Pagoda[5]arene with Large and Rigid Cavity for the Formation of 1∶2 Host–Guest Complexes and Acid/Base-Responsive Crystalline Vapochromic Properties

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022Pagoda[5]arene with Large and Rigid Cavity for the Formation of 1∶2 Host–Guest Complexes and Acid/Base-Responsive Crystalline Vapochromic Properties Xiao-Ni Han, Qian-Shou Zong, Ying Han and Chuan-Feng Chen Xiao-Ni Han Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of the Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Qian-Shou Zong College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing 314001 Google Scholar More articles by this author , Ying Han *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author and Chuan-Feng Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of the Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100870 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Pagoda[5]arene ( P5), which is composed of five 2,6-dimethoxylanthracene (2,6-DMA) subunits bridged by methylene groups at 1,5 positions, was conveniently synthesized in 43% by trifluoroacetic acid (TFA)-catalyzed one-pot condensation of commercially available 2,6-DMA and paraformaldehyde in 1,2-dichlorobenzene (o-DCB) at room temperature. P5 showed a highly symmetrical pentagonal structure with a large and deep electron-rich cavity, which made it form stable 1∶2 host–guest complexes with various aryl-pyridinium, bipyridinium, and stilbazolium salts in solution. Interestingly, P5 could complex two or even three guests with its intrinsic cavity and the pseudocavity formed by the adjacent macrocycles in solid states. Moreover, the strong intermolecular charge-transfer (ICT) interactions between P5 and the guests were observed. Especially, the complexes formed by P5 and the protonated pyridinium or bipyridinium salts showed acid/base-responsive color changes in solution and remarkable crystalline vapochromic properties by two kinds of processes: from ICT to no ICT and from ICT to enhanced ICT interaction or vice versa. We believe that the easily available P5 with the specific structure and intriguing 1∶2 host–guest complexation can find wide application in supramolecular assemblies and functional materials. Download figure Download PowerPoint Introduction Synthetic macrocyclic hosts have played a vital role in the development of macrocyclic and supramolecular chemistry.1,2 In particular, calixarenes,3–6 resorcinarene,7 cyclotriveratrylene,8 pillararenes,9–16 and their analogues have become some of the most studied and attractive synthetic macrocyclic hosts in recent decades because of their unique structures, ease of synthesis, and wide application in various research areas. Recently, various new types of macrocyclic arenes have attracted increasing interest,17–31 and the development of novel macrocyclic arenes with specific structural features and diversified host–guest complexation properties have also become a new focus and hot topic in macrocyclic and supramolecular chemistry. The inclusion of two or more guest molecules in one host’s cavity is not only a common phenomenon in biological systems but also an important challenge in supramolecular chemistry.32,33 The 1∶2 host–guest complexation based on cucurbit[8]uril,34–36 triptycene-derived macrotricyclic hosts,37,38 and others39–42 have been widely applied in a variety of research areas such as molecular machines, polymetric materials, hybrid nanomaterials, supramolecular nanoreactors, and so on.36,43–47 The host–guest complexations based on the macrocyclic arenes have also been studied extensively; however, few macrocyclic arenes with well-defined and large enough cavities that can complex two or more guest molecules have been reported so far.48,49 Although pillar[10]arene with double pillar cavities can complex two n-octyltrimethyl ammonium or paraquat guests,50,51 no interactions between the two guests could be found because of the encapsulation of two guests in two separated