Authors: Baolei Tang | Shiyue Tang | Cheng Qu | Kaiqi Ye | Zuolun Zhang | Hongyu Zhang
Source: CCS Chemistry (article, 2022-11-25)
Field: Materials Science — Polymers and Plastics

Side-Chain Engineering of Organic Crystals for Lasing Media with Tunable Flexibility Open AccessCCS ChemistryRESEARCH ARTICLES22 Dec 2022Side-Chain Engineering of Organic Crystals for Lasing Media with Tunable Flexibility Baolei Tang†, Shiyue Tang†, Cheng Qu, Kaiqi Ye, Zuolun Zhang and Hongyu Zhang Baolei Tang† State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, Jilin Province , Shiyue Tang† State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, Jilin Province , Cheng Qu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, Jilin Province , Kaiqi Ye State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, Jilin Province , Zuolun Zhang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, Jilin Province 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, Jilin Province https://doi.org/10.31635/ccschem.022.202202278 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Abundant high-performance organic crystals have been reported in the past decades; however, applications of crystalline materials are seriously restricted by their notorious brittleness. Recently, some organic crystals have been designed to be deformable in response to stress, light, heat, or humidity; nevertheless, the development of flexible organic crystals currently relies on a molecular framework design. So far there is no effective strategy for constructing organic crystals with tunable flexibility based on the same luminescent skeleton. Herein, we propose a side-chain engineering strategy aimed at facilely modulating the mechanical compliance of organic crystals. Subtly changing the side chains of a single-benzene π-skeleton greatly alters the mechanical behaviors while maintaining the unique optical functions of the produced crystals. Optical waveguides and amplified spontaneous emissions were measured to evaluate the application potentials of the flexible crystals as soft optical transducing media. We anticipate that the proposed strategy will be expanded to regulate the mechanical compliance of other organic crystals with unique emission properties. In addition, the applications attempted preliminarily here highlight the superiority of "crystal flexibility" in flexible optoelectronics for some special application scenarios that require complex and shape changeable optical circuits. Download figure Download PowerPoint Introduction Luminescent molecular crystals have been widely investigated in the past decades due to their unique features including long-range ordered packing structures, anisotropic physical properties, high carrier mobilities, intense luminescence, and so on.1–6 Although organic crystals with desired emission colors and high quantum yields can be synthesized via designing a π-conjugated skeleton of molecules, the intrinsically poor mechanical properties of organic crystals prevent crystalline materials from applications in soft optoelectronics and wearable devices. It is generally considered that crystalline and flexible are incompatible, that is, molecular crystals are brittle and tend to break into pieces when external pressure is applied. To explore the applications of molecular crystals, the development of organic crystals with tunable flexibilities is highly important and thus urgently demanded. Unlike polymer and metal materials, organic crystals with deformation capabilities require novel mechanisms. Very recently, some molecular crystals that can be reversibly (elastic) or irreversibly (plastic) deformed in response to light,7–9 heat,10,11 humidity,12 and mechanical force13,14 have been reported since the pioneering work of Ghosh and Reddy15 and Desiraju et al.16 Hayashi and coworkers17,18 reported elastic organic crystals with fluorescence, and they also investigated the effect of deformation on luminescence. Our group has successfully applied luminescent elastic organic crystals as soft, active optical waveguiding and/or lasing media,14,19–24 and this direction has been further carried forward by other research