High Performance and Circularly Polarized Mechanoluminescence From Noncentrosymmetric Crystals With Pronounced Macroscopic Polarity

ABSTRACT

Organic mechanoluminescent (ML) materials convert mechanical stimuli into light emission without photon excitation, offering opportunities for sustainable optoelectronics and flexible sensing. Yet, ambiguous mechanistic understanding limits structure to property relationships and hinders the rational design of high‐performance ML materials that combine durability, sensitivity, thermal tolerance, and polarization control. Herein, a general design strategy is presented based on chiral and supramolecular synergy, in which chirality enables symmetry breaking while directional supramolecular interactions amplify macroscopic polarity in noncentrosymmetric crystals. In such crystals, ML‐activity is governed by the net macroscopic rather than the molecular polarity. By constructing a zipper‐like supramolecular hydrogen‐bonding network, the resulting crystals show bright ML visible under ambient light, persistent emission after 90 min of friction, a low force threshold of 0.05 N, and thermal robustness up to 162°C. Modulating supramolecular interaction strength further enables tribochromic ML. Moreover, the chiral and polar crystal architecture affords circularly polarized ML (CPML), providing a mechanically driven route to polarized emission. To the best of our knowledge, this is the first example of purely organic CPML. This study establishes a general chiral‐polar supramolecular design strategy for high‐performance organic ML and supports their potential in advanced luminescent applications.

Organic mechanoluminescent crystals are designed through a chiral‐polar supramolecular strategy that clarifies the decisive role of macroscopic polarity in noncentrosymmetric crystals. A zipper‐like hydrogen‐bonding network generates bright ML visible under ambient light with exceptional durability (90 min friction), high sensitivity (0.05 N), and thermal robustness (162°C). The chiral polar crystals further enable the first organic circularly polarized mechanoluminescence (CPML).

1. Introduction

Mechanoluminescent (ML) materials, capable of directly converting mechanical stimuli such as grinding, compression, or deformation into visible light emission, represent a unique and promising class of smart luminescent systems [1, 2, 3, 4]. Since the first observation of ML‐activity in scraped sugar by Francis Bacon in 1605 [5], this phenomenon has commanded enduring scientific interest due to its inherent stimulus‐responsive nature and capacity for self‐reporting without external excitation [6, 7, 8]. To date, practical ML applications have been dominated by inorganic particles [8, 9, 10] and inorganic‐polymer composites [7, 11, 12, 13, 14]. These materials benefit from their high brightness, mechanical robustness, and excellent fatigue resistance, leading to wide exploration in such areas as stress visualization, tribo‐displays, and electronic skin technologies [15, 16, 17]. In contrast, organic ML materials offer compelling advantages, including vast structural diversity, facile molecular modification, good processability, and readily tunable emission properties [18]. However, the advancement of organic ML systems has been severely impeded by an incomplete comprehension of their underlying mechanistic principles. Despite extensive efforts, the lack of a unified mechanistic framework has critically hindered the establishment of reliable structure‐property relationships, rendering the rational and targeted manipulation of ML performance in organic solids highly challenging [18, 19].

The emergence of ML‐activity in organic solids is governed by several critical structural prerequisites. Substantial evidence has established that piezoelectric effects, arising from noncentrosymmetric crystal structures, play a pivotal role in activating ML‐activity [20, 21, 22]. However, molecular assembly into rigid crystals inevitably strengthens intermolecular interactions, often leading to aggregation‐caused quenching (ACQ) [6], which severely compromises the emission efficiency. A significant breakthrough was achieved when Chi [4, 22, 23, 24], Li [1, 18, 25, 26, 27, 28, 29] and co‐workers [30] introduced aggregation‐induced emission (AIE) characteristics [31] into organic ML systems. These seminal studies reconciled crystallinity with efficient solid‐state emission and, in turn, markedly accelerated progress in this field. Guided by this synergy between the AIE effect and noncentrosymmetric molecular packing, a variety of organic ML materials have been successfully developed, spanning derivatives based on tetraphenylethene [18, 32], carbazole [4, 22, 30], boronate esters [29], diphenylketone [20, 33], triphenylamine [25], and phenothiazine [27, 28, 30]. Crucially, although the vast majority of reported organic ML crystals belong to noncentrosymmetric space groups, it remains uncertain whether noncentrosymmetry alone is sufficient to induce ML‐activity. Furthermore, the decisive factors governing the ML behavior within noncentrosymmetric crystals are yet to be definitively identified.

