2024, Vol. 35 
b Key Laboratory of Environment-Friendly Composite Materials of the State Ethnic Affairs Commission, Gansu Provincial Biomass Function Composites Engineering Research Center, College of Chemical Engineering, Northwest Minzu University (Northwest University for Nationalities), Lanzhou 730000, China
The development of new macrocyclic hosts has always been a hotspot in supramolecular chemistry due to its wide range of applications such as in sensors [1,2], self-assembled materials [3–5], and molecular machines [6]. By far, many macrocyclic hosts, including crown ethers [7,8], cyclodextrins [9], calixarenes [10], cucurbiturils [11,12], pillararenes [13–21], etc., have been successfully developed and widely used. However, there is still a huge demand for novel macrocyclic hosts to supply more exact and effective binding for various target guests to achieve specific functions. To address this pressing need, a variety of efforts have been carried out in different ways, including modifying the existing macrocycle, developing new macrocycles, bi-macrocyclic hosts, and cages to adapt the target guests and supply exact binding [22–25]. All these approaches have their respective merits, while fused bi-macrocyclic hosts can supply two macrocycles to bind target guests. Moreover, the two macrocycles of the fused bi-macrocyclic host can supply an enrichment effect through the collaboration of the two cycles to bind more complicated guests or construct a more sophisticated supramolecular functional system [14,26–29]. Therefore, developing a novel fused bi-macrocyclic host and deeply investigating its host-guest interaction properties are very interesting and important.
As is well known, heavy metal ion contamination is one of the most severe environmental problems, which poses a great threat to the survival of living species [30–33]. For instance, Cr(Ⅵ) and its oxyanions such as dichromate ions (Cr2O72−) are widely utilized in chemical engineering including metallurgy, metal plating, pigments, and other fields [34,35], while Cr(Ⅵ) featured high toxicity, carcinogenicity, and so on. Moreover, since dichromate (Cr2O72−) is the most common source of chromium(Ⅵ), on the basis of the existing supramolecular strategies to recognize Cr2O72- [36–39], it is very important to develop new strategy that effectively bind Cr2O72− to improve the sensitivity and selectivity of Cr2O72− or its ion pairs. Significantly, the rapid development of macrocyclic chemistry supplied a bright opportunity to improve the sensing sensitivity and selectivity.
In light of the above, herein, a novel fused bi-macrocyclic chemosensor molecule BPN1 (Fig. 1a) has been rationally designed and synthesized. In BPN1, the electron-rich cavity of the pillar[5]arene was expected to bind the accompanying cation of the dichromate anions, meanwhile, the methoxy groups on the pillar[5]arene could supply multiple hydrogen bond interactions for Cr2O72−. In addition, the naphthalene-diimide (NDI) group and N-(2-amino-ethyl)-2-hexyl-thioethyl-amide moieties were employed to form a macrocycle with one phenyl group of pillar[5]arene. The obtained macrocycle possesses abundant supramolecular interaction sites, which also could bind other Cr2O72− anions. Therefore, one fused bi-macrocyclic host molecule BPN1 could bind several Cr2O72− anions. This is an enrichment effect and could improve the sensitivity of Cr2O72−. As we expected, based on the enrichment effect, the fused bi-macrocyclic host molecule BPN1 could detect Cr2O72− with high selectivity and high sensitivity.
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| Fig. 1. (a) The single-crystal structure of BAP and synthesis of the fused bi-macrocyclic BPN1. (b) Fluorescence intensity (λ ex = 360 nm, λ em = 425 nm) of BPN1 (1 × 10−4 mol/L) in the presence of various anions (50 equiv.) in DMSO/H2O (9: 1, v/v). | |
First, the pillar[5]arene-based fused bi-macrocyclic chemosensor BPN1 was synthesized by the cyclization of BAP with NDI (Fig. 1a) and the obtained BPN1 has been fully characterized (Figs. S10-S14 in Supporting information). Fortunately, a single crystal of ethyl mercapto-acetate functionalized pillar[5]arene (BPA) was obtained (Figs. S3-S5 in Supporting information). In the single crystal, the flexible chain of the BPA is on the outside of pillar[5]arene, which provides strong support for the flexible chain and NDI to form a ring on the outside of the pillar[5]arene. Furthermore, the size of the cavity of the pillar[5]arene is 5 Å [40], and the size of NDI is 6.62 Å [41]. Therefore, the obtained new ring is located on the outside of the pillar[5]arene cavity.
