Mechanistic insight into the Young′s modulus threshold for fracto-mechanoluminescence in manganese halides

Domain: physics
Published: 2026-01-08
Editorial outcome: major_revision

Abstract
Fracto-mechanoluminescence (FML), a subtype of mechanoluminescence, is the phenomenon of light emission triggered by the fracturing of solids under mechanical stimuli. Although many materials have been reported with FML, the underlying mechanism remains unclear, leveraging the fundamental prerequisites and design principles to achieve FML remain elusive. In this study, we systematically investigate a series of Mn halides and find that 12 out of 18 compounds exhibit bright and clearly detectable FML. Here we show that the occurrence of FML arises from the synergistic interplay among the crystal's elastic stiffness, local electromechanical coupling, and trap states, which collectively activate Mn2+ d−d transitions upon fracture. The enhancement of FML intensity is primarily governed by the enlargement of the effective fracture area, whereas the Young's modulus determines the fracture threshold and the tolerable stress range of crystals. Additionally, as-explored Mn halides exhibit improved X-ray imaging capabilities, which are further integrated into radiation warning and damage detection devices.

Introduction
For mechanoluminescence (ML), materials can be classified into three categories based on their response to mechanical stimuli: deformation luminescence, fracto-mechanoluminescence (FML) and triboluminescence1–3. Deformation luminescence can be further subdivided into two types: elastic-mechanoluminescence (EML) and plastic-mechanoluminescence (PML)4–8. EML occurs when the material undergoes reversible elastic deformation, during which the luminescence intensity typically exhibits a linear relationship with the applied stress, demonstrating good stress sensitivity7,9,10. As the stress increases beyond the elastic limit, the material enters an irreversible plastic deformation stage, involving local structural reconstruction and volume expansion, which gives rise to PML11,12. When the applied load continues to increase and causes the material to fracture, FML is triggered (Fig. 1a)13. This phenomenon is usually accompanied by the breaking of chemical bonds and localized plasma discharge14,15. It is an irreversible and non-repeatable process, typically resulting in permanent damage to the material.

The earliest record of FML dates back to 1605, when Francis Bacon observed that sugar crystals would emit light when they are fractured16,17. Since then, studies have shown that ≈50% of inorganic solids and 30% of organic crystals exhibit FML during the fracture process18–21. Representative materials include Mn(Ph3PO)2Br2, EuD4TEA, and Cu(NCS)(py)2(PPh3)2 (Fig. 1b)22–26. Although FML, as a sensitive indicator of stress concentration and crack propagation, has shown important application prospects in structural health monitoring and safety warning systems, there are still significant challenges in the study of its luminescence mechanism27,28. On one hand, the multiscaled dynamic structural evolution occurring during fracture is difficult to capture in real time, making mechanistic investigations heavily reliant on indirect experimental techniques and theoretical modeling29. On the other hand, the transient and non-repeatable nature of FML limits its applicability in controllable functional devices30. Recently, the hybrid metal halides have been reported to exhibit FML properties13–15. The former studies proposed that the hydrogen bonds are broken under the mechanical stimulation in H2(RR/SS-BDPP)MnBr4 crystals13. Then opposite charges were generated at the crystal fractures, which leads to electron bombardment and subsequent energy transfer to the inorganic luminescent center and induces the FML14,31. Studies on (C19H18P)2MnBr4 suggest a piezoelectric phase exhibits ideal EML32. Its emission mechanism is that piezoelectric polarization under mechanical stimulation causes charge carrier separation, and the separated electrons and holes are captured by traps, respectively32. Due to the influence of the internal piezoelectric potential, the band structure tilts and the trap depth becomes shallow, thereby releasing and recombining charge carriers32. The energy is then transferred to the luminescent center Mn2+ ions. When Mn2+ returns from the excited state 4T1 to the ground state 6A1, a radiative transition occurs, generating EML32. However, the specific effects of mechanical stress on electronic excitation and the role of hydrogen bond breaking in this process are still unknown33,34. In addition, the relationship between crystal symmetry and FML performance, the mechanism of electron excitation and energy transfer in the FML process, seem to be ignored.

In this work, we synthesized a series of zero-dimensional Mn halides composed of triphenylphosphine cations and [MnX4]2− (X=Cl, Br) tetrahedral units, among which 12 compounds exhibit pronounced FML characteristics. The results indicate that Young's modulus is associated with local piezoelectric response at the microscopic level and also affects the fracture threshold and stress range of the crystal. Trap states act as intermediate states that facilitate energy storage and release during the compression-induced fracture process, while the FML intensity is correlated with the effective fracture area. The crystals maintain enhanced FML performance even after undergoing a crushing-recovery-recrystallization cycle, offering a viable strategy for sustainable stress-sensing applications. Moreover, the Mn halides developed in this work also demonstrated enhanced performance in X-ray imaging, with a light yield of 64,500 photons MeV−1 and the detection limit of 96.20 nGy s−1, both surpassing those of currently available commercial scintillators. Our work elucidates the essential physical conditions necessary for FML generation in Mn halides and highlights their potential as multimodal phosphors for applications such as radiation warning and damage detection.

Results
Synthesis and structural characterization
The (ClMTP)2MnCl4, (HMTP)2MnCl4·H2O, (BTP)2MnCl4, (BTP)2MnCl4·H2O, (BTP)2MnBr4, (2ClBTP)2MnCl4, (4ClBTP)2MnCl4, (4MBTP)2MnCl4, (EOTP)2MnCl4 #1, (EOTP)2MnCl4 #2, (EOTP)2MnBr4 #1 and (EOTP)2MnBr4 #2 single crystals were prepared through the solvent evaporation methods at room temperature using triphenylphosphine cations with different functional groups. A simple schematic diagram of the single crystal synthesis process of Mn halides is shown in Supplementary Fig. 1. The structures of the organic cations used in this work are shown in Fig. 1c, d. The phenyl group is connected to the phosphine cation through a σ bond to form a stable structure, and its stability mainly comes from the steric hindrance effect and the strong C−C bond. We aim to construct multiple alternative phases and investigate the structure-properties relationship. Hereby, triphenylphosphine cations with different functional groups have been adopted35.

Their crystal structures were well refined based on the single crystal X-ray diffraction (SCXRD). The powder X-ray diffraction (PXRD) patterns of the as-prepared Mn halides are well indexed into the simulated ones (Supplementary Fig. 2). (BTP)2MnCl4 and (2ClBTP)2MnCl4 crystallizes in an orthorhombic system, while the others in monoclinic structures. For detailed crystallographic data, see Supplementary Tables 1-48. As shown in Supplementary Figs. 3a–14a, one Mn atom coordinates with four halogen atoms to form a [MnX4]2− (X=Cl, Br) tetrahedron. The tetrahedra of [MnX4]2− are separated from each other by organic cations, forming zero-dimensional structures. Since structural distortion affects the optical properties of materials, such as band gaps and luminescence behaviors, we calculated the polyhedral distortion index (D) of the [MnX4]2− tetrahedrons36. The calculation results show that the [MnX4]2− tetrahedrons in the structure are close to the ideal one (Supplementary Table 49)31,37. A small D usually indicates that the geometric structure of the material is relatively stable, and may have superior optoelectronic properties38. In all structures, the [MnX4]2− tetrahedrons are isolated from each other by the surroundings of organic cations, assuring their connection through weak hydrogen bonds. The hydrogen bonds between [MnX4]2− and organic cations are shown in Supplementary Figs. 3b–14b. There are free H2O molecules in the structures of (HMTP)2MnCl4·H2O and (BTP)2MnCl4·H2O. The organic cations, H2O molecules, and [MnCl4]2− tetrahedrons have O−H···Cl, C−H···O, and C−H···Cl hydrogen bonds in (BTP)2MnCl4·H2O and (HMTP)2MnCl4·H2O.

Compared with the freshly prepared samples, the PXRD patterns of the samples stored in air for several months are basically consistent, indicating that our synthesized samples exhibit high stability in air (Supplementary Fig. 2). The chemical valences of the synthesized Mn halides were further confirmed by X-ray photoelectron spectroscopy (XPS) analysis (Supplementary Figs. 15–26). The high-resolution X-ray photoelectron spectroscopy (HRXPS) core spectra of Mn 2p orbitals are shown in Supplementary Figs. 15b–26b. The core energy spectra of Mn were distinguished by the existence of two obvious peaks at about 641 and 653 eV, which correspond to Mn 2p3/2 and Mn 2p1/2 states.

Mechanical analysis of FML in Mn halide crystals
Interestingly, FML was observed when single crystals of (2ClBTP)2MnCl4, (4ClBTP)2MnCl4, (4MBTP)2MnCl4, (EOTP)2MnCl4 #1, (EOTP)2MnCl4 #2, (EOTP)2MnBr4 #1 and (EOTP)2MnBr4 #2 were pestled both in air and argon, which ruled out the possibility of FML through high-energy excitation of gas molecules in air (Fig. 2a, b, and Supplementary Movie 1)39. At the same time, we have also synthesized some reported Mn halides ((MTP)2MnCl4, (MTP)2MnBr4, (ETP)2MnCl4, (ETP)2MnBr4, (2MBTP)2MnCl4, and (HTPP)2MnBr4) constructed by triphenylphosphine cations, which also have FML properties (Supplementary Fig. 27)40–45. In these crystal structures, despite the presence of enough hydrogen bonds between organic cations and [MnX4]2− tetrahedrons, no FML phenomenon was observed when the (ClMTP)2MnCl4, (HMTP)2MnCl4·H2O, (BTP)2MnCl4, (BTP)2MnCl4·H2O, and (BTP)2MnBr4 single crystals were crushed. As shown in Fig. 2c and Supplementary Movie 2, we used an ultrahigh-speed camera to observe the FML behavior of Mn halide single crystals under stress in real time. Our results show that FML occurs at the exact moment of irreversible fracture of the crystals, and its intensity is positively correlated with the effective crack surface area. (EOTP)2MnBr4 #1 crystals @ PDMS (polydimethylsiloxane) film were prepared by screening single crystals of moderate particle size and embedding them in a PDMS matrix. As shown in Fig. 2d, when pressure is applied using a glass rod, a distinct green light emission is observed in the flexible film. Additionally, when writing the number “1” on the film with the glass rod, the written characters become clearly visible due to the FML (Supplementary Movie 3).

Different from the other metal halides that mainly emit light through self-trapped excitons, the luminescence of Mn halides usually originates from spin-forbidden transitions arising from d−d electron transitions46,47. The detailed energy absorption, nonradiative relaxation, and emission processes of Mn2+ in a tetrahedral or octahedral environment are shown in Supplementary Fig. 28. To gain a deeper understanding of the luminescence mechanism, the excited states of (EOTP)2MnBr4 #1 were analyzed based on the crystal field theory according to the Tanabe-Sugano diagram (Fig. 2e and Supplementary Table 50). Theoretically, the width of the excited state is primarily governed by the strength of the crystal field (Dq) and the Racah parameter (B) in the Mn2+ environment. Subsequently, the Dq (872 cm−1) and B (685 cm−1) parameters were calculated from the assigned excitation transitions48. The calculated Dq/B is 1.27, indicating that there is a weak crystal field around Mn2+. As a result, the energy level splitting of the d orbitals is not significant, and electrons can achieve transitions between different d orbitals without a large amount of energy input. Therefore, Mn halides are more likely to produce higher energy green FML phenomenon.

To further elucidate the mechanism underlying FML generation, atomic force microscopy (AFM) was employed to determine the elastic stiffness of the Mn halide crystals (Fig. 2f). The results show that the Young's modulus of Mn halides with explicit FML characteristics is generally lower than 10 GPa; while for crystals with a Young's modulus higher than 10 GPa, no obvious FML phenomenon was observed (Supplementary Figs. 29–46, Table 1). Meanwhile, we systematically studied the local piezoelectric effect of Mn halide single crystals using piezoelectric force microscopy (PFM). As shown in Supplementary Figs. 48 and 49, the PFM amplitude signals of (BTP)2MnCl4 (centrosymmetric) and (BTP)2MnBr4 (non-centrosymmetric), which do not exhibit FML characteristics, show only slight changes under different bias voltages30. In contrast, as shown in Supplementary Figs. 50–55, the centrosymmetric structures of (2ClBTP)2MnCl4, (4ClBTP)2MnCl4, (EOTP)2MnBr4 #1 and the non-centrosymmetric structures of (MTP)2MnCl4, (ETP)2MnCl4, (ETP)2MnBr4 all exhibit clearly enhanced PFM amplitude signals under different bias voltages49. During the voltage sweep, the amplitude-voltage curves of these crystals showed characteristics similar to butterfly loops, indicating that they all have considerable local electromechanical coupling effects50,51. The piezoelectric constants d33 of (2ClBTP)2MnCl4, (ETP)2MnCl4, (ETP)2MnBr4 and (4ClBTP)2MnCl4 single crystals are 1.0, 1.1, 1.3, and 1.4 pC N−1 (Supplementary Table 51). This trend underscores the synergistic interplay between lattice softness and local electromechanical coupling. Piezoelectric polarization reflects the material's ability to generate an electric polarization under mechanical stress, with the driving force originating from the applied mechanical load. A lower Young's modulus, corresponding to a softer lattice, facilitates ionic displacement, thereby enhancing piezoelectric polarization.

Furthermore, as shown in Supplementary Fig. 56, the thermoluminescence (TL) spectra obtained under mechanical grinding excitation reveal the existence of trap states in (2ClBTP)2MnCl4, (4ClBTP)2MnCl4, (EOTP)2MnBr4 #1, (MTP)2MnCl4, (ETP)2MnCl4, and (ETP)2MnBr4. The crystals were crushed at 77 K, which shows that the piezoelectric polarization induced by stress stimuli can excite electrons into the trap states. These trap states act as energy storage reservoirs, releasing stored electrons to emission centers during fracture. In contrast, (BTP)2MnCl4, which possesses a high Young's modulus (184.9 GPa), does not exhibit FML behavior, implying that excessive lattice rigidity suppresses the mechanical activation of trap states. In summary, the activation of FML requires not only sufficient mechanical flexibility but also the presence of trap states, which enable efficient trap activation under mechanical stimuli, thereby driving the luminescence process (Fig. 2g).

Optical analysis of FML in Mn halide crystals
To elucidate the FML behavior of Mn halide single crystals during the fracture, we employed a universal measuring machine to monitor their luminescence under applied stress (Fig. 3a and Supplementary Fig. 57). As shown in Fig. 3b and Supplementary Fig. 58, during a compression cycle, the crystals undergo multiple fractures events, each accompanied by a sharp emission pulse that corresponds to the applied stress curve. Typically, the first strong emission is the most representative; weaker signals that precede it may arise from crack propagation or minor fractures, while subsequent emissions originate from the re-fracture of already broken fragments. All the following analyses are therefore based on the first emission.

To quantify the effect of fracture area on FML intensity, we counted the number of fractured crystals and normalized the initial crystal size. This allowed us to evaluate the relationships among FML intensity, fracture area, applied stress, and the crystal's elastic stiffness. In Fig. 3c, the FML intensity of (2ClBTP)2MnCl4 single crystals increases progressively with crystal size in the 1−6 mm range. Since fracture area scales with initial size for crystals of similar shape and comparable numbers of fractures, this result indicates that the effective fracture area is the direct factor enhancing FML intensity. However, when the crystal size increased to 8−10 mm, this positive correlation no longer holds. This can be attributed to the fact that larger crystals are more prone to forming twins and interfacial defects, where fractures tend to occur preferentially. As a result, they absorb less energy during compression, leading to reduced FML intensity. These findings clearly demonstrate that enlargement of the effective fracture area is the primary factor governing FML enhancement. As shown in Fig. 3d, a correlation between the number of fractured crystals and FML intensity is observed for similarly sized (EOTP)2MnBr4 #1 crystals. Moreover, as the applied stress increases from 10 to 30 N, the FML spectra intensity of (EOTP)2MnBr4 #1 increases correspondingly as the crystals gradually fracture (Fig. 3e). This highlights the high sensitivity of FML to external mechanical stimuli.