The performance of organic ML materials is fundamentally dictated by molecular packing and the nature of intermolecular interactions [25, 28, 33, 34]. These supramolecular interactions not only determine the structural stability and rigidity of organic crystals but also play a crucial role in establishing crystal polarity [18, 19, 35] Nevertheless, in most organic systems, such interactions are relatively weak and readily perturbed, rendering organic crystals intrinsically vulnerable to thermal disorder. Consequently, many organic ML materials feature low melting points and exhibit pronounced nonradiative decay at elevated temperatures [36]. In addition, the rigid molecular backbones and dense packing required for efficient ML‐activity often come at the expense of mechanical toughness, causing brittleness, crystal fragmentation, and irreversible crystalline‐to‐amorphous transitions under repeated mechanical stimulation, which ultimately accelerates emission fatigue [15, 37] These intrinsic constraints underscore the urgent need for a deeper mechanistic understanding and generalizable supramolecular design principles to achieve organic ML materials that combine high brightness, durability, and thermal robustness.

The presence of chiral configurations is a sufficient condition for the construction of noncentrosymmetric materials. 1,1′‐Bi‐2‐naphthol (BINOL) in this work was selected as the chiral motif owing to its tunable packing modes [38, 39] and readily derivatized phenolic hydroxyl groups [40, 41], which together enable programmable intermolecular hydrogen bonding. Anthracene‐ and pyrene acyl chlorides were esterified with BINOL to afford five AIEgens (R/S‐BNPAC, R/S‐ACBNPAC, R/S‐BNPPy, R/S‐PyBNPPy, and R/S‐BNPAC‐1) by systematically varying the hydroxyl substitution patterns (Scheme 1).

SCHEME 1.

(a) Schematic illustration of the mechanistic origins of ML. (b) Molecular structures of the BINOL‐based AIEgens studied in this work and the corresponding schematic illustration of macroscopic polarity.

By regulating macroscopic polarity in noncentrosymmetric crystals through the directionality and type of intermolecular interactions, pronounced variations in ML performance were achieved. All polar crystals (R/S‐BNPAC, R/S‐ACBNPAC, and R/S‐BNPPy) are ML‐active. Among them, R/S‐BNPAC delivers exceptional brightness (visible under ambient light), high thermal stability (up to 162°C), excellent sensitivity (0.05 N), and outstanding repeatability (persistent emission after 90 min of continuous friction). Single‐crystal analyses further reveal that oriented hydrogen‐bonding chains both stabilize molecular conformations under external mechanical perturbation and generate a large macroscopic polarity. In contrast, R/S‐ACBNPAC and R/S‐BNPPy feature less directionally constrained hydrogen‐bonding motifs, allowing stress‐induced conformational rearrangements and giving rise to tribochromic ML‐activity. Notably, owing to the chiral crystal architectures, all ML emissions exhibit mechanically induced circularly polarized luminescence (CPL). To further validate the decisive role of macroscopic polarity, control systems featuring nonpolar packing (PyBNPPy) and antiparallel hydrogen‐bond arrangements (S‐BNPAC‐1 and rac‐BNPAC) were deliberately designed; all are ML‐inactive due to polarity cancellation (Scheme 1b). Collectively, these results establish concrete supramolecular design principles for high‐performance organic ML materials and represent a key step toward advanced smart luminescent systems.

2. Results and Discussion

The synthetic routes to the five BINOL‐based AIEgens are illustrated in Scheme S1. All compounds were purified by silica gel column chromatography, followed by recrystallization from a mixed CH₂Cl₂/n‐hexane solvent. Their molecular structures were fully characterized by ¹H and ¹³C NMR spectroscopy as well as high‐resolution mass spectrometry. Thermogravimetric analysis (TGA, Figure S1a) reveals that all compounds exhibit excellent thermal stability, with decomposition temperatures (T _d, defined at 5% weight loss) exceeding 300°C. Differential scanning calorimetry (DSC, Figure S1b) shows distinct melting points (T _m) of 230°C, 170°C, 250°C, 99°C, and 123°C for S‐BNPAC, S‐ACBNPAC, S‐BNPAC‐1, S‐BNPPy, and S‐PyBNPPy, respectively.

The photophysical properties of these compounds were systematically investigated (Figures S2 and S3). In dilute solution, S‐BNPAC, S‐ACBNPAC, S‐BNPPy, and S‐PyBNPPy exhibit low photoluminescence (PL) quantum yields (Φ _s) of 1%, 5%, 0.5%, and 0.1%, respectively. In contrast, their crystalline state quantum yields (Φ _c) increase markedly to 18%, 6%, 26%, and 36%, respectively, confirming their pronounced aggregation‐induced emission (AIE) characteristics. In the crystalline state, the rigid and sterically constrained packing of the anthracene/pyrene moieties within the BINOL scaffold effectively restricts intramolecular motions while suppressing detrimental π–π stackings, thereby significantly enhancing PL efficiency relative to dilute solution. Single crystals of S‐BNPAC, S‐ACBNPAC, and S‐BNPPy exhibit bright blue or green PL (Figure S4), whereas suitable single crystals of S‐PyBNPPy could not be obtained.

