Positive/Negative Phototropism: Controllable Molecular Actuators with Different Bending Behavior

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2021Positive/Negative Phototropism: Controllable Molecular Actuators with Different Bending Behavior Haoran Wang, Jiapeng Liu, Kaiqi Ye, Qiyao Li, Jianyu Zhang, Hao Xing, Peifa Wei, Jingbo Sun, Francesco Ciucci, Jacky W. Y. Lam, Ran Lu and Ben Zhong Tang Haoran Wang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study, The Hong Kong University of Science and Technology, Hong Kong Google Scholar More articles by this author , Jiapeng Liu Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Hong Kong Google Scholar More articles by this author , Kaiqi Ye State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun Google Scholar More articles by this author , Qiyao Li Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study, The Hong Kong University of Science and Technology, Hong Kong Google Scholar More articles by this author , Jianyu Zhang Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study, The Hong Kong University of Science and Technology, Hong Kong Google Scholar More articles by this author , Hao Xing Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study, The Hong Kong University of Science and Technology, Hong Kong Google Scholar More articles by this author , Peifa Wei Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study, The Hong Kong University of Science and Technology, Hong Kong Google Scholar More articles by this author , Jingbo Sun State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun Google Scholar More articles by this author , Francesco Ciucci Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Hong Kong Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Hong Kong Google Scholar More articles by this author , Jacky W. Y. Lam Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study, The Hong Kong University of Science and Technology, Hong Kong Google Scholar More articles by this author , Ran Lu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun Google Scholar More articles by this author and Ben Zhong Tang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study, The Hong Kong University of Science and Technology, Hong Kong Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000350 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Herein, a series of molecular actuators based on the crystals of (E)-2-(4-fluorostyryl)benzo[d]oxazole ( BOAF4), (E)-2-(2,4-difluorostyryl)benzo[d]oxazole ( BOAF24), (E)-2-(4-fluorostyryl)benzo[d]thiazole ( BTAF4), and (E)-2-(2,4-difluorostyryl)benzo[d]thiazole ( BTAF24) showed unique bending behavior under UV irradiation. The one-dimensional (1D) crystals of BOAF4 and BTAF4 bent toward light, whereas those of BOAF24 and BTAF24 bent away from light. Although the chemical structures of these compounds are similar, the authors found that F···H–C interaction played a key role in the different molecular packing in structures crystals, which led to the positive/negative phototropism of the actuators. Moreover, theoretical calculations were carried out to reveal the mechanical properties of the crystals. Taking advantage of these photomechanical properties, the authors achieved the potential application in pushing objects, as well as enriching and removing pollutants. Hence, the molecular actuators with different bending behavior could be fabricated by introducing different number of F atom, which may open a novel gate for crystal engineering. Download figure Download PowerPoint Introduction Nature endows organisms with tropisms for moving toward or away from a stimulus. For most plants, their stem exhibits positive phototropism to maximize photosynthetic energy for promoting growth.1,2 However, most roots and some vine shoot tips exhibit negative phototropism, allowing them to grow toward darkness, drill into the soil, and climb objects.3 The combination of positive and negative phototropism allows plants to grow in the correct direction, to make better use of light energy. Inspired by the response of natural organisms to external stimuli, various artificially intelligent materials, such as artificial muscles and flexible electronics, have been developed.4–10 Among them, photomechanical responsive materials have emerged as a research hot spot with unique advantages, including easy-to-achieve remote control and the miniaturization of devices.11,12 Especially, photomechanical molecular crystals exhibit some conspicuous merits over polymer-based