Flexible Organic Crystal with Two-Dimensional Elastic Bending and Recoverable Plastic Twisting for Circularly Polarized Luminescence

Abstract

Open AccessCCS ChemistryCOMMUNICATIONS20 Feb 2024Flexible Organic Crystal with Two-Dimensional Elastic Bending and Recoverable Plastic Twisting for Circularly Polarized Luminescence Wenjie Kuang†, Bo Jing†, Songgu Wu and Junbo Gong Wenjie Kuang† State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072 Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072 , Bo Jing† State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072 Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072 , Songgu Wu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072 Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072 and Junbo Gong *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072 Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072 https://doi.org/10.31635/ccschem.024.202303561 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Multidimensional mechanical flexible organic crystals with tunable optoelectronic properties hold significant promise for practical application in complicated environmental conditions. Herein, based on a newly designed "flexible" Schiff base small molecule with chirality, we presented a compatibly bendable and twistable organic single crystal with circularly polarized luminescence for the first time. First, the two-dimensional elastic bending of the chiral crystal was realized at both room and liquid nitrogen temperatures, along with recoverable plastic twisting at room temperature. Besides, circular dichroism and circularly polarized luminescence spectroscopy were employed to characterize the chiral enantiomer in solution and the solid state. Our design strategy provides a new perspective for the future construction of chiroptical flexible crystal materials. Download figure Download PowerPoint Introduction Organic single crystals, emerging as promising optoelectronic materials because of long-range ordered molecular arrangement, anisotropic physical properties, lower lattice defects, strong solid-state emission, and higher charge-carrier mobility, have been considered as one of the substitutable materials for next-generation optoelectronic devices.1–3 However, it is universally acknowledged that crystals are brittle, implying they are prone to abrasion and easily damaged when subjected to external strain, which poses limitations in practical fabrication and application. Significantly, since the initial identification of plastic organic single crystals in 2005 and elastic organic single crystals in 2012,4,5 chemists and material scientists have paid more attention to flexible organic crystal materials because they can relax internal stress through molecular reconfiguration,6–10 manifesting as bending,11–13 twisting14–16 and curling17,18 while maintaining macroscopic morphological integrity. Organic single crystals exhibit flexibility through two distinct mechanisms: elastic and plastic.19,20 The former describes the ability of crystals to recover to the initial state after deformation with the disappearance of strain, and the latter describes deformation that cannot be actively recovered.21 Particularly, elastic crystals deposited on a substrate could maintain deformation without external force due to the adhesive interaction between the surface of the substrate and the crystal, behaving like plastic crystals, referred to as pseudo-plastic.22,23 This remarkable nature of crystal brings light to solving the brittleness bottleneck of optical functional organic crystals.24 For example, Zhang and coworkers25 fabricated a highly elastic organic crystal for flexible optical waveguides based on Schiff base compounds. Naumov and colleagues26 developed a plastic organic crystal capable of multimode light propagation. Chandrasekar's lab integrated flexible micro-optical waveguides into organic photonic circuits.27 Although the past 5 years have witnessed the emergence of numerous photoluminescent organic single crystals showcasing elasticity and plasticity,28–35 rationally designing optically functionalized organic single crystals with multidimensional mechanical responses is still challenging because