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Synlett 2024; 35(01): 130-134
DOI: 10.1055/s-0042-1751486
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Functional Dyes

Anthracene-Functionalized Metallacage with Fluorescence Response Behavior to Anions

Zhi-Yong Zeng
a   School of Chemistry and Molecular Engineering, East China Normal University, 3663 N. Zhongshan Road, Shanghai 200062, P. R. of China
,
Xiaoli Zhao
a   School of Chemistry and Molecular Engineering, East China Normal University, 3663 N. Zhongshan Road, Shanghai 200062, P. R. of China
,
Junhai Huang∗
b   College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, P. R. of China
c   State Key Laboratory of New Drug and Pharmaceutical Process, Shanghai Institute of Pharmaceutical Industry, China State Institute of Pharmaceutical Industry, Shanghai 201203, P. R. of China
,
Jing Zhu
d   Department of Philosophy, East China Normal University, 500 Dongchuan Road, Shanghai 200241, P. R. of China
,
Tongxia Jin
a   School of Chemistry and Molecular Engineering, East China Normal University, 3663 N. Zhongshan Road, Shanghai 200062, P. R. of China
,
Lianrui Hu∗
a   School of Chemistry and Molecular Engineering, East China Normal University, 3663 N. Zhongshan Road, Shanghai 200062, P. R. of China
,
Wei-Tao Dou
a   School of Chemistry and Molecular Engineering, East China Normal University, 3663 N. Zhongshan Road, Shanghai 200062, P. R. of China
,
Lin Xu∗
a   School of Chemistry and Molecular Engineering, East China Normal University, 3663 N. Zhongshan Road, Shanghai 200062, P. R. of China
› Author Affiliations

This work was supported by the National Nature Science Foundation of China (No. 22103062), Shanghai Pujiang Program (No. 22PJ1402800), the Fundamental Research Funds for the Central Universities (2022QKT003), and the Open Research Fund of Shanghai Key Laboratory of Green Chemistry and Chemical Processes.
› Further Information
 
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Abstract

Functionalized metallacages have attracted tremendous attention in recent years due to their potential applications in optical sensing, catalysis, and recognition. A novel anthracene-functionalized metallacage was synthesized and characterized in detail by UV/vis spectroscopy, 1D/2D NMR, electrospray ionization time-of-flight mass spectrometry, and X-ray single crystal diffraction. This metallacage exhibited a specific fluorescence enhancement response to OH–, PO4 3–, and AcO– anions, and further analysis indicated that this was due to anion-induced metallacage disassembly.


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Key words

supramolecular chemistry - self-assembly - metallacages - fluorescence - anion recognition

The coordination-driven self-assembly strategy is considered to be an effective method for creating new substances and for realizing new functions at the molecular level. Since Verkade and co-workers[1] constructed the first metallacycle in 1983, various supramolecular metalloarchitectures with exquisite structures and diverse functions have been constructed successfully. Among them, metallacages have attracted extensive attention due to their specific cavity structure and easy synthesis. Numerous scientists have made outstanding contributions to the construction of novel metallacages. Notably, Fujita and co-workers[2] [3] stand out for their synthesis of a series of elegantly constructed octahedral and spherical molecular cage structures, such as M6L12 and M12L24, M24L48, M30L60, by using the molecular chimera strategy. The groups of Mukherjee,[4] Yoshizawa,[5] and Clever[6] have developed unique molecular cylinder, capsule, and asymmetric cage structures. Stang and co-workers[7] successfully employed a directional bonding strategy to construct a number of Pt-containing cages, and the Raymond group[8] has made significant contributions to supramolecular catalysis by using a symmetry-matching strategy to build a series of negatively charged metallacages. Nitschke and co-workers [9] [10] also contributed greatly to an enrichment of the structural diversity of metallacages by adopting a subcomponent assembly strategy to construct various topologies, including helices, tetrahedra, cubes, and octahedra. The Cui group[11] has also made noteworthy contributions to the design and construction of chiral cages. Su and co-workers[12] have also made significant strides in this field by building a series of octahedral cage structures with bimetallic catalytic centers. Moreover, the groups of Wang and Li[13] [14] have developed an impressive array of giant metallacages. These unique cages have opened up new possibilities for research and development across multiple disciplines with wide-ranging applications in areas such as molecular recognition,[15] [16] [17] catalysis,[18] [19] [20] [21] separation,[22] [23] drug delivery,[24] and optoelectronic materials,[25] [26] [27] among many other fields.

