TNF Induced Switching of Columnar Rectangular to Hexagonal Assemblies in a New Class of Triphenylene-Based Room Temperature Discotic Liquid Crystals

Article abstract of DOI:10.1021/acs.jpcb.7b06737

A straightforward synthesis of triphenylene-based oligomeric systems that self-organize into room temperature columnar structures is presented. The compounds with longer spacer length (m = 10 and 12) exhibit columnar rectangular (Col_r) mesophase whereas the compound with m = 8 exists in glassy Col_r state. Interestingly, the Col_r self-assembly of these compounds switches to columnar hexagonal (Col_h) on doping the compounds with 2,4,7-trinitrofluorenone (TNF). For the dopant concentration of 1:1 with respect to native compound, an intermediate transition state between Col_r and Col_h phase was observed which completely transformed into the hexagonal phase on increasing the concentration to 1:2 (compound: TNF) and afterward. Both the Col_r and Col_h self-assemblies have been well resolved by detailed X-ray analysis. These kind of oligomeric compounds generally possess a combination of desirable alignment properties analogous to monomeric compounds and long-lived glassy states similar to that of polymeric mesogens. In addition, charge hopping behavior is expected to increase in these compounds due to donor-acceptor interactions. Overall, these compounds can find possible potential applications in semiconductor devices.

Full text of DOI:10.1021/acs.jpcb.7b06737

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Article
TNF Induced Switching of Columnar Rectangular to
Hexagonal Assemblies in a New Class of Triphenylene-
Based Room Temperature Discotic Liquid Crystals
Monika Gupta, Santosh Prasad Gupta, and Santanu Kumar Pal
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b06737 • Publication Date (Web): 17 Aug 2017
Downloaded from http://pubs.acs.org on August 19, 2017
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TNF Induced Switching of Columnar Rectangular
to Hexagonal Assemblies in a New Class of
TriphenyleneꢀBased Room Temperature Discotic
Liquid Crystals
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Monika Gupta, Santosh Prasad Gupta, and Santanu Kumar Pal*
Department of Chemical Sciences, Indian Institute of Science Education and Research
(IISER) Mohali, Sectorꢀ81, Knowledge City, Manauliꢀ140306, India
ABSTRACT. A straightforward synthesis of triphenyleneꢀbased oligomeric systems that
selfꢀorganize into room temperature columnar structures is presented. The compounds with
longer spacer length (m = 10 and 12) exhibit columnar rectangular (Colr) mesophase whereas
the compound with m = 8 exists in glassy Colr state. Interestingly, the Colr selfꢀassembly of
these compounds switches to columnar hexagonal (Colh) on doping the compounds with 2, 4,
7ꢀtrinitrofluorenone (TNF). For the dopant concentration of 1:1 with respect to native
compound, an intermediate transition state between Colr and Colh phase was observed which
completely transformed into the hexagonal phase on increasing the concentration to 1:2
(compound: TNF) and afterwards. Both the Colr and Colh selfꢀassemblies have been well
resolved by detailed Xꢀray analysis. These kind of oligomeric compounds generally possess a
combination of desirable alignment properties analogous to monomeric compounds and longꢀ
lived glassy states similar to that of polymeric mesogens. In addition, charge hopping
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behaviour is expected to increase in these compounds due to donorꢀacceptor interactions.
Overall, these compounds can find possible potential applications in semiconductor devices.
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1. INTRODUCTION
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Recently, there has been substantial interest in the field of nonꢀconventional low molar mass
discotic liquid crystals (LCs) i.e. dimers and oligomers because of their fascinating
mesomorphic properties due to restricted molecular motions.1ꢀ4 These discotic LC oligomers
serve as ideal illustrative compounds for polymers due to ease of purification &
characterisation and remarkable similarities in their transitional behaviour.5 The physical
properties of these discotic LC oligomers are also notably different from their constituent
units. They are more prone to form columnar glasses which are mechanically stiff and have
potential applications as the anisotropic properties of mesophase and macroscopic alignment
of domains is vitrified and the columnar ordering is preserved in these systems.6ꢀ8 Therefore,
oligomeric discogens possess the combination of desirable alignment properties of monomers
with the longꢀlived glassy state of the polymers.
