A Polyaromatic Nano-nest for Hosting Fullerenes C60 and C70

Letter pubs.acs.org/OrgLett

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

A Polyaromatic Nano-nest for Hosting Fullerenes C60 and C70 Yihui Yang, Kunmu Cheng, Yao Lu, Dandan Ma, Donghui Shi, Yixun Sun, Mingyu Yang, Jing Li, and Junfa Wei* Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), Key Laboratory for Macromolecular Science of Shaanxi Province, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, P. R. China S Supporting Information *

ABSTRACT: A “Janus” type of hexa-cata-hexabenzocoronene with three triptyceno subunits fused symmetrically on the periphery of coronene has been synthesized using a covalent self-assembly strategy. The triptyceno subunits form a nanosized nest on one side of the aromatic plane with spacematching fullerenes such as C60 and C70 to afford shapecomplementary supramolecular complexes. The formation of the complexes in solution was confirmed by 1H NMR and fluorescence titration. Four complexes with C60 or C70 were obtained and studied by single-crystal X-ray diffraction analysis. In the crystal structure, the host shows a proper tunability to adjust its conformation in accordance with the shape of the guest. The different stoichiometric ratios and various stacking patterns of the complexes suggest the diversity of this nonplanar polyaromatic host in complexation with fullerenes. Scheme 1. One-Pot Synthesis of TBTTCa

H

exa-cata-hexabenzocoronene (contorted HBC, c-HBC), a unique large polycyclic aromatic hydrocarbon (PAH) or nanographene, surrounds a doubly concave structure (or a double-trefoil shape) as the outer phenyl rings alternately tilt above and below the plane demarcated by the coronene core.1,2 During the past decade, this nonplanar nanographene and its substituted derivatives have emerged as promising materials for their unique nonplanar shape and interesting electronic and self-assembly properties for applications in electronics and optoelectronics.3 For instance, c-HBC was revealed by Nuckolls et al. to form concave−convex complementary complexes with fullerene C60, and the complexes show a good performance in solar cells.4 Very recently, we established a facile and efficient covalent self-assembly (CSA) strategy for bottom-up construction of C3 symmetrically substituted c-HBC derivatives.5 This strategy provides opportunities for constructing c-HBCs with different appendixes on two opposite concaves by preinstalling proper functionalities on the starting reactants since the six outer benzo rings that form the down-concave and up-concave of the aromatic framework are separately introduced. Along this line, we envisioned that installing three triptycene moieties into one concave results in a “Janus” type of c-HBC, namely, tri-catatribenzotri-cata-tritriptycenocoronene (TBTTC) (Scheme 1), in which three axial benzene rings of the triptycene skeletons and the coronene core form a trigonal nanoscalar nest on one face. The most notable structural feature is that the nest has a cavity size suitable to trap a fullerene ball, such as C60 or C70 to form shape complementary supramolecules. This assumption finds its support from the DFT-optimized molecular structure, in which the nest matches well with fullerene C60 and C70 (Figures S26 and S27). Thus, such a polyaromatic nest provides a new host for supramolecular complexation of fullerenes. © XXXX American Chemical Society

a

Conditions: 10 mol % FeCl3, Ac2O, CH2Cl2, MeNO2, Ar, rt, 24 h; then excess FeCl3, 0 °C, rt, 6 h.

The synthesis of TBTTC was achieved via our CSA strategy from syn-triveratrylbenzene (TVB) and 2-formyltriptycene in 91% yield in one pot (Scheme 1).5 2-Formyltriptycene was prepared from 2-bromotriptycene by lithiation with n-BuLi followed by formylation with DMF (see the SI). The molecular nest shows sufficient solubility in common solvents, such as chloroform, dichloromethane, toluene, ethyl acetate, and carbon disulfide. The HR-MALDI-TOF mass spectrum displays one peak at 1309.44128, consistent with its formula (m/z = 1309.44234). The 1H NMR spectrum shows that the protons at the cove regions appear as two singlets at 9.31 and 8.66 ppm, respectively. The other two benzo moieties of triptycene subunits, which point upward and sideways twoard the TBTTC core, show two pairs of doublet−doublet signals at 7.32 and Received: January 28, 2018

