Contents
2 Mechano-responsive luminescent polymers based on physically doping
3 Mechano-responsive luminescent polymers based on chemically bonding
3.1 Mechano-responsive luminescent polymers based on intramolecular covalent-bond scission
3.2 Mechano-responsive luminescent polymers based on coordination-bond scission
3.3 Mechano-responsive luminescent polymers based on noncovalent-interaction disruption
1 Introduction
Polymer materials are widely used in packaging, coatings, biomedical supplies, textiles, adhesives, engineering composites and other fields because of their widely adjustable mechanical properties, low cost and easy processing in the manufacturing process, and become indispensable basic materials in human life and production process. Force stimulus-responsive luminescent polymers are a class of materials whose luminescent properties (such as luminescent color, fluorescence quantum yield or luminescent lifetime) change under the action of force stimulation. Compared with the conventional polymer matrix, the force-stimulus-responsive luminescence property endows this kind of material with a unique mechanical force indicating function, which has attracted wide attention in the fields of material damage detection, stress detection, sensing, non-destructive packaging and other applications
[1,2]. At the same time, the conduction of mechanical force in the polymer matrix leads to polymer chain slippage, recrystallization and even fracture, and the force stimuli-responsive luminescence also provides a feasible way to study the physical chemistry of polymerization at the molecular level (such as mechanical reinforcement, degradation, failure, etc.)
[3].
At present, mechanosensitive luminescent polymers are usually prepared by physical doping or chemical bonding, in which the groups whose luminescent properties can be changed under mechanical stimulation are introduced into the polymer matrix as mechanosensitive groups
[4]. The response mechanism of these materials is that under the action of force stimulation, the main chain of the polymer undergoes conformational changes, and the force is transmitted to the corresponding stress-sensitive group, resulting in changes in its molecular structure (such as intramolecular covalent bond cleavage) or supramolecular interaction (such as molecular conformation, molecular arrangement, phase structure, etc.), and then changes in luminescence properties
[5,6].
Therefore, the mechanoluminescence mechanism of the mechanosensitive group determines the stimulus-responsive luminescence performance of the polymer. Among them, the mechanical force can be effectively conducted to the luminescence mechanosensitive group through the polymer chain by the intramolecular covalent bond cleavage response of the luminescence system, but such systems generally require relatively high energy to trigger, and the reversibility is poor. In contrast, the mechanostimulus-responsive luminescence system that breaks the supramolecular interaction has lower trigger energy and higher reversibility, but the synthesis of some mechanosensitive groups is more difficult. In this paper, according to the luminescence principle of force-sensitive group, combined with the preparation methods of force-responsive luminescent polymers, the related research and application of force-responsive luminescent polymers were summarized, and the design ideas and development directions of future force-sensitive luminescent polymers were prospected.
2 Fabrication of mechanostimuli-responsive luminescent polymer by physical doping
The mechano-stimulus-responsive luminescence of organic small molecular materials is usually derived from the change of molecular conformation or molecular arrangement, which is a typical mechano-stimulus-responsive luminescent material that destroys the non-covalent interaction. Among them, direct dispersion, swelling-induced diffusion, or physical incorporation of molecular aggregates with vibration-induced emission (VIE) activity, aggregation-induced emission (AIE) activity, or easy to form excimers into polymer matrices is the simplest method to prepare mechanostimulus-responsive light-emitting polymers.
Ma et al. Doped typical VIE-molecule 9,14-diphenyl-9,14-dihydrodibenzo [a, C] -phenazine (DPAC) derivatives into polycaprolactone (PCL) and ethylene-vinyl acetate (EVA) copolymers to prepare force-stimulated responsive light-emitting polymer materials (Fig. 1)
[7]. When the polymer film is stretched by external force, the molecule can take on different conformations from bending to plane, so that the angle between the molecular configurations of the excited state is reduced, and the molecule shows continuous and significant force stimulus-responsive fluorescein behavior. Among them, the EVA polymer with good ductility can recover to the initial red fluorescence when the external force is removed, and its spectrum and scanning electron microscopy results also confirm its good reversibility. This design strategy, which combines continuous luminescence property changes with the internal environment of polymers, provides a new possibility for rapid and intuitive detection of microscopic changes in polymers.
AIE phenomenon was first proposed by Tang et al. In 2001. It refers to the phenomenon that compounds with weak or even quenched fluorescence aggregate with the increase of concentration in solution, and the fluorescence emission intensity increases gradually with the aggregation
[8,9]. In 2010, Park et al. Doped DBDCS, a cyanostyrene derivative with AIE activity, into poly (methyl methacrylate), and prepared a multi-stimulus responsive luminescent composite film capable of rapidly switching between blue and green light by using the conversion between two different crystal phases generated by the AIEgen under pressure, temperature, and solvent vapor stimulation conditions (Figure 2)
[10]. In 2014, Pucci et al. Physically doped typical AIEgen tetraphenylethylene (TPE) into styrene-butadiene-styrene block copolymer (SBS) and styrene-butadiene copolymer (SBR) elastomers.By changing the structure of the polymer matrix, the degree of rotation of the benzene ring in the TPE molecule is affected, and then the luminescence properties of the TPE are controlled, so that the polymer composite film with dynamic force stimulation response fluorescence characteristics is prepared
[11]. However, due to the AIE effect of TPE itself, the concentration of TPE in the composite film decreases with the thickness of the film during the stretching process, resulting in a decrease in the luminescence intensity of the film, which is also a common problem in the current direct physical doping preparation of mechano-stimulus-responsive luminescent polymers.