Till date, triphenylene (TP) is the most widely studied scaffold in the family of discotic
LCs and thus its derivatives are ideal workhorses for analysing the properties of discotic LC
oligomers and polymers as TP is easy to obtain in both mono as well as bifunctional forms.9ꢀ
11 Triphenylene oligomers have drawn particular attention as they show improved optical and
electronic properties.12ꢀ20 They are generally constructed by grafting triphenylenes onto
various functional frameworks. Most workers have concentrated on linear & starꢀshaped
oligomers containing three mesogenic moieties. However, fewer examples of starꢀshaped
oligomers containing four mesogenic moieties connected to a common nucleus have been
reported so far. The reported examples mainly include the triphenylene tetramers with
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siloxane core, spirocarbon spacer, triazole bridges, rigid aromatic Schiffꢀbases and hydrogenꢀ
bonding spacers.21ꢀ26 All these tetramers displayed excellent LC properties with lower phase
transition temperatures.
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In this report, we have designed new starꢀshaped oligomeric mesogens in which
azobenzene was chosen as rigid core attached to which are four TP units via varying flexible
alkyl spacers. LCs comprising photoꢀresponsive moieties such as azobenzene have been
established as promising materials for applications in photomechanics, instant displays,
reversible holographic storage and digital storage due to their high resolution and
sensitivity.27ꢀ29 Therefore, azobenzene will not only act as a linker for the triphenylene cores
but can also impart interesting properties to these oligomers due to its photoresponsive
behaviour. The oligomers synthesised in this study were found to selfꢀassemble in a columnar
rectangular fashion at roomꢀtemperature. We have further introduced 2, 4, 7ꢀ
trinitrofluorenone (TNF) in the columnar assemblies of these oligomers. TNF which is an
electronꢀacceptor moiety can form charge transfer (CT) interactions which can induce,
stabilize as well as extend the mesomorphic range of the native compound.30ꢀ33 Not only this,
it can also improve the charge carrier mobilities by facilitating charge transfer and thus
preventing recombination. In this process, intermediate excited chargeꢀ transfer (CT) states
are formed where the holes are present on the donor and electron at the acceptor molecule.
Thus, on applying electric field the electrical conductivity rises due to an increase in charge
migration.34ꢀ37 Oligomeric mesogens generally possess higher values of charge carrier
mobilities due to larger delocalisation of πꢀelectrons and thus a lower band gap. Further,
introducing TNF in these kind of oligomeric systems can lead to an increment in the charge
carrier mobilities. Therefore, the CT complexes of these oligomeric systems containing four
TP connected to an azobenzene core will be interesting from the device point of view.
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Surprisingly, we have found for the first time that columnar rectangular (Colr) selfꢀassembly
of these oligomers at roomꢀtemperature switches to columnar hexagonal (Colh) fashion on
doping the compounds with TNF. The combination of the various properties such as LC
phase at roomꢀtemperature, easy alignment by mechanical shearing, increased charge
hopping expected from the donorꢀacceptor charge transfer complexes and switching from
columnar rectangular to hexagonal phase makes these oligomers very advantageous for new
device applications.
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2. EXPERIMENTAL SECTION
2.1 Materials and Reagents
Chemicals and solvents were all of AR quality and were used without further purification.
Catechol, bromohexane, ferric chloride, 2ꢀbromoꢀ1,3,2ꢀbenzodioxaborole, cesium carbonate,
dibromooctane, dibromodecane, dibromododecane, potassium iodide, 5ꢀnitroisophthalic acid,
sodium hydroxide, potassium hydroxide, dextrose, hydrochloric acid, nꢀbutanone and
tetraoctyl ammonium bromide were all purchased from Sigma–Aldrich (Bangalore, India).
Column chromatographic separations were performed on silica gel (60ꢀ120 & 230ꢀ400
mesh). Thin layer chromatography (TLC) was performed on aluminium sheets preꢀcoated
with silica gel (Merck, Kieselgel 60, F254).
2.2 Instrumental
Structural characterization. Structural characterization of the compound was carried out
through a combination of infrared spectroscopy (Perkin Elmer Spectrum AX3), 1H NMR and
13C NMR (Bruker Biospin Switzerland Avanceꢀiii 400 MHz and 100 MHz spectrometers
respectively) and UVꢀVISꢀNIR spectrophotometer (Agilent Technologies UVꢀvisꢀNIR
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Spectrophotometer). NMR spectra were recorded using deuterated chloroform (CDCl3) as
solvent and tetramethylsilane (TMS) as an internal standard.
