Mechanochromic Behaviors Research Articles

A Rhodamine-Spiropyran Conjugate Empowering Tunable Mechanochromism in Polymers under Multiple Stimuli.

Mechanochromic functionality realized via the force-responsive mechanophores in polymers has great potential for damage sensing and information storage. Mechanophores with the ability to recognize multiple stimuli for tunable chromic characteristics are highly sought after for versatile sensing ability and color programmability. Nevertheless, the majority of mechanophores are based on single-component chromophores with limited sensitivity, or require additional fabrication technology for multi-modal chromism. Here, we report a novel multifunctional mechanophore capable of vividly detectable and tunable mechanochromism in polymers. This synergistic optical coupling relies on strategically fusing rhodamine and spiropyran (Rh-SP), and tethering polymer chains on both subunits. The mechanochromic behaviors of the Rh-SP-linked polymers under sonication and compression are thoroughly evaluated in response to changes in force and the light-controlled relaxation process. Non-sequential ring-opening of the two subunits under force is identified, endowing high-contrast mechanochromism. Light-induced differential ring-closing reactions of the two subunits, together with the acidichromism of the SP moiety, are employed to engineer elastomers with programmable and wide-spectrum colors. Our work presents an effective strategy for highly appreciable and regulable mechanochromic functionality, and also provides new insights into the rupture mechanisms of π-fused mechanophores, as well as how the stimuli history controls stress accumulation in polymers.

A Rhodamine‐Spiropyran Conjugate Empowering Tunable Mechanochromism in Polymers under Multiple Stimuli

AbstractMechanochromic functionality realized via the force‐responsive mechanophores in polymers has great potential for damage sensing and information storage. Mechanophores with the ability to recognize multiple stimuli for tunable chromic characteristics are highly sought after for versatile sensing ability and color programmability. Nevertheless, the majority of mechanophores are based on single‐component chromophores with limited sensitivity, or require additional fabrication technology for multi‐modal chromism. Here, we report a novel multifunctional mechanophore capable of vividly detectable and tunable mechanochromism in polymers. This synergistic optical coupling relies on strategically fusing rhodamine and spiropyran (Rh−SP), and tethering polymer chains on both subunits. The mechanochromic behaviors of the Rh−SP‐linked polymers under sonication and compression are thoroughly evaluated in response to changes in force and the light‐controlled relaxation process. Non‐sequential ring‐opening of the two subunits under force is identified, endowing high‐contrast mechanochromism. Light‐induced differential ring‐closing reactions of the two subunits, together with the acidichromism of the SP moiety, are employed to engineer elastomers with programmable and wide‐spectrum colors. Our work presents an effective strategy for highly appreciable and regulable mechanochromic functionality, and also provides new insights into the rupture mechanisms of π‐fused mechanophores, as well as how the stimuli history controls stress accumulation in polymers.

Impact of the terminal fluorination of bisthienylethene-based liquid crystals on the self-assembly and mechanochromic behaviors

Four novel bisthienylethene-based rod-like liquid crystals containing different numbers of terminal fluorine atoms were designed and the effect of the terminal fluorination on liquid crystalline self-assembly, optical characteristics in solid state and mechanochromic behaviors was explored. The transition from non-liquid crystal via monotropic liquid crystal with smectic A phase to enantiotropic liquid crystal with smectic A phase can be achieved and the temperature ranges of smectic A phase gradually increase by increasing the terminal fluorination, which might be due to the increase in intermolecular interactions by increasing the number of terminal fluorine atom. All the compounds exhibited AIE characteristic and distinct solid-state fluorescence with yellow or orange color. In addition, the non-fluorosubstituted and difluorosubstituted bisthienylethene-based liquid crystals exhibited reversible mechanochromism, whereas the monofluorosubstituted and trifluorosubstituted bisthienylethene-based liquid crystals exhibited extremely weak mechanochromism due to different intermolecular interaction, molecular symmetry and molecular conformation. These investigations implied that the incorporation of fluorine atom could result in interesting self-assemblies and photophysical properties in different states, which also provided the understanding of the relationship between chemical structures and properties.

