The Solid-State Multi-Color Fluorescence Switching from a [2.2]Paracyclophane-Based Triarylborane

The fluorophores, the fluorescence of which can be switched between multi bright colors in the solid state, show promising applications not only in the sophisticated multicolor display but also in the advanced encryption and anti-counterfeiting systems. However, it is very challenging to obtain such fluorophores. Herein, we disclose such an example, g-BPhANMe2-Cp, which contains an electron-donating dimethylamino (NMe2) and an electron-accepting [(2-dimesitylboryl)phenyl]acetyl at the pseudo-gem position of [2.2]paracyclophane skeleton. This molecule can display tricolor mechanochromic luminescence (MCL) due to the different responses of the mechanically ground amorphous state to heating and solvent-fuming. Owing to the absence of intermolecular π-π interactions in the solid state, the fluorescence efficiency is very high irrespective of its morphological state (ΦF=0.60-0.87). Moreover, this molecule also displays reversible acidochromic luminescence (ACL) by protonation and deprotonation of NMe2 with trifluoroacetic acid (TFA) and triethylamine (TEA), respectively. The protonated sample fluoresces (ΦF=0.31) at much shorter wavelength due to the interruption of intramolecular charge transfer process. Therefore, with the combination of tricolor MCL and ACL properties, the solid-state emission of g-BPhANMe2-Cp can be switched among four bright fluorescence colors of yellow, green, cyan and blue via treatment with appropriate stimulus.

Advances on fluorescence chemosensors for selective detection of water

Water, although an important part of everyday life, is acts as one of the most significant contaminants in various applications such as biomedical monitoring, chemical production, petroleum-based fuel and food processing. In fact, the presence of water in other solvents is a huge concern. For the quantification of trace water content, different methods such as Karl-Fischer, electrochemical, nuclear magnetic resonance, chromatography, and thermogravimetric analysis have been used. Although every technique has its own benefit, each one suffers from several drawbacks that include high detection costs, lengthy procedures and specialized operations. Nowadays, the development of fluorescence-based chemical probes has become an exciting area of research for the quick and accurate estimation of water content in organic solvents. A variety of chemical processes such as hydrolysis reaction, metal ions promoted oxidation reaction, suppression of the -C═N isomerization, protonation and deprotonation reactions, and molecular aggregation have been well researched in the last few years for the fluorescent detection of trace water. These chemical processes eventually lead to different photophysical events such as aggregation-induced emission (AIE), aggregation-induced emission enhancement (AIEE), aggregation-caused quenching (ACQ), fluorescent resonance energy transfer (FRET), charge transfer, photo-induced electron transfer (PET), excited state intramolecular proton transfer (ESIPT) that are responsible for the detection. This review presents a summary of the fluorescence-based chemosensors reported in recent years. The design of water sensors, sensing mechanisms and their potential applications are reviewed and discussed.

Stimuli-fluorochromic smart organic materials

Smart materials based on stimuli-fluorochromic π-conjugated solids (SFCSs) have aroused significant interest due to their versatile and exciting properties, leading to advanced applications. In this review, we highlight the recent developments in SFCS-based smart materials, expanding beyond organometallic compounds and light-responsive organic luminescent materials, with a discussion on the design strategies, exciting properties and stimuli-fluorochromic mechanisms along with their potential applications in the exciting fields of encryption, sensors, data storage, display, green printing, etc. The review comprehensively covers single-component and multi-component SFCSs as well as their stimuli-fluorochromic behaviors under external stimuli. We also provide insights into current achievements, limitations, and major challenges as well as future opportunities, aiming to inspire further investigation in this field in the near future. We expect this review to inspire more innovative research on SFCSs and their advanced applications so as to promote further development of smart materials and devices.