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Differential Scanning Calorimetry. DSC measurements were performed on Perkin Elmer
DSC 8000 coupled to a Controlled Liquid Nitrogen Accessory (CLN 2) with a scan rate of 5
°C/min.
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Polarized Optical Microscopy. Textural observations of the mesophase were performed with
Nikon Eclipse LV100POL polarising microscope provided with a Linkam heating stage (LTS
420). All images were captured using a Qꢀimaging camera.
X-ray Diffraction. Xꢀray diffraction (XRD) was carried out on powder samples using Cu Kα
(λ=1.54 Å) radiation from a source (GeniX 3D, Xenocs) operating at 50 kV and 0.6 mA. The
diffraction patterns were collected on a two module Pilatus detector.
2.3 Synthesis of oligomers 7
Synthesis of compounds 2, 3, 4 and 6 has been described in the earlier reports (Scheme 1).38ꢀ
42
For the synthesis of the target compound 7, compound 6 (1 equivalent) was dissolved in
aqueous KOH (1.1 equivalent) solution. To that solution, compound 4 (6 equivalents) was
added followed by the addition of tetraoctylammonium bromide in catalytic amounts. The
reaction mixture was refluxed under vigorous stirring for 5 hours & was then cooled to room
temperature. The compound was extracted with dichloromethane. The organic layer was
washed with brine & dried over anhydrous sodium sulphate. The chloroform was removed by
rotary evaporation and the resulting residue was purified by column chromatography over
silica gel using hexane & ethyl acetate as eluent. The synthesized compounds 7a-c were
characterized by 1H NMR, 13C NMR, IR, UVꢀvis and elemental analysis as shown below:
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Compound 7a
1H NMR (400 MHz, CDCl3, δ in ppm): 8.81 (t, 2H, J = 1.6 Hz), 8.77 (d, 4H, J = 1.6 Hz),
7.84 (s, 24H), 4.42 (t, 8H, J = 4, 8 Hz), 4.24 (m, 48H), 1.92 (m, 56H), 1.52 (m, 152H), 0.95
(m, 60H).
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13C NMR (400 MHz, CDCl3, δ in ppm): 165.24, 152.27, 148.96, 148.95, 148.90, 132.17,
123.60, 123.57, 107.33, 107.25, 69.70, 69.67, 69.61, 65.91, 31.72, 29.54, 29.47, 29.45, 29.35,
28.73, 26.20, 25.97, 25.88, 22.70, 14.10.
FTꢀIR (cmꢀ1): 3090.94, 2929.95, 2858.29, 1726.46, 1616.88, 1514.87, 1468.88, 1435.28,
1388.50, 1310.41, 1261.99, 1234.89, 1170.40, 1104.75, 1042.01, 923.59, 867.74, 837.26,
802.35, 758.78, 726.87, 684.06, 600.61.
UVꢀvis (nm): 251, 262, 270, 280, 309, 346, 444.
Elemental analysis (%): Calculated C 76.27 H 9.44 N 0.74. Found C 76.34 H 9.51 N 1.09.
Compound 7b
1H NMR (400 MHz, CDCl3, δ in ppm): 8.82 (s, 2H), 8.79 (s, 4H), 7.84 (s, 24H), 4.41 (t, 8H, J
= 8 Hz), 4.24 (t, 8H, J = 8 Hz), 1.95 (m, 48H), 1.85 (m, 8H), 1.59 (m, 40H), 1.41 (m, 128H),
0.95 (m, 60 H).
13C NMR (400 MHz, CDCl3, δ in ppm): 165.23, 148.95, 148.92, 123.58, 107.29, 69.69,
31.71, 29.66, 29.58, 29.54, 29.45, 29.37, 28.73, 26.25, 26.00, 25.87, 22.88, 14.10, 14.08.