materials, such as faster response time,13 higher Young’s modulus,14 and an ordered crystal structure that can be easily characterized by X-ray diffraction (XRD) techniques.6 In addition, an ordered crystal structure offers opportunities for rapid energy transfer between tightly packed molecules, and this could be used as a physical platform for actuation from the nanoscale to the macroscale.15,16 It has been found that the crystals based on photochemical active organic compounds, including azobenzenes, anthracenes, diarylethenes, and furylfulgides, are capable of bending,17–30 curling,31 twisting,31,32 crawling,33–37 and leaping5,38 in response to light. Recently, Naumov et al.39 reported that a smart cocrystal of probenecid and 4,4′-azopyridine, which could reversibly respond to multiple external stimuli (heat, UV light, and mechanical pressure) by twisting, bending, and elastic deformation without fracture. Bardeen et al.40 reported that the controllable movement of molecular crystals based on 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene by varying the angle of incident light. As far as we know, previous efforts to regulate the motion behavior of crystals mainly focused on changing external conditions such as controlling the crystal habits and the irradiation direction. Till now, controlling the motion modes such as positive and negative phototropism through tuning crystal structure has posed a significant challenge. In our previous work, we found that the introduction of chlorine at different position of benzene in styrylbenzoxazoles could affect the molecular packing structures in the crystals, which would further affect the topophotochemical reaction.41 Compared with chlorine, fluorine possesses a smaller atomic radius and a stronger electronegativity, so introducing fluorine into conjugated organic molecules might lead to a unique arrangement in crystals and interesting photomechanical behavior. With these considerations in mind, we intended to study the photoinduced mechanical motions of the molecular crystals based on fluorine-containing styrylbenzoxazoles [(E)-2-(4-fluorostyryl)benzo[d]oxazole ( BOAF4) and (E)-2-(2,4-difluorostyryl)benzo[d]oxazole ( BOAF24)] and styrylbenzothiazoles [(E)-2-(4-fluorostyryl)benzo[d]thiazole ( BTAF4) and (E)-2-(2,4-difluorostyryl)benzo[d]thiazole ( BTAF24); Scheme 1] so as to reveal the effect of the molecular structures and crystal habits on photomechanical behavior. It was found that the crystals of BOAF4, BOAF24, BTAF4, and BTAF24 with different habits showed photoinduced bending, curling, fragmentation, swelling, and photosalient behavior. Interestingly, the crystals of BOAF4 and BTAF4 showed photoinduced bending toward light, but the crystals of BTAF4 and BTAF24 showed oppositely photoinduced bending away from light. Scheme 1 | Molecular structures of fluorine-containing styrylbenzoxazoles and styrylebenthiazoles. Download figure Download PowerPoint Experimental Methods General information 1H NMR and 13C NMR spectra were recorded with a Mercury plus instrument (Varian. Palo Alto, California, USA) at 400 and 101 MHz using deuterated chloroform (CDCl3) and deuterium dimethyl sulfoxide (DMSO-d6) as the solvents. Fourier transform infrared (FT-IR) spectra were obtained with a Nicolet-360 FT-IR (Thermo Scientific, Middlesex County, Massachusetts, USA) spectrometer by the incorporation of samples into potassium bromide (KBr) disks. Mass spectra were measured with an AXIMA CFR matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer (compact, Shimadzu, Kyoto, Japan). The UV–Vis absorption spectra were obtained using a Mapada UV-1800pc spectrophotometer (Shimadzu, Kyoto, Japan). Fluorescence emission spectra were taken on a Cary Eclipse fluorescence spectrophotometer (Agilent, Santa Clara, California, USA). Fluorescence microscopy images were taken on a Fluorescence Microscope (Olympus Reflected Fluorescence System BX51, Olympus, Japan). XRD patterns were obtained on an Empyrean XRD (Malvern Instruments, London, UK), equipped with graphite monochromatized Cu-Kα radiation (λ = 1.5418 Å), employing a scanning rate of 0.00267°s−1 in the 2θ range of 2°–40°. The single crystals of BTAF4 and BTAF24 were obtained by slow evaporation from the solutions in dichloromethane (CH2Cl2)/petroleum ether (v/v = 1/4). The photodimerization products of D-BTAF4 and D-BTAF24 were gained via irradiating the microcrystals of BTAF4 and BTAF24 under 365 nm for 30 min, followed by recrystallization from petroleum ether. The single crystals of D-BTAF4 