Synthesis of organic-soluble conjugated polyrotaxanes by polymerization of linked rotaxanes

Article abstract of DOI:10.1021/ja9074437

(Figure Presented) Extensive research on the use of cyclodextrin for insulating π-conjugated polymer chains has been carried out. However, the resulting polyrotaxanes do not exhibit high and constant covering ratios and are generally insoluble in organic

Full text of DOI:10.1021/ja9074437

Published on Web 10/15/2009
Synthesis of Organic-Soluble Conjugated Polyrotaxanes by Polymerization of
Linked Rotaxanes
Jun Terao,*,† Susumu Tsuda,‡ Yuji Tanaka,‡ Kento Okoshi,*,§ Tetsuaki Fujihara,† Yasushi Tsuji,† and
Nobuaki Kambe*,‡
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto UniVersity, and
PRESTO, Japan Science and Technology Agency (JST), Nishikyo-ku, Kyoto 615-8510, Japan, Department of
Applied Chemistry, Graduate School of Engineering, Osaka UniVersity, Suita, Osaka 565-0871, Japan, and
Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan
Received September 3, 2009; E-mail: terao@scl.kyoto-u.ac.jp; okoshi.k.aa@m.titech.ac.jp; kambe@chem.eng.osaka-u.ac.jp
Scheme 1. Synthesis of PM R-CD-Based Polyrotaxane 6 by
Insulated molecular wires (IMWs) have attracted considerable
attention because of their potential applicability as next-generation
monomolecular electronic devices.1 Our objective is to develop organic
materials for use in nanoscale molecular electronics by interconnection
of nanoelectrodes via a chemical reaction with high reproducibility.2
In order to accomplish this, we have attempted to synthesize IMWs
with molecular structures that have the following ideal features: (1) a
π-conjugated polymer chain with a high degree of insulation; (2) a
rigid polymer chain with high linearity; (3) high solubility in organic
solvents essential for simple liquid solution processes; and (4) high
charge mobility. Extensive research on the use of cyclodextrin (CD)
for insulating π-conjugated polymer chains has been carried out
because of its commercial availability and its unique ability to form
inclusioncomplexeswithorganicmoleculesviahydrophilic-hydrophobic
interactions.3 In one of the methods for synthesizing IMWs from CD,
a solution of π-conjugated polymers is stirred in the presence of CD4
using the method developed by Harada et al.5 for the synthesis of a
molecular necklace. Another method involves the polymerization of
[2]-pseudorotaxanes generated in situ through the inclusion of conju-
gated monomers in CD6 or copolymerization of [2]-pseudorotaxanes
generated in situ with linker molecules.7 However, these IMWs do
not exhibit high and constant covering ratios and are generally insoluble
in organic solvents.8 Furthermore, they contain water molecules
because of the hydrophilic nature of CD. Thus, their potential for use
as an electronics material is reduced. Recently, we reported synthetic
routes to linked rotaxanes bearing a π-conjugated system as the guest
and permethylated (PM) R-CD as the host.9 Herein, we report new
methods for synthesizing polyrotaxanes (IMWs) through the polym-
erization of linked rotaxanes as the monomers.
Polymerization of Organic-Soluble Linked [2]-Rotaxane Monomer 5
monomer 5. Glaser polymerization of 5 was then carried out in the
presence of Cu(OAc)2 in pyridine at 50 °C. Results of time-course
studies and gel-permeation chromatography (GPC) revealed that 5
was completely consumed after 24 h. The mass-averaged molecular
weight (Mw) and polydispersity index (PDI) of the linked [2]-ro-
taxane polymer 6 were estimated to be 3.88 × 104 and 2.01,
respectively, using polystyrene as the calibration standard in the
analysis. The average degree of polymerization (n) was found to
be 25, indicating that the average contour length along the extended
rodlike chain is 50 nm.
1
As shown in Figure 2, the peaks in the H NMR spectrum of 6
were broader than those in the NMR spectrum of 5, indicating that
the rotaxane structure was maintained during polymerization. The
MALDI-TOF mass spectrum of 6 provided additional evidence for
the structural authenticity of this compound. As expected, all of the
An organic-soluble [1]-rotaxane 4, also classified as a linked
[2]-rotaxane, was synthesized by our previously reported method
based on intramolecular self-inclusion of a π-conjugated linear guest
unit into a lipophilic PM R-CD moiety in a hydrophilic solution.9a
Intramolecular self-inclusion of 1 in 1:1 CH3OH/H2O formed 2,
and 4 was obtained by subsequent capping with a stopper, 3
(Scheme 1). Crystals of 4 were grown from a mixture of DMSO
and H2O. The X-ray structure (lateral and top view) of 4 is shown
