Dynamically controllable emission of polymer nanofibers: Electrofluorescence chromism and polarized emission of polycarbazole derivatives

Article abstract of DOI:10.1002/chem.201201471

Electrochemical polymerization of a series of N-alkyl-2,7-di(2-thienyl) carbazoles in acetonitrile was performed to obtain conjugated polymers with fluorescence. Scanning electron and atomic force microscopies revealed that the surface morphology of the polymer films significantly depends on the alkyl chain lengths of the polymers. Particularly, a homopolymer bearing hexyl groups and copolymers with an average alkyl chain length of six carbon atoms show nanofiber morphology. The polymer nanofibers were stacked on a substrate electrode. The fluorescence of the polymer nanofiber film was tunable with application of voltage, with good repeatability. The X-ray diffraction pattern of the fibers showed the structural order. The polymer nanofibers thus prepared showed an electrochemically driven change in polarized photoluminescence.

Full text of DOI:10.1002/chem.201201471

FULL PAPER
DOI: 10.1002/chem.201201471
Dynamically Controllable Emission of Polymer Nanofibers:
Electrofluorescence Chromism and Polarized Emission of Polycarbazole
Derivatives
Kohsuke Kawabata and Hiromasa Goto*[a]
Abstract: Electrochemical polymeriza-
tion of a series of N-alkyl-2,7-di(2-thi-
ACHTUNGTRENNUNGenyl)carbazoles in acetonitrile was per-
formed to obtain conjugated polymers
with fluorescence. Scanning electron
and atomic force microscopies revealed
that the surface morphology of the
polyACHTUNGTRENNUNGmer films significantly depends on
the alkyl chain lengths of the polymers.
Particularly, a homopolymer bearing
hexyl groups and copolymers with an
average alkyl chain length of six
carbon atoms show nanofiber morphol-
ogy. The polymer nanofibers were
stacked on a substrate electrode. The
fluorescence of the polymer nanofiber
film was tunable with application of
voltage, with good repeatability. The
X-ray diffraction pattern of the fibers
showed the structural order. The poly-
mer nanofibers thus prepared showed
an electrochemically driven change in
polarized photoluminescence.
Keywords: electrochemistry · elec-
trochromism · luminescence · nano-
structures · polymerization
Introduction
duced in structures of monomers for electrochemical poly-
merization[7–10] because the initial step of electrochemical
polymerization is oxidation of monomers.[11,12] The mono-
mers with low oxidation potential can be polymerized to
produce low-bandgap polymers. To date, many kinds of elec-
trochemically synthesized polymers have been reported for
various applications, such as electrochromic devices,[13,14] or-
ganic light-emitting diodes,[15–17] and organic photovolta-
ics.[18–20] Electrochemical polymerization produces conjugat-
ed polymer films with various surface structures and mor-
phologies, which depend on the monomer structures and
polymerization conditions.[21–23] Surface structure and mor-
phology strongly influence the properties of polymeric mate-
rials. To control the surface structure and morphology of
electrochemically synthesized polymers, many techniques
have been developed, which can produce nanoarrays,[24]
nanotubes,[25] nanospheres,[26] nanocapsules,[26] and nanofib-
ers.[27] Such nanostructured conjugated polymers, especially
nanofibers, have attracted much attention for their function-
ality in application to optoelectrics.[28–33]
Photoluminescence (PL) and electroluminescence are light-
emitting processes occurring through relaxation of excited
electrons, which can be widely observed not only in synthet-
ic dyes,[1] but also in living creatures and plants, especially in
deep-sea fish,[2] since surprisingly some species can control
the emission of light.
p-Conjugated polymers showing PL and electrolumines-
cence have been developed for application in light-emitting
devices[3,4] and as photosensing materials.[5,6] Among many
synthetic methods for obtaining conjugated polymers, elec-
trochemical polymerization is a useful method for the prep-
aration of electroactive polymeric thin films. The electro-
chemical process is a noncatalytic reaction, and the polymer-
ization method provides good reproducibility because the
polymerization process can be easily controlled by monitor-
ing the current and integrated charge passed through elec-
trochemical cells. Monomers for electrochemical polymeri-
zation require no modification of chemical structure to
À
À
À
À
obtain reactive sites, such as Cl, Br, I, SnR3, and
Herein, we demonstrate electrochemical polymerization
and copolymerization of a series of N-alkyl-2,7-di(2-thienyl)-
carbazoles (TCzm; alkyl chain length m=2–8) in acetoni-
trile. The surface morphology of the polymers, observed
with scanning electron microscopy (SEM), depends on the
alkyl chain length. For the polymer with a hexyl group,
nanofiber formation is observed; these fibers exhibit fluores-
cence and show an electrochemically driven PL switching
function. Furthermore, the crystallinity and polarized emis-
sion of the polymer nanofiber film are examined.
