Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
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From Planar Macrocycle to Cylindrical Molecule: Synthesis and Properties of a Phenanthrene-Based Coronal Nanohoop as a Segment of [6,6]Carbon Nanotube
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Shengsheng Cui, Qiang Huang, Jinyi Wang, Hongxing Jia, Pingsen Huang, Shengda Wang, and Pingwu Du* Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Materials for Energy Conversion, iChEM, Department of Materials Science and Engineering, University of Science and Technology of China (USTC), 96 Jinzhai Road, Hefei, Anhui Province 230026, China S Supporting Information *
ABSTRACT: Herein, we explore phenanthrene as the building block to synthesize a hoop-shaped [6,6]carbon nanotube segment from a planar macocycle via a Diels−Alder reaction. The phenanthrene-based coronal nanohoop 7 was fully characterized by HR-MS, NMR, and other spectroscopies. In addition, its photophysical properties and the supramolecular interactions between 7 and fullerene C60 were investigated. This present work suggests an easily accessible Diels− Alder reaction strategy to synthesize cylindrical nanohoops.
R
rings.15−17 Recently, some other strategies have also been developed to synthesize hoop-shaped molecules using a rhodium-catalyzed intermolecular cross-cyclotrimerization reaction18 or 1,3-butadiene precursors.19,20 To construct aromatic moieties, there are many reported synthesis methods, such as the intra- and intermolecular Diels−Alder reaction,21−24 metal-catalyzed benzannulation,25,26 olefin metathesis,27,28 photocyclization and electrophilic cyclization,29,30 and flash-vacuum pyrolysis.31−34 Some of these methods have also been used to synthesize CNT fragments, such as conjugated macrocyclic rings by a [4 + 2] benzannulation reaction,35 and a hemispherical end-cap by flash-vacuum pyrolysis.36,37 However, the Diels−Alder reaction is seldom used to synthesize CNT fragments. Wegner and coworkers adopted rhodium-catalyzed [2 + 2 + 2] cycloaddition to convert the diyne moieties of a macrocycle precursor into benzene rings.38 Wang and co-workers reported the Diels− Alder reaction to form a cis-configurated L-shaped unit to gain conjugated macrocycles of naphthalene.20,39 To date, there is no study to prepare the armchair CNT fragments by the direct Diels−Alder reaction from a planar macrocyclic precursor. Herein, we report that a phenanthrene-based coronal nanohoop can be facilely synthesized by the Diels−Alder reaction from the aryleneethynylene macrocycle, which represents a new [6,6]carbon nanotube segment (Figure 1).
olling up graphene sheets can give rise to the hollow, cylindrical structure of carbon atoms known as carbon nanotubes (CNTs).1−3 CNTs have attracted much attention over the past three decades because of their excellent properties and great potential for future applications.4,5 However, the low quality of the CNTs obtained by the existing methods such as arc discharge (AD)6 and chemical vapor deposition (CVD)7 still remains an unsolved issue.5 To solve this problem, the bottom-up approach by organic synthesis can provide a possible solution. The growth of nanotube segments with specific sizes and chiralities will rely on hemispherical end-caps or hoop-shaped carbon macrocycles as the templates, which are small fragments of CNTs with definite structural features such as diameters, lengths, rigidity, and carbon arrangements.8−10 Various molecular templates, such as aromatic belts, hemispherical end-caps, and carbon nanoring molecules, have been proposed to grow uniform CNTs.11−14 Therefore, the synthesis of CNT fragments has attracted much attention due to their great potential in the synthesis of CNTs with specified length and diameter. The main difficulties of synthesizing hoop-shaped CNT fragments lie in two aspects: (1) how to achieve the cyclization to form strained macrocycles and (2) how to produce conjugated aromatic ring structures. Previous research progress has reported using unstrained macrocycle precursors with key moieties such as cyclohexadienes, 1,4-diphenylcyclohexanes, or cis-platinum complexes, followed by reductive or oxidative aromatization reactions to produce the corresponding aromatic © XXXX American Chemical Society
Received: June 14, 2019
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DOI: 10.1021/acs.orglett.9b02055 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
yield of ∼30%. This is much higher than those of many cycloparaphenylene-based macrocycles.10,16,40−42 The phenanthrene-based coronal nanohoop 7 was confirmed by 1H and 13C NMR spectra, mass spectrometry, and FT-IR (Figures S7−S9). The exact mass of 7 was measured by HR-MS, demonstrating a major peak at m/z = 2066.2906 (calculated for C150H168O6 [M]+: 2066.2874) (Figure 3a). As
Figure 1. Design of the phenanthrene-based coronal nanohoop 7 as a segment of [6,6]carbon nanotube.
The nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HR-MS), and steady-state spectroscopies were used to characterize the phenanthrene-based coronal nanohoop compound. The supramolecular interactions between the coronal nanohoop and C60 were tentatively investigated. The synthetic procedure of phenanthrene-based coronal nanohoop 7 is shown in Figure 2. Compound 3 was prepared
Figure 3. Characterization of phenanthrene-based coronal nanohoop 7: (a) MALDI-TOF-MS; (b) 1H NMR spectrum (400 MHz, CDCl3).
shown in Figure 3b, the aromatic region of the 1H NMR spectrum for compund 7 exhibits only five sets of peaks, suggesting the highly symmetric structure of 7 because the Diels−Alder reaction occurred on the same side of the flat macrocycle 5. The singlet located at 8.27 ppm should be from the protons at a sites of the phenanthrene units. The doublets at 7.99 and 7.61 ppm belong to the protons at b sites and c sites, respectively, according to the previous results for the trimeric macrocycle.35 The multiplets between 7.06 and 7.24 ppm are probably from the protons of the six benzene rings. As for the singlet at 1.69 ppm, this peak should originate from all the methyl groups because of their identical chemical environments. The remaining signals are assigned to the protons in alkoxy groups. DFT calculations for compound 7 were carried out at the theoretical level of BLYP/DNP using the DMol3 module.43,44 Geometrical optimization reveals that the twist angle between two adjacent edges in phenanthrene-based coronal nanohoop 7 averages as 64.42°. Furthermore, the strain energy is calculated to be 2.69 kcal/mol (Table S1) using the reported method45 (Figure S10), which is much lower than these theoretical values of cycloparaphenylenes because the phenanthrene moieties are not connected at the 2 and 7 positions.45 Such low strain energy also suggests that the uphill process in the Diels−Alder cyclization reaction can be easily overcome. Subsequently, steady-state spectroscopies (UV−vis absorption spectroscopy and fluorescence spectroscopy) and timeresolved fluorescent decay were performed to investigate the photophysical properties of the phenanthrene-based nanohoop 7. As shown in Figure 4a, the absorption band of 7 was observed at 300−390 nm, maximized at λmax = 306 nm (ε = 1.4 × 105 cm−1 M−1). Upon excitation at 350 nm, the fluorescence emission spectrum of 7 exhibited an emission band between 350 and 550 nm, maximized at 397 nm. For comparison, we also tested the absorption and fluorescence spectra of aryleneethynylene macrocycle 5 and found no significant
Figure 2. Synthesis procedures for phenanthrene-based coronal nanohoop 7.
by reduction of 3,6-dibromo-phenanthrenequinone 1 using Na2S2O4 followed by the Williamson reaction for etherification with 1-bromooctane in a solution containing 18-crown-6 and potassium carbonate. Then, compound 4 was synthesized from 3 with 2-methylbut-3-yn-2-ol via Sonogashira coupling catalyzed by Pd(PPh3)2Cl2 and CuI. Next, in situ Sonogashira coupling of the deprotected 4 led to cyclization, producing trimeric macrocycle 5, a flat aryleneethynylene macrocycle bearing phenanthrene units, in a relatively high yield (∼21%). All synthesis details can be found in the Supporting Information. The characterization data are shown in Figures S1−S6. Finally, the Diels−Alder reaction between trimeric macrocycle 5 and 2,5-dimethyl-3,4-diphenylcyclopentanone 6 was performed in diphenylether. After purification with preparative thin-layer chromatography, the target product, phenanthrene-based coronal nanohoop 7, was obtained with a B
DOI: 10.1021/acs.orglett.9b02055 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
induced electron transfer processes were observed in these supramolecular complexes.49,50 Inspired by these studies, the host−guest interaction behavior between 7 and fullerenes C60 is next investigated. When C60 was added into a solution of 7 in CH2Cl2, the color of the solution showed an obvious change from colorless to intense purple (Figure 5a). The possible
Figure 5. Change of (a) color and (b) mass spectrometry upon addition of C60 into the phenanthrene-based coronal nanohoop 7 solution.