Adjusting the dynamic and mechanochromic properties of aromatic disulfide epoxy vitrimers by unsaturation of epoxy monomers

AbstractDisulfide‐contained vitrimers that possess unprecedented functions, such as reprocessing and self‐healing, are considered to be one of the most promising materials since they also exhibit mechanochromic behaviors. A few reports have given the phenomenon of mechanochromism, but the relationship between their structure and mechanochromic behavior needed to further research. Herein, three aromatic disulfide‐contained epoxy vitrimers with different degree of unsaturation are prepared by changing the epoxy monomer. The effect of network structure on the dynamic and mechanochromic properties are investigated. With increasing the degree of unsaturation from the aliphatic to aromatic epoxy vitrimers, the Tg‐value increases and the chain mobility decreases, so the aliphatic epoxy vitrimers shows the lowest activation energy, the lowest temperature of malleability Tv, and the shortest relaxation times. In addition, these epoxy vitrimers all showes reversible mechanochromic behaviors due to the generation and coupling of sulfenyl radicals, which can be accelerated by the high temperature and solvent. Meanwhile, the epoxy structure has a great influence on the mechanochromic behavior, including the absorption wavelength and fading kinetics of grinding powders. These results can be utilized to design desired vitrimer materials for various applications that requires both the intrinsically self‐report damage and the dynamic properties such as recycling.

Impact of the phenyl on fluorescence properties of cyanostilbene modified tetraphenylethene compounds

Two novel cyanostilbene-modified tetraphenylethene derivatives were constructed by Williamson etherification, Suzuki coupling and Knoevenagel condensation reactions. Both compounds exhibited aggregation-induced emission and strong solid-state fluorescence behaviors with extremely high fluorescence quantum yield due to the distorted molecular conformation. Strangely, the compound without additional phenyl spacer did not show mechanochromic behavior, whereas, the compound with an additional phenyl spacer exhibited distinct mechanochromic behavior with the red-shifted emission wavelength from 480 nm to 503 nm upon grinding. The distinct differences in mechanochromic behaviors between the reported compounds could be attributed to that the additional phenyl spacer had benefits for the molecular planarization with external mechanical force. This investigation provided a method to construct mechanochromic material with aggregation-induced emission behavior and high solid-state fluorescence quantum yield and promoted the utilization in rewritable luminescent paper.

High-strength thermochromic and mechanochromic liquid-crystal elastomers with responsive shape memory and dynamic adhesion

Versatile CLCEs with a loosely crosslinking density were developed, which exhibit exceptional high mechanical performances and responsive functionalities including thermochromic, mechanochromic, shape memory, and dynamic adhesion behaviors.

Photophysical Properties of Pyrene‐Substituted nido‐Carborate Anion and the Change of Luminescence Species by Counter Cations in Solution and Solid States

AbstractThis study reports unique luminescent properties of pyrene‐substituted nido‐carborate anion (PYnCA) with two counter cations, tetrabutylammonium (NBu4+) and tetraphenylphosphonium (PPh4+). It is revealed that PYnCA shows twisted intramolecular charge transfer (TICT) emission by the change of the dihedral angle between pyrene and the CcageCcage bond in the carborane unit in the excited state. Although the structure of nCA is almost same as that of ortho‐carborane (oC), the photophysical processes of both pyrene‐substituted compounds are completely different. The study also succeeds in modulating the luminescence species by counter cations in both solution and solid states. To the best of the current knowledge, this is the first report that anion‐based luminescent materials can show the different luminescent species not only in solution but also in the mechanochromic behaviors depending on the type of counter cations. This fundamental research on nCA demonstrates anion‐based emitters as a new platform for stimuli‐responsive materials.

Fabricating Freestanding, Broadband Reflective Cholesteric Liquid-Crystal Networks via Topological Tailoring of the Sm-Ch Phase Transition.

