Mechanically induced chemiluminescence of xanthene-modified 1,2-dioxetane in polymers

Abstract

Xanthene-modified 1,2-dioxetane (Xa-Ad) derivatives were designed and synthesized using a facile protocol, and their mechanical activity was identified for the first time to the best of our knowledge. When these derivatives were incorporated into polymer main chains or crosslinked networks, the resultant polymers were mechanochemiluminescent (MCL). Bright chemiluminescence was observed when stretching the bulk polymer. Xa-Ad, as a new MCL mechanophore, was found according to DFT simulations and experimental results to exhibit a force threshold lower than that of bis(adamantyl) dioxetane, the only chemiluminescent mechanophore reported so far. The current study has not only increased the structural diversity of MCL mechanophores, but has also offered an efficient way to modulate the corresponding mechanochemical activity.

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Polymers have become indispensable in modern life, and in many applications their ability to withstand mechanical stress determines which polymer is chosen.¹ Understanding the influence of external mechanical stress on the mechanical properties of materials and providing failure warnings when the structural integrity of the materials is threatened have long been important areas of study.^2–4 However, failure in polymeric materials is complicated, where mechanical energy dissipation usually takes place at different scales ranging from nano- to macro-levels.^5,6 It has been well recognized that the macroscopic failure of a polymer under stress by necessity starts with a molecular event, whereby covalent bond scission is a key step to dissipate mechanical energy.^7–9 Despite great progress on analytical tools to probe for failures of polymers, such as optical or magnetic tweezers, microfibers, NMR, ESR and SPM spectroscopies, most of them have limited sensitivity and resolution to characterize the specific molecular details regarding the failure initiation events.^10,11

To address this limitation, molecular/chemical force probes with a leap in sensitivity for determining the precise location and timing of the rare polymer chain scission events and for achieving a fundamental uncovering of the failure mechanics of polymeric materials are in high demand.^3,4 For this purpose, polymer mechanochemistry has emerged as a promising approach for converting mechanical force into a visualized signal. It holds the potential to revolutionize the field by providing a new method for providing damage warning and assessing stress distribution in polymer materials.^12,13 Mechanochemiluminescence (MCL) is a suitable option for situations where sensitivity and time resolution are critical, and presents a method in which no excitation signal is required to visualize the signal and light emitted directly on bond scission.^14–17

So far, bis(adamantyl)dioxetane (Ad-Ad) is the only reported chemiluminescent mechanophore, achieved as a result of the synthetic elaborations of the labile chemiluminescent precursor, and was first discovered in 2012 by Sijbesma and Chen et al.¹⁸ Its blue MCL from the corresponding polymers is caused by the breakage of the dioxetane ring and generation of an excited-state ketone followed by its relaxation to the ground state ketone with the emission of a blue photon.¹⁸ Although the analytical scope of Ad-Ad-involved MCL now spans a broad range of soft polymers with sensitivity levels orders of magnitude higher than those of other optical force probes,^15–17 further applications of Ad-Ad have been restrained by its intrinsic limitations: (1) high force threshold for chemiluminescence, due to a relatively high activation energy barrier to decomposition of approximately 150 kJ mol⁻¹; (2) relatively faint blue light emission (λ_em = 420 nm) from Ad-Ad-containing polymers, and hence not easily recognized by the naked eye; and (3) complicated synthetic route and trivial modification of the dioxetane core. In this regard, it would be desirable to enrich the library of structures and opto-mechanical activities of MCL mechanophores, in particular those with relatively low force thresholds and increased MCL sensitivity levels. Such advancements would promote the application of MCL in force sensing and the study of fracture mechanisms in polymer materials.

Of the various dioxetane derivatives, we found that the xanthene-modified 1,2-dioxetane (Xa-Ad) displayed particularly bright CL, and a relatively low activation energy barrier to decomposition (approximately 46.4 kJ mol⁻¹) but was stable at room temperature.^19,20 These factors showed its potential as a new MCL mechanophore featuring a relatively low force threshold and increased sensitivity in optical signal output.¹⁸ Herein, we investigated the mechanical activities of Xa-Ad in polymethacrylates (PMAs) using DFT simulations, sonication experiments of polymer solutions and tensile tests of bulk films. Note the structural relationship between Xa-Ad and Ad-Ad, with one of the sterically hindering adamantyl groups of Ad-Ad replaced with a planar xanthene. Xa-Ad showed satisfactory MCL properties; compared to Ad-Ad, Xa-Ad showed a lower force threshold, higher chain scission rate and bright CL with high spatial and temporal resolutions (Scheme 1). This work has thus achieved a new kind of MCL mechanophore and is expected to provide further opportunities to broaden the applications of MCL polymer materials.

Scheme 1 Schematic illustration of the mechanical/thermal inductions of chemiluminescence in the indicated mechanophores.

