Bowl-Shaped Carbon Nanobelts Showing Size-Dependent Properties

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Bowl-shaped Carbon Nanobelts Showing Sizedependent Properties and Selective Encapsulation of C70 XUEFENG LU, Tullimilli Y. Gopalakrishna, Yi Han, Yong Ni, Ya Zou, and Jishan Wu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00683 • Publication Date (Web): 23 Mar 2019 Downloaded from http://pubs.acs.org on March 23, 2019

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Journal of the American Chemical Society

Bowl-shaped Carbon Nanobelts Showing Size-dependent Properties and Selective Encapsulation of C70 Xuefeng Lu,† Tullimilli Y. Gopalakrishna,† Yi Han,† Yong Ni,† Ya Zou,† and Jishan Wu*,† †Department

of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore

Supporting Information Placeholder ABSTRACT: The synthesis of carbon nanobelts (CNBs) with well-defined size and structure remains a challenging topic in nanocarbon chemistry and materials science. Herein, we report the synthesis, physical characterization and supramolecular properties of two fully conjugated bowl-shaped CNBs (CNB1/CNB2), in which six/eight cyclopenta- rings are fused onto a macrocycle containing three/four alternately linked 2,7-pyrenyl and 2,7-phenanthryl units. The existence of five-membered rings results in a bowl-shaped geometry, as revealed by X-ray crystallographic analysis and density functional theory calculations. Both molecules contain an alternate aromatic phenanthrene-co-quinoidal pyrene structure to satisfy Clar’s aromatic sextet rule. The smaller size CNB1 has a deeper bowl depth (d = 4.997 Å) than CNB2 (d = 3.682 Å) and cannot undergo bowl-to-bowl (BTB) inversion below 373 K in toluene. However, the larger size CNB2 shows a smaller BTB inversion barrier (~12 kcal/mol) at the coalescent temperature (248 K), which was estimated by variable-temperature NMR measurements. Both compounds exhibit a small energy gap and amphoteric redox behavior with multiple redox waves. The dications of CBN1 and CBN2, and the tetracation of CBN2, are experimentally accessible by chemical oxidation with NO•SbF6, all displaying unusual open-shell singlet diradical character with a small singlet-triplet energy gap (∆ES-T = -2.71 kcal/mol for CBN12+, -2.50 kcal/mol for CBN22+, and -2.00 kcal/mol for CBN24+). The dications are globally aromatic while the tetracations are globally anti-aromatic according to NMR measurements and theoretical calculations (anisotropy of the induced current density, nucleus independent chemical shift and 2D iso-chemical shielding surface). The small bowl-shaped CNB1 demonstrates selective encapsulation of fullerene C70 over C60, with a large association constant (Ka = 8.066 × 105 M-1 in toluene). However, the larger size CNB2 does not exhibit any encapsulation with both C60 and C70.

INTRODUCTION Compared to planar polycyclic aromatic hydrocarbons (PAHs),1 curved aromatic hydrocarbons such as buckybowls,2 carbon nano-rings (CNRs, single bond linked macrocyclic arene system),3 carbon nanobelts (CNBs, fully fused macrocyclic arene system),4 and saddles5 show additional outstanding physical and electronic properties, which depend on their size and shape. However, synthesis of these curved structures is usually not a straightforward task due to the builtin strain. Nevertheless, in the past decades, chemists have developed various bottom-up synthetic methods to access these curved aromatic molecules, and even fullerene C606 and carbon nanotubes (CNTs).7 One notable progress in recent years is the efficient synthesis of [n]cyclo-para-phenylenes ([n]CPPs),3 which can be regarded as single-stranded CNRs. Even more remarkably, Itami and co-workers very recently reported the successful synthesis of a series of double-stranded CNBs through iterative Wittig reactions followed by intramolecular Yamamoto coupling.4c,d These cylinder-like molecules contain large strain, making the synthesis extremely challenging. As to their electronic properties, they display local aromatic character with a relatively large band gap. Despite this success, it remains a very big challenge to synthesize fully conjugated CNBs with more extended structure. We were particularly interested in

