Narrowing Segments of Helical Carbon Nanotubes with Curved Aromatic Panels

Article abstract of DOI:10.1002/anie.201902893

Rigid molecular cylinders with a 1 nm diameter were synthesized by assembling arylene panels with Pt-mediated macrocylization. Chrysenylene panels that previously participated in tetrameric macrocyclization were contorted by the addition of two benzo groups on the sides to form dibenzochrysenylene, which allowed for a reduction in the numbers of participating panels to three. Consequently, narrowed cyclochrysenylene congeners were obtained. The narrowed chiral cylinders possessed width-dependent chiroptical properties. The magnetic transition dipole moment was dictated by the radius of a ring-current-like circle that was formed by local electric transition dipole moments on the cylinder.

Full text of DOI:10.1002/anie.201902893

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Narrowing segments of helical carbon nanotubes with curved
aromatic panels
Authors: Kanako Kogashi, Taisuke Matsuno, Sota Sato, and Hiroyuki
Isobe
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902893
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Narrowing segments of helical carbon nanotubes with curved
aromatic panels
Kanako Kogashi,[a] Taisuke Matsuno,[b] Sota Sato[b] and Hiroyuki Isobe[b]*
Abstract: Rigid molecular cylinders with a 1-nm diameter were
synthesized by assembling arylene panels with Pt-mediated
macrocylization. Chrysenylene panels that previously participated in
tetrameric macrocyclization were contorted by the addition of two
benzo groups on the sides to form dibenzochrysenylene, which
allowed for a reduction in the numbers of participating panels to three.
scarce.[11,13,14,15] We previously designed and synthesized the first
helical CNT segments, i.e., [4]cyclo-2,8-chrysenylenes ([4]CC),[11]
and revealed the anomalous properties of helical (P)/(M)-(12,8)-
congeners (q = 23.4°), such as unique chiroptical behaviors
(Figure 1).[5,16] However, examples of rigid chiral congeners are
still scarce,[5] and to deepen our understanding of the unique
Consequently, narrowed cyclochrysenylene congeners were obtained. cylinder chirality, a series of congeners with varying cylinder
The narrowed chiral cylinders possessed width-dependent chiroptical
properties. The magnetic transition dipole moment was dictated by
the radius of a ring-current-like circle that was formed by local electric
transition dipole moments on the cylinder.
widths and an identical chiral angle are indispensable.[17,18] We
herein report our first effort to alter the width of the helical CNT
segments. Contorting the arylene panels with steric constraints,
we succeeded in reducing the number of the participating panels
in
the
macrocyclization
process
for
[3]cyclo-3,11-
dibenzochrysenylene ([3]CdbC). A segment of (9,6)-CNT with q =
23.4° was thus obtained as a major product (Figure 1), and the
effects of narrowing were reflected in the physical properties, such
as isomerization behaviors and chiroptical properties.
a)
Rolled sheets of trigonal planar sp2-carbon atoms give rise to
unique carbonaceous cylinders known as carbon nanotubes
(CNT).[1,2] Chemistry-oriented recent investigations have resulted
in the emergence of discrete CNT segments and have begun
disclosing characteristics of CNTs as molecular entities.[ 3 , 4 , 5 ]
Being molecules, the finite CNT segments possess definite
structural features such as diameters, lengths, rigidity and carbon
arrangements.[5] These features are characterized and
categorized by vector descriptors of the carbon arrangements
(Figure 1). For instance, when the rigidity resides on the
cylindrical structure to fix the vector locations, the carbon
arrangements can be described by a so-called chiral vector Ch as
well as its integer indices known as a chiral index, (n,m).[2,6] The
CNTs and segments are then categorized as armchair, zigzag
and helical congeners with the relationships as n = m, m = 0 and
n ≠ m, respectively.[ 7 ] The physical characteristics of a CNT
depend on its chiral index[8] and, more specifically, on an angle
known as the chiral angle q between a unit vector and the chiral
vector (Figure 1).[9] The chiral angle is 30° for armchair congeners
and 0° for zigzag congeners,[10] and their in-between angles, 0° <
q < 30°, are associated with helical congeners.[ 11 ] Since the
synthesis of the first congener of [N]cyclo-para-phenylenes
([N]CPP) reported in 2008, a series of armchair segments with q
= 30° has dramatically accumulated to cover N = 5-16, 18, 20 and
21, which expanded the structural library of molecular segments
with this specific chiral angle.[4, 12 ] Synthetically, the armchair
library was expanded by the advent of a few privileged corner
units such as a cyclohexadiene unit, which were accumulated by
a number of relevant papers.[4] Contrarily, segments with chiral
angles other than q = 30° are much less explored and still
θ = 0°
chrysenylene = (3,2) unit
(n,0): zigzag
n = 16 as [4]CC
unit vectors
(9,6)
a1
θ
a2
(12,8)
chiral vector
θ
Ch = na1 + ma2
=
(
2
3
n
.
