Interconvertible oligothiophene nanorods and nanotapes with high chargecarrier mobilities

Article abstract of DOI:10.1002/chem.200901336

An experiment was conducted to show that a barbituric acid (BA)-functionalized oligothiophene undergoes self-aggregation and co-aggregation with a bismelamine receptor, affording nanorods and nanotapes with remarkable charge-carrier mobilities in their co

Full text of DOI:10.1002/chem.200901336

DOI: 10.1002/chem.200901336
Interconvertible Oligothiophene Nanorods and Nanotapes with High Charge-
Carrier Mobilities
Shiki Yagai,*[a, b] Tetsuro Kinoshita,[a] Yoshihiro Kikkawa,[c] Takashi Karatsu,[a]
Akihide Kitamura,[a] Yoshihito Honsho,[d] and Shu Seki[b, d]
Molecular designs leading functional p-conjugated oligo-
mers into well-defined nanoarchitectures through noncova-
lent interactions are crucial for the realization of miniatur-
ized organic electronic devices based on the bottom-up ap-
proach.[1] In particular, the bottom-up construction of elec-
tronically addressable 1D nanostructures has received a
great deal of attention, owing to their potential as effective
pathways for charge carriers.[1] If such nanostructures have
stimuli-responsive properties in their morphologies and
chromophore packing motifs, they could be used as smart
nanomaterials in which their electronic properties can be
controlled at will. Several fibrillar nanostructures have been
reported to exhibit morphological change induced by guest[2]
and solvent.[3] On the other hand, temperature is one of the
most convenient variables to control, but its use to regulate
well-defined nanostructures remains challenging,[4] because
most enthalpy-driven noncovalent assemblies in solution dis-
sociate into monomers upon increasing temperature. Here
we report thermally interconvertible semiconductive nano-
rods and nanotapes comprising p-conjugated oligothio-
phenes,[5,6] displaying high charge-carrier mobilities in their
condensed states (Scheme 1).
Recently, we have shown that merocyanine dyes equipped
with barbituric acid can be loaded into diverse self-organ-
ized nanoarchitectures upon complexation with flexible bis-
melamine receptors (BM, see Scheme 1) through comple-
mentary hydrogen-bonding interactions.[7–9] Our next chal-
lenge is to corroborate the validity of our strategy to extend-
ed p-conjugated systems and to offer more practical nano-
to microarchitectures, as functional modules for miniaturiz-
ing electronic devices. We prepared quaterthiophene (1a),
functionalized on one end by a barbituric acid (BA) head-
groups[10] and the other end by a tridodecyloxyphenyl
(TDP) tail (Scheme 1). Compound 1a was synthesized ac-
cording to Scheme S1 (in the Supporting Information) and
fully characterized.[11] Barbituric acid group is a well-known
ditopic triple-hydrogen-bonding module that complementar-
ily binds with diaminopyridine or melamine modules.[8]
However, the examples that demonstrate its capability to
lead p-conjugated molecules into well-defined nanostruc-
tures without assistance are scant.[12] The UV/Vis spectra of
1a (1ꢀ10ꢀ6 m<c<5ꢀ10ꢀ3 m) in methylcyclohexane (MCH)
showed the absorption maximum at l=439 nm, which was
strongly blue-shifted (n=3474 cmꢀ1) from that in dichloro-
methane (l=518 nm).[13] Furthermore, a significant drop in
the fluorescence intensity was observed.[13] These optical
changes are typical for the H-type aggregation of oligothio-
phenes in nonpolar and aqueous conditions,[6c,14] but not ob-
served for N,N’-dimethylated derivative 1b. This demon-
strates that the BA group of 1a plays a crucial role in the
aggregation. The H-aggregate showed a pronounced thermal
stability: it remained intact upon heating to 808C in MCH,
above which reversible transition to monomeric state was
observed.[13]
[a] Dr. S. Yagai, T. Kinoshita, Prof. Dr. T. Karatsu, Prof. Dr. A. Kitamura
Department of Applied Chemistry and Biotechnology
Graduate School of Engineering, Chiba University
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522 (Japan)
Fax : (+81)43-290-3039
E-mail: yagai@faculty.chiba-u.jp
[b] Dr. S. Yagai, Prof. Dr. S. Seki
PRESTO (Japan) Science and Technology Agency (JST)
4-1-8 Honcho Kawaguchi, Saitama (Japan)
[c] Dr. Y. Kikkawa
Photonics Research Institute
National Institute of Advanced Industrial Science
and Technology (AIST)
1-1-1 Higashi, Tsukuba, Ibaraki 305-8562 (Japan)
Dynamic light scattering of 1a in MCH (c=5ꢀ10ꢀ5 m)
showed the presence of large aggregates with hydrodynamic
diameters around 400 nm.[13] Atomic force microscopy
(AFM) of H-aggregated 1a exhibited uniform rodlike nano-
structures with hundreds of nanometers in length, which are
[d] Y. Honsho, Prof. Dr. S. Seki
Department of Applied Chemistry
Graduate School of Engineering, Osaka University
2-1, Yamadaoka, Suita, Osaka 565-0871 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901336.
