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[Yuichi Hirai](https://orcid.org/0000-0002-0252-1243), Stann Van Baaren, [Takahito Ohmura](https://orcid.org/0000-0001-7528-566X), [Takayuki Nakanishi](https://orcid.org/0000-0003-3412-2842), [Takashi Takeda](https://orcid.org/0000-0003-2510-4562), Yuichi Kitagawa, Yasuchika Hasegawa, Rémi Métivier, Clémence Allain

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[Bright Lanthanide            <sup>III</sup>            Triboluminescence despite Low Photoluminescence, and Dual Triboluminescence and Mechano‐Responsive Photoluminescence](https://mdr.nims.go.jp/datasets/1d8158cb-0256-4ed4-b7f6-a5b7e1bb3c0a)

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Bright LanthanideIII Triboluminescence despite Low Photoluminescence, and Dual Triboluminescence and Mechano‐Responsive Photoluminescencewww.advopticalmat.de2203139  (1 of 7) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbHBright LanthanideIII Triboluminescence despite Low Photoluminescence, and Dual Triboluminescence and Mechano-Responsive PhotoluminescenceYuichi Hirai,* Stann Van Baaren, Takahito Ohmura, Takayuki Nakanishi, Takashi Takeda,  Yuichi Kitagawa, Yasuchika Hasegawa, Rémi Métivier, and Clémence AllainDOI: 10.1002/adom.202203139reactions in organic solvents. Mechano-chromic luminescence (MCL) describes the deformation of molecular conforma-tions alongside changed optical properties in response to mechanical stimuli. Crystal-to-crystal/amorphous phase transitions produce shifts in the emission wavelength and/or changes in the emission intensities of organic and coordination compounds.[3] Despite the first description of blue-glowing sugar having been described in 1605, and the broad versatility of these mechanically-triggered phenomena, triboluminescence (TL)—fracture-induced luminescence—is not yet understood in terms of a general molecular design, and the underlying pho-tophysical processes remain unclear. TL is observed only in the moment in which force is applied and does not require a light source. Studying this instantaneous force-to-photon conversion is of critical not only from the perspective of fundamental science but also for industrial applications, which include irradiation-free force/stress sensors, security marking techniques, and health care devices.[4]Bright TL has been reported for various lanthanideIII (LnIII) complexes with dibenzoylmethide (dbm), thenoyl-trifluoroacet-ylacetonate (tta), and hexafluoroacetylacetonate (hfa) ligands, In pursuit of a new family of mechanically responsive luminescent materials, it is aimed to differentiate triboluminescence (TL) from photoluminescence (PL). A β-diketonate ligand with tert-butyl groups (2,2,6,6-tetramethylheptane-3,5-dionate: tmh) is selected to quench EuIII-centered PL via ligand-to-metal charge transfer, whereas tmh provides efficient photosensitization of TbIII ions. Bright TL is observed from the EuIII and TbIII homodinuclear complexes despite the fact that their PL quantum yields differed by a factor of >50. Nanomechanical tests reveal the ductility of the crystals, suggesting they are ideal for accumulating deformation energy before breakage. Furthermore, a TL/PL color difference is observed for a TbIII/EuIII heterodinuclear complex, and grinding results in mechanochromic luminescence (MCL); this is the first example of a dual TL- and MCL-active lanthanideIII coordination compound. The photophysical properties before, during, and after grinding are investi-gated and correlated with powder and single-crystal crystallographic data.Research ArticleY. Hirai, T. Nakanishi, T. TakedaNational Institute for Materials Science1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanE-mail: Hirai.Yuichi@nims.go.jpS. Van Baaren, R. Métivier, C. AllainPPSMUniversité Paris-SaclayCNRSENS Paris-Saclay4 Avenue des Sciences 91190, Gif-sur-Yvette, FranceT. OhmuraNational Institute for Materials Science1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, JapanY. Kitagawa, Y. HasegawaInstitute for Chemical Reaction Design and Discovery (WPI-ICReDD)Hokkaido UniversitySapporo, Hokkaido 001–0021, JapanY. Kitagawa, Y. HasegawaDivision of Applied ChemistryFaculty of EngineeringHokkaido UniversityKita-13 Jo, Nishi-8 Chome, Sapporo, Hokkaido 060–8628, JapanIntroductionSolid-state chemical and physical phenomena triggered by mechanical stimuli have attracted increasing attention in the last decade. Mechano-responsive gels[1] and solid-state coupling reac-tions[2] have provided new organic chemistry and materials sci-ence research avenues based on modifying conventional chemical The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.202203139.