DOI: 10.1002/asia.201500499
Full Paper
Luminescence
A Mechanochromic Luminescent Dye Exhibiting On/Off Switching by Crystalline–Amorphous Transitions Hiroaki Imoto, Kohei Kizaki, and Kensuke Naka*[a] Abstract: A mechanochromic luminescent dye based on a simple aminomaleimide skeleton was readily synthesized in a one-pot process. It exhibited an on/off mechanochromic luminescent switching property dependent on external stimuli, unlike a traditional mechanochromic color change. The green emission was turned on by grinding in a mortar and turned off by heating or treatment with dichloromethane. In the crystalline state, two molecules were stacked by cofacial p–p interactions, which caused concentration self-quench-
Introduction Mechanochromic luminescent materials[1] have attracted much attention owing to their potential applications in sensors,[2] security systems,[3] and memory devices.[4] The emission properties are changed by transformation of the molecular planarity and/or arrangement in the solid state with mechanical stimuli, such as grinding and pressing. In almost all cases, on the other hand, responses to mechanical input are emission color changes, which are insufficient in contrast to attain clear visibility for the naked eye. To overcome this problem, on/off mechanochromic luminescent switching system is required.[5] Some research into systems in which a mechanical stimulus turns on the emission and another stimulus turns it off have been reported toward sensing of mechanical stimuli.[6, 7] However, these studies employed complicated donor–accepter pairs[6] or metal–organic frameworks[7] to realize reversible on/off cycles. Thus, more simple system is necessary for more practical molecular design of on/off mechanochromic luminescent switching materials. To achieve this purpose, the material must be composed of a small molecule possessing a single luminophore. The molecules should be arranged in such a way as to cause self-quenching of the luminescence in the crystalline state to maintain the off state, and the arrangement should collapse under mechanical stimuli to turn on the emission. [a] Dr. H. Imoto, K. Kizaki, Prof. K. Naka Faculty of Molecular Chemistry and Engineering Graduate School of Science and Technology Kyoto Institute of Technology Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500499. Chem. Asian J. 2015, 10, 1698 – 1702
ing. The crystalline-to-amorphous transition induced by grinding removed cofacial p–p stacking, which led to intensive emission. Crystallizing processes recovered the cofacial p–p stacking, resulting in elimination of the emission. Theoretical calculations and X-ray diffraction analyses revealed that the dye molecule was distorted in the crystalline state; thus even a mechanical stimulus caused the crystalline-toamorphous transition.
However, such a molecule has never been reported, to the best of our knowledge, despite the simple concept. The most challenging matter lies in the balance of intermolecular interactions. Because intermolecular interactions for quenching, namely, p–p stacking, tend to construct strong long-range networks in the crystalline state, mechanical stimuli cannot break such structures. Thus, it is necessary to form intermolecular interactions that can cause self-quenching without network structures. Recently, we reported aggregation-induced emission (AIE)[8] of active aminomaleimide dyes.[9, 10] The aminomaleimide skeleton can be readily constructed through a one-pot process by using commercially available dimethyl acetylenedicarboxylate and primary amines.[10] For example, 2-anilino-N-phenylmaleimide (2) shows green emission in the solid state, whereas no emission was observed in solution. Free rotation of the secondary amine deactivates the exciton through a nonradiative pathway in good solvents, and the motion is frozen in the solid state to show the emission. In the course of screening of molecular structures, we found that 2-(m-trifluoromethylanilino)-N-(m-trifluoromethylphenyl)maleimide (1) exhibited no emission in the crystalline state. The packing structure of 1 was likely to cause concentrated self-quenching; thus we ground the crystals of 1 to change the molecular packing. Fortunately, the ground sample of 1 showed photoluminescence (PL) under UV irradiation, and the emission was erased by crystallization. This observation implies that 1 possesses an on/off mechanochromic switching property, unlike a traditional mechanochromic color change. Herein, we propose a novel on/off-type mechanochromic dye, and demonstrate its switching behavior. The mechanism of the unique mechanochromism was explored on the basis of X-ray diffraction (XRD) analyses.
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Full Paper Results and Discussion Synthesis The synthetic route of 1 is shown in Scheme 1. A mixture of dimethyl acetylenedicarboxylate and m-trifluoromethylaniline was heated at 150 8C for 4 h. The precipitates generated were collected by filtration and washed with methanol. Purification by recrystallization from dichloromethane (CH2Cl2) and methanol gave 1. The significant advantage of this process is that the reaction can be carried out without any solvents, additives, or complicated purification process.
addition, emission of the ground sample was also erased by the process in which small amounts of CH2Cl2 were added and subsequently dried in the mortar. These results mean that an on/off switching system is constructed by employing external stimuli: grinding turns on the emission and heating or CH2Cl2 treatment turns it off. To check the absence of decomposition, we measured the 1H NMR spectrum of the sample after grinding in a mortar and heating at 170 8C for 10 min, and there was no significant difference from the spectrum before the process (Figure S2 in the Supporting Information). Demonstration of successive switching was performed in the mortar. Treatment with CH2Cl2 and heating at 170 8C were employed to turn off the emission in the first four cycles and the last three cycles, respectively. The switching behavior was so clear that we were able to perceive the mechanical input by eye (Figure 2 a). PL measurements confirmed the switching performance (Figure 2 b). During the switching process, the
Scheme 1. Synthesis of 1.