Integrating functional building blocks into metallacages represents a compelling strategy to endow them with multifarious capabilities and to broaden their range of potential applications, thereby paving the way to the design and synthesis of advanced cage-based materials. For example, the integration of metal catalytic centers onto molecular backbones can produce cages with remarkable efficacy in catalysis.[28] Moreover, the attachment of chiral motifs onto ligands permits the construction of chiral cages, thus facilitating chiral recognition or chiral catalysis.[29]

The fabrication of metallacages containing fluorescent motifs[30] [31] imparts unique optical properties to the cages. Therefore, we envisioned a new type of fluorescent metallacage might be successfully constructed by using a ligand decorated with fluorescent motifs. Here, we describe a metallacage fashioned from an anthracene-modified triphenylamine skeleton. The cage displays intense stimuli responses to OH–, PO4 3–, and AcO–, due to their ability to replace the existing ligands within the cage, leading to rapid fluorescent enhancement, enabling the cage to interact selectively with AcO–, PO4 3–, and OH– among a wide range of anions.

Starting from the commercially available compound tris(4-bromophenyl)amine (S1), intermediate S3 was obtained through a palladium-catalyzed Suzuki reaction with pyridin-3-ylboronic acid (S2) (see the Supporting Information). Suzuki coupling of intermediate S3 with 9-anthrylboronic acid gave the required ligand 1. Cage 1 was synthesized by heating ligand 1 with Pd(NO3)2 in DMSO solution for eight hours (Scheme [1]).[32] The successful construction of the highly symmetric cage 1 was confirmed by means of 1H NMR, 13C NMR, 1H–1H correlation spectroscopy (COSY), diffusion-ordered (DOSY) NMR spectroscopy, ESI-TOF mass spectrometry, and X-ray single-crystal diffraction.[33]

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Scheme 1 Self-assembly of the metallacage cage 1 from ligand 1 and Pd2+

Compared with the free ligand 1, the chemical shift of the H(1), H(2), and H(4) atoms on the pyridine group of cage 1 were significantly shifted downfield (Figure [1a]). Specifically, the chemical shift of H(1) of the pyridine moiety shifted from δ = 8.92 to δ = 10.03 ppm, whereas that of H(2) shifted from δ = 8.54 to δ = 9.49 ppm, indicating a decrease in electron density on coordination of the pyridine group to the Pd2+. The DOSY NMR spectrum [see Supplementary Information (SI), Figure S9], confirmed the formation of a single species, as evidenced by the manifestation of a solitary diffusion coefficient of 10.17 × 10–10 m2/s. The structure of cage 1 was proved by high-resolution ESI-TOF MS, with m/z = 859.5978 and 1319.8873, corresponding to the multiply charged [M – 3NO3 –]3+ and [M – 2NO3 –]2+ molecular-ion signals, respectively. The resolved peaks agreed well with the simulated values (Figures [1b] and 1c), which confirmed the successful construction of cage 1.

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Figure 1 (a) 1H NMR spectra of ligand 1 and cage 1 (501 MHz, DMSO-d 6, 298 K). (b, c). ESI-TOF-MS of cage 1, observed value (top) and simulated value (bottom).

X-ray single-crystal diffraction was employed to further authenticate the cage structure.[33] Slowly diffusing ethyl acetate into a DMSO solution containing cage 1 over three weeks resulted in the successful formation of a crystal. Cage 1 crystallized in the P1 space group with a C4 symmetric unit containing half of the cage molecule, as shown in Figures [2a] and 2b. The crystal structure displays a Pd···Pd distance of 12.76 Å. The cage molecules are aligned parallel to one another through intermolecular CH⋯π interactions of an anthracene ring of one cage molecule and the pyridine ring of another cage molecule, as highlighted in Figure [2c]. This arrangement permits stacking of the cage molecules to form a complex porous structure (Figure [2d]).

In the UV/vis spectra, ligand 1 and cage 1 displayed a similar triple peak. However, whereas the maximum absorption peak of ligand 1 was detected at a wavelength of 351 nm, the cage exhibited a noticeable red shift to 371 nm due to its coordination with Pd2+, as shown in Figure [3a]. Because of the profound influence of the heavy atom effect of Pd2+, the fluorescence intensity of cage 1 was severely quenched relative to that of ligand 1 (Figure [3b]), resulting in a drastic reduction in the quantum yield from 66.77% for ligand 1 to a mere 0.33% for cage 1. It is worth mentioning that the excitation wavelength was carefully chosen at 425 nm for both ligand 1 and cage 1 because the 3D spectrum showed the highest emission intensity at this excitation wavelength (Figures [3c] and 3d).

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Figure 2 X-ray crystal structure of cage 1. (a and b) Main and top views of cage 1; (c and d) plane and three-dimensional stacked form of cage 1.
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Figure 3 (a) UV/vis absorption spectra of ligand 1 and cage 1. (b) Fluorescence emission spectra of ligand 1 and cage 1. (c and d). 3D spectra of ligand 1 and cage 1, respectively (ligand 1: c = 4.00 × 10–4 M; cage 1: c = 1.00× 10–4 M).

