A self-threading polythiophene: Defect-free insulated molecular wires endowed with long effective conjugation length

Article abstract of DOI:10.1021/ja107444m

Herein, we report on a self-threading polythiophene whose conjugated molecular wire is sheathed within its own cyclic side chains. The defect-free insulating layer prevents electronic cross-communication between the adjacent polythiophene backbone even in the solid film. Notably, the covalently linked cyclic side chains extend the effective conjugation length of the interior polythiophene backbone, which results in an excellent intrawire hole mobility of 0.9 cm² V^-1 s^-1.

Full text of DOI:10.1021/ja107444m

Published on Web 09/29/2010
A Self-Threading Polythiophene: Defect-Free Insulated Molecular Wires
Endowed with Long Effective Conjugation Length
Kazunori Sugiyasu,*,† Yoshihito Honsho,| Ryan M. Harrison,† Akira Sato,‡ Takeshi Yasuda,§
Shu Seki,| and Masayuki Takeuchi*,†
Macromolecules Group, Organic Nanomaterials Center (ONC), National Institute for Materials Science (NIMS),
Exploratory Materials Research Laboratory for Energy and EnVironment, NIMS, Materials Analysis Station, NIMS,
1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan, and Department of Applied Chemistry, Graduate School of
Engineering, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Received August 18, 2010; E-mail: SUGIYASU.Kazunori@nims.go.jp; TAKEUCHI.Masayuki@nims.go.jp
Abstract: Herein, we report on a self-threading polythiophene
whose conjugated molecular wire is sheathed within its own cyclic
side chains. The defect-free insulating layer prevents electronic
cross-communication between the adjacent polythiophene back-
bone even in the solid film. Notably, the covalently linked cyclic
side chains extend the effective conjugation length of the interior
polythiophene backbone, which results in an excellent intrawire
hole mobility of 0.9 cm2 V-1 s-1
.
Insulated molecular wires (IMWs) are conjugated polymers
molecularly covered with an insulating layer, which are structurally
analogous to electric cords in nanoscale.1,2 Due to the “insulation”
of the π-conjugated backbones, IMWs are expected to be applied
to various optoelectronic applications and nanotechnology.2,3 As
recently reviewed by Frampton and Anderson,1a IMWs have mostly
been prepared through polyrotaxane formation4 or polymer wrap-
ping approaches,5 both of which depend on intermolecular interac-
tions. Although a facile preparation procedure (i.e., just mixing two
components) is a great advantage, the formation of structural defects
is unavoidable because the supramolecular processes are governed
by the thermodynamic equilibrium. To address this issue, synthetic
approaches that afford defect-free IMWs have recently attracted
much attention.6 Particularly intriguing are the examples based upon
semiconducting polymers such as polypyrroles and polythiophenes,
which are as of yet scarcely developed.7,8 A charge carrier confined
within such one-dimensional transporting pathways not only leads
to various electronic applications but also enables elucidation of
the mechanisms of conduction.1,6a Herein, we report on a unique
polythiophene whose conjugated molecular wire is sheathed within
its own cyclic side chains. Notably, the covalently linked cyclic
side chains enhance the effective conjugation length (ECL) of the
interior polythiophene backbone, which results in an excellent
intrinsic hole mobility.
Figure 1. Synthetic route toward a self-threading polythiophene (PSTB):
(i) second generation Grubbs catalyst, DCM, RT; (ii) H2 gas, Pd/C, DCM/
MeOH, RT; (iii) NBS, CHCl3/AcOH, 0 °C; (iv) Ni(COD)2, bipy, PhMe/
DMF, 80 °C. (a) Lateral and (b and c) axial views of a self-threading
bithiophene monomer (1HSTB) in crystallized form.
The monomers are sashed with alkyl chains that doubly crossover
the two resorcinol units. Such a three-dimensional architecture is
expected to prevent π-π stacking interactions between adjacent
polythiophene backbones. 1HSTB was prepared in five steps
(overall yield ∼60%) from commercially available materials and
unambiguously characterized.9 A double ring closing olefin me-
tathesis (RCM) reaction is the key step that results in the cyclic
1
side chain production exclusively, which was monitored by a H
NMR spectroscopic method (Figure S1).9 In addition, this reaction
1
is highly trans-selective as confirmed by the H NMR spectrum
and X-ray crystal structure.9 The quantitative and regiospecific
reaction suggests the perfect fit of the insulating layer to the girth
of the bithiophene backbone.10
A synthetic strategy has been implemented that allows access
to a new class of sterically hindered thiophene derivatives. The
unique three-dimensional architecture of our monomer is reminis-
cent of rotaxanes; however, the “ring” and “axis” are covalently
connected. In other words, the molecular axis (i.e., bithiophene) is
threading through its own cyclic side chain; accordingly, we prefer
to refer to the monomer as a self-threading bithiophene (1HSTB
and 1STB).
X-ray crystallographic analysis of 1HSTB (after hydrogenation)
shows that about a half of the bithiophene surface is indeed
concealed (Figure 1a). Remarkably, the bithiophene backbone is
nearly planar (torsional angle ) 179.04°). This is noteworthy that
ordinary ꢀ-substitutions cause dihedral twists, which in turn decrease
the ECL.11 From 1STB as a monomer having solubilizing dodecyl
groups, we have synthesized regioregular (head-to-head) insulated
thiophene oligomers (nSTB, n ) 2-5) and a polymer (PSTB)
through bromination using NBS followed by Yamamoto reductive
