Congested position isomerism enhanced mechanoluminescence of triarylboranes

Abstract

The study of the mechanoluminescence enhancement (MLE) mechanism has been a challenging topic in the field of luminescent materials. Here, we implanted organoboron units with a steric hindrance effect into the molecular backbone and achieved the synthesis of MLE molecules using molecular engineering and crystal engineering. ortho-, meta-, and para-substituted organoboron compounds, namely o-NAB, m-NAB, and p-NAB, were synthesized, where o-NAB showed notable MLE, whereas m-NAB and p-NAB showed decreased fluorescence intensity upon mechanical activation without the assistance of solvent. Our study finds that the steric conformation resulting from different substitution positions plays a decisive role in the fluorescence performance. Grinding releases spatial stress in the o-NAB structure, thereby affecting the process of mechanoresponsive fluorescence transition. Our research provides a congested position strategy for constructing MLE molecules, which not only enhances the fundamental understanding of MLE mechanisms but also bears significant implications for future research.

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1. Introduction

Stimuli-responsive luminescent materials^1–4 where emission properties can change upon exposure to external stimuli such as lighting,^5–7 heating,^8–10 force^11–13 and chemical vapor^14–16 have attracted considerable interest due to their potential applications in security devices, chemical sensors, data storage, etc. Among them, mechanoresponsive luminogens based on metal complexes,¹⁷ organic molecules¹⁸ and polymers¹⁹ exhibit variable fluorescent properties (e.g., luminescence wavelength and luminescence intensity) under mechanical stimuli such as grinding, rubbing, pressing, and shearing, and have become a hotspot in the field of stimulus-responsive fluorescence. Their mechanism is primarily driven by molecular structure rearrangement (e.g., cleavage or formation of covalent bonds),^20,21 solid-state morphology transformation (e.g., crystal-to-amorphous,^22,23 crystal-to-crystal,²⁴ amorphous to crystal²⁵), intramolecular conformation change (e.g., conformational planarization),²⁶ and excited-state switching (e.g., from a locally excited (LE) state to an intramolecular charge transfer (ICT) state),²⁷etc. Although previous studies have made remarkable progress in exploring the mechanism of mechanoresponsive luminescence and the basic understanding has reached a certain depth, however, there is still a lack of effective technological means to characterize this complex process comprehensively and accurately, especially the transient, localized, or complex luminescence behavior, and thus the mechanoresponsive luminescence mechanism is still necessary.

Among mechanosensitive luminescent materials, there exists a mechanoluminescence enhancement (MLE) phenomenon, which can be widely used in practical applications and is the most valuable to be investigated due to the turn-on type photoluminescence upon stimulation by mechanical forces. However, the MLE phenomenon has rarely been reported because molecule design strategies must enable metastable crystal structures with low fluorescence quantum yields to respond sensitively to mechanical stimuli, undergo crystal transformations, and stabilize into highly luminescent crystalline states. Based on previous reports (Scheme S1, ESI†), MLE can be achieved through several practical strategies: planarization of molecular conformation,^28–31 breaking the aggregation state (e.g., H-aggregation,³² J-aggregation,³³ and other aggregation forms^34,35), aggregation-induced emission (AIE),^36,37 breaking of chemical bonds (e.g., covalent bond^38,39 and intermolecular interactions^40,41), generating lattice defects,⁴² induced crystallization,^43–45 B/N Lewis pairs^46,47 and so on. These methods make it difficult to achieve the targeted design of molecules to meet the precise modulation of mechanically stimulated fluorescence. In addition, although these luminescence mechanisms have been proposed, the relationship between fluorescent enhancement properties and the mechanism is still ambiguous. Therefore, it is necessary to design and synthesize new MLE molecules to investigate the mechanism of mechanically responsive luminescence enhancement.

