Metallo-responsive liquid crystalline monomers and polymers

Article abstract of DOI:10.1021/cm2011617

A series of liquid crystalline ligand-containing monomers with chemo-responsive properties has been prepared and studied. These molecules are functionalized derivatives of the 2,6-bisbenzimidazolylpyridine (Bip) ligand. Tailoring the size of the aromatic core and the length of two pairs of alkyl substituents allows the preparation of mesogens with a range of tunable thermal liquid crystalline transitions and phases. The binding of lanthanide- and/or transition metal salts to the ligand containing mesogens results in a transition from a liquid crystalline to an isotropic state. Acyclic diene metathesis of the monomers yielded either oligomers or polymers, dictated by whether the alkene is either terminal or nonterminal, respectively. Both oligomers and polymers exhibit enantiotropic liquid crystalline behavior and become isotropic in the presence of metal ions.

Full text of DOI:10.1021/cm2011617

ARTICLE
pubs.acs.org/cm
Metallo-Responsive Liquid Crystalline Monomers and Polymers
Blayne M. McKenzie,† Rudy J. Wojtecki,† Kelly A. Burke,† Cuiyu Zhang,‡ Antal Jꢀakli,‡ Patrick T. Mather,§
and Stuart J. Rowan†,*
†Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland,
Ohio 44106-7202, United States
‡Liquid Crystal Institute, Kent State University, Kent, Ohio 44240, United States
§Department of Biomedical and Chemical Engineering, Syracuse University, Syracuse, New York 13244, United States
S Supporting Information
b
ABSTRACT: A series of liquid crystalline ligand-containing monomers with chemo-
responsive properties has been prepared and studied. These molecules are functionalized
derivatives of the 2,6-bisbenzimidazolylpyridine (Bip) ligand. Tailoring the size of the
aromatic core and the length of two pairs of alkyl substituents allows the preparation of
mesogens with a range of tunable thermal liquid crystalline transitions and phases. The
binding of lanthanide- and/or transition metal salts to the ligand containing mesogens
results in a transition from a liquid crystalline to an isotropic state. Acyclic diene
metathesis of the monomers yielded either oligomers or polymers, dictated by whether
the alkene is either terminal or nonterminal, respectively. Both oligomers and polymers
exhibit enantiotropic liquid crystalline behavior and become isotropic in the presence of
metal ions.
KEYWORDS: ADMET, lanthanides, liquid crystal, stimuli-responsive, supramolecular
’ INTRODUCTION
of potential geometries and binding motifs, which can be utilized
to design the specific shapes necessary to retain, induce, or
otherwise impact liquid crystallinity. This approach enhances
the traditional thermotropic liquid crystal in two ways. First,
ligandscan be envisioned that, upon binding a metal ion, lose their
original liquid crystalline order and thus offer a useful method of
amplification for detection schemes and other applications.7
Second, ligands that bind metal ions and retain, change, or acquire
liquid crystalline order (metallomesogens) can also be envisaged.8
The 2,6-bisbenzimidazolylpyridine (Bip) ligand offers a ver-
satile scaffold from which highly functionalized derivatives can be
accessed (e.g., 1Et, Figure 1).9,10 Piguet and co-workers have
prepared a number of low molecular weight mesogenic Bip
derivatives and investigated the effects of metal binding on their
liquid crystalline properties.11 Noting that the incorporation of a
large metal center disrupts the organization of simpler Bip
mesogens,12 they decorated the Bip core with a highly alkylated
liquid crystallinity-enhancing moiety in the 50 positions, which
allowed access to a range of metallomesogens.13 Upon com-
plexation with a variety of Ln(NO3)3 salts, this Bip derivative
displayed columnar or cubic mesomorphism at elevated tem-
Small molecule and polymeric liquid crystalline materials have
applications in a multitude of areas, ranging from displays, optical/
electronic imaging, data storage, and stress/temperature sensing to
chemical/fire resistance and artificial muscle actuation.1 The de-
sign of typical molecular architectures that facilitate thermotropic
liquid crystalline behavior is well documented. Molecules with
structural anisotropy, i.e., a rigid moiety with high aspect ratio are
frequently utilized to impart the order inherent in a liquid crystal-
line phase. Functionalization of these cores with flexible alkane
chains of varying length then provides the structural complexity
necessary to hinder crystallization while preserving liquid crystal-
line order. As such, thermotropic liquid crystals are inherently
stimuli-responsive, as they can undergo a range of thermally
induced transitions between glassy/crystalline, liquid crystalline,
and isotropic states. Examples of thermotropic liquid crystalline
small molecules or polymers that are also designed to respond to
specific chemical stimuli are less prevalent.2
One approach to designing a chemo-responsive liquid crystal
involves incorporating a metal-binding ligand into the mesogenic
core. In addition to introducing additional, nonthermal, stimuli-
responsive behavior incorporation of metal ions into liquid
crystalline materials offers the opportunity to impart a number
of additional properties upon these systems. Chemical sensing,3
catalysis,4 and a host of biological applications5 highlight the many
practical properties of organic metal-coordinating species, while
some specific lanthanide complexes also exhibit interesting lumi-
nescent and paramagnetic properties.6 Additionally, the vast
number of transition and lanthanide metal ions offers a multitude
peratures. Further functionalization with the highly alkylated
0
moiety at the N1 positions increased the geometric screening of
the bulky metal complex, facilitating access to room-temperature
columnar metallomesogens.14 Thus, they have presented a room
Received: April 23, 2011
Revised:
June 13, 2011
Published: July 05, 2011
r
2011 American Chemical Society
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Chemistry of Materials
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Figure 1. Bip-containing monomer 1Et in the metal-free “trans/trans” ligand conformation and 1:1 complex with a metal ion, which adopts the “cis/cis”
ligand conformation, and schematic representations of the polymerization and proposed metallo-response of the monomer and polymer.
