Organic Luminescent Materials Research Articles

Smart aggregation‐induced emission polymers: Preparation, properties and bio‐applications

AbstractOwing to the outstanding photophysical properties, organic luminescent materials featuring aggregation‐induced emission (AIE) characteristics have attracted wide attention in various fields. Numerous researches focused on low‐mass AIE luminogens, and relatively less attention has been paid on AIE polymers and the related applications, in spite of the fact that AIE polymers exhibit excellent advantages of processability, multifunctional integration and synergistic effects. In this review, we briefly summarize and discuss the superiorities of AIE polymers in preparation, properties and bio‐applications, and the considerable progress in these aspects are introduced as well. Finally, the structure‐property relationship, challenges and opportunities are also discussed. Hopefully, this review will be a trigger for smart AIE polymer research and further broaden their applications.

Aggregation induced luminescence chromism molecule for preparation of physically unclonable function labels

Organic stimuli-responsive luminescent materials have attracted much attention due to their great potential in security fields such as fluorescent inks and information security. However, the widespread aggregation quenching effect limits the organics’ practical application in anti-counterfeiting technology. To overcome this challenge, this study developed a novel molecule 4-(4-phenyl-6-(pyren-1-yl)-1,3,5-triazin-2-yl)benzyl acrylate (PPTBA) which exhibits high luminescence efficiency in both monodisperse and aggregated states, with an obvious aggregation-induced emission color change. The copolymer P(NIPAM-PPTBA) was obtained by copolymerizing PPTBA and NIPAM (N-Isopropyl acrylamide). The polymer exhibits a blue-to-yellow luminescence chromism upon adding an appropriate amount of water. The polymer film can perfectly hide letters when combined with stamp technology and can be developed with water for general-purpose anti-counterfeiting verification. The polymer is utilized to prepare fluorescence physically unclonable functions (PUF) labels with a high-security level by using a convenient spraying technique. The random localization of polymeric domains yields unclonable surfaces with moderate uniformity, uniqueness, and good coding capacity. In particular, this PUF exhibits good temperature stability and photostability, and the luminescent hydrochromism of the polymer further enhances the encryption level. This work demonstrates the practicality and reliability of an organic light-emitting system for anti-counterfeiting applications and provides a realistic solution for preparing low-cost PUF.

Multi-stimuli, color switchable emitter by solvent induced polymorph synthesis

Polymorphism has drawn great attention in tuning organic solid-state luminescent materials by alteration of molecular packing structure and has considerable potential application in optoelectronic devices and fluorescence sensors. Herein, two polymorphs based on dicyanodistyrylbenzene (DCS) compound were obtained by solvent induced synthesis. We have comprehensively investigated photophysical properties (such as photoluminescence colors, lifetime and PL quantum yield) of the two polymorphs, blue one αDCFDCS (Aqua) and green one αDCFDCS (Green). The single crystal structure analysis and XRD powder pattern shows that there is a slightly packing dislocation in a-axis account for the difference between the two phases. Mechanical grinding can lead to an alternation in photoluminescence color from blue to green. The differential thermal analysis (DTA) measurement discloses αDCFDCS (Aqua) is the metastable state of the compound. Both of two polymorphs show thermochromic and can be reversibly switched upon heating and cooling, which is attributing to the crystal cell expansion with the temperature increasing revealed by in situ XRD. Moreover, fuming with NH3 also causes the color change via Lewis acid base reaction. Upon heating or being exposed to HCl, it will return to the original state. The results of the polymorphism not only provide an insight into molecular packing and emission properties, but also pave the way for understanding the solid luminescence change by the multi-stimuli response.

In-Situ versatile characterization and manipulation toward Amorphous-to-Crystalline phase transition via nanoparticles strategy for organic luminescent materials and its applications

In-situ characterization and manipulation toward the evolutions of optical properties during the amorphous-to-crystalline phase transition (ACPT) for the organic solid-state luminescent (OSL) materials are of great significance but yet still a big challenge. In this work, we demonstrated a strategy to conveniently in-situ characterize and monitor the optical changes caused by the ACPT for the OSL materials via the nanoparticle suspension (NPs). The freshly prepared NPs of OSL materials will spontaneously undergo the ACPT from the amorphous cluster to an orderly NPs crystalline structure along with the blue shift of emission while maintaining their particle size as well as the suspension’s excellent transmittance and homogeneity. Interestingly, the ACPT process for the OSL materials can be manipulated by the environment temperature, concentration and size of NPs, inorganic electrolyte, and surfactant. By the ACPT via NPs of OSL materials at different times, continuous tunable luminous color can be realized by a single component material, this versatile manipulation toward the ACPT of OLS nanomaterials is expected to be applied in anti-counterfeiting ink, photodynamic therapy, and other optical fields.

