Consideration of Molecular Structure in the Excited State to Design New Luminogens with Aggregation-Induced Emission

Self-Reporting of Folding and Aggregation by Orthogonal Hantzsch Luminophores Within a Single Polymer Chain

Self-reporting fluorescence methods for monitoring folding and aggregation of proteins have a long history in biochemistry. Placing orthogonal luminophores within individual synthetic polymer chains for self-reporting both folding (i.e., its intramolecular compaction to isolated single-chain nanoparticles, SCNPs) and unbidden aggregation (i.e., the intermolecular association of SCNPs) remains a great challenge. Herein, a simple and efficient platform to identify both single-chain compaction and intermolecular aggregation phenomena via photoluminescence is presented based on simultaneous synthesis through Hantzsch ester formation of orthogonal luminophores within the same polymer chain. Starting from non-luminescent β-ketoester-decorated chains, intramolecular compaction is visually detected through fluorescence arising from Hantzsch fluorophores generated as intra-chain connectors during folding. Complementary, intermolecular association is identified via aggregation-induced emission (AIE) from orthogonal luminophores displaying intense photoluminescence at redshifted wavelengths after formation of multi-SCNPs assemblies.

Innovative Synthetic Procedures for Luminogens Showing Aggregation-Induced Emission

As a consequence of their intrinsic advantageous properties, luminogens that show aggregation-induced emission (AIEgens) have received increasing global interest for a wide range of applications. Whereas general synthetic methods towards AIEgens largely rely on tedious procedures and limited reaction types, various innovative synthetic methods have now emerged as complementary, and even alternative, strategies. In this Review, we systematically highlight advancements made in metal-catalyzed functionalization and metal-free-promoted pathways for the construction of AIEgens over the past five years, and briefly illustrate new perspectives in this area. The development of innovative synthetic procedures will enable the facile synthesis of AIEgens with great structural diversity for multifunctional applications.

Nitro-Substituted Dipyrrolyldiketone BF2 Complexes as Electronic-State-Adjustable Anion-Responsive π-Electronic Systems

Nitro-substituted π-electronic molecules are fascinating because of their unique electronic and optical properties and the ease of their transformation into various functional derivatives. Herein, nitro-introduced dipyrrolyldiketone BF2 complexes as anion-responsive π-electronic molecules were synthesized, and their electronic properties and anion-binding abilities were investigated by spectroscopic analyses and theoretical studies. The obtained nitro-substituted derivatives showed solvent-dependent UV/vis spectral changes and high anion-binding affinities due to the easily pyrrole-inverted conformations and polarized pyrrole NH sites upon the introduction of electron-withdrawing moieties.

Carbazole-modified thiazolo[3,2-c][1,3,5,2]oxadiazaborinines exhibiting aggregation-induced emission and mechanofluorochromism

Two highly emissive carbazole-containing thiazole-fused oxadiazaborinines were designed and synthesized. These N,O-chelated organoboron dyes displayed large Stokes shifts and remarkable solvatofluorochromism in solutions, as well as good thermal stability and comparatively high photoluminescence quantum yields (up to 34%) in the solid state. The presence of a carbazole donor unit, linked with the oxadiazaborinine acceptor via a phenyl linker, restricted intramolecular rotation, leading to enhanced aggregation-induced emission properties of the compounds: in THF/water mixtures with a large water percentage, they demonstrated the formation of emissive nanoaggregates with an average size of 79 and 89 nm for complexes 2 and 3, respectively. The introduction of bulky tert-butyl groups attached to the carbazole moiety induced significant mechanofluorochromic properties of the compounds.

Molecular tuning of the crystallization-induced emission enhancement of diphenyl-dibenzofulvene luminogens

The absorption and emission properties of various diphenyl-dibenzofulvene (DP-DBF) derivatives were investigated, and their crystallization-induced emission enhancement (CIEE) performances were found to show a clear correlation with the twist angle around the C[double bond, length as m-dash]C bond of the DP-DBF structure.

