An Acid-Responsive Fluorescent Molecule for Erasable Anti-Counterfeiting

A tetraphenylethylene (TPE) derivative, TPEPhDAT, modified by diaminotriazine (DAT), was prepared by successive Suzuki-Miyaura coupling and ring-closing reactions. This compound exhibits aggregation-induced emission enhancement (AIEE) properties in the DMSO/MeOH system, with a fluorescence emission intensity in the aggregated state that is 5-fold higher than that of its counterpart in a dilute solution. Moreover, the DAT structure of the molecule is a good acceptor of protons; thus, the TPEPhDAT molecule exhibits acid-responsive fluorescence. TPEPhDAT was protonated by trifluoroacetic acid (TFA), leading to fluorescence quenching, which was reversibly restored by treatment with ammonia (on-off switch). Time-dependent density functional theory (TDDFT) computational studies have shown that protonation enhances the electron-withdrawing capacity of the triazine nucleus and reduces the bandgap. The protonated TPEPhDAT conformation became more distorted, and the fluorescence lifetime was attenuated, which may have produced a twisted intramolecular charge transfer (TICT) effect, leading to fluorescence redshift and quenching. MeOH can easily remove the protonated TPEPhDAT, and this acid-induced discoloration and erasable property can be applied in anti-counterfeiting.

Extending Conjugation of Linear Cyanostilbene Derivatives via a Pyridine Moiety for Multi-Stimuli-Responsive Fluorescence Organogels

In the design of effective gelators with aggregation-induced emission behavior, amide and cholesterol moieties are generally used to provide multiple driving forces for gelation. In this work, a series of linear cyanostilbene gelators with a pyridine moiety and different lengths of the alkoxyl group, i.e., CSpy-Cn, have been synthesized through nucleophilic substitution and Knoevenagel reaction. The direct connection of pyridine extends the conjugation of the cyanostilbene moiety, while the alkoxyl group can regulate the solubility of the compounds so that the compounds can serve as gelators for common solvents such as acetonitrile, dimethyl sulfoxide, and ethanol at ultra-low concentrations. At the same time, the cyanostilbene group makes the compounds undergo photoisomerization and emit fluorescence under UV light, while the pyridine group can serve as an acid-base responsive group due to easy protonation. The gels can respond to temperature, light, and organic acid/base. The fluorescence intensity and color can reversibly change during the gel-sol transitions. Finally, a thin film based on the CSpy-C8 xerogel has been prepared and utilized as a multi-stimuli-responsive fluorescence display for information storage and anti-counterfeiting.

Tunable Fluorescence via Self-Assembled Switching of AIE-Active Micelle-like Nanoaggregates

Chemical structures bearing a combination of aggregation-induced emission enhancement (AIEE) and intramolecular charge transfer (ICT) properties attracted the attention of many researchers. Recently, there is an increasing demand to pose tunable AIEE and ICT fluorophores that could present their conformation changes-related emission colors by adjusting the medium polarity. In this study, we designed and synthesized a series of 4-alkoxyphenyl-substituted 1,8-naphthalic anhydride derivatives NAxC using the Suzuki coupling reaction to construct donor-acceptor (D-A)-type fluorophores with alkoxyl substituents of varying carbon chain lengths (x = 1, 2, 4, 6, 12 in NAxC). To explain the observation that molecules with longer carbon chains revealed unusual fluorescence enhancement in water, we study the optical properties and evaluate their locally excited (LE) and ICT states by solvent effects combined with Lippert-Mataga plots. Then, we explored the self-assembly abilities of these molecules in water-organic (W/O) mixed solutions and observed the morphology of its nanostructure using a fluorescence microscope and SEM. The results show that NAxC, x = 4, 6, 12 show different degrees of self-assembly behaviors and corresponding aggregation-induced emission enhancement (AIEE) progresses. At the same time, different nanostructures and corresponding spectral changes can be obtained by adjusting the water ratio in the mixed solution. That is, NAxC compounds present different transitions between LE, ICT and AIEE based on the polarity, water ratio and time changes. We designed NAxC as the structure-activity relationship (SAR) of the surfactant to demonstrate that AIEE comes from the formation of micelle-like nanoaggregates, which causes a restriction of the transfer from the LE state to the ICT state, and micelle formation results in a blue-shift in emission and enhances the intensity in the aggregate state. Among them, NA12C is most likely to form micelles and the most obvious fluorescence enhancement, which will switch over time due to the nano-aggregation transition.

Mechano-induced photoluminescence colour change in an alkyltolane-terminated cyanostilbene

A novel alkyltolane-terminated cyanostilbene is synthesized. The photoluminescence colour changed depending on the precipitation method, with either a red shift or blue shift in the photoluminescence spectra observed upon mechanical grinding.