temperature “lanthanidomesogen” that balances the structural
anisotropy of the Bip core with the fluidity of 12 alkyl chains.
Other lanthanidomesogens have been prepared using different
binding motifs, such as simple alkyl carboxylates (i.e., lanthanide
soaps),15 porphyrins,15 and Schiff bases,15,16 while bipyridine,17
phenanthroline,18 terpyridine,19 and other pyridine-based ligands20
have been used to prepare metallomesogens with various transi-
tion metals.
Our group and others have utilized Bip derivatives to access a
range of stimuli-responsive metallo-supramolecular polymers,21
and we have investigated their potential in a number of material-
related areas, from chemical sensors,22 thermoplastic elastomers,23
photohealable materials,24 and stimuli-responsive gels,25 to pro-
cessable, robust poly(p-xylylene)26 and poly(p-phenylene ethynylene)
derivatives.27 As part of a long-term project to access new stimuli-
responsive structurally dynamic polymers,28 we targeted easy-to-
access,10 mesogenic Bip derivatives (1, 2, Scheme 1) that not only
feature relatively low clearing temperatures (75ꢀ125 °C), but can also
be readily incorporated into polymer and network architectures. In this
study, we report the synthesis and liquid crystalline properties of a range
of monomeric Bip derivatives that contain alkene functionalities, their
behavior in the presence of metal salts, and initial studies in to their
polymerization via acyclic diene metathesis chemistry (ADMet).29
H2O (0.10% trifluoroacetic acid, TFA); solvent B: acetonitrile (MeCN,
0.08% TFA); solvent program: 98% A, 2% B, ramp to 2% A, 98% B over
20 min, hold for 10 min. Elution time was reported as the signal at
365 nm unless otherwise noted. DSC and modulated DSC experiments
were performed on a TA Instruments Q2000 DSC equipped with a
refrigerated cooling system. The DSC cell resistance and capacitance
were calibrated using the Tzero calibration procedure (TA Instruments-
Waters LLC), and the cell constant and temperature were calibrated
using an indium standard. All samples were run under a flowing nitrogen
purge and were heated and cooled at a rate of 10 °C/min. Recrystalliza-
tion, melting, clearing, and liquid crystalline phase transition tempera-
tures of these monomers were determined from either the peak maxima
(for exothermic transitions) or minima (for endothermic transitions) on
the second heating or cooling traces. Modulated experiments were
heated and cooled at a rate of 3 °C/min, during which a linear heating
rate and a sinusoidal temperature waveform were applied to the sample
with a modulation amplitude of 1 °C and a modulation period of 30 s.
Polarized optical microscopy(POM) studies were performed using an
Olympus BX51 microscope equipped with 90° crossed polarizers, an
HCS402 hot stage from Instec, Inc., and a digital camera (14.2 Color
Mosaic Model from Diagnostic Instruments, Inc.). Images were ac-
quired from the camera at selected temperatures using Spot software
(Diagnostic Instruments, Inc.). Spatial dimensions were calibrated using
a stage micrometer with 10 μm line spacing. Either a 50x/0.5 NA or a
20x/0.4 NA achromat long working-distance objective lens (Olympus
LMPlanFI) was employed. The Instec hotstage was equipped with a
liquid nitrogen LN2-P cooling accessory for accurate temperature
control during both heating and cooling. Wide-angle X-ray data were
acquired on a Rigaku RINT 2000/PC series diffractometer with a Rigaku
PTC-30 programmable temperature controller. Photoluminescence
spectra were acquired on an Ocean Optics ACD1000-USB spectrometer
(λex = 377 nm) via a Y-shaped optical fiber. Size exclusion chromatog-
raphy (SEC) was performed on a Varian ProStar 210/215 equipped with
a Varian ProStar 350/352 Refractive Index Detector, a Viscotek Model
270 Dual Detector, and Varian OligoPore and ResiPore SEC/GPC
columns running tetrahydrofuran (THF) at a flow rate of 1.0 mL/min.