Their ML behaviors were subsequently examined. Upon mechanical stimulation, crystals of S‐PyBNPPy and S‐BNPAC‐1 are ML‐inactive (Videos S1 and S2), while S‐ACBNPAC and S‐BNPPy display weak ML activity, observable only in a dark environment (Videos S3 and S4). Strikingly, as shown in Figure 1a and Video S5, intense blue ML activity from S‐BNPAC crystals can be directly observed by the naked eye even under ambient incandescent lighting upon friction with a metal spatula. Interestingly, S‐BNPAC crystals exhibit a transitory ML afterglow, which is associated with delayed emission processes, as supported by the measured lifetime and delayed emission characteristics (Figures S5 and S6). Additionally, S‐BNPAC crystals still exhibit efficient ML under liquid nitrogen conditions (Video S6), which rules out friction‐induced heating as the origin of the ML emission. More intriguingly, S‐BNPAC further exhibits exceptional ML durability: bright emission persists even after continuous grinding for 90 min, despite the crystals being transformed into cotton‐like powders (Figure S7), with no detectable change in emission color. Such extraordinary durability is rare in organic ML systems and is comparable to that of inorganic phosphors [42, 43]. In contrast, S‐ACBNPAC powders display extremely poor ML durability, with emission fading within 2 min of continuous grinding (Figure S8a and Video S7), while the ML‐activity of S‐BNPPy disappears even more rapidly (<60 s; Figure S8b and Video S8). To further understand the origin of the distinct ML durability, the mechanical properties of the crystals were investigated by AFM nanoindentation measurements. The Young's moduli of S‐BNPAC, S‐ACBNPAC, and S‐BNPPy single crystals were 8.0 ± 1.5, 1.2 ± 0.1, and 1.0 ± 0.1 GPa, respectively (Figure S9). Notably, S‐BNPAC exhibits a significantly higher modulus than the other two systems, indicating a more tightly packed structure. In addition to the pronounced difference in durability, S‐ACBNPAC and S‐BNPPy exhibit distinct tribochromic ML‐activity. In their single‐crystal forms, both compounds emit blue‐green light under gentle mechanical stimulation; however, upon prolonged grinding, the resulting powders display yellow ML‐activity with a pronounced red shift in the emission color (Figure S10). This behavior is attributed to stress‐induced molecular slippage and reorganization during the grinding process.

FIGURE 1.

(a) ML of S‐BNPAC crystals under daylight and ML photographs recorded during continuous friction. (b) Photographs of S‐BNPAC crystals subjected to different impact forces. (c) ML images of S‐BNPAC under friction at elevated temperatures. (d–f) ML emission spectra of S‐BNPAC, S‐ACBNPAC, and S‐BNPPy crystals after different grinding times, respectively. (g–i) PXRD patterns of S‐BNPAC, S‐ACBNPAC, and S‐BNPPy crystals before and after grinding, respectively.