Figure 4. (a) Steady-state spectra of coronal nanohoop 7 (red) and aryleneethynylene macrocycle 5 (blue) in CH2Cl2. The solid lines are UV−vis absorption spectra, and the dash lines are fluorescence spectra. (b) Fluorescent photographs of 7 and 5 obtained in diluted CH2Cl2 solutions under a UV lamp (365 nm). Emission lifetime for (c) 7 and (d) 5 in CH2Cl2 measured at 397 nm.
supramolecular interaction was then studied by HR-MS spectrometry (Figure 5b). A new mass peak appeared at m/z 2787.2914 (calculated for C210H168O6 [C60@7]+: 2787.2908) suggesting the formation of a C60@7 complex with a ratio of 1:1. Furthermore, to provide more evidence of complex formation between 7 and C60, we measured the 1H NMR spectrum of the complex of coronal nanohoop 7@C60 (Figure S11). An obvious upfield-shift in the aromatic region can be found, confirming the supramolecular interaction between coronal nanohoop 7 and C60. The change of chemical shifts in the phenanthrene moieties is probably due to the change of the chemical environment for phenanthrene after complexing with C60. The complexation between 7 and C60 was next studied in a titration experiment measured by UV−vis spectroscopy. The result showed an isosbestic point at 323 nm (Figure 6a). Based on the change at 336 nm, a Job plot was obtained and the
changes in the peak shape, but in 7 both the absorption and emission band shifted to a much lower energy compared with aryleneethynylene macrocycle 5 (λmax (abs) = 351 nm, λmax (PL)= 460 nm). Therefore, the color of nanohoop 7 becomes intense blue, whereas aryleneethynylene macrocycle 5 is cyan in a diluted CH2Cl2 solution under a hand-held UV lamp (Figure 4b). This hypsochromic shift suggests that the radial πconjugation in 7 is decreased and the aromatic rings of compound 7 are not in the same plane. Previous studies have demonstrated that the maximum absorption peak of most CPPs with different sizes are located at 340 nm, which can be explained by the calculation results that the HOMO−LUMO transitions are not allowed and the maximum absorption probably originates from a combination of other transitions.15,46 Interestingly, since [6]CPP has completely forbidden transition, no appreciable fluorescence emission was detected from this compound.47 Based on the calculations, the strain energy of phenanthrene-based coronal nanohoop 7 is much lower than [6]CPP, indicating that their structures and photophysical properties are different. As a result, the nanohoop 7 presents intense blue fluorescence. According to the spectral data, the fluorescence quantum yield of 7 was measured to be ΦF = 45% (the anthracene solution in ethanol was chosen as the reference, ΦF = 27%). The time-resolved photoluminescence (TRPL) technique was used to study the excited state lifetimes of compounds 7 and 5 in degassed CH2Cl2. Upon excitation at 390 nm, the fluorescent lifetimes (τs) of 7 were determined to be τ1 = 0.36 ns, τ2 = 2.1 ns, τ3 = 9.2 ns at 397 nm by a fitting method of multiexponential decay curves (Figure 4c); these figures are comparable to those of compound 5 (τ1 = 0.42 ns, τ2 = 1.7 ns, τ3 = 9.7 ns, Figure 4d). All above characterization data confirmed the successful synthesis of the phenanthrene-based coronal nanohoop 7. Considering its concave cavity, this nanohoop 7 could be used as a supramolecular host for fullerene molecules. A few examples have been reported to use different hoop-shaped molecules to capture fullerenes.41,48−50 Especially, photo-
Figure 6. (a) UV−vis absorption features of different ratios of phenanthrene-based coronal nanohoop 7 and C60 for Job’s plot analysis in toluene. (b) Job’s plot of compound 7 and C60. (c) Fluorescence spectra of 7 (1.0 × 10−5 mol L−1) titrated with C60 in toluene at room temperature. The concentrations of C60 are (0.0− 3.0) × 10−5 mol L−1 from top to bottom. (d) Correlation of [C60] on the fluorescence intensity of 7 in toluene for calculating the Ka. R2 represents the standard deviation. C
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result showed that the value of absorption change reached the maximum value when the ratio of 7 to C60 reached a ratio of ∼1:1 (Figure 6b), thus confirming their 1:1 complexation. Based on fluorescence quenching experiments (Figure 6c), the binding constant Ka between 7 and C60 was calculated to be ∼3.9 × 104 M−1 (Figure 6d). In conclusion, phenanthrene-based coronal nanohoop 7 has been successfully synthesized as a [6,6]carbon nanotube segment, via a straightforward Diels−Alder reaction on an aryleneethynylene macrocycle. This facile synthesis strategy can provide a bottom-up route toward the segment compounds of carbon nanotubes with an excellent reaction yield. The target product was confirmed by different techniques, such as NMR and HR-MS techniques. Besides, its photophysical properties are well investigated. Furthermore, the supramolecular host−guest interaction between 7 and C60 exhibited the formation of a 1:1 complex and their binding constant is approximately 3.9 × 104 M−1 in toluene.
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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.9b02055.
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Experimental details, compound characterizations, and NMR spectra of the products (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Pingwu Du: 0000-0002-2715-0979 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Program of China (2017YFA0402800), the National Natural Science Foundation of China (51772285, 21473170).
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