Synthesis. Compounds 3, 4a-8a, 4d, 5d, and 1Et5 were synthesized
according to literature procedures.10
’ EXPERIMENTAL SECTION
Materials. All solvents, sodium bicarbonate, potassium carbonate, and
potassium hydroxide were purchased from Fisher Scientific. Deuterated
solvents were purchased from Norell, Inc. Tetrakis(triphenylphosphine)
palladium(0) was purchased from Strem Chemicals, Inc. Grubbs catalyst
third generation was prepared via literature procedures and used without
further characterization.30 All other reagents and solvents were purchased
from Aldrich Chemical Co. Reagents were used without further purifica-
tion. Solvents were distilled from suitable drying agents.
Instruments. NMR spectra were recorded on a Varian Inova
600 MHz NMR spectrometer (13C NMR = 150.8 MHz) using deuterated
solvents. Molecular weights of the materials were measured on a Bruker
AUTOFLEX III MALDI TOF/TOF mass spectrometer using HABA
[2-(4-hydroxyphenylazo)benzoic acid] as the matrix. HPLC data were
collected using a Varian ProStar with a reverse phase C18 silica analytical
column using the following procedure. Flow rate: 1 mL/min; solvent A:
Preparation and characterization details of 4-methoxy-N-propyl-2-
nitroaniline (4b), N-butyl-4-methoxy-2-nitroaniline (4c), 2,6-bis(50-methoxy-
10-propylbenzimidazol-20-yl)pyridine (5b), 2,6-bis(10-butyl-50-methoxy-
benzimidazol-20-yl)pyridine (5c), 4-bromo-N-butyl-2-nitroaniline (6c),
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Scheme 1. Synthesis of Bip-Containing Monomers. (A) Pd(0), K2CO3, THF, H2O, 75 °C, 18 h; (B) (i) DMF, r.t., 12 h; (ii)
Na2S2O4, DMF, EtOH, H2O, 85 °C, 12 h; (C) BBr3, DCM, r.t., 4 h; and (D) Br(CH2)nCHdCH2, (or TsO(CH2)3CHdCHCH3)
K2CO3, THF, MeOH, 75 °C, 12 h
N-butyl-2-nitro-4-(40-methoxy)phenylaniline (7c), and 2,6-bis(10-butyl-
(400-methoxy)phenylbenzimidazol-20-yl)pyridine (8c) are located in the
Supporting Information.
with DCM and water (3x 100 mL). The organic layers were combined
and the solvent removed in vacuo. The following is an example of the
characterization data for 1Et4. Similar data for the additional derivatives
of 1 and 2 are located in the Supporting Information.
General Procedure for the Preparation of 1 and 2. (C in Scheme 1)
The appropriate methoxy derivative (5 or 8) was dissolved in dry
dichloromethane (DCM, 100 mM) in a round-bottomed flask. The flask
was sealed with a septum and flushed with nitrogen (3x). While under
nitrogen, boron tribromide (5 equivalents) was added via syringe,
and the reaction was stirred for 4 h at room temperature. The
contents of the flask were then slowly added to stirring water (1:1
DCM:H2O) and neutralized to pH ∼7 with sodium bicarbonate. The
desired phenolic derivative was obtained by filtering the aqueous
suspension and drying the solids under vacuum. The following is an
example of the characterization data for the phenolic derivative of 5a.
Similar data for the additional derivatives of 5 and 8 are located in the
Supporting Information.
Phenolic Derivative of 5a. Precipitated from water to yield the
product as an off-white solid (98%); mp: >250 °C (decomp). 1H
NMR (600 MHz, CD3OD): δ (ppm) 8.61 (t, 1H, J = 7.8 Hz, PyrH),
8.45 (d, 2H, J = 8.0 Hz, PyrH), 7.82 (d, 2H, J = 8.0 Hz, ArH), 7.24 (d, 2H,
J = 7.8 Hz, ArH), 7.19 (s, 2H, ArH), 4.75 (q, 4H, J = 7.2 Hz, NCH2), 1.28
(t, 6H, J = 7.2 Hz, CH3). 13C NMR (150.8, CD3OD): δ (ppm) 156.3,
149.6, 141.5, 140.4, 131.3, 130.6, 127.1, 116.2, 113.2, 103.2, 50.8, 16.2.