To quantitatively evaluate the mechanical sensitivity of S‐BNPAC crystals, a series of collision experiments was conducted using an identical free‐fall impact setup, where weights of varying masses were released from controlled heights to strike the samples. Remarkably, distinct ML activity is readily visible under an applied force as low as 0.05 N (Figure 1b), highlighting an exceptionally high mechanical sensitivity. Beyond mechanical sensitivity, the thermal robustness of S‐BNPAC was systematically evaluated. Strikingly, S‐BNPAC crystals retain pronounced ML activity at temperatures as high as 162°C (Figure 1c). Although the ML activity is quenched at 180°C, the ML emission recovers upon cooling to room temperature (Video S9), indicating the absence of permanent structural degradation in the crystal packing. Variable‐temperature PL measurements further corroborate the outstanding thermal stability of S‐BNPAC crystals, with the emission intensity at 197°C remaining as high as 58% of the room temperature (Figure S11a). By comparison, the emission intensity of S‐ACBNPAC crystals drops abruptly to only 22% of the room‐temperature value at 127°C (Figure S11b), while S‐BNPPy crystals melt at approximately 100°C. Subsequently, the ML spectra of the ML‐active crystals were characterized. As shown in Figure 1d, S‐BNPAC crystals exhibit an ML emission maximum at 480 nm, corresponding to blue light visible to the naked eye. Notably, the ML peak is red‐shifted by ∼10 nm relative to the PL emission of 365 nm excitation. Besides, no wavelength‐dependent emission is observed under different excitation wavelengths, and the ML spectrum consistently exhibits a ∼10 nm red shift relative to the PL spectra at all excitation wavelengths (Figure S12). This shift is attributable to the distinct excitation and emission pathways of ML, which originate from crystal fracture or localized deformation sites rather than the surface. In this process, the emitted light propagates through the bulk crystal and undergoes partial reabsorption [23, 44, 45]. Importantly, even after continuous grinding for 30 min, a well‐defined ML peak remains detectable for S‐BNPAC, with only a gradual decrease in intensity and no discernible shift in emission wavelength. This excellent spectral stability further corroborates the exceptional mechanical robustness of S‐BNPAC crystals. In contrast, pronounced tribochromic ML‐activity is observed for S‐ACBNPAC and S‐BNPPy. Specifically, the ML emission of S‐ACBNPAC red‐shifts from 474 to 496 nm during grinding (Figure 1e), while a more substantial shift is observed for S‐BNPPy, from 460 to 503 nm (Figure 1f). Consistent with these spectral changes, both crystals display ML activity in their pristine single‐crystal states, which rapidly evolves into yellow emission after just 10 s of mechanical grinding (Figure S8). The machanofluorochromisn has also been observed in photoluminescence (Figures S13–S15). To elucidate the structural origin of these behaviors, powder X‐ray diffraction (PXRD) patterns were collected before and after mechanical grinding (Figure 1g–i). For S‐BNPAC, the PXRD patterns remain essentially unchanged after grinding with retained strong diffraction intensities, indicating that mechanical stimulation does not disrupt the intrinsic crystal packing but primarily reduces the crystal size on a macroscopic scale. By contrast, S‐ACBNPAC exhibits a markedly different response. Upon grinding, the original diffraction peaks at 2θ of 5.6°, 7.5°, 11.2°, 15.0°, 17.3°, and 22.7° completely disappear and are replaced by a new set of reflections, indicative of a grinding‐induced phase transition or polymorphic transformation, rather than a simple loss of preferred orientation. Even more pronounced structural disruption is observed for S‐BNPPy, as mechanical grinding not only substantially alters the diffraction pattern but also dramatically reduces overall diffraction intensity, indicating a severe loss of long‐range crystalline order, likely associated with partial amorphization.

To establish clear structure‐property relationships and to elucidate the mechanistic origin of ML‐activity, single‐crystal structures of the compounds were determined. A distinct crystallographic pattern emerged (Tables S1–S3): the chiral compounds (R/S‐BNPAC, R/S‐ACBNPAC, and R/S‐BNPPy) crystallize in noncentrosymmetric space groups, primarily the monoclinic P2₁, with S‐BNPAC‐1 adopting the orthorhombic P2₁2₁2₁. In stark contrast, their racemic counterparts (rac‐BNPAC, rac‐ACBNPAC, and rac‐BNPPy) consistently crystallized in the centrosymmetric triclinic space group P‐1. Given that enantiomeric pairs share identical intrinsic physicochemical properties, a representative enantiomer from each pair was selected for subsequent detailed analysis. Crucially, although all enantiopure compounds crystallize in noncentrosymmetric space groups, a direct consequence of axial chirality, thus implying potential piezoelectricity, ML activity was exclusively observed for R/S‐BNPAC, R/S‐ACBNPAC, and R/S‐BNPPy. This finding demonstrates that noncentrosymmetry alone is insufficient to induce ML‐activity. To further identify the structural factors governing ML‐activity within these noncentrosymmetric crystals, the electrostatic potential (ESP) distributions of the five molecules were calculated using density functional theory (DFT) methods (Figure S16). Notably, the negative charge density is predominantly localized on the ester carbonyl groups. In addition, the monosubstituted derivatives are more electron‐deficient than their disubstituted counterparts. Within this series, S‐BNPAC and S‐BNPAC‐1 exhibit relatively large molecular dipole moments of 3.7 D and 4.3 D, respectively. Despite these comparable and pronounced dipole moments, S‐BNPAC is ML‐active, whereas S‐BNPAC‐1 is not, indicating that the molecular polarity alone does not govern ML activity. Single‐crystal analysis further reveals that the ML response correlates with macroscopic polarity. As shown in Figure 2a, strong intermolecular O─H···O hydrogen bonds are formed between the phenolic hydroxyl groups and the ester carbonyl moieties in S‐BNPAC crystals. Remarkably, these hydrogen bonds display nearly identical geometries and are uniformly aligned along the crystallographic b axis, forming long‐range, unidirectional hydrogen‐bonding chains. This highly ordered arrangement generates a large macroscopic polarity along the b axis, which is considered a key contributor to the intense ML observed for S‐BNPAC crystals. Moreover, the anthracene moieties adopt a nearly parallel, domino‐like arrangement with an intermolecular centroid distance of 3.9 Å, indicative of significant π–π interactions. Together with the directional hydrogen‐bonding network, these interactions cooperatively construct a zipper‐like supramolecular architecture, which effectively locks molecular conformation and packing motifs (Figure 2b), thereby enabling the lattice to tolerate mechanical stimuli without catastrophic structural collapse or a phase transition. This robust supramolecular architecture accounts for the exceptional mechanical durability and thermal stability of S‐BNPAC under prolonged friction. In addition, owing to the X‐shaped conformation of the BINOL scaffold, the anthracene moieties pack in an optimized manner that avoids detrimental π–π overstacking associated with ACQ, while simultaneously maintaining sufficient intermolecular contacts to stabilize the lattice. The packing modes of R‐BNPAC and S‐BNPAC are mirror images (Figure S17), both exhibiting the same zipper‐like hydrogen‐bonded architecture.