Solvent-controlled E/Z isomerization vs. [2 + 2] photocycloaddition mediated by supramolecular polymerization

Control over the photochemical outcome of photochromic molecules in solution represents a major challenge, as photoexcitation often leads to multiple competing photochemical and/or supramolecular pathways resulting in complex product mixtures. Herein, we demonstrate precise and efficient control over the photochemical behaviour of cyanostilbenes in solution using a straightforward solvent-controlled approach based on supramolecular polymerization. To this end, we designed a π-extended cyanostilbene bolaamphiphile that exhibits tuneable solvent-dependent photochemical behaviour. Photoirradiation of the system in a monomeric state (in organic solvents) exclusively leads to a highly reversible and efficient E/Z photoisomerization, whereas a nearly quantitative [2 + 2] photocycloaddition into a single cyclobutane (anti head-to-tail) occurs in aqueous solutions. These results can be rationalized by a highly regular and preorganized antiparallel J-type arrangement of the cyanostilbene units that is driven by aqueous supramolecular polymerization. The presented concept demonstrates a novel approach towards solvent-selective and environmentally friendly photochemical transformations, which is expected to broaden the scope of supramolecular polymerization.

Aggregation-Induced Emission and White Luminescence from a Combination of π-Conjugated Donor-Acceptor Organic Luminogens

Two new star-shaped phenyl- and triazine-core based donor-acceptor (D-A) type conjugated molecules bearing triphenylamine end-capped arms were synthesized and characterized as imminent organic optoelectronic materials. Photophysical properties of the compounds were explored systematically via spectroscopic and theoretical methods. Because of the presence of donor-acceptor interactions, these luminogens display multifunctional properties, for instance, high extinction coefficient, large stokes shift, and pronounced solvatochromic effect. The compounds also exhibited phenomenon known as aggregation-induced emission on formation of nano-aggregates in the tetrahydrofuran-water mixture. The aggregate formation was confirmed by transmission electron microscopy, scanning electron microscopy, and dynamic light scattering analyses. Moreover, by controlling the electron withdrawing ability of the acceptor, complementary emissive fluorophores (blue and yellow) were achieved. These two complementary colors together span the entire range of visible spectrum (400-800 nm) and therefore when mixed in a requisite proportion generate white light in solution phase. These findings have potential for the progress of new organic white light radiating materials for applications in lighting and display devices.

Self-Assembled α-Cyanostilbenes for Advanced Functional Materials

In the specific context of condensed media, the significant and increasing recent interest in the α-cyanostilbene (CS) motif [ArCHC(CN)Ar] is relevant. These compounds have shown remarkable optical features in addition to interesting electrical properties, and hence they are recognized as very suitable and versatile options for the development of functional materials. This progress report is focused on current and future use of CS structures and molecular assemblies with the aim of exploring and developing for the next generations of functional materials. A critical selection of illustrative materials that contain the CS motif, including relevant subfamilies such as the dicyanodistyrylbenzene and 2,3,3-triphenylacrylonitrile shows how, driven by the self-assembly of CS blocks, a variety of properties, effects, and possibilities for practical applications can be offered to the scientific community, through different rational routes for the elaboration of advanced materials. A survey is provided on the research efforts directed toward promoting the self-assembly of the solid state (polycrystalline solids, thin films, and single crystals), liquid crystals, nanostructures, and gels with multistimuli responsiveness, and applications for sensors, organic light-emitting diodes, organic field effect transistors, organic lasers, solar cells, or bioimaging purposes.

A solvatochromic cyanostilbene derivative as an intensity and wavelength-based fluorescent sensor for water in organic solvents

A new solvatochromic fluorescent cyanostilbene derivative was developed for the qualitative and quantitative detection of water in THF and dioxane, respectively.

Aggregation-induced emission enhancement and living cell imaging of novel diarylanthracene conjugated dyes

Diarylanthracene conjugated dyes with good aggregation-induced emission enhancement (AIEE) properties were synthesized and their BSA nanoparticles were successfully used in HeLa cell imaging.

Development of a Cr(iii) ion selective fluorescence probe using organic nanoparticles and its real time applicability

Recognition of Cr(iii) ion using highly selective fluorescent organic nanoparticles N1 and its validation using DFT geometry optimization.

AIE active triphenylamine–benzothiazole based motifs: ESIPT or ICT emission

AIE active triphenylamine–HBT chromophores were synthesized via Suzuki coupling reaction and their emission properties due to ESIPT or ICT phenomena were investigated.

Clear piezochromic behaviors of AIE-active organic powders under hydrostatic pressure

A diphenylacrylonitrile derivative exhibiting the aggregation induced emission and piezochromic behavior with clear wavelength changes under hydrostatic pressure.

Aggregation-induced emission of diarylamino-π-carborane triads: effects of charge transfer and π-conjugation

Carborane-based donor–π–acceptor triads bearing triarylamine moieties showed dual emissions, which were assigned as a charge-transfer emission and an aggregation induced emission. The emission properties were affected by two main factors: solvent polarity and solubility.

