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Mechanochromic Wide-spectrum Luminescence based on A Monoboron Complex Yanyu Qi, Nannan Ding, Zhaolong Wang, Ling Xu, and Yu Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21617 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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ACS Applied Materials & Interfaces

Mechanochromic Wide-spectrum Luminescence based on A Monoboron Complex Yanyu Qi, Nannan Ding, Zhaolong Wang, Ling Xu and Yu Fang* Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, P. R. China

ABSTRACT: A reversible mechanochromic luminescent (MCL) material based on a simple tetrahedral monoboron complex (B-1) is described. Interestingly, in addition to amorphous powders (P), the compound could exist in three unique crystal states (A, B, C), showing efficient green-to-red luminescent colors, which is a result of wane and wax of dual emissions of the compound. Surprisingly, one of the emissions increases significantly with increasing temperature, fully offsetting the quenching effect of temperature assisted internal conversion process. The four states are fully inter-convertible through grinding and heating, allowing color writing/painting with a single ink. KEYWORDS: tetrahedral monoboron complex, mechanochromic luminescence (MCL), organic polymorphs, dual emissions, color writing

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INTRODUCTION Over the past decade, organic materials that exhibit strong emission in the solid state have attracted considerable amount of attention owing to their applications in organic light-emitting diodes (OLEDs),1 bioimaging,2 organic lasers,3 data security,4 photovoltaic cells,5 organic field effect transistors (OFETs),6 anti-forgery materials,7 photoelectronic sensors,8 etc. However, of all the organic photo-emissive materials, only a few shows stimulus-responsive luminescent properties, such as mechanochromic luminescence (MCL). Research on these materials, especially those with MCL properties has rapidly grown over the past decade due to strong interest in underlying mechanisms.9-13 To be MCL materials, the organic solids should be highly emissive, and, at the same time, the well-known aggregation-caused quenching (ACQ) effect must be avoided. For this reason, concepts such as aggregation induced emission (AIE) and crystallization-induced emission (CIE) were proposed and established.14-16 In both systems, the molecules only display strong luminescence in aggregated state or solids. However, changes in packing modes and conformations of the molecules could show significant effect upon their emissive properties, a basis for them to be MCL materials.17-19 Therefore, the most common strategy in developing unique fluorescence tunable materials is to modify molecules so that they can take different conformations and/or pack in various ways in solid states,20 suggesting fluorophores with AIE or CIE properties may find them as MCL materials.21-28 However, a challenge remains that is how the conformational and packing behavior of a designed molecule is predicated, and how the relevant solid emission is affected. In addition, high performance, full-spectrum (blue-to-red) MCLs are rarely reported. Therefore, effort in searching of structurally controllable new luminescent materials and understanding underlying mechanisms are of pivotal importance for the rational design of the smart luminescent materials. As a class of organic photoelectronic molecules, four-coordinate organoborons with rigid conjugated structures have attracted broad interest due to their intense fluorescent emission, structural diversity, easy modification and high carrier mobility.29-38 The key structural feature of these compounds is their central atom, B, which adopts sp3 hybrid orbitals, and displays a tetrahedral geometry. It is the non-planar structure of the boron complexes that may screen their dense packing, leading to the avoidance of the ACQ effect and also guaranteeing their solid-state emission. Consequently, four-coordinate organoboron complexes with rigid skeletons have been widely used for OLEDs,39 organic lasers,40 sensors,41 fluorescence imaging,42 solar cells,43 photo-responsive materials,44 etc. Moreover, the emissive properties 2 / 21


