Guest-responsive covalent frameworks by the cross-linking of liquid-crystalline salts: Tuning of lattice flexibility by the design of polymerizable units

Article abstract of DOI:10.1002/chem.201102422

Cross-linked polymers prepared by the in-situ polymerization of liquid-crystalline salts were found to work as solid-state hosts with a flexible framework. As a component of such hosts, four kinds of polymerizable amphiphilic carboxylic acids bearing alky

Full text of DOI:10.1002/chem.201102422

DOI: 10.1002/chem.201102422
Guest-Responsive Covalent Frameworks by the Cross-Linking of Liquid-
Crystalline Salts: Tuning of Lattice Flexibility by the Design of Polymerizable
Units
Yasuhiro Ishida,*[a, c] Hiroaki Sakata,[b] Ammathnadu S. Achalkumar,[a]
Kuniyo Yamada,[a] Yuki Matsuoka,[c] Nobutaka Iwahashi,[b] Sayaka Amano,[b] and
Kazuhiko Saigo[d]
Abstract: Cross-linked polymers pre-
pared by the in-situ polymerization of
liquid-crystalline salts were found to
work as solid-state hosts with a flexible
framework. As a component of such
hosts, four kinds of polymerizable am-
phiphilic carboxylic acids bearing alkyl
chains with acryloyloxy (A), dienyl
(D), and/or nonreactive (N) chain ends
ture, which was successfully polymer-
ized by 60Co g-ray-induced polymeri-
zation without serious structural disor-
dering to afford the salt of cross-linked
carboxylic acid (polymeric carboxylic
acid; PAAA, PANA, PDDD, and PDND) with
GRS. Owing to the noncovalency of the
interactions between the polymer
framework P and the template GRS, the
cross-linked polymers could reversibly
amount of GRS. In response to the de-
sorption and adsorption of GRS, the
cross-linked polymers dramatically
switched their nanoscale structural
order. A systematic comparison of the
polymers revealed that the choice of
polymerizable groups has a significant
influence on the properties of the re-
sultant polymer frameworks as solid-
state hosts. Among these polymers,
PDDD was found to be an excellent
solid-state host, in terms of guest-re-
leasing/capturing ability, guest-recogni-
tion ability, durability to repetitive
usage, and unique structural switching
mode.
(monomeric carboxylic acids; MAAA
,
M
ANA, MDDD, and MDND) were used.
release and capture
a
meaningful
The carboxylic acids were mixed with
an equimolar amount of a template
Keywords: chirality recognition
·
unit,
(1R,2S)-norephedrine
(guest
cross-linked polymers · intercala-
tion · liquid crystals · template syn-
thesis
amine; GRS), to form the corresponding
salts. Every salt exhibited a rectangular
columnar LC phase at room tempera-
Introduction
ductors, an so forth.[2] In addition, the dense integration of
components in solid-state hosts often enables their synerget-
ic motion in response to guest molecules, which would lead
to sophisticated stimuli-responsive materials reminiscent of
natural systems.[3–5] These solid-state hosts are usually con-
structed through the spontaneous assembly of small building
blocks by noncovalent interactions, such as hydrogen-bond-
ing, coordination, electrostatic, p–p, and CH–p interactions.
The reversibility of these noncovalent interactions plays an
indispensible role to create frameworks with resilience and
with few structural defects.[6] At the same time, however,
such reversibility inevitably limits the robustness, tunability,
and processability of the resultant frameworks.
Since the early stage of host–guest chemistry, solid-state
hosts have attracted a lot of attention.[1] Compared with ho-
mogeneous systems in dilute solutions, solid-state hosts find
a wide range of practical applications, including catalysts,
sensors, selectors, photonic/optoelectronic devices, ionic con-
[a] Dr. Y. Ishida, Dr. A. S. Achalkumar, K. Yamada
RIKEN Advanced Science Institute
2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)
Fax : (+81)48-467-8214
E-mail: y-ishida@riken.jp
[b] H. Sakata, N. Iwahashi, Dr. S. Amano
To overcome such limitation, a straightforward approach
is to covalently connect the components of these architec-
tures by in-situ polymerization. Unfortunately, this strategy
is not applicable to traditional solid-state hosts with crystal-
line structures, as rigid frameworks rarely undergo in-situ re-
actions.[7,8] On the other hand, liquid crystals (LCs) seem to
be quite promising precursors of such polymeric solid-state
hosts; molecules in LCs are aligned into an ordered struc-
ture with certain mobility, which often allows topologically
controlled reactions to proceed in high probability.[9] If a
two-component LC composed of a polymerizable matrix
Department of Chemistry and Biotechnology
Graduate School of Engineering, The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
[c] Dr. Y. Ishida, Dr. Y. Matsuoka
PRESTO, Japan Science and Technology Agency
4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan)
[d] Prof. K. Saigo
School of Environmental Science and Engineering
Kochi University of Technology, Miyanokuchi
Tosayamada-cho, Kami, Kochi 782-8502 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102422.
