Latticelike smectic liquid crystal phase in a rigid-rod helical polyisocyanide with mesogenic pendants

Article abstract of DOI:10.1021/ja201133d

We report a unique macromolecule consisting of a rodlike helical polyisocyanide backbone with a narrow molecular weight distribution and rigid mesogenic chiral pendants linked via a flexible spacer that exhibits lyotropic nematic and latticelike new smect

Full text of DOI:10.1021/ja201133d

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Latticelike Smectic Liquid Crystal Phase in a Rigid-Rod Helical
Polyisocyanide with Mesogenic Pendants
Takashi Kajitani,*,†,|| Hisanari Onouchi,† Shin-ichiro Sakurai,† Kanji Nagai,†,‡,^ Kento Okoshi,†
Kiyotaka Onitsuka,§ and Eiji Yashima*,†,‡
†Yashima Super-structured Helix Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology
Agency (JST), Japan
‡Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603,
Japan
§Department of Macromolecular Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan
S Supporting Information
b
smectic (Sm) LC phase in which the stiff helical polymers self-
ABSTRACT: We report a unique macromolecule consist-
ing of a rodlike helical polyisocyanide backbone with a
narrow molecular weight distribution and rigid mesogenic
chiral pendants linked via a flexible spacer that exhibits
lyotropic nematic and latticelike new smectic (lat-Sm)
liquid crystal phases at different concentrations. The un-
precedented lat-Sm phase is associated with the smectic
ordering of both the stiff polymer backbone and the rigid-
rod side groups. A detailed investigation of the films using
X-ray scattering and atomic force microscopy revealed a
novel tilted smectic layer structure of the polymer backbone
aligned perpendicular to the smectic layer of the mesogenic
pendants, which arrange in an antiparallel overlapping
interdigitated manner.
assemble into a layered structure with a uniform thickness on
length scales of tens of nanometers,7 whereas those with broad
MWDs form cholesteric or nematic phases.8
It has been considered that stiff main-chain polymers with
mesogenic pendants (i.e., combined main-chain and side-chain
LC polymers) have the potential to form new phases.9 However,
despite the wealth of phases produced by combining main-chain
and side-chain LC polymers, they have received little attention,
probably because of the lack of main-chain rigidity and controlled
molecular architectures.10 We now report a class of combined
main-chain and side-chain LC polymers represented by poly-1,
which contains a rodlike helical polyisocyanide with a narrow
MWD bearing rigid mesogenic 4-[(S)-2-methylbutoxycarbonyl]-
biphenyl residues linked via a flexible dodecyl spacer and an
(S)-alanine (Ala) moiety (Figure 1C). This polymer exhibits a
lyotropic nematic LC phase and a new latticelike smectic (lat-
Sm) LC phase.
he investigation of molecular self-assembly and self-
Our design approach builds upon our previous findings that
similar helical polyisocyanides that bear (S)- or (R)-Ala residues
with long alkyl chains as the pendants and have a controlled
molecular length and handedness (right- or left-handed helix)
can be prepared by helix-sense-selective living polymerization
with an achiral ethynediyl-bridged palladiumÀplatinum (PdÀPt)
complex as the initiator; the resulting polyisocyanides (poly-2;
Figure 1C) possess an almost perfect one-handed helix and form
an Sm LC phase, as directly observed by atomic force microscopy
(AFM) and revealed by X-ray diffraction (XRD).7c The poly-
isocyanides possess an unprecedented long persistence length
of 220 nm stabilized by intramolecular hydrogen-bonding
networks through the neighboring amide NÀH groups;11 this
value is the highest among all synthetic helical polymers
reported to date and comparable to those of biological, multi-
stranded helical polymers such as triple-stranded helical col-
lagen and schizophyllan.12
T
organization, resulting in new complex liquid crystal (LC)
phases, is among the most fascinating research areas.1 In parti-
cular, since Freiser’s prediction2 and the subsequent discovery of
the biaxial nematic (Nb) LC phase (Figure 1A),3 this emerging
topic has attracted significant attention because of its potential
device applications with the possibility of much faster switching.4
Another biaxial phase with an unusual layer structure, namely, the
biaxial smectic (SmAb) phase composed of orthogonally aligned
molecules in its layers (Figure 1B), was also discovered.5 These
biaxial phases have been observed not only in small-molecule-
based, plate5c,d and bent-core “banana-shaped” molecules,3cÀe,5b,5d
but also in polymeric systems consisting of flexible polymers with
platelike mesogenic pendants (side-chain LC polymers)3b,5a and
main-chain LC polymers.3a Although the first SmAb phase was
observed in a side-chain LC polymer upon cooling of its Nb
phase,5a a limited number of unique LC phases are available from
synthetic polymers.