Planar conjugated molecules with high fluorescence quantum yield and easy excimer formation are also an important class of luminescence mechanosensitive groups. In the free state, these molecules tend to aggregate face-to-face to form excimers. Under the action of external force stimulation, the excimer dissociates and becomes a monomer again, and the fluorescence is blue-shifted accordingly, thus realizing the force stimulation response luminescence. To achieve the force stimulus-responsive luminescence from excimer to monomer transition in polymers, it is necessary to pre-dope excimer aggregates in the polymer matrix in the initial state. In 2000, Weder et al. First used an excimer-forming cyanodiphenylethylene (OPV) oligomer derivative as a luminescence mechanosensitive group to directly disperse the luminescence mechanosensitive agglomerate into linear low density polyethylene through a swelling-induced diffusion process to prepare a mechanostimuli-responsive fluorochromic material (Fig. 3A, B)
[12]. Subsequently, Weder et al. Optimized the preparation process, using a higher concentration of light-emitting mechanosensitive groups, and using the method of rapid quenching of the blend and plasticizing of the polymer matrix to prevent the phase separation of the mechanosensitive groups and the polymer matrix, which significantly improved the contrast of the material's force-stimulated response fluorescence discoloration
[13]. In addition, Weder et al. Also confirmed that the introduction of long alkyl chains to reduce the polarity of the mechanosensitive group of cyano OPV derivatives can effectively reduce their solubility in polar matrices, increase the nucleation rate, and thus promote the formation of aggregates. This strategy to improve the sensitivity of mechanostimulus-responsive luminescence has shown good universality in a variety of polymer matrices
[14]. A similar luminescence strategy is also applicable to force-stimulus-responsive luminescent polymers doped with stilbene derivatives. Pucci et al. Dispersed 4,4 '-bis (2-benzoxazolyl) stilbene into polypropylene film by melt mixing (Figure 3C)
[15]. When the doping concentration is higher than 0.2 wt%, the excimer characteristic luminescence appears at 500 nm. When the film is stretched, the luminescence color changes from the green luminescence of the excimer to the blue luminescence of the monomer.
The polymer composite material prepared by the direct physical doping is subjected to external force stimulation such as pressure, stretching and the like, and the force stimulation in the matrix responds to the crystal phase transformation or the concentration change of the luminescent small molecule aggregate, so that the luminescent property of the polymer composite material is correspondingly changed. This preparation method has the advantages of simple process and low cost, but the molecular aggregates tend to be randomly distributed in the polymer matrix, which makes it difficult to study the mechanical properties or States of a specific region
[16,17]. At the same time, the mechanical properties of the polymer are also impaired due to the phase separation of the doped SRL molecular aggregates from the matrix during long-term use.
In order to solve the above problems, people have developed the preparation method with microcapsule as the carrier, which is to wrap the AIE small molecule solution in the microcapsule, and then physically dope the microcapsule into the polymer matrix. Under the action of force stimulation, the shell of the microcapsule is broken, AIE molecules are released, and the released AIE molecules are aggregated and emit bright fluorescence. On the basis of traditional microcapsule carriers, Moore et al. Further incorporated microcapsules containing AIE small molecules with different luminescent colors into polymer coating materials in a bottom-up order (Figure 4A)
[18]. When cracks with different depths penetrate the coating, different AIE small molecules or a combination of more than two AIE small molecules will be activated, so that the luminescence color of the material changes with the increase of the damage depth, so that the damage depth and the crack propagation can be detected intuitively according to the corresponding fluorescence color. This multi-layer design, which can visually assess the damage depth by simply reading the luminous color, is not only a direct and effective method for rapid detection of microcracks, but also a general damage detection strategy compatible with a variety of manufacturing processes and coating materials. On this basis, Tang et al. Further optimized the traditional microcapsule system and prepared a dual-function microcapsule system with optical indication and autonomous self-healing characteristics
[19]. The repair agent hexamethylene diisocyanate (HDI) was used as a solvent to encapsulate with AIEgens into a microcapsule carrier, which was then physically incorporated into the polymer coating (Fig. 4B, C). Under the action of external force, the microcapsules in the polymer coating are broken, and the internal AIEgens/HDI solvent is released to the damaged area, wherein the HDI can react with water to form a polyurea (PU) solid, and the PU solid is automatically filled into the gap to repair the damaged area; The AIEgens dissolved in HDI aggregated in the repaired area of PU after solidification, and the obvious blue fluorescent indicator could be seen by observing the repaired cracks under ultraviolet light, which provided a simple, economical and feasible new way for the intelligent anticorrosive coating based on microcapsules.
It can be seen that compared with the direct dispersion system, the preparation strategy using microcapsules as the carrier can not only achieve a higher contrast of force-stimulated response luminescence, but also create a significant contrast between the damaged and undamaged regions of the material. However, the research on the mechanical strength (such as rupture threshold) of microcapsules prepared by different methods and the durability of microcapsules in complex environments is not yet perfect, so under certain load or environmental conditions, microcapsules may not be able to accurately sense the surrounding stress and fail. In addition, once the microcapsule carrier is broken, it can not be restored, and it is difficult to recycle, which also limits its practical application
[16,17].
3 Preparation of Force-Stimulus-Responsive Luminescent Polymers by Chemical Bonding
In order to overcome the inherent defects of physical doping methods, in recent years, the strategy of chemically bonding luminescent mechanosensitive groups into polymer chains by modification or polymerization has attracted more and more attention. This can not only fundamentally improve the problems of uneven dispersion of mechanosensitive groups or poor compatibility with polymer matrix in physical doping, but also help to improve the stability and fatigue resistance of materials
[2]. More importantly, by using the chemical bonding strategy, the luminescent mechanosensitive group can be embedded into the specific position of the polymer molecular chain through different chemical reactions, which is more flexible than physical doping in terms of operability, and can further realize the study of the mechanical properties or States of specific regions at the molecular level, and has a wider range of applications
[20⇓ ~ 22].
3.1 Force-Stimulus-Responsive Luminescent Polymers Based on Intramolecular Covalent Bond Cleavage
Covalent bond is the most basic and the most solid existence in the molecular structure. The energy required to break the bond is generally high. The process of breaking the covalent bond under external stimuli is often accompanied by energy changes (such as the generation of responsive luminescence signals). Among them, the luminescent mechanosensitive group based on covalent bond cleavage usually contains weak covalent bonds, and this cleavage precedes the cleavage of the polymer backbone under force stimulation, so it can effectively visualize the force conduction process acting on the polymer chain, and lay a molecular foundation for the study of damage detection of polymers under mechanical force. Spiropyran derivatives, bis (adamantyl) -1,2-dioxetane, tetraphenylsuccinonitrile derivatives, and metal complexes have been widely studied. Force stimulus-responsive light-emitting polymers based on intramolecular covalent bond cleavage have also become a hot research topic in recent years.