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FULL PAPER
unit and a template unit is available, a new type of porous
material would be afforded by the in-situ polymerization of
the LC and subsequent removal of the template unit.
Indeed, pioneering works by Gin and co-workers have dem-
onstrated the utility of this concept; several nano-channeled
polymers with various applications, such as catalysts, nano-
composites, separation membranes, and so forth, have been
developed.[9f,10]
Most of the materials developed by Gin et al. were
formed by the cross-linking of lyotropic LCs.[10] Considering
the simplicity and ordering of supramolecular structures,
thermotropic LCs might be more advantageous than lyo-
tropic ones as the precursors of well-defined channels. At
the present time, however, nanochanneled polymers from
thermotropic LCs have been less systematically investigat-
ed.[11] Recently, we have reported a cross-linked polymer by
using a thermotropic LC salt, composed of an amphiphilic
polymerizable carboxylic acid (monomeric carboxylic acid
bearing three acryloyloxy groups; MAAA) with an enantio-
pure amino alcohol (Scheme 1).[12] Owing to the noncova-
lency of the interactions between the polymer framework
and the template, the resultant polymer was capable of re-
leasing and re-capturing certain amounts of amino alcohols.
Quite interestingly, the polymer showed unexpected dynam-
ic properties, despite of its covalently cross-linked frame-
work;[13] in response to the adsorption/desorption of the
template amino alcohol; the structure of the cross-linked
polymer was reversibly transformed between rectangular
columnar and amorphous states (Scheme 1c and d).
These encouraging results prompted us to expand the
scope of our system. However, our previous system based
on MAAA has the following problems and limitations, which
Scheme 1. Schematic representation for the structural switching of cross-linked polymers responsive to the desorption and adsorption of guest molecules.
a) LC fomration: i) dichloromethane; ii) slow evaporation. b) Polymerization: 60Co g-ray (1.0 kGyhꢀ1) for 100 h in vacuo at RT. c) Desorption: HCOOH
in EtOH (500 mm, 83 equiv) at 208C. d) Adsorption: G in MeOH (12 mm, 2 equiv) at 208C.
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Y. Ishida et al.
would hamper further application as a solid-state host.
1) Through the in-situ polymerization of the LC salt, the
original structure was disordered in a considerable level.[12a]
2) A non-negligible amount of template molecule (about
30%) could not be removed from the cross-linked polymer
by any means.[12b] 3) The structural switching mode of the
polymer was limited to only one type (rectangular colum-
nar$amorphous), unlike the case of other supramolecular
hosts, such as metal–organic frameworks.[13] Therefore, we
newly employed three kinds polymerizable carboxylic acids
as the matrix unit and systematically varied the polymeriz-
able (Scheme 1, MDDD, MANA, and MDND). Here we report a
detailed study on the properties of the resultant polymers,
which revealed that the modification of the polymerizable
parts had an unexpectedly large effect on their performance
as solid-state hosts.
us to investigate the chirality recognition ability of the resul-
tant polymers.
Preparation and characterization of LC salts M·GRS: The
carboxylic acids MAAA, MDDD, MANA, and MDND were mixed
with an equimolar amount of the amino alcohol GRS to give
the corresponding salts (Scheme 1a). Every salt M·GRS ex-
hibited an LC phase at room temperature, which was con-
firmed by the combinational use of polarized optical micros-
copy, differential scanning calorimetry, and X-ray diffraction
analysis (XRD). XRD studies revealed that the new salts
(MDDD·GRS, MANA·GRS, and MDND·GRS) took an LC structure
quite similar to that of MAAA·GRS (rectangular columnar;
space group, P2m; lattice parameters, 43ꢁ25–28 ꢂ), as un-
ambiguously confirmed by nine or more refractions ob-
served in a small-angle region (Figure 1).