the anticipated molecular skeleton and crystal structure requires collaborative consideration of mechanical compliance and optical properties. Due to its high optical sensitivity and resolution, circularly polarized light has aroused great interest in information anticounterfeiting, three-dimensional (3D) displays, light-emitting devices, and other aspects.36,37 A physical method to obtain circularly polarized light is to convert linearly polarized light through a quarter wave plate. In comparison, chemical methods could directly generate circularly polarized luminescence (CPL) via the effective coupling of photoluminescence and chirality, avoiding the energy loss caused by the wave plate transmission process in physical methods.38–43 Therefore, creating innovative chiral materials with CPL has become a cutting-edge field in the research of chiral science. Logically, endowing flexible organic single crystals with CPL properties can broaden the application scope of flexible materials. However, only one case of one-dimensional elastic small-molecular-weight organic crystals with CPL activity has been reported to our knowledge.44 In this regard, integrating multidimensional flexibility and CPL within one organic single crystal requires the following considerations: (1) The flexibility of the organic crystal originates from molecular sliding/rotating, cooperatively controlled by intermolecular interactions and packing structures. Flexible molecular conformation may be beneficial for the mechanical reconfiguration process in different directions of organic crystals, which may realize multidimensional flexibility even in extreme environments. (2) Photoluminescence in the solid state is required, which can be well-satisfied by aggregation-induced emission molecules. (3) Chirality, including molecular chirality, structural chirality, or macroscopic chirality, will probably induce the production of CPL. Given that most Schiff base crystals are flexible and have typical aggregation-induced emission properties, introducing a six-membered ring with conformational flexibility and chiral centers into Schiff base compounds could lead to the accomplishment of the purpose.45,46 Results and Discussion For a proof of concept, the newly designed Schiff base, (S/R)-4-methyl-2-(1-((1,2,3,4-tetrahydronaphthalen-1-yl)imino)ethyl)phenol (S/R-MTIEP) was synthesized in good yield via one-step reaction between (S/R)-1,2,3,4-tetrahydronaphthalen-1-amine and 1-(2-hydroxy-5-methylphenyl)ethan-1-one, possessing evident aggregation-induced emission characteristics (Figure 1a and Supporting Information Figures S1–S4). Yellow needle-like centimeter-scale S/R-MTIEP single crystals (Figure 1b), were obtained via both evaporation crystallization and sublimation crystallization, with the difference being that sublimation crystallization could generate thinner crystals. The phase purity of crystal samples prepared by these two approaches was confirmed by powder X-ray diffraction, revealing identical crystal forms( Supporting Information Figure S8). Since the optical and mechanical properties of the S/R-MTIEP enantiomer are consistent, we only present the characterization and discussion of S-MTIEP in this manuscript and put R-MTIEP in the Supporting Information for clarity. The absorption and fluorescence spectra of S-MTIEP were determined in solution and the solid (Figure 1c and Supporting Information Figure S9). The excitation spectrum was similar to the absorption spectrum in the solid state. Under 365 nm UV irradiation, the crystals of S-MTIEP showed a single-peak emission with a maximum emission wavelength of 530 nm, emitting greenish-yellow luminescence (ΦPL = 4.00%, τ = 1.32 ns, Supporting Information Figure S10). Figure 1 | (a) Molecular structure of S/R-MTIEP. (b) Micrographs of S/R-MTIEP single crystals under a standard white light source and 365 nm UV light. Scale bar, 1 mm. (c) UV–visible absorption spectra of solution (densely light dashed line) and powder (densely dark dashed line), excitation spectrum monitored at emission wavelength of 530 nm (loosely dashed line) and photoluminescence emission spectrum (solid line) correspond to S-MTIEP crystal with a CIE coordinate of (0.39, 0.56). (d) Schematic