Xanthene-modified 1,2-dioxetanes functionalized with, respectively, bis-α-bromo ester (Xa-Ad-Br) and bisacrylate (Xa-Ad-Vinyl) were synthesized as shown in Scheme 2a.^19,20 McMurry olefination proceeded smoothly on xanthone and adamantanone, to provide the corresponding alkenes. Olefins thus formed were submitted to the photooxidation [2 + 2] cycloaddition reaction, delivering Xa-Ad-Br and Xa-Ad-Vinyl, respectively (see ESI†). Xa-Ad-Br was used as an initiator for single-electron-transfer living radical polymerization (SET-LRP) of methyl acrylate (MA) to form Xa-Ad-centered linear PMA (Xa-Ad-PMA). Additionally, visible-light-induced radical polymerization of MA in the presence of Xa-Ad-Vinyl (0.5 wt%) as the crosslinker afforded a crosslinked PMA network (Cross-Xa-Ad-PMA). As controls, bis(adamantyl)-1,2-dioxetane functionalized with bis-α-bromo ester (Ad-Ad-Br) and mono-α-bromopropionyloxy-substituted Xa-Ad (Control-Xa-Ad) as the control initiators and the corresponding control linear PMAs (Ad-Ad-PMA, Control-Xa-Ad-PMA) were synthesized according to the literature (Table S1†).¹²

Scheme 2 (a) Synthetic route towards Xa-Ad-Br, Xa-Ad-Vinyl and the corresponding polymers Xa-Ad-PMA and cross-Xa-Ad-PMA. (b) Depictions of the chemical structures of control samples: Ad-Ad-PMA, mono-functionalized Control-Xa-Ad-Br, and the corresponding polymer Control-Xa-Ad-PMA.

Thermally induced chemiluminescences of Xa-Ad-Br and Ad-Ad-Br were compared first. As shown in Fig. 1, Xa-Ad-Br showed bright blue chemiluminescence emission when heated to 90 °C in air, with emissions in the wavelength range of 460–600 nm. Ad-Ad-Br displayed a similar emission behaviour (blue light in the range of 460–600 nm upon heating), suggesting similar CL mechanisms for Xa-Ad and Ad-Ad-Br.^18–22 Notably, appreciable chemiluminescence was observed for Xa-Ad-Br at a temperature of 90 °C, lower than the 180 °C value for Ad-Ad-Br, indicating that the introduction of xanthene could modulate the stability of the 1,2-dioxane ring. This result was probably due to the reduced activation energy derived from the less sterically hindering xanthene group.¹⁸

Fig. 1 Thermally induced chemiluminescence spectra of Xa-Ad-Br (a) and Ad-Ad (b) in air atmosphere. Insets show the corresponding optical images of the heating process.

Then, the linear PMAs (Xa-Ad-PMA, Ad-Ad-PMA and Control-Xa-Ad-PMA) with comparable molecular weights and low polydispersity levels (Table S1†) were subjected to pulsed sonication (1 s on/1 s off) in dilute tetrahydrofuran (THF, 10 mg mL⁻¹) at −5 °C. The relative rates constants for the chain scission under sonication were determined by monitoring the change in molecular weight with sonication time according to gel permeation chromatography (GPC) measurements (see ESI eqn (1)† for details).^23–25 As shown in Fig. S1a,† sonicated Xa-Ad-PMA yielded typical bimodal GPC curves, with the initial polymer molecular weight switching from 88 kDa (M_n0) to approximately 40 kDa. Its molecular weight then hardly changed with continued increasing sonication time, indicating that Xa-Ad-PMA underwent a selective fracture under sonication.^22,23 Similar GPC curves of Ad-Ad-PMA were obtained, as illustrated in Fig. S1b.† Further analysis of these GPC data yielded a chain scission rate of 2.45 (±1.54) × 10⁻³ kDa⁻¹ min⁻¹ for Xa-Ad-PMA, with this rate higher than that for Ad-Ad-PMA (2.03 (±5.64) × 10⁻³ kDa⁻¹ min⁻¹, Fig. 2). Under the same conditions, the control chain end polymer Control-Xa-Ad-PMA showed non-specific breakage during sonication (Fig. S2†), so that the mechanochemical nature of the chain scission process was confirmed. The comparison of the results of the sonication experiments with the three polymers thus illustrated that the introduction of xanthene was conducive to causing ring opening of 1,2-dioxetane in response to mechanical forces.

Fig. 2 Relative rates of scission of sonochemical polymers (a) Xa-Ad-PMA and (b) Ad-Ad-PMA. The data points were calculated each from the average of three separate experiments.