and D-BTAF24 were obtained in CH2Cl2 under petroleum ether steam by the vapor diffusion method. The above single crystals were selected for XRD analysis on an APEX II diffractometer (Bruker, Middlesex County, Massachusetts, USA), using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structures were solved by the direct methods and refined on F2 by full-matrix least-squares using the SHELXTL-97 program (George M. Sheldrick, Thomas R. Schneider, Lower Saxony, Germany). The nonhydrogen atoms (S, C, N, and F) were easily placed from the subsequent Fourier difference maps and refined anisotropically. The H atoms were introduced in the calculated positions and refined with fixed geometry with respect to their carrier atoms. Cambridge Crystallographic Data Centre (CCDC) 1814131, 1814889, 1850307, and 1850304 contain the supplementary crystallographic data for BTAF4, BTAF24, D-BTAF4, and D-BTAF24, respectively. The single-crystal data for BOAF4, BOAF24, D-BOAF4, and D-BOAF24 can be found in our previous work.42 Photomechanical behaviors were observed under a microscope, and the crystals were put on the glass substrate and irradiated by pocket lamp (365 nm, 3 W) for different times at 298 K. BOAF24 fibers were obtained by dropping the solution in tetrahydrofuran (THF; 1.0 × 10−2 mol/L) into sodium dodecyl sulfate aqueous solution (2 mg/mL) under stirring. THF was dried over sodium and benzophenone. CH2Cl2 was dried over calcium hydride. The other reagents were used without further purification. Computational methods We performed all the spin-unpolarized first-principle calculations using the Vienna ab initio simulation package (VASP, Vienna, Austria)43,44 with plane-wave basis set and a projector-augmented wave (PAW) approach.45 The exchange correlation is described using Perdew–Burke–Ernzerhof (PBE) functional46 under the generalized gradient approximation (GGA) scheme. To obtain an accurate elastic constant, we set the kinetic energy cutoff as large as 700 eV and sampled the Brillouin zone using Gamma-centered k-meshes with at least 5000 k points per reciprocal atom (pra). The structures were first fully relaxed to ensure that the energy difference in each self-consistent calculation (SCF) was smaller than 10−6 eV, and the maximum force was converged below 0.005 eV/Å. The elastic tensor was then calculated based on the fully relaxed structure by performing two magnitudes of displacement (0.01 and −0.01 Å) for each atom along each of the Cartesian directions. The bulk modulus and shear modulus were finally calculated based on the converged elastic stiffness tensor.47 Synthesis The syntheses of BOAF4, BOAF24, and BTAF24 were reported previously.41,48 BTAF4 was synthesized according to the reported procedure.41 The synthetic route and characterization data were shown in the Supporting Information. Results and Discussion Photomechanical behavior First, the slice-like crystals and rod-like crystals of BOAF4 were obtained from petroleum ether/CH2Cl2 (v/v = 3/1) in one test tube. In the case of the slice-like crystal with a low aspect ratio, the photoinduced swinging and swelling were observed ( Supporting Information Figure S1 and Video S1). Accompanied by the photomechanical effect, the emission of BOAF4 in crystal became stronger and blue-shifted gradually.42 The thinner slice-like crystal displayed a different behavior, that it could bend toward UV light ( Supporting Information Video S2 and Figure S2). Interestingly, for the irregular rod-like crystal of BOAF4 with high aspect ratio, comparable bending behavior was observed under UV irradiation. Besides exhibiting photoinduced swelling, it could roll up its tail and bend toward the UV light (Figure 1 and Supporting Information Video S3). In detail, the UV irradiation induced the emergence of cracks in the backlight side of the crystal, and the tail bent toward light. When the irradiation time was prolonged to 90 s, the tail was raised out of the plane, and the bending angle was ca. 135° without break. Figure 1. | Optical photographs of the irregular rod-like crystal of BOAF4 before and after irradiation vertically with 365-nm light for different time (the arrow indicates the irradiation direction). Download figure Download PowerPoint Therefore, BOAF4 crystals with smaller thicknesses or high aspect ratios tend to bend toward a light source. Notably, in our previous work, the needle-like crystals of (E)-2-(2,4-dichlorostyryl)benzo[d]oxazole ( BOACl24) bent away from the UV light,41 opposite to the rod-like crystals of BOAF4. To