Red-Emissive Organic Crystals of a Single-Benzene Molecule

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Cite This: J. Phys. Chem. Lett. 2019, 10, 1437−1442

Red-Emissive Organic Crystals of a Single-Benzene Molecule: Elastically Bendable and Flexible Optical Waveguide Bin Liu, Qi Di, Wentao Liu, Chenguang Wang,* Yue Wang, and Hongyu Zhang* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Qianjin Street, Changchun 130012, P.R. China

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S Supporting Information *

ABSTRACT: Organic crystals are easily cracked into pieces or powders under applied stress because of their intrinsic brittle nature. This undesired mechanical property directly limits their application in flexible optical and optoelectronic devices. Herein, we developed a compact single-benzene molecule dimethyl 2,5-bis((2-hydroxyethyl)amino)terephthalate, which was easily crystallized to form two polymorphs, A and B. Featuring a single-benzene π-system, both polymorphs A and B display red fluorescence in crystals. More importantly, crystals of polymorph A are flexible and can be elastically bent under mechanical force. Given these advantages, a flexible optical waveguide has been realized in the crystal of polymorph A with a bent shape, highlighting its potential application in flexible devices. In addition, the thermal transformation of crystals from polymorph A to polymorph B, which was accompanied by the change of optical property as well as mechanical elasticity, has been observed.

rganic crystals of π-conjugated molecules have attracted much attention in optical and optoelectronic applications, such as optical waveguides, solid-state lasers, field-effect transistors, and light-emitting diodes.1−3 However, owing to weak intermolecular interactions, organic crystals are generally brittle and hard and are easily cracked into pieces or powders when external stress is applied.4,5 This mechanical property largely limits the utility of organic crystals, particularly in flexible optical and optoelectronic devices. In this context, elastic organic crystals, which means the crystals are flexible and can be reversibly bent under mechanical force, have attracted increased attention in recent years.6−27 In this field, a pioneer contribution was given by Ghosh and Reddy in 2012.6 They reported that the needle cocrystals of caffeine, 4-chloro-3-nitrobenzoic acid, and methanol could be elastically bent under applied stress. Subsequently, on the basis of the study of a series of Nbenzylideneanilines, Ghosh and co-workers proposed the structural feature of elastic organic crystals: weak and dispersive intermolecular interactions as well as an interlocked packing structure.7 Very recently, a new mechanism of crystal elasticity was revealed by McMurtrie and co-workers.8 The intermolecular interactions of crystal copper(II) acetylacetonate which were dominated in one direction enabled the reversible rotation of the molecules, thus resulting in the crystal elasticity. Employing a π-extended molecule 1,4-bis[2-(4-methylthienyl)]-2,3,5,6-tetrafluorobenzene, Hayashi and Koizumi developed elastic crystals which were fluorescent and displayed mechanofluorochromism upon bending.9 Our group reported the elastic and fluorescent crystals of a Schiff base (E)-1-(4(dimethylamino)phenyl)iminomethyl-2-hydroxyl-naphthalene, and further achieved flexible optical waveguide in the crystals with bent shape.10 These works significantly advance the basic

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knowledge of elastic organic crystals and highlight the potential application of elastic crystals as flexible functional materials. However, the species of elastic crystals are still very limited. Moreover, the relationship between crystal elasticity and intermolecular interactions, i.e., the mechanism of elasticity, should be further clarified. To explore new elastic organic crystals, we pay much attention to π-conjugated molecules with compact structures, even singlebenzene π-systems. The recently developed single-benzene molecules display fascinating fluorescence properties and optical applications which are by no means inferior to the conventional π-extended molecules.28−32 Moreover, the single-benzene molecules have the advantages of simple chemical synthesis as well as easy crystallization to form large size crystals. In addition, the compact single-benzene structures simplify the intermolecular interactions in crystals, facilitating the study of the relationship between crystal elasticity and intermolecular interactions. Our group recently developed a single-benzene molecule dimethyl 2,5-bis(methylamino)terephthalate (compound 1, Scheme 1a) which impressively displays efficient red fluorescence in crystals.28 Herein, we slightly modify the molecular structure by introducing two hydroxyl groups, dramatically changing the mechanical property of crystals from brittle to flexible. The synthesized dimethyl 2,5-bis((2hydroxyethyl)amino)terephthalate (compound 2, Scheme 1a) is easily crystallized, and two red-emissive polymorphs A and B are obtained. Crystals of polymorph A are flexible and can be elastically bent under mechanical force. Moreover, the flexible Received: January 22, 2019 Accepted: March 11, 2019 Published: March 11, 2019 1437

