Achieving Bright Mechanoluminescence in a Hydrogen-Bonded Organic Framework by Polar Molecular Rotor Incorporation

Open AccessCCS ChemistryRESEARCH ARTICLE1 May 2022Achieving Bright Mechanoluminescence in a Hydrogen-Bonded Organic Framework by Polar Molecular Rotor Incorporation Qiuyi Huang†, Wenlang Li†, Zhan Yang, Juan Zhao, Yang Li, Zhu Mao, Zhiyong Yang, Siwei Liu, Yi Zhang and Zhenguo Chi Qiuyi Huang† PCFM Lab, GDHPPC Lab, Guangdong Engineering Technology Research Center for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of OEMT, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 , Wenlang Li† PCFM Lab, GDHPPC Lab, Guangdong Engineering Technology Research Center for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of OEMT, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 , Zhan Yang PCFM Lab, GDHPPC Lab, Guangdong Engineering Technology Research Center for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of OEMT, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 , Juan Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of OEMT, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275 , Yang Li Instrumental Analysis and Research Center, Sun Yat-sen University, Guangzhou 510275 , Zhu Mao PCFM Lab, GDHPPC Lab, Guangdong Engineering Technology Research Center for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of OEMT, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 , Zhiyong Yang PCFM Lab, GDHPPC Lab, Guangdong Engineering Technology Research Center for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of OEMT, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 , Siwei Liu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] PCFM Lab, GDHPPC Lab, Guangdong Engineering Technology Research Center for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of OEMT, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 , Yi Zhang PCFM Lab, GDHPPC Lab, Guangdong Engineering Technology Research Center for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of OEMT, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 and Zhenguo Chi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] PCFM Lab, GDHPPC Lab, Guangdong Engineering Technology Research Center for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of OEMT, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 https://doi.org/10.31635/ccschem.021.202100968 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Luminescent hydrogen-bonded organic frameworks (HOFs) have attracted increasing attention due to their corresponding luminescence that enables readily visualization of structural dynamics. HOFs with the mechanoluminescence (ML) property can emit light without photon excitation and are greatly attractive for advanced applications, but research in this area has been limiting. Herein, we report the first example of an ML-active flexible HOF with permanent porosity, named 8PCOM, assembled from polar molecular rotors with an aggregation-induced emission property. When responding to different solvent vapors, reversible structural transformations between ML-active and -inactive 8PCOM frameworks occur, including a single-crystal-to-single-crystal (SCSC) transformation. Thus, guest-induced breathing behaviors are mainly attributed to phenyl rotations of polar molecular rotors induced by external stimuli. During reversible structural transformations of various 8PCOM frameworks with different pores, the significant ML property is achieved successfully through supramolecular dipole moment regulation. Upon mechanical force, bright emission of the ML-active 8PCOM framework is observed without UV irradiation, and the ML-active crystals can be easily prepared and regenerated. This work not only provides a valuable strategy for engineering future multifunctional HOFs but also enriches the types and applications of existing luminescent porous materials. Download figure Download PowerPoint Introduction Hydrogen-bonded organic frameworks (HOFs) are an emerging class of multifunctional porous materials, constructed by discrete organic units through hydrogen-bonding interactions, further stabilized by π⋯π interactions, van der Waals (vdW) interactions, and other weak intermolecular interactions.1–4 HOFs show promising applications, including gas separation, catalysis, biological, and optical materials but suffer from porous instability due to weak noncovalent interactions.5–7 To establish permanent porosity of HOFs, a common strategy is to employ π-conjugated aromatic moieties as building blocks, which are usually good organic luminogens.1 