Based on our group's previous studies,^48,49 it has been observed that minor conformational changes within a donor–acceptor (D–A) system can result in significant photoluminescence spectral changes. The steric hindrance effect often affects various physical and chemical properties of materials due to molecular conjugation and space stacking, especially their mechanical properties.^50–52 Therefore, we propose to implant the steric hindrance factor into the D–A skeleton containing organoboron units to achieve the design and synthesis of MLE molecules using molecular engineering and crystal engineering. Specifically, three D–A organoboron compounds with different steric hindrance environments, 1,8-bis{[2-(dimesitylboranyl)phenyl]ethynyl}naphthalene (o-NAB), 1,8-bis{[3-(dimesitylboranyl)phenyl]ethynyl}naphthalene (m-NAB), and 1,8-bis{[4-(dimesitylboranyl)phenyl]ethynyl}naphthalene (p-NAB), were constructed by linking triarylborane units via alkyne groups at the 1,8-positions of naphthalene. Interestingly, due to the different ways in which their molecules are stacked, the three targeted triarylborane derivatives display different mechanoluminescent properties in the aggregated state. It can be observed that o-NAB-α has a pronounced MLE, whereas m-NAB and p-NAB show a decrease in luminescence intensity upon milling due to different molecular conformations (Scheme 1). Subsequently, with single-crystal data, excited-state spectra, and quantum chemical simulations, we confirmed that steric hindrance based on organoboron can generate the MLE phenomenon through a force-induced conformational change strategy. Finally, the efficient and reproducible MLE characterization of o-NAB suggests that these molecules can be used to accurately detect metal friction traces, highlighting the MLE molecular design concept and its potential application in photonics.

Scheme 1 The luminescence mechanism of most MLE phenomena.

2. Results and discussion

2.1 Synthesis and characterization

The synthesis of compounds o-NAB, m-NAB and p-NAB are depicted in Scheme S2 (ESI†). Treatment of the phenylethynyl-substituted bromophenyl compounds 1,8-bis[(2-bromophenyl)ethynyl]naphthalene (R2), 1,8-bis[(3-bromophenyl)ethynyl]naphthalene (R3), and 1,8-bis[(4-bromophenyl)ethynyl]naphthalene (R4) with ⁿBuLi (molar ratio = 1 : 2) in THF at −78 °C afforded the corresponding lithiation products, which then reacted with Mes₂BF to yield products o-NAB, m-NAB and p-NAB. They were fully characterized by ¹H NMR, ¹³C NMR, ¹¹B NMR spectra (Fig. S1–S3, ESI†) and single crystal X-ray diffraction. The chemical shift signals of compounds o-NAB,⁵³m-NAB and p-NAB in ¹¹B NMR were located at 74.1, 71.09 and 75.82 ppm, respectively (Fig. S2 and S3, ESI†), suggesting the formation of tri-coordinated structures of the boron center in the as-synthesized samples. The characteristic stretching vibration peak at 1029 cm⁻¹ in the Fourier transform infrared (FT-IR) spectra indicates the presence of B–C bonds in the structure (Fig. S5, ESI†). Compound o-NAB can obtain red crystals o-NAB-α and colorless crystals o-NAB-β under different crystallization conditions (Table S1, ESI†). The Mes₂B unit neighboring the alkynyl group shows different space packing in o-NAB-α and o-NAB-β crystals (Fig. 2). Among them, o-NAB-α has intramolecular C–H⋯π and π⋯π interactions, while o-NAB-β only has intramolecular C–H⋯π interactions. When the Mes₂B unit is present in the meta- or para-position of the alkynyl group, there is no significant intramolecular interaction in the m-NAB and p-NAB crystals. In addition, crystal o-NAB-α has abundant intermolecular interactions compared to o-NAB-β, m-NAB and p-NAB. All these tests implied the successful synthesis of three compounds. X-ray diffraction (XRD) patterns of o-NAB-α, o-NAB-β, m-NAB and p-NAB crystals show good agreement between the experimental and fitted values, indicating that synthesized samples are high purity and can be synthesized in bulk (Fig. S6, ESI†). The purity of these three samples was further confirmed by high-performance liquid chromatography (HPLC) in a mixture of H₂O/THF solvents (V_H2O : V_THF = 2 : 8), which was recorded at the peak absorption of 280 nm (Fig. S4, ESI†).