representation of two-dimensional elastic bending and plastic twisting of S/R-MTIEP single crystal. Download figure Download PowerPoint The single crystals of S/R-MTIEP displayed multidimensional flexibility, including two-dimensional (2D) elastic bending and recoverable plastic twisting (Figure 1d). First, we applied force at (100) and (001) crystal faces of the same single crystal of S-MTIEP to bend sequentially, and no noticeable cracking or defect was observed. Upon release, the single crystal rapidly recovered to its original state, indicating its 2D elastic bending property (Figure 2a–d and Supporting Information Figure S5). Besides, the single crystals of S-MTIEP could be repeatedly bent multiple times at (100) and (001) crystal faces, demonstrating the positive possibility for practical applicability ( Supporting Information Movies S1 and S2). It has been reported that good 2D bending ability means the crystal might further exhibit twisting ability.14 The single crystal of S-MTIEP was indeed twisted, forming a 3D torsional macroscopic configuration (Figure 2e and Supporting Information Figure S6). Unlike most reported plastic twisted organic crystals, the as-formed crystals could recover to the straight state without visible breakage and be twisted in the same or opposite direction once again ( Supporting Information Movies S5 and S6). The frequently used elastic materials such as rubber, would lose inherent elasticity and become significantly stiffer at low temperatures, so exploiting flexible materials with low-temperature resistance is crucial. Thus, we conducted the mechanical properties test on S/R-MTIEP single crystal in liquid nitrogen. Surprisingly, when using a needle to bend the (100) and (001) crystal faces, the crystal also showed good 2D bending and recovered rapidly after removing the needle (Figure 3a–l and Supporting Information Figure S7). Furthermore, after taking out the crystal that has been bent multiple times in liquid nitrogen, it still maintained the 2D elastic bending ability at room temperature ( Supporting Information Movies S3 and S4). Figure 2 | The multidimensional flexibility of one S-MTIEP single crystal, including elastic bending and recovering applied force at (100) crystal face (a, b), (001) crystal face (c, d), and recoverable plastic twisting (e) at room temperature. Scale bar, 1 mm. Download figure Download PowerPoint Figure 3 | The elastic bending and recovering of one S-MTIEP single crystal applied force at (100) crystal face (a–f) and (001) crystal face (g–l) at liquid nitrogen temperature. Scale bar, 4 mm. The portion of a glass slide immersed in liquid nitrogen is marked with a dashed line. Download figure Download PowerPoint To deeply understand the mechanical properties at both room and low temperatures, a variable-temperature single crystal X-ray diffraction analysis of S/R-MTIEP single crystal was performed, which crystallized in the monoclinic P21 space group, and there were two molecules in the asymmetric unit (Figure 4a and Supporting Information Table S1). At room temperature, the S-MTIEP molecules formed intramolecular hydrogen bonds (O–H···N, the distances of N···H: 1.782 and 1.812 Å). Different from generally reported π-conjugated Schiff base molecules with a planar conformation,15,25,47 the presence of a saturated six-membered ring led to the formation of a torsion angle of 127.33° and 118.38° within the S-MTIEP molecule (Figure 4a). Structurally, the π–π interactions facilitated the face-to-face parallel arrangement of S-MTIEP molecules to form densely packed molecular columns along the [010] direction, that is, the dominant growth direction (Figure 4c and Supporting Information Figure S11). Furthermore, adjacent columns were connected through intermolecular C–H···C, C–H···O, and other dispersed interactions to expand into a 3D packing structure (Figure 4b and Supporting Information Figure S13). The mechanism of elastic bending could be illustrated by a universal model.10 The narrow face (100) and the wide face (001), determined by the face indexing of a single crystal ( Supporting Information Figure S12), were bent by the external force, with the π-stacking columns playing a vital role in the