Aggregation-induced Emission Systems Research Articles

Uncovering the Aggregation-Induced Emission Mechanisms of Phenoxazine and Phenothiazine Groups.

Molecules with both aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) properties are potential organic light-emitting diode materials; however, the AIE and TADF mechanisms are still debatable. In this work, four molecules incorporating carbazole (Cz), phenoxazine (PXZ), and phenothiazine (PTZ) as donor groups to the diphenylsulfone acceptor were investigated. The experiment results indicate that a molecule containing Cz exhibits solely TADF properties, whereas molecules containing PXZ and PTZ demonstrate both TADF and AIE characteristics. As for DPS-PTZ, the result indicates that the thin-film environment restricts molecular twisting, consequently reducing nonradiative decay, thereby attributing to the AIE property by density functional theory and molecular dynamics simulation. As for DPS-PXZ, the result suggests that the restricted access to a conical intersection in a singlet excited via an expansion in the C-S-C angle is the pivotal factor for the AIE characteristic. The C-S-C angle twist of DPS-PXZ is impeded in the aggregate state and resulted in luminescence. Understanding the mechanisms serves as a valuable guide for the development of new AIE systems, enabling their application in various practical domains.

Tailoring Emission Color Shifts in Mechanofluorochromic-Active AIE Systems of Carbazole-Based D-π-A Conjugates: Impact of π Spacer Unit Variants.

The expanding field of organic electronics has led to the emergence of donor-π-acceptor (D-π-A) systems as promising candidates for various optoelectronic applications. Recent advancements in the design of organic fluorophores allow precise modulation of their optical properties in the solution and solid states, broadening their utility. Here, we synthesize and characterize two series of D-π-A systems based on carbazole-cyanostilbene compounds (3a-c and 4a-c), which feature differences in π-spacer groups and substitution on the acceptor cyanostilbene units. These compounds exhibit intramolecular charge transfer (ICT) characteristics, with 3c possessing a phenyl spacer and -CF3 substitution displaying notable solvatochromism and polarity-enhanced emission (PEE). Additionally, both series show aggregation-induced emission (AIE), with emission color shifts controlled by spacer groups. These compounds demonstrate diverse emissive behaviors, including locally excited (LE), twisted intramolecular charge transfer (TICT), and π-π stacked aggregated states, contributing to the AIE phenomenon. Compounds with phenyl spacers (3a-c) exhibit higher solid-state quantum yields than those of thiophene (4a-c). Furthermore, the impact of spacer groups and acceptor substitutions on the solid-state emissive properties and mechanofluorochromism (MFC) are highlighted. Compounds with phenyl spacers and substitution (3a, 3c) show a blue-shifted MFC, while those with thiophene (4a, 4c) display a red-shifted MFC. These findings offer insights into the development of advanced materials with tunable emission properties and high thermal stability for optoelectronic applications.

Assembly-Controlled Supramolecular Aggregation-Induced Emission Systems based on Amphiphilic Block Polymer Hosts

Fabrication of controlled supramolecular assembly and establishing the structure-function relationship was of great significance in supramolecular chemistry. Uniform fluorescence supramolecular polymeric vesicles and/or micelles with enhanced photophysical property were elaborately...

How does aggregation‐induced emission aggregate interdisciplinary research?

AbstractIt is a matter of debate whether the discipline independence in discipline formation narrows its interdisciplinarity. It is also less well understood how disruptive works emerge in investigative practice rather than a theory‐driven approach. Aggregation‐induced emission (AIE) is an atypical photophysical phenomenon, in which the whole (aggregate) is brighter than the sum of its parts (single molecule). Through measuring and computing the cognitive extent and evolution of research on AIE, including topics, epistemic‐social collaborative networks, interdisciplinarity, emergent concepts, core concept networks and knowledge flow, this study shows that a cross‐research scales concept and its practice can establish new bridges in the sciences and promote disruptive work. Focusing on mesoscale entities, scientists from many different branches of science are involved in theoretical research on mechanisms, as well as developing different AIE systems for applications. The data analysis in this study provides details showing how non‐reductionist concepts based on new scientific discoveries cross traditional disciplinary boundaries and aggregate interdisciplinary research. The emergence and evolution of the AIE field implies that scientists may be motivated to embrace nonreductionist ideas at different research scales, leading to a more permeable field boundary.

Theoretical Insights into Aggregation-Induced Emission with the Ionic π Fluorophore: The Importance of Choosing the Dimer QM Model in the ONIOM Study.