In this reaction, the hydroamination of a primary amine to give dimethyl acetylenedicarboxylate results in a trans adduct, and subsequently, trans–cis isomerization accompanied by condensation gives the 2-arylaminomaleimide derivative.[11] This synthetic procedure can be performed in one pot. Mechanochromism The crystals of 1, which were obtained by recrystallization from CH2Cl2 and methanol, showed no emission under UV irradiation at l = 365 nm. However, once these crystals were ground in a mortar, green emission was observed (lmax = 516 nm; Figure 1). The emission intensity of the ground sample was more than seven times higher than that of the crystalline sample. For crystallization, the ground sample was heated to 170 8C, just above its melting point (168 8C), for 10 min. After cooling to room temperature, the emission was eliminated. In
Figure 1. Photographs and PL spectra (excited at l = 365 nm) of 1 before and after grinding. Chem. Asian J. 2015, 10, 1698 – 1702
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Figure 2. On/off switching cycles of 1 detected with a) the naked eye (excited at l = 365 nm); b) the intensity of PL spectra at l = 516 nm (excited at l = 350 nm); and c) XRD patterns measured after grinding, heating, and CH2Cl2 treatment, and simulated powder XRD pattern of the crystal sample.
morphology of the solid samples was traced with XRD analysis (Figure 2 c). After every CH2Cl2 treatment and heating step, relatively defined peaks, which were corresponded well with the powder XRD pattern simulated by results of single-crystal XRD, were observed, although these peaks disappeared after every grinding step. This result indicated that grinding caused a crystalline-to-amorphous transition to turn on the emission, and that CH2Cl2 treatment and heating processes caused an amorphous-to-crystalline transition to turn it off. 1699
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Full Paper Optical Properties The optical properties of 1 were compared with those of 2, which was reported in our previous study as an AIE active dye (Table 1).[10] The absorption maxima of 1 were slightly blueshifted from those of 2 because electron-withdrawing CF3 units lowered the HOMO level, as estimated by DFT calculations performed with the Gaussian 09[12] suite of programs at the level of B3LYP/6-31G(d) theory. Solutions of 1 and 2 exhibited no emission because the excitons were deactivated by molecular motion, according to our previous reports.[9, 10]
Table 1. Optical properties and results of DFT calculations for dyes 1 and 2. Dye
labs,sol[a] [nm]
lPL,solid[b] [nm]
FPL,solid[c]
HOMO[d] [eV]
LUMO[d] [eV]
1 2
372, 280 380, 285
516 517
–[e] 0.05
¢6.38 ¢5.92
¢2.59 ¢2.18
Figure 4. Packing structures of 1: a) front view, b) side view, c) hydrogen bond and d) p–p interactions. Hydrogen atoms are omitted in a) and b) for clarity.
[a] Absorption in chloroform (c = 1.0 Õ 10¢4 m). [b] PL in the solid state. [c] Absolute PL quantum efficiencies in the solid state. PL spectra were measured under l = 350 nm irradiation for excitation. [d] Calculated at the B3LYP/6-31G(d) level of theory by using the Gaussian 09[12] suite of programs. [e] FPL of solid 1 was highly dependent on the preparation procedure for the ground samples.
Crystals of 1 showed no emission, whereas those of 2 showed green emission (l = 517 nm). The chemical structures, optical properties in solution, and calculated molecular orbitals were not significantly different between 1 and 2 (Table 1). In addition, grinding of the crystals of 2 did not change the PL properties. Therefore, we focused on the packing structures in the crystalline state.
because steric hindrance of the m-CF3 groups inhibits intermolecular CH···p interactions, and a noncovalent network structure cannot be constructed. Instead, antiparallel cofacial p–p stacking was observed (Figure 4 b and d). The p–p interactions resulted in concentrated self-quenching, so that 1 showed no emission in the crystalline state.[14] It is thought that mechanical stimuli collapsed the stacked structure to show emission without self-quenching through p–p interactions. To understand the driving force toward the collapse of the cofacial p–p interactions of 1, the torsion of the molecular planarity was evaluated on the basis of the interplanar angle of the maleimide and aminobenzene rings. The interplanar angles in the optimized structures were estimated by DFT calculations, and those in the single crystals were measured by XRD (Table 2). The angles of 1 and 2 in the crystalline states were
Single-Crystal XRD Analysis The results of single-crystal X-ray analysis on 2 are shown in Figure 3.[9, 13] Each of the hydrogen bonds (Figure 3 a) and CH···p interactions (Figure 3 b) were formed in a linear arrangement, which resulted in the robust network structure. The CH···p interaction enabled the molecules to be packed in a non-coplanar arrangement. This is why 2 shows intensive emission in the crystalline state without self-quenching. In the case of 1,[13] on the other hand, dimerization occurred through the double hydrogen bonds of NH¢O (Figure 4 a and c), and no CH···p interactions were observed. This is probably
Figure 3. Molecular packing structures of 2 in the single crystal : a) hydrogen bond and b) CH···p interactions. Chem. Asian J. 2015, 10, 1698 – 1702
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Table 2. Interplanar angles in dyes 1 and 2.[a]
Dye
Optimized structure[b] [8]
Single crystal[c] [8]
1 2
16.0 17.8
29.0 20.8
[a] The interplanar angles of the maleimide and secondary aminobenzene rings. [b] Estimated by DFT calculations at the level of B3LYP/6-31G(d) theory. [c] Measured by single-crystal XRD.
larger than those in the optimized structures by 13.0 and 3.08, respectively. This result implies that the molecular planarity of 1 was much more twisted in the crystalline state than that of 2. In addition, compound 1 did not form a robust noncovalent bond network. This is why the cofacial p–p stacking of 1 was readily collapsed by mechanical stimuli (Figure 5). Crystallizing processes, such as heating and CH2Cl2 treatment, recovered the p–p interactions, which quenched the emission. In the case of 2, no mechanochromic phenomenon was observed because molecules were tightly tied up in the network through hydrogen bonds and CH···p interactions without a large distortion of molecular planarity.
