Organic materials with hydrostatic pressure induced mechanochromic properties

Mechanochromic organic materials are a typical class of stimuli materials that has response to external physical stimuli such as shearing,grinding,and compressing etc.Organic compounds with mechanochromic characters in solid forms have attracted significant attention in the past decades due to their potential applications in sensors and memory devices.Diamond anvil cell is an emerging technology that can provide isotropic pressure in a tiny place.Thus a new stimuli method can be applied in investigating optical variation of mechanochromic materials.In this review,we focus on mechanoluminescence systems that are responsive to isotropic compression under high pressure and summarize the recent advances on organic materials studied by the diamond anvil cell.

Thickness-dependent wrinkling of PDMS films for programmable mechanochromic responses

We report a remarkable thickness-dependent wrinkling behavior of oxygen plasma-treated polydimethylsiloxane(PDMS)flms,in which an energy barrier separates the wrinkling mechanics into two regimes.For thick films,the film wrinkles with a constant periodicity which can be precisely predicted by the classic nonlinear finite mechanics.Reducing the film thickness below 1 mm leads to nonuniform wrinkles with an increasing periodicity which gives rise to random scattering and transparency changes under mechanical strains.By tuning the flm thickness,we were able to control both the quality and size of the periodic wrinkles and further design mechanochromic devices featuring briliant structural colors and programmable colorimetric responses.This work sheds light on the fundamental understanding of the wrinkling mechanics of bilayer systems and their intriguing mechanochromic applications.

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.

Recent Advances in Mechanism of AIE Mechanochromic Materials

Organic mechanochromic materials(also known as piezochromic materials),whose color or emission changes under mechanical force,have attracted great interest owing to their potential applications in pressure sensors,rewritable materials,optical storage,and security ink.Organic mechanochromic materials with aggregation-induced emission(AIE)features have better development prospects and research value owing to their excellent optical properties.To date,mechanochromism has mostly been realized by means of mechanical grinding.Nevertheless,the magnitude of the grinding force is usually uncontrollable and its direction is anisotropic,making it awkward to study the mechanism of mechanochromic materials.On the contrary,hydrostatic pressure,whose magnitude and direction are controllable,is a more valid and governable method to investigate the mechanism of mechanochromic materials,which can help us to construct a meaningful structure-property relationship and understand the latent origin of the mechanochromism.Furthermore,it is conducive to developing other mechanochromic material systems with desired chemical and physical properties.In this review,we focus on the recent progress in the mechanism of organic mechanochromic materials with AIE features under hydrostatic pressure.Four types of mechanisms are included:intermolecular interaction change,intramolecular conformation change,transformation from locally excited state to intramolecular charge-transfer state,and intra-and inter-molecular effects induced by hydrostatic pressure,respectively.

Progress in mechanochromic luminescence of gold(Ⅰ) complexes

Photophysical properties of organic and organometallic luminophors are closely related with their molecular packings, enabling the exploitation of stimuli-responsive functional luminescent molecules.Mechanochromic molecules, which can change their luminescence characteristics after mechanical stimulus, have received an increasing interest due to their promising applications in multifunctional sensors and molecular switches. During the past two decades, the development of gold(Ⅰ) chemistry has been attracting the attention of plenty of researchers. Indeed, a variety of gold(I) complexes with fascinating photophysical behaviors have been discovered. This review focuses on the research progress in the different types of mechanoluminochromic gold(Ⅰ) complexes, including mono-, bi-and multi-nuclear gold(Ⅰ)systems. Their interesting luminescence behaviors of these gold(Ⅰ)-containing luminogens upon mechanical stimulus and the proposed mechanisms of their observed mechanochromic luminescence are summarized systematacially. Moreover, this review will put forward an outlook about the possible opportunities and challenges in this significative scientific field.

A robust mechanochromic self-healing poly(dimethylsiloxane)elastomer

Simultaneously introducing mechanochromic and self-healing properties into polymers is almost a field unexplored,and the integration of these capabilities in one material has important theoretical and substantial significance.In this paper,a mechanochromic poly(dimethylsiloxane)(PDMS)elastomer with self-healing ability and superior mechanical properties is first reported.Spiropyran mechanophore and reversible hydrogen bonds are incorporated into the system to realize multi-stimuliinduced color change and self-healing ability,respectively.Upon uniaxial stretching,heating or UV irradiation,the elastomer exhibits a reversible color variation from yellow to purple,which can recover rapidly by white light illumination.Its excellent tensile strength(10.5 MPa)and elongation at break(785%)are distinctive among PDMS elastomers with no fillers.After heat treatment at 60℃for 24 h,the self-healing efficiency of strength can achieve 92.1%.This novel robust PDMS elastomer holds great promise for applications in visualized stress/strain sensing,self-healing biomaterials and wearable devices.

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.

