An Optical Waveguiding Organic Crystal with Phase-Dependent Elasticity and Thermoplasticity over Wide Temperature Ranges

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Oct 2021An Optical Waveguiding Organic Crystal with Phase-Dependent Elasticity and Thermoplasticity over Wide Temperature Ranges Jiacheng Cao, Huapeng Liu and Hongyu Zhang Jiacheng Cao State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 , Huapeng Liu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 and Hongyu Zhang *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 https://doi.org/10.31635/ccschem.020.202000565 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Elastic or plastic bendable organic crystals have attracted increasing attention in the field of crystal engineering. For the application of flexible materials, the applicable temperature range can not be ignored. However, studies on the flexible organic crystals reported so far have not involved the effect of temperature on the mechanical properties of these materials. Here, organic crystals of 9,10-bis(phenylethynyl)anthracene with phase-dependent mechanical properties over wide temperature ranges are reported. Phase A displays elastic bending ability under external stress in the temperature range of −196 to 150 °C. Phase B obtained from A through a heating-induced single-crystal-to-single-crystal (SCTSC) transition exhibits thermoplastic behavior (bendable at 120–240 °C): once heated above 120 °C, its shape was arbitrarily changeable. The newly formed shape could be preserved after cooling below 120 °C and the crystal could be reshaped when reheated over 120 °C. This unique property allows the organic crystals of phase B to be thermally processed into the desired shapes. Furthermore, the potential application in flexible optical devices was preliminarily evaluated by measuring the optical waveguide of the bent crystals. This study not only expands the crystal flexibility to thermoplastic bending, but also provides a strategy for tuning the mechanical property of organic single crystals by temperature-controlled phase transition. Download figure Download PowerPoint Introduction As functional materials, organic single crystals have attracted increasing attention because of their wide applications in light-emitting diodes,1,2 optical waveguides,3,4 and responsive materials for pressure,5 temperature,6,7 light,8,9 and so on. However, organic crystals are generally brittle and immutable, which extensively limits their application. Therefore, the exploration of organic crystals with mechanical properties such as plasticity and elasticity has become a cutting-edge field in the past 15 years. In 2006, Desiraju and co-workers10 found that 2-(methylthio)nicotinic acid crystals with an anisotropic packing structure could be bent plastically. In 2012, Ghosh and Reddy11 reported a kind of organic cocrystal that exhibited elastic bending ability under applied stress. Subsequently, elastic and plastic organic crystals have been rapidly developed and extensively studied by Saha and Desiraju,12,13 Naumov's group,14–16 Reddy's group,17,18 Hayashi et al.,19,20 Chandrasekar's group,21,22 and our group.23–28 Notably, flexible organic crystals compatible with optical and electrical functions have been reported recently, which advances the application of these flexible materials in optoelectronics.29–31 There are two key limiting factors in expanding the application of flexible organic crystals: single mechanical property and narrow applicable temperature range.32,33 The flexibility of organic crystals can be categorized into two types: irreversible plastic deformation and reversible elastic deformation. Most of the organic single crystals can be either elastically or plastically deformed, and it is difficult to incorporate two mechanical properties into the same organic crystal because the elastic and plastic deformations require isotropic and anisotropic packing, respectively.34,35 As for the real-world application of flexible materials, the effect of temperature on flexibility cannot be ignored. For example, most natural and synthetic rubbers lose their original mechanical property when the temperature is lower than −60 °C and melt when the temperature is higher than 300 °C.36 In the reported organic flexible crystals, research addressing applicable temperature is rare, and the limited examples concentrate on either low or high temperatures.11,33,37 Systematic study of crystal flexibility in a wide temperature range from low to high temperature is still not disclosed. Therefore, it is of great significance for the application of flexible materials to solve these two limitations. Here, we report an organic single crystal with phase-dependent mechanical properties based on a commercially available compound 9,10-bis(phenylethynyl)anthracene (Figure 1). The crystals of phase A were found to show elastic bending ability in the temperature range of −196 to 150 °C. Surprisingly, crystals of phase A underwent heating-induced single-crystal-to-single-crystal (SCTSC) transition and generated the crystals of phase B with thermoplastic bending behavior (bendable at 120–240 °C), a kind of mechanical property which is usually observed in linear polymers such as polyvinyl chloride, polystyrene, and polyoxymethylene.38 To the best of our knowledge, organic single crystals exhibiting thermoplastic bending have not been well demonstrated. Notably, the distinct properties, elasticity and thermoplasticity, are achieved on the same crystal through temperature-controlled SCTSC transition. Single-crystal X-ray diffractions (SCXRD) of phases A and B were carried out at −173, 25, and 120 °C to disclose the mechanism of this phase-dependent mechanical behavior. To demonstrate the utilization of these crystals in flexible devices, optical waveguides of phases A and B were tested in the bent state. This study not only expands the crystal flexibility to thermoplasticity, but also provides a strategy for tuning the mechanical property of organic crystals by temperature-controlled phase transition. Figure 1 | Molecular structure of 9,10-bis(phenylethynyl)anthracene; the diagram shows SCTSC transition from phase A to B and mechanical deformation of phases A and B at different temperatures. Download figure Download PowerPoint Results and Discussion Crystal structures of 9,10-bis(phenylethynyl)anthracene have been reported recently.39 Herein, phase A crystals were obtained by layering ethanol on the top of 9,10-bis(phenylethynyl)anthracene dispersed in dichloromethane for solvent diffusion. In general, the crystal is needle-shaped (length up to centimeter scale) with intense orange emission centered at 571 nm (Figures 2a and 2b). As shown in Figures 2c–2f, the crystal exhibits good elasticity. Upon applying stress at both ends of the crystal with a pair of tweezers, the straight crystal can be easily bent into a semicircle without breaking or cracking. Once the tweezers are withdrawn, the bent crystal quickly regains its original linear shape. The whole process is reversible and repeatable. The elastic nature of phase A was further confirmed as reflected by the linear correlation between stress and strain in the three-point bending experiment ( Supporting Information Figure S1). Figure 2 | Photographs of the crystal of phase A taken under daylight (a) and 365 nm UV light (b), and the elastic bending process of phase A (c–f). Photographs of the crystal of phase B taken under daylight (g) and 365 nm UV light (h), and the thermoplastic behavior of phase B under 160 °C (i–l). Download figure Download PowerPoint To explore the reasons for the elasticity, SCXRD of phase A at room temperature (r.t.) was performed. The crystal belongs to the monoclinic system in space group of C2/c, and there is one molecule existing in the asymmetric unit. The molecule takes a quite planar conformation with a small torsion angle of 4.33° between the benzene rings and the anthracene unit (Figure 3c). The molecules are stacked into a column along the [010] direction through intermolecular π⋯π interactions (cyan dotted line) with a vertical distance of 3.41 Å (Figure 3c). The π-stacked molecular columns are linked by CH⋯π interactions (2.76 and 2.77 Å, yellow dotted line), forming the (100) plane, which was confirmed by face indexing to be the bendable crystal face ( Supporting Information Figures S2 and S3). During