A dipeptide-based multicolored-switching luminescent solid material: when molecular assemblies meet mechanochemical reaction

Grinding-Induced Water Solubility Exhibited by Mechanochromic Luminescent Supramolecular Fibers

Most mechanochromic luminescent compounds are crystalline and highly hydrophobic; however, mechanochromic luminescent molecular assemblies comprising amphiphilic molecules have rarely been explored. This study investigated mechanochromic luminescent supramolecular fibers composed of dumbbell-shaped 9,10-bis(phenylethynyl)anthracene-based amphiphiles without any tetraethylene glycol (TEG) substituents or with two TEG substituents. Both amphiphiles formed water-insoluble supramolecular fibers via linear hydrogen bond formation. Both compounds acquired water solubility when solid samples composed of supramolecular fibers are ground. Grinding induces the conversion of 1D supramolecular fibers into micellar assemblies where fluorophores can form excimers, thereby resulting in a large redshift in the fluorescence spectra. Excimer emission from the ground amphiphile without TEG chains is retained after dissolution in water. The micelles are stable in water because hydrophilic dendrons surround the hydrophobic luminophores. By contrast, when water is added to a ground amphiphile having TEG substituents, fragmented supramolecular fibers with the same molecular arrangement as the initial supramolecular fibers are observed, because fragmented fibers are thermodynamically preferable to micelles as the hydrophobic arrays of fluorophores are covered with hydrophilic TEG chains. This leads to the recovery of the initial fluorescent properties for the latter amphiphile. These supramolecular fibers can be used as practical mechanosensors to detect forces at the mesoscale.

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.

Investigation of Mechanochromic Luminescence of Pyrene-based Aggregation-Induced Emission Luminogens: Correlation between Molecular Packing and Luminescence Behavior

To better understand the correlation between molecular structure and optical properties such as aggregation-induced emission (AIE) and mechanochromic luminescence (MCL) emission, two new pyrene-based derivatives with substitutions at the 4- and 5-positions (1HH) and at the 4-, 5-, 9-, and 10-positions (2HH) were designed and synthesized. Cyano groups were introduced at the periphery of the synthesized compounds (1HCN, 1OCN, 1BCN, 2HCN, 2OCN, and 2BCN) to investigate the influence of these groups on the emission properties of the pyrene derivatives both in solution and in the solid state. The fluorescence emission performance of these compounds in water/acetone mixtures was simultaneously studied, revealing outstanding aggregation-induced emission properties. The typical shift in emission maxima to higher values was attributed to J-aggregate formation in the aggregate state. Careful investigation of the crystal structures demonstrated abundant and intense intermolecular interactions, such as C-H…π and C-H…N hydrogen bonds, contributing to the remarkable mechanochromic luminescence performance of these compounds. The MCL properties of all the compounds were investigated using powder X-ray diffraction, and the remarkable mechanochromic properties were attributed to J-aggregate phenomena in the solid state. These results provide valuable insights into the structure-property relationship of organic MCL materials, guiding the design of efficient organic MCL materials.

Aggregation behaviour of pyrene-based luminescent materials, from molecular design and optical properties to application

Molecular aggregates are self-assembled from multiple molecules via weak intermolecular interactions, and new chemical and physical properties can emerge compared to their individual molecule. With the development of aggregate science, much research has focused on the study of the luminescence behaviour of aggregates rather than single molecules. Pyrene as a classical fluorophore has attracted great attention due to its diverse luminescence behavior depending on the solution state, molecular packing pattern as well as morphology, resulting in wide potential applications. For example, pyrene prefers to emit monomer emission in dilute solution but tends to form a dimer via π-π stacking in the aggregation state, resulting in red-shifted emission with quenched fluorescence and quantum yield. Over the past two decades, much effort has been devoted to developing novel pyrene-based fluorescent molecules and determining the luminescence mechanism for potential applications. Since the concept of "aggregation-induced emission (AIE)" was proposed by Tang et al. in 2001, aggregate science has been established, and the aggregated luminescence behaviour of pyrene-based materials has been extensively investigated. New pyrene-based emitters have been designed and synthesized not only to investigate the relationships between the molecular structure and properties and advanced applications but also to examine the effect of the aggregate morphology on their optical and electronic properties. Indeed, new aggregated pyrene-based molecules have emerged with unique properties, such as circularly polarized luminescence, excellent fluorescence and phosphorescence and electroluminescence, ultra-high mobility, etc. These properties are independent of their molecular constituents and allow for a number of cutting-edge technological applications, such as chemosensors, organic light-emitting diodes, organic field effect transistors, organic solar cells, Li-batteries, etc. Reviews published to-date have mainly concentrated on summarizing the molecular design and multi-functional applications of pyrene-based fluorophores, whereas the aggregation behaviour of pyrene-based luminescent materials has received very little attention. The majority of the multi-functional applications of pyrene molecules are not only closely related to their molecular structures, but also to the packing model they adopt in the aggregated state. In this review, we will summarize the intriguing optoelectronic properties of pyrene-based luminescent materials boosted by aggregation behaviour, and systematically establish the relationship between the molecular structure, aggregation states, and optoelectronic properties. This review will provide a new perspective for understanding the luminescence and electronic transition mechanism of pyrene-based materials and will facilitate further development of pyrene chemistry.