À
B(OR)2. Furthermore, the electrochemical method re-
quires no cumbersome purification process.
Generally, electron-rich heteroaromatic rings, such as pyr-
role, furan, thiophene, and 3,4-ethylenedioxythiophene,
which have relatively low oxidation potentials, are intro-
[a] K. Kawabata, Prof. H. Goto
Faculty of Pure and Applied Sciences
University of Tsukuba
Tsukuba, Ibaraki 305-8573 (Japan)
Fax : (+81)29-8534490
E-mail: gotoh@ims.tsukuba.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201471.
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Results and Discussion
in the current response intensity in sequential cycles indicat-
ed the progress of the polymerization reaction. Oxidation
and reduction current signals at about 0.5 and 0.3 V, respec-
tively, corresponded to oxidation and reduction of a p-con-
jugated system of the resulting polymer deposited on the
electrode. Lowering of the oxidation and reduction current
onsets with an increase in number of cycles indicated exten-
sion of effective conjugation
An electrochemical oxidative coupling reaction between
TCzm molecules afforded a-position linkage between thio-
phene rings, to give the corresponding conjugated polymers
and random copolymers (PTCzm and PTCz
ly), as shown in Scheme 1.
ACHTUNGTREN(NUGN k-l), respective-
length in the polymer back-
bone. The growth of the conju-
gated polymer chains is also
confirmed by in situ UV/Vis ab-
sorption spectroscopy during
electrochemical polymerization
of TCz6 (Figure S1 in the Sup-
porting Information).
The infrared (IR) absorption
spectrum of PTCz6 in the neu-
tral state shows absorption
bands similar to those of the
monomer (TCz6), although the
bands are slightly broadened
(Figure S2 in the Supporting In-
formation). However, an ab-
sorption band of TCz6 at
806 cmÀ1 due to the C H bend-
ing vibration (dCÀH) of the thio-
phene unit was shifted to
À
Scheme 1. Electrochemical polymerization and copolymerization of TCzm.
792 cmÀ1 after electrochemical
polymerization. This band at 792 cmÀ1 is characteristic of
À
the C H bending vibration (dCÀH) of 2,5-substituted thio-
phenes.[36,37] Therefore, the energy shift indicated the occur-
rence of a coupling reaction between monomers at a posi-
tions of thiophene units during electrochemical polymeriza-
tion. Furthermore, an absorption band of TCz6 at
À1
À
3100 cm , which is characteristic of the a-position C H
stretching vibration of a thiophene ring, disappeared after
polymerization, whereas an absorption band of TCz6 at
À1
À
3070 cm , due to the b-position C H stretching vibration
(nCÀH) of thiophene, remained after the polymerization with
a slight redshift of 5 cmÀ1. This confirms that electrochemi-
cal polymerization of TCz6 afforded PTCz6 through a cou-
pling reaction between a positions of thiophene units of
TCz6.
Figure 1. Cyclic voltammograms for electrochemical polymerization of
TCz6 obtained by using a platinum disk working electrode, a platinum
wire counter electrode, and an Ag/Ag+ reference electrode. The electro-
lyte solution contained 0.1m tetrabutylammonium perchlorate and
The PTCzm films were insoluble in acetonitrile and ace-
tone, and partially soluble in tetrahydrofuran and chloro-
form. Therefore, the molecular weights of the polymer films
were not measurable with gel permeation chromatography.
We examined the molecular weight of the polymer films by
MALDI-TOF mass spectrometry (MS). Figure 2 shows
1.0 mm TCz6 in acetonitrile. The scan rate was 100 mVsÀ1
.
Figure 1 shows cyclic voltammograms for the electro-
chemical polymerization of TCz6 by using a platinum disk
working electrode. The oxidation current onset of the first
cycle corresponded to the oxidation potential of TCz6 of
0.67 V (vs. Ag/Ag+; the other monomers show the same oxi-
dation potential). Oxidation potentials of related molecules
were evaluated experimentally in a previous report,[34] and
reported theoretically with DFT calculations.[35] An increase
MALDI-TOF mass spectra of PTCz6, PTCz
ACHTUNGTRENN(UNG 5-7), and
PTCz(4-8). The MALDI-TOF mass spectrum of PTCz6
AHCTUNGTRENNUNG
shows a certain pattern with a periodicity of 413 of m/z,
which corresponds to the molecular weight of the monomer
unit (413.13). In the measurements, peaks up to m/z 5375,
which corresponds to 13-mers, were detected. The other
homoACTHUNRTGNEUNGpolymers also show periodic patterns due to molecular
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Emission of Polymer Nanofibers
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conditions, and are introduced equally into the resulting co-
polymers.
