English

• Calixarenes [1,2], a type of macrocycles with -CH₂- or -CHR- bridges linked at meta positions of aryls, have been prevailed in chemical community for nearly one century and in supramolecular chemistry for several decades. Replacing the bridging -CH₂- or -CHR- between aryls with other atoms such as oxygen, nitrogen, sulfur, silicon, the so called heterocalixarenes [3] or heterocalixaromatics [4] might be endowed with unique structures and fine tuning properties. Because of easy synthesis, thiacalixarenes are popular in supramolecular chemistry [5,6]. Oxa- [7,8] and aza-calixarenes [9,10], though reported several decades ago [11,12], did not invoke wide attention until synthesis breakthrough via nucleophilic aromatic substitution reaction (S_NAr) with mild reaction conditions, metal catalyst free and wide functional group tolerance by Wang [13] and Katz [14,15] in the early part of the 21^st century. Unlike their calix[4]arene counterparts with various and flexible conformations, most oxa- and aza-calix[4]arenes adopted 1,3-alternate conformations both in solution and solid state, which greatly limited their applications. For example, great achievements were obtained in the field of fluorescent sensors with calixarenes [16,17]. However, there were only limited examples based on oxa- [18–21] and aza-calix[4]arenes [22]. Considering their easy synthesis, relatively fixed but still adjustable conformations, and facile installation of signal units and binding sites, oxa- and aza-calix[4]arenes might also serve as excellent platforms for this mission after suitable modification. This field is still underdeveloped and holds great potentials.

  Usually, there are two ways of functionalization of azacalix[4]arene skeletons: functionalizing the precursors for azacalix[4]arene frameworks and post-macrocyclization derivation. Most post-macrocyclization derivation methods focused on the aromatic rings. Here we introduced a new strategy of derivation: functionalization at the bridging NH sites via formation of imidazobenzimidazole fused heterocycles. To the best of our knowledge, this is the first example of modification on the bridging NH sites.

  Benzimidazoles and the relating imidazobenzimidazoles or benzobisimidazoles, are important heterocyclic pharmacophores [23–25]. They showed binding ability toward anions [26–28], cations [29,30], and DNA minor groove [31]. Here, we fused two imidazobenzimidazole motifs into an aza-calix[4]arene skeleton to integrate four N atoms on one rim, which we wished to capture metal cations via cooperative metal-ligand coordination interactions.

  Recently, by installing four anthracenyls onto a tetra(amino)oxacalix[4]arene framework with 1,3-alternate conformation via formation of four imine bonds to form a coordination pocket (Scheme 1a), we developed a colorimetric and "turn-on" fluorometric sensor for Cu²⁺ with high selectivity and strong anti-interference ability [32]. Similarly, we wished to functionalize a tetra(amino)azacalix[4]arene backbone via an analogous condensation reaction with anthracene-9-carbaldehyde to investigate the effect of different bridging atoms (Scheme 1b). Much to our surprise, we isolated an imidazobenzimidazole fused aza-calix[4]arene (Scheme 2), rather than the expected imine bonds linked product. The bridging NHs were involved in the reaction, which was confirmed via X-ray single crystal analysis (vide infra). With this finding in hand, we then attached pyrenyl fluorophores to this skeleton, which we wished to explore its cation selective sensing ability using fluorescence (FL) method. Due to its easy formation of an excimer emission band [33,34], pyrenyl was selected as a reporting unit for our fluorescent sensor.

  Scheme 1

  

  Scheme 1.  (a) Synthesis of anthracenyl-modified oxa-calix[4]arene and illustration of Cu²⁺ sensing; (b) hypothesized anthracenyl-modified azacalix[4]arene.

  DownLoad: Full-Size Img PowerPoint

  Scheme 2

  

  Scheme 2.  Synthetic routes for the imidazobenzimidazole fused aza-calix[4]arenes 7, 9, 11, and control compound 13.