The role of Young's modulus in FML requires further clarification. In Fig. 3f, as the Young's modulus of (2ClBTP)2MnCl4 (9.30 GPa), (ETP)2MnCl4 (4.86 GPa), (ETP)2MnBr4 (2.97 GPa), (EOTP)2MnBr4 #1 (2.72 GPa), (MTP)2MnCl4 (1.10 GPa) and (4ClBTP)2MnCl4 (0.178 GPa) decreases, their FML intensity does not show a significant regular change, although low Young's modulus facilitates the generation of local piezoelectric fields (Fig. 3g). This suggests that FML behavior is governed by multiple factors. Considering the breaking stress of the crystals and the first occurrence of FML, higher FML intensity generally corresponds to larger fracture values (Fig. 3c). Those high-quality crystals with fewer defects require greater force to break and emit bright FML, while poor-quality crystals with more defects fracture at lower forces and display weaker FML. In addition, we found that Young's modulus primarily influences the tolerable stress range of the crystals, corresponding to the “slope” of the fracture force-FML intensity relationship. This highlights its synergistic role in modulating, rather than determining, the FML behavior. Overall, Young's modulus exerts a multifaceted yet cooperative influence on FML by regulating the initiation conditions and effective stress window, thereby reflecting its integrated and multilevel contribution to the overall mechanism.

Based on the combined PFM and TL results, we propose a refined mechanism (Fig. 3h). In Mn halide single crystals with relatively low Young's modulus, applied stress induces local piezoelectric polarization that facilitates carrier trapping and the filling of trap states. Upon fracture, the carriers stored in the traps are released and recombine, and the released energy subsequently excites Mn2+ ions through radiative or nonradiative channels, resulting in the characteristic Mn2+ d−d transition (4T1 → 6A1) and producing pronounced FML. Obviously, the FML intensity is affected by electron-photon processes, such as carrier migration and recombination efficiency, which result in intensity not directly related to Young's modulus. A lower Young's modulus allows the crystal to store more energy from the piezoelectric field, thereby enhancing carrier mobility and facilitating radiative recombination at luminescent centers. This cooperative interaction among elastic stiffness, local electromechanical coupling, and trap dynamics collectively drives the FML process, underscoring that Young's modulus serves a modulatory and synergistic, rather than a purely deterministic, role in the underlying mechanism52,53.

As shown in Supplementary Fig. 59, after six cycles under 60 N stress, the FML intensity decreases significantly as the crystal size diminishes (Supplementary Fig. 60). When stress was gradually increased from 0 to 60 N, the FML intensity decayed more slowly over the six cycles (Supplementary Fig. 61). This is due to the gradual miniaturization or pulverization of the crystals (Supplementary Fig. 62). However, optical sensitivity declined with progressive fragmentation. Interestingly, the pulverized material dissolved well in methanol and recrystallized into high-quality single crystals upon solvent evaporation. The recrystallized crystals retained phase purity and FML responsiveness (Supplementary Figs. 63 and 64). This demonstrates not only the reproducibility of the material but also provides a practical strategy for sustainable sensing applications. Through a crushing-recovery-recrystallization cycle, the crystals can be reused, offering favorable prospects for long-term structural health monitoring and stress-sensing.

Optoelectronic properties of Mn halides
To clarify the basic photoluminescence (PL) properties of as-synthesized Mn halides, we measured their ultraviolet-visible (UV-Vis) absorption spectra, photoluminescence excitation (PLE) and PL spectra. As shown in the UV-Vis absorption spectra, the absorption peak is attributed to the electronic transition of Mn2+. According to the Tauc plot of the UV-Vis absorption spectra, their optical band gaps were calculated to be 2.37−3.07 eV (Supplementary Fig. 65). Figure 4a shows the photos of the Mn halide single crystals under natural and UV lights. Under the excitation of UV light, green light emission can be observed in the synthesized Mn halides (Supplementary Figs. 66 and 67)54,55. The photoluminescence quantum yield (PLQY) of as-synthesized Mn halides ranges from 5.51% to 99.52% (Supplementary Fig. 68). Table 1 summarizes the PL properties of various Mn halides, highlighting differences in excitation wavelength and PLQY that are influenced by their composition and structures. These variations provide insights into how crystal field strength, coordination environment, and symmetry affect the optoelectronic properties of Mn2+ centers.

To investigate the emission mechanism and optoelectronic properties, the PL spectra of the as-synthesized Mn halides were measured from 80 to 360 K. As shown in Fig. 4b, the emission band exhibits a blue shift with increasing temperature due to lattice thermal expansion, which reduces the crystal field splitting energy of Mn2+ ions, shifting the 4T1(G) state to higher energy (Supplementary Figs. 69 and 70). The Huang-Rhys factor (S) and phonon energy (ħω) of (EOTP)2MnBr4 #1 are fitted to be 5.34 and 28.54 meV, indicating the strong electron-phonon coupling in (EOTP)2MnBr4 #1 (Fig. 4c). The temperature-dependent PL decay lifetime of (EOTP)2MnBr4 #1 were 0.33, 0.32, and 0.31 ms at 80, 180, and 280 K, respectively, which were fitted by a single exponential decay function (Fig. 4d). Different from the other metal halides, the fluorescence decay time of synthesized Mn halides range from 0.34 to 3.40 ms, with the long PL decay lifetime attributed to the spin-forbidden nature of d−d transitions rather than the self-trapped emissions (Supplementary Fig. 71)56,57. (HMTP)2MnCl4·H2O (0.34 ms) and (BTP)2MnCl4·H2O (0.36 ms), containing free water molecules, exhibit shorter PL decay lifetimes. This is mainly due to the high vibration frequency of the −OH in the water molecule (3000−3600 cm−1), which readily undergoes multi-phonon coupling with the energy of the excited state. Such high-energy vibrational modes significantly enhance the nonradiative relaxation process, thereby shortening the PL decay lifetime. The PL decay times of (BTP)2MnBr4, (EOTP)2MnBr4 #1, and (EOTP)2MnBr4 #2 are 0.38, 0.49, and 0.43 ms, respectively. In comparison, the PL decay lifetimes of the other manganese chloride samples all exceed 1 ms. This is because Cl− has a higher electronegativity and a smaller radius than Br−, resulting in a shorter Mn−Cl bond than the Mn−Br bond. The stronger crystal field of [MnCl4]− leads to a larger d-level splitting, reducing the probability of radiative transitions from the excited state and the nonradiative relaxation rate. Our research demonstrates that FML relies on the coupled process of carrier capture and release in traps. Therefore, Mn halides with longer PL decay lifetimes generally correspond to more efficient FML responses.

The distances between [MnX4]2− tetrahedrons of our samples are shown in Supplementary Fig. 7235. However, despite large distances of 10.37 Å and 11.39 Å in (HMTP)2MnCl4·H2O and (BTP)2MnCl4·H2O, their PLQY are only 5.51% and 21.27%, indicating that the distance between the luminescent centers is not the sole factor affecting luminescence efficiency. On the other hand, the Mn-based compounds are usually magnetic materials. When the distances between Mn2+ are short (not exceeding 5 Å), ferromagnetism, antiferromagnetism, and paramagnetism may appear in Mn halides (Fig. 4e)58,59. We further studied the magnetic properties of the Mn halides, and the Curie-Weiss temperatures θ and Curie constants C reflected the absence of Mn2+ coupling (Fig. 4f, g and Supplementary Table 52)58,59. This feature indicates that the magnetism between Mn2+ does not contribute to the high PLQY of our samples36. The hydrogen bonds between the organic cations and the [MnX4]2− luminescent centers also play a pivotal role in the optoelectronic properties of Mn halides38. The strong electronegativity of Cl atoms (hydrogen bond acceptors) can effectively attract the electron density on the C−H bonds (hydrogen bond donors) and enhance the transition of electrons from organic cations to [MnX4]2− luminescence centers (Supplementary Figs. 73 and 74). Therefore, we believe that the distance between [MnX4]2− tetrahedra is not the only factor affecting the luminescence efficiency. The hydrogen bonding effect and the electron transfer mechanism it induces also play an important role in determining the optical properties of the Mn halides.

To reveal the excited state evolution of Mn halides after excitation, the internal conversion time of (EOTP)2MnBr4 #1 was studied by femtosecond transient absorption spectroscopy (fs-TAS). As shown in Fig. 4h, the fs-TAS spectra were recorded with a scanning delay time ranging from 0.3 to 100 ps, and a detection range from 300 to 700 nm. The spectral characteristics include three different types of signals: photo-induced bleaching (PB), stimulated emission (SE), and photo-induced absorption (PIA)60,61. Among them, the PIA feature appears as a strong positive signal in the range of 330−610 nm, while an obvious negative signal is observed near 610−650 nm, which is attributed to PB or SE. As shown in Fig. 4i, the PIA signal is established rapidly in a very short time, and the signal intensity begins to increase at 0.8 ps, indicating that the formation of carriers or excitons has a fast kinetic process. Subsequently, from 1.1 to 1.9 ps, the PIA signal gradually stabilizes, implying that the excitons begin to relax or transfer to a different metastable state. At 100 ps, the PIA signal still exists, suggesting that the excited state has a long PL decay lifetime, which may be related to the d−d transition or trap state of Mn2+. These transient features reveal the complex and multi-step excited state dynamics of (EOTP)2MnBr4 #1 on an ultrafast timescale.

To further study the structure-properties relationship of Mn halides, the partial density of states (pDOS), VBM, and CBM associated partial charge densities of (BTP)2MnCl4 were calculated by the Perdew-Burke-Ernzerhof (PBE) method62. The density functional theory (DFT)-D3 method was added as the dispersion correction63,64. The pDOS for (BTP)2MnCl4 is shown in Fig. 4j. The VBMs of (BTP)2MnCl4 are contributed by the Cl 3p orbital and the Mn 3d orbital, while the CBMs are mainly contributed by the orbitals from the organic cations, as seen in the partial charge densities in Fig. 4k. Due to the inherent electronic structure of zero-dimensional metal halides, their electronic bands are usually flat, which makes it possible to localize and relax strong electrons in the structure, resulting in efficient luminescence.

Scintillation properties of Mn halides
Mn halides have also been gradually explored for use in X-ray imaging. Their crystal structures and energy band characteristics enable them to effectively absorb high-energy radiation during X-ray detection and convert it into visible light signals, achieving improved imaging capabilities65. Fig. 5a shows the Mn halide crystals under natural light and X-ray irradiation. Also, under UV and X-ray excitation, they show the same green emission (Supplementary Fig. 75). Light yield, a key parameter reflecting a scintillator's ability to convert high-energy radiation into visible light, was compared between (EOTP)2MnBr4 #1 and commercial scintillator Ce:LuAG. (EOTP)2MnBr4 #1 exhibits a light yield of 64,500 photon MeV−1, 2.58 times higher than Ce:LuAG (25,000 photon MeV−1) and surpassing other traditional scintillators (Fig. 5b, c).

To further evaluate the X-ray irradiation resistance of the synthesized Mn halides, we measured the radioluminescence (RL) spectra of samples under long-term and high-dose X-ray irradiation. At a dose rate of 26.5 Gy min−1 for 1800 s (total dose about 795 Gy), (EOTP)2MnBr4 #1 showed only a 3.43% decrease in luminescence intensity, demonstrating high X-ray tolerance (Fig. 5d). The resistance measurements on (HMTP)2MnCl4·H2O, (BTP)2MnCl4, (BTP)2MnBr4, and (EOTP)2MnCl4 #1, also exhibit high stability, confirming their strong anti-irradiation feature (Supplementary Figs. 76–79). The RL spectra measured at varying X-ray dose rates show a linear relationship, with a detection limit of 96.20 nGy s−1, 57 times lower than the standard for medical X-ray diagnosis (5.5 μGy s−1) (Fig. 5e, Supplementary Fig. 80). We list the detection limits of some organic-inorganic hybrid Mn halides reported in recent years in Fig. 5f and Supplementary Table 53. The luminescence of Mn2+ has a long carrier recombination time, which leads to the high light yield under X-ray excitation. (EOTP)2MnBr4 #1 exhibits different luminescence dynamics under different excitation conditions. Under UV light excitation, its PL decay lifetime is 0.49 ms, which conforms to the typical first-order exponential decay model. Under high-energy X-ray excitation, its RL decay lifetime is extended to 1.35 ms, indicating that the radiative recombination path is more complicated and involves multiple electronic transition processes, so a double exponential function is required for fitting (τ1 = 0.53 ms, τ2 = 1.71 ms) (Fig. 5g). As shown in Fig. 5h, the TL spectrum of (EOTP)2MnBr4 #1 under X-ray excitation exhibit two distinct emission peaks at 155 and 219 K, which can be attributed to trap states with different depths. Similar TL features were also observed in other synthesized Mn halides under X-ray excitation, confirming the ubiquitous presence of trap states in these materials (Supplementary Fig. 81)13. These trap states release trapped electrons during heating and then undergo radiative recombination, which affects the luminescence process under X-ray excitation. This result is consistent with the double exponential characteristics of the RL decay lifetime, indicating that some electrons can recombine quickly (corresponding to τ1), while the other part undergoes delayed recombination after being released through the trap state (corresponding to τ2).

Finally, to evaluate its potential application in low-dose rate and high-resolution X-ray imaging. We mixed (EOTP)2MnBr4 #1 powders with PDMS to assemble a flexible (EOTP)2MnBr4 #1 powders @ PDMS film. The modulation transfer function (MTF) revealed a spatial resolution of 10.28 line pairs per millimeter (LP mm−1, one line pair consists of one black line and its adjacent white line) at an MTF value of 0.2 (Fig. 5i). X-ray images of a standard resolution plate confirmed its high spatial resolution (Fig. 5j). We used the as-assembled films to shoot photos under a self-made imaging system (Fig. 5k). As shown in Fig. 5l, (EOTP)2MnBr4 #1 powders @ PDMS film showed clear imaging ability at low X-ray dose rates. These results confirm that our explored Mn halides not only exhibit FML phenomena but also have great potential in the field of radiography.

Applications of multimodal response Mn halides
Based on the high PLQY, visible FML response and high X-ray radiative light yield of (EOTP)2MnBr4 #1, we further applied it to an intelligent response device that integrates radiation warning and damage detection (Fig. 6a, b). As shown in Fig. 6c, d, based on the sensitive optical response characteristics of (EOTP)2MnBr4 #1 to multiple excitation methods (UV light, X-rays and mechanical stimulation), we constructed a multimodal intelligent response system that integrates optoelectronic sensing modules. When (EOTP)2MnBr4 #1 produces obvious PL or RL under UV or X-ray irradiation, the photoresistor can capture the corresponding luminescence signal and convert it into an electrical signal, thereby triggering the light-emitting diode (LED) to light up. The on-off state of the LED can be used to intuitively judge the irradiation state in the environment. In addition, when (EOTP)2MnBr4 #1 is stimulated by mechanical force, a transient FML is generated. This transient optical signal can also be quickly responded to by the photoresistor and drive the LED to light up. Due to the discontinuity and short-term nature of FML, the generated signal presents a flickering feature, which can effectively distinguish mechanical damage events (Supplementary Movies 4 and 5). This response mechanism provides an alternative visualization path for damage detection. In summary, Mn halides show considerable advantages in multimodal optical response, and their potential applications in intelligent optoelectronic devices, such as radiation warning and damage detection, provide alternative ideas for the design of multifunctional materials and practical engineering applications.

Methods
Materials
The following chemicals were used as purchased without further purification: manganese chloride (MnCl2, 99%, Energy Chemical), manganese bromide (MnBr2, 98%, Adamas), methanol (MeOH, 99.5%, Macklin), ethyl alcohol (EtOH, 95%, Macklin), hydrochloric acid (HCl, 36.0 wt.% in H2O, Aladdin), hydrobromic acid (HBr, 40.0 wt.% in H2O, Macklin), hypophosphorous acid (H3PO2, 50 wt.% in H2O, Macklin), N,N-dimethylformamide (DMF, 99.90%, Rhawn), (chloromethyl)triphenylphosphonium chloride (C19H17ClP, ClMTP-Cl, 95%, Leyan), (hydroxymethyl)triphenylphosphonium chloride (C19H18ClOP, HMTP-Cl, 95%, Leyan), butyltriphenylphosphonium chloride (C22H24ClP, BTP-Cl, 98%, Leyan), butyltriphenylphosphonium bromide (C22H24BrP, BTP-Br, 98%, Leyan), (2-chlorobenzyl)triphenylphosphonium chloride (C25H21Cl2P, 2ClBTP-Cl, 98%, Leyan), (4-chlorobenzyl)triphenylphosphonium chloride (C25H21Cl2P, 4ClBTP-Cl, 98%, Leyan), (4-methylbenzyl)triphenylphosphonium chloride (C26H24ClP, 4MBTP-Cl, 95%, Leyan), (2-ethoxy-2-oxoethyl)triphenylphosphonium chloride (C22H22ClO2P, EOTP-Cl, 95%, Leyan), (2-ethoxy-2-oxoethyl)triphenylphosphonium bromide (C22H22BrO2P, EOTP-Br, 98%, Macklin), methyltriphenylphosphonium chloride (C19H18ClP, MTP-Cl, 95%, Leyan), methyltriphenylphosphonium bromide (C19H18BrP, MTP-Br, 95%, Leyan), ethyltriphenylphosphonium chloride (C20H20ClP, ETP-Cl, 95%, Leyan), ethyltriphenylphosphonium bromide (C20H20BrP, ETP-Br, 95%, Leyan), (2-methylbenzyl)triphenylphosphonium chloride (C26H24ClP, 2MBTP-Cl, 95%, Leyan), hexyltriphenylphosphonium bromide (C24H28BrP, HTPP-Br, 95%, Leyan), and polydimethylsiloxane (PDMS, Sylgard 182).