Ultrasound responsive organogels based on cholesterol-appended quinacridone derivatives with mechanochromic behaviors

A series of cholesterol-appended quinacridone (QA) derivatives 1a-1d have been synthesized,in which 1b and 1c could form stable organogels in a wide range of organic solvents upon ultrasound irradiation.Field emission scanning electronic microscope (FESEM) and transmission electron microscopy (TEM) of xerogels or precipitates indicated that 1b and 1c formed 1D fibrous nanostructure,while 1a assembled into 3D flower-like microstructures.The ultrasound-induced organogel process was characterized by kinetic UV-vis and photoluminescence spectroscopic methods suggesting the formation of ?-? aggregates in the gel state.Experimental results demonstrated that the ultrasound could promote molecules to contact frequently in the solution and induce semistable initial aggregates,which propagate to form nano/micro superstructures.The aggregation model was optimized by semiempirical AM1 calculation suggesting the hierarchical self-assembly process.In addition,the formed xerogel film exhibited mechanochromic property,and the phase transition process was accompanied by the fluorescence changes between yellowish green and orange.

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.

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.

Design of High-Contrast Mechanochromic Materials Based on Aggregation-Induced Emissive Pyran Derivatives Guided by Polymorph Predictions

High-contrast mechanochromic(MC)materials are prominent candidates for sensor,security,and memory applications;however,the development of materials with a large luminescence change(Δλem>100 nm)under external stimuli is challenging.Considering that polymorphic molecules usually exhibit reversible mechanochromism,polymorph prediction is adopted for thefirst time to guide the design of high-contrastMC materials in this study.We designed and synthesized a series of donor–π–acceptor pyran derivatives bearing different electron donors and acceptors as model systems.The polymorph prediction indicated that 4-dicyanomethylene-2,6-distyryl-4H-pyran and 4H-pyran-4-one derivatives had the potential to crystallize in both monomer andπ-dimer aggregates,while barbituric acid-based compound tended to pack tightly in all aggregated states.The experimental results agreed well with the prediction that the derivatives potentially possessing both monomer andπ-dimer aggregate structures exhibit excellent MC behavior,whereas the fluorescence difference for the barbituric acid-based compounds is minimal.Moreover,a compound with an excellent fluorescence difference of three colors during reversible mechanochromism was chosen as the candidate for an optical recording material and security ink.This work proposes an effective method to guide the design of stimuli-responsive materials,which may open promising avenues for the development of high-contrast MC molecules.

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.

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.

Multifunctional 2-(2-hydroxyphenyl)benzoxazoles: Ready synthesis,mechanochromism, fluorescence imaging, and OLEDs

Triphenylamine(TPA)-containing 2-(2-hydroxyphenyl)benzoxazoles(2a-2c)have been synthesized via a highly efficient rhodium-catalyzed C–H/C–H cross-coupling reaction.Compound 2a is a novel mechanofluorochromic material with blue-shifted mechanochromic properties.Compounds 2b and 2c presented opposite mechanochromic trends.For 2b,the enol-form emission enhanced,and the ketoform emission blue-shift after grinding.In contrast,2c exhibited the weak enol-form emission disappeared and the keto-form emission slightly red-shift after grinding treatments.The estrone-containing2b-based water-dispersed nanoparticles(NPs)exhibit apparent dual-emission and were applied for fluorescence images.In addition,bis(TPA)-containing 2c-based devices exhibit dual-emission with good performance and a singlet exciton yield of 92%,which breaks through the theoretical upper limit of 25%in conventional fluorescent OLEDs.This is one of the highest exciton utilization values recorded for the ESIPT molecules with a dual emission system.

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.

A Versatile BenzimidazoleSubstituted Spirolactam Mechanophore:When Excited State Intramolecular Proton Transfer Couples with Rhodamine

Exploration of multicolor mechanochromic bulk polymers based on a single mechanophore is a big challenge to date.Herein,we report a versatile benzimidazole-substituted spirolactam mechanophore where excited state intramolecular proton transfer(ESIPT)coupled with rhodamine.The mechanophore was facilely synthesized and then covalently linked to polyurethane(PU)chains.The PU film containing the mechanophore(1@PU)showed cooperative photochromism upon irradiation involving simultaneous enhancement of normalized enol and rhodol emissions based on a cooperative ESIPT process and the ring-opening reaction of spirolactam.Moreover,the film exhibited dual-mode multicolor mechanochromism upon stretching and compression.The normalized intensity of enol emission increased and the fluorescence turned from light green to cyan after stretching,then red coloration appeared from colorless after compressing.Control experiments and density functional theory calculations confirmed that the stretch-induced increase of enol emission was attributed to torsion of the dihedral angle between xanthene and benzimidazole in the mechanophore via force-induced disaggregation and direct force action on the isolated mechanophore.Torsion of the dihedral angle and the ring-opening reaction of spirolactam in a single mechanophore occurred sequentially during compression,resulting in an observed red coloration.This study might provide a glimpse into the design of novel multicolor mechanochromic mechanophores.

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.

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.

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.