Synthesis of Fluorophore Based Functional Material for Selective Detection of Al3+ Ion in Water and Decoding the AIEE Property of Its Hydrosol

A designed aggregation-induced emission enhancement (AIEE) active fluorescence probe 2,3-Bis-[(2-hydroxy-napthalen-1-ylmethylene)-amino]-but-2-enedinitrile (L) was synthesized via one step condensation method. The probe shows swift sensitivity and selectivity toward Al3+over other relevant metal ions and also exhibits significant AIEE phenomena in methanol/water mixture. Significant enhancement of fluorescence intensity is triggered via chelation-enhanced fluorescence through complex (Al3+-L) formation. A 2:1 metal to ligand ratio is observed from Job's plot based on UV - Vis absorption titration and detection limit (LOD) is found as low as 31.14 nM. Moreover, 1H NMR titrations and fluorescence reversibility by adding Al3+ and EDTA sequentially had been performed to establish the binding site of sensor complex (Al3+-L). Time-resolved photoluminescence, dynamic light scattering, optical microscopy, and on-site visualization studies have been performed to understand the AIEE mechanism of L in different volume percentage of water and methanol mixture. An INHIBIT molecular logic gate has been constructed utilizing the fluorescence behavior of the probe, L in presence of Al3+ and strong chelating ligand EDTA.

Aggregation-induced emission of a bis(imino)acenaphthene zinc complex with tetraphenylethene units

Using bis(imino)acenaphthene (BIAN) zinc(II) and palladium(II) complexes with tetraphenylethene (TPE) units as bulky aryl groups, Zn-2 and Pd-2 have been designed and developed, and their photophysical properties in solution and in the solid state have been investigated. Both in solution and in the solid state Zn-2 and Pd-2 show two photoabsorption bands in the ranges of 300 nm to 350 nm and 450 nm to 600 nm, which are assigned to the π-π* transition originating from both the TPE units and naphthalene units and the intraligand charge transfer (ILCT) between the TPE units and the BIAN unit, respectively. Density functional theory (DFT) calculations demonstrated that for Zn-2 the highest occupied molecular orbitals (HOMO) are localized on the TPE units, while the lowest unoccupied molecular orbitals (LUMO) are localized on the BIAN unit, leading to the appearance of a photoabsorption band on the ILCT. The emission from Zn-2 was quenched in solution, but appeared as phosphorescence at around 600 nm by photoexcitation at the ILCT band in the solid state as well as in the aggregated state, which was formed by the addition of n-hexane as a poor solvent to the dichloromethane (DCM) solution. The aggregate formation of Zn-2 in the DCM/n-hexane (10 wt%/90 wt%) solution was confirmed by the Tyndall scattering and scanning electron microscopy (SEM) measurements, demonstrating the aggregation-induced emission (AIE) characteristics of Zn-2. On the other hand, Pd-2 was non-emissive in the solid state and in the aggregated state as well as in solution. Moreover, the DCM-inclusion complexes of Zn-2 and Pd-2 were obtained and their photophysical properties were investigated. It was found that the photoluminescence quantum yield (Φ_PL-solid) values of Zn-2 and Zn-2-DCM in the solid state are less than 1%. Single-crystal X-ray structural analysis of Zn-2-DCM revealed the absence of intermolecular π-π interactions. Consequently, it was suggested that the low Φ_PL-solid value of Zn-2 is mainly due to the radiationless relaxation of the excitons by dynamic rotation of the phenyl groups of the TPE units, even in the solid state and in the aggregation state.

Carbazole-based aggregation-induced phosphorescent emission-active gold(I) complexes with various phosphorescent mechanochromisms

A series of carbazole-containing gold(I) complexes modified with different substituents were successfully designed and synthesized, and their molecular structures were characterized by nuclear magnetic resonance spectroscopy and mass spectrometry. The aggregation-induced behaviors of these gold(I) complexes were studied by ultraviolet/visible and photoluminescence spectroscopy. Meanwhile, their mechanical force-responsive emissive properties were also investigated via solid-state photoluminescence spectroscopy. Interestingly, all these gold(I)-based luminogenic molecules were capable of exhibiting aggregation-induced phosphorescent emission phenomena. Furthermore, their solids of three gold(I) complexes displayed contrasting mechano-responsive phosphorescence features. More specifically, trifluoromethyl or methoxyl-substituted luminophores 1 and 3 demonstrated mechanochromic behaviors involving blue-shifted phosphorescence changes, and their mechanoluminochromic phenomena were reversible. However, the solid-state phosphorescence of phenyl-substituted luminophor 2 was not sensitive to external mechanical force.

