Aggregation‐Induced Emission with Long‐Lived Room‐Temperature Phosphorescence from Methylene‐Linked Organic Donor–Acceptor Structures

Aggregation‐caused quenching (ACQ), where excited‐state and/or ground‐state electronic structures are altered to exhibit an increased proclivity for non‐radiative decay for the aggregates, is largely responsible for the lack of fluorescence and phosphorescence in molecular solids in general. Here we show that ACQ could be effectively circumvented by constructing an aromatic system with a methylene‐linker, where the system exhibits typical aggregation‐induced emission (AIE) with long‐lived room‐temperature phosphorescence, since the tetrahedral structure in the solid state may significantly reduce strong intermolecular interactions contributing to ACQ.     

Quantum chemical insights into the aggregation induced emission phenomena: A QM/MM study for pyrazine derivatives

There have been intensive studies on the newly discovered phenomena called aggregation induced emission (AIE), in contrast to the conventional aggregation quenching. Through combined quantum mechanics and molecular mechanics computations, we have investigated the aggregation effects on the excited state decays, both via radiative and nonradiative routes, for pyrazine derivatives 2,3‐dicyano‐5,6‐diphenylpyrazine ( DCDPP ) and 2,3‐dicyanopyrazino phenanthrene ( DCPP ) in condensed phase. We show that for DCDPP there appear AIE for all the temperature, because the phenyl ring torsional motions in gas phase can efficiently dissipate the electronic excited state energy, and get hindered in aggregate; while for its “locked”‐phenyl counterpart, DCPP , theoretical calculation can only give the normal aggregation quenching. These first‐principles based findings are consistent with recent experiment. The primary origin of the exotic AIE phenomena is due to the nonradiative decay effects. This is the first time that AIE is understood based on theoretical chemistry calculations for aggregates, which helps to resolve the present disputes over the mechanism. © 2012 Wiley Periodicals, Inc.     

Aggregation‐Induced Emission Rotors: Rational Design and Tunable Stimuli Response

A novel molecular design strategy is provided to rationally tune the stimuli response of luminescent materials with aggregation‐induced emission (AIE) characteristics. A series of new AIE‐active molecules (AIE rotors) are prepared by covalently linking different numbers of tetraphenylethene moieties together. Upon gradually increasing the number of rotatable phenyl rings, the sensitivity of the response of the AIE rotors to viscosity and temperature is significantly enhanced. Although the molecular size is further enlarged, the performance is only slightly improved due to slightly increased effective rotors, but with largely increased rotational barriers. Such molecular engineering and experimental results offer more in‐depth insight into the AIE mechanism, namely, restriction of intramolecular rotations. Notably, through this rational design, the AIE rotor with the largest molecular size turns out to be the most viscosensitive luminogen with a viscosity factor of up to 0.98.     

Theoretical insight into the aggregation induced emission phenomena of diphenyldibenzofulvene: a nonadiabatic molecular dynamics study

The diphenyldibenzofulvene (DPDBF) molecule appears in two forms: ring open and ring closed. The former fluoresces weakly in solution, but it becomes strongly emissive in the solid phase, exhibiting an exotic aggregation-induced emission phenomenon. The latter presents a normal aggregation quenching phenomenon, as is expected. We implement nonadiabatic molecular dynamics based on the combination of time-dependent Kohn-Sham (TDKS) and density functional tight binding (DFTB) methods with Tully's fewest switches surface hopping algorithm to investigate the excited state nonradiative decay processes. From the analysis of the nonadiabatic coupling vectors, it is found that the low frequency twisting motion in the ring open DPDBF couples strongly with the electronic excitation and dissipates the energy efficiently. While in the closed form, such motion is blocked by a chemical bond. This leads to the nonradiative decay rate for the open form (1.4 ps) becoming much faster than the closed form (24.5 ps). It is expected that, in the solid state, the low frequency motion of the open form will be hindered and the energy dissipation pathway by nonradiative decay will be slowed, presenting a remarkable aggregation enhanced emission phenomenon.     