4
=
θ
(
°
n
5
=
,
-1
n
3
):
chiral angle
6
0
°
,
a
1
rmch
8
,
√ 3 m
m + 2n
θ = tan–1
2
0
a
,
i
r
2
1
a
s
(angle between a1 and Ch)
C
PP)
b)
R
R
4
3
R
R = hexyl
R
[4]CC
all A isomer = (12,8)-[4]CC
[3]Cdb
C
all A isomer = (9,6)-[3]Cdb
C
Figure 1. Helical carbon nanotube structures. a) Vector descriptors with
representative helical segments shown in red.[2,6] b) Molecular entities of the
segmental carbon nanotubes of the helical congeners. The first helical CNT
molecule ([4]CC) and the narrowed [3]CdbC molecule. These two molecules
share a common chrysenylene moiety and an identical chiral angle (23.4°). The
[3]CdbC molecule was narrowed by panel contortion with benzo-extensions on
both sides.
[a]
[b]
K. Kogashi
Department of Chemistry, Tohoku University
Aoba-ku, Sendai 980-8578, Japan
Dr. T. Matsuno, Dr. S. Sato, Prof. Dr. H. Isobe
Department of Chemistry, The University of Tokyo,
JST, ERATO, Isobe Degenerate π-Integration Project
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
E-mail: isobe@chem.s.u-tokyo.ac.jp
Arylene panel structures play key roles in deciding the
numbers of panels participating in macrocyclization,[19] and during
our synthetic study of a sextuply helical molecule,[20] we realized
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201902893
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that a contorted aromatic panel of dibenzochrysene (denoted dbC)
could tune the panel numbers to form a narrow cylindrical
macrocycle composed of chrysenylene units. Thus, in the dbC
panel, two boryl linkers were first installed at its 3,11-positions via
an 8-step synthetic process starting from a commercially available
compound (Figure 2). Bromoanisole 1 was iodized[21] to furnish
acetylene with aryl termini via Sonogashira coupling, and the
subsequent four steps were performed without fully purifying the
synthetic intermediates to afford 3 in 47% yield. The diborylated
dbC precursor 4 for macrocyclization was then synthesized via ICl-
promoted cyclization, Mizorogi-Heck coupling and Miyaura
borylation.[22] As was measured in the theoretical structure (Figure
2a), the dbC panel was contorted to have the 3,11-substituents
bent out of plane at an angle of 140°.[23] We then examined the
macrocyclization with the contorted dbC panel. The panel
contortion successfully reduced the number of dbC panels
involved in the Pt-mediated macrocyclization,[11,24] and trimeric
[3]CdbC was exclusively obtained in 23% yield (see MS spectrum
in Figure 2b). The trimeric macrocyclization is geometrically
reasonable: in trimeric, six-cornered Pt-complex (5), the sum of
the internal angles is 690° (Figure 2a), which is close to the ideal
value of 720° for a hexagon.
a)
1) CF3CO2Ag (1.1 eq.)
I2 (1.1 eq.)
CHCl3/MeOH
Br
Br
rt, 15 h, 85%
MeO
OMe
MeO
2)
(0.50 eq.)