  DownLoad: Full-Size Img PowerPoint

  The tetra(nitro)azacalix[4]arene skeleton was previously synthesized by Siri group via a fragment "3 + 1" strategy [35,36]. We tried a more efficient one-pot method. As shown in Scheme 2, N¹, N³-diisobutyl-4,6-dinitrobenzene-1,3-diamine 1 was first reduced into N¹, N⁵-diisobutylbenzene-1,2,4,5-tetraamine 2 under catalytic hydrogenation conditions, which was used without further purification and characterization. Then four folds of S_NAr reactions between equimolar amounts of compound 2 and 1,5-difluoro-2,4-dinitrobenzene 3 in refluxing THF using K₂CO₃ as a base afforded the symmetrically substituted aza-calix[4]arene 4 in 30% yield in a single step via a "2 + 2" manner. Only the primary amines were involved in this reaction and the secondary amines remained unchanged. The nitro groups on aza-calix[4]arene 4 were subsequently reduced again in the presence of Zn in CH₂Cl₂ and AcOH and the product was used without separation. Finally, the tetra(amino)azacalix[4]arene 5 condensed with four equivalents of anthracene-9-carbaldehyde 6 and then dehydroarylated using air as an oxidation reagent at 60 ℃, affording the imidazobenzimidazole [25,37] fused aza-calix[4]arene 7 in 21% yield. This method proceeded smoothly also for benzaldehyde 8 and pyrene-1-carbaldehyde 10 to provide 9 (48%) and 11 (13%) over two steps, respectively. The relatively low yield might result from the strong oxidation propensity of the amines on 5. Thus, four pyrenyl fluorophores were successfully attached to the aza-calix[4]arene skeleton at the bridging positions. The structures of the imidazobenzimidazole fused aza-calix[4]arenes 7, 9, and 11 were characterized thoroughly via ¹H NMR, ¹³C NMR, high resolution mass spectroscopy, and X-ray single crystal analysis (only for 7 and 11, vide infra). Pyrenyl modified benzimidazole 13 [27] was also synthesized from the condensation reaction of pyrene-1-carbaldehyde 10 and o-benzenediamine 12 in refluxing C₂H₅OH, which was used as a control compound for cation sensing studies.

  A single crystal of 7 (CCDC: 2209686) suitable for X-ray analysis was obtained by slow evaporation of a CHCl₃/CH₃CN = 5:1 solution. As shown in Fig. 1, a distorted 1,3-alternate conformation is observed, which is common for most oxa- [7,8] and aza-calix[4]arenes [9,10]. The two benzene rings with isobutylamino groups are almost parallel with a small dihedral angle of 11.85° and a centroid-centroid distance of 5.788 Å, while the two imidazobenzimidazole rings diverge with a large dihedral angle of 115.80° and a centroid-centroid distance of 6.725 Å. The two anthracenyls attached to the same imidazobenzimidazole are almost coplanar and vertical to the imidazobenzimidazole plane. An interesting packing mode was observed. They existed as dimers via off-set π-π stacking interactions (Fig. S13 in Supporting information) between imidazobenzimidazole rings (centroid-centroid distance: 3.541 Å) and anthracenyls (centroid-centroid distance: 5.500 Å and inter anthracenyl distance: about 3.4 Å). Each dimer further interacts with other four dimers via off-set π-π stacking interactions between the pendent anthracenyls (Fig. S14 in Supporting information).

  Figure 1

  

  Figure 1.  X-ray single crystal structure of anthracenyl modified imidazobenzimidazole fused aza-calix[4]arene 7: (a) side view; (b) top view. Hydrogen atoms are omitted for clarity.