2.2 Photophysical properties

To study the mechanoresponsive luminescence properties of crystals o-NAB-α, o-NAB-β, m-NAB and p-NAB, their steady-state photoluminescence (PL) properties and quantum yields were tested before and after grinding (Fig. 1). The crystal o-NAB-α has two different emission peaks at 455 nm and 530 nm, and after grinding, the intensity of the emission peak at 455 nm is enhanced nearly 10-fold, while the absolute quantum yield is increased from 0.7% in the original crystal to 12.6% in the grinding powder, which is an increase of about 18-fold. The o-NAB-α is one of the known organoboron materials with significant MLE phenomenon.^34,46,47 However, after grinding, the intensity of the emission peak at 437 nm of the crystal o-NAB-β with the same molecular structure decreased and red-shifted to 446 nm, and the quantum yield decreased from 23.5% to 15.3%. These differences illustrate that differences in the structural stacking of compounds with the same molecular structure affect the luminescent properties of the molecular mechanical response. That is to say, appropriate stacking structures generated by the large sterically hindered Mes₂B group offer the possibility of “turn on” luminescent molecules. For both crystals m-NAB and p-NAB, their luminescence intensity decreased after grinding. The difference is that crystal m-NAB shows a slight decrease in the intensity of the emission peaks after grinding, and the emission peaks split from a single peak into double peaks (411 and 428 nm), whereas crystal p-NAB only showed a significant decrease of nearly three times the luminescence intensity. All these results show that the luminescence intensity of the milled crystals tends to decrease when the Mes₂B group is changed from ortho-substitution to meta- or para-substitution, suggesting that the position of the Mes₂B substituent has an important effect on the luminescence of the molecular aggregates.

Fig. 1 PL spectra of o-NAB-α (a), o-NAB-β (b), m-NAB (c) and p-NAB (d) taken before and after grinding under 365 nm excitation. (e) The photophysical properties and photographs of four crystals show their states before and after grinding under sunlight and 365 nm UV irradiation.

2.3 Mechanism insights of MLE

To explore the reason for the MLE phenomenon of o-NAB-α, the attenuation total reflection infrared spectroscopy (ATR-FTIR) method was used to identify o-NAB-α and o-NAB-β before and after crushing (Fig. S11, ESI†). The main characteristic peaks of the obtained map were assigned, it can be observed that there is no significant change in wavelength. Therefore, the structure of molecules in the crystal has not changed. Furthermore, solid-state nuclear magnetic resonance spectra (¹³C CP-MAS NMR, Fig. S12 and S13, ESI†) of o-NAB-α were tested in both crystalline and grinding powder state. ¹³C CP-MAS NMR spectra show that the chemical shifts of the carbon element of o-NAB-α were almost unchanged (Fig. S12, ESI†), indicating that the skeletal structure of the samples did not change before and after grinding. However, ¹¹B solid-state nuclear magnetic resonance (¹¹B CP-MAS NMR) spectra showed that the peak positions of elemental boron in crystalline o-NAB-α were 51.56 (a) and 68.92 (b), whereas the peak positions of elemental boron in the grounded o-NAB-α powders were 51.63 and 68.96. The peak intensity ratio of a and b has changed from 3.68 to 2.55, indicating changes in the chemical environment of the boron center, and to some extent also reflecting the change in the molecular conformation.⁵⁴

