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[Yuichi Hirai](https://orcid.org/0000-0002-0252-1243), Anna Wrona-Piotrowicz, Janusz Zakrzewski, Magdalena Ciechańska, [Takahito Ohmura](https://orcid.org/0000-0001-7528-566X), [Takashi Takeda](https://orcid.org/0000-0003-2510-4562), [Takayuki Nakanishi](https://orcid.org/0000-0003-3412-2842), Rémi Métivier, Clémence Allain

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[Mechanofluorochromism and self-recovery of alkylsilylpyrene-1-carboxamides](https://mdr.nims.go.jp/datasets/8777b80a-2e1c-43e3-b7e2-f65d2f3989e6)

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Mechanofluorochromism and self-recovery of alkylsilylpyrene-1-carboxamidesMaterials for optical, magnetic and electronic devicesJournal of Materials Chemistry Crsc.li/materials-c PAPER  Yuichi Hirai, Rémi Métivier, Clémence Allain  et al .  Mechanofluorochromism and self-recovery of alkylsilylpyrene-1-carboxamides ISSN 2050-7526Volume 12Number 614 February 2024Pages 1911–22661952 |  J. Mater. Chem. C, 2024, 12, 1952–1957 This journal is © The Royal Society of Chemistry 2024Cite this: J. Mater. Chem. C,2024, 12, 1952Mechanofluorochromism and self-recovery ofalkylsilylpyrene-1-carboxamides†Yuichi Hirai, *a Anna Wrona-Piotrowicz, b Janusz Zakrzewski, bMagdalena Ciechańska, b Takahito Ohmura,d Takashi Takeda, aTakayuki Nakanishi, a Rémi Métivier *c and Clémence Allain *cA family of (alkylsilyl)pyrene-1-carboxamides exhibits similar mechanofluorochromic responses upongrinding. However, their spontaneous fluorescence recovery processes are distinct despite theirsimilarity in chemical structures and crystal packings. Fluorescence spectroscopy, crystallography, andnanomechanical tests revealed that the chromic direction is dominated by the packing motif, while thefluorescence recovery is driven by the intermolecular interactions and the reversibility of deformation.IntroductionMechanofluorochromism (MFC), the phenomenon whereorganic and coordination compounds exhibit reversible changesin their emission properties under mechanical stimuli, hasemerged as a captivating area of research in materials science,chemistry, and photonics.1–3 Thus, it has garnered considerableattention owing to its potential applications in sensors tovisualise the location/magnitude of the stress and informationencryption systems for anticounterfeiting technologies.4,5The ability to modulate the emission properties of com-pounds through mechanical deformation can be applied to thedesign of smart materials with tuneable and responsive lumi-nescent behaviour. When subjected to external mechanicalstimuli, such as compression, shear, grinding, or stretching,MFC materials experience changes in discrete molecular con-formations, intermolecular interactions, and correspondingelectronic and vibrational properties, which in turn manifestas shifts in their emission wavelength, intensity, and decaydynamics.6,7 One of the primary drivers of MFC is the reversiblebreaking and formation of non-covalent interactions, such asp–p stacking, hydrogen bonding, or metallophilic interactions,which play a crucial role in the molecular packing and electroniccommunication within the material.8–10 In addition to the dis-ruption or re-establishment of these interactions under mechan-ical stress, the presence of twisted intramolecular charge transferstates and aggregation-induced emission (AIE) units can alsocontribute to the MFC response, thereby enhancing the emissionefficiency and spectral shifts upon deformation.11–13 Thus, under-standing the fundamental principles governing MFC and eluci-dating structure–property relationships are crucial for designingand optimising materials with tailored MFC behaviour.To investigate these relationships, we focused on a series ofpyrene-derived compounds by introducing different substituents torealise diverse steric and electronic structures.14–16 These structuralmodifications induce changes in the intermolecular interactionsand molecular flexibility/rigidity, which influenced the mechanicalresponse and consequent luminescent properties. While relevantresearch primarily focuses on large-wavelength shifts and multi-functionality,17–20 there have been rare comprehensive demonstra-tions of the intricate interplay between molecular structures andtheir dynamic mechanical/photophysical responses in a bulk solidstate, including spontaneous recovery processes.21In this study, we focused on a family of pyrene-derivedcarboxamides with alkylsilyl groups22 because of their reportedmonomer-like purple fluorescence in the solid state and thecharacteristic ‘‘half-stacking’’ arrangement, which allowsFig. 