Sky‐Blue Aggregation‐Induced Delayed Fluorescence Luminogens with High Horizontal Dipole Orientation for Efficient Organic Light‐Emitting Diodes†

Comprehensive SummaryEfficient and stable blue luminescent organic materials are highly demanded in the field of organic light‐emitting diodes (OLEDs) but still remain challenging. In this work, two new sky‐blue luminescent molecules comprised of electron acceptor of benzophenone and electron donors of spiro[acridine‐9,9'‐fluorene] and carbazole are designed and synthesized, and their thermal stability, electrochemical behaviors, photophysical properties, carrier transport ability and electroluminescence performance are investigated. They exhibit distinct merits of aggregation‐induced delayed fluorescence with high solid‐state photoluminescence quantum yields, balanced bipolar carrier transport and high horizontal dipole ratios. Highly efficient OLEDs using both molecules as emitters are obtained, which radiate strong sky‐blue light and provide outstanding external quantum efficiencies of up to 33.3% with small roll‐offs.

Anti-Oxidation Agents to Prevent Dye Degradation in Organic-Based Host-Guest Systems Suitable for Luminescent Solar Concentrators.

Luminescent solar concentrators (LSCs) have been extensively studied as they offer a practical solution to increase the efficiency of silicon-based photovoltaics (PVs). In this context, the use of natural and organic luminescent materials is desirable in order to obtain sustainable and environmentally friendly devices. Moreover, solution-processable organic host-guest systems based on Foerster Resonant Energy Transfer (FRET) processes offer the possibility to exploit a low-cost technique to obtain an efficient energy downshift from the UV-visible to red or deep red emissions in order to concentrate the radiation in the area of maximum efficiency of the PV device. Nevertheless, organic materials are subjected to photodegradation that reduces their optical properties when exposed to UV light and oxygen. In this work, we incorporated two different antioxidant molecules (i.e., octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (Octa) and L-ascorbic acid (L-Asc)) in a three-dye host-guest system and studied the corresponding optical properties after prolonged irradiation times in air. It was found that the presence of the antioxidants, especially L-Asc, slowed the system's photodegradation down whilst at the same time retaining high emission efficiencies and without interfering with the cascade Resonant Energy Transfer processes among the dyes inserted in the nanochannels of the host.

A supramolecular assembly strategy towards organic luminescent materials.

Supramolecular organic luminescent materials with different dimensionalities usually exhibit different optical properties as well as their potential applications in various fields. Recent reports showed that non-covalent interactions are useful tools to obtain diverse luminescent materials due to their dynamicity and reversibility, including π-π stacking, host-guest interactions, hydrophobic effects, hydrogen bonding, electrostatic effects and so on. In this review, we summarized recent progress in zero-, one-, two-, three-dimensional and disordered organic luminescent materials using the aforementioned strategies, in order to provide a solution for designing luminescent materials with specific structures and morphologies. The relationship between assembly behavior and luminescent properties is discussed in detail, along with the existing difficulties hindering the development of supramolecular assembly systems and future research directions.

As Fiber Meets with AIE: Opening a Wonderland for Smart Flexible Materials.

Aggregation-induced emission luminogens (AIEgens) have recently been developed at a tremendous pace in the area of organic luminescent materials by virtue of their superior properties. However, the practical applications of AIEgens still face the challenge of transforming AIEgens from molecules into materials. Till now, many AIEgens have been integrated into fiber, endowing the fiber with prominent fluorescence and/or photosensitizing capacities. AIEgens and fiber complement each other for making progress in flexible smart materials, in which the utilization of AIEgens creates new application possibilities for fiber, and the fiber provides an excellent carrier for AIEgens towards realizing the conversion from molecule to materials and an ideal platform to research the aggregate state of AIEgens in mesoscale and macroscale. This review begins with a brief summary of the recent advances related to some typical AIEgens with various functions and the technology for the fabrication of AIEgen-functionalized fiber. The most representative applications are then highlighted by focusing on energy conversion, personal protective equipment, biomedical, sensor, and fluorescence-related fields. Finally, the challenges, opportunities, and tendencies in future development are discussed in detail. This review hopes to inspire innovation in AIEgens and fiber from the view of mesoscale and macroscale.

Experimental and theoretical studies of the effect of molecular conformation on the photophysical properties in the pyrene system

Disordered motion is an intrinsic property of molecules. The use of controllable molecular motion can not only achieve interesting molecular geometry, but can also induce novel opto-electronic behaviour. Herein, a set of pyrene-based compounds were synthesized to realize an observable molecular motion. 2,5-Dimethoxyphenyl unit (s) were introduced at the pyrene core, which lead to different molecular conformations. Furthermore, the detailed molecular geometries of the compounds were investigated by NMR spectroscopy and DFT calculations. In addition, the photophysical properties of these compounds were investigated in order to understand the relationship between the molecular conformations and the photophysical properties. The diversiform of molecular conformations can cause a broadened FWHM emission in organic luminescence materials.