In order to test the enrichment effect of the BPN1, host-guest interactions were investigated by adding 50 equiv. of various anions (F−, Cl−, Br−, I−, NO3−, ClO3−, IO3−, SO42−, CO32−, ClO4−, MnO42−, SCN−, CrO42−, Cr2O72−, AsO2−, H2AsO4−, 0.1 mol/L, water solution) into the solution (DMSO/H2O, 9:1, v/v) of BPN1 (1 × 10−4 mol/L), respectively. As shown in Fig. 1b and Fig. S15 (Supporting information), only Cr2O72− could induce the fluorescence decrease of the BPN1 solution, while other anions could not induce a similar response. These results indicated that BPN1 possesses selective recognition properties for Cr2O72−. In addition, in the presence of other anions (F−, Cl−, Br−, I−, NO3−, ClO3−, IO3−, SO42−, CO32−, ClO4−, MnO4−, SCN−, AsO2−, and H2AsO4− (50 equiv.)), Cr2O72− (50 equiv.) was added to BPN1 (1 × 10−4 mol/L) and fluorescence experiments were carried out (Fig. S16 in Supporting information) [42]. The results show that the existence of the other anions does not interfere with BPN1 recognition of Cr2O72−. According to competing experiments (Fig. 1b) and coexisting anions experiments, coexisting anions could not influence the Cr2O72− recognizing process, which demonstrated the good anti-interference ability of BPN1 on selective sensing Cr2O72−. Furthermore, according to the fluorescent titration experiments (Fig. S17 in Supporting information), with the gradual fluorescence intensity of the BPN1 solution at 425 nm decreased by degrees. In addition, based on the fluorescence titration spectra and calculation via the 3σ/m method (Figs. S18-S20 in Supporting information) [43], the lowest limit of detection (LOD) of the BPN1 to Cr2O72− is 1.27 × 10−7 mol/L, which indicated that the BPN1 has a high sensitivity for detection of Cr2O72−. Meanwhile, as shown in Table 1 [44–52], compared with reported Cr2O72− sensing materials, the BPN1 possesses considerably higher sensitivity.
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Table 1 Comparison of LOD with some reported of materials for Cr2O72−. |
To deeply understand the Cr2O72− recognition mechanism of the fused bi-macrocyclic compound BPN1, a series of researches including HR-MS, 1H NMR titration, FT-IR, SEM, as well as density functional theory (DFT) had been carried out. In the beginning, the HR-MS had been employed to evaluate the stoichiometry of the host-guest complexes, in the HR-MS (Fig. S22 in Supporting information), the negative ion pattern shows the peak at m/z = 840.1957, which agree with the [BPN1+ K2Cr2O7-H]2− and could be attributed to the BPN1 forming an ions pair complex with K2Cr2O7 (BPN1: K2Cr2O7 = 1:1). Meanwhile, the peak at m/z = 472.5684 agree with the [BPN1+ 2Cr2O7]4−, which could be attributed to the formation of BPN1+ 2Cr2O72− complex (BPN1: Cr2O72− = 1:2). Simultaneously, the peak at m/z = 339.0240 agree with the [BPN1+ 3Cr2O7]6−, which could be attributed to the formation of BPN1+ 3Cr2O72− complex (BPN1: Cr2O72− = 1:3). These results show that the fused bi-macrocyclic compound BPN1 can bind to multiple Cr2O72− and have certain enrichment effect.
In the 1H NMR titration experiments (Fig. 2), after the gradual addition of 10 equiv. of Cr2O72− into the BPN1 solution, the proton signal of H1 on the outer ring showed upfield shift, which is due to the anion-π interaction between the NDI group and Cr2O72−. In addition, the proton H1 signal on the outer ring is also shifted upfield, indicating that there may be hydrogen bonds between the alkyl chain on the outer ring and Cr2O72−. Both H2 and H4 on the pillar[5]arene cavity are shifted to high fields, indicating that there may also be hydrogen bonds and anion-π interactions between the pillar[5]arene cavity and Cr2O72−. Moreover, in the corresponding FT-IR spectra, upon the addition of Cr2O72− into the BPN1, the -NH signal shifted from 3398 cm−1 to 3427 cm−1, while the C=O signal shifted from 1705 cm−1 to 1690 cm−1. These shifts could be attributed to Cr2O72- binding with -NH to form -N-H Cr2O72− hydrogen bonds (Fig. S23 in Supporting information).