Poly-113 has a backbone structure and Ala-based pendant
subunit identical to those in poly-2 and shows a similar circular
dichroism (CD) spectrum in the main-chain imino chromophore
region (Figure 2A). The Cotton effect sign and intensity at
On the other hand, biological helical polymers such as DNA
form complex LC structures [e.g., a two-dimensional smectic
(2D-Sm) phase when DNA is complexed with a cationic lipo-
some in a lyotropic system6] originating from their main-chain
stiffness with a controlled helical sense. Rodlike helical polymers
with inherent chain stiffness (large persistence length) and a
narrow molecular weight distribution (MWD) can exhibit a
Received: February 5, 2011
Published: May 31, 2011
r
2011 American Chemical Society
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Figure 1. (A, B) Packing structures of platelike molecules in the (A)
biaxial nematic (Nb) and (B) biaxial smectic (SmAb) phases. (C)
Structures and schematic illustration of polyisocyanide (poly-1) com-
posed of a stiff helical main chain with a controlled helicity (blue)
bearing rodlike mesogenic pendants (green). The structure of poly-2
is also shown.
Figure 3. X-ray analyses of poly-1. (A) SAXS and (B) WAXD patterns
of an oriented poly-1 film prepared from a concentrated nematic LC
benzene solution (∼15 wt %). The vertical direction nearly corresponds
to the helix axis. The reflections were indexed with a monoclinic lattice
(a = 4.90 nm, b = 2.24 nm, c = 1.35 nm, γ = 115ꢀ), suggesting a 15/4
helical structure. (C) SR-SAXS pattern of a film of Pd-eliminated,
nonoriented poly-1 (Mn = 15.9 Â 104) prepared from a concentrated
smectic LC CHCl3 solution. The spacing of an inner reflection (dSR-
SAXS) was 10.09 nm. (D) Schematic illustration of the proposed packing
structure of poly-1 (tetramer) in the oriented polymer film based on the
WAXD and SAXS measurement results. The main-chain carbon atoms
are shown in yellow.
forms a nematic phase, as evidenced by its indisputably clear
schlieren texture (Figure2B) that exclusively consists of two-brush
disclinations,5d which is an important diagnostic for the presence
of a biaxial phase; however, we did not have concrete evidence for
the biaxial nematic phase rather than a nematic phase at present.
An increase in the concentration produced a fan-shaped texture
being typical of an Sm LC phase (Figure 2C).14 Furthermore, this
mesomorphic texture was retained after complete removal of the
solvent, resulting in a polymer film with the Sm layer structure, as
evidenced by its XRD and AFM measurement results (see below).
Polarized IR spectra of an electric-field-induced oriented film
of poly-1 with the incident light polarized parallel ( ) and
perpendicular (^) to the applied electric field (Figure S2)15
revealed the relative orientations of the helical backbone and the
mesogenic pendant groups of the combined polyisocyanide
(Figure 2D,E). Sharp signals assigned to the amide NH and
Figure 2. Spectroscopic and optical properties of helical poly-1. (A) CD
(a) and absorption (b) spectra of poly-1 in CHCl3 (0.2 mg/mL) at 25
ꢀC. (B, C) Polarized optical micrographs of (B) the schlieren texture of
the nematic LC phase and (C) the fanlike texture of the new lat-Sm LC
phases of poly-1 in CHCl3. (D, E) Polarized FT-IR spectra of an
oriented poly-1 film with the incident light polarized parallel (||, red)
and perpendicular (^, blue) to the applied electric field. The film was
prepared on a CaF2 substrate from a concentrated LC toluene solution
in an electric field (6000 V/cm).
∼360 nm for poly-1 (Δεfirst = À22.2) suggest that the polymer
likely possesses an almost perfect left-handed helical structure.7c
Polarized optical microscopy of a concentrated solution of
poly-1 in CHCl3 (∼15 wt %) demonstrated that the polymer
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carbonyl stretching (amide I) bands at 3265 and 1632 cmÀ1
,
respectively, suggested the formation of intramolecular hydro-
gen-bonding networks.7c,11a The dichroic ratios ([A /A^]) for
the selected functional groups of poly-1 (Table S1) indicated that
the the polar amide NÀH (νNH) and carbonyl groups (amide I)
were arranged parallel to the electric-field applied to the poly-1
film, resulting in the uniaxial orientation of the rodlike helical
backbone parallel to the direction of the applied electric field. On
the other hand, the aromatic ester CÀO (νCÀO-II) and ether
CÀOÀC (νCÀOÀC) groups tended to be oriented perpendicular
to the applied electric field (Figure 2E). Thus, the long axes of the
mesogenic pendants were likely aligned perpendicular to the
helical backbone axis.
