1,5-Disila-s-indacene-based emitters: a molecular design towards efficient blue OLEDs

We designed three 1,5-dihydro-1,5-disila-s-indacene-based emitters with blue/deep-blue fluorescence emissions by introducing trifluoromethyl phenyl units into the π-extended bisilole fluorophores. When these silole derivatives were applied in electroluminescent devices, the 2, 3, 6, and 7 tetra-substituted 1,5-disila-s-indacene-based device gave the hitherto highest maximum external quantum efficiency (5.43%) for a silole-based blue OLED.

Designing High-Sensitivity Mechanochromic Luminescent Materials Through Friction-Induced Crystallization Strategy

Despite recent significant breakthroughs in novel mechanochromic luminescent (ML) materials, developing a high-sensitivity ML material is still challengeable. Herein, a "friction-induced crystallization" strategy is proposed to realize highly sensitive transformations of luminescent signal, through an integration of polymeric chains and an aggregation-sensitive luminescent core, which act as mechanical sensors and fluochromic actuators, respectively. The coupling of these two components enables the material to crystallize in response to shear friction, thereby exhibiting blue-shift fluorescence due to a more restricted relaxation pathway. This study underscores a high-sensitivity ML material based on the precise regulation of molecular-scale motions, and also expands the scope and potential of ML materials toward user-friendly, interactive wearable devices.

Pressure effects on both fluorescent emission and charge transport properties of organic semiconductors: a computational study

External pressure can regulate the photophysical property and charge transport performance of organic semiconductors, however, the underlying mechanism at the microscopic level is still elusive. Using thermal vibrational correlation function coupled quantum mechanics/molecular mechanics and full quantum charge transfer rate theory, we systematically explore the influence of pressure on fluorescence emission and charge transport behaviours of representative cyclooctatetrathiophene (COTh). It is found that, upon pressurization, the intramolecular configurations of COTh became more twisted, leading to the blue-shifted emission. The fluorescence quantum efficiency (FQE) of COTh crystals decreases monotonically in a wide pressure range of 0-4.38 GPa, because the increase of intermolecular electronic energy transfer rate constant (keet) is larger than the decrease of internal conversion rate constant (kic), and the variation of keet is dominant. The decrease in kic is attributed to the decreasing reorganization energy, reflecting the suppression of the low-frequency flipping vibrations of four thiophene rings and the high-frequency stretching vibrations of central cyclooctatetraene, while the keet increase is due to the simultaneous increase in exciton coupling and spectra overlap. Moreover, we predicted that the hole mobility of COTh increases monotonically by nearly an order of magnitude from 0.39 to 3.00 cm2 V-1 s-1 upon compression, because of the increase in transfer integral and the decrease of charged reorganization energy. Furthermore, its hole mobility exhibits obvious anisotropy. Our work systematically builds the external pressure, molecular packing, luminescence and transport properties relationships of organic semiconductors and provides theoretical guidance for the rational design of pressure responsive organic semiconductors with excellent photoelectric performance.

Turning enol* emission of SBOH via restricting twisted intramolecular charge transfer behavior by pressure

Stimuli-responsive luminogens with aggregation-induced emission and excited state intramolecular proton transfer (ESIPT) properties have applications in storage devices, anti-counterfeiting, imaging, and sensors. Nevertheless, group rotation appears in twisted intramolecular charge transfer (TICT) state, resulting in decreased fluorescence intensity. Inhibiting TICT remains a challenge based on their intrinsic molecular configuration. Herein, we present a simple facile pressure-induced method to restrict the TICT behavior. Steady-state spectroscopy measurement shows that fluorescence enhancement and color shifts can be achieved under high pressure. Combined with in situ high-pressure ultrafast spectroscopy and theoretical calculations, the TICT behavior was restricted in two aspects. The ESIPT process was damaged, hence more particles stored in the E* state, and transferred to the TICT state hardly. Also, the rotation of (E)-dimethyl5-((4-(diethylamino)-2-hydroxybenzylidene)amino)isophthalate (SBOH) was restricted, significantly increasing the fluorescence intensity. This approach provides a new strategy for the development of stimulus-responsive materials.