Results and Discussion
Design of matrix and core units: In our previous work, a
polymerizable amphiphilic carboxylic acid bearing three
acryloyloxy groups (MAAA) was used as the matrix unit.[13,14a]
To clarify the effect of polymerizable functional groups on
the properties of the resultant polymers, an analogue of
M
AAA, in which the acryloyloxy groups were replaced with
conjugated dienyl groups (MDDD), was employed; polymeriz-
able LCs bearing 1,3-dienyl groups have been recently re-
ported by Gin and co-workers, in which the characteristic
structure of 1,3-dienyl groups, similar to typical alkyl chains
in polarity and size, has been shown to bring significant ben-
efits, such as the stabilization of LCs.[14b] In addition, the
effect of the cross-linked density was also investigated by
using analogous carboxylic acids with two polymerizable
groups (MANA and MDND).[14c–e,15]
As template units for the present template polymerization
of two-component LCs, a number of amino alcohols are ap-
plicable. In fact, we have recently reported that the salts of
trisACHTUNGTRENNUNG(alkyloxy)benzoic acid derivatives with various amino al-
cohols have a general tendency to exhibit thermotropic LC
phases.[12,13,16] In this work, (1R,2S)-norephedrine (guest
amine; GRS) was selected, because an abundant amount of
data have been accumulated by our group on the structure
and properties of the LC salts of GRS with amphiphilic car-
boxylic acids. In addition, easy access to its antipode
((1S,2R)-norephedrine, GSR) and diastereoisomers ((1R,2R)-
and (1S,2S)-pseudonorephedrine, GRR and GSS) would allow
Figure 1. XRD patterns of the salts of a) MAAA, b) MANA, c) MDDD, and
d) MDND with GRS: i) before and ii) after g-ray irradiation.
The lattice size of these LC salts changed with a clear ten-
dency, depending on the number of acryloyloxy groups in
the carboxylic acid unit (Table 1). Among three types of the
Table 1. Structure and phase behavior of the LC salts M·GRS before and after g-ray irradiation.
Salt Molecular structure of M
Clearing point [8C][a]
Lattice parameters, aꢁb [ꢂ][b]
A
D
Mr
monomer
polymer
monomer
polymer
M
AAA·GRS
3
2
0
0
0
0
3
2
843.1
787.1
663.0
667.0
73
77
99
>200[c]
>200[c]
>200[c]
>200[c]
43.1ꢁ27.6
43.0ꢁ26.4
42.9ꢁ24.8
43.0ꢁ24.9
42.1ꢁ26.7
41.7ꢁ26.3
41.9ꢁ24.2
42.2ꢁ24.6
MANA·GRS
M
MDND·GRS
DDD·GRS
103
[a] Determined by DSC on the first heating process. [b] Calculated from the d-spacings of the XRD reflections attributable to the (010) face and the
overlapping of the (110) and (200) faces. [c] Transition into an isotropic melt was not observed up to 2008C.
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Liquid Crystals
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chain termini groups (acryloyloxy, dienly, and alkyl groups),
acryloyl
acryloyl
ACHTUNGTRENNUNG
A
,
3; MANA, 2; MDDD, 0; MDND, 0), the b axis (=the shorter
side) of the lattice became shorter (MAAA, 27.6; MANA, 26.4;
M
DDD, 24.8; MDND, 24.9 ꢂ). In contrast to this, the length of
the a axis (=the longer side) was hardly influenced by the
structure of the carboxylic acid units (MAAA, 43.1; MANA
,
43.0; MDDD, 42.9; MDND, 43.0 ꢂ). Considering this tendency,
the polymerizable end groups of the alkyl chains are likely
to be mainly located on the a-side of the lattice, so that the
bulkiness of the acryloyloxy group contributes only to the
elongation of the length of the b axis (Figure 2).
Figure 3. FT-IR spectra of the salts of a) MAAA, b) MANA, c) MDDD, and
d) MDND with GRS: i) before and ii) after g-ray irradiation.
However, it is worth noting that structural preservation in
the cases of the carboxylic acids with dienyl groups was rela-
tively better than those bearing acryloyloxy groups (Fig-
ure 1a vs. c and b vs. d). In general, the cross-linking of LC
mesogens induced the congestion of polymerizable moieties,
which are usually polar/rigid functionalities, to hinder dense
molecular packing. However, the characteristic structure of
the 1,3-dienyl group, less polar and less sterically demanding
compared to usual polymerizable groups, would be favora-
ble to retain the original molecular packing even after the
polymerization
Figure 2. Schematic representation for the possible arrangement of mole-
cules in the LC salts (M·GRS) with a rectangular columnar structure
(P2m).
In-situ polymerization of LC salts M·GRS: The LC salts
M·GRS thus obtained were then subjected to radical poly-
merization (Scheme 1b). For the cross-linking of LC materi-
als, photoinitiated polymerization has been widely used.
However, we employed 60Co g-ray-induced polymerization
due to the following advantages:[12b,17,18] 1) g-Ray-induced
polymerization does not require any radical initiator, which
sometimes induces undesired disordering of LC structures.
