All-benzene carbon nanocages: Size-selective synthesis, photophysical properties, and crystal structure

Article abstract of DOI:10.1021/ja509880v

The design and synthesis of a series of carbon nanocages consisting solely of benzene rings are described. Carbon nanocages are appealing molecules not only because they represent junction unit structures of branched carbon nanotubes, but also because of their potential utilities as unique optoelectronic π-conjugated materials and guest-encapsulating hosts. Three sizes of strained, conjugated [n.n.n]carbon nanocages (1, n = 4; 2, n = 5; 3, n = 6) were synthesized with perfect size-selectivity. Cyclohexane-containing units and 1,3,5-trisubstituted benzene-containing units were assembled to yield the minimally strained bicyclic precursors, which were successfully converted into the corresponding carbon nanocages via acid-mediated aromatization. X-ray crystallography of 1 confirmed the cage-shaped structure with an approximately spherical void inside the cage molecule. The present studies revealed the unique properties of carbon nanocages, including strain energies, size-dependent absorption and fluorescence, as well as unique size-dependency for the electronic features of 1-3.

Full text of DOI:10.1021/ja509880v

Subscriber access provided by UNIV OF PADOVA
Article
All-benzene Carbon Nanocages: Size-selective Synthesis,
Photophysical Properties, and Crystal Structure
Katsuma Matsui, Yasutomo Segawa, and Kenichiro Itami
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja509880v • Publication Date (Web): 31 Oct 2014
Downloaded from http://pubs.acs.org on November 2, 2014
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 9
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
All-benzene Carbon Nanocages: Size-selective Synthesis,
Photophysical Properties, and Crystal Structure
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
Katsuma Matsui,a Yasutomo Segawa,*,a,b and Kenichiro Itami*,a,b,c
a Graduate School of Science, Nagoya University, Japan, Chikusa, Nagoya 464-8602, Japan. b JST ERATO, Itami Molecular
Nanocarbon Project, Nagoya University, Japan. c Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Japan,
ABSTRACT: The design and synthesis of a series of carbon nanocages consisting solely of benzene rings are described. Carbon nanocages
are appealing molecules not only because they represent junction unit structures of branched carbon nanotubes, but also because of their
potential utilities as unique optoelectronic π-conjugated materials and guest-encapsulating hosts. Three sizes of strained, conjugated
[n.n.n]carbon nanocages (1: n = 4, 2: n = 5, 3: n = 6) were synthesized with perfect size-selectivity. Cyclohexane-containing units and 1,3,5-
trisubstituted benzene-containing units were assembled to yield the minimally strained bicyclic precursors, which were successfully converted
into the corresponding carbon nanocages via acid-mediated aromatization. X-ray crystallography of 1 confirmed the cage-shaped structure
with an approximately spherical void inside the cage molecule. The present studies revealed the unique properties of carbon nanocages,
including strain energies, size-dependent absorption and fluorescence, as well as unique size-dependency for the electronic features of 1–3.
the conditions we developed for CPP-to-CNT (ring-to-tube)
conversion could be applied for the bottom-up synthesis of
INTRODUCTION
Carbon nanotube (CNT) segments and molecular nanocarbons
have captivated the attention of the broad scientific community
from both fundamental and application perspectives.1 For example,
there is an enormous expectation for CNT segment molecules to
serve as templates for structurally uniform CNTs.2 In addition, due
to their unique arrangement of π-systems, CNT segment molecules
are extremely interesting functional small molecules in its own right.
However, due to the general difficulty in building up significant
strain energy during their synthesis and to the lack of appropriate
strategies and component-assembling reactions, it was only until
recently that the chemistry of CNT segment molecules expanded
rapidly.1-3
branched CNTs using carbon nanocages as templates. Moreover,
carbon nanocages should have vast utility in host-guest chemistry
as well as in the field of organic electronics.18
n–2
n–2
n–8
n–2
[n]cycloparaphenylene
([n]CPP)
[n.n.n]carbon nanocage
1 (n = 4), 2 (n = 5), 3 (n = 6)
One of the most featured CNT segment molecules in recent
years are cycloparaphenylenes (CPPs),3 which are ring-shaped
molecules that correspond to the shortest sidewall segment of
armchair CNTs (Figure 1). Since the first successful synthesis of
CPP by Bertozzi and Jasti in 2008,4 the groups of Itami, Jasti,
Yamago, Isobe, Müllen, Wegner, and Wang reported the synthesis
of various sizes of [n]CPPs (n = 5–16, 18),5–7 functionalized
CPPs,8–10 and CPP-related carbon nanorings.11–13 In 2013, our
group finally showed that CNTs can be synthesized in a diameter-
selective fashion by using CPPs as templates.14 In addition to being
used as carbon nanotube templates, CPPs themselves are useful
functional molecules. Representative interesting properties of
CPPs include the size-dependent photophysical properties15 and
their host-guest ability.16
straight
carbon nanotube
branched
carbon nanotube
Figure 1. [n]Cycloparaphenylene ([n]CPP) and [n.n.n]carbon
nanocages (1–3) as segments of straight and branched carbon
nanotubes.