Figure 3A,B shows small-angle X-ray scattering (SAXS) and
wide-angle X-ray diffraction (WAXD) patterns, respectively, for an
oriented poly-1 film prepared from a concentrated LC benzene
solution in an electric field (Figure S2). These revealed apparent
meridional and equatorial reflections, leading to a 15/4 helical
structure [i.e., a 15 units per 4 turns (15/4) helix]7c with a
monoclinic lattice (parameters: a = 4.90 nm, b = 2.24 nm, c =
1.35 nm, γ = 115ꢀ) (Table S2 and Figure 3D). These XRD
patterns identified a bilayer-type architecture9a,16 in which the
mesogenic pendants (lpendant = 6.2 nm) are arranged in an
antiparallel overlapping interdigitated manner, giving a length
difference of 4.45 nm between the polyisocyanide backbone layers.
Synchrotron-radiation SAXS (SR-SAXS) of a poly-1 film17
prepared from a concentrated smectic LC CHCl3 solution demon-
strated a weak inner reflection with a spacing of 10.09 nm as
well as a strong outer reflection of 4.45 nm attributable to the
interdigitated lateral packing of the rod polymer (Figure 3C).
We also found that the observed inner spacing became shorter
(8.70 and 8.50 nm) when lower-molecular-weight poly-1s (Mn =
12.7 Â 104, Mw/Mn = 1.09 and Mn = 12.2 Â 104, Mw/Mn = 1.10,
respectively) were used (Figure S3). Thus, the inner spacing of
10.09 nm corresponds to the smectic layer distance of the
polymer backbones.
An AFM image of a Pd-eliminated poly-1 thick film prepared
from a concentrated CHCl3 solution (1.0 mg/mL) on highly
oriented pyrolytic graphite (HOPG)7c,18 clearly revealed a
smectic-like self-assembly of the polymer chains with a controlled
spacing (Figure 4A). The layer structure was nearly perpendi-
cular to the direction of the polymer main-chain axes. The
average layer spacing increased with the increasing Mn of the
polymers from 14 to 16 nm (Mn = 12.7 Â 104 and 15.9 Â 104,
respectively) (Figure 4A and Figure S4B). However, these layer
spacings as estimated using AFM were significantly different
from those determined using SR-SAXS (dSR-SAXS < dAFM). We
then performed AFM measurements on thin films of Pd-elimi-
nated poly-1 prepared from a dilute benzene solution (0.005 mg/
mL) on HOPG, which also showed a 2D-Sm-like assembly of the
polymer chains with average layer spacings of 21 and 16 nm
(Figure 4B and Figure S4C, respectively). The difference in the
layer spacings for the thick and thin films may be due to the
difference in the interactions between the polymers and the
HOPG substrate. In addition, the average main-chain lengths
(Ln) of these polyisocyanides (Figure S5) were indeed longer than
the layer spacings determined by SR-SAXS. Thus, we consider that
poly-1 forms a Sm-C-like tilted layer structure in the lat-Sm phase.
On the basis of the results presented here, we propose that the
polyisocyanides form a platelike structure composed of a rigid
helical backbone with the lateral mesogenic pendants oriented
perpendicular to the polymer backbone axis. Furthermore, the
Figure 4. Self-assembled smectic layer structures of poly-1. (A, B) AFM
phase images of (A) a thick film and (B) self-assembled 2D Pd-eliminated
poly-1 (Mn = 15.9 Â 104) onHOPG. The yellow bars and red dotted lines
indicate the individual polymer chains and 2D smectic layers with respect
to the polymer backbone axis, respectively. (C) Schematic illustration of a
possible layer structure of poly-1 in the lat-Sm phase.
mesogenic pendants most likely form the bilayer smectic structure
inan antiparallel overlapping interdigitated manner. The SR-SAXS
and AFM results support the tilted smectic layer structure of the
polymer backbones perpendicular to the layer plane of the
mesogenic pendants. Consequently, a class of combined main-
chain and side-chain LC polymers (i.e., the rodlike helical poly-
isocyanide poly-1) forms the highly ordered latticelike smectic
(lat-Sm) LC phase in a concentrated LC solution (Figure 4C).
This unprecedented phase behavior was generated by careful
design of the constituent polymer, which incorporates rodlike
mesogenic pendants linked via a flexible spacer to an exceptionally
rigid rod helical backbone with a controlled rod length.