Syntheses of Mn halide single crystals
Synthesis of (ClMTP)2MnCl4 single crystals. 1.0 mmol MnCl2 was dissolved in 8 mL MeOH and stirred at room temperature until the solution was clarified. 1.0 mmol ClMTP-Cl was added and stirred for 30 min in a magnetic stirrer. The solution slowly evaporated at room temperature. After three days, mass green flake crystals can be harvested.

Synthesis of (HMTP)2MnCl4·H2O single crystals. 2.0 mmol MnCl2 and 1.0 mmol HMTP-Cl were dissolved in 8 mL MeOH and stirred for 15 min at room temperature, and then added 0.5 mL HCl and 50 μL H3PO2. The solution was allowed to evaporate slowly at room temperature. After three days, block crystals can be harvested.

Synthesis of (BTP)2MnCl4 single crystals. 1.0 mmol MnCl2 was dissolved in 8 mL MeOH and stirred at room temperature until the solution was clarified. 1.0 mmol BTP-Cl and 0.5 mL HCl were added stirred for 30 min in a magnetic stirrer. The solution slowly evaporated at room temperature. After three days, green flake crystals can be harvested.

Synthesis of (BTP)2MnCl4·H2O single crystals. Similar to the synthesis conditions of (BTP)2MnCl4, the difference is that EtOH is used instead of MeOH, and then 50 μL H3PO2 is added to the reaction solution. The solution slowly evaporates at room temperature. After two days, mass green bulk (BTP)2MnCl4·H2O single crystals can be harvested.

Synthesis of (BTP)2MnBr4 single crystals. 1.0 mmol MnBr2 was dissolved in 8 mL MeOH and stirred at room temperature until the solution was clarified. 1.0 mmol BTP-Br and 0.5 mL HBr were added and stirred for 30 min in a magnetic stirrer. The solution slowly evaporated at room temperature. After three days, green flake crystals can be harvested.

Synthesis of (2ClBTP)2MnCl4 single crystals. Similar to the synthesis method of (BTP)2MnCl4 single crystals, the green block (2ClBTP)2MnCl4 single crystals can be obtained by replacing 1.0 mmol BTP-Cl with 2.0 mmol 2ClBTP-Cl at room temperature.

Synthesis of (4ClBTP)2MnCl4 single crystals. Similar to the synthesis method of (2ClBTP)2MnCl4 single crystals, the green block (4ClBTP)2MnCl4 single crystals can be obtained by replacing 2.0 mmol 2ClBTP-Cl with 2.0 mmol 4ClBTP-Cl at room temperature.

Synthesis of (4MBTP)2MnCl4 single crystals. Similar to the synthesis method of (4ClBTP)2MnCl4 single crystals, the (4MBTP)2MnCl4 single crystals can be obtained by replacing 2.0 mmol 4ClBTP-Cl with 2.0 mmol 4MBTP-Cl at room temperature.

Synthesis of (EOTP)2MnCl4 #1 and (EOTP)2MnCl4 #2 single crystals. Similar to the synthesis method of (4MBTP)2MnCl4 single crystals, replace 2.0 mmol 4MBTP-Cl with 1.0 mmol EOTP-Cl at room temperature. There are two kinds of structures in the obtained crystals that can be distinguished under UV light irradiation. Under UV light irradiation, (EOTP)2MnCl4 #1 is green light, and the emission of (EOTP)2MnCl4 #2 under UV light is yellow-green.

Synthesis of (EOTP)2MnBr4 #1 and (EOTP)2MnBr4 #2 single crystals. Similar to the synthesis method of (EOTP)2MnCl4 #1 and (EOTP)2MnCl4 #2 single crystals, by replacing MnCl2 with MnBr2, replacing EOTP-Cl with EOTP-Br at room temperature. Under UV light irradiation, (EOTP)2MnBr4 #1 is green light, and the emission of (EOTP)2MnBr4 #2 under UV light is yellow-green.

Preparation of (EOTP)2MnBr4 #1 @ glass film. The as-synthesized single crystals of (EOTP)2MnBr4 #1 were ground into powders in an agate mortar. Then, (EOTP)2MnBr4 #1 powders were dissolved in DMF to prepare a clear solution of 0.5 M. The mixed solution was spin-coated onto a glass substrate. The as-deposited film was dried at room temperature for 2 h to remove any residual solvent and to form the (EOTP)2MnBr4 #1 @ glass film.

Preparation of (EOTP)2MnBr4 #1 powders @ PDMS film. The film can be obtained by stirring 0.05 g (EOTP)2MnBr4 #1 powders with 0.5 g PDMS matrix resin and 0.05 g curing agent in a petri dish to mix evenly and solidify at 60 °C for 4 h.

Single-crystal X-ray diffraction. The crystal structures of (ClMTP)2MnCl4, (HMTP)2MnCl4·H2O, (BTP)2MnCl4, (BTP)2MnCl4·H2O, (BTP)2MnBr4, (2ClBTP)2MnCl4, (4ClBTP)2MnCl4, (4MBTP)2MnCl4, (EOTP)2MnCl4 #1, (EOTP)2MnCl4 #2, (EOTP)2MnBr4 #1 and (EOTP)2MnBr4 #2 are determined by the X-ray crystallography analysis. The diffraction data was collected by ω−2θ scanning using Mo-K α ray (λ=0.71073 Å) and Cu-K α ray (λ=1.5406 Å) purified by graphite monochromator (Bruker Smart APEX-II CCD, Germany). All intensity data is corrected by Lorentz and polarization factors, as well as absorption (MULTI-SCAN program SADABS). Using Olex2, the structure was solved with the SHELXT structure solution program using Intrinsic Phasing and refined with the SHELXL refinement package using Least Squares minimization66–68.

Powder X-ray diffraction. The powder diffraction patterns were carried out on a DX-2700B X-ray Diffractometer system equipped with a D/teX detector using a Ni-filtered Cu K α radiation source and operating at 40 kV and 20 mA. Typical PXRD scans were performed in the 5°−50° (2θ) range, with a step size of 0.02°.

Optoelectronic measurements. Room temperature PLE and PL measurements, the temperature-dependent PL spectra were performed on a HORIBA Jobin Yvon Fluorolog-3 spectrofluorometer using a Xe lamp, and PLQY measurements were performed on Edinburgh FLS1000 Quanta-φ integrating sphere using the two-curve method. UV-visible absorption spectra were obtained to study the optical absorption properties of the compounds. Measurements were performed on polycrystalline powder samples using a Shimadzu UV3600, where BaSO4 was used as the reference sample. The time-resolved PL decay lifetime and temperature-dependent lifetime of the as-synthesized compounds were monitored on a HORIBA Jobin Yvon Fluorolog-3 spectrofluorometer using a time-correlated single photon counting module. HORIBA Jobin Yvon NanoLEDs (pulsed light-emitting diodes) were used as the excitation source. The components of the Mn halides were analyzed by XPS (Thermo Fisher Scientific, USA).

Ultrahigh-speed camera. To capture the transient luminescence precisely at the moment of fracture, the single crystal was mounted on a custom-designed sample stage and manually fractured under applied pressure. The fracture-induced emission was recorded in real time using a Phantom Flex 4K ultrahigh-speed camera (4096×2160 pixels, maximum frame rate: 1000 fps, minimum exposure time: 5 μs), enabling direct visualization of the emission dynamics at the fracture instant.

AFM. Fix the single crystal with a flat and smooth surface on the sample stage of the AFM to ensure that the single crystal is stable during the measurement. Select a suitable AFM probe and calibrate the force constant of the probe by the resonant frequency method or the thermodynamic method. The AFM probe is gradually pressed down to the surface of the single crystal, and the force applied by the probe and the deformation of the single crystal surface are recorded. Then, by analyzing the force-displacement curves, the Derjaguin-Muller-Toporov (DMT) model is used to fit the data and calculate the Young's modulus. By measuring the Young's modulus in different areas, the uniformity and stress distribution of the single crystal surface can be evaluated. By calculating the Young's modulus at three different sites on the surface of every single crystal, the average value was taken as the average Young's modulus of the materials.

PFM. Local piezoelectric responses were investigated using PFM on the same AFM system equipped with a conductive tip (SCM-PIT-V2, Bruker). An AC voltage of 300 mV at a frequency of 1.0 kHz was applied between the tip and the sample to induce electromechanical deformation. Both vertical and lateral PFM amplitude and phase signals were recorded, and amplitude-voltage loops were obtained by performing DC bias sweeps from −10 V to 10 V to evaluate the local electromechanical coupling. Each measurement was repeated at multiple locations to confirm reproducibility.

TL measurements. TL measurements were performed on an LTTL-3DS multifunctional defect fluorescence spectrometer. Single-crystal samples were mounted on the instrument sample holder and pre-cooled to 100 K. Samples were irradiated with the built-in X-ray source (50 kV, 80 μA, 4 W) for 100 s or mechanically ground under liquid nitrogen to populate trap states. Then, the glow curve was recorded by linearly heating the sample from 100 to 300 K at a constant rate of 1 K s−1 while collecting emission with an integration time of 1 s per data point.

FML measurements. FML experiments were conducted by compressing individual single crystals (1−10 mm in size) using a universal mechanical compression system under controlled conditions. The compression was applied vertically at a constant rate of 5 mm min−1, with the maximum applied load ranging from 0 to 60 N to induce fracture. The emitted ML during fracture was detected using a photomultiplier tube (PMT) to monitor light intensity in real time. Simultaneously, the emission spectra were recorded using an Ocean Optics fiber-optic spectrometer with an integration time of 20 s. All measurements were performed in ambient conditions, and the setup ensured precise synchronization between mechanical loading and optical signal acquisition to capture the transient FML response.

SHG. SHG measurements were conducted using a Q-switched Nd:YAG laser (1064 nm, 5 ns, 10 Hz) as the excitation source. The SHG intensity was compared with a standard KDP reference under identical conditions to evaluate relative efficiency.

Piezoelectric constant measurements. To measure piezoelectric constants d33, the quasi-static method (ZJ-3A) was used on the Mn halide single crystals. Crystal size: 5×5×2 mm3.

Magnetic measurements. The temperature dependence of the magnetization was measured in a vibrating sample magnetometer attached to a physical property measurement system for the temperature range 10−300 K.

Femtosecond transient absorption spectroscopy. Laser pulses (800 nm, 50 fs pulse length, 1 kHz repetition rate) are generated by a Ti: sapphire femtosecond laser source (Hurricane, Spectra-Physics). Excitation power and spot size measurements are used to determine the excitation fluence (pump wavelength: 310 nm).

Radioluminescence and X-ray imaging. The emitted RL spectra were collected using a monochromator equipped with a PMT (R928P PMT, Hamamatsu). X-ray imaging experiments were conducted using an X-ray source operated at 60 kV and 200 μA. The resulting images were recorded directly with a digital camera. The light yield was determined by comparing the integrated RL intensity of the sample with that of a commercial reference scintillator (Ce: LuAG) under identical conditions, using a calibrated photodetector system coupled with a monochromator. The detection limit was evaluated based on the signal-to-noise ratio (SNR=3) under low-dose irradiation, using a dosimeter to quantify the X-ray dose rate and a sensitive CCD detector to monitor emission intensity.