further research the positive and negative phototropism behavior of crystals under UV irradiation, BOAF24 was designed and synthesized for comparison with BOAF4.48 BOAF24 formed block, ribbon-like, and rod-like crystals in petroleum ether/CH2Cl2 (v/v = 3/1). The long block irregular crystal of BOAF24 could swing upon UV irradiation, and small pieces jumped away from the body. When prolonging the UV irradiation, more cracks were observed in the cross sections of the crystal ( Supporting Information Video S4). We also observed the energy accumulation state in the photomechanical process that Naumov et al.6 proposed. As shown in Supporting Information Video S5, after turning off the UV lamp at 23 s, the crystal of BOAF24 lay still at that time but began to break after 1 s. This proves that the photosalient behavior occurs after an induction period during which energy accumulates. Furthermore, the 1D crystal bent away from irradiation and the thinner crystals markedly exhibited more significant bending compared with the thicker ones ( Supporting Information Video S6 and Figure S3a). When the crystals were vertically irradiated by UV light from the downside, the right side of the elongated crystal marked by the red circle bent up and could also bend down via turning around the irradiation direction. Meanwhile, the crystal part marked with the yellow circle gave a slight movement as it was thicker. Also, the bouquet-like crystals consisting of lots of ribbons displayed similar behavior ( Supporting Information Figure S3b). Except for the bending behavior, the bouquet of ribbon-like crystals of BOAF24 could even jump under UV irradiation ( Supporting Information Video S7). To clearly observe the bending direction of the crystals, one rod-like crystal of BOAF24 was selected. As shown in Supporting Information Video S8 and Figure 2, it clearly bent away from the light source upon being irradiated vertically by 365-nm light. When the irradiation direction was reversed, the bent crystal straightened. Prolonging the irradiation time, the crystal jumped at 11 s and continued to bend backward toward UV light. Such bending away from the light source was repeated several times, meaning that the photoinduced bending process is reversible. Figure 2. | Optical photographs of rod-like crystals of BOAF24 before and after irradiation with 365-nm light for different time (the arrows indicate the irradiation direction). Download figure Download PowerPoint For deep understanding of the different photomechanical effects of BOAF4 and BOAF24, the crystals of their analogs BTAF4 and BTAF24 were obtained. As anticipated, a rod-like crystal of BTAF4 bent toward UV light, and the bent crystal could be straightened when the irradiation direction was changed to the right side (Figure 3). Meanwhile, the enhanced fluorescence intensity was also observed ( Supporting Information Figure S4).49–51 We also obtained the rod-like, block, and slice-like crystals of BTAF24 from cyclohexane/CH2Cl2 (v/v = 4/1). Under UV irradiation, the rod-like and slice-like crystals bent away from the light source, whereas the block crystals just jumped without any bending. As shown in Supporting Information Figure S5, after irradiation with 365-nm light from upside vertically, the straight rod-like crystal bent backward toward the light source. When the irradiation direction changed to the downside, the bent crystal could be straightened and further bent upward by prolonging the irradiation time. The backward bending of UV light was repeated several times ( Supporting Information Video S9). The microcrystals of BTAF24 also showed “turn on” emission under UV irradiation ( Supporting Information Figure S6). Furthermore, the block crystals of BTAF24 could jump away from their original positions when the UV irradiation time was 3 s, and small pieces exploded out of the body ( Supporting Information Video S10). The slice-like crystal of BTAF24 bent backward toward the light source in the beginning of the irradiation, followed by splitting into several parts perpendicular to the long axis of the crystal ( Supporting Information Video S11). Figure 3. | Optical photographs of the rod-like crystals of BTAF4 before and after irradiation with 365-nm light for different time (the arrows indicate the irradiation directions). Download figure Download PowerPoint Mechanism study In our previous work, we found that a photoinduced [2 + 2] cycloaddition reaction took place in the molecular crystals of BOAF4 and BOAF24, which was the driving force