Since luminescence is a sensitive and visual signal, HOFs with distinct luminescence properties are applicable to chemical sensing, detection, molecular recognition, barcode identification, and so on.8–15 Numerous remarkable luminescence characteristics, for example, stimuli-responsive fluorescence8,10,14,15 and ultralong phosphorescence,12,13 have been observed in HOFs. To the best of our knowledge, all the luminescence emitted from previously reported HOFs is excited by photons, namely photoluminescence (PL). However, HOF materials capable of emitting light without optical excitation have not yet been reported, and remains a significant challenge regarding the development of HOFs. Mechanoluminescence (ML), which was first found by Bacon in 1605,16 is a distinctive type of luminescence simply induced by mechanical stimuli such as scratching, shaking, compressing, and rubbing to solids.17–19 Mechanical force widely exists in nature and everyday life of mankind and is very easy to be obtained as an environmentally friendly excitation mode, thus endowing ML materials with intriguing applications in advanced anticounterfeiting, intelligent sensors, information storage, health care, and so on.20–24 Generally, ML takes places at the crack surfaces of crystals due to electron bombardment19; therefore, the facile acquisition and regeneration of crystals are essential for achieving the ML property. Fortunately, HOFs are highly crystalline, even single-crystalline; thus, they can be prepared readily and regenerated by simple recrystallization.11,25–27 HOFs are generally constructed using compounds containing carboxyl groups or amine groups that can induce strong hydrogen bonds. In comparison, weak hydrogen bonds are more flexible, poorly directional, and reversible, thus allowing the fabrication of HOFs with better flexibility, as well as the tendency to produce polymorphs. Moreover, polymorphism of HOFs provides a powerful tool for revealing the relationship between the inherent mechanism of ML and detailed structures of frameworks. In this regard, HOFs exhibit a considerable advantage for developing ML-active porous materials over other types of optical fiber materials such as microstructured optical fibers (MOFs). Therefore, HOFs with interesting ML properties have the potential to extend the roles of existing porous materials and should be a significant advancement in the design and synthesis of porous luminescent systems. Herein, we report a flexible HOF (named 8PCOM) with permanent porosity exhibiting the outstanding luminescence property of ML. To the best of our knowledge, 8PCOM is the first example of a HOF with the ML property, presenting substantial progress in fabricating luminescent pure organic porous materials. By virtue of its flexible nature, derived from the polar molecular rotor, the ML property of 8PCOM frameworks can be controlled by regulating supramolecular dipole moments. Upon force excitation, bright emission from the ML-active 8PCOM framework was observed without UV irradiation, and its crystals could be prepared readily and regenerated. Experimental Methods Materials Bis(4-bromophenyl)methanone, titanium tetrachloride (TiCl4), and tetrakis(triphenylphosphine) palladium(0) (Pd(PPh3)4) were purchased from J&K Scientific. Zinc dust was purchased from Aladdin Industrial Co. (Shanghai, China). (4-Acetylphenyl)boronic acid was purchased from Sukailu Co. (Suzhou, Jiangsu, China). Potassium carbonate (K2CO3), tetrahydrofuran (THF), dichloromethane (DCM), toluene (TOL), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), acetone (ACT), and methanol (MeOH) were purchased from Guangzhou Dongzheng Co. (Guangzhou, Guangdong, China) as an analytical grade. All these materials were used as received without further purification. Synthesis of the polar molecular rotor TPE-4PCOM 1,1,2,2-tetrakis(4-bromophenyl)ethene (2.50 g, 3.86 mmol) and (4-acetylphenyl)boronic acid (3.99 g, 24.3 mmol) were dissolved in THF (80.0 mL). Then a 2 M aqueous K2CO3 solution (5.00 mL) was added. The resulting mixture was stirred for 15 min in nitrogen at room temperature. Subsequently, Pd(PPh3)4 (0.50 mg) was added, and the mixture was stirred at 80 °C for 20 h, during which the reaction was completed. The crude product was purified by silica gel column chromatography with DCM/n-hexane (v/v = 1:1) as the eluent. Compound TPE-4PCOM was obtained as a yellow-green solid (1.70 g, 60% yield). 1H NMR (500 MHz, CDCl3, δ): 8.00 (d, J = 8.4 Hz, 1H), 7.67 (d, J = 8.4 Hz, 1H), 7.46 (d, J = 8.3 Hz, 1H), 7.22 (d, J = 8.3 Hz, 1H), 2.62 (s, 1.5H). 