Computational methods. The calculations were performed based on the DFT using the Vienna Ab-Initio Simulation Package 63,69. We relaxed the crystal structures and calculated the band structures using the PBE generalized gradient approximation and using the DFT-D3 method to correct the van der Waals interaction. The cutoff energy was set to 612 eV in the relaxation calculation and was set to 408 eV in the DOS and band structure calculation. In structural relaxation, the atomic force convergence criterion was set to 0.00257 eV Å−1.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript presents an extensive and compelling study of Mn halide-based materials that exhibit strong fracto-mechanoluminescence (FML) properties, governed by Young’s modulus. The authors successfully link low mechanical stiffness (<10,000 MPa) with the emergence of FML, and they explore the underlying luminescence mechanisms, crystallographic features, and applications in structural health monitoring and radiation sensing. The multimodal functionality —including stress detection and X-ray imaging—highlights the promising potential of these materials for smart optoelectronic devices.\nDespite the overall merit of the work, a critical emission undermines its practical relevance for long-term or repeatable structural health monitoring applications: the issue of particle size reduction and material reuse is not addressed. Since FML is fundamentally induced by fracturing events, the material is physically consumed in each use. This inherently raises questions about:\n• The evolution of crystal size with repeated use or multiple fracture cycles.\n• Reusability or recyclability of the crystals in sensor platforms.\n• Whether miniaturization or pulverization compromises the optical output or sensitivity over time.\nWithout addressing these concerns, the proposed manuscript may suffer from poor durability. Major revision is needed.", "reviewer_2": "The manuscript provides valuable and broad insight into FML-active Mn halides, but several mechanistic aspects require clarification. Specifically, the role of low Young’s modulus needs to be more precisely defined, whether it merely facilitates crystal fracture or directly enhances FML by increasing the electric field strength. Additionally, the origin of stress-dependent FML intensity remains unclear, whether it is due to increased fractured area or amounts of bonding breakage. Clarifying whether ML occurs prior to fracture (e.g., at the elastic limit) would help decouple mechanical sensitivity from intrinsic FML activation. Finally, the influence of crystal symmetry on fracto-induced electric fields, and whether the Cl electronegativity effect is supported by halide-comparative trends, warrants further discussion to reinforce the theoretical mechanism.\n1. What is the exact role of structural softness (i.e., low Young’s modulus) in enabling FML? Does it simply facilitate easier fracture of the crystals, or does it directly enhance FML intensity? Is the generation of a stronger electric field upon fracture primarily enabled by lower stiffness, or does a soft lattice also allow for more favorable electronic or band structure changes?\n2. Given that fracto-mechanoluminescence (FML) arises from crystal fracture, what is the underlying reason for the observed increase in ML intensity with increasing applied mechanical stress (as shown in Figure 2e)? Is the increase in luminescence due to a larger fracture area, a greater number of fractured bonding, or another factor? This aspect remains somewhat unclear in the current discussion.\n3. To clarify the effect of Young’s modulus on FML behavior, would it be possible to provide analytical data comparing the FML response before and after the fracture event? For example, does the material exhibit any mechanoluminescence at the elastic limit (i.e., just before fracture), or is the emission only triggered after irreversible fracture? If the elastic modulus is correlated with crystal fracture, it may be more closely related to the sensitivity of the ML response rather than being a direct indicator of a favorable ML-active environment.\n4. If the electric field generated at the fracture surface is the driving force of FML, what is the role of structural centrosymmetry or non-centrosymmetric in this context? Previous reports suggest that non-centrosymmetric structures facilitate ML due to polarization-induced potentials. However, is such symmetry-breaking still relevant for fracto-induced electric fields, which presumably arise from charge separation rather than built-in polarization? (9p, 19 line)\n5. In Supplementary Figure 57, the effect of Cl electronegativity on PLQY is presented. Has this hypothesis been further supported by a systematic comparison across other halides (e.g., Br-based analogs)? Are there observable trends in PLQY or related optical properties that can be correlated with halide electronegativity? This further strengthens the theoretical framework of the electronegativity effect of halide ions.", "reviewer_3": "Recommendation: Publish after critical issues are addressed.\nThis manuscript presents a comprehensive study on wide range of Mn halides exhibiting FML, PL, and RL, contributing insights into multimodal luminescent materials. Furthermore, this work tried to elucidate mechanism in Mn halide systems based on the FML, PL properties. However, to further strengthen the manuscript, several mechanistic aspects and relationship with spectroscopic evidence should be clarified. The author should carefully connect the FML phenomenon with possible influence of fractured surface area, which could be electric field, under the specific stress. Based on this, the role of Young’s modulus should be demonstrated in FML process. Establishing the correlation between the structural characteristics of each complex and their associated kinetics could be important for elucidating the fundamental principles underlying FML, which should include the connection between photoexcited luminescence and FML induced by fracture-generated electric fields. To meet the high standards and authoritative quality of Nature Communications, we recommend publication only after these major issues have been thoroughly addressed.\n1. As discussed on page 12, lines 10–11, the authors suggest that the breakage of hydrogen bonds during fracture induces interfacial electric fields by releasing charges. If the number of broken bonds is proportional to the generated electric field, could the observed increase in FML intensity with applied mechanical stress (Fig. 2e), at constant crystal size, be attributed to an increase in fracture surface area, or is it a consequence of low Young’s modulus enhancing the efficiency of electric field generation? The authors are encouraged to clarify the role of Young’s modulus.\n2. The electric field generated upon fracture should ideally be normalized to the surface area at the fracture interface. As surface area significantly affects the local electric field strength even under the same applied force, it would be valuable if the authors could attempt to quantify this relationship or provide insight into how the fractured surface area is correlated with FML intensity. If author establish a common condition for surface effects, a clear trend between FML and Young’s modulus can be identified.\n3. The absorption and PL spectra shown in Supplementary Figures 50 and 51 indicate that some Mn halide samples exhibit strong absorption bands in the long-wavelength region (~700 nm), while the PL emission is centered at shorter wavelengths (~515 nm). The authors should explain the energy transfer mechanism responsible for this apparent anti-Stokes-type emission. Is this related to specific ligand-to-metal charge transfer or crystal-field splitting? Furthermore, how does this photophysical behavior connect to the observed FML response?\n4. On page 17, line 10, the authors suggest the long PL decay times (0.07–3.31 ms) solely to spin-forbidden d–d transitions of Mn²⁺ ions. However, such a wide lifetime range could also result from other mechanisms, including indirect bandgap transitions or trap-mediated processes. The authors should discuss whether d–d transitions alone sufficiently explain the lifetime behavior across all samples, and if not, what role other relaxation pathways might play. What causes the variation in each of the observed decay times, and how do these differences influence the FML behavior?\n5. While most of the emission characterization (PL, RL) focuses on photo-excited pathways, FML is attributed to field-induced excitation via mechanical fracture. The authors are encouraged to elaborate on how the optical behavior observed under light excitation or X-ray excitation correlates mechanistically with the FML process. Establishing a clear link between photoluminescence properties and electrically-driven FML activation would strengthen the theoretical foundation and broaden the impact of this work."}, "author_response": "The manuscript presents an extensive and compelling study of Mn halide-based materials that exhibit strong fracto-mechanoluminescence (FML) properties, governed by Young′s modulus. The authors successfully link low mechanical stiffness (<10,000 MPa) with the emergence of FML, and they explore the underlying luminescence mechanisms, crystallographic features, and applications in structural health monitoring and radiation sensing. The multimodal functionality—including stress detection and X-ray imaging—highlights the promising potential of these materials for smart optoelectronic devices. Despite the overall merit of the work, a critical emission undermines its practical relevance for long-term or repeatable structural health monitoring applications: the issue of particle size reduction and material reuse is not addressed. Since FML is fundamentally induced by fracturing events, the material is physically consumed in each use. This inherently raises questions about:\nResponse: We sincerely thank the reviewer for this valuable comments. Since fracto-mechanoluminescence (FML) is inherently associated with fracture events, materials consumption during each use is unavoidable. We do really appreciate that the reviewer also recognizes this hard issue.\nMoreover, to address the very helpful comments, we conducted cyclic stress experiments on (EOTP) MnBr (1) and recyclability assessments based on (2ClBTP) MnCl , (4ClBTP) MnCl (EOTP) MnBr (1), (MTP) MnCl , (ETP) MnCl , and (ETP) MnBr . The new results suggest that the materials possess good recrystallization capability, indicating their value of reusability (Supplementary Figures 59-64).\n1) Particle size reduction: A single crystal of (EOTP) MnBr (1) with a size of approximately 10 mm was selected for cyclic stress experiments. After six stress cycles, the crystal size was significantly reduced (final crystal size was less than 1 mm or smaller), both under a constant load of 60 N and under a variable load of 0−60 N. The degree of crystal fragmentation became more pronounced with the magnitude of applied stress and the number of cycles.\n2) Material reuse: Here, we selected six materials, (2ClBTP) MnCl , (4ClBTP) MnCl (EOTP) MnBr (1), (MTP) MnCl , (ETP) MnCl , and (ETP) MnBr , to demonstrate the recyclability of the crystals. Fragmented or miniaturized materials can be recrystallized from methanol to obtain single crystals with excellent morphology. The resulting single crystals retain excellent phase purity and sensitive FML response. This process not only demonstrates the reproducibility of the materials but also provides a practical strategy for long-term or sustainable sensing applications, enabling the crystals to be reused through a crushing-recovery-recrystallization cycle.\nAll in all, the phenomenon of mechanoluminescence inherently faces challenges related to long-term use; however, the facile synthesis of the Mn halide materials presented in this work partially compensates for the value of mechanoluminescence towards certain specialized applications.\nRevision: The specific revisions have been addressed point by point to the reviewer′s comments.\nComment 1. The evolution of crystal size with repeated use or multiple fracture cycles.\nResponse: To explore the evolution of crystal sizes, we selected (EOTP) MnBr (1) single crystal with sizes close to 10 mm and subjected them to two types of cyclic stress experiments: (i) six cycles under a constant load of 60 N, and (ii) variable loading from 0 to 60 N in increments of 10 N. As shown in Supplementary Figures 60 and 62, the crystal size was markedly reduced after six stress cycles under both constant and variable loading conditions, with the final size shrinking to less than 1 mm or even small. These results suggest that applying high stresses, combined with an increased number of loading cycles, should lead to more pronounced crystal pulverization (Supplementary Figures 60 and 62).\nRevision: We have added photos of the crystals after the fracture cycling experiments in the revised manuscript.\n“As shown in Supplementary Figure 59, after six cycles under 60 N stress, the FML intensity decreases significantly as the crystal size diminishes (Supplementary Figure 60). When stress was gradually increased from 0 to 60 N, the FML intensity decayed more slowly over the six cycles (Supplementary Figure 61). This is due to the gradual miniaturization or pulverization of the crystals (Supplementary Figure 62).”\nSupplementary Figure 60. Crystal morphology of (EOTP) MnBr (1) after six times of cyclic application of 60N pressure.\nSupplementary Figure 62. Crystal morphology of (EOTP) MnBr (1) after gradually applying pressure of 10−60 N.\nComment 2. Reusability or recyclability of the crystals in sensor platforms.\nResponse: In response to the reviewer′s insightful comment, we performed additional experiments to evaluate the reusability and recyclability. The results suggest that although the crystals show certain limitations in reusability, they exhibit acceptable recyclability (Supplementary Figures 63 and 64).\n1) Reusability: During the application of the sensor platform, we observed that the crystals gradually became smaller and eventually pulverized after repeated use, which led to a significant decrease in light output sensitivity.\n2) Recyclability: These crystals exhibit good recyclability. Once pulverized into powders, they can be readily dissolved in methanol, and single crystals can be recovered through a simple recrystallization process. Supplementary Figure 63 shows the crystal morphology and PXRD patterns before and after recrystallization, demonstrating their excellent recyclability. Furthermore, Supplementary Figure 64 presents the fracture luminescence properties of the recycled crystals, indicating that they can still be effectively employed in the sensing platform, thereby extending their service life.\nHereby, although this material, like traditional known mechanoluminescent materials, faces challenges related to sustained use after fracture, its ease of reprocessing distinguishes it from previous materials and represents a significant advantage.\nRevision: We have added a description of the recyclability of the crystals in the revised manuscript.\n“Interestingly, the pulverized material dissolved well in methanol and recrystallized into high-quality single crystals upon solvent evaporation. The recrystallized crystals retained excellent phase purity and FML responsiveness (Supplementary Figures 63 and 64). This demonstrates not only the reproducibility of the material but also provides a practical strategy for sustainable sensing applications. Through a crushing-recovery-recrystallization cycle, the crystals can be reused, offering promising prospects for long-term structural health monitoring and stress sensing.”\nSupplementary Figure 63. a Photos of (2ClBTP) MnCl , (4ClBTP) MnCl (EOTP) MnBr (1), (MTP) MnCl , (ETP) MnCl , and (ETP) MnBr single crystals before and after recrystallization in natural light. b The powder X-ray diffraction patterns before and after recrystallization of (2ClBTP) MnCl , (4ClBTP) MnCl (EOTP) MnBr (1), (MTP) MnCl , (ETP) MnCl , and (ETP) MnBr single crystals are compared with the simulated X-ray diffraction patterns.\nSupplementary Figure 64. The curves of recrystallized (EOTP) MnBr (1) single crystal fracto-mechanoluminescence intensity changing with applied mechanical force stimulations.\nComment 3. Whether miniaturization or pulverization compromises the optical output or sensitivity over time.\nResponse: The fracture cycling stress experiments have been carried out to explore this concern. We employed (EOTP) MnBr (1) single crystals with dimensions of approximately 10 mm and performed two sets of cyclic stress experiments: (i) six cycles under a constant load of 60 N, and (ii) variable loading from 0 to 60 N with incremental steps of 10 N. The results indicate that FML sensitivity inevitably decreases significantly with increasing applied stress and cycle numbers, which could be attributed to the gradual pulverization of crystals (with the final crystal size reduced to less than 1 mm or even smaller). Hereby, we do really acknowledge that miniaturization or pulverization compromises the optical output or sensitivity over time. (Supplementary Figures 59 and 61).\nOn one hand, larger mechanical stress and higher cycle numbers result in more severe pulverization; therefore, the crystals should still function until the luminescence intensity falls below the specified sensitivity threshold.\nOn the other hand, although repeated fracture cycling reduces the FML intensity, the material holds significant application value in stress-sensing scenarios involving specialized threshold-related measurements where is not suitable for the elastic-mechanoluminescence and plastic-mechanoluminescence materials.\nRevision: We have added a description of the effect of crystal pulverization on light output in the revised manuscript.\n“As shown in Supplementary Figure 59, after six cycles under 60 N stress, the FML intensity decreases significantly as the crystal size diminishes (Supplementary Figure 60). When stress was gradually increased from 0 to 60 N, the FML intensity decayed more slowly over the six cycles (Supplementary Figure 61). This is due to the gradual miniaturization or pulverization of the crystals (Supplementary Figure 62).”\nSupplementary Figure 59. Variation of fracto-mechanoluminescence intensity of (EOTP) MnBr (1) with the cyclic application of 60 N mechanical stimulus for six times.\nSupplementary Figure 61. Variation of fracto-mechanoluminescence intensity of (EOTP) MnBr (1) with the gradual application of mechanical stimulus from 0 to 60 N.\nReviewer #2:\nThe manuscript provides valuable and broad insight into FML-active Mn halides, but several mechanistic aspects require clarification. Specifically, the role of low Young′s modulus needs to be more precisely defined, whether it merely facilitates crystal fracture or directly enhances FML by increasing the electric field strength. Additionally, the origin of stress-dependent FML intensity remains unclear, whether it is due to increased fractured area or amounts of bonding breakage. Clarifying whether ML occurs prior to fracture (e.g., at the elastic limit) would help decouple mechanical sensitivity from intrinsic FML activation. Finally, the influence of crystal symmetry on fracto-induced electric fields, and whether the Cl electronegativity effect is supported by halide-comparative trends, warrants further discussion to reinforce the theoretical mechanism.