2) Unlike the case of UV light, g-rays are highly penetrant
and promote the polymerization in a homogeneous manner
without depending on the depth from the surface.
Regardless of the structure of the carboxylic acid unit, the
g-ray irradiation converted the LC salts into hard solid
masses that were insoluble in common organic solvents and
nonmeltable up to 2008C. Through the irradiation, the
acryroyloxy and dienyl groups were consumed almost quan-
titatively, as confirmed by the disappearance of their charac-
teristic IR absorptions (Figure 3, filled circles). The resultant
cross-linked polymers were mechanically milled and washed
with dichloromethane; the unreacted carboxylic acid and
the template unit GRS were not detected in the washing.
For every salt, an XRD study showed that the original
rectangular structure was disordered to some extent, as gen-
erally observed in the cross-linking of LC architectures.
Desorption and adsorption of the guest GRS on the cross-
linked polymers P: With four kinds of cross-linked polymer
frameworks (PAAA, PANA, PDDD, and PDND) in hand, we next
investigated their properties as solid-state hosts. To clarify
the characteristics of these four polymers, we at first studied
very simple consecutive processes, the desorption and ad-
sorption of the template amino alcohol GRS on the cross-
linked polymers (Scheme 1c and d).
Owing to the noncovalency of the interactions between
the template unit GRS and the polymer matrices P, GRS
could be easily extracted from the as-prepared holo-poly-
mers (holo-P·GRS) to afford the corresponding apo-polymers
(apo-P) by treatment with
a solution of formic acid
(Table 2). The apo-polymers apo-P thus obtained readily ad-
sorbed the original guest GRS, when treated with a solution
of GRS to afford the reconstituted polymers (rec-P·GRS).
Through the desorption and adsorption, the mass transfer of
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Y. Ishida et al.
Table 2. Capacity and structural change of P on the desorption and adsorption of GRS
.
Salt
Amount of GRS [%][a]
Lattice parameters, aꢁb [ꢂ][b]
holo
apo
rec
holo
apo
rec
P
AAA·GRS
100
100
100
100
N
25
4
5
G
95
98
76
77
42.1ꢁ26.7
41.7ꢁ26.3
41.9ꢁ24.2
42.2ꢁ24.6
n.d.ꢁn.d.
n.d.ꢁn.d.
41.5ꢁ26.5
41.6ꢁ26.0
41.2ꢁ24.1
n.d.ꢁ24.5
PANA·GRS
PDDD·GRS
PDND·GRS
n.d.ꢁ23.0
3
n.d.ꢁca. 24[c]
[a] With respect to the carboxyl groups in the cross-linked polymer (mol%), which were calculated based on the change of the concentration of GRS in
the supernatant (determined by HPLC). Values in the parentheses represent the amounts of GRS desorbed from (ꢀ) and adsorbed onto (+) the poly-
mers. [b] Calculated from the d-spacings of the XRD reflections attributable to the (010) face and the overlapping of the (110) and (200) faces. [c] Calcu-
lated from a highly broadened XRD reflection.
GRS was monitored by the HPLC analysis of the superna-
tants. Hereafter, the amounts of GRS are expressed with re-
spect to the carboxyl groups in the cross-linked polymers
(mol%). In all cases of these four polymers, the desorption
and adsorption attained an equilibrium within 100 h, in spite
of quite mild conditions (Figure S2 in the Supporting Infor-
mation).[19]
Efficiency of desorption/adsorption of the guest GRS: In the
desorption process, holo-PANA·GRS, holo-PDDD·GRS, and holo-
P
DND·GRS released GRS almost quantitatively [PANA, 100!
4% (ꢀ96%); PDDD, 100!5% (ꢀ95%); PDND, 100!3%
(ꢀ97%)], while the desorption from holo-PAAA·GRS realized
satisfactory but a little less conversion [PAAA, 100!25%
(ꢀ75%)]. FT-IR studies clearly showed that the desorption
process involved only a simple ion-exchange reaction (am-
monium carboxylate!carboxylic acid); at the equilibrium
stage, the IR absorptions attributable to ammonium carbox-
ylate disappeared, whereas those of free carboxylic acid
newly emerged (Figure 4, a–d; i vs. ii).
In the following adsorption process, the apo-polymers
composed of acryloyloxy groups (apo-PAAA and apo-PANA
)
incorporated almost quantitative amounts of GRS, respec-
tively [PAAA, 25!95% (+70%); PANA, 4! 98% (+94%)].