Furthermore, single crystal X-ray diffraction analysis was used to understand the MLE phenomenon. The four crystals have different molecular stacking structures and many intramolecular or intermolecular interactions, such as C–H⋯H–C, C–H⋯π or π⋯π interactions (Fig. 2, Fig. S14 (ESI†), Table 1). Crystal o-NAB-α has four intermolecular C–H⋯H–C distances ranging from 2.333 to 2.388 Å and six C–H⋯π distances ranging from 2.814 to 2.887 Å, one intramolecular C–H⋯π distance that is 2.876 Å and two significant intramolecular π⋯π interactions that are 3.360 and 3.466 Å. Crystal o-NAB-β has five intermolecular C–H⋯π interactions varying from 2.838 to 2.883 Å and two intramolecular C–H⋯π interactions that are 2.686 and 2.775 Å. Compared to crystal o-NAB-β, crystal o-NAB-α has more intermolecular C–H⋯H–C and C–H⋯π interactions. By using o-NAB-α and o-NAB-β crystal structures to predict their crystal morphology, it shows these two crystals are clearly different (Fig. S15, ESI†). Crystal m-NAB only has five intermolecular C–H⋯π distances varying from 2.746 to 2.849 Å. Crystal p-NAB has two intermolecular C–H⋯H–C distances that is 2.392 Å and four C–H⋯π distances vary from 2.785 and 2.886 Å. The conformation of o-NAB-α is notably different from that in the other three crystals, featuring intramolecular face-to-face stacking between the benzene and naphthalene rings. Time-dependent density functional theory (TD-DFT) calculations help to directly characterize and reveal these weak interactions in o-NAB-α, o-NAB-β, m-NAB, and p-NAB.^55–57 The IRI is based on the Hessian matrix of the electron density and density functional theory (DFT), where IRI analysis uses the eigenvalues (such as λ₂) of the electron density matrix and electron density (ρ) to describe interatomic interactions, revealing attraction or repulsion, with different color intensities representing the types and strengths of these interactions. As shown in Fig. 2e–h, we conducted interaction region indicator (IRI) analysis on the four crystals conformation, and it can be seen more clearly that the repulsive forces in o-NAB-α mainly exist in the Mes group (aromatic ring P2) and the naphthylacetylene group, while the others mainly exist between alkyne bonds (Fig. S16, ESI†). Therefore, it is reasonable to believe that during the grinding process, intermolecular forces are disrupted and stress in the crystal is released, causing the separation of the P2 ring from the naphthyl group.

Fig. 2 Intramolecular and intermolecular interactions (Å) in o-NAB-α (a), o-NAB-β (b), m-NAB (c) and p-NAB (d) crystals, respectively (C–H⋯H–C interaction: blue; C–H⋯π interaction: pink; π⋯π interaction: yellow; P1, P2, P1′, P2′: representing the aromatic ring at the corresponding position). Isosurface map of the interaction region indicator (IRI) of interaction for o-NAB-α (e), o-NAB-β (f), m-NAB (g), and p-NAB (h), where green represents the interaction area.

Table 1 Summary of intermolecular and intramolecular interactions, interaction region indicator (IRI) analysis, radiative rate constants (k_r) and non-radiative rate constants (k_nr) in the o-NAB-α, o-NAB-β, m-NAB and p-NAB crystal structures. The sign(λ₂)ρ function displays the type and intensity of interactions by projecting different colors onto the isosurface