The surface morphology of the polymer films electrode-
posited on indium tin oxide (ITO) glass electrodes was ob-
served with SEM. Generally, electrochemically polymerized
polymer films have flat, porous, rugged, globular or cauli-
flower-like surface morphology. Figure 3 shows SEM images
Figure 2. MALDI-TOF mass spectra of a) PTCz6, b) PTCzACTHNUTRGNEUG(N 5-7), and
c) PTCzACHTUNGTRENNUNG(4-8). Insets are magnifications of the spectra.
distribution of the monomer units (Figure S3 in the Support-
ing Information). The peak intensity of the spectra de-
creased with an increase in the m/z value. This result indi-
cates that the MALDI-TOF MS measurements for the elec-
trosynthesized polymer can evaluate the molecular weights
of the low-molecular-weight fraction of the polymers. On
the other hand, MALDI-TOF mass spectra of the copoly-
mers PTCzACHTUNGTRENNUNG(5-7) and PTCzACHUTNGTREN(NGUN 4-8) show periodic patterns with
two types of periodicity. A long periodicity corresponds to
an average molecular weight of two kinds of monomer
Figure 3. SEM images of surfaces of a) PTCz2, b) PTCz3, c) PTCz4,
d) PTCz5, e) and f) PTCz6, g) PTCz7, h) PTCz8, i) PTCz
j) PTCz(4-8) films electrodeposited on ITO glass electrodes.
ACHTUNGTRENNUNG(5-7), and
AHCTUNGTRENNUNG
of the polymer films. The surface morphology of the poly-
mer films significantly depends on the alkyl chain length.
PTCz2, 4, and 8 show some platelike structures, whereas
PTCz5 and 7 show cauliflower-like structures with fine fiber
structures. On the other hand, PTCz6 shows an entangled
nanofiber structure (Figure 3e and f). The nanofibers have
diameters of 100–200 nm and lengths of approximately
10 mm. The nanofibers stack on the substrate to form a film.
The PTCz6 nanofiber film is insoluble in acetonitrile, but
the entangled nanofibers are, to some extent, untangled
with sonification in acetonitrile. Figure 4a and c show
atomic force microscopy (AFM) images of untangled nano-
fibers on a glass substrate and their 3D profile, respectively.
Figure 4b shows a cross-sectional profile of the nanofiber.
units. For example, in the case of PTCzACTHNUTRGNE(NUG 5-7), the average
molecular weight of two monomer units of TCz5 (=399.1)
and TCz7 (=427.1) is 413.1, which is the same value as that
of TCz6. A short periodicity corresponds to differences of
molecular weights between two monomer units (28.0 for
PTCzACHTUNGTRENNUNG(5-7) and 56.1 for PTCzHCATUNGTRENN(UGN 4-8)). The periodic patterns
with the short periodicity show symmetric distribution,
which indicates that the two monomers have the same reac-
tivity under these electrochemical random copolymerization
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H. Goto and K. Kawabata
by electrochemical polymerization show no (or quite weak)
fluorescence properties because of residual dopants (ions) in
the films, well-aggregated structures of the polymers, and
undesired crosslinks between conjugated main chains, which
result in quenching of PL. Therefore, few papers have re-
ported the PL and electroluminescence properties of elec-
trosynthesized conjugated polymer films despite a large
number of studies on electrosynthesized polymers.[38] How-
ever, the PTCzm films in this study show PL properties in
their neutral state. Figure 5a and b show visual images of
the PTCz6 film under irradiation by visible white light and
UV light, respectively. Under white light, the neutral area
(Figure 5a, left) of the polymer film appeared yellow, and
the oxidized area (Figure 5a, right) appeared black. On the
other hand, under incident UV light, the neutral area (Fig-
ure 5b, left) showed fluorescent light-green emission, where-
as the oxidized area (Figure 5b, right) showed no emission.