DFT/TDDFT insight into the impact of ring size of the NHC chelating unit of high effective phosphorescent Platinum (II) complexes

Uncovering the photodeactivation mechanisms of unique N‐heterocyclic carbene (NHC)‐based transition metal complexes is favorable for designing more high‐efficiency phosphorescent materials. In this work, four bidentate platinum (II) complexes with NHC‐chelate are investigated by the density functional theory (DFT) and time‐dependent density functional theory (TDDFT) to probe into how the ring size of NHC‐chelate unit influences on electronic structures and the phosphorescent properties. To illustrate the photodeactivation mechanisms clearly, three significant photodeactivation processes (radiative decay process, temperature‐independent and temperature‐dependent nonradiative decay processes) were taken into consideration. We stated that radiative decay rate constants k_r slightly increased with declined number of NHC‐chelate ring, owing to the gradually larger SOC matrix elements between the T₁ state and S_n states. Combining the temperature‐independent with temperature‐dependent nonradiative decay processes, the nonradiative decay rate k_nr is Pt‐4 (five‐membered) < Pt‐3 (six‐membered) < Pt‐2 (seven‐membered) < Pt‐1 (eight‐membered). The calculated results testify that the decrease of size of the NHC chelating unit is a reliable insurance to improve the quantum yield. The designed complex Pt‐4 with five‐membered NHC‐ring can serve as a highly efficient phosphorescent material in the future. The results indicated controlling the ring size of NHC‐chelate is a feasible method to tune phosphorescence properties of Pt (II) complexes.     

Bridged Stilbenes: AIEgens Designed via a Simple Strategy to Control the Non‐radiative Decay Pathway

To broaden the application of aggregation‐induced emission (AIE) luminogens (AIEgens), the design of novel small‐molecular dyes that exhibit high fluorescence quantum yield (Φ_fl) in the solid state is required. Considering that the mechanism of AIE can be rationalized based on steric avoidance of non‐radiative decay pathways, a series of bridged stilbenes was designed, and their non‐radiative decay pathways were investigated theoretically. Bridged stilbenes with short alkyl chains exhibited a strong fluorescence emission in solution and in the solid state, while bridged stilbenes with long alkyl chains exhibited AIE. Based on this theoretical prediction, we developed the bridged stilbenes BPST[7] and DPB[7], which demonstrate excellent AIE behavior.     

Synthesis and Fluorescent Properties of Tetra‐Biphenyl N‐Substituted Phthalimides

A series of tetra(biphenyl‐4‐yl)phthalimide (TBPPI ) derivatives with different N‐substituents (n‐butyl, phenyl, p‐methyl phenyl, and p‐acetyl phenyl moieties for compounds 7 – 10 , respectively) were prepared to examine their fluorescent behavior under various conditions. The chemical structure of compound 7 has been successfully confirmed by single crystal X‐ray diffraction analysis. The photoluminescence (PL ) spectra in different ratios of CH ₂Cl ₂/EtOH mixture solutions revealed that compounds 7 and 8 exhibited both aggregation‐induced emission (AIE ) and aggregation‐caused quenching (ACQ ) behaviors, while compounds 9 and 10 displayed AIE and aggregation‐induced emission enhancement (AIEE ) properties, respectively.     

聚集诱导发光现象的理论研究

本文系统介绍了本课题组发展的分子辐射跃迁和无辐射跃迁速率常数的热振动关联函数理论方法的最新进展及其在聚集诱导发光领域的典型应用. 基于第一性原理计算, 定量考察了位阻、温度、聚集等因素对分子体系发光性质的影响. 从微观角度给出了分子聚集诱导发光机理： 分子激发态的无辐射能量衰减通道主要是对应于低频模式的芳香环扭转和高频模式的碳碳伸缩振动. 当位阻增加、温度降低或者分子聚集时, 芳香环的转动受限, 无辐射能量衰减通道被抑制, 导致无辐射跃迁速率常数降低, 而其对辐射跃迁速率常数影响不大, 从而提高分子的荧光量子产率, 荧光增强.     

Self‐Assembly of Highly Stable Zirconium(IV) Coordination Cages with Aggregation Induced Emission Molecular Rotors for Live‐Cell Imaging

The self‐assembly of highly stable zirconium(IV)‐based coordination cages with aggregation induced emission (AIE) molecular rotors for in vitro bio‐imaging is reported. The two coordination cages, NUS‐100 and NUS‐101, are assembled from the highly stable trinuclear zirconium vertices and two flexible carboxyl‐decorated tetraphenylethylene (TPE) spacers. Extensive experimental and theoretical results show that the emissive intensity of the coordination cages can be controlled by restricting the dynamics of AIE‐active molecular rotors though multiple external stimuli. Because the two coordination cages have excellent chemical stability in aqueous solutions (pH stability: 2–10) and impressive AIE characteristics contributed by the molecular rotors, they can be employed as novel biological fluorescent probes for in vitro live‐cell imaging.     

