Mesomorphic triphenylene polyanion-surfactant complexes

Article abstract of DOI:10.1016/j.molliq.2020.114770

Triphenylene derivatives have been reported as one of the prominent members of discotic liquid crystalline systems. This report gives an account of the mesomorphic behavior of complexes involving triphenylene and double tail surfactants synthesized through the ionic self-assembly route. The lamellar structure of these complexes has been determined using X-ray diffraction, differential scanning calorimetry and polarizing optical microscopy. The amphotropic nature of these complexes has also been studied.

Full text of DOI:10.1016/j.molliq.2020.114770

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Mesomorphic triphenylene polyanion-surfactant complexes
K. Swamynathan, Sreeja Sasidharan, V.A. Raghunathan, Sandeep
Kumar
PII:
S0167-7322(20)37012-4
DOI:
https://doi.org/10.1016/j.molliq.2020.114770
MOLLIQ 114770
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
24 June 2020
20 October 2020
10 November 2020
Please cite this article as: K. Swamynathan, S. Sasidharan, V.A. Raghunathan, et al.,
Mesomorphic triphenylene polyanion-surfactant complexes, Journal of Molecular Liquids
(2020), https://doi.org/10.1016/j.molliq.2020.114770
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Mesomorphic Triphenylene Polyanion-Surfactant Complexes
K. Swamynathan1; Sreeja Sasidharan1; V.A.Raghunathan1; Sandeep Kumar1, 2*
Address: 1. Raman Research Institute, Soft Condensed Matter, C. V. Raman Avenue,
Bangalore – 560080, India.
2. Department of Chemistry, Nitte Meenakshi Instittue of Technology (NMIT),
Yelahanka, Bangalore, India.
E-mail: skumar@rri.res.in; Fax: +91 80 23610492; Tel: +91 80 23610122.
Abstract:
Triphenylene derivatives have been reported as one of the prominent members of discotic
liquid crystalline systems. This report gives an account of the mesomorphic behavior of
complexes involving triphenylene and double tail surfactants synthesized through the ionic
self-assembly route. The lamellar structure of these complexes has been determined using
X-ray diffraction, differential scanning calorimetry and polarizing optical microscopy. The
amphotropic nature of these complexes has also been studied.

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1. Introduction:
Soft materials are one of the prominent categories of functional materials, thanks to their
mechanical response and sensitivity towards small perturbations [1]. Soft materials are
prevalent in the form of blood, cells, and macromolecules, etc. [2]. Many soft matter systems
are built up from molecular building blocks by self-assembly [3]. Liquid crystals (LCs)
constitute one of the major classes of soft materials where anisotropy in structure and
intermolecular interactions form the basis of their organization [4,5]. Non-conventional liquid
crystals have been reported in the literature involving modifications in the shapes of molecules
[6]. Noncovalent interactions known for their specificity can induce interesting properties and
functions in these systems. Metal containing LCs, called metallomesogens, show mesomorphic
behavior due to coordination interactions with metals [7]. Charge transfer and hydrogen
bonding interactions have been employed to induce mesomorphic behavior in materials [8]. The
influence of hydrogen bonding in the self-organization of materials is widely known in
biological systems performing diverse functions [9]. Ionic interactions have been one of the
least explored strategies in the construction of novel self-organized materials. The formation of
supramolecular complexes using ionic interactions called ionic self-assembly (ISA) has been
recently reported to create nanostructured materials [10]. The ionic self-assembly has been
found to follow a cooperative mechanism. These ionic self-assembled materials have been
shown to have liquid crystalline order [11]. Cooperative assembly of surfactants with opposite
charged tectons opens up a facile noncovalent route for formation of various functional
nanomaterials [12]. There are reports of ionic self-assembled complexes of disc shaped

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perylene systems which exhibit amphotropic properties [13]. The ISA technique has been
explored recently for inducing different stimuli responsive character in assemblies [14]. The
properties of ISA complexes can be fine-tuned by an appropriate choice of opposite charged
tectons [15,16]. ISA approach has been used to form dye-polyelectrolyte complexes leading to
ordered fibrillar structures [17]. Polyoxometallo systems have been used in ISA strategy to form
complexes with surfactants, which exhibit amphotropic behavior [18]. Further, it has been
reported that polyoxometallo surfactant complexes show photoluminescence, electro-optical
switching, and conducting properties[18].
ISA is useful in synthesizing hybrid materials involving both organic and inorganic tectons
[16,19]. Azobenzenes have been widely known for their photoisomerization properties [20].
Azobenzene based photosensitive scaffolds have been used as tectons to form ISA complexes.
The resulting complexes were found to be glassy in nature with good optical anisotropy [21].
Biological tectons such as nucleobases and polypeptides have been found to show lamellar
phases on complexation with surfactants [22]. ISA strategy was recently used for making
modification of electrodes for sensing applications [23].
Discotics constitute one of the potential functional materials for usage in electronic
devices due to their excellent optical and electrical properties [24]. The large anisotropies in
their electronic conductivity and hole mobility have made them very popular in the field of
material chemistry [25,26]. Triphenylene has been one of the most widely reported motifs for
liquid crystalline [27] and self-assembling behaviors [28]. Monomeric as well as oligomeric
triphenylene motifs are known to exhibit mesomorphic behavior including nematic and
columnar phases[29].