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Full Paper
Figure 5. Schematic illustration of the mechanism of on/off switching mechanochromism.
Dispersion in Polymer Matrices For further observation of the emission behavior, the PL spectra of 1 were measured in polymer matrices (Figure S3 in the Supporting Information), in which the molecular motions were restricted and the molecules could be dispersed without p–p stacking. Polystyrene (PS), poly(methyl methacrylate) (PMMA), and ethylene vinyl acetate copolymer (EVA) were used as matrix polymers. To prepare the samples, compound 1 (3.8 wt %) was added to these polymers. The dye-dispersed samples showed light-blue emissions at l = 477–484 nm (Figure 6 a). The emission was slightly blueshifted in comparison to the ground solid sample (lmax = 516 nm), possibly because the planarity of 1 was distorted in the polymer matrices. The XRD pattern of the PS sample displays no defined signal (Figure 6 b), which suggests that no significant crystalline aggregation occurs in the polymer matrix. This result supports the idea that the prevention of crystallization leads to the emission of 1.
interactions in the crystalline state, leading to emission. On the other hand, compound 1 formed cofacial p–p interactions without network structures in the crystalline state and the molecular planarity of 1 was twisted. Thus, a mechanical stimulus readily collapsed the cofacial p–p stacking, and the crystallineto-amorphous transition caused the emission. Crystallizing processes recovered the cofacial p–p interactions for the elimination of the emission. The simple molecular structure, easy synthetic procedure, visually defined on/off contrast, and good reproducibility are great advantages for applications. The molecular design proposed in this study is a promising route to mechanical sensing materials.
Experimental Section Materials Dimethyl acetylenedicarboxylate was purchased from Tokyo Chemical Industry Co., Ltd. m-(Trifluoromethyl)aniline was purchased from Wako Pure Chemical Industry, Ltd. Dichloromethane and methanol were purchased from Nacalai Tesque, Inc. EVE was provided by Nissan Chemical Industries, Ltd. All chemicals were used without any purification. Compound 2 was prepared by following a procedure reported in the literature.[10]
Measurements 1
H NMR (400 MHz) spectra were recorded on a Bruker DPX-400 spectrometer, and samples were analyzed in CDCl3 by using Me4Si as an internal standard. The following abbreviations were used: s: singlet, d: doublet, m: multiplet. HRMS was recorded on a JEOL JMS-SX102A spectrometer. The UV/Vis spectra were recorded on a Jasco spectrophotometer (model V-670 KKN; Jasco, Tokyo, Japan). PL spectra were obtained on an FP-8500 (JASCO) instrument. XRD patterns were recorded on a Smart Lab (Rigaku) diffractometer with CuKa radiation (l = 1.5406 æ) in q/2q mode at room temperature. The 2q scans were collected at 0.018 intervals, and the scan speed was 28(2q) min¢1.
X-ray Crystallographic Data for Single-Crystalline Products
Figure 6. a) Photographs of polymer dispersion films under irradiation at l = 365 nm, and b) XRD patterns of crystals and a film of 1 (3.8 wt %) in PS.
Conclusion We developed a novel on/off switching mechanochromic luminescent dye, 1. The intensive green emission was triggered by grinding and erased by heating or CH2Cl2 treatment. The intermolecular noncovalent bonds in the crystals were one of the key factors in the present unique optical property. Molecules of 2 formed a robust network structure without cofacial p–p Chem. Asian J. 2015, 10, 1698 – 1702
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The single crystal was mounted on glass fibers with epoxy resin. Intensity data were collected at room temperature on a Rigaku RAXIS RAPID II imaging plate area detector with graphite monochromated MoKa radiation. The crystal-to-detector distance was 127.40 mm. Readout was performed in the 0.100 mm pixel mode. The data were collected at room temperature to a maximum 2q value of 55.08. Data were processed by using the PROCESSAUTO[15] program package. An empirical or numerical absorption correction[16] was applied. The data were corrected for Lorentz and polarization effects. A correction for secondary extinction[17] was applied. The structure was solved by heavy-atom Patterson methods[18] and expanded by using Fourier techniques.[19] Some non-hydrogen atoms were refined anisotropically, whereas the rest were refined isotropically. Hydrogen atoms were refined by using the riding model. The final cycle of full-matrix least-squares refinement on F2 was based on observed reflections and variable parameters. In the case of the crystalline product recrystallized from acetone, the final cycle of full-matrix least-squares refinement on F was based on observed reflections and variable parameters. All calculations were performed by using the CrystalStructure[20, 21] crystallo-
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Full Paper graphic software package. Crystal data and more information on Xray data collection are summarized in Tables S1 and S2 in the Supporting Information.