© 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the  Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.Adv. Optical Mater. 2023, 11, 2203139http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadom.202203139&domain=pdf&date_stamp=2023-02-08www.advancedsciencenews.com www.advopticalmat.de2203139  (2 of 7) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbHand these complexes also exhibit photoluminescence (PL) upon UV irradiation.[5] As the absorption coefficients of LnIII ions are intrinsically small (εLn < 1 cm−1 M−1)[6] because the f– transitions are parity-forbidden, the introduction of organic chromophores is essential to achieve efficient PL.[7] Thus, ligand-to-metal photosen-sitized energy transfer (PSET) is a reasonable explanation for the brightness of LnIII TL (Figure 1a, “1st generation”).[8] Nevertheless, we previously reported that the LnIII TL does not always have the same characteristics of the corresponding PL, and we observed distinct PL/TL color differences in TbIII/EuIII mixed coordination polymers (Figure  1a, “2nd generation”).[9] Therefore, leveraging the intrinsic photophysical differences between PL and TL should lead to a theoretical breakthrough and the development of a new family of mechano-responsive luminescent materials.The aim of this study was to distinguish TL and PL by designing a LnIII complex with quenched PL but enhanced TL (Figure  1a, “3rd generation”), which is an emerging option to clarify the PL/TL difference simply by the emission intensity. Based on our hypothesized “f–f priority rule,” which predicts that TL is more likely to be dominated by direct f–f transi-tions than ligand-mediated processes,[10] we focused on three aspects: i) inefficient PSET, ii) low coordination geometry, and iii) high mechanical sensitivity (Figure  1b). To avoid effi-cient PSET, EuIII ions were combined with 2,2,6,6-tetrameth-ylheptane-3,5-dionate (tmh) ligands (Figure  1c). The ligand-to-metal charge transfer (LMCT) states of EuIII-tmh systems are known to quench the EuIII-centered emission, resulting in low PL quantum yields (≈1%). Moreover, the low-symmetry seven-coordinate structures created by bulky tmh ligands are ideal for elevating the f–f transition probability.[11] As Clegg et al. concluded based on their study of the TL of Ln(tmh)3(DMAP) (Ln: Tb or Sm, DMAP: 4-dimethylaminopyridine),[12] LnIII-Adv. Optical Mater. 2023, 11, 2203139Figure 1.  a) PL/TL properties of LnIII complexes. b) Dinuclear LnIII mechanophore design. c) Ln2(tmh)6(dpdf) (Ln = Eu or Tb) structures. 21951071, 2023, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202203139 by Cochrane Japan, Wiley Online Library on [04/05/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensewww.advancedsciencenews.com www.advopticalmat.de2203139  (3 of 7) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbHtmh systems are potentially mechano-responsive; this is prob-ably because of the face-to-face tert-butyl groups arrangement in the crystal packing system. A rigid and bent organic linker (2,7-bis(diphenylphosphoryl)-9-9-dimethylfluorene: dpdf)[13] was adopted to generate intermolecular steric repulsion. Based on a report of a dinuclear LnIII complex with a biphenylene bridging ligand containing a twisted C– bond between phenyl groups,[14] we deduced that a bent non-rotatable linker should reduce the degrees of freedom of the discrete dinuclear unit and promote the formation of a mechanically strained packing structure.1. Results and DiscussionThe dinuclear complex Eu2(tmh)6(dpdf) was prepared using the method shown in Scheme S1 (Supporting Information). Tb2(tmh)6(dpdf) which has a high PSET efficiency was also synthesized for comparison. Block crystals of both the EuIII and TbIII complexes were obtained, and single crystal X-ray analyses revealed an identical structure consisting of two LnIII