In understanding the mechanism of aggregation-induced emission (AIE), the multilevel ONIOM framework has been demonstrated as one of the efficient tools that can capture the essential mechanistic information by choosing a single fluorophore as the quantum mechanics (QM) model and putting all surrounding molecules in the low-level region. Recently, the ionic styryl-pyridine salt (namely, SPH) has been reported as a new class of AIEgen with a high fluorescence yield. In the SPH crystal, a pair of ionic SPH molecules are closely stacked with each other in an antiparallel, head-to-tail pattern, thus the choice of QM models (an individual or dimeric structure) becomes critical in the ONIOM study. Herein we report the AIE mechanism of the ionic SPH at the QM ((TD)-CAM-B3LYP) and ONIOM(QM:MM) levels. As usual, the fluorescence quenching of SPH in tetrahydrofuran (THF) solution is attributed to a nonradiative relaxation via the central C═C bond rotation, with a rather low barrier of 2.7 kcal/mol. In crystals, either with a monomer or dimer model, the fluorescence quenching channel is found to be restricted due to the obvious C═C rotation barriers. Compared with the monomer model, the dimer model, by treating the orbital interaction of the two SPH molecules at the QM level, provides significantly increased barriers and a red-shifted emission wavelength that better matches the experimental value. In addition, the calculated exciton coupling in the fluorescence emission state can be discovered only by a dimer model. The findings here emphasize not only the importance of choosing a proper model in the ONIOM study of AIE but also expanding our understanding of novel AIE systems.

Regulating the proximity effect of heterocycle-containing AIEgens

Proximity effect, which refers to the low-lying (n,π*) and (π,π*) states with close energy levels, usually plays a negative role in the luminescent behaviors of heterocyclic luminogens. However, no systematic study attempts to reveal and manipulate proximity effect on luminescent properties. Here, we report a series of methylquinoxaline derivatives with different electron-donating groups, which show different photophysical properties and aggregation-induced emission behaviors. Experimental results and theoretical calculation reveal the gradually changed energy levels and different coupling effects of the closely related (n,π*) and (π,π*) states, which intrinsically regulate proximity effect and aggregation-induced emission behaviors of these luminogens. With the intrinsic nature of heterocycle-containing compounds, they are utilized for sensors and information encryption with dynamic responses to acid/base stimuli. This work reveals both positive and negative impacts of proximity effect in heterocyclic aggregation-induced emission systems and provides a perspective to develop functional and responsive luminogens with aggregation-induced emission properties.

Aggregation-Induced Catalysis: Asymmetric Catalysis with Chiral Aggregates.

So far, there have been 4 methods to control chirality including the use of chiral auxiliaries, reagents, solvents, and catalysts documented in literature and textbooks. Among them, asymmetric catalysts are normally divided into homogeneous and heterogeneous catalysis. In this report, we present a new type of asymmetric control-asymmetric catalysis via chiral aggregates that would not belong to the above categories. This new strategy is represented by catalytic asymmetric dihydroxylation reaction of olefins in which chiral ligands are aggregated by taking advantage of typical aggregation-induced emission systems containing tetrahydrofuran and H2O cosolvents. It was proven that the chiral induction can be enhanced from er of 78:22 to 97:3 simply by changing the ratios of these 2 cosolvents. The formation of chiral aggregates of asymmetric dihydroxylation ligands, (DHQD)2PHAL and (DHQ)2PHAL, has been proven by aggregation-induced emission and a new analytical tool-aggregation-induced polarization established by our laboratory. In the meanwhile, chiral aggregates were found to be formed either by adding NaCl into tetrahydrofuran/H2O systems or by increasing concentrations of chiral ligands. The present strategy also showed promising reverse control of enantioselectivity in the Diels-Alder reaction. This work is anticipated to be extended broadly to general catalysis, especially to asymmetric catalysis in the future.

Nonconventional 1,8-Diazafluoren-9-One Aggregates for Green Light Enhancement in Hybrid Biocompatible Media.

Organic aggregates currently play a prominent role, mainly for their unique optoelectronic properties in the aggregated state. Such properties can be related to the aggregates’ structure and the molecular packing mode. In the literature, we have well-established models of H and J aggregates defined based on the molecular exciton model. However, unconventional aggregates, the most unrecognized forms, have been generating interest among researchers recently. Within unconventional aggregation, aggregation-induced emission systems (AIE) are considered. In the present work, we discuss the effect of the forming of unconventional aggregation together with the change in dye concentration on the surface energy characteristics of the materials. All materials were prepared as hybrid biocompatible thin films where the matrix is TiO2 or TiO2/carbon nanowalls (CNWs) with the incorporated dye in the form of 1,8-diazafluoren-9-one (DFO). Using the time-resolved emission spectra and the determination of surface parameters from contact angle measurements, we indicated the correlation between the changes in such parameters and the concentration of DFO dye in two types of TiO2 and TiO2/CNW structures. To examine the propensity of DFO for aggregation, the internal energy of the dye was assessed in several aggregate structures using Quantum chemistry calculations. The results emphasize that DFO is an attractive structure in the design of new fluorophores due to its low molecular weight, the presence of a nitrogen atom that provides good coordination properties, and the ability to form hydrogen bonds. Our studies show that when using suitable matrices, i.e., rigid media, it forms the preferred forms of aggregates in the excited state, characterized by high emission efficiency in the band maximum of around 550 nm.