developing new double- or multi-stranded π-conjugated macrocycles with novel properties and less strain compared with the all-benzenoid, cylinder-like CNBs. One of our targets is bowl-shaped CNB, which possesses features of both buckybowl and CNT fragment. Synthesis of this kind of nanocarbon is more practical due to diminished strain, although it is still quite significant. We expect that fusion of π-conjugated cyclopenta- (CP-) rings onto a planar single-stranded macrocycle may work because the five-membered rings can induce curvature and result in both bowl- and belt-shaped structure. In this context, herein, we report the synthesis and physical properties of two bowl-shaped nanobelts, CNB1 and CNB2, in which six/eight cyclopenta- rings are fused onto a macrocycle containing three/four alternately linked 2,7-pyrenyl and 2,7-phenanthryl units (Figure 1). The CNBs can be drawn in different resonance forms, for example, an open-shell hexa/octa- radical form with twelve/sixteen aromatic sextet rings (the hexagons shaded in blue) (form A/A'), a closed-shell form in which the spins are coupled through the phenanthrene units with six/eight aromatic sextet rings (form B/B'), and a closedshell form in which the spins are coupled through the pyrene units with nine/twelve

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R R

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Figure 1. Representative resonance forms of the bowl-shaped carbon nanobelts CNB1 and CNB2.

ensure sufficient solubility. The key intermediates hexaaldehyde 3a and octa-aldehyde 3b were synthesized by Suzuki coupling reaction between equimolar amounts of 1 (see synthesis in the Supporting Information (SI)) and 210 using Pd2(dba)3-[(t-Bu)3PH]BF4 as catalyst, and isolated by recycling preparative GPC in 32% and 19% yield, respectively. Treatment of 3a and 3b with 2mesitylmagnesium bromide at room temperature gave the respective hexa-alcohol and octa-alcohol, which then underwent BF3•Et2O mediated intramolecular Friedel-Crafts alkylation to afford the ring-cyclized products 4a and 4b. The final oxidative dehydrogenation step, using 2,3-dichloro-5,6dicyano-p-benzoquinone (DDQ) as oxidant, provided the final target compounds CNB1 and CNB2 in 85% and 82% yield (over three steps from 3a/3b), respectively, which are stable and can be purified by normal silica gel column chromatography. All new products were well characterized by 1H/13C NMR and high-resolution mass spectra (see SI).

aromatic sextet rings (form C/C'). Thus, the electronic structure, global (anti)aromaticity, and radical character of both the neutral CNBs and their charged species are of great interest. In addition, such type of curved nanocarbon may show size and shape-selective encapsulation of fullerenes, like many other non-planar fullerene acceptors.8 That is, the bowlshaped curved structure and defined cavity inside the CNBs may enable them to act as a host for the fullerene guests. In this article, we will report the synthesis, size-dependent physical properties, and selective encapsulation of fullerenes of this new type of bowl-shaped CNB molecules. RESULTS AND DISCUSSION Synthesis. The bowl-shaped carbon nanobelts CNB1 and CNB2 were synthesized according to Scheme 1. The synthetic strategy is similar to our previous reports.9 Bulky mesityl groups are used to block the most reactive sites at the CP rings to obtain stable materials, and the long octyloxy chains are attached onto the 9,10-positions of the phenanthrene units to R C8H17O

R

R

OC8H17 OHC

OHC

a'

g

H

Mes

CHO

R H

Mes

R h

Mes

a

CHO Br

1

f b

Br

i

+

ii

BPin

2

Mes

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H

Mes

iii

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Mes

Mes

CHO

OHC

PinB

R

c

R

R

n

R CHO

OHC

3a (n=1), 32% 3b (n=2), 19%

R

R

R

R R = OC8H17

H

Mes

H

n

4b (n=2)

O

i k

Mes

4a (n=1)

m o p

R

R

R

j l

n

n Mes

R

Mes

CNB1 (n=1), 85%, from 3a CNB2 (n=2), 82%, from 3b

Scheme 1. Synthetic route for the bowl-shaped carbon nanobelts CNB1 and CNB2. Reagents and conditions: i) Pd2(dba)3, [(t-Bu)3PH]BF4, NaHCO3, THF/H2O, 80 oC; ii) a) MesMgBr, THF, RT; b) BF3·OEt2, DCM, RT; iii) DDQ, toluene, RT. Mes: mesityl.