Facile Synthesis of 2-Methylenebenzothiazoles from Benzothiazole Salts and 4-Hydroxycoumarins by Ball Milling

A simple and practical method for the synthesis of 2-methylenebenzothiazoles via the coupling of a benzothiazole salts and 4-hydroxycoumarins have been described herein. The reaction showed a good substrate range and functional compatibility under mechanochemical conditions, and proceeded without catalysts or solvents. The 2-methylenebenzothiazoles products were achieved via the formation of a C=C double bond and exhibited strong luminescence and typical aggregation-induced emission (AIE) properties, indicating their potential for use as fluorescent materials.

Large Stokes shift benzothiazolium cyanine dyes with improved intramolecular charge transfer (ICT) for cell imaging applications

Intramolecular Charge Transfer (ICT) is a crucial photophysical phenomenon that can be used to improve the Stokes' shift in fluorescent dyes. The introduction of molecular asymmetry is a promising approach to mitigate significant drawbacks of the symmetric cyanine dyes due to their narrow Stokes' shifts (Δλ < 20 nm). In this feature article, we discuss recent progress towards improving the Stokes' shift (Δλ > 100 nm) in benzothiazolium-based fluorophore systems via efficient ICT and recent discoveries related to potentially useful live cell imaging applications of these asymmetric cyanine dyes. This article explores three interesting asymmetric benzothiazolium dye designs (D-π-A, π-A and D-π-2A) in detail while discussing their optical properties. The key advantage of these probes is the synthetic tunability of the probe's photophysical properties and cellular selectivity by simply modifying the donor (D) or the acceptor (A) group in the structure. These new asymmetric ICT fluorophore systems exhibit large Stokes' shifts, high biocompatibility, wash-free staining, red to NIR emission and facile excitation with commercially available laser wavelengths.

Mechanofluorochromism of 2-Biarylyl Cinchoninic Acids with High Sensitivity and Large Mechanochromic Shift

In recent years, organic mechanofluorochromism (MFC) materials have attracted wide attention in many fields. However, the exploration of MFC materials with high-contrast, high-sensitivity and high-responsiveness remains a challenge. Herein, a series of MFC materials with 2-biarylyl cinchoninic acid skeleton were successfully established, which are based on interconversion of classical/ frustrated Brönsted pairs. These compounds have the mechanochromic shift of up to 115 nm, as well as the property of stunning sensitivity and multiple responses to external mechanical force stimuli. The luminescence properties can be easily tuned by changing the substituents.

Deep-Red Emissive Squaraine-AIEgen in Elastomer Enabling High Contrast and Fast Thermoresponse for Anti-Counterfeiting and Temperature Sensing

Two challenges remain for organic thermoresponsive materials; one is to develop high-performance red-emissive thermoresponsive materials, while another is to simultaneously achieve high contrast ratio (CR), fast and reversible thermoresponse in a single element. Herein, we not only develop a new deep-red emissive squaraine-based AIEgen (TPE-SQ12) based on a pyrylium end group, which is suitable for fabricating high-performance thermoresponsive materials, but also show an effective approach to improve both CR (∼ten times increase) and response time (less than 3 seconds), that is, molecularly dispersing AIEgen into an elastomer, attributed to the significantly expanded free volume of elastomer upon increasing the temperature that can activate the AIEgen intramolecular movements more pronouncedly. Double encryption and temperature mapping systems have been separately established by using our designed elastomer/TPE-SQ12 film, showing the great potential for anti-counterfeiting and temperature sensing. Finally, white emission is further achieved by co-doping TPE-SQ12 with cyan dye into elastomer, which enables fluorescent thermochromism for improving the temperature mapping ability.