MS (MALDI): m/z 400.2 (M+H+). HPLC: 14.0 min.
2,6-Bis(50-(n-buten-300 -yl-1-oxy)-10-ethylbenzimidazol-20-yl)pyridine (1Et4).
Purified via column chromatography on silica (CHCl3:MeOH 100:0
to 95:5) to yield the product as a white solid (78%). TLC (silica, 95:5
CHCl3:MeOH) Rf = 0.29; mp: 120.3 °C. 1H NMR (600 MHz,
CDCl3): δ (ppm) 8.31 (d, 2H, J = 7.8 Hz, PyrH), 8.02 (t, 1H, J =
8.1 Hz, PyrH), 7.35 (d, 2H, J = 8.4 Hz, ArH), 7.33 (d, 2H, J = 1.2 Hz,
ArH), 7.03 (dd, 2H, J = 8.4, 1.2 Hz, ArH), 5.95 (m, 2H, H2CdCH),
5.16 (dd, 4H, J = 40.2, 10.2 Hz, H2CdCH), 4.78 (q, 4H, J =
7.2 Hz, NCH2), 4.11 (t, 4H, J = 6.6 Hz, OCH2), 2.61 (q, 4H, J =
6.6 Hz, OCH2CH2), 1.36 (t, 6H, J = 7.2 Hz, CH3). 13C NMR
(150.8 MHz, CDCl3): δ (ppm) 155.6, 149.8, 149.7, 143.4, 137.8,
134.4, 130.5, 125.2, 116.9, 114.5, 110.5, 102.8, 67.8, 39.8, 33.7, 15.6.
MS (MALDI): m/z 508.9 (M+H+). HPLC: 15.4 min.
ADMet Polymerizations. 1Et5 or 1Et5Me and Grubbs catalyst third
generation (5 mol %), were placed in a vial with a stir bar and dry,
degassed DCM, which had been previously passed over basic alumina.
The reaction was sealed with a septum and allowed to stir at room
temperature for 3 h. During the first hour, argon was bubbled through
the solution for 1 min every 10 min. The solvent was then removed in
vacuo, and SEC analysis was performed on the crude material. For DSC
and POM characterization, the crude reaction was purified via passing
over a short plug of silica in DCM.
Formation of Metal Complexes. Equimolar amounts of the
appropriate Bip derivative and metal salt were dissolved in dichloro-
methane and acetonitrile, respectively. These solutions were mixed, and
the solvents removed in vacuo. The complexes were then redissolved in
dichloromethane for purposes of casting films.
General Procedure for the Preparation of 1 and 2. (D in Scheme 1)
The appropriate phenolic derivative of 5 or 8 was added to a dry round-
bottomed flask with the appropriate alkyl bromide or alkyl tosylate (5
equivalents), potassium carbonate (5 equivalents), and a 4:1 mixture of
dry, degassed tetrahydrofuran and methanol (250 mM [Bip]). The
reaction was refluxed at 75 °C for 12 h in an oil bath. The solvents were
then removed in vacuo, and the dried solids were separated via extraction
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Table 1. Selected Thermal Transitions of Monomersa
transition
T (°C)
transition
T (°C)
1Et4
1Et5
1Et5Me
1Et6
1Et7
1Et7
1Et11
2Et5
2Bu5
Cr f I
Cr f I
Cr f I
Cr f I
Cr f I
120.3
106.5
110.1
78.1
I f N
I f N
I f N
I f N
I f N
N f SC
I f Cr
I f Cr
I f N
61.8
97.7
94.5
64.0
102.1
93.2
72.2
Cr f I
Cr f I
Cr f I
104.2
216.4
178.4
102.0
159.3
150.2
a Transition temperatures taken from minima/maxima on DSC (mDSC
for 1Et7 cooling) heating/cooling traces. I-isotropic; N-nematic; SC-
smectic C; and Cr-Crystalline.
monomers undergoes a cold crystallization and melting upon
heating, but that upon cooling, each derivative, except 1Et11
(which features the longest alkene chains), exhibits a small
exothermic transition rather than an obvious recrystallization
exotherm. Consistent with what is observed in many other
mesogenic systems the terminal olefins derivatives show a distinct
odd/even effect in the temperature at which the isotropic to
nematic transition occurs (Table 1). However, in this case the
compounds with an even number of carbons on the alkyl chains
exhibit a transition in the low 60s °C, while the odd carbon chain
derivatives are much more stable exhibiting transitions in the mid
90s °C. This suggests that in this case the molecular anisotropy
and hence molecular order is enhanced by having an odd number
of carbons attached to the oxygen. Interestingly, 1Et5Me, the
derivative decorated with six carbon nonterminal olefins, behaves
similarly to 1Et5 rather than 1Et6, suggesting that the location of
the alkene within the chain has a greater effect on thermal
behavior than the additional methyl unit. Thus, it appears that
the alkene in the side chains of the mesogens may play an
important role in the stabilizing interactions between these units.