13C NMR (126 MHz, CDCl3, δ): 197.70, 144.99, 143.48, 140.67, 138.01, 135.85, 132.06, 128.91, 126.93, 126.70, 26.65. High-resolution electrospray ionization (HRESI) (m/z): [M]+ calcd for C58H45O4, 805.33124; found, 805.33184. Preparation of single crystals The single crystals of 8PCOM-DCM, 8PCOM-DMSO, 8PCOM-DMF, and 8PCOM-ACT suitable for single-crystal X-ray diffraction (SXRD) analyses were grown by slow solvent evaporation of saturated solutions of TPE-4PCOM in DCM, DMSO, DMF, and ACT at room temperature for 10 days, respectively. The single crystals of 8PCOM-TOL and 8PCOM-MeOH suitable for SXRD analyses were grown by slow solvent evaporation of saturated solutions of TPE-4PCOM in DCM/TOL (v/v = 2∶1) and DCM/MeOH (v/v = 2∶1) at room temperature for 10 days, respectively. Further experimental details and characterizations are available in the Supporting Information. Results and Discussion Design strategy for the polar molecular rotor Generally, HOFs are assembled from symmetric building blocks with hydrogen-bonding synthons, of which geometries favor the construction of regular porous structures ( Supporting Information Figure S1). In contrast, the ML phenomenon is typically observed in polar molecules with lower symmetric structures ( Supporting Information Figure S2). From the perspective of molecular design, it seems there is a conflict by incorporating ML property into HOFs. To eliminate the conflict, four rotatable polar groups, namely, acetophenone moieties, were introduced in a tetraphenylethylene (TPE) backbone with an aggregation-induced emission (AIE) characteristic.28–31 As a result, a dynamic AIE-active polar molecular rotor, TPE-4PCOM ( Supporting Information Figures S3–S6), was achieved, used as the building block of 8PCOM (Figure 1 and Supporting Information Figure S7).32 Figure 1 | The design strategy for the ML-active flexible 8PCOM framework using a polar molecular rotor as the organic building block. Download figure Download PowerPoint The AIE property is favorable for the enhancement of ML brightness, thereby providing an effective strategy for developing high-performance ML-active materials.33–36 Notably, the acetophenone moieties did not just serve as hydrogen-bond acceptors but played a vital role in triggering the ML phenomenon of 8PCOM. Upon external stimuli, rotations between phenyl rings in TPE and phenyl rings in acetophenone moieties around the C–C single bond occurred, leading to huge differences in relative locations of the methyl groups and oxygen atoms in the acetophenone moieties. Therefore, the orientations of these oxygen atoms relative to the ethylene core plane in TPE-4PCOM could be different from each other. Consequently, asymmetry molecular conformations were achieved from an organic molecule with a symmetrical chemical structure, resulting in an asymmetry pore structure formation and a non-centrosymmetric crystalline structure, which contributed significantly to the ML activity of 8PCOM.19 Most importantly, as the only electron-withdrawing units in TPE-4PCOM, acetophenone moieties with different rotation angles could greatly affect the dipole moment of the polar molecular rotor ( Supporting Information Figure S8), which further brought about a dramatic impact on the supramolecular dipole moment of 8PCOM, when the orientations of the oxygen atoms in acetophenone moieties tended to be consistent with each other. 8PCOM exhibited a high value of supramolecular dipole moment, which accounted for a much stronger electric field upon external force. Accordingly, the ML phenomenon could be observed in the HOF for the first time. Single-crystal structures of 8PCOM Through a simple solvent evaporation method, six types of single crystals of 8PCOM (with solvent inclusion), viz, 8PCOM-DCM, 8PCOM-TOL, 8PCOM-DMSO, 8PCOM-DMF, 8PCOM-ACT, and 8PCOM-MeOH, were obtained and amenable to SXRD analyses (Figure 2a and Supporting Information Table S1). The SXRD results reveal that, for each single-crystal structure of 8PCOM, each TPE-4PCOM molecular rotor interacts with four neighboring ones through numerous hydrogen-bonding interactions, thus, extending into a single porous layer. The layers were further stacked together by interlayer hydrogen bonds, building a three-dimensional (3D) porous architecture with one-dimensional (1D) channels. Figure 2 | Investigation of single-crystal structures of 8PCOM. (a) SXRD structures of the six types of 8PCOM frameworks. H atoms and solvent molecules in these voids are omitted for clarity. Color code: bluish-green, C; vermilion, O. (b) Visualization