o‐Carborane‐Based Anthracene: A Variety of Emission Behaviors

An o‐carborane‐based anthracene was synthesized, and single crystals, with incorporated solvent molecules, were obtained from the CHCl₃, CH₂Cl₂, and C₆H₆ solutions. The anthracene ring in the crystal is highly distorted by the formation of a π‐stacked dimer between the anthracene units. The crystals exhibited a variety of emission behaviors such as aggregation‐induced emission (AIE), crystallization‐induced emission (CIE), aggregation‐caused quenching (ACQ), and multichromism.     

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.     

Hexaphenylbenzene‐Based, π‐Conjugated Snowflake‐Shaped Luminophores: Tunable Aggregation‐Induced Emission Effect and Piezofluorochromism

In this work, two rigid, multiple tetraphenylethene (TPE)‐substituted, π‐conjugated, snowflake‐shaped luminophores BT and BPT were facilely synthesized by using a 6‐fold Suzuki coupling reaction. These molecules are constructed based on the nonplanar structure of propeller‐shaped hexaphenylbenzene (HPB) or benzene as core groups and TPE as end groups. As a result, they reserve the intrinsic aggregation‐induced emission (AIE) property of the TPE moiety. Meanwhile, both fluorescence quantum yield and piezochromic behavior in the solid state can be tuned or switched by inserting the phenyl bridges through changing the twisting conformation. The more extended structure BPT showed a much stronger AIE effect and higher Φ_F,f in the solid state in comparison with that of BT. Furthermore, an excellent optical waveguide application of these molecules was achieved. However, the revisable piezofluorochromic behavior has only appeared when BT was ground using a pestle and treated with solvent.     

Self‐Assembly and Aggregate‐Induced Enhanced Emission of Amphiphilic Fluorescence Dyes in Water and in the Solid State

1‐Cyano‐1,2‐bis(biphenyl)ethene (CNBE) derivatives with a hexa(ethylene glycol) group as an amphiphilic side chain were synthesized and the self‐assembling character and fluorescence behavior were investigated. The amphiphilic derivatives showed aggregate‐induced enhanced emission (AIEE) in water and in the solid state. The fluorescence quantum yield increased as the rigidity of the aggregates increased (i.e., in ethyl acetate<in water<in the solid state). As determined from measurements of fluorescence spectra, fluorescence quantum yields, and fluorescence lifetimes, a key factor for the enhanced emission is suppression of the nonradiative decay process arising from restricted molecular motion. Additionally, the difference in the emission rate constant is not negligible and can be used to interpret the difference in fluorescence quantum yield in water and in the solid state.     

Towards meso‐Ester BODIPYs with Aggregation‐Induced Emission Properties: The Effect of Substitution Positions

Three meso‐ester boron dipyrromethene (BODIPY) dyes have been synthesized and functionalized with aggregation‐induced emission (AIE)‐active tetraphenylethene or triphenylethene moieties. It was found that functionalizing at the different positions of the BODIPY core resulted in the final dye having different emission properties in response to aggregation: from aggregation‐induced quenching (ACQ) to being AIE active. X‐ray crystallographic analysis was thus performed to provide an explanation for these differences.     

Fluorescent Bioprobes: Structural Matching in the Docking Processes of Aggregation‐Induced Emission Fluorogens on DNA Surfaces

Whereas most conventional DNA probes are flat disklike aromatic molecules, we explored the possibility of developing quadruplex sensors with nonplanar conformations, in particular, the propeller‐shaped tetraphenylethene (TPE) salts with aggregation‐induced emission (AIE) characteristics. 1,1,2,2‐Tetrakis[4‐(2‐triethylammonioethoxy)phenyl]ethene tetrabromide (TPE‐ 1 ) was found to show a specific affinity to a particular quadruplex structure formed by a human telomeric DNA strand in the presence of K⁺ ions, as indicated by the enhanced and bathochromically shifted emission of the AIE fluorogen. Steady‐state and time‐resolved spectral analyses revealed that the specific binding stems from a structural matching between the AIE fluorogen and the DNA strand in the folding process. Computational modeling suggests that the AIE molecule docks on the grooves of the quadruplex surface with the aid of electrostatic attraction. The binding preference of TPE‐ 1 enables it to serve as a bioprobe for direct monitoring of cation‐driven conformational transitions between the quadruplexes of various conformations, a job unachievable by the traditional G‐quadruplex biosensors. Methyl thiazolyl tetrazolium (MTT) assays reveal that TPE‐ 1 is cytocompatible, posing no toxicity to living cells.     