the bending process, the molecular layer structure expands or contracts along the crystallographic [010] direction by increasing or decreasing the π⋯π distance. The (100) planes are connected by interlayer CH⋯π interactions (2.87 and 2.82 Å, purple dotted lines) constructing the crystal packing with isotropic intermolecular interactions and crosswise structure that match the requirements for crystal elasticity. Figure 3 | Torsion angle between benzene rings and anthracene unit in the crystal of phase A and the intermolecular interactions (yellow dotted line for CH⋯π and cyan dotted line for π⋯π) in (100) plane at −173 °C (a), 25 °C (c), and 120 °C (e). The molecular packing structure of phase A in (001) plane at −173 °C (b), 25 °C (d), and 120 °C (f). Arrows indicate the direction of the movement of the molecules during the bending process. Download figure Download PowerPoint In addition, the ultralow temperature elasticity of the crystal of phase A was tested. A crystal was fixed between two coverslips and placed in liquid nitrogen (LN) followed by changing the angle of the coverslip to bend the crystal ( Supporting Information Figure S4). To our delight, in LN it exhibited elasticity equivalent to that in air. When the crystal was removed from LN to air, its elasticity was retained at r.t. To reveal the mechanism of the elasticity of phase A in LN, SCXRD was performed at −173 °C on the same single crystal that was analyzed at r.t. The data summarized in Supporting Information Table S1 show that the unit cell parameters of the crystal are little affected by temperature. The three crystallographic axes were slightly shortened (<0.3%) upon freezing the crystal, and the variations were relatively isotropic. The cell volume was contracted by only 0.69%, and the β angle remained almost unchanged. The effect of cold contraction is so weak that neither the strength of the π⋯π interaction nor the strength of the CH⋯π interaction changes obviously (Figures 3a–3d). Thus, the molecular conformation and packing feature are well maintained during the cooling process, and the crystal retains its original elasticity in LN. The SCXRD of phase A at 120 °C further demonstrated that the cell parameters and crystal structures are insensitive to temperature (Figures 3e and 3f and Supporting Information Table S1). The unique elastic property of phase A at r.t. and –196 °C inspired us to test the high-temperature mechanical behavior. Hence, the crystals of phase A were placed on a hot stage and heated from r.t. to 240 °C (melting point of the crystal is ca. 260 °C) at a rate of 10 °C min–1. During the heating process, the crystals retained elasticity until the temperature approached 160 °C, at which point the crystals underwent a visible phase transition accompanied by a hypochromic effect of fluorescence (Figures 2g and 2h). The formed crystal, named phase B, exhibited plastic bending upon stress application. As shown in Figures 2i–2l, compression by tweezers bent the linear crystal. The crystal retained a curved shape after releasing the stress and could be reshaped after heating above 120 °C again. In Supporting Information Movie S2, the reversible process was confirmed. These results indicate the thermoplastic nature of phase B. The thermoplastic bending ability is demonstrated by bending the crystals into various shapes under heating conditions (Figure 2l). To investigate the SCTSC process during heating, variable-temperature powder X-ray diffractions (PXRD) were carried out in a circular manner, from r.t. to 160 °C and then from 160 °C to r.t. ( Supporting Information Figure S5). When the temperature was <140 °C, the crystal phase transition does not take place as evident from an unchanged PXRD pattern. Once the temperature rises to 160 °C, there is a significant change in the peak position of the PXRD pattern, which means that the crystal phase was transformed from A to B. During the cooling process, phase B retains its crystal phase. To know the critical temperature at which the crystal of phase B achieves thermoplasticity, further experiments were conducted. The crystals of phase A were heated to 160 °C for 20 min until the SCTSC transition was