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of the compounds can be easily tuned by modifying the structures of the chelating ligand or changing the substituents on boron.30, 34 In addition, it is also confirmed that the alteration of the molecular rigidity may also affect their photophysical properties. For example, Šket and co-workers45 reported that varying solid-state emissions were achieved from the same BF2 complex by controllable alteration of its molecular arrangements through external stimuli. Recently, Chujo and co-workers46 successfully achieved the switching and tuning of luminescence properties by modulating the packing mode of a flexible, fused, azomethine–boron complex. In the photophysical studies of tetrahedral monoboron complexes, we found, for the first time, that some of the monoboron complexes exhibit dual emissions at temperatures below the melting point of some polar solvents, of which the two emissions are respectively ascribed to originate from the local excited (LE) state and the relaxed excited (RE) state.29 Interestingly, temperature decrease enhances the emission from LE state, but weakens that from RE state, suggesting looser environment favors RE emission. Accordingly, we suspect that the unique dual emission may also be observed in solid state, since the mobility of the monoboron complexes is highly restricted, a situation very similar to that frozen in solution. Discovery of dual emission in solid state of the tetrahedral monoboron complexes is expected to be valuable for the development of MCL materials since the packing and the environmental rigidity of the building molecules can easily be tuned via grinding and heating. Accordingly, the dual emissions of a number of tetrahydral monoboron complexes with HQ as chelating ligand in solid state were examined, and it was found that only the one with 5,7-diiodo-8-hydroxyquinoline (HQ) as chelating ligand and hexyl benzene as monodentate ligands showed the expected dual emission and much pronounced MCL property when compared with the others, especially with the one lacking iodo-substitutes (DH8) and the one without alkyl chains (DB8I) (Figure S1 and Figure S2). Herein we present a new mechanism underlying MCL phenomenon. The structure of the monoboron complex, B-1, is presented in Figure 1a. As expected, the complex is luminescent, and the reasons behind can be ascribed to its tetrahedral geometry which endows it with torsional molecular conformation. It is the non-planar structure that could depress the undesired close π-stacking and screens ACQ effect as confirmed by single crystal structure described in Figure S3 in the Supporting Information. Moreover, the two phenyl rings are rotatable owing to their single-bond connection to the central atom, boron, which may allow B-1 to pack in different ways, making MCL potentially possible. As reported in an earlier publication,29 the conformational change owing to the rotation of the phenyl 3 / 21



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structures can be roughly described by two dihedral angles, α and β, which stand for the angles between one of the phenyl rings and the rigid boron containing chelating structure (Figure 1b). To verify the predicted MCL phenomenon, we grew crystals of B-1 in different conditions, and managed to obtain three polymorphs with distinct emission colors. Moreover, the obtained polymorphs exhibit typical crystal-to-crystal transformations, and well-related mechanochromic luminescence. These findings allow us to explore the relationship between the dual emission of the molecule and the MCL property of the molecule-based crystals, laying foundation for the creation of new stimuli-responsive luminescent molecular systems via utilization of the new principles.

Figure 1. Chemical structure (a), dihedral angles (b) and the optical images (c-f) of B-1 taken under daylight. Note: (c) B-1 in amorphous state (powder); (d) Crystal state, A-form; (e) Crystal state, B-form; (f) Crystal state, C-form.

RESULTS AND DISCUSSION The synthetic procedure for B-1 is described in Scheme S1. The compound was fully characterized by 1H

NMR,

13C

NMR,

11B

NMR and HRMS measurements, as well as elemental analysis. Relevant data

and information are provided in the Supporting Information. The UV−vis absorption and emission spectra of B-1 in dry CHCl3 at a concentration of 20 μM were recorded in air at room temperature, and the results are presented in Figure S4. Reference to the spectra reveals that (1) the absorption of the compound is characterized by a few peaks or bands, of which a noticeable one appears at 424 nm (ε = 2050 M−1 cm−1), and (2) the profile of the fluorescence emission spectrum is structure-less with maximum appears around 523 nm. Further fluorescence emission measurements in solvents of different polarities revealed that B-1 possesses little solvatochromic property (Figure S5). Interestingly, careful examination revealed four states of B-1 as depicted in Figure 1c-1f. Specifically, 4 / 21