Strong Solid‐state Luminescence Enhancement in Supramolecular Assemblies of Polyoxometalate and “Aggregation‐Induced Emission”‐active Phospholium

Two new supramolecular fluorescent hybrid materials, combining for the first time [M₆O₁₉]^2? (M=Mo, W) polyoxometalates (POMs) and aggregation‐induced emission (AIE)‐active 1‐methyl‐1,2,3,4,5‐pentaphenyl‐phospholium ( 1⁺ ), were successfully synthesized. This novel molecular self‐assembling strategy allows designing efficient solid‐state emitters, such as (1)₂[W₆O₁₉] , by directing favorably the balance between the AIE and aggregation‐caused quenching (ACQ) effects using both anion‐π⁺ and H‐bonding interactions in the solid state. Combined single‐crystal X‐ray diffraction, Raman, UV‐vis and photoluminescence analyses highlighted that the nucleophilic oxygen‐enriched POM surfaces strengthened the rigidity of the phospholium via strong C?H???O contacts, thereby exalting its solid‐state luminescence. Besides, the bulky POM anions prevented π–π stacking interactions between the luminophores, blocking detrimental self‐quenching effects.     

AIE Polymers: Synthesis,Properties, and Biological Applications

Aggregation‐caused quenching (ACQ) is a general phenomenon that is faced by traditional fluorescent polymers. Aggregation‐induced emission (AIE) is exactly opposite to ACQ. AIE molecules are almost nonemissive in their molecularly dissolved state, but they can be induced to show high fluorescence in the aggregated or solid state. Incorporation of AIE phenomenon into polymer design has yielded various polymers with AIE characteristics. In this review, the recent progress of AIE polymers for biological applications is summarized.

     

Photophysics of Fluorescent Contact Sensors Based on the Dicyanodihydrofuran Motif

Fluorescent molecular rotors have been used for measurements of local mobility on molecular length scales, for example to determine viscosity, and for the visualization of contact between two surfaces. In the present work, we deepen our insight into the excited-state deactivation kinetics and mechanics of dicyanodihydrofuran-based molecular rotors. We extend the scope of the use of this class of rotors for contact sensing with a red-shifted member of the family. This allows for contact detection with a range of excitation wavelengths up to ∼600 nm. Steady-state fluorescence shows that the fluorescence quantum yield of these rotors depends not only on the rigidity of their environment, but – under certain conditions – also on its polarity. While excited state decay via rotation about the exocyclic double bond is rapid in nonpolar solvents and twisting of a single bond allows for fast decay in polar solvents, the barriers for both processes are significant in solvents of intermediate polarity. This effect may also occur in other molecular rotors, and it should be considered when applying such molecules as local mobility probes.     

Planar AIEgens with Enhanced Solid‐State Luminescence and ROS Generation for Multidrug‐Resistant Bacteria Treatment

Planar luminogens have encountered difficulties in overcoming intrinsic aggregation‐caused emission quenching by intermolecular π‐π stacking interactions. Although excited‐state double‐bond reorganization (ESDBR) can guide us on designing planar aggregation‐induced emission (AIE) luminogens (AIEgens), its mechanism has yet been elucidated. Major challenges in the field include methods to efficiently restrict ESDBR and enhance AIE performance without using bulky substituents (e.g., tetraphenylethylene and triphenylamine). In this study, we rationally developed fluoro‐substituent AIEgens with stronger intermolecular H‐bonding interaction for restricted molecular motions and increased crystal density, leading to decreased nonradiative decay rate by one order of magnitude. The adjusted ESDBR properties also show a corresponding response to variation in viscosity. Furthermore, their aggregation‐induced reactive oxygen species (ROS) generations have been discovered. The application of such planar AIEgen in treating multidrug‐resistant bacteria has been demonstrated in a mouse model. The relationship between ROS generation and distinct E/Z‐configurational stacking behaviors have been further understood, providing a design principle for synthesizing planar AIEgen‐based photosensitizers.