Numerous biological systems in nature provide much inspiration for humanity to master diverse coloration strategies for creating stimuli-responsive materials and display devices, such as to access gorgeous structural colors from well-defined photonic structures. Cholesteric liquid crystals (CLCs) are a fascinating genre of photonic materials displaying iridescent colors responsive to circumstance changes; however, it is still a big challenge to design materials with broadband color variation as well as good flexibility and freestanding capacity. Herein, we report a feasible and flexible strategy to fabricate cholesteric liquid-crystal networks (CLCNs) with precise colors across the entire visible spectrum through molecular structure tailoring and topology engineering and demonstrate their application as smart displays and rewritable photonic paper. Influences of chiral and achiral LC monomers on the thermochromic behaviors of CLC precursors as well as on the topology of the polymerized CLCNs are systematically investigated, demonstrating that the monoacrylate achiral LC facilitated the formation of a smectic phase-chiral phase (Sm-Ch) pretransitional phase in the CLC mixture and improved the flexibility of the photopolymerized CLCNs. High-resolution multicolor patterns in one CLCN film are generated through photomask polymerization. In addition, the freestanding CLCN films show perceivable mechanochromic behaviors and repeated erasing-rewriting performances. This work opens avenues toward the realization of pixelated colorful patterns and rewritable CLCN films promising in technology fields ranging from information storage and smart camouflage to anti-counterfeiting and smart display.

Excitation Wavelength-Dependent Charge Stabilization in Highly Interacting Phenothiazine Sulfone-Derived Donor–Acceptor Constructs

Prolonging the lifetime of charge-separated states (CSS) is of paramount importance in artificial photosynthetic donor-acceptor (DA) constructs to build the next generation of light-energy-harvesting devices. This becomes especially important when the DA constructs are closely spaced and highly interacting. In the present study, we demonstrate extending the lifetime of the CSS in highly interacting DA constructs by making use of the triplet excited state of the electron donor and with the help of excitation wavelength selectivity. To demonstrate this, π-conjugated phenothiazine sulfone-based push-pull systems, PTS2-PTS6 have been newly designed and synthesized via the Pd-catalyzed Sonogashira cross-coupling followed by [2 + 2] cycloaddition-retroelectrocyclization reactions. Modulation of the spectral and photophysical properties of phenothiazine sulfones (PTZSO2) and terminal phenothiazines (PTZ) was possible by incorporating powerful electron acceptors, 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (exTCBD). The quadrupolar PTS2 displayed solvatochromism, aggregation-induced emission, and mechanochromic behaviors. From the energy calculations, excitation wavelength-dependent charge stabilization was envisioned in PTS2-PTS6, and the subsequent pump-probe spectroscopic studies revealed charge stabilization when the systems were excited at the locally excited peak positions, while such effect was minimal when the samples were excited at wavelengths corresponding to the CT transitions. This work reveals the impact of wavelength selectivity to induce charge separation from the triplet excited state in ultimately prolonging the lifetime of CCS in highly interacting push-pull systems.

Mechanochromic, thermoresponsive and triboluminescence behaviors of one divinyl thioxanthene based AIE luminogen with variable conformations

A conformation variable thioxanthene chromophore was employed for the first time to construct a muti-stimuli-responsive luminescent material, which generates three polymorphs with distinct mechano-, thermochromic and triboluminescence behaviors.

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.

D-A-D’ type phenothiazine modified bisarylic methanone derivatives exhibiting fluorescent response to mechanical force and reversible tricolor switching

Two D-A-D′ type bisarylic methanone derivatives PTZ-BZ-Cz and PTZ-BZ-TPA, which consisted of two different electron donors and the same electron acceptor were designed and synthesized. Theoretical calculations revealed that both possessed asymmetric donor-acceptor structures and highly distorted spatial conformations. Optical researches illustrated that both exhibited an evident intramolecular charge transfer (ICT) characteristics and effective emission in the solid state. Interestingly, the mechanochromic behaviors were found. The XRD indicated that the reason for the mechanochromic phenomenon was crystalline-to-amorphous transition. It should be noted that PTZ-BZ-Cz exhibited high contrast and tricolor switching mechanochromic properties under external force stimuli. This novel phenomenon is very interesting for the development of mechanochromic materials that can simultaneously achieve high-contrast and multicolor switching. The current work provided a basis for the design of organic molecules featuring both high contrast and multicolor switching.

Recent Advances in Mechanochromism of Metal-Organic Compounds.