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Scheme 1. (a) Single-Benzene Molecules 1 and 2 and Crystal Photographs of Polymorphs A and B of Compound 2 under Daylight and UV Light; (b) Synthesis of Compound 2

Figure 1. (a) Normalized absorption (solid lines) and fluorescence spectra (broken lines) of compound 2 in solutions. (b) Fluorescence spectra of crystals of compound 2. To avoid the possible effect of different self-absorption (different thickness of crystal) on the fluorescence spectra, a crystal of polymorph A with thickness of 0.064 mm and a crystal of polymorph B with thickness of 0.062 mm were used for the fluorescence measurements under the identical conditions of excitation and detection.

optical waveguide has been successfully realized in the crystals of polymorph A with bent shape. In addition, the thermal transformation of crystals from polymorph A to polymorph B, which was accompanied by the change of optical property as well as mechanical elasticity, has been observed. In comparison to compound 1, the crystals of which are brittle and hard, we introduce two hydroxyl groups to compound 2 aiming to tune the intermolecular interactions and thus change the mechanical properties. Indeed, the multiple intermolecular hydrogen bonds between the hydroxyl groups are critical to get the good crystal elasticity of polymorph A (vide infra). The synthesis of compound 2 is very easy (Scheme 1b). One-step condensation between dimethyl 1,4-cyclohexanedione-2,5dicarboxylate and ethanolamine under air atmosphere readily produced compound 2 in 56% yield. The detailed synthetic procedure and 1H and 13C NMR spectra are shown in the Supporting Information. Crystallization of compound 2 in different processes provided two polymorphs A and B (Scheme 1a). By layering petroleum ether on the top of acetone solution and subsequent solvent diffusion at room temperature, the needle-shaped crystals of polymorph A were obtained. In contrast, the block shape crystals of polymorph B were prepared by vacuum sublimation at 190 °C. The identity and purity of crystals of the two polymorphs were confirmed by elemental analysis and NMR spectra (see the Supporting Information). The photophysical properties of compound 2 were investigated in various solvents (Figure 1a and Table 1). The

Table 1. Photophysical Data for Compound 2 in Solutions and Crystals solution or crystals

λabsa (nm)

λem (nm)

ΦFb

1,4-dioxane CHCl3 CH2Cl2 DMSO polymorph A polymorph B

480 473 473 484

587 592 584 599 622 640

0.26 0.22 0.28 0.23 0.08 ≤0.01

τ (ns)

kr (×108 s−1)

knr (×108 s−1)

8.6 7.1 8.7 7.7 6.1

0.30 0.31 0.32 0.30 0.13

0.86 1.1 0.83 1.0 1.5

a The longest absorption maximum wavelengths. bAbsolute fluorescence quantum yields determined by a calibrated integrating sphere system within ±3% error.

absorption (λabs) and emission (λem) maxima are around 480 and 590 nm, respectively, which are insensitive to the solvent polarity. While featuring large Stokes shift of about 110 nm, compound 2 shows intense orange fluorescence with quantum yield (ΦF) of about 0.25 in solutions. These photophysical properties are similar to those of compound 1 in solutions.28 In crystals, both polymorphs A and B display red emissions with λem of 622 and 640 nm, respectively, which are significantly longer than the solutions (Figure 1b and Table 1). Although the ΦF is not very great (ΦF = 0.08 for polymorph A, ΦF ≤ 0.01 for polymorph B), that the single-benzene π-system displays such long wavelength fluorescence is impressive. In addition, the 1438

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Figure 2. Elastic bending of crystals of polymorph A: (a−f) reversible bending−relaxation process with a pair of tweezers; (g−h) photographs of a crystal wound around a glass tube under daylight and UV light.

