Differential response of cholesterol based pyrimidine systems with oxyethylene type spacers to gelation and mesogen formation in the presence of alkali metal ions

Article abstract of DOI:10.1039/c4sm02792b

A new series of lipophilic cholesteryl derivatives of 2,4,6-trichloro-pyrimidine-5-carbaldehyde has been synthesized. Oxyethylene spacers of variable lengths were inserted between the hydrogen bonding promoting pyrimidine core and the cholesteryl tail in order to understand their effect on the self-assembly of these compounds. Only compound 1a with the shortest spacer formed a gel in organic solvents such as n-butanol and n-dodecane. While other members (1b and c) having longer spacers led to sol formation and precipitation in n-butanol and n-dodecane respectively. The self-assembly phenomena associated with the gelation process were investigated using temperature-dependent UV-Vis and CD-spectroscopy. The morphological features of the freeze-dried gels obtained from different organic solvents were examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The solid phase behaviours of these molecules and their associated alkali metal ion complexes were explored using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The molecular arrangements in the xerogel and in the solid state were further probed using a wide-angle X-ray diffraction (WAXD) technique. Analysis of the wide-angle X-ray diffraction data reveals that this class of molecules adopts a hexagonal columnar organization in the gel and in the solid state. Each slice of these hexagonal columnar structures is composed of a dimeric molecular-assembly as a building block. Significant changes in the conformation of the oxyethylene chains could be triggered via the coordination of selected alkali metal ions. This led to the production of interesting metal ion promoted mesogenic behaviour.

Full text of DOI:10.1039/c4sm02792b

Soft Matter
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Differential response of cholesterol based
pyrimidine systems with oxyethylene type spacers
to gelation and mesogen formation in the presence
of alkali metal ions†
Cite this: Soft Matter, 2015, 11, 1945
a
abc
Sougata Datta and Santanu Bhattacharya*
A new series of lipophilic cholesteryl derivatives of 2,4,6-trichloro-pyrimidine-5-carbaldehyde has been
synthesized. Oxyethylene spacers of variable lengths were inserted between the hydrogen bonding
promoting pyrimidine core and the cholesteryl tail in order to understand their effect on the self-
assembly of these compounds. Only compound 1a with the shortest spacer formed a gel in organic
solvents such as n-butanol and n-dodecane. While other members (1b and c) having longer spacers led
to sol formation and precipitation in n-butanol and n-dodecane respectively. The self-assembly
phenomena associated with the gelation process were investigated using temperature-dependent UV-
Vis and CD-spectroscopy. The morphological features of the freeze-dried gels obtained from different
organic solvents were examined by scanning electron microscopy (SEM) and atomic force microscopy
(AFM). The solid phase behaviours of these molecules and their associated alkali metal ion complexes
were explored using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The
molecular arrangements in the xerogel and in the solid state were further probed using a wide-angle X-
ray diffraction (WAXD) technique. Analysis of the wide-angle X-ray diffraction data reveals that this class
of molecules adopts a hexagonal columnar organization in the gel and in the solid state. Each slice of
these hexagonal columnar structures is composed of a dimeric molecular-assembly as a building block.
Significant changes in the conformation of the oxyethylene chains could be triggered via the
coordination of selected alkali metal ions. This led to the production of interesting metal ion promoted
mesogenic behaviour.
Received 16th December 2014
Accepted 13th January 2015
DOI: 10.1039/c4sm02792b
www.rsc.org/softmatter
2,8,14–17
gelators
comprising certain low-molecular weight mole-
Introduction
cules are particularly useful because of their anisotropic
Fabrication of self-assembled supramolecular architectures by properties in contrast to their solution and crystal states. In
inter-molecular H-bonding and other weak non-covalent inter- addition, self-assemblies of the low molecular-weight organo-
actions (e.g., van der Waals, p–p stacking and solvophobic gelators (LMOGs) are thermo-reversible in most of the cases,
effect, charge-transfer (CT), metal–ligand coordination, etc.) i.e., they show reversible sol-to-gel transitions when their
among the various molecular building blocks is dependent on a heated solutions are cooled below the respective gelation
2,8,14–21
number of essential parameters. One of the most important of temperatures.