pseudocavities. Design and synthesis of macrocyclic arenes with large and rigid cavities for encapsulating two or more guests simultaneously remains a considerable challenge. Previously, Ogoshi et al.9 first reported a cyclic pentamer of 1,4-dimethoxybenzene and named it pillar[5]arene (Figure 1a). Early in 2020, Gaeta et al.30 reported a new macrocyclic arene of prism[5]arene (Figure 1a), which was composed of five 2,6-dimethoxynaphthalene subunits. Both pillar[5]arene and prism[5]arene could form 1∶1 host–guest complexes with various organic guests. In this paper, we report a new family member of pagoda[n]arenes, pagoda[5]arene ( P5, (Figure 1b)), which is composed of five 2,6-dimethoxylanthracene (2,6-DMA) subunits bridged by methylene groups at the 1,5 positions. P5 containing five anthracene rings shows a regular pentagonal electron-rich cavity that is larger and deeper than those of pillar[5]arene9 and prism[5]arene30 (see Supporting Information Table S2), which can make P5 encapsulate two identical cationic guests in the cavity simultaneously to form 1∶2 host–guest complexes in solution. Interestingly, it was found that besides the inclusion of two guests in the cavity of P5, one more guest could be encapsulated in the pseudocavities formed by the adjacent macrocycles of P5 in the solid state. Moreover, the strong intermolecular charge-transfer (ICT) interactions between the electron-rich P5 and the electron-deficient guests have also been observed. Especially, the complexes formed by P5 and the protonated pyridinium or bipyridinium salts exhibited acid/base-responsive switchable behaviors with color changes in solution visible to the naked eye and showed remarkable acid/base-responsive crystalline vapochromic properties by two kinds of processes: from ICT to no ICT and from ICT to enhanced ICT interaction or vice versa, which could not only enrich the scope of vapochromic crystals but also have potential applications in environmental monitoring and chemical sensing. Figure 1 | (a) Structures of pillar[5]arene and prism[5]arene. (b) Structure and calculated potential profile of pagoda[5] arene. Download figure Download PowerPoint Experimental Methods In this study, all reagents were obtained from commercial sources unless otherwise indicated. Commercial reagents were used without further purification. Pagoda[5]arene ( P5) was designed and synthesized by the one-pot condensation reaction with a yield of 43% as shown in Scheme 1 (see the detailed procedure in Supporting Information). 1H and 13C NMR spectra as well as 1H NMR titrations were recorded on the Brucker AVANCE III 400 MHz NMR spectrometer or the Brucker AVANCE III 500 MHz NMR spectrometer (Bruker, Beijing, China). Electrospray ionization mass spectra (ESI-MS) and atmospheric-pressure chemical ionization MS (APCIMS) were recorded on the Thermo Fisher Exactive high-resolution liquid chromatography MS (LC-MS) spectrometer (Thermo Fisher, Beijing, China). UV–vis absorption spectra were taken on a LAMBDA 950 UV–vis spectrophotometer (Perkin Elmer, Beijing, China). More experimental details and characterization of products are available in Supporting Information. Scheme 1 | Synthesis of P5. Download figure Download PowerPoint Results and Discussion Synthesis and structure of P5 Previously, we obtained pagoda[4]arene ( P4) and its isomer i-pagoda[4]arene (i- P4) by the trifluoroacetic acid (TFA)-catalyzed one-pot condensation of 2,6-DMA and paraformaldehyde in dichloromethane.31 To obtain a new family member of pagoda[n]arenes with larger cavities, we tried the condensation in different templated solvents such as 1,2-dichlorobenzene (o-DCB), cyclohexane, and chlorocyclohexane. It was found that when o-DCB was used as solvent, P5 as a cyclic pentamer of 2,6-DMA could be obtained as the major product in 43% yield (Scheme 1). Although the cyclic tetramers as minor products were produced as well, P5 could be conveniently purified by recrystallization of the mixture from dichloromethane