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Triphenylene has been one of the most explored discotic systems thanks to its simpler
synthesis and potential for mesomorphism [30], However, there is still a lot of room in
designing noncovalent routes for induction of mesomorphic behavior. Disc-shaped molecules,
such as benzene, perylene, tricycloquinazoline, and anthraquinone, have been reported to form
complexes with surfactants via ISA that have been found to exhibit amphotropic behavior [31–
34]. Here we report the preparation of novel triphenylene-surfactant complexes using ISA.
Polarizing optical microscopy (POM) and X-ray diffraction measurements show that these
complexes have a lamellar structure. Further, the amphotropic nature of these complexes has
also been studied.
2. Experimental:
2.1. Equipments and measurements
The chemical structures of the compounds were determined using a combination of
spectroscopic techniques. Infrared spectra were recorded using a Shimadzu FTIR–8400
1
spectrophotometer. H NMR spectra were recorded on a Bruker AMX 400 spectrometer using
tetramethylsilane (TMS) as an internal standard. Elemental analysis was carried out for all the
intermediate and final compounds using a Carlo-Erba 1106 elemental analyzer. A Perkin-
Elmer, Pyris 1 differential scanning calorimeter (DSC) was used to determine the transition
temperatures and the associated enthalpy values for all the mesogenic compounds. In all cases,
heating and cooling rates were 5 ºC/min. The mesophase textures were observed using an
Olympus BX50 polarizing optical microscope equipped with a Mettler FP82HT heating stage
and a Mettler FP90 central processor. X-ray diffraction (XRD) measurements on powder

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samples were carried out using a PANalytical Empyrean diffractometer using Cu-Kα (λ = 1.54
Å) beam. The samples were held in Lindemann capillaries (0.7mm diameter)
2.2. Synthesis:
Synthesis of hexamethoxy triphenylene 1 [35]:
To a suspension of FeCl3 (36.96 g, 227.85 mmol, 9.3 equiv) in CH2Cl2 (200 mL) and H2SO4 (0.5
mL), was added dropwise a solution of veratrole (9.4 mL, 73.5 mmol, 3 equiv) in CH2Cl2 (100
mL). The reaction mixture was stirred at room temperature for 3 h and quenched by slow
addition of methanol (300 mL) at 0 °C. The mixture was further stirred for 0.5 h and the obtained
precipitate was filtered, washed with methanol, and dried in vacuum. The grey solid (7.75 g, 78
%) was used in the next step without further purification. 1H NMR (500 MHz, CDCl3) δ 7.78 (s,
6H), 4.13 (s, 18H).
Synthesis of hexahydroxy triphenylene 2 [35]:
A solution of 1 (3.00 g, 7.2 mmol, 1 equiv) in a mixture of glacial CH3COOH (100 mL) and 48
wt% aqueous HBr (100 mL) was bubbled with argon for 0.5 h. The grey suspension was stirred
at reflux under argon for 12 h. The mixture was cooled down to room temperature and then
filtered, washed with cold water and dried. The grey solid was recrystallized from
CH3COOH/H2O (3:2, 200 mL) by treating with activated carbon, affording gray crystals (1.12
1
g, 48 %), which were stored under argon. H NMR (500 MHz, DMSO-d6) δ 9.31 (bs, 6H,),
7.59 (s, 6H).
Synthesis of triphenylene hexaacetate 3 [36]:

