Impact of Polymer Matrix on Polymer Mechanochromism from Excited State Intramolecular Proton Transfer

Mechanochromic polymers based on non-covalent changes have attracted much attention recently.Herein,we report the impact of inter/intramolecular hydrogen bonds on polymer mechanochromism from the excited state intramolecular proton transfer (ESIPT) process.PhMz-NH2-OH and PhMz=2A are designed and obtained by simple and high-yield synthesis,and are connected into polyurethane and poly(methyl acrylate-co-2-ethylhexyl acrylate),respectively.In the initial state,the PhMz-NH2-OH@PU sample shows blue fluorescence from the excited enol form (E*) excitons,owing to intermolecular hydrogen bonds that interrupt the ESIPT reactions but the PhMz=2A@PMA-2-EA sample expresses cyan fluorescence belonging to the excited keto form (K*) emission,implying that the intramolecular hydrogen bonds matter.Furthermore,under stretching,external force can tune the emission of the PhMz=2A@PMA-2-EA sample from K* to E* state.Though external force can putatively still promote a bond rotation,ESIPT reactions remain equivalently interrupted in both the relaxed and stressed states in a hydrogen-bond donating environment.DFT calculation confirms the force-induced increase in dihedral angle for the transition of ESIPT-on/off.Thus,PhMz-NH2-OH@PU and PhMz=2A@PMA-2-EA showed disparate initial ESIPT states and further different responses/sensitivity to force.This study reports a novel and efficient strategy for enriching mechanochromic investigation and extending the applications of ESIPT reactions.

Mechanochromism of polyurethane based on folding-unfolding of cyano-substituted oligo(p-phenylene)vinylene dimer

The incorporation of mechanophores,motifs that transform mechanical stimulus into chemical reaction or optical variation,allows creating materials with stressresponsive properties.The most widely used mechanophore generally features a weak bond,but its cleavage is typical an irreversible process.Here,we showed that this problem can be solved by folding–unfolding of a molecular tweezer.We systematically studied the mechanochromic properties of polyurethanes with cyano-substituted oligo(p-phenylene)vinylene(COP)tweezer(DPU).As a control experiment,a class of polyurethanes containing only a single COP moiety(MPU)was also prepared.The DPU showed prominent mechanochromic properties,due to the intramolecular folding–unfolding of COP tweezer under mechanical stimulus.The process was efficient,reversible and optical detectable.However,due to the disability to form either intramolecular folding or intermolecular aggregation,the MPU sample was mechanical inert.

Multifunctional AIE Molecules Possessing Long-lasting Room Temperature Phosphorescence, Thermally Activated Delayed Fluorescence and Dual-mode Mechanochromism

Two simple donor-acceptor multifunctional pure organic light-emitting molecules[(9H-carbazol-9-yl)(4-hydroxyphenyl)-methanone(CzMP)and(4-hydroxyphenyl)(10H-phenothiazin-10-yl)methanone(PTZMP)]with distinct aggregation-induced emission(AIE)properties were synthesized.Surprisingly,CzMP showed a long room temperature phosphorescence lifetime(>900 ms),and PTZMP exhibited triple emission of prompt fluorescence(PF),room temperature phosphorescence(RTP)and thermally activated delayed fluorescence(TADF).Furthermore,CzMP effectively responded to mechanical external forces and solvent fumigation,exhibiting dual-mode mechanochromic luminescence(MCL)including multiple fluorescence color shifts and phosphorescence switching.Time-dependent density functional theory(TDDFT)calculations were investigated to explain different luminescence properties of the two molecules,and the single crystal of CzMP was obtained and analyzed to demonstrate the unique molecular stacking pattern and strong intermolecular interactions in close association with phosphorescence emission.The multifunctional luminescent properties of the emitters explored in this work could be more effectively applied to a wide range of applications,such as information encryption and anti-counterfeiting.

A Versatile Tetraphenylethene Derivative Bearing Excitation Wavelength Dependent Emission,Multistate Mechanochromism,Reversible Photochromism,and Circularly Polarized Luminescence and Its Applications in Multimodal Anticounterfeiting

Multimodal anticounterfeiting has become increasingly challenging in modern society to guarantee information security and the safety of property.In this study,a versatile cholesterol-containing tetraphenylethene derivative is shown to have multiple optical properties,including stimuli-responsive fluorescence,reversible photochromism,excitation wavelength dependent luminescence,and circularly polarized luminescence.After the application of diverse processing methods(writing,screen painting,drawing,and pyrography),we found that this molecule can serve as an anticounterfeiting toolbox to provide rich anticounterfeiting effects through the synergistical use of multiple optical properties.This work offers important insight for designing novel small organic molecules for advanced multimodal anticounterfeiting technology.