CH/π-interaction-driven self-assembly of tetraphenylethylene derivatives into the face to face arrangement

For tetraphenylethene (TPE) derivatives, it is difficult to determine the arrangement of the molecules in the aggregation state because disordered aggregation usually occurs. To solve the problem, we have explored a novel and facile strategy to investigate the aggregation mode of a TPE derivative framework in which the two neighboring ortho carbons of two phenyl moieties at the same ethylene carbon were linked with an alkoxyl chain (C4) (denoted as TPEC4). The XRD measurements on the particles obtained in a DMSO/H₂O mixture (f_w = 60%) showed sharp peaks which is consistent with the simulated XRD patterns on the basis of a single crystal structure of TPEC4, indicating well-ordered molecular packing in the aggregated state. The CH/π-interaction and solvophobicity driven self-assembly behaviour of the compound was observed in the DMSO/H₂O mixture. A face to face molecular packing structure that arises from quadruple intermolecular CH/π-interactions of the tetraphenylethylenes is the key motif for self-assembly in solution. The unique blue-red shifted emission in the DMSO/H₂O mixture associated with aggregated behaviour of the compound was also investigated. This discovery will provide the basis for theoretical research and the rational design of TPE-based luminogens.

The CH/π interaction can drive the TPE motif to adopt a face-to-face crystalline packing in this TPE benzene rings linked with an alkoxyl chain (C4) during aggregation.

Restriction of Conformation Transformation in Excited State: An Aggregation-Induced Emission Building Block Based on Stable Exocyclic C=N Group

The development of aggregation-induced emission (AIE) building block and deciphering its luminescence mechanism are of great significance. Here a feasible strategy for the construction of AIE unit based on E-Z isomerization (EZI) of exocyclic C=N double bond is proposed. Taking [1,2,4]thiadiazole[4,3-a]pyridine (TZP) derivative as an example, its aryl-substituted derivative (TZPP) shows obvious AIE character. The analysis of spectral data and theoretical calculations indicates that fast structural relaxation of TZPP in the emissive state plays a key role in a low fluorescence quantum yield in dilute solution, which should be caused by the small energy gap between locally excited (LE) state and twisted intramolecular charge transfer state. When in solid state, the bright emission with LE state characteristic reappears due to the large shift barrier of geometry transformation. As a potential building block for AIEgens with special heterocyclic structure, these findings would open up opportunities for developing various functional materials.

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A new aggregation-induced emission building block

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A novel AIE mechanism with spectral measurements and theoretical calculations

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Available starting materials resulting in convenient synthesis and modification

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A stable exocyclic C=N double bond in heterocycles

Recent progress in the molecular imaging of therapeutic monoclonal antibodies

Therapeutic monoclonal antibodies have become one of the central components of the healthcare system and continuous efforts are made to bring innovative antibody therapeutics to patients in need. It is equally critical to acquire sufficient knowledge of their molecular structure and biological functions to ensure the efficacy and safety by incorporating new detection approaches since new challenges like individual differences and resistance are presented. Conventional techniques for determining antibody disposition including plasma drug concentration measurements using LC-MS or ELISA, and tissue distribution using immunohistochemistry and immunofluorescence are now complemented with molecular imaging modalities like positron emission tomography and near-infrared fluorescence imaging to obtain more dynamic information, while methods for characterization of antibody's interaction with the target antigen as well as visualization of its cellular and intercellular behavior are still under development. Recent progress in detecting therapeutic antibodies, in particular, the development of methods suitable for illustrating the molecular dynamics, is described here.