1 Chemical structures of pyrene-derived carboxamides with alkylsilylgroups, and a schematic of the stacking model.a National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, 3050044,Japan. E-mail: hirai.yuichi@nims.go.jpb Department of Organic Chemistry, Faculty of Chemistry, University of Łódź,Tamka 12, 91-403 Łódź, Polandc Université Paris-Saclay, ENS Paris-Saclay, CNRS, PPSM, 4 Avenue des Sciences,91190, Gif-sur-Yvette, France. E-mail: remi.metivier@ens-paris-saclay.fr,clemence.allain@ens-paris-saclay.frd National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, 3050047,Japan† Electronic supplementary information (ESI) available. CCDC 2285534–2285543:Me3, 2285544–2285553: Me2Et and 2285523–2285532: Et3. For ESI and crystal-lographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3tc03968dReceived 31st October 2023,Accepted 19th December 2023DOI: 10.1039/d3tc03968drsc.li/materials-cJournal ofMaterials Chemistry CPAPEROpen Access Article. Published on 19 January 2024. Downloaded on 6/27/2024 6:10:24 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View Issuehttps://orcid.org/0000-0002-0252-1243https://orcid.org/0000-0001-6930-6989https://orcid.org/0000-0002-3403-1126https://orcid.org/0000-0001-6510-1716https://orcid.org/0000-0003-2510-4562https://orcid.org/0000-0003-3412-2842https://orcid.org/0000-0001-5612-8327https://orcid.org/0000-0002-2908-264Xhttp://crossmark.crossref.org/dialog/?doi=10.1039/d3tc03968d&domain=pdf&date_stamp=2024-01-03https://doi.org/10.1039/d3tc03968dhttps://doi.org/10.1039/d3tc03968dhttps://rsc.li/materials-chttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3tc03968dhttps://pubs.rsc.org/en/journals/journal/TChttps://pubs.rsc.org/en/journals/journal/TC?issueid=TC012006This journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. C, 2024, 12, 1952–1957 |  1953realising MFC by increasing or decreasing the effective p-orbitals overlaps (Fig. 1). We investigated the fluorescenceresponsivity from a broad perspective using crystallography,spectroscopy, and nanomechanics to guide the design anddevelopment of novel materials with enhanced MFC properties.Results and discussionsN-tert-butyl-2-(alkylsilyl)pyrene-1-carboxamides (alkyl = (CH3)3,(CH3)2(C2H5) and (C2H5)3) were synthesised following areported method22 and denoted as Me3, Me2Et, and Et3, respec-tively. The pristine powders exhibited purple fluorescence, andthe recorded profiles (Fig. 2, black lines) were the same as thosereported previously; lem2 was in the range of 405–410 nm, and aslightly broader spectrum was observed in the case of Et3 wherethe vibrational structure is attenuated. As mentioned already,the ‘‘half-stacking’’ arrangement of the p-orbitals of Me3 con-tributed to the monomeric emission bands, even in the solidstate. Thus, the same stacking is expected for Me2Et, and theless-dominant monomeric emission bands in Et3 indicates thelarger overlapping of p-orbitals. Upon mechanical grindingusing a pestle and a mortar, a purple-to-blue bathochromicMFC was observed for all compounds, which indicates theformation of aggregated conformations with new intermolecularinteractions. MFC spectroscopy was performed following thereported procedures to determine the mechanical responsivityincluding spontaneous self-recovery processes in the solid state(Fig. S1, ESI†).16 An increased broad emission band (lem3 B460 nm) and accompanying near-ultraviolet (UV) shoulder bands(lem1 B 388 nm) with different degrees of contribution wereobserved (Fig. 2, blue lines). The relative intensity of the near-UVbands was minimal immediately after grinding and increasedover time, particularly for Me2Et and Et3 (Fig. S2, ESI†). Theshoulder bands should stem from the residues of the pristineand/or the ground forms after undergoing fast spontaneousmolecular rearrangements under ambient conditions. In thisstudy, the self-recovery rates were compared qualitatively by thespectral intensity ratios of the ground forms at lem2 and lem3,which resulted in the order of Me3 o Me2Et { Et3. The spectralfeature was almost preserved for Me3 