In 2012, we designed and proposed all-benzene carbon
nanocages,
three-dimensional
cage-shaped
conjugated
hydrocarbons consisting solely of sp2-carbons and covalent bonds,
as a new family of molecular nanocarbons (Figure 1).17 The
[n.n.n]carbon nanocages consist of three [n]paraphenylene bridges
connected by two benzene-1,3,5-triyl bridgeheads. As carbon
nanocages represent junction units of branched CNTs (Figure 1),
We herein report the size-selective synthesis of [4.4.4]carbon
nanocage (1) and [5.5.5]carbon nanocage (2), adding to
[6.6.6]carbon nanocage (3), which we previously reported.17 With
these carbon nanocages of different sizes in hand, it was possible to
uncover the size effect in photophysical properties. In addition, the
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 9
first X-ray crystal structure of carbon nanocage (1) is reported. It energies per carbon atom. Thus, the strain energies for 1–3 and
1
2
3
4
5
6
7
8
should be noted that, Yamago recently disclosed another class of
carbon nanocages, which display interesting properties.19
[6]–[16]CPP per carbon atom versus diameter are plotted in
Figure 3, showing that the carbon nanocages share the same class of
strained macrocycles with CPP.20 These results indicate that similar
sizes of carbon nanocages and CPPs are almost equally strained,
and we thus envisioned that the synthetic strategy for middle-size
CPPs should be applicable to the synthesis of these carbon
nanocages.
RESULT AND DISCUSSION
1. Structural Features of Carbon Nanocages
Investigated by DFT Calculation
9
To gain insight into the structural features of carbon nanocages,
DFT calculations of 1–3 were conducted by Gaussian09 program
with the B3LYP/6-31G(d) level of theory. Optimized structures of
1–3 are shown in Figure 2. Symmetries of the optimized structures
are D3 (1, 3) and C3h (2), the difference of which comes from the
even or odd number of bridged phenylene rings. The cages contain
approximately spherical voids. The distances between the two tri-
substituted benzene rings of 1, 2, and 3 are 13.0 Å, 15.8 Å, and 18.4
Å, respectively and their radii estimated from the top view are 7.2 Å,
8.5 Å, and 10.1Å, respectively as shown in Figure 2. The sizes of the
[n.n.n]carbon nanocages are in between those of the
corresponding [2n+1]CPP and [2n+2]CPP.15a
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
Figure 3. Strain energies of carbon nanocages 1–3 (square) and [6]–
[16]CPP (filled circle) per carbon atom versus diameters (B3LYP/6-
31G(d)).
2. Synthesis of Carbon Nanocages
2-1. Strategy and Overview
The methodology we have developed for CPP synthesis uses
cyclohexane moieties as bent benzene precursors. The
combination of cis-diphenylcyclohexane and phenylene diboronic
acid produced strain-free macrocycles, which could then be
transformed to the corresponding [9]–[16]CPPs (Figure 4, eq.
1).5a–e For the synthesis of smaller CPPs such as [7] and [8]CPP,
the phenylcyclohexanone derivative was employed to form
triangular macrocycles (eq. 2).5f
Figure 2. Optimized structures of carbon nanocages 1–3 from top
(left) and side (right) view with strain energies (SE).
Based on hypothetical homodesmotic reactions, we estimated
the strain energies (SE) for 1–3 to be 96.0 kcal·mol–1, 80.0
kcal·mol–1, and 67.2 kcal·mol–1, respectively (see the Supporting
Information for details). Although these values are comparable to
those of very small CPPs (ex. SE[6]CPP = 96.0 kcal·mol–1),11a the real
stain on the cage molecules should be estimated by the strain
ACS Paragon Plus Environment

Page 3 of 9
Journal of the American Chemical Society
Scheme 1. Synthesis of carbon nanocage 3.
X
1
2
3
4
5
6
7
8
9
ref 5a–e
X
MOMO
(1)
Br
Br
RO
MOMO
X
[9]–[16]CPP
[7],[8]CPP
RO
X
Br
cyclic precursors
MOMO
X
(2)
Br
ref 5f
MOMO
Pd
RO
X
MOMO
OMOM
OMOM
4
O
MOMO
B(pin)
ref 17
X
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
(3)
(pin)B
B(pin)
6
RO
5
X
Br
RO
[6.6.6]
carbon nanocage
MOMO
OMOM
bicyclic precursors
X
X
X
(4)
MOMO
this work
RO
MOMO
[4.4.4], [5.5.5]
carbon nanocage
O
OMOM
Ni
H+
OMOM
OMOM
3
Figure 4. Outline of synthesis of [n]cycloparaphenylenes and
[n.n.n]carbon nanocages. X = halogen or boryl group. R = H or
methoxymethyl.