coupling. Using the isolated nSTBs as standards for analytical GPC,
the molecular weight of PSTB was determined to be 14.4 kD (PDI
† Macromolecules Group, NIMS.
‡ Materials Analysis Station, NIMS.
§ Exploratory Materials Research Laboratory for Energy and Environment,
NIMS.
| Osaka University.
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C O M M U N I C A T I O N S
with P3DDT; a red shift of λmax by 3200 cm-1 and an appearance
of a shoulder at 2.0 eV were confirmed due to the strong π-π
stacking of polythiophene in the solid state.15 In clear contrast,
PSTB showed almost identical absorption spectra between the
solution and solid state (Figure 2d, red lines, ∆λmax ) 210 cm-1).
This characteristic was also confirmed for oligomeric nSTBs. The
plot between E at λmax vs 1/nring in the film state was fitted with an
intercept of 2.01 and a coefficient of 3.68, which are similar to the
values determined in solution (see Figures 2b and S9).9 These results
indicate that electronic cross-communication between the adjacent
polythiophene backbones is indeed impeded by the “insulating”
layer even in the solid state.
Although the semiconducting backbone is molecularly “in-
sulated”, the spin-coated film of PSTB is electrochemically
active and exhibits a reversible redox peak at E1/2 ) 340 mV
versus Fc/Fc+.9 Thus, the film of PSTB can be doped with I2
vapor. The absorption spectrum of the I2-doped PSTB film is
typical of the bipolaronic species. Exhalation of I2 from the film
(i.e., dedoping) was accompanied by UV/vis/NIR spectral
changes, which matches well to the polaron-bipolaron model
for charge delocalized π-platforms.9 These doping/dedoping
processes are completely reversible, demonstrating the chemical
stability of PSTB. Using a four-probe method, the conductivity
of the I2-doped PSTB film was determined to be 2 × 10-3
S
cm-1. This value was poorer by 2 orders of magnitude than that
of the I2-doped P3DDT film measured under the same conditions.
Considering the long ECL (i.e., delocalized π-conjugation) of
PSTB demonstrated above, the low conductivity at the bulk state
should originate from the limited interchain transfer integral of
PSTB.1,16 In fact, a time-resolved microwave conductivity
Figure 2. (a) Normalized absorption spectra of nSTBs (n ) 1-5), PSTB
(red), and fluorescence spectrum of PSTB (dotted red line). (b) Correlation
between electronic transition energies (E) and inverse ring numbers (1/
nring) of nSTBs. (c) Photographs of the solutions of nSTBs (n ) 1-5),
PSTB, and P3DDT from left to right under room (top) and UV (bottom)
light. (d) Absorption spectra of PSTB (red lines) and P3DDT (black lines)
in solutions (solid lines) and spin-coated films (dotted lines).
(TRMC) method proved that PSTB intrinsically has an excellent
9,17
intrachain hole mobility of 0.9 cm2 V-1 s-1
.
We expect that
PSTB shows a significant difference in the contributions between
the interchain and intrachain charge carrier transports in its
conduction mechanism.18,19
In conclusion, we have succeeded in the synthesis of a self-
threading polythiophene (PSTB). The polyrotaxane-like three-
dimensional architecture has led to a unique polythiophene
backbone featuring extended π-electron delocalization sheathed
within defect-free “insulating” layers. Studies on structure-proper-
ty relationships of PSTB, e.g. effects of the “ring” size10 and
covering ratio, are now underway. We believe that our poly-
thiophene could find use not only as molecular electric cords in
nanointegrated circuits but also as anisotropic conducting
materials when aligned.
) 1.4), which corresponds to 32 repeating thiophene rings (DP )
16).9 A computer generated model of PSTB revealed its rod-like
structure with a diameter of 1.2 nm, which shows good agreement
with the height profiles of its AFM images (Figure S8).9
Figure 2a displays absorption spectra of STB derivatives. Unlike
common oligothiophenes, nSTBs feature a vibronic shoulder at
longer wavelengths, implying they possess rigid frameworks. As
shown in Figure 2b, the short oligomeric nSTBs showed a good
linear relationship between the transition energy (E) and the inverse
ring number (1/nring), which was fitted by eq 1.
Acknowledgment. Authors thank Dr. Nakayama and Dr. Kubo
in NIMS for AFM measurements and “Nanothchnology Network
Project” of MEXT. K.S. thanks Mr. Yuki Ouchi (NIMS) for helpful
discussions and Shorai Foundation for Science and Technology,
The Association for the Progress of New Chemistry, and KAKENHI
(No. 20750097) for financial support.
E(eV) ) 2.04 + 3.68/nring
(1)
The coefficient for the STB backbone appeared to be 3.68, which
is close to that of a nonsubstituted oligothiophene (3.73) and much
larger than that of oligo(3-alkylthiophenes) (3.15).12 Fluorescence
spectral measurement revealed that PSTB exhibits a very small
Stokes shift and a better fluorescence quantum yield (790 cm-1
and 0.61, respectively) compared to those of P3DDT13 (4970 cm-1
and 0.41, respectively) in DCM at rt. These results clearly indicate
that the symmetrial double crossovers restrain the rotational motion
of the bithiophene’s dihedral angle, thereby enhancing the ECL.
We expect that our molecular design concept can endow the IMWs
with a similar effect that is generally realized by ladder-type
conjugated systems (i.e., the planarization effect).14
Supporting Information Available: Synthesis and characterization
of nSTBs and PSTB, AFM image, spectral and electrochemical data,
and TRMC profiles of PSTB. This material is available free of charge
via the Internet at http://pubs.acs.org.
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