Combined aggregation induced emission (AIE), photochromism and photoresponsive wettability in simple dichloro-substituted triphenylethylene derivatives

Tetraphenylethene-based mononuclear aggregation-induced emission (AIE)-active mechanofluorochromism gold(I) complexes with different auxiliary ligands

In this study, a series of tetraphenylethene-containing gold(I) complexes with different auxiliary ligands have been synthesized. These complexes were characterized using a variety of techniques including nuclear magnetic resonance spectroscopy, mass spectrometry, and single crystal X-ray diffraction. Their aggregation-induced emission (AIE) behaviors were investigated through ultraviolet/visible and photoluminescence spectrum analyses, and dynamic light scattering measurements. Meanwhile, their mechanofluorochromic properties were also studied via solid-state photoluminescence spectroscopy. Intriguingly, all these mononuclear gold(I) molecules functionalized by tetraphenylethene group demonstrated AIE phenomena. Furthermore, five gold(I) complexes possessing diverse auxiliary ligands exhibited distinct fluorescence changes in response to mechanical grinding. For luminogens 2-5, their solids showed reversible mechanofluorochromic behaviors triggered by the mutual transformation of crystalline and amorphous states, while for luminogen 1, blue-green-cyan three-color solid fluorescence conversion was realized by sequential mechanical grinding and solvent fumigation. Based on this stimuli-responsive tricolored fluorescence feature of 1, an information encryption system was successfully constructed.

Responsive Organic Fluorescent Aggregates Based on Ion-π Interactions Away from Fluorescent Conjugated Groups

Responsive organic luminescent aggregates have a wide range of application fields, but currently there is still a lack of reasonable molecular design strategies. Introducing ion-π interactions into molecules can effectively alter their luminescent properties. However, current research typically focuses on ion localization at luminescent conjugated groups with the strong interaction forces. In this work, we introduce the flexible alkoxy chain spacers between fluorescent conjugated groups and ion-π interaction sites, and then adjust the fluorescence performance of the molecule by changing the strength of ion-π interactions. Bromine ion-based molecules with strong ion-π interactions exhibit high and stable fluorescence quantum yields in crystals and amorphous powders under the external stimuli. Hexafluorophosphate ion-based molecules with weak ion-π interactions have the high fluorescence quantum yield in crystals and very low fluorescence quantum yield in amorphous powders, showing variable fluorescence intensities under external stimuli. This demonstrates a new class of responsive organic luminescent solid-state materials.

Z-selective radical difunctionalization of aromatic alkynes: synthesis of multi-substituted triarylethenes

An efficient method for the radical difunctionalization of aromatic alkynes has been developed, resulting in the synthesis of a range of valuable triarylethenes. This approach utilizes strategically designed aryldiazonium salts with tertiary alcohol substitution as bifunctional reagents, along with cost-effective cuprous chloride as a catalyst. The method demonstrates remarkable Z-selectivity and is capable of gram-scale preparation. Additionally, a novel spin-trapping reagent has been developed based on the synthesized product.

An AIEE-active Triphenylethylene Derivative with Photoresponsive Character for Latent Fingerprints Detection via a Simple Soaking Method

Latent fingerprints (LFPs) is one of the most important physical evidence in the criminal scene, playing an important role in forensic investigations. Therefore, developing highly sensitive and convenient materials for the visualization of LFPs is of great significance. We designed and synthesized an organic fluorescent molecule TP-PH with aggregation-induced enhanced emission (AIEE) activity. By simply soaking, blue fluorescent images with high contrast and resolution are readily developed on various surfaces including tinfoil, steel, glass and plastic. Remarkably, LFPs can be visualized within 5 min including the first-, second- and tertiary-level details. In addition, TP-PH exhibits interesting photoactivated fluorescence enhancement properties. Under irradiation of 365 nm UV light with a power density of 382 mW/cm2, the fluorescence quantum yield displays approximately 21.5-fold enhancement. Mechanism studies reveals that the photoactivated fluorescence is attributed to the irreversible cyclodehydrogenation reactions under UV irradiation. This work provides a guideline for the design of multifunctional AIEE fluorescent materials.