Also of note is that each of these monomers exhibits a Tg between
ꢀ9.8 and 11.7 °C. As these materials vitrify below room
temperature, they will exhibit liquid crystalline fluidity at useful
temperatures. Although it should be noted that all the monomers
are monotropic, exhibiting liquid crystallinity only upon cooling.
The DSC trace of 1Et11 shows a cold crystallization peak and three
endothermic transitions observed upon heating. The POM of this
material shows no observable change until it melts at 104.5 °C,
which corresponds to the last of the three endotherms. At this
point, we have no explanation for the other two endotherms at
87.2 and 90.1 °C other than the existence of solidꢀsolid phase
transitions. Upon cooling, it does show the expected recrystalliza-
tion exotherm (102.0 °C), likely due to the significant length (11
carbons) of the alkene chains enhancing crystallization (Figure 2.)
To further probe the smaller transitions on cooling and
determine if they are, in fact, evidence of the formation of liquid
crystalline phases in these monomers, polarized light optical
microscopy (POM) studies were performed on each ligand.
Upon heating through the melting points determined by DSC,
the micrographs showed the removal of birefringence to a dark
state, consistent with an isotropic, liquid-like phase. Subsequent
cooling through the temperature of the small transition observed
in the DSC cooling traces, however, yielded a distinct Schlieren-
Figure 2. (a) DSC traces of the 1Et monomers and (b) expanded scale
to reveal small exotherms upon cooling of 1Et4, 1Et5, 1Et5Me, 1Et6, and
1Et7. Cooling traces are on top in panel (a) (exothermic up). Heating
and cooling rate is 10 °C/min.
’ RESULTS AND DISCUSSION
Synthesis. Starting from commercially available materials, we
prepared the acid chloride (3) and o-nitroaniline derivatives (4).
These materials were subsequently reacted to yield 5 via meth-
odology we recently reported.10 The new class of mesogenic
monomers (1) was prepared from 5 in two steps by deprotection
of the aryl methoxy groups with BBr3, followed by reaction of the
appropriate halo-alkene with the resulting bis-phenol under
Williamson ether synthesis conditions. The total synthesis of
the Bip ligand derivatives (1) is carried out in five (or six for
1
Et5Me) steps from commercially available materials (Scheme 1)
and allows access to the desired products in good overall yield
(50ꢀ67%) and on a practical scale (>2 g). Using similar
methodology, the synthesis of the aryl extended Bip derivatives
(2), which feature a core with higher structural anisotropy,10
requires one additional step from commercial materials (Scheme 1)
but also allows access to the targeted species in similar yield (52ꢀ
64%) on a practical scale.
Liquid Crystalline Properties of Derivatives of 1. Initially,
our efforts concentrated on the series of Bip ligands based on core
1 with ethyl substituents on the nitrogen (10-position, Figure 1)
and linear alkenes ranging from four to eleven carbons in
length on the 50 oxygen (1Et, Scheme 1). The presence of thermal
transitions indicative of liquid crystalline behavior in these
materials was first investigated by differential scanning calorime-
try (DSC) (Figure 2). The DSC traces reveal that each of these
like texture in the micrographs for 1Et4, 1Et5, 1Et5Me, and 1Et6
suggestive of a nematic liquid crystalline phase (Supporting
,
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Figure 3. (A) WAXS of 1Et5 (top trace) and 1Et7 (bottom trace) after cooling to room temperature from the isotropic state at 10 °C/min, revealing only
an amorphous halo in the former and a small peak consistent with a smectic arrangement in the latter. (b) POM images showing the birefringent textures
of 1Et7 upon cooling from isotropic to nematic and nematic to smectic states. (c) Modulated DSC thermogram (exothermic up) of the overall heat flow
revealing two small exothermic transitions upon cooling (93.2 and 72.6 °C). (d) Temperature dependence of the transmitted light intensity through a
5 μm thick film of 1Et7 with antiparallel rubbed polyimide coating for planar alignment integrated over about 1 mm2 area. The polyimide thickness is
roughly 50 nm.