OMOM
MOMO
Meanwhile, for the synthesis of carbon nanocages, the route was
designed so that unstrained bicyclic precursors could be
synthesized by the combination of cyclohexane-containing units
and trisubstituted benzene units instead of the paraphenylene unit
(eq 3). Through this synthetic route, [6.6.6]carbon nanocage (3)
was successfully synthesized.17 To synthesize smaller carbon
nanorings (1 and 2), a phenylcyclohexanone derivative was chosen
for the small bent unit (eq 4).
MOMO
OMOM
MOMO
7
Strain energy (ΔH): 2.1 kcal·mol–1
2-3. Synthesis of [4.4.4]Carbon Nanocage (1) and
[5.5.5]Carbon Nanocage (2)
For the synthesis of smaller carbon nanocages 1 and 2, small
bicyclic macrocycles, shown in Scheme 2, were designed as possible
precursors for these cages. These macrocyclic precursors differ
from 7 in that the three cyclohexane moieties are directly attached
to the central trisubstituted benzene rings. The strain energies of
the precursors for 1 and 2 are 12.4 kcal·mol–1 and 11.6 kcal·mol–1,
respectively, which are greater than the 2.1 kcal·mol–1 value found
for 7 (see the Supporting Information for details). Those
molecules are more strained compared to 7 probably due to the
steric hindrance at the 1,3,5-tricyclohexylbenzene moieties.
2-2. Synthesis of [6.6.6]Carbon Nanocage (3)
Scheme 1 outlines the synthesis of [6.6.6]carbon nanocage (3).
The palladium-catalyzed Suzuki–Miyaura coupling reaction
between L-shaped unit 4 and 1,3,5-triborylbenzene 5 provided a
trifurcated unit 6, a molecule with C3 symmetry. Compound 6 was
then subjected to a threefold homocoupling reaction mediated by
Ni(cod)2/2,2'-bipyridyl (cod = 1,5-cyclooctadiene) to produce
bicyclic macrocycle 7. Treatment of 7 with acid afforded
[6.6.6]carbon nanocage (3) through conversion of the cyclohexane
moieties to benzene rings.17
Scheme 2. Retrosynthesis of carbon nanocages 1 and 2.
1
2
RO
OR
RO
RO
OR
OR
RO
RO
OR
RO
OR
OR
OR
OR
RO
OR
RO
RO
OR
RO
OR
RO
OR
RO
Strain energy (ΔH)
12.4 kcal·mol–1
11.6 kcal·mol–1
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 9
Based on these considerations, [4.4.4]carbon nanocage (1) and Synthesis of the bicyclic precursors and carbon nanorings 1 and
1
2
3
4
5
6
7
8
[5.5.5]carbon nanocage (2) were synthesized as follows. The
requisite trifurcate unit was synthesized from a new cyclohexane
unit, 4-(4-chlorophenyl)-4-hydroxycyclohexan-1-one (9), which
can be prepared from 1-bromo-4-chlorobenzene and 1,4-
cyclohexanedione monoethyleneketal in three steps.5f Thus, 3,5-
dibromophenyllithium, generated by the lithiation of 1,3,5-
tribromobenzene (8) with n-BuLi, was reacted with 9 to provide
10. The crude mixture was subjected to methoxymethyl (MOM)
protection to afford cis compound 11 in 60% yield after two steps.
The corresponding trans isomer was also formed, but this could be
easily separated from 11 by silica-gel column chromatography.
Subjecting 11 to an identical reaction sequence (lithiation,
carbonyl addition, MOM protection) provided the two-arm
compound 13 in moderate yield. After isolation, 13 was then
subjected to a similar sequence, yielding the trifurcated framework
14 in 72% yield. Finally, MOM protection of 14 proceeded
smoothly to yield 15 in 94% yield. The trifurcated and fully cis
structure of 15 was unambiguously confirmed by X-ray
crystallography (Scheme 3), the single crystal for which was
obtained from hot 2-propanol solution. Then, the Miyaura
borylation of 15 was conducted with the Pd2(dba)3/X-Phos
catalyst to yield 16, a viable coupling partner, in 93% yield. In order
to synthesize nanocage 2, a one-benzene extended coupling unit
17 was prepared by the Suzuki–Miyaura coupling of triborylated
compound 16 and 1,4-dibromobenzene in the presence of
PdCl2(dppf) catalyst and Ag2O (62% yield).