Recent Developments of Photodeformable Polymers: From Materials to Applications

Photodeformable polymer materials have a far influence in the fields of flexibility and intelligence. The stimulation energy is converted into mechanical energy through molecular synergy. Among kinds of photodeformable polymer materials, liquid crystalline polymer (LCP) photodeformable materials have been a hot topic in recent years. Chromophores such as azobenzene, α-cyanostilbene, and 9,10-dithiopheneanthracene have been widely used in LCP, which are helpful for designing functional molecules to increase the penetration depth of light to change physical properties. Due to the various applications of photodeformable polymer materials, there are many excellent reports in intelligent field. In this review, we have systematized LCP containing azobenzene into 3 categories depending on the degree of crosslinking liquid crystalline elastomers, liquid crystalline networks, and linear LCPs. Other structural, typical polymer materials and their applications are discussed. Current issues faced and future directions to be developed for photodeformable polymer materials are also summarized.

Realizing Photoswitchable Mechanoluminescence in Organic Crystals Based on Photochromism

Organic mechanoluminescent (ML) materials possessing photophysical properties that are sensitive to multiple external stimuli have shown great potential in many fields, including optic and sensing. Particularly, the photoswitchable ML property for these materials is fundamental to their applications but remains a formidable challenge. Herein, photoswitchable ML is successfully realized by endowing reversible photochromic properties to an ML molecule, namely 2-(1,2,2-triphenylvinyl) fluoropyridine (o-TPF). o-TPF shows both high-contrast photochromism with a distinct color change from white to purplish red, as well as bright blue ML (λ_ML  = 453 nm). The ML property can be repeatedly switched between ON and OFF states under alternate UV and visible light irradiation. Impressively, the photoswitchable ML is of high stability and repeatability. The ML can be reversibly switched on and off by conducting alternate UV and visible light irradiation in cycles under ambient conditions. Experimental results and theoretical calculations reveal that the change of dipole moment of o-TPF during the photochromic process is responsible for the photoswitchable ML. These results outline a fundamental strategy to achieve for the control of organic ML and pave the way to the development of expanded smart luminescent materials and their applications.

Construction of Photoresponsive 3D Structures Based on Triphenylethylene Photochromic Building Blocks

Photoresponsive materials have been widely used in sensing, bioimaging, molecular switches, information storage, and encryption nowadays. Although a large amount of photoresponsive materials have been reported, the construction of these smart materials into precisely prescribed complex 3D geometries is rarely studied. Here we designed a novel photoresponsive material methyl methacrylate containing triphenylethylene (TrPEF₂-MA) that can be directly used for digital light processing (DLP) 3D printing. Based on TrPEF₂-MA, a series of photoresponsive 3D structures with reversible color switching under ultraviolet/visible light irradiations were fabricated. These complex photoresponsive 3D structures show high resolutions (50 μm), excellent repeatability (25 cycles without fatigue), and tunable saturate color degrees. Multicomponent DLP 3D printing processes were also carried out to demonstrate their great properties in information hiding and information-carrying properties. This design strategy for constructing photoresponsive 3D structures is attractive in the area of adaptive camouflage, information hiding, information storage, and flexible electronics.

Multicolor-Luminescence Including White Light by Photomodulation of Supramolecular Assemblies in Aqueous Media

Photo-responsive supramolecular systems offer intriguing functional aspects which have led to their applications in diverse fields such as optoelectronics and biomedicine. However, the modulation of the luminescence output in a spatiotemporal fashion by photo-controlled transformation still remains a challenging task. Herein, we report the controlled regulation of the emission color of supramolecular assemblies of amphiphilic cyanostilbenes (CSs) in water through in situ photomodulation employing UV and sunlight. Due to their aggregation-induced emission (AIE) features, the CS chromophores in the supramolecular assemblies exhibited bright greenish-yellow emission. Photoirradiation predominantly led to the formation of a cyclized product exhibiting aggregation-caused quenching (ACQ) features and having efficient cyan-blue emission in water but severely quenched emission in the solid state. Hence, starting from a unicomponent scaffold, photomodulation provided tunable emission ranging from greenish-yellow to cyan-blue including white light in water. Furthermore, using the contrasting AIE and ACQ behavior of the components in the photoirradiated mixtures, we were able to design rewritable fluorescent inks and encryption in solid films indicating the practical utility of these systems.