Entry		o-NAB-α	o-NAB-β	m-NAB	p-NAB
Number of intermolecular interactions	C–H⋯H–C	4	0	0	2
	C–H⋯π	6	5	5	4
Number of intramolecular interactions	C–H⋯π	1	2	0	0
	π⋯π	2	0	0	0
Number of molecules wrapped around the central molecule		8	6	6	8
Functional groups interaction, sign(λ2)ρ (a.u.)		Mes–Naph, +0.01	Mes–Mes, +0.01; ethynyl–ethynyl, +0.011	Mes–Mes, +0.01; ethynyl–ethynyl, +0.012	Mes–Mes, +0.01; Ethynyl–Ethynyl, +0.013
CT through space (Naph → Mes)		Strong	Weak	Weak	Weak
k r (s^−1)	Crystal	1.2 × 10^7	1.4 × 10^8	5.9 × 10^8	2.7 × 10^8
	Powder	7.4 × 10^7	1.4 × 10^8	3.3 × 10^8	2.5 × 10^8
k nr (s^−1)	Crystal	1.7 × 10^7	4.5 × 10^6	3.2 × 10^6	4.4 × 10^6
	Powder	5.1 × 10^6	7.7 × 10^6	1.9 × 10^6	6.6 × 10^6

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To understand the excited-state dynamics, the lifetimes of all crystals before and after grinding were further tested and were all at the ns level, implying that their luminescence was fluorescence emission (Tables S2, S3 and Fig. S17–20, ESI†). Moreover, the radiative rate constants (k_r) and non-radiative rate constants (k_nr) of four crystals before and after grinding were offered in Table 1. After grinding, the k_r value of crystal o-NAB-α increases from 1.2 × 10⁷ s⁻¹ to 7.4 × 10⁷ s⁻¹, while the k_nr decreases from 1.7 × 10⁷ s⁻¹ to 5.1 × 10⁶ s⁻¹, indicating an inhibition of the non-radiative pathway. For o-NAB-β, the k_r remains 1.4 × 10⁸ s⁻¹, but the k_nr increases from 4.5 × 10⁶ s⁻¹ to 7.7 × 10⁶ s⁻¹, showing enhanced non-radiative transitions. However, under external force, the crystal environment is disrupted, and the molecular conformation transforms into a less hindered and more stable amorphous state (closer to the stable conformation of the monomer), resulting in different excitation state characteristics and ultimately leading to distinct mechanoluminescence performances for o-NAB-α and o-NAB-β.

To explore the fluorescence mechanism of the amorphous state after friction, we investigated the fluorescence properties of o-NAB, m-NAB, and p-NAB in a THF/water mixture at various water fractions (f_w). As shown in Fig. 3a and b, o-NAB exhibits typical aggregation-induced emission (AIE) characteristics, which can be attributed to the restriction of molecular rotation by the Mes₂B group in the solid state, while emission intensity of m-NAB and p-NAB gradually decreases with the increase of water content (Fig. S21 and S22, ESI†). As depicted in Fig. 3c, normalized PL spectra have been compared with grinding o-NAB-α powder, grinding o-NAB-β powder and o-NAB aggerate from the 1.0 × 10⁻⁵ mol L⁻¹ THF/H₂O mixture with high water fraction of 95%. It is worth noting that the emission peak of o-NAB at 95% concentration almost overlaps with the emission peaks of o-NAB-α (446 nm) and o-NAB-β (455 nm) after grinding. The low concentration o-NAB aggregates in the solution are very similar to the state of o-NAB-α and o-NAB-β after grinding, and the amorphous state is exactly the “metastable state” that the two crystals strive to achieve after grinding (Fig. S26, ESI†). This suggests that the milled powder and diluted aggregates may have similar molecular stacking environments. Based on the above results, only when the Mes₂B groups are fixed at the ortho-position, steric hindrance groups induce the formation of a metastable conformation and lead to restricted intramolecular motion. Due to the molecular structure, which includes steric hindrance groups BMes₂, polycrystalline phenomena can be formed. Additionally, substitution at different positions in the ortho-, meta-, and para-position results in different stable conformations, leading to varying solid state luminescence efficiencies. Grinding can cause a transition from one crystalline phase to another, but it does not completely break the material down into individual molecules. The metastable structure of the conformational restricted state makes it easier for the energy of the excited state to be released through radiation, and non-radiative transitions are suppressed. As a result, both the fluorescence quenching molecule conformation in o-NAB-α crystals and the strong luminescent molecule conformation in o-NAB-β crystals become the same conformation of amorphous state after grinding. The molecular conformation of o-NAB-α and o-NAB-β crystals changes before and after grinding leading to the differences in luminescence.