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DOI: 10.1021/acs.orglett.8b00306 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters 6.87 ppm and at 7.66 and 7.23 ppm. The bridgehead and methoxy protons appear as two singlets at 5.75 and 4.19 ppm. The simplicity of the spectrum indicates a time-averaged C3 symmetry in solution. In the UV−vis spectrum, it exhibits three absorption bands which are characteristic of aromatic hydrocarbons around λmax 393 (β band, ε = 1.84 × 105 M−1 cm−1), 424 (p band, ε = 0.78 × 105 M−1 cm−1), and 472 nm (α band, ε = 3.62 × 103 M−1 cm−1),1b−d similar to the calculated data and the analogues we previously reported. The calculated characteristic peaks are in agreement with the experimental spectrum (Figures S1−S3). The emission spectrum exhibits a strong green fluorescence with three identical maxima at λmax 510, 550, and 580 nm with a Stokes shift of nearly 115 nm. The designed structure was unambiguously confirmed by Xray crystallographic study (Figure 1). In the crystal structure,

Figure 2. 1H NMR spectral change of TBTTC in toluene-d8 with addition of (a) C60 and (b) C70 in the presence of TBTTC (4 × 10−4 M) at 298 K.

the methoxy groups shift downfield. The signals (e, f) ascribable to the sideward phenyls of triptycene shift slightly, while the signals (g, h) of the upward rings shift downfield. Apparently, these results illustrate that the fullerene ball is indeed embedded into the nest in solution, and thus, the ring current effect of aromatic surfaces of fullerene causes the downfield shifts of the protons positioned in deshielding regions (i.e., the upward benzo-moieties) and the highfield shifts of those in shielding regions.8 The binding constant, calculated by the relatively largest absolute downfield shift, is (4.93 ± 0.47) × 103 M−1 (Figure S5). Job’s plots derived from the titration data indicate the predominance of a 1:1 stoichiometry of the binding event (Figure S6). Similar complexation behaviors of C70 with TBTTC and 1:1 stoichiometry were observed under the same conditions. The larger shifts for the aromatic protons and smaller shifts for the bridgehead and methoxy protons as well as the increased binding constant (6.94 ± 0.89) × 103 M−1 (Figures S8 and S9) might be attributed to the “standing egg” conformation of the C70 ellipsoid in the nest, which indicate its equator is higher than that of C60 as revealed by X-ray study of the solid complex. The UV−vis spectral titration of a solution of TBTTC against solutions of C60 with variable concentrations at 298 K in toluene indicated that electronic spectroscopy is insensitive to the complexation owing to a simple sum of the spectra of two individual chromophores (Figures S10 and S11). This is similar to that of porphyrin/C60 reported by Reed et al.9 In the fluorescence titration10 (Figures S12−S15), the fluorescence emissions of TBTTC at 509 nm are gradually quenched upon increasing the concentration of fullerenes. C70 is more conspicuous than C60 for fluorescence quenching. Job’s plots also elucidate the complexation of 1:1 stoichiometry between TBTTC and C60 or C70, indicating fluorescence titration can be used as a monitoring method to support the complexation. To further shed light on the nature of the recognition of fullerenes by the host, we turned our efforts to demonstrate its supramolecular chemistry with C60 and C70 in the solid state. Years of endeavors gave us four complexes of TBTTC with C60 and with C70 with 1:1 and 2:1 stoichiometric ratios of host− guest, and their single crystals qualified for X-ray diffraction (for details, see the SI). It should be pointed out that, although

Figure 1. X-ray structure of the nest TBTTC (solvent molecules are omitted for clarity; oxygen is depicted in red).

the host is not strictly symmetrical. The aromatic core, in analogy with the c-HBCs reported by Nuckolls et al.2a and us, has the double-concave structure in which three outer benzene rings (V) of veratryl moieties tilt below the coronene plane with the formation of one concave, while the benzene rings (IV) associated with triptycene subunits tilt above the coronene plane, forming the opposite concave, thus giving the desired “Janus” architecture demarcated by the aromatic plane. The inbetween benzene rings (II) adopt boat conformations with the average splay angle at cove-region is 41.6°, as in the case of dodecasubstituted c-HBCs reported by us.5 Such an arrangement allows three upward benzene rings (VI) of triptycenylenes to be nearly vertical to the hub ring (I) (on average 94.0°), forming the aromatic sidewalls of the nest. The centroid−centroid distance between each of the two upward rings is 12.52 Å on average, which encompasses the circular opening of the nest with a diameter of 14.46 Å. The depth of the nest as defined by the height from the plane of the hub at the bottom to the top rim of phenyl wall (VI) is 5.85 Å if the van der Waals radius of carbon (1.7 Å) is not taken into account.6 These results indicate that TBTTC should be a suitable host or receptor of fullerenes C60, whose diameter is 7.1 Å. Keeping the host in hand, we initially investigated its supramolecular chemistry with fullerenes C60 and C70 in solution. Three titrimetric experiments were carried out to characterize the supramolecular behaviors of TBTTC with fullerenes in solution. The 1H NMR titration experiments7 in toluene-d8 solution at 298 K (Figure 2) show diverse changes of the resonance peaks of TBTTC in the presence of C60. The cove protons (b and c) and the bridgehead protons (d) shift to high-field upon addition of the guest C60. The protons (a) of B