Sample Preparation 2-(m-Trifluoromethylanilino)-N-(m-trifluoromethylphenyl)maleimide (1): A mixture of dimethyl acetylenedicarboxylate (0.394 g, 2.77 mmol) and m-trifluoromethylaniline (1.77 g, 11.0 mmol) were heated at 150 8C. After stirring for 4 h, the reaction mixture was cooled to ¢10 8C in a freezer. The generated dark-brown precipitates were collected by filtration and washed with methanol. The isolated residue was subjected to recrystallization from dichloromethane and methanol to give 1 as a yellow solid (0.244 g, 0.609 mmol, 22 %). M.p. 167.8–168.4 8C; 1H NMR (400 MHz, CDCl3): d = 7.76 (s, 1 H), 7.68–7.56 (m, 4 H), 7.48–7.46 (m, 3 H), 7.40 (d, J = 8.4 Hz, 1 H), 5.76 ppm (s, 1 H); the solubility of 1 was too low to measure a 13C NMR spectrum; HRMS (FAB): m/z calcd for C18H10F6N2O2 [M] + : 400.0646; found: 400.0653; elemental analysis calcd (%) for C18H10F6N2O2 : C 54.01, H 2.52, N 7.00; found: C 53.81, H 2.49, N 7.18. Dye-dispersed polymer films: A polymer (100 mg of EVA, PS, or PMMA) was added to a solution of 1 (4.0 mg, 0.01 mmol) in toluene (3 mL). Each of the solutions were cast onto a silicon wafer. Films were formed at 70 8C and heated at 130 8C for 20 min to give 3.8 wt % dye-dispersed polymer films.
Acknowledgements We thank Dr. K. Matsukawa and Dr. S. Watase of Osaka Municipal Technical Research Institute for single-crystal X-ray diffraction measurements. Keywords: dyes/pigments · luminescence · maleimides · mechanochromism · X-ray diffraction [1] a) Y. Sagara, T. Kato, Nat. Chem. 2009, 1, 605; b) Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu, J. Xu, Chem. Soc. Rev. 2012, 41, 3878; c) X. Zhang, Z. Chi, Y. Zhang, S. Liu, J. Xu, J. Mater. Chem. C 2013, 1, 3376. [2] For examples of recent papers, see: a) D.-H. Park, J. Hong, I. S. Park, C. W. Lee, J.-M. Kim, Adv. Funct. Mater. 2014, 24, 5186; b) J. R. Hemmer, P. D. Smith, M. van Horn, S. Alnemrat, B. P. Mason, J. R. deAlaniz, S. Osswald, J. P. Hooper, J. Polym. Sci. Part B 2014, 52, 1347; c) F. Cellini, S. Khapli, S. D. Peterson, M. Porfiri, Appl. Phys. Lett. 2014, 105, 061907. [3] a) H. Sun, S. Liu, W. Lin, K. Y. Zhang, W. Lv, X. Huang, F. Huo, H. Yang, G. Jenkins, Q. Zhao, W. Huang, Nat. Commun. 2014, 5, 3601; b) X. Zhu, R. Liu, Y. Li, H. Huang, Q. Wang, D. Wang, X. Zhu, S. Liu, H. Zhu, Chem. Commun. 2014, 50, 12951. [4] a) M. Irie, T. Fukaminato, T. Sasaki, N. Tamai, T. Kawai, Nature 2002, 420, 759; b) C. E. Olson, M. J. R. Previte, J. T. Fourkas, Nat. Mater. 2002, 1, 225; c) S. J. Lim, B. K. An, S. D. Jung, M. A. Chung, S. Y. Park, Angew. Chem. Int. Ed. 2004, 43, 6346; Angew. Chem. 2004, 116, 6506; d) S. Hirata, T. Watanabe, Adv. Mater. 2006, 18, 2725.
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[5] X. Luo, J. Li, C. Li, L. Heng, Y. Q. Dong, Z. Liu, Z. Bo, B. Z. Tang, Adv. Mater. 2011, 23, 3261. [6] a) J. Luo, L.-Y. Li, Y. Song, J. Pei, Chem. Eur. J. 2011, 17, 10515; b) M.-J. Teng, X.-R. Jia, S. Yang, X.-F. Chen, Y. Wei, Adv. Mater. 2012, 24, 1255; c) M. S. Kwon, J. Gierschner, S.-J. Yoon, S. Y. Park, Adv. Mater. 2012, 24, 5487. [7] J.-K. Sun, C. Chen, L.-X. Cai, C.-X. Ren, B. Tan, J. Zhang, Chem. Commun. 2014, 50, 15956. [8] For reviews on AIE, see: a) Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Commun. 2009, 4332; b) D. Ding, K. Li, B. Liu, B. Z. Tang, Acc. Chem. Res. 2013, 46, 2441; c) R. Hu, N. L. C. Leung, B. Z. Tang, Chem. Soc. Rev. 2014, 43, 4494; d) J. Mei, Y. Hong, J. W. Y. Lam, A. Qin, Y. Tang, B. Z. Tang, Adv. Mater. 2014, 26, 5429. [9] T. Kato, K. Naka, Chem. Lett. 2012, 41, 1445. [10] K. Kizaki, H. Imoto, T. Kato, K. Naka, Tetrahedron 2015, 71, 643. [11] a) N. D. Heindel, V. B. Fish, T. F. Lemke, J. Org. Chem. 1968, 33, 3997; b) N. D. Heindel, J. Org. Chem. 1970, 35, 3138. [12] Gaussian 09, Revision C.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, ©. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. [13] CCDC 1058098 (1) and 888730 (2) contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centre. [14] J. Gierschner, M. Ehni, H.-J. Egelhaaf, B. M. Medina, D. Beljonne, H. Benmansour, G. C. Bazan, J. Chem. Phys. 2005, 123, 144914. [15] PROCESS-AUTO, RAXIS Data-Processing Software, Rigaku Corporation, Tokyo, Japan, 1996. [16] T. Higashi, FS_ABSCOR, Program for Absorption Correction, Rigaku Corporation: Tokyo, Japan, 1995. [17] A. C. Larson, Crystallographic Computing (Ed.: F. R. Ahmed), Munksgaard, Copenhagen, 1970, pp. 291 – 294. [18] P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman, R. de Gelder, R. Israel, J. M. M. Smits, The DIRDIF-99 Program System, Technical Report of the Crystallography Laboratory, University of Nijmegen, Nijmegen, The Netherlands, 1999. [19] P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman, S. GarciaGranda, R. O. Gould, J. M. M. Smits, C. Smykalla, PATTY, The DIRDIF Program System, Technical Report of the Crystallography Laboratory, University of Nijmegen, Nijmegen, The Netherlands, 1992. [20] CrystalStructure 3.8: Crystal Structure Analysis Package, Rigaku Americas, The Woodlands, TX, 2000. [21] J. R. Carruthers, J. S. Rollett, P. W. Betteridge, D. Kinna, L. Pearce, A. Larsen, E. Gabe, CRYSTALS Issue 11; Chemical Crystallography Laboratory, Oxford, UK, 1999.