\nResponse: We sincerely thank the reviewer for the constructive comments. As for the concerns, they are addressed as following:\n1) The role of low Young′s modulus (Fig. 3f and Fig. 3g): Mn halide crystals with a Young′s modulus below 10000 MPa generally exhibit pronounced fracto-mechanoluminescence (FML), indicating that a low Young′s modulus is a prerequisite for FML generation. Piezoresponse force microscopy (PFM) measurements further reveal that, under the same bias voltage, crystals with smaller Young′s modulus display larger PFM amplitudes, suggesting stronger local electro-mechanical coupling. Enhanced elastic flexibility facilitates the formation of local polarization and internal electric fields, thereby promoting FML. In addition, the fracture force-FML intensity experiments conducted on crystals with different Young′s modulus ((2ClBTP) MnCl (9299.52 MPa), (ETP) MnCl (4859.07 MPa), (ETP) MnBr (2974.09 MPa), (EOTP) MnBr (1) (2718.67 MPa), (MTP) MnCl (1102.19 MPa) and (4ClBTP) MnCl (177.51 MPa)) further demonstrated the multi-level regulation effect of Young′s modulus (Figs. 3f and 3g). Young′s modulus not only defines the stress threshold for FML generation but also governs the applicable stress window of the crystal: crystals with lower Young′s modulus can sustain a broader stress range, whereas those with higher Young′s modulus are prone to fracture under relatively lower stresses. In summary, the role of Young′s modulus in FML could be not singular but rather a multifaceted, integrated effect encompassing triggering conditions, intensity regulation, and the applicable threshold and stress range.\n2) The origin of stress-dependent FML intensity (Fig. 3c, Fig. 3d, and Fig. 3e): In (2ClBTP) MnCl crystals, small crystals (1−6 mm) exhibited progressively stronger FML with increasing fracture area. When the crystal size increased to 8−10 mm, the presence of twins and interfacial defects caused fractures to occur preferentially at defect sites, which reduced the effective fracture area and weakened the FML intensity. These results indicate that FML intensity is primarily determined by the effective fracture area that contributes to light emission. In (EOTP) MnBr (1) crystals, the FML intensity showed an approximately linear dependence on applied stress in the range of 10−30 N. Moreover, for crystals of the same size, FML intensity increased significantly with the number of fracture surfaces. Overall, these findings demonstrate that larger external forces can induce larger effective fracture areas, thereby enhancing the FML intensity.\n3) Whether ML occurs prior to fracture (Fig. 2c and Supplementary Video 2): The FML process was recorded using an ultrahigh-speed camera (4096×2160 pixels, maximum frame rate: 1000 fps, minimum exposure time: 5 μs). As far as the limit of the camera, it looks like that FML occurs at the moment of crystal fracture, and the fracture area is positively correlated with the FML intensity.\n4) The influence of crystal symmetry (Fig. 5h, Supplementary Figures 48-56): We carried out piezoelectric force microscopy (PFM) measurements and found that crystal symmetry is not the determining factor for the generation of FML. Although existing studies suggest that non-centrosymmetric structures in Mn halides are more susceptible to FML (Adv. Mater. 2024, 36, 2309906), we found that the occurrence of FML relies on the synergistic effects of a low Young′s modulus (specifically below 10000 MPa), local electromechanical coupling, and the presence of trapped states, which together trigger the Mn2+ d−d transitions during stress-induced fracture.\n5) The Cl electronegativity effect (Supplementary Figure 74): As shown in supplementary Figure 74, as Cl is gradually replaced by Br, the PL intensity of (EOTP) MnCl Br (1) shows a significant improvement, among which the PLQY of (EOTP) MnBr (1) is significantly higher than that of (EOTP) MnCl (1). This trend is consistent with the difference in halogen electronegativity: Br⁻ has a lower electronegativity than Cl⁻, and its weaker ligand field effect and stronger electronic polarization effect help to reduce non-radiative relaxation, thereby enhancing the radiative recombination efficiency. Therefore, it can be confirmed that the change trend of PLQY is reasonably related to the halide electronegativity, providing experimental evidence to support the theoretical framework of the halide electronegativity effect.\nRevision: A detailed account of the modifications is provided individually in the corresponding replies to each comment.\nComment 1. What is the exact role of structural softness (i.e., low Young′s modulus) in enabling FML? Does it simply facilitate easier fracture of the crystals, or does it directly enhance FML intensity? Is the generation of a stronger electric field upon fracture primarily enabled by lower stiffness, or does a soft lattice also allow for more favorable electronic or band structure changes?\nResponse: The role of structural softness (i.e., lower Young′s modulus) in enabling FML looks like to make the crystals easier to fracture. The stronger electric field generated during fracture is mainly due to the fact that the soft lattice also allows more favorable electronic or band structure changes. The detailed analysis is as following (Fig. 3f, Fig. 3g, Fig. 5h, and Supplementary Figures 48-56):\n1) The exact role of structural softness in enabling FML: AFM measurements reveal that Mn halides with a Young′s modulus below 10000 MPa exhibit distinct FML, indicating that reduced structural rigidity is essential for efficient FML generation. As shown in Supplementary Figures 48-55, the PFM measurements further reveal that, under the same bias voltage, crystals with smaller Young′s modulus display larger PFM amplitudes, suggesting stronger local electro-mechanical coupling. Enhanced elastic flexibility facilitates the formation of local polarization and internal electric fields, thereby promoting FML. In addition, the fracture force-FML intensity experiments conducted on crystals with different Young′s modulus ((2ClBTP) MnCl (9299.52 MPa), (ETP) MnCl (4859.07 MPa), (ETP) MnBr (2974.09 MPa), (EOTP) MnBr (1) (2718.67 MPa), (MTP) MnCl (1102.19 MPa) and (4ClBTP) MnCl (177.51 MPa)) further demonstrated the multi-level regulation effect of Young′s modulus (Figs. 3f and 3g). Young′s modulus not only defines the stress threshold for FML generation but also governs the applicable stress window of the crystal: crystals with lower Young′s modulus can sustain a broader stress range, whereas those with higher Young′s modulus are prone to fracture under relatively lower stresses.\nIn summary, the role of Young′s modulus in FML is not singular but rather a multifaceted, integrated effect encompassing triggering conditions, intensity regulation, and the applicable threshold and stress range.\n2) The generation of a stronger electric field upon fracture: The stronger electric field generated during fracture primarily originates from the lower elastic stiffness of the materials. A softer lattice is more susceptible to deformation under external stress, which induces pronounced local polarization and enhances the electric field at the moment of fracture. In addition, a soft lattice not only reduces the elastic stiffness but also provides favorable conditions for electronic state modulation: a weaker crystal field decreases the splitting of d orbitals, lowering the energy required for electronic transitions, while trap states can be more readily activated under stress and contribute to radiative recombination. Therefore, the stronger electric field is mainly attributed to the electro-mechanical coupling effect induced by lower elastic stiffness, whereas the band tunability of the soft lattice further facilitates the participation of electrons and trap states. Together, these factors synergistically enable efficient FML.\nAll in all, the role of Young′s modulus in FML lies in reducing the fracture threshold of the crystal, thereby facilitating its fracture.\nRevision: In the revised manuscript, we have added a discussion on the exact role of lower Young′s modulus and the stronger electric field generated during fracture influence the FML behavior.\n“The results show that the Young′s modulus of Mn halides with explicit FML characteristics is generally lower than 10000 MPa; while for crystals with a Young′s modulus higher than 10000 MPa, no obvious FML phenomenon was observed (Supplementary Figures 29-46, Table 1). This implies that reduced structural rigidity, as reflected by a lower Young′s modulus, is a prerequisite for enabling efficient FML in these materials.”\n“In addition, by comparing the Young′s moduli of (2ClBTP) MnCl (9299.52 MPa), (4ClBTP) MnCl (177.51 MPa), and (EOTP) MnBr (1) (2718.67 MPa), it can be found that under the same bias voltage, the smaller the Young′s modulus, the larger the PFM amplitude. This trend reveals the intrinsic connection between the elastic stiffness of the crystal and the local electro-mechanical coupling effect, indicating that regulating the Young′s modulus may be an effective way to optimize the FML behavior.”\n“Furthermore, as shown in Fig. 5h and Supplementary Figure 56, the results of thermoluminescence (TL) spectra under X-ray excitation indicate that there are trap states in the structures of (BTP) MnCl , (2ClBTP) MnCl , (4ClBTP) MnCl , (EOTP) MnBr (1), (MTP) MnCl , (ETP) MnCl , and (ETP) MnBr , which can serve as energy storage reservoirs to compensate electrons to the emission center. However, (BTP) MnCl , which has an extremely high Young′s modulus (184890.81 MPa), does not exhibit FML properties, suggesting that excessive elastic stiffness inhibits the effective participation of trap states in the FML process.”\n“In summary, the realization of FML relies not only on the presence of trapped states but also requires the material to possess appropriate mechanical flexibility to ensure that the trapped states can be effectively activated by mechanical stimulation and drive the luminescence process (Fig. 2g).”\n“The role of Young′s modulus in FML requires further clarification. In Fig. 3f, as the Young′s modulus of (2ClBTP) MnCl (9299.52 MPa), (ETP) MnCl (4859.07 MPa), (ETP) MnBr (2974.09 MPa), (EOTP) MnBr (1) (2718.67 MPa), (MTP) MnCl (1102.19 MPa) and (4ClBTP) MnCl (177.51 MPa) decreases, their FML intensity does not show a significant regular change, although low Young′s modulus facilitates the generation of local piezoelectric fields. This suggests that FML behavior is governed by multiple factors. Considering fracture stress together with the first FML response (Fig. 3g), a general positive correlation between FML intensity and fracture force is evident. Those “perfect” crystals with fewer defects require greater force to break and emit bright FML, while “imperfect” crystals with more defects fracture at lower forces and display weaker FML. In addition, we find the Young′s modulus mainly affects the tolerable stress range of the crystals, which means the “slope” of the force-FML intensity relationship. This underlines the significant impact of Young′s modulus on the FML mechanism. In summary, Young′s modulus exerts a multifaceted influence on FML, simultaneously governing its initiation conditions, the corresponding threshold and stress window, thereby reflecting an integrated and multi-level effect.”\nSupplementary Figure 48. Piezoelectric force microscopy of (BTP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 49. Piezoelectric force microscopy of (BTP) MnBr single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 50. Piezoelectric force microscopy of (2ClBTP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 51. Piezoelectric force microscopy of (4ClBTP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 52. Piezoelectric force microscopy of (EOTP) MnBr (1) single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 53. Piezoelectric force microscopy of (MTP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 54. Piezoelectric force microscopy of (ETP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 55. Piezoelectric force microscopy of (ETP) MnBr single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nFig. 5 | Scintillation properties of synthesized Mn halides. h X-ray excited thermoluminescence spectrum of (EOTP) MnBr (1) measured at a heating rate of 1 K s−1.\nSupplementary Figure 56. X-ray excited thermoluminescence spectrum of a (BTP) MnCl , b (2ClBTP) MnCl , c (4ClBTP) MnCl , d (MTP) MnCl , e (ETP) MnCl , and f (ETP) MnBr measured at a heating rate of 1 K s−1.\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. f Fracto-mechanoluminescence intensity of Mn halides with different Young′s modulus.\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. g Relationship between fracto-mechanoluminescence intensity and fracture force of Mn halides.\nComment 2. Given that fracto-mechanoluminescence (FML) arises from crystal fracture, what is the underlying reason for the observed increase in ML intensity with increasing applied mechanical stress (as shown in Figure 3e)? Is the increase in luminescence due to a larger fracture area, a greater number of fractured bonding, or another factor? This aspect remains somewhat unclear in the current discussion.\nResponse: Based on the new experimental results (ultrahigh-speed camera, TL and PFM measurements, and stress-intensity analysis), the observed enhancement of FML with increasing mechanical stress could be attributed to several interrelated mechanisms. Among them, the enlargement of the effective fracture area appears to play a predominant role, while the increase in broken bonds, the enhancement of local electric fields, and the activation of trap states are likely to work synergistically to further amplify the FML response (Fig. 2, Fig. 3, Fig. 5h, Supplementary Figures 48-56, and Supplementary Video 2).\n1) Enlargement of effective fracture area: As the fracture area increases, the FML intensity is significantly enhanced, indicating that newly formed fracture surfaces directly contribute to luminescence. Each new fracture surface acts as an independent local emission unit, and the additive effect of multiple surfaces leads to stronger overall emission.\n2) Increase in broken bonds: The formation of new fracture surfaces involves more broken chemical bonds, generating additional carrier recombination sites and thus improving luminescence efficiency.\n3) Enhancement of local electric fields: A softer lattice is more easily deformed at the moment of fracture, producing stronger local electric fields. These fields facilitate the release of trapped carriers and promote their transfer to Mn2+ energy levels, thereby increasing the probability of radiative recombination.\n4) Activation and compensation of trap states: TL measurements reveal the presence of trap states capable of storing carriers. Stress and electric fields generated during fracture can effectively activate these traps, releasing carriers that compensate the luminescence centers and enhance transient emission.\nIn summary, larger mechanical stress primarily enhances FML by increasing the effective fracture area, while the strengthening of local electric fields and the activation of trap states during fracture further improve emission efficiency. These effects act synergistically to achieve stronger FML.\nRevision: We have added a discussion of the underlying reasons for the increased FML intensity in the revised manuscript.\n“As shown in Fig. 2c and Supplementary Video 2, we used an ultrahigh-speed camera to observe the FML behavior of Mn halide single crystals under stress in real time. Our results show that FML occurs at the exact moment of irreversible fracture of the crystals, and its intensity is positively correlated with the effective crack surface area.”\n“To quantify the effect of fracture area on FML intensity, we counted the number of fractured crystals and normalized the initial crystal size. This allowed us to evaluate the relationships among FML intensity, fracture area, applied stress, and Young′s modulus. In Fig. 3c, the FML intensity of (2ClBTP) MnCl single crystals increases progressively with crystal size in the 1−6 mm range. Since fracture area scales with initial size for crystals of similar shape and comparable numbers of fractures, this result indicates that the effective fracture area is the direct factor enhancing FML intensity. However, when the crystal size increased to 8−10 mm, this positive correlation no longer holds. This can be attributed to the fact that larger crystals are more prone to forming twins and interfacial defects, where fractures tend to occur preferentially. As a result, they absorb less energy during compression, leading to reduced FML intensity. These findings clearly demonstrate that enlargement of the effective fracture area is the primary factor governing FML enhancement. As shown in Fig. 3d, a correlation between the number of fractured crystals and FML intensity is observed for similarly sized (EOTP) MnBr (1) single crystals. Moreover, within the stress range of 10−30 N, the FML spectra of (EOTP) MnBr (1) exhibited an approximately linear dependence of intensity on applied stress as the crystals gradually fracture (Fig. 3e). This highlights the high sensitivity of FML to external mechanical stimuli.”\nFig. 2 | Mechanical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. c Fracto-mechanoluminescence process of (EOTP) MnBr (1) single crystal under ultrahigh-speed camera (4K at up to 1000 fps).\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. c Fracto-mechanoluminescence intensity of (2ClBTP) MnCl single crystals with different sizes.\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. d Relationship between fracture area and luminescence intensity of (EOTP) MnBr (1) single crystal.\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. e Fracto-mechanoluminescence spectra of (EOTP) MnBr (1) under different applied mechanical forces.\nComment 3. To clarify the effect of Young′s modulus on FML behavior, would it be possible to provide analytical data comparing the FML response before and after the fracture event? For example, does the material exhibit any mechanoluminescence at the elastic limit (i.e., just before fracture), or is the emission only triggered after irreversible fracture? If the elastic modulus is correlated with crystal fracture, it may be more closely related to the sensitivity of the ML response rather than being a direct indicator of a favorable ML-active environment.\nResponse: To try to address this comment, we used an ultrahigh-speed camera (4096×2160 pixels, maximum frame rate: 1000 fps, minimum exposure time: 5 μs) and a universal measuring machine to investigate the FML behavior of Mn halide single crystals during fracture.\nWe recorded the FML processes of (2ClBTP) MnCl , (4ClBTP) MnCl (EOTP) MnBr (1), and (ETP) MnBr under applied stress using ultrahigh-speed camera. The pictured videos suggest that even as the material approaches its elastic limit, no detectable luminescence is observed during elastic deformation. Instead, pronounced FML is initiated only at the moment of irreversible fracture, indicating that the luminescence is closely associated with crack formation (Fig. 2c, Fig. 3b, Supplementary Video 2, and Supplementary Figure 58).\nRevision: We have added analysis of FML responses before and after breakage events to the revised manuscript.\n“As shown in Fig. 2c and Supplementary Video 2, we used an ultrahigh-speed camera to observe the FML behavior of Mn halide single crystals under stress in real time. Our results show that FML occurs at the exact moment of irreversible fracture of the crystals, and its intensity is positively correlated with the effective crack surface area.”\n“As shown in Fig. 3b and Supplementary Figure 58, during a compression cycle, the crystals undergo multiple fractures events, each accompanied by a sharp emission pulse that corresponds to the applied stress curve. Typically, the first strong emission is the most representative, weaker signals that precede it may arise from crack propagation or minor fractures, while subsequent emissions originate from the re-fracture of already broken fragments. All following analyses are therefore based on the first emission.”\nFig. 2 | Mechanical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. c Fracto-mechanoluminescence process of (EOTP) MnBr (1) single crystal under ultrahigh-speed camera (4K at up to 1000 fps).\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. b Variation of fracto-mechanoluminescence intensity of (EOTP) MnBr (1) under gradual application of a 30 N mechanical load.\nSupplementary Figure 58. The curves of (EOTP) MnBr (1) single crystal fracto-mechanoluminescence intensity changing with applied mechanical force stimulations.\nComment 4. If the electric field generated at the fracture surface is the driving force of FML, what is the role of structural centrosymmetry or non-centrosymmetric in this context? Previous reports suggest that non-centrosymmetric structures facilitate ML due to polarization-induced potentials. However, is such symmetry-breaking still relevant for fracto-induced electric fields, which presumably arise from charge separation rather than built-in polarization?