2
,8,14–22
these parameters for determining the states and properties of
Low molecular-weight organogelators (LMOGs)
the organized structures is the geometry of the involved mole- liquid-crystals (LCs)
and
23–25
are the topics of growing attention
and supramolecular because of their numerous potential applications. They have
been recognized as high technology materials because of their
1–8
1,5,6,9–13
cules. Liquid-crystalline materials
diverse utility in various elds, including the design and
a
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, synthesis of nanomaterials,26,27 biomaterials, sensors,
28
29–32
Karnataka, India. E-mail: sb@orgchem.iisc.ernet.in; Fax: +91-80-23600529; Tel:
33,34
stimuli-responsive materials
and in molecular elec-
+
b
91-80-22932664
35–37
38,39
tronics
and catalysis.
In addition to their self-organizing
Jawaharlal Nehru Centre for Advanced Scientic Research, Jakkur, Bangalore 560
potential in the solid/gel or solution state, additional properties
0
64, India
4
0
41
42
c
such as luminescence, redox activity, sensing, self-heal-
J. C. Bose Fellow, Department of Science and Technology, New Delhi, India
43
ing, etc. may be induced by incorporating metal ions inside the
†
Electronic supplementary information (ESI) available: Complete synthetic
procedures, compound characterization data for the cholesteryl derivatives pre-designed building blocks, leading to functional or multi-
described in the text, UV-Vis spectra, DSC proles and wide-angle X-ray
diffraction data. See DOI: 10.1039/c4sm02792b
44
functional materials.
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82
Various types of organic molecules are used in recent years a hexagonal columnar mesophase. A similar observation was
to achieve gelation involving non-covalent interactions as the obtained on addition of sodium triate to the crown ether
driving force. Strong and directional hydrogen bonding receptor of benzo-15-crown-5 benzoate [6,7,9,10,12,13,15,16-
4
5–49
interactions are the main driving force for amide,
urethane, urea and sugar derived gelators.
octahydro-1,4,7,10,13-pentaoxa-benzocyclopentadecen-2-ylmethyl-
5
0
51
52–55
83
However, 3,4,5-tris(p-dodecyloxybenzyloxy)benzoate]. Kato and co-workers
van der Waals force is oen the dominant interaction among reported a triphenylphosphine oxide (TPPO) compound bearing
5
6
the steroid derivatives. Cholesteryl derivatives have been 3,4,5-tridodecyloxybenzyloxy moieties, capable of exhibiting a
84
exploited as versatile so materials, some of which exhibit columnar liquid-crystalline phase. Interestingly, complexation
+
bilayer membrane forming properties, chiral recognition, with the Li ion stabilized the columnar phases due to the ion–
organogelation and liquid-crystalline behaviour depending dipole interaction, whereas a cubic phase was observed in the
5
7–61
+
upon the self-assembly process.
ally originate due to the presence of the cholesterol backbone derivatives with oxyethylenes as side-chains exhibiting a unique
in these molecules. In one of the earlier reports, Weiss and alkali metal ion induced discotic nematic (N ) to hexagonal
) phase transformation. There are other instances
of various aromatic group appended cholesterol-based gela- of utilization of oligo-(oxyethylene) units as a spacer in the
These properties gener- presence of the Na ion. Kohmoto's group explored triphenylene
D
85
co-workers explored the structure and property relationships columnar (Col
h
6
2
tors. Shinkai's group demonstrated the formation of helical modulation of properties of various supramolecular and bio-
supramolecular aggregates by azobenzene appended choles- logically active systems as well.
86–90
6
3
terol-derived gelators. The same authors have also shown
Fenniri introduced an interesting molecular scaffold based
remarkable stabilization of crown-based organogels in the on 4-chloro-2,6-bis(octadecylamino)pyrimidine-5-carbalde-
presence of a bipyridinium-based guest by the synergistic hyde, which is capable of generating brous lamellar struc-
9
1
effect of H-bonding, hydrophobic and charge-transfer inter- tures in both dry solid and gel states. Subsequently, we have
6
4
actions. Cholesteryl derivatives bearing a crown ether or developed a family of organogelators based on L-amino acid
azobenzene moiety were shown to display the unique
characteristics of cholesteric liquid-crystals and organo-
gelation by photo-irradiation or via crown ether–metal ion
6
5–67
interactions.