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| Fig. 2. 1H NMR titration spectra (400 MHz, DMSO-d6, 298 K) of BPN1 and BPN1 + Cr2O72−. | |
In order to deeply understand and visualization the binding mechanism of fused bi-macrocyclic compounds BPN1 and Cr2O72−, the host-guest interaction was studied by density functional theory (DFT) [53]. First, a separate theoretical calculation of the host BPN1 was performed, and isosurface maps of intramolecular interactions were obtained at ωB97XD/6–311+G(d, p) level [54,55]. All DFT calculations uses 6–311G(d, p) for BPN1 and 6–311+G(d, p) for Cr2O72− with implicit solvation model. The BNPI optimized structure is shown in Fig. 3a, and the NDI part of the side ring is parallel to the methoxybenzene part of the pillar[5]arene cavity. Using Multiwfn to visualize the weak intramolecular interaction of BPN1 [56–58], it can be seen that this is due to the strong van der Waals force (π-π interaction) between the NDI on the side ring and the methoxybenzene part of the pillar[5]arene, the distance between the two paralleled groups is 3.4 Å (Fig. 3a). As shown in Fig. 3b, ESP (the electrostatic potential maps) of BPN1 also shows that the methoxy group in the pillar[5]arene cavity and the alkyl chain on the side ring have a strong positive charge and are easy to combine with electron-rich groups such as anions. Meanwhile, the π plane and a large number of heteroatoms of the NDI ring on the side ring can provide abundant supramolecular interaction sites, forming anion-π interactions and multiple H-bond interactions.
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| Fig. 3. (a) Side view the structure optimization. (b) The ESP (the electrostatic potential maps) of free BPN1, [BPN1+ K2Cr2O7], [BPN1+ 2Cr2O72−], [BPN1+ 3Cr2O72−] at the ωB97XD/6–311+G(d, p) level. | |
Next, to deeply study the specific binding mode between the BNP1 and the Cr2O72−, the DFT study was carried out based on the 1H NMR titration and FT-IR spectra under different binding ratio of host and guest. As shown in Fig. 3, according to the DFT results, the relative positions of the host-guest binding were determined. It can be seen from Fig. 3a that the host-guest complex with a binding ratio of 1:2 has the least configuration change compared with the host BPN1 alone. To understand the complexation mode of fused bi-macrocyclic host BNP1 with the Cr2O72− guest, a knowledge of the charge distribution and reactive sites is crucial and can be gauged through the ESP. Among the three different complexes formed by weak interactions, the electron density inside the cavity of the pillar[5]arenes was significantly decreased, thus allowing for the formation of new host- guest complexes with anionic guests via anion-π interactions (Fig. 3b). Besides, the frontier molecular orbitals (highest occupied molecular orbital and lowest unoccupied molecular orbital, HOMO, and LUMO) of the complexes with different host-guest binding ratios by DFT is obtained. In Fig. S24 (Supporting information), the HOMO orbital of BPN1 is mainly distributed on the pillar[5]arene cavity. In [BPN1 + K2Cr2O7], the HOMO orbitals are mainly distributed in the pillar[5]arene cavity near the side ring, in addition, they are also distributed on the NDI. These changes are caused by the redistribution of electrons. The HOMO-LUMO gap of [BPN1 + K2Cr2O7] is 5.64 eV which is greater than BPN1 (5.10 eV). In [BPN1+ 2Cr2O72−], the LUMO orbital was transferred from the NDI portion of BPN1 to the side ring, most likely because of the host-guest interaction between Cr2O72− and the side ring. The frontier molecular orbitals also of [BPN1+ 3Cr2O72−] are mainly distributed in pillar[5]arenes cavity and the HOMO-LUMO gap is 5.15 eV. On observing the values of the [BPN1+ 2Cr2O72−] and [BPN1+ 3Cr2O72−] molecules, [BPN1+ 2Cr2O72−] are smaller than [BPN1+ 3Cr2O72−]. All these results suggest that the [BPN1+ 2Cr2O72−] have a greater affinity and thus have stronger interactions. In addition, to clearly demonstrate the strength of the interaction between the host and the guest, the binding free energy is also calculated. The binding free energies were obtained by subtracting the sum of individual host and guest self-consistent field energies from that of the complex [59]. As shown in Table 2, the binding free energies of BPN1+ Cr2O72−, BPN1+ 2Cr2O72−, BPN1+ 3Cr2O72− are −88.76, −455.778, −588.031 kcal/mol, respectively. The results indicated that the binding free energies of BPN1 and Cr2O72− enhanced with increasing the binding ratio of BPNI and Cr2O72−. This result indicated that the binding of host and guest is more stable in a higher binding ratio, which agrees with the "enrichment effect".