3.1.1 Force Stimulus Responsive Luminescent Polymers Based on Spiropyran Derivatives
Spiropyran (SP) derivatives are a class of ring-opening response luminescent systems with spiropyran as the skeleton, which can undergo ring-opening isomerization under ultraviolet light or force stimulation to become merocyanine (MC) structures with fluorescent properties (Figure 5); Under the stimulation of visible light, the merocyanine structure undergoes a reverse reaction and reverts to the original non-fluorescent spiropyran ring structure
[23]. Moore et al. First used spiropyran as a crosslinking agent to covalently bond into the main chain of poly (methyl acrylate) and poly (methyl methacrylate), and prepared a force stimulus-responsive luminescent polymer which can be used to monitor the change of stress
[24].
In recent years, with the in-depth study of spiropyran-based mechanosensitive systems, some mechanoresponsive luminescent polymers based on intermolecular interactions, such as F Förster resonance energy transfer (FRET), have been developed with higher luminescent efficiency and sensitivity
[25⇓ ~ 27].
Wang et al. Synthesized a force-stimulus-responsive luminescent polymer based on the FRET process
[28]. With nitrobenzoxadiazole (NBD) as the energy donor and spiropyran as the mechanically activated energy acceptor, the side chain of poly (methyl methacrylate) containing NBD was grafted onto the backbone of poly (butyl acrylate) through the spiropyran group (Fig. 6), so that the distance between the donor and acceptor was limited to 10 nm, resulting in a comb polymer PBA-SP-P (MMA-co-NBD). Under uniaxial tensile force, the luminescence color of the sample can be changed from green to red through an effective FRET process, and quickly changed to red after failure, realizing a force-stimulus-responsive luminescence process with high luminescence efficiency and high contrast.
In order to meet the needs of practical applications, researchers have also further improved the force-stimulated response luminescence properties of spiropyran-based mechanosensitive polymer materials by optimizing the linking mode of mechanosensitive groups in the polymer matrix, the structure of polymer matrix and processing methods.
When spiropyran is used as a force damage warning group in high performance structural materials, it is difficult to effectively activate the ring opening reaction of spiropyran in glassy polymers at room temperature, resulting in the brittle failure of matrix materials usually occurring before the force stimulus-responsive luminescence of spiropyrane
[29⇓⇓⇓ ~ 33]. In order to solve this problem, Silberstein et al. Developed the bisphenol structure of dihydroxyspiropyran, which was directly introduced into the main chain of bisphenol a polycarbonate (BPA-PC) as a bisphenol comonomer by polycondensation.The typical low-yield SP functionalization step was effectively avoided, resulting in a glassy polymer SP-BPA-PC (fig. 7) that can achieve mechanochromism at room temperature by applying a tensile load, and achieved a differentiated response to strain rate and strain size
[34]. The mechanochromic property can effectively track the surface deformation of the sample, and is an ideal damage sensing material.
Park et al. Developed a synthetic strategy combining responsive polymers with a layered flexible framework, and reported a nanoparticle-microporous (NP-MP) structured force-stimulus-responsive color-changing polymer composite with spiropyran as the force-sensitive group, poly (dimethylsiloxane) as the matrix, and silica nanoparticles as the filler (Fig. 8)
[35]. Among them, the layered structure can disperse the stress from the macroscopic scale to the smaller nanoscale structure, causing stress concentration in the microporous region decorated by silica nanoparticles. Compared with the composites without porous structure, the force sensitivity and stretchability of the porous structure are significantly improved. At the same time, the pores inside the material can quickly store and release energy, so that the material shows multiple reversible cycles and highly stable force-stimulated color change response. This method of stress focusing through matrix structure optimization can be further extended to other composites with force-sensitive clusters to enhance the sensitivity and stretchability of their force stimulus response.
In order to achieve a distinctive response to stress in different directions, Gualandi et al. Prepared a copolymer nanofiber containing spiropyran groups by electrospinning
[36]. Due to the directional arrangement of the spiropyran copolymer chains, the nanofibers can achieve the force-stimulus-responsive discoloration of spiropyran at a low strain of 5% (fig. 9). Such nanofibers with highly oriented chains were composited with a polydimethylsiloxane matrix to obtain a stress/strain elastic force stimulus-responsive luminescent composite P (SP-alt-C10) that is highly sensitive to the stress direction and reversible. This structural optimization strategy by constructing anisotropic mechanosensitive clusters provides a feasible method to achieve stress response in different directions.
Naphthopyran is also considered to be a potential luminescence mechanosensitive group because of its ring-opening reaction ability similar to spiropyran. Moore et al. studied the effect of the position of the active substituent, i.e., the site of stress application, on the naphthopyran force-stimulated response luminescence activity (Fig. 10)
[37]. Both density functional theory calculations and experimental results confirmed that only the 5-substituted naphthopyran, whose C — O pyran bond cleavage direction was in good agreement with the direction of external mechanical force, could be transformed into merocyanine derivatives under force stimulation, thus showing force stimulus-responsive color change activity. This result lays a foundation for the application of naphthopyran mechanosensitive groups in mechanostimuli-responsive luminescent polymers.
At present, great progress has been made in the luminescent efficiency of the luminescent mechanosensitive group based on spiropyran derivatives, but there are still some problems, such as limited conversion rate of ring-opening isomerization, high compatibility requirements with the polymer matrix, uneven distribution in the polymer network, and difficulty in achieving response luminescence in the low strain region
[38]. Therefore, further reducing the response threshold of the opening/closing reaction of the spiropyran bond and improving the compatibility between the force-responsive luminescent group and the polymer matrix through structural optimization will provide effective guidance for the development of such materials in the future.
3.1.2 Mechano-stimulus-responsive chemiluminescent polymers based on bis (adamantyl) -1,2-dioxetane derivatives
Bis (adamantyl) -1,2-dioxetane can be decomposed into two adamantanone groups (Fig. 11) under chemical reaction or thermal stimulation, and one of them is in the electronically excited state and produces blue light with a wavelength of 420 nm during the deexcitation process. Its highly sensitive luminescence performance is widely used in the field of bioassay
[39,40]. In 2012, Sijbesma et al. First introduced bis (adamantyl) -1,2-dioxetane into the backbone of methyl acrylate to prepare a force-stimulus-responsive luminescent polymer with self-luminescent characteristics
[41]. The polymer does not emit light in the initial state, but produces blue fluorescence after being stimulated by mechanical force.