Jung and co-workers showed signicant
stabilization of asymmetric bolaamphiphilic sugar-based
hydrogelators in the presence of alkyl diammonium ions as
6
8
guests by the synergistic effect of H-bonds. Qi et al.
synthesized and investigated achiral benzo-21-crown-7-
substituted bis-urea LMOGs which hierarchically assembled
into helical brils and further developed into bundles and

nally formed a stable gel in acetonitrile. The gel–sol tran-
+
sition could be modulated by K ion binding to the crown
ethers, pseudorotaxane formation with secondary ammo-
nium ions and chloride anion binding with the urea units.
6
9
Several non-covalent and non-classical approaches have
been adopted for the production of liquid-crystalline mate-
5
,6,70–73
rials.
Metallo-mesogens are among the best examples for
the fabrication of a variety of materials. These systems not only
contain different metal species, but also exhibit a variety of
7
0–72,74
phases.
Previously, luminescence properties were intro-
duced by the graing of organic luminophores, exhibiting
7
5–77
optical properties,
or by the anchorage of platinum
7
8
complexes, as triplet emitters. Various chelating agents were
also integrated on the platforms such as polypyridine frag-
ments or crown ethers leading to functional metallo-mesogens
7
9,80
aer metal ion coordination.
Similar to stimuli-responsive
8
1
gels, ion-responsive liquid crystals of organic molecules
are of recent interest because of their switching to different
liquid-crystalline structures which result in interesting
Scheme 1 Synthetic scheme of cholesteryl derivatives of 2,4,6-tri-
ꢀ
chloro-pyrimidine-5-carbaldehyde: (i) POCl
3
, DMF, rt, 15 min, 100 C,
8
2–85
ꢀ
properties.
4 h, yield ¼ 60%; (ii) p-TsCl, dry pyridine, DMAP, dry CHCl
3
, 0 C, 6 h,
yield ¼ 92%; (iii) HO(CH
2
CH O)
2
n
H (n ¼ 2, 3 and 4), dry 1,4-dioxane,
It has been demonstrated that the monoesters of triethylene
reflux, 4 h, yield ¼ 80–90%; (iv) phthalimide, diisopropyl azodi-
carboxylate (DIAD), triphenylphosphine, dry DMF, rt, 4 h, yield ¼ 95–
glycol with 3,4,5-tris(n-dodecan-1-yloxy)-benzoic acid are crys-
+
talline solids. Complexation with the Li ion, however, desta-
9
2
7%; (v) hydrazine monohydrate, pyridine, rt, 24 h, yield ¼ 82–85%; (vi)
bilizes their crystalline phase and drives the same to self-
,4,6-trichloro-pyrimidine-5-carbaldehyde (2), N,N-diisopropylethyl-
2 2
assemble into a cylindrical architecture which nally results in amine, dry CH Cl , rt, 6 h, yield ¼ 80–85%.
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9
2
derivatives of 2,4,6-trichloro-pyrimidine-5-carbaldehyde.
Gelation experiment
They self-assemble in hydrocarbons to result in b-sheet type
extensive hydrogen-bonded organizations which nally lead
to the formation of various interesting gel superstructures
depending upon the choice of the L-amino acid based spacer.