Figure 3. Crystal structure of polymorph A. (a) Multiple intermolecular hydrogen bonds (marked by broken line) between one molecule and six neighboring ones. The hydrogen bond distance of O···H is shown. (b) Molecular packing structure along crystallographic c-axis.

solutions, of which the kr is maintained at about 0.30 × 108 s−1 and the knr is around 1.0 × 108 s−1 in various solvents, polymorph A has a much smaller kr of 0.13 × 108 s−1, a larger knr of 1.5 × 108 s−1, and thus a lower ΦF. The decrease of kr and increase of knr of polymorph A are attributed to the H-aggregation with slippage angle of 43.8° in the crystal (Figure S4a). For polymorph B, the

absorption edge of polymorph B even reaches to about 640 nm, corresponding to a very small band gap of 1.94 eV (Figure S1). To deeply understand the emission property of compound 2, we measured the fluorescence lifetime (τ) and further determined the radiative (kr) and nonradiative (knr) decay rate constants (Table 1 and Figures S2 and S3). In comparison to the 1439

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Figure 4. Optical waveguides of the crystals of polymorph A with a straight shape (a−d) and a bent shape (e−h). (a and e) Photographs of the crystal under UV light. (b and f) Fluorescence microscopy images collected upon excitation of the crystal at different positions. (c and g) Emission spectra measured at one end of the crystal with various distances (0−5 mm) between the end and the excitation positions. (d and h) The Itip/Ibody decays of the crystals of polymorph A with a straight shape or a bent shape, respectively. The optical loss coefficients (α) were determined by a single-exponential fitting of the function Itip/Ibody = A exp(−αD), where Itip and Ibody are the fluorescence intensities of out-coupled and incident light, respectively, and D is the distance between the excited site and the end of the crystal for collecting emission.

crystals with good elasticity. This result also demonstrates that the enrichment of intermolecular hydrogen bonds is a reasonable strategy to develop new elastic crystals. Notably, because the compact single-benzene structure of compound 2 simplifies the intermolecular interactions, the discussion of crystal elasticity is convenient and much easier than other elastic crystals with complicated molecular structures. For a similar reason, the single-benzene elastic crystal should be an ideal model for the molecular dynamics stimulations which have been recently employed to theoretically predict crystal elasticity.22 Because the crystals of polymorph A are elastic as well as fluorescent, their potential optical applications were investigated in both straight and bent shapes. We initially tested the optical waveguide property of the crystal in a straight shape (Figure 4a). When different positions of the crystal were excited with a pulse laser (Nd:YAG, 355 nm), the emissions were generated and propagated (optical waveguide) to the two ends of the crystal (Figure 4b). A set of emission spectra were recorded at one end of the crystal while changing the distance between the excitation position and the detection end (Figure 4c). Upon fitting these data, the optical loss coefficient, which was a crucial parameter for evaluating the optical waveguide property, was determined to be 0.351 dB mm−1 at 641 nm (Figure 4d). Notably, this loss coefficient is quite small for organic crystals and comparable to or even better than some representative polymers (Table S2), revealing the good performance of the optical waveguide. We further performed the optical waveguide measurements in a bent crystal (Figure 4e). As shown in Figure 4f, the flexible optical waveguide was impressively realized. The optical loss coefficient of bent crystal was estimated to be 0.376 dB mm−1 at 641 nm (Figure 4h), which was near to that of the unbent crystal. Moreover, upon repeatedly bending and relaxing the identical crystal, its optical waveguide property, in terms of emission

slippage angle of H-aggregation is up to 54.9° (Figure S4b), which should further suppress kr and facilitate knr (the values are not determined because of the weak fluorescence) and thus result in a much lower ΦF. The large needle crystal of polymorph A shows good elasticity. As shown in Figure 2a−d, compression by a pair of tweezers causes a straight crystal to be easily bent into a half loop shape without breaking or cracking. Upon relaxation of the force, the crystal quickly regains the original straight shape (Figure 2e). This bending−relaxation process is reversible (Figure 2f), revealing the elastic nature of the crystal. The good elasticity is further highlighted by winding a crystal around a glass tube (Figure 2g,h). To quantitatively evaluate the mechanical bendability of the crystals, we determined the largest expansion/contraction ratio of the outer/inner arc (the maximum elastic strain, εmax) of bent crystals without cracking by using a reported method.10 The method involved winding the crystals with various thicknesses around a cylinder (diameter of 4.62 mm) to form a half loop; the largest applicable thickness (0.101 mm) corresponded to the εmax. Thus, the calculated εmax of polymorph A of 2.14% was comparable to those of the reported elastic crystals of Nbenzylideneanilines (about 2%)7 and Schiff base (2.26%).10 The macroscopic crystal elasticity was further correlated with the microscopic intermolecular interactions. In polymorph A, facilitated by the hydroxyl groups and ester moieties, one molecule interacts with six neighboring ones through up to 20 intermolecular hydrogen bonds (Figure 3a). Upon bending the crystal, in which the crystal would be expanded or contracted along the crystallographic c-axis as revealed by X-ray diffraction (Figure S5), such multiple intermolecular hydrogen bonds should be rather “soft” for easy deformation (Figure 3b), thus avoiding cracking the crystal. In other words, the multiple hydrogen bonds arranged in various directions endow the 1440