TMS
Br
PdCl2(PPh3)2 (6 mol%)
CuI (10 mol%)
DBU (6.0 eq.)
H2O (0.40 eq.)
toluene, 60 °C, 47 h, 67%
2
1
1)
(2.0 eq.)
R
pinB
R
Pd(OAc)2 (5 mol%)
PPh3 (10 mol%), K2CO3 (4.0 eq.)
H2O/1,2-dimethoxyethane, 80 °C, 12 h
2) ICl (1.2 eq.), CH2Cl2, –78 °C, 12 h
OH
HO
3) PdCl2(PPh3)2 (8 mol%)
NaOAc (4.0 eq.)
N,N-dimethylacetamide, 120 °C, 47 h
4) BBr3 (2.3 eq.), CH2Cl2, rt, 4 h
47% (4 steps)
3
R
R = hexyl
R
1) Tf2O (3.0 eq.)
pyridine/CH2Cl2, rt, 2 h, 96%
11
Bpin
3
pinB
2) PdCl2(dppf)·CH2Cl2 (10 mol%)
(Bpin)2 (2.2 eq.)
KOAc (6.0 eq.)
1,4-dioxane, 80 °C, 90 h, 54%
4
R
PtCl2(cod) (1.0 eq.)
CsF (6.0 eq.)
140°
≡
88°
142°
THF, 65 °C, 24 h
oDCB, 100 °C, 3 h
4
(PM6)
Trimeric Pt-complex 5
(PM6)
b)
[3]Cdb
C
PPh3 (10 eq.)
[3]Cdb
C
oDCB, rt, 0.5 h
100 °C, 24 h
23% (2 steps)
[4]Cdb
C
1500
2000
m/z
Figure 2. Synthesis of [3]CdbC. a) Synthetic scheme. Theoretical structures of
4 and 5 were obtained from PM6 calculations using methyl models. b) MALDI
MS spectrum of the crude materials from the final macrocyclization step of 4.
Trimeric [3]CdbC was obtained as a mixture of atropisomers,
and we succeeded in separating and identifying all of the isomers
(Figures S1-S3).[5,13, 25 ] We isolated four stereoisomers using
1
cholesterol-loaded silica gel columns, and their H NMR spectra
revealed the presence of two diastereomers composed of
enantiomer pairs. The presence of four total stereoisomers
indicated the presence of coplanar biaryl linkages and,
consequently, persistent cylindrical shapes.[5] The numbers of
aromatic 1H resonances for the two diastereomers were 6 and 18
(Figure S2), which allowed us to assign them as (9,6)-[3]CdbC (D3
point symmetry) and (8,7)-[3]CdbC (C2 point symmetry),
respectively. Among the diastereomers, (9,6)-[3]CdbC possessed
the chiral angle (24.3°) identical to that of (12,8)-[4]CC and was
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10.1002/anie.201902893
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one panel smaller/narrower than the [4]CC congeners (Figure 1).
As shown in Figure 3, we isolated and identified all of the possible
isomers arising from panel orientations (AAA/BBB and AAB/BBA)
of the trimeric cylindrical cycloarylenes.[5]
400
300
(M)-(9,6)-[3]Cdb
C
200
100
0
R
R
350
400
450
500
550
600
nm
R
R
–100
–200
R
R
Δε
R
R
(P)-(9,6)-[3]Cdb
C
–300
–400
R
R
150
100
50
(M)-(8,7)-[3]Cdb
C
R
R
(P)-(9,6)-[3]Cdb
C
(M)-(9,6)-[3]Cdb
C
8.3% yield
8.3% yield
tf = 3.64, Fb = 61%, Fa = 68% tf = 3.64, Fb = 61%, Fa = 68%
D3 point symmetry D3 point symmetry
0
350
400
450
500
550
600
nm
–50
–100
–150
Δε
(P)-(8,7)-[3]Cdb
C
R
R
R
R
R
R
R
R
Figure 4. Experimental and theoretical CD spectra. Dotted lines show
theoretical spectra obtained by TDDFT calculations. See also Figure S3 for
reference UV-vis spectra.