groups.9,25–29 In addition, the elastically bendable and plastically twistable emissive crystals can precisely control the polarization direction through fine tuning the twisted angle of the crystal.30,31 Furthermore, by using the flexibility of organic crystals, Chandrasekar and coworkers have opened a new application direction, that is, flexible single-crystal photonic circuit, based on elastic bendable organic luminescent crystals.32 All these achievements elucidate the great potential of flexible organic crystals as active materials in the coming wearable optoelectronic devices. Organic crystals with different deformations of elasticity and plasticity reported so far are obtained mainly by constructing different π-systems, which usually cause dramatic change of emissions. Although the deformation mechanism of organic crystals remains a debate, clearly, the mechanical properties of organic crystals are directly related to the molecular arrangements as well as intermolecular interactions. Hence, crystal engineering is a good choice to grow organic crystals with elastic and plastic deformation abilities, and several organic polymorphs with different mechanical behaviors have been achieved.33 Unfortunately, most organic compounds can generate only one stable crystalline phase, indicating the limitation of the methodology of crystal engineering. The emissions of organic materials are dominated by the π-conjugated skeleton, and the side substituents, particularly carbon chains, have little effect on the luminescence of crystals. This implies that there is another way, namely, side-chain engineering, to regulate the mechanical compliance of organic crystals without damaging the emission performance. In this case, the π-electron cores serve as luminescent structures and side chains optimize the packing structures as well as intermolecular interactions, thereby modulating the mechanical behaviors. In the previous work, we designed a tetra-substituted benzene framework that could readily yield highly efficient red-emissive crystals with unique optical functions.34 The molecules bearing this single-benzene skeleton exhibited high crystallinity and could form high-quality crystals with potential applications as optical transducing media.35 However, these crystals were generally brittle and underwent fracture or crack upon loading force. Herein, we report the modulation of mechanical compliance of organic crystals via a side-chain engineering strategy based on a tetra-substituted single-benzene core. Compounds 1− 4 with different side chains connected to nitrogen atoms were newly synthesized. They readily generated high-quality organic crystals (Cry- 1, Cry- 2, Cry- 3, and Cry- 4) with intense red emissions. Although these compounds have quite similar chemical structures, their crystals 1− 4 show distinct mechanical properties. Cry- 1 can be plastically bent along two different directions perpendicular to the crystal growth axis, while Cry- 2 and Cry- 3 undergo elastic bending when external transverse forces are loaded. Thick flake Cry- 4 breaks into pieces upon bending, which is indicative of its brittle character. X-ray single-crystal structural analyses of crystals 1− 4 have been carefully investigated to disclose the effects of side chains on the mechanical properties. The potentials of Cry- 1, Cry- 2, and Cry- 3 as active materials in flexible optical devices have been preliminarily evaluated by checking the optical waveguides and amplified spontaneous emissions (ASE) in one-dimensional (1D) and two-dimensional (2D) plastically bent (for Cry- 1) and elastically bent (for Cry- 2 and Cry- 3) states. Experimental Methods General information The starting materials were purchased and used without purification. Elemental analyses were performed on a flash EA 1112 spectrometer (Agilent Technologies, Santa Clara, California, America). Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was performed on a Shimadzu AXIMA-CFR MALDI-TOF mass spectrometer (Shimadzu Corporation SHIMADZU CORPORATION, Kyoto, Japan). Ultraviolet–visible (UV–vis) absorption spectra were recorded by a Shimadzu UV-2550 spectrophotometer (Shimadzu Corporation SHIMADZU CORPORATION, Kyoto, Japan). The emission spectra were recorded by a Shimadzu RF-5301 PC spectrometer (Shimadzu Corporation SHIMADZU CORPORATION, Kyoto, Japan) or a Maya2000 Pro CCD spectrometer (Ocean Optics, Dunedin, Florida, America). The absolute fluorescence quantum yields were measured on an Edinburgh FLS920 spectrometer (Edinburgh Instruments, Livingston, Scotland) combined with a calibrated integrating sphere. Three-point bending tests were carried out using an Instron 5944 universal testing system with a capacity of 10 N Instron 2530 load cell. Scanning electron microscopy (SEM) images were obtained on the FEI Quanta 450 field emission scanning electron microscope operated at 3 kV. All the measurements were carried out at room temperature under ambient conditions. The active optical waveguides The crystal was irradiated by the third harmonic (355 nm) of a Nd: YAG (yttrium-aluminum-garnet) laser at a repetition rate of 10 Hz and a pulse duration of about 10 ns. The energy of the laser was in the range of 96–210 kW/cm2. The beam was focused into a small dot by using a concentrated piece. The emission was detected at one end of the crystal using a Maya2000 Pro CCD spectrometer (Ocean Optics, Dunedin, Florida, America). The measurements were carried out in air at room temperature. The laser test The crystal was irradiated by the third harmonic (355 nm) of a Nd: YAG laser at a repetition rate of 10 Hz and pulse duration of about 10 ns. The energy of the pumping laser was adjusted by using the calibrated neutral density filters. The beam was focused into a strip whose shape was adjusted to 3.3 × 0.6 mm2 by using a cylindrical lens and a slit, and the light strip was completely absorbed by the crystals. The edge emission and photoluminescence spectra of the crystals were detected using a Maya2000 Pro CCD spectrometer (Ocean Optics, Dunedin, Florida, America). The measurements were carried out in air at room temperature. Single-crystal X-ray diffraction Single-crystal X-ray diffraction (SCXRD) data were collected on a Rigaku RAXIS-PRID diffractometer (Rigaku Corporation, Kyoto, Japan) using the ω-scan mode with graphite monochromator Mo Kα radiation. The structures were solved with direct methods using the Olex2 programs and refined with full-matrix least squares on F2. Non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms were calculated and refined isotropically. The crystallographic data have been deposited in Cambridge Crystallographic Data Centre (CCDC) and assigned to CCDC codes 2123436 (Cry -1), 2123437 (Cry -2), 2123438 (Cry -3), and 2123439 (Cry -4). Calculation details Density functional theory (DFT) and time-dependent (TD) DFT calculations were performed using the Gaussian 16 program package (revision A. 03).36 All geometry optimizations of minima were calculated by DFT using B3LYP37 as a functional and 6-31G(d,p)38 as a basis set with Grimme's D3BJ dispersion correction. The optimized geometries were confirmed to be local minima by performing frequency calculations. The lowest-energy vertical transitions were calculated (singlets and triplets) by TD-DFT at the B3LYP/6-31G(d,p) level based on the optimized ground-state structures. TD-DFT geometry optimizations of the S1 states were performed at the B3LYP/6-31G(d,p) level using the optimized ground-state geometries as starting coordinates. No symmetry constraint was used in any of the calculations. The transition electric dipole moments were analyzed by Multiwfn (Version 3.8 dev).39 All visualization of geometric configuration and molecular orbitals were performed by visual molecular dynamics (VMD; version 1.9.3).40 Synthesis of compounds 1– 4 Dimethyl 2,5-bis((2-methoxyethyl)amino)terephthalate (1) To a suspension of dimethyl 2,5-dioxocyclohexane-1,4-dicarboxylate (1.14 g, 5.00 mmol) in methanol (50 mL) was added 2-methoxyethylamine (0.75 g, 10 mmol) and acetic acid (5 mL) in one portion, and the resulting mixture was refluxed for 24 h. After cooling to room temperature, the mixture was filtered and the solids were washed with methanol (20 mL) to afford 1.24 g (3.64 mmol, 73%) of product as orange powder. 