molecule rearrangement process. Specifically, the distance between S-MTIEP molecules of the outer and the inner arcs was expanded and compressed, accompanied by the rotating of molecules, to maintain the overall structure (Figure 4e and Supporting Information Figure S14). In addition, the corrugated structures inhibited the long-range sliding of molecules, and numerous weak intermolecular interactions dissipated internal stress as a buffer during the bending. From 293 K to 193 K, the b axis of the S-MTIEP crystal was contracted by 2.01%, whereas the a and c axes varied slightly, and the cell volume was contracted by 2.26%. In structure, the molecular conformation and arrangement were identical to that at room temperature ( Supporting Information Figure S15). Furthermore, the slightly strengthened π···π interactions (the vertical distance from 2.996 Å to 2.912 Å) further enhanced the resistance of the π-stacking columns to external forces (Figure 4e).47 For plastic twisting, the twisting force to which the end of the single crystal is subjected will break weak intermolecular interactions and cause the movement of the molecules parallel to the (010) crystal plane. Nonetheless, the strong π–π interactions along the [010] direction will prevent the crystal structure from collapsing, as well as the weak intermolecular interactions are easy to break and reform, which may be responsible for plastic twisting. The intermolecular interactions in the (010) plane were crucial for understanding the recoverability of plastic deformation. For this reason, the intermolecular interactions were further visualized quantitatively through energy framework analysis (Figure 4d and Supporting Information Figures S16 and S17).48 The size of the blue tube represents the total energy of intermolecular interactions. We noted that the interactions along the [100] direction were significantly stronger than the [001] direction due to the formation of hydrogen bonds. We speculated that when opposite twisting forces were applied, the molecules tended to reform more stable and directional hydrogen bonds rather than move randomly; thus, fully restored to the initial state. Figure 4 | The crystal structure of S-MTIEP. (a) The asymmetric unit of the S-MTIEP single crystal. Molecular packing structure viewed along the a-axis (b) and b-axis (c) directions. (d) Energy framework of the S-MTIEP single crystal, viewed along the [010] direction. (e) Molecular packing structure viewed along the c-axis at room and low temperatures. Download figure Download PowerPoint The ground state chirality and excited state chirality of S/R-MTIEP were comprehensively characterized by circular dichroism (CD) and CPL spectra. The mirror-symmetry CD absorption spectra of both the solution (Figure 5a) and the powders (Figure 5b) of S/R-MTIEP displayed a significant Cotton effect, with S-MTIEP and R-MTIEP, exhibiting a positive and negative Cotton effect, respectively. In detail, an ethanol solution of S/R-MTIEP displayed three peaks at 276, 330, and 402 nm; the solid powder of S/R-MTIEP also exhibited three peaks at 288, 315, and 367 nm, consistent with the absorption spectra, indicating chirality in the discrete and aggregated states. Additionally, we tested the CD and linear dichroism (LD) of the S/R-MTIEP crystal: The LD signal was far lower than the CD signal, which indicated that the LD could be ignored in this crystal system ( Supporting Information Figure S18). Then about the chirality of S/R-MTIEP in the excited state, we found that the CPL signal could not be detected in the clear solution owing to the aggregation-induced emission feature of the S/R-MTIEP molecule. The CPL emission spectra of S/R-MTIEP crystals were mirror-symmetry in the wavelength range from 450 to 655 nm, which, well corresponded to the photoluminescence emission spectra (Figure 5c). The level of CPL could be evaluated using the photoluminescence dissymmetry factor (gPL), defined as gPL = 2 × (IL − IR)/(IL + IR), where IL and IR refer to the intensity of left-handed and right-handed CPL. The calculated value of gPL of S/R-MTIEP crystals in the photoluminescence wavelength range of S/R-MTIEP is approximately 5 × 10−4 (Figure 5d), equivalent to the