To obtain a clear picture of the dissociation pathway and the mechanosensitivity of 1,2-dioxetane derivatives with different regiochemistries, DFT calculations on model compounds M-Xa-Ad and M-Ad-Ad (Fig. S2a†) were carried out by using the constrained geometries simulate external force (CoGEF) method.^26,27 The results served as the input for a kinetic model which allowed for a calculation of the bond rupture rate constant as a function of the applied force, or the force as a function of the lifetime of the bond.²⁷ Starting from the equilibrium geometry of each molecule, the constrained distance between methyl carbon atoms was increased in increments of 0.02 Å and the energy of the molecule was minimized at each step. The rupture force (F_max), a reliable indicator of mechanochemical activity, was calculated from the maximum slope along the potential curve toward bond rupture. Although DFT calculations have been found in general to substantially overestimate the energy barrier of the mechanical dissociation process, the process of molecular fracture can be understood by stretching the optimal configuration. The change in distance between the ester and ether bonds in the molecular structure also were recorded (Fig. S3†). M-Xa-Ad showed a selective ring opening break of the 1,2-dioxetane ring during progressive stretching and was successfully converted to a ketone structure with a maximum estimated rupture force of 3.93 nN, which was distinctly lower than the force required for M-Ad-Ad (4.43 nN). The geometry was optimized using density functional theory (DFT) and the distances of the labeled bonds were recorded (Fig. S4†). The results showed that the four-membered dioxetane ring was relatively weak and could be broken selectively under mechanical force.

To shed more light on MCL from Xa-Ad-containing polymer materials, the rubber-like Cross-Xa-Ad-PMA films were subjected to optomechanical tests using a rheometer equipped with two rotating drums at a controlled rate of 8 s⁻¹, which allowed for uniform extensional deformation and facilitated recording of luminescence with a high-speed camera. Appreciable blue light was observed upon deformation in the dark. As illustrated in Fig. 3a, at the early stages of deformation, the luminescence intensity gradually increased upon stretching, corresponding to the activation of the 1,2-dioxetane group. Later frames showed a strongly increased luminescence intensity combined with a pronounced localization of scission at the location of the fracture. After fracture, no further luminescence was detected. The results implied that the transient nature of the emission allowed chain scission events to be monitored in real time at spatial resolution. In addition, compared to Cross-Ad-Ad, Cross-Xa-Ad showed a lower force threshold and stronger luminescence intensity during stretching (Fig. S5 and S6†). In contrast, the physically mixed film (crosslinked PMA mixed with Xa-Ad-Br) did not show not MCL under stretching, which again proved the mechanical nature of this kind of blue chemiluminescence from the Xa-Ad mechanophore.

Fig. 3 Hencky stress and light intensity versus time during stretching of bulk films. Graphs of Hencky stress and light intensity versus time (left) and optical images and intensity analysis of the sample during stretching (right) of (a) Cross-Xa-Ad-PMA, (b) Cross-Xa-Ad-PMA containing DPA (0.05 wt%) and (c) Cross-Xa-Ad-PMA containing DTBD (0.05 wt%).

To sensitize the MCL from deformed Cross-Xa-Ad-PMA, we used fluorescent 9,10-diphenylanthracene (DPA) and 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) as energy transfer acceptors to harvest more light as well as to red-shift the emission wavelength. The composite films were obtained by adding the respective acceptor (0.05 wt%) during polymerization. As shown in Fig. 3b and c, bright light with blue and yellow colors was observed at the fracture, respectively. And recorded successfully using a camera. Thus, Xa-Ad containing PMAs exhibited satisfactory MCL properties with facilely tunable emission colors. As a complementary MCL mechanophore to Ad-Ad, Xa-Ad featuring a higher cleavage rate and lower rupture force may be used to more sensitively probe mechanochemical chain scission with high resolution in space and time, and contribute to the capability of providing exceptionally detailed insight into the origins and mechanisms of failure in polymeric materials.

Conclusions

Xanthene-modified 1,2-dioxane (Xa-Ad) derivatives were designed and synthesized, and could serve as a new kind of MCL mechanophore. Sonication of a solution of Xa-Ad-centered linear PMA demonstrated a selective cleavage of the 1,2-dioxetane ring with its chain scission rate higher than that of the reported Ad-Ad-centered PMA. According to DFT simulation results, the introduction of xanthene to form Xa-Ad can reduce the activation energy of 1,2-dioxetane, thus lowering the force threshold, compared to the Ad-Ad. Benefiting from the facile synthetic protocol and excellent chemiluminescence properties, the bulk films of Xa-Ad crosslinked PMAs exhibited bright blue chemiluminescence under mechanical force. Incorporation of energy transfer acceptors into the polymeric material was found to allow for the modulation of the intensity and color of the MCL. The results therefore identified the MCL activities of Xa-Ad for the first time to the best of our knowledge, and demonstrated Xa-Ad to be a complementary MCL mechanophore to Ad-Ad with the ability to produce a sensitive MCL signal under low rupture force. This work is expected to provide a broad perspective for the development and applications of new MCL materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grants 22275068, 21734006, and 21975178) and the Open Project of the State Key Laboratory of Supramolecular Structure and Materials.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3py00786c

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