Synthesis, aromatization and cavitates of an oxanorbornene-fused dibenzo[de,qr]tetracene nanobox

Oxanorbornene-fused double-stranded macrocycles, represented by kohnkene, are not only synthetic precursors toward short segments of zigzag carbon nanotubes but also typical cavitands processing an intrinsic cavity. However, their capability to bind guest molecules in solution remained unexplored. Herein we report a new member of oxanorbornene-fused double-stranded macrocycles, which is named a nanobox herein because of its shape. Reductive aromatization of this oxanorbornene-fused nanobox leads to observation of a new zigzag carbon nanobelt by high resolution mass spectroscopy. The fluorescence titration and NMR experiments indicate that this nanobox encapsulates C70 in solution with a binding constant of (3.2 ± 0.1) × 106 M−1 in toluene and a high selectivity against C60 and its derivatives. As found from the X-ray crystallographic analysis, this nanobox changes the shape of its cross-section from a rhombus to nearly a square upon accommodating C60.


Introduction
Oxanorbornene-fused double-stranded macrocycles, represented by kohnkene (Fig. 1a), have received considerable attention not only because they are synthetic precursors toward short segments of zigzag carbon nanotubes 1-6 but also because they are typical cavitands processing well dened cavities as a result of the intrinsic curvature of the oxanorbornene moieties. 7 Kohnkene was synthesized by Kohnke, Stoddart and coworkers in 1987 during the targeted synthesis of [12]cyclacene, 8,9 and named aer its creator. Partial deoxygenation of kohnkene with low valent titanium gave dideoxykohnkene 10 ( Fig. 1a), which has a cavity shaped like a Celtic cross. The recently revived interest in the synthesis of carbon nanobelts, [11][12][13] particularly, the efforts to synthesize zigzag carbon nanobelts [14][15][16] by Diels-Alder reactions [17][18][19][20] gave rise to new members of oxanorbornene-fused double-stranded macrocycles, 21 such as 1 17 and 2 20 (Fig. 1a). Because these doublestranded macrocycles are shaped like boxes, they are named oxanorbornene-fused nanoboxes herein. Similar to kohnkene and dideoxykohnkene, these nanoboxes all have a well-dened cavity although they lack hydrogen atoms pointing inward the cavity. However, their potential to function as molecular containers is largely unexplored. It is found that kohnkene has a cavity too small to accommodate any molecular guest, while dideoxykohnkene can only accommodate a molecule of water in its cavity in the crystal state. Some oxanorbornene-fused nanoboxes (e.g. 1) are found to have their cavities occupied by cocrystallized solvent molecules, 17,21 while the others (e.g. 2) are found to have empty cavities in the crystals because access to the cavity is blocked by bulky substituting groups. 18,20 On the other hand, none of these oxanorbornene-fused macrocycles have demonstrated the capability of accommodating guest molecules in solution.
Herein we report a new oxanorbornene-fused nanobox (3 in Fig. 1b), which contains four dibenzo[de,qr]tetracene subunits. Density functional theory (DFT) calculations indicate that the cavity of 3 has a cross-section shaped like a square and can accommodate a fullerene, such as C 70 (Fig. 1b). As detailed below, reductive aromatization of 3 led to observation of the corresponding zigzag carbon nanobelt 4 (Fig. 1b) by high resolution mass spectroscopy, and the capability of 3 to bind a fullerene (C 60 or C 70 ) in both solution and crystal states was demonstrated using different techniques. gave the deoxygenated product (8) in a yield of 83%. In contrast, the attempts to deoxygenate 8 under other conditions including NaI/trimethylsilyl iodide (TMSI) 17,18 and NH 4 ReO 4 /P(OPh) 3 24 led to decomposition of 7 or a very low yield of 8. Having a diene moiety (in the furan ring) on one end and a potential benzyne (to be formed by desilylation and elimination of triate) as the dienophile on the other end, 8 was used as a bifunctional building block to construct the nanobox through Diels-Alder reactions. The reaction of 8 with an excess of CsF (5 equivalents) in a dilute solution in THF and acetonitrile at 50 C gave the cyclic tetramer (3) in a yield of 8% together with the cyclic trimer (9) in a yield of 9%. The yields of 3 and 9 are higher than those of the reported oxanorbornene-fused nanoboxes from a bisbenzyne precursor and a bisfuran in a two-step manner 18 (e.g. 4% for compound 1 17 and 2% for compound 2 20 ). The pale-yellow solids of 3 and 9 both form colorless solutions in CH 2 Cl 2 , which both exhibit blue luminescence upon irradiation with UV light. As shown in Fig. S2 in the ESI, † the absorption and photoluminescence spectra of 3 are very similar to those of 9, respectively, but have higher intensity, in agreement with the fact that 3 has more dibenzo[de, qr]tetracene subunits than 9.