FIGURE 2.

(a) Molecular packing in the single crystal of S‐BNPAC, highlighting the zipper‐like hydrogen‐bonded chains and π–π interactions. (b) Schematic illustration of the zipper‐like hydrogen‐bonded architecture accounting for the durable ML. (c, d) Molecular packing in the single crystals of S‐BNPAC and S‐BNPPy. (e) Schematic illustration of friction‐induced structural evolution changes in the crystal leading to tribochromic behavior. (f) Design rules for high‐performance organic ML materials. For clarity, hydrogen atoms are omitted; carbon and oxygen atoms are shown in gray and red, respectively.

In contrast, in S‐ACBNPAC crystals (Figure 2c), esterified phenolic hydroxyl groups eliminate available hydrogen‐bond donors and thus prevent the formation of O─H···O hydrogen‐bond chains. Similarly, no extended hydrogen‐bond chain is observed in S‐BNPPy crystals (Figure 2d). Consequently, these lattices are more susceptible to stress‐induced structural rearrangements or grinding‐triggered phase transitions, leading to pronounced tribochromic ML responses. Notably, despite the absence of robust hydrogen‐bond chains, S‐ACBNPAC and S‐BNPPy crystals still exhibit macroscopic polarity along the b axis, originating from the net vectorial alignment of ester carbonyl dipoles and other polar functionalities (Figures S18 and S19). Based on these findings, general design rules for high‐performance organic ML materials are summarized in Figure 4f: (i) a noncentrosymmetric crystal structure with pronounced macroscopic polarity is required, and (ii) intermolecular interactions should be deliberately engineered to simultaneously ensure lattice robustness and a controllable stress response.

FIGURE 4.

(a) Intermolecular hydrogen bonds between parallel‐aligned molecules in the S‐BNPAC single crystal. (b) Comparison of ML and FL spectra of S‐BNPAC crystals. (c, e) Intermolecular hydrogen bonds between antiparallel‐aligned molecules in rac‐BNPAC and S‐BNPAC‐1 single crystals. (d, f) Corresponding ML and FL spectra of rac‐BNPAC and S‐BNPAC‐1 crystals. λex = 365 nm. For clarity, hydrogen atoms are omitted; carbon, oxygen, and nitrogen atoms are shown in gray, red, and purple, respectively.

A quantitative analysis of intermolecular interactions was performed to elucidate how supramolecular organization governs ML performance. For each single crystal, a representative molecule was extracted, and its interactions with surrounding neighbors were quantified in terms of both contact number and interaction strength. As can be seen from Table S4 and Figure 3, the supramolecular interaction landscapes of S‐BNPAC, S‐ACBNPAC, and S‐BNPPy crystals differ markedly. In S‐BNPAC, abundant intermolecular hydrogen bonds and π–π interactions coexist, constructing a highly organized supramolecular framework (Figure 3a). The hydrogen bonds exhibit a short donor–acceptor distance of 1.9 Å and a near‐linear angle of 161°, consistent with strong and highly directional interactions that effectively constrain molecular conformations and suppress intermolecular motions. Meanwhile, adjacent anthracene units adopt a slipped π–π stacking geometry with a distance of 3.9 Å and a small dihedral angle of 8.6° (Figure S20), providing moderate π–π interactions while avoiding excessive face‐to‐face overlap. Within a cluster of ten neighboring molecules, ten hydrogen bonds and ten π–π interactions are identified (Figure 3b), collectively generating a dense and well‐connected supramolecular network. This interaction pattern stabilizes the packing against mechanical perturbation, rationalizing the exceptional ML brightness and durability of S‐BNPAC. In contrast, S‐ACBNPAC is predominantly stabilized by much weaker C─H···O contacts between phenyl C─H groups and carbonyl oxygen atoms (2.7 to 2.9 Å, Figure 3c). These interactions are markedly bent, with a C─H···O angle of ∼113°, indicating reduced directionality and strength. Accordingly, the packing of a ten‐molecule cluster of ACBNPAC molecules is mainly maintained by ten such weak C─H···O interactions (Figure 3d).