In spite of numerous variations and the number of small-
molecule-based LC phases, it is difficult for such small molecules
to form a complicated latticelike smectic structure because of the
size limit. Realization of the lat-Sm phase is significant because it
opens up the possibility of fabricating new types of liquid-
crystalline electro-optical devices. We anticipate that the com-
bined main-chain and side-chain LC polymers may form further
unprecedented, complicated packing structures in an LC system
because various combinations of smectic structures between the
rodlike backbones and the pendant layers are possible. We are
currently investigating the interplay between the helical structure
and the dynamic behavior of the nematic and lat-Sm phases.
’ ASSOCIATED CONTENT
S
Supporting Information. Full experimental details, FT-
b
IR data, SAXS and WAXD data, and AFM images. This material
is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
(9) (a) Watanabe, J.; Tominaga, T. Macromolecules 1993,
26, 4032–4036. (b) Zhang, J. J.; Ge, A.; McCreight, K. W.; Ho, R.-M.;
Wang, S.-Y.; Jin, X.; Harris, F. W.; Cheng, S. Z. D. Macromolecules 1997,
30, 6498–6506.
Corresponding Author
kajitani@riken.jp; yashima@apchem.nagoya-u.ac.jp
(10) (a) Schaefer, K. E.; Keller, P.; Deming, T. J. Macromolecules
2006, 39, 19–22. (b) Chen, X.-F.; Shen, Z.; Wan, X.-H.; Fan, X.-H.;
Chen, E.-Q.; Ma, Y.; Zhou, Q.-F. Chem. Soc. Rev. 2010, 39, 3072–3101.
(11) (a) Okoshi, K.; Nagai, K.; Kajitani, T.; Sakurai, S.-i.; Yashima, E.
Macromolecules 2008, 41, 7752–7754. For helical polyisocyanopeptides
stabilized by intramolecular hydrogen bonds, see ref 8b and:(b)
Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk, N.
A. J. M. Chem. Rev. 2001, 101, 4039–4070.
Present Addresses
Advanced Science Institute, RIKEN, 2-1 Hirosawa, Wako,
Saitama 351-0198, Japan.
^Department of Applied Chemistry, Graduate School of Engineer-
ing, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan.
(12) Sato, T.; Teramoto, A. Adv. Polym. Sci. 1996, 126, 85–161.
(13) Left-handed helical poly-1 was prepared by living polymeriza-
tion of the corresponding monomer with the PdÀPt catalyst in
tetrahydrofuran (THF) at 55 ꢀC for 20 h (Scheme S1), which produced
both perfectly right- and left-handed helical polyisocyanides with
different molecular weights and sufficiently narrow MWDs. The left-
handed helical poly-1 was easily fractionated with acetone as an acetone-
insoluble fraction and used in the present study; the number-average
molecular weight (Mn) and its distribution (Mw/Mn) were 1.76 Â 105
and 1.06, respectively, as determined by size-exclusion chromatography
(SEC) in THF containing 0.1 wt % tetra-n-butylammonium bromide
(TBAB) as the eluent with standard polystyrenes. (Figure S1).
(14) Dierking, I. Textures of Liquid Crystals; Wiley-VCH: Weinheim,
Germany, 2003.
(15) Okoshi, K.; Kajitani, T.; Nagai, K.; Yashima, E. Macromolecules
2008, 41, 258–261.
(16) Schenning, A. P. H. J.; Fransen, M.; Meijer, E. W. Macromol.
Rapid Commun. 2002, 23, 265–270.
(17) Poly-1 was treated with CuCl in piperidine to eliminate the
terminal Pd residues, and the resulting Pd-eliminated poly-1 was
fractionated using preparative SEC (Mn = 1.59 Â 105, Mw/Mn = 1.07)
prior to the SR-SAXS measurements (Scheme S2).7c
(18) (a) Sakurai, S.-i.; Okoshi, K.; Kumaki, J.; Yashima, E. Angew.
Chem., Int. Ed. 2006, 45, 1245–1248. (b) Kumaki, J.; Sakurai, S.-i.;
Yashima, E. Chem. Soc. Rev. 2009, 38, 737–746.
’ ACKNOWLEDGMENT
We thank Professors Atsushi Takano and Yushu Matsushita
(Nagoya University) for their help with the SR-SAXS measure-
ments. This work was supported in part by a Grant-in-Aid for
Scientific Research from the Japan Society for the Promotion of
Science (JSPS) and JST. K.N. expresses his thanks for a JSPS
Research Fellowship for Young Scientists (6683).