after 30 min at roomtemperature, whereas the fluorescence spectra of Me2Et andEt3 showed a remarkably diminished emission band at 460 nmand concomitant increase in the UV emission for the sameduration (Fig. 2, red lines). All compounds exhibited pristine-like spectra with an additional peak at approximately 388 nmafter heating (Fig. 2, green lines), which are denoted as ‘‘recov-ered’’ in the remainder of the text. This indicates the mobility ofthe compounds in the solid state for reverting back to thepristine form and beyond without the aid of organic vapoursor solvents. Differential scanning calorimetry (DSC) of pristinepowders also indicated the lack of phase transitions under thecondition of the MFC recovery processes (Fig. S3, ESI†).To correlate the molecular arrangement and MFC response,the crystal structures of all compounds were determined usingsingle-crystal X-ray diffraction (XRD) measurements and thoseof Me3 were re-determined. The space group was determined tobe P21/c with half-stacking features, and the coincidence ofsimulated and experimental powder XRD (PXRD) patternsconfirmed the phase purity of the pristine powders (Fig. S4,ESI†). The packing manner was categorised as a herringbonemotif, which has atomic contact ratios %C� � �H/%C� � �C 4 4.5,via the Hirshfeld surface analysis23–25 for all molecules (Fig. S5,ESI†). This corroborates the purple-to-blue bathochromic MFCbehaviours of Me3, Me2Et, and Et3, where the monomer-likeconformation shifts to a dimer-/aggregated-like conformationunder mechanical stimuli.16 However, the similar values ofatomic contacts between adjacent molecular surfaces andpacking motifs still do not explain the distinct kinetic gap inthe self-recovery processes that spontaneously occur afterunloading. An energy framework analysis was also performedusing CrystalExplorer to quantitatively visualise the topology ofthe intermolecular interactions in these crystals.26–29 The three-dimensional (3D) topologies illustrated the strongest inter-action energy (EInterMol of approximately �70 kJ mol�1) betweenhead-to-head pairs that involve NH� � �OQC hydrogen bondingin all cases, which dominates the crystal packing manner(Fig. 3). In addition to the 30–39% larger EInterMol betweenhalf-stacked pairs compared with others, Et3 formed anotherinteraction (�27.5 kJ mol�1) between its head-to-head pairswithout NH� � �OQC hydrogen bonding. Therefore, relativelystronger and multidimensional interactions between moleculescould retain fewer monomeric fluorescence features of Et3 anddrive the faster reconstruction of molecular packing in theground form.Following those crystallographic characteristics, themechanical properties of the crystals were also investigatedFig. 2 Normalised emission spectra of Me3, Me2Et, and Et3 at roomtemperature (RT) (black: pristine, blue: ground, red: 30 min at RT aftergrinding, green: 30 min at RT and 30 min heating at 50 1C after grinding,lex = 340 nm, in the solid state), and chemical structures and photographsunder UV irradiation (left: pristine, right: ground, lex = 365 nm). Emissionbands at approximately 388, 405–410 and 460 nm are highlighted byyellow shadows.Paper Journal of Materials Chemistry COpen Access Article. Published on 19 January 2024. Downloaded on 6/27/2024 6:10:24 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3tc03968d1954 |  J. Mater. Chem. C, 2024, 12, 1952–1957 This journal is © The Royal Society of Chemistry 2024using nanoindentation experiments (see Fig. S6 in ESI† fortesting procedures). The load–displacement (p–h) curves exhib-ited similar responses without sudden bursts (so-called ‘‘pop-ins’’),30 which is common in brittle molecular crystals (Fig. 4a).The reduced Young’s modulus (Er) and hardness (H) wereestimated from more than 300 data sets for each moleculeusing the Oliver–Phar method (Fig. 4b and c, left).31 Theestimated values indicated that Me3 was the softest amongthe studied compounds, as it exhibited relatively low Er and Hvalues (3.9 and 0.26 GPa, respectively). The slow recovery natureof softer compounds was also supported by the pioneeringstudy on MFC using polymorphic difluoroboron avobenzonecrystals with distinct plasticity.21 In this study, additionally, theelasticity index (H/E) and geometric mean ((EH)1/2) were intro-duced to reflect the type of deformation and include bothstiffness and hardness (Fig. 