2 is shown in Scheme 4. Bicyclic macrocycle 18, the primary
precursor of 1, was synthesized in 9% yield by the threefold
homocoupling of 15 mediated by Ni(cod)2/1,10-phenanthrolin.
Compound 18 could also be prepared by a palladium-catalyzed
cross-coupling of 15 and its boronate counterpart 16, albeit in
somewhat lower yield (7%). Palladium-catalyzed cross-coupling
reaction between 16 and 17 produced a larger bicyclic macrocycle
19 in 11% yield. The final cyclohexane-to-benzene transformation
(aromatization) was achieved by treatment of 18 and 19 with
NaHSO4·H2O and o-chloranil in m-xylene/DMSO under reflux
conditions, furnishing [4.4.4]carbon nanocage (1) and
[5.5.5]carbon nanocage (2) in 29% and 53% yield, respectively.
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
1
The formation of these nanocages was confirmed by H and 13C
NMR spectroscopy, HH COSY, HMQC, and high-resolution
1
MALDI-TOF mass spectrometry. It is worth noting that the H
NMR chemical shifts for the singlet on the 1,3,5-benzenetriyl
moieties of 1, 2, and 3 are 7.65, 7.79, and 7.85 ppm, respectively,
showing a downfield shift with an increase in size; the same
tendency as observed in CPPs.4–7
Scheme 4. Synthesis of carbon nanocages 1 and 2.
MOMO
Br
MOMO
Br
X
MOMO
X
MOMO
OMOM
OMOM
OMOM
MOMO
OMOM
MOMO
OMOM
OMOM
Scheme 3. Preparation of trifurcate units.
MOMO
X
Br
Cl
MOMO
Br
15
(X = Cl)
16
(X = B(pin))
Cl
17
Br
Br
MOMO
Br
(a)
(a)
8
RO
61%
9%
7%
11%
MOMO
(a)
(b)
(c)
OR
+
Cl
OMOM
Br
(b)
Br
10 (R = H)
OMOM
MOMO
MOMO
OMOM
11 (R = MOM)
MOMO
OMOM
MOMO
63%
2 steps
MOMO
OMOM
MOMO
Cl
O
9
12 (R = H)
OMOM
(b)
93%
13 (R = MOM)
MOMO
OMOM
X
n
OMOM
MOMO
X
OMOM
MOMO
(c)
OMOM
OMOM
n
MOMO
MOMO
72%
OMOM
OR
MOMO
OMOM
MOMO
MOMO
OMOM
OMOM
MOMO
18
19
X
n
29%
52%
(d)
(d)
14 (R = H, X = Cl, n = 1)
(b), 94%
(d), 93%
(e), 62%
15 (R = MOM, X = Cl, n = 1)
16 (R = MOM, X = B(pin), n = 1)
17 (R = MOM, X = Br, n = 2)
X-ray Crystal Structure of 15
Reaction conditions: (a) bromide (8 or 11), n-BuLi, Et2O, –78 °C;
then 9. (b) iPr2NEt, MOMCl, CH2Cl2, rt. (c) n-BuLi, Et2O/THF = 2:1,
–78 °C; then 9. (d) B2(pin)2, Pd2(dba)3, X-Phos, KOAc, 1,4-dioxane,
110 °C. (e) 1,4-dibromobenzene, PdCl2(dppf), K2CO3, Ag2O,
toluene/water, 80 °C. MOM = methoxymethyl, B(pin) = 4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl,
dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl, dppf
diphenylphosphinoferrocene.
X-Phos
=
2-
1,1'-
=
1
2
ACS Paragon Plus Environment

Page 5 of 9
Journal of the American Chemical Society
Reaction conditions: (a) Ni(cod)2, 1,10-phenanthroline, THF,
reflux. (b) Pd(OAc)2, S-Phos, K3PO4, 1,4-dioxane/water, 100 °C. (c)
1
2
3
4
PdCl2(dppf), K2CO3, toluene/water, 80 °C. (d) NaHSO4·H2O, o-
2-
chloranil,
m-xylene/DMSO,
150
°C.
dicyclohexylphosphino-2',6'-dimethoxybiphenyl.
S-Phos
=
5
6
7
3. X-ray Crystal Structure of Carbon Nanocage 1
The cage-shaped structure of [4.4.4]carbon nanocage (1) was
unambiguously confirmed by X-ray crystallography (Figure 5). A
single crystal was obtained from a THF solution of 1 through slow
addition of Et2O vapor at room temperature. THF and Et2O
molecules were observed within the crystal structure for every cage
molecule with disordering. The shape of 1 was slightly distorted in
the solid state with respect to the calculated structure; the cavity
inside became ellipsoidal rather than spherical (see the Supporting
Information for details). Figure 6 shows the packing modes of 1 in
the bc plane and ac plane. These figures clearly show that nanocage
1 packs tightly, with paraphenylene bridges filling the grooves of
adjacent molecules. Similar to CPPs, no significant π-π interaction
was observed in the solid state of 1, most likely because the walls of
cage 1 are bent (convex-to-convex π-stacking is unfavorable).