Sequential-Stimuli Induced Stepwise-Response of Pyridylpyrenes

Stimuli-responsive materials, especially multi-stimuli-responsive materials, can sense external stimuli such as light, heat, and force, have shown great potential in drug delivery, data storage, encryption, energy-harvesting, and artificial intelligence. Conventional multi-stimuli-responsive materials are sensitive to each independent stimulus, causing losses in the diversity and accuracy of the identification for practical application. Herein, a unique phenomenon of sequential-stimuli induced stepwise-response generated from elaborately designed single-component organic materials is reported, which shows large bathochromic shifts up to 5800 cm-1 under sequential stimuli of force and light. In contrast to multi-stimuli-responsive materials, the response of these materials strictly relies on the sequence of stimuli, allowing logicality, rigidity, and accuracy to be integrated into one single-component material. The molecular keypad lock is built based on these materials, pointing promising to a future for this logical response in significant practical applications. This breakthrough gives a new drive to classical stimuli-responsiveness and provides a fundamental design strategy for new generations of high-performance stimuli-responsive materials.

Control of Fluorescence of Organic Dyes in the Solid-State by Supramolecular Interactions

Fluorescent organic dyes play an essential role in the creation of new "smart" materials. Fragments and functional groups capable of free rotation around single bonds can significantly change the fluorescent organic dye's electronic structure under analyte effects, phase state transitions, or changes in temperature, pressure, and media polarity. Dependencies between steric and electronic structures become highly important in transition from a solution to a solid-state. Such transitions are accompanied by a significant increase in the dye molecular structure's rigidity due to supramolecular associates' formation such as H-bonding, π···π and dipole-dipole interactions. Among those supramolecular effects, H-bonding interactions, first of all, lead to significant molecular packing changes between loose or rigid structures, thus affecting the fluorescent dye's electronic states' energy and configuration, its fluorescent signal's position and intensity. All the functional groups and heteroatoms that are met in the organic dyes seem to be involved in the control of fluorescence via H-bonding: C-H···N, C-H···π, S = O···H-C, P = O···H, C-H···O, NH···N, C - H···C, C - H···Se, N-H···O, C - H···F, C-F···H. Effects of molecular packing of fluorescent organic dyes are successfully used in developing mechano-, piezo-, thermo- fluorochromes materials for their applications in the optical recording of information, sensors, security items, memory elements, organic light-emitting diodes (OLEDs) technologies.

Acetonitrile-induced structure fine-tuning of a trinuclear zinc complex showing multistimuli responsive luminescence

A trinuclear zinc complex (1) exhibits mechanochromic and acidochromic luminescence properties with five-color switching. The structure of complex 2 shows that the acetonitrile molecules induce fine-tuning of the structures compared with 1.