The 1 H NMR of 3 in the downeld region shows ve singlets ( Fig. S31 †), which are assigned to the corresponding protons on the basis of the ROESY 2D NMR (Fig. S32 †). The 1 H NMR of 9 in the downeld region ( Fig. S29 †), slightly different from that of 3, shows three singlets and two doublets due to observation of the coupling between two meta protons on the same benzene ring.
As revealed by the calculations at the B3LYP-D3 level of DFT with the 6-31g(d) basis set, 3 is of C 4h symmetry and has a slightly bent square cross-section with essentially at p-planes of dibenzo[de,qr]tetracene ( Fig. 2a), while 9 is of C 3h symmetry and its cross-section is shaped like a Reuleaux triangle with bent pplanes of dibenzo[de,qr]tetracene (Fig. 2b). The 9,10-dihydro-9,10-epoxyanthracene moiety at each corner of 3 exhibits the same bond angle of 104.7 between the two blue C-C bonds as shown in Fig. 2d, while that of 9 exhibits a slightly smaller bond angle of 103.0 at each corner. On the basis of the hypothetical homodesmotic reactions shown in Fig. S17 in the ESI, † the strain energy of 3 and 9 is estimated as 7 kcal mol À1 and 16 kcal mol À1 , respectively. Although 3 is less strained than 9, 3 was obtained in a slightly lower yield presumably because the formation of 3 requires one more Diels-Alder cycloaddition. Single crystals of 3 were obtained by slow diffusion of isopropanol vapor into its solution in CH 2 Cl 2 and 1,2-dichloroethane. X-ray crystallography revealed a triclinic unit cell containing one molecule of 3 and cocrystallized 1,2-dichloroethane and CH 2 Cl 2 . 25 It is found that 3 in the crystal has a roughly rhombic cross-section ( Fig. 2c) with four different bond angles (101.9-105.7 ) in the 9,10-dihydro-9,10epoxyanthracene moieties as shown in Fig. 2d. The different bond angles (Fig. 2d) and the different dihedral angles between benzene rings in the 9,10-dihydro-9,10-epoxyanthracene moieties in 3 (both DFT-calculated model and crystal structure) and 9 indicate that the oxanorbornene units in these doublestranded macrocycles are not completely rigid but exible to some degree.