and ethanol. However, if the condensation occurred in other bulky solvents, little or none of the cyclic pentamer was obtained, which showed that o-DCB might play a template role in the synthesis of the cyclic pentamer. Moreover, we also tried the condensation in mixed solvents of dichloromethane (DCM) and o-DCB with different percentages (see Supporting Information Table S1). It was found that when DCM was used as solvent, only cyclic tetramers were obtained. With the increase of the percentage of o-DCB, the yield of P5 increased while the yields of cyclic tetramers decreased. Luckily, we obtained the crystal of P5 containing two o-DCB molecules (see Supporting Information Figure S21). It was found that P5 could encapsulate two o-DCB molecules in its cavity by multiple CH⋯Cl and CH⋯π interactions. And the size of the o-DCB molecule fit well with the cavity of P5, indicating that o-DCB could be used as a templating solvent for the cyclic pentamer. Furthermore, it was found that when TFA was added to the solution of P5 in o-DCB, none of P5 was converted into other cyclomers even when the solution was heated to 100 °C, indicating that P5 was a stable product in o-DCB. The structure of P5 was well characterized by its 1H and 13C NMR, high-resolution mass spectrometry (HRMS) spectra, and X-ray crystal structure (see Supporting Information). The 1H NMR spectrum of P5 showed only three singlet signals and two doublet signals (Figure 2a), corresponding to the anthracene ring protons H1, H2, and H3, the methylene proton H4, and methyl proton H5, which were assigned by the two-dimensional (2D) 1H–1H correlation spectroscopy (COSY) spectra (Figure 2a). Moreover, only nine signals were observed in 13C NMR spectrum of P5 (see Supporting Information Figure S2). These results were all consistent with the high-symmetrical structure of P5. Moreover, the variable-temperature 1H NMR of P5 showed no obvious changes to the characteristic methylene proton signals between −50 and 50 °C (see Supporting Information Figure S9). Figure 2 | (a) 1H NMR spectrum (500 MHz, 298 K) of P5 in CDCl3. Inset: 1H–1H COSY spectrum (500 MHz, 298 K) of P5 in CDCl3. (b) Crystal structure of P5 from side and top views. Hydrogen atoms were omitted for clarity. Download figure Download PowerPoint A single yellow crystal of P5 suitable for X-ray analysis was obtained by slow evaporation of the solution in dichloromethane. As shown in Figure 2b, P5 was the cyclic pentamer that five 2,6-DMA subunits were all connected to by methylene linkages at the 1,5 positions. Moreover, P5 showed a symmetrical cylindrical structure from the side view and a highly symmetrical regular pentagonal structure from the top view with the angles between the adjacent anthracene planes all close to 108°. The side length of P5 was 7.8 Å, which was longer than those of pillar[5]arene (5.8 Å) and prism[5]arene (6.5 Å). Besides, the distance between the vertex and center of the opposite aromatic face was up to 11.8 Å, while the cavity of P5 was deepened to 8.0 Å due to the large 2,6-DMA moiety. The large and deep cavity of P5 could benefit from its specific binding ability toward the guests. Moreover, we obtained the crystal structure of P5 containing two isopropyl ether molecules inside the cavity as well (see Supporting Information Figure S11), in which P5 showed similar highly symmetric regular pentagonal structure. It was further found that P5 molecules could stack to form channel-like architecture in the solid states (see Supporting Information Figures S22–S25). Host–guest complexations in solution Since P5 showed a large and deep cavity, and the five anthracene rings formed a rigid regular pentagonal electron-rich cavity evidenced by the potential calculation (Figure 1b),52 it was expected that P5 could complex various 4-aryl-pyridinium, 2-aryl-pyridinium, bipyridinium, and stilbazolium salts (Scheme 2) with multiple collaborative noncovalent interactions. Especially, P5 could show the ability to