Synthesis, Optoelectronic, and Supramolecular Properties of a Calix[4

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

Synthesis, Optoelectronic, and Supramolecular Properties of a Calix[4]arene−Cycloparaphenylene Hybrid Host Paolo Della Sala, Carmen Talotta, Amedeo Capobianco, Annunziata Soriente, Margherita De Rosa, Placido Neri, and Carmine Gaeta* Dipartimento di Chimica e Biologia “A. Zambelli”, Università di Salerno, Via Giovanni Paolo II 132, I-84084 Fisciano, Salerno, Italy

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S Supporting Information *

ABSTRACT: A novel hybrid host has been obtained by fusion of the calix[4]arene skeleton with a cycloparaphenylene (CPP) ring. The CPP-bridged calix[4]arene 1 combines the optoelectronic and structural properties of the CPP rings with the recognition abilities of the calix[4]arene hosts. Thus, calix-CPP 1 shows an unexpected selectivity for the Li+ cation over Na+, as a result of more favorable cation···π interactions of Li+ with the CPP bridge and its better size complementarity.

T

oday, calixarenes1 are considered a useful platform for designing macrocyclic hosts with novel and intriguing supramolecular properties.2 Among the calixarene hosts, a central role has been played by hybrid systems which are obtained by fusion of the calixarene skeleton with portions of different macrocyclic hosts. Often, they have provided true benchmarks in supramolecular chemistry; see, for instance, calix-crowns,3 calix-spherands,4 calixpyrrole-resorcinarene.5 Generally, the hybrid system combines the recognition features of both the precursor hosts. As a recent example, Sessler and coworkers have reported6 an ion-pair host derived by combination of a calix[4]pyrrole as the anion receptor and a calix[4]arene as the cation recognition site. Carbon nanorings7 are a class of macrocyclic compounds constituted by arene rings linked by single bonds. Among them, the [n]cycloparaphenylene8 ([n]CPP, Figure 1) macrocycles

absorbance, which increases with the number of phenylene rings,8 whereas their emission is significantly red-shifted as the number of benzene rings decreases. Finally, the quantum efficiency of [n]CPPs also decreases as n decreases.8 The size-tunable optoelectronic properties of [n]CPPs make them attractive for applications in organic electronics and optics.9 The optoelectronic properties are not the only factor that prompts chemists to study [n]CPP macrocycles. In fact, [n]CPPs have also shown interesting recognition properties. Thanks to their π-rich inner cavity, CPPs show affinity for πpoor guests, such as C6010 and pyridinium salts.11 Prompted by these considerations, we have designed the hybrid system 1 (Scheme 1), in which the calix[4]arene Scheme 1. Synthesis of Calix[4]-CPP 1

Figure 1. Most common members of the CPPs (left) and calixarene families.

macrocycle is combined with a curved cycloparaphenylene unit (calix[4]-CPP). We have envisioned that in this way the optoelectronic properties of CPPs could be combined with the recognition abilities of the calixarene moiety. Normally, the synthesis of CPP is obtained by macrocyclization of precursors that incorporate rigidly curved

exhibit size-dependent optoelectronic properties.8 In particular, in the [5−12]CPP series, a narrowing of the HOMO−LUMO gap is observed as the number of the aromatic unit decreases, in sharp contrast with respect to linear [n]parapolyphenylenes in which the HOMO−LUMO gap is narrowed as the number of the aromatic unit increases.8 [n]CPPs possess a high optical © XXXX American Chemical Society

