Synthesis of pentadecaphenylenes, their inclusion properties, and nanostructure formation with C60

Article abstract of DOI:10.1039/c3cc42922a

Synthesis of macrocyclic pentadecaphenylene was carried out via electron-transfer oxidation of the corresponding Lipshutz cuprate. Pentadecaphenylene self-aggregated in solution to form a gel. Furthermore, it incorporated C₆₀ in its cavity to produce a fibrous inclusion complex which showed a high gelation ability in benzene.

Full text of DOI:10.1039/c3cc42922a

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Synthesis of pentadecaphenylenes, their inclusion
properties, and nanostructure formation with C60†
Cite this: DOI: 10.1039/c3cc42922a
a
a
M. Jalilur Rahman,z Hideyuki Shimizu,z Yasuyuki Araki,b Hiroshi Ikedac and
Received 19th April 2013,
Accepted 25th May 2013
Masahiko Iyoda*a
DOI: 10.1039/c3cc42922a
www.rsc.org/chemcomm
Synthesis of macrocyclic pentadecaphenylene was carried out via
electron-transfer oxidation of the corresponding Lipshutz cuprate.
Pentadecaphenylene self-aggregated in solution to form a gel. Further-
more, it incorporated C60 in its cavity to produce a fibrous inclusion
complex which showed a high gelation ability in benzene.
The study of fully conjugated cyclic oligophenylenes with well-defined
molecular diameters has become important for gaining specific
information concerning structural, electronic, and optical properties
especially in relation to the ‘‘inner’’ and ‘‘outer’’ domains.1 These
macrocycles with both long alkyl sidechains and shape-persistent,
non-collapsible, and fully p-conjugated backbones are useful for
building columnar 1D nanotubes, 2D porous surface networks,
and 3D inclusion complexes via self-assembly.2 Furthermore, cyclic
oligoarylenes with well-defined size and inner cavities are of interest
because of their ability to act as a host in host–guest chemistry and
potential use in optoelectronic devices.1f,3 Recently, Yamago and
co-workers have reported that cyclo[10]paraphenylene encapsulates
C60.4 In the course of our research, we have reported the synthesis
of nonaphenylenes and dodecaphenylenes via electron-transfer oxida-
tion of Lipshutz cuprate intermediates and the formation of nano-
Fig. 1 Hexaoctyl- (1a) and hexabutylpentadecaphenylenes (1b) and the parent
pentadecaphenylene (1c).
structures from hexadodecyloxynonaphenylene.5,6 Herein, we report 3 under Suzuki coupling conditions proceeded smoothly to produce
the synthesis of much larger triangular pentadecaphenylenes 1a and bis(trimethylsilyl)dialkylquinquephenyls 4a and 4b in 74% and 62%
1b (Fig. 1) and the self-aggregation and formation of a nanostructure yields, respectively. The trimethylsilyl groups of 4a and 4b were
of 1a as well as the complexation of 1a with C60.
converted into bromo and iodo groups using NBS and ICl, respec-
Pentadecaphenylenes 1a and 1b were synthesized by modifying tively, to afford dihalodialkylquinquephenyls 5a–c in 85–96% yields.
our procedure for synthesizing cyclic oligophenylenes (Scheme 1).5a Although the electron-transfer oxidation of Lipshutz cuprates derived
The reaction of dialkyldiiodobenzenes 2a and 2b with a boron reagent from diiodide 5c produced 1a in 20% yield, the yield was increased to
28% by using dibromide 5a as the starting material. Since 2-iodo-2-
a Department of Chemistry, Graduate School of Science and Engineering,
methylpropane formed by the reaction of 5c with t-BuLi quenches
lithioaromatics in the reaction cage easier than 2-bromo-2-methyl-
Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan.
