Stacked 1,3,5-tris[(2,5-dimethylphenyl)ethynyl]benzenes: Dimer and conjugated microporous polymer

Article abstract of DOI:10.1016/j.tetlet.2011.08.067

A new [2.2]paracyclophane compound consisting of two 1,3,5-tris[(2,5- dimethylphenyl)ethynyl]benzenes stacked in proximity to each other. The compound exhibited a unique absorption band (cyclophane band) and an emission from the phane state, both of which

Full text of DOI:10.1016/j.tetlet.2011.08.067

Tetrahedron Letters 52 (2011) 5504–5507
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Stacked 1,3,5-tris[(2,5-dimethylphenyl)ethynyl]benzenes: dimer
and conjugated microporous polymer
⇑
⇑
Yasuhiro Morisaki , Masayuki Gon, Yuichi Tsuji, Yuichi Kajiwara, Yoshiki Chujo
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new [2.2]paracyclophane compound consisting of two 1,3,5-tris[(2,5-dimethylphenyl)ethynyl]ben-
zenes stacked in proximity to each other. The compound exhibited a unique absorption band (cyclophane
Received 19 July 2011
Revised 10 August 2011
Accepted 11 August 2011
Available online 2 September 2011
band) and an emission from the phane state, both of which were derived from the
p–p stacking of the
poorly extended conjugation systems of 1,3,5-tris[(2,5-dimethylphenyl)ethynyl]benzene. In addition, a
conjugated microporous polymer (CMP) that comprises pseudo-para-substituted [2.2]paracyclophane
was prepared. The obtained CMP is regarded as a polymer, in which 1,3,5-tris[(2,5-dimethylphenyl)ethy-
nyl]benzenes are infinitely stacked to form a network structure. The CMP exhibited a type I nitrogen gas
sorption profile and an H4-like hysteresis loop, and possessed the slit-like mesopores with a BET surface
Keywords:
[2.2]Paracyclophane
p–p Stacking
Conjugated microporous polymer
area of 501 m2 gÀ1
.
Ó 2011 Elsevier Ltd. All rights reserved.
Introduction
Our previous work showed that the incorporation of [2.2]para-
cyclophane into a conjugated polymer backbone results in the for-
[2.2]Paracyclophane, in which two benzene rings are connected
with two ethylenes at the para-positions, has attracted the atten-
tion of researchers owing to its structure, reactivity, and physical
properties. The distance between the bridge carbon atoms is
approximately 2.8 Å, and that between the centers of the two ben-
mation of a
polymers containing [2.2]paracyclophane as a repeating unit have
extended conjugation lengths via the stacked -electron systems,
p
-stacked structure in a single polymer chain.4 The
p
and despite the stacked structures in the polymer main chain, they
zene rings is approximately 3.1 Å; thus, a p–p interaction exists in
[2.2]paracyclophane because this distance is shorter than the sum
of the van der Waals radii of an sp2 carbon (3.40 Å). Since the first
report on the practical synthesis of [2.2]paracyclophane,1 a variety
of cyclophane derivatives have been prepared, and their unique
structural and electronic properties arising from the longitudinal
p–p interaction between the co-facial aromatic rings have been
investigated in detail.2
By the introduction of two p-electron systems into a [2.2]para-
cyclophane skeleton, two chromophores are constrained to be
stacked in proximity to each other. The electronic structure of such
a [2.2]paracyclophane compound depends on its
p-conjugation
length, orientation, and the overlapping position of the stacked
chromophores.3 For example, the photoluminescence spectrum of
stilbene dimer 1 is broad and featureless with a low fluorescence
quantum efficiency (UPL), while that of compound 2 shows a vibra-
tional structure with a high UPL. The emission of 1 is derived from
the phane state, that is, excimer-like emission, while that of 2 is
derived from the monomer (chromophore) state (Fig. 1).
⇑
Corresponding authors.
E-mail address: ymo@chujo.synchem.kyoto-u.ac.jp (Y. Morisaki).
Figure 1. Emission mechanism of cyclophane compounds.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.067