form C rather than the form B. This is reasonable considering that there are nine aromatic sextet rings in form C while there are only six in form B. According to Clar’s aromatic sextet rule,12 form C is more favourable. The bond length analysis also suggests that the open-shell radical forms (such as form A) has negligible contribution to the ground-state structure, and indeed, our spin-unrestricted density functional theory (DFT) calculations of both CNB1 and CNB2 revealed closedshell nature. This is also reasonable considering that conversion from the closed-shell form C to an open-shell diradical form will only gain one aromatic sextet ring, which is not sufficient to compensate the energy required to break a π bond. Single-crystal structure of CNB2 could not be obtained after many efforts and thus its geometry was optimized by DFT calculations at B3LYP/6-31G(d,p) level of theory, which also gave a bowl-shaped carbon nanobelt backbone (Figure 2d-e). CNB2 shows larger cavity (diameter of the inner most hub: 14.204 Å and diameter of the outer most rim: 26.000 Å), more shallow bowl depth (3.682 Å), and thus less strain compared with CNB1. The side-wall plane is tilted about 32o relative to the mean plan of the inner-most hub. Bond length analysis predicted that the bond b in the CP rings linking the phenanthrene units is obviously longer than that of bond a linking the pyrene units, again indicating that the form C' makes a major contribution. HOMA value calculations support the same.

(c)

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1.394

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(b)

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rim

1.4 41

(a)

1.4 19

Ground-state geometry. Single crystals of CNB1 were successfully grown by slow diffusion of acetonitrile into the chloroform solution under ambient conditions and its crystallogrpahic structure is shown in Figure 2a-b, which clearly reveals a bowl-shaped carbon nanobelt structure.11 The diameter of the inner-most hub and the outer-most rim is about 9.916 Å and 19.818 Å, respectively, and distance between the mean planes of the inner-most hub and the outer-most rim (i.e., the depth of the bowl) is about 4.997 Å, indicating a large strain. The side-wall plane is tilted about 45.3o relative to the mean plan of the inner-most hub. The bond a (1.388/1.384/1.374/1.353/1.353/1.419 Å) in the CP rings linking the pyrene units is obviously shorter than that of a typical C(sp2)–C(sp2) single bond (1.45 Å) and close to that of in a typical olefin (1.33-1.35 Å), while the bond b (1.493/1.476/1.505/1.496/1.476/1.477 Å) in CP rings linking the phenanthrene units is obviously longer than that of a typical C(sp2)-C(sp2) double bond in benzene (1.39 Å) and slightly longer than that of a typical C(sp2)–C(sp2) single bond (1.45 Å), all suggesting formation of a dominate quinoidal indeno[2,1-a:1',2'-i]pyrene structure as shown in the resonance form C in Figure 1. The harmonic oscillator model of aromaticity (HOMA) value calculations on individual rings also imply a more aromatic (larger HOMA values) phenanthrene unit versus a less aromatic (smaller HOMA values) quinoidal pyrene unit. That means, the antiferromagnetic spin-spin coupling in CNB1 prefers to the

19.

C1

C13 C14

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1.4 66

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3 1

C81

3.682 A

Figure 2. X-ray crystallographic structure of CNB1 and optimized (B3LYP/6-31G(d,p)) structure of CNB2: (a)/(d) top view; (b)/(e) side view; (c)/(f) selected bond lengths (in Å) of the backbone. All substituents are omitted for clearance. The red numbers in the individual rings (c/f) are calculated HOMA values based on the bond lengths.