4c, right).32,33 Thus, the indenta-tion hardness of Me3 was dominated by a more irreversible(plastic) character than the reversible (elastic) ones present inMe2Et and Et3. Although no general mathematical relation-ships yet exists between stiffness and density, highly densematerials are more prone to show higher Er values.34 In thiscase, the higher void volume of Me3 (Vvoid = 285, 266, and 267 Å3for Me3, Me2Et, and Et3, respectively) possibly resulted in softerand more irreversible characteristics than in the other com-pounds because Me3 and Me2Et show similar EInterMol distribu-tions and all compounds have similar crystal densities.Therefore, microscopic mechanical parameters may governsome of the self-recovery trends after unloading, which explainsthe relatively slow fluorescence recovery in Me3.Finally, we performed in situ temperature-variable spectro-scopy on the recovered solids and pristine powders and crystal-lography on the single crystals in the range of 123–393 K toconsider the solid-state mobility of the molecules in terms offluorescence signals and absolute molecular configurations.The temperature-dependent emission spectra of the recoveredsamples revealed nearly identical profiles and thermalresponses for Me3 and Me2Et (Fig. 5a, top). In addition to thesmall overlap of effective p-orbitals in the pristine forms,mechanical grinding further dislocated the half-stacked pairs,thereby resulting in the typically structured fluorescence ofmonomeric pyrenes at low temperatures. By contrast, therecovered form of Et3 showed ratiometric spectral changesbetween the VIS (lem B 410–419 nm) and UV (lem B388 nm) bands. The intensity ratios (IVIS/IUV) decreased by15-folds upon heating, accompanying a hypsochromic shift of9 nm of the VIS peak (Fig. S7, ESI†). Pristine Et3 also showed asimilar response with a smaller contribution of the UV band(Fig. 5a bottom), which indicated the existence of effectivestacking in both the pristine and recovered forms. Such atemperature-dependent fluorescence response based on theinterchromophoric interactions in a bulk solid material wasalso reported in a pyrene-based metal–organic framework.35Single-crystal XRD (SCXRD) experiments at different tem-peratures demonstrated the evolution of the unit-cell para-meters upon heating (Fig. 5b and Tables S1–S3, ESI†). Thecontraction/expansion of crystals is generally observed under acontrolled pressure using a diamond anvil cell (DAC), and theFig. 3 CE-B3LYP energy frameworks of Me3, Me2Et, and Et3 from differ-ent perspectives representing the net interaction energy EInterMol (cut-off:25.0 kJ mol�1) at 123 K. The blue cylinders connect the centres of mass ofadjacent molecules, and the diameter of cylinders are proportional to themagnitutdes of EInterMol. Opaque red and green shadows highlight theinteractions between parallel half-stacked pairs and non-parallel head-to-head pairs without NH� � �OQC hydrogen bonding, respectively.Fig. 4 (a) Selected load–displacement curves and in situ scanning probemicroscopy two-dimensional images after indentations (20 mm � 20 mm).(b) Distribution map of Young’s modulus and the contact depth (4300indentations for each molecule). (c) Young’s modulus versus the hardnessplot (left) and the geometric mean versus the elasticity index plot (right).Journal of Materials Chemistry C PaperOpen Access Article. Published on 19 January 2024. Downloaded on 6/27/2024 6:10:24 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3tc03968dThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. C, 2024, 12, 1952–1957 |  1955magnitude of cell expansion in this study (3–5% in the Me3c-axis, Me2Et a-axis, and Et3 a-axis at 393 K relative to 123 K) is inthe same order as that reported for pyrene-1-carbaldehyde (PA)at several GPa in DAC (relative to the atmospheric pressure).36As previously mentioned for the emission of PA at a higherpressure, the higher emission intensity at lower temperature inthis study may be explained by the suppression of vibrationalrelaxation processes or AIE due to the higher crystal density.37The blue-shifted emission of Et3 at higher temperatures alsocoincides with the shorter emission wavelengths of PA underlower pressures. The intramolecular torsion angles (ytorsion) andenergy frameworks (Fig. S8 and S9, ESI†) correlate the similarspectral shapes of Me3 at 123 K and 393 K with a slight