Manuscript received: May 15, 2015 Accepted article published: June 12, 2015 Final article published: July 7, 2015
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Luminescence
A Mechanochromic Luminescent Dye Exhibiting On/Off Switching by Crystalline–Amorphous Transitions Hiroaki Imoto, Kohei Kizaki, and Kensuke Naka*[a] Abstract: A mechanochromic luminescent dye based on a simple aminomaleimide skeleton was readily synthesized in a one-pot process. It exhibited an on/off mechanochromic luminescent switching property dependent on external stimuli, unlike a traditional mechanochromic color change. The green emission was turned on by grinding in a mortar and turned off by heating or treatment with dichloromethane. In the crystalline state, two molecules were stacked by cofacial p–p interactions, which caused concentration self-quench-
Introduction Mechanochromic luminescent materials[1] have attracted much attention owing to their potential applications in sensors,[2] security systems,[3] and memory devices.[4] The emission properties are changed by transformation of the molecular planarity and/or arrangement in the solid state with mechanical stimuli, such as grinding and pressing. In almost all cases, on the other hand, responses to mechanical input are emission color changes, which are insufficient in contrast to attain clear visibility for the naked eye. To overcome this problem, on/off mechanochromic luminescent switching system is required.[5] Some research into systems in which a mechanical stimulus turns on the emission and another stimulus turns it off have been reported toward sensing of mechanical stimuli.[6, 7] However, these studies employed complicated donor–accepter pairs[6] or metal–organic frameworks[7] to realize reversible on/off cycles. Thus, more simple system is necessary for more practical molecular design of on/off mechanochromic luminescent switching materials. To achieve this purpose, the material must be composed of a small molecule possessing a single luminophore. The molecules should be arranged in such a way as to cause self-quenching of the luminescence in the crystalline state to maintain the off state, and the arrangement should collapse under mechanical stimuli to turn on the emission. [a] Dr. H. Imoto, K. Kizaki, Prof. K. Naka Faculty of Molecular Chemistry and Engineering Graduate School of Science and Technology Kyoto Institute of Technology Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500499. Chem. Asian J. 2015, 10, 1698 – 1702
ing. The crystalline-to-amorphous transition induced by grinding removed cofacial p–p stacking, which led to intensive emission. Crystallizing processes recovered the cofacial p–p stacking, resulting in elimination of the emission. Theoretical calculations and X-ray diffraction analyses revealed that the dye molecule was distorted in the crystalline state; thus even a mechanical stimulus caused the crystalline-toamorphous transition.
However, such a molecule has never been reported, to the best of our knowledge, despite the simple concept. The most challenging matter lies in the balance of intermolecular interactions. Because intermolecular interactions for quenching, namely, p–p stacking, tend to construct strong long-range networks in the crystalline state, mechanical stimuli cannot break such structures. Thus, it is necessary to form intermolecular interactions that can cause self-quenching without network structures. Recently, we reported aggregation-induced emission (AIE)[8] of active aminomaleimide dyes.[9, 10] The aminomaleimide skeleton can be readily constructed through a one-pot process by using commercially available dimethyl acetylenedicarboxylate and primary amines.[10] For example, 2-anilino-N-phenylmaleimide (2) shows green emission in the solid state, whereas no emission was observed in solution. Free rotation of the secondary amine deactivates the exciton through a nonradiative pathway in good solvents, and the motion is frozen in the solid state to show the emission. In the course of screening of molecular structures, we found that 2-(m-trifluoromethylanilino)-N-(m-trifluoromethylphenyl)maleimide (1) exhibited no emission in the crystalline state. The packing structure of 1 was likely to cause concentrated self-quenching; thus we ground the crystals of 1 to change the molecular packing. Fortunately, the ground sample of 1 showed photoluminescence (PL) under UV irradiation, and the emission was erased by crystallization. This observation implies that 1 possesses an on/off mechanochromic switching property, unlike a traditional mechanochromic color change. Herein, we propose a novel on/off-type mechanochromic dye, and demonstrate its switching behavior. The mechanism of the unique mechanochromism was explored on the basis of X-ray diffraction (XRD) analyses.