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Figure 5. Thermal transformation from polymorph A to polymorph B. (a) Simulated X-ray diffraction (XRD) patterns of polymorphs A and B based on the single-crystal structures, and the measured XRD pattern of the heated polymorph A. (b) Photographs of polymorph A under daylight and UV light, before and after heating.

a flexible optical waveguide has been realized in the crystal of polymorph A with bent shape, demonstrating its potential application in flexible devices. In addition, we achieved the thermal transformation of crystals from polymorph A to polymorph B, which was accompanied by the change of crystal color and fluorescence, as well as the loss of crystal elasticity. Overall, this work highlights the advantage of single-benzene molecules as well as the potential utility of elastic crystals and will inspire the development of more, and more advanced, crystalline materials based on structurally compact molecules.

maximum and intensity, did not change (Figure S6). The flexible optical waveguide with bending-insensitive character demonstrates the potential utility of the crystal in flexible optical and optoelectronic devices.10−13,33 Polymorphs A and B are obtained by solution processing at room temperature and by vacuum sublimation at 190 °C, respectively. This fact implies that polymorph B may be a thermal stable phase. Indeed, the differential scanning calorimetry (DSC) measurements directly proved this speculation: only one melting peak for polymorph B while an additional endothermic peak located at about 180 °C was observed for polymorph A (Figure S7). These results inspired us to investigate the thermal transformation between two polymorphs. After the crystal of polymorph A was heated from room temperature to 180 °C at a rate of 10 °C min−1, its X-ray diffraction (XRD) pattern was well overlapped with polymorph B in the region of the first-order diffraction (Figure 5a). Therefore, the thermal transformation from polymorph A to polymorph B was achieved. The disappearance of XRD peaks in the region of 20°−30° was due to the following reasons: (1) The peaks in this region corresponded to the high-order diffraction; the peak intensities fall off rather drastically with 2θ. (2) The effect of preferred orientation may be relatively significant because of the limitation of the sample amount. Moreover, this thermal transformation from polymorph A to polymorph B was accompanied by the change of color and fluorescence: the color of the crystal became dark, and the fluorescence intensity significantly decreased (Figure 5b). These observations were consistent with the photophysical spectra measurements. Notably, the thermal transformation causes the loss of crystal elasticity: the initial crystal of polymorph A, which is flexible and elastically bendable, becomes brittle and easily cracked after heating. This is because (1) the intermolecular interactions of polymorph B are different from those of polymorph A and (2) some crystal lattice defects may be generated during the transformation. In summary, a single-benzene molecule dimethyl 2,5-bis((2hydroxyethyl)amino)terephthalate has been readily synthesized via a one-step condensation. Its compact molecular structure facilitates crystallizations of two polymorphs, A and B. Featuring a single-benzene π-system, both polymorphs A and B impressively display red fluorescence in crystals. More importantly, crystals of polymorph A are flexible and can be elastically bent under mechanical force. Given these advantages,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00196. Detail chemical synthesis, additional data of photophysical properties, X-ray crystallographic analysis, optical waveguide properties, and thermal properties (PDF) Crystallographic data for polymorph A (CIF) Crystallographic data for polymorph B (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.W.). *E-mail: [email protected] (H.Z.). ORCID

Yue Wang: 0000-0001-6936-5081 Hongyu Zhang: 0000-0002-0219-3948 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51622304 and 51773077). REFERENCES

(1) Wang, C.; Dong, H.; Jiang, L.; Hu, W. Organic semiconductor crystals. Chem. Soc. Rev. 2018, 47, 422−500. (2) Zhang, X.; Dong, H.; Hu, W. Organic Semiconductor Single Crystals for Electronics and Photonics. Adv. Mater. 2018, 30, 1801048. (3) Liu, H.; Cheng, X.; Bian, Z.; Ye, K.; Zhang, H. Four organic crystals displaying distinctively different emission colors based on an ESIPT-active organic molecule. Chin. Chem. Lett. 2018, 29, 1537− 1540.