E-mail: iyoda@tmu.ac.jp; Fax: +81-42-677-2525; Tel: +81-42-677-2731
b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
propane does, the electron-transfer oxidation of Lipshutz cuprates
prepared from dibromides 5a and 5b afforded linear oligophenyls as
by-products in lower yields.7 Electron-transfer oxidation of Lipshutz
cuprates prepared from dibromide 5b produced 1b in 30% yield.
B3LYP/6-31G(d) calculations on 1c (R = H) showed that it has a
C3 symmetric triangular structure with the torsional angle involving
the o-terphenylene and biphenylene units being 361–521 (ESI†).
Although it is a nonplanar aromatic hydrocarbon with nonpolar
2-1-2 Katahira, Aoba-ku, Sendai 980-8577, Japan
c Department of Applied Chemistry, Graduate School of Engineering, Osaka
Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
† Electronic supplementary information (ESI) available: Experimental procedures
and characterization data for 1a,b, 4a,b, 5a,b, and 2 : 1 1aꢀC60 fibers. Fig. S1–S24.
Cartesian coordinates of the optimized structure of 1c. See DOI: 10.1039/
c3cc42922a
‡ These authors contributed equally to this work.
c
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Fig. 2 (a) Photograph of a gel of 1a and (b) SEM image of fibers of 1a.
spectra of the film and fiber were different from that of the THF
solution due to electronic interaction in the excited state (ESI†). The
fluorescence spectra of the film (lem = 413 nm, FF = 0.71) and fiber
(414 nm, FF = 0.62) were red shifted compared to that of the THF
solution (388 and 404 nm, FF = 0.99). From excitation spectra, it was
observed that the 1a film (lex = 364 nm) and fiber (371 nm) had lower
excitation energies than the THF solution (320 nm) (ESI†). Thus, we
concluded that excimer emission occurred in the solid state.
Although the fiber and film showed similar absorption and emis-
sion behavior, their X-ray diffraction (XRD) patterns were quite different
(ESI†). The XRD pattern of the fiber had reflections at 2y = 2.941, 4.501,
6.201, and 6.621 (d = 30.0, 19.6, 14.2, and 13.3 Å, respectively), with many
weak reflections in the 2y range of 8.01–15.61, indicating moderate
crystallinity in the solid state. The XRD pattern of the film from
benzene indicated an amorphous structure, whereas that of the film
from THF exhibited an intense reflection at 2y = 2.841 (d = 31.1 Å) and
broad reflections in the 2y range of 4.81–6.01, indicating the formation
of a roughly aligned hexagonal structure.
As expected from B3LYP/6-31G(d) calculations, pentadecaphenylene
1a formed an inclusion complex with C60 in benzene-d6. The 1H NMR
spectrum of a 1 : 1 mixture of 1a and C60 in benzene-d6 at 25 1C (0.55 ꢂ
10ꢁ3 M) showed an upfield shift of the aromatic protons by 0.01 ppm
in comparison to the chemical shifts of 1a itself (ESI†). A Job plot
prepared from UV-Vis spectra at 410 nm in toluene with a total
concentration of 5.0 ꢂ 10ꢁ4 M indicated that a 1 : 1 complex
formed, although the binding constant of 1a with C60 could not
be accurately determined from the analysis of the 1H NMR
spectra due to the competition between the self-aggregation of
1a and its complexation with C60.9
Scheme 1 Synthesis of hexaoctyl- (1a) and hexabutylpentadecaphenylenes
(1b) and the formation of 1aꢀC60 1 : 1 and 2 : 1 complexes.
alkyl side chains, 1a exhibited self-aggregation behavior at low
temperatures in hydrocarbon solvents such as cyclohexane,
benzene, and toluene. Nanophase separation of the inner
cavity, outer domain, and long alkyl side chains causes the
self-aggregation behavior of 1a.