e

f

c

d

hand, for the smaller size nanobelt CNB1, the protons a and a' on the mesityl groups are well separated at room temperature and even upon heating (up to 323 K in THF-d8 and 373 K in toluene-d8, Figures S1-S2 in SI), indicating a much larger BTB inversion barrier. Optical and electrochemical properties. Both CNB1 and CNB2 are black in the solid state, and green when dissolved in toluene or dichloromethane (DCM). The absorption spectra of CNB1 and CNB2 in toluene are shown in Figure 4a. Compound CNB2 displays a major vibronic absorption with maximum wavelength (λmax) at 730 nm, which is more intense and red-shifted compared to compound CNB1 (λmax = 705 nm). This can be explained by the fact that the nanobelt CNB2 extends the π-conjugation system in the backbone as compared with the nanobelt CNB1. However, the absorption onset of CNB2 exhibits a slightly blue shift compared to that of CNB1, which could be due to the more contorted structure of CNB1 which perturbs the electronic structure of aromatic rings, as observed in the [n]CPP series.15 The optical energy gap (Egopt) was estimated to be 1.24 eV and 1.36 eV for CNB1 and CNB2, respectively, from the lowest-energy absorption onset. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements of CNB1 and CNB2 were carried out in dry DCM by using 0.1 M tetra-nbutylammonium hexfluorophosphate as the supporting electrolyte (Figure 4b and Figure S5). CNB1 displayed five quasi-reversible oxidation waves with E1/2ox = -0.03, 0.26, 0.55, 0.72 and 0.88 V (vs. Fc+/Fc) and five quasi-reversible reduction waves with E1/2red = -1.36, -1.52, -1.72, -1.88, and 1.97 V. CNB2 exhibited even more overlapped redox waves. The HOMO/LUMO energy levels of CNB1 and CNB2 were determined to be -4.71/-4.75 and -3.62/-3.57 eV, respectively, from the onset of the first oxidation/reduction wave, and the electrochemical energy gaps (EgEC) are estimated to be 1.09 and 1.18 eV, respectively, in consistent with the trend of the optical energy gaps.

a' a

b

a'

a

Chemical Shift (ppm)

Figure 3. Variable-temperature 1H NMR spectra (aromatic region) of CNB2 in THF-d8 (referring to Scheme 1 for labelling).

Both CNB1 and CNB2 showed sharp 1H NMR spectra in THF-d8 or toluene-d8 even under heating (Figure 3 and Figures S1-S3), indicating that both of them have a closedshell ground state, in consistent with the DFT calculation. Notably, for the larger size nanobelt CNB2, cooling of the solution in THF-d8 down to 233 K led to a splitting of the one averaged peak for the aromatic protons on the mesityl groups into two peaks (a and a') (Figure 3 and Figure S3 in SI). Such a phenomenon indicates that the bowl structure can undergo a quick bowl-to-bowl (BTB) inversion at room temperature, while the process is slowed down at lower temperatures. The bowl inversion rate constant k at different temperatures was estimated by careful line-shape analysis13 and fitting of the data by Eyring equation (ln(k/T) = - ΔHǂ/R•1/T + ln(kB/h) + ΔSǂ/R) gave thermodynamic parameters ΔHǂ = 56 ± 2 kJ/mol and ΔSǂ = 20 ± 8 J/(mol•K) (Table S1 and Figure S4, see details in the SI). Accordingly, free energy of activation ΔGǂ was estimated to be 51 ± 4 kJ/mol (~12 kcal/mol) at the coalescence temperature 248 K. This barrier is slight larger than the parent corannulene (~11.5 kcal/mol).14 On the other (b)

705

Current

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315

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Abs.

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Figure 4. (a) UV-vis-NIR spectra of CNB1 and CNB2 in toluene solutions, insets are the magnified onset absorption bands; (b) cyclic voltammograms of CNB1 and CNB2 in DCM solutions; (c,d) absorption spectra of the neutral, dication and tetracation of CNB1 and CNB2 in DCM solutions, using NO•SbF6 as oxidant.


Oxidative titration of CNB1 and CNB2 was conducted by using oxidant NO•SbF6 in dry DCM and the absorption spectra of the major oxidized species are shown in Figure 4c-d. For CNB1, the titration did not stop at the monocation stage, and thus only the spectrum of the dication was recorded (λmax = 930 nm, Figure 4c). Addition of more oxidant did not generate the trication or tetracation. For CNB2, the titration gave its dication and tetracation, both showing long-wavelength absorption in the NIR region (λmax = 1026 and 1000 nm, respectively, Figure 4d). Addition of more oxidant did not generate the pentacation or hexacation. c d

f

CNB1

C8H17O

b

OC8H17

c

a f

a a' i

298 K

C7H15

12.0

11.0

O

OC8H17 OC8H17

C8H17O Mes

Mes

a

f

*

243 K

c

10.0

9.0

i

a' 8.0

7.0

6.0

Mes

i

*

CNB12+

b

e d

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a'

Mes

5.0

i

d 4.0

1

e

* 0

-1

-2

-3

-4

-5

-6

Chemical Shift (ppm)

Figure 5. Partial 1H NMR spectra of CNB1 (measured at 298 K) and its dication (measured at 243 K) in CD2Cl2. The peaks labeled as * indicate residue solvents.

