Optically active, lyotropic liquid crystalline poly(diphenylacetylene) derivative: Hierarchical chiral ordering from isotropic solution to anisotropic solid films

Article abstract of DOI:10.1039/c2cc34513g

The side chain chirality of a poly(diphenylacetylene) derivative was transferred and amplified spontaneously from solution to a bulk film due to lyotropic liquid crystallinity.

Full text of DOI:10.1039/c2cc34513g

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Optically active, lyotropic liquid crystalline poly(diphenylacetylene)
derivative: hierarchical chiral ordering from isotropic solution to
anisotropic solid filmsw
Daehoon Lee,a Hyojin Kim,a Nozomu Suzuki,b Michiya Fujiki,*b Chang-Lyoul Lee,c
Wang-Eun Leea and Giseop Kwak*a
Received 23rd June 2012, Accepted 25th July 2012
DOI: 10.1039/c2cc34513g
The side chain chirality of a poly(diphenylacetylene) derivative
was transferred and amplified spontaneously from solution to a
bulk film due to lyotropic liquid crystallinity.
To address this subject, a highly soluble, lyotropic liquid
crystalline, optically active polymers are highly desired. Our
synthetic strategy follows this molecular design concept. The
attachment of an alkyl side chain to one side phenyl ring of
poly(diphenylacetylene) (PDPA) via a silylene linkage imparts
high solubility to afford extremely high molecular weight
polymer derivatives.4 Most importantly, these polymer derivatives
show lyotropic liquid crystallinity due to the high molecular
weight and chain rigidity.5 Thus, we thought that it is possible
to synthesize a target polymer in the desired direction simply by
introducing a chiral alkyl group into the side phenyl ring of the
PDPA derivative. The (S)-2-methylbutyl group was chosen as the
chiral source to synthesize a optically active, liquid crystalline
PDPA derivative (2MB-PDPA in Scheme 1). The polymer film
that was drop-cast from a toluene solution exhibited very unusual
fibril bundles on the surface. In addition, this polymer showed
significantly different CD signals in solution and film.
Comparative CD spectral analysis clarified the chirality transfer
and amplification from a dispersed single polymer chain to a
bulk solid film in a hierarchical structure. Herein we will
describe the influence of liquid crystallinity on the morphology
and chiroptical properties of the final film products.
External and internal chiral sources often make (liquid)
crystalline molecules undergo asymmetric structural changes
during self-assembly in a mirror-symmetry-breaking process,
leading to circular dichroism (CD) in the solid state.1 Many
research groups have reported that conjugated oligomeric
and polymeric molecules with a rigid backbone and planar
geometry provide super-helical fibrils and nanometer-order
sized aggregates with a one-handed screw sense in the presence
of a chiral source.2 The highly ordered structures were
spontaneously generated to construct super-figured frame-
works with a hierarchical structure. As one of the most outstanding
works, Akagi et al. demonstrated spontaneous generation of a
unique artificial gigantic figure due to hierarchical helical
ordering at levels of isolated polymers, supramolecular
polymers, and upwards.3 They elegantly synthesized a super-
helical fibril and bundle of unsubstituted polyacetylenes in a
chiral nematic liquid crystal reaction field. On the other hand,
the chiral structure and chiroptical properties of the single
polymer chain could not be examined because the final
product is insoluble in organic solvents and completely
infusible. Therefore, if a certain chiral polymer dissolves well
in an organic solvent and has lyotropic liquid crystallinity, it
may allow a study on how the chirality of a single polymer
chain transfers to an anisotropic self-assembly with a highly
ordered structure.
The synthetic procedure for the monomer is shown in
Scheme 1: chlorodimethyl-(S)-2-methylbutylsilane (1) was
employed as a starting compound for the chiral source, and
reacted with a lithiated 1,2-diphenylacetylene to provide
the present monomer (2MB-DPA: [a]D = +11.751, c =
1.00 g dLÀ1, in CHCl3). Polymerization using the TaCl5–n-Bu4Sn
a Department of Polymer Science, Kyungpook National University,
1370 Sankyuk-dong, Buk-gu, Daegu 702-701, Korea.
E-mail: gkwak@knu.ac.kr; Fax: +82-53-950-6623;
Tel: +82-53-950-7558
b Graduate School of Materials Science, Nara Institute of Science and
Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan
c Advanced Photonics Research Institute, Gwangju Institute of Science
and Technology, 1 Oryong-dong, Buk-gu, Gwangju 500-712, Korea
w Electronic supplementary information (ESI) available: Detailed
experimental methods, XRD (Fig. S1), TEM (Fig. S2), temperature-
variable CD and UV-vis spectra (Fig. S3), CD and LD spectra (Fig. S4
and S5), polarized UV-vis spectra (Fig. S6), CD spectra and DLS on
aggregation (Fig. S7), PL and CPL spectra (Fig. S8). See DOI:
10.1039/c2cc34513g
Scheme 1 Synthesis of monomer (2MB-PDA) and polymer
(2MB-PDPA).
c
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Chem. Commun., 2012, 48, 9275–9277 9275