Regulation of Multicolor Fluorescence Changes Found in Donor-acceptor-type Mechanochromic Fluorescent Dyes

The regulation of multicolor fluorescence changes in mechanochromic fluorescence (MCF) remains a challenging task. Herein, we report the regulation of MCF using a donor-acceptor structure. Two crystal polymorphs, BTD-pCHO(O) and BTD-pCHO(R) produced by the introduction of formyl groups to an MCF dye, respond to a mechanical stimulus, allowing a three-color fluorescence change. Specifically, the orange-colored fluorescence of the metastable BTD-pCHO(O) polymorph changed to a deep-red color in the amorphous-like state to finally give a red color in the stable BTD-pCHO(R) polymorph. This change occurred by mechanical grinding followed by vapor fuming. The two different crystal packing patterns were selectively regulated by the electronic effect of the introduced functional groups. The two types of selectively formed crystals in BTD(F)-pCHO bearing fluorine atoms, and BTD(OMe)-pCHO bearing methoxy groups, respond to mechanical grinding, allowing for the regulation of multicolor MCL from a three-color change to two different types of two-color changes.

Multicolor Mechanofluorophores for the Quantitative Detection of Covalent Bond Scission in Polymers

The fracture of polymer materials is a multiscale process starting with the scission of a single molecular bond advancing to a site of failure within the bulk. Quantifying the bonds broken during this process remains a big challenge yet would help to understand the distribution and dissipation of macroscopic mechanical energy. We here show the design and synthesis of fluorogenic molecular optical force probes (mechanofluorophores) covering the entire visible spectrum in both absorption and emission. Their dual fluorescent character allows to track non-broken and broken bonds in dissolved and bulk polymers by fluorescence spectroscopy and microscopy. Importantly, we develop an approach to determine the absolute number and relative fraction of intact and cleaved bonds with high local resolution. We anticipate that our mechanofluorophores in combination with our quantification methodology will allow to quantitatively describe fracture processes in materials ranging from soft hydrogels to high-performance polymers.

Multicolor mechanochromism of a multinetwork elastomer that can distinguish between low and high stress

We report our findings on a multicolor mechanochromic elastomer that is able to discriminate between low and high stress. The key point of our design depends on the introduction of two UV-inert mechanophores into different polymer networks.

Effect of Alkyl Groups in Pyrene Chromophore on the Mechanical Response of Pyrene-Octafluoronaphthalene Co-Crystals

Changes in the photophysical properties of pyrene (Py)-octafluoronaphthalene (OFN) co-crystals (Py⋅OFN) upon mechanical stimuli are described herein. The Py⋅OFN co-crystal showed a mechano-induced bathochromic shift in emission, and a similar tendency was observed for the 1,3,6,8-tetramethylpyrene-OFN co-crystal. These shifts are due to disruption of the microscopic molecular orientation in the co-crystal, which allows for excimer formation. In sharp contrast to the parent Py⋅OFN and methyl-substituted Py-OFN co-crystals, no mechano-induced bathochromic shift was observed when longer alkyl chains were introduced to the 1-, 3-, 6-, and 8-positions of the Py chromophore. This photophysical opposability against mechanical stimuli could be explained by the orthogonally oriented alkyl groups on the Py ring, which existed between two Py cores like pillars. This fixed OFN to maintain the face-to-face alternatively stacked structure of the co-crystal and thus prevented the formation of the Py excimer. The pillar effect demonstrated herein provides a rational design for co-crystalline systems that are photophysically stable against mechanical stresses.

Mechanochromic Luminescence from Crystals Consisting of Intermolecular Hydrogen-Bonded Sheets

Introduction of functional groups that can form intermolecular hydrogen bonds into highly-emissive luminophores is a promising way to induce mechanochromic luminescence. Herein, we report that a 9,10-bis(phenylethynyl)anthracene derivative featuring two amide groups forms green-emissive crystals based on two-dimensional hydrogen-bonded molecular sheets. Mechanical grinding changed the emission from green to yellow, owing to a transition from a crystalline to an amorphous phase. Infrared spectroscopy revealed that mechanical stimuli disrupted the linear hydrogen-bonding formation. A thermal treatment recovered the original green photoluminescence.

A 4-N,N-dimethylaminoaniline salicylaldehyde Schiff-base solution-solid dual emissive fluorophore: An aggregation-induced turquoise emission characteristics in liquid as a fluorescent probe for Zn2+ response; a strong near-infrared emission in solid state and application for optical data storage

A new Schiff-base 1 based on 4-N,N-dimethylaminoaniline salicylaldehyde is developed. It possesses unique solution-solid dual emission behaviour with emission color: an aggregation-induced bright turquoise emission in liquid and strong near-infrared emission in the solid state. Interestingly, on the one hand, compound 1 is promising a ratiometric fluorescent probe for Zn²⁺ ions detection in the aqueous solution with high sensitivity, selectivity, and relatively low detection limit. On the other hand, based on its inner stimuli-responsive nature, outstanding thermostability and photostability, 1 should be a very promising candidate for the write-once read-many optical data storage medium.