However, the self-assembly of the 2,4,6-trichloro-pyrimidine-
First, the gelation ability of these compounds 1a–c was evalu-
ated in different organic solvents by the “stable-to-inversion
protocol of a test tube” method. 1a immobilized organic
solvents such as n-butanol (Fig. 1) and n-dodecane. The
minimum gelator concentration (mgc) associated with the gel
formation in n-butanol (3.6 mM) was lower compared to that in
n-dodecane (4.6 mM). The characteristics of compounds 1a–c in
different solvents are listed in Table S1 (ESI†). They, however,
precipitated from their solutions in t-butanol or n-hexane and
5
-carbaldehyde derivative containing a rigid cholesterol
residue is still unknown. Herein, we report the rst synthesis
of pyrimidine derivatives which contain cholesterol residue
attached to the pyrimidine core via oligo-oxyethylene spacers
of various lengths (Scheme 1). The length of the oxyethylene
spacer was varied in order to understand the effect of such
spacers on the gelation process. The main objective of the
present work is to design and synthesize a multifunctional
material that is capable of showing gelation and mesogenic
behaviour induced by an external stimulus. For this purpose,
cholesterol units were covalently incorporated into the
supramolecular building block to achieve gelation. At the
same time, the insertion of oxyethylene chains as a spacer
between the cholesterol unit and the pyrimidine moiety offers
an opportunity to exploit alkali metal ion induced propaga-
tion of the liquid-crystalline behaviour in the solid state. To
the best of our knowledge this is the rst report which shows
modulation of the gelation properties by varying the length of
the oxyethylene spacer. This is also the rst instance where it
has been possible to achieve gelation and manifest alkali
metal ion induced liquid crystallinity from the same series of
compounds.
2 2 3
were found to be soluble in each of CH Cl , CHCl , THF,
benzene and toluene. Surprisingly, none of the other members
in this series with longer oxyethylene spacers induced gelation
in any of these solvents. An increase in the length of the oxy-
ethylene spacer probably enhanced the extent of degrees of
freedom of the cholesterol moiety, resulting in ‘looser’ packing
of the cholesteryl derivatives. Compounds 1b and 1c with longer
oxyethylene spacers (n ¼ 3 and 4) resulted in solutions and
precipitates in n-butanol and n-dodecane respectively. This
result clearly indicates that the increase in the length of the
oxyethylene spacer enhances the polarity of these compounds.
Hence, the solubility associated with the spacer lengths of this
series of compounds also plays an important role in the gelation
process.
Morphological behaviour
To reveal the microstructures of organogels of 1a in n-butanol
and n-dodecane, scanning electron microscopy (SEM) and
atomic force microscopy (AFM) were performed. SEM images of
the xerogels of 1a obtained from n-butanol (Fig. 2a) and n-
dodecane (Fig. 2b) showed brous structures of high aspect
ratio. The AFM images of the xerogels of 1a obtained from n-
Results and discussion
Synthesis
A series of cholesteryl derivatives of 2,4,6-trichloro-pyrimidine- butanol and n-dodecane also exhibited brous structures of
-carbaldehyde was synthesized (Scheme 1). Cholesterol was high aspect ratio and diameters ranged from 100–200 nm
5
converted to cholesteryl p-toluenesulfonate (tosylate) (3) in the (Fig. 2c and d). However, it is important to note that drying of
presence of pyridine and a catalytic amount of DMAP. Then, the gel samples during xerogel preparation may induce artifacts
cholesteryl tosylate was further heated in the presence of as well.
an excess of individual oxyethylene diols to obtain various
alcohols (4a–c). These alcohols were converted to the corre-
UV-Vis and CD-spectroscopy
sponding phthalimide derivatives (5a–c) using triphenyl-
In order to understand the nature of inter-molecular interac-
phosphine and diisopropyl azodicarboxylate (DIAD) in DMF
tions in the supramolecular aggregates, a temperature-depen-
based on the Mitsunobu reaction. These phthalimide deriva-
dent UV-Vis absorption spectroscopic study was performed (S1a
tives were subsequently transformed to the corresponding
92
and b, ESI†). We avoided the spectroscopic investigation at
amines (6a–c) using hydrazine monohydrate in pyridine.
Finally, the resulting amines were treated with 2,4,6-trichloro-
pyrimidine-5-carbaldehyde in the presence of diisopropyle-
thylamine in CH
2 2
Cl to get the nal products (1a–c). Each new
compound and the intermediates were characterized by FT-IR,
1
13
H and C-NMR spectroscopy and ESI-MS (see the Experi-
mental section, ESI†).
In the following sections, we report the physical character-
ization of different modes of aggregation of different
compounds (1a–c) given in Scheme 1, in various organic
solvents and in the solid state. We also investigate the role of the
oxyethylene spacer in the self-assembly process of the above
molecules and document their properties herein.
Fig. 1 Photographs showing the temperature induced gel-to-sol
transition of 1a (3.6 mM) in n-butanol.