Received: October 1, 2018

A

DOI: 10.1021/acs.orglett.8b03134 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters elements.12,13 In fact, CPP macrocycles are highly curved nanohoops which suffer from the high strain of the macrocycle12 that prevents the direct macrocyclization from linear paraphenylenes.12 Thus, on the basis of these considerations, we have planned the synthesis of 1 reported in Scheme 1. The key step is the Suzuki−Miyaura cross-coupling/macrocyclization12b between the curved element 3, incorporating a cyclohexa-2,5-diene moiety as a masked benzene ring,12b and bis[(p-bromobenzyloxy)]-calix[4]arene derivative 2 (Supporting Information (SI)). When 2 and 3 were reacted in the presence of Pd(OAc)2, S-Phos, and K3PO4 in DMF/H2O, the macrocycle 4 was isolated in 5.8%14 yield after column chromatography. The HR MALDIFT-ICR mass spectrum of 4 (SI) showed a molecular ion peak [M + Na]+ at m/z 1167.6645 (calcd for C80H88NaO6, 1167.6473). The 1D and 2D NMR spectra of 4 confirm the structural assignment (SI). The reduction of 4, with SnCl2 in the presence of HCl and THF as the solvent, at room temperature for 7 h, gave 1 in 14% yield.14 The HR MALDI-FT-ICR mass spectrum of 1 (SI) was obtained in the presence of the cationizing agent NaTFA and showed a molecular ion peak [M + Na]+ at m/z 1105.6147 (calcd for C78H82NaO4, 1105.6105). In order to confirm the structural assignment of 1, we performed a MALDI-CID MS/MS experiment. The CID mass spectrum of sodiated 1 ([1 + Na]+) is shown in Figure 2. The fragmentation

Figure 3. Methylene regions of the 1H NMR spectra (600 MHz, CDCl3) of 1 at (a) 253 K or (b) 298 K. (c) Cone paco interconversion due to the OMe through the annulus passage in 1. (d) Normalized UV/ vis absorption (red) and emission (blue) spectra of 1 in CH2Cl2 (λexc = 300 nm).

observed at 393 K, indicative of a fast OMe through-the-annulus passage. An energy barrier of 14.3 kcal/mol was estimated for this conformational process.15 Finally, the 1H NMR spectrum of 1 at 393 K (TCDE, 300 MHz) showed an ArCH2Ar AX system at 4.05/3.21 ppm, an OMe singlet at 3.34 ppm, and an OCH2 singlet at 4.66 ppm, in accordance with the symmetry of its structure (SI). The paco and cone structures of 1 were optimized at the B3LYP/6-31G(d) level of theory (Figure 4 and SI), including

Figure 2. HR MALDI-FT-ICR-CID mass spectrum of 1 in the presence of cationizing agent NaTFA (bottom) and LiTFA (top). Figure 4. DFT-optimized structures of 1 in (left) cone conformation; (middle) detail of the benzene ring A; (right) partial cone conformation.

behavior is in accordance with the loss of the CPP bridge due to a homolytic dissociation of the two OCH2−CPP bonds. In summary, the collisional activation of sodiated 1 (Figure 2) exclusively generates tBuC[4](OMe)2(O•)2@Na+ at m/z 697.421760 (calcd for C46H58NaO4, 697.422731; accuracy = 1.40 ppm). An anagous behavior was observed in the presence of LiTFA (Figure 2, top). The 1H NMR spectrum of 1 in CDCl3 at 298 K shows broad ArCH2Ar signals, indicative of a conformational mobility of the two anisole rings (Figure 3), due to the OMe through-theannulus passage (Figure 3c). By decreasing the temperature to 253 K, the ArCH2Ar signals decoalesced into two AX systems attributable to two conformations of 1, namely, the cone and partial cone ones (Figure 3a; see SI), frozen out in the NMR time scale and assigned by 1D and 2D NMR studies (see SI). The integration of 1H NMR signals reveals that the cone and partial cone (paco) conformation of 1 are present in a 46/54 ratio. By increasing the temperature, the 1H NMR spectrum of 1 (TCDE 300 MHz) shows a coalescence for the ArCH2Ar signals at 323 K (SI), whereas a complete sharpening of all signals was