Synthesis,Crystal Structure and Luminescent Mechanochromism of a Square-planar Pt(Ⅱ) Complex Based on 1,10-Phenanthroline Derivative

A new square-planar platinum(Ⅱ) complex, [Pt(Me3SiC≡Cphen)(C≡CC6H4Cl-3)2](1), was synthesized by using 3-trimethylsilylethynyl-1,10-phenanthroline(Me3SiC≡Cphen) and 3-chlorophenylacetylene ligands, and its structure was characterized by single-crystal X-ray crystallography. 1 crystallizes in triclinic, space group P1 with a = 6.9524(5), b = 12.7796(11), c = 17.5846(10) ?, α = 78.484(5), β = 88.700(5), γ = 75.600(6)°, V = 1482.23(19) ?3, Z = 2, C33H24Cl2N2 PtSi, Mr = 742.62, Dc = 1.664 g/cm3, F(000) = 724, μ(Mo Ka) = 4.979 mm-1, R = 0.0369 and wR = 0.0786. In 1, adjacent molecules along the a axis are stacked in a columnar structure through π···π interactions, and such neighboring columnar structures are connected with each other by C–H···π hydrogen bonds to form a 2D supramolecular network. 1 exhibits reversible luminescence mechanochromic property with the luminescence red shift in a range of ca. 146~182 nm. Extremely large red-shifts of luminescence spectra suggest that luminescent mechanochromic property of 1 is due to the formation of aggregate via Pt–Pt interaction during the mechanical grinding. Based on this property, a simple device was developed and used for rewritable data storage.

Mononuclear aggregation-induced emission (AIE)-active gold(Ⅰ)-isocyanide phosphors: Contrasting phosphorescent mechanochromisms and effect of halogen substitutions on room-temperature phosphorescence nature

Developing phosphors with long-lifetime(millisecond scale or even longer) solid state room temperature phosphorescence(RTP) feature has attracted considerable attention. However, to date, stimuli-responsive phosphors with RTP nature are still rare due to the absence of effective guidelines for the exploitation of luminophors synchronously possessing stimuli-responsive and RTP characteristics. In this work,a series of mononuclear gold(Ⅰ) complexes are reported. All these complexes exhibit various solid-state RTP properties, and phosphor 1-Cl exhibits long-lived RTP behavior. The effect of halogen atoms on the RTP nature of these complexes is investigated in detail. Furthermore, the introduction of different types of halogen atoms can effectively regulate the phosphorescent mechanochromism phenomena of these gold(Ⅰ)-containing complexes. In addition, these phosphors display typical aggregation-induced emission(AIE) effect except for phosphor 5-CCl, which lacks hydrogen-bonding interactions compared with the other four phosphors. This work will be very helpful to the development of mechanical-force-responsive AIE phosphors with lasting RTP.

Crystallization-induced phosphorescence, remarkable mechanochromism, and grinding enhanced emission of benzophenone-aromatic amine conjugates

Pure organic luminogens with efficient room temperature phosphorescence（RTP） and remarkable mechanochromism are highly desired in view of their fundamental significance and technical applications. Herein, four twisted pure organic luminogens based on benzophenone and aromatic amines were synthesized and their photophysical properties were thoroughly investigated. They exhibit crystallization-induced phosphorescence（CIP）, giving bright fluorescence and phosphorescence dual emission in crystals. Upon grinding, they become amorphous and emit predominantly red-shifted fluorescence, demonstrating remarkable mechanochromism. Furthermore, three of them even demonstrate greatly enhanced emission upon grinding, which is rarely observed in twisted D-A structured luminogens.

Remarkable mechanochromism and force-induced thermally activated delayed fluorescence enhancement from white-light-emitting organic luminogens with aggregation-induced emission

The development of organic materials with white-light emission and thermally activated delayed fluorescence(TADF)properties in the solid state remain a challenge.Herein,a series of white-light-emitting organic luminogens have been developed and are found to show aggregation-induced delayed fluorescence(AIDF)characteristics.The AIDF emitters present dual-emission consisted of prompt fluorescence and TADF in the crystalline state.Their white-light emissions can be easily tuned by altering the chemical structure and connecting position of the heterocyclic aromatic substituent.Under the stimuli of mechanical force and solvent vapor,the compounds exhibit remarkable and reversible mechanochromism,in which their emission colors are switchable between white and yellow.Upon grinding,they also display linearly tunable luminescence colors,as well as force-induced TADF enhancement,which may be associated with the more compact molecular packing and the restriction of intramolecular motions.The results from time-resolved emission scanning and theoretical calculation suggest that the dual-emission of the AIDF luminogens likely results from the twisted intramolecular charge transfer transitions of the molecules,and the reversible mechanochromism properties probably stem from the interconversion of the quasi-axial and the quasi-equatorial conformations.