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Fig. 2 (a and b) SEM and (c and d) AFM images of the xerogels of 1a
obtained from n-butanol and n-dodecane respectively at concentra-
tions of 3.6 and 4.6 mM respectively.
higher concentrations because of the high scattering under
such conditions due to the opaque nature of the gel. Since
aggregates still persisted at lower concentrations, the temper-
ature-dependent UV-Vis spectra of 1a were recorded using less
19
concentrated solution. The UV-Vis spectrum of 1a in n-butanol
at a concentration of 0.046 mM showed a broad peak in the
range of 220–240 nm which may be attributed to the n–p*
93
transition of the C]N and C]C bonds (Fig. S1a, ESI†). In
addition, two sharp peaks near 275 and 315 nm were also Fig. 3 Temperature-dependent changes in the CD-spectra of 1a in (a)
n-butanol and (b) n-dodecane at a concentration of 0.046 mM. Insets
show the plot of CD (mdeg) vs. temperature at 345 nm.
observed which most likely originated from the p–p* transition
of the aromatic ring and n–p*transition of the formyl substit-
93
uent respectively. The intensity of these absorption spectral
bands monotonically increased with the increase in tempera-
ꢀ
intensity at 345 nm vs. temperature for the disruption of the
self-assembly to smaller aggregates (inset, Fig. 3a). A similar
pattern was also observed in the temperature-dependent CD-
spectra of 1a in n-dodecane, thus elucidating the presence of
similar motifs in the self-assembly process (Fig. 3b).
ture and nally became saturated at approximately 60 C. This
was due to a likely consequence of the weakening of the non-
19
covalent interactions between the chromophores. This resul-
ted in the breaking of larger aggregates to form aggregates of
smaller sizes. A similar pattern was also observed in the
temperature-dependent UV-Vis spectra of 1a in n-dodecane
Thermal response to mesophase behaviour in the presence of
alkali metal ions
(
Fig. S1b, ESI†).
The manifestation of the supramolecular chirality at the
molecular level induced by the presence of the cholesterol
moiety due to the formation of a self-assembly associated with
the sol-to-gel transition process was investigated by circular
dichroism (CD) spectroscopy (Fig. 3a and b). Fig. 3a illustrates
the temperature-dependent changes in the CD spectra of 1a in
n-butanol at 0.046 mM. It exhibited two positive Cotton effects
near 345 and 288 nm indicating a clockwise orientation of the
The solid state behaviours of these cholesteryl derivatives, 1a–c
possessing di-, tri- and tetra-oxyethylene units and their asso-
ciated alkali metal ion complexes were investigated by various
physical methods including polarizing optical microscopy
(
POM), solid phase differential scanning calorimetry (DSC) and
98–100
wide-angle X-ray diffraction (WAXD) experiments.
Alkali
metal complexes of this series of compounds were prepared by
evaporation of the THF solution of each derivative with appro-
priate stoichiometry of the perchlorate salt of the alkali metal.
94,95
dipole moments in the supramolecular aggregates.
The two
positive peak intensities near 345 and 288 nm monotonically
decreased with the increase in temperature presumably because
of the loss of the supramolecular chirality. This indicates that
the supramolecular chirality of the self-assembled aggregates is
In the POM experiment, each sample was rst heated to
ꢀ
obtain an isotropic melt and then cooled down slowly (5
C
ꢁ
1
min ) to room temperature. Various types of phases appeared
9
6,97
responsible for the observed CD-signal.
The transition
during cooling of the isotropic melts of compounds 1a–c at 65,
ꢀ
ꢀ
temperature (ca. 45 C) was calculated from the plot of the CD
7
0 and 80 C respectively (Fig. 4a–c). Interestingly, dramatic
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ꢀ
exhibited single endothermic transitions at 75 and 85
C
respectively. In cooling cycles compound 1a showed double
ꢀ
exothermic transitions at 58 and 66 C similarly to the heating
cycle. However, single exothermic transitions were observed at
ꢀ
7
1 and 81 C respectively for compounds 1b and 1c.