Smart luminescent materials, which can respond to the changing of external environment (light, electricity, force, temperature, etc.), have always been one of the research hotspots. Mechanochromism refers to the materials whose emission color or intensity can be altered under the stimulation of external mechanical force. This kind of smart materials have been widely used in data storage, information encryption and sensors due to its simple operation, obvious and rapid response. The introduction of metal atoms in metal-organic compounds brings about fascinating metalophilic interactions and results in more interesting and surprising mechanochromic behaviors. In this mini-review, recent advances in mechanochromism of metal-organic compounds, including mono-, di-, multinuclear metal-organic complexes and metallic clusters are summarized. Varies mechanisms are discussed and some design strategies for metal-organic compounds with mechanochromism are also presented.

Reversible Mechanochromisms via Manipulating Surface Wrinkling.

Mechanochromic structural-colored materials have promising applications in various domains. In this Letter, we report three types of reversible mechanochromisms in simple material systems by harnessing mechano-responsive wrinkling dynamics including (i) brightness mechanochromism (BM), (ii) hue change mechanochromism (HCM), and (iii) viewable angle mechanochromism (VAM). Upon stretching, the BM device exhibits almost a constant hue but reduces light brightness due to the postbuckling mechanics-controlled deformation, while the HCM device can change the hue from blue to red with almost constant intensity because of the linear elastic mechanics-controlled deformation. The VAM device shows a constant hue because of the thin film interference effect. However, the viewable angles decrease with increasing applied strain owing to the light scattering of wrinkles. All of the mechanochromic behaviors exhibit good reversibility and durability. We clearly elucidated the underlying mechanisms for different mechanochromisms and demonstrated their potential applications in smart displays, stretchable strain sensors, and antipeeping/anticounterfeiting devices.

Aggregation-induced delayed fluorescence molecules with mechanochromic behaviors for efficient blue organic light-emitting diodes

Aggregation-induced delayed fluorescence (AIDF) materials combining the advantages of aggregation-induced emission and thermally activated delayed fluorescence have attracted considerable attention. However, blue AIDF materials with satisfactory electroluminescence efficiencies are still few. In this work, two blue luminescent molecules (XT-DPDBA and XT-BDPDBA) with evident AIDF characteristics and high photoluminescence quantum yields are developed by using rigid electron acceptor xanthenone and weak electron donor 10-dihydrodibenzo[b,e][1,4]azasiline. Both molecules exhibit prominent mechanochromic luminescence (MCL) behaviors originating from conformational interconversion between quasi-axial and quasi-equatorial conformations. The quasi-equatorial conformation has small singlet–triplet energy splitting and thus delayed fluorescence characteristic. Efficient blue organic light-emitting diodes are fabricated based on both molecules, providing high external quantum efficiencies of up to 27% and small efficiency roll-offs. These blue AIDF molecules with MCL property have good application potentials in various optical and electronic devices.

Intense mechanoluminescence in an organic donor-acceptor crystal: Grinding induced crystal-to-crystal phase transformation

Mechanochromic luminescent (MCL) materials with sensitive and reversible responses have attracted extensive attention in many fields. Herein, we report a donor-acceptor (D-A) phenothiazine derivative (CzS-CN), whose crystals show obvious hypochromatic shifts upon anisotropic grinding. In this work, three different crystals based on the phenothiazine derivative were prepared by solvent evaporation method. Due to the different solvents used during crystallization, these three crystals show different molecular arrangements and stacking methods, and thus they present different shapes, including needle-like (crystal-A), flake-like (crystal-B) and block-like (crystal-C). In addition, these three kinds of crystals showed mechanochromic luminescent property with a remarkable change in emission colors from yellow to yellow-green after grinding. To our excitement, these crystals can be partially recovered by heating at 120 °C for 5min or recrystallized in solvents. Detailed spectroscopic and structural analyses revealed that the hypochromatic shifted emission under grinding and the recovery of fluorescence under recrystallization or heating are attributed to the crystal-to-crystal transformation. On the basis of these crystals, reversible crystal-to-crystal phase transitions in response to external stimuli (such as heating and recrystallization) are achieved, which is ultimately attributed to the stacking mode and molecular structure. This work reports a kind of MCL material, which shows the phase transition from crystal to crystal, and reveals some important information to better understand the relationship between molecular packing and mechanochromic behaviors.