Meanwhile, the apo-polymers of dienyl groups (apo-PDDD
Figure 4. FT-IR spectra of a) PAAA, b) PANA, c) PDDD, and d) PDND : i) the
original state (holo-polymer), ii) after the desorption of GRS (apo-poly-
mer), and iii) after the adsorption of GRS (reconstituted polymer). For
each spectrum, the amount of GRS incorporated is shown with respect to
the carboxyl groups in the polymer (mol%).
and apo-PDND) showed saturation at a moderate level [PDDD
,
5!76% (+71%); PANA, 3!77% (+74%)]. FT-IR studies
again showed that the adsorption progressed by a simple
ion-exchange reaction (carboxylic acid ! ammonium car-
boxylate), accompanied with no detectable side reactions,
such as the condensation of the carboxylic acid with the
Structural switching mode through the desorption/adsorp-
tion of the guest GRS: As we have reported, the desorption
and adsorption of the guest GRS induced the “melting” and
“crystallization” of PAAA (Scheme 1c and d).[13] If the struc-
tural switching mode can be controlled by the cross-linking
manner, it should further expand the scope of the present
system. Therefore, the nanoscaled structures of the three
new polymers (PDDD, PANA, and PDND) at the equilibrium
stages of desorption and re-adsorption were investigated by
XRD (Figures 5 and 6).
During the desorption, the polymer frameworks of acryl-
oyloxy groups (PAAA and PANA) were transformed into an
amorphous-like structure. After the completion of the de-
sorption, reflections originally observed at a small-angle
region, characteristic of a rectangular columnar structure,
became undetectable (Figure 5a and b; ii). The only detecta-
amine and the aminolysis of the poly
(Figure 4, a–d; ii vs. iii).
ACHTUNGTREN(NUNG acrylate) moieties
Overall, meaningful amounts of GRS could be cleanly des-
orbed (ꢀ75 to ꢀ97%) from and adsorbed (+70 to +94%)
to all of the four polymers by simple treatment with an
acidic solution and a GRS solution, respectively (Table 2). As
described in the introductory part, our polymer PAAA could
not release the original template quantitatively (up to about
70% with respect to the carboxyl groups in the polymer),
which would be a fatal drawback as a solid-state host;[12,13]
the present study revealed that a small modification of the
carboxylic acid unit, that is reducing cross-link density (PANA
and PDND) or changing polymerizable groups (PDDD), could
solve this problem.
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Figure 5. 2D XRD images of a) PAAA, b) PANA, c) PDDD, and d) PDND : i) the original state (holo-polymer), ii) after the desorption of GRS (apo-polymer),
and iii) after the adsorption of GRS (reconstituted polymer). For each image, the amount of GRS incorporated is shown with respect to the carboxyl
groups in the polymer (mol%).
ble reflection was a wide-angle halo peak due to the loose
packing of the paraffin chains (the d-spacing=about 3.5 ꢂ),
which was inert toward the removal of GRS (Figure S1, a
and b, ii). Unlike the polymer frameworks of the acryloyloxy
groups, those of the dienyl groups (PDDD and PDND) exhibit-
ed a structural periodicity with a d-spacing of about 20 ꢂ,
even after the removal of GRS. Upon completion of the de-
sorption, apo-PDDD showed a small but sharp reflection at a
small-angle region, besides a wide-angle halo scattering due
to the alkyl-chain packing (Figure 6c; ii). The corresponding
reflection was also observed in the case of PDND, although its
peak width highly broadened, compared with that of PDDD
(Figure 6d; ii).
In the adsorption process, all of the polymers recovered
their structural order. However, the efficiency of their struc-
tural reconstruction was highly influenced by the cross-
linked density. In the cases of the highly cross-linked poly-
mers PAAA and PDDD, an ordered rectangular columnar struc-
ture with lattice parameters essentially same as those of the
original polymer was recovered (Table 2); a set of reflec-
tions characteristic of a rectangular columnar structure ap-
peared again, of which the intensities were slightly lower
but comparable to those of the pristine polymers (Figure 6a
and c; iii). In contrast, the polymers with lower cross-linked
density (PANA and PDND) were not capable of recovering the
original structure to such a high level. In the case of PANA
,
most of the reflections owing to a rectangular columnar
structure were recovered. At the same time, however, a
broad peak newly emerged at the 2q region of 3–48, indicat-
ing a certain structural disordering during the desorption–
adsorption cycle (Figure 6b; iii). Further considerable disor-
dering was observed in the reconstitution of PDND. After the
adsorption, the two reflections attributable to the (010) and
(110)/ACHTNUTRGNE(NUG 200) faces were significantly broadened, and other
high-order reflections were barely detectable (Figure 6d; iii).