changein EInterMol (D = 0.4%) and relatively large ytorsion relaxation.In addition to the approximately 5% decrease in EInterMol for half-stacked pairs and head-to-head pairs with NH� � �OQC hydrogenbonding in Me2Et and Et3, respectively, the EInterMol betweenhead-to-head pairs without NH� � �OQC interaction in Et3decreased by 25% after heating, which drove the anisotropicexpansion of the cell and the appearance of a monomericemission peak at a higher temperature. Therefore, the intra-and inter-molecular arrangements may change in either orderedor disordered solids depending on the temperature, thus result-ing in characteristic emission spectral shifts in accordance withthe overlap of the stacking.These dynamic molecular conformation changes were alsoconfirmed by the temperature-variable FT-IR spectra (Fig. 5c).Amide-I (ca. 1630 cm�1, CQO stretch), amide-II (ca. 1530 cm�1,N–H bend/C–N stretch), and amide-A (ca. 3000–3500 cm�1, N–Hstretch) bands were identified,38–40 and the shifts in amide-Iand II bands (D B +30 and �40 cm�1, respectively) and theappearance of the monomer’s amide-A bands (ca. 3420 cm�1)upon heating above their melting points indicated the breakageof hydrogen bonding, which is in accordance with the spectro-scopic and crystallographic results.ExperimentalSynthesisN-tert-butyl-2-(alkylsilyl)pyrene-1-carboxamides (alkyl = (CH3)3:Me3, (CH3)2(C2H5): Me2Et and (C2H5)3: Et3) were synthesizedfollowing the reported methods.22 See ESI† for 1H- and 13C-NMR, FTIR, and DSC data. ATR-FTIR spectra were performedwith a Nicolet 6700 spectrometer equipped with a deuteratedtriglycine sulphate detector. The spectra were obtained byadding together 64 scans at a resolution of 2 cm�1. DSCanalyses were performed using a DSC 2500 Discovery, TAInstruments, under a nitrogen atmosphere for all samples ata heating and cooling rate of 10 1C min�1.Room-temperature MFC spectroscopyEmission and excitation spectra of pristine powder sampleswere recorded on a HORIBA Jobin-Yvon Fluorolog FL3-221spectrometer using a short path length optical quartz cell(20/C/Q/0.2, Starna), and the spectra were corrected for theresponse of the detector system. The pristine powder (o1 mg)was ground using an agate mortar, and the resulting amor-phous solid adhered on a pestle surface was immediatelytransferred to a quartz plate by smearing to record the spectrawithin ca. 1 min and minimize the effect of self-recovery atroom temperature. Due to the small amount of sample and theshort time of measurements for the optimized experiments,emission decay lifetimes and quantum yields of the groundforms were not discussed in this study.In situ temperature-dependent fluorescence spectroscopyPristine powders and recovered solids were placed in a THMS600temperature control stage equipped with a LNP-96 liquid nitro-gen pump and a T96-S controller (Linkam Scientific InstrumentsLtd). Emission spectra were recorded using an array spectro-meter MCPD-9800 and a quantum efficiency measurementsystem QE-2100 (Otsuka Co., Ltd) in the range between 123 Kand 393 K (150 K min�1).SCXRD experimentsSingle crystals were mounted on a glass capillary using Ara-ldites epoxy adhesive. The measurements were performed on aFig. 5 Temperature-dependent (a) fluorescence spectra (top: recovered,bottom: pristine), (b) relative unit-cell axes lengths (values at 393 K areindicated), and (c) ATR-FTIR spectra (DHBD: doubly hydrogen-bondeddimers, SHBD: single hydrogen-bonded dimers) of Me3, Me2Et, and Et3.Paper Journal of Materials Chemistry COpen Access Article. Published on 19 January 2024. Downloaded on 6/27/2024 6:10:24 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3tc03968d1956 |  J. Mater. Chem. C, 2024, 12, 1952–1957 This journal is © The Royal Society of Chemistry 2024Rigaku Synergy Custom system with a Hybrid Pixel ArrayDetector HyPix-Arc 150 using graphite monochromated Mo-Karadiation. Correction for decay and Lorentz-polarization effectswere made using empirical absorption correction, solved bydirect methods, and expanded using Fourier techniques. Non-hydrogen atoms were refined anisotropically. Hydrogen atomswere refined using the riding model except for the ones on theamide donor nitrogen atoms which were refined freely. Allcalculations were performed using the Rigaku CrysAlis(Pro)crystallographic software package. CIF data was confirmed bycheckCIF/PLATON service. CCDC data (2285534–2285543: Me3,2285544–2285553: Me2Et, 2285523–2285532: Et3).