An AEE-active Polymer Containing Tetraphenylethene and 9,10-Distyrylanthracene Moieties with Remarkable Mechanochromism

A polymer （poly（9,10）anthracenevinylene-alt-4,4＇-（9,9-bis（4-（4＇-（1,2,2＇-triphenyviny）phenoxy）butyl）-9H- fluorene-2,7-diyl） dibenzaldehyde）, P1） was successfully synthesized through the Wittig-Horner reaction by employing fluorene and 9,10-distyrylanthracene moieties as building blocks for backbone and tetraphenylethenes as pendant groups. Photophysical and thermal properties of the resulting polymeric emitter were fully characterized by ultraviolet-visible （UV- Vis） absorption and photoluminescence （PL） spectra, thermogravimetric analysis （TGA） and differential scanning calorimetry （DSC）. While P1 emits an orange-light centered at 567 nm in dilute tetrahydrofuran （THF） solution, the solid powder of the polymer exhibits strong yellow emission peaked at 541 nm. It is also found that the as-synthesized polymer shows unique property of aggregation-enhanced emission （AEE）. In addition, P1 possesses high thermal stability with a decomposition temperature （Td,5%） of 430 ℃ and high morphological stability with a glass transition temperature （Tg） of 171℃. Under the stimulus of mechanical force, the emission of P1 can be changed from yellow to red （△λmax = 61 rim）, showing a remarkable mechanochromism. The results from XRD analysis suggest that such mechanochromic phenomenonof PI is probably caused by the destruction of crystalline structure, which leads to the conformational planarization of the distyrylanthracene moieties forming by the polymerization and the increase of molecular conjugation of the backbone.

Emerging interactively stretchable electronics with optical and electrical dual-signal feedbacks based on structural color materials

The booming development of wearable devices has aroused increasing interests in flexible and stretchable devices.With mechanosensory functionality,these devices are highly desirable on account of their wide range of applications in electronic skin,personal healthcare,human–machine interfaces and beyond.However,they are mostly limited by single electrical signal feedback,restricting their diverse applications in visualized mechanical sensing.Inspired by the mechanochromism of structural color materials,interactively stretchable electronics with optical and electrical dual-signal feedbacks are recently emerged as novel sensory platforms,by combining both of their sensing mechanisms and characteristics.Herein,recent studies on interactively stretchable electronics based on structural color materials are reviewed.Following a brief introduction of their basic components(i.e.,stretchable electronics and mechanochromic structural color materials),two types of interactively stretchable electronics with respect to the nanostructures of mechanochromic materials are outlined,focusing primarily on their design considerations and fabrication strategies.Finally,the main challenges and future perspectives of these emerging devices are discussed.

Dihydroanthracene bridged bis-naphthopyrans:A multimodal chromophore with mechano-and photo-chromic properties

Mechanochromophores based on bichromic molecular switches,such as bis-naphthopyanes,allow multimodal mechanochromic behavior beyond the typical binary response from single chromophores,which is important for distinguishing between multiple stress states through discrete changes in color.Spontaneously generated persistent and distinguishable multi-colors from activated bis-naphthopyanes remain challenging.And the versatility of bis-mechanophore design for advanced optical molecular systems and the fundamental insights into the corresponding mechano-reactivity are not enough.Here,we identify a dihydroanthracene bridged bis-naphthopyrans as a multimodal mechanochromophore in polymers.Bridging two pyrans with the sterically constrained dihydroanthracene is helpful to control the steric effect for the favorable formation of a distinctly appreciable bis-merocyanine(bis-MC)product.By varying the length of the polymer chains,the force delivered to the mechanophore is modulated,resulting in a gradient change in the relative distribution of two distinctly colored MC products and a multicolor mechanochromism.Mechanical activation of this bis-naphthopyanes proceeds via a mechanistically distinct pathway compared to the photochemical process.In addition,the bulk films can also achieve pronounced color changes when subjected to mechanical force.This study thus further expands the molecular diversity of mechanochromophores and tune the multimodal switch properties of bis-naphthopyrans based polymers.