Tetraphenylethene-Modified Colorimetric and Fluorescent Chemosensor for Hg2+ With Aggregation-Induced Emission Enhancement, Solvatochromic, and Mechanochromic Fluorescence Features

A tetraphenylethene (TPE)-modified rhodanine derivative was successfully designed and prepared, and this luminophor showed intramolecular charge transfer nature from the TPE unit to the rhodanine-3-acetic acid unit. Interestingly, this luminogen not only exhibited typical aggregation-induced emission enhancement (AIEE) behavior but also showed good cell imaging performance. Remarkably, this AIEE-active TPE-containing rhodanine derivative possessed noticeable solvatochromic fluorescence effect involving multiple fluorescent colors of green, yellow-green, yellow, orange, and red. Meanwhile, this fluorescigenic compound displayed reversible mechanochromic fluorescence behavior based on the mutual transformation of between stable crystalline and metastable amorphous states. On the other hand, this multifunctional fluorophor could selectively and sensitively detect Hg2+ in an acetonitrile solution. Furthermore, this chemosensor could also be used to detect Hg2+ on test paper strips.

Hydrophilic Tetraphenylethene-Based Tetracationic Cyclophanes: NADPH Recognition and Cell Imaging With Fluorescent Switch

A hydrophilic TPE-based tetracationic cyclophane TPE-cyc was synthesized, which could capture intracellular Nicotinamide adenine dinucleotide phosphate and fuel the antioxidative ability of tumor cells to detoxify reactive oxygen species (ROS). Meanwhile, upon the reduction by cellular GSH, TPE-cyc could light up tumor cells, acting as a GSH-responsive fluorescent switch to image cells with high resolution.

Tetraphenylethene–anthracene-based fluorescence emission sensor for detection of water with photo-induced electron transfer and aggregation-induced emission characteristics

As a fluorescent sensor for water over a wide range from low to high water content regions in organic solvents, we have designed and developed a PET (photo-induced electron transfer)/AIE...

Fluorescent polymer films based on photo-induced electron transfer for visualizing water

As fluorescent materials for visualization, detection, and quantification of a trace amount of water, we have designed and developed a PET (photo-induced electron transfer)-type fluorescent monomer SM-2 composed of methyl methacrylate-substituted anthracene fluorophore-(aminomethyl)-4-cyanophenylboronic acid pinacol ester (AminoMeCNPhenylBPin) and achieved preparation of a copolymer poly(SM-2-co-MMA) composed of SM-2 and methyl methacrylate (MMA). Both SM-2 and poly(SM-2-co-MMA) exhibited enhancement of the fluorescence emission with the increase in water content in various solvents (less polar, polar, protic, and aprotic solvents) due to the formation of the PET inactive (fluorescent) species SM-2a and poly(SM-2-co-MMA)a, respectively, by the interaction with water molecules. The detection limit (DL) of poly(SM-2-co-MMA) for water in the low water content region below 1.0 wt% in acetonitrile was 0.066 wt%, indicating that poly(SM-2-co-MMA) can act as a PET-type fluorescent polymeric sensor for a trace amount of water in solvents, although it was inferior to that (0.009 wt%) of SM-2. It was found that spin-coated poly(SM-2-co-MMA) films as well as 15 wt% SM-2-doped polymethyl methacrylate (PMMA) films produced a satisfactory reversible fluorescence off–on switching between the PET active state under a drying process and the PET inactive state upon exposure to moisture, which is demonstrated by the fact that the both the films are similar in hydrophilicity to each other from the measurement of the water contact angles on the polymer film surface. Herein we propose that PET-type fluorescent polymer films based on a fluorescence enhancement system are one of the most promising and convenient functional dye materials for visualizing moisture and water droplets.

Photo-induced electron transfer (PET)-type fluorescent polymer films based on a fluorescence enhancement system have been prepared as one of the most promising and convenient functional dye materials for visualizing moisture and water droplets.