Under the same circumstances, interesting and drastic
changes were observed in the DSC proles of 1b–c in the pres-
ence of alkali metal ions (Fig. S3a–d, ESI†). Fig. S3a and b (ESI†)
show DSC thermograms during heating and cooling of 1b and
1
c in the presence of one equiv. of LiClO . The heating cycles
4
exhibited multiple weak transitions in the range of 150–175 and
ꢀ
1
40–185 C. On the other hand, small peaks associated with
exothermic transitions of liquid-crystalline phases appeared in
ꢀ
the cooling cycles in the range of 120–170 and 125–190 C.
+
Similar patterns were obtained in the case of Na -complexes of
1b and 1c (Fig. S3c and d, ESI†). Such kinds of multiple tran-
sitions were observed for bent core liquid-crystalline
compounds based on a 1,3-dihydroxybenzene core containing a
101
cholesteryl 6-oxyhexanoate wing. Park et al. also found similar
types of multiple transitions in the DSC experiment of Cu(II)
102
complexes of certain pyridyl ligands.
Structural analysis and molecular modelling
Wide-angle X-ray diffraction (WAXD) was carried out to acquire
information about the packing patterns in the supramolecular
assembly present in the xerogel and in the solid state. In the
WAXD pattern of the xerogel of 1a derived from n-butanol, three
reections corresponding to the d-spacings of 24, 15 and 12 A˚
+
Fig. 4 POM images of (a) 1a, (b) 1b, (c) 1c, (d) 1b-Na (0.5 equiv.), (e)
+
+
+
2+
1
(
b-Na (1 equiv.), (f) 1b-Li (0.5 equiv.), (g) 1b-Li (1 equiv.), (h) 1b-Mg
+
+
+
1 equiv.), (i) 1c-Na (0.5 equiv.), (j) 1c-Na (1 equiv.), (k) 1c-Li (0.5 were obtained (Fig. 5). The relative positions of these reections
+ 2+
equiv.), (l) 1c-Li (1 equiv.) and (m) 1c-Mg (1 equiv.) obtained on
cooling isotropic phases. Magnification 20ꢃ.
at O1, O3 and O4 are in good agreement with the (100), (110) and
200) planes, respectively. This indicates the formation of a
columnar hexagonal phase (Col ) with a lattice constant of 27.7
(
h
˚
A.
5,6,71,103,104
Similarly, the xerogel of 1a obtained from n-dodec-
changes in the birefringence and the morphology of the mes-
ophases were observed with 1b and 1c in the presence of 0.5 and
ane showed three peaks associated with the d-spacings of 19 A˚ ,
˚
˚
1
2 A (1 : O3) and 9 A (1 : O4), indicating the presence of a
+
+
1
equiv. of Li or Na ions presumably because of the
h
hexagonal columnar structure (Col ) with a lattice constant of
complexation with the oxyethylene chain (Fig. 4d–g and i–l). In
contrast, compound 1a underwent an apparent decomposition
under these conditions. All of these samples remained as
˚
2
1.9 A (Fig. 5). The WAXD data of the xerogels are summarized
ꢀ
viscous liquid above ꢂ130 C and the liquid-crystalline nature
of the samples was conrmed by scratching. Addition of excess
alkali metal salt disrupted the mesophase formation.
2
+
Interestingly, the presence of Mg resulted in the decom-
position of these compounds probably due to the high charge
2
+
density of the Mg ion (Fig. 4h and m). It is also important to
ꢀ
note that compounds 1a–c became isotropic below 100
C
whereas alkali metal complexes transformed to isotropic melts
ꢀ
above 150 C under POM. This observation is in accordance
84
with the results obtained by Kato and co-workers.
In the rst heating cycle of the DSC experiment, the sample
was heated to remove the anisotropic history of the sample. The
ꢀ
ꢁ1
resulting melt was cooled down at a rate of 5 C min . Aer the
sample reached room temperature, new heating and cooling
cycles were performed under the same conditions as before. The
DSC prole of compound 1a showed a broad endothermic
ꢀ
transition consisting of two closely placed peaks at 64 and 70 C
Fig. 5 WAXD plots of xerogels of 1a obtained from n-butanol and n-
respectively (Fig. S2, ESI†). However, compounds 1b and 1c dodecane.