Insight into the Clustering-Triggered Emission and Aggregation-Induced Emission Exhibited by an Adamantane-Based Molecular System.

Chemical clustering of a nonemissive and non-AIEgen of Cb-Ph endowed a molecular system (Ad-4CP) with unique dual emissions in the solution state, a typical clustering-triggered emission (CTE), and high emission efficiency in the aggregated state, an aggregation-induced emission (AIE). The CTE was ascribed to intramolecular charge transfer (CT); however, the AIE was ascribed to both intra- and intermolecular CTs. The two-level CTs make the Ad-4CP exhibit remarkable excitation-dependent emissions. We believe that the present work not only delivers a peculiar molecular system with both CTE and AIE properties but also provides an example on how molecular engineering promotes the rational design of CTE and AIE systems via clusterization of suitable structural units.

Influence of Wallach’s Rule on Chiral AIE Systems and Its Application in Cryptographic Information Storage

Influence of Wallach’s Rule on Chiral AIE Systems and Its Application in Cryptographic Information Storage

Insights on isomeric emitters with thermally activated delayed fluorescence: Comparison between solvent and crystal state

Thermally activated delayed fluorescence (TADF) molecules with aggregation-induced emission (AIE) properties have attracted great attention in recent studies. In this study, three isomeric emitters DPAC-4PYPM, DPAC-TRZ and DPAC-6PYPM with TADF are studied in toluene and crystal with the combination of polarizable continuum model (PCM) and quantum mechanics and molecular mechanics (QM/MM) method. Results show that tiny difference of intramolecular interaction is induced due to the variation in the acceptor group, thus similar geometries and stacking patterns in crystal are obtained for three molecules. Our calculation also indicates that the energy diagram is quite different for three molecules in both toluene and crystal state, while the participation of higher triplet excited states provide additional decay channels for the reverse intersystem crossing (RISC) process, which favors the generation of TADF. In addition, the bending vibrations of the phenyl in the donor and the stretching vibrations of the C=C and C-H bonds are suppressed due to the intermolecular interactions in crystal state, thus block the excited-state energy consumption pathway. It indicates that all three molecules are typical AIE systems. Our calculation results agree with experimental measurements and provide more useful information for TADF emitters with AIE properties.

Novel Quinolizine AIE System: Visualization of Molecular Motion and Elaborate Tailoring for Biological Application.

Molecular motions are ubiquitous in nature and they immutably play intrinsic roles in all actions. However, exploring appropriate models to decipher molecular motions is an extremely important but very challenging task for researchers. Considering aggregation-induced emission (AIE) luminogens possess their unique merits to visualize molecular motions, it is particularly fascinating to construct new AIE systems as models to study molecular motion. Herein, a novel quinolizine (QLZ) AIE system was constructed based on the restriction intramolecular vibration (RIV) mechanism. It was demonstrated that QLZ could act as an ideal model to visualize single-molecule motion and macroscopic molecular motion via fluorescence change. Additionally, further elaborate tailoring of this impressive core achieved highly efficient reactive oxygen species production and realized fluorescence imaging-guided photodynamic therapy applications, which confirms the great application potential of this new AIE-active QLZ core. Therefore, this work not only provides an ideal model to visualize molecular motion but also opens a new way for the application of AIEgens.

Novel Quinolizine AIE System: Visualization of Molecular Motion and Elaborate Tailoring for Biological Application**

AbstractMolecular motions are ubiquitous in nature and they immutably play intrinsic roles in all actions. However, exploring appropriate models to decipher molecular motions is an extremely important but very challenging task for researchers. Considering aggregation‐induced emission (AIE) luminogens possess their unique merits to visualize molecular motions, it is particularly fascinating to construct new AIE systems as models to study molecular motion. Herein, a novel quinolizine (QLZ) AIE system was constructed based on the restriction intramolecular vibration (RIV) mechanism. It was demonstrated that QLZ could act as an ideal model to visualize single‐molecule motion and macroscopic molecular motion via fluorescence change. Additionally, further elaborate tailoring of this impressive core achieved highly efficient reactive oxygen species production and realized fluorescence imaging‐guided photodynamic therapy applications, which confirms the great application potential of this new AIE‐active QLZ core. Therefore, this work not only provides an ideal model to visualize molecular motion but also opens a new way for the application of AIEgens.