Recent Trends in the Design, Synthesis, Spectroscopic Behavior, and Applications of Benzazole-Based Molecules with Solid-State Luminescence Enhancement Properties

Molecules that exhibit solid-state luminescence enhancement, i.e. the rare property to be more strongly emissive in the solid state than in solution, find an increasing number of applications in the fields of optoelectronic and nanophotonic devices, sensors, security papers, imaging, and theranostics. Benzazole (BZ) heterocycles are of particular value in this context. The simple enlargement of their π-electron system using a -C=C-Ar or -N=C-Ar moiety is enough for intrinsic solid-state luminescence enhancement (SLE) properties to appear. Their association with a variety of polyaromatic motifs leads to SLE-active molecules that frequently display attractive electroluminescent properties and are sensitive to mechanical stimuli. The excited-state intramolecular proton transfer (ESIPT) process that takes place in some hydroxy derivatives reinforces the SLE effect and enables the development of new sensors based on a protection/deprotection strategy. BZ may also be incorporated into frameworks that are prototypical aggregation-induced enhancement (AIE) luminogens, such as the popular tetraphenylethene (TPE), leading to materials with excellent optical and electroluminescent performance. This review encompasses the various ways to use BZ units in SLE systems. It underlines the significant progresses recently made in the understanding of the photophysical mechanisms involved. A brief overview of the synthesis shows that BZ units are robust building blocks, easily incorporated into a variety of structures. Generally speaking, we try to show how these small heterocycles may offer advantages for the design of increasingly efficient luminescent materials.

Aggregation induced emission (AIE) materials for mitochondria imaging

Mitochondria are energy producing organelle of the eukaryotic cells. The main activities of mitochondria monitored by various marker molecules are autophagy detection, estimation of Reactive Oxygen Species (ROS), mitochondrial death and Photodynamic therapy in cancer cells. Due to the advantages of specificity and sensitivity, aggregation induced emission (AIE) is now popular for the mitochondria labeling. In this chapter, we would like to discuss three major types of AIEgens probe used in mitochondrial staining. There are three different types of AIEgens available for mitochondrial detection and sensing based on their different structural motifs. The first type of AIEgens is tetraphenylethene (TPE) based molecules. Due to simple engineering architecture, TPE based AIEgens are widely employed in bioimaging applications. AIEgen such as triphenylphosphine (TPP), and triphenylamine (TPA) are also employed as a novel building block. These are successfully used as exceptional lipid droplet (LD)-specific bio probes in cell imaging, assurance of cell combination, and photodynamic cancer cell removal. The third group is the miscellaneous AIEgens probe involved in mitochondria imaging.

Hydrophilic AIE-Active Tetraarylethenes for Fluorescence Sensing and Super-Resolution Imaging of Amyloid Fibrils from Hen Egg White Lysozyme

Hen egg white lysozyme (HEWL) is frequently applied as a model protein for research on protein folding, unfolding, and fibrillization identified by featured fluorescent probes. Here, a series of hydrophilic, pH-sensitive tetraarylethene (TAE)-type AIEgens are synthesized via a geminal cross-coupling (GCC) reaction and evaluated for their capabilities of fluorescence sensing and super-resolution localization imaging of HEWL fibrils. With superior optical and sensing properties, the selected TAE-type AIEgen probe is weakly emissive in aqueous media, without dependence on the pH value and buffer concentration, but exhibits "turn-on" fluorescence upon interaction with HEWL amyloid fibrils in a spontaneous and reversible way that just meets the requirement of fluorescence random switching for super-resolution imaging. The selected probe has the strongest fluorescence response to HEWL amyloid fibrils exhibiting a limit of detection of 0.59 nmol/L and enables super-resolution fluorescence imaging of amyloid aggregates with a high resolution of 40 nm.

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.

Novel Solid-State Emissive Polymers and Polymeric Blends from a T-Shaped Benzodifuran Scaffold: A Comparative Study

Two novel polyimines were synthesized from a benzodifuran based diamino monomer and two dialdehydes bearing bulky groups and a flexible spacer. The polymers display tuned luminescence performance according to the presence of half-salen groups. The effect of the intramolecular bond on the emission properties were examined. Two model compounds, replicating the same emissive Schiff base cores, were synthetized. From the models, dye-doped blends in the fluorophore/matrix ratio, resembling the polymers, were produced. Amorphous thin films of the covalent polymers and the polymeric blends were obtained by spin-coating technique. The Photoluminescent (PL) response of the different macromolecular systems were qualitatively and quantitatively examined and compared.

