechanism, the emission behavior of xanthan gum was investigated. Xanthan gum shown obvious aggregation-induced emission (AIE) characteristics in concentration solution. Moreover, xanthan gum has shown potential values in intracellular imaging and can be used as a biosensor for detecting Fe³⁺ and Cu²⁺ in human serum.

Wood-Derived Carbon Materials and Light-Emitting Materials

Wood is a sustainable and renewable material that naturally has a hierarchical structure. Cellulose, hemicellulose, and lignin are the three main components of wood. The unique physical and chemical properties of wood and its derivatives endow them with great potential as resources to fabricate advanced materials for use in bioengineering, flexible electronics, and clean energy. Nevertheless, comprehensive information on wood-derived carbon and light-emitting materials is scarce, although much excellent progress has been made in this area. Here, the unique characteristics of wood-derived carbon and light-emitting materials are summarized, with regard to the fabrication principles, properties, applications, challenges, and future prospects of wood-derived carbon and light-emitting materials, with the aim of deepening the understanding and inspiring new ideas in the area of advanced wood-based materials.

Photoluminescence of Tilapia skin collagen: Aggregation-induced emission with clustering triggered emission mechanism and its multiple applications

There is an urgent need for natural sources of aggregation-induced emission (AIE) materials which have good water solubility, biocompatibility, and can be produced in large quantities. Here, Tilapia skin collagen (Tsc) is a very abundant protein in nature, with solid-phase and solution-state fluorescence emission effect and its multiple applications was explored. Due to Tsc was in high concentration or aggregation state which shown AIE property. This obvious emission can be account for clustering-triggered emission (CTE) mechanism. The photoluminescence property of Tsc not only provide a deeper understanding of the emission characteristics of proteins, but also has important guiding significance for further elucidating the basis of fluorescence properties.

Clustering-triggered emission of poly(vinyl) alcohol

PVA can emit blue light under UV light and the mechanism of this fluorescence was studied in this paper. PVA can be added to other materials to broaden their properties. The fluorescence of PVA has great application prospects.

Functional Polymer Systems with Aggregation-Induced Emission and Stimuli Responses

Functional polymer systems with stimuli responses have attracted great attention over the years due to their diverse range of applications. Such polymers are capable of altering their chemical and/or physical properties, such as chemical structures, chain conformation, solubility, shape, morphologies, and optical properties, in response to single or multiple stimuli. Among various stimuli-responsive polymers, those with aggregation-induced emission (AIE) properties possess the advantages of high sensitivity, fast response, large contrast, excellent photostability, and low background noise. The changes in fluorescence signal can be conveniently detected and monitored using portable instruments. The integration of AIE and stimuli responses into one polymer system provides a feasible and effective strategy for the development of smart polymers with high sensitivity to environmental variations. Here, we review the recent advances in the design, preparation, performance, and applications of functional synthetic polymer systems with AIE and stimuli responses. Various AIE-based polymer systems with responsiveness toward single physical or chemical stimuli as well as multiple stimuli are summarized with specific examples. The current challenges and perspectives on the future development of this research area will also be discussed at the end of this review.