Fluorescence-based monitoring of the pressure-induced aggregation microenvironment evolution for an AIEgen under multiple excitation channels

The development of organic solid-state luminescent materials, especially those sensitive to aggregation microenvironment, is critical for their applications in devices such as pressure-sensitive elements, sensors, and photoelectric devices. However, it still faces certain challenges and a deep understanding of the corresponding internal mechanisms is required. Here, we put forward an unconventional strategy to explore the pressure-induced evolution of the aggregation microenvironment, involving changes in molecular conformation, stacking mode, and intermolecular interaction, by monitoring the emission under multiple excitation channels based on a luminogen with aggregation-induced emission characteristics of di(p-methoxylphenyl)dibenzofulvene. Under three excitation wavelengths, the distinct emission behaviors have been interestingly observed to reveal the pressure-induced structural evolution, well consistent with the results from ultraviolet-visible absorption, high-pressure angle-dispersive X-ray diffraction, and infrared studies, which have rarely been reported before. This finding provides important insights into the design of organic solid luminescent materials and greatly promotes the development of stimulus-responsive luminescent materials.

The preparation of novel AIE fluorescent microspheres by dispersion polymerization

An approach to prepare monodisperse polystyrene microspheres with aggregation-induced emission (AIE) characteristics has been developed which shows promising applications in fluorescence-encoding. The micron-sized, monodisperse polystyrene microspheres with AIE molecules were perfectly synthesized by two-stage dispersion polymerization. Fluorescent AIE monomer was synthesized by Suzuki reaction, confirmed by nuclear magnetic resonance (NMR). These AIE fluorogens (AIEgens) exhibited unique properties such as bright green emission in solid state and increased emission in tetrahydrofuran (THF) solution with the increase of water content. The influence of the AIE molecules concentration to microspheres synthesis was well investigated. The reaction conditions were optimized to obtain the functional polystyrene microspheres with a size distribution around 3%. The novel microspheres were characterized by scanning electron microscopy (SEM), confocal fluorescence microscope and flow cytometry. According to these results, two-stage dispersion polymerization was proved to be an efficient pathway for the preparation of AIE fluorescent and functionalized microspheres, which could be used in many biomedical industries.

Origin of Nonmonotonical Variation of Luminescence Efficiency under Pressure in Organic Molecule

The nonmonotonical variation of luminescence efficiency under extra pressure occurs frequently in organic molecules; however, the mechanism behind this is still elusive. Using a theoretical protocol combining thermal vibration function rate formalism coupled quantum mechanics/molecular mechanics models, we explored the relationship between extra pressure, molecular packing, and fluorescent quantum efficiency (FQE) of the representative 1,2,3,4-tetraphenyl-1,3-cyclopentadiene (TPC). It is found that the first increase and then decrease of FQE in TPC crystalline aggregates upon pressurization is cooperatively attributed to the continuous reduction of the radiative decay rate constant and nonmonotonical change of the nonradiative decay rate constant (k_ic). The initial decrease of k_ic originates from the effective suppression of electron-vibration coupling and the Duschinsky rotation effect by extra pressure, whereas the following increase of k_ic comes from the surge of nonadiabatic electronic coupling and the reduction of adiabatic excitation energy upon further compression. This study can provide a theoretical basis for the rational design and performance control of the piezochromic luminescent materials.