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The solution of hexahydroxy triphenylene (2g) in acetic anhydride (50ml) and 5 drops of
H2SO4 was refluxed overnight. The hot solution was poured in ice and the precipitate was
filtered under vacuum. The precipitate was washed with a large amount of water until the
washings were neutral and dried. The dried precipitate was dissolved in chloroform and
reprecipitated using methanol which was used for further reaction. 1H NMR (500 MHz, CDCl3)
δ 8.21 (s, 6H), 2.4 (s, 18H).
General procedure for preparation of ISA complexes 4a-d [33,34]:
The triphenylene hexaacetate (1eq) was dispersed in 10N NaOH solution. The dispersion was
stirred for 15 minutes. The solution of surfactant (appropriate equivalents as per no of acetates
in the core) in 10N NaOH solution was added to the above prepared solution. The mixture was
stirred for 48 hrs at room temperature. After 48hrs there was precipitate floating in the medium.
The mixture was centrifuged in order to remove the precipitate (as they were sticky cannot be
filtered through filter paper). The precipitate collected after centrifugation was subjected to
water washing multiple times in order to remove the excess base present in the precipitate. The
obtained precipitate was dried under high vacuum. The precipitate was dissolved in a suitable
organic solvent (chloroform). The chloroform solution of complexes was washed excessively
with water, dried, and used for experiments.

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3. Results and discussion:
3.1 Characterization of complexes:
The synthesis of the triphenylene based ISA complex is depicted in Scheme 1.
The surfactants used for complexation was dialkyl dimethyl ammonium bromides (Figure1).
The synthesized complexes were characterized using IR, NMR, and elemental analysis. The IR
spectroscopy was used to confirm the complexation as the process involves cleavage of ester
linkage from acetates and complexing with cationic surfactants. The IR spectrum of the
complexes shows the disappearance of the C=O vibration (1715cm-1) of esters which was
present in the hexaacetate of triphenylene that confirms the complete formation of the complex
(Figure 2). The complex formation was further validated from NMR spectra (Figure 3).
(Figure 1)
(Scheme 1)
(Figure 2)
(Figure 3)
The stoichiometry of the complexes was calculated from elemental analysis. The
amount of nitrogen matches with expected values within acceptable errors as reported
previously in similar polyelectrolyte surfactant complexes[37][38].

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3.2 Thermotropic behaviour
Mesomorphic properties of the complexes were studied using POM, DSC, and SAXS. POM
images show focal conic textures typical of smectic phases (Figure 4). The mesomorphic
behavior was confirmed through calorimetry studies. The DSC thermograms suggest that all the
complexes show a smectic phase in addition to different crystals to crystal transitions. The
enthalpy values are higher than typical values observed in case of crystal to mesophase
transitions probably because of ionic complexes, similar values are being reported in
metallomesogens[39]
(Figure 4)
The DSC thermograms of the complexes 4a-d are given in Figure 5. The temperature and
corresponding enthalpy changes are given in Table 1. The smectic nature of the mesophase was
validated via X-ray diffraction studies. The diffractograms show peaks whose d spacings are in
the ratio1:1/2:1/3 (Figure 6). The d-spacings and corresponding indexation are given in Table 2.
A similar kind of smectic behavior was reported for other discotic systems-surfactant complexes
in the literature[31,33,34].
(Figure 5)
(Table 1)
(Figure 6)
(Table 2)

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3.3 Lyotropic behavior
The ionic self-assembled complexes were analyzed for lyotropic behavior in the presence of
water. The isolated complex was mixed with water at appropriate weight ratios and checked for
liquid crystallinity. The diffraction pattern of as- prepared water -complex system was found to
be same as that of powdered complex which suggests that water molecules cannot swell the
material as required for lyotropic systems which can be accounted as follows, since the complex
is already in a charge neutralized state, the added water cannot swell the complex. In order to
explore their lyotropic nature, Triphenylene sodium salt and surfactant were taken in a 1:6
molar ratio and mixed with water. After equilibration, for 48 hours the samples were checked
for mesomorphic behavior. The samples were found to show lamellar structure which was
confirmed by POM, XRD & CRYO SEM (Figure 7). POM images of the samples show the
oily streak texture which is typical of the lamellar phase. The XRD data of the water –complex
mixture shows reflections which are in ratio 1: 1/2: 1/3. The temperature-dependent diffraction
pattern has been given in the figure. The spacing value suggests that the double tail surfactants
form bilayer where the charged discs complexes with them in the lamellar medium to get
charge neutralized state.
(Figure 6)
The complexes previously reported based on hexa anionic anthraquinones and double tail
surfactants were reported to form a smectic phase with similar spacings [34]. The diffractograms
of the thermotropic and lyotropic systems suggest that there is not much difference in the
spacings. This shows that there is only a slight swelling of bilayers in lyotropic systems
(approximately one layer of water molecules). The arrangement of molecules remains the same

