Self-assembly of taper- and wedge-shaped maleimide derivatives: Synthesis and structure-property relationship

Article abstract of DOI:10.1016/j.molliq.2019.02.054

Herein, we describe design and synthesis of five new amphiphilic systems, viz. 10b-c and 11a-c composed of maleimide at the focal point and alkoxy phenyl ring at the periphery. Their self-assembling behaviour was examined systematically by varying the length of hydrophobic part and aromatic core segment. Evidently, on increasing the aromatic core length, maleimide based amphiphiles relatively emerge as two distinct molecular structures, i.e. wedge- and tapered-shaped. The mesomorphic studies of the two series reveal that, tapered-shaped maleimide derivatives assemble into a smectic A phase with an interesting phasmidic arrangement, whereas the wedge-shaped molecules exclusively form a prospective supramolecular hexagonal columnar mesophase through the intermolecular hydrogen-bonding via maleimide head group. These self-assembled materials could demonstrate high-sensitivity towards various external stimuli.

Full text of DOI:10.1016/j.molliq.2019.02.054

Accepted Manuscript
Self-assembly of taper- and wedge-shaped maleimide derivatives:
Synthesis and structure-property relationship
D.R. Vinayakumara, Sandeep Kumar, S. Krishna Prasad, Airody
Vasudeva Adhikari
PII:
S0167-7322(18)35350-9
DOI:
Reference:
https://doi.org/10.1016/j.molliq.2019.02.054
MOLLIQ 10455
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
16 October 2018
28 January 2019
10 February 2019
Please cite this article as: D.R. Vinayakumara, S. Kumar, S.K. Prasad, et al., Self-assembly
of taper- and wedge-shaped maleimide derivatives: Synthesis and structure-property
relationship, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.02.054
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ACCEPTED MANUSCRIPT
Self-assembly of Taper- and Wedge-Shaped Maleimide
Derivatives: Synthesis and Structure-property
Relationship
D. R. Vinayakumara,a Sandeep Kumar, b S. Krishna Prasad,c Airody Vasudeva Adhikari.*a
a Organic Materials Laboratory, Department of Chemistry, National Institute of Technology
Karnataka, Surathkal, Mangalore 575 025, India.
b SCM group, Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore -
560 080, India.
c Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore-560 013, India.
Corresponding Author: Airody Vasudeva Adhikari
E mail: avachem@gmail.com; avachem@nitk.ac.in
Abstract
Herein, we describe design and synthesis of five new amphiphilic systems, viz. 10b-c
and 11a-c composed of maleimide at the focal point and alkoxy phenyl ring at the periphery.
Their self-assembling behaviour was examined systematically by varying the length of
hydrophobic part and aromatic core segment. Evidently, on increasing the aromatic core
length, maleimide based amphiphiles relatively emerge as two distinct molecular structures,
i.e. wedge- and tapered-shaped. The mesomorphic studies of the two series reveal that, tapered-
shaped maleimide derivatives assemble into a smectic A phase with an interesting phasmidic
arrangement, whereas the wedge-shaped molecules exclusively form a prospective
supramolecular hexagonal columnar mesophase through the intermolecular hydrogen-bonding
via maleimide head group. These self-assembled materials could demonstrate high-sensitivity
towards various external stimuli.
Keywords
Supramolecular chemistry; Amphiphiles; Hydrogen bonding; Heterocycles; Thermotropic
liquid crystals
1. Introduction
In this advanced era, novel liquid crystalline (LC) materials are being successfully
employed in the wide range of fields from electronics to biomedical, because of their unique
properties [1–6]. In fact, LCs are regarded as “soft matters” which possess molecular order,
yet fluidic assemblies by the dint of variety of non-covalent interactions. Now, supramolecular
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liquid crystals are considered to be fascinating soft materials, wherein small molecular
components associate to form complex nanoscale architectures through interactions such as
hydrogen bonding, dipolar interactions, Coulombic forces, π-π interactions, metal
complexation, ionic attentions or combination of these [7–9]. These nanosegregated structures
are highly dynamic, their properties are typically depending on the nature of constituent
building blocks as well as the type of forces by which they assembled. Such mesogens are
promising functional materials in numerous applications mainly due to their high response
towards the external stimuli [10–19].