Absolute PL quantum yields of the polymer in the oxidized
and neutral states are 0.32% (internal) and 0.30% (exter-
nal), and 0.0035% (internal) and 0.0032% (external), re-
spectively. The polymer in the neutral state exhibited an ap-
proximately 100 times higher value of the quantum yield
than that of the polymer in the oxidized state. The differ-
ence in fluorescence intensity of the polymer between its
oxidized and neutral states was also clearly observable by
sight.
Figure 4. a) Tapping-mode AFM image, b) cross-sectional profile, and
c) 3D profile of PTCz6 nanofibers on a glass substrate.
The PTCz6 film was also observed with a fluorescence
microscope at 450–490 nm excitation wavelength (Fig-
ure 5c). On the neutral part, the PL emission from the
nanofibers was observed, whereas on the oxidized part there
was no PL emission. This quenching of the polymer film in
the oxidized state is due to formation of radical cations (po-
larons) and dications (bipolarons) along the conjugated
backbone generated by electrochemical oxidation.[39–41] In
the neutral state, ground-state molecules are photoexcited
by absorption of light at around 425 nm, which corresponds
to the p–p* transition of the conjugated main chain. Then,
the photoexcited molecules relax through a radiative path-
way to emit green light (lem,max =535 nm; Figure S6 in the
Supporting Information). In the oxidized state, nonradiative
In these images, two fibers are crossed on the substrate. The
length of the longer fiber was approximately 7 mm, and its
diameter estimated from the cross-sectional profile was ap-
proximately 150 nm. This nanofiber was formed by self-ag-
gregation during the electrochemical polymerization pro-
ACHTUNGTRENNUNGcess. Furthermore, copolymers PTCzACHTUNRTGENNG(UN 5-7) and PTCzACHTNUGTNER(NUGN 4-8),
which both have an average alkyl chain length of six, also
show nanofiber formation (Figure 3i and j). However, poly-
mer films prepared from TCz6 and TCz4 or TCz8 with dif-
ferent constituent ratios show that nanofiber formation in
the resulting polymers disappears with deviation of the aver-
age alkyl chain length from six (Figures S4 and S5 in the
Supporting Information). These
results indicate that the chemi-
cal structures of monomers
strongly affect the aggregation
form of the resulting polymers.
In this case, the relative propor-
tion between flexible alkyl
chain length and rigidity of the
conjugated backbone is an im-
portant factor for nanofiber for-
mation.
The entire series of PTCzm
films shows the same UV/Vis–
near infrared (NIR) absorption
spectrum despite various alkyl
chain lengths. Generally, conju-
Figure 5. Photographs of PTCz6 nanofiber film deposited on an ITO glass electrode, which is half neutral
(left) and half oxidized (right), a) under visible white light and b) under incident UV light. c) Fluorescence mi-
croscopy image of the surface of the PTCz6 film at the boundary between neutral (left) and oxidized (right)
gated polymer films prepared parts.
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Emission of Polymer Nanofibers
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exciton decay occurs at polaronic quenching centers, thereby
resulting in a depression of PL intensity.
The UV/Vis–NIR absorption and PL spectra of the
PTCz6 film changed repeatedly during electrochemical
redox processes. Figure 6 shows UV/Vis–NIR absorption
Figure 7. a) Visual images and b,c) in situ PL spectra of the PTCz6 depos-
ited on an ITO glass electrode at various applied potentials (vs. Ag/Ag+)
during electrochemical b) oxidation and c) reduction processes.
Figure 6. In situ UV/Vis–NIR spectra of PTCz6 deposited on an ITO
glass electrode at various applied potentials (vs. Ag/Ag+) during electro-
chemical a) oxidation and b) reduction processes (arb.=arbitrary).
Figure 7 shows the PL spectra of the PTCz6 film with an
excitation wavelength of 425 nm at various applied poten-
tials with a three-electrode system (vs. Ag/Ag+). In the re-
duced state, the PTCz6 film emitted green light at 535 nm,
the intensity of which declined with an increase in the ap-
plied potential. Then, in the oxidized state at 0.6 V (Ag/Ag+
), the polymer exhibited no PL because the formation of po-
larons and bipolarons in the conjugated main chain of the
polymer promoted nonradiative decay rather than a radia-
tive pathway for the energy relaxation of the photoexcited
state. In the reduction process, the PL intensity was gradual-
ly recovered with a decrease in the applied potential.