1H NMR (500 MHz, CDCl3, δ): 7.33 (s, 2H), 7.00 (s, 2H), 3.88 (s, 6H), 3.66 (t, J = 5.5 Hz, 4H), 3.43 (s, 6H), 3.36 (t, J = 5.4 Hz, 4H). 13C NMR (126 MHz, CDCl3, δ): 168.23, 141.22, 117.02, 114.28, 71.12, 58.95, 51.89, 43.46. MS m/z: [M]+ calcd for 340.16; found, 360.28. Anal. calcd (%) for C16H24N2O6: C, 56.46; H, 7.11; N, 8.23. Found: C, 56.59; H, 7.09; N, 8.11. Dimethyl 2,5-bis((2-ethoxyethyl)amino)terephthalate (2) To a suspension of dimethyl 2,5-dioxocyclohexane-1,4-dicarboxylate (1.14 g, 5.00 mmol) in methanol (50 mL) was added 2-ethoxyethylamine (0.89 g, 10 mmol) and acetic acid (5 mL) in one portion, and the resulting mixture was refluxed for 24 h. After cooling to room temperature, the mixture was filtered and the solids were washed with methanol (20 mL) to afford 1.43 g (3.89 mmol, 78%) of product as orange powder. 1H NMR (400 MHz, CDCl3, δ): 7.34 (s, 2H), 7.00 (s, 2H), 3.88 (s, 6H), 3.70 (s, 4H), 3.57 (q, J = 7.0 Hz, 4H), 3.37 (s, 4H), 1.25 (t, J = 7.0 Hz, 6H). 13C NMR (126 MHz, CDCl3, δ): 168.21, 141.23, 117.06, 114.38, 69.02, 66.53, 51.84, 43.66, 15.19. MS m/z: [M]+ calcd for 368.19; found, 368.25. Anal. calcd (%) for C18H28N2O6: C, 58.68; H, 7.66; N, 7.60. Found: C, 58.61; H, 7.62; N, 7.55. Dimethyl 2,5-bis((3-hydroxypropyl)amino)terephthalate (3) To a suspension of dimethyl 2,5-dioxocyclohexane-1,4-dicarboxylate (1.14 g, 5.00 mmol) in methanol (50 mL) was added 3-amino-1-propanol (0.75 g, 10 mmol) and acetic acid (5 mL) in one portion, and the resulting mixture was refluxed for 24 h. After cooling to room temperature, the mixture was filtered and the solids were washed with methanol (20 mL) to afford 1.11 g (3.26 mmol, 65%) of product as orange powder. 1H NMR (500 MHz, DMSO, δ): 7.21 (s, 2H), 6.72 (s, 2H), 4.58 (t, J = 5.0 Hz, 2H), 3.83 (s, 6H), 3.53 (dd, J = 11.2, 5.9 Hz, 4H), 3.15 (d, J = 3.9 Hz, 4H), 1.74 (p, J = 6.5 Hz, 4H). 13C NMR (126 MHz, DMSO, δ): 168.01, 140.92, 116.56, 113.79, 59.04, 52.45, 40.63, 32.37. MS m/z: [M]+ calcd for 340.38; found, 340.45. Anal. calcd (%) for C16H24N2O6: C, 56.46; H, 7.11; N, 8.23. Found: C, 56.53; H, 7.01; N, 8.21. Dimethyl 2,5-bis((3-methoxypropyl)amino)terephthalate (4) To a suspension of dimethyl 2,5-dioxocyclohexane-1,4-dicarboxylate (1.14 g, 5.00 mmol) in methanol (50 mL) was added 3-methoxypropylamine (0.89 g, 10 mmol) and acetic acid (5 mL) in one portion, and the resulting mixture was refluxed for 24 h. After cooling to room temperature, the mixture was filtered and the solids were washed with methanol (20 mL) to afford 1.31 g (3.56 mmol, 71%) of product as orange powder. 1H NMR (400 MHz, CDCl3, δ): 7.33 (s, 2H), 3.88 (s, 6H), 3.53 (t, J = 6.0 Hz, 4H), 3.37 (s, 6H), 3.27 (t, J = 6.7 Hz, 4H), 1.98–1.92 (m, 4H). 13C NMR (126 MHz, CDCl3, δ): 168.39, 141.21, 116.66, 114.06, 70.57, 58.66, 51.80, 40.82, 29.47. MS m/z: [M]+ calcd for 368.43; found, 368.52. Anal. calcd (%) for C18H28N2O6: C, 58.68; H, 7.66; N, 7.60. Found: C, 58.63; H, 7.58; N, 7.65. Results and Discussion Photophysical properties Compounds 1− 4 (Figure 1a) were synthesized simply through a one-step reaction according to the reported method.34 The reaction mixtures were purified by recrystallization, which produced the targets with good isolated yields (65%−78%). The molecular structures of 1− 4 were fully characterized by 1H and 13C NMR, mass spectrometry, elemental analysis, and finally by SCXRD. All the compounds display intense orange fluorescence (Figure 1b) in organic solvents with good quantum yields (Φ) of 0.34−0.47 ( Supporting Information Table S1). As outlined in Figure 1c, the absorption and emission spectra of 1− 4 in dichloromethane (DCM, 1 × 10−4 M) are quite similar (λem = 571, 572, 575, and 577 nm for 1, 2, 3, and 4, respectively), indicating that the photophysical properties of individual molecules 1− 4 are determined by the single-benzene framework. Compounds 1− 4 in different solvents with varied polarity show no solvent polarity effects ( Supporting Information Figure S1). In addition, the optical profiles of 1 in aprotic solvent (DCM) and protic solvent (methanol) are comparable, as shown in Supporting Information Figure S2. This observation indicates that the