Reductive aromatization of the oxanorbornene-fused nanoboxes
Because reductive aromatization of 3 and 9 can in principle result in the corresponding zigzag carbon nanobelts, the deoxygenation reactions of 3 and 9 were tested under different conditions including H 2 SnCl 4 , TiCpCl 2 /Zn, 23 NaI/TMSI, 26 TiCl 4 / LiAlH 4 , 27 and NH 4 ReO 4 /P(OPh) 3 . 24 Among these conditions, only treatment with H 2 SnCl 4 28 (freshly prepared from anhydrous SnCl 2 and concentrated HCl) at 120 C under an atmosphere of N 2 was able to convert 3 to the corresponding carbon nanobelt (4), which was detected by MALDI-TOF mass spectroscopy from the crude product. As shown in Fig. 3, when 3 was treated with H 2 SnCl 4 in toluene at 120 C for 10 minutes, the mass spectrum from the reaction mixture indicated the formation of partially deoxygenated products C 136 H 112 O 3 and C 136 H 112 O 2 . When the reaction time was prolonged, 4 (C 136 H 112 ) gradually became the major product. The observed molecular ion peak (m/z of 1745.8791) and isotope patterns ( Fig. 3 and S24 †) are in good agreement with the molecular formula of C 136 H 112 (m/z of 1745.8792). When the crude product was cooled to room temperature and exposed to air, the mass spectrum exhibited a new peak of m/z ¼ 1779.8811 for C 136 H 114 O 2 , which likely resulted from photo-induced oxygenation of 4 by molecular oxygen in air followed by protonation. This peak increased and the peak of 4 decreased quickly as exposure to air was prolonged (Fig. 3). This indicates low stability of 4 toward oxidation by air. Temperature was found important to the reduction of 3 with H 2 SnCl 4 . When treated with H 2 SnCl 4 at room temperature for 5 hours, 3 was almost completely recovered. When treated with H 2 SnCl 4 at 80 C for 5 hours, 3 was partially recovered and a partially deoxygenated product (C 136 H 112 O 2 ) was observed by mass spectroscopy. In contrast, other conditions either led to only partial deoxygenation or gave over-reduced products that exhibited molecular ion peaks in the mass spectra in agreement with hydrogenated carbon nanobelts. Unfortunately, our attempts to isolate 4 were not successful. When the reaction mixture was extracted and concentrated, the molecular ion peak for 4 disappeared and the product became less soluble, likely due to oligomerization and oxidation. The nanobelt 4 is less stable than the successfully synthesized zigzag carbon nanobelts 19,20 likely because 4 has fewer aromatic sextets.

Cavitates of the nanobox with fullerenes
Host-guest chemistry of the nanobox 3 in solution with different fullerenes including C 60 , C 70 , [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) and indene-C 60 bisadduct (ICBA) was  studied using different techniques. From a solution containing a 1 : 1 mixture of 3 and C 60 or a derivative of C 60 in toluene, the high resolution MALDI-TOF mass spectra (Fig. S20-S22 in the ESI †) revealed both free 3 and the corresponding cavitate: C 60 33 (m/z: 2530.8703), PCBM33 (m/z: 2720.9606), or ICBA33 (m/z: 2762.9836). In contrast, from the solution of 3 and C 70 (1 : 1) in toluene, only the molecular ion peak for C 70 33 (m/z: 2650.8664) was observed in the mass spectrum (Fig. S23 †). This suggests that 3 binds C 70 more strongly than C 60 . Upon gradual addition of a fullerene to the solution of 3, the intensity of the blue uorescence of 3 decreased dramatically as shown in Fig. 4, S9 and S11. † The Job's plots with an unchanged total concentration of 3 and fullerene in toluene showed a maximum uorescence change when the ratio of 3:fullerene reached 1 : 1 as shown in Fig. S3-S6. † On the basis of the 1 : 1 stoichiometry, the binding constant (K a ) of 3 for C 60 is determined as (3.3 AE 0.8) Â 10 4 M À1 at room temperature by tting the data from three independent uorescence titration experiments using the equation: 29,30 where F and F 0 are the uorescence intensity of nanobelts with and without addition of C 60 , respectively; k f and k s are proportionality constants for the complex and nanobox 3, respectively; and K a is the binding constant of nanobox 3 for C 60 . Using the same method, the binding constants of 3 for C 60 derivatives, PCBM and ICBA, are determined as (3.3 AE 0.9) Â 10 4 M À1 and (3.1 AE 0.7) Â 10 4 M À1 , respectively, which are essentially the same as that of C 60 33 likely as a result of arranging the substituting groups outside the cavity of 3. The binding constant of 3 for C 70 at room temperature is determined using the same method as (3.2 AE 0.1) Â 10 6 M À1 , which is larger than that of C 60 33 by two orders of magnitude. The binding constant of 3 for C 70 in toluene is larger than the reported values of [10]cycloparaphenylene ((8.4 AE 0.3) Â 10 4 M À1 , measured from UV-vis titration), 31 [11]cycloparaphenylene ((1.5 AE 0.1) Â 10 5 M À1 , measured from UV-vis titration), 31 and [4]cyclo(2,11-hexa-perihexabenzocoronene) (1.07 Â 10 6 M À1 , measured from uorescence quenching) 32 but lower than that of porphyrin nanohoops (2 Â 10 7 M À1 , measured from UV-vis titration) 33 in the same solvent. Moreover, 3 exhibits a higher selectivity between two fullerenes (C 70 /C 60 ¼ 97) than [10]cycloparaphenylene (C 60 /C 70 ¼ 33), 30,31 the porphyrin nanohoop (C 60 /C 70 ¼ 15), 33 and (12,8)- [4]cyclo-2,8-anthanthrenylene (C 70 /C 60 ¼ 1.3 in o-dichlorobenzene) 34 as well as the self-assembled capsule (C 70 /C 60 ¼ 21 in C 2 H 2 Cl 4 as measured from UV-vis titration or C 70 /C 60 ¼ 4.2 in C 2 H 2 Cl 4 as measured by isothermal titration calorimetry). 35 In order to study the encapsulation of C 60 and C 70 by 3 with NMR spectroscopy, o-C 6 D 4 Cl 2 , a better solvent for fullerenes, was used. Addition of excessive C 60 (3.5 eq.) into the solution of 3 in o-C 6 D 4 Cl 2 led to a broadened and slightly down-eld shied peak for H-a as shown in Fig. 5. In contrast, addition of 0.4 equivalent of C 70 into the solution of 3 resulted in two apparently different sets of peaks, which are attributed to the free host (3) and the complex (C 70 33), respectively. In the presence of excessive C 70 (2.0 eq.), the peaks for the free host disappeared and only the peaks for C 70 33 were observed. The same 1 H NMR spectrum (Fig. 5) was observed when 3.5 eq. of C 70 was added Fig. 4 Fluorescence spectra of (a) 3 (5.0 Â 10 À7 mol L À1 ) in toluene titrated with C 60 (from 0 to 5.0 Â 10 À5 mol L À1 ) and (b) 3 (1.0 Â 10 À7 mol L À1 ) in toluene titrated with C 70 (from 0 to 5.0 Â 10 À6 mol L À1 ) at room temperature. into a solution that already contained 3 and 3.5 equivalent of C 60 in o-C 6 D 4 Cl 2 . These results indicate that 3 binds C 60 weakly in o-C 6 D 4 Cl 2 with a fast exchange at the NMR time scale but binds C 70 strongly with a slow exchange under the same conditions. In agreement with the NMR experiments, from the solution containing a 1 : 1 : 1 mixture of 3, C 60 and C 70 in o-C 6 H 4 Cl 2 , the high-resolution mass spectrum revealed the complex of C 70 33 only. These results indicate selective encapsulation of C 70 by 3 in the presence of C 60 . From the NMR titration experiments (Fig. S36 in the ESI †), the binding constant of 3 for C 70 in o-C 6 H 4 Cl 2 at room temperature is determined as 2.5 Â 10 4 M À1 , which is smaller than the binding constant in toluene presumably because C 70 and 3 have a higher degree of solvation in o-C 6 H 4 Cl 2 than in toluene.