Furthermore, Figure 6b shows the presence of a linear void channel,
suggesting that 1 may be useful as gas-storage material if solvent-
free crystals of 1 could be obtained.
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
Figure 6. Packing structures of 1. (a) bc plane, (b) ac plane.
4. Cyclic voltammetry of Carbon Nanocages
The cyclic voltammetry of carbon nanocages 1–3 was next
measured. The cyclic voltammograms are shown in Figure 7 along
with the oxidation potentials Eox. The values of Eox are determined
from the center voltage between the peak for the oxidation of the
sample and the peak for the reduction of the oxidized sample. All
the carbon nanocages showed reversible oxidation waves indicative
of the stability of the cationic species generated in these cage
skeletons. The first oxidation potential increases as the number of
paraphenylene units increases.
Eox = +0.92 V
3
Eox = +0.87 V
2
Figure 5. ORTEP drawing of 1·THF·Et2O with 50% thermal
ellipsoids. All hydrogen atoms, THF and Et2O molecules are omitted
for clarity.
Eox = +0.84 V
1
1"
0.8"
0.8
0.6"
0.6
0.4"
0.4
0.2"
0.2
0"
0
1
E / V (vs Fc/Fc+)
Figure 7. Cyclic voltammograms of 1–3 in n-Bu4NPF6/CH2Cl2 at
room temperature.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 9
5. Photophysical Properties of Carbon Nanocages
HOMO/HOMO–1→LUMO/LUMO+1 transitions in which the
1
2
3
4
5
6
7
8
pairs of HOMO/HOMO-1 and LUMO/LUMO+1 are
degenerated. Complicated transitions including ten orbitals from
HOMO–4 to LUMO+4 were found at around 330 nm. It can be
explained that the blue-shift of shoulder peaks in absorption and
fluorescence maxima is due to increasing of the HOMO-LUMO
gap21 as the size of the nanocage increases, as shown in Figure 9(a).
The photophysical properties (UV-vis absorption and
fluorescence) of carbon nanocages 1–3 were also investigated
(Figure 8); the results are summarized in Table 1. Absorption
maxima (λabs) were observed at 316 nm, 321 nm, and 325 nm for 1,
2, and 3, respectively, with molecular absorption coefficients (ε)
of 9.4 × 104 cm–1 M–1, 1.8 × 105 cm–1 M–1, and 1.9 × 105 cm–1 M–1,
respectively. The broadness of the spectra (380–420 nm) may
imply the existence of weak absorption peaks. The absorption
wavelengths of 1–3 are red-shifted with increasing cage sizes. This
trend is in sharp contrast to that of CPPs where they all exhibit
absorption maxima at 338–339 nm regardless of their ring sizes.15
On the other hand, the fluorescence maxima, determined by the
negative second derivatives (see the Supporting Information for
details), are gradually blue-shifted as the cage size increases. This
tendency is the same as the case of CPPs. All carbon nanocages
exhibit intense blue fluorescence with high fluorescence quantum
yields (ΦF = 0.66 (1), 0.76 (2), 0.87 (3)) as shown in Figure 8b.
(a)
LUMO+2
–1.39 eV
–1.59 eV
–1.44 eV
–1.57 eV
9
–1.48 eV
–1.57 eV
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
LUMO,
LUMO+1
HOMO,
HOMO–1
–5.37 eV
–5.39 eV
–5.40 eV
–5.45 eV
HOMO–2
–5.48 eV
–5.50 eV
1
2
3
(b)
LUMO
LUMO+1
LUMO+2
HOMO
HOMO–1
HOMO–2
(c)
Figure 8. (a) UV-vis absorption (solid line) and fluorescence (broken
line) spectra of 1 (red), 2 (green), and 3 (blue) in chloroform. (b)
Fluorescence of chloroform solution of 1.
LUMO+2
Table 1. Photophysical data for [n.n.n]carbon nanocages.a
LUMO,LUMO+1
absorption
fluorescence
d
λabsb [nm]
λemc [nm]
ΦF
compound
HOMO,HOMO–1
HOMO–2
1
2
3
316
321
325
b
427, 454, 483
418, 445, 476
414, 439, 471
0.66
0.76
0.87
a
c
In chloroform. The highest absorption maxima are given.
Emission maxima determined by negative second derivative are given. d
Absolute fluorescence quantum yields determined by a calibrated
integrating sphere system within ±3%.