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the green powder was obtained after the precipitation of B-1 from diethyl ether with n-pentane. Recrystallization of B-1 in powder state could result in two different crystals depending on the solvents used, of which A is a green flake-like crystal that was prepared by layering methanol onto the top of a dichloromethane (DCM) solution, and B, a yellow plate-like crystal which was obtained through slow evaporation of the n-hexane/DCM solution at room temperature. The powder and the two kinds of crystals prepared display similar UV−vis absorption profiles (diffuse reflectance method) before 500 nm, but unlike powder B-1, A and B show small absorptions at longer wavelengths, of which the absorption peaks of A appears around 523 nm while that of B around 556 nm (Figure S6 and Table 1). In addition, the fluorescence excitation spectra of the four forms are provided in Figure S7, which are generally in good agreement with the absorption spectra shown in Figure S6. Table 1. Selected photophysical properties of B-1 in different states a

a

Powder

Form-A

Form-B

Form-C

Solution

Description

green powder

green crystal

yellow crystal

orangish-red crystal

--------

Preparation

as prepared

recrystallization from DCM/methanol

recrystallization from DCM/n-hexane

thermal annealing

in CHCl3

UV (nm)

498b

500b, 523

500b, 556

500b, 516, 556

424

PL (nm)c

500

500, 557, 604

500, 563, 604

500, 570, 604

520

ΦF/%d

2.28

3.97

5.84

18.88

11.66

τ (ns) e

0.56

0.51, 2.49, 2.53

0.48, 2.50, 2.54

0.42, 2.56, 2.62

2.1

kr (106 s−1)

17.4

29.8, 15.9, 15.7

43.3, 21.2, 21.0

142.0, 73.8, 72.1

55.52

knr (106 s−1)

745.9

722.1, 385.7, 379.6

697.5, 342.4, 338.7

609.9, 316.9, 309.6

420.7

Experimental data: b Absorption terminus. c Emission maximum. d Absolute fluorescence quantum yield. e Fluorescence lifetimes measured

in powder or crystal state adopting a front face method using EPLED-405 picosecond pulsed-light-emitting diode as an excitation source.

Being treated at 70 °C for 30 min, the two crystal forms of B-1 could be turned into another kind of crystal (C), which is orangish red, and looks like platelet. The terminus of the broad absorption of this crystal appears at 500 nm, and, moreover new absorptions between 504 and 606 nm appear as well (Figure S6). The differences of the four types of B-1 can also be seen from the color changes (Figure 1). Wide-angle powder X-ray diffraction (XRD) measurements confirmed the crystalline structures of A, B, C in solid state as shown in Figure S8. Clearly, the XRD trace of the powder sample showed many diffraction peaks, but those of the crystals depicted only a few peaks originated from the (002), (006), (008) and (0010) lattice planes, suggesting well-structured single-crystalline eminence. 5 / 21



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Fluorescence emission spectra and fluorescence microscopy images under UV illumination (λex = 365 nm) of the amorphous and the three crystals are presented in Figure 2. It is seen that the emission spectrum of the powder B-1 is characterized by a broad and structureless band with maximum emission around 500 nm. A exhibits a bright green color (inset b (A), Figure 2) with a main emission at ~500 nm, and two small emissions at 557 nm and 604 nm, respectively. The intensity ratios of the two weak ones to the main one, I570/I500 and I604/I500, are less than 0.5 (inset a in Figure 2). For B, the band at ~500 nm is reduced but the one at ~604 nm increases significantly which is accompanied with a red shift of the band of ~563 nm, resulting in a yellow color (inset b (B) in Figure 2). The intensity ratios of the emissions are greater than 1, but less than 2.5. Remarkably, C shows orange–red color (inset b (C) in Figure 2) under UV light with its main emissions appearing at ~570 nm and 604 nm, respectively. In this case, the emission at 500 nm becomes very weak as demonstrated by the large intensity ratios (25) of I570/I500 and I604/I500.

Figure 2. Steady-state fluorescence emission spectra of the as-prepared fluorophore, B-1, in powder or crystal states (λex=400 nm, room temperature). Inset: (a) Plots of the intensity ratios with specific wavelengths of the fluorescence emissions of B-1 recorded in different states; (b) Fluorescence microscopy images of B-1 in powder or crystal states recorded at room temperature; (c) Normalized fluorescence emission spectra (at 500 nm) of the as-prepared fluorophore, B-1, in powder or crystal states (λex=400 nm, room temperature).