Polymer mechanochemistry-enabled pericyclic reactions

Polymer mechanochemical pericyclic reactions are reviewed with regard to their structural features and substitution prerequisites to the polymer framework.

Anti-Stokes Stress Sensing: Mechanochemical Activation of Triplet-Triplet Annihilation Photon Upconversion

The development of methods to detect damage in macromolecular materials is of paramount importance to understand their mechanical failure and the structure-property relationships of polymers. Mechanofluorophores are useful and sensitive molecular motifs for this purpose. However, to date, tailoring of their optical properties remains challenging and correlating emission intensity to force induced material damage and the respective events on the molecular level is complicated by intrinsic limitations of fluorescence and its detection techniques. Now, this is tackled by developing the first stress-sensing motif that relies on photon upconversion. By combining the Diels-Alder adduct of a π-extended anthracene with the porphyrin-based triplet sensitizer PtOEP in polymers, triplet-triplet annihilation photon upconversion of green to blue light is mechanochemically activated in solution as well as in the solid state.

An all-photonic full color RGB system based on molecular photoswitches

On-command changes in the emission color of functional materials is a sought-after property in many contexts. Of particular interest are systems using light as the external trigger to induce the color changes. Here we report on a tri-component cocktail consisting of a fluorescent donor molecule and two photochromic acceptor molecules encapsulated in polymer micelles and we show that the color of the emitted fluorescence can be continuously changed from blue-to-green and from blue-to-red upon selective light-induced isomerization of the photochromic acceptors to the fluorescent forms. Interestingly, isomerization of both acceptors to different degrees allows for the generation of all emission colors within the red-green-blue (RGB) color system. The function relies on orthogonally controlled FRET reactions between the blue emitting donor and the green and red emitting acceptors, respectively.

Stimuli-responsive multicolor luminescent materials typically react on external triggers of physical nature, but photonically controlled systems which allow for remote operation were not realized. Here the authors use light as the stimulus of a responsive luminescent material which eliminates the need for physical access.

Multicolor Mechanochromism of a Polymer/Silica Composite with Dual Distinct Mechanophores

The development of a multicolor mechanochromic polymer/silica composite is achieved by using two distinct types of mechanochromophores. The multicolor mechanochromism of the composite containing diarylbibenzofuranone in silica-rich domains and naphthopyran in the polymer-rich domain is observed. The obtained composite shows blue, green, and orange colors according to the intensity of applied mechanical stimuli, solvent addition, and lapse of time. This unique multicolor mechanochromic behavior is evaluated by solid-state UV-vis absorption spectroscopy, ab initio steered molecular dynamics simulations, and computed minimum energy paths on force-modified potential energy surfaces. The unique mechanochromism is attributed to the difference in properties, activated colors, and domain locations between the two mechanochromophores.

Hydrochromism behaviors of solid forms of chelerythrine hydrochloride

Salt formation has the potential to produce smart stimulus–response fluorescent materials without modifying the main chemical structures, especially for unstable stem-nucleus structures, and this is investigated in this work.

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.

Spontaneous proton transfer in a series of amphoteric molecules under hydrostatic pressure

Hydrostatic pressure has induced intermolecular proton transfer in the crystals of a series of amphoteric molecules, which results in significant color changes.

Fluorescence response of cruciform D–π–A–π–D phenothiazine derivatives to mechanical force

Three kinds of crystals of two phenothiazine derivatives transformed into similar amorphous powders, in which the short-range π-stacking can be deduced by single-crystal structure.