Understanding the solid state luminescence and piezochromic properties in polymorphs of an anthracene derivative

Luminescent molecular crystals have gained significant research interest for optoelectronic applications. However, fully understanding their structural and electronic relationships in the condensed phase and under external stimuli remains a significant challenge. Here, piezochromism in the molecular crystal 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene (BP4VA) is studied using a combination of density functional theory (DFT) and time-dependent TD-DFT. We investigate the effects that molecular packing and geometry have on the electronic and phonon structure and the excited state properties in this archetypal system. We find that the luminescence properties are red-shifted with the transition from a herringbone to a sheet packing arrangement. An almost continuous red-shift in the band gap is found with the application of an external pressure through the enhancement of π-π and CH-π interactions, and is a mechanism in fine tuning an emissive response. The analysis of the phonon structure of the molecular crystal suggests restriction of motion in the herringbone packing arrangement, with motion restricted at higher pressure. This is supported by the Huang-Rhys factors which show a decrease in the reorganisation energy with the application of pressure. Ultimately, a balance between the decrease in reorganisation energies and the increase in exciton coupling will determine whether nonradiative decay is enhanced or decreased with the increase in pressure in these systems.

Ultrahigh Aggregation Induced Emission Efficiency in Multitwist-Based Luminogens under High Pressure

Increasing aggregation induced emission (AIE) efficiency is of fundamental interest as it directly reflects performance of multitwist-based luminogens in bioimaging and in the photoelectric device field. However, an effective and convenient methodology to increase AIE efficiency significantly remains a challenge. Here, we present a general strategy to increase AIE efficiency of multitwist-based luminogens by pressure, resulting in a 120.1-fold enhancement of the AIE intensity of tris[4-(diethylamino)phenyl]amine (TDAPA) under high pressure compared to that of the traditional method. AIE efficiency of TDAPA increases from 0.5% to 46.1% during compression. Experimental and theoretical investigations reveal that the AIE efficiency enhancement originates from intramolecular vibration and the twisted intramolecular charge transfer are suppressed under high pressure. High AIE efficiency under high pressure provides an important inspiration for improving performance of multitwist-based luminogens in the lighting and biomedical fields.

Ultrafast Aggregation-Induced Tunable Emission Enhancement in a Benzothiadiazole-Based Fluorescent Metal-Organic Framework Linker

Aggregation-induced emission enhancement (AIEE) is a process recently exploited in solid-state materials and organic luminophores, and it is explained by tight-molecular packaging. However, solution-phase AIEE and its formation mechanism have not been widely explored. This work investigated AIEE phenomena in two donor-acceptor-donor-type benzodiazole-based molecules (the organic building block in metal-organic frameworks) with an acetylene and phenyl π-conjugated backbone tapered with a carboxylic acid group at either end. This was done using time-resolved electronic and vibrational spectroscopy in conjunction with time-dependent density functional theory (TD-DFT) calculations. Fluorescence up-conversion spectroscopy and time-correlated single-photon counting conclusively showed an intramolecular charge transfer-driven aggregate emission enhancement. This is shown by a red spectral shift of the emission spectra as well as an increase in the fluorescence lifetime from 746 ps at 1.0 × 10^-11 to 2.48 ns at 2.0 × 10^-3 M. The TD-DFT calculations showed that a restricted intramolecular rotation mechanism is responsible for the enhanced emission. The femtosecond infrared (IR) transient absorption results directly revealed the structural dynamics of aggregate formation, as evident from the evolution of the C≡C vibrational marker mode of the acetylene unit upon photoexcitation. Moreover, the IR data clearly indicated that the aggregation process occurred over a time scale of 10 ps, which is consistent with the fluorescence up-conversion results. Interestingly, time-resolved results and DFT calculations clearly demonstrated that both acetylene bonds and the sulfur atom are the key requirements to achieve such a controllable aggregation-induced fluorescence enhancement. The finding of the work not only shows how slight changes in the chemical structure of fluorescent chromophores could make a tremendous change in their optical behavior but also prompts a surge of research into a profound understanding of the mechanistic origins of this phenomenon. This may lead to the discovery of new chemical strategies that aim to synthesize novel chromophores with excellent optical properties for light-harvesting applications.