reported value of chiral materials of organic small molecules (ranges from 10−5 to 10−3) at room temperature.49 We considered that the imperfect CPL spectra might be attributed to the inherent weak signal of small organic molecules and the aggregate inhomogeneity of the crystals sample,50 but the mirror-image CPL spectra of the S/R enantiomer with opposite chirality demonstrated the reliability of CPL data. It should be noted that twisted crystals with macroscopic chirality might have positive impacts on chiroptical properties. Therefore, we envisaged that preparing twisted crystals with controllable chirality and pitch through top-down or bottom-up approaches would be fascinating in future research. Figure 5 | Chiroptical properties of S/R-MTIEP. (a) CD absorption spectra of S/R-MTIEP in ethanol solution. (b) CD absorption spectra of S/R-MTIEP powders. (c) CPL emission spectra of S/R-MTIEP crystals. (d) CPL dissymmetry factor gPL of S/R-MTIEP crystals versus wavelength. Download figure Download PowerPoint With the first case of micro-flexible organic waveguides reported, the optical waveguides of flexible organic crystals have attracted widespread attention.51–55 Therefore, we explored the optical waveguide performance of as-prepared chiral organic crystals. When using 365 nm UV light as the excitation source, we found that the fluorescence emission at the tip of S/R-MTIEP crystals was brighter than that in the middle part, indicating it might have optical waveguide characteristics ( Supporting Information Figure S19). Using a point laser source to excite at different positions in the straight and bent S/R-MTIEP single crystal and record a series of emission spectra at one tip of the crystal (Figure 6a,b and Supporting Information Figure S20). As the propagation distance increased, the emission intensity gradually decreased due to larger optical loss over longer propagation distances. By fitting the collected spectral data, the optical loss coefficients at 530 nm of straight S-MTIEP and R-MTIEP single crystals were 0.213 and 0.220 dB mm−1. In the bent state, the single crystal of S/R-MTIEP still exhibited optical waveguide performance, and the optical loss coefficients were 0.260 and 0.257 dB mm−1, respectively (Figure 6c,d and Supporting Information Figure S20). The results demonstrated that the single crystal of S/R-MTIEP had the potential for use as optical waveguide materials in both states. However, the bending deformation hurt the light transmission ability attributed to defects generation or changes in molecular stacking. Figure 6 | Photoluminescence spectra, collected at the tip of the S-MTIEP single crystal in the straight state (a) and bent state (b) by changing the distance between the point of excitation and the tip of the crystal. The single-exponential fits of Itip/Ibody versus the distance between the point of excitation and the tip of the crystal for the emission wavelength at 530 nm in the S-MTIEP single crystal in the straight state (c) and bent state (d). Download figure Download PowerPoint Conclusion We proposed a molecular design strategy of flexible Schiff base compounds with chirality via introducing a nonplanar 1,2,3,4-tetrahydronaphthalene and chiral center. The single crystals obtained realized multidimensional flexibility both at room and liquid nitrogen temperatures, including 2D elastic bending and recoverable plastic twisting. This is the first time to endow multidimensional flexible organic molecular single crystals with CPL, highlighting the potential talent of the application under complicated surroundings. In contrast to the widespread use of π-conjugated planar rigid molecules, this contribution put forward an approach based on nonplanar flexible molecules to manufacture photoluminescent flexible crystals, which broadens the horizons for future development of flexible functional crystal materials. Supporting Information Supporting Information is available and includes chemicals and instruments, detailed experimental procedures, crystallographic and spectroscopic characterization, and additional graphs. 