\nResponse: To investigate the possible effect of crystal symmetry on the fracture-induced electric field, we conducted a systematic comparative study using three non-centrosymmetric crystals: (MTP) MnCl , (ETP) MnCl , and (ETP) MnBr , and three centrosymmetric crystals: (2ClBTP) MnCl , (4ClBTP) MnCl , (EOTP) MnBr (1). The piezoelectric force microscopy results indicate that crystals exhibiting FML show stress-induced local polarization and electric field responses, irrespective of whether they are centrosymmetric or not. These findings suggest that the fracture-induced electric field is not directly related to crystal symmetry. Instead, it is the low Young′s modulus that facilitates carrier release from trap states at the moment of fracture, the soft lattice rather than symmetry breaking is the critical factor that promotes efficient FML generation (Supplementary Figures 48-55).\nRevision: We have added relevant descriptions of the structural symmetry in the revised manuscript.\n“Meanwhile, we systematically studied the local piezoelectric effect of eight Mn halide single crystals using piezoelectric force microscopy (PFM). As shown in Supplementary Figures 48 and 49, the PFM amplitude signals of (BTP) MnCl (centrosymmetric) and (BTP) MnBr (non-centrosymmetric), which do not exhibit FML characteristics, show only slight changes under different bias voltages.30 They do not display typical butterfly loop behavior, indicating the absence of a significant piezoelectric response. In contrast, as shown in Supplementary Figures 50-55, the centrosymmetric structures of (2ClBTP) MnCl , (4ClBTP) MnCl , (EOTP) MnBr (1) and the non-centrosymmetric structures of (MTP) MnCl , (ETP) MnCl , (ETP) MnBr all exhibit significantly enhanced PFM amplitude signals under different bias voltages and show nonlinear changes with the electric field.49 During the voltage sweep, the amplitude-voltage curves of these crystals showed characteristics similar to butterfly loops, indicating that they all have considerable local electro-mechanical coupling effects.50, 51 In addition, by comparing the Young′s moduli of (2ClBTP) MnCl (9299.52 MPa), (4ClBTP) MnCl (177.51 MPa), and (EOTP) MnBr (1) (2718.67 MPa), it can be found that under the same bias voltage, the smaller the Young′s modulus, the larger the PFM amplitude. This trend reveals the intrinsic connection between the elastic stiffness of the crystal and the local electro-mechanical coupling effect, indicating that regulating the Young′s modulus may be an effective way to optimize the FML behavior.”\nSupplementary Figure 48. Piezoelectric force microscopy of (BTP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 49. Piezoelectric force microscopy of (BTP) MnBr single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 50. Piezoelectric force microscopy of (2ClBTP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 51. Piezoelectric force microscopy of (4ClBTP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 52. Piezoelectric force microscopy of (EOTP) MnBr (1) single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 53. Piezoelectric force microscopy of (MTP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 54. Piezoelectric force microscopy of (ETP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 55. Piezoelectric force microscopy of (ETP) MnBr single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nComment 5. In Supplementary Figure 73, the effect of Cl electronegativity on PLQY is presented. Has this hypothesis been further supported by a systematic comparison across other halides (e.g., Br-based analogs)? Are there observable trends in PLQY or related optical properties that can be correlated with halide electronegativity? This further strengthens the theoretical framework of the electronegativity effect of halide ions.\nResponse: Based on the hypothesis that the electronegativity of halide ions affects PLQY, we further carried out halogen regulation experiments in the (EOTP) MnCl Br (1) (x = 0−4). As shown in Supplementary Figure 74, as Cl is gradually replaced by Br, the PLQY of (EOTP) MnCl Br (1) shows a significant improvement, among which the PLQY of (EOTP) MnBr (1) is significantly higher than that of (EOTP) MnCl (1). This trend is consistent with the difference in halogen electronegativity: Br− is less electronegative than Cl−. Its weaker ligand field interaction and stronger electron polarization effect suppress nonradiative relaxation. This, in turn, enhances radiative recombination efficiency. Therefore, the observed changes in PLQY can be reasonably correlated with halide electronegativity. These results provide experimental evidence supporting the theoretical framework of electronegativity effects (Supplementary Figure 74).\nRevision: We further discuss the effect of Cl electronegativity on PLQY in the revised Supporting Information.\n“Notably, the hydrogen bonds between the organic cations and the [MnX ]2− luminescent centers also play a pivotal role in the optoelectronic properties of Mn halides.3 The strong electronegativity of Cl atoms (hydrogen bond acceptors) can effectively attract the electron density on the C−H bonds (hydrogen bond donors) and enhance the transition of electrons from organic cations to [MnX ]2− luminescence centers (Supplementary Figure 73). In contrast, the O−H···Cl hydrogen bonds may reduce the optical performance. This is because the electronegativity of the O atom is stronger than that of the Cl atom, which makes the electron cloud around the O atom more concentrated. It is not conducive to the transfer of electrons from the organic cations to the [MnX ]2− luminescence centers. In addition, the bond length of O−H···Cl is longer than that of C−H···Cl, which leads to structural imbalance within the molecule and further limits the effective migration of electrons. Hereby, our studies demonstrate that the distance between [MnX ]2− tetrahedra is not the only factor affecting the luminescence efficiency. The effect of hydrogen bonding and the associated electron transfer mechanisms also play a critical role in determining the optical properties.”\n“In response to the hypothesis that halide electronegativity affects PLQY, we further conducted halogen manipulation experiments on the (EOTP) MnCl Br (1). As shown in supplementary Figure 74, as Cl is gradually replaced by Br, the PL intensity of (EOTP) MnCl Br (1) shows a significant improvement, among which the PLQY of (EOTP) MnBr (1) is significantly higher than that of (EOTP) MnCl (1). This trend is consistent with the difference in halogen electronegativity: Br⁻ has a lower electronegativity than Cl⁻, and its weaker ligand field effect and stronger electronic polarization effect help to reduce non-radiative relaxation, thereby enhancing the radiative recombination efficiency. Therefore, it can be confirmed that the change trend of PLQY is reasonably related to the halide electronegativity, providing experimental evidence to support the theoretical framework of the halide electronegativity effect.”\nSupplementary Figure 73. Schematic diagram of hydrogen bond donors (from organic cations) and hydrogen bond acceptors (from luminescent centers).\nSupplementary Figure 74. a Photos of (EOTP) MnCl Br (1) (x = 0−4) single crystals under natural and ultraviolet light. b The powder X-ray diffraction patterns of (EOTP) MnCl Br (1) (x = 0−4) are compared with the simulated X-ray diffraction patterns. c The photoluminescence spectra of (EOTP) MnCl Br (1) (x = 0−4).\nReviewer #3:\nThis manuscript presents a comprehensive study on wide range of Mn halides exhibiting FML, PL, and RL, contributing insights into multimodal luminescent materials. Furthermore, this work tried to elucidate mechanism in Mn halide systems based on the FML, PL properties. However, to further strengthen the manuscript, several mechanistic aspects and relationship with spectroscopic evidence should be clarified. The author should carefully connect the FML phenomenon with possible influence of fractured surface area, which could be electric field, under the specific stress. Based on this, the role of Young′s modulus should be demonstrated in FML process. Establishing the correlation between the structural characteristics of each complex and their associated kinetics could be important for elucidating the fundamental principles underlying FML, which should include the connection between photoexcited luminescence and FML induced by fracture-generated electric fields. To meet the high standards and authoritative quality of Nature Communications, we recommend publication only after these major issues have been thoroughly addressed.\nResponse: Thank you very much for the constructive comments. The comments concerning several mechanistic aspects and their relationship with spectroscopic evidence have been summarized as below:\n1) The fracto-mechanoluminescence (FML) phenomenon with possible influence of fractured surface area (Fig. 3c, Fig. 3d, and Fig. 3e): We used an ultrahigh-speed camera (4096×2160 pixels, maximum frame rate: 1000 fps, minimum exposure time: 5 μs) and a universal measuring machine to study the FML behavior of Mn halide single crystals during fracture. In (2ClBTP) MnCl crystals, small crystals (1−6 mm) exhibited progressively stronger FML with increasing fracture area. When the crystal size increased to 8−10 mm, the presence of twins and interfacial defects caused fractures to occur preferentially at defect sites, which reduced the effective fracture area and weakened the FML intensity. These results indicate that FML intensity is primarily determined by the effective fracture area that contributes to light emission. In (EOTP) MnBr (1) crystals, the FML intensity showed an approximately linear dependence on applied stress in the range of 10−30 N. Moreover, for crystals of the same size, FML intensity increased significantly with the number of fracture surfaces. Overall, these findings demonstrate that larger external forces can induce larger effective fracture areas, thereby enhancing the FML intensity.\n2) The role of low Young′s modulus (Supplementary Figures 48-55, Fig. 3f, and Fig. 3g): Mn halide crystals with a Young′s modulus below 10000 MPa generally exhibit pronounced FML, indicating that a low Young′s modulus is a prerequisite for FML generation. Piezoelectric force microscopy (PFM) measurements further reveal that, under the same bias voltage, crystals with smaller Young′s modulus display larger PFM amplitudes, suggesting stronger local electro-mechanical coupling. Enhanced elastic flexibility facilitates the formation of local polarization and internal electric fields, thereby promoting FML. In addition, the fracture force-FML intensity experiments conducted on crystals with different Young′s modulus ((2ClBTP) MnCl (9299.52 MPa), (ETP) MnCl (4859.07 MPa), (ETP) MnBr (2974.09 MPa), (EOTP) MnBr (1) (2718.67 MPa), (MTP) MnCl (1102.19 MPa) and (4ClBTP) MnCl (177.51 MPa)) further demonstrated the multi-level regulation effect of Young′s modulus (Figs. 3f and 3g). Young′s modulus not only defines the stress threshold for FML generation but also governs the applicable stress window of the crystal: crystals with lower Young′s modulus can sustain a broader stress range, whereas those with higher Young′s modulus are prone to fracture under relatively lower stresses.\nIn summary, the role of Young′s modulus in FML is not singular but rather a multifaceted, integrated effect encompassing triggering conditions, intensity regulation, and the applicable threshold and stress range.\n3) The fundamental principles underlying FML (Fig. 3h): Based on PFM and thermoluminescence (TL) studies, we propose the following mechanism: in Mn halide single crystals with low Young′s modulus, applied stress induces local polarization and internal electric fields, which promote carrier trapping and filling of trap states. When the fracture process begins, the strong electric field caused by the fracture leads to the swift release and recombination of trapped carriers. The energy released during carrier recombination excites Mn2+ ions through radiative or non-radiative processes, triggering the Mn2+ d−d transition (4T →6A ) and generating intense FML.\n1 1\nRevision: The revisions are documented comprehensively and presented alongside the respective responses to the comments.\nComment 1. As discussed on page 12, lines 10−11, the authors suggest that the breakage of hydrogen bonds during fracture induces interfacial electric fields by releasing charges. If the number of broken bonds is proportional to the generated electric field, could the observed increase in FML intensity with applied mechanical stress (Fig. 3e), at constant crystal size, be attributed to an increase in fracture surface area, or is it a consequence of low Young′s modulus enhancing the efficiency of electric field generation? The authors are encouraged to clarify the role of Young′s modulus.\nResponse: The additional experiments suggest that the stress-induced enhancement of FML intensity is primarily associated with the enlargement of the fracture surface area (Supplementary Figures 48-55, Fig. 3f, and Fig. 3g).\nThrough systematic and repeated experiments, we have clarified the multi-level regulatory effect of Young′s modulus on FML performance. First, AFM measurements reveal that Mn halides with a Young′s modulus below 10000 MPa exhibit distinct FML, indicating that reduced structural rigidity is essential for efficient FML generation. As shown in Supplementary Figures 48-55, the PFM measurements further reveal that, under the same bias voltage, crystals with smaller Young′s modulus display larger PFM amplitudes, suggesting stronger local electro-mechanical coupling. Enhanced elastic flexibility facilitates the formation of local polarization and internal electric fields, thereby promoting FML. In addition, the fracture force-FML intensity experiments conducted on crystals with different Young′s modulus ((2ClBTP) MnCl (9299.52 MPa), (ETP) MnCl (4859.07 MPa), (ETP) MnBr (2974.09 MPa), (EOTP) MnBr (1) (2718.67 MPa), (MTP) MnCl (1102.19 MPa) and (4ClBTP) MnCl (177.51 MPa)) further demonstrated the multi-level regulation effect of Young′s modulus (Figs. 3f and 3g). Young′s modulus not only defines the stress threshold for FML generation but also governs the applicable stress window of the crystal: crystals with lower Young′s modulus can sustain a broader stress range, whereas those with higher Young′s modulus are prone to fracture under relatively lower stresses.\nIn summary, the role of Young′s modulus in FML is not singular but rather a multifaceted, integrated effect encompassing triggering conditions, intensity regulation, and the applicable threshold and stress range.\nRevision: In the revised manuscript, we added a description of the role of Young′s modulus.\n“The results show that the Young′s modulus of Mn halides with explicit FML characteristics is generally lower than 10000 MPa; while for crystals with a Young′s modulus higher than 10000 MPa, no obvious FML phenomenon was observed (Supplementary Figures 29-46, Table 1). This implies that reduced structural rigidity, as reflected by a lower Young′s modulus, is a prerequisite for enabling efficient FML in these materials.”\n“In addition, by comparing the Young′s moduli of (2ClBTP) MnCl (9299.52 MPa), (4ClBTP) MnCl (177.51 MPa), and (EOTP) MnBr (1) (2718.67 MPa), it can be found that under the same bias voltage, the smaller the Young′s modulus, the larger the PFM amplitude. This trend reveals the intrinsic connection between the elastic stiffness of the crystal and the local electro-mechanical coupling effect, indicating that regulating the Young′s modulus may be an effective way to optimize the FML behavior.”\n“The role of Young′s modulus in FML requires further clarification. In Fig. 3f, as the Young′s modulus of (2ClBTP) MnCl (9299.52 MPa), (ETP) MnCl (4859.07 MPa), (ETP) MnBr (2974.09 MPa), (EOTP) MnBr (1) (2718.67 MPa), (MTP) MnCl (1102.19 MPa) and (4ClBTP) MnCl (177.51 MPa) decreases, their FML intensity does not show a significant regular change, although low Young′s modulus facilitates the generation of local piezoelectric fields. This suggests that FML behavior is governed by multiple factors. Considering fracture stress together with the first FML response (Fig. 3g), a general positive correlation between FML intensity and fracture force is evident. Those “perfect” crystals with fewer defects require greater force to break and emit bright FML, while “imperfect” crystals with more defects fracture at lower forces and display weaker FML. In addition, we find the Young′s modulus mainly affects the tolerable stress range of the crystals, which means the “slope” of the force-FML intensity relationship. This underlines the significant impact of Young′s modulus on the FML mechanism. In summary, Young′s modulus exerts a multifaceted influence on FML, simultaneously governing its initiation conditions, the corresponding threshold and stress window, thereby reflecting an integrated and multi-level effect.”\nSupplementary Figure 48. Piezoelectric force microscopy of (BTP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 49. Piezoelectric force microscopy of (BTP) MnBr single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 50. Piezoelectric force microscopy of (2ClBTP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 51. Piezoelectric force microscopy of (4ClBTP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 52. Piezoelectric force microscopy of (EOTP) MnBr (1) single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 53. Piezoelectric force microscopy of (MTP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 54. Piezoelectric force microscopy of (ETP) MnCl single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nSupplementary Figure 55. Piezoelectric force microscopy of (ETP) MnBr single crystal. a Piezoelectric amplitude 1 under different bias voltages. b Piezoelectric amplitude 2 under different bias voltages. c Topography image. d Amplitude butterfly loop.\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. f Fracto-mechanoluminescence intensity of Mn halides with different Young′s modulus.\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. g Relationship between fracto-mechanoluminescence intensity and fracture force of Mn halides.\nComment 2. The electric field generated upon fracture should ideally be normalized to the surface area at the fracture interface. As surface area significantly affects the local electric field strength even under the same applied force, it would be valuable if the authors could attempt to quantify this relationship or provide insight into how the fractured surface area is correlated with FML intensity. If author establish a common condition for surface effects, a clear trend between FML and Young′s modulus can be identified.\nResponse: In our study, ultrahigh-speed camera imaging (4096×2160 pixels, maximum frame rate: 1000 fps, minimum exposure time: 5 μs) combined with stress-intensity analysis suggests that the FML intensity is closely associated with the effective fracture surface area. The FML intensity increases with fracture area for (2ClBTP) MnCl crystals of 1−6 mm, confirming that enlarged fracture surfaces directly enhance emission. For larger crystals (8−10 mm), this correlation is weakened due to twins and interfacial defects that reduce the effective fracture area. Similar trends observed in (EOTP) MnBr (1) further support the view that the effective fracture area plays a central role in governing FML intensity, while structural defects may suppress emission by limiting the usable surface area (Fig. 2c, Fig. 3c, Fig. 3d, Fig. 3e, and Supplementary Video 2).\nRevision: We have added a discussion of the relationship between fracture surface area and FML intensity in the revised manuscript.\n“As shown in Fig. 2c and Supplementary Video 2, we used an ultrahigh-speed camera to observe the FML behavior of Mn halide single crystals under stress in real time. Our results show that FML occurs at the exact moment of irreversible fracture of the crystals, and its intensity is positively correlated with the effective crack surface area.”