of the channel surface of 8PCOM-DCM by gray/yellow (inner/outer) curved planes produced with a probe of 1.2 Å. Download figure Download PowerPoint In particular, single crystals of 8PCOM-DCM include four crystallographically independent TPE-4PCOM molecular rotors ( Supporting Information Figure S9 and Tables S4 and S5). Due to complex molecular arrangement during the molecular assembly process, four types of quadrangular pores (a, c, b, and d pores) with various sizes were formed in a single layer of the 8PCOM-DCM framework when the shortest distances between the oxygen atom centers on the diagonals were regarded as pore sizes (Figure 3a). Each of these pores was assembled by four TPE-4PCOM molecules. In the same single layer, additional four types of quadrangular pores were observed, named b′, d′, a′, and c′ pores, which were adjacent to the a, c, b, and d pores, respectively (Figure 3b). The a and a′ pores were related by inversion symmetry, which was also true for b and b′, c and c′, as well as d and d′ pores. However, in the second adjacent single layer, the relative positions of different pores had changed, compared with that in the first layer (Figure 3c). Due to ML being a kind of solid-state luminescence, specifically, aggregation-state luminescence, it was more reasonable to study ML properties from the supramolecular aspect. Therefore, deep investigations on the single-crystal structures of different 8PCOM frameworks were carried out to reveal how many channels existed in each of these 8PCOM frameworks, meaning how many porous structures existed. The different porous structures were considered as supramolecular units to explore the ML properties for later studies ( Supporting Information Figures S10–S13). Finally, two different void spaces were determined in the four-layer packing structure, marked as abab (the first/third layers correspond to a pore and the second/fourth layers corresponded to b pore, abab presents the same size as baba, b′a′b′a′, and a′b′a′b′) and cdcd (the first/third layers corresponded to c pore and the second/fourth layers corresponded to d pore, cdcd presented the same size as dcdc, d′c′d′c′, and c′d′c′d′). Figure 3 | Investigation of the single-crystal structure of 8PCOM-DCM. (a) Schematic representation of the four-layer stacking manner of 8PCOM-DCM. The first (b) and second (c) single layers in the 8PCOM-DCM framework along the b axis of its single-crystal structure. Molecules are colored by symmetry equivalence. The shortest distances between oxygen atom centers on the diagonals, including vdW radii, are regarded as their corresponding sizes. H atoms are omitted for clarity. Download figure Download PowerPoint Henceforth, the different void spaces of abab and cdcd were denoted as α pore and β pore, respectively, which are regarded as supramolecular units of 8PCOM-DCM. Each of the two supramolecular units was constructed by 16 molecules composed of four layers, with a pore size of 11.958 × 6.394 Å2 for α pore and pore size of 10.704 × 7.448 Å2 for β pore. As a result, two types of 1D channels constructed by the α and β pores were formed in the single-crystal structure of 8PCOM-DCM (Figure 2b). The solvent-accessible void space ratio for 8PCOM-DCM was 11.8%, calculated using the Platon V-191114 ( http://www.platonsoft.nl/) in the absence of solvents.37 Moreover, the supramolecular unit composition of 16 molecules was adopted for all the pore structures in different 8PCOM frameworks. We found that 8PCOM-TOL (α′ pore and β′ pore) and 8PCOM-MeOH (α″ pore and β″ pore) also exhibited two types of pore structures with different sizes, whereas the other three frameworks presented respective regular pores ( Supporting Information Table S2). For instance, 8PCOM-DMF displayed a regular pore structure with a pore size of 11.190 × 7.264 Å2. Therefore, the 8PCOM frameworks showed remarkable changes in pores when responding to different solvent conditions, indicating that 8PCOM could adapt to different guests, thereby enabling frameworks with different pores. ML activity investigation Under UV-light illumination, all 8PCOM frameworks exhibited remarkable solid-state PL emission ( Supporting Information Figures S14–S16 and Table S3). Transparent rod-like single crystals of 8PCOM-DCM could be obtained readily. Under 365 nm irradiation, 8PCOM-DCM single crystals exhibited strong blue fluorescence with an impressive fluorescence quantum yield (ΦF) of 81%. Encouragingly, a significant ML phenomenon was found in the HOF of 8PCOM-DCM. When the single crystals of 8PCOM-DCM