Microcrystal Electron Diffraction Elucidates Water-Specific Polymorphism-Induced Emission Enhancement of Bis-arylacylhydrazone

Aggregation-induced emission (AIE) phenomena have gained intense interest over the last decades because of its importance in solid-state emission. However, the elucidation of a working mechanism is difficult owing to the limited characterization methods on solid-state molecules, further complicated if dynamic structural changes occur. Here, a series of bis-arylacylhydrazones (BAHs) were synthesized, for which their AIE properties are only turned on by the reversible adsorption of water molecules. We used microcrystal electron diffraction (MicroED) to determine the molecular structures of two BAHs directly from bulk powders (without attempting to grow crystals) prepared in the absence or presence of water adsorption. This study reveals the unambiguous characterization of the dependence of crystal packing on the specific cocrystallization with hydrates. The structural analysis demonstrates that water molecules form strong hydrogen bonds with three neighboring BAH-1, resulting in the almost complete planarization and restriction of the intramolecular rotation of the molecule. MicroED plays an important role in providing a decisive clue for the reversible polymorphism changes induced by the adsorption of water molecules, regulating emissive properties.

Luminogens for Aggregation-Induced Emission via Titanium-Mediated Double Nucleophilic Addition to 2,5-Dialkynylpyridines: Formation and Transformation of the Emitting Aggregates

New luminogens for aggregation-induced emission (AIE), which are characterized by a branched cross-conjugated 2,6-bis(1,2,2-triarylvinyl)pyridine motif, have been synthesized exploiting the one-pot Ti-mediated tetraarylation of 2,6-bis(arylethynyl)pyridines. Thin layer solid-state emitters were prepared by spin-coating of the luminogens, while AIE-colloidal dispersions were investigated in terms of optical density and scattering behaviour. This has given insight into particle size distributions, time evolution of the aggregation and the influence of different functionalization patterns on the luminescence of molecular aggregates. In particular, a combination of extinction spectroscopy and dynamic light scattering is being proposed as a powerful method for investigating the dynamic aggregation process in AIE-type colloids.

Mechanism of Different Piezoresponsive Luminescence of 2,3,4,5-Tetraphenylthiophene and 2,3,4,5-Tetraphenylfuran: A Strategy for Designing Pressure-Induced Emission Enhancement Materials

Mechanoresponsive luminescent materials have attracted widespread attention for their potential applications, especially for these behaving pressure-induced emission enhancement (PIEE). Designing and seeking systems with high-efficiency PIEE are desirable and crucial for material science. Here, the mechanisms of different piezoresponsive luminescence of 2,3,4,5-tetraphenylthiophene (TPT) and 2,3,4,5-tetraphenylfuran (TPF) crystals are explored. The experimental results combined with density functional theory (DFT) theory calculation indicate that the PIEE phenomenon is possibly exhibited in V-shape arrangement for the reason of the weak π-π interactions. This study not only gains deep insight into the relationship between optical properties and structural evolution but also puts forward a strategy for designing PIEE materials from the point of molecular arrangement.

Aggregation-tuned dual emission of silole derivatives: synthesis, crystal structure, and photophysical properties

Novel silole derivatives are highly emissive in the microcrystalline state and the dual emission can be tuned by the aggregation states.

Pressure-Induced Emission Enhancement and Multicolor Emission for 1,2,3,4-Tetraphenyl-1,3-cyclopentadiene: Controlled Structure Evolution

Mechanoresponsive luminescent (MRL) materials have attracted considerable attention because of their potential applications in mechanical sensors, memory chips, and security inks; MRL materials possessing high efficiency and multicolor emission qualities are especially interesting. In this Letter, we found 1,2,3,4-tetraphenyl-1,3-cyclopentadiene (TPC) crystal exhibited both pressure-induced emission enhancement (PIEE) and multicolor behavior. In addition, infrared spectroscopy analysis indicated that the ring-opening reaction of the phenyl ring occurred when pressure was beyond 24.7 GPa. The reaction was promoted from 24.7 to 35.9 GPa, which resulted in the redder irreversible color change for the sample released from 35.9 GPa than from 24.7 GPa. The results regarding the mechanoresponsive behavior of TPC offered a deep insight into PIEE and multicolor properties from the structural point of view and inspired the idea of capturing different colors by hydrostatic pressure, which will facilitate the design of and search for high-performance MRL materials.