Information). POM images taken below Tg confirm that this
phase remains intact upon further cooling in each sample.
Wide-angle X-ray scattering (WAXS) experiments support
the hypothesis that the transitions exhibited upon cooling in
the DSC and POM data are from an isotropic phase to a more
ordered nematic liquid crystalline phase. For example, the
diffraction pattern of the powdered monomer 1Et5 before any
thermal treatment shows an array of reflections consistent with a
crystalline solid, while upon heating, only an amorphous halo is
visible. Cooling the sample again past the transition at 97.7 °C
does not initially yield the return of crystalline reflections but
rather the presence of a broad amorphous peak and the absence
of long-range order, again in agreement with a nematic liquid
crystalline material (Figure 3a, top). After several minutes at
room temperature, ordered reflections begin to develop as the
material crystallized, aided by the Tg of the material being below
room temperature (about 9.5 °C). Crystallization was complete
by WAXS after about 80 min at room temperature (Supporting
Information). WAXS and POM experiments performed on the
heptene-functionalized monomer (1Et7) showed a more com-
plex behavior (Figure 3). As before, the POM micrographs show
the absence of birefringence upon heating through the melting
temperature observed by DSC (102.1 °C). Upon cooling
through the small DSC exotherm (93.2 °C), a Schlieren-like
texture appears, akin to that observed in the other monomers.
Further cooling, however, reveals a fan-like texture, consistent
with a smectic liquid crystalline phase (Figure 3b). Modulated
DSC (mDSC, Figure 3c) was performed on the 1Et7 monomer
near the temperature of the change in texture observed during
POM. Gratifyingly, this more sensitive technique revealed a
second exotherm (72.6 °C) at a lower temperature than the
first, consistent with the transition observed by POM. The
presence of two liquid crystalline phases upon cooling was
further supported by the measurement of the light intensity
transmitted through a 5 μm thick sample placed between crossed
polarizers. At a cooling rate of 1 °C/min, a double transition is
observed at 75.5 and 70.6 °C. The liquid crystalline phase is still
visible via this technique upon further cooling to room tempera-
ture (Figure 3d).
Electric current measurements on 1Et7 indicated a somewhat
ionic dielectric response. The dielectric measurements in the
planar alignment cell show very low dielectric constant (barely
above the square of the refractive index), except for the 1/
frequency-type decrease, which is due to the dc conductivity that
increases upon heating. The very low dielectric constant normal
to the electric fields in the nematic phase is lower than the
dielectric constant in the isotropic phase, which shows that the
material has a positive dielectric anisotropy. This may indicate
that the most stable conformation is not the bent-shaped cis/cis,
but rather the more linear trans/trans conformation (Figure 1),
as is observed in the crystal structure of similar Bip derivatives,9a
or that the arms are twisted with respect to each other.
WAXS experiments (Figure 3a) were then carried out to
determine if the final liquid crystalline phase upon cooling was,
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Figure 4. POM images displaying the optical response of 1Et5 with Eu(NO3)3 6H2O at 95 °C. Top left image is the initial state. The next six images
3
depict the advancing front of Eu(NO3)3 over 60 s. Bottom right image was repositioned to show the Eu(NO3)3 crystal.
in fact, a smectic phase. A reflection was indeed observed (about
29.3 Å), which corresponds to a distance slightly less than that of
the energy minimized end-to-end distance (34.7 Å) of 1Et7 in its
extended trans configuration. The location of this peak is con-
sistent with the presence of a smectic C phase with a tilt angle of
approximately 32° from normal (Supporting Information). We
hypothesize that the longer alkene chains used to decorate 1Et7
compared to those on the other 1Et ligands induce enhanced two-
dimensional order, and thus upon cooling this monomer adopts a
smectic arrangement after passing through a nematic region. As
with the 1Et5 derivative, 1Et7 also has a Tg below room tempera-
ture (about ꢀ8.0 °C) and is also metastable, crystallizing over a
period of days at room temperature.
counterparts. DSC and POM experiments suggest that 2Et5 is,
in fact, highly crystalline with a melting point of 216.4 °C (c.f.
106.5 °C for the related “short” derivative, 1Et5). The n-butyl
functionalized derivative, however, displays different0 behavior.