  DownLoad: Full-Size Img PowerPoint

  A 1,3-alternate conformation was also found for the single crystal of 11 (CCDC: 2209690), which was obtained via slow evaporation of a THF/C₂H₅OH = 3:1 solution (Fig. 2a). The dihedral angles between benzene rings and imidazobenzimidazole rings are 38.98° and 124.63°, respectively. The centroid-centroid distances are 6.226 Å and 6.794 Å, respectively. The conformation is not so distorted as that for 7. Two pyrenyls attached to one imidazobenzimidazole ring are almost on the same plane (with a dihedral angle of 8.54°) and perpendicular to the imidazobenzimidazole ring, while the other two pyrenyls on the other imidazobenzimidazole ring are a little twisted with a dihedral angle of 24.48° and extend toward opposite directions. 11 also existed as dimers in the solid state. But the intermolecular interactions are different. Three folds of off-set π-π stacking interactions between pyrenyls and imidazobenzimidazole rings, together with four folds edge-to-face T-shape π-π interactions [38,39] or CH-π hydrogen bonding [40] between pyrenyls (with centroid-centroid distances of 6.727 Å and 6.458 Å) account for the dimerization (Fig. 2b). Different to 7, further packing of the dimers of 11 was dominated mainly by two folds of edge-to-face T-shape π-π interactions, with centroid-centroid distances of 6.794 Å and 7.616 Å, respectively (Fig. S16 in Supporting information).

  Figure 2

  

  Figure 2.  X-ray single crystal structure of pyrenyl modified imidazobenzimidazole fused aza-calix[4]arene 11: (a) side view; (b) illustration of intermolecular interactions in a dimer. Hydrogen atoms are omitted for clarity.

  DownLoad: Full-Size Img PowerPoint

  We further tested the cation sensing ability of the fluorophores modified 7 and 11. The cations were screened via fluorescence spectroscopy. When excited with λ_ex = 393 nm, more or less changes in fluorescence emission spectra were observed for the anthracenyl modified 7 (Fig. S17 in Supporting information) upon addition of different cations: it showed little selectivity. Upon mixing 11 with Zn(ClO₄)₂ in CHCl₃, the solution changed from colourless to light yellow. However, addition of ClO₄⁻ salts of other cations such as Al³⁺, Ca²⁺, Co²⁺, Cu²⁺, Fe²⁺, Hg²⁺, Li⁺, Mg²⁺, Mn²⁺, Cd²⁺, and Ni²⁺ led to negligible changes (Fig. S18a in Supporting information). UV-vis measurements (Fig. 3a) further revealed that new absorption bands at λ = 306 nm and λ = 420 nm appeared upon addition of 10 equiv. of Zn²⁺. Addition of the above mentioned other cations only led to minor changes in the absorption bands at λ = 275 nm and λ = 361 nm. UV-vis titration studies (Fig. 3b) indicated that the absorption bands at λ = 275 nm and λ = 361 nm decreased and two new absorption bands at λ = 306 nm and λ = 420 nm evolved with gradual addition of Zn²⁺. The titration curves showed two clear isosbestic points at λ = 322 nm and λ = 396 nm, indicating formation of a well-defined 11–Zn²⁺ complex. Job's plot via UV-vis measurements by fixing the total concentration of 11 and Zn²⁺ at 1 × 10⁻⁵ mol/L revealed a 1:1 stoichiometry for the complex 11–Zn²⁺ (Figs. S19 and S20 in Supporting information) [41]. Nonlinear fitting the above UV-vis titration data at λ = 430 nm into a 1:1 binding mode yielded an association constant K of 1.1 × 10⁵ L/mol (Fig. 3c) for the complex 11–Zn²⁺ [42]. In the concentration range from 1 × 10⁻⁶ mol/L to 1 × 10⁻⁵ mol/L, a good linear relationship (Fig. 3c, R² = 0.988) between ΔA and the concentration of Zn²⁺ was found.