Recently, liquid crystals formed through hydrogen-bonding interaction have attracted
intense scientific attention in the area of soft materials [20–26]. Generally, the mode of H-
bonding in LCs can be divided into close-type and open-type. The former type of H-bonding
generates closed complexes, while the latter leads to form ribbon or layer kind of structures
[9]. Till now, a number of H-bonded motifs such as heterocycles, polyalcohols, carboxylic
acids, amides, amines and properly functionalized carbonyl structures, have been used to obtain
supramolecular liquid crystals [27]. Amid, heterocyclic systems are of particular importance
due to their ability of forming stable and specific H-bonds, imparting lateral and longitudinal
dipoles to overall structure [9]. Moreover, electron deficient nature of heterocycles impel the
important electronic communication throughout the structure when they appended with
electron donating moieties. This feature also provides a good optoelectronic property to the
molecules, such materials can potentially be used as active components in OLEDs, solar cells,
TFT, etc [11].
In the literature, heterocycles capable of forming H-bonding such as pyridone [28],
melamine [24,29], 1H-pyrazole [21,23,30], barbituric acid [31,32], folic acid [33–35], 1H-
imidazole [36–38] and some factionalized heterocycles [22,39–41] have been reported as
potential structural motifs to achieve supramolecular liquid crystals. In this context, we recently
explored cyanopyridone as an eminent electron deficient motif with proper donor-acceptor
configuration to realize the luminescent liquid crystalline material for OLED application [42–
45]. In continuation of our effort on finding different heterocyclic systems for developing the
various functional liquid crystalline materials, herein we report two new series of maleimide
based LCs carrying trialkoxy phenyl rings. The two different taper and wedge-shaped
amphiphiles, exhibiting supramolecular LC behaviour have been designed, synthesized and
characterized systematically to understand the structure-property relationship in view of their
potential application in electronic devices (Figure 1).
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Figure 1. Newly designed amphiphilic LCs
2. Results and discussion
2.1 Synthesis and characterization
The synthetic route for the preparation of newly designed maleimide derivatives is
described in Scheme 2. The required key-precursors, i.e. trialkoxy benzyl bromides, 5b-c [46]
and trialkoxy benzaldehydes, 6a-c [47] were synthesized by adopting well-documented
procedures with optimized conditions (Scheme 1). Synthesis of first series of targeted
compounds 10b-c involves a simple O-alkylation of 4-hydroxybenzaldehyde (7) with 5b-c,
followed by the Wittig condensation of aldehydes 8b-c with maleimide derivative 9, which
was prepared by the Michael addition reaction of triphenylphosphine to maleimide. Further,
trialkoxybenzylidenyl maleimides 11a-c were obtained by using Wittig’s protocol. The
reaction, under the reflux condition in methanol, yielded the desired product with an (E)-
configuration exclusively (yield of 40-60 %). The structures of the newly synthesized
compounds were determined by FTIR, 1H-NMR, 13C-NMR spectral and elemental analyses.
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Scheme 1. Synthesis of key-intermediates. Reagents and conditions: (a) Cat. H2SO4, EtOH,
reflux, overnight, 89 %; (b) n-alkylbromide, K2CO3, KI, DMF, 80 ℃, 12 hrs, 71-85 %; (c)
LiAH4, anhydrous THF, 0 ℃, 15 hrs, 86-90 %; (d) PBr3, dry DCM, 4 hrs, 70-85 %; (e) PCC,
dry DCM, 3 hrs, 63-86 %
Scheme 2. Synthesis of maleimide derivatives. Reagents and conditions: (f) K2CO3, DMF, 70
℃, overnight, 78-81 %; (g) CH3OH, reflux, 10 hrs, 45-40 %; (h) CH3OH, reflux, 10 hrs, 51-60
%
2.2 Experimental methods
General procedure for the synthesis of precursor aldehydes, 8b-c
A mixture of 4-hydroxybenzaldehyde (7) (1 equiv.), trioctyloxybenzyl bromide (5a)
(1.1 equiv.) and anhydrous K2CO3 (2 equiv.) in 10 volume of dry DMF was heated to 70 ℃
under an argon atmosphere for overnight. Then, the mixture was poured into ice cooled water
and the obtained precipitate was filtered. The crude was recrystallized from cold methanol to
get the pure product.