Y. Morisaki et al. / Tetrahedron Letters 52 (2011) 5504–5507
5505
Scheme 1. Synthesis of compound 5.
Figure 2. (A) UV–vis absorption spectra of compounds
5
and
8
in CH2Cl2
in CH2Cl2
(1.0 Â 10À5 M). (B) Photoluminescence spectra of
5
and 8
Scheme 2. Synthesis of compound 8.
(1.0 Â 10À5 M) excited at 313 nm.
emit high UPL due to the emission from the monomer (chromo-
phore) state. Thus, [2.2]paracyclophane can be used as a scaffold-
reacted, 1-iodo-3,5-bis[(2,5-dimethylphenyl)ethynyl]benzene
was mainly obtained in 32% isolated yield (Scheme 2). 1,3-Diio-
do-5-[(2,5-dimethylphenyl)ethynyl]benzene and were also
6
ing on which various
p-electron systems can be stacked, leading
5
to a better understanding of their through-space interactions. In
this Letter, we report on the synthesis, characterization, and optical
properties of a new [2.2]paracyclophane compound comprising of
two 1,3,5-tris[(2,5-dimethylphenyl)ethynyl]benzenes stacked in
proximity to each other. In addition, we prepared a conjugated
microporous polymer (CMP) that consists of pseudo-para-substi-
tuted [2.2]paracyclophane. The obtained CMP is regarded as a
polymer, in which 1,3,5-tris[(2,5-dimethylphenyl)ethynyl]ben-
zenes are infinitely stacked to form a network structure; the syn-
thesis and characterization are also described.
obtained as by-products, which were readily removed by standard
SiO2 column chromatography (Supplementary data). 1,3,5-
Tris[(2,5-dimethylphenyl)ethynyl]benzene-stacked compound
8
was prepared using pseudo-para-diethynyl[2.2]paracyclophane 7
as a building block.6,7 Thus, the reaction of 6 with 7 in the presence
of a catalytic amount of PdCl2(PPh3)2/PPh3/CuI afforded 8 in 64%
isolated yield (Scheme 2). Owing to the [2.2]paracyclophane
framework, two 1,3,5-tris[(2,5-dimethylphenyl)ethynyl]benzenes
are constrained to partially overlap one another and to be stacked
in proximity by two ethylene chains.
The structures of 5 and 8 were confirmed by 1H and 13C NMR
spectroscopy, and the spectra are shown in the Supplementary
data. In the 1H NMR spectrum of 5 (Fig. S1), the signals of xylene
protons appeared at 7–7.5 ppm, while the signals of the aromatic
protons of 8 were observed at around 6.5 ppm (Fig. S7). These sig-
nals in the higher magnetic field are assigned to the aromatic pro-
tons in the [2.2]paracyclophane moiety produced because of the
ring current effect.
Results and discussion
The synthesis of 1,3,5-tris[(2,5-dimethylphenyl)ethynyl]ben-
zene 5 was carried out as the model compound of the monolayer
unit. As shown in Scheme 1, Sonogashira–Hagihara coupling5 of
1,3,5-triiodobenzene 3 with 3 equiv of ethynylxylene 4 proceeded
smoothly to obtain 5 in 81% isolated yield. When 2 equiv of 4 was

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Y. Morisaki et al. / Tetrahedron Letters 52 (2011) 5504–5507
The UV–vis absorption spectra of 5 and 8 in CH2Cl2 (1.0 Â 10
À5 M) are shown in Figure 2A. The spectrum of 5 exhibited vibra-
tional structures with peak top wavelengths of 292, 304, and
313 nm. This spectrum was similar to that of tris(phenyleth-
ynyl)benzene, and it red-shifted by approximately 10 nm.8 It is
reported that such a red-shift was observed with the donor substi-
tution; the absorption spectra of tris(phenylethynyl)benzenes are
red-shifted by substitution of the donor methoxy group.9 The
shape of the absorption spectrum of 8 was identical to that of 5
(Fig. 2A), and the molar extinction coefficient (
e) of 8 was almost
twice that of 5 due to the -stacked dimer. In the spectrum of 8,
p
however, a typical cyclophane band was observed at around
375 nm as a broad peak with low absorbance.
The photoluminescence (PL) spectra of 5 and 8 in CH2Cl2
(1.0 Â 10À5 M) were obtained with an excitation wavelength of
313 nm (Fig. 2B). We confirmed that this concentration was suffi-
ciently diluted (Fig. S9 in Supplenentary data) without intermolec-
ular interactions. Compound 5 exhibited a sharp PL spectrum with
a vibrational structure, which was also similar to the spectrum of
tris(phenylethynyl)benzene. The absolute PL quantum efficiency
Figure 3. N2 adsorption–desorption isotherm of CMP P1 at 77 K.
(UPL) of 5 was calculated to be 0.24. The p-stack structure caused
a red-shift of the PL spectrum and a decrease in UPL; thus, the PL
spectrum of 8 was broad and featureless, and the kPL,max of 8 was
observed at around 396 nm with a UPL of 0.16. These results indi-
cate that emission by 8 occurs from the phane state rather than the
monomer state due to the poorly extended conjugation system of
at 70 °C under vacuum for 24 h, CMP P1 was obtained in 114% iso-
lated yield. Generally, the isolated yield of a CMP prepared by pal-
ladium-catalyzed coupling is over 100% owing to the unreacted
halogens. In the case of P1, it was found to contain 3.40 wt % of
iodine according to the elemental analysis.
the stacked
p-electron system, that is, the p-electron system of
5. The PL spectra of 5 and 8 in the solid state are shown in Figures
S10A and S11A, respectively. Due to the intermolecular interaction,
both peaks were red-shifted in comparison with those of the solu-
tions. On the other hand, 5 and 8 were well-dispersed in the poly-
styrene matrix (cast film including 1.0 Â 10À2 wt % of 5 and 8), and
their spectra were similar to those in solutions (Figs. S10B and
S11B, respectively).
By stacking 1,3,5-tris[(2,5-dimethylphenyl)ethynyl]benzenes
repeatedly, a network polymer can be prepared. Recently, network
polymers consisting of rigid-rod conjugated frameworks possess-
ing micropores, which are called conjugated microporous poly-
mers (CMPs), have been extensively studied.10,11 As shown in
Scheme 3, the treatment of 3 with 7 in the presence of a catalytic
amount of Pd(PPh3)4 and CuI at 80 °C for 72 h yielded a pale yellow
precipitate. The reaction was quenched with 1.0 N aqueous HCl,
and the obtained precipitate was washed with CHCl3 and MeOH
(each for 24 h) using a Soxhlet extractor. Then, after it was heated
The structure of P1 was confirmed by solid-state CP MAS 13C
NMR spectrum and FTIR spectrum, as shown in Figures S12 and
S13, respectively (Supplementary data). In addition, the character-
ization of P1 was carried out by scanning electron microscopy
(SEM, Fig. S16), X-ray diffraction analysis (XRD, Fig. S18), and
nitrogen sorption analysis. Figure 3 shows the nitrogen adsorp-
tion–desorption isotherm of P1 measured at 77 K. The isotherm
was categorized as the type I nitrogen gas sorption profile
Scheme 3. Synthesis of CMP P1.