’ REFERENCES
(1) (a) Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray,
G. W., Spiess, H. W., Vill, V., Eds.; Wiley-VCH: Weinheim, Germany,
1998. (b) Tschierske, C. J. Mater. Chem. 2001, 11, 2647–2671. (c) Kato,
T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45, 38–68.
(d) Goodby, J. W.; Saez, I. M.; Cowling, S. J.; G€ortz, V.; Draper, M.; Hall,
A. W.; Sia, S.; Cosquer, G.; Lee, S.-E.; Raynes, E. P. Angew. Chem., Int. Ed.
2008, 47, 2754–2787.
(2) Freiser, M. J. Phys. Rev. Lett. 1970, 24, 1041–1043.
(3) (a) Kricheldorf, H. R.; Domschke, A. Macromolecules 1996,
29, 1337–1344. (b) Severing, K.; Saalw€achter, K. Phys. Rev. Lett. 2004,
92, No. 125501. (c) Madsen, L. A.; Dingemans, T. J.; Nakata, M.;
Samulski, E. T. Phys. Rev. Lett. 2004, 92, No. 145505. (d) Acharya, B. R.;
Primak, A.; Kumar, S. Phys. Rev. Lett. 2004, 92, No. 145506. (e) Prasad,
V.; Kang, S.-W.; Suresh, K. A.; Joshi, L.; Wang, Q.; Kumar, S. J. Am.
Chem. Soc. 2005, 127, 17224–17227.
(4) Luckhurst, G. R. Thin Solid Films 2001, 393, 40–52.
(5) (a) Leube, H. F.; Finkelmann, H. Makromol. Chem. 1991,
192, 1317–1328. (b) Pratibha, R.; Madhusudana, N. V.; Sadashiva,
B. K. Science 2000, 288, 2184–2187. (c) Hegmann, T.; Kain, J.; Diele, S.;
Pelzl, G.; Tschierske, C. Angew. Chem., Int. Ed. 2001, 40, 887–890.
(d) Yelamaggad, C. V.; Prasad, S. K.; Nair, G. G.; Shashikala, I. S.; Rao,
D. S. S.; Lobo, C. V.; Chandrasekhar, S. Angew. Chem., Int. Ed. 2004,
43, 3429–3432.
(6) R€adler, J. O.; Koltover, I.; Salditt, T.; Safinya, C. R. Science 1997,
275, 810–814.
(7) (a) Yu, S. M.; Conticello, V. P.; Zhang, G.; Kayser, C.; Fournier,
M. J.; Mason, T. L.; Tirrell, D. A. Nature 1997, 389, 167–170.
(b) Okoshi, K.; Kamee, H.; Suzaki, G.; Tokita, M.; Fujiki, M.; Watanabe,
J. Macromolecules 2002, 35, 4556–4559. (c) Onouchi, H.; Okoshi, K.;
Kajitani, T.; Sakurai, S.-i.; Nagai, K.; Kumaki, J.; Onitsuka, K.; Yashima,
E. J. Am. Chem. Soc. 2008, 130, 229–236.
(8) (a) Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook,
R.; Lifson, S. Science 1995, 268, 1860–1866. (b) Cornelissen, J. J. L. M.;
Donners, J. J. J. M.; de Gelder, R.; Graswinckel, W. S.; Metselaar, G. A.;
Rowan, A. E.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. Science 2001,
293, 676–680. (c) Watanabe, J.; Kamee, H.; Fujiki, M. Polym. J. 2001,
33, 495–497. (d) Kim, J.; Novak, B. M.; Waddon, A. J. Macromolecules
2004, 37, 8286–8292. (e) Nagai, K.; Sakajiri, K.; Maeda, K.; Okoshi, K.;
Sato, T.; Yashima, E. Macromolecules 2006, 39, 5371–5380. (f) Okoshi,
K.; Sakurai, S.-i.; Ohsawa, S.; Kuniaki, J.; Yashima, E. Angew. Chem., Int.
Ed. 2006, 45, 8173–8176. (g) Liu, J.; Lam, J. W. Y.; Tang, B. Z. Chem.
Rev. 2009, 109, 5799–5867. (h) Nagai, K.; Okoshi, K.; Sakurai, S.-i.;
Banno, M.; Azam, A. K. M. F.; Kamigaito, M.; Okamoto, Y.; Yashima, E.
Macromolecules 2010, 43, 7386–7390.
9159
dx.doi.org/10.1021/ja201133d |J. Am. Chem. Soc. 2011, 133, 9156–9159