Tunable aggregation-induced emission, solid-state fluorescence, and mechanochromic behaviors of tetraphenylethene-based luminophores by slight modulation of substituent structure

Herein, we report the synthesis of two AIE-active tetraphenylethene-derived luminogens with diethylamino (TPEDA) or pyrrolidino group (TPEPL) as terminal substituents, and systematically investigate the influence of diethylamino and pyrrolidino groups as terminal substituents on their photophysical properties and mechanochromic behavior. Single-crystal structural analyses verified that the free alkyl chains of the diethylamino groups were beneficial to loosen the intermolecular stacking. They exhibited distinct luminescence behaviors in the aggregated state. Compared with TPEDA, TPEPL displayed a long-wavelength solid-state fluorescence due to the more extended π-conjugation system across its intermolecular packing structure. However, TPEDA exhibited mechanochromic behavior that manifested in a larger red-shift (25 ​nm), which was attributed to its looser stacking mode. The work not only presents a better understanding of structure-property relationship, but also offers a new method for modulating intermolecular stacking and developing mechanochromic materials.

Synthesis of C3-Symmetric Triazine-Based Derivatives: Study of their AIEE, Mechanochromic Behaviors, and Detection of Picric Acid and Uric Acid in Aqueous Medium

A smart accessible D-A with a tripod shape of triazine derivatives such as TZOME, TZSET, and TZNET was synthesized through simple Suzuki–Miyaura coupling and cyano cyclization reaction. Remarkably, TZNET has excellent optical properties compared to TZOME and TZSET. In particular, TZNET exhibits enhanced emission, induced through aggregation and mechanochromic luminescence. More interestingly, AIEE-fluorescent TZNET has a turnoff fluorescence sensor for picric acid with an LOD of 203 nM and a turn-on sensor for uric acid with an LOD of 209 nM. To the best of our knowledge, this is the first dual-coupled sensor for picric acid and uric acid in an aqueous medium.

Improved mechanochromism and mechanoluminescence in fluoro-substituted N^N^C-coordinating cyclometalated platinum(II) complexes

Mechanochromism and mechanoluminescence of a series of fluoro-substituted chiral Pt(N^N^C)Cl complexes have been investigated. With the grafting of F substitutes, the absorption and emission in the solution give an obvious blue-shift. The molecular stacking of solids may also be affected, leading to different mechanochromic and mechanoluminescent behaviors for obtained yellow or orange solid powders of cyclometalated platinum(II) complexes. Remarkably, yellow crystallites of (−)-3FC display a sensitive and contrast mechanochromic and mechanoluminescent phenomenon, with color changing from yellow to orange and luminescence changing from bright green to red (Δλem ≈ 125 nm). The ground solids can be recovered to original crystallites by adding a drop of acetone. Furthermore, the mechanochromic and mechanoluminescent behaviors can be reversible for multiple cycles without perceivable performance degradation.

Synthesis and fluorescent properties of quinoxaline derived ionic liquids

Ionic liquids (ILs) have attracted increasing attention since last few decades due to their high molecular design abilities and wide applications in different fields. In this study, four novel fluorescent isoquinolino [2,1-a]quinoxalin-5-ium ILs were designed and synthesized via a two-step process including a simple dual Schiff's base formation and a subsequent [RhCp∗Cl2]2-catalyzed oxidative C–H activation/annulation reaction. The as-synthesized ILs were extensively characterized using FT-IR, 1H-NMR, 13C-NMR, 19F-NMR, HSQC-NMR, HMBC-NMR and HR-MS. Their photophysical properties were determined by steady-state fluorescence spectroscopy. The results demonstrate that all these ILs showed dual or triple emissions, large stokes shift (90 nm) and mechanochromic behaviors. Basing on solvatochromism and titration experiments, it is thought that the emission bands of the ILs are raised from their local excited states, charge transfer states or excited state proton transfer of cations, while the substitute effect of these quinoxaline derived ILs on their stokes shifts is negligible.