completed, and the crystals of phase B, obtained by heating, were then cooled to r.t. and heated again. Here, we found that the crystals of phase B exhibited plasticity at 120 °C whereas the crystal of phase A exhibited elasticity at this temperature ( Supporting Information Movie S1). This experiment demonstrates that the crystal of phase B exhibits thermoplastic bending behavior, and the critical temperature is approximately 120 °C. To reveal the mechanism of thermoplasticity, SCXRD of phase B was initially performed at r.t. As expected, phases A and B have the same components but adopt totally different molecular arrangements. Phase B belongs to the orthorhombic system in space group of Pbcn. In the asymmetric unit, there is one molecule that takes a rather distorted molecular conformation with a torsion angle of 25.8° between the benzene rings and the anthracene unit (Figure 4). Along the crystallographic [001] direction, molecules are stacked into a column by intermolecular π⋯π interaction. Compared with phase A, the π-stacking molecules in phase B are parallel but not completely overlapped, and the vertical distance is 3.65 Å (Figure 4c, cyan dotted line). The π-stacked columns are connected by CH⋯π interactions (2.76 and 2.78 Å, yellow dotted line), forming the crystallographic (010) plane which was determined as the bendable plane through face indexing ( Supporting Information Figure S6). The layered structures are stacked along the [010] direction with an interlayer distance of 4.283 Å, forming slip planes which are the basis for plastic bending (Figure 4d).32 Figure 4 | Torsion angle between the benzene rings and anthracene unit in the crystal of phase B and the intermolecular interactions (yellow dotted line for CH⋯π and cyan dotted line for π⋯π) in (010) plane at −173 °C (a), 25 °C (c), and 120 °C (e). The molecular packing structure and interlayer distance (purple dotted line) of phase B in (100) plane at −173 °C (b), 25 °C (d), and 120 °C (f). Download figure Download PowerPoint As mentioned earlier, the critical temperature at which phase B exhibits thermoplastic bending is approximately 120 °C. To explain this phenomenon, SCXRD was then performed on phase B at −173 and 120 °C, respectively. Based on the data summarized in Supporting Information Table S2, the crystallographic b- and c-axis are expanded while the a-axis is compressed upon increasing temperature, and the magnitude of the change is different. Thus, the intermolecular distances vary extensively when the temperature changes (Figures 4a, 4b, 4e, and 4f). For instance, the distance of CH⋯π and π⋯π interactions in the layered structures dramatically increased from 2.67 to 2.81 Å (+5.2%) and 3.46 to 3.73 Å (+7.8%), respectively. More importantly, the interlayer distance, which is crucial for the plane sliding, significantly increased by 3.9% (4.189–4.353 Å) from −173 to 120 °C. The increase in temperature leads to the enlargement of interlayer distance, which creates more space between (010) planes and makes the plane sliding easier and thereby facilitates the generation of plastic bending. The anisotropic expansion of the crystal axes during heating is the core reason for the thermoplastic bending of the crystal of phase B. Block-shaped crystals with the same component and molecular arrangement as that of phase B were obtained by layering petroleum ether on the top of a tetrahydrofuran (THF) solution of 9,10-bis(phenylethynyl)anthracene for solvent diffusion ( Supporting Information Figure S7). However, these naturally grown crystals of phase B do not have the possibility of elastic or plastic bending. The moduli of phase B obtained by different methods are similar ( Supporting Information Figure S8), indicating that the properties of those mechanical materials are identical. However, it is harder for the thick crystals to show bending strain than thin crystals. The crystal of phase B obtained by SCTSC transition retains the long needle shape of the crystal of phase A, which contributes to the thermoplastic bending of phase B. The crystal tips of phase A showed a brighter emission than the body under a 365 nm UV lamp, which indicates an efficient optical confinement exists