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General procedure for the synthesis of target compounds 10b-c and 11a-b
An equimolar mixture of aldehyde (1 equiv.) and triphenylphosphoranylidenyl
succinimide (9) (1 equiv.) in 10 volume of methanol was reflexed for 10 hrs. After completion
of the reaction, the mixture was allowed to cool at room temperature. The obtained white
precipitate was filtered and washed with methanol. Then, the crude was purified by silica gel
column chromatography (100-200 mesh size) eluting with a mixture of ethyl acetate and pet-
ether (60-70 ℃) to yield final compounds.
2.3 Self-assembling properties
The self-assembling behaviour of newly synthesized compounds was systematically
studied by using DSC, POM and XRD analytical techniques. The thermal properties were
investigated by DSC scan at the rate of 5 ℃/min under a nitrogen atmosphere. The compound,
10b bearing triple octyloxy chains showed two distinct crystalline states, which clear at 100.21
℃on heating. In cooling scan, a trivial transition peak was observed at 89.69 ℃with an enthalpy
of -2.0 kJmol-1 which could be assigned to an I-LC transition and on further cooling, the
compound has crystallized. In the DSC scan of longer chain analogue, i.e. 10c, two
endothermic peaks corresponding to Cr-Cr transition and a small LC-I transition (100.34 ℃)
were observed. The mesophase was reappeared at 99.14 ℃ (-1.39 kJmol-1) on cooling cycle
and no sign of crystallization was noticed till room temperature.
The observed thermotropic transitions were further confirmed by polarized optical
microscope and they were identified to be quite consistent with DSC results. As shown in
Figure 2, both the compounds displayed birefringent pseudo focal-conic texture on slow
cooling from their respective isotropic phase, which are commonly encountered textures for
smectic A phases of calamitic and bent-core LC systems [25,48,49].
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Figure 2. DSC thermograms of (a) 10b and (b) 10c recorded at a rate of 5 ℃/min under a
nitrogen atmosphere (red and blue traces represent heating and cooling scans, respectively)
Figure 3. Microphotographs of 10b captured on slow cooling at (a) 85 ℃and (b) 77 ℃;
textures of 10c obtained at (c) 98 ℃and (d) 25 ℃
Table 1. a Thermal transitions and associated enthalpy changes
Compd.
Phase sequence
Heating
Cooling
I 89.69 (-1.20) Sm A b 71.29 (-16.60)
Cr
10b
10c
Cr1 56.67 (4.97) Cr2 100.21 (45.24) I
Cr1 75.83 (53.90) Cr2 84.74 (27.03) Sm I 99.14 (-1.39) Sm A c
A 100.34 (1.44) I
11a
11b
Cr1 57.37 (22.78) Cr2 124.25 (29.89) I I 115.39 (-29.42) Cr2
b
Cr1 63.22 (5.62) Cr2 124.16 (32.71) I
I 122.75 (-3.65) Colh 112.76 (-28.28)
Cr2
11c
Cr1 33.38 (14.84) Cr2 118.42 (23.52) I I 113.27 (-23.33) Cr2 30.60 (-16.67)
Cr1
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a
Peak temperatures/℃ (enthalpies/kJmol-1) obtained by endothermic and exothermic
b
c
cycles at the rate of 5 ℃/min; monotropic mesophase; Crystallization transition was
not noticed till RT; Cr, Cr1 and Cr2, are different crystalline states; SmA = Smectic A
mesophase; Colh = columnar hexagonal mesophase; I = isotropic liquid state
In order to get further insight into the molecular self-assembling in the smectic phase
of taper-shaped amphiphilic systems, we recorded XRD spectra of 10c at different temperatures
throughout its LC phase range. The obtained spectra are depicted in Figure 4 and the relevant
data are summarized in Table 2. At all the four temperatures, the XRD profile has evidently
two common features: a sharp and intense peak at low angles and a diffuse broad one at wider
angles. These features are typical of a layered smectic phase, a fact corroborated by POM
textures. For further description we consider the profile at the lowest temperature, i.e. at T =
55 ℃, shown in Figure 4, as the additional features become more clear at this temperature.