accommodate two identical cationic guests in the cavity simultaneously, which would enable wide potential application of P5 in supramolecular chemistry. The host–guest complexations in solution were investigated using the 1H NMR spectroscopic method ( Supporting Information Figures S26–S41 and S43–S88). We first studied the complexation of P5 with a series of cationic guests G1– G10 with single positive charge. As shown in Figures 3a–3c, when P5 (1.00 mM) and 2.0 equiv of protonated 4-phenylpyridinium guest G1 were mixed in 1∶1 (v/v) CDCl3 and acetone-d6, the 1H NMR spectrum of the mixture showed only one set of different signals from those of the free host and free guest, suggesting that a new complex was formed, and the complexation between P5 and G1 was a fast-exchange process. Especially, it was found that all the protons Ha–He of G1 showed broadened and upfield shifts due to the shielding effect of aromatic rings of P5. The 1H NMR spectroscopic titrations further afforded quantitative estimate of complexation between P5 and G1 by monitoring the changes of the chemical shifts. The results showed that 1∶2 complex between P5 and G1 was formed in solution by a mole-ratio plot (see Supporting Information Figure S42). Accordingly, the association constants (Ka) for the 1∶2 complex were calculated to be (9.55 ± 0.15) × 103 M−1 (Ka1) and (1.16 ± 0.09) × 102 M−1 (Ka2) using the BindFit software,53–55 which showed that Ka1 was larger than Ka2, probably due to the electrostatic repulsion of the two guests in the cavity. The ESI mass study was also used to characterize the formation of G12@P5 (see Supporting Information Figure S89). Consequently, the peak at m/z 1708.6235 for [ G12@P5-PF6]+ was found, which provided further evidence of formation of the 1∶2 stable complex. Figure 3 | 1H NMR spectra (500 MHz, acetone-d6/CDCl3 = 1∶1, v/v, 298 K) of (a) free G1, (b) P5 and 2.0 equiv of G1, and (c) free P5. [P5]0 = 1.0 mM. Download figure Download PowerPoint Similar to the case of G1, we found that P5 could also form 1∶2 complexes with 4-aryl-pyridinium G2– G6 in solution by the mole-ratio plots (see Supporting Information). For the 1∶2 complex of G22@P5, the association constants were calculated to be (1.00 ± 0.11) × 104 (Ka1) and (2.39 ± 0.10) × 102 M−1 (Ka2), in which the first association constant was two orders of magnitude larger than the second one. When the methyl group of G2 was replaced by a propyl group as G3, it was found that the association constants were calculated to be (5.00 ± 0.08) × 104 (Ka1) and (4.59 ± 0.13) × 102 M−1 (Ka2), respectively, which were larger than those of G2, and the Ka1 value was also two orders of magnitude larger than Ka2. Furthermore, we studied the complexation of P5 toward 4-aryl-N-methylpyridinium salts G4– G6 with different substituents on the aryl group. The association constants were calculated by the BindFit software, and the results are summarized in Table 1. It was found that when the electron-donating methoxy group was introduced to the aryl group of 4-aryl-N-methylpyridinium at the para-position as G4, the association constants were (1.53 ± 0.03) × 103 M−1 (Ka1) and (3.83 ± 0.23) × 102 M−1 (Ka2), which were much smaller than those of G2, probably because of the lower electron deficiency of the guest when the electron-donating methoxy group was introduced. Conversely, when halogen atoms were introduced to the 4-aryl-N-methylpyridinium salts, the association constants between P5 and the guests increased to (4.78 ± 0.08) × 104 M−1 (Ka1) and (3.05 ± 0.02) × 102 M−1 (Ka2) for G5, and (6.34 ± 0.12) × 104 M−1 (Ka1) and (5.65 ± 0.04) × 102 M−1 (Ka2) for G6, respectively. Table 1 | Association Constants (Ka) and Cooperativity Factors (α) for 1∶2 Complexes Formed by P5 and the Guests at 298 K Guest Solvent Ka1 (M−1) Ka2 (M−1) Cooperativity Factor α (α = 4Ka2/Ka1) G1 Acetone-d6/CDCl3(1∶1, v/v) (9.55 ± 0.15) × 103 (1.16 ± 0.09) × 102 0.05 G2 Acetone-d6/CDCl3(1∶1, v/v) (1.00 ± 0.11) × 104 (2.39 ± 0.10) × 102 