†PXRD experimentsPXRD patterns were recorded on a Rigaku SmartLab X-rayDiffractometer using a single-crystal Si specimen holder toconfirm the crystallinity of the pristine powders (Fig. S4, ESI†).Energy framework analysisThe intermolecular interaction energy was calculated at theB3LYP/6-31+G(d,p) level of theory using CrystalExplorer (version21.5).27 The interaction energy was calculated for a target singlemolecule with all molecules having any atom within 3.8 Å.Nanoindentation testsThe target crystals were glued on a glass plate with a Crystal-bondt 509 mounting adhesive using a micromanipulator(Fig. S6, ESI†). Nanoindentations were carried out on a Tri-boindenter system (Hysitron Inc. Triboindenter TI950)equipped with a 601 3-sided pyramidal tip on fused quartz(Hysitron Inc. Ti-0038). All experiments were performed in theload-controlled mode using a loading/unloading time of 10 sand a hold time of 10 s at peak load (50–400 mN). More than 4sets of 81 indentations (9 � 9 array) were performed on eachcrystal. Young’s modulus and hardness were estimated fromthe unloading curves using the reported eq. 1 and eq. 2 in Fig.S6 (ESI†).31 In situ SPM images were taken using the same probeafter indentations.ConclusionsIn this work, we elucidated the structure–property relationshipsgoverning the MFC behaviour using pyrene-derived carboxa-mide with alkylsilyl and tert-butyl groups. All compoundsexhibited similar purple-to-blue bathochromic MFC upongrinding. However, the spontaneous fluorescence recovery rateswere significantly different, despite their slight differences inthe chemical components and molecular packing motifs.Single-crystal XRD experiments and Hirshfeld and energy fra-mework analyses indicated that the chromic direction wasdetermined by the packing motif, whereas the self-recoveryprocess was not simply dominated by the steric hinderancebut by the distribution of intermolecular interaction energies.Nanomechanical tests also revealed the reversibility of defor-mation after unloading, and the ‘‘mobility’’ of molecular solidswas demonstrated by temperature-dependent spectroscopy andcrystallography either in crystalline or ground/annealed samples.The outcomes of our study contribute to a broader and deeperunderstanding of the MFC behaviour, including spontaneousfluorescence recovery processes, and inspire further explorationof innovative materials and devices based on simple organicsystems.Author contributionsYH: conceptualisation, methodology, crystallography, spectro-scopy, funding acquisition, writing; AW-P: organic synthesis,characterization; JZ: organic synthesis, characterization; MC:organic synthesis, characterization; TO: nanoindentation experi-ments; TT: single-crystal and powder XRD experiments; TN: in situtemperature-dependent fluorescence spectroscopy; RM: concep-tualisation, review and editing; CA: conceptualisation, fundingacquisition, review and editing.Conflicts of interestThere are no conflicts to declare.AcknowledgementsThis work was supported by Japan Society for the Promotion ofScience (JSPS) Grant-in-Aid for Grant Number 22K14661 andthe European Research Council (ERC) under the EuropeanUnion’s Horizon 2020 research and innovation program(grant agreement no. 715757 MECHANO-FLUO to C. A.). Y. H.*gratefully acknowledges Dr Shiro Funahashi (National Institutefor Materials Science, Japan) for single crystal X-ray analyses andDr Bartłomiej Kost (Centre of Molecular and MacromolecularStudies, Polish Academy of Sciences, Poland) for DSC and FTIRmeasurements.References1 Y. Sagara and T. Kato, Nat. Chem., 2009, 1, 605–610.2 Y. Sagara, S. Yamane, M. Mitani, C. Weder and T. Kato, Adv.Mater., 2016, 28, 1073–1095.3 Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu andJ. Xu, Chem. Soc. Rev., 2012, 41, 3878–3896.4 K. Muthamma, D. Sunil and P. Shetty, Mater. Today Chem.,2020, 18, 100361.5 P. Shi, D. Deng, C. He, L. Ji, Y. Duan, T. Han, B. Suo andW. Zou, Dyes Pigm., 2020, 173, 107884.6 D. Avobenzone, G. Zhang, J. Lu, M. Sabat and C. L. Fraser,J. Am. Chem. Soc., 2010, 132, 2160–2162.7 M. Raisch, W. Maftuhin, M. Walter and M. Sommer, Nat.Commun., 2021, 12, 1–10.8 Y. Sagara, H. Traeger, J. Li, Y. Okado, S. Schrettl, N. Tamaokiand C. Weder, J. Am. Chem. 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