for the photomechanical effects. Particularly, in the process of photoinduced movement, the crystals of BOAF4 and BOAF24 were “turned on” from nonemissive to emissive. BOAF4 and BOAF24 were typical aggregation-caused quenching (ACQ) molecules,42 so the crystals were nonemissive before UV irradiation. Although the photodimerization broke the molecular conjugation to some extent, the intensified emission of the crystals of BOAF4 and BOAF24 was observed upon UV irradiation due to the intramolecular and intermolecular through-space conjugation.42 Moreover, 1H NMR spectral changes of BTAF24 before and after UV irradiation for different times suggested that photodimerization occurred in the microcrystals. As depicted in Supporting Information Figure S7, after irradiating the microcrystals of BTAF24 with UV light for 2 min, a double emerged at 5.20 ppm, which could be ascribed to the protons in newly formed cyclobutane. Meanwhile, the new peaks at 6.93, 7.11, and 7.60 ppm ascribed to the protons in the photodimerization product (named as D-BTAF24) were also detected. The similar 1H NMR spectral changes of BTAF4 were observed during the UV irradiation of the microcrystals ( Supporting Information Figure S8). This suggested that photodimerization was also the driving force for the transformation from light into mechanical energy in crystals of BTAF4 and BTAF24. To further demonstrate the driving force of the photomechanical behavior of the crystals, we collected the single-crystal data of BOAF4, BOAF24, BTAF4, and BTAF24 as well as their dimers of D-BOAF4, D-BOAF24 (α- and β-types), D-BTAF4, and D-BTAF24. Among them, the single-crystal structures of BOAF4, BOAF24, D-BOAF4, and D-BOAF24 (α- and β-types) had been reported in our previous work.42 Notably, Schmidt52 found that the distance of “olefin pair” (<4.2 Å) as well as the angles of θ2 and θ3 (close to 90°) depicted in Supporting Information Chart S1 were geometric criteria for photodimerization. In the single crystal of BOAF4, the distance in “olefin pair” was 3.896 and 3.681 Å ( Supporting Information Figure S9a), and θ2 and θ3 were ca. 77.00°/69.99° and 87.97°/83.04°, respectively ( Supporting Information Table S3). Therefore, such molecular packing made the photodimerization of BOAF4 accessible. Subsequently, the single-crystal structure of D-BOAF4 was analyzed. For D-BOAF4, isomers I and II were achieved via photocycloaddition ( Supporting Information Figure S9b). Taking isomer I as an example, the bond lengths of the newly formed C–C bond in the four-membered ring were 1.578 and 1.579 Å. Therefore, each carbon atom in the original C=C had to move ca. 1.1–1.3 Å during photodimerization. Meanwhile, the newly formed cyclobutane impelled the benzoxazole and benzene to move away from the four-membered ring, like a bird spreads its wings. As a result, the distances between the outermost F and C atoms reached 7.132 and 7.811 Å. Similar results were found from the single-crystal structures of BOAF24, BTAF4, and BTAF24 ( Supporting Information Figures S9b, S10, and S11). Since the molecular volume was amplified upon dimerization, the strain was yielded, leading to the mechanical movements of crystals. More interestingly, based on the single crystal of BOAF24, it seems that [2 + 2] cycloaddition will yield only β-type dimer, but α-type D-BOAF24 was also obtained ( Supporting Information Figure S9b). The ratio of the amount of β- and α-type D-BOAF24 was about 20:1 via column chromatography. Since BOAF24 molecules were arranged in a head-to-head packing in the crystal, the β-type D-BOAF24 was the main product as a dynamic product.53 The possible reason for the formation of α-type D-BOAF24 was shown in Supporting Information Figure S12. Under UV irradiation, molecules 1 and 2 are unparalleled and far from each other at the distance of 8.513 Å. However, the strains generated by [2 + 2] cycloaddition of other molecules satisfied with Schmidt’s criteria squeezed the distance between molecules 1 and 2, thus affording α-type D-BOAF24. On the contrary, the dimerization of BOAF4, BTAF4, and BTAF24 was only concerned with the prearrangement of the molecules with the distance <4.2 Å for “olefin pairs” in crystals. It is time to discuss why BOAF4 and BTAF4 bent toward UV light, while BOAF24 and BTAF24 exhibited backlight bending performance. In 2003, Ikeda et al.54 and Yu et al.55 found that monodomain and polydomain liquid crystal elastomer (LCE) films showed different light-induced bending behavior, and the monodomain LCE bent along the