Aggregate Science: From Structures to Properties

Molecular science entails the study of structures and properties of materials at the level of single molecules or small interacting complexes of molecules. Moving beyond single molecules and well-defined complexes, aggregates (i.e., irregular clusters of many molecules) serve as a particularly useful form of materials that often display modified or wholly new properties compared to their molecular components. Some unique structures and phenomena such as polymorphic aggregates, aggregation-induced symmetry breaking, and cluster excitons are only identified in aggregates, as a few examples of their exotic features. Here, by virtue of the flourishing research on aggregation-induced emission, the concept of "aggregate science" is put forward to fill the gaps between molecules and aggregates. Structures and properties on the aggregate scale are also systematically summarized. The structure-property relationships established for aggregates are expected to contribute to new materials and technological development. Ultimately, aggregate science may become an interdisciplinary research field and serves as a general platform for academic research.

Direct Observation of Aggregation-Induced Emission Mechanism

The mechanism of aggregation-induced emission, which overcomes the common aggregation-caused quenching problem in organic optoelectronics, is revealed by monitoring the real time structural evolution and dynamics of electronic excited state with frequency and polarization resolved ultrafast UV/IR spectroscopy and theoretical calculations. The formation of Woodward-Hoffmann cyclic intermediates upon ultraviolet excitation is observed in dilute solutions of tetraphenylethylene and its derivatives but not in their respective solid. The ultrafast cyclization provides an efficient nonradiative relaxation pathway through crossing a conical intersection. Without such a reaction mechanism, the electronic excitation is preserved in the molecular solids and the molecule fluoresces efficiently, aided by the very slow intermolecular charge and energy transfers due to the well separated molecular packing arrangement. The mechanisms can be general for tuning the properties of chromophores in different phases for various important applications.

Principles of Aggregation-Induced Emission: Design of Deactivation Pathways for Advanced AIEgens and Applications

Twenty years ago, the concept of aggregation-induced emission (AIE) was proposed, and this unique luminescent property has attracted scientific interest ever since. However, AIE denominates only the phenomenon, while the details of its underlying guiding principles remain to be elucidated. This minireview discusses the basic principles of AIE based on our previous mechanistic study of the photophysical behavior of 9,10-bis(N,N-dialkylamino)anthracene (BDAA) and the corresponding mechanistic analysis by quantum chemical calculations. BDAA comprises an anthracene core and small electron donors, which allows the quantum chemical aspects of AIE to be discussed. The key factor for AIE is the control over the non-radiative decay (deactivation) pathway, which can be visualized by considering the conical intersection (CI) on a potential energy surface. Controlling the conical intersection (CI) on the potential energy surface enables the separate formation of fluorescent (CI:high) and non-fluorescent (CI:low) molecules [control of conical intersection accessibility (CCIA)]. The novelty and originality of AIE in the field of photochemistry lies in the creation of functionality by design and in the active control over deactivation pathways. Moreover, we provide a new design strategy for AIE luminogens (AIEgens) and discuss selected examples.

Solid-State Emissive Aroyl-S,N-Ketene Acetals with Tunable Aggregation-Induced Emission Characteristics

N-Benzyl aroyl-S,N-ketene acetals can be readily synthesized by condensation of aroyl chlorides and N-benzyl 2-methyl benzothiazolium salts in good to excellent yields, yielding a library of 35 chromophores with bright solid-state emission and aggregation-induced emission characteristics. Varying the substituent from electron-donating to electron-withdrawing enables the tuning of the solid-state emission color from deep blue to red.

Spatiotemporal Dynamics of Aggregation-Induced Emission Enhancement Controlled by Optical Manipulation

We present spatiotemporal control of aggregation-induced emission enhancement (AIEE) of a protonated tetraphenylethene derivative by optical manipulation. A single submicrometer-sized aggregate is initially confined by laser irradiation when its fluorescence is hardly detectable. The continuous irradiation of the formed aggregate leads to sudden and rapid growth, resulting in bright yellow fluorescence emission. The fluorescence intensity at the peak wavelength of 540 nm is tremendously enhanced with growth, meaning that AIEE is activated by optical manipulation. Amazingly, the switching on/off of the activation of AIEE is arbitrarily controlled by alternating the laser power. This result means that optical manipulation increases the local concentration, which overcomes the electrostatic repulsion between the protonated molecules, namely, optical manipulation changes the aggregate structure. The dynamics and mechanism in AIEE controlled by optical manipulation will be discussed from the viewpoint of molecular conformation and association depending on the laser power.

