Synthesis of an organic-soluble π-conjugated [1]rotaxane

Article abstract of DOI:10.1246/cl.2009.76

An organic-soluble π-conjugated [1]rotaxane has been synthesized by intramolecular self-inclusion of a lipophilic permethylated α-cyclodextrin bearing a rigid π-conjugated system as a guest moiety. End-capping has been achieved successfully by connecting

Full text of DOI:10.1246/cl.2009.76

76
Chemistry Letters Vol.38, No.1 (2009)
Synthesis of an Organic-soluble ꢀ-Conjugated [1]Rotaxane
Susumu Tsuda,1 Jun Terao,ꢀ2 and Nobuaki Kambeꢀ1
1Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871
2Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510
(Received October 16, 2008; CL-080993; E-mail: terao@scl.kyoto-u.ac.jp)
An organic-soluble ꢀ-conjugated [1]rotaxane has been syn-
thesized by intramolecular self-inclusion of a lipophilic per-
methylated ꢁ-cyclodextrin bearing a rigid ꢀ-conjugated system
as a guest moiety. End-capping has been achieved successfully
by connecting an aniline moiety without using bulky stoppers.
The structure of the [1]rotaxane was determined by 2D NMR
spectroscopy.
ꢀ-Conjugated systems constitute a core technology for next-
generation electronic materials such as organic light-emitting
diodes (OLEDs), organic thin-film field-effect transistors, and
fluorescent probes. Recently, particular attention has been paid
to insulated ꢀ-conjugated systems with high stability, high
solubility, and high fluorescence quantum yield arising from
the decreased ꢀ–ꢀ interaction among the ꢀ-conjugated systems
and/or their separation from the external environment.1 Various
water-soluble rotaxanes2 having insulated ꢀ-conjugated systems
have been prepared using cyclodextrins (CDs) as a protective
cylindrical sheaths.3 For example, [2]rotaxanes have been syn-
thesized by the inclusion of a ꢀ-conjugated system into a CD
in aqueous medium followed by the end-capping of the complex
with two water-soluble bulky stoppers. Tian et al. synthesized a
[1]rotaxane4,5 by forming an intramolecular self-inclusion com-
plex of an azobenzene-linked ꢂ-CD and subsequent end-capping
with a water-soluble bulky stopper for a light-driven molecular
machine. We report herein a new synthetic method of rotaxanes
having high organic solubility and high coverage of a ꢀ-conju-
gated system (axial guest) with a macrocyclic host.
Scheme 1. Synthesis of a modified PM ꢁ-CD 3.
Our strategy to fabricate a [1]rotaxane is based on intramo-
lecular self-inclusion of lipophilic permethylated ꢁ-cyclodextrin
(PM ꢁ-CD) bearing a diphenylacetylene derivative as a rigid
ꢀ-conjugated system and on a subsequent end-capping with a
nonbulky ꢀ-conjugated unit.
The substitution reaction of 6-O-monotosyl PM ꢁ-CD 16
with 2-iodo-5-acetamidophenol7 gave a modified PM ꢁ-CD
iodide 2 in 98% yield. The desired modified PM ꢁ-CD 3 was
prepared using a sequential Sonogashira coupling reaction of 2
with trimethylsilylacetylene and 1,4-diiodobenzene in 67% yield
(Scheme 1). Detailed procedures and the spectral data of these
compounds are described in Supporting Information.8
The intramolecular self-inclusion phenomenon of 3 has been
confirmed by CPK model and been examined by 1H NMR
employing different solvents and concentrations. As shown in
Figure 1, the NMR spectrum of 3 in CDCl3 at room temperature
reveals the exclusion of the diphenylacetylene moiety from the
cavity of the PM ꢁ-CD. The spectrum in CD3OD at room tem-
perature indicates the presence of a mixture of 3 and its supramo-
lecular complex (pseudo[1]rotaxane) 30. The intensity of new
Figure 1. The aromatic region of 400 MHz 1H NMR spectra of
3 in several solvents at rt. 1) CDCl3; 2) CD3OD (soon after dis-
solved); 3) CD3OD after heating at 60 ꢁC for 60 min and cooling
to rt; 4) D2O:CD3OD = 1:1 after heating at 60 ꢁC for 60 min and
cooling to rt.
peaks (a0–e0) increased on standing at room temperature over-
night or by warming up to 60 ꢁC and then cooling to room tem-
perature indicating the slow equilibrium process at room temper-
ature. 3 was converted to the supramolecular complex 30 in D2O:
CD3OD = 1:1 and disappeared completely. The evidence that
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.1 (2009)
77
Table 1. Electronic spectra and fluorescence quantum yieldsa
Absorption
(ꢃmax/nm)
Emission
(ꢃmax/nm)
Sample
ꢄsolution
ꢄsolid
4
5
328
338
398
396
0.89
0.71
0.68
0.06
aSpectra were recorded in CHCl3. Absolute quantum yields
were determined by a calibrated integrating sphere system.
ane was confirmed by 2D TOCSY, COSY, and ROESY NMR.
The NOEs between protons on the diphenylacetylene moiety
and the internal protons of the PM ꢁ-CD were observed. The
details are described in Supporting Information.8
In order to examine the shielding effect of PM ꢁ-CD, we
compared the fluorescence quantum yield of 4 with that of the
corresponding uninsulated compound 5 (Table 1). As expected,
there is a significant fluorescence enhancement in 4 especially in
solid state suggesting that encapsulation of the chromophore by
PM ꢁ-CD is essential to attain efficient fluorescence properties.
In conclusion, an organic-soluble [1]rotaxane was prepared
via intramolecular self-inclusion of PM ꢁ-CD bearing a diphen-
ylacetylene moiety and subsequent end-capping with an aniline
unit by the Suzuki–Miyaura coupling. The present study re-
vealed that bulky stoppers are not necessary when [1]rotaxane
consist of PM ꢁ-CD as a macrocyclic host and a rigid conjugated
system as the guest unit are linked each other.
Scheme 2. Synthesis of [1]rotaxane 4 and uninsulated com-
pound 5 by cross-coupling reaction in different solvents.
the NMR spectra of 30 at different concentrations in D2O:
CD3OD = 1:1 showed no new peaks ascribable to oligomeric
and/or polymeric supramolecular complexes may support intra-
molecular self-inclusion complex (pseudo[1]rotaxane) 30.
The formation of 30 resulted in the following up- or down-
field shifts of aromatic protons in 30, Ha{a (ꢂ0:25), Hb{b
0
0
References and Notes
1
2
3
M. J. Frampton, H. L. Anderson, Angew. Chem., Int. Ed. 2007, 46,
1028.
For nomenclature of rotaxanes, see: A. Yerin, E. S. Wilks, G. P. Moss,
A. Harada, Pure Appl. Chem. 2008, 80, 2041.
0
(+0.56), Hc{c (+0.13), Hd{d (+0.49), and He{e (+0.09 ppm).
0
0
0
The remarkably large downfield shift of Hd{d suggests that the
protons are located very close to the ꢁ-1,4-glucosidic oxygen
atoms of PM ꢁ-CD.9
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In order to fix pseudo[1]rotaxane 30 by capping the end of
the guest moiety with a ꢀ-conjugated unit, 30 was treated with
aniline boronic ester under Suzuki–Miyaura coupling conditions
in H2O:CH3OH = 1:1 solution (Scheme 2). The desired [1]ro-
taxane 4 was purified by silica gel column chromatography
and was obtained in pure form in high yield (80%).8 This [1]ro-
taxane is soluble in various organic solvents such as methanol,
ethyl acetate, chloroform, toluene, and DMF. It is known that
the decomplexation of [1]rotaxane through ‘‘flipping’’ mecha-
nism is often observed owing to large flexibility of PM ꢁ-CD
in comparison to that of native ꢁ-CD.10 However, [1]rotaxane
4 was stable in CDCl3 for more than seven days without decom-
plexation. The corresponding uninsulated compound 5 was in-
tentionally synthesized by the reaction of 3 with aniline boronic
ester in DMF instead of 1:1 solution of H2O and CH3OH.
Kaneda et al. succeeded in synthesizing dimeric cyclic
[2]rotaxane via end capping of dimeric cyclic inclusion com-
pound of a para substituted azophenol-linked PM ꢁ-CD by azo
coupling using sterically hindered naphthol derivative.9 In our
[1]rotaxane synthesis, however, MALDI-TOF mass spectrum
exhibited only the peak at m=z 1558 corresponding to [4 +
Na]þ. No evidence for the formation of dimeric cyclic [2]rotax-
ane was detected by MALDI-TOFMS and GPC analysis. It is
quite interesting that a pseudo[1]rotaxane was selectively gener-
ated from ortho substituted diphenylacetylene-linked PM ꢁ-CD
via intramolecular self-inclusion. The structure of this [1]rotax-
4
5
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Published on the web (Advance View) December 13, 2008; doi:10.1246/cl.2009.76