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The Journal of Physical Chemistry Letters (4) Saha, S.; Mishra, M. K.; Reddy, C. M.; Desiraju, G. R. From Molecules to Interactions to Crystal Engineering: Mechanical Properties of Organic Solids. Acc. Chem. Res. 2018, 51, 2957−2967. (5) Ahmed, E.; Karothu, D. P.; Naumov, P. Crystal Adaptronics: Mechanically Reconfigurable Elastic and Superelastic Molecular Crystals. Angew. Chem., Int. Ed. 2018, 57, 8837−8846. (6) Ghosh, S.; Reddy, C. M. Elastic and Bendable Caffeine Cocrystals: Implications for the Design of Flexible Organic Materials. Angew. Chem., Int. Ed. 2012, 51 (41), 10319−10323. (7) Ghosh, S.; Mishra, M. K.; Kadambi, S. B.; Ramamurty, U.; Desiraju, G. R. Designing Elastic Organic Crystals: Highly Flexible Polyhalogenated N-Benzylideneanilines. Angew. Chem., Int. Ed. 2015, 54, 2674−2678. (8) Worthy, A.; Grosjean, A.; Pfrunder, M. C.; Xu, Y.; Yan, C.; Edwards, G.; Clegg, J. K.; McMurtrie, J. C. Atomic resolution of structural changes in elastic crystals of copper (II) acetylacetonate. Nat. Chem. 2017, 10, 65−69. (9) Hayashi, S.; Koizumi, T. Elastic Organic Crystals of a Fluorescent π-Conjugated Molecule. Angew. Chem., Int. Ed. 2016, 55, 2701−2704. (10) Liu, H.; Lu, Z.; Zhang, Z.; Wang, Y.; Zhang, H. Highly Elastic Organic Crystals for Flexible Optical Waveguides. Angew. Chem., Int. Ed. 2018, 57, 8448−8452. (11) Huang, R.; Wang, C.; Wang, Y.; Zhang, H. Elastic Self-Doping Organic Single Crystals Exhibiting Flexible Optical Waveguide and Amplified Spontaneous Emission. Adv. Mater. 2018, 30, 1800814. (12) Liu, H.; Bian, Z.; Cheng, Q.; Lan, L.; Wang, Y.; Zhang, H. Controllably realizing elastic/plastic bending based on a room temperature phosphorescent waveguiding organic crystal. Chem. Sci. 2019, 10, 227−232. (13) Hayashi, S.; Yamamoto, S. Y.; Takeuchi, D.; Ie, Y.; Takagi, K. Creating Elastic Organic Crystals of π-Conjugated Molecules with Bending Mechanofluorochromism and Flexible Optical Waveguide. Angew. Chem., Int. Ed. 2018, 57, 17002−17008. (14) Ghosh, S.; Mishra, M. K.; Ganguly, S.; Desiraju, G. R. Dual Stress and Thermally Driven Mechanical Properties of the Same Organic Crystal: 2,6-Dichlorobenzylidene-4-fluoro-3-nitroaniline. J. Am. Chem. Soc. 2015, 137, 9912−9921. (15) Horstman, E. M.; Keswani, R. K.; Frey, B. A.; Rzeczycki, P. M.; LaLone, V.; Bertke, J. A.; Kenis, P. J. A.; Rosania, G. R. Elasticity in Macrophage-Synthesized Biocrystals. Angew. Chem., Int. Ed. 2017, 56, 1815−1819. (16) Brock, A. J.; Whittaker, J. J.; Powell, J. A.; Pfrunder, M. C.; Grosjean, A.; Parsons, S.; McMurtrie, J. C.; Clegg, J. K. Elastically Flexible Crystals have Disparate Mechanisms of Molecular Movement Induced by Strain and Heat. Angew. Chem., Int. Ed. 2018, 57, 11325− 11328. (17) Gupta, P.; Karothu, D. P.; Ahmed, E.; Naumov, P.; Nath, N. K. Thermally Twistable, Photobendable, Elastically Deformable, and SelfHealable Soft Crystals. Angew. Chem., Int. Ed. 2018, 57, 8498−8502. (18) Owczarek, M.; Hujsak, K. A.; Ferris, D. P.; Prokofjevs, A.; Majerz, I.; Szklarz, P.; Zhang, H.; Sarjeant, A. A.; Stern, C. L.; Jakubas, R.; Hong, S.; Dravid, V. P.; Stoddart, J. F. Flexible ferroelectric organic crystals. Nat. Commun. 2016, 7, 13108. (19) Hayashi, S.; Asano, A.; Kamiya, N.; Yokomori, Y.; Maeda, T.; Koizumi, T. Fluorescent organic single crystals with elastic bending flexibility: 1, 4-bis (thien-2-yl)-2, 3, 5, 6-tetrafluorobenzene derivatives. Sci. Rep. 2017, 7, 9453. (20) Hayashi, S.; Koizumi, T.; Kamiya, N. Elastic Bending Flexibility of a Fluorescent Organic Single Crystal: New Aspects of the Commonly Used Building Block 4,7-Dibromo-2,1,3-benzothiadiazole. Cryst. Growth Des. 2017, 17, 6158−6162. (21) Hayashi, S.; Koizumi, T. Mechanically Induced Shaping of Organic Single Crystals: Facile Fabrication of Fluorescent and Elastic Crystal Fibers. Chem. - Eur. J. 2018, 24, 8507−8512. (22) Chen, C. T.; Ghosh, S.; Reddy, C. M.; Buehler, M. J. Molecular mechanics of elastic and bendable caffeine co-crystals. Phys. Chem. Chem. Phys. 2014, 16, 13165−13171.