Converse transitions between the micelles and the vesicles of pyrrolidone-based AIE amphiphilic copolymers in polar and apolar solvents

Herein, a new family of aggregation-induced emission (AIE) amphiphilic copolymers, named poly(N-(2-methacryloyloxyethyl)pyrrolidone)-b-poly(lauryl methacrylate-co-1-ethenyl-4-(1,2,2-triphenylethenyl)benzene), PNMP_x-b-P(LMA_y-co-TPE_z), was developed by the reversible addition–fragmentation chain transfer (RAFT) polymerization method. The polymerization degree x of the NMP segment was kept constant at 35, whereas that of the LMA segment ranged from 9 to 55 with the polymerization degree ratio y/z of the LMA and TPE segments being around 9. As a result, the PNMP_x-b-P(LMA_y-co-TPE_z) copolymer gradually transformed from being water soluble to oil soluble with an increase in the length of the P(LMA_y-co-TPE_z) segment. Moreover, these copolymers could form self-organized normal and reverse assemblies in both water and n-dodecane. Various morphologies, including spherical micelles, worm-like micelles and vesicles, were confirmed by the transmission electron microscopy (TEM) observation. Specifically, the micelle-to-vesicle transition via worm-like micelles occurred in the aqueous solution upon increasing the length of the P(LMA_y-co-TPE_z) segment, whereas the reverse transition occurred in n-dodecane. Because of the presence of the AIE-active TPE segment, both the aqueous and the n-dodecane solutions of PNMP_x-b-P(LMA_y-co-TPE_z) were highly luminescent, and their fluorescence quantum yields significantly depended on the polarity of the solvent and the morphology of the assemblies. Due to the strong luminescence properties of PNMP_x-b-P(LMA_y-co-TPE_z) assemblies, these AIE-active amphiphilic copolymers acted as excellent bioimaging probes with high efficiency.

A series of AIE-active amphiphilic copolymers, PNMP_x-b-P(LMA_y-co-TPE_z), were developed as bioimaging probes. Converse transitions from spherical micelles to vesicles via wormlike micelles of them in water and n-dodecane were happened, respectively.

Facile synthesis of AIEgens with wide color tunability for cellular imaging and therapy

Luminogens with aggregation-induced emission (AIE) characteristics are nowadays undergoing explosive development in the fields of imaging, process visualization, diagnosis and therapy. However, exploration of an AIE luminogen (AIEgen) system allowing for extremely wide color tunability remains challenging. In this contribution, the facile synthesis of triphenylamine (TPA)-thiophene building block-based AIEgens having tunable maximum emission wavelengths covering violet, blue, green, yellow, orange, red, deep red and NIR regions is reported. The obtained AIEgens can be utilized as extraordinary fluorescent probes for lipid droplet (LD)-specific cell imaging and cell fusion assessment, showing excellent image contrast to the cell background and high photostability, as well as satisfactory visualization outcomes. Interestingly, quantitative evaluation of the phototherapy effect demonstrates that one of these presented AIEgens, namely TTNIR, performs well as a photosensitizer for photodynamic ablation of cancer cells upon white light irradiation. This study thus provides useful insights into rational design of fluorescence systems for widely tuning emission colors with high brightness, and remarkably extends the applications of AIEgens.

Mechano-fluorochromic behavior of AEE polyurethane films and their high sensitivity to halogen acid gas

Three polyurethanes with different contents of tetraaryl-buta-1,3-diene derivatives in the soft segment (STMPU-25/STMPU-50/STMPU-75) have been synthesized and found to present aggregation-enhanced emission features. The fluorescence intensity of polymer films was greatly enhanced with increasing tensile stress. Also, polyurethanes with higher aggregation-induced emission fluorogen content had stronger mechano-fluorochromic behavior in the same tension state. Moreover, the resulting polyurethane films possessed high sensitivity for halogen acid gas, suggesting their potential applications in environmental monitoring fields.