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 (grant no. 22178254), the Key R&D Program of Hebei Province, China (grant no. 21282602Z), and the Tianjin Natural Science Foundation, China (grant no. 21JCZJC00400). Acknowledgments The authors wish to acknowledge Dr. Chen Yifu for his suggestion on the selection of research fields. W.K. thanks Prof. Dr. Zhang Xin for his assistance in optical characterization. References 1. Liu D.; Liao Q.; Peng Q.; Gao H.; Sun Q.; De J.; Gao C.; Miao Z.; Qin Z.; Yang J.; Fu H.; Shuai Z.; Dong H.; Hu W.High Mobility Organic Lasing Semiconductor with Crystallization-Enhanced Emission for Light-Emitting Transistors.Angew. Chem. Int. Ed.2021, 60, 20274–20279. Google Scholar 2. Briseno A. L.; Tseng R. J.; Ling M.-M.; Falcao E. H. L.; Yang Y.; Wudl F.; Organic on Flexible Google Scholar C.; J.; R. L.; of in Organic Google Scholar of on of Google Scholar and for the of Flexible Organic Chem. Int. Google Scholar D.; J.; F.; A. Y.; L.; H.; Y.; Zhang H.; Sun C.; C.; R. Naumov and of Molecular Google Scholar to to Crystal of Organic Chem. Google Scholar A. J.; A. A. J.; J.; Flexible Molecular Google Scholar D.; Naumov of in a Molecular Google Scholar C.; Y.; C.; of in Elastic of Google Scholar Sun Elastic Google Scholar Liu Y.; Yang Zhang J.; Wu Gong and of with Bending Google Scholar and Bending in Molecular on a Google Scholar Engineering of Chem. Google Scholar Liu H.; Z.; C.; Zhang Z.; Zhang Flexible Organic Crystal with and and Chem. Int. Google Scholar Chen J.; Y.; Liu H.; Zhang Y.; Flexible Crystal with 2D Elastic Plastic Twisting and Bending Chem. Google Scholar H.; Chen Wu Z.; J.; Sun J.; and of Molecular by of a Chem. Int. Google Scholar D.; R. of Organic Molecular Crystal Chem. Int. Google Scholar Zhang Sun C.; Liu Y.; C.; J.; Wu Gong of Elastic and 2D Plastic of Molecular with of Google Scholar A. J.; J.; C.; C.; Flexible of Molecular by and Chem. Int. Google Scholar Google Scholar A. Flexible Organic and Chem. Int. Google Scholar L.; Naumov of Organic Chem. Int. Google Scholar Organic of a Chem. Int. Google Scholar Liu H.; Z.; Zhang Z.; Y.; Zhang Elastic Organic for Flexible Chem. Int. Google Scholar L.; J.; Naumov a Organic Crystal as an Chem. Int. Google Scholar J.; A. Organic from Different Flexible Google Scholar D.; Y.; Elastic Organic of with Bending and Flexible Chem. Int. Google Scholar J.; Liu H.; Zhang Organic Crystal with and over Google Scholar H.; Y.; Flexible and Organic Crystal Google Scholar J.; A. of a as a Chem. Int. Google Scholar C.; L.; Liu Zhang Y.; Sun L.; J.; Q.; Zhang Y.; Yang Y.; Y.; Sun Y.; L.; C.; Sun C.; Z.; Chen Ling H.; J.; of Elastic Molecular Crystal for Flexible Organic Google Scholar A. in a 2D Flexible Organic of Google Scholar J.; Y.; Z.; Hu Y.; Z.; H.; Elastic Organic Crystal with Google Scholar C.; Zhang Z.; Zhang Engineering of Organic for Lasing with Google Scholar Gong Z.; Zhang Yang D.; Zhang J.; Y.; H.; Chen Z.; Liu in Circularly Polarized Molecular and China Google Scholar Y.; J.; Liu Polarized Luminescence in and Google Scholar E. F.; L.; A. J.; Polarized Luminescence by in a of CPL Organic from Chem. Google Scholar L.; C.; L.; Liu Wu of and the Emission and Circularly Polarized Chem. Int. Google Scholar Z.; J.; of Chem. Int. Ed.2021, 60, Google Scholar J.; D.; F.; Y.; L.; of Circularly Luminescence Circularly Chem. Int. Ed.2021, 60, Google Scholar F.; Z.; with Circularly Polarized of Google Scholar F.; of for Chem. Int. Google Scholar Q.; L.; Naumov Zhang Flexible Organic Crystal with Circularly Polarized Chem. Int. Google Scholar Elastic Organic Flexible Chem. Int. Google Scholar Q.; Y.; L.; C.; J.; Liu Y.; Emission as for Google Scholar Liu H.; Zhang Z.; Zhang Organic Crystal with at an and at Chem. Int. Google Scholar J.; J.; J.; D.; A Program for and of Molecular Google Scholar Peng Q.; Polarized Luminescence in Organic Google Scholar Y.; H.; with Circularly Polarized Luminescence and Chem. Google Scholar Y.; Y.; from Organic in Google Scholar and Google Scholar Yang Y.; and Polarized Chem. Int. Google Scholar and within Molecular Chem. Int. Google Scholar C.; D.; from Information Google Scholar Information Chemical polarized authors wish to acknowledge Dr. Chen Yifu for his suggestion on the selection of research fields. W.K. thanks Prof. Dr. Zhang Xin for his assistance in optical characterization. times