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and 318 nm, respectively. The gCD value of 2MB-PDPA in
solution at 385 nm was as large as 0.69 Â 10À3 indicating the
optical activity of the isolated polymer chain. Previously, Aoki
and Tang independently synthesized PDPA derivatives
containing chiral pinanyl and menthyl groups, respectively,
in the side chain.6 These polymers in solution also showed
similar CD spectra with extremum at approximately 385 nm to
that of 2MB-PDPA. One of the interesting findings by Aoki’s
research group was that the PDPA containing a pinanyl group
showed a strong Cotton effect, whereas the corresponding
disubstituted polymers with only one phenyl ring showed very
weak CD signals. This significant difference in the Cotton
effect between the PDPA derivative and other types of
disubstituted polymers was unexpected because the latter
polymers had not only the same chiral side group as that of
the former one but also the same geometric structure. This
suggested that PDPA derivatives with two phenyl rings are
favorable for generating optical activity in disubstituted
acetylene polymers.6 The reason has not been described in
detail and the origin of CD of PDPAs is still unclear. In
general, the magnitude of the Cotton effects of dynamic helical
polymers tends to decrease with increasing temperature, which
is derived from a change in population of the right- and left-
handed helices. The present polymer in solution, however, did
not show a significant decrease in the gCD value over a wide
temperature range of 20 1C (gCD B 0.69 Â 10À3) to 80 1C
(B0.61 Â 10À3) (Fig. S3, ESIw). This indicates that the chiral
conformation responsible for the optical activity is thermally
stable in solution. Therefore, it would be reasonable to say
that the present polymer has a quite thermally stable chiral
conformation in solution due to the rigid backbone and large
steric hindrance between the bulky side phenyl rings although
achiral PDPAs could show an optical activity (induced CDs)
after introduction of chiral substituents via polymer reactions
and also in chiral solvents, clearly suggesting that a class of
PDPAs are not static helical polymers.7
Fig. 1 (a) SEM image and (b) polarized optical, (c) optical,
(d) polarized fluorescence microscope images of 2MB-PDPA in drop-
cast film.
catalyst under suitable conditions gave an optically active
polymer, 2MB-PDPA, with a high molecular weight of
4.2 Â 106 g molÀ1 in a yield of 40%.
Films drop-cast from a toluene solution of 2MB-PDPA left
very unusual bundles of fibrils on the surface, as shown in the
scanning electron microscope (SEM) image (Fig. 1a). The
bundles of fibrils gave chevron morphology, which were
spread consecutively from the core to the edge of the film.
Although no screw sense of the fibril could be determined,
the shape and diameter of the bundles were quite uniform.
Similarly, optical microscope images (Fig. 1b–d) clearly
showed anisotropic textures. The X-ray diffraction (XRD) of
the drop-cast film revealed a very sharp diffraction peak at a
small angle of 6.41, corresponding to a distance of 13.7 A,
which was assigned to the lamellar layer distance (Fig. S1,
ESIw). The high resolution transmission electron microscope
(HR-TEM) image clearly showed lamellar structure in the
bulk solid (Fig. S2, ESIw). The hierarchical structure with an
anisotropic texture and lamellar layer in the drop-cast film results
from the intrinsic lyotropic liquid crystallinity characteristic of
PDPA derivatives with side alkyl chains.5
In the preparation of a drop-cast film from lyotropic liquid
crystalline polymers, the polymer chains should necessarily
undergo an isotropic-to-liquid crystal phase transition at a
critical concentration during slow solvent evaporation.5 As
shown in Fig. 1, 2MB-PDPA left a well-constructed morphology
on the drop-cast film due to the disorder-to-order phase transition.
Moreover, a higher-order structure such as a helix may be
obtained during the phase transition owing to the internal
chiral source of the (S)-2-methylbutyl group in the side chain.1
Namely, the polymer chains in the ideal solution should be
completely dispersed and kinetically free whereas the polymer
chains in the spin- and drop-cast films should be quenched
and self-assembled, respectively. Accordingly, comparative
CD spectral analysis of the polymer in solution, spin- and
drop-cast films, may provide more detailed information on the
conformational change and chirality transfer from the dispersed
single polymer chains to the self-assembled polymer chains. As
shown in Fig. 2, the polymer in solution and films showed quite
different CD signals. In both the spin- and drop-cast films, the
first Cotton band of the solution at 385 nm shifted to a shorter
wavelength of approximately 378 nm and a new CD band
appeared at 437 and 443 nm, respectively. The CD and linear
dichroism (LD) spectra of the films indicated that the
Fig. 2 shows the normalized CD spectra of 2MB-PDPA in
three different phases of solution, spin- and drop-cast films.
The toluene solution displayed a strong (plus) CD signal