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Organic Letters many elegant receptors for C60 or C70 have been reported,11 only a few were found to encapsulate both C60 and C70.12 The X-ray diffraction outcomes for these complexes are depicted in Figures 3−5 and the SI.

Figure 4. Side-view of partial packing diagrams of: (a) the dimer of TBTTC·C60; (b) (TBTTC)2·C60; (c) TBTTC·C70; (d) (TBTTC)2· C70.

Figure 3. X-ray structure of complex (top view): (a) TBTTC·C60; (b) (TBTTC)2·C60; (c) TBTTC·C70; (d) (TBTTC)2·C70. (Solvent molecules are omitted for clarity; oxygen is depicted in red.)

with a twist angle of 39.5°. Noticeably, the binding of C60 makes the host molecules more curved than the free host: the depth of the nest increases to 6.12 Å, whereas the diameter of the circular opening reduces to 13.95 Å. In contrast, the depth and the diameter of the unfilled nest are 5.51 and 14.75 Å, meaning the unfilled nest is less curved than the filled one. The separation from the concave surface of C60 to the hub plane of the filled host molecule is 3.05 Å, shorter than that of the unfilled host (3.24 Å). These data demonstrate the difference between two surfaces of such a “Janus” molecule. Carbon atoms in sidewall phenyl rings of filled host are in van der Waals contact with the C60 ball with the shortest distance of 3.57 Å. Consequently, both host molecules adjust their conformation to come closer to each other and to encapsulate the C60 ball tightly. The complex molecules are stacked in one-dimensional strands parallel to the c axis in a head-to-tail columnar arrangement. The individual stacks, which are surrounded triangularly by three antiparallel neighbors, form a honeycomb arrangement (Figures 5b and S21). The diameter of cylindrical channels formed by the honeycomb arranged columns is 10.08 Å. In the 1:1 complex with C70 (Figure 3c), the host is C3 symmetrical and the C70 ellipsoid is well-centered in the nest, as a standing-egg in a tori (the C70 exhibits crystallographic disorder with rapid oscillation of the carbon atoms). The major axis (or C5 axis) of C70 is coincident with the C3 axis of the host (Figure 4c). The interplanar distance of the concave surface of C70 to the hub plane is 2.49 Å, slightly shorter than that of C60, suggesting a stronger π−π interaction between the host and C70. Meanwhile, the circular opening of the nest reduces to 14.27 Å. Noticeably, the host molecule of one complex contacts the C70 in the adjacent complex with the back surface; two hosts arrange coaxially with a twist angle of 36.85°, thus forming a head-to-tail columnar arrangement alternating TBTTC and C70 (Figures 5c and S23) and showing a divalent supramolecular assembly.14 The antiparallel columns also form