R
R
R
R
(P)-(8,7)-[3]Cdb
C
(M)-(8,7)-[3]Cdb
C
The molecular structure of (9,6)-[3]CdbC was revealed by the
crystallographic analysis of a single crystal. Although extensive
efforts to crystallize an enantiomer failed, growing a single crystal
of a racemic mixture was feasible.[ 26 ] Thus, we mixed an
equimolar amount of (P)- and (M)-(9,6)-[3]CdbC in
tetrahydrofuran/ethanol and grew a single crystal by slow
evaporation at 25 °C. As shown in Figure 5, the crystal structure
revealed the cylindrical shape of the molecule with a diameter of
1.06 nm, which is geometrically expected for (9,6)-CNT (dt = 1.04
nm).[2] An enantiomer pair was entangled in a thread-in-bead
manner, which may have facilitated the effective crystal growth
from the racemate.[10] The enantiomer pairs were aligned to form
a column of cylinders, which were often noted with cylindrical
molecules.[10,13,27]
2.4% yield
2.3% yield
tf = 3.89, Fb = 59%, Fa = 66% tf = 3.89, Fb = 59%, Fa = 66%
C2 point symmetry C2 point symmetry
Figure 3. Structures of [3]CdbC isomers. R = hexyl. See ref. 6 for the vector
descriptors.
Absolute configurations of [3]CdbC were determined by CD
spectra with the aid of theoretical calculations (Figure 4). Despite
an expected underestimation of optical gaps,[11] time-dependent
density functional theory (TDDFT) calculations of (P)-isomers
reproduced one of the spectra to allow us to assign the
handedness of (9,6)- and (8,7)-[3]CdbC. The TDDFT calculations
performed at the B3LYP/6-31G(d,p) theory level reasonably
reproduced the other chiroptical properties, such as intense
rotatory strengths (~10–37 erg esu cm G–1; Supporting Information).
The chiroptical properties were further investigated by in-depth
analyses to disclose unique width-dependency (see below).
An important structural feature for the cylinder chirality, i.e.,
rigidity, was also confirmed by experiments. The panel rotations
in the [3]CdbC molecules were thus severely hindered by a large
rotational barrier. When we heated (P)-(9,6)-[3]CdbC at 200 °C in
solution, no isomerization was observed even after 24 h (Figure
S5). Previous [4]CC molecules have been shown to be belt-
persistent at ambient temperature, but their half-life has been
estimated to be as short as 2 sec at 180 °C.[10,11] Thus, these
results confirmed a tougher cylinder shape in narrower [3]CdbC
cylinders.
a)
b)
c)
Figure 5. Crystal structures of (9,6)-[3]CdbC. a) Molecular structure of (P)-
isomer (top view). The diameter was measured as the distance between the two
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10.1002/anie.201902893
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midpoints of the C-C bonds located at the opposite side of the wall. b) Racemic
entanglement (side view). c) Columnar assembly of enantiomer pairs.
(P)-(9,6)-[3]Cdb
C
m
Finally, an in-depth analysis of the theoretical data allowed
us to reveal the width-dependent origins of the chiroptical
properties. Previously, with the wider (12,8)-[4]CC cylinders, we
showed an intense magnetic transition dipole moment (MTDM;
m) as a key feature of the chiroptical behaviors. With the present
µlocal
180°
µtotal
narrower (9,6)-[3]CdbC cylinders,
a detailed decomposition
analysis of the total electric transition dipole moment (ETDM;
µtotal) deepened our understanding of the width-dependent
chiroptical properties.[28] When the µtotal vector was decomposed
into local contributions, we obtained three local µlocal vectors
located near lines tangent to the [3]CdbC cylinder (Figure 6). Their
unique locations near a horizontal plane of the cylinder resulted
in a small total µtotal vector on the order of 10–18 esu cm. Because
the three µlocal vectors were further balanced out by the C2
symmetric AAB panel orientations, the (8,7)-isomer possessed a
smaller µtotal vector that was 1/3 that of the (9,6)-isomer.