Given the substantial difference in the quantum efficiencies of ligand 1 and cage 1, as well as the stimuli response of metallacages to anions,[34] [35] we investigated the potential application of cage 1 in anion recognition. Initially, we measured the anion-recognition ability of ligand 1 and found that in DMSO solution, ligand 1 remained unresponsive to all the tested anions, as revealed by UV/vis and fluorescence spectroscopy (Figures [4a] and 4b). Specifically, two equivalents of potassium or sodium salts carrying various anionic species (BF4 –, PF6 –, F–, Cl–, Br–, I–, NO3 –, CO3 2–, SO4 2–, HPO4 2–, H2PO–, OH–, PO4 3–, and AcO–) were individually added to a DMSO solution of ligand 1 to confirm the recognition ability toward the anions separately. However, as shown in Figure [4c], the maximum absorption peak of the solution cage 1 exhibited a slight blue shift upon the addition of two equivalents of OH–, PO4 3–, or AcO– anion. Moreover, the fluorescence intensity of cage 1 in DMSO solution exhibited a marked increase on exposure to two equivalents of OH–, PO4 3–, or AcO–anion (Figure [4d]). Notably, the fluorescence enhancement observed for PO4 3– and OH– was slightly less than that observed for AcO–. On increasing the amount of the anion to eight equivalents, a similar phenomenon was observed in the solution of cage 1, which exhibited a specific fluorescence enhancement response to OH–, PO4 3–, and AcO– anions (Figures [4e] and 4f).

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Figure 4 (a, b) UV/vis absorption and fluorescence responses of ligand 1 (4.00 × 10–4 M) to 2.0 equivalents of various anions (BF4 –, PF6 –, F–, Cl–, Br–, I–, NO3 –, CO3 2–, SO4 2–, HPO4 2–, H2PO–, OH–, PO4 3–, and AcO–) in DMSO solution (λex = 425 nm). (c, d) UV/vis absorption and fluorescence responses of cage 1 (1.00 × 10–4 M) to 2.0 equivalents of various anions in DMSO solution (λex = 425 nm). (e) The fluorescence responses of cage 1 (1.00 × 10–4 M) to 8.0 equivalents of various anions in DMSO solution (λex = 425 nm). (f) The fluorescence intensity at 538 nm of cage 1 (1.00 × 10–4 M) to 8.0 equivalents of various anions in DMSO solution (λex = 425 nm).

It is worth mentioning that the fluorescence intensity of the solution of cage 1 at 538 nm was significantly enhanced compared with that in the presence of only two equivalents of OH–, PO4 3–, and AcO–. Again, this response was accompanied by a blue shift in the maximum absorption peak (SI; Figure S10).

In addition, the fluorescence intensity of cage 1 was slightly enhanced in the presence of F–, Cl–, Br–, I–, CO3 2–, SO4 2– and HPO4 2–, H2PO– anions. Among these eight anions, the most significant change in fluorescence intensity was observed upon the addition of SO4 2–. As a model for mechanistic study, further analysis was focused on the response of cage 1 toward SO4 2–. 1H NMR titration revealed that the chemical shift of cage 1 remained relatively unchanged, even after the introduction of various amounts of SO4 2– (SI; Figure S11). This strongly suggests that SO4 2– does not cause disassembly of cage 1. This fluorescence response might be caused by a replacement by SO4 2– of the NO3 – ion present in the cage, which affects the charge characteristics of the ligand on the cage, leading to a slight increase in fluorescence intensity.

To elucidate the mechanism behind the fluorescence response of cage 1 toward OH–, PO4 3–, and AcO–, a detailed mechanistic investigation was performed. Close examination of Figures [5a] and 5b reveals that the UV/vis absorption spectrum displayed a notable blue shift from 371 nm to 353 nm and an augmentation in fluorescence intensity as AcO– anions were gradually added. This increase in fluorescence intensity stabilized once the amount of AcO– reached eight equivalents (SI; Figure S12). It can be inferred from Figure [5c] that the fluorescence intensity of cage 1 exhibits a strong linear correlation with the amount of AcO– anions up to six equivalents, suggesting that cage 1 is capable of accurately quantifying the concentration of AcO– over a wide range (Figure [5c]). This observed fluorescence enhancement phenomenon might be attributed to disassembly of cage 1 into ligand 1 upon interaction with AcO– ions. Due to the effect of metal ions released by the disassembly, the fluorescence intensity from the disassembled system is slightly less than that exhibited by ligand 1 alone at the same concentration.