To explore the origins of the emissions of B-1 based polymorphs, fluorescence lifetime measurements 6 / 21


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were conducted, and the results are depicted in Figure S9. Analysis of the decays as recorded reveals that the average fluorescence lifetimes (τf) of the three emissions (~500nm, 570 nm, 604 nm) in respective of the polymorphs examined are ~0.5 ns, ~2.6 ns and ~2.6 ns, respectively (Table 1), demonstrating that the two emissions appearing at longer wavelengths should have the same origin. Thus, it is tentatively concluded that B-1 could exist in two different conformations, and each polymorph under test possesses different compositions of the two kinds of B-1 (Figure S3). In other words, variation of the ratio of the two components will result in different colors and fluorescence images. Unlike in solid, B-1 stays only in one form in solution, as confirmed by the fact that its fluorescence decay could be fully fitted with a single exponential function (Figure S10). The lifetime of B-1 in solution at room temperature is ~2.1 ns, a value in between of the two lifetimes determined in solid state, suggesting that two conformations of B-1 in solid state are actually results of dense packing. To further support the conclusion as drawn, we further examined the phase purity of the three B-1 crystals via conducting high-resolution XRD measurements at room temperature. The obtained rocking curves of the (002) diffractions of the as-grown crystals are depicted in Figure S11. It is seen that the rocking curves are not only sharp, possess a fair degree of symmetry, but also relatively narrow. Meanwhile, the widths at the half maximums of the curves are 0.144°, 0.149° and 0.152°, respectively. These facts demonstrate the good phase purity of the bulk crystals as grown.47-51 This is further supported by the pure fluorescence color of the respective bulk samples of the crystals under study. As described above, different phases of the B-1 crystals show highly unique fluorescent colors. In other words, phase impurity in any of the three B-1 crystals ought to have a mottled color under UV light. Figure S12 shows an example of fluorescence photo of a mixed phase sample of B-1, which was produced using a laboratory tablet press. This can be taken as an indirect evidence to support the phase purity of our crystal samples. The fluorescence emission efficiencies (Φf) of the four solids (P, A, B, C) and B-1 in CHCl3 were also determined, and the values are 2.28%, 3.97%, 5.84%, 18.88% and 11.66%, respectively. Clearly, C is much more efficient than the others, demonstrating that one of the B-1 states is more emissive. In addition, compared with the emission of amorphous B-1, crystalized B-1 is more emissive, indicating that the compound as produced possesses significant crystallization-induced emission enhancement (CIEE) property.52 To make sense of the structural origins of the two states of B-1 in the polymorphs under examination, 7 / 21



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single crystal X-ray diffraction studies were performed at 153 K. Spatial relations of two molecules of B-1 in corresponding crystals and the relevant dihedral angles (α, β) are depicted in Figure 3. Table S1 and Table S2 show selected bond lengths and angles relevant to the molecular packing structures presented in Figure 3. Figure S13 shows the definition of the dihedral angles (α1, β1, α2, β2). Clearly, the three crystals are all belonging to P21/n space group, and are monoclinic. As expected, boron atom locates in the center of the molecule and is chelated by HQ, producing a rigid five membered heterocycle structure which is fused with the π-unit of HQ. Moreover, together with the two carbon atoms coming from the benzene derivative, boron and its four coordinating atoms exhibits a distorted tetrahedral geometry.

Figure 3. Dihedral angles and intermolecular non-covalent interactions in form A at 153 K (a) (e), form B at 100 K (b) (f), form B at 153 K (c) (g), and form C at 153 K (d) (h). Note: The data depicted in the Table are the specific dihedral angles of the relevant crystals. 8 / 21