Understanding AIE and ACQ phenomenon of organometallic iridium(Ⅲ) complexes by simple cationization engineering

Understanding the relationship between structure and properties is critical to the development of solidstate luminescence materials with desired characteristics and performance optimization. In this work, we elaborately designed and synthesized a pair of mononuclear iridium(Ⅲ) complexes with similar structures but different degrees of cationization. [Ir2-f][2PF_(6)] with two counterions is obtained by simple Nmethylation of the ancillary ligand of [Ir1-f][PF_(6)] which is a classic cationic iridium(Ⅲ) complex. Such a tiny modification results in tremendously different optical properties in dilute solutions and powders.[Ir1-f][PF_(6)] exhibits weak light in solution but enhanced emission in solid-state as well as poly(methyl methacrylate) matrix, indicative of its aggregation-induced emission(AIE) activity. On the sharp contrary, [Ir2-f][2PF_(6)] is an aggregation-caused quenching(ACQ) emitter showing strong emission in the isolated state but nearly nonemissive in aggregation states. Benefiting from the appealing characteristics of mechanochromic luminescence and AIE behavior, [Ir1-f][PF_(6)] has been successfully applied in reversible re-writable data recording and cell imaging. These results might provide deep insights into AIE and ACQ phenomenon of iridium(Ⅲ) complexes and facilitate the development of phosphorescent materials with promising properties.

Emission or scattering?Discriminating the origin of responsiveness in AIEgen-doped smart polymers using the TPE dye

Aggregation-induced emission(AIE)luminogens are attractive dyes to probe poly-mer properties that depend on changes in chain mobility and free volume.When embedded in polymers the restriction of intramolecular motion(RIM)can lead to their photoluminescence quantum yield(PLQY)strong enhancement if local microviscosity increases(lowering of chain mobility and free volume).Nonethe-less,measuring PLQY during stimuli,i.e.heat or mechanical stress,is technically challenging;thus,emission intensity is commonly used instead,assuming its direct correlation with the PLQY.Here,by usingfluorescence lifetime as an absolutefluorescence parameter,it is demonstrated that this assumption can be invalid in many commonly encountered conditions.To this aim,different poly-mers are loaded with tetraphenylenethylene(TPE)and characterized during the application of thermal and mechanical stress and physical aging.Under these con-ditions,polymer matrix transparency variation is observed,possibly due to local changes in refractive index and to the formation of microfractures.By combin-ing different characterization techniques,it is proved that scattering can affect the apparent emission intensity,while lifetime measurements can be used to ascertain whether the observed phenomenon is due to modifications of the photophysi-cal properties of AIE dyes(RIM effect)or to alterations in the matrix optical properties.

Luminescent d-f transition Yb(Ⅱ)-containing azacryptates with high photoluminescence quantum yields

Emission tunable d-f transition lanthanide complexes have wide applications in many fields such as lighting and photoredox catalysis.Compared with Ce(Ⅲ)and Eu(Ⅱ)complexes,which have achieved near-unity photoluminescence quantum yields(PLQYs),Yb(Ⅱ)complexes still suffer from low reported PLQYs not exceeding 10%.In this work,two luminescent Yb(Ⅱ)-containing azacryptates YbI_(2)-N8M6(N8M6=4,7,13,16,21,24-hexamethyl-1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane)and YbI2-N8E6(N8E6=4,7,13,16,21,24-hexaethyl-1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane)were synthesized and characterized.YbI2-N8M6 in solid powder exhibits blue emission with a maximum emission wavelength(λm)of 464 nm and a PLQY of 76%,while YbI_(2)-N8E6 in solid powder exhibits green emission with a λ_(m) of 537 nm and a PLQY up to93%.Moreover,YbI_(2)-N8M6 exhibits mechanochromic property,showing an emission color change from deep-blue to cyan under mechanical grinding.These results will inspire the studies about luminescent Yb(Ⅱ)complexes and their further applications.

Vinyl-functionalized multicolor benzothiadiazoles: design,synthesis, crystal structures and mechanically-responsive performance

Benzothiadiazole(BTD) has been extensively used as a building block in optical materials. In this work, a class of π-conjugated BTD-based luminogens BTD-685, BTD-580, BTD-675 and BTD-565 were designed by varying donor units and facilely synthesized by Heck coupling reaction. It was found that their emission in solid state covered the regio from orange, red to near infrared fluorescence. Investigation on photo-physical property manifested that they had strong solvatochromic behavior except symmetric 2-vinylpyridine substituted BTD-565. Crystal X-ray diffraction analysis revealed that they involved in multi weak intermolecular interactions. And loose molecular packing implied that they can be easily rearranged under external force stimuli.Indeed, all compounds showed reversible mechanically-responsive behavior in solid state. Interestingly, the vinylpyridinecontaining BTD-565 exhibited hypochromatic mechanochromic behavior, whereas others showed bathochromic mechanochromism behavior. It was worth mentioning that BTD-675 had self-recovery behavior after grinding. The powder X-ray diffraction study showed that these chromic processes may be mainly attributed to the reversible morphological changes between crystalline and amorphous phase upon grinding or fuming. These observations suggested that the vinyl-functionalized benzothiadiazoles can be considered as a type of excellent candidates in mechanically-responsive chromic materials.