alignment direction of the azobenzene mesogens., In 2006, they found that the initial alignment of photoactive mesogens significantly affected the bending behavior of the LCE films: the homogeneous films bent toward the irradiation direction of the actinic UV light, while the homotropic films bent away from the light source.56 Thus, we proposed that the arrangement of molecules in the crystal has a crucial impact on the bending behavior. First, it was proven that the strain would be generated from the phototropic surface of the crystal under the UV irradiation.57 As shown in Figure 4a, the [2 + 2] cycloaddition mainly occurred on the (010) face of the crystal of BOAF4, which was the largest superficial area. Before UV irradiation, the molecules were longitudinally aligned along the long axis of the crystal. The molecular length decreased from 12.268 ( BOAF4) to ca. 11.091 Å (the average value of the two isomers of D-BOAF4 upon photodimerization). However, the molecular width of D-BOAF4 was increased from 3.56 (the average value of the distance of two BOAF4 molecules) to ca. 7.476 Å. Hence, on the (010) face of the BOAF4 crystal, the force along the longitudinal direction (FL) made the crystal shrink, while the force along the transverse direction (FT) lead to the crystal’s expansion. Therefore, on the (010) face of the crystal was extended along the transverse direction, while FL compressed it in the longitudinal direction. As a result, the crystals of BOAF4 exhibited phototropic bending behavior. Similarly, the [2 + 2] cycloaddition reaction of BOAF24 took place on the (011) face of the crystal, leading to the decrease of the molecular length from 12.286 to 11.046 Å (the average value of β-type D-BOAF24) and the increase of the molecular width from an average of 3.863 to 6.822 Å (the average width of β-type D-BOAF24). However, the molecular alignment of BOAF24 was different from that of BOAF4 in crystal. Molecules of BOAF24 aligned perpendicularly to the long axis of the crystal (Figure 4b). On the (011) face, the crystal was compressed along the transverse direction and extended along the longitudinal direction, exhibiting backlight bending behavior. Similar observations were also found for BTAF4 and BTAF24. Figure 4. | Schematic illustration of the bending of (a) BOAF4 and (b) BOAF24 under UV irradiation. Download figure Download PowerPoint Theoretical calculations To investigate the different molecular packing within crystals, Hirshfeld surface and two-dimensional (2D) fingerprint plots of crystal stacking based on F···H–C interaction in crystals of BOAF4, BOAF24, BTAF4, and BOAF24 were initially calculated ( Supporting Information Figure S13). For BOAF4, there was only one kind of H-bond of C(3)–H(3)···F(1) with a distance of 2.74 Å, and the F···H–C interaction (the colored part in Figure 5a) makes the molecule align along the long axis of the crystal. However, except for the similar H-bond of C(4)–H(4)···F(1) (2.69 Å), four additional H-bonds of C(14)–H(14)···F(1) (2.69 Å), C(3)–H(3)···F(2) (2.88 Å), C(3)–H(3)···F(2) (2.87 Å), and C(12)–H(12)···F(2) (2.97 Å) were formed in the crystal of BOAF24. Besides, the Hirshfeld surface map based on BOAF24 shows that the F···H–C interaction tends to form a plane (Figure 5c). Thus, the multiple H-bonds induced the molecules to align perpendicularly to the long axis of the crystal. Furthermore, F···H–C interactions in BOAF24 account for 23.0% of the total intermolecular interactions, which is twice as much as that of BOAF4 (12.2%) ( Supporting Information Table S8). Therefore, we believe that the F···H–C interaction plays a key role in the different molecular packing in the crystals. The analysis of F···H–C interactions in the single crystals of BTAF4 and BTAF24 suggested similar results to those of BOAF4 and BOAF24 (Figures 5b and 5d). Figure 5. | The Hirshfeld surfaces (mapped over di) and the distances of F···H–C interaction of (a) BOAF4, (b) BTAF4, (c) BOAF24, and (d) BOAF24. Download figure Download PowerPoint To explore the mechanical properties of crystals, theoretical calculations were carried out with the density functional theory using the VASP software. Calculations of the elastic properties of crystals were conducted by imposing small strains along a specific direction to obtain the stiffness tensor. As listed in Supporting Information Table S9, the Pugh ratios obtained for the crystals of BOAF4, BOAF24, BTAF4, and BTAF24 were 2.7280, 2.1115, 2.2372, and 1.9556, respectively. All of the values were larger than 1.75, indi