were scraped by a stainless-steel spoon at room temperature, bright blue emission was evident with naked eyes without UV irradiation, revealing the fascinating ML characteristic (Figures 4a and 4b and Supporting Information Video S1). Notably, after being destroyed by mechanical force, the ML-active crystals of 8PCOM-DCM could be regenerated promptly and facilely by exposure to DCM vapor or simple recrystallization. In contrast to 8PCOM-DCM, no detectable ML property was exhibited among the other 8PCOM frameworks. Figure 4 | ML activity investigation. (a) Schematic illustration of the ML phenomenon of 8PCOM-DCM upon external force and its regeneration process, along with an ML photograph of 8PCOM-DCM at room temperature. (b) ML spectra of 8PCOM frameworks at room temperature. (c) Illustration of different oxygen-atom orientations of the acetophenone moieties with respect to the ethylene core plane in the molecular rotor. (d) SXRD structures of the α pore (left) and β pore (right) of 8PCOM-DCM, where the rightward and leftward oxygen atoms of the acetophenone moieties are circled in blue and red, respectively. H atoms and DCM molecules in the pores are omitted for clarity. Color code: bluish-green, C; vermilion, O. (e) Molecular-rotor dipole moments (hollow) and supramolecular dipole moments (solid) of different 8PCOM frameworks (upper). Electrostatic potential diagrams of molecular rotors in 8PCOM-DCM and 8PCOM-TOL (lower), indicating the oxygen atoms present a higher electron density (in red) than the methyl groups (in blue), which can also be used to determine the oxygen-atom orientations of the acetophenone moieties. Download figure Download PowerPoint To gain an in-depth understanding of the relationship between the ML activity and HOF structures, single-crystal structures of 8PCOM frameworks were investigated in detail. With regard to different 8PCOM frameworks, due to phenyl ring-rotation of the polar molecular rotor in different solvent conditions, marked changes in the orientations of oxygen atoms in the acetophenone moieties were noted, thereby leading to significant differences between supramolecular dipole moments of the frameworks, which could have a crucial impact on their ML activities. For uniformity, the plane of the ethylene core in TPE-4PCOM was taken as a reference; in this case, for the whole molecular conformation, four types of orientations relative to the oxygen atoms in the acetophenone moieties existed that included up and down (perpendicular to the ethylene core plane), as well as left and right (parallel to the ethylene core plane) (Figure 4c and Supporting Information Figure S20) faces. For the single-crystal structure of 8PCOM-DCM, among the four molecular rotors with different conformations, one of them possessed four oxygen atoms presenting the same upward orientation, and one possessed three upward oxygen atoms (Figure 4e and Supporting Information Table S6 and Figures S17–S19 and S21). Such a high degree of orientation consistency enabled these two molecular rotors with large molecular-rotor dipole moments of 7.6015 and 8.3748 Debye (D), respectively ( Supporting Information Table S8). More importantly, the complex assembly manner of the molecular rotors in the HOF structure of 8PCOM-DCM, as shown in Figure 3a, was favorable in achieving large supramolecular dipole moments. For both α and β pore structures in 8PCOM-DCM, there were four more rightward oxygen atoms than leftward oxygen atoms, while the number of upward oxygen atoms was the same as that of downward oxygen atoms (Figure 4d and Supporting Information Table S7). Consequently, their supramolecular dipole moments increased dramatically up to 29.9404 and 28.9500 D, respectively (Figure 4e), which might play a dominant role in the ML property of 8PCOM-DCM. By contrast, for each crystallographically independent molecular rotor in the single-crystal structure of 8PCOM-TOL, two oxygen atoms existed in the acetophenone moieties, oriented in opposite directions; thus, their impacts on the dipole moment could cancel each other out to a certain extent. Consequently, molecular rotors in 8PCOM-TOL presented relatively small dipole moments of 2.9235, 3.5220, 3.9549, and 3.9868 D. Therefore, although the unit-cell parameters and molecular-rotor packing of 8PCOM-TOL was quite similar to that of 8PCOM-DCM, the supramolecular dipole moments of 8PCOM-TOL (13.6491 and 12.5776 D) were still relatively small, compared with those of 8PCOM-DCM. Moreover, for the pore structures of other 8PCOM frameworks, not only the number of upward