  Figure 3

  

  Figure 3.  (a) UV–vis absorption spectra of tetrapyrenyl modified compound 11 (1 × 10⁻⁵ mol/L in CHCl₃) upon addition of various metal ions (10 equiv.); (b) UV-vis titration curves of 11 (fixed at 1 × 10⁻⁵ mol/L in CHCl₃) with 0~10 equiv. of Zn²⁺ (bulk solution: 2 × 10⁻³ mol/L in CH₃CN), 298 K; (c) nonlinear fitting (red curve) of the absorption data at λ = 430 nm into a 1:1 binding mode to determine the association constant K_a for complex 11–Zn²⁺. The insets show the linear fitting result in the concentration range from 1 × 10⁻⁶ mol/L to 1 × 10⁻⁵ mol/L (blue line).

  DownLoad: Full-Size Img PowerPoint

  The selective sensing ability of 11 toward Zn²⁺ was further investigated via FL measurements. 11 itself in CHCl₃ showed weak yellow fluorescence under λ = 365 nm UV lamp. With addition of 10 equiv. of Zn²⁺, a bright yellow fluorescence was observed (Fig. S18b). However, addition of other cations did not lead to apparent changes. FL measurements revealed a remarkably strong emission band at λ = 522 nm and a shoulder band at λ = 500 nm upon addition of Zn²⁺ (Fig. 4a) when excited with λ_ex = 381 nm. 11 itself showed a modest emission band at λ = 481 nm and a shoulder band at λ = 520 nm. Addition of other cations led to partial quenching of the fluorescence. Titration curves (Fig. 4b) further revealed that the fluorescence was quenched with gradual addition of the first 1.5 equiv. of Zn²⁺. With further addition of Zn²⁺, the emission band at λ = 522 nm evolved. The limit of detection (LOD) was calculated to be 2.3 × 10⁻⁶ mol/L (Fig. S21 in Supporting information).

  Figure 4

  

  Figure 4.  (a) FL emission spectra of pyrenyl modified compound 11 (1 × 10⁻⁵ mol/L in CHCl₃) upon addition of various metal ions (10 equiv.), λ_ex = 381 nm; (b) FL titration curves of 11 (fixed at 1 × 10⁻⁵ mol/L in CHCl₃) with 0~10 equiv. of Zn²⁺ (bulk solution: 2 × 10⁻³ mol/L in CH₃CN), 298 K. The insert shows FL intensity changes at λ = 522 nm with addition of 0~10 equiv. of Zn²⁺.

  DownLoad: Full-Size Img PowerPoint

  The selective sensing of Zn²⁺ showed strong anti-interference in the presence of other cations. Whatever addition of 10 equiv. of Zn²⁺ to a solution of compound 11 and other cations in CHCl₃, or addition of other cations to a solution of complex 11 and Zn²⁺, similar changes in solution colour, UV-vis (Fig. S22 in Supporting information) and FL (Fig. S23 in Supporting information) spectra were observed as if other metal ions did not exist.

  The sensing process is completely reversible as exemplified by alternate addition of Zn²⁺ and F⁻. Addition of two equivalents of F⁻ to a solution of complex 11–Zn²⁺ could restore the UV-vis absorption almost completely to the initial state and the solution colour faded. This process could be circulated several times by continuous alternate additions of Zn²⁺ and F⁻ (Figs. S24 and S25 in Supporting information).

  Our sensor 11 is almost insoluble in water. Following a simple protocol developed by Cragg et al. [43], we showed that 11 could, in principle, be used to analyse Zn²⁺ in aqueous samples. A mixture of our sensor 11 and 2~10 equiv. of Zn(ClO₄)₂ in 3 mL 1:1 CHCl₃-H₂O in a sealed tube was heated to reflux for 24 h. After evaporation of the solvent under reduced pressure, the residue was re-dissolved in 1:1 CHCl₃-CH₃CN and subjected to fluorescence measurements. A good linear relationship between the FL intensity at λ = 522 nm and the concentration of Zn²⁺ (Fig. S29 and Fig. S30 in Supporting information) was revealed.