0.09 G3 Acetone-d6/CDCl3(1∶1, v/v) (5.00 ± 0.08) × 104 (4.59 ± 0.13) × 102 0.04 G4 Acetone-d6/CDCl3(1∶1, v/v) (1.53 ± 0.03) × 103 (3.83 ± 0.23) × 102 1.00 G5 Acetone- d6/CDCl3(1∶1, v/v) (4.78 ± 0.08) × 104 (3.05 ± 0.02) × 102 0.03 G6 Acetone-d6/CDCl3(1∶1, v/v) (6.34 ± 0.12) × 104 (5.65 ± 0.04) × 102 0.04 G7 Acetone-d6/CDCl3(1∶1, v/v) (1.25 ± 0.02) × 104 (0.64 ± 0.04) × 102 0.02 G8 Acetone-d6/CDCl3(1∶1, v/v) (2.21 ± 0.07) × 102 (1.16 ± 0.14) × 102 2.10 G9 Acetone-d6/CDCl3(1∶1, v/v) (3.23 ± 0.02) × 104 (1.82 ± 0.06) × 102 0.02 G10 Acetone-d6/CDCl3(1∶1, v/v) (2.06 ± 0.07) × 104 (0.78 ± 0.17) × 102 0.02 G11 Acetone-d6/CDCl3(1∶1, v/v) (7.21 ± 0.15) × 104 (2.01 ± 0.09) × 102 0.01 G12 Acetone-d6/CDCl3(1∶1, v/v) (1.28 ± 0.07) × 105 (8.20 ± 0.07) × 103 0.26 G13 Acetone-d6/CDCl3(1∶1, v/v) (3.96 ± 0.05) × 104 (4.56 ± 0.01) × 102 0.05 G14 Acetone-d6/CDCl3(1∶1, v/v) (5.00 ± 0.05) × 104 (5.28 ± 0.12) × 102 0.04 G15 Acetone-d6/CDCl3(1∶1, v/v) (7.78 ± 0.11) × 103 (7.00 ± 0.02) × 102 0.36 G16 Acetone-d6/CDCl3(1∶1, v/v) (6.71 ± 0.12) × 104 (2.16 ± 0.04) × 103 0.01 Scheme 2 | Structures of guests G1–G16. Download figure Download PowerPoint Moreover, we also studied the complexation ability of P5 with 2-aryl-pyridinium salts G7– G9 with a single positive charge. It was found that P5 could form 1:2 complexes with G7– G9 in solution as well. The association constants were calculated by the BindFit software, and the results are summarized in Table 1. Similar to the cases of 4-aryl-pyridinium salts, the Ka1 values for 2-aryl-pyridinium salts were normally on the order of 104 M−1 and two orders of magnitude larger than the Ka2 values. However, the Ka1 of (2.21 ± 0.07) × 102 M−1 for G8 was much smaller than those of G7 and G9, probably due to the lower electron deficiency of the guest when the electron-donating methoxy group was introduced to the aryl group. Similarly, the association constants for the 1:2 complex between P5 and 1-methyl-4,4′-bipyridinium G10 were calculated to be (2.06 ± 0.07) × 104 M−1 (Ka1) and (1.16 ± 0.14) × 102 M−1 (Ka2). In addition, the cooperativity factors (α = 4Ka2/Ka1) were calculated and summarized in Table 1. It was found that the cooperativity factors for the guests, except G4 and G8, were all smaller than 1.00, suggesting a negative allosteric cooperativity because of the charge repulsion between the two encapsulated guests. We further investigated the complexation of P5 toward 4,4′- and 2,2′-bipyridinium salts G11–G 14 with double positive charges. Although bipyridinium guests were much more electron-deficient, and there were stronger electrostatic repulsions between the guests, it was interestingly found that the 1∶2 complexes between P5 and G11– G14 could also form in solution by the mole-ratio plots (see Supporting Information). Formation of the 1:2 complexes was probably because the five electron-rich anthracene walls of P5 could strongly bind with the electron-deficient guests and even overcome the electrostatic repulsion between the guest molecules in the cavity. By the BindFit software, the association constants were calculated as well, and the results are summarized in Table 1. It was found that P5 could effectively complex the bipyridinium salt G11–G 14 in solution. The Ka1 values were all larger than 104 M−1, and Ka2 values were larger than 102 M−1. Moreover, we found that π-extended stilbazolium guests G15 and G16 could form 1∶2 complexes with P5 in solution as well by the mole-ratio plots (see Supporting Information). The association constants for G152@P5 were found to be (7.78 ± 0.11) × 103 (Ka1) and (7.00 ± 0.02) × 102 M−1 (Ka2), and they were smaller than those of G162@P5 with (6.71 ± 0.12) × 104 (Ka1) and (2.16 ± 0.04) × 103 M−1 (Ka2), probably because the triarylamine group of G16 showed strong interaction with the pyridinium group of the guest in the cavity. In addition, ESI studies provided further evidence for formation of the 1∶2 stable complexes between P5 and G2– G16 (see Supporting Information Figures