Multi-photoresponsive triphenylethylene derivatives with photochromism, photodeformation and room temperature phosphorescence

A series of triphenylethylene derivatives exhibited multi-photoresponsive properties, including photochromism, photodeformation and room temperature phosphorescence (RTP), which are strongly related to molecular conformations and packing in the aggregated states. Accordingly, these properties can be subtly adjusted by substituents to the center double bond and/or peripheral phenyl moieties. The introduction of bromine atom was beneficial to photochromism and photodeformation properties as a result of the additional C-H⋯Br interactions and electron-withdrawing property. Also, it can promote the RTP effect via heavy atom effect, resulting in persistent afterglow lasting up to 150 min as detected by chemiluminescent imaging system.

Suppression of reversible photocyclization reaction induced fluorescence enhancement: a theoretical study

The fluorescence intensity of SIP-2 and DPI under different environments are mainly related to the competition between the PC reaction and aggregation induced restriction of phenyl ring rotational motions.

Aggregation-Induced Emission Luminogens with Photoresponsive Behaviors for Biomedical Applications

Fluorescent biomedical materials can visualize subcellular structures and therapy processes in vivo. The aggregation-induced emission (AIE) phenomenon helps suppress the quenching effect in the aggregated state suffered by conventional fluorescent materials, thereby contributing to design strategies for fluorescent biomedical materials. Photoresponsive biomedical materials have attracted attention because of the inherent advantages of light; i.e., remote control, high spatial and temporal resolution, and environmentally friendly characteristics, and their combination with AIE facilitates development of fluorescent molecules with efficient photochemical reactions upon light irradiation. In this review, organic compounds with AIE features for biomedical applications and design strategies for photoresponsive AIE luminogens (AIEgens) are first summarized briefly. Applications are then reviewed, with the employment of photoresponsive and AIE-active molecules for photoactivation imaging, super-resolution imaging, light-induced drug delivery, photodynamic therapy with photochromic behavior, and bacterial targeting and killing being discussed at length. Finally, the future outlook for AIEgens is considered with the aim of stimulating innovative work for further development of this field.

Recent Development of Photodeformable Crystals: From Materials to Mechanisms

Photodeformable materials are a class of molecules that can convert photon energy into mechanical energy, which have attracted tremendous attention in the last few decades. Owing to their unique photoinduced deformable properties, including fast light-response and diverse mechanical behaviors, photodeformable materials have exhibited great potential in many practical applications such as actuators, photoswitches, artificial muscles, and bioimaging. In this review, we sort out the current state of photodeformable crystals and classify them into six categories by molecular structures: diarylethenes, azobenzenes, anthracenes, olefins, triarylethylenes, and other systems. Three distinct light-responsive mechanisms, photocyclization, trans-cis isomerization, and photodimerization, are revealed to play significant roles in the molecular photodeformation. Their corresponding photodeformable behaviors such as twisting, bending, hopping, bursting, and curling, as well as the potential applications, are also discussed. Furthermore, the challenges and prospective development directions of photodeformable crystals are highlighted.

A Tri-state Fluorescent Switch with "Gated" Solid-state Photochromism Induced by an External Force

Mr. Chen Qian, Dr. Zhimin Ma, Mr. Jianwei Liu, Mrs. Xue Zhang, Prof. Shitao Wang and Prof. Zhiyong Ma. In this article, we report a newly designed molecule composed of a dihydroazulene (DHA) group and a phenothiazine (PTZ) moiety, which achieves aggregation-induced emission enhancement (AIEE), mechanochromism and "gated" solid-state photochromism upon stimulation by an external force. Grinding loosens intermolecular interactions in the crystal and causes a red-shift of fluorescence from 570 nm to 600 nm. Meanwhile, the ring-opening reaction of DHA unit is activated by grinding and a remarkable photochromism could be observed from the grinded powder. The reddish emission of the grinded powder peaked at 600 nm weakened gradually and finally became dark, and a new absorption band at 470 nm emerged in the absorption spectra. Time-dependent density functional theory (TD-DFT) calculation results reveal that the intramolecular intramolecular charge-transfer (ICT) process is replaced by a locally excited (LE) emission on the DHA group, which leads to the quenching of fluorescence. Its impressive photochromic property inspired us to a simple but effective way to develop an encryption system which can let the correct information be displayed upon external stimulation.