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in both, in the presence as well as the absence of water. By considering all our results we propose
the model for this system as given in Figure 9. A similar kind of arrangement was proposed for
anthraquinone-surfactant complexes. The electron density profile was constructed based on X-
ray diffraction results for this system, the observations and conclusions arrived based on electron
density profiles have been summarized below.
3.4: Electron density profiles
Since the electron density of the lamellar phase is a periodic function, it can be expanded in a
Fourier series [40]
ρ (z) = ∑h αh Fh Cos(hq0z),
Where qo = 2π/d, d being the lamellar periodicity, h is the order of the reflection, Fh and αh are
the magnitude and phase of the reflection. For a centrosymmetric motif, αh takes the values +1
or -1, which reduces the possible combination of phases to 2n for n number of reflections.
The electron density obtained using the above equation with the phase combination (- + -) for
the three observed reflections is shown in Figure 8. It is very similar to the electron densities of
lipid and surfactant bilayers, reported in the literature [41,42]. The minimum at z = 0
corresponds to the bilayer mid-plane and the peaks correspond to the thin polar region
containing the water and the ionic species. A schematic of the structure suggested by the
electron density model is given in figure 9.
(Figure 8)
(Figure 9)

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4. Conclusions:
We report the synthesis of triphenylene-surfactant complexes through ionic self-assembly. The
complexes were found to be mesomorphic. The smectic nature of the complexes was confirmed
through POM and XRD. The complexes are further explored for lyotropic behavior in water.
The water-swelled systems also show lamellar structures. We propose a model for the structure
of the complexes based on the electron density profiles calculated from the diffraction data.
5. Acknowledgments;
The authors thank Prof. V. Lakshminarayanan for his useful suggestions, Mrs. Vasudha, K,
N, and Dr. Srinivasa. H.T, Dr.Vijayaraghavan for technical support and Dr. A. JayaKumar &
Dr. Madhukar. S. Ms. Meera Thomas, Mr. Marichandran, Mr. Irla Sivakumar, for their valuable
help in preparing the manuscript.

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Figure 1: Chemical structure of surfactants used
Scheme 1: Synthesis triphenylene ionic self-assembled complexes: (i) FeCl3; DCM, 0 °C to
RT, 3 hrs; (ii) HBr, ACOH reflux 24hrs; (iii) Ac2O, H2SO4 (catalytic), reflux overnight; (iv)
10 N NaOH, R2N (CH3)2Br, RT, 24hrs (R=C12H25,C14H29 C16H33; C18H37)

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Figure 2: IR spectra of compounds 3 and 4

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Figure 3: 1H- NMR of 4c in DMSO-d6

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Figure 4: POM images of TP-IPA complexes: 4a at 95°C; 4b at 95 °C; 4c at 120°C; 4d
at90°C

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Figure 5: DSC thermogram of complexes 4a-d

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Table 1. Phase transition temperatures (℃) and the corresponding energy changes (Jg-1)
Compd.
Phase sequence
Heating
Cooling
Cr1 20.69 (46.82) Sm 157.47 (2.26)
I
I 153.86(--2.24)Sm
4a
4b
4c
Cr1 40.66 (51.40) Cr2 56.32(8.04)
Sm 119.65 (4.82) I
I 107.98 (-5.55) Sm 53.12
(18.45)Cr
Cr2 74.8(24.76) Sm 149.37(4.01)I
I 136.16 (--5.83)Sm 54.63(-
22.40) Cr
Cr 83.45 (67.88) Sm 143.69 ( 1.07)
I
I 120.93(-2.01) Sm
9.58)Cr
62.15 (-
4d

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Table2: d spacings of complexes 4a-d
COMPOUND
4a
Phase (temperature
in °C)
d
Index
(Å)
27.31
4.45
10
60
Smectic
(120°C)
31.37
15.77
4.49
10
20
70
10
20
30
70
10
20
30
80
4b
4c
Smectic
(95°C)
32.27
16.16
10.91
4.53
Smectic
(80°C)
34.47
17.20
11.49
4.26
4d
Smectic
(95°C)

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105
104
103
102
105
104
103
102
4d
4c
0
10
20
30
0
10
20
30
2
2
Figure 6: X-ray diffractograms of IPA complexes: 4a (95°C); 4b (90°C); 4c (120°C); 4d
(90°C)

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Figure 7: SAXS diffraction, POM & Cryo SEM images of 4a-water (70 wt %) system

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Figure 8: Electron density profile for (-+-) combination for 4b-water (70 wt %) system
Figure 9: Proposed model for organization of polyanion-surfactant complexes.

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