By increasing the length of the alkane chains at the N1 position,
liquid crystalline order can be observed. DSC experiments reveal
that upon heating 2Bu5 melts at 178.4 °C and upon cooling
exhibits a small exotherm at 150.2 °C (c.f. the shorter 1Bu5 is an
oil). POM micrographs taken at temperatures near this exotherm
suggest a transition from an isotropic melt to a nematic liquid
crystalline phase (see Supporting Information for DSC and
POM data of 2). These derivatives offer the option to incorpo-
rate crystalline or high-temperature liquid crystalline monomers
into polymers, copolymers, blends, and networks, further en-
hancing the tunability of our design (Table 1).
Chemo-Response with Metal Salts. As part of our project to
develop metallo-responsive liquid crystalline materials, we initi-
ally targeted mesogens that would become isotropic upon binding
metal ions. It has been demonstrated9ꢀ14 that Bip ligands bind to
a variety of metal ions; thus, we carried out experiments to
investigate the effects of this binding on the liquid crystalline
behavior of these mesogens. Initial DSC and POM results have
shown that addition of one equivalent of Eu(NO3)3 (which has
been shown to form a 1:1 complex with Bip ligands)31 to 1Et5
results in the switching of the system from nematic to isotropic.10
Additional experiments (Supporting Information) suggest that
this system becomes isotropic after exposure to only ∼0.6
equivalents of Eu(NO3)3. In a more direct demonstration of
the metallo-response itself, a slide of 1Et5 was prepared, thermally
treated as outlined above to yield a liquid crystalline phase, and
Cognizant of the prior work by Piguet and co-workers,11ꢀ14
and by the fact that we are interested in accessing polymeric liquid
crystalline Bip compounds, where solubility can be an issue at
higher degrees of polymerization, we were curious about what
potential0effects that increasing the length of the alkyl substituents
in the N1 positions would have on the liquid crystalline behavior
of our monomers. To that end, we synthesized derivatives with
0
n-propyl, n-butyl, and n-hexyl chains in the N1 positions and
pentene chains in the 50 positions (1Pr5, 1Bu5, 1Hx5). The n-butyl
functionalized derivative was also prepared with a heptene chain
in the 50 positions (1Bu7). While none of these monomers
displayed liquid crystalline behavior, presumably due to changes
in the aspect ratio of the monomer, their facile synthesis bodes
well for both enhancing the solubility and tuning liquid crystalline
properties of polymeric derivatives through copolymerization
and/or blending with the mesogenic derivatives.
Liquid Crystalline Properties of Derivatives of 2. In addi-
10
tion to easily tuning the length of the N and 50 alkyl/alkenyl
then observed via POM as a crystal of Eu(NO3)3 6H2O (mp:
3
substituents on these monomers, our synthetic scheme allows for
the preparation of larger aromatic cores by introducing a phenyl
ring between the Bip ligand and the 50-substituent (2, Scheme 1).
To probe potential liquid crystalline properties of these “ex-
tended” monomers, derivatives with ethyl and n-butyl chains on
40 °C) was placed upon the sample. Subsequent micrographs
depict the advancing front of the molten metal salt as a border
between birefringent (liquid crystalline) and dark (isotropic)
regions (Figure 4). Similar isotropic behavior was observed when
complexes of 1Et5 or 1Et5Me were prepared with Eu(ClO4)3
(known to bind Bip derivatives in a 1:3 ratio), Cu(OTf)2, and
Zn(OTf)2, which bind in 1:2 metal:ligand ratios.25
10
the N positions and pentene chains on the 50 oxygen (2Et5 and
2
Bu5) were also prepared. These derivatives have a signifi-
cantly larger aromatic surface, and as such, it was anticipated
that they would be generally more crystalline than their “short”
Polymerization via Acyclic Diene Metathesis (ADMet). As
we are interested in accessing metallo-responsive polymers, we
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have also investigated the ADMet polymerization (Scheme 2) of
these monomers. Several attempts to polymerize 1Et5 using a
variety of documented metathesis conditions and catalysts
yielded only oligomeric species with a maximum average degree
of polymerization (DP) of 3.5 with specific short oligomers
discernible via SEC (Figure 5a). Having ruled out other potential
issues such as binding of the catalyst by the ligand and reduced
solubility of the oligomers, we investigated that the possibility of
side reactions occurring from decomposition products of the
catalyst. Grubbs and co-workers published an elegant explana-
tion of (and solution to) this problem whereby the formation of
degradation products of the catalyst is hindered by using
substrates with nonterminal olefins.32 Thus, we attempted to
polymerize 1Et5Me via the same method as above and were able to
access a polymer of DP = 25 (Mn ∼ 14000 g/mol, PDI = 2.5) by
SEC (Figure 5a). It should be noted that because of the
metathesis mechanism, 1Et5 and 1Et5Me yield essentially the same
polymer structure, with the former releasing ethylene and the
latter butylene (Scheme 2).