FIGURE 3.

Intermolecular interactions between a selected molecule and its surrounding neighbors in the single crystals of (a) S‐BNPAC, (c) S‐ACBNPAC, and (e) S‐BNPPy. Statistical analysis of the distances and orientations of intermolecular interactions within ten‐molecule clusters in the single crystals of (b) S‐BNPAC, (d) S‐ACBNPAC, and (f) S‐BNPPy. Numbers in parentheses indicate the corresponding interaction counts.

A similar but more pronounced trend is observed for S‐BNPPy crystals. Although relatively strong O─H···O hydrogen bonds are present in the S‐BNPPy packing, featuring distances below 1.9 Å and angles up to 166° (Figure 3e), these interactions fail to develop into a continuous, long‐range network. Meanwhile, the pyrene moieties adopt two distinct π–π packing motifs, namely head‐to‐tail stacking and V‐shaped arrangements. The former features an intermolecular separation of 3.7 Å and a small dihedral angle of 3.1° (Figure 3f, Figure S21), consistent with a nearly coplanar stacking geometry. In contrast, the latter displays a much larger dihedral angle of ∼42°, characteristic of a pronounced V‐type π–π interaction (Figure S22). For a ten‐molecule cluster of S‐BNPPy, only five hydrogen bonds and five π–π interactions are identified (Figure 3f), reflecting a substantially reduced interaction density. This sparse supramolecular connectivity cannot effectively preserve the original packing motif under mechanical stimulation, thereby facilitating grinding‐induced structural reorganization and the emergence of new packing arrangements. Such reorganization accounts for the pronounced tribochromic and mechanochromic luminescence observed for S‐ACBNPAC and S‐BNPPy.

Beyond the strength and robustness of intermolecular interactions, the symmetry and polarity of crystal packing are equally critical in dictating ML behavior. The ML‐inactive PyBNPPy crystallizes in the noncentrosymmetric space group P2₁2₁2₁ yet belongs to the nonpolar point group 222 (Figure S23) [46]. The three mutually perpendicular twofold rotation axes enforce complete cancellation of macroscopic polarity, thereby precluding ML despite the absence of inversion symmetry. To assess the contribution of macroscopic polarity in noncentrosymmetric crystals, racemates and S‐BNPAC‐1 were employed as controls. In enantiopure S‐BNPAC, uniform molecular handedness enforces a consistent spatial orientation of hydrogen‐bond donors and acceptors, promoting unidirectionally aligned hydrogen‐bonded chains (Figure 4a). This directional supramolecular arrangement generates macroscopic polarity and enables pronounced ML (Figure 4b). By contrast, racemic crystals preferentially adopt antiparallel hydrogen‐bonding motifs driven by enantiomeric complementarity, leading to cancellation of directional polar components and the loss of macroscopic polarity (Figure 4c). Moreover, rac‐ACBNPAC and rac‐BNPPy crystallize in centrosymmetric, nonpolar point groups (Figures S24 and S25). Consistently, no ML is observed for these centrosymmetric crystals (Figure 4d). Collectively, these results indicate that a noncentrosymmetric crystal structure is necessary for ML but not sufficient.

To further test whether macroscopic polarity is the decisive factor for ML within noncentrosymmetric lattices, noncentrosymmetric yet nonpolar crystals were deliberately constructed as critical controls. In S‐BNPAC‐1, strong intermolecular hydrogen bonds are also present and assemble into extended chains. However, adjacent hydrogen‐bonded columns are arranged with opposite orientations while maintaining comparable interaction strengths, leading to cancellation of the macroscopic polarity. Symmetry analysis reveals that S‐BNPAC‐1 crystallizes in the noncentrosymmetric yet nonpolar space group P2₁2₁2₁. As expected, these noncentrosymmetric yet nonpolar crystals do not exhibit ML, despite showing efficient solid‐state PL. Overall, the comparative structural and ML analyses underscore the critical role of macroscopic polarity rather than inversion symmetry alone. Chirality enables a noncentrosymmetric lattice, providing the structural prerequisite for ML, while macroscopic polarity, rather than the molecular dipole moment, governs the ML behavior (Table S5). Importantly, unidirectionally oriented hydrogen‐bond chains not only generate large macroscopic polarity but also help preserve favorable packing under mechanical and thermal stimuli, thereby enabling intense emission, exceptional durability, and high thermal tolerance.