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Table 2 The total energy (E) of host (BPN1) and guest (Cr2O72-) and host-guest binding energies (BE). |
For visually showing the multiple supramolecular interactions, the noncovalent interaction picture was got by the IGMH approach [60,61]. IGMH analysis has been performed by considering both guest molecules as the fragment and the host molecule as another one, and the obtained δginter green isosurface highlights the interaction region between the corresponding fragments (Fig. 4). The noncovalent interaction analysis can provide details to distinguish different weak noncovalent interactions, such as van der Waals, hydrogen bonds, and steric repulsion, and describe their intensity quantitatively. The color-coding scheme is as follows: red regions mean strong repulsive, strong attractive interactions are shown in blue while green regions represent electrostatic interactions. 3D isosurfaces and scatter diagrams for complexes are illustrated (Fig. S27 in Supporting information). These green patches are syllabified visualizing the interactions between BPN1 and Cr2O72−, like anion-π interactions and multiple hydrogen bonds. The above results support our proposal of an "enrichment effect" in chemosensor design to enhance host-guest interaction.
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| Fig. 4. IGMH analysis of (a) free BPN1, (b) [BPN1+ K2Cr2O7], (c) [BPN1+2Cr2O72−], (d) [BPN1+3Cr2O72−] at the ωB97XD/6–311+G(d, p) level. | |
Moreover, by SEM experiments with different ratios of host and guest, as shown in Fig. 5, BPN1 exists in a regular spherical structure, while when 1 equiv. Cr2O72− is added, BPN1+ Cr2O72− shows agglomerated nanosphere structure; when 2 equiv. Cr2O72− is added, BPN1+ 2Cr2O72− shows aggregated a three-dimensional rod-like structure; while adding 3 equiv. Cr2O72−, BPN1+ 3Cr2O72− showed a three-dimensional sponge-like structure. With the increase of Cr2O72−, the host and guest are gradually crosslinked, and then the morphology changes from nanospheres to three-dimensional sponge network structure. These results clearly show that the bi-macrocyclic host BPN1 and Cr2O72− can bind by three different ratios, and the electron microscope morphology of the three binding modes can predict that different ratios of host-guest complexes have different modes of action. The above results were consistent with DFT calculations, and indicated that the "enrichment effect" in chemosensor design really enhanced the host-guest interactions.
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| Fig. 5. SEM images showing the morphology of (a) BPN1; (b) BPN1+ Cr2O72−; (c) BPN1+ 2Cr2O72−; (d) BPN1+ 3Cr2O72−. | |
In conclusion, a novel bi-macrocyclic chemosensor BPN1 was synthesized and developed. The BPN1 could highly sensitively and selectively recognize Cr2O72− by the fluorometric response. The host and guest are gradually crosslinked, and then the morphology changes from nanospheres to three-dimensional sponge network recognition mechanism of BPN1 toward Cr2O72− was deeply studied by experiments and DFT. As a result, the sensitive and selective sensing mechanism of BPN1 for Cr2O72− was based on the enrichment effect which was supplied by the bi-macrocycle of the chemosensor BPN1 through multiple intermolecular hydrogen bonding and electrostatic attractions. This work provided an easy way to improving the sensitivity and selectivity of chemosensor by inducing enrichment effect.
Declaration of competing interestThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
AcknowledgmentsThis work was supported by the National Natural Science Foundation of China (NSFC, Nos. 22065031, 22061039, 22001214, 22165027), the top-notch talent project in Gansu province, the Key R & D Program of Gansu Province (No. 21YF5GA066), Gansu Province College Industry Support Plan Project (No. 2022CYZC-18), Natural Science Foundation of Gansu Province (Nos. 2020–0405-JCC-630, 20JR10RA088), The star of innovation (No. 2023CXZX-244).
Supplementary materialsSupplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.109281.
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