Aggregation-induced enhanced emission-type cruciform luminophore constructed by carbazole exhibiting mechanical force-induced luminescent enhancement and chromism

One D-A type cruciform luminophore MDCS-BC based on carbazole has been prepared. We observed that this compound exhibits unique intramolecular charge-transfer (ICT) and typical aggregation-induced enhanced emission (AIEE) properties with the solid-state luminescence efficiency of 0.184. Moreover, this luminophore shows a significant stimuli-induced emission enhancement and chromism effect with good reversibility. Upon grinding, the fluorescence color of the as-prepared samples can change from blue (454 nm) to green (504 nm). What is unexpected is that the fluorescence efficiency of the initial powder is dramatically increased from 0.184 to 0.424 upon grinding. The results of PXRD, DSC and spectral properties studies show that the mechanical force-induced luminescence enhancement and chromism behavior of MDCS-BC originates from the transition between crystal and amorphous morphology, and the large red-shift and the emission enhancement inducing by grinding may be attributed to the planarization of the molecular conformation and subsequent planar ICT process.

Color polymorphism in organic crystals

Color polymorphism is an interesting property of chemical systems which present crystal polymorphs of different colors. It is a rare phenomenon, with only a few examples reported in the literature hitherto. Nevertheless, systems exhibiting color polymorphism have many potential applications in different domains, such as pigment, sensor, and technology industries. Here, known representative chemical systems showing color polymorphism are reviewed, and the reasons for them to present such property discussed. Also, since some of the concepts related to color polymorphism have been frequently used imprecisely in the scientific literature, this article provides concise, systematic definitions for these concepts.

Vapochromic crystals: understanding vapochromism from the perspective of crystal engineering

Vapochromic materials, which undergo colour and/or emission changes upon exposure to certain vapours or gases, have received increasing attention recently because of their wide range of applications in, e.g., chemical sensors, light-emitting diodes, and environmental monitors. Vapochromic crystals, as a specific kind of vapochromic materials, can be investigated from the perspective of crystal engineering to understand the mechanism of vapochromism. Moreover, understanding the vapochromism mechanism will be beneficial to design and prepare task-specific vapochromic crystals as one kind of low-cost 'electronic nose' to detect toxic gases or volatile organic compounds. This review provides important information in a broad scientific context to develop new vapochromic materials, which covers organometallic or coordination complexes and organic crystals, as well as the different mechanisms of the related vapochromic behaviour. In addition, recent examples of supramolecular vapochromic crystals and metal-organic-framework (MOFs) vapochromic crystals are introduced.

Rational design of a colorimetric and fluorescence turn-on chemosensor with benzothiazolium moiety for cyanide detection in aqueous solution

A novel colorimetric and fluorescence turn-on chemosensor TBB with benzothiazolium moiety has been explored, which exhibited the high selectivity for cyanide ion (CN^-) in THF-H₂O (2:8, v/v) mixture. The aqueous solution of sensor TBB was scarcely emissive. In the presence of CN^- ion, the nucleophilic addition of CN^- with the benzothiazolium CN bond of TBB produced the new species TBB-CN, consequently resulting in the intense orange-red emission by aggregation-induced emission (AIE) effect. Meanwhile, the color of solution was changed from orange-yellow to light yellow. The sensing mechanism was verified by Mass spectrometry, NMR analysis and DFT calculations.

Tetraphenylethene-based tetracationic dicyclophanes: synthesis, mechanochromic luminescence, and photochemical reactions

Two TPE-based tetracationic dicyclophanes 1 and 2 exhibited a classic AIE but contrasting mechanochromic luminescence under grinding or hydrostatic pressure in solid state, and photochemical reactions induced by photoirradiation in solution state.