Fig. 3 (a) PL spectra of o-NAB in THF/H₂O mixtures with different volume fractions of H₂O (λ_ex = 365 nm, C = 10 μM). (b) Plot of PL intensity and emission wavelength of o-NAB in THF/H₂O mixtures with different f_w. Inset: Photographs in THF and different THF/H₂O mixtures taken under the 365 nm irradiation. (c) Normalized PL spectra of grinding powder about o-NAB-α and o-NAB-β, and o-NAB in THF/H₂O mixture with a water fraction of 95% (10⁻⁵ mol L⁻¹), the illustration shows the mutual transformation between o-NAB-α, o-NAB-β and amorphous state.

To better understand the mechanism behind this phenomenon, we resorted to femtosecond time-resolved transient absorption (TA) spectroscopy to examine the charge separation processes involved.^58–61 In the TA measurements with a pump–probe configuration, the pump was selected at 400 nm and the probe was from a white-light continuum in the range of 500–700 nm (Fig. 4). The representative TA spectra recorded on o-NAB-α crystals, grinding o-NAB-α powder, grinding o-NAB-β powder, and o-NAB-β crystals exhibit similar broad profiles of photoinduced absorption (Fig. 4a). However, pronounced changes can be detected in their relaxation kinetics (Fig. 4b). To eliminate the subtle influence of probing wavelengths on relaxation lifetimes, we performed a global fitting using 7 kinetic traces with a 10-nm interval in the range of 580–640 nm. The fitting results are collected in Table S5 (ESI†). Before grinding, the average lifetimes of o-NAB-α and o-NAB-β crystals are ∼314 ps and ∼478 ps, respectively. Markedly, a reverse trend of lifetime change was observed after grinding, i.e., the former was prolonged to ∼402 ps (by ∼28%) while the latter was shortened to ∼409 ps (by ∼14%). The convergence to ∼405 ps for both grinding power samples suggests that they have similar amorphous forms.

Fig. 4 (a) TA spectra (pump at 400 nm) taken at several representative probe delays for o-NAB-α crystals, grinding o-NAB-α powder, grinding o-NAB-β powder, and o-NAB-β crystals. (b) The global fitting results of TA kinetics for o-NAB-α crystals, grinding o-NAB-α powder, grinding o-NAB-β powder, and o-NAB-β crystals.

To gain a deep insight into the influence MLE mechanism of molecular conformation in crystals, we carried out theoretical calculations derived from the crystallographic data (Fig. 5). Through ground-state and excited-state optimizations, we obtained the stable conformations of both the S₀ and S₁ states, enabling us to simulate their emission-related excited-state properties. Comparing the relative energies of the S₀ states, we find that the amorphous state has a lower energy than the o-NAB-α and o-NAB-β phases. This explains why both crystals tend to transition into the amorphous state after their structures are disrupted by friction. The natural transition orbitals (NTOs) of the S₁ excited state, as shown in Fig. 5b, indicate that the NTO orbitals involved in the fluorescence process are closely associated with the Mes₂B structure. Notably, in the NTOs of o-NAB-α, there is a significant overlap between the Mes₂B fragment and the anthracene moiety, suggesting that the S₁ → S₀ deactivation corresponds to an intramolecular CT transition between the naphthalene ring skeleton (bridged by alkyne bonds) and the Mes₂B groups. By analyzing the transition density matrix heatmaps, we can observe that significant local areas exhibit CT phenomena in the o-NAB-α conformation. Through the analysis of excited-state charge transfer (CT) characteristics (Fig. 5b), it is demonstrated that in o-NAB-α, a through-space (Naph → Mes) interaction exists, indicating that non-radiative transitions primarily occur via CT. The other two electronic transitions are expressed globally, highlighting the important regulatory capability of conformations in boron-containing luminescent materials. In contrast, no significant CT was observed in the molecular conformations of m-NAB and p-NAB (Fig. S27, ESI†). The fluorescence oscillator strengths for o-NAB-α, the amorphous powder, and o-NAB-β are 0.028, 0.082, and 0.177, respectively. This indicates that the radiative transition of the amorphous powder is stronger than that of o-NAB-α but weaker than that of o-NAB-β. Electrostatic potential calculations (Fig. S28, ESI†) further demonstrate that variations in substituent positions and spatial stacking lead to significant differences in the electron density distributions of the four crystals. Compared with o-NAB-β, p-NAB, and m-NAB, the electron density on the aromatic ring of o-NAB-α is more dispersed.^62,63 The above computational results jointly contribute to the metastable, fluorescence-quenched state of the o-NAB-α crystal. By disrupting this crystal structure, enhanced fluorescence emission can be achieved.

