Ultrahigh toughness of stretchable ratiometric mechanofluorescent polyurethane elastomers enhanced by dual slide-ring motion of polyrotaxane cross-linkers and daisy chain backbones†

Tu Thi Kim Cuc^a, Yun-Chen Tso^a, Ting-Chi Wu^a, Pham Quoc Nhien^b, Trang Manh Khang^a, Bui Thi Buu Hue^b, Wei-Tsung Chuang^c and Hong-Cheu Lin*^ad
aDepartment of Materials Science and Engineering, National Yang Ming Chiao Tung University, Hsinchu 300093, Taiwan. E-mail: linhc@nycu.edu.tw
^bDepartment of Chemistry, College of Natural Sciences, Can Tho University, Can Tho City 94000, Vietnam
^cNational Synchrotron Radiation Research Center, Hsinchu 300092, Taiwan
^dCenter for Emergent Functional Matter Science, National Yang Ming Chiao Tung University, Hsinchu 300093, Taiwan

First published on 2nd August 2024

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Introduction

In recent times, the upgraded toughness, stretchability and durability of advanced polymeric materials have attracted significant attention due to the limitations of stress and strain in large deformations of conventional polymeric materials.^1–6 The mechanical properties of polymeric materials depend on numerous factors, such as the molecular weight, composition of monomers, types of cross-linkers, etc., which should be manipulated skillfully to achieve desirable properties of the polymers. Among them, cross-linkers are recognized as some of the most vital components, as they form covalent bonds between polymer chains, enhancing cohesive interactions.^7–10 Notably, polyrotaxanes (PRXs) are necklace-shaped mechanically interlocked polymers with supramolecular self-assembled structures consisting of linear polymers and multiple threaded cyclic molecules accompanied by bulky terminal moieties at chain ends, which have been effectively designed for functioning as novel functional polymers and soft materials with movable cross-linkers, which endow them with advantageous mechanical characteristics.^11–15 To date, a number of research studies have focused on PRXs containing α-cyclodextrins (α-CDs, as cyclic molecules) and polyethylene glycols (PEGs, as axle polymers) for the development of bulk polymeric materials (i.e., elastomers, gels, etc.) with ideal mechanical toughness and softness, which can be attributed to the superior molecular dynamics of rotating and sliding motions of interlocked α-CD cross-linkers within topological constraints of axial PEGs.^16–28

So far, many more mechanically interlocked molecules (MIMs) with magnificent topologies and specific dynamic characteristics have been developed to be modifiable platforms for the advent of artificial molecular machines and molecular switches, which display controllably internal translations and/or circumrotations of movable components within MIMs under external stimuli.^29–33 Daisy chain rotaxanes have been identified as ideal molecular structures for the construction of functional artificial molecular machines, as they can undergo stimuli-triggered switchable conformation changes in response to electrochemical, photochemical or chemical inputs.^34–39 In particular, bistable [c2] daisy chain rotaxanes are composed of two macrocyclic molecules covalently linked to two linear axles that are cross-threaded mutually and terminated with bulky end groups simultaneously. By incorporating distinct binding sites for cyclic wheels onto linear fragments, the controllable slipping of subunits can lead to reversible contractile and extensile motions, enabling them to act as artificial molecular muscles.^40–46 Hence, the technique of introducing daisy chain molecules into polymeric materials in order to generate molecular machine-based polymeric materials with stimuli-responsive behaviors and macroscopic motions as well as to enhance the intrinsic mechanical features of parent polymers has recently emerged and is also anticipated as an intriguing challenge.

To date, organic fluorescent materials with photoluminescence (PL) switching capabilities in response to different external stimuli, including temperature, light, mechanical force, acid/base conditions, magnetic field, electric field, etc., have attracted much attention due to their promising applications in sensors, memory devices, security inks, and so forth.^47–53 Particularly, dual fluorescent materials possessing controllable ratiometric fluorescence properties have exhibited great potential as advanced fluorescent probes due to the reduction in environmental interference through the self-calibration of ratiometric detections, highly sensitive signal responses, facile recognitions, visualizations, etc.^54–60 The commonly employed strategies for the construction of ratiometric fluorescent materials are mostly reliant on internal charge transfer (ICT), excited state intramolecular proton transfer (ESIPT), photoinduced electron transfer (PET), through-bond energy transfer (TBET), and Förster resonance energy transfer (FRET) processes.

Additionally, advanced polymeric materials sensitive to external mechanical forces as a class of mechanoresponsive materials are quite appealing and essential thanks to their potential applications in sensors and displays, wherein mechanical forces are exploited as external stimuli to specifically trigger effective chemical transformations of mechanophores that are covalently installed within polymer skeletons for the disclosure of unusual mechanochemical phenomena as well as the achievements of preferred material functions.^61–64 Mechanophores are optically mechano-active moieties containing mechanically labile bonds, which reveal the alterations of their photophysical properties with regard to mechanical forces, and have an important role in the development of force-responsive intelligent polymeric materials.^65–67 Consequently, a number of mechanophores have been well-considered and embedded into different polymer backbones, such as derivatives of dioxetanes,⁶⁸ diarylbibenzofuranones,⁶⁹ tetraarylsuccinonitriles,⁷⁰ spiropyrans,⁷¹ naphthopyrans,^72,73 rhodamines,^74,75 π-extended Diels–Alder adducts,⁷⁶ and others. Being special stimuli-responsive polymeric materials, shape memory polymers are able to preserve their temporary shapes and restore to original shapes through the application of certain external stimuli, including temperature, light, magnetism, electricity, radio frequency, and so on.^77–80 Although shape memory effects are the innate properties of polymeric materials, divergent polymers with distinct molecular structures and components exhibit various degrees of shape memory behaviors. Polyurethanes (PUs) are known as the most talented candidates of shape-memory polymers with a widespread applications, which are frequently used for the fabrication of novel mechano-responsive polymeric materials due to their adaptable structures, outstanding mechanical features, broad range of transition temperatures for shape memory behaviors, biocompatibility, biodegradability, and so forth.^81–88