Iridium(III) Complex Radical and Corresponding Ligand Radical Functionalized by a Tris(2,4,6-trichlorophenyl)methyl Unit: Synthesis, Structure, and Photophysical Properties

Organic radical luminescent materials with doublet excited state character based on tris(2,4,6-trichlorophenyl)methyl (TTM) have attracted extensive attention in recent years. However, how they affect the phosphorescent iridium(III) complex characterized by the triplet excited state has not been studied yet. Herein, a new iridium(III) complex radical (Ir-TTM) and corresponding ligand radical (ppy-TTM) with a TTM unit have been designed and synthesized, and their radical properties were confirmed by the single crystal structure and EPR spectra. Notably, the ligand radical ppy-TTM shows an efficient red light emission, whereas the iridium complex radical Ir-TTM emits no light, which resulted from the intramolecular quenching effect of the TTM radical unit on the iridium luminescence center. DFT calculations demonstrate that the lowest doublet (D1) excited state of ppy-TTM shows an intramolecular charge transfer character from the 2-phenylpyridine moieties to the TTM unit, whereas the D1 of Ir-TTM exhibits a significant charge transfer character from the iridium luminescence center moieties to the TTM unit, which further explains the luminescence quenching mechanism of the phosphorescent iridium complex radical.

Organic persistent luminescence imaging for biomedical applications

Persistent luminescence is a unique visual phenomenon that occurs after cessation of excitation light irradiation or following oxidization of luminescent molecules. The energy stored within the molecule is released in a delayed manner, resulting in luminescence that can be maintained for seconds, minutes, hours, or even days. Organic persistent luminescence materials (OPLMs) are highly robust and their facile modification and assembly into biocompatible nanostructures makes them attractive tools for in vivo bioimaging, whilst offering an alternative to conventional fluorescence imaging materials for biomedical applications. In this review, we give attention to the existing limitations of each class of OPLM-based molecular bioimaging probes based on their luminescence mechanisms, and how recent research progress has driven efforts to circumvent their shortcomings. We discuss the multifunctionality-focused design strategies, and the broad biological application prospects of these molecular probes. Furthermore, we provide insights into the next generation of OPLMs being developed for bioimaging techniques.

Organic luminescent radical-based film probes with reversible doublet state switching for ultrasensitive nerve agents simulant vapor detection

Donor-acceptor (D-A) type organic luminescent materials with intramolecular charge transfer (ICT) mechanism constitute a significant class of fluorescent probes for nerve agents sensing. However, blue-shifted emissions from nerve agents due to the protonation on donor moieties of the probes are still rarely studied and reported so far. Herein, by incorporation of high luminescence and photostability open-shell organic radicals with proton-responsive characteristics, for the first time, we report two D-A type radical-based probes, which enable high sensitivity, selectivity and good reusability nerve agents simulant (diethyl chlorophosphite, DCP) vapor detection via an uncommon blue-shift emission response. Specifically, the sensing mechanism is ultimately deciphered to be the DCP induced protonation of the donor of the radicals, whereas present fully reversal D-A electronic structure models and the nature of the doublet excited states. This sensing system offers new insights into the rational design and utility of luminescent radicals for more broad applications.

Rational design strategies for nonconventional luminogens with efficient and tunable emission in dilute solution

The design and synthesis of organic nonconventional luminescent (NL) materials with efficient emission have attracted significant attention. However, these materials usually suffer from the fact of low photoluminescent quantum efficiency (Φ), poor tunability of emission wavelength, and non-luminescence in dilute solution, limiting their further application. Herein, rational molecular design strategies for NL compounds with efficient and tunable emission in dilute solution were presented via theoretical calculation and practical verification. Firstly, increasing the ratio of π → π* transition in the excited state to suppress the n → π* transition, inhibit intersystem crossing, and avoid the generation of triplet excitons which can be easily quenched by water or oxygen. Secondly, introducing strong intramolecular hydrogen bonds to suppress intramolecular rotation and inhibit non-radiative transitions. Besides, D-π-A structure can effectively adjust the luminescence color. Based on the proposed design strategies, two novel compounds named LJ-1 and LJ-2 were synthesized, both compounds can emit bright green fluorescence in dilute solution with Φ of up to 59.8 %, and their photophysical properties are pH-dependent. The results showed that pH had a great impact on the strength of intramolecular hydrogen bonds and the degree of conjugation. For example, the emission wavelength of LJ-1 exhibited red-shift as the increase of pH value. Furthermore, LJ-1 can be used for visual detection of Ag+ in water solution in naked eye and the precise localization of lysosomes. We believe that our work will greatly broaden the application range of nonconventional luminogenic compounds in future.