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Figure 5 (a, b) UV absorption and fluorescence emission spectra of cage 1 in response to various amounts of AcO–. (c) Fluorescence intensity plot at 538 nm of cage 1 (1.00 × 10–4 M) in solution with continuous addition of AcO– (0–6 equiv). (d) Change in the 1H NMR of cage 1 after the addition of various amounts of AcO–. (e) Schematic representation of the anion-response mechanism.

The disassembly behavior of cage 1 upon the addition of AcO– was confirmed by 1H NMR titration experiments. As the amount of AcO– increased, the intensity of the peak for the H(1)′ atom of cage 1 (δ = 9.48 ppm) diminished gradually, whereas the signal for H(1) of ligand 1 (δ = 8.53 ppm) gradually intensified. When the amount of AcO– reached four equivalents, the cage H(1)′ signal completely disappeared, whereas the ligand H(1) signal continued to increase in intensity and only stabilized when the amount of AcO– reached eight equivalents. As the amount of AcO– was increased from two to four equivalents, some new peaks (labeled with stars) belonging to neither ligand 1 nor cage 1 appeared (Figure [5d]). This indicates that cage 1 does not dissociate directly to ligand 1 upon the addition of AcO–, but instead forms an intermediate state before completely breaking down to ligand 1 and Pd2+ (Figure [5e]). With the addition of OH– or PO4 3–, cage 1 likewise undergoes a similar disassembly process to that observed with AcO– (SI; Figures S14–S17). In addition, the disassembly rate of cage 1 is influenced by the type of anion introduced into the system.

In conclusion, we designed and synthesized a metallacage based on a triarylamine, and we characterized its structure by various techniques, including 1H NMR, 13C NMR, COSY, DOSY, ESI-TOF MS, and X-ray single-crystal diffraction. As a result of the introduction of the anthracene moiety, the cage exhibits responsive behavior toward OH–, PO4 3–, and AcO– in terms of a fluorescence enhancement. This functionalized cage could lay a foundation for further designs of novel smart fluorescence-responsive materials.


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Conflict of Interest

The authors declare no conflict of interest.

Supporting Information

    Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/s-0042-1751486.
  • Supporting Information

Corresponding Authors

Junhai Huang
College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science
Shanghai 201620
P. R. of China   

Lianrui Hu
School of Chemistry and Molecular Engineering, East China Normal University
3663 N. Zhongshan Road, Shanghai 200062
P. R. of China   

Lin Xu
School of Chemistry and Molecular Engineering, East China Normal University
3663 N. Zhongshan Road, Shanghai 200062
P. R. of China   

Publication History

Received: 09 May 2023

Accepted after revision: 24 July 2023

Article published online:
07 September 2023

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Zoom Image
Scheme 1 Self-assembly of the metallacage cage 1 from ligand 1 and Pd2+
Zoom Image
Figure 1 (a) 1H NMR spectra of ligand 1 and cage 1 (501 MHz, DMSO-d 6, 298 K). (b, c). ESI-TOF-MS of cage 1, observed value (top) and simulated value (bottom).
Zoom Image
Figure 2 X-ray crystal structure of cage 1. (a and b) Main and top views of cage 1; (c and d) plane and three-dimensional stacked form of cage 1.
Zoom Image
Figure 3 (a) UV/vis absorption spectra of ligand 1 and cage 1. (b) Fluorescence emission spectra of ligand 1 and cage 1. (c and d). 3D spectra of ligand 1 and cage 1, respectively (ligand 1: c = 4.00 × 10–4 M; cage 1: c = 1.00× 10–4 M).
Zoom Image
Figure 4 (a, b) UV/vis absorption and fluorescence responses of ligand 1 (4.00 × 10–4 M) to 2.0 equivalents of various anions (BF4 –, PF6 –, F–, Cl–, Br–, I–, NO3 –, CO3 2–, SO4 2–, HPO4 2–, H2PO–, OH–, PO4 3–, and AcO–) in DMSO solution (λex = 425 nm). (c, d) UV/vis absorption and fluorescence responses of cage 1 (1.00 × 10–4 M) to 2.0 equivalents of various anions in DMSO solution (λex = 425 nm). (e) The fluorescence responses of cage 1 (1.00 × 10–4 M) to 8.0 equivalents of various anions in DMSO solution (λex = 425 nm). (f) The fluorescence intensity at 538 nm of cage 1 (1.00 × 10–4 M) to 8.0 equivalents of various anions in DMSO solution (λex = 425 nm).
Zoom Image
Figure 5 (a, b) UV absorption and fluorescence emission spectra of cage 1 in response to various amounts of AcO–. (c) Fluorescence intensity plot at 538 nm of cage 1 (1.00 × 10–4 M) in solution with continuous addition of AcO– (0–6 equiv). (d) Change in the 1H NMR of cage 1 after the addition of various amounts of AcO–. (e) Schematic representation of the anion-response mechanism.