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Full Paper Results and Discussion Synthesis The synthetic route of 1 is shown in Scheme 1. A mixture of dimethyl acetylenedicarboxylate and m-trifluoromethylaniline was heated at 150 8C for 4 h. The precipitates generated were collected by filtration and washed with methanol. Purification by recrystallization from dichloromethane (CH2Cl2) and methanol gave 1. The significant advantage of this process is that the reaction can be carried out without any solvents, additives, or complicated purification process.
addition, emission of the ground sample was also erased by the process in which small amounts of CH2Cl2 were added and subsequently dried in the mortar. These results mean that an on/off switching system is constructed by employing external stimuli: grinding turns on the emission and heating or CH2Cl2 treatment turns it off. To check the absence of decomposition, we measured the 1H NMR spectrum of the sample after grinding in a mortar and heating at 170 8C for 10 min, and there was no significant difference from the spectrum before the process (Figure S2 in the Supporting Information). Demonstration of successive switching was performed in the mortar. Treatment with CH2Cl2 and heating at 170 8C were employed to turn off the emission in the first four cycles and the last three cycles, respectively. The switching behavior was so clear that we were able to perceive the mechanical input by eye (Figure 2 a). PL measurements confirmed the switching performance (Figure 2 b). During the switching process, the
Scheme 1. Synthesis of 1.
In this reaction, the hydroamination of a primary amine to give dimethyl acetylenedicarboxylate results in a trans adduct, and subsequently, trans–cis isomerization accompanied by condensation gives the 2-arylaminomaleimide derivative.[11] This synthetic procedure can be performed in one pot. Mechanochromism The crystals of 1, which were obtained by recrystallization from CH2Cl2 and methanol, showed no emission under UV irradiation at l = 365 nm. However, once these crystals were ground in a mortar, green emission was observed (lmax = 516 nm; Figure 1). The emission intensity of the ground sample was more than seven times higher than that of the crystalline sample. For crystallization, the ground sample was heated to 170 8C, just above its melting point (168 8C), for 10 min. After cooling to room temperature, the emission was eliminated. In
Figure 1. Photographs and PL spectra (excited at l = 365 nm) of 1 before and after grinding. Chem. Asian J. 2015, 10, 1698 – 1702
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Figure 2. On/off switching cycles of 1 detected with a) the naked eye (excited at l = 365 nm); b) the intensity of PL spectra at l = 516 nm (excited at l = 350 nm); and c) XRD patterns measured after grinding, heating, and CH2Cl2 treatment, and simulated powder XRD pattern of the crystal sample.
morphology of the solid samples was traced with XRD analysis (Figure 2 c). After every CH2Cl2 treatment and heating step, relatively defined peaks, which were corresponded well with the powder XRD pattern simulated by results of single-crystal XRD, were observed, although these peaks disappeared after every grinding step. This result indicated that grinding caused a crystalline-to-amorphous transition to turn on the emission, and that CH2Cl2 treatment and heating processes caused an amorphous-to-crystalline transition to turn it off. 1699
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Full Paper Optical Properties The optical properties of 1 were compared with those of 2, which was reported in our previous study as an AIE active dye (Table 1).[10] The absorption maxima of 1 were slightly blueshifted from those of 2 because electron-withdrawing CF3 units lowered the HOMO level, as estimated by DFT calculations performed with the Gaussian 09[12] suite of programs at the level of B3LYP/6-31G(d) theory. Solutions of 1 and 2 exhibited no emission because the excitons were deactivated by molecular motion, according to our previous reports.[9, 10]
Table 1. Optical properties and results of DFT calculations for dyes 1 and 2. Dye
labs,sol[a] [nm]
lPL,solid[b] [nm]
FPL,solid[c]
HOMO[d] [eV]
LUMO[d] [eV]
1 2
372, 280 380, 285
516 517
–[e] 0.05
¢6.38 ¢5.92
¢2.59 ¢2.18
Figure 4. Packing structures of 1: a) front view, b) side view, c) hydrogen bond and d) p–p interactions. Hydrogen atoms are omitted in a) and b) for clarity.
[a] Absorption in chloroform (c = 1.0 Õ 10¢4 m). [b] PL in the solid state. [c] Absolute PL quantum efficiencies in the solid state. PL spectra were measured under l = 350 nm irradiation for excitation. [d] Calculated at the B3LYP/6-31G(d) level of theory by using the Gaussian 09[12] suite of programs. [e] FPL of solid 1 was highly dependent on the preparation procedure for the ground samples.