Figure 9. (a) Energy diagrams of the frontier MOs of 1–3. (b)
Pictorial representations of the frontier MOs of 1 calculated at the
B3LYP/6-31G(d) level of theory. (c) Components of frontier MOs of
1–3.
To shed light on the nature of the electrochemical and
photophysical properties of the carbon nanocages, time-dependent
DFT (TD-DFT) calculations of the optimized structures of 1–3 at
the same calculation level (B3LYP/6-31G(d)) were performed. In
line with CV measurement, the smaller cage has higher HOMO
energy levels. Forbidden transitions at the highest wavelength
regions (377 nm (1), 372 nm (2), 371 nm (3)) were found as
We then questioned why the HOMO-LUMO gap increases as
the size of nanocage increases. In the case of CPPs, where the same
tendency is observed, we explained that the bending effect is the
main contributor to the HOMO-LUMO gap.15b Careful
investigation of the frontier orbitals of carbon nanocages provides
us different reasons. Depicted in Figure 9(b) are six representative
ACS Paragon Plus Environment

Page 7 of 9
Journal of the American Chemical Society
REFERENCES
frontier orbitals of 1. These orbitals are the combination of the
1
2
3
4
5
6
7
8
orbitals of three independent paraphenylenes and two bridgehead
benzene rings as shown in Figure 9(c). Because two orbitals are
degenerate at the HOMO of benzene, two orbitals (HOMO and
HOMO–1) are created by two of the three paraphenylene orbitals
and two benzene orbitals. The other HOMO of paraphenylene
remains as HOMO–2 of the cage. The energy of HOMO–2 is
raised as the size of the nanocage increases, because the length of
paraphenylene is the main contributor in this case. The energy
difference between HOMO–2 and HOMO/HOMO–1 reflects the
degree of interaction between paraphenylenes and central benzene
rings, which is probably affected by dihedral angles of the central
benzene and connecting phenylene (25.2° (1), 25.7° (2), 29.3°
(3)). The energy levels of unoccupied orbitals (LUMO, LUMO+1,
and LUMO+2) can also be explained in a similar manner. We
conclude that the characteristic blue-shift of the absorption and
fluorescence spectra of [n.n.n]carbon nanocages comes from the
orbital interactions between three [n]paraphenylene bridges with
two benzene bridgeheads.
1
Bunz, U. H. F.; Menning, S.; Martin, N. Angew. Chem., Int. Ed. 2012,
51, 7094.
2
(a) Fort, E. H.; Donovan, P. M.; Scott, L. T. J. Am. Chem. Soc. 2009,
131, 16006. (b) Bodwell, G. J. Nat. Nanotechnol. 2010, 5, 103. (c)
Jasti, R.; Bertozzi, C. R. Chem. Phys. Lett. 2010, 494, 1. (d) Yu, X.;
Zhang, J.; Choi, W. M.; Choi, J.-Y.; Kim, J. M.; Gan, L.; Liu, Z. Nano
Lett. 2010, 10, 3343. (e) Fort, E. H.; Scott, L. T. J. Mater. Chem.
2011, 21, 1373. (f) Scott, L. T.; Jackson, E. A.; Zhang, Q.; Steinberg,
B. D.; Bancu, M.; Li, B. J. Am. Chem. Soc. 2012, 134, 107.
For reviews, see: (a) Omachi, H.; Segawa, Y. Itami, K. Acc. Chem.
Res. 2012, 45, 1378. (b) Hirst, E. S.; Jasti, R. J. Org. Chem. 2012, 77,
10473. (c) Yamago, S.; Kayahara, E.; Iwamoto, T. Chem. Rec. 2014,
14, 84.
9
3
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
4
5
Jasti, R.; Bhattacharjee, J.; Neaton. J. B.; Bertozzi, C. R. J. Am. Chem.
Soc. 2008, 130, 17646.
(a) Takaba, H.; Omachi, H.; Yamamoto, Y.; Bouffard, J.; Itami, K.
Angew. Chem., Int. Ed. 2009, 48, 6112. (b) Omachi, H.; Matsuura,
S.; Segawa, Y. Itami, K. Angew. Chem., Int. Ed. 2010, 49, 10202. (c)
Segawa, Y.; Miyamoto, S.; Omachi, H.; Matsuura, S.; Sénel, P.;
Sasamori, T.; Tokitoh, N. Itami, K. Angew. Chem., Int. Ed. 2011, 50,
3244. (d) Segawa, Y.; Sénel, P.; Matsuura, S.; Omachi, H. Itami, K.
Chem. Lett. 2011, 40, 423. (e) Ishii, Y.; Nakanishi, Y.; Omachi, H.;
Matsuura, S.; Matsui, K.; Shinohara, H.; Segawa, H.; Itami, K. Chem.