The advantage of bis (adamantyl) -1,2-dioxetane over other luminophores is that it has a self-luminous characteristic without an excitation light source, and the luminous intensity is proportional to the number of cleaved covalent bonds. Furthermore, the activation of bis (adamantyl) -1,2-dioxetane is directly visualized and enables real-time monitoring of chain scission events with high time resolution.
On this basis, Chen et al. Developed a series of thermoplastic elastomers by introducing bis (adamantyl) 1,2-dioxetane into the main chain of the polymer, and systematically studied the failure mechanism of the polymer, which broadened the application of force-responsive light-emitting polymers based on dioxetane derivatives in the field of polymer composites
[42,43]. Compared with the dim blue luminescence of the adamantanone radical, the luminescence signal can be further amplified and the response force threshold can be reduced by transferring the energy to a suitable acceptor through the FRET process,It is a significant and efficient sensitized luminescence strategy to make the location and time of bond breakage in the failure process have higher resolution and improve the sensitivity of bond breakage detection. For example, Chen et al. Used the nanosheet filler in the composite-fluorescent conjugated microporous polymer as an acceptor to study the transition process of material luminescence from blue light to orange light under force stimulation, which opened up a new way to study the failure mechanism of polymer nanocomposites at the molecular level
[27]. Chen et al. Also used 4,7-di (thiophen-2-yl) benzo [C] [1,2,5] thiadiazole as an energy acceptor, and realized the quantitative distinction between force-induced chain slip and chain breakage in polyurethane/siloxane composite films by using the proportional relationship between the number of dioxetane bond breaks and the cumulative luminescence intensity
[44]. Combined with the high sensitivity and resolution of adamantane mechanosensitive group in space and time, Chen et al. Also used the luminescence of 4,7-di (thiophen-2-yl) benzo [C] [1,2,5] thiadiazole receptor to convert the stress distribution and chemical bond breakage during the failure process of polymethyl acrylate/cellulose nanocrystal composite into visual optical signals
[45]. In addition, Chen et al. Also introduced the red light Eu (Ⅲ) complex Eu (TTA) 3 (Aphen) into poly (methyl acrylate) as a side chain and energy acceptor
[46]. The polymer film shows strong red emission under force stimulation. Compared with the pure organic energy acceptor, the metal complex with antenna effect can effectively promote the energy transfer process in the polymer network through sensitized luminescence, further reduce the response threshold, and improve the sensitivity of bond cleavage detection.
Chen et al. Prepared a covalently crosslinked polyolefin using 1,2-dioxetane as a crosslinking agent and a luminescent stress probe
[47]. Benefiting from the low crosslinking density and the hydrogen bonds introduced by the amide side groups into the system, the crosslinked network has high chain mobility and a large number of non-covalent interactions, which synergistically endow the covalent crosslinked network with good elasticity and self-healing ability. More importantly, the 1,2-dioxetane probe can not only indicate the process of chemical bond breakage under tensile stress, but also characterize the repair efficiency in the self-repair process by sensing the stress generated by chain rewinding on the fracture surface. This study expands the application range of 1,2-dioxetane mechanosensitive groups in different polymer systems (Fig. 12).
3.1.3 Force Stimulus Responsive Luminescent Polymers Based on Tetraphenylsuccinonitrile Derivatives
Conventional mechanosensitive clusters based on covalent bond cleavage tend to generate more stable chemical structures under force stimulation, resulting in poor response reversibility. In order to solve this problem, Otsuka et al. Reported a dynamic covalent color-changing luminescence system that produces metastable free radicals under the action of force stimulation, and the system can autonomously return to a stable thermodynamic original state without by-products after removing the external force
[48]. The force-stimulated luminescence of this system originates from the pink diarylacetonitrile (DAAN) radical with yellow fluorescence emission generated by the homogeneous cleavage of the central dynamic C — C bond of the tetraphenylsuccinonitrile (TASN) derivative under mechanical force (Fig. 13). TASN usually exists as a unimolecular dimer at room temperature, but in solution at mild temperature, it exists in dynamic equilibrium with the DAAN radical produced by homogeneous cleavage
[49⇓ ~ 51]. When TASN was attached to the polymer chain, the color of the polymer changed from white to pink under sunlight and its fluorescent color changed from blue to yellow under ultraviolet lamp under force stimulation. Therefore, TASN is a potential luminescence mechanosensitive group with high sensitivity and can be used for quantitative detection.
In 2018, Otsuka et al. Reported a polyurethane gel system with TASN as the crosslinking point. After swelling-freezing treatment, the gel cube sample showed a color change from colorless to pink, accompanied by yellow fluorescence (Fig. 14 a, B, C)
[52]. Electron paramagnetic resonance (EPR) spectroscopy confirmed that the luminescence process originated from the DAAN radical generated by the dissociation of TASN due to the internal stress caused by freezing after the gel was swollen by solvent. Therefore, the visualization of the internal stress generated in the gel network was achieved by using the force-stimulated response luminescence property of TASN with low response threshold. On this basis, Otsuka et al. Further improved the stability of the gel, the efficiency of force conduction and the sensitivity of the response to internal stress by increasing the content of static crosslinking points in the dynamic crosslinking network, which provided a new idea for the detection of gel internal stress at low temperature
[53]. At the same time, Otsuka et al. also introduced TASN into polymer/silsesquioxane composites with adjustable mechanical properties (Fig. 14d). The rigid covalent network not only improves the dissociation rate of TASN, but also endows these materials with high visibility and high sensitivity of force-stimulated response luminescence behavior
[51].
In recent work, Otsuka et al. introduced DAAN as a side chain into the polystyrene backbone (Figure 15A), effectively increasing the compatibility of the DAAN group in the polymer matrix
[54]. Under the action of force stimulation, the free radicals generated by the breakage of the polymer main chain are transferred to DAAN to generate DAAN free radicals, and the high-sensitivity detection of the breakage of the polymer main chain is realized through a fluorescence signal; At the same time, the free radicals generated on the main chain are effectively removed, the possible chain decomposition is inhibited, and the thermal stability of the material is improved. This work not only helps to clarify the fracture mechanism of polymer materials without mechanosensitive groups in the backbone, but also provides a guiding direction for improving the properties of materials.
Similar to the structure of TASN, difluorenylsuccinonitrile (DFSN) cleaves the central C — C bond in response to force stimulation, generating a cyanofluorene (CF) radical in pink with yellow fluorescence (Fig. 15B). Otsuka and Ito et al. Designed polyurethanes with DFSN mechanosensitive groups in the main chain and methacrylate groups in the side chain (Fig. 15b)
[55]. Under force stimulation, DFSN generates active CF radicals, and the color of the polymer changes to pink, resulting in yellow fluorescence. At the same time, free radicals also initiate the crosslinking reaction of side chains, which makes the material stronger and stronger under pressure. This design and synthesis strategy of force load response, force damage warning and material self-reinforcement is also applicable to other polymer matrix networks
[56]. More importantly, such force-sensitive groups not only play an important role in the construction of macroscopic polymer multifunctional materials, but also can sense small stresses and be used to visualize the crystallization process of polymers under microscopic conditions (Fig. 15C)
[57].
3.1.4 Diels-Alder adduct based mechanostimulus-responsive luminescent polymer
Diels-Alder adducts are a kind of typical mechanosensitive groups which respond to mechanical stimulation by cleaving the cyclohexene bond to produce the corresponding alkene and conjugated diene derivatives, and have important research significance in the field of polymer mechanochemistry. Boydston et al. Synthesized a linear polymer containing an anthracene-maleimide Diels-Alder adduct and a star three-arm polymer, respectively, in order to study the correlation between the chain scission rate involved in mechanochemistry and the polymer structure (fig. 16)
[58]. Under the action of ultrasound, the reverse Diels-Alder reaction produces anthracene chromophore, which produces fluorescence signal. This work proves from the kinetic point of view that the chain scission rate of polymer is determined by the length of any two arms emanating from the core.It provides quantitative guidance for improving the mechanochemical reaction rate of macromolecules by controlling the overall structure of polymers.
In polymeric mechanochemistry, in order to study the conduction efficiency of mechanical force from the polymer to the weak chemical bond in the mechanosensitive group, Stevenson et al. Introduced four mechanosensitive group isomers into the central region of the poly (methyl acrylate) backbone by changing the attachment site and steric structure on the mechanoactive group of the furan/maleimide Diels-Alder adduct (Fig. 17)
[59]. The results show that the rate of reverse Diels-Alder reaction of polymer in solution under ultrasonic irradiation is mainly determined by the attachment sites on the mechanosensitive group. The orientation relationship between the direction of the scissile covalent bond and the force vector in the force-sensitive cluster determines the degree of difficulty in breaking the covalent bond, and the force conduction efficiency is the highest when the direction of the covalent bond is consistent with that of the force vector. This study proves that different proportions of mechanosensitive group isomers can be mixed in polymers to adjust the overall mechanical properties of materials, and also provides a new supplement to dynamic covalent chemistry.
3.1.5 Ladder alkane-based force-stimulated response luminescent polymer
Biological systems in nature can sense and respond to force stimuli in complex ways, and Xia et al., inspired by the structure of lipid natural products, designed a force-stimulation-responsive polyladene non-conjugated polymer
[60]. Under the action of ultrasound, the unstable cyclobutane ladder series units in polyladene undergo ring-opening reaction, and a series of σ bonds that can be strained by force are converted into π bonds, so that the original discontinuous insulating non-conjugated system is expanded into a continuous semiconductor large conjugated system. In addition, the extended conjugated long chains can be further self-assembled into semiconductor nanowires under the action of ultrasonic waves, so that the solution gradually becomes dark and finally turns black. This kind of system, in which the macromolecular structure is reconstructed by force stimulation to completely change its inherent characteristics, provides a new idea for the design of force-responsive polymers.
On the basis of the above work, in order to solve the problems of difficult synthesis and easy interruption of the polymer chain expansion process, Xia et al. Used the benzotetraene unit to replace the original tertiene unit (Fig. 18), and the prepared force stimulus response polymer also achieved controllable polymerization expansion under the action of ultrasound, and the color deepened with the extension of the force action time
[61]. This work simplifies the mechanochemical activation step of ladder alkanes and further enriches the types of ladder alkane polymers.
3.1.6 Other force-stimulus-responsive luminescent polymers based on intramolecular chemical bond cleavage
In addition to the widely used mechanoresponsive luminescent groups discussed above, some potential luminescent systems with similar chemical bond cleavage mechanisms are also suitable for the construction of mechanoresponsive luminescent polymers.
Excited state proton transfer (ESIPT) is a process in which the proton on the donor in the excited state of a molecule is transferred to the adjacent acceptor through an intramolecular hydrogen bond, usually accompanied by significant fluorescence emission
[62]. Based on this principle, Weder et al. Reported a 2- (2 '-hydroxyphenyl) benzoxazole derivative, which was introduced into the molecular chain of polymethyl acrylate by esterification (fig. 19)
[63]. Under the action of ultrasonic wave, the luminescence of polymer solution gradually changed from initial blue light to ESIPT characteristic broadband emission centered at 500 nm, which proved that mechanical force could induce the dissociation of C — O ester bond of aromatic ester in 2- (2 '-hydroxyphenyl) benzoxazole.And the reduced 2- (2 '-hydroxyphenyl) benzoxazole group can be used as a luminescence mechanosensitive group to realize the response luminescence to mechanical stimulation through an ESIPT process. As a typical representative of ESIPT luminescence, this molecule has potential universality when used in polymer luminescence mechanosensitive groups.
In addition to spiropyran derivatives, rhodamine derivatives are also an important class of ring-opening luminescent mechanosensitive groups. Jia et al. Reported a rhodamine derivative Rh-OH with force-stimulated reversible color-changing luminescence characteristics in the solid state (Fig. 20)
[64]. Rh-OH was covalently bonded into the polyurethane elastomer matrix to limit the isomerization of the rhodamine group, and it was found that the covalently bonded film, when subjected to an applied pressure,It can effectively induce the ring-opening reaction of Rh-OH from the twisted spirolactam structure in the ring-closed state to the planar zwitterion in the ring-opened state, thus showing the force-stimulated response fluorescence color change behavior, while no change was found in the control sample. The film can be restored to the initial state by heating at 100 ° C for a few minutes or standing at room temperature for a period of time. The reversible force-stimulus-responsive light-emitting film can promote the practical application of such materials. At the same time, this synthetic strategy of using the polymer network to restrict the isomerization of rhodamine groups also provides a new direction for the preparation of rhodamine-like force-stimulus-responsive luminescent polymers.
3.2 Force-Stimulus-Responsive Luminescent Polymers Based on Coordination Bond Cleavage
Coordination bond is a special kind of covalent bond, and the energy required to break the coordination bond involving metal ions is less than that of the general covalent bond in most cases. In addition, lanthanide metal elements with antenna effect can produce strong metal ion center luminescence at d-f transition. Therefore, the corresponding lanthanide metal complexes become an important class of luminescence mechanosensitive groups
[65⇓⇓ ~ 68]. On this basis, the ligand is attached to the end of the polymer chain, and the metal ion dynamically connects the polymer chain through the coordination bond, and the prepared telechelic polymer becomes an important class of force-responsive luminescent materials.
He et al. Prepared a europium-containing luminescent polymer hydrogel by introducing a europium-iminodiacetate complex into a hydrophilic polydimethylacrylamide matrix (Fig. 21)
[69]. Under the action of ultrasonic wave, the luminescence intensity of the hydrogel sample originated from trivalent europium ion is obviously reduced due to the dissociation of the europium-iminodiacetate complex; When the ultrasonic source was removed, the luminescence of the sample recovered rapidly within 2 min. They also further studied the fluorescent color change behavior of the hydrogel material under compressive stress. The red luminescence intensity of the hydrogel decreases rapidly after compression, and the luminescence intensity can recover rapidly after releasing the stress. Weder et al. Assembled a ligand with europium salt and telechelic poly (ethylene-co-butene) and 2,6-bis (1 '-methylbenzimidazolyl) pyridine (Mebip) at the end of the polymer chain to form a metallosupramolecular polymer with a crosslinked network
[70]. Under the action of the ultrasonic wave, due to the dissociation of the ligand, the dilute polymer solution with the original luminous color of saturated red turns into blue, and the metal supramolecular polymer is reassembled within a few minutes after the ultrasonic wave stops.The solution returned to the original luminescent color, the whole process was completely reversible, and the degree of dissociation increased with the length of the applied ultrasonic pulse. Compared with the conventional covalent bond breaking process, the fast reversible force stimulation response luminescence characteristic shows obvious advantages. When the Mebip ligand is replaced by a dipicolinate (dpa) ligand with stronger coordination ability, the dpa ligand does not dissociate when exposed to ultrasound, and the luminescence color does not change significantly.This proves that the force stimulus response properties of this kind of materials can be adjusted by changing the strength and dynamic characteristics of the metal-ligand coordination bond to achieve reversible and irreversible mechanically induced reactions. In addition, this metallosupramolecular polymer also has the ability to induce metal exchange reaction by force stimulation, and the film is sonicated in a solution containing Fe
2+.The Eu
3+ in the metallo-supramolecular polymer undergoes an exchange reaction with Fe
2+, and the color of the film gradually changes to deep purple, while the morphology and mechanical properties are basically unchanged, and this irreversible color change can also be triggered by mechanical forces other than ultrasonic waves.
From the above results, it can be seen that the luminescence system responding by intramolecular chemical bond cleavage generally requires relatively high energy to trigger, and the specificity and reversibility of the triggering conditions are poor. For example, spiropyran-containing stimulus-responsive luminescent materials may also respond to luminescence by breaking weak covalent bonds triggered by light and thermal stimuli. At the same time, additional conditions need to be satisfied in order to achieve a reversible reaction. With the decrease of bond energy, the force-stimulated response luminescence process based on the cleavage of coordination bond shows obvious advantages in reducing the response threshold and improving the reversibility. Therefore, it can be speculated that when the bond energy is further reduced and the non-covalent bond is destroyed, such as the force stimulus-responsive luminescence system of intermolecular supramolecular interaction, the trigger energy will be further reduced, and its reversible recovery will be greatly improved.
3.3 Force-Stimulus-Responsive Luminescent Polymers Based on Disrupted Noncovalent Interactions
Force-stimulus-response luminescence system based on non-covalent bond cleavage refers to the system dominated by intermolecular interaction forces (mainly including hydrogen bonding, dipole-dipole interaction, π-π interaction, van der Waals force, etc.). Under the action of force stimulation, the non-covalent interaction in the system is destroyed, resulting in a responsive luminescence signal
[5]. Unlike the process of breaking covalent bonds to emit light, breaking noncovalent bonds requires less force
[71⇓ ~ 73]. Therefore, as a force stimulus-responsive light-emitting unit, such systems generally have a lower response threshold. Moreover, the way of compounding this kind of system with polymer matrix is more flexible and diverse. This system will be reviewed from the perspective of material preparation in the following.
When the intramolecular torsion angle of donor-acceptor molecules with twisted structure changes, the absorption or emission spectra of the molecules usually change accordingly. Based on this principle, Sommer et al. Designed an electron donor-acceptor supramolecular mechanosensitive group based on molecular conformational transition, diphenylpyrrolopyrroledione (PhDPP), and covalently bonded it into the polystyrene matrix
[74]. In the free state, the mechanosensitive molecule is in a highly distorted conformation due to the steric hindrance between the paired benzene ring donor and pyrrolopyrroledione acceptor. When uniaxial force is applied to the polymer sample, the mechanical force induces the decrease of the torsion angle between the donor-acceptor groups in the mechanosensitive group molecule and the increase of the electron transport ability between the donor-acceptor groups, resulting in the red shift of the mechanosensitive luminescence. This force-stimulated response luminescence process, which depends on the conformational transition of the force-sensitive group molecule, responds quickly when stress is applied and returns to the initial state quickly after the stress is released, so that the force-stimulated response luminescence wavelength of the material changes continuously with the stress and has good reversibility (Fig. 22).
Similarly, it is also a simple and effective strategy to prepare force-responsive light-emitting polymers by inserting AIEgens into the main chain of the polymer as a force-sensitive group or into the polymer matrix as a side chain, and using the responsive light-emitting signal generated by the change of the relative position between AIEgens molecules under mechanical stimulation
[17]. For example, Yang et al. incorporated AIEgen into a non-conjugated polyurethane matrix to obtain an elastic polymer with aggregation-induced emission enhancement (AEE) characteristics
[75,76]. Under the action of stretching force, with the increase of strain, the fluorescence intensity of the polymer sample increases first and then decreases due to the aggregation and dissociation of AIE groups. The strain-sensitive force stimulus-response luminescence process reflects the process of relative slippage and rearrangement of polymer chain segments under the action of stretching force, and compared with the force stimulus-response luminescence process of breaking covalent bonds,It not only has a lower response threshold, but also provides a visual research method for the phase transition process of force-induced polymer crystallization, which has important enlightenment for the development of new force-stimulated luminescent polymers with low response threshold.
In addition to AIEgens, force-responsive light-emitting polymers can also be constructed by chemically bonding excimer-prone planar conjugated molecules into polymer molecular chains. Weder et al. Prepared ureido-4-pyrimidone (UPy) -terminated oligomer UPy-OPV-UPy 2 (Fig. 23) by using the typical planar conjugated molecule 1,4-bis (α-cyano-4 (12-hydroxydodecyloxy) styryl) -2,5-dimethoxybenzene (OPV) as the luminescence mechanosensitive group
[77]. Under the action of multiple self-complementary hydrogen bonds, these oligomers self-assemble into supramolecular polymers. Among them, the OPV group in the center of UPy-OPV-UPy 2 spontaneously aggregates in the supramolecular polymer chain through π-π stacking to form excimer, and exhibits excimer red luminescence. When the surface of the sample is scratched, the color of the scratched part changes from red to yellow, indicating that the distance between the OPV parts in the supramolecular polymer segment has changed. With the advantages of dynamic supramolecular self-assembly, high sensitivity, and low response threshold, the force stimulus-responsive luminescence process of excimer-monomer interconversion has also attracted attention in other polymer systems.
On the basis of ion-pair complexes, Sijbesma et al. Introduced 8- (2-hydroxyethoxy) pyrene-1,3,6-trisulfonic acid (HEPTS) as an end group into polyurethane, and developed a force-stimulus-responsive luminescent telechelic polymer PU-HEPTS (Fig. 24)
[78]. PU-HEPTS forms aggregates through HEPTS ionic end groups in nonpolar media, and the degree of aggregation in solution depends on the concentration of HEPTS end groups and the polarity of the solvent, and the effect of solvent polarity is much greater than that of concentration. In N, N-dimethylformamide solution, only the luminescence peak of the monomer was observed at 426 nm due to the high solubility of the HEPTS end group in the solvent. However, in the poor solvent tetrahydrofuran and the film, the emission peak shifts to 510 nm due to the formation of aggregates. Under the stretching force, the emission peak ratio of the monomer in the PU-HEPTS film gradually increased, while the emission spectrum of the control sample containing only free HEPTS did not change significantly, which proved that the coupling of HEPTS attached to the polyurethane end and polyurethane promoted the overall mechanical activation of the aggregate.
In order to further constrain the relative position of the excimer-forming groups in space, Weder et al. Covalently attached the perylene diimide (PDI) chromophore to the main chain of the polymer through a short alkoxy chain, endowing the polymer with macroscopic force-stimulated response luminescence properties (Fig. 25)
[79]. During the stretching process of the prepared film sample, the folded PDI dimer caused by the intermolecular interaction in the polymer backbone will gradually open, and the luminescence color will gradually change from orange excimer luminescence to green monomer luminescence. In contrast, polymer PMA-P, which contains only a single PDI group, can only form interchain excimers, while polymer PMA-L, which contains two PDI groups connected by an alkoxyl chain, mainly forms intrachain excimers. Therefore, the excimer formation of PMA-L requires a lower concentration of PDI, and the PMA/PMA-L blend has better thermal stability and reversible mechano-stimulus-responsive luminescence properties than the PMA/PMA-P blend.
By adding oligomeric flexible hinge segments at both ends of the PDI chromophore that can form excimers, Li et al. further constrained its spatial position, introduced it into the side chain of the comb polymer, and used the folding/unfolding process of the PDI excimer monomer to achieve high-contrast two-color fluorescence conversion (Fig. 26)
[80]. In the initial state of the prepared liposome gel, the PDI dimer is a compact folded structure, and the PDI dimer shows obvious orange excimer luminescence; When the gel absorbs the organic solvent and expands 9 times in volume, the folded dimer unfolds in the cross-linked gel and emits intense bright green PDI monomer luminescence. The force stimulation response luminescence phenomenon generated by the expansion-contraction of the gel can maintain 30 expansion-contraction switching cycles, has remarkable reversible performance and anti-fatigue performance, effectively avoids the process of covalent bond breakage and recombination in the traditional force sensitive group, and minimizes the loss of the molecular structure level.
Sagara et al. Devoted to the development of cyclophane derivatives with symmetric or asymmetric structure and aggregation-dependent luminescence, and proved that cyclophane is a reliable and effective stimulus-responsive luminescent material
[81⇓⇓⇓⇓⇓⇓ ~ 88]. On the basis of bis-1,6-bis (phenylethynyl) -pyrene fluorophore cyclophane, Sagara et al. Chemically bonded this cyclophane structure into the polyurethane elastomer backbone
[89]. Under the action of tensile force, the polyurethane sample shows a transition from excimer-dominated cyan luminescence to monomer-dominated blue luminescence (fig. 27). Because the force stimulation is to change the molecular conformation of the mechanosensitive group, that is, the intramolecular interaction between the luminescent groups, it does not involve the breakage and formation of chemical bonds, and it is the force stimulation response luminescence generated by the mechanosensitive group at the single molecular level. Therefore, the change of luminous color and deformation during the whole stretching and recovery process are instantaneous and reversible, and this response characteristic can be maintained after 25 cycles of cyclic stretching. This work proves that the conformational change of cyclophane structure can affect the non-covalent interaction between multiple luminescent groups in the molecule, thus affecting their luminescence at the single molecule level. Therefore, cyclophane based supramolecular luminescent mechanosensitive groups have great potential in the construction of mechanoresponsive luminescent polymers with low response threshold, high sensitivity and reversibility.
The excimer-monomer conversion luminescence mechanosensitive group based on supramolecular interaction has a simple molecular structure, and on this basis, by changing the structure of the luminophore, the turn-on type force stimulation response luminescence with low response threshold and high contrast can also be realized. Takeda et al. Reported a supramolecular luminescent mechanosensitive group based on intermolecular charge transfer (CT), using pyrene (Py) as the charge donor and naphthalimide (NDI) as the charge acceptor, and covalently linking the donor and acceptor at a stoichiometric ratio of 1:2 through a copper-catalyzed alkyne/azide click reaction (Fig. 28)
[90]. Subsequently, the luminophores were covalently incorporated into the polycaprolactone (PCL) matrix by ring-opening polymerization of ε-caprolactone. The fluorescence of pyrene is effectively quenched in the initial state of the prepared film. During the stretching process, the fluorescence intensity derived from pyrene gradually increases as the pyrene-naphthalimide intermolecular charge transfer pair is destroyed. However, due to the crystallinity and limited molecular mobility of PCL, the reversibility based on the CT interaction force sensitive group is not ideal. Therefore, the system needs to be further improved to meet the reversibility detection in practical applications.
Rotaxane is a kind of supramolecular group with molecular shuttle function, and its "wheel" and "axis" parts will change their relative positions in space under force stimulation (Fig. 29), resulting in the destruction of its supramolecular interaction, thus producing responsive luminescence
[91]. Compared with spiropyran and other luminescent mechanophores based on covalent bond cleavage, the molecular shuttle structure of rotaxane can ensure that it is not affected by heat, light and other stimuli, and has obvious luminescence specificity in response to mechanical stimuli, which has advantages that other materials do not have to a certain extent.
Sagara et al. Chemically bonded donor and acceptor groups based on CT interactions into polyurethane elastomers (less than 1 wt%) as the "wheel" and "axis" of the rotaxane, respectively (Figure 30 a)
[26,92]. When the polymer is stretched, the donor and acceptor groups in the rotaxane are spatially separated, and the material exhibits efficient light-up type force-stimulus-response luminescence. By changing the structure of the donor, the polyurethane elastomer realizes the green and red emission of the force stimulus response turn-on type respectively; On this basis, Sagara et al. also chemically bonded rotaxane molecules containing blue, green, and red luminescent groups into the polyurethane matrix in a molar ratio of 8:16:5, so that the material achieved force-stimulated white light emission (Fig. 30b)
[93]. The force stimulus-responsive luminescence process has instantaneous reversible response characteristics, and the CIE coordinates of the emission spectrum can remain unchanged after 20 cycles of stretching. By replacing the fluorescent group and selectively combining a plurality of light-emitting mechanosensitive groups in one material, the light-emitting color of the polymeric material in response to stimulation can be easily adjusted, which is a major feature of the rotaxane light-emitting mechanosensitive groups and also makes up for the vacancy of the mechanically induced white light elastomer material. However, up to now, the synthesis of rotaxane mechanosensitive groups is still difficult, which hinders the further promotion of this kind of mechanoresponsive luminescence system to a certain extent.
Besides rotaxane system, catenane is also a kind of potential supramolecular luminescence mechanosensitive group with three-dimensional molecular structure. Takeuchi et al. Reported a catenane molecule consisting of a double-chain porphyrin energy acceptor and a dithiophene unit energy donor connected by four alkyl chains
[94]. This locked hydrocarbon structure is covalently bonded into the polysiloxane network. Before stretching, the distance between the donor-acceptor units in the film sample is controlled within the F Förster energy transfer radius, and the donor-acceptor can carry out an effective FRET process to show the luminescence of the acceptor; When the elongation reaches 60%, the FRET efficiency decreases and the fluorescence intensity of the donor increases significantly. Upon strain release, the film is able to recover the initial FRET efficiency and luminescence properties. Because the interlocked three-dimensional structure limits the distance between the energy donor and acceptor, the main principle of force-stimulated luminescence is the change of the spatial orientation between the donor and acceptor groups, which leads to the change of the relative direction of the transition dipole moment. This FRET system, which is sensitive to mechanical stimulation, is different from the previous concept of changing the distance between energy donor and acceptor molecules, and is expected to play a role in the in-depth analysis of the conformational changes of polymer chains after mechanical stimulation (Fig. 31).
4 Conclusion and prospect
The unique property of mechanosensitive light-emitting polymers, which can convert mechanical force into optical signals, makes these materials play an important role in both theoretical research and practical applications. However, these materials still have the following problems: (1) Some luminescent mechanosensitive groups themselves have limitations in stimulus-responsive luminescent properties, such as low conversion rate, low sensitivity, poor stability, poor self-recovery reversibility, etc., and there are still some difficulties in practical application; (2) When the existing force stimulus-responsive luminescent materials are subjected to mechanical forces with different properties (such as different directions), the luminescent properties are relatively single, which can not be identified or detected and distinguished, and there are shortcomings in the field of intelligent materials; (3) Some luminophores have poor specificity in stimulus response and may be activated by other stimulus conditions, which is complex in the analysis of practical problems and requires in-depth exploration of the correlation between different activation modes and regulation; (4) The microenvironment and mechanical properties of polymer matrix materials have a great influence on the mechanical stimulus response luminescence properties, but the available matrix materials are still limited to a certain extent, and the compatibility with some luminescence mechanosensitive groups is poor, so it is necessary to further explore polymer matrix materials with good mechanical properties and better compatibility; (5) There are still some problems in the preparation of some highly sensitive mechanosensitive groups, such as cumbersome synthesis steps and low yield, and in the preparation process of the polymer matrix, some processing steps or the introduction of additives may hinder the response process of the mechanosensitive groups. There is a need to optimize the processing flow to avoid adverse effects on the force-stimulus-responsive luminescence properties of the final product. In the future work, efforts should be made to develop simpler and more efficient synthesis strategies through more ingenious molecular structure design on the original basis, to explore the luminescence mechanism in depth, and to expand the structural design of multilayer, core-shell, three-dimensional and other composite force stimulus-responsive luminescence systems.Improve the luminescence contrast and response sensitivity, and optimize the force stimulation response luminescence performance, so that this kind of material can break through the limitation of traditional synthesis methods and achieve innovative breakthroughs in large-scale commercial applications.
National Natural Science Foundation of China(61605138)
Shanxi Scholarship Council of China(2021-057)
Natural Science Foundation of Shanxi Province(20210302123144)
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