Three more peaks, one at low angles, sharp albeit very weak, a diffuse one at low angles and
another diffuse peak at large angles, again weak. The identification of second reflection in the
lower angle is easy since its spacing is exactly half of that of the fundamental (first reflection).
Thus, it is the second harmonic, which is barely seen at T = 95 ℃, but picks up intensity as the
temperature is lowered. The presence of another diffuse peak at the spacing ~16.5 Å is
somewhat intriguing. In the light of the fact that, this value matches the longitudinal dimension
of the core portion of the molecules, we propose that, the core regions of the neighbouring
molecules completely overlap and the phasmidic chains stick out as the two termini of this
dimer pair (Figure 5). Thus, the thickness of this partially interdigitated layer would be the
dimension of the core plus the twice the length of terminal chain. With the length of 12-carbons
being ~14 Å, the determined dimer length of 45 Å matches with the experimentally obtained
layer thickness.
Since the bifurcation between the core and the hydrocarbon chain is better defined in
the phasmidic arrangement, the overlapped core region can be considered to be a sub-layer,
giving rise to its own X-ray diffraction. This results in the peak at ~16.5 Å, which is the
dimension of the core. However, unlike the layer interfaces determined by the extreme ends of
the dimer, there is enough liberation possible for the overlapped region of the core resulting in
a spread of the diffraction pattern. The diffuse peak in higher angle region corresponding to
~3.42 Å, is commonly seen in columnar phases of discotic molecules. Except in systems
wherein the terminal chains have different character, such as hydrocarbon/fluorocarbon or
hydrocarbon/cholesteryl [50,51], it is not seen in smectic phases. Extending the argument made
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for the case of discotic systems, we propose that, peak spacing 3.42 Å arises due to the
correlation between the core-parts of the neighbouring molecules. As expected, this correlation
improves on lowering the temperature, and from the peak width, it is seen that, about 6
molecules are correlated. As observed commonly, increasing the layer spacing on lowering the
temperature, owing to stretching of the chains (moving towards all-trans). However the
consequent negative thermal expansion (d/dT) layer spacing of ~ 0.09 Å/℃is quite large than
usual smectics [52].
Table 2. XRD indexation of mesogen 10c
Compound dobs [Å] at 25 ℃ dobs [Å] at 55 ℃ dobs [Å] at 75 ℃ dobs [Å] at 95 ℃
45.48
24.81
16.51
4.35
43.32
21.59
16.37
4.41
40.66
20.32
16.08
4.43
39.11
19.64
16.03
4.46
10c
3.38
3.42
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Figure 4. XRD pattern obtained for 10c at various temperatures on cooling from I phase
Figure 5. Schematic illustration of SmA mesophase structure of 10c
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On reducing the hydrophobic part in the design, we obtained a wedge configured
molecules (11a-c), wherein three alkoxy groups attached to phenyl ring connected with polar
maleimide ring. DSC traces of all the three compounds with varied aliphatic chain lengths
reveal that, they exhibit polymorphic nature as a function of temperature (Figures S1-S2).
Among them, the midst member, i.e. 11b has shown a monotropic mesophase behaviour as
apparent in its first cooling cycle of DSC scan, which was further confirmed by immediate
reheating and cooling scans as shown in Figure 6b. Under POM, slow cooling from its I phase
brings about a bright mosaic pattern which is a typical indication of presence of columnar
organization (Figures 6c-d) [53].
Figure 6. DSC traces of 11b: (a) first heating and cooling scan (red and blue traces represent
heating and cooling scans, respectively) and (b) second cycle; POM textures of 11b captured
at (c) at 120 ℃and (d) 111 ℃
To understand the structure of columnar mesophase of 11b, we carefully recorded the
XRD spectra within its short thermal range on slow cooling from I phase as depicted in Figure
7. For instance, the spectrum obtained at 125 ℃ displayed three reflections at the Bragg’s d-
spacings 27.57, 16.01 and 13.44 Å with the reciprocal ratio of 1: √3: 2 in the lower angle region.