emissions of 1− 4 do not originate from the excited-state intramolecular proton-transfer emission, similar to another single-benzene emitter.41 By solution diffusion of biphasic DCM-methanol (good/poor solvents) solutions, compounds 1− 3 produced needle-like red crystals (Cry- 1, Cry- 2, and Cry- 3), whereas 4 generated flake-shaped red crystals (Cry- 4). Compared to the solution samples, the crystals show distinct redshifted absorption and emission spectra with moderate Φ in the range of 0.21−0.32, demonstrating that the general aggregation effect of redshift on luminescence works well for the present system. In addition, the emission peaks of crystals 1− 4 (λem = 598, 609, 617, and 605 nm for Cry- 1, Cry- 2, Cry- 3, and Cry- 4, respectively) are different (Figure 1d), reflecting the difference in molecular arrangements of the packing structures. Under 365 nm UV light irradiation, the crystalline samples 1– 4 exhibited bright red emissions, as shown in Figure 1e,f. Moreover, the thermal properties of 1− 4 were characterized by differential scanning calorimetry (DSC) under a nitrogen atmosphere at a heating rate of 10 °C/min ( Supporting Information Figure S3). No obvious phase transitions occur for crystals 1− 4 according to the DSC curves. The melting points of 1− 4 by DSC measurements are 162, 126, 188, and 100 °C, respectively. Figure 1 | (a) Chemical structures of compounds 1−4; (b) photographic image of compound 1 in DCM solution under UV irradiation (365 nm); (c) absorption and emission spectra of 1−4 (black color, 1; red color, 2; green color, 3; blue color, 4) in DCM solutions; (d) absorption and emission spectra of 1−4 (black color, 1; red color, 2; green color, 3; blue color, 4) in crystalline states; and photographic images of crystals 1−4 in daylight (e) and under 365 nm UV irradiation (f). Download figure Download PowerPoint Mechanical properties Introducing different side chains to the amine groups did not change the electronic structure of this tiny π-system as reflected by the identical absorption and emission properties of 1− 4 in various solutions. In sharp contrast, the mechanical properties of crystals 1− 4 are completely different and very sensitive to the side-chain substituents. The needle-shaped Cry- 1, possessing a rectangular cross section, underwent permanent deformations when external stress was applied on either the (001) or (010) plane. As shown in Figure 2a, when a straight crystal was pressed by a needle against a pair of forceps, bending deformations took place at points 1 and 2. The crystal kept this deformed shape when the force was released, implying a plastic-deformation nature. Another curve formed at 3 further bending the straight of the deformed After a bending, the straight crystal Although several sharp formed bending, the crystal a as by images (Figure To the plastic another Cry- 1 was bent along the (001) to form a crystal (Figure In addition, Cry- 1 could also be bent along the of that is, exhibited As shown in Figure a straight crystal was bent along the (001) and the (010) and as a and was that some elastic bendable crystals with of the bending direction undergo plastic deformation upon we thus the bending of Cry- crystals with of and along the direction were and forces were applied on the (001) and (010) at different As shown in Supporting Information Figure these crystals were of bending plastically along different the plastic bending of Cry- to the plastic deformation Cry- 1 could be twisted into either or (Figure The plastic mechanical properties were further confirmed by bending straight crystal of Cry- 1 with a of about was in and the two crystal in were to loading forces on the (001) and (010) to test the mechanical properties. As the two pieces exhibited very similar when loading forces along different the was with pressure along the of underwent elastic deformation ( by a into plastic deformation ( took place (Figure The of the identical crystal quite similar mechanical when external stress was on the (010) (Figure All these that Cry- 1 a plastic bending Figure 2 | (a) The plastically bending of crystal 1 by a pair of and a (b) images of the