Y. Morisaki et al. / Tetrahedron Letters 52 (2011) 5504–5507
5507
according to the IUPAC classification,12 and it was associated with
References and notes
an H4-like hysteresis loop, indicating the existence of narrow slit-
like pores. Thus, the incorporation of the step structure of the
[2.2]paracyclophane moiety formed the slit-like mesopores, and
the Brunauer–Emmett–Teller (BET) surface area (SBET) was esti-
mated to be 501 m2 gÀ1. On the other hand, it has been reported
that the CMP P2 with phenylene instead of [2.2]paracyclophane
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possesses a larger surface area of 834 m2 gÀ1 11a,c,13
The two-
.
dimensional network in P1 probably arises from the step and
stacked structure of the [2.2]paracyclophane unit in addition to
the three-dimensional network, while the three-dimensional net-
work structure is effectively created in P2 by the bending of the
struts, which leads to a larger surface area. The differences in these
structures reflect their morphologies.11a The SEM image of P1
reveals the aggregates of masses and plates (Fig. S16), while that
of P2 shows the aggregates of spherical particles (Fig. S17).14
Conclusion
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dimethylphenyl)ethynyl]benzenes. The compound exhibited
unique absorption band (cyclophane band) and emission from
the phane state, both of which were the result of the stacking
a
p–p
of the poorly extended conjugation systems of 1,3,5-tris[(2,5-
dimethylphenyl)ethynyl]benzene. We have prepared a CMP, in
which 1,3,5-tris[(2,5-dimethylphenyl)ethynyl]benzenes were
stacked. The CMP exhibited a type I nitrogen gas sorption profile
and an H4-like hysteresis loop, and possessed the slit-like mesop-
ores with a BET surface area of 501 m2 gÀ1. This [2.2]paracyclo-
phane-based CMP showed a morphology that was different from
that of the benzene-based CMP. The effect of the [2.2]paracyclo-
phane moiety on the CMP is currently under investigation. The
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Acknowledgments
Financial support from The Inamori Foundation is gratefully
acknowledged. This work was also partially supported by the
Grant-in-Aid for Young Scientists (A) (No. 21685012) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan. Y.T. appreciates Research Fellowships from the Japan Society
for the Promotion of Science for Young Scientists. The authors are
grateful to Professor Susumu Kitagawa, Associate Professor Takashi
Uemura, and Dr. Nobuhiro Yanai (Department of Synthetic
Chemistry and Biological Chemistry, Kyoto University) for solid
state CP MAS 13C NMR spectroscopy and valuable discussions.
12. Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.;
Rouquérol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603–619.
13. Although CMP P2 has already been synthesized, we prepared it from 3 with
1,4-diethynylbenzene. The SBET value of P2 we synthesized was 822 m2 gÀ1
.
The 13C NMR CP MAS and FTIR spectra (Figs. S14 and S15), SEM image
(Fig. S15), XRD pattern (Fig. S19), and nitrogen adsorption–desorption
isotherm (Fig. S20) of P2 are shown in Supplementary data.
14. The morphology of P2 we synthesized (Fig. S17) was also almost identical to
the reported one; see Ref. 11a.
Supplementary data
Supplementary data (synthetic details, 1H and 13C NMR spectra)
associated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2011.08.067.