\n“To quantify the effect of fracture area on FML intensity, we counted the number of fractured crystals and normalized the initial crystal size. This allowed us to evaluate the relationships among FML intensity, fracture area, applied stress, and Young′s modulus. In Fig. 3c, the FML intensity of (2ClBTP) MnCl single crystals increases progressively with crystal size in the 1−6 mm range. Since fracture area scales with initial size for crystals of similar shape and comparable numbers of fractures, this result indicates that the effective fracture area is the direct factor enhancing FML intensity. However, when the crystal size increased to 8−10 mm, this positive correlation no longer holds. This can be attributed to the fact that larger crystals are more prone to forming twins and interfacial defects, where fractures tend to occur preferentially. As a result, they absorb less energy during compression, leading to reduced FML intensity. These findings clearly demonstrate that enlargement of the effective fracture area is the primary factor governing FML enhancement. As shown in Fig. 3d, a correlation between the number of fractured crystals and FML intensity is observed for similarly sized (EOTP) MnBr (1) single crystals. Moreover, within the stress range of 10−30 N, the FML spectra of (EOTP) MnBr (1) exhibited an approximately linear dependence of intensity on applied stress as the crystals gradually fracture (Fig. 3e). This highlights the high sensitivity of FML to external mechanical stimuli.”\nFig. 2 | Mechanical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. c Fracto-mechanoluminescence process of (EOTP) MnBr (1) single crystal under ultrahigh-speed camera (4K at up to 1000 fps).\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. c Fracto-mechanoluminescence intensity of (2ClBTP) MnCl single crystals with different sizes.\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. d Relationship between fracture area and luminescence intensity of (EOTP) MnBr (1) single crystal.\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. e Fracto-mechanoluminescence spectra of (EOTP) MnBr (1) under different applied mechanical forces.\nComment 3. The absorption and PL spectra shown in Supplementary Figures 50 and 51 indicate that some Mn halide samples exhibit strong absorption bands in the long-wavelength region (~700 nm), while the PL emission is centered at shorter wavelengths (~515 nm). The authors should explain the energy transfer mechanism responsible for this apparent anti-Stokes-type emission. Is this related to specific ligand-to-metal charge transfer or crystal-field splitting? Furthermore, how does this photophysical behavior connect to the observed FML response?\nResponse: We consider that the energy transfer mechanism underlying this apparent anti-Stokes emission may be associated with crystal field splitting. At the same time, it should be noted that this photophysical behavior has not yet been directly correlated with the observed FML response (Supplementary Figure 67).\nTo clarify whether there is an anti-Stokes process between this long-wave absorption and short-wave emission, we examined the PL spectra of (BTP) MnBr , (EOTP) MnBr (1) and (EOTP) MnBr (2) under 270 nm and 700 nm excitation, respectively. The results show that only a very weak emission signal can be obtained under 700 nm excitation, with an intensity close to the background noise. However, after normalization, its emission peak position remains consistent with that under 270 nm excitation, showing an apparent anti-Stokes emission characteristic. Considering that (EOTP) MnBr (1) does observe a clear thermoluminescence spectra under X-ray excitation, this suggests that additional energy storage and re-release processes may be involved in the system. Therefore, the energy transfer mechanism between long-wave absorption and short-wave emission is probably not a direct anti-Stokes transition in the traditional sense, but is more likely to be closely related to the charge transfer state coupling from ligand to metal, the crystal field splitting effect, and the excitation-emission process assisted by thermal energy release. FML was not observed in (BTP) MnBr , indicating that this photophysical behavior is irrelevant to the FML response.\nRevision: We have added relevant instructions in the revised Support Information.\n“In the absorption spectra shown in Supplementary Figure 65, (BTP) MnBr , (EOTP) MnBr (1) and (EOTP) MnBr (2) exhibit pronounced absorption bands in the long-wavelength region of 650−750 nm, while their PL emission centers are located in the shorter-wavelength range of 510−530 nm. To clarify whether this long-wavelength absorption and short-wavelength emission involve an anti-Stokes process, we measured the PL spectra of them under 270 nm and 700 nm excitation. Under 700 nm excitation, only a very weak emission signal was observed, with intensity close to the background noise. However, after normalization, the emission peak position coincided with that under 270 nm excitation, giving rise to an apparent anti-Stokes-like feature (Supplementary Figure 67). Considering that (EOTP) MnBr (1) indeed shows clear TL peaks under X-ray excitation, it is likely that additional energy storage and release processes are involved. Therefore, the observed long-wavelength absorption and short-wavelength emission are unlikely to arise from a conventional anti-Stokes transition. Instead, they may originate from a more complex energy-transfer mechanism, possibly involving ligand-to-metal charge-transfer states, crystal-field splitting effects, and thermally assisted excitation-emission pathways.”\nSupplementary Figure 67. Photoluminescence spectra of a (BTP) MnBr , b (EOTP) MnBr (1), c (EOTP) MnBr (2) under excitation of 270 and 700 nm. Normalized photoluminescence spectra of d (BTP) MnBr , e (EOTP) MnBr (1), f (EOTP) MnBr (2) under excitation of 270 and 700 nm.\nComment 4. On page 17, line 10, the authors suggest the long PL decay times (0.07−3.31 ms) solely to spin-forbidden d−d transitions of Mn2+ ions. However, such a wide lifetime range could also result from other mechanisms, including indirect bandgap transitions or trap-mediated processes. The authors should discuss whether d−d transitions alone sufficiently explain the lifetime behavior across all samples, and if not, what role other relaxation pathways might play. What causes the variation in each of the observed decay times, and how do these differences influence the FML behavior?\nResponse: We consider that the d−d transitions of Mn2+ may provide a reasonable explanation for the lifetime behavior observed in all samples (Fig. 5g, Fig. 5h, and Supplementary Figure 71).\nWe re-examined all samples on the same equipment and refitted the PL decay curves to correct some values with large deviations. As shown in Supplementary Figure 71, the PL decay times after re-measurement ranges from 0.34 to 3.40 ms. The PL decay times of (HMTP) MnCl ·H O and (BTP) MnCl ·H O are 0.34 and 0.36 ms respectively. (HMTP) MnCl ·H O and (BTP) MnCl ·H O containing free water molecules exhibit shorter PL decay times. The main reason is that the vibration frequency of the −OH group in the water molecule is very high (~3000-3600 cm⁻1), which easily undergoes multi-phonon coupling with the energy of the excited state. This high-energy vibrational mode significantly enhances the non-radiative relaxation process, thereby shortening the PL decay times. The PL decay times of (BTP) MnBr , (EOTP) MnBr (1) and (EOTP) MnBr (2) are 0.38, 0.49 and 0.43 ms, respectively. In comparison, the PL decay times of the other manganese chloride samples are all over 1 ms. This is because Cl⁻ has a higher electronegativity and a smaller radius than Br⁻, resulting in a shorter Mn−Cl bond than the Mn−Br bond. The stronger crystal field of [MnCl ]2⁻ increases the d-level splitting, reducing the probability of radiative transitions from the excited state and the nonradiative relaxation rate, resulting in a significantly longer PL decay time than [MnBr ]2⁻.\nOur studies show that FML intrinsically relies on the coupled process of carrier capture in traps and their release by fracture. Therefore, Mn halides with longer PL decay times generally correspond to more efficient FML responses, as carriers spend longer in traps and are more responsive to the fracture-induced local electric field.\nRevision: We have provided an in-depth discussion of the broad lifetime range of Mn halides in the revised manuscript.\n“Different from the other metal halides, the fluorescence decay time of synthesized Mn halides range from 0.34 to 3.40 ms, with the long PL decay lifetime attributed to the spin-forbidden nature of d−d transitions rather than the self-trapped emissions (Supplementary Figure 71).57, 58 (HMTP) MnCl ·H O (0.34 ms) and (BTP) MnCl ·H O (0.36ms) containing free water molecules exhibit shorter PL decay lifetimes. This is mainly due to the high vibration frequency of the −OH in the water molecule (3000−3600 cm−1), which easily undergoes multi-phonon coupling with the energy of the excited state. Such high-energy vibrational modes significantly enhance the non-radiative relaxation process, thereby shortening the PL decay lifetime. The PL decay times of ((BTP) MnBr , (EOTP) MnBr (1) and (EOTP) MnBr (2) are 0.38, 0.49, and 0.43 ms, respectively. In comparison, the PL decay lifetimes of the other manganese chloride samples all exceed 1 ms. This is because Cl− has a higher electronegativity and a smaller radius than Br−, resulting in a shorter Mn−Cl bond than the Mn−Br bond. The stronger crystal field of [MnCl ]− leads to a larger d-level splitting, reducing the probability of radiative transitions from the excited state and the nonradiative relaxation rate, resulting in significantly longer PL decay times than [MnBr ]−. Our research demonstrates that FML fundamentally relies on the coupled process of carrier capture and release in traps. Therefore, Mn halides with longer PL decay lifetimes generally correspond to more efficient FML responses.”\n“(EOTP) MnBr (1) exhibits significantly different luminescence dynamics under different excitation conditions. Under UV light excitation, its PL decay lifetime is 0.49 ms, which conforms to the typical first-order exponential decay model. Under high-energy X-ray excitation, its RL decay lifetime is significantly extended to 1.35 ms, indicating that the radiative recombination path is more complicated and involves multiple electronic transition processes, so a double exponential function is required for fitting ( = 0.53 ms,  = 1.71 ms) (Fig. 5g). As shown in Fig. 5h, two obvious luminescence peaks appear in the TL spectrum of (EOTP) MnBr (1) under X-ray excitation, located at 155 K and 219 K, respectively, corresponding to trap states of two different depths. These trap states release trapped electrons during heating and then undergo radiative recombination, which significantly affects the luminescence process under X-ray excitation. This result is consistent with the double exponential characteristics of the RL decay lifetime, indicating that some electrons can recombine quickly (corresponding to  ), while the other part undergoes delayed recombination after being released through the trap state (corresponding to  ).”\nSupplementary Figure 71. The photoluminescence decay curves of the synthesized Mn halides.\nFig. 5 | Scintillation properties of synthesized Mn halides. g The radioluminescence decay curve of (EOTP) MnBr (1) at room temperature. h X-ray excited thermoluminescence spectrum of (EOTP) MnBr (1) measured at a heating rate of 1 K s−1.\nComment 5. While most of the emission characterization (PL, RL) focuses on photo-excited pathways, FML is attributed to field-induced excitation via mechanical fracture. The authors are encouraged to elaborate on how the optical behavior observed under light excitation or X-ray excitation correlates mechanistically with the FML process. Establishing a clear link between photoluminescence properties and electrically-driven FML activation would strengthen the theoretical foundation and broaden the impact of this work.\nResponse: The PL lifetime (single-exponential decay) and RL decay behavior (bi-exponential decay) clearly demonstrate that the luminescence of Mn halides primarily originates from spin-forbidden d−d transitions of Mn2+ ions in tetrahedral coordination environments. Thermoluminescence (TL) spectra further reveal that energy traps are commonly present in the crystal structures of these materials, which can store charge carriers over extended periods following photo- or X-ray irradiation. During mechanical fracture, the instantaneous generation of cracks induces local lattice distortions, which in turn create strong localized electric fields. These fields can effectively activate the trap states, releasing the stored carriers. The released electrons or holes subsequently migrate to Mn2+ emission centers and recombine radiatively via the same spin-forbidden d−d transition pathway as in PL or RL. Thus, although FML is driven by mechanically induced electric fields, PL by photon excitation, and RL by high-energy radiation, all three processes converge on the same emission channel, namely the Mn2+ d−d radiative transition. This connection indicates that the luminescence mechanism of Mn halides is essentially unified: trap-state dynamics govern the storage and release of carriers, while the spin-forbidden d−d transitions of Mn2+ ions constitute the ultimate emission pathway under all excitation modes. It is precisely this “trap state, local electric field, Mn2+ emission” coupling mechanism that enables PL, RL, and FML to exhibit comparable optical behaviors under different conditions, thereby providing a solid physical basis for understanding the nature of mechanically induced luminescence (Fig. 3h, Fig. 5g, and Fig. 5h).\nRevision: We have added relevant descriptions in the revised manuscript.\n“Based on PFM and TL studies, we propose a refined mechanism (Fig. 3h). In Mn halide single crystals with low Young′s modulus, applied stress induces local polarization and internal electric fields, which promote carrier trapping and filling of trap states. When the fracture process begins, the strong electric field caused by the fracture leads to the swift release and recombination of trapped carriers. The energy released during carrier recombination excites Mn2+ ions through radiative or non-radiative processes, triggering the Mn2+ d−d transition (4T →6A ) and generating intense FML. In this process, the relatively low Young′s modulus not only facilitates the formation of controllable local stress and electric fields under mechanical load but also promotes efficient carrier transitions to the Mn2+ 4T level, resulting in high-intensity FML.”\n“(EOTP) MnBr (1) exhibits significantly different luminescence dynamics under different excitation conditions. Under UV light excitation, its PL decay lifetime is 0.49 ms, which conforms to the typical first-order exponential decay model. Under high-energy X-ray excitation, its RL decay lifetime is significantly extended to 1.35 ms, indicating that the radiative recombination path is more complicated and involves multiple electronic transition processes, so a double exponential function is required for fitting ( = 0.53 ms,  = 1.71 ms) (Fig. 5g). As shown in Fig. 5h, two obvious luminescence peaks appear in the TL spectrum of (EOTP) MnBr (1) under X-ray excitation, located at 155 K and 219 K, respectively, corresponding to trap states of two different depths. These trap states release trapped electrons during heating and then undergo radiative recombination, which significantly affects the luminescence process under X-ray excitation. This result is consistent with the double exponential characteristics of the RL decay lifetime, indicating that some electrons can recombine quickly (corresponding to  ), while the other part undergoes delayed recombination after being released through the trap state (corresponding to  ).”\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. h Schematic illustration of the fracto-mechanoluminescence mechanism in Mn halides.\nFig. 5 | Scintillation properties of synthesized Mn halides. g The radioluminescence decay curve of (EOTP) MnBr (1) at room temperature. h X-ray excited thermoluminescence spectrum of (EOTP) MnBr (1) measured at a heating rate of 1 K s−1."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors addressed all comments/questions arised by Reviewers. The manuscript can be now published as is.", "reviewer_2": "The authors have carried out an extensive set of experiments and have addressed the majority of the concerns raised in the previous review round. I commend their efforts and the breadth of the data presented, which substantially improve the manuscript. However, one critical issue still remains, and I recommend that acceptance should be considered only after this point is satisfactorily clarified.\nThe authors claim that FML is governed by Young’s modulus. While the additional experiments support the view that a lower Young’s modulus enhances local electromechanical coupling and facilitates polarization upon fracture, direct evidence for this mechanism remains insufficient. The fundamental origin of FML is the polarization generated at the fracture interface, which induces the electric field that drives luminescence. Even in materials with a Young’s modulus greater than 10,000 MPa, fracture should still create polarization at crack surfaces. Why, then, is no FML observed in these cases?\nIf FML is truly controlled by Young’s modulus, then Young’s modulus must directly regulate the strength of the polarization electric field. In such a scenario, one would expect a more systematic, possibly linear correlation between Young’s modulus and FML intensity, which is not evident in the current data. At present, the authors provide evidence of surface potential variations with modulus, but the actual electric field generated at the moment of fracture has not been demonstrated.\nEstablishing this link is crucial, as local potential differences may modulate the magnitude of the electric field, but they should not fundamentally suppress the electron–hole polarization that arises from bond rupture during fracture. Clarifying this mechanism would significantly strengthen the central claim that Young’s modulus governs FML, and it is essential before the manuscript can be considered for publication. I understand that this may be a challenging task, but it needs to be resolved in order to rule out additional possible contributions arising from chemical bond rupture or compositional effects. Only then can the authors convincingly claim that the observed FML is truly dependent on Young’s modulus.", "reviewer_3": "The authors have carried out extensive experiments and presented reasonable results to clarify the mechanism of FML. I believe that the manuscript could be considered for publication once one major point is adequately addressed.\nThe authors claim that FML is governed by Young’s modulus, and specifically argue that a lower Young’s modulus strengthens the polarization electric field. This interpretation may be valid in cases where lower modulus leads to larger fracture surfaces; however, if the fracture surface area is the same, can Young’s modulus alone account for the observed enhancement of FML?\nAs the authors themselves describe, polarization is generated by the breaking of hydrogen bonds during fracture. If the fracture surface area is the same, the number of broken bonds—and hence the amount of separated charges—should also be the same, independent of Young’s modulus. In this case, the polarization electric field generated at the interface should be of the same magnitude. Since the electric field strength is fundamentally determined by the amount of separated charges, it seems difficult to attribute the enhancement solely to Young’s modulus.\nTherefore, rather than directly governing the generation of the polarization electric field, the role of Young’s modulus may lie in providing a synergistic contribution under the fracture-induced electric field. For example, a lower modulus could facilitate carrier transport or radiative recombination at emission centers by lowering energetic barriers. Clarifying this possible synergistic effect would provide a stronger and more convincing rationale. Since the central claim of the manuscript is the correlation between Young’s modulus and FML, explicitly addressing this aspect would significantly strengthen the manuscript."