At room temperature 1a weakly aggregated in toluene and
benzene; however, much stronger aggregation was observed at lower
temperatures. The aggregation ability was investigated using
1H NMR spectroscopy in toluene-d8 in the temperature range of
ꢁ20 to 20 1C and with different concentrations (0.15–2.26 ꢂ 10ꢁ3 M)
of 1a (ESI†). Assuming a monomer–oligomer equilibrium,
the association constant (Ka) at ꢁ20 1C was found to be 5.28 ꢂ
103 Mꢁ1. The self-aggregation of 1a is enthalpy driven and entropy
opposed. The large negative enthalpy (ꢁ87.8 kJ molꢁ1) is attributed
to the van der Waals interaction in both the nonplanar pentadeca-
phenylene moiety and the alkyl side chains. In addition, the change
in entropy was negative and very large (ꢁ276 J Kꢁ1 molꢁ1) which
indicates some ordering of the long alkyl side chains due to
solvophobic interaction. In contrast to 1a, 1b is much less soluble.
Therefore, further studies on the self-aggregation and nanostructure
formation abilities were performed only on 1a.
From the self-aggregation behavior of 1a in benzene and toluene,
1a was expected to form either one-dimensional (1D) or two-dimen-
sional (2D) nanostructured morphologies, although 1a neither has a
planar framework nor polar side groups. When a hot solution of 1a
in benzene (15 mg mlꢁ1) was allowed to stand at room temperature,
a white gel formed (Fig. 2a). As shown in SEM images (Fig. 2b), the
gel of 1a appeared to be a bundle of nanofibers with widths in the
range of 200–500 nm and lengths of more than 100 mm. A 1H NMR
spectrum of the gel of 1a formed in benzene-d6 showed a small
up- and downfield shift of aromatic and alkyl protons, respectively,
owing to the self-aggregation of 1a.8
Moreover, 1a and C60 combined to form a nanostructure,
although the binding constant of 1a with C60 is small. When a
hot purple solution of a mixture of 1a (6.9 mg) and C60 (1.4 mg,
0.5 molar equiv.) in benzene (1 ml) was allowed to stand at
room temperature, a dark brown gel, composed of 1a and C60 in
a ratio of 2 : 1, formed (Fig. 3a). Similarly, a mixture of 1a and
C
60 (1 molar equiv.) in benzene (1 ml) yielded a gel. The UV-Vis
spectra of the 2 : 1 and 1 : 1 gels of 1aꢀC60 in benzene showed an
intense broad CT-absorption band at 465 nm as well as very
broad tailing up to 800 nm (Fig. 3b).
When hexane was added to the 2 : 1 and 1 : 1 1aꢀC60 gels in
benzene, a 2 : 1 1aꢀC60 fiber was obtained in 86 and 81%,
respectively, as a brownish black solid.10 Optical and SEM
images of the xerogel showed a fibrous structure with widths
in the range of 100–400 nm and lengths >100 mm (Fig. 4a and b).
Much smaller amounts of 1a and C60 compared to that of 1a by
The UV-Vis absorptions of a film (lmax = 316 nm), a fiber
(321 nm), and a THF solution (1.63 ꢂ 10ꢁ6 M, 318 nm, and log e 5.27)
of 1a occurred at similar values of lmax owing to a small intermole-
cular p–p interaction in the ground state, whereas the emission
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Lipshutz cuprate intermediates derived from dihaloquinquephenyl
derivatives in a one-pot reaction. In solution 1a and 1b exhibited a
strong UV-Vis absorption at 318 nm and intense blue emission at
B400 nm with a very high quantum efficiency (FF E 100%). Despite
the rigid non-planar framework and non-polar side arms, hexaoctyl-
pentadecaphenylene 1a showed self-aggregation behavior (nanophase
separation) in hydrocarbon solvents such as benzene and toluene, to
afford a gel composed of fibers of 1a. Since there is a large inner cavity
in 1a, it can act as a host molecule in solution to form a 1 : 1 complex
with C60. Moreover, from a mixture of 1a and C60 in benzene, a dark
brown gel composed of entangled fibers of the 2 : 1 1aꢀC60 complex,
which showed an intense CT-absorption band at 465 nm with tailing
up to 800 nm, formed. The gelation ability of 1aꢀC60 was much higher
than that of 1a by itself. The 2 : 1 sandwich complex aligned lamellarly
and stacked parallel to the direction of the fiber. In the presence of
additional C60 molecules, C60 was intercalated between the 1a–C60–1a
sandwich complex to form 1.5 : 1 to 1 : 1.5 complexes (1.5 : 1 to 1 : 1.5
1aꢀC60 gels). It is noteworthy that an increase in additional C60
molecules enhances the gelation ability.