solvent (dichloromethane) effects, which were modeled through the polarizable continuum model. B3LYP computations predict the partial cone to be more stable than the cone conformation by less than 0.4 kcal/mol. Thus, cone and paco can be considered nearly isoenergetic conformations, within computational accuracy, in line with the NMR results. A close inspection of the optimized paco structure of 1 (Figure 4) reveals the presence of stabilizing C−H···π interactions16 between the tert-butyl group of the inverted anisole ring with the aromatic rings of the CPP bridge (SI). Additionally, a C−H···π interaction (C−H···πcentroid distance 3.28 Å)16 was detected between a methoxy group and the distal inverted aromatic ring of 1. The analysis of the DFT structure of 1 reveals that the CPP bridge in 1 presents a strain not uniformly distributed over the phenylene chain but mainly located on the benzene ring A (Figure 4). The CPP strain is also transmitted to the connected calixarene Ar rings which are consequently forced to be almost B

DOI: 10.1021/acs.orglett.8b03134 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters parallel in both cone and paco conformers (Figure 4). The absorption spectrum of 1 (Figure 3d) is characterized by an intense and broad band, extending for ca. 1.5 eV and peaking at 310 nm (ε = 35000 M−1 cm−1). The absorption maximum is blue-shifted with respect to the case of unsubstituted [n]CPPs,17a whose lowest energy absorption is usually found at ≈340 nm,17a but significantly red-shifted if compared to the absorption maximum of 273 nm 17b of the linear pquinquephenyl.17b This is largely expected, given the disruption of conjugation caused by the insertion of the aliphatic units of calixarene into the nanoring of 1.18 According to timedependent (TD) DFT computations (SI), the absorption band results from three optical transitions falling at 346, 285, and 275 nm (Table S1), with oscillator strengths amounting to 0.35, 1.04, and 0.58, respectively, for the paco conformer. Differently from regular [n]CPPs where the S1 ← S0 transition is forbidden, the HOMO → LUMO excitation is optically allowed in 1 due to the loss of symmetry. The most intense transition (predicted at 285 nm) consists of HOMO → LUMO+1 and HOMO−4 → LUMO excitations (Figure 5 and Table S1). The

Figure 6. Fluorescence spectra of 1 (5.4 μM) upon addition of PF6− salts of Li+ (left) and Na+ (right) (0.1−3.0 equiv) in CHCl3/CH3CN (9:1, v/v).

in close contact with the oxygen atoms. A sharpening of the ArCH2Ar 1H NMR signals (SI) of 1 was observed, upon addition of Li+ or Na+, as PF6− salts, indicative of the blockage of the cone and paco conformations, with respect to the NMR time scale (600 MHz, 298 K), when the sodium or lithium cation was complexed. The binding constants for the formation of the complexes Na+⊂1 and Li+⊂1, in a CHCl3/CH3CN (9:1, v/v) mixture, were determined by curve fitting analysis of the fluorescence data (SI and insets in Figure 6). Calix[4]-CPP 1 shows a greater affinity for Li+ cation with a Li+/Na+ selectivity ratio of about 5 (KNa+⊂1 = 4.7 ± 0.2 × 104 Μ−1 and KLi+⊂1 = 2.50 ± 0.07 × 105 Μ−1). Thus, differently by calix-crown and calixspherand hybrid systems, which show a greater affinity for Na+ cation over Li+,4 in the case of calix[4]-CPP 1, this selectivity is reversed. Naturally, the cation···π interaction between the Li+/ Na+ cation and the benzene units of the CPP bridge in 1 could play a crucial role in the stabilization of the respective complexes. It is well-known that in the gas phase20 the energy binding of Li+ to benzene is 38 kcal/mol,20 significantly higher than that experienced by Na+ cation of 28 kcal/mol.20 Consequently, in accordance with the literature data, we could explain the greater affinity of 1 for the Li+ cation on the basis of a more favorable interaction between the Li+ cation with the benzene rings of the CPP bridge. Interestingly, when the larger K+ cation was used, no hint of interaction between the calix[4]-CPP 1 and K+ (as KPF6) was detected by fluorescence titration. Thus, the stability of the M+⊂1 complexes follow the order Li+ > Na+, whereas K+ is not complexed. These results suggest also that the size complementarity between host 1 and cationic guest plays a crucial role in the stability of the complexes. Probably, the small size of the CPP bridge at the lower rim of 1 does not allow the access of larger cations into its binding site. In conclusion, we have here introduced a novel class of hybrid hosts in which the calixarene skeleton is fused with a cycloparaphenylene ring. Our studies show that the CPPbridged calix[4]arene 1 combines perfectly the recognition abilities of the calix-skeleton (e.g., metal cation recognition) with the optoelectronic and structural properties of the cycloparaphenylene derivatives. In fact, calix-CPP 1 shows an unusual Li+ selectivity due probably to a more favorable cation···π interaction between the cation and the benzene rings of the CPP bridge and to a better size complementarity between 1 and the smaller cation. We do believe that further examples of calix-CPP hybrids will provide novel interesting supramolecular properties.