  However, with our control compound 13, addition of all the above mentioned cations caused no apparent colour change (Fig. S26 in Supporting information), minor changes in UV-vis (Fig. S27 in Supporting information) and FL spectra (Fig. S28 in Supporting information). The high Zn²⁺-selective sensing ability of 11 might come from cooperative action of four pre-organized N atoms on one rim, which captured the Zn²⁺ with an appropriate diameter. The conformation flexibility of the host itself might also account for the selective sensing process. Upon coordination with Zn²⁺, the pyrenyls on different imidazobenzimidazole heterocycles were drawn close and oriented in a face-to-face manner, which might be the origin of the excimer emission.

  In summary, imidazobenzimidazole fused aza-calix[4]arenes were synthesized by condensation reactions of a tetra(amino)azacalix[4]arene and various aromatic aldehydes. For the first time, the bridging NH sites were involved in the functionalization of the aza-calix[4]arene framework. Various aromatic motifs including anthracenyl and pyrenyl fluorophores could be introduced. Distorted 1,3-alternate conformations were observed for the imidazobenzimidazole fused aza-calix[4]arenes in the solid state. They existed as unique dimers via multi-fold of π-π stacking interactions between planar fluorophores. The pyrenyl modified product could be used as a colorimetric and "off-on" fluorometric sensor for Zn²⁺. Job's plot via UV-vis measurements revealed a 1:1 stoichiometry and an association constant of 1.1 × 10⁵ L/mol was determined. The selective sensing showed strong anti-interference in the presence of various other cations. The sensing process is completely reversible as proved via alternate addition of Zn²⁺ and F⁻. Aza-calix[4]arene skeleton with relatively fixed but still adjustable conformation proved itself to be a good platform for fabricating fluorescent sensors.

  Declaration of competing interest

  The 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.

  Acknowledgment

  This work is supported by National Natural Science Foundation of China (Nos. 21971223 and 21772178).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108294.

  
  
• 1. [1]

     C.D. Gutsche, Calixarenes: An Introduction, 2nd Ed., RSC Pub., Cambridge, 2008.

  2. [2]

     P. Neri, J.L. Sessler, M.X. Wang, Calixarenes and Beyond, Springer International Publishing, 2016.

  3. [3]

     B. König, M.H. Fonseca, Eur. J. Inorg. Chem. (2000) 2303–2310.

  4. [4]

     M.X. Wang, Acc. Chem. Res. 45 (2012) 182–195. doi: 10.1021/ar200108c

  5. [5]

     N. Morohashi, F. Narumi, N. Iki, T. Hattori, S. Miyano, Chem. Rev. 106 (2006) 5291–5316. doi: 10.1021/cr050565j

  6. [6]

     R. Kumar, Y.O. Lee, V. Bhalla, M. Kumar, J.S. Kim, Chem. Soc. Rev. 43 (2014) 4824–4870. doi: 10.1039/c4cs00068d

  7. [7]

     W. Maes, W. Dehaen, Chem. Soc. Rev. 37 (2008) 2393–2402. doi: 10.1039/b718356a

  8. [8]

     R. Hudson, J.L. Katz, Oxacalixarenes, in: P. Neri, J.L. Sessler, M.X. Wang (Eds.), Calixarenes and Beyond, Springer International Publishing, 2016, pp. 399–420.

  9. [9]

     D.X. Wang, M.X. Wang, Azacalixaromatics, in: P. Neri, J.L. Sessler, M.X. Wang (Eds.), Calixarenes and Beyond, Springer International Publishing, 2016, pp. 363–398.

  10. [10]

      H. Tsue, K. Ishibashi, R. Tamura, Top. Heterocycl. Chem. 17 (2008) 73–96.

  11. [11]

      N. Sommer, H.A. Staab, Tetrahedron Lett. 7 (1966) 2837–2841. doi: 10.1016/S0040-4039(01)99870-3

  12. [12]

      G.W. Smith, Nature 198 (1963) 879. doi: 10.1038/198879a0

  13. [13]

      M.X. Wang, H.B. Yang, J. Am. Chem. Soc. 126 (2004) 15412–15422. doi: 10.1021/ja0465092