Fig. 5 (a) Jablonski diagrams showing calculated vertical energies for S₀ → S₁ and S₁ → S₀ transitions and configuration energies associated with the amorphous state; (b) natural transition orbitals (NTO) images and the transition density matrix of the S₁ state for o-NAB-α, amorphous state and o-NAB-β, calculated at B3LYP/6-31G(d). Herein, f represents the oscillator strength; “Hole” and “Particle” of the NTO represent the hole and the electron moieties, respectively, and the bottom demonstrates the transition density matrix heat maps.

The conformational changes caused by steric hindrance functional groups have a significant impact on “turn on–off” photoluminescence, the MLE characteristics of o-NAB-α have been proven to have good reversibility and the ground emission position has good reproducibility through multiple repeated “grinding and CH₂Cl₂ fuming” processes (Fig. 6a). The fluorescence intensity of the original o-NAB-α sample exhibits decreases and enhancement in repeatability during the grinding and CH₂Cl₂ fuming process (Fig. 6b), and the strongest emission position remains within the range of 453–457 nm, indicating a relatively stable luminescence color. To illustrate the structural transformation that occurred during the grinding process, powder X-ray diffraction (PXRD) testing was performed on o-NAB-α. As shown in Fig. 6c, the crystallinity of o-NAB-α crystals significantly decreases after grinding, while there is a certain recovery in crystallinity after smoking. The (1, 0, −1), (0, 1, 1), (1, 0, 1) and (1, 1, 0) crystal planes disappear after grinding, while the (0, 1, 2), (1, 1, −1), (1, 1, 2) and (0, 1, 3) crystal planes reappear after CH₂Cl₂ fuming. The gradual disappearance of (0,1,1), (1, 2, 0), (1, 2, −1) and (2, 1, −1) crystal facets was observed when o-NAB-α crystals in the grinding state were tested by single-crystal X-ray diffraction (Fig. 6d). The XRD crystal planes changing information further indicates the structural change that occurs in o-NAB-α crystals during the grinding process. Thus, the MLE behavior may be attributed to the transformation from a crystalline phase to a partially amorphous state.^64,65 The amorphous state fluorescence can be recovered to a “turn off” state during CH₂Cl₂ fuming, indicating that this amorphous molecular conformation is metastable.

Fig. 6 (a) Reversible switching emission of original o-NAB-α crystals by repeating “grinding – CH₂Cl₂ fuming” cycles. (b) Reversible switching emission wavelength and intensity of original o-NAB-α crystals by repeating “grinding – CH₂Cl₂ fuming” cycles. (c) Powder XRD patterns of o-NAB-α in the form of original crystals, grinding powder and grinding powder fumed by CH₂Cl₂. (d) Comparison of simulated and experimental XRD patterns about o-NAB-α through single-crystal X-ray diffraction.