Sci. 2012, 3, 2340. (f) Sibbel, F.; Matsui, K.; Segawa, Y.; Studer, A.;
Itami, K. Chem. Commun. 2014, 50, 954.
CONCLUSION
In summary, three sizes of strained, conjugated carbon
nanocages consisting solely of covalently bound benzene rings were
synthesized. Cyclohexane-containing units and 1,3,5-trisubstituted
benzene-containing units were assembled through stepwise
addition to yield the minimally strained bicyclic precursors, which
were in turn successfully converted into the corresponding carbon
nanocages via acid-mediated aromatization. X-ray crystallography
of 1 confirmed the cage-shaped structure with an approximately
spherical void. Electrochemical and photophysical measurements
and DFT studies revealed the unique properties of carbon
nanocages, including strain energies, size-dependent absorption
and fluorescence spectra, and degeneration of HOMO/HOMO–1
and LUMO/LUMO+1. With three sizes of carbon nanocages in
hand, further investigations are underway in our laboratory,
including determination of the guest-encapsulating properties of
carbon nanocages and the possibility of bottom-up chemical
synthesis of branched carbon nanotubes using these unique
molecules.
6
7
(a) Yamago, S.; Watanabe, Y.; Iwamoto, T. Angew. Chem., Int. Ed.
2010, 49, 757. (b) Iwamoto, T.; Watanabe, Y.; Sakamoto, Y.; Suzuki,
T.; Yamago, S. J. Am. Chem. Soc. 2011, 133, 8354. (c) Kayahara, E.;
Sakamoto, Y.; Suzuki, T.; Yamago, S. Org. Lett. 2012, 14, 3284. (d)
Kayahara, E.; Iwamoto, T.; Suzuki, T.; Yamago, S. Chem. Lett. 2013,
42, 621. (e) Kayahara, K.; Patel, V. K.; Yamago, S. J. Am. Chem. Soc.
2014, 136, 2284.
(a) Sisto, T. J.; Golder, M. R.; Hirst, E. S.; Jasti, R. J. Am. Chem. Soc.
2011, 133, 15800. (b) Xia, J.; Jasti, R. Angew. Chem., Int. Ed. 2012,
51, 2474. (c) Darzi, E. R.; Sisto, T. J.; Jasti, R. J. Org. Chem. 2012, 77,
6624. (d) Evans, P. J.; Darzi, E. R.; Jasti, R. Nat. Chem. 2014, 6, 404.
(a) Sisto, T. J.; Tian, X.; Jasti, R. J. Org. Chem. 2012, 77, 5857. (b)
Golder, M. R.; Wong, B. M.; Jasti, R. J. Am. Chem. Soc. 2012, 134,
19709.
(a) Nishiuchi, T.; Feng, X.; Enkelmann, V.; Wagner, M.; Müllen, K.
Chem. Eur. J. 2012, 18, 16621. (b) Golling, F. E.; Quernheim, M.;
Wagner, M.; Nishiuchi, T.; Müllen, K. Angew. Chem., Int. Ed. 2014,
53, 1525. (c) Tran-Van, A.-F.; Huxol, E.; Basler, J. M.; Neuburger,
M.; Adjizian, J.-J.; Ewels, C. P.; Wegner, H. A. Org. Lett. 2014, 16,
1594. (d) Huang, C.; Huang, Y.; Akhmedov, N. G.; Popp, B. V.;
Petersen, J. L. Wang, K. K. Org. Lett. 2014, 16, 2672.
8
9
ASSOCIATED CONTENT
Supporting Information. Experimental procedures, spectra of new
compounds, and details of computational studies. CCDC 1006531
(1·THF·Et2O), 1006666 (15). This material is available free of charge
via the Internet at http://pubs.acs.org.
10 Ishii, Y.; Matsuura, S.; Segawa, Y.; Itami, K. Org. Lett. 2014, 16,
2174.
11 (a) Omachi, H.; Segawa, Y.; Itami, K. Org. Lett. 2011, 13, 2480. (b)
Yagi, A.; Segawa, Y.; Itami, K. J. Am. Chem. Soc. 2012, 134, 2962.
(c) Matsui, K.; Segawa, Y.; Itami, K. Org. Lett. 2012, 14, 1888. (d)
Yagi, A.; Venkataramana, G.; Segawa, Y.; Itami, K. Chem. Commun.
2013, 50, 957.
12 Iwamoto, T.; Kayahara, E.; Yasuda, N.; Suzuki, T.; Yamago, S. Angew.
Chem., Int. Ed. 2014, 53, 6430.
13 (a) Hitosugi, S.; Nakanishi, W.; Yamasaki, T.; Isobe, H. Nat.