Achieving highly efficient aggregation-induced emission, reversible and irreversible photochromism by heavy halogen-regulated photophysics and D-A molecular pattern-controlled photochemistry of through-space conjugated luminogens

It is extremely challenging but desirable to regulate the photophysical and photochemical processes of aggregation-induced emission luminogens (AIEgens) in distinct states in a controllable manner. Herein, we design two groups of AIEgens based on a triphenylacrylonitrile (TPAN) skeleton with through-space conjugation (TSC) property, demonstrate controlled regulation of photophysical emission efficiency/color and photochemical photochromic and photoactivatable fluorescence behaviours of these compounds, and further validate design principles to achieve highly efficient and emission-tuning AIEgens and to accomplish photo-dependent color switches and fluorescence changes. It is surprisingly found that the introduction of heavy halogens like bromine into a TPAN skeleton dramatically enhances the emission efficiency, and such an abnormal phenomenon against the heavy-atom effect is attributed to the specific through-space conjugation nature of the AIE-active skeleton, effective intermolecular halogen-bond-induced restriction of intramolecular motions, and heavy atom-induced vibration reduction. The incorporation of two electron-donating amino groups into the TPAN skeleton cause the luminogens to undergo a bathochromic shifted emission due to the formation of a D-A pattern. Apart from the regulation of photophysical processes in the solid state, the construction of the D-A pattern in luminogens also results in extremely different photochemical reactions accompanying reversible/irreversible photochromism and photoactivatable fluorescence phenomena in a dispersed state. It is revealed that photo-triggered cyclization and decyclization reactions dominantly contribute to reversible photochromism of the TPAN family, and the photo-induced cyclization-dehydrogenation reaction is responsible for the irreversible color changes and photoactivatable fluorescence behaviours of the NTPAN family. The demonstrations of multiple-mode signaling in photoswitchable patterning and information encryption highlight the importance of controlled regulation of photophysics and photochemistry of fused chromic and AIE-active luminogens in distinct states.

Aggregation-Induced Emission Molecule Microwire-Based Specific Organic Vapor Detector through Structural Modification

An optical organic vapor sensor array based on colorimetric or fluorescence changes quantified by spectroscopy provides an efficient method for realizing rapid identification and detection of organic vapor, but improving the sensitivity of the optical organic vapor sensor is challenging. Here, AIE/polymer (AIE, ggregation-induced emission) composites into microwires arrays are fabricated as organic vapor sensors with specific recognition and high sensitivity for different vapors using the capillary-bridge-mediated assembly method. Such organic vapor sensor successfully detects organic vapor relying on a swelling-induced fluorescence change of the AIE/polymer composites, combating the unique property of AIE molecules and vapor absorption-induced polymer swelling. A series of AIE/polymer composites into microwires arrays with four different groups on the AIE molecule and four different side chains on the polymer is fabricated to detect four different organic vapors. The mechanism for improved sensitivity of the AIE/polymer composites microwires arrays sensors is the same because of the similar polarity between the group of AIE molecules and the vapor molecules. Molecular design of the side chains of the polymer and the groups of AIE molecules based on the polarity of the targeted vapor molecule can enhance the sensitivity of the sensors to the subparts per million level.

Fluorescence sensors for detection of water based on tetraphenylethene–anthracene possessing both solvatofluorochromic properties and aggregation-induced emission (AIE) characteristics

TPE-(An-CHO)4 has been developed as an SFC (solvatofluorochromism)/AIEE (aggregation-induced emission enhancement)-based fluorescence sensor for detection of water over a wide range from low to high water content regions in solvents.