bent crystal 1; (c) the plastically bent crystal of 1 along the (001) (d) plastically bent crystal of 1 along (001) and (010) (e) plastically twisted crystal of 1; of different and curve of the bending test of crystal 1 along crystallographic (001) and (010) Download figure Download PowerPoint Cry- 2 and Cry- 3 have a similar needle shape with Cry- 1; however, they to external force to Cry- As in Figure the straight crystals of 2 and 3 bent upon by a pair of and finally formed a without or They the straight shape the pressure was released, indicating that the deformations were The bending and could be without any obvious of the crystal as reflected by the images (Figure Cry- 2 and Cry- 3 are intrinsically To this bending were carried out for Cry- 2 and Cry- As shown in Figure the using different samples for Cry- 2 and Cry- 3) of crystals are the elastic deformation of Cry- 2 and Cry- The elastic of Cry- 2 and Cry- 3 are calculated to be and the range of the reported elastic organic crystals, indicating that Cry- 3 elastic bending deformation Cry- 2. However, flake-shaped Cry- 4 a when external stress was applied to this crystal ( Supporting Information Figure In this the mechanical properties of crystals 1− 4 be related to the intermolecular as well as the molecular packing Hence, side-chain engineering of a single-benzene framework mechanical behaviors of the produced organic crystals. Figure 3 | (a) of the elastic crystal 2; (b) images of a bent crystal of 2; (c) the of the elastic crystal 3; (d) images of a bent crystal of 3; and by bending of crystals 2 (e) and 3 (f). elastic Download figure Download PowerPoint structures do the side chains the mechanical properties of the present To elucidate this crystal structures of 1– 4 were solved by and were carefully in of molecular and intermolecular interactions. Cry- 1 and Cry- 4 to a system and and there is one individual in the crystal The tetra-substituted benzene in Cry- 1 and Cry- 4 a completely due to the intramolecular and with of for Cry- 1 and for Cry- 4 ( Supporting Information Figure The and chains from the in a with the vertical of and ( Supporting Information Figure respectively. Cry- 2 and Cry- 3 to a system and and one the in crystals. Cry- 1 and Cry- 4, the tetra-substituted benzene of individual molecules have a and structure to the intramolecular of for Cry- 2 and for Cry- The of and in Cry- 2 and Cry- 3 are on of the benzene with a vertical of and respectively. to these side-chain in this work have little effect on molecular atoms and and two (Figure and intermolecular are formed in crystals that the molecular packing structures and thus the mechanical properties of the crystals. Cry- 1, one 4 through intermolecular Supporting Information Figure the crystallographic (001) (Figure which has been confirmed by to be the of Cry- 1 ( Supporting Information Figure The (001) along the direction to form the (010) by forces carbon chains (Figure the crystal growth direction, the molecules into a structure through intermolecular and interactions. The molecular packing shown in Figure indicates that there are two along the crystal growth The molecular are in a and there is no or of molecules in Cry- upon loading force on either the (010) or (001) long-range and permanent molecular due to the of the intermolecular by the of novel which in the plastic deformation of Cry- intermolecular with in the range of are also in Cry- 2, which the crystallographic (Figure and to the of molecular that are further through forces to the (001) plane. (001) is the bendable and is the growth direction, as determined by ( Supporting Information Figure the molecular packing structure in the is quite important for the mechanical of Cry- 2. As shown in Figure molecular are connected by intermolecular to form a which is an packing structure for elastic crystals. a transverse force was applied on the (001) the and in the are expanded and by molecular Supramolecular chemistry ; Flexibility (engineering) ; Chemistry ; Crystallography ; Crystal structure ; Management Synthesis and properties of polymers ; Nonlinear Optical Materials Research ; Silicone and Siloxane Chemistry