A General Descriptor ΔE Enables the Quantitative Development of Luminescent Materials Based on Photoinduced Electron Transfer

Photoinduced electron transfer (PET) is one of the most important mechanisms for developing fluorescent probes and biosensors. Quantitative prediction of the quantum yields of these probes and sensors is crucial to accelerate the rational development of novel PET-based functional materials. Herein, we developed a general descriptor (ΔE) for predicting the quantum yield of PET probes, with a threshold value of ∼0.6 eV. When ΔE < ∼0.6 eV, the quantum yield is low (mostly <2%) due to the substantial activation of PET in polar environments; when ΔE > ∼0.6 eV, the quantum yield is high because of the inhibition of PET. This simple yet effective descriptor is applicable to a wide range of fluorophores, such as BODIPY, fluorescein, rhodamine, and Si-rhodamine. This ΔE descriptor enables us not only to establish new applications for existing PET probes but also to quantitatively design novel PET-based fluorophores for wash-free bioimaging and AIEgen development.

High-Performance Quinoline-Malononitrile Core as a Building Block for the Diversity-Oriented Synthesis of AIEgens

In vivo fluorescent monitoring of physiological processes with high-fidelity is essential in disease diagnosis and biological research, but faces extreme challenges due to aggregation-caused quenching (ACQ) and short-wavelength fluorescence. The development of high-performance and long-wavelength aggregation-induced emission (AIE) fluorophores is in high demand for precise optical bioimaging. The chromophore quinoline-malononitrile (QM) has recently emerged as a new class of AIE building block that possesses several notable features, such as red to near-infrared (NIR) emission, high brightness, marked photostability, and good biocompatibility. In this minireview, we summarize some recent advances of our established AIE building block of QM, focusing on the AIE mechanism, regulation of emission wavelength and morphology, the facile scale-up and fast preparation for AIE nanoparticles, as well as potential biomedical imaging applications.

Room-Temperature Phosphorescence-active Boronate Particles: Characterization and Ratiometric Afterglow-sensing Behavior by Surface Grafting of Rhodamine B

We found that boronate particles (BP), as a self-assembled system prepared by sequential dehydration of benzene-1,4-diboronic acid with pentaerythritol, showed greenish room-temperature phosphorescence (RTP). This emission was observed in both solid and dispersion state in water. To understand the RTP properties, X-ray crystallographic analysis, and density functional theory (DFT) and time-dependent DFT at M06-2X/6-31G(d,p) level were performed using 3,9-dibenzo-2,4,8,10-tetraoxa-3,9-diboraspiro[5.5]undecane (1) as a model compound. Our interest in functionalizing the RTP-active particles led us to graft Rhodamine B onto their surface. The resulting system emitted a dual afterglow via a Förster-type resonance energy transfer process from the BP in the excited triplet state to Rhodamine B acting as an acceptor fluorophore. This emission behavior was used for ratiometric afterglow sensing of water content in THF with a detection limit of 0.28 %, indicating that this study could pave the way for a new strategy for developing color-variable afterglow chemosensors for various analytes.

Mechanochromic Luminescence from Crystals Consisting of Intermolecular Hydrogen-Bonded Sheets

Introduction of functional groups that can form intermolecular hydrogen bonds into highly-emissive luminophores is a promising way to induce mechanochromic luminescence. Herein, we report that a 9,10-bis(phenylethynyl)anthracene derivative featuring two amide groups forms green-emissive crystals based on two-dimensional hydrogen-bonded molecular sheets. Mechanical grinding changed the emission from green to yellow, owing to a transition from a crystalline to an amorphous phase. Infrared spectroscopy revealed that mechanical stimuli disrupted the linear hydrogen-bonding formation. A thermal treatment recovered the original green photoluminescence.