Photochromism and Fluorescence Switch of Furan-Containing Tetraarylethene Luminogens with Aggregation-Induced Emission for Photocontrolled Interface-Involved Applications

It is extremely challenging to design photocontrolled molecular switches with absorption and fluorescence dual-mode outputs that are suited for a solid surface and interface. Herein, we report a group of furan-containing tetraarylethene derivatives with unique photophysical behavior of aggregation-induced emission (AIE) and distinct photochemical reaction-triggered photochromic behaviors by combining a photoactive furan or benzofuran group and an AIE-active triphenylethene molecule. The introduction of a furyl or benzofuryl group into the AIE luminogen endows the molecules with significant reversible photochromism and solid-state fluorescence. The coloration and decoloration of these molecules can be switched by respective irradiation of UV and visible light in a reversible way, and the photochromic changes are accompanied by a switch-on and switch-off of the solid-state fluorescence. It is revealed that the photocontrolled cyclization and cycloreversion reactions are responsible for the reversible photochromism and fluorescence switching based on experimental data and theoretical analysis. Both the position and conjugation of the introduced photoactive units have significant influence on the color and strength of the photochromism, and the simultaneous occurrence of photoinduced fluorescence change in the solid state is perfectly suited for surface-involved applications. The demonstrations of dual-mode signaling in photoswitchable patterning on a filter paper and anti-counterfeiting of an anti-falsification paper strongly highlight the unique advantage of these photochromic molecules with an aggregation-induced emission characteristic in various practical applications. This work proposes a general strategy to design photochromic molecules with AIE activity by introducing photoactive functionals into an AIEgen and demonstrates incomparable advantage in dual-mode signaling and multifunctional applications of these molecules.

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.

Regiodivergent Synthesis of Bis(4-oxycoumarin)-based Dioxabicycles: Exploration of [4 + 4] (Heterocyclo)reversion/addition and 1,5-Hydrogen Shift Photochromism

Two isomeric dioxabicyclic molecular skeletons were constructed by employing the concepts of divergent synthesis. A base-mediated and an acid-catalyzed pseudo-three-component reaction of two equivalents of 4-hydroxycoumarin and (Z)-3-chloro-3-phenylacrylaldehyde yielded the corresponding bis(4-oxycoumarin)-based 2,6- and 2,8-dioxabicycles, respectively. The prepared colorless 2,6-dioxabicycles turned red upon UV irradiation and underwent the reverse reaction when exposed to visible light. The photochromism was proposed to proceed via a sequential [4 + 4] (heterocyclo)addition/reversion and 1,5-hydrogen shift on the basis of photogenerated product-trapping experiments.

Functional polyimides based on diamine containing diarylethylene moieties and their photochromic mechanism studies

Diarylethylene (DTE) bearing polyimides showed reversible photochromic behaviors and photoluminescence “on/off” effects, expanded application conditions of photochromic polymers and enhanced tolerance as electronic devices in extreme environments.

Polymorph-Dependent Luminescence Response to Acid Vapors and Its Application in Safety Protection of File Information

A Schiff base, (E)-1-(phenylimino)methyl-2-hydroxylnaphthalene (PIHN), was found to form both nonluminescent and luminescent polymorphs. A unique luminescence "turn on" behavior was observed when the nonluminescent polymorph was fumed with the vapor of aliphatic acids with two or three carbons in the main chain, while the luminescent polymorphs almost did not change the emission color under the same condition. As we know, this is the first report on polymorph-dependent acid response which discloses the influence of crystalline phase on acid-responsive behavior. The formation of hydrogen bonds between PIHN and aliphatic acid is proposed to be the reason for the responsive behavior of the nonemissive polymorph, and such a mechanism is different from the common protonation mechanism. A novel safety protection method of file information has been developed based on the polymorph-dependent luminescence response of PIHN. In addition, we disclose that a crystalline form could show multiple responsive behaviors toward different acids, which benefits the further design of novel acid sensors, such as the sensors that can qualitatively analyze the species of the acid source in an acidic environment.

Assembling-Induced Emission: An Efficient Approach for Amorphous Metal-Free Organic Emitting Materials with Room-Temperature Phosphorescence