Thus, all of the four polymers generally changed the
shape of their framework through the desorption and ad-
sorption processes. However, the dynamic behavior of the
polymers was highly influenced by the cross-linked density
and the polymerizable functional group. In other words, we
might be able to tune the flexibility of polymers derived
from LC salts by the proper control of the following two
factors. 1) Polymerizable functionality; the polymers of
dienyl groups kept a certain structural order even after the
desorption of the template (PDDD and PDND), whereas those
of acryloyloxy groups turned into an amorphous-like state
(PAAA and
PANA). 2) Cross-linked density; to construct
frameworks with a satisfactory reconstitution ability, it was
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Y. Ishida et al.
Figure 6. XRD patterns of a) PAAA, b) PANA, c) PDDD, and d) PDND: i) the
original state (holo-polymer), ii) after the desorption of GRS (apo-poly-
mer), and iii) after the adsorption of GRS (reconstituted polymer). For
each pattern, the amount of GRS incorporated is shown with respect to
the carboxyl groups in the polymer (mol%).
essential to introduce three polymerizable groups per one
carboxylic acid unit (PAAA and PDDD). Among these polymer
frameworks, the highly cross-linked ones (PAAA and PDDD
)
showed a superior structural reconstitution ability, which
seem more attractive than the less cross-linked types (PANA
and PDND).
Figure 7. Schematic representation for the possible structural-switching
patterns of a rectangular columnar lattice.
Intermediate states of the structural switching: To gain fur-
ther insights into the dynamics of PAAA and PDDD, the inter-
mediate structures of these polymers between the holo- and
apo-states were thoroughly studied. For the estimation of
structural order during the desorption and adsorption pro-
cesses, we monitored the intensity of two strong reflections,
attributable to the (010) face (2q=3.3–3.78, d-spacing=24–
27 ꢂ) and the overlapping of the (110) and (200) faces (2q=
3.8–4.28, d-spacing=21–23 ꢂ). Considering their Miller indi-
ces, these reflections should correspond to the structural
order along the a and b axes of the rectangular lattice, re-
spectively. Depending on the ordering/disordering along the
two directions, the structural switching of the rectangular
lattice can be classified into four extreme cases, as depicted
in Figure 7.
From these observations, we can deduce that the framework
has similar “rigidity” in the directions of the a and b axes, so
that the structural disordering/re-ordering proceeds in a di-
rectionally independent manner (Figure 7d and Scheme 2,
left).
Quite interestingly and unexpectedly, the transformation
of PDDD was more directionally dependent than that of
P
AAA. At the initial stage of the desorption, the reflection
owing to the (110)/(200) faces showed a more drastic change
than did the (010) face; the reflection of the (110)/(200)
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
faces became almost undetectable even when the conversion
was not sufficient (ca. 70% desorption, Figure 8b; iv). In
contrast to this, the reflection of the (010) face was still visi-
ble at the equilibrium of the desorption process, at which an
almost quantitative amount of GRS was removed (95% de-
sorption, Figure 8b; v). Through the adsorption process, the
reflection of the (010) face changed in proportion to the ad-
In the case of PAAA, the two reflections changed their in-
tensity synchronously with each other, in response to the
amount of GRS incorporated in the polymer (Figure 8a). At
the equilibrium stage of the desorption, no structural order
remained in either of the directions, suggestive of the trans-
formation into an amorphous-like structure (Figure 8a; v).
sorption, whereas that of the (110)/
ACHTUNGERTN(NUNG 020) faces gained its in-
tensity only at the final stage. Thus, PDDD had a tendency to
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Figure 9. XRD patterns of a) PAAA and b) PDDD on the repetitive desorp-
tion/adsorption of GRS
.
state hosts attain reversible switchings of their ordered
structures by an “error-correction” process, characteristic of
supramolecular architectures based on noncovalent interac-
tions between components.[6]In contrast, covalently cross-
linked frameworks, such as the present polymers, lack such
an error-correction ability, in principle. Therefore, it might
be intriguing to check the durability of the present polymers
to the repetitive desorption and adsorption of the guest GRS.