oxygen atoms was the same as that of the downward oxygen atoms, but also the number of leftward oxygen atoms was identical to the number of rightward oxygen atoms. Accordingly, a remarkable decrease in their supramolecular dipole moments was noticeable. In the case of 8PCOM-DMSO, its molecular-rotor dipole moments were comparable with that of 8PCOM-DCM. However, once its pore structure was assembled, the supramolecular dipole moment of 8PCOM-DMSO decreased profoundly to only 0.0032 D, resulting in an ML-inactive framework. Hence, the general rule between the ML activity and supramolecular dipole moment was determined by the orientations of oxygen atoms in the acetophenone moieties that could be confirmed in all the 8PCOM frameworks. Correspondingly, the first ML-active framework of 8PCOM-DCM was achieved successfully through supramolecular dipole moment regulation when responding to the guest of DCM molecules. Single-crystal-to-single-crystal transformation According to thermogravimetric analysis data ( Supporting Information Figure S22), DCM molecules in the voids of the 8PCOM-DCM framework could be released by heating to 140 °C. Due to the framework flexibility caused by the polar molecular rotors, the framework of 8PCOM-DCM was transformed into a new crystal structure after losing its guest molecules ( Supporting Information Figure S23). By carefully heating 8PCOM-DCM single crystals until all DCM molecules were removed, new transparent single crystals, namely, 8PCOM-H, were obtained successfully. When cooled to room temperature, the 8PCOM-H could well retain the single crystallinity, achieved by SXRD measurement, revealing that 8PCOM-H exhibited two types of narrow pores with sizes of 9.852 × 3.692 Å2 (α‴ pore) and 9.753 × 7.003 Å2 (β‴ pore), and a small void ratio of 4.0%. Therefore, during the DCM removal process, SCSC transformation from 8PCOM-DCM with large pores to 8PCOM-H with narrow pores occurred (Figure 5a and Supporting Information Figure S24), confirming the framework flexibility of 8PCOM ( Supporting Information Figure S33). Furthermore, in consideration of the porous nature of 8PCOM-H revealed by its single-crystal structure, gas adsorption measurements were carried out. The CO2 gas adsorption/desorption isotherms of 8PCOM-H verified its permanent porosity ( Supporting Information Figure S32)38; notably, the hysteretic desorption behavior also supported its framework flexibility (Figure 5b).39 On the other hand, 8PCOM-H hardly presented N2 adsorption, likely due to the small quadrupole moment of N2 and weak interactions between N2 and the framework.40 Figure 5 | The SCSC transformation was determined by SXRD analysis. (a) Structural transformations from the large α pore of 8PCOM-DCM to the narrow α‴ pore of 8PCOM-H. Upward and downward oxygen atoms of the acetophenone moieties are circled in dark gray. Rightward oxygen atoms are circled in blue. Leftward oxygen atoms are circled in black (new ones compared with that of 8PCOM-DCM) and red. (b) CO2 and N2 adsorption/desorption isotherms for 8PCOM-H. (c) Intermolecular interactions of the areas outlined in 8PCOM-DCM (left) and 8PCOM-H (right) of (a) with gray circles. (d) Intermolecular interactions of the area outlined in 8PCOM-DCM of (a) with a blue parallelogram. Part of the molecular structures is omitted for clarity. Color code: bluish-green, C; vermilion, O; white, H; bright green, Cl. Download figure Download PowerPoint Guest-induced breathing behaviors with controlled ML activity during 8PCOM-DCM transitions During the SCSC transformation from 8PCOM-DCM to 8PCOM-H, due to the phenyl rotations of the molecular rotors in response to DCM removal, significant changes in oxygen-atom orientations of the acetophenone moieties in the pore structures were observed. As mentioned before, the four acetophenone moieties in the two pores of 8PCOM-DCM were originally perpendicular to the ethylene core plane. During the DCM removal process, the four acetophenone moieties exhibited apparent rotations to the left, making the number of leftward oxygen atoms identical to the rightward oxygen atoms in the two pores of 8PCOM-H ( Supporting Information Figures S25–S27 and Tables S7 and S9–S12). Subsequently, 8PCOM-H presented very small supramolecular dipole moments of 3.2003 D (α‴ pore) and 3.1295 D (β‴ pore), accounting for its ML inactivity. Furthermore, in the single-crystal structure of 8PCOM-H, numerous intermolecular interactions, including C=O⋯π, C−H⋯O, and π−H⋯π interactions, were observable (Figure 5c and Supporting Information Figure S24), respons