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in Table S2 (ESI†). Interestingly, the longest repeat distance
It is reported that poly(ethylene glycol) (PEG) and poly-
associated with the rst diffraction peak of the xerogel of 1a ethylene oxide (PEO) acquire helical and coiled conforma-
1
05
˚
obtained from n-butanol is much larger than that obtained tions and the average width of the single helix of PEO is 7.6 A
106
from n-dodecane. The higher hydrophobicity of n-dodecane from its STM measurements. The oxygen atoms in the oxy-
presumably drives the oxyethylene spacer to loop inside which ethylene chains coordinate with alkali metal ions by ion–dipole
interactions, leading to straightening of the bent, coiled PEO
The modes of self-assembly in the solid state of compounds segments. Since the alkali metal ion coordinates with the ‘O’
a–c and their alkali metal complexes were also investigated by atoms of the oxyethylene chain, the ClO
WAXD [Fig. 6 and S4 (ESI†)]. Samples were annealed at their inter-disk region in order to stabilize the Col
107
leads to shortening of the repeat distance.
85
ꢁ
1
4
prefers to stay in the
structure in the
h
associated isotropic temperatures before recording their wide- alkali metal complexes (Fig. 7).
angle X-ray diffractograms. In the WAXD plot of 1a, three
Interestingly, the basic hexagonal columnar structural
peaks appeared corresponding to the d-spacings of 22, 14 and patterns of 1b and 1c remained unaffected in the presence of
˚
1
1 A [Fig. 6a and b and S4a (ESI†)]. These reections were one equiv. of LiClO
4
[Fig. 6a and S4a (ESI†)]. This result is in
84
indexed in sequence as the (100), (110), and (200) planes, due agreement with previous ndings of Kato and co-workers.
˚
to a columnar hexagonal lattice with the lattice parameter of a However, the d-spacings appeared at 24.5 and 24.7 A associated
25.4 A˚ .
5,6,71,103,104
Similarly compounds 1b and 1c showed with the (100) reection of 1b and 1c increased to 24.9 and 27 A˚
¼
4
hexagonal columnar structures with the lattice parameters of respectively in the presence of one equivalent of LiClO (Fig. 6a
˚
˚
2
8.3 and 28.5 A respectively [Fig. 6a and b and S4a (ESI†)]. The and b). Hence, the lattice parameter increased to 28.8 A for 1b
+
˚
WAXD data of the compounds 1a–c are summarized in and 31.2 A for 1c aer Li coordination. The WAXD data of the
Table S3 (ESI†).
+
Li -complexes of 1a–c are summarized in Table S3 (ESI†). This
observation emphasizes that the oxyethylene units remain in
coil-like conformation under ion-free conditions and get
extended by the coordination of the alkali metal ions increasing
the diameter of the disc (Fig. 7). This observation is in good
agreement with the results reported by Kohmoto and co-
85
+
workers. The WAXD data of the Li -complexes of compounds
+
+
Fig. 6 (a) WAXD plots of 1a–c and 1b-Li (1 equiv.) and 1c-Li (1 equiv.)
in the solid state; (b) d-spacings associated with (100) and (110) Fig. 7 A proposed model of a hexagonal columnar (Col
h
) type aggre-
+
reflections of the hexagonal columnar structure of 1a–c and 1b-Li (1 gation pattern composed of dimeric assemblies as building blocks and
+
+
equiv.) and 1c-Li (1 equiv.).
elongation of oxyethylene units due to coordination with Li .
1950 | Soft Matter, 2015, 11, 1945–1953
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Soft Matter
1
a–c are summarized in Table S3 (ESI†). Similar results were of the xerogels conrms the presence of dimeric molecular
+
also obtained for the Na -complexes (Table S4, ESI†). The lattice assemblies. This eventually propagates in the orthogonal
parameter increased to 28.5 A for 1b and 29.9 A for 1c aer Na
+
˚
˚
direction to develop into hexagonal columnar structures (Colh).
coordination. It is important to note that the maximum elon- In our previous report, we have shown that graing of hydro-
gation of the d-spacing was observed for 1c in the presence of 1 carbon chains with the 2,4,6-trichloro-pyrimidine-5-carbalde-
+
equiv. of Li . This has also been reected in the POM image of hyde template leads to the formation of a lamellar self-assembly
+
92
1
c-Li (Fig. 4l).