ions, six tmh ligands, and one bridging ligand (Figure S1a, left, Supporting Information). The crystal structure belongs to the non-centrosymmetric space group Fdd2, which can generate a piezoelectric current on the cleavage upon fracturing. The dinu-clear surface is surrounded by the bulky tert-butyl groups. The π-aromatic system of the bridging ligand barely contributes to specific intermolecular contacts, such as edge-to-face C–H⋯π and face-to-face π⋯π interactions (Figure S1a, right, Supporting Information). Hirshfeld surface (HS) analysis[15]—which gen-erates quantitative crystal packing descriptions—indicated that 87% of the entire surface is dominated by H⋯H contacts (Figure S1b, Supporting Information). The high ρ indicator[16] (ρ  = %C⋯H/%C⋯C = 96), curvedness plots without flat sur-face patches, and featureless shape-index plots[15,17] also suggest Adv. Optical Mater. 2023, 11, 2203139Figure 2.  PL and TL spectra and corresponding energy-transfer diagrams of a) Eu2(tmh)6(dpdf) and b) Tb2(tmh)6(dpdf) (colored arrows: dominant excitation processes). The insets are photographs acquired under white light or UV illumination, or during fracturing. 21951071, 2023, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202203139 by Cochrane Japan, Wiley Online Library on [04/05/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensewww.advancedsciencenews.com www.advopticalmat.de2203139  (4 of 7) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbHthat face-to-face stacking does not occur (see Figure S1c, Sup-porting Information).Eu2(tmh)6(dpdf) and Tb2(tmh)6(dpdf) showed weak red (Φtotal  < 1%) and intense green (Φtotal  = 56%) PL upon UV irradiation owing to the quenching via LMCT states for EuIII ions and the enhancement via PSET for TbIII ions, respectively (Figure 2, solid lines). Despite the significant difference in PL quantum yields (>50-fold), bright TL, recognizable by the naked eye under room light, was observed for both compounds (Figure 2, dashed lines). The TL intensities of Eu2(tmh)6(dpdf) and Tb2(tmh)6(dpdf) were similar (1061 and 1820 photon counts, respectively). The TL spectral profiles are identical to the PL spectral profiles, which implies that TL and PL occur from the same excited states (5D0 and 5D4 for the EuIII and TbIII ions, respectively), without distortion of the coordination geom-etry around the LnIII ions. As the crystallinity and crystal grain size of the samples are not quantified, we did not estimate the “light yield” of the TL, which is typically measured using a drop-tower method with a reference sample.[18]The faint PL and intense TL of Eu2(tmh)6(dpdf) are coun-terintuitive to the conventional idea that efficient PSET plays a key role in bright LnIII TL. Thus, the TL of Eu2(tmh)6(dpdf) instead correlates with the bombardment of electrons[19] that are effectively generated across cracks in the crystal belonging to a non-centrosymmetric space group. This results in the direct population of the EuIII 5D0 excited state, followed by effi-cient 5D0→7FJ transitions (J  = 0–4, Φf-f  = 75%) owing to the low-symmetry seven-coordinate geometry. This mechanism is consistent with the aforementioned “f–f priority rule,” and PSET does not play an important role in the formation of the LnIII excited states (Figure 2, right). As TL requires the crystal deformation and the excited species are generated across growing cracks,[20] intermolecular face-to-face tert-butyl groups arrangement should enhance the mechano-responsiveness on the molecular scale by suppressing the formation of robust hydrogen-bonded networks. Mononuclear Ln(tmh)3(DMAP)[12] has a similar packing arrangement. Moreover, we reported the high TL activity of LnIII-hfa coordination polymers in which fluorine atoms face each other.[9]Nanoindentation tests were also performed to provide the mechanical fingerprint of the crystals (Figure 3a). Although the appearance of the crystals differed, smooth load–displace-ment (p–h) curves were obtained across the series without the “pop-ins” that usually appear as a result of rapid crack initiation and propagation.[21] Here, such behaviors were not identified, implying that the applied deformation energy was absorbed by the crystal before fracture. The reduced Young’s modulus (Er) and hardness (H) of the crystals were estimated using Adv. Optical Mater. 