The two polymers (PAAA and PDDD) were subjected to two
cycles of desorption and adsorption under conditions similar
to those described above [Figure 9i (holo-P·GRS)—desorp-
tion!ii (apo1st-P)—adsorption!iii (rec1st-P·GRS)—desorp-
Figure 8. XRD patterns of a) PAAA and b) PDDD on the desorption (i–iv)
and adsorption (v–xi) of GRS. For each pattern, the amount of GRS incor-
porated is shown with respect to the carboxyl groups in the polymer
(mol%).
retain the structural order along the b axis more persistently
than that along the a axis throughout the desorption and ad-
sorption of GRS; PDDD is considered to reversibly change its
shape between a rectangular columnar structure and a la-
mellar structure (Figure 7b and Scheme 2, right).
tion!iv
(apo2nd-P)—adsorption!v
(rec2nd-P·GRS)].[20]
Through the cycles, the nanoscaled structures of the poly-
mers were monitored by XRD analyses. To our delight, the
polymers showed an excellent durability to the repetitive
usage, in which the frameworks switched their shape reversi-
bly between two states; the polymers in the second cycle
showed XRD patterns almost identical to those of the corre-
Tolerance to repetitive desorption/adsorption of the guest
GRS: As described in the introductory part, traditional solid-
Scheme 2. Schematic representation for the structural switching of PAAA and PDDD
.
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Y. Ishida et al.
sponding polymers in the first cycle (Figure 9a and b; ii vs.
iv and iii vs. v).
PAAA, regarding directional dependence. In the case of PDDD,
it always recovered the reflection of the (010) face inde-
pendent of the guests, while the recovery of the reflection of
the (110)/ACTHNUTRGNEUG(N 200) faces was highly sensitive to the matching/
mismatching of the guests (Figure 10b; i vs. ii–iv). Thus, the
These observations clearly proved that PAAA and PDDD
have satisfactory tolerance to repetitive desorption and ad-
sorption; the history of the polymers gives essentially no in-
fluence on the capability of switching their structures. Most
likely, the cross-linked density of the present system, three
polymerizable groups per one carboxylic acid unit, was suit-
able to cope with both flexibility and durability. Such knowl-
edge provides us a clue to design next-generation covalent
frameworks based on the present concept.
Ability to distinguish the shape of stereoisomeric guest mol-
ecules: As reported in our previous papers, apo-PAAA could
incorporate not only the original template GRS, but also
other various amines.[12] In addition, apo-PAAA was capable
of recognizing subtle structural difference between the origi-
nal template GRS and its stereoisomers (GSR, GRR, and GSS),
so that the structural order of the reconstituted PAAA was in-
fluenced by the shape of guests.[13] These observations indi-
cate the potential utility of our system as guest-responsive
materials with a chirality recognition ability. In order to
prove the generality and expansivity of this phenomenon,
the new polymer PDDD, exhibiting similar but different struc-
tural switching property from that of PAAA, was also em-
ployed for the same experiment.
Thus, apo-PAAA and apo-PDDD were treated with the solu-
tions of GRS, GSR, GRR, and GSS. To fairly evaluate the struc-
tural reconstitution efficiency, the adsorption was stopped
before equilibrium, when the uptake reached a specific
value (PAAA (58ꢁ2)%; PDDD (49ꢁ2)%).[21] The nanoscaled
structure of the resultant polymers was analyzed by XRD.
Here, we again focused on the intensities of the two charac-
teristic reflections in XRD patterns owing to the (010) and
Figure 10. XRD patterns of a) PAAA and b) PDDD reconstituted with
i) GRS, ii) GSR, iii) GRR, and iv) GSS. For each pattern, the amount of G
captured by the polymer through the adsorption process is shown with
respect to the carboxyl groups in the polymer (mol%).
guest specificity of PDDD was found to be expressed in the
order along the a axis rather than in the order along the b
axis. In contrast to this, the guest specificity of PAAA was re-
flected in the orders along both the a and b axes, since the
two reflections synchronously varied their intensity depend-
ing on the matching of guests (Figure 10a; i vs. ii–iv).
(110)/ACHTUNGTRENNUNG(200) faces, which represent the degree of the structur-
al order along the b and a axes, respectively.
Although such a difference in the structural switching
mode was observed between PAAA and PDDD, both of these
polymers showed an ability to distinguish the original tem-
plate GRS from its stereoisomers (GSR, GRR, and GSS). Most
likely, “best fitting” was achieved only when GRS was incor-
porated to the apo-polymers, because the cavities of the
apo-polymers were tailored to accommodate the original
As we had expected, PDDD as well as PAAA showed an abil-
ity to recognize the difference between the guest molecules
(Scheme 3); structural reconstitution by the matched guest
(GRS) proceeded more efficiently than by the mismatched
guests (GSR, GRR, and GSS). Quite interestingly, PDDD
showed a different structural switching mode from that of
Scheme 3. Reconstitution of apo-PAAA and apo-PDDD by the adsorption of GRS, GSR, GRR, and GSS.