Pure organic emitting materials with room-temperature phosphorescence (RTP), showing large Stokes shifts with long emitting lifetime, low preparation cost, good processability, and wide applications in analysis, bioimaging, organic light emitting diode, and so forth, have been drawing great attentions recently. Related to the design strategy for metal-free RTP materials, the phosphors containing heavy atoms (Br, I, etc.) and other heteroatoms (O, S, etc.) to facilitate the singlet-to-triplet intersystem crossing (ISC) to populate the triplet are usually employed. Besides this factor, the pathways of nonradiative relaxation are inhibited as much as possible. Crystalline packing was the commonly used strategy to engender the rigid environment to suppress the nonradiative decay, and thus to enhance the RTP emission. However, crystal RTP materials might usually be provided with not good enough repeatability and processability, which would restrict their specific practical applications special for biosystem. Instead, amorphous metal-free RTP materials could overcome such deficiencies. Recently, great efforts have been devoted to develop challengeable amorphous metal-free materials and expand their potential applications. This Account mainly focuses on the recent progress on amorphous pure organic RTP system, focusing on the rigid effect to restrict the nonradiative decay to induce or enhance the RTP emission via supramolecular interactions such as host-guest interaction and hydrogen-bonding rigid matrix. Typical host-guest assembling and supramolecular polymer systems, hydrogen-binding copolymers, and small molecules for RTP emission, as well as the heavy-atom free assembling systems for RTP emission are well illustrated in this Account. In the summary, we also give some future perspectives and research direction of the area of pure organic RTP systems, such as enhancement of emission quantum yield, emission color tuning, possible device applications, and the remaining challenge. Moreover, based on these amorphous RTP material examples and beyond, we herein would like to conclude and propose a new concept as "Assembling-Induced Emission", the key thought of which systems is "control molecular motions, then control emission" via supramolecular dynamic assembling. This assembling-induced emission strategy is applicable in many emissive assembling systems besides such amorphous RTP materials introduced in this Account. We hope this concept will be a helpful guide for understanding the emissive mechanism and constructing strategy of various emissive materials.

Synthesis and Characterization of Liquid-Crystalline Tetraoxapentacene Derivatives Exhibiting Aggregation-Induced Emission

A series of new tetrakis(dialkoxyphenyl) dicyanotetraoxapentacene derivatives (1 a-c) were prepared by reaction of the appropriate terphenyl diols with tetrafluoroterephthalonitrile in good yields. Compounds 1 b and 1 c, which bear hexyloxy and decyloxy side chains, exhibited columnar hexagonal mesophases, as shown by polarized optical microscopy, variable-temperature powder X-ray diffraction, and differential scanning calorimetry. Single-crystal X-ray diffraction of methoxy-substituted 1 a revealed that the dicyanotetraoxapentacene core is highly planar, consistent with the notion that these molecules are able to stack in columnar mesophases. A detailed photophysical characterization showed that these compounds exhibit aggregation-induced emission in solution, emission in nonpolar solvents, weak emission in polar solvents, and strong emission in the solid state both as powder and in thin films. These observations are consistent with a weakly emissive charge-transfer state in polar solvents and a more highly emissive locally excited state in nonpolar solvents.

Rational design of red AIEgens with a new core structure from non-emissive heteroaromatics

The development of new aggregation-induced emission (AIE) systems is a hot research topic, from which functional materials with diversified structures and properties are derived. Here, based on rare, non-emissive and highly electron-withdrawing heteroaromatics of 1,4,5,8-tetraazaanthracene (TAA), experimental and theoretical studies reveal that attaching phenyl rotors to TAA is crucial to creating a new N-type AIE core structure. Furthermore, by covalent attachment of electron-donating aromatic amines to the peripheries of the AIE core, red AIEgens could be obtained readily, which exhibit excellent photostability for long-term lysosome tracking. This work not only provides a new strategy to design heterocycle-containing AIEgens from non-emissive heteroaromatics but also stimulates more their applications as bio-imaging materials.

Crystallization-Induced Emission Enhancement of a Deep-Blue Luminescence Material with Tunable Mechano- and Thermochromism

Organic luminescent materials with the ability to reversibly switch the luminescence when subjected to external stimuli have attracted considerable interest in recent years. However, the examples of luminescent materials that exhibit multiresponsive properties are rarely reported. In this work, a new stimuli-responsive dye P1 is designed and synthesized with two identical chromophores of naphthalimide, one at each side of an amidoamine-based spacer. This amide-rich molecule offers many possibilities for forming intra- and intermolecular hydrogen bond interactions. Particularly, P1 has an intrinsic property of cocrystallizing with methanol. Compared with the pristine P1 sample, the as-prepared two-component cocrystalline material displays an exceptive deep-blue emission, which is extremely rare among naphthalimide-based molecules in the solid state. Furthermore, the target material exhibits an obvious mechanochromic fluorescent behavior and a large spectral shift under force stimuli. On the other hand, the cocrystalline material shows an unusual "turn off" thermochromic luminescence accompanied by solvent evaporation. Moreover, using external stimuli to reversibly manipulate fluorescent quantum yields is rarely reported to date. The results demonstrate the feasibility of a new design strategy for solid-state luminescence switching materials: the incorporation of solvents into organic compounds by cocrystallization to obtain a crystalline state luminescence system.