FTꢀIR (cmꢀ1): 3096.6, 2929.47, 2857.75, 1726.35, 1616.93, 1514.77, 1468.61, 1435.52,
1388.47, 1310.23, 1262.48, 1235.31, 1170.73, 1042.42, 923.77, 867.46, 837.10, 802.68,
758.82, 726.10, 683.92, 600.68.
UVꢀvis (nm): 251, 261, 270, 280, 308, 346, 443.
Elemental analysis (%): Calculated C 76.54 H 9.58 N 0.72. Found C 76.16 H 9.61 N 0.99.
Compound 7c
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1H NMR (400 MHz, CDCl3, δ in ppm): 8.84 (t, 2H, J = 2, 1.2 Hz), 8.80 (d, 4H, J = 1.6 Hz),
7.85 (s, 24H), 4.42 (t, 8H, J = 4, 8 Hz), 4.25 (t, 48H, J = 8 Hz), 1.96 (m, 48H), 1.85 (m, 8H),
1.60 (m, 40H), 1.40 (m, 144H), 0.96 (m, 60H).
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13C NMR (400 MHz, CDCl3, δ in ppm): 165.27, 152.32, 148.95, 148.94, 132.24, 127.90,
123.59, 107.27, 69.68, 65.98, 31.73, 29.74, 29.71, 29.63, 29.54, 29.46, 29.38, 28.73, 26.26,
26.01, 25.89, 22.70, 14.11.
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FTꢀIR (cmꢀ1): 3100.3, 2926.32, 2855.88, 1726.58, 1615.40, 1515.54, 1469.16, 1434.76,
1389.7, 1314.5, 1264.4, 1234.79, 1170.57, 1046.2, 925.23, 869.62, 840.18, 800.93, 761.68,
722.43, 683.18, 604.68.
UVꢀvis (nm): 251, 261, 270, 280, 309, 346, 443.
Elemental analysis (%): Calculated C 76.80 H 9.72 N 0.70. Found C 76.62 H 9.76 N 0.97.
2.4 Preparation of CT complexes. The amount of TNF and oligomer was calculated
according to molar ratios. The calculated amount was dissolved in dichloromethane and both
the solutions were mixed. The resulting solution which was of deep brown colour was then
allowed to evaporate by leaving the solution to stand overnight. The final traces of solvent
were removed under high vacuum.
3. RESULTS AND DISCUSSION
Synthesis and structural characterization
The target compounds were obtained by following the synthetic route as illustrated in
Scheme 1. Synthesis of compounds 2, 3, 4 and 6 has been described in the earlier reports.38ꢀ42
The final compounds 7 were prepared by reacting the triphenylene derivative 4 and
azobenzene tetracarboxylic acid
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in presence of potassium hydroxide and
tetraoctylammonium bromide using water as solvent under reflux for around 7 hours (see
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experimental section for details). All the compounds were characterised by H NMR, 13C
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NMR, IR, UVꢀvis and elemental analysis (ESI, Figure S1ꢀS8).
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C6H13
O
OC6H13
HO
OC6H13
9
OC6H13
OC6H13
(i)
(ii)
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C6H13
O
OC6H13
OC6H13
C6H13
O
OC6H13
OC6H13
C6H13
O
C6H13O
1
2
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(iii)
m(H2C) O OC6H13
Br
HOOC
HOOC
HOOC
HOOC
COOH
(iv)
N
NO2
+
N
C6H13
C6H13
m = 8, 10, 12
O
OC6H13
COOH
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O
OC6H13
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OC6H13
C6H13
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C6H13
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OC6H13
OC6H13
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C6H13
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O
OC6H13
OC6H13
O
C6H13
O
O
n
n
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N
O
O
O
n
O
O
C6H13
C6H13
O
7
n
O
OC6H13
O
OC6H13
OC6H13
7a: n = 8
7b: n = 10
7c: n = 12
C6H13
O
C6H13
O
OC6H13
C6H13
O
OC6H13
Scheme 1. Synthesis of the target compounds 7. Reagents and conditions: (i) FeCl3, CH2Cl2,
H2SO4, RT, 1 h, 70%; (ii) CatꢀBꢀBr, CH2Cl2, RT, 48 h, 40%; (iii) Cs2CO3, KI, butanone, 80
°C, 18 h, 85%; (iv) NaOH, dextrose, 80 °C, HCl, 70%; (v) KOH, H2O, TOAB, reflux, 5h,
50%.