(a) Polyurethanes with higher content of TABDAA2 presented better positive correlation between fluorescence and strain. (b) Sensing mechanism of STMPU-50 for detection of HCl.

Journey of Aggregation-Induced Emission Research

Highly efficient luminescent materials in solid states are promising candidates for the development of organic optoelectrical materials and devices and chemical and biological sensors. Aggregation-induced emission (AIE), a novel photophyscial phenomena coined in 2001 where the aggregate formation enhances the light emission, has drawn great attention because it provides a fantastic platform for the development of these useful luminescent materials. After 17 years of AIE research, diverse AIE luminogens with tunable color and high quantum yields have been explored, which finds diverse applications from optics and electronics to energy and bioscience. Most importantly, the concept of AIE has gradually changed people's thinking way about the aggregation of luminogen and put forth a revolution of luminogen research both conceptually and technically. This perspective revisits our journey of AIE research, discusses our current understanding of the AIE mechanism, debates current challenges, and looks for the potential breakthroughs in this exciting research area.

Pressure-Induced Emission Enhancement of Carbazole: The Restriction of Intramolecular Vibration

Pressure-induced emission enhancement (PIEE), a novel phenomenon in the enhancement of the solid-state emission efficiency of fluorophores, has been arousing wide attention in recent years. However, research on PIEE is still in the early stage. To further pursue more enhanced efficiency, discovering and designing more PIEE systems would be urgently desirable and of great importance. In this Letter, we found that carbazole presented a conspicuous emission enhancement under high pressure up to 1.0 GPa. In situ high-pressure infrared spectroscopy and angle-dispersive X-ray diffraction analysis combined with Hirshfeld surface theory calculation indicated that the PIEE of carbazole was attributed to the decrease of the nonradiation vibration process. This phenomenon mainly resulted from restriction of the N-H stretching vibration by increased N-H···π interactions under high pressure. Our study puts forward a mechanism of PIEE related to the restriction of intramolecular vibration, which provided deep insight into the essential role of intermolecular interaction in fluorescence emission properties.

Aggregation induced emission: Concluding Remarks

This article presents the Closing Remarks of the Faraday Discussion on aggregation induced emission (AIE) held in Guangzhou, China in November 2016. The history of the AIE phenomenon is summarized, from its discovery and mechanistic studies to real-life applications in optoelectronics, environmental monitoring and biomedical research. The paper concludes with comments on the future perspectives of the field.

Aggregation-induced emission in precursors to porous molecular crystals

Trigonal fluorinated pyrazoles assemble into porous molecular crystals and show solid-state fluorescence. However, their emission behavior in solution is dramatically different.

Blue-shifted aggregation-induced emission of siloles by simple structural modification and their application as nitro explosive chemosensors

By substituting methyl groups at ortho positions of peripheral tetraphenyl rings on the silacyclopentadiene ring, intramolecular rotations were successfully controlled and the photophysical properties were varied.

Tunable Supramolecular Interactions of Aggregation-Induced Emission Probe and Graphene Oxide with Biomolecules: An Approach toward Ultrasensitive Label-Free and "Turn-On" DNA Sensing

Aggregation-induced emission (AIE) probes have shown great potential applications in fluorescent sensing of biomolecules, and the integration of AIE probe and graphene oxide (GO) attracts intense interest in developing new tools for label-free and "turn-on" fluorescent biomolecular analysis. Herein, an ultrasensitive label-free and "turn-on" DNA sensing is realized by tuning the supramolecular interactions of AIE probe and GO with DNA. The investigation of supramolecular interactions of AIE probes and GO with DNA demonstrate that AIE probe with short alkyl chains substitute shows highest binding affinity with DNA strand, and GO with low oxidation degree possesses strong binding interactions to ssDNA and the highest fluorescence quenching efficiency. As a result, the optimized AIE probes and GO-based fluorescent sensor can selectively detect the target DNA sequence and exhibits the detection limitation as low as 0.17 × 10^-9 m. It is believed that the research efforts will provide an efficient approach to improve the performance of DNA sensing assay and an indepth understanding of the supramolecular interactions of AIE probes and GO with DNA, and thus facilitate their extended applications in biosensors and biomedicine.