To examine the repeatability of the electrochromic prop-
erties, the changes in absorption intensity at 425, 565, and
1300 nm and PL intensity at 535 nm of the PTCz6 film
during repeated potential cycles were investigated
(Figure 8). The potentials were applied sequentially between
À0.2 and 0.6 V (vs. Ag/Ag+) with a scan rate of 100 mVsÀ1
for 400 s. The absorption and PL intensities responded to
the periodic potential cycles. In the absorption intensity, the
PTCz6 film showed good repeatability. After the first cycle,
the PL intensity was restored to 90% of that from the initial
stage. After the second cycle it showed approximately 99%
repeatable restoration from the previous intensities. After
24 cycles, the reversibility slightly decreased to 82% of the
initial intensity. These results demonstrate that the polymer
spectra of PTCz6 film at various applied potentials with a
three-electrode system. At À0.2 V (vs. Ag/Ag+), in the re-
duced state, the polymer shows only an absorption band at
425 nm due to the p–p* transition of the conjugated main
chains. The 425 nm absorption band decreased and blue-
shifted to 407 nm with an increase of the potential, and new
absorption bands (doping bands) at around 565 and
1300 nm appeared, which correspond to polarons (radical
cations) and bipolarons (dications), respectively, generated
by the oxidation of the conjugated backbone. The blueshift
of the p–p* transition band was observed due to shortening
of the effective conjugation length of the main chain by po-
larons and bipolarons.[40] On the other hand, during the re-
duction process, the polaron and bipolaron bands disap-
peared and the 425 nm band regained its original intensity.
Notably, a large optical change was observed at 0.4–0.5 V in
the oxidation process, whereas a gradual change was ob-
served during the reduction process. In other words, a non-
linear spectral change in the intensities was observed during
the electrochemical process. Dyer et al. reported these phe-
nomena for electrochromic poly(dialkoxythiophene)s,[13] and
referred to a cooperative intramolecular domino effect,
which is similar to the “twiston” described by Leclerc for
the thermochromism of poly(alkylthiophene)s and poly-
ACHTUNGTRENNUNG
(alkoxythiophene)s.[42]
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H. Goto and K. Kawabata
Furthermore, X-ray diffrac-
tion (XRD) measurements for
the PTCz6 nanofiber film de-
posited on an ITO glass elec-
trode were performed to exam-
ine the ordered structure of the
polymer chains in the nanofib-
ers. Figure 10 shows XRD pat-
terns of the PTCz6 film and an
ITO glass electrode substrate
(the peak at 21.38 is due to a
characteristic diffraction from
the ITO glass electrode). The
XRD pattern of the PTCz6
film shows well-defined peaks
at around 2.7 and 4.48 (=2q),
which correspond to 32.1 and
20.1 ꢁ d spacings, respectively.
These d spacings are in good
agreement with theoretically
calculated distances between
adjacent alkyl chains perpendic-
Figure 8. Time dependence of UV/Vis–NIR absorption intensity at 425, 565, and 1300 nm and PL intensity at
535 nm of PTCz6 film during repeated potential cycles. The potential was repeatedly applied between À0.2
and 0.6 V (vs. Ag/Ag+) with a scan rate of 100 mVsÀ1 for 400 s.
shows dynamically controllable change in emission with
good repeatability. It is well known that the 3- and 6-posi-
tions of carbazole are reactive even after substitution or pol-
ymerization at the 2- and 7-positions. However, within the
potential range up to 0.65 V (vs. Ag/Ag+), PTCz6 exhibited
a repeatable redox behavior even after 20 potential cycles
unless a potential larger than 0.8 V (vs. Ag/Ag+) was ap-
plied, which led to an unfavorable oxidative cross-linking re-
action between the 3- and 6-positions of the carbazole units
of adjacent polymer chains, thereby resulting in unrepeata-
ble redox switching (Figure S8 in the Supporting Informa-
tion).
Polarized PL emission from the PTCz6 nanofiber was ob-
served visually by polarized fluorescence microscopy. Fig-
Figure 9. Polarized fluorescence microscopy images of a PTCz6 nanofiber
with the polarizer direction a) perpendicular and b) parallel to a PTCz6
nanofiber.
ure 9a and
b show polarized fluorescence microscopy
images of the PTCz6 nanofibers under incident UV light
with different polarizer directions. In the polarizer direction
perpendicular to the nanofiber, PL emission from the nano-
fiber was brighter than that in the parallel direction. This in-
dicated that conjugated main chains of PTCz6 align perpen-
dicular to the longitudinal axis of the nanofiber because
linear conjugated polymers usually have transition dipole
moments along the conjugated backbone. The molecular
orientation in the nanofibers of this study was different
from that of conjugated polymer nanofibers prepared by
electrospinning, in which conjugated main chains were ori-
ented along the longitudinal axis of the fibers.[33,43] The mo-
lecular orientation reported herein was similar to that of
polythiophene nanofibers fabricated by using cast film, a
whisker method, or shadow masks, in which the polymer
chains were packed with their longitudinal axis perpendicu-
lar to the nanofiber axis.[31,44,45]
Figure 10. XRD patterns of PTCz6 nanofiber film deposited on an ITO
glass electrode (top) and an uncoated ITO glass electrode (bottom).