Interestingly, supramolecular rotaxane-based mechanoluminophores, which contain cyclic molecules with attached luminophores and dumbbell-shaped molecules comprising electronically matched quenchers along with bulky stopper groups, have been proposed lately.^89–93 Linear PU elastomers covering these rotaxane-based mechanoluminophores exhibited rapidly reversible on/off photoluminescence behavior upon application and removal of mechanical forces and the emission colors could be changed through the selection of luminophores. Importantly, photoluminescence activation was independent of any covalent bond fractures; however, it is totally dependent on the short-term separation of luminophores and quenchers upon the application of external mechanical forces. In addition, an appealing slide-ring cross-linker polyrotaxane holding ca. 22 cyclic molecules of α-CD and a polyethylene glycol axle with two fluorophoric naphthalimide stopper moieties to be combined with the mechanochromic rhodamine derivative in PU frameworks to achieve polyrotaxane-based PU films with ratiometric fluorescence emissions upon stretching has been obtained.⁹⁴ Notably, the negligible added amount (ca. 1 wt%) of polyrotaxane cross-linkers could intensely improve the toughness and stretchability of the resultant PU film, which could be attributed to shuttling motions created by α-cyclodextrin molecules threading on polyethylene glycol backbones, leading to effective dissipation of applied mechanical forces to increase breakage strains and toughness. Moreover, the reversible fluorescence emissions between red rhodamine and green napthalimide derivatives of the PU film upon mechanical stretching and relaxation processes through energy transfer pathways were detected via photoluminescence measurements. Significantly, the extended and contracted conformations of [c2] daisy chain rotaxanes were integrated into mechanofluorophoric PU backbones to obtain interesting mechanical and optical properties upon external forces, which have been reported recently.⁹⁵ Especially, the inherent stretchable capabilities and toughness of PU films consisting of very minor contents (ca. 0.03% molar amounts of total monomers) of [c2] daisy chain moieties (as backbones) with long-range sliding motions were apparently improved. Furthermore, another daisy chain component (also as backbones) has been incorporated to advanced shape-memory polymeric materials in the study by Qu's group.⁴⁶

In an attempt to boost the mechanical properties of elastic polymers by dual introduction of daisy chain molecules as artificial molecular muscles accompanied by polyrotaxanes as slide-ring cross-linkers, a series of stretchable mechanofluorophoric polyurethane (PU) elastomers containing daisy chain units (as backbones) along with polyrotaxanes (as cross-linkers) were constructed and their mechanical and mechanofluorescence properties were examined in this study. Particularly, the designed [c2] daisy chain rotaxanes with their extended and contracted states involve mechanofluorophoric rhodamine moieties as bulky terminated groups and α-CD-based polyrotaxanes are built through threading processes of α-CDs on polyethylene glycols with fluorophoric naphthalimide derivatives as hefty stoppers to prevent their de-threading processes, which have been inserted into the scaffolds of polyurethane elastomers to enhance their inherent mechanical properties and achieve ratiometric fluorescence emissions of MIM-functionalized PU films under tensile forces. Interestingly, with the addition of small amounts of daisy chain rotaxanes (as backbones) and polyrotaxanes (as cross-linkers), the toughness and stretchable capabilities of mechanofluorophoric MIM-based PU films can be strongly enhanced owing to the daisy chain moieties acting as molecular muscles and pulley effects of polyrotaxane cross-linkers assisting energy dissipation. Furthermore, the switchable fluorescence emissions via energy transfer processes between green-emissive naphthalimide donors and ring opening yellow-orange-emissive rhodamine acceptors of PU films upon mechanical stretching can be observed. Besides, the shape memory characteristics and fluorescence emission recoveries of stretched PU elastomers can also be explored upon thermal treatment in order to further confirm their real-world applications for the foundation of advanced devices and materials.

Results and discussion

Molecular design and synthesis

As shown in Scheme S3 (ESI†), according to our previous publication,⁹⁵ the targeted mechanofluorophoric [c2] daisy chain rotaxane DRh/E with an extended conformation was prepared through the self-sorting manner of the macrocyclic host-functionalized dibenzo-24-crown-8 (DB24C8) ring with the secondary ammonium guest moiety, where the mutually threaded pseudo-rotaxane dimer, i.e., pseudo-[c2] daisy chain 15 (without end-capped groups), was produced upon hydrogen-bonding interactions (in chloroform as the aprotic and less polar solvent). Then, the common end-capping reactions between 15 and mechano-fluorophoric rhodamine stopper 4 were proceeded via the Cu(I)-catalyzed Huisgen alkyne-azide 1,3-dipolar cycloaddition reaction in chloroform under mild conditions to collect intermediate [c2] daisy chain rotaxane 16 containing the initial dibenzylammonium (DBA) recognition site. Thereafter, the N-methyltriazolium (MTA) unit as another binding site of the [c2] daisy chain rotaxane was provided by the methylation of the triazole moiety on 16 with methyl iodide (in acetonitrile) followed by the ion-exchange process with an overdose of saturated ammonium hexafluorophosphate solution (in acetone), resulting in the formation of the desired bistable [c2] daisy chain rotaxane DRh/E in its extended state equipped with two types of recognition sites at distinct positions (see Scheme S3, ESI†), where DB24C8 wheels were placed at DBA stations initially. Particularly, the ROESY-NMR spectrum of the extended [c2] daisy chain rotaxane DRh/E in acetone-d₆ exhibited several cross-peaks, proposing the interactions between phenyl protons and ethylene protons of DB24C8 wheels, which has been reported in our recent publication.⁹⁵ Nevertheless, no cross-peaks between triazolium protons and ethylene protons of DB24C8 rings could be perceived in the ROESY-NMR spectrum, thus clearly indicating that DB24C8 macrocycles were initially located on the primary –NH₂⁺ recognition sites of DRh/E. Based on the verification from the HRMS and NMR spectra,⁹⁵ the force-sensitive fluorescence [c2] daisy chain rotaxane DRh/E has been effectively synthesized by the self-assembly of dimer 14 upon the formation of hydrogen-bonding interactions between the secondary ammonium salt and the DB24C8 wheel combined with the classic end-capping strategy of the CuAAC click reaction.