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Further examination of the results shown in Figure 3 reveals that B-1 displays different conformations in the polymorphs examined. In the unit cell of A, the two dihedral angles of one component molecule equal 72.78° (α1) and 88.03° (β1), respectively, but the other shows values of 108.94°(α2) and 84.26° (β2), respectively, indicating, spatially, the two molecules in the cell are very different from each other. Furthermore, dimers are formed between the two components (Figure 3a), which are connected via intermolecular C-H…π interactions with carbon atom-phenyl ring distance of ~3.7 Å (Figure 3e). For B, the values are 108.09° (α1), 82.76° (β1), 71.99°(α2), and 87.25° (β2), respectively, and the distance of intermolecular C-H…π interactions is ~3.8 Å (Figure 3c and 3g). Similarly, C is formed based on weaker intermolecular C-H…π interactions (~3.9 Å) between carbon atom-phenyl ring and the dihedral angles are 88.23° (α1), 73.95° (β1), 72.99° (α2) and 83.71° (β2), respectively (Figure 3d and 3h). In addition, crystalline lattice densities of 1.523, 1.521 and 1.520 g/cm3 (Table S2) were observed for A, B and C, respectively. These results indicate that the molecules of B-1 in C state are more relaxed than in other states, owing to lower density and weaker intermolecular interaction, which may explain why C emits at longer wavelengths. Theoretical calculations revealed the excited state (S1) of B-1 exhibits different conformations compared with its ground state (Figure S14). In addition, the HOMO and LUMO orbital plots are shown in Figure S15. It is clearly seen that the electrons of B-1 in its HOMO orbital are mainly distributed in the HQ unit, the boron atom and the two iodine atoms, whereas the electron densities on the two iodine atoms and that on one of the two phenyl rings decreased significantly upon B-1 being excited to its LUMO orbital, illustrating the importance of iodine substitution upon the observed MCL property. Figure S16 also shows the different conformations of A, B and C obtained from TD-DFT calculations, of which the obtained crystal structures were used as the input files. This result is in good agreement with the data depicted in Figure 3, which also confirms our hypothesis-the molecules of B-1 possess different mobility in different crystals. Further study reveals that for each crystal, the dihedral angles are temperature dependent (Figure 3b and 3f), suggesting the crystal may emit differently at different temperatures. To confirm the speculation, temperature-dependent fluorescence emission measurements, with B as an example, were conducted. The results are presented in Figure 4. Reference to the spectra shown in Figure 4a reveals that the intensity of the fluorescence emission at 500 nm increases gradually along with decreasing temperature, but the emissions at 562 and 604 nm, on the other hand, decrease. The 9 / 21



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observations are further confirmed by the plots shown in Figure 4b, which explains why the sample behaves more like B-1 in powder state at lower temperatures as the emission around 500 nm starts to dominate (Figure 2). In fact, the fluorescence color of the sample changes from yellow to green with decreasing temperature from 293 K to 77 K (Figure 4c), a result of enhanced emission around 500 nm and slightly decreased emission at longer wavelengths. The increase in the fluorescence emission around 500 nm is not surprising, as decreasing temperature tends to block the competitive radiation-less decay channel of the excited state of the fluorophore, resulted in the enhancement of the efficiency of radiation decay. The reason that the fluorescence emission at longer wavelengths reduces with decreasing temperature may be considered as a result of decreased amount of the molecules existing in the conformation relevant to the emission. In other words, increasing temperature favors B-1 existing in the conformation relevant to the longer-wavelength emission, a statement confirmed by further temperatureeffect studies presented later. A similar phenomenon was reported by our previous work,29 as well as other groups in the studies of the photophysical behaviors of fluorescent carborane derivatives.53-55

Figure 4. Temperature-dependent fluorescence property of polymorphous B-1 in crystal state. Note: (a) Fluorescence spectra recorded with decreasing temperature from 293 to 77 K (λex = 400 nm); (b) Plots of the emission intensity ratios with specific wavelengths against the temperature relevant to a; (c) The CIE chromaticity coordinates of the fluorescence color changing process; (d) Fluorescence spectra recorded with increasing temperature from 293 to 373 K (λex = 400 nm); (e) Plots of the emission intensity ratios with specific wavelengths against the temperature relevant to d; (f) The CIE chromaticity coordinates of the fluorescence color changing process observed in d.