Aggregation-induced emission of siloles

Aggregation-induced emission (AIE) is a unique and significant photophysical phenomenon that differs greatly from the commonly acknowledged aggregation-caused emission quenching observed for many π-conjugated planar chromophores. The mechanistic decipherment of the AIE phenomenon is of high importance for the advance of new AIE systems and exploitation of their potential applications. Propeller-like 2,3,4,5-tetraphenylsiloles are archetypal AIE-active luminogens, and have been adopted as a core part in the design of numerous luminescent materials with diverse functionalities. In this review article, we elucidate the impacts of substituents on the AIE activity and shed light on the structure-property relationship of siloles, with the aim of promoting the judicious design of AIE-active functional materials in the future. Recent representative advances of new silole-based functional materials and their potential applications are reviewed as well.

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.

Concentration-dependent and light-responsive self-assembly of bolaamphiphiles bearing α-cyanostilbene based photochromophore

In this paper, a new bolaamphiphile bearing 1-cyano-1,2-bis(phenyl)ethene (CNBE) has been synthesized. The self-assembly of this molecule in aqueous solution is concentration-dependent. Two distinct morphologies, monomolecular layered lamellas and helical nanofibres have been obtained with the as-prepared molecular configuration. Note worthily, the helical nanofibres provide an experimental evidence for the pure twisted structure in the liquid crystals, which is theoretically proposed by De Gennes. Due to the photoisomerization of CNBE, the self-assembled nanostructures undergo morphological changes upon irradiation. Although various nanostructures were observed in the solution-state, only nanofibres were obtained after the solution was cast on a substrate, which was attributed to a strong dewetting effect. This work illustrates concentration-dependent and light-responsive self-assembly and provides a novel avenue for fabricating smart soft materials.

Carbazole-functionalized polyphenylene-decorated solid state emissive D–A–D molecules: reduced donor–acceptor interaction and enhanced emission in the solid state

Reducing intermolecular donor–acceptor interactions through steric effects can prevent ACQ and lead to enhanced emission in the solid state.

Bent-core liquid crystalline cyanostilbenes: fluorescence switching and thermochromism

Incorporation of the cyanostilbene unit in a bent-core liquid crystal-type structure gives rise to fluorescent systems whose emission is very sensitive to different environments.

Tuning the fluorescence of aggregates for end-functionalized polymers through varying polymer chains with different polarities

Two pyrazoline initiators and their four end-functionalized polymers have been prepared and their tunable emission behaviors have been investigated.

Cu-catalysed direct C–H (hetero)arylation of [1,2,4]triazolo[4,3-a]pyridine to construct deep-blue-emitting luminophores

An efficient protocol for the synthesis of 3-aryl-[1,2,4]triazolo[4,3-a]pyridines has been developed via Cu-catalysed direct C–H (hetero)arylation. The resulting compounds exhibit deep-blue emission with high quantum yields, photostability, and thermal stability.

9,10-Bis(N-alkylindole-3-yl-vinyl-2)anthracenes as a new series of alkyl length-dependent piezofluorochromic aggregation-induced emission homologues

AIE active 9,10-bis(N-alkylindole-3-yl-vinyl-2)anthracene homologues (IACn) could be facilely prepared and exhibit alkyl length-dependent piezofluorochromic properties whose spectral shifts increase with the decrease of N-alkyl chain lengths.

Tetraphenylethene end-capped [1,2,5]thiadiazolo[3,4-c]pyridine with aggregation-induced emission and large two-photon absorption cross-sections

New two-photon absorption dyes (TPEPT1 and TPEPT2) were designed and synthesized. These materials show different 2PA properties and unique AIE effects, in which TPEPT1 with higher brightness was used to perform a cell-imaging proposal.

2-{4-[Bis(4-bromo-phen-yl)amino]-benzyl-idene}malono-nitrile

In the crystal structure of the title compound, C22H13Br2N3, the two bromo-phenyl rings are rotated out of the plane of the central benzyl-idene ring by 68.7 (1) and 69.3 (1)°. Both cyano substituents are located nearly in the plane of the benzylidene ring, with the mean plane of the methylmalononitrile group being inclined to this ring by 5.8 (1)°. In the crystal, the mol-ecules are linked by weak C-H⋯N hydrogen bonds into layers parallel to the bc plane.

Polymorphism-dependent and piezochromic luminescence based on molecular packing of a conjugated molecule

We report a unique example of polymorphism-dependent and piezochromic fluorescence, based on 2-(anthracen-9-yl)-4,5-diphenyl-1H-imidazole.

Excimer and exciplex emissions of 1,8-naphthalimides caused by aggregation in extremely polar or nonpolar solvents

1,8-Naphtalimdes showed aggregation induced emission in both extremely polar or nonpolar solvents, which is attributed to the intermolecular-excimer and intramolecular-exciplex formation.