Emissive tetraphenylethylene (TPE) derivatives in a dissolved state tightly fastened by a short oligo(ethylene glycol) chain

The requirements for strong emission from a typical AIEgen in solution were quantitatively examined both by photochemical experiments and theoretical computations.

Photopolymerization with AIE dyes for solid-state luminophores

The photoinitiating activities of MPAs/ONI were evaluated. The AIE emission of MPAs occurred during photocuring. MPAs showed potential as fluorescent molecular probes to monitor the progress of photopolymerization.

Restriction of Flip-flop Motion as a Mechanism for Aggregation-Induced Emission

Although the restriction of intramolecular motion (RIM) has been accepted as a general working mechanism for the aggregation-induced emission (AIE) phenomenon, some new mechanisms, such as suppression of Kasha's rule (SOKR), has also been proposed to explain the AIE of boron difluorohydrazone (BODIHY) derivatives. However, the understanding of the relation and difference between RIM and SOKR mechanisms is limited. To address this issue, we performed a theoretical study on the excited state decay of a series of BODIHY derivatives. Surprisingly, we found that the first excited state of BODIHY derivatives is a bright state and contradicts with the SOKR mechanism. Importantly, we proposed a new mechanism, termed as restriction of flip-flop motion, to explain the AIE of BODIHY derivatives. This mechanism involves the formation of an umbrella-like minimal energy conical intersection through flip-flop motion, which is easily accessible in low-viscosity solvents and will be restricted in high-viscosity solvents.

Recent Progress of BODIPY Dyes With Aggregation-Induced Emission

With the development of organic optoelectronic materials and bioimaging technology, to exploit organic luminescent materials with high luminescent efficiency in aggregation-state has become a research hotspot. BODIPYs have become one of the research objects of this kind of material because of their obvious advantages. This review focuses on the design and synthesis of AIE-type BODIPYs, the mechanism of AIE properties and their applications in recent years. Through classification, analysis, and summary, this review aims to explore the structure-activity relationship of AIE-type BODIPYs and to provide ideas for the further design and potential applications of AIE-active fluorescent materials.

Recent Advances in Aggregation-Induced Electrochemiluminescence

The emergence of the rising alliance between aggregation-induced emission (AIE) and electrochemiluminescence (ECL) is defined as aggregation-induced electrochemiluminescence (AIECL). The booming science of AIE has proved to be not only distinguished in luminescent materials but could also inject new possibility into ECL analysis. Especially in the aqueous phase and solid state for hydrophobic materials, AIE helps ECL circumvent the dilemma between substantial emission intensity and biocompatible media. The wide range of analytes makes ECL an overwhelmingly interesting analytical technique. Therefore, AIECL has gained potential in clinical diagnostics, environmental assays, and biomarker detections. This review will focus on introduction of the novel concept of AIECL, current applied luminophores, and related applications developed in recent years.

Spiro-Functionalized Diphenylethenes: Suppression of a Reversible Photocyclization Contributes to the Aggregation-Induced Emission Effect

Many aggregation-induced emission (AIE) materials are featured by the diphenylethene (DPE) moiety which exhibits rich photophysical and photochemical activities. The understanding of these activities behind AIE is essential to guide the design of fluorescent materials with improved performance. Herein by fusing a flexible DPE with a rigid spiro scaffold, we report a class of novel deep-blue material with solid-state fluorescent quantum yield (Φ_F) up to 99.8%. Along with the AIE phenomenon, we identified a reversible photocyclization (PC) on DPE with visible chromism, which is, on the contrary, popularized in solutions but blocked by aggregation. We studied the steric and electronic effects of structural perturbation and concluded that the PC is a key process behind the RIMs (restriction of intramolecular motions) mechanism for these materials. Mitigation of the PC leads to enhanced fluorescence in solutions and loss of the AIE characteristics.