To be clear, for the excited polymorph B, the state corresponding to the initial conformation is named 10 / 21


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as local excited (LE) state (emission around 500 nm) and the one corresponding to the preferential conformation as relaxed excited (RE) state (emission around 570 and 604 nm). This hypothesis can also be confirmed by the results obtained via reversibility test. As shown in Figure 4a and 4b, both the intensity and intensity ratios of the emissions changed reversely upon increasing temperature from 77 K to 293 K. For increasing temperature test, changing of B to C was observed, but cooling to room temperature did not result in the expected C to B change (Figure 4d, 4e and 4f). In other words, direct C to B transformation is not observed by simple cooling treatment. To explore the crystal transition further, differential scanning calorimetric (DSC) measurements were performed, and the results are presented in Figure S17. With reference to the trace of B, an exothermic peak around 70 °C, a main endothermic peak around 173 °C, and a small endothermic peak around 157 °C were observed, of which the exothermic process should be originated from the crystal transformation from B to C, and the endothermic processes may be well related with the melting of crystal C. Similar DSC phenomena were also observed when powder B-1 and A were used as test samples (Figure S17). As seen, the DSC trace of C is characterized by a single endothermic peak which is, no doubt originated from the melting of the sample. Melting of the B-1-related samples including P around 175 °C was directly confirmed by melting point measurements (~177 °C).

Figure 5. Reversible phase transformation of B-1 in different states as monitored by changes in fluorescent colors (ex=365 nm). Note: (1) the change could be induced by heating and grinding; (2) the photographs of A and B only represent the fluorescent colors, pure form A and B can be obtained by recrystallization.

Moreover, transformation of P or A to C with increasing temperature was further confirmed by 11 / 21



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fluorescence and PXRD measurements (Figure S18, Figure S19 and Figure S20). These results demonstrate that, thermodynamically, C is more stable than A and B, which explains why C cannot be simply changed into A or B. However, through hard grinding, the fluorescence of C can be turned into firstly B, then A, and finally P at room temperature as the fluorescence color of the sample changes from orangish red to yellow and then to yellow-green under UV light (Figure 5), a typical mechanochromic property56-69, which could be a result of conformational changes of the complex. The fluorescence of the crystals can be re-generated by simply heating the powder as obtained (Figure S21). Remarkably, the grinding and heating processes can be repeated for many times. Inspired by the observations, color writing with a single ink was tested. Figure 6 shows a typical result from the test, of which a 2-methyltetrahydrofuran (MTHF) solution of B-1 was used as the ink. The writing was performed at different temperatures. It is clearly seen that bright green, bright yellow and bright red were achieved via simple variation of the solution and the paper temperature. Color painting was also realized as depicted in the TOC of this contribution. Fortunately, the B-1 related polymorphs are thermochemically stable as they showed little change in air after they have been stored, away from light, for more than four months (Figure S22). Moreover, they are also photochemically stable. This is because continuous irritation with light (~400 nm, 2.5 h) resulted in little change in the emission intensity of different states of B-1 (Figure S23). These stabilities lay foundation for them to find practical applications.

Figure 6. Color writing with a single ink (a MTHF solution of B-1, 110-3 mol/L). Note: a) Writing with a cool solution at room temperature; b) Writing with a hot solution (70C) at room temperature; c) Writing with the hot solution on a hot paper. 12 / 21


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CONCLUSIONS In summary, a four-coordinate monoboron complex B-1 is presented as a fluorescent emitter with molecular-conformation-dependent solid-state fluorescence and remarkable MCL properties. The unusual properties of the fluorophore as generated are ascribed to the different conformations of the molecules in different crystals. Importantly, based on the luminescent, temperature-dependent wide spectrum emission of B-1, color writing and color painting with a single ink were realized, laying foundation for color printing in the future. We believe that the conformation-property relationship as revealed would open a new avenue to the design of new MCL materials. Considering the potential applications of stimuli-responsive fluorescent materials in mechano-sensing, data storage, security writing, probing, color printing, etc., further studies on B-1 related new MCL systems are presently underway in our laboratory.