Vinylpyridine-and vinylnitrobenzene-coating tetraphenylethenes：Aggregation-induced emission (AIE) behavior and mechanochromic property

Two functional tetraphenylethylene derivatives modified by vinylpyridine and vinylnitrobenzene,respectively, were synthesized by Heck coupling reaction. Their optical behaviors were investigated. The results showed they had AIE activity in solution. The property in solid state displayed that both of them had reversible mechanochromism. Upon grinding, their fluorescence spectra showed around 13–40nm red-shift, and could return to the original state after solvent fuming. We believe that this work will be helpful for the design of stimuli-responsive materials in future.

Mechanically controlled FRET to achieve high-contrast fluorescence switching

Organic luminescent materials with the ability to reversibly switch the luminescence when subjected to external stimuli have attracted considerable interest in recent years. However, luminescent materials with mechanochromic and photochromic dualresponsive properties are rarely reported. Hererin, we designed and synthesized a molecule P1 with dipeptide as a spacer to link rhodamine B and spiropyran moieties. P1 exhibited efficient photochromic properties both in solution and solid state. Highcontrast independent fluorescence switch was also realized under the stimulus of external force. Moreover, two-step ring opening reaction and subsequent fluorescence resonance energy transfer process between the donor-acceptor pairs within one single molecule achieved successive color switch by mechanical control. Therefore, this behavior of P1 made it a promising candidate for high-contrast and sensitive optical recording and mechanical sensing system.

Rhodamine Mechanophore Functionalized Mechanochromic Double Network Hydrogels with High Sensitivity to Stress

Mechanochromic hydrogels, a new class of stimuli-responsive soft materials, have potential applications in a number of fields such as damage reporting and stress/strain sensing. We prepared a novel mechanochromic hydrogel using a strategy that has been developed to prepare dual-network(DN) hydrogels. A hydrophobic rhodamine derivative(Rh mechanophore) was covalently incorporated into a first network as a cross-linker. This first network embedded with Rh mechanophore within the DN hydrogel was pre-stretched. This guaranteed that the stress could be transferred extensively to the Rh-crosslinked first network once the hydrogel was under an applied force. Interestingly, we found that the threshold stress required to activate the mechanochromism of the hydrogel was less than 200 kPa, and much less than those in previous reports. Moreover, because of the excellent sensitivity of the hydrogel to stress, the DN hydrogel exhibited reversible freezing-induced mechanochromism. Benefiting from the sensitivity of Rh mechanophore to both p H and force, the DN hydrogel showed p H-regulated mechanochromic behavior. Our experimental results indicate that the preparation strategy we used introduces sensitive mechanochromism into the hydrogel and preserves the advantageous mechanical properties of the DN hydrogel. These results will be beneficial to the design and preparation of mechanochromic hydrogels with high stress sensitivity, and foster their practical applications in a number of fields such as damage reporting and stress/strain sensing.

Aggregation-induced phosphorescence and mechanochromic luminescence of a tetraphenylethene-based gold(Ⅰ) isocyanide complex

In this study,a new twisting gold（Ⅰ） isocyanide complex based on tetraphenylethene（TPE）,TPE-NC-Au.was designed and synthesized.It exhibits aggregation induced phosphorescence（AIP） characteristics,owing to the incorporation of Au moiety and conformation rigidification in the aggregated states.Moreover,the emission color of the crystalline solid of TPE-NC-Au changes from blue（454 nm） to green（500 nm） in response to mechanical grinding,due to the combined effects of conformation planarization,enhanced π…π stacking,as well as the emergence of aurophilic interactions in the ground amorphous state.Notably,the emission color can be restored upon solvent fuming,associating with the reconstruction of crystalline lattices.The AIP and switchable mechanochromism of TPE-NC-Au make it suitable for potential applications in bioimaging,sensing,and optoelectronic devices.

Synthesis and characterization of an unexpected mechanochromicbistricyclic aromatic ene

An unexpected bistricyclic aromatic ene AF was synthesized in a tin(Ⅱ)chloride-mediated reductive aromatization reaction.The obtained AF showed a highly overcrowded structural conformation as revealed by X-ray crystallography.Interestingly,AF exhibited reversible high-contrast mechanochromism and thermochromism between pale and red color.The obvious chromism is likely ascribed to the conformation transformation and trace amount of diradical species formation upon stimulus.