}, "author_response": "We are grateful for the reviewer′s recognition of our revised manuscript and for the valuable suggestions offered in the second round of review.\n(1) Clarification of ambiguities in the previous manuscript: We do sincerely apologize for the misunderstanding raised by the previous statements. Considering the experimental results and the similar comments raised by Reviewer #3, we would like to clarify that the role of Young′s modulus lies in providing a synergistic contribution under the fracture-induced electric field, rather than directly governing the generation of the polarization field. The detailed mechanism discussion will be presented in section (4).\n(2) Absence of FML in crystals with a Young′s modulus greater than 10000 MPa: The occurrence of FML depends not only on the presence of fracture-induced polarization but also on the mechanical softness of the lattice and the conditions for effective trap activation. As shown in Supplementary Table 51, we investigated the piezoelectric constants d of Mn halide single crystals with different Young′s modulus, including (2ClBTP) MnCl (9299.52 MPa), (ETP) MnCl (4859.07 MPa), (ETP) MnBr (2974.09 MPa), and (4ClBTP) MnCl (177.51 MPa). The results show that the piezoelectric constants d is affected by the elastic modulus, and the piezoelectricity is almost not detected in the PFM experiment in crystals with elastic modulus greater than 10000 MPa, such as (BTP) MnBr , (BTP) MnCl . A softer lattice (lower Young′s modulus) allows greater ionic displacement, leading to stronger piezoelectric polarization. A low Young′s modulus allows for greater lattice deformation, providing a prerequisite for the storage of energy during compression which is released during fragmentation. Consequently, in crystals with high Young′s modulus, even though transient polarization may occur at the fracture site, the resulting FML could be too weak to be detected due to the limited elastic energy, weak local piezoelectric response.\n(3) Relationship between Young′s modulus and the strength of the polarization electric field: We do also agree that it would be ideal to directly characterize the magnitude and temporal profile of the local electric field generated at the moment of fracture. However, the electric field upon fracture is instant and too hard to be detected. We have also tried our best to reach out to different research groups, but no suitable equipment was available. Hereby, we consider measuring the piezoelectric field before fracture as an alternative. If the electric field at the moment of fracture comes from the uneven breakage of the chemical bond, then the positive and negative charge centers should not coincide before the break, and the polarization caused by the piezoelectric field is likely to be one of the reasons. As shown in Supplementary Table 51, the piezoelectric constants d of (2ClBTP) MnCl , (ETP) MnCl , (ETP) MnBr and (4ClBTP) MnCl single crystals are 1.0, 1.1, 1.3, and 1.4 pC N−1, respectively. The piezoelectric polarization reflects the ability of a material to generate polarization under mechanical stress, where the driving force arises from the applied stress. A smaller Young′s modulus, corresponding to a softer lattice, facilitates ionic displacement and thus enhances piezoelectric polarization. On the other hand, it looks like FML also involves trap assistance. As shown in Supplementary Figure 56, we additionally measured the TL spectra obtained at low temperature after trap filling induced by mechanical grinding. The results further confirm that the piezoelectric polarization induced by stress stimuli can excite electrons into the trap states. These trap states act as energy storage reservoirs, releasing trapped electrons to the emission centers during fracture. Obviously, the FML intensity is affected by electron-photon processes, such as carrier migration and recombination efficiency, which results in intensity not directly related to Young′s modulus. Consequently, the combined evidence from piezoresponse force microscopy (PFM), piezoelectric constants d measurements, and thermoluminescence (TL) analyses provides compelling indirect support for the synergistic role of Young′s modulus in modulating FML.\nSupplementary Table 51. Piezoelectric constants of Mn halide single crystals.\nⅡ Ⅲ\nΙ Average\nYoung′s\nd d\n33 33\nmodulus\nCompound d d\n33 33\n(pC (pC (MPa)\n(pC N−1) (pC N−1)\nN−1) N−1)\n(2ClBTP) MnCl 1.0 1.1 1.0 1.0 9299.52\n2 4\n(ETP) MnCl 1.1 1.1 1.2 1.1 4859.07\n2 4\n(ETP) MnBr 1.3 1.3 1.3 1.3 2974.09\n2 4\n(4ClBTP) MnCl 1.4 1.5 1.4 1.4 177.51\n2 4\n(4) Mechanism of FML: The consistency among the PL, RL, and FML spectra indicates that these emission processes should originate from the same d−d transition of Mn2+ ions. The differences should be their excitation pathways. As shown in Supplementary Figure 56, we additionally measured the TL spectra obtained at low temperature after trap filling induced by mechanical grinding. The results further confirm that mechanical grinding can achieve the charging effect of the carriers into the traps through piezoelectric effect, thus the thermoluminescence performance can be triggered by increasing the temperature. Based on the combined AFM, PFM and TL results, we consider the FML process can be divided into three consecutive stages: a) Prior to Crystal Fracture: In Mn halide single crystals with relatively low Young′s modulus, the applied stress readily induces local piezoelectric polarization, thereby promoting carrier trapping and trap filling; b) At the Moment of Fracture: Upon fracture, the carriers stored in the trap are released and recombination; c) After Crystal Fracture: The released energy subsequently excites Mn2+ ions through radiative or nonradiative pathways, triggering the d−d transition (4T →6A ) and producing observable FML. A lower Young′s modulus allows the crystal to store more energy from the piezoelectric field, thus enhancing carrier mobility and facilitating radiative recombination at luminescent centers.\nSupplementary Figure 56. Mechanical grinding excited thermoluminescence spectrum of a (2ClBTP) MnCl , b (4ClBTP) MnCl , c (EOTP) MnBr (1), d (MTP) MnCl , e (ETP) MnCl , and f (ETP) MnBr .\n2 4 2 4 2 4 2 4 2 4 2 4\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. h Schematic illustration of the fracto-mechanoluminescence mechanism in Mn halides.\nCollectively, the synergistic interplay among elastic stiffness, local electromechanical coupling, and trap-state dynamics governs the FML process, highlighting the crucial regulatory role of Young′s modulus in its intrinsic mechanism.\nRevision:\n(1) The manuscript title has been modified to more accurately capture the interplay between Young′s modulus and FML, thereby eliminating potential ambiguity.\n“Observation of Fracto-mechanoluminescence by the Synergistic Effect of Young′s Modulus in Manganese Halides”\n(2) The discussion of the correlation between Young′s modulus and FML in the Abstract has been revised for greater clarity and coherence.\n“We demonstrate that the occurrence of FML arises from the synergistic interplay among the crystal′s elastic stiffness, local electromechanical coupling, and trap states, which collectively activate Mn2+ d−d transitions upon fracture.”\n(3) The explanation of the relationship between Young′s modulus and FML in the Introduction has been expanded and improved.\n“The results indicate that Young′s modulus is associated with local piezoelectric response at the microscopic level and also affects the fracture threshold and stress range of the crystal. Trap states act as intermediate states that facilitate energy storage and release during the compression induced fracture process, while the FML intensity is correlated with the effective fracture area.”\n(4) The Results and Discussion section has been substantially refined to offer a more comprehensive and precise interpretation of how Young′s modulus influences the FML mechanism.\n“The piezoelectric constants d of (2ClBTP) MnCl , (ETP) MnCl , (ETP) MnBr and (4ClBTP) MnCl single crystals are 1.0, 1.1, 1.3, and 1.4 pC N−1 (Supplementary Table 51). This trend underscores the synergistic interplay between lattice softness and local electromechanical coupling. Piezoelectric polarization reflects the material′s ability to generate an electric polarization under mechanical stress, with the driving force originates from the applied mechanical load. A lower Young′s modulus, corresponding to a softer lattice, facilitates ionic displacement, thereby enhancing piezoelectric polarization.”\nSupplementary Table 51. Piezoelectric constants of Mn halide single crystals.\nⅡ Ⅲ\nΙ Average\nYoung′s\nd d\n33 33\nmodulus\nCompound d d\n33 33\n(pC (pC (MPa)\n(pC N−1) (pC N−1)\nN−1) N−1)\n(2ClBTP) MnCl 1.0 1.1 1.0 1.0 9299.52\n2 4\n(ETP) MnCl 1.1 1.1 1.2 1.1 4859.07\n2 4\n(ETP) MnBr 1.3 1.3 1.3 1.3 2974.09\n2 4\n(4ClBTP) MnCl 1.4 1.5 1.4 1.4 177.51\n2 4\n“Furthermore, as shown in Supplementary Figure 56, the thermoluminescence (TL) spectra obtained under mechanical grinding excitation reveals the existence of trap states in (2ClBTP) MnCl , (4ClBTP) MnCl , (EOTP) MnBr (1), (MTP) MnCl , (ETP) MnCl , and (ETP) MnBr . The crystals were crushed at 77 K, it shows that the piezoelectric polarization induced by stress stimuli can excite electrons into the trap states. These trap states act as energy storage reservoirs, releasing stored electrons to emission centers during fracture. In contrast, (BTP) MnCl , which possesses an extremely high Young′s modulus (184890.81 MPa), does not exhibit FML behavior, implying that excessive lattice rigidity suppresses the mechanical activation of trap states. In summary, the activation of FML requires not only the sufficient mechanical flexibility but also the presence of trap states, which enables efficient trap activation under mechanical stimuli, thereby driving the luminescence process.”\nSupplementary Figure 56. Mechanical grinding excited thermoluminescence spectrum of a (2ClBTP) MnCl , b (4ClBTP) MnCl , c (EOTP) MnBr (1), d (MTP) MnCl , e (ETP) MnCl , and f (ETP) MnBr .\n2 4 2 4 2 4 2 4 2 4 2 4\n“Based on the combined PFM and TL results, we propose a refined mechanism (Fig. 3h). In Mn halide single crystals with relatively low Young′s modulus, applied stress induces local piezoelectric polarization that facilitates carrier trapping and the filling of trap states. Upon fracture, the carriers stored in the traps are released and recombine, and the released energy subsequently excites Mn2+ ions through radiative or nonradiative channels, resulting in the characteristic Mn2+ d−d transition (4T →6A ) and producing pronounced FML. Obviously, the FML intensity is affected by electron-photon processes, such as carrier migration and recombination efficiency, which results in intensity not directly related to Young′s modulus. A lower Young′s modulus allows the crystal to store more energy from the piezoelectric field, thereby enhancing carrier mobility and facilitating radiative recombination at luminescent centers. This cooperative interaction among elastic stiffness, local electromechanical coupling, and trap dynamics collectively drives the FML process, underscoring that Young′s modulus serves a modulatory and synergistic, rather than a purely deterministic, role in the underlying mechanism.”\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. h Schematic illustration of the fracto-mechanoluminescence mechanism in Mn halides.\n(5) The Conclusions section has been rewritten to better emphasize and summarize the synergistic role of Young′s modulus in influencing FML behavior.\n“Our investigations demonstrate that the occurrence of FML in Mn halides originates from the synergistic interplay among elastic stiffness, local electromechanical coupling, and trap-state dynamics. The interplay between lattice softness and local electro-mechanical coupling enables efficient trap activation and carrier recombination upon fracture.”\nAll in all, we have made every effort to improve the manuscript to the best of our ability. Based on your suggestions and those of Reviewer 3, we have revised and refined the paper. We hope that the revised version could now be accepted.\nReviewer #3:\nThe authors have carried out extensive experiments and presented reasonable results to clarify the mechanism of FML. I believe that the manuscript could be considered for publication once one major point is adequately addressed.\nThe authors claim that FML is governed by Young′s modulus, and specifically argue that a lower Young′s modulus strengthens the polarization electric field. This interpretation may be valid in cases where lower modulus leads to larger fracture surfaces; however, if the fracture surface area is the same, can Young′s modulus alone account for the observed enhancement of FML?\nAs the authors themselves describe, polarization is generated by the breaking of hydrogen bonds during fracture. If the fracture surface area is the same, the number of broken bonds—and hence the amount of separated charges—should also be the same, independent of Young′s modulus. In this case, the polarization electric field generated at the interface should be of the same magnitude. Since the electric field strength is fundamentally determined by the amount of separated charges, it seems difficult to attribute the enhancement solely to Young′s modulus.\nTherefore, rather than directly governing the generation of the polarization electric field, the role of Young′s modulus may lie in providing a synergistic contribution under the fracture-induced electric field. For example, a lower modulus could facilitate carrier transport or radiative recombination at emission centers by lowering energetic barriers. Clarifying this possible synergistic effect would provide a stronger and more convincing rationale. Since the central claim of the manuscript is the correlation between Young′s modulus and FML, explicitly addressing this aspect would significantly strengthen the manuscript.\nResponse: We sincerely appreciate the reviewer′s thoughtful evaluation of our revised manuscript and the constructive suggestions provided during the second round of review. We believe that these valuable comments have greatly contributed to improving the clarity, depth, and overall quality of our work. In light of this, we have refined our discussion to clarify that Young′s modulus primarily acts as a synergistic factor rather than a direct source of polarization.\n(1) The observed enhancement of FML: The exact same fracture area and the identical chemical bonds involved in the breaking are ideal, which is difficult to control under existing experimental conditions. If the fracture chemical bonds are identical, Young′s modulus alone could not account for the observed enhancement of FML. This is attributed to possible trap processes during the transition; the fragmentation of stress stimuli can excite electrons into the trap state. These trap states act as energy storage reservoirs, releasing trapped electrons to the emission centers during fracture. In such a case, the intrinsic electric field at the fracture interface should be theoretically comparable. Therefore, the enhanced luminescence observed in crystals with lower Young′s modulus likely originates from a synergistic effect rather than a direct modulation of the polarization field.\n(2) The role of Young′s modulus in FML mechanism: A lower Young′s modulus corresponds to a mechanically softer lattice that can more readily deform under external stress, thereby facilitating the induction of local piezoelectric polarization and enhancing carrier trapping and trap filling prior to fracture. During crystal fracture, the carriers stored in the trap are released and recombination. The released energy subsequently excites Mn2+ ions through radiative or nonradiative pathways, triggering the d−d transition responsible for the observed FML. Specifically, a lower Young′s modulus enhances lattice deformability and reduces the energetic barrier for carrier migration, thereby promoting carrier transport and radiative recombination at Mn2+ emission centers under the fracture induced electric field. Collectively, the FML behavior is governed by the cooperative interplay among elastic stiffness, local electromechanical coupling, and trap-state dynamics, where Young′s modulus acts as a key modulator that bridges mechanical softness and electronic excitation processes. The revised manuscript now explicitly highlights this synergistic role of Young′s modulus to better reflect the underlying mechanism.\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. h Schematic illustration of the fracto-mechanoluminescence mechanism in Mn halides.\nRevision:\n(1) The title of the manuscript was revised to more accurately reflect the relationship between Young′s modulus and FML and to avoid ambiguity.\n“Observation of Fracto-mechanoluminescence by the Synergistic Effect of Young′s Modulus in Manganese Halides”\n(2) The discussion of the relationship between Young′s modulus and FML in the Abstract has been refined and clarified.\n“We demonstrate that the occurrence of FML arises from the synergistic interplay among the crystal′s elastic stiffness, local electromechanical coupling, and trap states, which collectively activate Mn2+ d−d transitions upon fracture.”\n(3) The Results and Discussion section has been revised to provide a more detailed and precise analysis of the correlation between Young′s modulus and the FML mechanism.\n“Based on the combined PFM and TL results, we propose a refined mechanism (Fig. 3h). In Mn halide single crystals with relatively low Young′s modulus, applied stress induces local piezoelectric polarization that facilitates carrier trapping and the filling of trap states. Upon fracture, the carriers stored in the traps are released and recombine, and the released energy subsequently excites Mn2+ ions through radiative or nonradiative channels, resulting in the characteristic Mn2+ d−d transition (4T →6A ) and producing pronounced FML. Obviously, the FML intensity is affected by electron-photon processes, such as carrier migration and recombination efficiency, which results in intensity not directly related to Young′s modulus. A lower Young′s modulus allows the crystal to store more energy from the piezoelectric field, thereby enhancing carrier mobility and facilitating radiative recombination at luminescent centers. This cooperative interaction among elastic stiffness, local electromechanical coupling, and trap dynamics collectively drives the FML process, underscoring that Young′s modulus serves a modulatory and synergistic, rather than a purely deterministic, role in the underlying mechanism.”\nFig. 3 | Optical study on fracto-mechanoluminescence phenomenon of Mn halide single crystals. h Schematic illustration of the fracto-mechanoluminescence mechanism in Mn halides.\n(4) The Conclusions section has been updated to more clearly summarize the role of Young′s modulus in the FML mechanism.\n“Our investigations demonstrate that the occurrence of FML in Mn halides originates from the synergistic interplay among elastic stiffness, local electromechanical coupling, and trap-state dynamics. The interplay between lattice softness and local electro-mechanical coupling enables efficient trap activation and carrier recombination upon fracture.”"}, {"round": 3, "reviewer_comments": {"reviewer_2": "The authors clearly responded to all reviewers' comments. Therefore, it can be published in this journal without further revisions.", "reviewer_3": "The authors have substantially revised the manuscript and have fully addressed the major concerns raised during the previous review round. I am satisfied that the revisions adequately resolve the outstanding issues, and I believe the manuscript is now suitable for publication in its current form."}, "author_response": "We thank the reviewer for the supportive assessment. We are glad that our revisions have fully resolved the comments and that the manuscript is now regarded as ready for publication.\nWe gratefully acknowledge the reviewer’s supportive remarks. We are pleased that the revised manuscript has resolved all concerns and is considered acceptable for publication."}]

Decision letter