Synthesis of an insulated molecular wire by click polymerization

Article abstract of DOI:10.1039/c1cc13012a

We developed a new method for the synthesis of an organic-soluble insulated molecular wire (IMW) with permethylated cyclodextrin (PMCD); this method involves click polymerization of linked [2]rotaxane containing azide and alkynyl groups at both ends of a

Full text of DOI:10.1039/c1cc13012a

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COMMUNICATION
Synthesis of an insulated molecular wire by click polymerizationwz
Jun Terao,*a Kazuya Kimura,a Shu Seki,*b Tetsuaki Fujiharaa and Yasushi Tsujia
Received 23rd May 2011, Accepted 21st June 2011
DOI: 10.1039/c1cc13012a
We developed a new method for the synthesis of an organic-
soluble insulated molecular wire (IMW) with permethylated
cyclodextrin (PMCD); this method involves click polymerization
of linked [2]rotaxane containing azide and alkynyl groups at
both ends of a p-conjugated guest.
Scheme 1 Synthetic route to PMCD-based IMW via click polymer-
The copper(I)-catalysed azide–alkyne reaction, commonly
referred to as a ‘‘click reaction’’, has a number of remarkable
advantages: (1) high efficiency, (2) atom economy, and (3) high
tolerance of functional groups. This reaction has attracted
significant attention owing to its potential applicability in
organic synthesis. Polymer chemists have also utilized this
reaction to synthesize dendritic, hyper-branched, and linear
macromolecules.1,2 Although a few studies have been reported
on the synthesis of p-conjugated polymers employing the click
reaction,3–6 to the best of our knowledge, there is no report on
the synthesis of insulated molecular wires (IMWs) having a
p-conjugated polymer chain as a backbone.7 IMWs have
generated considerable interest for next-generation mono-
molecular electronic devices.8 Presently, we are interested in
the synthesis of IMWs9–12 having the following ideal features:
(1) a p-conjugated polymer chain with a high degree of
insulation, (2) a rigid polymer chain with high linearity, (3)
high solubility in organic solvents, (4) high charge mobility,
and (5) high regioregularity. We have recently developed a new
method for the synthesis of an organic-soluble IMW by Glaser
or Sonogashira polymerization of a lipophilic permethylated
a-cyclodextrin (PMCD)-based linked [2]- or [3]rotaxane
monomer formed by self-inclusion in hydrophilic solvents.13–16
We intended to apply click chemistry to the synthesis of
p-conjugated IMWs since the click reaction provides an
aromatic 1,2,3-triazole skeleton and proceeds efficiently in
hydrophilic solvents, which promotes the formation of a
self-inclusion complex (PMCD-based rotaxane monomer) by
a hydrophilic–hydrophobic interaction.17–19 We disclose herein
new methods for the synthesis of a defect-free PMCD-based
ization.
IMW (3) via click polymerization of a self-inclusion complex
(2) containing azide and alkynyl groups at the ends of the
monomer (Scheme 1).
p-Conjugated guest-linked PMCD (1) was synthesized from
monotosyl PMCD (4)20 via five steps, as shown in Scheme 2.
Reaction of 4 with 2-iodo-5-nitrophenol resulted in an iodo-
benzene linked-PMCD (5) in 98% yield. Sonogashira coupling
of 5 with (4-ethynylphenylethynyl)trimethylsilane and subse-
quent reduction of the nitro group gave an ethynyltolan-linked
PMCD (7) isolated in 94% yield. 7 was then treated with
tBuONO and Me3SiN3, which was followed by deprotection
of the trimethylsilyl group to give 1 in 48% isolated yield via
these two steps.
Intramolecular self-inclusion of p-conjugated guest-linked
PMCD 1 was confirmed by a space-filling model and examined
1
by H NMR using different solvents and concentrations. As
shown in Fig. 1, 1H NMR analysis of 1 in CDCl3 at room
temperature revealed that the p-conjugated guest moiety was
excluded from the cavity of the PMCD, whereas the spectrum
in (CD3)3COD at room temperature indicated the presence of
a mixture of 1 and its inclusion complex (linked [2]rotaxane) 2.
When D2O : (CD3)3COD = 2 : 1 was used as the solvent, 1 was
completely converted to 2. Formation of linked [2]rotaxane (2)
resulted in the downfield shifts of aromatic protons in 2, i.e.,
a Department of Energy and Hydrocarbon Chemistry,
Graduate School of Engineering, Kyoto University, Kyoto 615-8510,
Japan. E-mail: terao@scl.kyoto-u.ac.jp; Fax: +81-75-383-2516;
Tel: +81-75-383-2514
b Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan.
E-mail: seki@chem.eng.osaka-u.ac.jp; Tel: +81-6-6879-4586
w This article is part of the ChemComm ‘Emerging Investigators 2012’
themed issue.
z Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc13012a
Scheme 2 Synthesis of p-conjugated guest-linked PMCD (1).
Chem. Commun., 2012, 48, 1577–1579 1577
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Fig. 2 (a) GPC analysis of monomer 1 and IMW 3; (b) MALDI-TOF
mass spectrum of IMW 3.
PMCD, the structure of inclusion complex 9 was characterized
by 2D ROESY NMR (Fig. S1 in the ESIz).
Next, we conducted polymerization of monomeric linked
[2]rotaxane (2) via
a Cu-catalyzed click reaction in
H2O : tBuOH = 2 : 1 solution. As shown in Fig. 2a, GPC
analysis revealed that monomer 2 almost completely disappeared
after 10 h and formation of the IMW 3 (Mw = 1.00 ꢀ 105,
n˜ = 70) was confirmed. The MALDI-TOF mass spectrum of 3
provided additional evidence for the structural authenticity of
this compound. As expected, all of the observed peaks in the
mass spectrum corresponded to singly charged molecular ions
containing more than ten repeating units (Fig. 2b). Studies
conducted using space-filling models (Fig. S2, ESIz) revealed
that the covering ratio of PMCDs in the backbone of 3 was
approximately 80%.
1
Fig. 1 Aromatic region of 400 MHz H NMR spectra of 1 and/or 2
in several solvents at 25 1C (1) CDCl3, (2) (CD3)3COD, (3) 1 : 1 D2O :
(CD3)3COD, and 2 : 1 D2O : (CD3)3COD.
Ha–A (+0.53 ppm), Hb–B (+0.41 ppm), Hc–C (+0.02 ppm),
Hd–D (+0.41 ppm), and He–E (+0.20 ppm). The remarkably
large downfield shifts of Ha–A, Hb–B, or Hd–D suggest that the
protons are located very close to the a-1,4-glucosidic oxygen
atoms of PMCD or methoxy groups by self-inclusion.21
Confirmation of the fixed inclusion state by capping the end
of the guest moiety with a p-conjugated unit bearing an azide
group was further supported by the synthesis of compound 60,
which was formed by deprotection of the silyl group of 6 after
60 was treated with p-azidobenzoic acid under Cu-catalysed
click reaction conditions in H2O : tBuOH = 2 : 1 solution
(Scheme 3). Formation of the fixed [2]rotaxane (9)22 was
inferred from the MALDI-TOF mass spectrum, which displayed
a strong signal at m/z 1642 corresponding to the [9 + Na]+
ion. Notably, no evidence for the formation of the uninsulated
compound (90) was detected by NMR analysis of the crude
product. Compound 9 was purified by silica gel column
chromatography and obtained in pure form in 69% yield.
Owing to NOEs between the protons in the p-conjugated guest
moiety and the H3 and H5 protons in the interior of the
To synthesize uninsulated polymer as a reference for
IMW 3, we carried out click polymerization of uninsulated
monomer 1 in a hydrophobic (iPr2NH) instead of a hydrophilic
(H2O : tBuOH = 2 : 1) solution under similar click reaction
conditions to those used for the synthesis of 3. The uninsulated
polymer was obtained in low yield along with a significant
amount of undesired products, probably via 1,4- and/or
1,5-addition, since it is known that the click reaction sometimes
does not proceed selectively and efficiently in non-aqueous
solutions.23,24 We then polymerized 10 by Sonogashira
reaction, which efficiently proceeds in hydrophobic solution, and
succeeded in synthesizing uninsulated polymer 30 (Scheme 4)
(Mw = 1.00 ꢀ 105, n˜ = 16) having the same main chain structure
formed by click polymerization of 1.
To evaluate the covering effect of PMCD, we compared the
absolute fluorescence quantum yield of 3 and 30 using a
calibrated integrating sphere system. As expected, IMW 3
(Fsolution = 0.37, Fsolid = 0.06) indicated higher photo-
luminescence efficiencies as compared to uninsulated polymer
30 (Fsolution = 0.07, Fsolid = 0.02) in both solid and solution
states, suggesting that to attain efficient fluorescence properties,
encapsulation of the chromophore by PMCD is essential.
Next, we examined the potential for photo-induced charge
separation and the transport properties of the IMW in the
Scheme 3 Synthesis of fixed [2]rotaxane (9).
1578 Chem. Commun., 2012, 48, 1577–1579
Scheme 4 Synthesis of uninsulated polymer 30.
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In conclusion, we developed new methods for the syntheses
of highly organic-soluble IMWs with PMCD lines in the same
direction by carrying out click polymerization of linked
[2]rotaxane monomers. The obtained IMWs had high solubility
in organic solvents and a high covering ratio, rigidity, and
photoluminescence efficiency. Experiments are currently being
performed to evaluate the behaviour of the new material in
molecular electronics.29
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Chem. Commun., 2012, 48, 1577–1579 1579