The structure of C 60 33 was unambiguously determined by X-ray crystallographic analysis of the single crystals of C 60 -33$2(C 2 H 4 Cl 2 )$2(C 6 H 5 CH 3 ), 25 which were grown by slow diffusion of isopropanol vapor into the solution of C 60 and 3 in o-dichloroethane (C 2 H 4 Cl 2 ) and toluene (C 6 H 5 CH 3 ). However, our attempts to grow single crystals of C 70 33 suitable for X-ray diffraction were not successful. As shown in Fig. 6a, the cross section of 3 in the crystal of C 60 33 is close to a square, which has a side length of about 13.1Å as measured from the distance between the center carbon atoms of dibenzo[de,qr]tetracene on the opposite sides. Sixteen short intermolecular C-to-C contacts in the range of 3.11-3.36Å are observed between 3 and C 60 . The cross-section of 3 in C 60 33 has inner angles of 92.2 and 86.0 as measured from the dihedral angle between the two benzene rings in each 9,10-dihydro-9,10-epoxyanthracene moiety (see Fig. 2d). Another nding from comparing the structures of 3 in the crystals with and without C 60 is that the yellow benzene rings in 3 bend inward to form concave-convex p-p interactions with C 60 as shown in Fig. 6b. As a result, the two yellow benzene rings in the same dibenzo[de,qr]tetracene unit form a dihedral angle of 13.7 . Further detailed analysis on the Hirshfeld surface 36 of C 60 in the complex shows short contacts (CH-p interactions) between the t-butyl group of 3 and C 60 (Fig. S1 in the ESI †). Fig. 6c shows the molecular packing in a unit cell of C 60 33$2(C 2 H 4 Cl 2 )$2(C 6 H 5 CH 3 ), where two adjacent molecules of 3 exhibit face-to-face p-stacking with a distance of 3.26Å between p-planes (dened by the tetracene moiety). In contrast, no p-p interactions are observed between molecules of C 60 , which are in fact separated by the t-butyl groups of 3. The p-p stacking between molecules of 3 and the relatively high HOMO energy level of 3 (À5.18 eV as calculated at the B3LYP/6-31-g(d) level of DFT) suggest that the crystals of C 60 3 3 can, in principle, function as hole-transporting organic semiconductors for application in phototransistors or photodetectors 37 on the basis of photo-induced electron transfer 38 from 3 to C 60 . Unfortunately, our preliminary efforts to dropcast or dip-coat a solution of C 60 33 onto a substrate failed to give lms suitable for device fabrication.

Conclusions
In summary, the above study has put forth a new oxanorbornene-fused nanobox (3), which contains four dibenzo [de, qr]tetracene subunits. It was synthesized through one-pot iterative Diels-Alder reactions, which also gave a Reuleaux triangle-shaped double-stranded macrocycle (9). Reductive aromatization of 3 with H 2 SnCl 4 led to observation of the corresponding zigzag carbon nanobelt by high resolution mass spectroscopy. The host-guest chemistry of 3 with different fullerenes was studied in both solution and crystal states using different techniques. The uorescence titration experiments indicate that 3 encapsulates C 70 in toluene with a binding constant of (3.2 AE 0.1) Â 10 6 M À1 and a high selectivity against C 60 and its derivatives. The NMR titration experiments indicate that 3 encapsulates C 70 with a slow exchange at the NMR time scale and a binding constant of 2.5 Â 10 4 M À1 in o-dichlorobenzene. The X-ray crystallographic analysis shows that 3 changes the shape of its cross-section from a rhombus to nearly a square upon accommodating C 60 . On the basis of the above results, 3 is the rst member of oxanorbornene-fused doublestranded macrocycles demonstrating the capability to accommodate molecular guests in solution. This aromatic nanobox may nd potential application for crystallization of fullerene derivatives, which is still a challenge in fullerene chemistry due to the limited solubility of fullerenes and the geometrical similarities of the carbon spheroids. 39-41 Fig. 6 Crystal structure of C 60 33$2(C 2 H 4 Cl 2 )$2(C 6 H 5 CH 3 ): (a) top view of C 60 33; (b) side view of C 60 33; (c) molecular packing in a unit cell. C 60 is shown in violet with a space-filling model; in panels a and b, carbon and oxygen atoms in 3 are shown as ellipsoids at the 50% probability level, and hydrogen atoms are removed for clarity; in panel c, molecules of 3 are shown with stick models, and co-crystallized solvent molecules are shown with space-filling models.

Data availability
All the data are provided in ESI. †

Author contributions
H. Chen and Q. Miao conceived the project, and Q. Miao directed the project. H. Chen performed most of the experiments and calculations, and Z. Xia contributed to the uorescence tritration experiments and data analysis. Q. Miao and H. Chen wrote the manuscript, and all authors checked the manuscript.

Conflicts of interest
There are no conicts of interest to declare.