  14. [14]

      J.L. Katz, M.B. Feldman, R.R. Conry, Org. Lett. 7 (2005) 91–94. doi: 10.1021/ol047840t

  15. [15]

      J.L. Katz, K.J. Selby, R.R. Conry, Org. Lett. 7 (2005) 3505–3507. doi: 10.1021/ol051180q

  16. [16]

      N. Kumar, P.X. Qui, I. Leray Roopa, M.H. Ha-Thi, Calixarene-based fluorescent molecular sensors, in: J.L. Atwood (Ed.), Comprehensive Supramolecular Chemistry Ⅱ, Elsevier B.V., 2017, pp. 197–226.

  17. [17]

      R. Kumar, A. Sharma, H. Singh, et al., Chem. Rev. 119 (2019) 9657–9721. doi: 10.1021/acs.chemrev.8b00605

  18. [18]

      A. Peterson, M.L. Ludvig, J. Martonova, et al., Supramol. Chem. 32 (2020) 313–319. doi: 10.1080/10610278.2019.1659269

  19. [19]

      V. Mehta, M. Athar, P.C. Jha, et al., New J. Chem. 41 (2017) 5125–5132. doi: 10.1039/C7NJ01111C

  20. [20]

      H.X. Wang, Z. Meng, J.F. Xiang, et al., Chem. Sci. 7 (2016) 469–474. doi: 10.1039/C5SC03511B

  21. [21]

      M. Panchal, M. Athar, P.C. Jha, et al., RSC Adv. 6 (2016) 53573–53577. doi: 10.1039/C6RA05707A

  22. [22]

      G. Canard, J.A. Edzang, Z. Chen, et al., Chem. Eur. J. 22 (2016) 5756–5766. doi: 10.1002/chem.201505089

  23. [23]

      Y. Bansal, O. Silakari, Bioorg. Med. Chem. 20 (2012) 6208–6236. doi: 10.1016/j.bmc.2012.09.013

  24. [24]

      S. Choudhary, M. Arora, H. Verma, M. Kumar, O. Silakari, Eur. J. Pharmacol. 899 (2021) 174027. doi: 10.1016/j.ejphar.2021.174027

  25. [25]

      M. Sweeney, D. Conboy, S.I. Mirallai, F. Aldabbagh, Molecules 26 (2021) 2684. doi: 10.3390/molecules26092684

  26. [26]

      Z. Xu, N.J. Singh, S.K. Kim, et al., Chem. Eur. J. 17 (2011) 1163–1170. doi: 10.1002/chem.201002105

  27. [27]

      A. Kushwaha, S.K. Patil, D. Das, New J. Chem. 42 (2018) 9200–9208. doi: 10.1039/C8NJ01011K

  28. [28]

      A.L. Brazeau, K. Yuan, S.B. Ko, I. Wyman, S. Wang, ACS Omega 2 (2017) 8625–8632. doi: 10.1021/acsomega.7b01631

  29. [29]

      N. Kaur, G. Dhaka, J. Singh, New J. Chem. 39 (2015) 6125–6129. doi: 10.1039/C5NJ00683J

  30. [30]

      M. Barwiolek, A. Wojtczak, A. Kozakiewicz, et al., New J. Chem. 42 (2018) 18559–18568. doi: 10.1039/C8NJ03801E

  31. [31]

      P. Guo, A.A. Farahat, A. Paul, D.W. Boykin, W.D. Wilson, Chem. Sci. 12 (2021) 15849–15861. doi: 10.1039/D1SC04720E

  32. [32]

      S.G. Wang, Y. Pang, M. Xue, Y. Yang, New J. Chem. 45 (2021) 19219–19223. doi: 10.1039/D1NJ04034K

  33. [33]

      J.B. Birks, Rep. Prog. Phys. 38 (1975) 903–974. doi: 10.1088/0034-4885/38/8/001

  34. [34]

      F.M. Winnik, Chem. Rev. 93 (1993) 587–614. doi: 10.1021/cr00018a001

  35. [35]