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Liquid Crystals
FULL PAPER
template (GRS). We should specifically mention here that
the present polymers were capable of recognizing a quite
subtle structural deviation of guests, reminiscent of en-
zymes; among the mismatched guest molecules, GRR has the
most similar shape to that of the original template GRS, in
which the only difference is the configuration of the C(2)
methyl group.
rational design of covalent frameworks with desired proper-
ties.
1) To reduce the structural disordering during the in-situ
polymerization of the LC architecture, dienyl groups
(PDDD and PDND) were advantageous than the acryloyl-
AHCTUNGERTNoGNUN xy groups (PAAA and PANA).
Taking into account of the accurate template-memory
effect, together with the tunability of lattice flexibility de-
pending on the polymerizable units, the present system
could lead to sophisticated sensors, actuators, and gate devi-
ces.
2) For the quantitative removal of the template, the reduc-
tion of cross-linked density (PANA, and PDND) or the em-
ployment of dienyl groups as polymerizable functionali-
ties (PDDD) was effective.
3) For the creation of frameworks with a sufficient structur-
al reconstitution ability, it was essential to introduce
three polymerizable groups per one carboxylic acid unit
(PAAA, and PDDD).
Origin of difference between PAAA and PDDD as solid-state
hosts: As described above, the cross-linked polymers PAAA
and PDDD were found to have favorable properties as solid-
state hosts, regarding structural reconstitution ability, dura-
bility to repetitive usage, and guest-recognition ability. It is
worth noting that the choice of the polymerizable function-
alities (acryloyloxy or dienly groups) brought an unexpect-
edly large effect on the properties of the resultant polymer,
particularly in the structural switching mode on the desorp-
tion and adsorption of the guest molecules (Scheme 2). Al-
though analytical methods to study the structure of cross-
linked polymers are generally limited, it might be important
to examine the difference between PAAA and PDDD as solid-
state hosts. Particularly, the molecular arrangement in the
4) The polymers of dienyl groups (PDDD and PDND) kept cer-
tain structural order even after the desorption of the
template, whereas those of acryloyloxy groups (PAAA and
PANA) turned into an amorphous-like state.
Among these polymers, PDDD was found to be an excel-
lent solid-state host, in terms of structural order, guest-re-
leasing ability, guest-recognition ability, durability to repeti-
tive usage, and a unique structural switching mode. Consid-
ering the beneficial features characteristic of covalently
cross-linked systems, such as processability, physical/chemi-
cal robustness, kinetic stability, and size/shape tunability,[22]
the present system could lead us to further facile manipula-
tion and more profound understanding of host–guest sys-
tems in the solid state.
precursors of these polymers (MAAA·GRS and MDDD·GRS
)
would give us some insights.
Supposing the molecular packing in these LC salts as
shown in Figure 2, the polymerizable functionalities are
prone to crowd along with the a axis of the rectangular lat-
tice. Taking account of this localization and bulkiness/polari-
ty of the polymerizable functionalities, the properties of
PAAA and PDDD might be elucidated as follows. In the case of
MDDD bearing dienly groups with less polar and less sterical-
ly demanding structure, the interdigitation of chain termini
would easily take place, which allows the polymer network
to grow along the a axis. Therefore, the resultant polymer
PDDD is expected to retain the ordered structure along the b
aixs more persistently than along the a axis. In contrast to
this, the acryloyloxy groups in MAAA are bulkier and more
polar, which would cause the steric/electrostatic repulsion
with each other. As a result, the polymer network is antici-
pated to develop in a nondirectional manner.
Acknowledgements
We acknowledge Dr. Toshimi Shimizu (AIST) and Dr. Hiroyuki Minami-
ˇ ´
kawa (AIST) for XRD measurements. We thank Aleksandra Jelicic for
the synthesis of the polymerizable amphiphilic carboxylic acid MDDD and
the cross-linked polymer holo-PDDD·GRS
.
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1
change was observed in H NMR analysis.
[19] The rates of the desorption and adsorption were highly influenced
by the concentrations of formic acid and GRS in solutions, respec-
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adsorption (see reference [20]), we intentionally used the relatively
diluted solutions to retard the rates, which was suitable to monitor
the time course by HPLC. Additionally, in the case of the adsorp-
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quantification of GRS, since the change in the concentration became
too small.
[20] In the repetitive desorption and adsorption, the concentrated solu-
tions of formic acid and GRS (15 and 20 times more concentrated
than those for the standard conditions) were used, respectively, in
order to accelerate the reaction rates. For experimental details, see
Supporting Information.
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higher affinity to the original GRS than its stereoisomers (GSR, GRR
,
G
SS). Although the selective binding implies the potential of these
polymers as chiral selectors, such applications will be reported else-
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Received: August 4, 2011
Published online: December 6, 2011
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