S90–S104), which were all consistent with the results in solution. Host–guest complexations in the solid state We obtained the single crystals of the complexes between host P5 and some of the guests (see Supporting Information Figures S10 and S12–S20 and Tables S3–S13), and also studied the host–guest complexations in solid states. As shown in Figure 4a, the crystal structure of complex G32@P5 clearly revealed that two molecules of G3 were fully threaded in the cavity of P5 with a “head-to-tail” arrangement, which avoided the strong electrostatic repulsion between the pyridinium subunits. The two encapsulated guest molecules were antiparallel to each other and showed π⋯π stacking interactions with an average distance of 3.36 Å. Besides, there existed multiple C–H⋯π interactions between the host and G3 with the distances of 2.87 (A), 2.83 (B), 2.78 (C), 2.88 (D), 2.78 (E), 2.77 (F), 2.88 (G), 2.85 (H), 2.85 (I), and 2.86 Å (J), respectively. Moreover, C–H⋯F hydrogen-bonding interactions between the hexafluorophosphate and P5 were also found, which played a key role in the formation of the stable complex. Figure 4 | Top and side views of crystal structures of (a) G32@P5, (b) G42@P5, (c) G82@P5, and (d) G112@P5. Blue dotted lines represent the noncovalent interactions. PF6− counterions and hydrogen atoms not involved in the noncovalent interactions were omitted for clarity. Download figure Download PowerPoint Moreover, we also obtained the crystal structure of G42@P5, and found that two molecules of G4 were threaded in the cavity of P5 with a “head-to-tail” arrangement as well (Figure 4b). Multiple C–H⋯π interactions between P5 and G4 with the distances of 2.87 (A), 2.79 (B), 2.77 (C), 2.79 (D), 2.87 (E), 2.79 (F), 2.82 (G), 2.64 (H), 2.77 (I), 2.76 (J), 2.64 (K), 2.75 (L), and 2.86 Å (M), respectively, could be found. For complex G82@P5 (Figure 4c), although two G8 molecules were encapsulated with a “head-to-tail” arrangement as well and showed π⋯π stacking interactions with an average distance of 3.36 Å, the two guests were not fully threaded in the cavity of host P5. There existed few C–H⋯π interactions between the host and G8 with the distances of 2.88 (A), 2.78 (B), 2.79 (C), 2.87 (D), and 2.87 Å (E), respectively, which might also explain the small association constants of the complex G82@P5 in solution. Luckily, the crystal structure of complex G112@P5 was also obtained. As shown in Figure 4d, the crystal structure clearly revealed that the two molecules of G11 were both encapsulated in the cavity of P5, which further proved formation of the 1∶2 complex between P5 and guest G11 with double positive charges. There existed multiple C–H⋯π interactions between the host and the encapsulated guests with the distances of 2.79 (A), 2.81 (B), 2.71 (C), 2.89 (D), 2.79 (E), 2.80 (F), 2.82 (G), 2.69 (H), 2.79 (I), 2.67 (J), 2.84 (K), and 2.76 Å (L), respectively. Moreover, it was also found that the hexafluorophosphate anions could form multiple C–H⋯F hydrogen bonds not only with host P5 but also with the two bipyridinium guests in the cavity (see Supporting Information Figure S25), which played an important role in formation of the stable complex. However, unlike the cases of the guests with single positive charge in the cavity of P5, there was no π⋯π stacking interaction between the two encapsulated guests, probably due to the strong electrostatic repulsion of the two electron-deficient bipyridinium guests. We further obtained the crystal structure of the complex between P5 and G1. As shown in Figure 5a, P5 encapsulated two guests in its intrinsic cavity, and the two guests in the cavity took a “head-to-tail” arrangement, which avoided the electrostatic repulsion between the two protonated pyridinium units. Moreover, the two encapsulated guests were antiparallel to each other and showed π⋯π stacking interactions with an average distance of 3.36 Å. There also existed multiple C–H⋯π interactions between P5 and the encapsulated guests with the