Tetraphenylethene-Induced Free Volumes for the Isomerization of Spiropyran toward Multifunctional Materials in the Solid State

Solid-state photochromic materials with reversible and adjustable optical properties are very appealing because of their wide prospects in advanced functional materials. Yet, it remains a significant challenge to develop such materials in the solid state. In this study, a tetraphenylethene derivative (SP-TPE-SP)-based solid-state photoswitch, which exhibits reversible photochromism in the solid state, was constructed. Efficient photoswitching between SP-TPE-SP and its photoisomer MC-TPE-MC is assisted by the large free volumes caused by the nonplanar molecular structures of the TPE1 moieties and the intramolecular π-π stackings between the two MC moieties. The free volumes are large enough to allow for the transport of HCl gas molecules for an acidochromic response. Furthermore, the morphology of the SP-TPE-SP solid surface can be regulated by ultraviolet light irradiation. The contact angles of the SP-TPE-SP solid surface can be decreased, changing from 97 to 82°. Therefore, SP-TPE-SP with a rather simple molecular structure is appealing for advanced multifunctional materials.

Regulating the emission of tetraphenylethenes by changing the alkoxyl linkage length between two neighboring phenyl moieties

Alkoxyl linkages with different carbon lengths are employed to link the two neighboring ortho carbons of the two phenyl moieties at the same ethylene carbon of the tetraphenylethene framework, resulting in successful regulation of the molecular conformation and in turn the emission properties.

Gated photochromic molecules with AIEgen: turn-on the photochromism with an oxidation reagent

A couple of gated photochromic molecules TrPEP and TrPEPO with AIEgen have been rationally designed and synthesized. No photochromism is detected for TrPEP whilst TrPEPO shows obvious photochromic properties in the solution state. By adding equimolar H2O2 aqueous solution to the TrPEP solution, the photochromic properties would be quickly turned on. The oxidation reagent acts as a gate to switch the photochromic properties by switching the triphenylphosphine group to a triphenylphosphine oxide group. Both TrPE and TrPEO display typical AIE phenomena. Different intensive emission bands with the emission maxima of 500 nm and 455 nm are detected before (TrPEP) and after (TrPEPO) oxidization in solid states. Combining the AIEgens, photochromic ON/OFF states can be easily indicated by the different emission colors in the solid state. Single crystal analyses and TD-DFT calculations were carried out to further investigate the photophysical and photochromic properties of these compounds. These new triphenylethylene derivatives provide a new strategy to achieve gated photochromic materials with simple chemical structures and gate indicators.

Multiple yet Controllable Photoswitching in a Single AIEgen System

Seeking new methods to obtain elaborate artificial on-demand photoswitching with multiple functionalities remains challenging. Most of the systems reported so far possess only one specific function and their nonemissive nature in the aggregated state inevitably limit their applications. Herein, a tailored cyanostilbene-based molecule with aggregation-induced emission characteristic was synthesized and was found to exhibit efficient, multiple and controllable photoresponsive behaviors under different conditions. Specifically, three different reactions were involved: (i) reversible Z/E isomerization under room light and thermal treatment in CH₃CN, (ii) UV-induced photocyclization with a concomitant dramatic fluorescence enhancement, and (iii) regio- and stereoselective photodimerization in aqueous medium with microcrystal formation. Experimental and theoretical analyses gave visible insights and detailed mechanisms of the photoreaction processes. Fluorescent 2D photopattern with enhanced signal-to-background ratio was fabricated based on the controllable "turn-on" and "turn-off" photobehaviors in different states. The present study thus paves an easy yet efficient way to construct smart multiphotochromes for unique applications.