Commun. 2011, 2, 492. (b) Hitosugi, S.; Yamasaki, T.; Isobe, H. J.
Am. Chem. Soc. 2012, 134, 12442. (c) Matsuno, T.; Kamata, S.;
Hitosugi, S.; Isobe, H. Chem. Sci. 2013, 4, 3179.
AUTHOR INFORMATION
Corresponding Author
*ysegawa@nagoya-u.jp, itami@chem.nagoya-u.ac.jp
ACKNOWLEDGMENT
This work was supported by the ERATO program from JST (K.I.) and
the Funding Program for Next Generation World-Leading Researchers
from JSPS (K.I.). K.M. is a recipient of JSPS fellowship for young
scientists. We thank Dr. Hiroshi Ueno (Nagoya Univ.) for assistance of
cyclic voltammetry, and Dr. Ayako Miyazaki (Nagoya Univ.) for
fruitful discussion and critical comments. Calculations were performed
using the resources of the Research Center for Computational Science,
Okazaki, Japan. ITbM is supported by the World Premier International
Research Center (WPI) Initiative, Japan.
14 Omachi, H.; Nakamura, T.; Takahashi, E.; Segawa, Y.; Itami, K. Nat.
Chem. 2013, 5, 572.
15 (a) Segawa, Y.; Omachi, H.; Itami, K. Org. Lett. 2010, 12, 2262. (b)
Segawa, Y.; Fukazawa, A.; Matsuura, S.; Omachi, H.; Yamaguchi, S.;
Irle, S.; Itami, K. Org. Biomol. Chem. 2012, 10, 5979. (c) Nishihara,
T.; Segawa, Y.; Itami, K.; Kanemitsu, Y. J. Phys. Chem. Lett. 2012, 3,
3125. (d) Fujitsuka, M.; Cho, D. W.; Iwamoto, T.; Yamago, S.;
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 9
Majima, T. Phys. Chem. Chem. Phys. 2012, 14, 14585. (e) Camacho,
17 Matsui, K.; Segawa, Y.; Namikawa, T.; Kamada, K.; Itami, K. Chem.
Sci. 2013, 4, 84.
18 (a) Seel, C.; Vögtle, F. Angew. Chem., Int. Ed. Engl. 1992, 31, 528.
(b) Hof, F.; Craig, S. L.; Nuckolls, C.; Rebek, J. Angew. Chem., Int.
Ed. 2002, 41, 1488.
19 Kayahara, E.; Iwamoto, T.; Takaya, H.; Suzuki, T.; Fujitsuka, M.;
Majima, T.; Yasuda, N.; Matsuyama, N.; Seki, S.; Yamago, S. Nat.
Commun. 2013, 4, 2694.
20 In the case of Yamago’s cage (ref 19), the strain energy per carbon
atom is 1.25 kcal·mol–1, which is almost the same as that of [9]CPP
(1.22 kcal·mol–1).
1
2
3
4
5
6
7
8
C.; Niehaus, T. A.; Itami, K.; Irle, S. Chem. Sci. 2013, 4, 187. (f)
Nishihara, T.; Segawa, Y.; Itami, K.; Kanemitsu Y. Chem. Sci. 2014,
5, 2293.
16 (a) Iwamoto, T.; Watanabe, Y.; Sadahiro, T.; Haino, T.; Yamago, S.
Angew. Chem., Int. Ed. 2011, 50, 8342. (b) Xia, J.; Bacon, J. W.; Jasti,
R. Chem. Sci. 2012, 3, 3018. (c) Iwamoto, T.; Watanabe, Y.; Takaya,
H.; Haino, T.; Yasuda, N.; Yamago, S. Chem. Eur. J. 2013, 19, 14061.
(d) Nakanishi, Y.; Omachi, H.; Matsuura, S.; Miyata, Y.; Kitaura, R.;
Segawa, Y.; Itami, K.; Shinohara, H. Angew. Chem., Int. Ed. 2014,
53, 3102. For CPP-related hosts, see: (e) Isobe, H.; Hitosugi, S.;
Yamasaki, T.; Iizuka, R. Chem. Sci. 2013, 4, 1293. (f) Hitosugi, S.;
Iizuka, R.; Yamasaki, T.; Zhang, R.; Murata, Y.; Isobe, H. Org. Lett.
2013, 15, 3199. (g) Sato, S.; Yamasaki, T.; Isobe, H. Proc. Natl. Acad.
Sci. U.S.A. 2014, 111, 8374.
9
21 It was revealed that the blue-shift of the fluorescence spectra of CPPs
is attributed to the increasing of the HOMO-LUMO gap. See ref 15c.
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
ACS Paragon Plus Environment

Page 9 of 9
Journal of the American Chemical Society
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
9
ACS Paragon Plus Environment