Through the neutralization of DBA binding sites in the switchable extended [c2] daisy chain DRh/E, the shuttling movement of DB24C8 rings from DBA to MTA stations would take place to acquire the corresponding [c2] daisy chain rotaxane DRh/C in the contracted form (see Scheme S3, ESI† and Fig. 1a), which was also affirmed by HRMS and NMR spectra in the previous study.⁹⁵ Accordingly, it was indicated that the synthesized [c2] daisy chain rotaxane DRh/E performed the reversible molecular switching of the extended DRh/E and the contracted DRh/C forms upon reciprocating movements of DB24C8 wheels between two distinct binding sites of DBA and MTA stations under respective acid/base conditions (see Scheme S3, ESI†). Subsequently, both comparative [c2] daisy chain rotaxanes DRh/E and DRh/C were covalently inserted into polyurethane (PU) skeletons to obtain polymeric materials PURh-DRh/E and PURh-DRh/C (i.e., extended and contracted forms), respectively. Benefitting from distinctive molecular structural natures of both decoupled [c2] daisy chains, it could be assumed that they would expose different responses under mechanical force stimuli, in which the extended daisy chain moiety in PURh-DRh/E would mainly tighten its compact conformation to mimic the ends of sliding motions with little artificial muscle functions. Meanwhile, the contracted daisy chain component in PURh-DRh/C would undergo obvious slipping movements to facilitate artificial muscle functions and thus to reach the ultimate conformations similar to the tightened forms of PURh-DRh/E. Moreover, non-interlocked analogue 18 was also synthesized by the same procedure as displayed in Scheme S4 (ESI†).

In addition, polyrotaxanes were necklace-like mechanically interlocked molecules to be utilized as slide-ring cross-linkers to construct the desired polymer networks with artificial muscle functions, in which multiple cyclic α-cyclodextrin molecules (α-CDs) were threaded onto two kinds of linear diamino-polyethylene glycol PEG backbones (i.e., PEG10 and PEG20 with respective M_W = 10000 and 20000) through self-assembly processes, and naphthalimide thiocyanate end-capping units were appended to chain ends to obstruct cyclic components from slipping off (see Scheme S6, ESI† and Fig. 1a). Besides, ¹H NMR analyses were utilized to characterize numbers of α-CDs in these two PEG chains (M_W = 10000 and 20000). Accordingly, it was verified that approximately 20 and 50 α-CDs were threaded onto corresponding chains of PEG10 and PEG20 to achieve polyrotaxanes PR20CD and PR50CD, respectively (see Fig. S14 and S16, ESI†).

As described in Fig. 1b, step-growth polymerization processes catalyzed by dibutyltin dilaurate (DBTDL) were carried out by integrating polyrotaxane cross-linkers with both extended/contracted states of the bistable [c2] daisy chain in the backbones to acquire the targeted mechanofluorophoric PU materials. These mechano-responsive PU elastomers consist of various amounts of tetra-ethylene glycol (TEG) and hexamethylene diisocyanate (HDI) as monomers, diol-functionalized rhodamine derivative Rh (as a mechano-fluorophore), triethanolamine (TEA, as a cross-linker), daisy chain molecules (DRh/E and DRh/C, as artificial molecular muscle units) and polyrotaxanes (PR20CD and PR50CD as slide-ring cross-linkers). The detailed synthetic procedures and related chemical characterization of all intermediate and targeted compounds are presented in the ESI.† Moreover, the stoichiometric ratios of reactive –OH and –NCO groups were proposed to control complete monomer reactions to generate amide units in the PU films. Additionally, the control PU elastomers PURh(s) and PURh-Rh₂ were prepared, which only included the mono-rhodamine mechano-fluorophore Rh along with bis-rhodamine mechanofluorophore Rh₂ as the analogous substituent (without any artificial muscle function) of both extended/contracted [c2] daisy chain rotaxanes DRh/E and DRh/C embedded in polymeric backbones for comparisons.

Chemical and thermal characterization of polyurethane (PU) films

Fourier transform infrared (FTIR) measurements were first carried out for the demonstration of the reactions between –NCO and –OH groups to donate amide units in polymeric films through the poly-condensation reaction. As presented in Fig. S1a (ESI†), the stretching vibration of –OH groups in TEG could be detected around 3300 cm⁻¹, and the characteristic band at 2250 cm⁻¹ related to –NCO groups in HDI was also observed. Nevertheless, after polymerization, the wide band around 3300 cm⁻¹ became cramped, the specific band at 2250 cm⁻¹ faded, the N–H stretching vibration transpired at 3300 cm⁻¹, together with the amide I and II bands at 1675 and 1560 cm⁻¹, respectively, signifying that the reactions between –NCO and –OH groups were nearly completed. The thermal features of PU films could be surveyed by thermogravimetric analysis (TGA) (see Fig. S1b, ESI†), wherein TGA curves revealed high stabilities of PU elastomers up to 200 °C. Furthermore, the differential scanning calorimetry (DSC) profiles of PURh-DRh/C-PR50CD films before and after stretching were demonstrated and are shown in Fig. S1c and d (ESI†), wherein the glass transition temperature (T_g) was enhanced from −13.9 to −8.1 °C after stretching (ca. 7000% strain). The reasonable explanation of increased T_g might be the strain-induced crystallization (which significantly enhanced the tensile strength and toughness for polymers)⁹⁶ accompanied by the enriched hydrogen-bonding of stress-aligned polymer chains upon tensile forces, which would be consistent with the XRD results.