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Emission of Polymer Nanofibers
FULL PAPER
cates that the proportion of the alkyl chain length relative
to the conjugated backbone is important for nanofiber for-
mation. The nanofibers thus prepared showed dynamically
controllable electrofluorescence chromism. This could be
applied to fluorescent-type passive indicator and display de-
vices. Furthermore, PTCz6 nanofibers showed polarized
emission, in which the polarizing direction was perpendicu-
lar to the fiber axis. XRD measurements of PTCz6 showed
that the polymer chains possessed crystalline order in the
nanofibers. This nontemplate bottom-up formation of nano-
fibers is due to self-assembly of growing polymers during
electrochemical polymerization. Thus, electrochemical poly-
merization can be a straightforward method to deposit
nanostructured polymers on electrode substrates under cer-
tain conditions and with appropriate molecular shape of the
monomers, from the viewpoint of bottom-up technology.
Figure 11. Schematic illustration of a possible packing structure of PTCz6
chains into nanofibers.
Experimental Section
ular to the main chain, which are alternately substituted in
opposite directions, and between adjacent main chains
(Figure 11). The XRD pattern also shows two small peaks at
6.2 and 17.88 (=2q), which correspond to 14.3 and 5.0 ꢁ
d spacings, respectively. The 5.0 ꢁ d spacing is assignable to
the p-stacking distance between the main chains. These re-
sults lead to a possible structure of polymer chain packing
of PTCz6 in the nanofiber, as shown in Figure 11. However,
Materials: TCzm was synthesized according to the method reported pre-
viously.[34] The synthetic routes to the monomers are summarized in the
Supporting Information.
Electrochemical polymerization: Electrochemical polymerization and co-
polymerization of TCzm were carried out by repeated potential cycling
with a three-electrode system, which consisted of an indium tin oxide
(ITO)-coated glass or platinum disk working electrode, an Ag/Ag+ refer-
ence electrode, and a platinum wire counter electrode. The electrolyte
solution consisted of tetrabutylammonium perchlorate (0.1m) and mono-
mers (1.0 mm) in acetonitrile. The scan rate was 100 mVminÀ1. In the
case of electrochemical copolymerization, the concentration of each
PTCzACHTUNGTRENNUNG(5-7) and PTCzACHTUTGNREN(NUNG 4-8) nanofiber films show no such
XRD pattern and polarized emission, thus indicating lower
crystallinity in the form of the nanofibers. In electrochemical
polymerization, the solubility of the growing polymer de-
creases as the polymerization reaction proceeds, which re-
sults in nucleation on the electrode followed by a solid-state
polymerization reaction. The solubility of PTCzm depends
on the alkyl chain length and the degree of polymerization,
and the polymer has good intermolecular p–p interaction
due to the good coplanarity and symmetry of carbazole and
bithiophene units in the backbone. Therefore, aggregation
occurs as a result of the competition between the solubility
and intermolecular p–p interaction of the polymers, which
determines the surface morphology. The competition can be
a crucial factor for nanofiber formation of PTCzm. To date,
many fluorescent conjugated polymer nanofibers have been
reported, most of which are hybrid materials with organic
polymer matrices or an inorganic template.[33,43,46] In con-
trast, the PTCz6 nanofiber is made purely from a conjugated
polymer that has fluorescence and structural order, which
can be applied to new optoelectronic devices.
monoACTHNUTRGENUmGN er was 0.5 mm.
Acknowledgements
We would like to thank the Chemical Analysis Center of the University
of Tsukuba and the Glass Workshop of the University of Tsukuba. K.K.
is a research fellow of the Japan Society for the Promotion of Science.
The research was supported by a Japan Society for the Promotion of Sci-
ence (JSPS) Grant-in-Aid for Scientific Research (No. 22550161). We are
grateful to JASCO Corporation for measurements of photoluminescence
quantum yields of the polymer films.
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ACHTUNGTRENNUNG
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Received: April 28, 2012
Revised: August 9, 2012
Published online: October 2, 2012
15072
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15065 – 15072