distances of 2.78 (A), 2.89 (B), 2.82 (C), 2.88 (D), 2.81 (E), 2.87 (F), 2.85 (G), 2.87 (H), 2.68 (I), 2.89 (J), and 2.71 Å (K), respectively. Interestingly, it was further found that the four adjacent macrocycles of P5 could form one pseudocavity with electron-rich anthracene rings as the walls, which could complex two guests of G1 in the solid state as well (Figure 4a). Comparatively, P5 only encapsulated two guests of G3 in its intrinsic cavity, probably because of the steric effect of the propyl group of the guest. Figure 5 | Top and side views as well as packing mode of the crystal structures: (a) G13@P5, (b) G23@P5, (c) G103@P5, and (d) G153@P5. Blue dotted lines represent the noncovalent interactions. PF6− counterions and hydrogen atoms not involved in the noncovalent interactions were omitted for clarity. Download figure Download PowerPoint The crystal structure of the complex between P5 and G2 showed that each P5 could also complex three molecules of G2 in the solid state (Figure 5b). Besides encapsulating two guest molecules by the intrinsic cavity of P5, the pseudocavity formed by the three adjacent macrocycles could also complex one more guest, which was different from the case of G13@P5. It was further found that in G23@P5, the two guests in the cavity of P5 took a “head-to-tail” arrangement as well, and the π⋯π stacking interaction between the aromatic rings of the two guests with an average distance of 3.29 Å was shown. Moreover, there also existed multiple C–H⋯π interactions between the P5 and G2 with the distances of 2.66 (A), 2.89 (B), 2.82 (C), 2.86 (D), 2.85 (E), 2.83 Å (F), 2.78 Å (G), 2.74 Å (H), and 2.87 Å (I), respectively. Similarly, in the cases of guests G10 and G15, each P5 could also complex three guest molecules in the solid states by both its intrinsic cavity and the pseudocavity formed by the three adjacent macrocycles (Figures 5c and 5d). It was further found that for both G10 and G15, the two identical guests in the cavity of P5 adopted the “head-to-tail” arrangement as well. There existed multiple C–H⋯π interactions between the host and G10 with the distances of 2.90 (A), 2.89 (B), 2.79 (C), 2.80 (D), 2.88 (E), 2.82 (F), 2.83 (G), 2.84 (H), and 2.76 Å (I), respectively. For complex G153@P5, there existed multiple C–H⋯π interactions between the host and G15 with the distances of 2.71 (A), 2.88 (B), 2.81 (C), 2.81 (D), 2.85 (E), 2.70 Å (F), 2.76 (G), 2.89 (H), and 2.80 Å (I), respectively. Besides, multiple C–H⋯F hydrogen-bonding interactions between the hexafluorophosphate and P5 could be found in G103@P5 and G153@P5, and they also played an important role in the formation of the stable complexes. ICT interactions between P5 and the guests Since P5 contains five electron-rich anthracene rings, and the tested guests are electron-deficient, strong ICT interactions between P5 and the guests was found. We first studied the photophysical properties of P5 in solution. It was found that the solution of P5 was yellow, and the absorption of P5 showed a clear redshift compared with that of 2,6-DMA (see Supporting Information Figure S105). Besides, P5 showed a broad emission band at 430–550 nm with maximum peak at 453 nm, which exhibited strong blue fluorescence (see Supporting Information Figure S106). We further found that when P5 (3.0 mM) and G1 (6.0 mM) were mixed in CH3CN and CHCl3 (1∶1, v/v), a significant color change occurred, and an orange solution was immediately obtained (Figure 6) due to the ICT interaction between electron-rich P5 and the electron-poor pyridinium salts. Meanwhile, a broad charge-transfer absorption band at 450–600 nm that was different from the spectrum of either P5 or G1 was observed. Figure 6 | UV–vis spectra of P5 (3.0 mM) and G1 (6.0 mM), and the complex of P5 and G1 in CHCl3/CH3CN (1∶1, v/v) at 298 K. Inset: optical images showed the solution color changes because of CT interaction between P5 and G1. Download figure Download PowerPoint Similar to the case of G1, the complexation