CD and CPL spectra were recorded for the ML‐active crystals to probe their chiroptical characteristics in ground and excited states. The CD spectra of R/S‐BNPAC, R/S‐ACBNPAC, and R/S‐BNPPy exhibit significant Cotton effects with clear mirror symmetry in the range of 200‐450 nm, consistent with their corresponding UV–vis absorption profiles (Figure 5). Notably, the split CD features in the 200‐275 nm region are attributed to the binaphthyl moieties [47, 48], indicative of Davydov splitting arising from strong excitonic coupling within the chiral supramolecular arrangement [49, 50]. At longer wavelengths, distinct Cotton effects from 275 to 450 nm coincide with the absorption bands of anthracene and pyrene, and can be ascribed to chiral perturbation of the corresponding electronic transitions imparted by the axially chiral BINOL scaffold [51, 52]. The presence of well‐defined chiroptical activity associated with the emissive chromophores further motivates investigation of CPL.

FIGURE 5.

CD spectra of (a) R/S‐BNPAC, (b) R/S‐ACBNPAC, and (c) R/S‐BNPAPy crystal powders.

To enable solid‐state chiroptical characterization under mechanical stimulation, a simple sandwich‐type film device was fabricated by hot‐pressing ML‐active crystalline powders between two PET films at 80°C. The resulting flexible films exhibit robust ML responses under external force, and the ML trajectories were recorded using in situ camera observation (Figure 6a). Taking S‐BNAC as an example, bright blue emission is readily generated upon gentle writing with a hard pen. As shown in Figure 6b (top), the handwritten pattern “CPML”, denoting circularly polarized mechano‐luminescence, is clearly visualized through force‐triggered emission. Similar light‐tracking behavior is also observed for S‐ACBNPAC and S‐BNPPy‐based films. The CPL and CPML properties of these films were then systematically evaluated. As shown in Figure 6c–e, mirror‐image CPL signals are observed for R/S‐BNPAC, R/S‐ACBNPAC, and R/S‐BNPPy under 365 nm excitation, with emission maxima at 450, 470, and 525 nm, respectively. The dissymmetry factors [g _lum = 2(I _L − I_R)/(I _L + I_R)] [53, 54, 55], are on the order of (2–10) × 10^{− 3} for the polycrystalline samples (Figure S26), confirming efficient transfer of ground‐state chirality to the emissive‐state polarization.

FIGURE 6.

Schematic illustration of the sandwich‐type ML device for tracking motion trajectories. (b) ML display for S‐BNPAC, S‐ACBNPAC, and S‐BNPPy‐based films. (c–e) CPL spectra of R/S‐BNPAC (c), R/S‐ACBNPAC (d), and R/S‐BNPPy (e) under 365 nm excitation. (f–h) CPML spectra of R/S‐BNPAC (f), R/S‐ACBNPAC (g), and R/S‐BNPPy (h) under mechanical stimulation.

Remarkably, CPL can also be generated under mechanical stimulation without external photoexcitation. CPML signals are detected for R/S‐BNPAC, R/S‐ACBNPAC, and R/S‐BNPPy films, with emission maxima at 480, 490, and 500 nm, respectively (Figure 6f–h). The spectral disparity between CPL and CPML originates from differences in the corresponding emissive species or environments accessed under photoexcitation versus mechanical activation in the excitation pathway. CPML has been previously realized in inorganic‐organic hybrid systems [56, 57]. In contrast, this work represents the first example of CPML achieved in a purely organic system.

3. Conclusion

In summary, a class of chiral, polar organic crystals is developed that synergistically integrates molecular chirality, macroscopic polarity, and supramolecular regulation to realize bright, wear‐resistant, thermally robust, and tribochromic CPML. Single‐crystal analysis reveals that noncentrosymmetry alone is insufficient to enable ML; instead, within noncentrosymmetric lattices, the emergence of macroscopic polarity plays the decisive role. In R/S‐BNPAC, ordered hydrogen‐bond chains generate a large macroscopic polarity while simultaneously stabilizing the crystal packing against mechanical and thermal perturbations, thereby resulting in stable ML. Accordingly, S‐BNPAC crystals display high brightness visible under ambient lighting, retain ML even after prolonged grinding (>90 min), respond to mechanical stimuli with a low force threshold of 0.05 N, and remain ML‐active up to 162°C. By contrast, crystals lacking directional supramolecular constraints are prone to stress‐induced structural rearrangements, leading to weakened emission or pronounced tribochromic ML response. Moreover, the chiral crystal architectures empower the direct generation of CPML. Collectively, these results establish a feasible supramolecularly programmable strategy for the construction of organic ML materials and highlight the cooperative roles of macroscopic polarity and directional intermolecular interactions in achieving high brightness, durability, and thermal robustness.