Precise Molecular Design for High-Performance Luminogens with Aggregation-Induced Emission

Precise design of fluorescent molecules with desired properties has enabled the rapid development of many research fields. Among the different types of optically active materials, luminogens with aggregation-induced emission (AIEgens) have attracted significant interest over the past two decades. The negligible luminescence of AIEgens as a molecular species and high brightness in aggregate states distinguish them from conventional fluorescent dyes, which has galvanized efforts to bring AIEgens to a wide array of multidisciplinary applications. Herein, the useful principles and emerging structure-property relationships for precise molecular design toward AIEgens with desirable properties using concrete examples are revealed. The cutting-edge applications of AIEgens and their excellent performance in enabling new research directions in biomedical theranostics, optoelectronic devices, stimuli-responsive smart materials, and visualization of physical processes are also highlighted.

Halogenated tetraphenylethene with enhanced aggregation-induced emission: an anomalous anti-heavy-atom effect and self-reversible mechanochromism

Halogenated tetraphenylethene derivatives show a unique anti-heavy-atom effect where introducing heavy halogens like bromine greatly improves the fluorescence quantum yield upon aggregation, contrary to the classic heavy-atom effect. The unique self-reversible mechanochromism of brominated TPE is attributed to re-generation of halogen-halogen bonding after its breakage.

Toward a simple way for a mechanochromic luminescent material with high contrast ratio and fatigue resistance: Implication for information storage

In this work, we present the synthesis and photoluminescence (PL) behaviour of a new compound, DHNC. The molecular design includes twisted conformation and the incorporation of electron donor (D) and acceptor (A) pairs, which endows the compound with both twisted intramolecular charge transfer (ICT) and aggregation-induced emission (AIE) properties. Importantly, the compound exhibits mechanochromic luminescence (MCL): The emission of the crystalline powder shows strong green emission but turns into orange-red with an obvious quenching effect after grinding, demonstrating a high contrast ratio. The emission of the ground sample can be rejuvenated though recrystallization by either immersion or fumigation in common organic solvents. The emission can be reversibly switched between two states for more than 10 cycles, showing fatigue resistance. In a quantitative mechanical experiment, the DHNC-loaded film has a remarkable emission loss with the external force up to 67.9 Mpa, showing high sensitivity. An archetype of information storage is developed based on this MCL material, which uses mechanical force to write information and organic vapour to erase. Letters and cartoon pictures can be written and erased repeatedly on the DHNC-loaded film, indicating high contrast ratio and fatigue resistance.

A mechano-responsive fluorescent xerogel based on an anthracene-substituted acylhydrazone derivative

AHBP-8 can form a stable organogel and enhanced fluorescence emission has been observed after gelation. The AHBP-8 xerogel exhibited favorable mechano-fluorochromism behavior.

Mechanochromic luminescence based on a phthalonitrile-bridging salophen zinc(ii) complex

Here, we showcase the impressive stimuli-responsive properties of a luminescent zinc(ii)–salophen complex CN-Zn, highlighting a reversible mechanochromic property.

Development of an intramolecular charge transfer-type colorimetric and fluorescence sensor for water by fusion with a juloidine structure and complexation with boron trifluoride

An intramolecular charge transfer-type optical sensor fused with a juloidine structure and complexed with boron trifluoride can detect and determine water over a wide concentration range.

Two new quinoline-based regenerable fluorescent probes with AIE characteristics for selective recognition of Cu2+ in aqueous solution and test strips

Two novel highly selective quinoline-based fluorescent probes (1 and 2) with an aggregation induced emission (AIE) feature have been designed and synthesized for the rapid analysis of Cu2+ ions in aqueous media and on paper strips with a fluorescence quenching mechanism. Moreover, probes 1 and 2 exhibit excellent sensitivity and anti-interference for Cu2+ detection, and the detection limits are as low as 1.3 × 10-8 M and 8.5 × 10-8 M, respectively, which are much lower than the allowable standard of Cu2+ (∼20 μM) in drinking water (EPA). More importantly, these two probes were successfully applied for the determination of Cu2+ in real aqueous samples and fabrication of simple device test strips for rapid and on-site detection of Cu2+ ions. Interestingly, they can also be regenerated by adding an excess of S2-. Additionally, the crystallographic structure of probe 1 was confirmed through a single crystal X-ray study. Job's plot analysis and ESI-MS spectroscopic studies reflect the 1 : 1 complexation of the 1-Cu2+ and 2-Cu2+ complexes. Furthermore, DFT/TDDFT calculations were performed in order to help in understanding the electronic properties of the complexes and the chelation-induced quenching mechanism.