in Figure 1. It is apparent that the PM R-CD moiety has been
displaced from the tolan core and that the π-conjugated guest is
almost planar, while slight torsion is observed around its main axis.
Studies performed using space-filling models revealed that the
covering ratio of PM R-CD in the π-conjugated backbone of 4 was
∼60%. The acetamide and amino groups of 4 were converted to
ethynyl groups in four steps to yield the linked [2]-rotaxane
Figure 1. Lateral (space-filling model) and top (stick model) view of 4.
Figure 2. (a) 1H NMR spectra (400 MHz, CDCl3) of (bottom) 5 and (top)
6. (b) MALDI-TOF mass spectrum of 6. (c) Fluorescence micrograph of 6
(yellow scale bar: 1 mm).
† Kyoto University and JST.
‡ Osaka University.
§ Tokyo Institute of Technology.
9
16004 J. AM. CHEM. SOC. 2009, 131, 16004–16005
10.1021/ja9074437 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
observed peaks in the mass spectrum correspond to singly charged
molecular ions containing more than 20 repeating units. In addition,
the rotaxane polymer was found to be soluble in various organic
solvents such as ethyl acetate, chloroform, toluene, and DMF. The
absolute photoluminescence (PL) efficiency of 6 in the solid state was
measured to be 14% using an integrating sphere system.
fingerprint texture, while 10 forms only an isotropic liquid even at
high concentrations. The observed cholesteric phase can be at-
tributed to the presence of PM R-CD in 9, as a nematic LC phase
should be observed if a polymer chain threads through achiral
macrocycles. To the best of our knowledge, this is the first report
of the formation of a cholesteric LC phase whose chirality originates
from the inclusion of the polymer main chain into the CD cavity.
Both 9 and 10 with the aforementioned global conformations
successfully solidify into films upon evaporation of the solvent from
the solution (Figure S2). The striking difference in the birefringence
of these films indicates that 9 forms an anisotropic phase in
concentrated solutions because of the rigidity of its main chain,
which in turn is a result of the covering effect of PM R-CD.
In order to increase the covering ratio and structural regularity of
the IMWs, we employed linked symmetrical [3]-pseudorotaxane 8 as
the monomer.9b The formation of 8 by sequential intramolecular self-
inclusion of 7, a phenylene-ethynylene oligomer having two PM
R-CDs, was confirmed from the 1H NMR spectra recorded under the
same conditions as in the case of the following polymerization (1:1:
0.1 CD3OD/D2O/TMEDA, 50 °C). From the CPK model of 8, the
covering ratio was estimated to be ∼95%. Subsequently, Glaser
polymerization of 8 was carried out in the presence of CuI. Although
the polymerization of 8 was markedly slower than that of 5 because
of the high coverage of the former, linked polyrotaxane 9 (Mw ) 5.77
× 104, PDI ) 1.51, n ) 21) was formed, as confirmed by GPC and
MALDI-TOF mass spectroscopy. In order to examine the covering
effect of PM R-CD, the uncovered polymer 10 (Mw ) 5.64 × 104,
PDI ) 1.34, n ) 21) was synthesized as a reference via polymerization
of 7 in CH2Cl2 instead of 1:1 CH3OH/H2O solution. The absolute
quantum yields of 9 and 10 were determined using a calibrated
integrating sphere system. The fluorescence quantum yields of 9
(Φsolution ) 0.64, Φsolid ) 0.23) revealed that its fluorescence efficiency
was better than that of 10 (Φsolution ) 0.58, Φsolid ) 0.06), particularly
in the solid state.
Figure 3. Polarized optical micrograph of 9 in CHCl3 solution (∼15 wt
%) sealed in a glass capillary tube; the micrograph was obtained at 25 °C
under crossed polarizers. The LC phase was separated from the isotropic
phase in equilibrium, and its cholesteric pitch was maintained for more
than 3 months. Black scale bar: 50 µm.
Scheme 2. Syntheses of the Fully Covered Polymer 9 and the
Uncovered Polymer 10 by Polymerization of 7 in Different Solvents
In conclusion, we have developed a new method for synthesizing
polyrotaxanes (IMWs) through the polymerization of structurally
defined rotaxane monomers. The IMWs thus formed have a high
covering ratio, rigidity, and PL efficiency; furthermore, they are
readily soluble in a variety of organic solvents.
Acknowledgment. This research was supported the PRESTO
program of the Japan Science and Technology Agency (JST).
Supporting Information Available: Experimental procedures, com-
pound characterization data, and crystallographic data for 4 (CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
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