Mechanical properties of polyurethane (PU) films

It is well-known that the outstanding mechanical strength accompanied by excellent tensile and ductile capabilities is the most essential parameter for PU elastomeric films for their applications in mechanofluorophoric stretchable sensors to identify fluorescence emission changes upon elongation adjustments. In order to estimate the mechanical strength of PU elastomeric films, tensile tests were conducted and the tensile stress (δ) and strain (ε) were computed by using the following equation: stress (δ) = force (N)/area (A); strain (ε) = (ΔL/L_o) × 100%, where ΔL denotes the displacement in length after stretching and L_o represents the original length. For the discovery of an ultimate stoichiometric proportion to construct PU films with delightful mechanical properties, the feed molar ratios of components Rh (mono-rhodamine mechano-fluorophore) and TEA (cross-linker) were examined as shown in Table 1, proposing that ca. 0.25 mmol (5 eq.) of TEA in the reaction system facilitated the creation of PURh(s) films with the highest tensile stress (6.85 MPa) and allowed breaking elongation (over 3400%) as shown in Fig. 2a, where the standard PURh(s) film possessing a medium toughness (174 MJ m⁻³) is possible to be further optimized. Accordingly, based on PURh(s) films, the artificial molecular muscle and slide-ring motion functions via dual combinations of extended/contracted [c2] daisy chain rotaxanes (i.e., DRh/E and DRh/C) and polyrotaxane cross-linkers PR20CD and PR50CD (containing 20 and 50 α-CDs, respectively) into PU scaffolds were investigated to upgrade the toughness of PU elastomers. Since poor film qualities were induced with less TEA amount in the range of 0–0.05 mmol (0–1 eq.) for PURh(s)-1 and PURh(s)-2 as shown in Table 1, the molar amount of 0.25 mmol (5 eq.) in TEA could achieve PURh(s) films with the highest tensile stress (6.85 MPa). For the molar amounts of 0.10 mmol (2 eq.) and larger than 0.25 mmol (5 eq.) in TEA for PURh(s)-3, PURh(s)-6 and PURh(s)-7 as shown in Fig. 2a, these tensile stresses and strains were inferior to both of the standard PURh(s) films. Though the molar amounts of 0.15 and 0.20 mmol (3 and 4 eq.) in TEA in Table 1 gave the longer tensile strains, smaller tensile stresses were obtained in contrast to that of the PURh(s) film. Also, by increasing the molar amounts from 0.05 (1 eq.) to 0.06 (1.2 eq.) and 0.07 mmol (1.4 eq.) in rhodamine for PURh(s)-9 and PURh(s)-10 as shown in Table 1, the diminished breaking stresses and elongations were observed in contrast to both of the standard PURh(s) films as shown in Fig. 2a. Thus, the optimal molar amount of 0.05 mmol (1 eq.) in rhodamine for the standard PURh(s) film was confirmed. In addition, since the PU film could not be shaped (remained as a powder form) without any TEG as an ingredient for PURh(s)-8 as shown in Table 1, the film revealed better mechanical properties in the presence of ca. 4.45 mmol (89 eq.) of TEG in the standard PURh(s) film. Furthermore, the influence of diverse strain rates, i.e., 1, 10, and 100 mm s⁻¹, on the mechanical properties of the PURh(s) film was investigated as described in Fig. 2b, in which longer strains were achieved with slower strain rates, recommending that the slowest strain rate of 1 mm s⁻¹ was chosen to yield the longest tensile length accompanied by later studies of expected mechano-responsive properties in PU films. In general, the fracture toughness of any polymeric materials was directly proportional to the loads applied before the failure of the specimens, which were related to both tensile stresses and strains. With increasing strain rates, the stiffness of the polymer films would be obtained, so the loads (along with their Young's moduli) required to disrupt the specimens became enhanced. However, the increasing strain rates might encounter higher brittleness to result in the earlier fractures and reduced their toughness of polymeric materials. Moreover, the effects of increasing strain rates might augment their ultimate strengths, intensify Young's moduli, drop down breaking elongations, and diminish ductilities.

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Afterwards, the molar ratios of both extended/contracted [c2] daisy chain rotaxanes DRh/E and DRh/C along with their analogous substituent of bis-rhodamine mechanofluorophore Rh₂ (i.e., as the control compound without any artificial muscle function) implanted in the backbone frameworks were also investigated in all PU films to acquire their probable force-active elastic deformation properties. Therefore, the distinct mechanical properties of various PU films with different proportions of chemical components are displayed in Fig. S2a and b and Tables S1–S3 (ESI†), where the total molar numbers of mechanofluorophoric rhodamine moieties for Rh, Rh₂, DRh/E, and DRh/C were maintained as 0.05 mmol (1 eq.) in all PU films. The effects of diverse [c2] daisy chain amounts installed in polymer networks on the mechanical properties of PU films were inspected and explained in the stress–strain curves of PU films with a strain rate of 1 mm s⁻¹ (see Fig. S2a and b, ESI†). By enhancing the additional amounts of both extended/contracted [c2] daisy chain rotaxane DRh/E and DRh/C components up to 0.0025 mmol (0.05 eq.) as shown in Tables S1 and S2 (ESI†), respectively, the progressively improved mechanical strengths with the higher stretchable capabilities could be perceived (see Fig. S2a and b, ESI†), where PURh-DRh/E and PURh-DRh/C films (with corresponding amounts of DRh/E and DRh/C = 0.0025 mmol (0.05 eq.)) possessed the best mechanical properties. Nonetheless, the tensile strains and toughness of acquired PU films in Fig. S2a and b and Tables S1 and S2 (ESI†) were reduced by additions of [c2] daisy chain amounts over 0.0025 mmol (0.05 eq.) due to intricate and huge molecular structures of [c2] daisy chains to hinder polymerization processes and decrease their polymer chain lengths. In comparison with the standard PURh(s) film in Fig. 2c and e, all PURh-DRh/E, PURh-DRh/C and PURh-Rh₂ films showed higher stretchabilities and toughness owing to the larger flexibilities of longer alkyl spacers and sliding motions of daisy chains. Remarkably, in contrast to the PURh-DRh/E film containing extended [c2] daisy chain DRh/E with shorter slipping lengths (see Fig. 1b), the PURh-DRh/C film containing contracted [c2] daisy chain DRh/C with preeminent long-range sliding motions revealed evidently higher stretchabilities and toughness as shown in Fig. 2c and e due to dissimilar slipping distances of [c2] daisy chains DRh/E and DRh/C, advising the key contribution of muscle-like contracted MIM structures to the enhancements of the innate mechanical features of elastomer PURh-DRh/C. Besides, both PURh-DRh/E and PURh-DRh/C films containing extended/contracted [c2] daisy chain rotaxane DRh/E and DRh/C components possessed better mechanical properties than those of the comparative PU film of PURh-Rh₂ containing control compound Rh₂ without any artificial muscle functions, except a slightly better tensile strain than that of PURh-DRh/E (see Fig. 2c–e and Tables S1–S3, ESI†). As shown in Fig. 2c–f, the enriched mechanical behaviors of PU elastomers containing both extended/contracted [c2] daisy chain rotaxanes DRh/E and DRh/C with different sliding motions of macrocyclic movable units attached to polymer skeletons were verified. Accordingly, these enhanced mechanical features of MIM-based PU films could be attributed to the slipping motions of [c2] daisy chains as artificial molecular muscles to offer longer elongations, higher stresses and larger toughness.