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COMMUNICATION
tance of 7.3ꢁ0.3 nm estimated
by
cross-sectional
analysis
would be a reliable width of
rods (panel i in Figure 1b). In
some parts, bilayered rods were
observed with orthogonal (line
ii) or parallel (line iii) orienta-
tion between layers. The former
bilayers provides the height of
7.0ꢁ0.4 nm for the upper
layers
(full
height=13ꢁ
0.4 nm), which corresponds to
the reliable longitudinal diame-
ter of rods because the height
of the bottom layers (6.0ꢁ
0.3 nm) may involve alkyl
chains deformed by the interac-
tion with graphite surface
(panel ii in Figure 1b). These
analyses revealed that 1a self-
Scheme 1. Schematic representation of interconversion between nanorod and nanotape.
assemble into
a
cylindrical
nanostructure with a diameter
arranged in monolayered fashion (Figure 1a). Owing to the
side-by-side interaction between rods, the body-to-body dis-
of 7 nm. On the other hand, the bilayers with the parallel
orientation provide the average height of 12ꢁ0.4 nm (panel
iii in Figure 1b), indicative of the closest (hexagonal) pack-
ing of cylindrical rods. The interlayer spacing is roughly esti-
mated to be 6 nm.
Of reported crystal structures of BA derivatives,[8b] one-di-
mensional tapelike and ribbonlike motifs are most compara-
ble to the organization of 1a (Figure S6).[13] However, these
flat structures do not fit with the observed cylindrical mor-
phology. Furthermore, molecular modeling demonstrated
that the distances between neighboring oligothiophene seg-
ments in tapes and ribbons exceed 9 and 6 ꢂ, respectively,
which do not allow the strong electronic interactions shown
by the UV/Vis study. X-ray diffraction of thin films of 1a re-
vealed a hexagonal columnar packing with a lattice constant
of a=6.6 nm (Figure 1c). Assuming a density of 1 gcmꢀ3,
the number of molecules per cylindrical stratum of 3.5 ꢂ
(typical p–p stacking distance) is 6.8. Furthermore, the UV/
Vis spectrum of the thin film confirmed the presence of the
chromophore packing to be identical to that observed in so-
lution (H-type aggregation). We thus propose the oligomeri-
zation of 1a into a supermacrocyclic architecture (rosette)
through double hydrogen bonds, which subsequently p
stacks into nanorods (Figure 1c).[12b,15,16] Molecular mechan-
ics calculations provided energetically stable hexameric ro-
settes (1a)6 with diameters of 7–8 nm, depending on the
conformation of TDP tails (Figure 1c).
Figure 1. a) AFM height image of self-assembled 1a spin-cast from hot
(>608C) MCH solution (c=5ꢀ10ꢀ5 m) onto highly-oriented pyrolytic
The addition of 1 equiv of BM12 to a yellow MCH solu-
tion of H-aggregated 1a (c>5ꢀ10ꢀ5 m) induced a very slow
spectral change indicating the disruption of H-aggregates
(lmax =439!510 nm, Figure 2a).[17] The spectral change
almost finished after 15 h in the case of 1ꢀ10ꢀ4 m solution
(Figure 2b), and the resulting red solution contained poly-
meric assemblies whose hydrodynamic diameters range
from 50 to 100 nm.[13] These results corroborates the forma-
graphite (HOPG).
z
scale=20 nm. Inset shows
a
magnified image.
!
b) Cross sectional analysis along the lines i–iii in a). The triangles ( ) in
panel i indicate the body-to-body distance between rods (7.3 nm). The
!
triangles ( ) in panel ii indicate the height of the upper layer (7.0 nm).
The distance between dotted lines in panel iii corresponds to the interlay-
er spacing (6.0 nm). c) X-ray diffraction pattern of a thin film of 1a (pre-
pared from 1ꢀ10ꢀ4 m solution) and a proposed packing motif of 1a to
form hexagonally-ordered nanorods.
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S. Yagai et al.
Figure 3. a) AFM height image of coaggregated 1a·BM12 spin-cast from
MCH (c=5ꢀ10ꢀ5 m) on HOPG. z scale=7 nm. b) Cross sectional analy-
sis along the lines i and ii in a). c) X-ray diffraction pattern of a thin film
of 1a·BM12 (prepared from 5ꢀ10ꢀ3 m solution) and a proposed packing
motif of 1a·BM12 to form a lamellar structure.