4. Experimental Details

4.1. Materials

R‐ and S‐binaphthol, R‐ and S‐[1,1′‐binaphthalene]‐2,2′‐diamine, anthracene‐9‐carboxylic acid, triethylamine (ultradry), pyrene‐1‐carboxylic acid, and 4‐dimethylaminopyridine (DMAP) were purchased from Aladdin. Solvents including dichloromethane, methylbenzene, tetrahydrofuran, thionyl chloride, petroleum ether, and ethyl acetate were obtained from Sinopharm Chemical Reagent Co., Ltd. Polyethylene terephthalate (PET) films were purchased from JD Supermarket and used as received.

4.2. Measurements

¹H NMR and ¹³C NMR spectra were measured on a Bruker AV 400 spectrometer at 298 K in CDCl₃ or DMSO‐d₆ . High‐resolution mass spectra were measured on an Ion Spec 4.7 Tesla FTMS instrument. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed using a Perkin‐Elmer Diamond TG/DTA under a nitrogen atmosphere with a heating rate of 10°C min⁻¹. Crystallographic data were collected using a Rigaku CCD diffractometer using Cu Kα (λ = 1.54184 Å) radiation. The relevant single crystal structure data in this paper are freely available in the Cambridge Crystal Structure Database. Powder XRD was measured on D8 Advance using Cu Kα radiation with a scanning rate of 10°C min⁻¹. Photoluminescence quantum yield (PLQY) and fluorescence spectra were recorded by a Fluorolog‐QM spectrometer. ML spectra were collected from an Acton SP2750 spectrometer with a CCD (SPEC‐10, Princeton) as a power detector. The Young's modulus was measured using a FastScan Bio AFM under an applied force of 100 nN. CD spectra of crystals were measured in clear slices of quartz cuvettes (light path length 10 mm) on a JASCO J‐1500 CD spectrometer. The measurement of CPL was performed on the JASCO CPL‐300 spectrophotometer. CPML measurements were performed using a JASCO CPL‐300 spectrometer equipped with a motor‐driven rotor purchased from Taobao. Mechanoluminescence was generated by the rotor impacting the sample, and the process was conducted inside the CPL‐300 chamber, with the CPL‐300 serving as the detector for circularly polarized light. Throughout the measurements, the shutter of the CPL‐300 remained closed to eliminate interference from photoexcitation. All measurements were conducted at room temperature (25°C) under ambient atmospheric conditions.

4.3. Density Functional Theory (DFT) Calculations

DFT calculations were carried out using the Gaussian 16 program at the B3LYP/6‐31G(d,p) level. The geometries were directly adopted from the corresponding single‐crystal X‐ray diffraction structures. To preserve the experimentally observed molecular conformations and packing‐related features, no further geometry optimization was performed.

4.4. Preparation of a Large‐Area Sandwich‐Type ML Device

The mechanoluminescent (ML) doped crystalline powders (0.5 g) were placed between two polyethylene terephthalate (PET) films. The assembly was then subjected to hot pressing, during which the crystalline powders were encapsulated between the two PET films to form a flexible ML composite film.

4.5. Single‐Crystal Growth

Single crystals of R‐ or S‐BNPAC were grown by a liquid–liquid diffusion method. Typically, R‐ or S‐BNPAC (40 mg) was dissolved in dichloromethane (DCM, 1 mL) as the good solvent. n‐Hexane (4 mL) was then slowly added to form a solvent bilayer. The vial was sealed and kept in a refrigerator to allow slow solvent diffusion. After one week, rod‐like single crystals suitable for X‐ray diffraction were obtained.

Single crystals of ACBNPAC and BNPAC‐1 were grown using the same procedure. For BNPPy, the liquid–liquid diffusion method was also employed, except that toluene was used as the good solvent instead of DCM. Racemic crystals were obtained by crystallization from a 1:1 mixture of the corresponding R and S enantiomers under identical conditions.

Deposition numbers 2455081 (for R‐BNPAC), 2455010 (for S‐BNPAC), 2468647 (for rac‐BNPAC), 2468657 (for R‐ACBNPAC), 2520961 (for S‐ACBNPAC), 2468661 (for S‐BNPAC‐1), 2520965 (for rac‐ACBNPAC), 2520972 (for R‐BNPPy), 2520974 (for S‐BNPPy), and 2520976 (for rac‐BNPPy) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/data_request/cif.

Funding

This work was financially supported by the National Natural Science Foundation of China (52350337, 52233016, 21975156, U22A20250, and 52473185).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: adma73834‐sup‐0001‐SuppMat.docx.

Supporting File 2: adma73834‐sup‐0002‐VideoS1‐S9.zip.

Supporting File 3: adma73834‐sup‐0003‐DataFile.zip.

Data Availability Statement

The data that supports the findings of this study are available in the supplementary material of this article.