Consequently, the optimal molar ratio of the PURh-DRh/C film in Table S2 (ESI†) was selected to prepare PU films possessing ideal mechanical properties with the desired strain and rough stress values (ca. 4267% and 10.65 MPa, respectively), which was also employed as the most favorable recipe to be further considered in the following experiments. With the purpose of obtaining ultrahigh toughness and excellent stretchabilities of daisy chain rotaxane-based PU elastomers, polyrotaxanes were utilized to replace the traditional cross-linker TEA as novel slide-ring cross-linkers inside PU films.⁹⁴ The effects of the addition of polyrotaxanes PR20CD and PR50CD (containing 20 and 50 α-CDs threaded onto PEG chains with M_W = 10000 and 20000, respectively) on the mechanical properties of PU elastomers were investigated and are plotted in Fig. 2c–f, wherein the toughness of polyrotaxane-based PU films was impressively boosted due to the excellent stretchabilities initiated by the pulley effects of polyrotaxane cross-linkers with greater elongations. As shown in Fig. 2c and e, PURh(s)-PR20CD and PURh(s)-PR50CD films containing polyrotaxane cross-linkers PR20CD and PR50CD (ca. 1.0 wt%) revealed the largest tensile strains and highest toughness in contrast to PURh(s), PURh-DRh/E, PURh-DRh/C, and PURh-Rh₂ films. However, PURh-DRh/C films illustrated the highest tensile stress containing contracted [c2] daisy chain DRh/C among all PU films as shown in Fig. 2c and e due to its shorter sliding length than those of polyrotaxanes PR20CD and PR50CD. Specifically, PU elastomers comprising the slide-ring cross-linker PR50CD hold the better mechanical properties than those of PU films containing the slide-ring cross-linker PR20CD, which might be due to the presence of larger numbers of α-CDs (i.e., 50 α-CDs) in the longer PEG chain (i.e., M_W = 20000) for polyrotaxane PR50CD to offer more effective pulley effects. Since both kinds of slide-ring components, including extended/contracted [c2] daisy chain rotaxanes DRh/E and DRh/C along with polyrotaxanes PR20CD and PR50CD, could improve the mechanical properties of PU films separately, the simultaneous contributions of both artificial muscle functions and pulley effects enhanced by dual slide-ring motion of polyrotaxane cross-linkers and daisy chain backbones are demonstrated in Fig. 2d and f. Similar to the previous result of PURh(s)-PR20CD and PURh(s)-PR50CD films in Fig. 2c and e, all PU films containing polyrotaxane PR50CD (solid lines) acquired better mechanical properties than those containing PR20CD (dash lines) due to more and longer slide-ring motions of PR50CD as shown in Fig. 2d and f. As expected, contracted daisy chain rotaxane DRh/C-based PU films (i.e., PURh-DRh/C-PR20CD and PURh-DRh/C-PR50CD) demonstrated the highest toughness (with the artificial muscle function) and Rh₂-based PU films (i.e., PURh-Rh₂-PR20CD and PURh-Rh₂-PR50CD) displayed the worst toughness (without the artificial muscle function) among all PU films as shown in Fig. 2f by following the same trend as shown in Fig. 2e. As a result, PURh-DRh/C-PR50CD revealed a perfect strain value over 6000% without fracture together with a stress over 15.93 MPa; thus, a notable toughness over 557 MJ m⁻³ was summarized according to Fig. 2f. Compared with a toughness of 174 MJ m⁻³ acquired in the standard PURh(s) film, this ultrahigh toughness over 557 MJ m⁻³ (>3 times higher than the standard film) without any rupture confirmed the major tasks of dual slide-ring motion of [c2] daisy chain components and polyrotaxane cross-linkers in the augmented mechanical properties of targeted PU elastomers. In summary, all PU films containing the [c2] daisy chain with the extended form (E) revealed reduced tensile stresses than those counterparts containing the [c2] daisy chain with the contracted form (C), such as PURh-DRh/E < PURh-DRh/C (without polyrotaxane) as shown in Fig. 2c, along with PURh-DRh/E-PR20CD < PURh-DRh/C-PR20CD (with a shorter polyrotaxane) and PURh-DRh/E-PR50CD < PURh-DRh/C-PR50CD (with a longer polyrotaxane) as shown in Fig. 2d. Besides, as displayed in Fig. S3a and b (ESI†), the PURh-DRh/E film of the extended form (E) had a lower glass transition temperature (i.e., T_g = −10.1 °C) than that of the PURh-DRh/C film of the contracted form (C) with a T_g of −8.6 °C. Thus, the elongated structure of separated axles in the extended form (E) for the [c2] daisy chain DRh/E as shown in Fig. 1a seemed longer to form more coiled amorphous regions (with a lower T_g value) compared with the more well-packed structure of overlapped axles in the contracted form (C) for the [c2] daisy chain DRh/C to reveal higher rigidity (with a higher T_g value) in all respective PU films. Accordingly, the higher tensile stress of the PURh-DRh/C film in contrast to that of the PURh-DRh/E film could be matched well with the DSC data in Fig. S3a and b (ESI†). Moreover, the medium stress–strain curves in tensile stresses and strains of all prepared PU samples were also studied and are demonstrated in Fig. S4 and S5 (ESI†), where different batches of the same PU samples presented alike mechanical properties. Furthermore, the Young's moduli (i.e., elastic moduli) of obtained PU films with different monomer components as shown in Fig. 2c and d could also be determined and are shown in Fig. 3 along with Table S7 (ESI†). In contrast to PURh(s) and PURh-Rh₂ films, both PURh-DRh/E and PURh-DRh/C films revealed higher elastic moduli (see Fig. 3a and Table S7, ESI†), which could be due to the enhanced rigidities of PURh-DRh/E and PURh-DRh/C films through the integration of extended/contracted [c2] daisy chain rotaxanes DRh/E and DRh/C into PU skeletons. As a result, the PURh-DRh/C film possessed the highest Young's modulus of 7.79 MPa. However, either standard PU films containing polyrotaxanes as slide-ring cross-linkers, i.e., PURh(s)-PR20CD and PURh(s)-PR50CD, as shown in Fig. 3a or PU films comprising dual types of polyrotaxane cross-linkers PR20CD and PR50CD together with extended/contracted daisy chain rotaxanes (DRh/E and DRh/C) or the Rh₂ unit as shown in Fig. 3b revealed the lower Young's moduli than those of respective PURh(s), PURh-Rh₂, PURh-DRh/E and PURh-DRh/C films. The possible reasons might be that the incorporation of polyrotaxanes as slide-ring cross-linkers (PR20CD and PR50CD) possessing long chain molecular structures and weak intermolecular forces into PU frameworks enabled resultant PU films to become more flexible. Definitely, all PU films consisting of the slide-ring cross-linker PR50CD disclosed lower elastic moduli compared with PU films involving the polyrotaxane cross-linker PR20CD (i.e., with PEG chain of M_W = 10000) owing to the longer PEG chain (i.e., M_W = 20000) for the polyrotaxane cross-linker PR50CD to afford more stretchable polymeric materials.