Tetraphenylcyclopentadiene Derivatives: Aggregation-Induced Emission, Adjustable Luminescence from Green to Blue, Efficient Undoped OLED Performance and Good Mechanochromic Properties

Silole derivatives, the first reported and famous AIEgens, are a series of Si-containing conjugated rings with the σ*-π* conjugation, and this unique electronic structure imparts them high electron affinity and fast electron mobility, but not ideal blue luminogens due to their relatively long conjugation length. By replacing the Si atom with the C one, six new AIEgens without the σ*-π* conjugation effect are successfully synthesized based on a tetraphenylcyclopentadiene core. In addition to the sky-blue emission (λ_EL = 492 nm) with L_max , η_C,max , and η_P,max up to 24 096 cd m^-2 , 6.80 cd A^-1 , and 4.07 lm W^-1 , respectively, the careful control of the conjugation degree by changing the linkage mode, results in the blue one (λ_EL = 440 nm) with relatively good performance (L_max : 8721 cd m^-2 and η_C,max : 3.40 cd A^-1 ), indicating that the replacement of the Si atom by C one is an alternative design strategy to yield blue even deep-blue AIEgens with good device performance. Meanwhile, their reversible mechanochromic properties are realized with apparent fluorescence changes between deep-blue and green emissive colors, offering them additional promising applications in optoelectronic devices.

Crystal induced phosphorescence from Benz(a)anthracene microcrystals at room temperature

Pure organic compounds that are also phosphorescent at room temperature are very rare in literature. Here, we report efficient phosphorescence emission from aggregated hydrosol of Benz(a)anthracene (BaA) at room temperature. Aggregated hydrosol of BaA has been synthesized by re-precipitation method and SDS is used as morphology directing agent. Morphology of the particles is characterized using optical and scanning electronic microcopy (SEM). Photophysical properties of the aggregated hydrosol are carried out using UV-vis, steady state and time resolved fluorescence study. The large stoke shifted structured emission from aggregated hydrosol of BaA has been explained due to phosphorescence emission of BaA at room temperature. In the crystalline state, the restricted intermolecular motions (RIM) such as rotations and vibrations are activated by crystal lattice. This rigidification effect makes the chromophore phosphorescent at room temperature. The possible stacking arrangement of the neighboring BaA within the aggregates has been substantiated by computing second order Fukui parameter as local reactivity descriptors. Computational study also reveals that the neighboring BaA molecules are present in parallel slipped conformation in its aggregated crystalline form.

Adaptive soft molecular self-assemblies

Adaptive molecular self-assemblies provide possibility of constructing smart and functional materials in a non-covalent bottom-up manner. Exploiting the intrinsic properties of responsiveness of non-covalent interactions, a great number of fancy self-assemblies have been achieved. In this review, we try to highlight the recent advances in this field. The following contents are focused: (1) environmental adaptiveness, including smart self-assemblies adaptive to pH, temperature, pressure, and moisture; (2) special chemical adaptiveness, including nanostructures adaptive to important chemicals, such as enzymes, CO2, metal ions, redox agents, explosives, biomolecules; (3) field adaptiveness, including self-assembled materials that are capable of adapting to external fields such as magnetic field, electric field, light irradiation, and shear forces.

Bioinspired diphenylalanine with aggregation-induced emission in deep ultraviolet range

Diphenylalanine showing aggregation-induced emission (AIE) at 282 nm is demonstrated. The luminescent intensity increases rapidly with the occurrence of aggregation. Time-resolved and temperature-dependent PL spectra are used to interpret the AIE effect.

Silicon effect of dendritic polyphenyl derivatives: enhancement of aggregation-induced emission and emission color adjustment

Polyphenyl compounds could not emit strong fluorescence and adjust the emission colors. The “silicon effect” appeared to resolve this problem.