AUTHOR INFORMATION Corresponding Authors Email for Y. F.: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the funding from the Natural Science Foundation of China (21527802, 21673133, 21820102005), 111 project (B14041) and Program for Changjiang Scholars and Innovative Research Team in University (IRT-14R33) and the Fundamental Research Funds for the Central Universities of China (2016TS050). We also thank Prof. Gang He (Xi’an Jiaotong University) for helpful discussion. We are grateful to Dr. Huaming Sun for his help with X-ray crystallographic analysis, and Miss Sophie Xiaohua Fang for her helps in the polish of English expressions.

ASSOCIATED CONTENT Supporting Information. Experimental section, synthetic procedures, characterization data, NMR spectra, single crystal structures and crystallographic data, thermogravimetric analysis, theoretical calculations, PXRD measurements, UV-vis absorption spectra and some fluorescence spectra.

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Composites:

Mechanochromic

Luminescence

Phenomenon

and

Thermoelectric

Performance. ACS Appl. Mater. Interfaces 2018, 10, 5603−5608. [60] Xue, P.; Ding, J.; Wang, P.; Lu, R. Recent Progress in the Mechanochromism of Phosphorescent Organic Molecules and Metal Complexes. J. Mater. Chem. C 2016, 4, 6688–6706. [61] Genovese, D.; Aliprandi, A.; Prasetyanto, E. A.; Mauro, M.; Hirtz, M.; Fuchs, H.; Fujita, Y.; Uji-I, H.; Lebedkin, S.; Kappes, M.; De Cola, L. Mechano- and Photochromism from Bulk to Nanoscale: Data Storage on Individual Self-Assembled Ribbons. Adv. Funct. Mater. 2016, 26, 5271–5278. [62] Ariga, K.; Mori, T.; Hill, J. P. Mechanical Control of Nanomaterials and Nanosystems. Adv. Mater. 2012, 24, 158–176. [63] Xue, S.; Qiu, X.; Sun, Q.; Yang, W. Alkyl Length Effects on Solid-State Fluorescence and Mechanochromic Behavior of Small Organic Luminophores. J. Mater. Chem. C 2016, 4, 1568–1578. [64] Ma, Z.; Ji, Y.; Lan, Y.; Kuang, G.; Jia, X. Two Novel Rhodamine-Based Molecules with Different Mechanochromic and Photochromic Properties in Solid State. J. Mater. Chem. C 2018, 6, 2270– 2274. [65] Li, J.; Zhou, J.; Mao, Z.; Xie, Z.; Yang, Z.; Xu, B.; Liu, C.; Chen, X.; Ren, D.; Pan, H.; Shi, G.; Zhang, Y.; Chi, Z. Transient and Persistent Room-Temperature Mechanoluminescence from a 19 / 21



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White-Light-Emitting AIEgen with Tricolor Emission Switching Triggered by Light. Angew. Chem. Int. Ed. 2018, 57, 1−6. [66] Ishizuki, K.; Aoki, D.; Goseki, R.; Otsuka, H. Multicolor Mechanochromic Polymer Blends That Can Discriminate between Stretching and Grinding. ACS Macro Lett. 2018, 7, 556–560. [67] Xiong, J.; Wang, K.; Yao, Z.; Zou, B.; Xu, J.; Bu, X. Multi-Stimuli-Responsive Fluorescence Switching from a Pyridine-Functionalized Tetraphenylethene AIEgen. ACS Appl. Mater. Interfaces 2018, 10, 5819–5827. [68] Cheng, X.; Wang, K.; Huang, S.; Zhang, H.; Zhang, H.; Wang, Y. Organic Crystals with Near-Infrared Amplified Spontaneous Emissions Based on 2’-Hydroxychalcone Derivatives: Subtle Structure Modification but Great Property Change. Angew. Chem. Int. Ed. 2015, 54, 8369–8373. [69] Mukherjee, S.; Thilagar, P. Stimuli and Shape Responsive ‘Boron-containing’ Luminescent Organic Materials. J. Mater. Chem. C 2016, 4, 2647–2662.

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Mechanochromic Wide-spectrum Luminescence ... - ACS Publications

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