Pure dication of CNB1 was isolated, and interestingly, its NMR spectrum was broadened at room temperature and became sharper with decreasing temperature (Figure S6), indicating an open-shell singlet ground state.16 Indeed, the diradical character (y0) of CNB12+ was calculated to be 61.9% (UCAM-B3LYP/6-31G(d,p)). The 1H NMR spectrum of CNB12+ recorded at 243 K clearly showed that the inner-rim protons d (0.52 ppm), e (-0.43 ppm) and f (-2.71 ppm) were 1H

(b)

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NICS(0) +16.97 ppm

NICS(0) -7.99 ppm

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NICS(0) 0.04 ppm

NICS(0) -10.27 ppm

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Z-axis (Å)

(a)

highly shielded, and the protons of mesityl group (a and a’) were also slightly shielded, while the outer-rim protons were slightly de-shielded (Figure 5), indicating a change from a local aromatic nanobelt to a global aromatic system. To deep understand the possible global (anti)aromaticity, anisotropy of the induced current density (ACID),17 nucleus independent chemical shift (NICS)18 and 2D iso-chemical shielding surface (ICSS)19 calculations were conducted. ACID pot of the neutral compound CNB1 displays six local clockwise ring current flows (Figure 6a), implying local aromaticity. While ACID plot of its dication CNB12+ shows clockwise diatropic ring current flow along the periphery (Figure 6b). 2D ICSS map also reveals that the inner protons are shielded (positive ICSS(1)zz values) while the outer protons are de-shielded (negative ICSS(1)zz values) (Figure 6g). In addition, the center of the macrocycle has a negative NICS(0) value of -10.27 ppm. Therefore, the conjugated backbone of CNB12+ with total 94 π (4n+2) electrons has a global aromatic character, satisfying Hückel rule. On the other hand, the NICS(0) value for CNB1 is nearly zero (-0.49 ppm), and the 2D ICSS map (Figure 6f) also exhibits local aromatic character for the spacers only. The dication and tetracation of CNB2 did not show any NMR signals for the backbone at room temperature and even after cooling to 193 K. In fact, the diradical character (y0) of CNB22+ and CNB24+ was calculated to be 86.7% and 77.6%, respectively, which is larger than that of CNB12+, explaining the more significant NMR broadening. Similarly, ACID plots, NICS values and 2D ICSS maps disclose that neutral CNB2 has a local aromatic character (Figure 6c, h; NICS(0) = 0.04 ppm), its dication with 126 πconjugated electrons displays global aromatic character (Figure 6d, i; NICS(0) = -7.99 pm), and its tetracation with 124 πconjugated electrons exhibits global anti-aromatic character (Figure 6e, j; NICS(0) = +16.97 ppm), again, following the Hückel rule.

Y-axis (Å)

e

Z-axis (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Z-axis (Å)

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X-axis (Å)

X-axis (Å)

X-axis (Å)

Figure 6. Calculated ACID plots (contribution from π electrons only) of CNB1 (a), CNB12+ (b), CNB2 (c), CNB22+ (d) and CNB24+ (e). The magnetic field is perpendicular to the XY plane and points out through the paper. The blue and red arrows indicate the counter-clockwise (paramagnetic) and clockwise (diamagnetic) current flow, respectively. Calculated 2D-ICSS maps of CNB1 (f), CNB12+ (g), CNB2 (h), CNB22+ (i) and CNB24+ (j). The top and bottom images are mapped at XY and XZ planes, respectively. For all calculations, the octyloxy groups are replaced by methoxy groups. See magnified ACID plots in SI.