Crystals of 1 showed no emission, whereas those of 2 showed green emission (l = 517 nm). The chemical structures, optical properties in solution, and calculated molecular orbitals were not significantly different between 1 and 2 (Table 1). In addition, grinding of the crystals of 2 did not change the PL properties. Therefore, we focused on the packing structures in the crystalline state.
because steric hindrance of the m-CF3 groups inhibits intermolecular CH···p interactions, and a noncovalent network structure cannot be constructed. Instead, antiparallel cofacial p–p stacking was observed (Figure 4 b and d). The p–p interactions resulted in concentrated self-quenching, so that 1 showed no emission in the crystalline state.[14] It is thought that mechanical stimuli collapsed the stacked structure to show emission without self-quenching through p–p interactions. To understand the driving force toward the collapse of the cofacial p–p interactions of 1, the torsion of the molecular planarity was evaluated on the basis of the interplanar angle of the maleimide and aminobenzene rings. The interplanar angles in the optimized structures were estimated by DFT calculations, and those in the single crystals were measured by XRD (Table 2). The angles of 1 and 2 in the crystalline states were
Single-Crystal XRD Analysis The results of single-crystal X-ray analysis on 2 are shown in Figure 3.[9, 13] Each of the hydrogen bonds (Figure 3 a) and CH···p interactions (Figure 3 b) were formed in a linear arrangement, which resulted in the robust network structure. The CH···p interaction enabled the molecules to be packed in a non-coplanar arrangement. This is why 2 shows intensive emission in the crystalline state without self-quenching. In the case of 1,[13] on the other hand, dimerization occurred through the double hydrogen bonds of NH¢O (Figure 4 a and c), and no CH···p interactions were observed. This is probably
Figure 3. Molecular packing structures of 2 in the single crystal : a) hydrogen bond and b) CH···p interactions. Chem. Asian J. 2015, 10, 1698 – 1702
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Table 2. Interplanar angles in dyes 1 and 2.[a]
Dye
Optimized structure[b] [8]
Single crystal[c] [8]
1 2
16.0 17.8
29.0 20.8
[a] The interplanar angles of the maleimide and secondary aminobenzene rings. [b] Estimated by DFT calculations at the level of B3LYP/6-31G(d) theory. [c] Measured by single-crystal XRD.
larger than those in the optimized structures by 13.0 and 3.08, respectively. This result implies that the molecular planarity of 1 was much more twisted in the crystalline state than that of 2. In addition, compound 1 did not form a robust noncovalent bond network. This is why the cofacial p–p stacking of 1 was readily collapsed by mechanical stimuli (Figure 5). Crystallizing processes, such as heating and CH2Cl2 treatment, recovered the p–p interactions, which quenched the emission. In the case of 2, no mechanochromic phenomenon was observed because molecules were tightly tied up in the network through hydrogen bonds and CH···p interactions without a large distortion of molecular planarity.
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Full Paper
Figure 5. Schematic illustration of the mechanism of on/off switching mechanochromism.
Dispersion in Polymer Matrices For further observation of the emission behavior, the PL spectra of 1 were measured in polymer matrices (Figure S3 in the Supporting Information), in which the molecular motions were restricted and the molecules could be dispersed without p–p stacking. Polystyrene (PS), poly(methyl methacrylate) (PMMA), and ethylene vinyl acetate copolymer (EVA) were used as matrix polymers. To prepare the samples, compound 1 (3.8 wt %) was added to these polymers. The dye-dispersed samples showed light-blue emissions at l = 477–484 nm (Figure 6 a). The emission was slightly blueshifted in comparison to the ground solid sample (lmax = 516 nm), possibly because the planarity of 1 was distorted in the polymer matrices. The XRD pattern of the PS sample displays no defined signal (Figure 6 b), which suggests that no significant crystalline aggregation occurs in the polymer matrix. This result supports the idea that the prevention of crystallization leads to the emission of 1.
interactions in the crystalline state, leading to emission. On the other hand, compound 1 formed cofacial p–p interactions without network structures in the crystalline state and the molecular planarity of 1 was twisted. Thus, a mechanical stimulus readily collapsed the cofacial p–p stacking, and the crystallineto-amorphous transition caused the emission. Crystallizing processes recovered the cofacial p–p interactions for the elimination of the emission. The simple molecular structure, easy synthetic procedure, visually defined on/off contrast, and good reproducibility are great advantages for applications. The molecular design proposed in this study is a promising route to mechanical sensing materials.
Experimental Section Materials Dimethyl acetylenedicarboxylate was purchased from Tokyo Chemical Industry Co., Ltd. m-(Trifluoromethyl)aniline was purchased from Wako Pure Chemical Industry, Ltd. Dichloromethane and methanol were purchased from Nacalai Tesque, Inc. EVE was provided by Nissan Chemical Industries, Ltd. All chemicals were used without any purification. Compound 2 was prepared by following a procedure reported in the literature.[10]
Measurements 1
H NMR (400 MHz) spectra were recorded on a Bruker DPX-400 spectrometer, and samples were analyzed in CDCl3 by using Me4Si as an internal standard. The following abbreviations were used: s: singlet, d: doublet, m: multiplet. HRMS was recorded on a JEOL JMS-SX102A spectrometer. The UV/Vis spectra were recorded on a Jasco spectrophotometer (model V-670 KKN; Jasco, Tokyo, Japan). PL spectra were obtained on an FP-8500 (JASCO) instrument. XRD patterns were recorded on a Smart Lab (Rigaku) diffractometer with CuKa radiation (l = 1.5406 æ) in q/2q mode at room temperature. The 2q scans were collected at 0.018 intervals, and the scan speed was 28(2q) min¢1.
X-ray Crystallographic Data for Single-Crystalline Products
Figure 6. a) Photographs of polymer dispersion films under irradiation at l = 365 nm, and b) XRD patterns of crystals and a film of 1 (3.8 wt %) in PS.