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In addition, a broad halo appears at wide angle region ascribed to liquid-like motion of the
alkyl chains. The lower angle reflection could be index to (10), (11), (20) planes of two
dimensional hexagonal geometry with the lattice parameter a = 31.83 Å. Further, the number
of molecules (Z) occupied in each hexagonal unit cell was estimated using the XRD results,
assuming the density value for these molecules is 1 gcm-3 [54]. For this calculation, a much
needed parameter, i.e. the height or thickness of the columnar stratum is unknown. This is
because of the absence of a wide angle reflection responsible for intracolumnar spacing as a
consequence of the lack of long-range periodicity along the columnar axis. Nevertheless, we
deduced the value of Z by using the alkyl halo (ha), and it was found to be three. The results
indicated that, each slice of the columns has composed of three molecules and this value is
consistent with the calculated large lattice area, S = 877.69 Å2 and volume, V = 3923.29 Å3.
Figure 7. XRD patterns obtained for Colh phase for 11b at (a) 123 ℃and (b) 125 ℃
As mentioned earlier, it is clear that, each columnar stratum of Colh phase of 11b,
comprises three wedge-shaped molecules. Since tip of these molecules is bearing a polar
maleimide group, capable of forming H-bonding, they can assemble comfortably to form a
supramolecular disk by means of H-bonded interaction. The resulting H-bond driven
macrocyclic structure is depicted in Figure 8. These superstructures form a long-range
columnar assemblies in the hexagonal fashion. On the basis of XRD results, we proposed a
schematic model for the self-assembly of wedge-shaped maleimide 11b, as illustrated in Figure
9.
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Figure 8. H-bond driven supramolecular disc of 11b
Table 3. XRD indexation of mesogen 11b
Compd. Phase
dobs [Å]
dcal [Å]
Miller
Indices
(hk)
Lattice parameter a [Å],
Lattice area S [Å2],
Molecular volume V
[Å3]
(T/℃)
Colh (123) 27.61
27.61
15.76
(10)
(20)
a = 31.88
S = 880.24
10b
13.83
4.56 (ha)
V = 4013.9
Z = 3.2
Colh (125) 27.57
26.034
15.91
13.78
(10)
(11)
(20)
a = 31.83
S = 877.69
V = 3923.29
Z = 3.1
16.01
13.44
4.47 (ha)
dobs, observed spacings; dcal, calculated spacings from the lattice parameter a for the
hexagonal lattice; ha, alkyl chains correlation peak value; Z, number of molecules per
columnar unit
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Figure 9. Schematic illustration of columnar hexagonal self-assembly of 11b
3. Conclusion
In summary, two new series of amphiphilic derivatives with maleimide head group
were designed and synthesized. In the new designs, the aromatic core-length and peripheral
chain length were rationally varied. Their thermotropic mesomorphism was investigated using
standard techniques. It was observed that, the length of hydrophobic segment has significant
influence on their LC behaviour. Further, two molecules with tapered-shape have been shown
to exhibit a room temperature mesophase with a notable smectic order, irrespective of length
of alkyl chains. While, the only midst alkyl member of wedge-shaped derivatives demonstrated
an imperative supramolecular columnar phase with hexagonal lattice geometry. From the XRD
analysis, we concluded that totally three wedge-shaped molecules occupied in each slice of
Colh of 11b, which may be attributed to the possible close-type intermolecular H-bonding
through polar maleimide functionality. Conclusively, the showcased results would be the basis
for designing new unconventional LC materials for their future applications.
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Acknowledgement
The authors are thankful to NITK, Surathkal and SCM group, RRI, Bangalore, India, for
providing necessary laboratory facilities. The authors are also grateful to Dr. Vijayaraghavan
for recording the NMR spectra, Mrs. K. N. Vasudha for their technical support. VK is grateful
to NITK for providing fellowship.
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Graphical abstract
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Highlights
 New series of tapered and wedge shaped molecules have been designed and synthesized
 Tapered-shaped molecules exhibits stable enantiotropic smectic mesomorphism
 Wedge-structures shown to possess supramolecular columnar mesophase by H-bonding
 The present work mainly emphasizes on the structure-LC property relationship
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