Journal of the American Chemical Society Variable-temperature (VT) ESR measurements of the dication of CNB1 and the dication, tetracation of the CNB2 in DCM showed a featureless broad signal (ge = 2.0024-2.0029) and the intensity decreased as the temperature was lowered (Figure 7a, c, e), indicating that they can be regarded as diradical dications or diradical tetracation. Fitting of the data by the Bleaney-Bowers equation20 gave a singlet-triplet energy gap (∆ES-T) of 2.71 kcalmol-1 (CNB12+), 2.50 kcalmol-1 (CNB22+) and 2.0 kcalmol-1 (CNB24+), respectively (Figure 7b, d, f).

approximately 8.066 × 105 M-1 (Figure 8b and see details in SI).21 (a)

1.4

0 eq 0.125 eq 0.25 eq 0.375 eq 0.5 eq 0.625 eq 0.75 eq 0.875 eq 1.0 eq 1.25 eq 1.5 eq 1.75 eq 2.0 eq 2.5 eq 3.0 eq

Absorbance

1.2 1.0 0.8 0.6

744 nm

0.4 0.2

200 0 -200 -400 -600 -800 326

(c) 1500

327

328

329 330 Field (mT)

331

Intensity (a.u.)

1000 500 0 -500

-1000 -1500 326

(e) 3000

327

328

329 330 Field (mT)

CNB24+

2000

0 -1000 -2000 -3000 326

327

328

329 330 Field (mT)

400

400

500

600

700

800

900

1000

1100

Wavelength (nm)

500

(b)

ES-T = -2.71 kcal/mol

0.00

300 200 100 0 100 120 140 160 180 200 220 240 260 Temperature (K)

(d) 800 CNB22+

-0.04

Ka = 8.066 x 105 M-1

-0.08

R2 = 0.99292

-0.12 -0.16

700 600

-0.20

500 400

0.0


0 100 120 140 160 180 200 220 240 260 Temperature (K)

1600 243 K (f) 213 K 1400 183 K 173 K 1200 163 K 158 K 1000 153 K 148 K 800 143 K 138 K 600 133 K 128 K 400 331 332

CNB24+

To further shed light on the supramolecular interaction between the CNB1 and C70, the binding behaviour was also monitored by 1H NMR spectrum in toluene-d8 solution at 298 K. Upon addition of one equivalent C70, the inner-most hub protons (d, e and f) and the mesityl group protons (a and a') all are shifted to low field, while the outer-most rim protons (b and c) are slightly shifted to high field (Figure 9). The formation of supramolecular complexes in solution was also confirmed by 2D NOSEY NMR spectroscopy (Figure S10). Apparently, these results suggest that the fullerene C70 is indeed embedded into the bowl in solution, and thus, the ring current effect of aromatic surfaces of fullerene causes down-field shifts of the protons positioned in de-shielding region (i.e., the inner-most hub protons) and high-field shifts of those in shielding region.22 Unfortunately, no single crystals suitable for X-ray analysis could be obtained after many attempts.


120

140

160 180 200 220 Temperature (K)

240

260

Figure 7. VT ESR spectra and fitted I×T-T curves by using Bleaney-Bowers equation of compounds CNB12+ (a, b), CNB22+ (c, d) and CNB24+ (e, f) in DCM solution.

Binding behaviour with fullerenes. Considering that both CNB1 and CNB2 have defined concave cavity, they may act as supramolecular hosts for π-conjugated molecules with a convex surface, such as fullerenes C60 or C70. UV-vis-NIR titration experiments of CNB1 and CNB2 with C60 and C70 were first carried out. No appreciable change was observed when CNB1 or CNB2 was mixed with C60 (Figure S6). Also, there was no obvious change was observed when CNB2 was mixed with C70 (Figure S7). However, strong supramolecular interaction between CNB1 and C70 was clearly observed. In particular, a substantial decrease in absorption intensity of CNB1 at 705 nm was observed upon addition of C70 (Figure 8a), indicating that the existence of intermolecular electronic interactions between CNB1 and C70. Judging from the Job’s plot, the UV-vis-NIR spectra at 705 nm in toluene showed the maximum absorption change for a 1:1 mixture of CNB1 and C70 (Figure S8), indicating that a 1:1 stoichiometry between CNB1 and C70 predominates in solution. The 1:1 complex was also observed by MALDI-TOF mass spectrometry (Figure S9), showing the peak at m/z = 3511.8 (calculated for C268H198O6 [C70@CNB1]: 3511.5), thus confirming the formation of a 1:1 complex. On basis of the UV-vis-NIR titration experiments, the binding constant Ka of CNB1 with C70 in toluene was estimated to be