The X-ray study reveals that TBTTC forms a concave− convex complementary complex with C60 in a 1:1 stoichiometry and the C60 ball is held in the nest, though the host is not symmetrical and the C60 ball is not well-centered in the nest (Figure 3a). The separation between the centroid of C60 and the hub ring plane of TBTTC is 6.66 Å, with the depth of the nest (6.49 Å) increased upon complexation. Compared with the free molecule, the aromatic core of TBTTC in the complex becomes more curved to strengthen the π−π interaction with C60. Simultaneously, the distances between the centroid of each axial benzene ring and the diameter of the circular opening reduce to 11.95 Å on average and 13.79 Å. Three upward benzene rings come close to the host C60 molecule at 3.42, 3.25, and 3.33 Å, respectively. In the packing diagram, each of the two molecules of the complex are self-associated with a slipped cofacial dimer with an inversion center (Figure 4a). Inside the dimer, two C60 balls apart away from each other by the center−center distance of 10.25 Å, slightly longer than that in pure C60 crystals (9.94 Å);13 a strong aromatic C−H···π interaction between a hydrogen at the upward benzo units of the host of one complex molecule and the C60 of another complex molecule with an H-to-plane (a six-membered ring of C60) distance of 2.33 Å might play a more important role in stabilizing the dimer. The neighboring dimers are orthogonally assembled into a columnar arrangement along the c-axis (Figures 5a and S19). Each of the four neighboring columns constitutes a channel with a pore diameter of 7.25 Å. In the 2:1 complex of TBTTC and C60, two host molecules and one C60 ball assemble an interesting “sandwich”-type structure with perfect C3 symmetry: one host encapsulates a well-centered C60 ball (Figure 3b), while another is in contact with the C60 from the side opposite its empty nest (or filled by solvent molecules). Remarkably, the sandwiched C60 and two host molecules share their C6 or C3 axes. Two hosts are parallel to each other, with their hub planes in a skew conformation C


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Organic Letters

Figure 5. Selected packing patterns and channels view along thec-axis of (a) TBTTC·C60; (b) (TBTTC)2·C60 (c) TBTTC·C70; (d) (TBTTC)2·C70.

Accession Codes

a honeycomb network viewed from the ab-plane with the diameter of cylindrical channels is 9.34 Å. The “sandwich” complexation pattern is found again in the X-ray structure of the 2:1 complex of TBTTC and C70 (Figure 3d,d). The two host molecules that sandwich C70 parallel to each other and share the same C3 symmetric axis, similar to its C60 analogue. Most remarkably, the sandwiched C70 ellipsoid “lies” in the nest of one host. This is different from the orientation of C70 in the 1:1 complex, wherein the C70 fits its major axis to the C3 axis of the host molecule. We assume that such a lying down ellipsoid rotates around its minor axis (the C2 axis) in the nest like a stir-bar in a bottle, resulting in the C3symmetrical conformation of the host as well as the flattened ellipsoidal appearance of the sandwiched C70 (Figure 4d). Distances from the centroid of C70 to the hub ring plane of two host molecule are 6.47 and 6.51 Å, respectively. The π−π interplanar distances between C70 and the two hosts are 3.03 and 3.16 Å. The shortest distance of carbon atoms in sidewall phenyl rings to the C70 cage is 3.18 Å. Analogous features, e.g., the head-to-tail arrangement, antiparallel columnar packing, and honeycomb network, are also found in this case (Figures 5d and S25); the diameter of the channels is 11.09 Å. In summary, we designed and synthesized a novel “Janus”type 3-fold symmetrical c-HBC derivative with a nano-nest formed on one side of the aromatic plane using our CSA strategy. X-ray structural investigation reveals that such a nonplanar PAH provides a suitable structural preorganization, i.e., the nano-nest, for binding fullerenes C60 and C70. 1H NMR and fluorescence titrations reveal the TBTTC forms 1:1 host− guest complexes with C60 or C70 in solution. Furthermore, four supramolecular complexes with 1:1 and 2:1 stoichiometric ratios of host−guest with miscellaneous complexation and stacking patterns were found in the single-crystal structure by tuning its conformation to fit the shape and size of fullerenes. These results should be of importance in enrichment of supramolecular chemistry of fullerenes and development of optoelectronic materials concerning fullerenes.

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CCDC 1559896−1559897 and 1560053−1560055 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Junfa Wei: 0000-0002-0827-9822 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Foundation of Natural Science of China (Grant Nos. 21572124, 21272145, 21602128, and 21702131) and the Natural Science Basic Research Plan in Shaanxi Province of China (Program Nos. 2016ZDJC-03 and 2017JQ2010). We are also grateful to Prof. Zhaohui Wang at the Institute of Chemistry, Chinese Academy of Sciences, and Prof. Rui Cao, Prof. Wenliang Wang, and Prof. Shiwei Yin at Shaanxi Normal University for their helpful advice.

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S Supporting Information *

GThe Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00306. General experimental procedure, synthetic details, and characterization including NMR and MS spectra for all products and intermediate compounds, X-ray crystallographic data for products TBTTC, and physicochemical data (PDF) D


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A Polyaromatic Nano-nest for Hosting Fullerenes C60 and C70

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