Thermal Behavior
Thermal behavior of all the compounds was investigated with the help of Polarizing Optical
Microscopy (POM) and Differential Scanning Calorimetry (DSC). Compound 7a was a
transparent glassy material and displayed a glass to isotropic transition at 77 °C on heating,
whereas, on cooling it remained isotropic till room temperature. For compounds 7b and 7c,
the presence of thermotropic LC phase at room temperature was confirmed with the highly
birefringent textures and their shearability under polarizing optical microscopy (POM)
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observation (Figure 1). Compounds 7b & 7c exhibited isotropization at 40.3 °C & 43.5 °C
respectively. However, on further cooling, the mesophase did not appear immediately till
room temperature (ESI, Figure S9). This was a general observation for all the compounds and
the mesophase only reappears after keeping the isotropic melt for few hours. A possible
explanation for this behaviour could be that the triphenylene molecules are connected via
relatively short spacers to a rigid core and do not have enough freedom in a short time span to
selfꢀassemble into columns on subsequent cooling from isotropic state.
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Figure 1. Optical photomicrograph of compound (a) 7b and (c) 7c obtained with a polarized
optical microscope at 25 °C (crossed polarizers, magnification × 50). (b) & (d) represent the
respective textures after shearing the film. (scale bar = 10 ꢁm).
To further explore mesophase behaviour of the synthesized compounds SAXS/WAXS
experiments were carried out. As concluded from POM observations, compounds 7b & 7c
were showing LC behaviour at room temperature whereas compound 7a was glassy in nature.
However, in XRD studies it was found that diffractograms of all the three compounds were
similar (Figure 2). This indicates that the selfꢀassembly of molecules in 7a is similar to that in
7b and 7c, however, the motion of alkyl chains is frozen in 7a. Inꢀorder to find out the
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detailed structure of the assembly of compound 7, we have carried out a thorough analysis on
diffractogram of compound 7c. This analysis could explain the structure of assembly of
compound 7b and 7a, because the diffraction pattern for both of them are similar to 7c.
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Figure 2. Xꢀray diffraction pattern corresponding to mesophases of compound (a) 7a, (b) 7b
and (c) 7c indexed on columnar rectangular (Colr) phase. Right panel shows the respective
2D diffraction patterns.
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The diffraction pattern of compound 7c exhibits many peaks as shown in Figure 2. In
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particular, it showed hc peak of spacing of about 3.93 Å in the wide angle region which
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appears due to πꢀπ interaction and correspond to face to face separation between TP groups.
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Hence, it confirms the columnar nature of the structure of assembly of compound 7c and
indicates that TP cores are assembled on top of each other to form columns which are further
arranged on a 2D lattice. This hc peak gives the value of one of the lattice parameter, i.e., c =
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3.93 Å. However, the reported faceꢀtoꢀface distance between two πꢀconjugated molecules is
in the range of 3.5ꢀ3.7 Å which suggests that the TP cores are packed with a tilting angle of
23.37 ± 3.69 degrees with respect to the columnar axis. The XRD pattern further exhibit four
peaks in the small angle region. Interestingly, the dꢀspacing of the first and third peak as well
as that of second and fourth peak are in the ratio of 1:1/2, indicating the lamellar nature of the
structure in two different directions. We have also performed the SAXS experiment at lower
q region but could not see any additional peak in that region which could be assigned to
lower hk values (Figure S10). Inꢀorder to understand the detailed arrangement of columns on
2D lattice and to facilitate their indexing, GIWAXS/SAXS experiments were also carried out
on oriented thin films obtained through mechanical shearing on an ITO coated glass substrate
(Figure 3). The GISAXS pattern of the sample showed partially aligned peaks in the small
angle region (Figure 3a). Azimuthal plot of the first two peaks of the small angle shows that
they are orthogonal to eachꢀother (Figure 3b). Further, each of the first two peaks is twoꢀfold
confirming that the structure is rectangular.
Therefore, combining the information from SAXS/WAXS and GISAXS, it is clear that TP
cores massed on top of each other and form columns and the arrangement of columns on 2D
lattice exhibit lamellar ordering in two orthogonal directions in the columnar plane. Based on
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above analysis, a possible model representing the selfꢀassembly in compounds 7 is shown in
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Figure 4.