Effects of Through-Bond and Through-Space Conjugations on the Photoluminescence of Small Aromatic and Aliphatic Aldimines.

Through-bond conjugation (TBC) and/or through-space conjugation (TSC) determine the photophysical properties of organic luminescent compounds. No systematic studies have been carried out to understand the transition from aromatic TBC to non-aromatic TSC on the photoluminescence of organic luminescent compounds. In this work, a series of small aromatic and aliphatic aldimines were synthesized. For the aromatic imines, surprisingly, N,1-diphenylmethanimine with the highest TBC is non-emissive, while N-benzyl-1-phenylmethanimine and N-cyclohexyl-1-phenylmethanimine emit bright fluorescence in aggregate states. The aliphatic imines are all emissive, and their maximum emission wavelength decreases while the quantum yield increases with a decrease in steric hindrance. The imines show concentration-dependent and excitation-dependent emissions. Theoretical calculations show that the TBC extents in the aromatic imines are not strong enough to induce photoluminescence in a single molecule state, while the intermolecular TSC becomes dominant for the fluorescence emissions of both aromatic and aliphatic imines in aggregate states, and the configurations and spatial conformations of the molecules in aggregate states play a key role in the formation of effective TSC. This study provides an understanding of how chemical and spatial structures affect the formation of TBC and TSC and their functions on the photoluminescence of organic luminescent materials.

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 (τp = 394.1 ms; Φp = 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.

Ion−π interactions for constructing organic luminescent materials

AbstractOrganic luminescent materials are one of the research hotspots worldwide. The luminescent efficiency of these materials can be tuned by introducing noncovalent interactions. Ion−π interactions, mainly including anion−π interactions and cation−π interactions, are relatively new but powerful noncovalent interactions with extensive applications in supramolecular chemistry. The use of ion−π interactions to fabricate highly luminescent materials is initiated since they were introduced to develop aggregation‐induced emission luminogens in 2017. Then, this research field is undergoing through challenging but very prospective developments. In this minireview, we start with a brief introduction of ion−π interactions and then make a summary of the reports on organic fluorescence based on ion−π interactions and room temperature phosphorescence induced by ion−π interactions. The applications of organic luminescent materials based on ion−π interactions in bioimaging, imaging‐guided anticancer and antibacterial treatment are also discussed. The challenges and prospects of using anion−π and cation−π interactions for constructing organic luminogens are presented in the final part. It is expected that this minireview will provide some guidelines for fabricating novel organic luminescent materials and further extend the application potential of ion−π interactions.

Recent Advances in Organic Room-Temperature Phosphorescence of Heteroatom (B/S/P)-Containing Chromophores

Recent Advances in Organic Room-Temperature Phosphorescence of Heteroatom (B/S/P)-Containing Chromophores

Three-Component Cycloaddition of Nitriles: Construction of Bicyclic 4-Aminopyrimidines and Their Photophysical Studies.

A base-induced synthesis of bicyclic 4-aminopyrimidines by the cycloaddition of three types of nitriles is reported. The scope of the method is demonstrated with 44 examples. Products are found to have luminescence properties and show potential applications as organic luminescent layer materials.

Influence of Sulfur Atoms on TADF Properties from Through-Space Charge Transfer Excited States.

The harnessing of heavy atom effect of chalcogen elements offers a way for boosting the thermally activated delayed fluorescence (TADF) of purely organic luminescent materials that can harvest triplet excitons. However, the conformational and electronic variations induced by the heavy and large atoms may also have adverse effects on the TADF properties. Herein, the design, synthesis, and structures of a new type of through-space charge transfer (TSCT) emitters containing benzothiazino[2,3,4-kl]phenothiazine (DPTZ) as the donor unit are reported. The influences of S atoms on the emission properties have been systematically investigated by means of theoretical simulations, electrochemical and spectroscopic studies. Although the presence of π-stacking interactions and calculated spin-orbit coupling (SOC) values are beneficial for TSCT-TADF properties, the triplet TSCT states are uplifted to above the locally excited (LE) state of the acceptor moieties. As a result, the new emitters display longer delayed fluorescence lifetimes (τDF ) of 255.0-114.3 μs and lower PLQYs of 45-61 % in comparison with the O-containing congeners (τDF =26.9-6.8 μs; PLQYs=74-71 %). This work highlights that a full consideration of various effects is essential when making use of heavy chalcogen atoms for the design of TADF emitters.

Highly Emissive Luminogens in Both Solution and Aggregate States

Highly Emissive Luminogens in Both Solution and Aggregate States