Mechanophoric and mechanofluorophoric properties of polyurethane (PU) films

Additionally, the mechanical force-activated isomerizations from ring-closing to ring-opening forms of rhodamine derivatives as mechanofluorophores which implanted in different polymeric architectures to display fluorescence emission switching have been well-known recently; thus, the alteration of fluorescence emission intensities upon stretching of the prepared PU films (i.e., the standard film PURh(s) and the contracted daisy chain rotaxane-based PU elastomer PURh-DRh/C) containing rhodamine mechanofluorophores were investigated and are shown in Fig. S6 (ESI†). The primitive PURh(s) and PURh-DRh/C films before stretching exposed weak blue fluorescence emissions ca. 450 nm correlated with the presence of urethane linkages in PU films,⁹⁷ while new peaks at 572 nm arose with yellow-orange fluorescence emissions of ring-opening rhodamine derivatives and their emission intensities augmented steadily upon continual stretching. As shown in Fig. 4a, these yellow-orange emission enhancements of PURh(s) and PURh-DRh/C films indicated that external mechanical forces were conveyed through polymer backbones to rhodamine moieties and stimulated ring-opening reactions efficiently.

Importantly, the key intention of the utilization of polyrotaxane as cross-linkers (with pulley effects) possessing green-emitting naphthalimide moieties to be decorated with yellow-orange mechanofluorescent rhodamine derivatives in PU skeletons was not only to promote the inherent mechanical properties of PU films, but also to discriminate the ratiometric fluorescence emissions via mechanically controlled energy transfer processes between the donor–acceptor pairs upon stretching. The schematic illustration of dual fluorescence emission changes and plausible energy transfer manners manipulated by tensile forces for MIM-based PU films containing green-emissive naphthalimide stoppers on polyrotaxanes as energy donors and yellow-orange mechano-fluorescent rhodamine moieties as energy acceptors is propounded in Fig. 4b. As signified by the stress–strain curves of MIM-based PU films in Fig. 2d, PURh-DRh/C-PR20CD and PURh-DRh/C-PR50CD films (comprising dual slide-ring motion of the contracted daisy chain molecule and polyrotaxane cross-linkers) possessed the highest tensile strains to support the occurrence of strong yellow-orange emissions of rhodamine derivatives at 572 nm in these PU films upon large degrees of chemical transformations of rhodamine derivatives. Accordingly, the PL spectra and ratiometric fluorescence emissions of green-emissive naphthalimide at 504 nm and yellow-orange-emissive rhodamine units at 572 nm of PURh-DRh/C-PR20CD and PURh-DRh/C-PR50CD films with various strains could be examined and are illustrated in Fig. 5a–d, in which the broad green-emission band of naphthalimide unit at 504 nm was identified initially, and the green naphthalimide emission weakened progressively in conjunction with a mechanofluorescence yellow-orange-emission band of rhodamine moieties at 572 nm boosted persistently during mechanical stretching processes.

Markedly, the magnification of yellow-orange rhodamine mechanofluorescence emissions at 572 nm was inversely proportional to the reduction of green naphthalimide emission at 504 nm under tensile forces of PURh-DRh/C-PR20CD and PURh-DRh/C-PR50CD films as shown in Fig. 5a–d, and the brightest yellow-orange rhodamine emissions were perceived at a 7000% strain with severe depletions of ca. 57.8 and 55.9% green naphthalimide emission intensities, respectively. The reasonable clarification for the attenuation of green naphthalimide fluorescence emissions might be responsible for the appearance of energy transfer processes between green-emissive naphthalimide donors and yellow-orange mechanofluorescent rhodamine acceptors. Thus, the energy transfer efficiencies of 22.7 and 17.7% in respective PURh-DRh/C-PR20CD and PURh-DRh/C-PR50CD films could also be identified by subsequent time-resolved photoluminescence (TRPL) experiments, which was owing to the longer chain of PR50CD to dilute the green naphthalimide donor emission in the PURh-DRh/C-PR50CD film to induce a lower energy transfer efficiency of 17.7%. Likewise, the PL spectra and ratiometric fluorescence emission behaviors of PURh(s)-PR20CD and PURh(s)-PR50CD films at different strains could also be inspected and are shown in Fig. S7 (ESI†). In addition, the mechanofluorescence emission variations of PURh-DRh/C-PR50CD films were recorded with dynamic strains up to 7000% at a strain rate of 1 mm s⁻¹ and offered in Movie S1 in the ESI.† Also, the photographs of the PURh-DRh/C-PR50CD film under different elongation examinations are demonstrated in Fig. 5e. Thoroughly, the yellow-orange emission of the PURh-DRh/C film was detected ca. 1000% strain and obtained the highest yellow-orange emission ca. 4000% upon stretching, whereas PURh-DRh/C-PR50CD at a 1000% strain still revealed the original naphthalimide emission, and the yellowish emission could be discerned at 2000% strain upon the mechanical force-caused ring-opening reaction of rhodamine moieties (with a mixing color of naphthalimide green emission and rhodamine yellow-orange emission). Moreover, the strongest yellow-orange fluorescence emission of PURh-DRh/C-PR50CD was achieved ca. 7000% strain and the breakage occurred ca. 8000%, describing that the pulley effects of slide-ring polyrotaxane cross-linkers were ascribed to the decays of mechanical tensile forces. Besides, the energy transfer process revealed in the PURh-DRh/C-PR50CD film upon stretching could be further affirmed by the spectral overlap between the emission spectra of naphthalimide donors and the absorbance spectra of rhodamine acceptors accompanied by TRPL measurements. Therefore, the ultraviolet-visible light (UV-vis) spectra of the standard PURh(s) film and the contracted daisy chain rotaxane-based PU elastomer PURh-DRh/C incorporated with polyrotaxane cross-linkers PR20CD and PR50CD (i.e., PURh(s), PURh(s)-PR20CD, PURh(s)-PR50CD, PURh-DRh/C, PURh-DRh/C-PR20CD, and PURh-DRh/C-PR50CD films) were monitored to display absorption spectral changes of naphthalimide and rhodamine derivatives before and after stretching. As demonstrated in Fig. S8b–d (ESI†), the new absorption band at ca. 540 nm was attributed to the ring-opening rhodamine derivatives in these PU films after stretching, wherein the visible color changes of PU films from almost transparent to pink color could be distinguished by the naked eye. Appealingly, the absorption spectrum (λ_abs = 540 nm) of ring-opening rhodamine units in the stretched PURh-DRh/C-PR50CD film was partially overlapped with the green emission spectrum (λ_em = 504 nm) of naphthalimide derivatives in the unstretched PURh-DRh/C-PR50CD film (see Fig. S9a, ESI†), supporting the energy transfer processes of the released emission energies from naphthalimide donors to be absorbed by the ring-opening rhodamine acceptors in the PURh-DRh/C-PR50CD film upon mechanical forces.

Furthermore, the time-resolved photoluminescence (TRPL) spectra, fluorescence lifetime values and energy transfer efficiencies of PURh-DRh/C-PR20CD and PURh-DRh/C-PR50CD films (consisting of dual slide-ring motion) before and after stretching were also evaluated to prove the existence of the energy transfer process from the naphthalimide donor to the ring-opening rhodamine acceptor. As shown in Fig. S9b (ESI†), the stretched PURh-DRh/C-PR20CD and PURh-DRh/C-PR50CD films possessed shorter lifetime values (τ_DA = 8.09 and 7.92 ns after energy transfer) of naphthalimide donors than those of primitive PURh-DRh/C-PR20CD and PURh-DRh/C-PR50CD films (τ_D = 10.46 and 9.62 ns without energy transfer), proposing that energy transfer processes between naphthalimide donors and ring-opening rhodamine acceptors existed in PURh-DRh/C-PR20CD and PURh-DRh/C-PR50CD films after stretching. Consequently, the energy transfer efficiencies (E, %) of stretched PURh-DRh/C-PR20CD and PURh-DRh/C-PR50CD films were found to be 22.7 and 17.7%, respectively, using the equation E = 1 − (τ_DA/τ_D), where τ_DA and τ_D represent the lifetime values of the stretched and un-stretched PU films, respectively, which indicated the attractive ratiometric fluorescence altering features of mechanoresponsive MIM-based polymeric materials with the integrations of dual fluorescence emissions into their polymeric scaffolds. Additionally, in order to intuitively demonstrate force-induced fluorescence emission changes in PU films, as presented in Fig. 5f, PURh-DRh/C-PR50CD samples were ground with a pestle or scratched with a blunt object to exhibit yellow-orange emissions at the damaged areas, indicating that the ring-opening isomers of rhodamine moieties in PU films were formed under force perturbations.

XRD analyses

Except for the macroscopic mechanical tests, SAXS/WAXS analyses were also explored to examine the microstructure of elastomer PURh-DRh/C-PR50CD (consisting of dual slide-ring motion) in tensile tests with different pre-strains as shown in Fig. 6a–d, respectively. Without stretching, the 1D SAXS curve of the PURh-DRh/C-PR50CD film has no obvious peak value (see Fig. 6a), which indicated that the elastomer possessed a loose amorphous unoriented structure. When the strain increased, the hard domains of the elastomer aggregated, inducing the 1D SAXS curve to rise with one major q value ca. 0.06 Å⁻¹, which was attributed to an average inter-domain distance of 10.5 nm between oriented microphase-separated domains. As the strain reached 7000%, the partial dissociation of the hard domain structure resulted in the 1D SAXS curve to decline, and the weaker intensity might be correlated to the slowly ruptured structure and a smaller thickness originated from the rearrangement of the strain-induced structure in the stretched PURh-DRh/C-PR50CD film. The 2D SAXS patterns of the un-stretched PURh-DRh/C-PR50CD film presented a uniform circular shape which indicated the isotropic structure of the elastomer at a 0% strain, where the hard segments were randomly dispersed in PU matrixes (see Fig. 6b). The scattering circle was gradually distorted and transformed into two scattering spots oriented along the stretching direction with the increasing strain, proposing that the hard domains continued to orientate along the stretching direction, the orientation degrees of the hard segments in the stretching direction were amplified, and the aligned microstructures were induced under tensile procedures.

Moreover, the structural transformation of the elastomer from static to tensile was proven by WAXS. Fig. 6c and d exhibit the 1D WAXS and 2D WAXS profiles of PURh-DRh/C-PR50CD films under various strains, respectively. As stretched up to 7000% strain, the intensity of the scattering peak in the stretched PURh-DRh/C-PR50CD film was weakened (see Fig. 6c), which could be ascribed to the partial hydrogen bond dissociation, the increased orientation of the molecular structure, and the reduced thickness. In particular, three diffraction peaks at 2θ values of ca. 17, 18 and 19 deg (at 0% strain) in the unstretched state gradually became one main peak at 2θ ≈ 18 deg (after stretching) as shown in Fig. 6c, indicating the stacked structure with a period of about 4.2 Å. Besides, the isotropic scattering halo of the 2D WAXS pattern became concentrated and changed into two intense arcs in the meridional direction with increasing strains, suggesting the preferred orientation associated with the periodic distance of PU backbones with stretching-induced hydrogen bonding in the tensile direction (see Fig. 6d). These results designated that the introduction of the movable crosslinking obstructed the shish-kebab orientation during stretching and was an essential reason for the mechanical properties of the elastomer to maintain a high toughness at a high Young's modulus, as well as the aggregated domains of slipping-oriented polyrotaxanes were steadily deformed and the hard segments of PU domains were gradually aggregated under higher strains.

Photoluminescence (PL) and shape recovery analyses

Subsequently, the disappearances of the mechanofluorescence emission in stretched PU films after fracture were observed by the investigation of PL spectra for ruptured PURh-DRh/C-PR50CD films at ambient temperature within 5 days. As exhibited in Fig. S10a and b (ESI†), when the mechanical force was removed, the yellow-orange fluorescence emissions of mechanofluorophoric rhodamine moieties at 572 nm decayed gradually and diminished to a minimum after 5 days at room temperature. Consequentially, the green fluorescence emissions of naphthalimide units (λ_em = 504 nm) were regained progressively up to 51.9% recovery after 5 days, offering that the incomplete repossession of the green emission at 504 nm was incompatible with the obvious reduction of yellow-orange mechano-fluorescence at 572 nm, which might be associated with the diminished thickness of the cracked specimen after breakage, bringing about the unfeasible regaining of the entire fluorescence emission. However, upon the immersion of the colored segments into a hot water bath ca. 100 °C directly, the yellow-orange color dissipated swiftly and these polymeric samples approximately restored to their original green fluorescence emission within 180 min, identifying the proficiently reversible mechanofluorescence characteristics of rhodamine moieties in these PU films (see Fig. S10c and d, ESI†). The efficacious reversibility could be determined by the examination of the PL spectra in PU films under several stretching-heating cycles as shown in Fig. S11a and b (ESI†), providing that mechanical force-active PU films disclosed reversible PL intensities after three stretching–heating cycles with trivial decays of fluorescence emission intensities.