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Figure 3. (a) GIWAXS & GISAXS (inset) pattern from a thin oriented film obtained through
mechanical shearing of compound 7c on ITO coated glass substrate. (b) Azimuthal plot of the
first two peaks, (40) and (04). Both peaks are orthogonal to each other and each one is twoꢀ
fold in nature. (c) Optical micrograph of the oriented film of compound 7c as viewed between
crossed polarizers.
Figure 4. Schematic of (A) the structure of compound 7, modelled as four triphenyl (TP)
discs (in blue colour) connected with azobenzene group (in red color) via flexible spacers.
(B) A possible arrangement of columns on 2D rectangular lattice where centre of azo group
forms a rectangular lattice. Box in the black colour show the unit cell of rectangular lattice.
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7
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Addition of TNF to compound 7, transforms the rectangular lattice into hexagonal lattice.
Green dotted parallelogram with modified dimensions represents the unit cell of hexagonal
lattice. (C) 3D arrangement of structure representing the columnar rectangular (Colr) phase.
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The compounds 7 are discotic mesogenic quadric based on TP linked with azobenzene
group via flexible alkyl spacers. The compound could be modelled as four TP discs (blue in
colour) tethered via flexible alkyl chains to an azobenzene group (red in colour). The model
shows that the centre of the azo group forms the rectangular lattice (Figure 4). The rectangle
with black boundary shows the unit cell of the lattice. The unit cell also exhibits the lamellar
of the column layers in two orthogonal directions as suggested by SAXS/WAXS and
GISAXS analysis (Figure 5). The lamellar periodicity is a/4 and b/4 in two orthogonal
directions of rectangle, respectively, indicating that the indexes of first two peaks would be
(40) and (04) and also suggests that the allowed reflection would be h, k = 4n (Figure 5a).
Other reflection of unꢀoriented pattern is indexed accordingly and structure of the assembly is
ꢄ
Colr. The dꢀspacing are calculated according to equation 1/ꢀꢁꢂꢃ = ℎꢄ/ꢅꢄ + ꢆꢄ/ꢇꢄ where h
and k are the miller indices. The calculated lattice parameters are a = 81.28 Å and b = 65.20
Å. The details are summarized in the Table 1. This assignment of hk values was done to
represent the actual symmetry of the molecular arrangement. The present indexing indicates
that the unit cell has fourꢀfold lamellar ordering of the column layer with periodicities a/4 and
b/4 along the direction of a and b, respectively. This is consistent with most possible
arrangement of the molecule in the rectangular (Colr) phase (Figure 4) and also the symmetry
of the unit cell of Colr phase (Figure 5a). Deducing the hk values to lower numbers (i.e. 10
and 01) will lead to lattice dimensions of about 1/4th of the present value and will correspond
to about one TP unit of the oligomer. The symmetry of unit cell and its arrangement in the
rectangular lattice cannot be justified in that case.
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Table 1. The observed and calculated dꢀspacings and planes, correlation length of the
diffraction peaks of the columnar rectangular (Colr) phase observed in compound 7c with the
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lattice parameters: a = 81.28 Å, b = 65.20 Å and c = 3.93 Å. dexp is the observed dꢀspacing.
9
ꢍ
dcal is the calculated dꢀspacing according to formula
=
+
ꢍꢍ. ξ is the correlation
ꢈ
ꢎ
ꢂ
ꢏ
ꢐ
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ꢍ
ꢍ
ꢉ
ꢊꢋꢌ
length.
dꢀspacing
dexp (Å)
dꢀspacing
dcal (Å)
20.32
16.30
10.16
8.15
Planes (hkl)
ξ
ξ/ dexp
4 0
0 4
8 0
20.32
16.30
10.25
8.18
6.53
5.40
4.95
4.57
3.93
124.27
111.84
6.11
6.86
0 8
12 0
0 12
16 0
8 12
hc
6.77
5.43
5.08
4.79
23.23
5.91
The correlation length (ξ) which provides an indication of ordering within the mesophases
was calculated using ξ = [k 2π]/[(ꢂq)]; equivalent to Scherrer’s equation. Here, k is the shape
factor whose typical value is 0.89, λ is the wavelength of the incident Xