A Single Crystal with Multiple Functions of Optical Waveguide

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A Single Crystal with Multiple Functions of Optical Waveguide, AIE and Mechanochromism Yan Li, Zhiyong Ma, Aisen Li, Weiqing Xu, Yuechao Wang, Hong Jiang, Kang Wang, Yong Sheng Zhao, and Xinru Jia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00195 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 23, 2017

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ACS Applied Materials & Interfaces

A Single Crystal with Multiple Functions of Optical Waveguide, AIE and Mechanochromism Yan Li,‡, # Zhiyong Ma,†, # Aisen Li,§ Weiqing Xu,§ Yuechao Wang,† Hong Jiang,† Kang Wang,ǁ Yongsheng Zhaoǁ and Xinru Jia*,† †

Beijing National Laboratory for Molecular Sciences and Key Laboratory ofPolymer Chemistry

and Physics of the Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871,China ‡

College of Materials Science and Engineering, Hebei University of Engineering, Handan

056038, China §

State Key Laboratory for Supramolecular Structure and Materials, Institute of Theoretical

Chemistry, Jilin University, Changchun 130012, China ǁ

Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences,

Beijing 100190, China. KEYWORDS: single crystal, optical waveguide, mechanochromism, hydrostatic pressure, AIE

ABSTRACT A novel single crystal PyB is produced in a high yield by a simple method of connecting a pyrene unit and a rhodamine B moiety together. PyB shows multiple functions of

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AIE, low-loss optical waveguiding and tricolored mechanochromism. The crucial point for fabricating such a multifunctional single crystal is selecting C=N group as a spacer, which simplifies the synthetic procedure, confines the molecular conformation to develop single crystals, and allows to dynamically observe the color variation in-situ and quantitatively analyze the effect of applied pressures. Such a simple approach may be extended to other fluorophores, thus provides a new opportunity for the real world application of mechanochromic materials as mechanical sensors, optical encoding, and optoelectronic devices, etc.

INTRODUCTION As well known, most luminophores with strong luminescence in solution become weak- or nonemissive in the solid states due to the notorious aggregation caused quenching (ACQ),1-3 which has greatly limited the applications of organic luminescent materials. To overcome the ACQ effect, Tang et al developed an effective methodology named aggregation induced emission (AIE).4 Since then, AIE materials have attracted considerable attention and found extensive applicaitons in optoelectronics, such as light-emitting diodes,5-9 optical waveguides,10-14 lasers,1517

solar cells18 and mechanical sensors.19, 20 The single crystalline 1D micro- or nanostructures derived from the assembled organic

emissive molecules exhibit significantly improved optoelectronic properties due to their intrinsic lightweight, free grain boundary and structure-function tunability.21, 22 These features allow such materials to serve as superior media to propagate light in a predetermined pattern.23 Although several examples of 1D organic assemblies with different shapes, including microtubes,24,

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microwires26 and nanorods,27 have demonstrated excellent optical waveguiding nature, the


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design and fabrication of high quality single crystals with low optical loss remain a great challenge. For practical applications of optoelectronic devices, organic materials with adjustable and stimuli-responsive luminescent emission and intensity are highly demanded and have been hotly studied. Mechanical force, as the credible power source and environmental friendly energy, can offer the materials with novel luminescent properties.28-30 In general, the chemical structure change and more commonly, molecular packing modes variation govern the mechanochromic luminescence of a given molecular system in the condensed phase.31-33 For example, in Prof. Misra’s work, end group and donor-acceptor interaction will greatly influence the mechanochromic behavior.1,2 However, the preparation of mechanochromic materials is relatively complicated and the simple, efficient preparation method is scarcely reported. Moreover, the comprehensive study of the underlying mechanism at the molecular level still needs further exploration.33 In addition, detection and quantitative analysis of dynamic color change and emission variation dependence on the mechanical forces remain in the preliminary stage, although it is crucial for their usage in real world. To overcome such problems, seeking a simple method to prepare a color switchable single crystal for probing the alternation of molecular assemblies and chemical structures may be much informative to reveal the structureproperty relationship. In particular, it is beneficial to quantify the effect of applied forces on the molecular materials. Tian’s group synthesized a novel mechanochromic molecule of 9, 10bis((E)-2-(pyrid-2-yl)vinyl) anthracene (BP2VA).34 Due to the different molecular packing patterns of polymorphs, BP2VA showed distinct difference in fluorescence emissions. Their study provided a further insight to understand the luminescence changes triggered by external pressure. Zhang and Zou et al. demonstrated the tautomerization of a benzo[1,3]oxazine OX-1 in


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the crystal state under hydrostatic pressure.35 With the diamond anvil cell (DAC) equipment, they dynamically observed the real-time optical images of the OX-1 crystal in the pressure increase and decrease processes. Up to now, althouth a few mechanochromic single crystals have been reported,36, 37 most of them are from serendipitous discoveries, which certainly limits to advance our knowledge for clarifying and understanding the mechanisms of mechanochromism.

Figure 1 The chemical structure of PyB and PySB. Herein, we report a novel single crystal PyB composed of two chromophores of a pyrene unit and a rhodamine B moiety (Figure 1). One notable merits of this molecule is the adoption of C=N group as a linker. Schiff base derivatives have been explored extensively as ions sensors,38 catalysts,39 and pharmacological component,40 but rarely investigated in mechanochromism. Nevertheless, in our case, the introduction of C=N group simplifies the preparation process to meet the requirement for industrial production and practical application, and endows the molecules with strong crystallizability, which ensures the single crystal with high quality and the reversible mechanochromism. This single crystal is superior to the reported examples with complex synthesis and simple properties. Additionally, it is very suitable for the dynamic observation of color change and quantitative analysis of applied mechanical force via the diamond anvil cells (DAC) technique, which is helpful for comprehensive understanding of mechanochromic mechanism. To the best of our knowledge, PyB is a rarely reported single


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crystal prepared by a simple way, which shows multiple functions including optical waveguide, AIE and mechanochromism. EXPERIMENTAL SECTION All the starting materials and solvents were purchased from commercial suppliers and directly used as received. RhB-NH2 was synthesized according to the literature.41 1H NMR with d6DMSO or CDCl3 as the solvent was recorded on 400 MHz (Bruker ARX400) spectrometer. ESI high resolution mass-spectra (HRMS) were acquired on a Bruker Apex IV FTMS mass spectrometer. Steady fluorescence spectra were performed on the Hitachi F-7000 fluorescence spectrophotometer. Wide angle X-rays diffraction (WAXD) experiments were measured on a Philips X’Pert Pro diffractometer with a 3 kW ceramic tube as the X-ray source (Cu Kα) and an X’celerator detector. Differential scanning calorimetry (DSC) measurement was carried out by using TA instruments Q100 DSC. Confocal images of single crystals were captured using A1Rsi confocal laser scanning microscope with excitation wavelength of 405nm. Fluorescence lifetime and quantum yield were measured on the LifeSpec Red Spectrometer and Nanolog FL32iHR NanoLog infrared fluorescence spectrometer, respectively. Synthesis of PyB: Py-CHO (230.3 mg, 1 mmol) and RhB-NH2 (484.6 mg, 1 mmol) were both placed in 50 ml round-bottomed flask. Ethanol (16 ml, HPLC grade) was then added and the solution was refluxed for 24h, followed by standing for another 2h at room temperature. The product was gained as a yellow solid after filtration. Yield: 92%. 1H NMR (400 MHz, d6DMSO), δ/ppm: 1.04 (t, 12H, CH3, Et), 3.24 (q, 8H, CH2, Et), 3.42 (t, 2H, CH2, NCH2CH2N), 3.54 (t, 2H, CH2, NCH2CH2N), 6.28–6.40 (m, 6H, C6H3), 6.99–7.06 (m, 1H,C6H4), 7.49–7.54 (m, 2H, C6H4), 7.81–7.84 (m, 1H, C6H4), 8.09–8.38 (m, 8H, pyrene), 8.94 (s, 1H,pyrene), 8.97


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(s, 1H, CHN).13C NMR (101 MHz, CD2Cl2), δ/ppm: 168.37, 161.28, 154.35, 153.80, 149.24, 133.04, 132.64, 131.64, 131.01, 130.03, 129.27, 128.78, 128.37, 127.76, 127.06, 126.50, 126.13, 125.91, 125.16, 124.92, 123.93, 123.42, 122.95, 108.41, 106.12, 98.09, 65.21, 60.47, 44.68, 41.75, 21.59, 14.34, 13.25, 12.72.HR-ESI-MS Calcd. For C47H44N4O2: 696.34643. Found: 697.35348 [M+H]+. Synthesis of PySB: PyB (620 mg, 0.89 mmol) was dissolved in 1,2-dichloroethane (12ml). Afterwards, sodium triacetoxy borohydride (223 mg, 1.05 mmol) was added to the solution in ice bath and stirred for 2h. The crude product was then obtained after the evaporation of the solvent and purified by column chromatography (dichloromethane: petroleum ether = 1:2 v/v in the presence of triethylamine). Yield: 60%. 1H NMR (400 MHz, CDCl3), δ/ppm: 1.07 (t, 12H, CH3, Et), 2.63 (t, 2H, CH2, NHCH2CH2N), 3.20 (q, 8H, CH2, Et), 3.41 (t, 2H, CH2, NHCH2CH2N), 4.28 (s, 2H, CH2NH), 6.12 (d, 2H, C6H3), 6.34 (s, 2H, C6H3), 6.40 (d, 2H, C6H3), 7.05 (m, 1H,C6H4), 7.43 (m, 2H, C6H4), 7.87–7.89 (m, 1H, C6H4),7.94–8.14 (m, 8H, pyrene), 8.24-8.27 (m, 1H,pyrene).13C NMR (101 MHz, CDCl3), δ/ppm: 168.70, 153.73, 153.31, 148.74, 134.08, 132.38, 131.31, 131.22, 130.87, 130.41, 128.93, 128.76, 128.00, 127.49, 127.33, 126.83, 126.72, 125.74, 124.87, 124.66, 123.81, 123.39, 122.78, 108.08, 105.62, 97.76, 65.06, 51.00, 48.06, 45.97, 44.26, 40.28, 12.56.HR-ESI-MS Calcd. For C47H46N4O2: 698.36208. Found: 699.36861 [M+H]+. Preparation of PyB single crystal: PyB single crystal was obtained by slowly evaporating the mixed solvent of dichloromethane and ethanol. Optical waveguide test: A 375 nm UV laser was used to measure the microarea PL spectra of single microrod on a glass coverslip. The excitation laser was filtered with a band-passfilter


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(330–380 nm), and focused on the microrod with an objective (5×, N.A.= 0.15). The collected emission was then guided to a grating spectrometer (Acton SP-2358) and recorded by a thermalelectrically cooled CCD (Princeton Instruments, ProEm: 1600B). Single-crystal X-ray diffraction: The single-crystal X-ray diffraction data was performed on a Rigaku AFC-10/Saturn 724+ CCD detector diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Ǻ) through the multiscan modes. The structures were solved by direct methods using SHELXS–9742 and refined by full–matrix least–squares methods on F2 with Olex-2,43 and finally checked using PLATON. CCDC 1471001 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk. DAC technique: Hydrostatic pressure experiments were carried out using BGI-type diamond anvil cell (DAC) equipment with silicone oil as pressure-transmitting medium. The diameter of diamond anvils was 0.3 mm. T301 stainless steel gaskets with thickness of 0.25 mm and center hole of 0.1 mm were preindented. The pressure determination was conducted on a Horiba Jobin Yvon JY-T64000 spectrometer using a pre-implanted ruby chip according to the R1 fluorescence technique. The excitation wavelength was 514.5 nm and the output power was 7 mW. The in situ PL spectra measurements were performed on a Jobin Yvon iHR320 spectrometer equipped with a fluorescent microscope in the reflection mode. Mercury lamp was used as the excitation source and 365nm was selected as excitation wavelength. The fluorescent images was snapshot by putting the DAC containing the ruby and single crystal on the fluorescent microscope (IX71, Olympus, 20×, nemerical aperture=0.4). All experiments were carried out at room temperature. RESULTS AND DISCUSSION


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The pristine PyB was obtained directly from one step reaction as a powder with a high yield of 92% (Scheme S1). For comparison, a control compound PySB was prepared through the reduction of C=N double bond by NaBH(OAc)3. The single crystal of PyB was easily obtained by slowly-evaporating its mixed solvent of dichloromethane/ethanol. AIE Property. PyB displayed a pronounced AIE behavior (Figure S1).4 It emitted a very weak blue light with the quantum yield (ΦF) of only 0.4% in a dilute ethanol solution, which was probably caused by the photo-induced electron transfer (PET) process derived from imine nitrogen to the excited pyrene motiety.44-47 Upon addition of water to the system, PET was suppressed because of the H-bond interaction between O-H in H2O as the H-bond donor and N in PyB as the H-bond acceptor. As a result, the fluorescence emission was dramatically enhanced with the maximum ΦF of 37.9% at the water fraction of 50%, which was about 95-fold higher than that of its dilute ethanol solution. The drastic increase of ΦF convincingly proved the AIE property of PyB. Similar phenomenon for other C=N containing molecules or pyrene derivatives was also observed by the pioneers.48 For example, Kathiravan and coworkers reported a pyrenecontaining schiff base (KB-1). It expressed obvious AIE property due to the inhibition of PET by the addition of water.46 However, unlike the traditional AIE/AIEE systems,49-52 further addition of water in our system resulted in not only the fluorescence emission red-shifting from 460nm to 476nm, but also the quantum efficiency rapidly decreasing to 4.8% at the water content of 90%, probably owing to the formation of various states of aggregates.53 The aggregates formed in 80% and 90% (fw) aqueous solution were then examined by dynamic light scattering (DLS, Figure S2), which stated kinds of aggregates. The ΦF was improved to 9.8% for the amorphous solid of PyB, further proving the AIE activity.


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Figure 2. (a) Bright field (top) and microarea emission images of a rod-like single crystal of PyB (Exciting at six different positions with a 375 nm focused laser, respectively. Scale bar: 100µm); (b) Fluorescence spectra of the rod tip. The Itip/Ibody decays of 440nm (c) and 497nm (d) from 1 to 6. Optical Waveguide. Unexpectedly, we found that the microrod-like single crystal PyB exhibited unique optical waveguide ability (Figure 2a). While excited by a focused laser (375 nm) at different positions along its length, bright blue emission was found from the edge and both ends of the rod irrespective of the excited position, which was a typical characteristic of optical waveguide materials. In addition, with the guided distance increasing, the emission color of the rod tip became dark and turned to green. From the fluorescence spectra (Figure 2b), we also observed the decay of fluorescent intensity with distances, and the more rapid decrease of emission at 440nm compared to that at 497nm, indicating that this interesting blue-to-green transformation was probably caused by the self-absorption of blue light during the light


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propagation process. UV-Vis spectrum gave the direct evidence of the self-absorption. As shown in Figure S3, PyB had a broad absorption from 270 nm to 500 nm and the absorption of 440 nm was obviously higher than that of 497 nm. The reabsorption was further verified by the calculation of optical loss coefficient (α) using the Equation (1): Iout/Iin = Aexp(-αL)

(1)

where Iout and Iin were the fluorescent intensities of out-coupled and incidence light, respectively, and L was the propagation distance.54 The α for 440 nm and 497 nm were calculated to be 0.00669 dBµm-1 and 0.00351 dBµm-1 (Figure 2c and d), respectively, indicating that the blue light at 440 nm had a relative higher optical loss arising from reabsorption. It should be noted that the calculated α values are about ten-fold lower than those of the similar 1D microwires.55 The smooth surface, large size and high crystal quality may contribute to the excellent waveguiding property, thus declaring great potential of PyB single crystals for the future sophisticated optical communications. Mechanochromism. As expected, PyB showed a characteristic color variation upon mechanical action. As shown in Figure S4, the pristine PyB powder gave a very broad emission from 400 nm to 700 nm with a center at 492 nm, which might be due to the complicated aggregation states of pristine PyB from the fast-cooling reaction system. However, the developed large crystals exhibited a narrow fluorescent emission at 447nm with a deep-blue color, corresponding to the partially-overlapped excimer I of pyrene (Figure 3b).56 Upon grinding by a spatula, the deep-blue color transferred to bright-green and the emission red-shifted to 507 nm, coincident with the sandwich-like packed excimers II of pyrene. Accordingly, the quantum yield ФF increased from 3.52% to 9.80% and the average lifetime τpw changed from


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Figure 3. (a) Mechanofluorochromism of PyB and the color recovery after annealing and ethanol treating (λex = 365nm); (b) Fluorescence spectra of PyB at different solid states; (c) WAXD patterns (i to v represent the original, ground, red powders, and the red samples after annealing and ethanol treatment, respectively); (d) DSC profiles of the pristine powder of PyB (black line) and the ground powder (red line). 6.73 ns to 20.30 ns before and after grinding (Table S1). These results demonstrated that the highly ordered structure was disturbed by the anisotropic grinding, leading to the pyrene groups slippage to form stable excimer II. Further grinding in situ afforded a red colored powder with a new fluorescent emission at 578 nm, which was attributed to the emission of the ring-opening isomer of rhodamine B with a more planar and conjugated structure evidenced by the literature reports and our previous study.31 The force-induced color switch could be fully restored by


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simple annealing and solvent treatment (Figure 3a, b). The red powder was converted rapidly to green by heating the sample at 150oC for 2 min, and fully returned to the original deep-blue with the aid of ethanol. Correspondingly, the fluorescence emission reappeared at 492 nm and 445 nm, respectively, indicating the crystal structure was thermodynamically stable and could be restored simply. Such reversibility was quite different from our previous results. For example, the fluorescence of single crystal M-4-B could be recovered from reddish to bluish-green by heating, but could not be fully restored to the original deep-blue either by heating or by solvent treating. The different reversibility between PyB and M-4-B was probably caused by the different crystallization capacity, where C=N linker might perform important functions. Futhermore, the reversible color change of PyB could be repeated for several times without any fatigue (Figure S5), further desmontrating the excellent crystallizability of PyB. Wide-angle X-ray diffraction (WAXD) was conducted to primarily know the microstructure of the pristine powders under different treating conditions (Figure 3c). Before grinding, the pristine PyB exhibited numerous sharp and intense reflection peaks. This pattern was accorded well with the simulated one based on PyB crystal data (Figure S6), suggesting that the pristine powder was mainly composed of the microcrystals of PyB. These reflection peaks disappeared for the bright-green and red samples after in-situ grinding, which illustrated that the bathochromic shift of fluorescence emission was induced by the structural transformation from crystalline form to amorphous state. After heating and ethanol treatment, plenty of sharp peaks reappeared and agreed well with that of the pristine sample, suggesting that the amorphous sample was metastable and could achieve a stable crystalline state by recrystallization. Differential scanning calorimetry (DSC) results further confirmed the phase change (Figure 3d). The pristine PyB only showed one sharp endothermic peak at 202.4oC on the heating cycle,


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relating to the melting of the crystalline phase. Nevertheless, the ground sample displayed a coldcrystallization transition peak at 155.7 oC before the broad melting peak at 198.2 oC during the first heating cycle, implying that the metastable amorphous powder could convert to a more stable crystalline phase by annealing.57 All abovementioned observations clarified that the anisotropic grinding induced the transition of molecular assemblies from a stable crystalline phase to a metastable amorphous state, which altered the packing of the pyrene excimers, leading to the increase of fluorescent quantum yield and the change of emission colors.

Figure 4. (a) Fluorescence images of PyB single crystal under different hydrostatic pressures; (b,c) Corresponding PL spectra during pressurizing and depressurizing process. In order to dynamically observe the color variation in-situ and quantify the dependence of emission transformation on the external force, hydrostatic pressure was applied to the PyB single


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crystals via the DAC equipment, where silicone oil was used as a pressure transmitting medium. As shown in Figure 4a, the pristine single crystal was transparent and deep-blue under the 365nm UV-light. There was no continuous color variation during the pressurizing process, instead, an obvious two-stage change process, blue to green and then to dark red from the fluorescent images, was observed following the increase of hydrostatic pressure. Initially, no apparent change was found in both the fluorescent images and spectra as the pressure below 1.22 GPa. With the applied pressure increased to 2.61 GPa, the single crystal turned to cyan, accompanied by the red-shift of emission from 446 to 507 nm (Figure 4a, b). In this period, the changes of both emission wavelength and fluorescent intensity were quite similar to that of sample upon grinding, indicating PyB single crystals might experience almost identical changes upon different mechanical stimuli. Thereafter, the emission was constantly red-shifted, but the luminescence intensity was gradually weakened. Accordingly, the deep-blue single crystal became green and then to dark red when the applied pressure was raised to 5.12 GPa. Eventually, the single crystal underwent a sequential color variation from deep-blue to cyan and green, and an abrupt change to dark-red with a large red-shifted emission of 130 nm from 446 nm to 507 nm and to 576 nm. Upon releasing the pressure, the single crystal could be restored to cyan (Figure 4a, c) rather than deep-blue. This incomplete recovery might be caused by the lattice disturbance under such a high pressure. Single Crystal Analysis. To explore the relationship between molecular structure and spectroscopic behavior, and to gain further insight into the mechanochromic mechanism, we analyzed the single crystal structure of PyB (Table S2). The single crystal is attributed to a triclinic system, with a space group of P-1(No.2). The molecular geometry of PyB in the crystal is highly twisted with pyrene and rhodamine B nearly perpendicular to each other. The pyrene


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unit and imine group locate basically at the same plane with a dihedral angle of 8.55oC (Figure 5a). In each unit cell, two pyrenyl groups are partially overlapped with the vertical distance of 3.43 Å, suggesting the formation of a J-type aggregate. Thus the two molecules are interacted by π···π stacking and weak C-H···π and C-H···O bonds (Figure 5b). In such a way, pyrene units are in the restrained state, which corresponds to the literature report that the partially overlapped arrangement of pyrene groups is usually achieved either in a viscous liquid or in a crystalline state.58, 59 It is very interesting to note that PyB is polymorphism because our single crystal is distinctly different from that reported by Prof. Das and coworkers,47 who gained a monoclinic crystal system with a space group of C2/c for the same structured compound to investigate its ion-sensing property in the solution state. Viewed along the a axis (Figure S7b), PyB molecules align to a column-like mode, and the adjacent molecules are antiparallelly packed with the pyrene groups in the center and rhodamine B moieties around of the column. While observed from the b axis (Figure S7c and Figure 5c), it is more clear to visualize a column-like pattern.