at 385 nm as a first Cotton band. The second (plus), third
(minus), and fourth (plus) Cotton bands appeared at 351, 338
Fig. 2 CD and UV-vis spectra of 2MB-PDPA in solution (c = 5.0 Â
10À4 mol LÀ1 in toluene), spin-cast (thickness B200 nm) and drop-cast
(B5 mm) films.
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contribution of LD caused by macroscopic anisotropy was
almost negligible (Fig. S4 and S5, ESIw). 2MB-PDPA shows
dual UV-visible absorption bands characteristic of PDPA
derivatives. All CD bands of 2MB-PDPA in the solution
and films showed mismatch in the dual UV absorption bands.
Here, there are two questions: the first is why the CD spectra
of solution and films differ from each other. The second is why
the CD bands do not match the UV absorption band. To
answer these questions, the origin of the shorter wavelength
CD signals (385 nm band in solution; 378 nm bands in films)
and the longer wavelength CD (no band in solution; 437 nm
band in spin-cast film; 443 nm band in drop-cast film) signals
should be discussed first. Owing to the intrinsic lyotropic
liquid crystallinity, the present polymer can give a sheared
film.5 The polarized absorption spectra of 2MB-PDPA in the
sheared film indicated that the 430 nm absorption band was
ascribed to p–p* transition of conjugation structure extended
to a direction parallel to the main chain, while the 370 nm
band was due to a p–p* transition of localized electrons
within the aromatic mesogenic units attached in a direction
perpendicular to the main chain (Fig. S6, ESIw). Noticeably,
an isosbestic point appeared at 377 nm, which almost matches
the 378 nm CD extremum wavelength, indicating axial
chirality between the main chain and resonant side phenyl
rings. On the other hand, the newly appeared longer
wavelength CD signals in the films indicate the formation of
a one-handed screw sense helical structure in the polymer
backbone. The induced helicity on the polymer backbone in
the films probably arose from the restricted conformational
freedom in solid state. It should also be noted that the longer
wavelength CD band of the drop-cast film is red-shifted
slightly and is little stronger than that of the spin-cast film.
This means that the conjugation of the main chain was
extended further, and the magnitude of the one-handed
screw sense is greater in the self-assembled film than in the
quenched film. The larger helicity of the polymer main chain in
the drop-cast film may be due to a further restricted chain
conformation in the denser space of the assembled film. This
comparative CD spectral analysis clarified that the axial
chirality between the main chain and resonant side phenyl
rings of a single polymer chain in solution is transferred to the
polymer backbone due to an assembly effect, resulting in a
one-handed helical sense, thereby producing a CD signal in
the absorption region of the polymer backbone. The chirality
transfer was reconfirmed by CD and DLS measurement in an
aggregation experiment (Fig. S7 and the description, ESIw).
The solution and drop-cast film of 2MB-PDPA showed
photoluminescence (PL) maximum at 505 and 524 nm, respectively
(Fig. S8, ESIw). The emission colors of the solution and films
appeared to be sky blue and greenish yellow, respectively.
The photoemission decay of this polymer obeyed a biexpo-
nential decay. The average PL lifetime was slightly longer
in films (0.81 ns, monitored at 530 nm) than in solution
(0.69 ns, monitored at 500 nm). This suggests that the polymer
chain mobility is highly restricted in the glassy solid state. The
solution and films also showed quite intense circular photo-
luminescence (CPL) with a maximum at 503 and 522 nm,
respectively (Fig. S8, ESIw), which matched the PL band. The
signs of the CPL signals were identical to those of the CD
bands. The absolute magnitude of the gCPL value of the
solution (B0.60 Â 10À3) at 503 nm was similar to that of
the gCD value (B0.69 Â 10À3). These results suggest that the
CPL bands are solely derived from the same chiral sources as
that of the CD bands.
In summary, we successfully synthesized a highly soluble,
optically active, liquid crystalline poly(diphenylacetylene)
derivative. Very unusual bundles of fibrils were left on the
surface of the drop-cast film. Comparative CD spectral
analysis clarified that the chirality of the side alkyl group
was transferred and amplified spontaneously from an isotropic
solution to a bulk film during the self-assembly process due to
lyotropic liquid crystallinity. These results may explain the
naturally occurring, highly ordered, super-helical structures in
terms of chirality and liquid crystallinity.
This research was supported by Kyungpook National
University Research Fund, 2012. This work was supported
by the Basic Science Research Program through the National
Research Foundation of Korea (NRF) grants funded by
the Korea government (MEST) (2012-0000633, 2012-
0001714, 2012-0005143). This work was also supported by
Grant-in-Aid for Scientific Research (B) (22350052) from
MEXT, Japan.
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c
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