Fine Tuning of Emission Behavior, Self-Assembly, Anion Sensing, and Mitochondria Targeting of Pyridinium-Functionalized Tetraphenylethene by Alkyl Chain Engineering

Compared to the many studies that focus on the development of novel molecular frameworks pertaining to functionalized fluorescent materials, there is lesser emphasis on side chains even though they have a significant impact on the properties and applications of fluorescent materials. In this study, a series of pyridinium-functionalized tetraphenylethene salts (TPEPy-1 to TPEPy-4) possessing different alkyl chains are synthesized, and the influence of chain length on their optical performance and applications is thoroughly investigated. By changing the alkyl chain, the fluorogens exhibit opposite emission behavior in aqueous media because of their distinct hydrophobic nature, and their solid-state emission can be fine-tuned from green to red owing to their distinct molecular configuration. In addition, by increasing the chain length, the microstructure of the self-assembled fluorogens converts from microplates to microrods with various emission colors. Moreover, TPEPy-1 exhibits dual-mode fluorescence "turn-on" response toward NO₃^- and ClO₄^- in aqueous media because the anions induce the self-assembly of fluorogens. Furthermore, the fluorogens display cellular uptake selectivity while the proper alkyl chain impels the fluorogens to penetrate the cell membrane and accumulate in the mitochondria with high specificity.

Supramolecular Aggregates of Tetraphenylethene-Cored AIEgen toward Mechanoluminescent and Electroluminescent Devices

Luminescent materials possessing both the mechanoluminescence (MCL) and electroluminescence (EL) properties are the quest for sensing and optoelectronic applications. We report on the synthesis of a new tailor-made luminogen, 1,2-bis(4-(1-([1,1'-biphenyl]-4-yl)-2,2-diphenylvinyl)phenyl)-1,2-diphenylethene (TPE 5), using Suzuki coupling reaction with high yield. An aggregation-induced emission (AIE) active complex TPE 5 forms supramolecular spherical aggregates at the air-water interface of a Langmuir trough. As a consequence, a large enhancement of luminescence is obtained from the mono- and multilayer Langmuir-Blodgett films of TPE 5 owing to the AIE effect. The luminogen TPE 5 exhibits a reversible MCL response, displaying photoluminescence switching due to change in the crystalline states under external stimuli. The unique feature of luminescence enhancement upon aggregate formation is utilized for the fabrication of light-emitting diodes with low threshold voltage using supramolecular aggregates as the active layer. This work demonstrates an efficient strategy for obtaining controlled supramolecular aggregates of AIEgen with a potential in the dual applications of MCL and EL.

Controlled Photoerasable Fluorescent Behaviors with Dithienylethene-Based Molecular Turnstile

The precise control of molecular interaction and motion is a powerful strategy in the creation and development of intelligent materials. We demonstrate here a simple concept and approach to integrate intramolecular photochromic property with intermolecular aggregation-induced emission behaviors, with the aim to construct a new type of photoswitchable luminescent materials. This strategy is realized by the dithienylethene-bridged bispyridinium salt as photochromic molecular turnstile, and their subsequent fabrication into optically functional materials is reported. By restricting the rotation of central chemical bonds, the obtained molecular turnstile not only exhibits photocontrolled fluorescence emission through solvent exchange but is also capable of transforming into photowritable and photoerasable films in polymeric matrix with good recyclability. This functional molecular turnstile provides convenient routes to construct photochromic nanomaterials with controlled photophysical behaviors.

Tetraphenylethene– and diphenyldibenzofulvene–anthracene-based fluorescence sensors possessing photo-induced electron transfer and aggregation-induced emission enhancement characteristics for detection of water

RS-1 and RS-2 have been developed as PET/AIEE hybrid fluorescence sensors for detection of water in the low and high water content regions in solvents.