2.4 Friction trace detection through MLE

Scratch detection is very common and important in the machine industry. Generally, the existence of metal scratches is detected using a metalloscope, but fluorescence microscopes have advantages such as lower noise and easier resolution. Therefore, it is of practical significance and progressiveness to use MLE molecules to realize this detection. Considering the unique mechanical response and reversible recovery characteristics of organic molecules, compound o-NAB was attempted as a scratch detection substance for metals (Fig. 7a). When o-NAB is dissolved in soluble organic solvents, brushed on the metal surface when rubbed with a standard scratch-free metal block and irradiation with a 365 nm ultraviolet lamp, obvious metal scratches can be seen.

Fig. 7 (a) Demonstration of metal scratch, brushing o-NAB solution, 365 nm irradiation, after rubbing and 365 nm irradiation again. (b) The detection of scratches on actual metal blocks is achieved by capturing images using a fluorescence microscope and reproducible under CH₂Cl₂ fuming (Δ21: is the new fluorescence position generated by image 2 relative to image 1, Δ42: is the new fluorescence position generated by image 4 relative to image 2).

To demonstrate its practicality, we attempted actual metal blocks and conducted microscopic fluorescence testing on fine metal scratches (Fig. 7b). By using a fluorescence microscope to magnify small scratches, it can be observed that the fluorescence of scratches at the grinding site is significantly enhanced under 365 nm light irradiation. When comparing the first rubbing image (2) with brushing o-NAB solution (1), obtain a new fluorescence position image and record it as Δ(21). Comparing the second rubbing image (4) with the first rubbing image (2), obtaining a new fluorescence position image and recording it as Δ(42). From images Δ(21) and Δ(42), it can be seen that after the second rubbing, the scratches presented are more obvious and then the fluorescence intensity at non-scratch areas is enhanced. More importantly, due to the excellent reversibility of this friction enhanced luminescence, with the help of solvent fuming, it is convenient to achieve repeated detection of scratches. The results indicate that o-NAB material with rational design and MLE characteristics can be used for scratch detection applications.

3. Conclusions

In summary, we have designed and synthesized a class of alkynyl-bridged boron-containing compounds, and the compound o-NAB exhibits MLE phenomenon. By leveraging intramolecular and intermolecular interactions, the molecule undergoes significant conformational changes under mechanical stress, effectively modulating CT-related conformations to achieve MLE luminescence. Through characterizations of the polycrystalline structures, luminescent properties, ultrafast transient absorption spectra, and theoretical calculations for o-NAB, the mechanism of this friction PL enhancement phenomenon has been deeply explored. Research has found that the involvement of boron, through congested crystal structures and its electronic effects, enables stress release upon grinding, facilitating a repeatable “turn-on/off” capability. Our work offers a new strategy based on stress release for designing efficient organic MLE materials for mechanoresponsive and display applications.

Author contributions

Conceptualization and writing – original draft: Yangbin Xie and Yujie Zhou. Data curation and validation: Yan-Ting Zhang and Hanting Zhou. Methodology: Zhenghua Ju. Supervision: Shenlong Jiang. Writing – review & editing: Chun-Lin Sun, Jincai Wu, Qun Zhang, and Xiaobo Pan.

Data availability

All relevant data are within the manuscript and its ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work has been supported by the National Natural Science Foundation of China (22171111, 22471110, 22075117, 92256202, 22173090, and 91950207), the National Key Research and Development Program of China (2023YFA1506804, 2022YFA1503300, 2016YFA0200602, and 2018YFA0208702), the Fundamental Research Funds for the Central Universities (lzujbky-2023-15 and lzujbky-2023-ey03), the Innovation Program for Quantum Science and Technology (2021ZD0303303), and the Anhui Initiative in Quantum Information Technologies (AHY090200). We also acknowledge the supercomputing center of Lanzhou University for DFT calculations.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2413338, 2413344 and 2413345. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5tc00180c

‡ Y. Xie and Y. Zhou contributed equally to this work.

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