within the crystal body. According to the previous reports40–42 and through further experiments, we confirmed that the straight and bent crystals of phase A exhibited optical waveguide behavior. As shown in Figure 5a, a straight single crystal was exposed to 355 nm UV laser, and the emission spectrum was recorded corresponding to the radiation position from one tip to the other. As the distance between the tip and the illumination position increased, the propagation distance of the light increased accordingly, and the emission intensity of the tip thus decreased. It is worth noting that the main peak at 571 nm had more attenuation than the shoulder peak at 616 nm. This situation was due to the self-absorption of the crystal, which caused more light loss at 571 nm (Figures 5d–5f). According to literature procedures,43 the calculated optical loss coefficient of the straight crystal was 0.329 dB mm−1 ( Supporting Information Figure S9). We bent the straight crystal and analyzed it via the same method (Figure 5b). The calculated optical loss coefficient of the elastically bent crystal was 0.347 dB mm−1. Finally, the crystal was plastically bent at 160 °C, and the optical waveguide was tested after the crystal cooled (Figure 5c). Surprisingly, the calculated optical loss coefficient of the plastically bent crystal was 0.379 dB mm−1, which means the plastic bending does not cause additional optical loss. Good flexibility and optical waveguiding capacity indicate that the present crystals have potential applications in flexible optical devices. Figure 5 | Optical waveguides of phase A with the straight (a) and elastically bent (b) shapes, and phase B with plastically bent shape. Fluorescence images collected upon excitation of different positions of single crystal under 355 nm laser beam. Emission spectra are measured at the tip of a single crystal and the distances between this tip and the excitation positions are constantly changed (d–f). Download figure Download PowerPoint Conclusion We report an organic crystal with phase-dependent elasticity and thermoplasticity over wide temperature ranges. Phase A with isotropic intermolecular interactions as well as a crosswise packing structure exhibits elastic deformation in the temperature range of −196 to 150 °C. Phase B obtained from A via a heating-induced SCTSC transition shows thermoplastic behavior (plastically bendable between 120 and 240 °C) due to the lamellar packing structure with temperature-dependent interlayer distance. The temperature-controlled phase transition provides the possibility for a single crystal to achieve a variety of mechanical properties, which may significantly improve the applicability of the materials. Notably, the thermoplasticity, a well-known mechanical property for linear polymer, has been realized in an organic single crystal. Our results not only expand the flexibility of organic crystals to thermoplastic bending, but also pave a way to tune the mechanical property of organic crystals by temperature-controlled phase transition. Supporting Information Supporting Information is available. Conflict of Interest There is no conflict of interest to report. Funding Information This work was supported by the National Natural Science Foundation of China (no. 51773077). References 1. Ahmed E.; Briseno A. L.; Xia Y. N.; Jenekhe S. A.High Mobility Single-Crystal Field-Effect Transistors from Bisindoloquinoline Semiconductors.J. Am. Chem. Soc.2008, 130, 1118–1119. Google Scholar 2. Mun S.; Park Y.; Lee Y. E. K.; Sung M. M.Highly Sensitive Ammonia Gas Sensor Based on Single-Crystal Poly(3-hexylthiophene) (P3HT). Organic Field Effect Transistor.Langmuir2017, 33, 13554–13560. Google Scholar 3. Chandrasekar R.Organic Photonics: Prospective Nano/Micro Scale Passive Organic Optical Waveguides Obtained from π-Conjugated Ligand Molecules.Phys. Chem. Chem. Phys.2014, 16, 7173–7183. Google Scholar 4. Zhang C.; Zhao Y.; Yao J.Optical Waveguides at Micro/Nanoscale Based on Functional Small Organic Molecules.Phys. Chem. Chem. Phys.2011, 13, 9060–9073. Google Scholar 5. Ma Z.; Wang Z.; Meng X.; Ma Z.; Xu Z.; Ma Y.; Jia X.A Mechanochromic Single Crystal: Turning Two Color Changes into a Tricolored