Bringing Inherent Charges into Aggregation-Induced Emission Research.

Charged organic molecules, such as DNA, RNA, proteins, and polysaccharides, are ubiquitous and indispensable in natural living systems, which possess specific biological functions to interact with oppositely charged species via electrostatic attraction. The molecules with inherent charges typically differentiate themselves from the neutral ones with unique attributes (e.g., ionic interactions and high polarity), thereby playing a pivotal role in a broad spectrum of areas, including supramolecular chemistry, structural biology, and materials science. It is thus of great importance to explore and develop various charged organic systems for biomimicry and the creation of functional materials. In 2001, our group reported a peculiar luminogen that exhibited weak emission in solution but had significantly enhanced emission in aggregates, and we, for the first time, coined this phenomenon as aggregation-induced emission (AIE). The AIE concept significantly changes the cognition of the scientific community toward classic photophysical phenomena. Since the discovery of this unusual luminescence phenomenon, AIE luminogens (AIEgens) have attracted extensive attention from researchers in a plethora of disciplines because of their high brightness in aggregates, large Stokes shift, excellent photostability, and good biocompatibility. In the past 10 years, our laboratory has expended a great amount of effort to bring inherent charges into AIE research and acquired fruitful achievements.In this Account, we summarize the progress of charged AIE systems primarily made by our laboratory. We start with a brief introduction to charged AIEgens and then discuss their design strategies from molecular and topological perspectives, respectively. Next, we review the unique properties of charged AIEgens, including D-A interactions, anion-π+ interactions, and intermolecular electrostatic interactions, with an emphasis on how they differentiate themselves from the neutral analogs. On the one hand, positively charged AIEgens exhibit unique photophysical properties by forming typical donor-acceptor structures to manipulate the emission wavelength or initiate ultralong persistent luminescence. On the other hand, positively charged AIEgens exhibit unique physiochemical properties, such as an adjustable targeting capability toward biological targets and a strong capability for the generation of reactive oxygen species. Furthermore, we showcase the applications of charged AIEgens in imaging and diagnosis, photodynamic therapy, gas separation, and solar desalination. Finally, we conclude this Account with a summary and some perspectives regarding the existing challenges and future directions. We hope that this Account can spark new ideas and inspire scientists from different disciplines to explore this nascent yet promising research area.

Aggregation-Induced Synthesis (AIS): Asymmetric Synthesis via Chiral Aggregates.

A new chiral aggregate-based tool for asymmetric synthesis has been developed by taking advantage of chiral aggregates of GAP (Group-Assisted Purification) reagents, N-phosphonyl imines. This tool was proven to be successful in the asymmetric GAP synthesis of functionalized 2,3-dihydrobenzofurans by reacting salicyl N-phosphonyl imines with dialkyl bromomalonates in various cosolvent systems. The chiral induction can be controlled by differentiating between two asymmetric directions simply by changing the ratios of cosolvents which are commonly adopted in AIE (aggregation-induced emission) systems. The formation of chiral aggregates was witnessed by a new analytical tool—aggregation-induced polarization (AIP). The present synthetic method will be broadly extended for general organic synthesis, particularly, for asymmetric synthesis and asymmetric catalysis in the future.

Molecular mechanism of aggregation‐induced emission

AbstractDeep understanding of the inherent luminescence mechanism is essential for the development of aggregation‐induced emission (AIE) materials and applications. We first note that the intermolecular excitonic coupling is much weaker in strength than the intramolecular electron‐vibration coupling for a majority of newly termed AIEgens, which leads to the emission peak position insensitive to excitonic coupling, hence the conventional excitonic model for J‐aggregation cannot effectively explain their AIE phenomena. Then, using multiscale computational approach coupled with our self‐developed thermal vibration correlation function rate formalism and transition‐state theory, we quantitatively investigate the aggregation effect on both the radiative and the nonradiative decays of molecular excited states. For radiative decay processes, we propose that the lowest excited state could convert from a transition dipole‐forbidden “dark” state to a dipole‐allowed “bright” state upon aggregation. For the radiationless processes, we demonstrate the blockage of nonradiative decay via vibration relaxation (BNR‐VR) in harmonic region or the removal of nonradiative decay via isomerization (RNR‐ISO) or minimum energy crossing point (RNR‐MECP) beyond harmonic region in a variety of AIE aggregates. Our theoretical work not only justifies a plethora of experimental results but also makes reliable predictions on molecular design and mechanism that can be experimentally verified. Looking forward, we believe this review will benefit the deep understanding about the universality of AIE phenomenon and further extending the scope of AIE systems with novel applications.