A Mechanochromic and Photochromic Dual-Responsive Co-assembly with Multicolored Switch: A Peptide-Based Dendron Strategy

In this contribution, we report a unique co-assembly composed of pyrene and spiropyran that were linked separately at the focal point of the same peptide-based dendron (Phe-Glu), in which the dendrons offer driving forces for the coaggregation. A series of co-assemblies with different weight ratios (Py-Phe-Glu/SP-Phe-Glu) were prepared and the morphology could be tuned. It is found that the resulting stable co-assembled organogel is double switchable triggered by light and heat. TEM revealed that, in the xerogel, Py-Phe-Glu formed rigid rod nanofibers with large diameters and acted as a rigid sketelon where the gracile interwoven fibrous structure of SP-Phe-Glu grew. More interestingly, the original powder of the co-assembled xerogel (1.0 mg/0.1 mg) not only displayed a sequential high-contrast tricolored switch from dark blue to bright cyan and to red under external force but also presented multistate accessible photochromic properties. Such mechanochromic and photochromic behaviors of the xerogel are mainly due to the transition of different excimers of pyrene and the force/photoinduced ring-opening reaction of spiropyran. It is rarely reported that self-assembled soft materials achieve mechanochromic and photochromic dual-responsive behaviors with a high-contrast multicolored switch. We believe the co-assembly strategy based on polypeptide dendrons can be extended to other systems for establishing novel intelligent fluorescent 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.

Reversibly Switching Molecular Spectra

Manipulation of light transmission/absorbance and reflection/emission has a great significance in smart windows and displaying media like liquid crystal. Here, we report the usage of an external electric field to reversibly switch the molecular spectra of a model molecule on the basis of its interaction with an electroresponsible polymer brush. Both the UV-vis absorbance spectrum and the fluorescence emission spectrum of the model molecule were confirmed to be electroswitchable. The electroswitchable spectra were experimentally demonstrated to be induced by the electroswitchable statuses of medium anionic poly-allyloxy hydroxypropyl sulfonate (poly-AHPS) brush. Insightfully, the molecular aggregated status of model proflavine molecules could be electrically controlled via the electroresponsible poly-AHPS brushes and then the molecular spectra of the model proflavine molecule also could be electrically and controllably shifted. The success in the manipulation of molecular spectra opens up a wide range of applications not only for displaying but also for nonlinear optics, in vivo imaging, sensors, and environmental inspection.

Mechanically triggered reversible stepwise tricolor switching and thermochromism of anthracene-o-carborane dyad

A novel single organic molecule–carborane conjugate, CAN, was synthesized in a high yield via a modified nickel-catalyzed cross-coupling reaction incorporating an anthracene unit and an o-carborane moiety.

A novel single organic molecule–carborane conjugate, CAN, was synthesized in a high yield via a modified nickel-catalyzed cross-coupling reaction incorporating an anthracene unit and an o-carborane moiety. CAN exhibits multiple functions of tricolored mechanochromism and mechanically triggered thermochromism. The fluorescence could be switched from blue to bright yellow then to pink by grinding. The robust and reversible thermochromic process was triggered by the mechanical force. The locally excited (LE) state emission, intermolecular excimer formation and twisted intermolecular charge transfer (TICT) are the primary origins of this tricolor switching property. High temperature sensitivity of the heavily ground CAN powders contribute to the mechanical force induced TICT emission enhancement and color switching.

Multi-functional bis(alkynyl)gold(iii) N⁁C complexes with distinct mechanochromic luminescence and electroluminescence properties

A new class of highly luminescent bis(alkynyl)gold(iii) N⁁C complexes has been utilized as emissive materials in OLEDs and exhibits mechanochromic behaviors.

A new class of donor–acceptor type luminescent bis(alkynyl)gold(iii) N⁁C complexes has been synthesized and characterized. These gold(iii) complexes not only exhibit high photoluminescence quantum yields of up to 0.81, but also interesting mechanochromic luminescence behaviors that are reversible. Upon grinding, a dramatic luminescence color change from green to red can be observed in solid samples of the gold(iii) complexes, and the mechanochromic luminescence can be readily tuned via a judicious selection of substituents on the pyridine ring. In addition, solution-processable OLEDs based on this class of complexes with EQE values of up to 4.0% have been realized, representing the first demonstration of bis(alkynyl)gold(iii) N⁁C complexes as emissive materials in solution-processable OLEDs.

A mechano- and thermoresponsive luminescent cyclophane

The first fluorescent cyclophane with mechano- and thermoresponsive solid-state fluorescence characteristics is reported. The new cyclophane comprises two 9,10-bis(phenylethynyl)anthracene moieties that are bridged by tetraethylene glycol spacers. The stimuli-responsiveness is based on molecular assembly changes.