Figure 5. Isodensity surface plots of the frontier Kohn−Sham orbitals of the paco conformer of 1.

latter gives rise to a weak charge transfer from the calixarene aromatic rings to the CPP moiety (see Figure 5). Finally, an additional transition is predicted to occur at 275 nm, mainly HOMO → LUMO+2 in character (Figure 5 and Table S1), consistent with the shoulder observed at 280 nm. Identical conclusions are found for the cone conformer (see Table S1 and Figure S25). The fluorescence spectrum of 1 (Figure 3d) exhibits an intense peak at 382 nm, blue-shifted with respect to native [n]CPPs,17a but is red-shifted with respect to linear pquinquephenyl (344 nm).17b The quantum yield of 1 (Φ = 0.26, measured using anthracene as standard) is significantly lower than that reported for linear p-quinquephenyls (Φ ≈ 0.89)17b but far higher than that of [5]CPP, which indeed does not exhibit appreciable fluorescence.19 Similar to absorption, in 1, emission is also facilitated by the loss of symmetry, which makes S0 ← S1 a bright transition, thus making the optical properties of 1 similar to those of alkyl-tethered CPPs.19 The recognition and sensing properties of 1 toward Li+ and Na+ cations were investigated by fluorescence titration. Upon titration with NaPF6 and LiPF6, in CHCl3/CH3CN (9:1, v/v), the fluorescence intensity of 1 peaked at 382 nm and progressively decreased (Figure 6 and SI), whereas no change was observed in the UV−vis absorption spectrum. The HR MALDI-FT-ICR mass spectra of 1 in Figure 2 confirmed a 1:1 stoichiometry for both the complexes Na+⊂1 and Li+⊂1. 1D and 2D NMR (SI) studies clearly indicate that calix[4]-CPP 1 binds the cation M+ (Li+ or Na+) at the lower rim

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03134. C

DOI: 10.1021/acs.orglett.8b03134 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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(18) Li, P.; Sisto, T. J.; Darzi, E. R.; Jasti, R. Org. Lett. 2014, 16, 182− 185. (19) Kayahara, E.; Patel, V. K.; Yamago, S. J. Am. Chem. Soc. 2014, 136, 2284−2287. (20) For a recent review, see: Dougherty, D. D. Acc. Chem. Res. 2013, 46, 885−893.

Details of experimental procedures, 1D and 2D NMR spectra, fluorescence and NMR binding studies, TDDFT results for the cone conformation, and Cartesian coordinates of the DFT-optimized structures (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Carmen Talotta: 0000-0002-2142-6305 Amedeo Capobianco: 0000-0002-5157-9644 Annunziata Soriente: 0000-0001-6937-8405 Margherita De Rosa: 0000-0001-7451-5523 Placido Neri: 0000-0003-4319-1727 Carmine Gaeta: 0000-0002-2160-8977 Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.orglett.8b03134 Org. Lett. XXXX, XXX, XXX−XXX