Conclusion We developed a novel on/off switching mechanochromic luminescent dye, 1. The intensive green emission was triggered by grinding and erased by heating or CH2Cl2 treatment. The intermolecular noncovalent bonds in the crystals were one of the key factors in the present unique optical property. Molecules of 2 formed a robust network structure without cofacial p–p Chem. Asian J. 2015, 10, 1698 – 1702
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The single crystal was mounted on glass fibers with epoxy resin. Intensity data were collected at room temperature on a Rigaku RAXIS RAPID II imaging plate area detector with graphite monochromated MoKa radiation. The crystal-to-detector distance was 127.40 mm. Readout was performed in the 0.100 mm pixel mode. The data were collected at room temperature to a maximum 2q value of 55.08. Data were processed by using the PROCESSAUTO[15] program package. An empirical or numerical absorption correction[16] was applied. The data were corrected for Lorentz and polarization effects. A correction for secondary extinction[17] was applied. The structure was solved by heavy-atom Patterson methods[18] and expanded by using Fourier techniques.[19] Some non-hydrogen atoms were refined anisotropically, whereas the rest were refined isotropically. Hydrogen atoms were refined by using the riding model. The final cycle of full-matrix least-squares refinement on F2 was based on observed reflections and variable parameters. In the case of the crystalline product recrystallized from acetone, the final cycle of full-matrix least-squares refinement on F was based on observed reflections and variable parameters. All calculations were performed by using the CrystalStructure[20, 21] crystallo-
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Full Paper graphic software package. Crystal data and more information on Xray data collection are summarized in Tables S1 and S2 in the Supporting Information.
Sample Preparation 2-(m-Trifluoromethylanilino)-N-(m-trifluoromethylphenyl)maleimide (1): A mixture of dimethyl acetylenedicarboxylate (0.394 g, 2.77 mmol) and m-trifluoromethylaniline (1.77 g, 11.0 mmol) were heated at 150 8C. After stirring for 4 h, the reaction mixture was cooled to ¢10 8C in a freezer. The generated dark-brown precipitates were collected by filtration and washed with methanol. The isolated residue was subjected to recrystallization from dichloromethane and methanol to give 1 as a yellow solid (0.244 g, 0.609 mmol, 22 %). M.p. 167.8–168.4 8C; 1H NMR (400 MHz, CDCl3): d = 7.76 (s, 1 H), 7.68–7.56 (m, 4 H), 7.48–7.46 (m, 3 H), 7.40 (d, J = 8.4 Hz, 1 H), 5.76 ppm (s, 1 H); the solubility of 1 was too low to measure a 13C NMR spectrum; HRMS (FAB): m/z calcd for C18H10F6N2O2 [M] + : 400.0646; found: 400.0653; elemental analysis calcd (%) for C18H10F6N2O2 : C 54.01, H 2.52, N 7.00; found: C 53.81, H 2.49, N 7.18. Dye-dispersed polymer films: A polymer (100 mg of EVA, PS, or PMMA) was added to a solution of 1 (4.0 mg, 0.01 mmol) in toluene (3 mL). Each of the solutions were cast onto a silicon wafer. Films were formed at 70 8C and heated at 130 8C for 20 min to give 3.8 wt % dye-dispersed polymer films.
Acknowledgements We thank Dr. K. Matsukawa and Dr. S. Watase of Osaka Municipal Technical Research Institute for single-crystal X-ray diffraction measurements. Keywords: dyes/pigments · luminescence · maleimides · mechanochromism · X-ray diffraction [1] a) Y. Sagara, T. Kato, Nat. Chem. 2009, 1, 605; b) Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu, J. Xu, Chem. Soc. Rev. 2012, 41, 3878; c) X. Zhang, Z. Chi, Y. Zhang, S. Liu, J. Xu, J. Mater. Chem. C 2013, 1, 3376. [2] For examples of recent papers, see: a) D.-H. Park, J. Hong, I. S. Park, C. W. Lee, J.-M. Kim, Adv. Funct. Mater. 2014, 24, 5186; b) J. R. Hemmer, P. D. Smith, M. van Horn, S. Alnemrat, B. P. Mason, J. R. deAlaniz, S. Osswald, J. P. Hooper, J. Polym. Sci. Part B 2014, 52, 1347; c) F. Cellini, S. Khapli, S. D. Peterson, M. Porfiri, Appl. Phys. Lett. 2014, 105, 061907. [3] a) H. Sun, S. Liu, W. Lin, K. Y. Zhang, W. Lv, X. Huang, F. Huo, H. Yang, G. Jenkins, Q. Zhao, W. Huang, Nat. Commun. 2014, 5, 3601; b) X. Zhu, R. Liu, Y. Li, H. Huang, Q. Wang, D. Wang, X. Zhu, S. Liu, H. Zhu, Chem. Commun. 2014, 50, 12951. [4] a) M. Irie, T. Fukaminato, T. Sasaki, N. Tamai, T. Kawai, Nature 2002, 420, 759; b) C. E. Olson, M. J. R. Previte, J. T. Fourkas, Nat. Mater. 2002, 1, 225; c) S. J. Lim, B. K. An, S. D. Jung, M. A. Chung, S. Y. Park, Angew. Chem. Int. Ed. 2004, 43, 6346; Angew. Chem. 2004, 116, 6506; d) S. Hirata, T. Watanabe, Adv. Mater. 2006, 18, 2725.
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Manuscript received: May 15, 2015 Accepted article published: June 12, 2015 Final article published: July 7, 2015
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