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Figure 5. (a) ORTEP drawings (CCDC 1471001). Hydrogen atoms are omitted for clarity. Inset: Cofocal Laser Scanning Microscope image of PyB single crystals (Scale bar: 100µm); The stacking mode in an unit cell (b) and along b axis (c). DFT Calculation. Computer simulation and calculations afforded a deeper understanding for transformation of the packing mode in PyB single crystals induced by hydrostatic pressure. Density functional theory (DFT) calculations under different pressure were performed in VASP Package based on the structure data of PyB single crystal. Table S3 and S4 gave the optimized lattice parameters, the external pressure, the interplanar distance and the projected area of adjacent pyrene groups under different volume of 1.0, 0.9 and 0.8. These calculated results illustrated that the adjacent pyrene groups trended to move closer vertically and had a slight slippage to each other with the external stress increasing. Accordingly, further compressing the


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single crystal by higher pressure would generate a much tighter structure with higher molecular density. Therefore, we suspect that the J-type aggregate is strengthened initially due to the enhanced π···π interaction, which facilitates to form the excimer II of pyrene. Such effect results in the increase of fluorescent intensity and the change of emission from deep-blue to cyan, similar with that of sample upon grinding. However, higher pressure compels the pyrene groups to pack more close with each other, causing the gradual red-shift of emission and the dramatic decrease of fluorescence intensity, which is natural in the hydrostatic pressure-induced color switchable materials.34, 60 As a comparison, we also examined the mechanical-responsive behavior of the control compound of PySB. It could not form single crystal in spite of kinds of solvent and methods applied. Only two colors change was observed, a blue color emitted at 460 nm and a red color lighted at 581 nm after heavily grinding in situ (Figure S9). Both the original and ground powder exhibited the amorphous phase with broad and diffused signals in the WAXD profiles (Figure S10). By the aforementioned results, we confirm that the C=N group in PyB plays a key role in controlling the molecular conformation and endows it with enhanced crystallization capacity. This conclusion is also supported by our following study with C=N group as a crystallization enhancer. Such a result is encouraging because the Schiff base derivatives are simple to be obtained, which provides a new way for industrial production and practical applications of mechanochromic materials. CONCLUSION


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In conclusion, a novel single crystal PyB with C=N group as a linker was designed and obtained in a high yield. PyB exhibited mutiple functions of effective photo waveguiding with low optical loss, AIE behavior and reversible tricolored switch either by grinding or under hydrostatic pressure. Both mechanical stimuli promoted the formation of pyrene excimer II and the isomerization of rhodamine B moiety, which caused the increase of fluorescent intensity and transformaion of emission colors from deep-blue to green and to red. Continuously pressurizing the single crystal, however, resulted in dramatic decrease of fluorescent intensity due to the enhanced π-π interaction. The multiple funtions are all derived from the highly-ordered single crystal architecture, which is easily realized by using C=N group as a crystallizaiton enhancer. Such a simple approach can also be extended to other fluorophores besides pyrene, thus provides a new opportunity for the scale-up fabrication of multifunctional single crystals which may find useful applications in mechanical sensors, optical encoding, optoelectronic devices, quantitative determination, and so forth. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. The synthetic procedure of PyB and PySB, fluorescent images and spectra of PyB solution, lifetimes, quantum yields, single crystal structure data and packing mode of PyB, computer simulations, mechanochromism and WAXD patterns of PySB. (PDF) CCDC 1471001 (CIF) AUTHOR INFORMATION


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Corresponding Author *Email: [email protected] Fax: +86-010-62751708 Author Contributions #

These authors contributed equally.

Funding Sources This work is financially supported by the National Natural Science Foundation of China (21174005 and 21274004) to X.-R. Jia and Natural Science Foundation of Hebei Province (B2016402030) to Tao Chang. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank Dr. Yangang Bi (Peking University) and Dr. Yongli Yan (Institute of Chemistry, Chinese Academy of Sciences) for the help of single crystal analysis and optical waveguide measurement, respectively. REFERENCES (1)

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A Single Crystal with Multiple Functions of Optical Waveguide

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