Besides the partial mechanofluorescence recovery features of rhodamine moieties in these stretched PU films by heating, PUs are also famous as a class of shape memory polymers with the restorations of their initial states from stretched states upon heating due to the removal of force induced hydrogen bonded domains. Fundamentally, elastomeric polymers could be easily stretched in their elastic regions, and the mechanical strains could be assembled as they were quenched beneath their glass transition temperatures (T_g). Then, by external stimuli (such as heating), they could release the stored strains and return their original shapes. Hence, the thermo-mechanical cycle to survey the shape memory behavior of the PURh-DRh/C-PR50CD film was carried out and is revealed in Fig. 7 and Movie S2 (ESI†), in which a transparent color and green fluorescence emission in the original state of the PURh-DRh/C-PR50CD film could be visualized, and an obvious pink color and yellow-orange emission became observable in this PU film after stretching. Later on, the stretched PURh-DRh/C-PR50CD film was frozen by employing liquid nitrogen to sustain the temporary shape and the yellow-orange emission of ring-opening rhodamine units under the tensile condition. Thereafter, by dipping in a hot water bath ca. 100 °C, the temporary shape of the stretched PURh-DRh/C-PR50CD film was reverted to its pristine shape (ca. >90%), recommending the probable industrial applications of this obtained PU elastomer to shape memory materials. Moreover, the reversibilities of the mechanical properties of PURh-DRh/C-PR50CD film could be confirmed by the investigations of its stress–strain curves under 3 stretching-heating cycles as shown in Fig. S11c and d (ESI†), wherein the hysteresis occurred and the Young's moduli of the resultant PU elastomer decreased gradually in each stretching–heating cycle, leading to a reduction of ca. 30% of its stress and toughness. In brief, the designed force-triggered mechanofluorescent PU elastomers exposed both reversible PL intensities (with respective recoveries of PL intensity ratios as 85%, 77% and 66% as shown in Fig. S11a and b, ESI†) and mechanical features (with respective recoveries of stress at 6000% strain as 95%, 86% and 70% as shown in Fig. S11c and d, ESI†) after 3 stretching–heating cycles. These results could be assigned to the unfinished reversibilities between ring-opening and ring-closing processes of mechanofluorophoric rhodamine derivatives as shown in Fig. S11a and b (ESI†) along with the imperfect recoveries of polymeric structures as shown in Fig. S11c and d (ESI†) (i.e., both the recoveries of mechanofluorophoric rhodamine units and hydrogen bonds as well as the re-allocations in positions of α-CDs for slide-ring cross-linkers).

Conclusions

Various mechanofluorescent PU films were constructed through the step-growth polymerization of diverse components, i.e., slide-ring polyrotaxane cross-linkers PR20CD and PR50CD, along with the mechanofluorophoric rhodamine derivative Rh and extended/contracted [c2] daisy chains DRh/E and DRh/C in the backbones. Upon stretching, in contrast to extended [c2] daisy chain DRh/E-based PU films with short-range slipping movements, the elongated gliding motions of contracted [c2] daisy chain DRh/C-based PU films could release concealed chains to enhance their stretchable capabilities. Consequently, PURh-DRh/C films exhibited remarkable ductility and toughness due to the promising artificial muscle function of the contracted [c2] daisy chain DRh/C in PU frameworks. In addition, all PU films containing the polyrotaxane cross-linker PR50CD revealed better mechanical properties than those containing PR20CD owing to more and longer slide-ring motions of PR50CD. Accordingly, PURh-DRh/C-PR50CD films incorporating negligible amounts of the contracted [c2] daisy chain DRh/C (ca. 0.05% molar amount of all diol monomers) and polyrotaxane cross-linker PR50CD (ca. 1 wt%) could intensely boost the mechanical properties with miraculous stretchable capabilities with a tensile stress of ∼16 MPa and a strain of >6000%, accompanied by a ultrahigh toughness of over 557 MJ m⁻³, which could be attributed to dual contributions of slide-ring motions of the contracted [c2] daisy chain DRh/C and pulley effects of the movable CD-based polyrotaxane cross-linker PR50CD. Besides, the Young's moduli of achieved PU films with different monomer components could also be compared, and PURh-DRh/C films (without polyrotaxane) possessed the highest Young's modulus of 7.79 MPa. Notably, the ratiometric fluorescence features of reduced green-emitting naphthalimide and enhanced ring-opening yellow-orange-emitting rhodamine moieties at 504 and 572 nm, respectively, were observed in our mechanically optimized PURh-DRh/C-PR50CD film upon stretching, which also exhibited reversible dual fluorescence conversion characteristics upon stretching and relaxation processes. The energy transfer efficiency from the green-emitting naphthalimide donor to the yellow-orange-emitting mechanofluorophoric rhodamine acceptor was evaluated to be ca. 17.7% by TRPL measurements. Moreover, the stretching deformation of PU films was inspected by using X-ray diffraction (XRD) techniques to verify the correlated morphological properties of stretching states in the oriented PU films. Furthermore, the designed MIM-functionalized PU films revealed splendid shape recapture behavior and reversible ratiometric mechano-responsive fluorescence emission changes upon heating. Finally, the potential applications of highly stretchable mechanically interlocked molecule (MIM)-based polymers contributed by artificial muscle functions of contracted [c2] daisy chains (as backbones) and pulley effects of polyrotaxanes (as cross-linkers) with ultrahigh toughness and ratiometric mechano-fluorescence properties can be extended to numerous fields of advanced functional polymeric materials.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful for the funding from the National Science and Technology Council, Taiwan (through grant no. MOST 110-2221-E-A49-003-MY3, NSTC 111-2622-E-A49-031, and MOST 111-2634-F-A49-007) and this work is also supported by the Center for Emergent Functional Matter Science of National Yang Ming Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc01839g

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