Nonetheless, MTDM |m| size of these two isomers were almost
identical. Thus, the three local µlocal moments on the tangent lines
were aligned in a cyclical manner with an identical direction,
irrespective of panel orientations, and intensified the current-
induced m. As can be seen from the top view in Figure 6, the
horizontal components of the µlocal vectors are similar between the
(9,6)- and (8,7)-isomers, and the widths of the two cylinders are
almost identical (1.04 nm vs. 1.03 nm). Therefore, when an
MTDM is generated by a transient current circulating on the
cylinder through a combination of µlocal, the vector size of m is
determined by the relationship of m µ r ´ µlocal and is therefore
dependent on the cylinder width. The relative size of the MTDM
of (P)-(9,6)-[3]CdbC against (P)-(12,8)-[4]CC was 0.64 (Figures S6
and S8), which is comparable to the relative size of the cylinder
widths (1.04 nm / 1.38 nm = 0.75). As the alignment of µlocal on a
horizontal plane is essential for the unique and intense MTDM,
chiroptical behaviors of an infinite CNT with a higher freedom of
µlocal directions may differ from the present finite congeners.[29] As
was disclosed by our previous study, another important feature of
the π-conjugated systems of the rigid cylinders is the relative
directions of the two moments (µtotal and m): these two moments
were nearly aligned in an antiparallel manner to maximize the
rotatory strength.[30]
|µtotal| = 1.49×10–18 esu cm
|m| = 6.45×10–20 erg G–1
|r| = dt/2 = 0.518 nm
(P)-(8,7)-[3]Cdb
C
m
170°
µlocal
µtotal
|µtotal| = 0.445×10–18 esu cm
|m| = 6.30×10–20 erg G–1
|r| = dt/2 = 0.515 nm
Figure 6. Decomposition analysis of the chiroptical properties of [3]CdbC. Local
ETDM (µlocal) (left; 1 a.u. = 4 Å) and total ETDM (µtotal)/MTDM (m) (right; 1 a.u.
= 1 Å). See also Figures S6 and S7.
In contrast to armchair CNT segments with a variety of
[N]CPP-like congeners,[4] structural variations of helical CNT
segments are much less explored, despite their unique chiral
structures.[5] This study demonstrated that a method to narrow the
chiral cylinders can be devised by adopting curved arylene panels
in the Pt-mediated macrocyclization.[10-15,24,31] Width variations of
the chiral molecular cylinders allowed us to deepen our
understanding of their unique chiroptical properties.[18] Notably, a
transient cyclic current can be induced on the chiral cylinders
upon photo-excitation, which intensifies current-induced magnetic
dipole moments in a width-dependent manner. Although the
present narrowed congeners deepened our understanding of
chiroptical properties of aromatic cylinders, the molecules were
not of particular interest for use as circularly polarized
luminescence (CPL) materials.[16,32] Requisite properties such as
large R values (see above) and efficient fluorescence were
missing with the present congeners (Figure S4).[16,33,34] To further
utilize the chiroptical properties such as CPL of the chiral cylinders,
the widening of rigid helical CNT segments is an intriguing next
subject to be explored in the future. Together with the lengthening
of the cylinders,[13] unique chiroptical materials may be developed.
Acknowledgements
This study is partly supported by JST ERATO (JPMJER1301) and
KAKENHI (18J10155, 16K05681, 16K04864 and 17H01033). We
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10.1002/anie.201902893
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were granted access to the X-ray diffraction instruments in
SPring-8 BL38B1 beamline (no. 2018A1061).
Keywords: chirality • circular dichroism • cylinders •
macrocycles • nanotubes
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10.1002/anie.201902893
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
A method to narrow rigid cylinder
cycloarylenes was devised and
revealed width-dependent chiroptical
properties.
Kanako Kogashi, Taisuke Matsuno, Sota
Sato and Hiroyuki Isobe*
Page No. – Page No.
Narrowing segments of helical
carbon nanotubes with curved
aromatic panels
This article is protected by copyright. All rights reserved.