Aggregation-induced emission: a review of promising cyano-functionalized AIEgens

The aggregation-induced emission (AIE) is a photochemical process described in 2001, where the aggregation of specific kinds of organic compounds enhances the emission of light performed by these organic compounds. Since then, this phenomenon had attracted much interest because of its potential application in optics, electronics, energy and bioscience. In this review, the main concepts of AIE are going to be explained through the mechanistic decipherment of these photophysical processes. Additionally, some AIE systems will be discussed, describing the phosphorescence enhancement induced in organic molecules by this effect, but we will be focusing on the cyano-containing AIEgens, its recent advances and the driving forces that lead to the AIE effect in these cyano-containing molecules.

New pyrene-based butterfly-shaped blue AIEgens: Synthesis, structure, aggregation-induced emission and their nondoped blue OLEDs

Developing the blue emitters with excellent electroluminescence (EL) performance is crucial for full-color displays and solid-state lighting. Based on the synthetic strategy for exploring highly efficient luminescent solids by constructing aggregation-induced emission (AIE) systems, two new pyrene-based butterfly-shaped blue AIEgens, Py(5,9)BTPE and Py(5,9)BTriPE, have been successfully designed and synthesized by integrating two tetraphenylethylene (TPE) or triphenylethylene (TriPE) groups into pyrene ring. The adoption of the pyrene unit with a large optical bandgap, the use of the TPE or TriPE group with AIE characteristic, the integration of the TPE or TriPE group into pyrene ring at 5,9-positions, and the highly twisted molecular conformations of present AIE systems endow the two compounds with bright blue emission in the solid state. These foregoing features ensure that the new pyrene-based compounds can serve as excellent blue-emitting materials in OLEDs. A non-doped OLED based on Py(5,9)BTPE exhibited a sky-blue emission (Von: 3.2 V) with a CIE coordinates of (0.19, 0.28), a maximum external quantum efficiency (EQEmax) of 3.35%, a current efficiency (CE) of 6.51 cd A−1 with a low efficiency roll-off that represents one of the highest value reported for sky-blue AIE-based OLEDs. An analogous device with Py(5,9)BTriPE as an emitting layer displayed a pure-blue emission (CIE: 0.16, 0.17) with a moderate EQEmax of 1.27% and a CE of 1.63 cd A−1 with a low efficiency roll-off. Furthermore, the EL emissions of the two devices display almost no change with the increasing of the current density, indicating their good color stability.

Restriction of Access to the Dark State: A New Mechanistic Model for Heteroatom-Containing AIE Systems.

Restriction of intramolecular motion (RIM), as the working mechanism of aggregation-induced emission (AIE), cannot fully explain some heteroatom-containing systems. Now, two excited states are taken into account and a mechanism, restriction of access to dark state (RADS), is specified to elaborate RIM and complete the picture of AIE mechanism. A nitrogen-containing molecule named APA is chosen as a model compound; its weak fluorescence in solution is ascribed to the easy access from the bright (π,π*) state to the close-lying dark (n,π*) state. By either metal complexation or aggregation, the dark state is less accessible due to restriction of the molecular motion leading to the dark state and elevation of the dark state energy, thus the bright state emission is restored. RADS is powerful in elucidating the AIE effect of molecules with excited states favoring non-radiative decay, including overlap-forbidden states such as (n,π*) and CT states, spin-forbidden triplet states, and so on.

Non-aromatic annulene-based aggregation-induced emission system via aromaticity reversal process

Aggregation-induced emission (AIE) is a photophysical phenomenon correlated closely with the excited-state intramolecular motions. Although AIE has attracted increasing attention due to the significant applications in biomedical and optoelectronics, an in-depth understanding of the excited-state intramolecular motion has yet to be fully developed. Here we found the non-aromatic annulene derivative of cyclooctatetrathiophene shows typical AIE phenomenon in spite of its rotor-free structure. The underlying mechanism is investigated through photoluminescence spectra, time-resolved absorption spectra, theoretical calculations, circular dichroism as well as by pressure-dependent fluorescent spectra etc., which indicate that the aromaticity reversal from ground state to the excited state serves as a driving force for inducing the excited-state intramolecular vibration, leading to the AIE phenomenon. Therefore, aromaticity reversal is demonstrated as a reliable strategy to develop vibrational AIE systems. This work also provides a new viewpoint to understand the excited-state intramolecular motion behavior of lumiongens.