Fluorescent Sensors Based on Aggregation-Induced Emission: Recent Advances and Perspectives

Fluorescent sensors with advantages of excellent sensitivity, rapid response, and easy operation are emerging as powerful tools in environmental monitoring, biological research, and disease diagnosis. However, conventional fluorophores featured with π-planar structures usually suffer from serious self-quenching in the aggregated state, poor photostability, and small Stokes' shift. In contrast to conventional aggregation-caused quenching (ACQ) fluorophores, the newly emerged aggregation-induced emission fluorogens (AIEgens) are featured with high emission efficiency in the aggregated state, which provide unique opportunities for various sensing applications with advantages of high signal-to-noise ratio, strong photostability, and large Stokes' shift. In this review, we will first briefly give an introduction of the AIE concept and the turn-on sensing principles. Then, we will discuss the recent examples of AIE sensors according to types of analytes. Finally, we will give a perspective on the future developments of AIE sensors. We hope this review will inspire more endeavors to devote to this emerging world.

2,5-bis(4-alkoxycarbonylphenyl)-1,4-diaryl-1,4-dihydropyrrolo[3,2-b]pyrrole () AIEgens: tunable RIR and TICT characteristics and their multifunctional applications

Novel propeller-like AIEgens with tunable emission were readily prepared and used as a fluorescent thermometer and selective chemosensor for Cd(ii) detection.

Aggregation-induced emission luminogens (AIEgens) have attracted extensive interest for their outstanding luminescence properties in the aggregated state and even in the solid state. In this work, we developed a series of novel AIEgens based on 2,5-bis(4-alkoxycarbonylphenyl)-1,4-diaryl-1,4-dihydropyrrolo[3,2-b]pyrrole (AAPP). The AIEgens can be facilely synthesized through a single-step reaction under mild conditions with satisfactory yields. Interestingly, AAPP was found to have multiple luminous mechanisms that result in variable fluorescence properties. The propeller-like structure of AAPP enables a restricted intramolecular rotation (RIR) process which significantly enhances its fluorescence in the aggregated state (i.e. AIE fluorescence). In addition, there is a donor–acceptor interaction between the heterocycle center and the alkoxycarbonyl units in AAPP which allows a typical twisted intramolecular charge transfer (TICT) process in the dispersed state, resulting in strong fluorescence emissions in non-polar or low-polarity solvents but fluorescence quenching in high-polarity solvents. Due to the tunable RIR and TICT processes and the multiple fluorescence, AAPP compounds exhibit multifunctional applications: (1) as a reversible fluorescent thermometer, AAPP exhibited excellent fatigue resistance. There was a good linear relationship between its fluorescence intensity and temperature from 10 °C to 60 °C. (2) The desethyl AAPP derivative (CAPP) was successfully applied in the detection of Cd(ii) in aqueous solution at neutral pH, and showed a 500-fold fluorescence “turn-on” response to Cd(ii) with good selectivity.

Wettability with Aggregation-Induced Emission Luminogens

Aggregation-induced emission luminogens (AIEgens) have become an emerging field since the concept of AIE was proposed in 2001. Recently, AIEgens have attracted considerable attention due to their abnormal non-emissive fluorescent behavior in solution but strongly emissive behavior in the aggregate state. By utilizing the inherent hydrophobicity, AIEgens can be used to monitor the crystal formation and dewetting behavior in the self-assembly process. More importantly, some stimuli-responsive AIE-active surfaces have been successfully fabricated. In this perspective review, we outline the advances of surface wettability of AIEgens and its applications.

Introductory lecture: recent research progress on aggregation-induced emission

Since the discovery of the aggregation-induced emission (AIE) phenomenon in 2001, research on AIE molecules has drawn much attention, and this area has been expanding tremendously. This brief review will focus on recent advances in the science and application of AIE molecules, including new mechanistic understanding, new AIE molecules for sensing and imaging, stimuli-responsive AIE molecules and applications of AIE molecules for OLEDs. Moreover, this review will give a perspective on the possible opportunities and challenges that exist in the future for this area.

Aggregation-induced emission (AIE) characteristic of water-soluble tetraphenylethene (TPE) bearing four sulfonate salts

Aggregation-induced emission (AIE) characteristic of water-soluble tetraphenylethene (WS-TPE), was investigated by the addition of organic solvent into WS-TPE aqueous solution.