3.0

2.0

[C70] x 105 (M)

Figure 8. (a) UV-vis-NIR absorption spectra of CNB1 in the presence of C70 (0.0-3.0 equiv) in toluene. (b) Plot of absorption changes at 705 nm versus [C70] in toluene for calculating the Ka value. R2 is the coefficient of determination.

100

332

1.0

300 200

IxT (a.u.k)

1000

331

300

600

332 243 K 213 K 183 K 173 K 163 K 158 K 153 K 148 K 143 K 138 K 133 K 128 K 123 K 118 K

CNB22+

0.0

700

A-A0 at 705 nm

Intensity (a.u.)

400

(b) 800 CNB12+

IxT (a.u.k)

243 K 213 K 183 K 173 K 163 K 158 K 153 K 148 K 143 K 138 K 133 K 128 K 123 K 118 K

600

IxT (a.u.k)

(a) 800 CNB12+

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 10

C8H17O

OC8H17

a f

b

e d

Mes

f

* **

a'

c Mes

1 eq C70

cd Mes

b

e

a' a

CNB6 OC8H17

C8H17O

* **

OC8H17

C8H17O Mes

Mes

f

e

c

b d

a'

a

0 eq C70

10.0

9.8

9.6

9.4

9.2

9.0

8.8

8.6

8.4

8.2 8.0 7.8 7.6 Chemical Shift (ppm)

7.4

7.2

7.0

6.8

6.6

6.4

6.2

1H

Figure 9. NMR spectral change of CNB1 in toluene-d8 with addition of one equivalent C70 at 298 K, the peaks labeled as * indicate residue solvents.


6.0

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CONCLUSIONS In summary, two fully conjugated, bowl-shaped carbon nanobelts with different size (CNB1 and CNB2) were successfully synthesized by a macrocycle formation followed by periphery fusion strategy. Due to the fusion of fivemembered rings, both molecules have a curved, bowl-shaped geometry. X-ray crystallographic analysis and DFT calculations reveal that both CNB1 and CNB2 have a closed-shell ground electron state, with a dominant alternate aromatic phenanthreneco-quinoidal pyrene structure to maximize the number of aromatic sextet rings. Their dications and tetracations, however, show significant open-shell diradical character, with small singlet-triplet energy gaps. Global (anti)aromaticity was observed for these charged species, which also follows the Hückel rule. In addition, these compounds displayed size and shape dependent optical, electrochemical and dynamic properties. Moreover, the small-size bowl-shaped CNB1 showed selective encapsulation of fullerene C70 in toluene, with a large binding constant Ka of 8.066 × 105 M-1, demonstrating the potential of using bowl-shaped carbon nanobelts as size- and shape-selective host molecules for various guest molecules. Our studies shed some light on the design and synthesis of new carbon nanobelts with tunable properties. ASSOCIATED CONTENT Supporting Information. Synthetic procedures and characterization data of all new compounds; details for all physical characterizations and theoretical calculations; and additional spectroscopic and X-ray crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; ACKNOWLEDGMENT We acknowledge financial support from the MOE Tier 3 programme (MOE2014-T3-1-004) and NRF Investigatorship Award.

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(21) Conners, K. A. Binding Constants: The Measurement of Molecular Complex Stability, Wiley-VCH, New York, 1987. (22) Zhou, Z. M.; Qin, Y. K.; Xu, W.; Zhu, D. B. Supramolecular Association Behavior of a Strong C60 Receptor with Conjugated

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Table of Contents

C60 & C70

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CNB2

CNB1

CNB1: Selective Encapsulation of C70!

ACS Paragon Plus Environment

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Bowl-Shaped Carbon Nanobelts Showing Size-Dependent Properties

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