Aggregation-induced emission: the origin of lignin fluorescence

Aggregation-induced emission plays a role in the origin of lignin fluorescence owing to the agglomeration of carbonyl groups and restriction of intramolecular rotation.

A dialdehyde–diboronate-functionalized AIE luminogen: design, synthesis and application in the detection of hydrogen peroxide

A dialdehyde–diboronate-functionalized tetraphenylethene (TPE-DABF) was reported as a H₂O₂-specific AIE luminogen.

Mechanochromic Luminescence of Aggregation-Induced Emission Luminogens

Mechanochromic (MC) luminogens have found promising applications in mechanosensors, security papers, and optical storage for their change in emission behaviors in response to mechanical stimuli. Examples on MC luminescent materials are rare before the discovery of MC luminescence in aggregation-induced emission (AIE) luminogens. The twisted conformations of AIE luminogens (AIEgens) with appropriate crystallization capability afford loosely packing patterns, which facilitates their phase transformation in the solid state. The amorphous films of AIEgens exhibit enhanced emission intensity upon pressurization due to the increased molecular interactions, whereas crystals of AIEgens exhibit MC luminescence due to their amorphization by mechanical stimuli. AIEgens enrich the type of MC luminogens but those showing high emission contrast and multicolor emission switching and those working in a turn-on emission mode are seldom reported. Disclosure of the design strategy of high performance MC luminogens and exploration of their high-tech applications may be the future research directions for MC luminogens.

Insight into the strong aggregation-induced emission of low-conjugated racemic C6-unsubstituted tetrahydropyrimidines through crystal-structure-property relationship of polymorphs

Racemic C6-unsubstituted tetrahydropyrimidines (THPs) are a series of fluorophores with a strong aggregation-induced emission (AIE) effect. However, they do not possess the structural features of conventional AIE compounds. In order to understand their AIE mechanism, here, the influences of the molecular packing mode and the conformation on the optical properties of THPs were investigated using seven crystalline polymorphs of three THPs (1-3). The racemic THPs 1-3 have low-conjugated and highly flexible molecular structures, and hence show practically no emission in different organic solvents. However, the fluorescence quantum yields of their polymorphs are up to 93%, and the maximum excitation (λex) and emission (λem) wavelengths of the polymorphs are long at 409 and 484 nm, respectively. Single-crystal structures and theoretical calculation of the HOMOs and LUMOs based on the molecular conformations of these polymorphs indicate that the polymorphs with the shortest λex and λem values possess a RS-packing mode (R- and S-enantiomers self-assemble as paired anti-parallel lines) and a more twisted conformation without through-space conjugation between the dicarboxylates, but the polymorphs with longer λex and λem values adopt a RR/SS-packing mode (R- and S-enantiomers self-assemble as unpaired zigzag lines) and a less twisted conformation with through-space conjugation between the dicarboxylates. The molecular conformations of 1-3 in all these polymorphs are stereo and more twisted than those in solution. Although 1-3 are poorly conjugated, the radiative rate constants (kr) of their polymorphs are as large as conventional fluorophores (0.41-1.03 × 108 s-1) because of improved electronic conjugation by both through-bond and through-space interactions. Based on the obtained results, it can be deduced that the strong AIE arises not only from the restriction of intramolecular motion but also from enhanced electronic coupling and radiatively-favored inter-crossed local excitation (LE) and intramolecular charge transfer (ICT) excitation states. The abnormal molecular structures, easily-controllable self-assembly of the R- and S-enantiomers, and the strong AIE effect make THPs very useful fluorophores for applications and theoretical research.

Effect of stacking mode on the mechanofluorochromic properties of 3-aryl-2-cyano acrylamide derivatives

Three structurally simple 3-aryl-2-cyano acrylamide derivatives (o-MPCPA, m-MPCPA and p-MPCPA) were synthesized, which exhibited different optical properties due to their distinct face-to-face stacking mode.