2023, 11, 2203139Figure 3.  a) Nanomechanical testing schematic. Optical micrographs of pristine crystals (top left), in situ surface probe microscopy images after indentations (top right, 20 × 20 µm), and load–displacement curves of b) Eu2(tmh)6(dpdf) and c) Tb2(tmh)6(dpdf). 21951071, 2023, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202203139 by Cochrane Japan, Wiley Online Library on [04/05/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensewww.advancedsciencenews.com www.advopticalmat.de2203139  (5 of 7) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbHthe Oliver-Pharr method[22] to be 5.8 ± 0.44  GPa and 0.28 ± 0.024 GPa, respectively, for Eu2(tmh)6(dpdf), and 5.3 ± 0.29 GPa and 0.27 ± 0.018  GPa, respectively, for Tb2(tmh)6(dpdf). Thus, the target crystals can be categorized as “ductile” materials, even in comparison with organic compounds,[23] which are gen-erally described as “soft” materials.Further grinding with a pestle and mortar reduced the TL intensity and crystallinity, and TL was not observed in the end (hereafter, the “ground form”). Note that fracturing was realized using a spatula, and the ground form with a pestle. Diffraction patterns obtained from synchrotron radiation wide-angle X-ray scattering measurements confirmed that the grinding process only lowers the crystallinity of the sample and does not induce a transition to another crystalline phase (Figure S4, Supporting Information). Interestingly, the PL properties of the ground and pristine forms were different (Figure S5, Supporting Information). In particular, the relative intensities of the UV–vis excitation bands (5D2  ← 7F0 at 464  nm) were inverted for Eu2(tmh)6(dpdf). As the PL intensity ratio for the magnetic (5D0→7F1 at 592  nm) and electric dipole transitions (5D0→7F2 at 612 nm) did not change, the non-radiative process, including the state transitions, must have been suppressed in the ground form. We suggest that this was because of deformation of the intermolecular packing structure, which can alter the ligand–metal spacing and corresponding LMCT band energy levels.[24] Indeed, a comparison between the diffuse reflectance spectra of Eu2(tmh)6(dpdf) and Gd2(tmh)6(dpdf) demonstrated that the characteristic low-lying band of Eu2(tmh)6(dpdf) disappeared after grinding (Figure S6, Supporting Information).Taking advantage of the sensitivity of the PL to mechan-ical grinding, the EuIII/TbIII heterodinuclear complex EuTb(tmh)6(dpdf) was also prepared as a TL- and MCL-active LnIII mechanophore. MCL has not often been reported for LnIII complexes because the dominant emission bands of homo-LnIII complexes are inert to the external environment (although their Stark-splittings may change). Moreover, the responses of mixed-LnIII complexes to temperature and pres-sure condition variation,[14,25] but not mechanical stimuli, have been examined. Grinding changed EuTb(tmh)6(dpdf) PL from yellow to orange-yellow (Figure 4a,b, dashed and solid blue lines, respectively) because the relative contribution of the EuIII-centered emission was lower for the pristine sample. The corresponding Commission Internationale de l’Eclairage (CIE) 1931 chromaticity coordinates were ≈(0.46, 0.51) and (0.48, 0.48) for pristine and ground forms, respectively. To the best of our knowledge, the only mechano-optically coupled LnIII PL is the sonication-induced mechanochemical transforma-tion of LnIII-supramolecules accompanied by a white-to-blue or yellow-to-red PL color change owing to competing organic fluorescence.[26] Nevertheless, there are no reports of MCL based on ratiometric LnIII emission (red (λem,Eu  = 612  nm)/green (λem,Tb = 549 nm)) realized simply by grinding. Similar to Eu2(tmh)6(dpdf) and Tb2(tmh)6(dpdf), bright yellow TL was also confirmed upon fracturing the EuTb(tmh)6(dpdf) crystals (Figure 4b, black line). As a slight change in TL color, assigned to dynamic axial-to-equatorial molecular conformation change, was reported for a phenothiazine derivative,[27] we conducted a continuous TL/PL measurements (Figure S7 and Table S2, Supporting Information). The emission color did not change over time for either TL (IEu/ITb = 1.00 ± 0.06 for 10 successive TL spectra) or