Figure 2. a) Time-dependent UV/Vis spectral change of 1a in MCH (c=
1.0ꢀ10ꢀ4 m) at 258C induced by the addition of 1 equiv of BM12.
b) Time-dependent change of e at l=510 nm in a) and the color change
of the solution (yellow at t=0 and red at t=15 h). c) Temperature-depen-
dent (60–1008C) UV/Vis spectra of an equimolar mixture of 1a and
BM12 (c=1.0ꢀ10ꢀ5 m) in MCH. d) Change of e at l=439 nm (lmax of H-
aggregates) by temperature.
imum transient conductivities (fSm in cm2 Vꢀ1 sꢀ1) of 1.0ꢀ
10ꢀ4 for 1a and 0.67ꢀ10ꢀ4 for 1a·BM12 (Figure 4). Negligi-
ble influence of the TRMC signal under SF6 atmosphere in-
tion of supramolecular copolymers 1a·BM12 through com-
plementary triple hydrogen bonds.[7,18,19] AFM of the coag-
gregates displayed flat, tapelike nanostructures with an aver-
age thickness of 1.7ꢁ0.1 nm and the width of ꢂ5.5 nm (Fig-
ure 3a and panel i in Figure 3b). Most tapes exist as two-ply
strips (the height of brighter tapes is 3.3ꢁ0.2 nm, see panel
ii in Figure 3b), implying that p stacking is operative be-
tween tapes upon drying. This trend leads to the formation
of a multilamellar superstructure with an interlayer spacing
of 3.2 nm upon increasing concentration, as revealed by the
X-ray diffraction pattern of a solvent-free film (Figure 3c).
Taking the width of nanotapes (ꢂ5.5 nm) into account, the
inclination by an angle of ꢂ388 is suggested for the stacking
of nanotapes within a lamella. The UV/Vis spectrum of the
solvent free-film showed slight increase of absorption at
around l=450 and 600 nm,[13] revealing the presence of in-
terchain electronic interactions between oligothiophene seg-
ments.
Figure 4. Transient conductivities observed for films of a) 1a and
b) 1a·BM12
under
an
excitation
at
l=355 nm
by
1.8ꢀ
1016 photonscmꢀ2 pulseꢀ1. The samples were prepared from cyclohexane
solutions (gels) of c=4ꢀ10ꢀ3 m.
The morphological features of the present oligothiophene
assemblies imply that they should act as efficient pathways
for mobile charge carriers.[6a,d,h,20] Intrinsic charge-carrier
mobilities of hexagonally-packed nanorods of 1a and lamel-
larly-packed nanotapes of 1a·BM12 were thus evaluated by
flash-photolysis time resolved microwave conductivity (FP-
TRMC) measurements.[21] For this purpose, the thin films of
the assemblies were prepared on quartz substrate, and irra-
diated with a l=355 nm laser pulse. The irradiation resulted
in the generation of long-lived charge carriers showing max-
dicates the p-type semiconducting properties of the materi-
als. The quantum efficiencies of the charge-carrier genera-
tion (f) were determined as 1.5ꢀ10ꢀ4 for 1a and 1.2ꢀ10ꢀ4
for 1a·BM12, by the integration of photocurrent under an
excitation of the materials overcoated onto an interdigitated
electrode at l=355 nm.[13] Thus, hole mobility (m+) of
1a·BM12 was calculated as 0.57 cm2 Vꢀ1 sꢀ1, whereas for 1a
the value of one-dimensional hole mobility (m+
) of
1D
1.3 cm2 Vꢀ1 sꢀ1 was estimated by taking its morphological
feature into account.[13] These values are one-order of mag-
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Nanorods and Nanotapes with High Charge Carrier Mobilities
COMMUNICATION
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[10] Oligothiophenes functionalized with barbituric acids or melamines
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Interestingly, H-aggregation of 1a was observed by UV/
Vis spectroscopy (lmax =510!439 nm) upon heating a MCH
solution of coaggregates 1a·BM12 (c=1ꢀ10ꢀ5 m) to 708C
(Figure 2c,d). Above 808C, the resulting H-aggregates start-
ed to dissociate into monomers. AFM images of the samples
prepared from the solution heated at 808C indeed show the
formation of nanorods instead of nanotapes.[13] Released
BM12 molecules are thus considered to exist in a molecular-
ly-dissolved state. The mixture of nanorods of 1a and mono-
meric BM12 regenerated nanotapes when the solution was
kept at room temperature. Although the formation of nano-
rods from the molecularly dissolved 1a is an enthalpy-
driven process, the formation of nanorods from coaggregates
(1a·BM12) upon heating is apparently an entropy-driven
process, in which the release of closely-packed BM12 con-
tributes to the reorganization of 1a.[23]
In summary, we have shown that a BA-functionalized oli-
gothiophene undergoes self-aggregation and co-aggregation
with a bismelamine receptor, affording nanorods and nano-
tapes with remarkable charge-carrier mobilities in their con-
densed states. Furthermore, we have demonstrated that se-
lective formation of either of two nanostructures is possible
by regulating the temperature of the coaggregated solution.
Such smart self-assemblies composed of electronically ad-
dressable components contribute to a step forward toward
switchable electronic nanomaterials.
Keywords: conductivity · conjugation · hydrogen bonds ·
nanostructures · self-assembly
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Received: May 19, 2009
Published online: July 27, 2009
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