Switch.Angew. Chem. Int. Ed.2016, 55, 519–522. Google Scholar 6. Chen P.; Zhang H.; Niu L.; Zhang Y.; Chen Y.; Fu H.; Yang Q.A Solid-State Fluorescent Material Based on Carbazole-Containing Difluoroboron β-Diketonate: Multiple Chromisms, the Self-Assembly Behavior, and Optical Waveguides.Adv. Funct. Mater.2017, 27, 1700332. Google Scholar 7. Zhu Q.; Yang W.; Zheng S.; Sung H.; Williams I. D.; Liu S.; Tang B.Reversible Thermo-Stimulus Solid-State Fluorescence-Colour/On-Off Switching and Uses as Sensitive Fluorescent Thermometers in Different Temperature Ranges.J. Mater. Chem. C2016, 4, 7383–7386. Google Scholar 8. Wang H.; Chen P.; Wu Z.; Zhao J.; Sun J.; Lu R.Bending, Curling, Rolling, and Salient Behavior of Molecular Crystals Driven by [2+2] Cycloaddition of a Styrylbenzoxazole Derivative.Angew. Chem. Int. Ed.2017, 56, 9463–9467. Google Scholar 9. Terao F.; Morimoto M.; Irie M.Light-Driven Molecular-Crystal Actuators: Rapid and Reversible Bending of Rodlike Mixed Crystals of Diarylethene Derivatives.Angew. Chem. Int. Ed.2012, 51, 901–904. Google Scholar 10. Reddy C. M.; Padmanabhan K. A.; Desiraju G. R.Structure–Property Correlations in Bending and Brittle Organic Crystals.Cryst. Growth Des.2006, 6, 2720–2731. Google Scholar 11. Ghosh S.; Reddy C. M.Elastic and Bendable for the of Organic Chem. Int. Ed.2012, 51, Google Scholar Saha S.; Desiraju G. and in Crystal of Elastic Google Scholar Saha S.; Desiraju G. of Am. Chem. Google Scholar L.; P.; S.; Ahmed E.; J.; A.; through a Bendable Organic Crystal as an Optical Chem. Int. Google Scholar Ahmed E.; P.; Elastic and Molecular Chem. Int. Google Scholar L.; J.; P.; M.; S.; M. W.; and of Molecular Single-Crystal Optical Waveguides.Adv. Funct. Google Scholar G. Reddy C. Organic Crystals by in Supramolecular Am. Chem. Google Scholar M.; Ghosh S.; N.; G. D.; Reddy C. M.; of Structure in a Molecular Google Scholar Hayashi S.; Bending of a Fluorescent Organic Single Crystal: of the Google Scholar Hayashi S.; S. Y.; D.; Y.; Elastic Organic Crystals of π-Conjugated with Bending and Optical Chem. Int. Google Scholar M.; A. S.; S. S.; Reddy C. M.; Chandrasekar Single-Crystal Organic Waveguides and Chem. Int. Google Scholar M.; P.; L.; Ahmed E.; Ghosh S.; P.; Chandrasekar of Organic Single-Crystal Optical Chem. Int. Google Scholar Liu H.; Lu Z.; Zhang Z.; Wang Y.; Zhang Elastic Organic Crystals for Optical Chem. Int. Google Scholar Wang C.; Wang Y.; Zhang Organic Single Crystals Optical and Google Scholar Liu Q.; Liu W.; Wang C.; Wang Y.; Zhang Organic Crystals of a Bendable and Optical Chem. Google Scholar Lu Z.; Zhang Y.; Liu H.; K.; Liu W.; Zhang Waveguiding Organic Single Crystals and Bending Chem. Int. Google Scholar Liu H.; Lu Z.; Tang C.; Zhang Z.; Zhang Organic Single Crystal with and and Chem. Int. Google Scholar Liu Lu Z.; Tang Liu H.; Liu H.; Zhang Z.; K.; Zhang Organic Crystal with Temperature Elasticity as a Chem. Int. Google Scholar Ghosh S.; M. K.; S. Desiraju G. Elastic Organic Chem. Int. Google Scholar A.; A.; M. C.; Xu Y.; C.; K.; of Changes in Elastic Crystals of Google Scholar Hayashi S.; Organic Crystals of a Fluorescent π-Conjugated Chem. Int. Ed.2016, 55, Google Scholar Desiraju G. to Am. Chem. Google Scholar Liu H.; K.; Zhang Z.; Zhang Organic Crystal with Elasticity at an Temperature and at Chem. Int. Google Scholar Ghosh S.; M. K.; S.; Desiraju G. and Driven of the Organic Crystal: Am. Chem. Google Scholar Liu H.; Z.; Q.; L.; Wang Y.; Zhang Bending Based on a Waveguiding Organic Google Scholar Zhao K.; Zhang H.; Yang Y.; P.; Zhang H.; C.; C. S.; M.; Chen of the to Google Scholar Ghosh S.; Reddy C. of with and and Google Scholar of Chem. Int. 13, Google Scholar A.; Wang J.; Xu S.; Z.; Y.; Xu W.; Behavior for Two of Based on Google Scholar Zhao Y.; Fu H.; A.; Ma Y.; Q.; Yao and of Organic Chem. Google Scholar Q.; Q.; Y.; Y.; Y.; C.; Z.; Sun C.; Yao J.; Zhao in Organic 4, Google Scholar Zhao Y.; Xu J.; A.; Fu H.; Ma Y.; L.; Yao J.Optical Based on Organic and Chem. Int. Google Scholar Y.; Ma Z.; A.; Xu W.; Wang Y.; H.; Wang K.; Zhao Y.; Jia X.A Single Crystal with Multiple of Optical and Mater. Google Scholar Information crystals

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