(23) Saha, S.; Desiraju, G. R. Using structural modularity in cocrystals to engineer properties: elasticity. Chem. Commun. 2016, 52, 7676− 7679. (24) Saha, S.; Desiraju, G. R. σ-Hole and π-Hole Synthon Mimicry in Third-Generation Crystal Engineering: Design of Elastic Crystals. Chem. - Eur. J. 2017, 23, 4936−4943. (25) Mishra, M. K.; Kadambi, S. B.; Ramamurty, U.; Ghosh, S. Elastic flexibility tuning via interaction factor modulation in molecular crystals. Chem. Commun. 2018, 54, 9047−9050. (26) Mishra, M. K.; Mishra, K.; Asif, S. S.; Manimunda, P. Structural analysis of elastically bent organic crystals using in situ indentation and micro-Raman spectroscopy. Chem. Commun. 2017, 53, 13035−13038. (27) Saini, A. K.; Natarajan, K.; Mobin, S. M. A new multitalented azine ligand: elastic bending, single-crystal-to-single-crystal transformation and a fluorescence turn-on Al (III) sensor. Chem. Commun. 2017, 53, 9870−9873. (28) Tang, B.; Wang, C.; Wang, Y.; Zhang, H. Efficient Red-Emissive Organic Crystals with Amplified Spontaneous Emissions Based on a Single Benzene Framework. Angew. Chem., Int. Ed. 2017, 56, 12543− 12547. (29) Huang, R.; Liu, B.; Wang, C.; Wang, Y.; Zhang, H. Constructing Full-Color Highly Emissive Organic Solids Based on an X-Shaped Tetrasubstituted Benzene Skeleton. J. Phys. Chem. C 2018, 122, 10510−10518. (30) Tang, B.; Liu, H.; Li, F.; Wang, Y.; Zhang, H. Single-benzene solid emitters with lasing properties based on aggregation-induced emissions. Chem. Commun. 2016, 52, 6577−6580. (31) Beppu, T.; Tomiguchi, K.; Masuhara, A.; Pu, Y.-J.; Katagiri, H. Single Benzene Green Fluorophore: Solid-State Emissive, WaterSoluble, and Solvent- and pH-Independent Fluorescence with Large Stokes Shifts. Angew. Chem., Int. Ed. 2015, 54, 7332−7335. (32) Shimizu, M.; Takeda, Y.; Higashi, M.; Hiyama, T. 1,4Bis(alkenyl)-2,5-dipiperidinobenzenes: Minimal Fluorophores Exhibiting Highly Efficient Emission in the Solid State. Angew. Chem., Int. Ed. 2009, 48, 3653−3656. (33) Catalano, L.; Karothu, D. P.; Schramm, S.; Ahmed, E.; Rezgui, R.; Barber, T. J.; Famulari, A.; Naumov, P. Dual-Mode Light Transduction through a Plastically Bendable Organic Crystal as an Optical Waveguide. Angew. Chem., Int. Ed. 2018, 57, 17254−17258.

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