Scheme 2. ADMet Polymerization of 1Et5 and 1Et5Me to Yield
Oligomeric and Polymeric Liquid Crystalline Materials
Having now successfully prepared both polymers and oligo-
mers of 1Et5, we then investigated their thermal properties.
Thermal and optical analysis of oligo(1Et5) revealed evidence
of liquid crystalline behavior that differs significantly from that of
the monomer. For example, unlike the monomer, which is
metastable and monotropic (only exhibits liquid crystalline
phases on cooling), DSC experiments (Figure 5b, top pair)
suggest that the oligomer exhibits enantiotropic behavior, with
transitions on heating that appear to correspond to a Tg at 65 °C
and a clearing point at 155 °C. POM images (Figure 5c, left)
taken in the temperature ranges suspected of liquid crystalline
behavior reveal textures indicative of slightly smaller liquid
crystalline domains than the monomer, as would be expected
for a higher molecular weight molecule. This trend in smaller
domain size appears to continue with poly(1Et5Me), where a
grainy birefringent texture is observed in the POM (Figure 5c,
right), while the DSC traces (Figure 5b, bottom pair) show no
discernible liquid crystalline transitions for this material over the
observed temperature range (ꢀ90 to 200 °C).We suspect that
the higher molecular weight polymer degrades before reaching its
liquid crystalline to isotropic clearing temperature. We ascribe
the dramatic differences in the LC behavior of the monomers and
Figure 5. (a) SEC traces of oligo(1Et5) (0) and poly(1Et5Me) (9). (b) DSC traces of oligo(1Et5) (top pair) and poly(1Et5Me) (bottom pair) (cooling
traces on top and exothermic up). (c) POM micrographs of oligo(1Et5) (left image T = 135 °C) and poly(1Et5Me) (right image T = 209 °C). (d) Film of
poly(1Et5Me) cross-linked with Cu(OTf)2.
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Chemistry of Materials
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the oligomers or polymers at least in part to the reduced
crystallinity of the polydispersed oligomers and polymers. The
higher thermal stability of the LC phases in the polymer can also
be ascribed to multivalent interactions between the mesogens
within the polymer chain stabilizing the LC order. The addition
of Eu(NO3)3 to poly(1Et5Me) disrupts the liquid crystalline
behavior, akin to that observed in the monomer, demonstrating
that the polymer retains the metallo-responsive characteristics of
the monomer. Interestingly, addition of a small amount of metal
ion (e.g., Cu(OTf)2, 10:1 Bip:Cu) that can bind two Bip ligands
and thus cross-link poly(1Et5Me) not only disrupts the liquid
crystallinity of the polymer but also results in free-standing films
of the material after solution casting (Figure 5d). Such films are
not accessible with either the metal-free polymer, a solid which
displays no film-forming behavior, or the metal complexes of the
monomers or oligomer.
’ AUTHOR INFORMATION
Corresponding Author
*E-Mail: stuart.rowan@case.edu.
’ ACKNOWLEDGMENT
This material is based upon work supported by the National
Science Foundation (CHE-0704026, DMR-0758631, and DMR-
0964765) and MRI-0821515 (for the purchase of the MALDI-
TOF/TOF). R.J.W. acknowledges support through the NASA
Graduate Student Research Program (NNX08AY62H). K.A.B.
acknowledges the support of a Graduate Research Fellowship
from the National Science Foundation (DGE-0234629).
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’ CONCLUSIONS
We have prepared a series of easy-to-access and scalable Bip-
containing monomers with chemo-responsive liquid-crystalline
properties. The derivatives thatdisplayliquidcrystalline transitions
do so only upon cooling from the isotropic phase and are thus
monotropic, with transition temperatures (I f N, orI f N f SC)
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yielding enantiotropic liquid crystalline oligomers (oligo(1Et5))
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dictated by the nature of the terminal alkene. These oligomers/
polymers are also metallo-responsive in a similar manner to their
monomeric counterparts but are now enantiotropic and show a
dramatically reduced tendency to crystallize relative to the mono-
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appropriate metal salts, yielding materials that can be prepared as
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responsive materials, e.g., such as metallo-responsive liquid crystal-
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phosphine oxide containing molecules (such as those found in
pesticides and nerve gas agents)22 opening the door for Bip-
containing polymers to be utilized to access structurally dynamic
polymers that are multiresponsive.28
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’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic details and charac-
b
terization data of 1-2, 4-8, and the phenolic derivatives of 5 and 8,
exemplary H NMR and 13C NMR spectra and POM micro-
1
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and POM titrations of 1Et5 with Eu(NO3)3. This material is
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