      L. Lavaud, S. Pascal, K. Metwally, et al., Chem. Commun. 54 (2018) 12365–12368. doi: 10.1039/C8CC05851B

  36. [36]

      Z. Chen, R. Haddoub, J. Mahe, et al., Chem. Eur. J. 22 (2016) 17820–17832. doi: 10.1002/chem.201602288

  37. [37]

      A.A. Farahat, S. Iwamoto, M. Roche, D.W. Boykin, J. Heterocycl. Chem. 58 (2021) 2280–2286. doi: 10.1002/jhet.4353

  38. [38]

      C.A. Hunter, J.K.M. Sanders, J. Am. Chem. Soc. 112 (1990) 5525–5534. doi: 10.1021/ja00170a016

  39. [39]

      R. Thakuria, N.K. Nath, B.K. Saha, Cryst. Growth Des. 19 (2019) 523–528. doi: 10.1021/acs.cgd.8b01630

  40. [40]

      M. Nishio, CrystEngComm 6 (2004) 130. doi: 10.1039/b313104a

  41. [41]

      J. Polster, H. Lachmann, Spectrometric Titrations: Analysis of Chemical Equilibria, Wiley-VCH, 1989.

  42. [42]

      P. Thordarson, Chem. Soc. Rev. 40 (2011) 1305–1323. doi: 10.1039/C0CS00062K

  43. [43]

      E. Manandhar, P.J. Cragg, K.J. Wallace, Supramol. Chem. 26 (2014) 141–150. doi: 10.1080/10610278.2013.835050

• 

  Scheme 1  (a) Synthesis of anthracenyl-modified oxa-calix[4]arene and illustration of Cu²⁺ sensing; (b) hypothesized anthracenyl-modified azacalix[4]arene.

  下载: 全尺寸图片 幻灯片
  

  Scheme 2  Synthetic routes for the imidazobenzimidazole fused aza-calix[4]arenes 7, 9, 11, and control compound 13.

  下载: 全尺寸图片 幻灯片
  

  Figure 1  X-ray single crystal structure of anthracenyl modified imidazobenzimidazole fused aza-calix[4]arene 7: (a) side view; (b) top view. Hydrogen atoms are omitted for clarity.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  X-ray single crystal structure of pyrenyl modified imidazobenzimidazole fused aza-calix[4]arene 11: (a) side view; (b) illustration of intermolecular interactions in a dimer. Hydrogen atoms are omitted for clarity.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  (a) UV–vis absorption spectra of tetrapyrenyl modified compound 11 (1 × 10⁻⁵ mol/L in CHCl₃) upon addition of various metal ions (10 equiv.); (b) UV-vis titration curves of 11 (fixed at 1 × 10⁻⁵ mol/L in CHCl₃) with 0~10 equiv. of Zn²⁺ (bulk solution: 2 × 10⁻³ mol/L in CH₃CN), 298 K; (c) nonlinear fitting (red curve) of the absorption data at λ = 430 nm into a 1:1 binding mode to determine the association constant K_a for complex 11–Zn²⁺. The insets show the linear fitting result in the concentration range from 1 × 10⁻⁶ mol/L to 1 × 10⁻⁵ mol/L (blue line).

  下载: 全尺寸图片 幻灯片
  

  Figure 4  (a) FL emission spectra of pyrenyl modified compound 11 (1 × 10⁻⁵ mol/L in CHCl₃) upon addition of various metal ions (10 equiv.), λ_ex = 381 nm; (b) FL titration curves of 11 (fixed at 1 × 10⁻⁵ mol/L in CHCl₃) with 0~10 equiv. of Zn²⁺ (bulk solution: 2 × 10⁻³ mol/L in CH₃CN), 298 K. The insert shows FL intensity changes at λ = 522 nm with addition of 0~10 equiv. of Zn²⁺.

  下载: 全尺寸图片 幻灯片
•