PL (IEu/ITb = 1.48 (pristine) or 1.47 (fractured)). Alongside the ductile nanomechanical properties of the crys-tals, MCL was observed when the applied force exceeded a cer-tain limit and/or the particle size was sufficiently decreased to deactivate TL (Figure 4c). It is important to emphasize that the magnetic and electric dipole transition ratios (Ielectric/Imagnetic) of Eu2(tmh)6(dpdf) were identical for PL (pristine/ground) and TL. Therefore, the same excited states were populated and the local geometry around the EuIII ions was not deformed by fracturing or grinding. In contrast, the different EuIII-to-TbIII Adv. Optical Mater. 2023, 11, 2203139Figure 4.  a) Micrographs of EuTb(tmh)6(dpdf) before and after fracturing and grinding. b) PL excitation/emission spectra (λexc = 360 nm; λobs = 549/612 nm for excitation; dotted/solid lines: pristine/ground forms) and TL spectra. c) Schematic showing molecular packing and TL/PL proper-ties. The size of each symbol on the graph indicates the emission inten-sity; Imin = minimum intensity for detection. 21951071, 2023, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202203139 by Cochrane Japan, Wiley Online Library on [04/05/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensewww.advancedsciencenews.com www.advopticalmat.de2203139  (6 of 7) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbHintensity ratios (IEu/ITb) for EuTb(tmh)6(dpdf) indicate dis-tinct TL and PL excitation probabilities and energy-transfer efficiencies.3. ConclusionWe presented a new molecular strategy for LnIII-based mechan-ophores. By introducing bulky β-diketonate and rigid fluorene-based bridging ligands, we produced bright TL even though the ligand-excited PL quantum yield was negligible (< 1%). This suggested the existence of direct mechanically stimulated LnIII excitation mechanisms in TL, while photo-irradiation of the ligands is essential to efficiently sensitize LnIII ions in PL. Fur-thermore, nanomechanical testing indicated that the ductility that allows crystals to restore the deformation energy could be critical for TL activity from a mechanical point of view. More-over, a EuIII/TbIII heterodinuclear complex showed not only PL/TL color difference but also MCL, which implies that LnIII coordination materials with two or more metal ions—such as dimers, clusters, and polymers—also potentially exhibit MCL. As typical MCL materials are organic or d-block transition metal complexes with broad emission bands, sharp f-centered emissions expand the application scope of MCL-active com-pounds and should promote solid-state photophysics research. The presented strategy for LnIII mechanophores will advance the development of next-generation mechano-responsive lumi-nescent materials, and further nanomechanical studies will provide deeper insights into the relationships between the structure and mechanical and photophysical properties of such materials.Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.AcknowledgementsThis work was supported by Japan Society for the Promotion of Science (JSPS) Overseas Research Fellowship, Grant-in-Aid for Grant Numbers 22K14661, 17H04873, 21H02031, 21K18969, 20H02748 and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No. 715757 MECHANO-FLUO). Thanks are given to Dr. Kei Yanagisawa for the help in sample preparation. Y.H. gratefully acknowledges Dr. Mitsuhiro Hirai for SR-WAXS measurements and Dr. Shiro Funahashi for single crystal X-ray analyses.Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available in the supplementary material of this article.Keywordseuropium, nanoindentation, photoluminescence, terbium, triboluminescenceReceived: January 1, 2023Revised: January 19, 2023Published online: February 8, 2023[1]  T. Matsuda, R. Kawakami, R. Namba, T. Nakajima, J. P. Gong, Sci-ence 2019, 363, 504.[2]  a) K. Kubota, T. Seo, K. Koide, Y. Hasegawa, H. Ito, Nat. Commun. 2019, 10, 111; b) K. Kubota, Y. Pang, A. Miura, H. Ito, Science 2019, 366, 1500.[3]  a) Y. Sagara, T. Kato, Nat. Chem. 2009, 1, 605; b) Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu, J. Xu, Chem. Soc. Rev. 2012, 41, 3878; c) Y. Sagara, S. Yamane, M. Mitani, C. Weder, T. Kato, Adv. Mater. 2016, 28, 1073.[4]  a) D. 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