Mechanoresponsive Luminescent Molecular Assemblies: An Emerging Class of Materials

The possibility to change the molecular assembled structures of organic and organometallic materials through mechanical stimulation is emerging as a general and powerful concept for the design of functional materials. In particular, the photophysical properties such as photoluminescence color, quantum yield, and emission lifetime of organic and organometallic fluorophores can significantly depend on the molecular packing, enabling the development of molecular materials with mechanoresponsive luminescence characteristics. Indeed, an increasing number of studies have shown in recent years that mechanical force can be utilized to change the molecular arrangement, and thereby the optical response, of luminescent molecular assemblies of π-conjugated organic or organometallic molecules. Here, the development of such mechanoresponsive luminescent (MRL) molecular assemblies consisting of organic or organometallic molecules is reviewed and emerging trends in this research field are summarized. After a brief introduction of mechanoresponsive luminescence observed in molecular assemblies, the concept of "luminescent molecular domino" is introduced, before molecular materials that show turn-on/off of photoluminescence in response to mechanical stimulation are reviewed. Mechanically stimulated multicolor changes and water-soluble MRL materials are also highlighted and approaches that combine the concept of MRL molecular assemblies with other materials types are presented in the last part of this progress report.

Adaptive soft molecular self-assemblies

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

Remarkable substitution influence on the mechanochromism of cyanostilbene derivatives

Well-designed full-color emission, AIE, CIEE-active luminogens and high contrast mechanochromic behaviour.

Mechanochromism triggered fluorescent color switching among polymorphs of a natural fluorescence pigment

The solid-state study of emodin demonstrates the feasibility of building new MCL systems via controlling the solid-state transition of natural fluorescent dyes.

Exploring optical mechanotransduction in fluorescent liquid crystal elastomers

Carbazole-based liquid single elastomers: optical mechanotransduction under ambient conditions.

A Novel Mechanochromic and Photochromic Polymer Film: When Rhodamine Joins Polyurethane

A rhodamine-based molecule, Rh-OH, is synthesized. Rh-OH exhibits a reversible mechanochromic luminescent character but a passivating response to UV light. An elastomeric polymer film based on polyurethane with embedded Rh-OH is prepared via a polycondensation reaction. The film shows mechanochromic and photochromic properties with reversible color change, which originates from the isomerization of the Rh-OH molecule from a twisted spirolactam in the ring-closed form to a planarized zwitterionic structure in the ring-open state.

Multistimuli-Responsive Luminescence of Naphthalazine Based on Aggregation-Induced Emission

Stimuli-responsive luminescent materials, which are dependent on changes in physical molecular packing modes, have attracted more and more interest over the past ten years. In this study, 2,2-dihydroxy-1,1-naphthalazine was synthesized and shown to exhibit different fluorescence emission in solution and solid states with characteristic aggregation-induced emission (AIE) properties. A remarkable change in the fluorescence of 2,2-dihydroxy-1,1-naphthalazine occurred upon mechanical grinding, heating, or exposure to solvents. According to the characterization by solid-state fluorescence spectroscopy, X-ray crystallography, differential scanning calorimetry, and X-ray powder diffraction, the fluorescence change could be attributed to transitions between two structurally different polymorphs. These significant properties could also give 2,2-dihydroxy-1,1-naphthalazine more potential applications as a multifunctional material.

Optical mechanotransduction with carbazole-based luminescent liquid single-crystal elastomers

Carbazole-based liquid single-crystal elastomers (LSCEs) are valuable fluorescent flexible materials to perform optical mechanotransduction under ambient conditions. Indeed, the covalent incorporation of carbazole derivatives into nematic LSCEs allows to tune their luminescence on demand under mechanical control in a quick and reversible fashion. Specifically, the fluorescence intensity for these materials can be switched back and forth in less than a second. Moreover, such a process can be performed several times without detecting any sign of fatigue in the system. In addition, these materials show excellent resistance to aging; 2 years after their preparation they exhibit the very same mechanofluorescent behavior as when freshly prepared. In fact, the here reported fluorescent systems are highly sensitive; the application of a force of 70 mN decreases the fluorescence in the elastomeric material by 7%. Thus, mechanical forces are attractive external stimuli to modulate the fluorescence of nematic elastomers rapidly and reversibly enabling thereby mechanotransduction.