in the xerogel. Hence, the present work sheds new light on the
Hence, it is clearly evident that the liquid-crystalline states of eld of self-assembly, in which the structure of the supramo-
the compounds alone do not stabilize. That is why cooling of lecular assembly could be modulated by the choice of the
the isotropic liquid leads to direct crystallization to more substituents. This is also a rare instance in which the solvent
stabilized forms. However, the liquid-crystalline states are polarity results in a signicant change in the conformation of
stabilized due to the ion–dipole interaction in the case of their the oxyethylene spacer of gelator 1a. This was manifested in the
alkali metal complexes.
Since the structures of the steroid amphiphiles are not disk- upon changing the solvent from n-dodecane to n-butanol.
like, one may assume that a column slice comprises more than Insertion of an oxyethylene type spacer offers an opportunity
form of an increase in the diameter of the disk in the xerogel
one molecule. The number (n) of molecules constituting a to tune their solid state properties by the addition of alkali
+
+
single disk of the column could be estimated according to metal ions. Complexation of 1b–c (n ¼ 3 and 4) with Li or Na
1
03,104
eqn (1).
results in dramatic changes in the optical birefringence and the
morphology of the mesophases as revealed by polarized optical
microscopy (POM). It may be noted that differential scanning
calorimetry (DSC) traces of the alkali metal ion complexes are
entirely different from those of the compounds alone. The
hexagonal columnar structure of the aggregates present in the
2
n ¼ (a )(O3/2)(hrN
A
/M)
(1)
is
where the notation ‘a’ is the hexagonal lattice parameter, N
Avogadro's number and M is the molecular mass of the
amphiphile. The thickness (h) of a slice is about 5.5 A, calcu-
A
˚
+
mesophases of these compounds and their associated Li and
lated on the basis of wide-angle X-ray diffractograms [Fig. S4b
+
Na complexes have been analyzed by WAXD. Elongation of the
103,104
ꢁ3
(ESI†)].
Assuming a density (r) of 1 g cm , the estimated
coil like oxyethylene spacers occurs presumably due to the
number of molecules (n) in a disk in the columnar hexagonal
state is approximately 2 in each case (Fig. 7). Hence each disk of
the hexagonal columnar structure is composed of dimeric
assemblies of the amphiphilic molecules since pyrimidine
derivatives are well-known to form complementary H-bonded
+
+
coordination with Li or Na . However, the basic pattern of the
hexagonal columnar organizations remains unaffected. This is
the rst example of a series of oxyethylene chain integrated
cholesteryl derivatives which could be employed in tuning their
solid state properties by recognition of specic alkali metal
ions.
91,92,108–110
structures.
Previously, we have shown possible complementary
hydrogen-bonding interactions of similar pyrimidine deriva-
tives, which occur through the participation of sp -hybridized
2
Acknowledgements
ring nitrogen atoms, –NH, and aldehyde groups. On the basis of
these results a model of formation of the dimeric assemblies
leading to the hexagonal columnar structure is proposed
This work was supported by the Department of Science and
Technology (J. C. Bose Fellowship to S. B.), Government of
India, New Delhi, India. S. D. thanks CSIR for a senior research
fellowship.
91,92,108–110
(Fig. 7).
Conclusions
Notes and references
In summary, we have described the syntheses of a new family of
cholesterol appended 2,4,6-trichloropyrimidine-5-carbaldehyde
derivatives, 1a–c, tailored with variable lengths of oxyethylene
spacers. Only 1a with the shortest spacer (n ¼ 2) acts as a gelator
in organic solvents such as n-butanol and n-dodecane. The self-
assembly of 1a in these solvents is driven by both hydrogen
bonding and lipophilic interactions among the gelator mole-
cules. To the best of our knowledge this is the rst report where
it has been possible to modulate the gelation properties by
varying the lengths of the oxyethylene type spacer.
The mode of aggregation of 1a in organic solvents has been
investigated by temperature-dependent UV-Vis and CD spec-
troscopy. Morphological investigations of the xerogels under
SEM and AFM reveal the presence of brous structures with
high aspect ratio. Wide-angle X-ray diffraction (WAXD) analysis
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