Fig. 3 (a) Photograph of a 2 : 1 1aꢀC60 gel. (b) UV-Vis spectra of a 2 : 1 1aꢀC60 gel,
a 1 : 1 1aꢀC60 gel, and C60 in benzene.
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas ‘‘Organic Synthesis based on
Reaction Integration’’ (No. 2105). We thank Dr M. Hasegawa
(Kitasato University) for the structural calculations on 1c.
Fig. 4 (a) Optical micrograph and (b) SEM image of a 2 : 1 1aꢀC60 fiber.
Notes and references
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3 For a recent review, see: F. Davis and S. Higson, Macrocycles -
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6 ‘Electron-transfer oxidation of Lipshutz cuprates’ is a typical organic
synthetic method based on reaction integration: J.-I. Yoshida,
K. Saito, T. Nokami and A. Nagaki, Synlett, 2011, 1189–1194.
7 For electron-transfer oxidation of Lipshutz cuprates, see:
(a) Y. Miyake, M. Wu, M. J. Rahman and M. Iyoda, Chem. Commun.,
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8 The upfield shift (B0.06 ppm) of aromatic protons and the down-
field shift (B0.01 ppm) of octyl protons except for the benzyl proton
indicate the self-aggregation of 1a (ESI†).
Fig. 5 (a) Structure of the 2 : 1 1aꢀC60 complex. (b) Possible lamellar arrangement
of the 2 : 1 1aꢀC60 complex. (c) Possible lamellar arrangement of the 1 : 1 1aꢀC60
complex.
itself formed gels in benzene, also indicating the formation of a
more entangled fibrous structure.
XRD profiles of the 2 : 1 1aꢀC60 xerogel showed a strong reflection
at 2y = 2.161 (d = 40.9 Å) with weak reflections at 2y = 4.321 (d = 20.4 Å)
and 6.481 (d = 13.6 Å). XRD profiles of the 1:1 1aꢀC60 xerogel exhibited
a similar but more broad reflections (ESI†). Although 1a fibers had
moderate crystallinity, the 2 : 1 1aꢀC60 xerogel (fiber) mainly had a
lamellar form, and we concluded that a giant 1a–C60–1a sandwich
complex, aligned lamellarly and stacked in the direction of the fiber,
formed (Fig. 5a and b). In the case of the 1 : 1 complex, the excess C60
molecules were intercalated between the 1a–C60–1a sandwich
complex to form a gel (Fig. 5c). Interestingly, in the 2 : 1 1aꢀC60 fiber
prepared from hexane–benzene molecules aligned hexagonally and
stacked in the direction of the fiber formed (ESI†).
9 Fluorescence lifetime measurements of the 1aꢀC60 complex from a
mixture of 1a (0.15 ꢂ 10ꢁ3 M) and C60 (0.15 ꢂ 10ꢁ3 M) in toluene
revealed that 5–20% of the 1aꢀC60 complex formed in solution.
10 By mixing 1a with 0.5, 0.75, 1.0, and 1.5 molar equiv. of C60 in hot
benzene, dark brown gels composed of 2 : 1, 1.5 : 1, 1 : 1, and 1 : 1.5
1aꢀC60 formed, and almost all C60 was incorporated in the gels.
In summary, we synthesized large triangular pentadecaphenylenes
1a and 1b in moderate yields via electron-transfer oxidation of
c
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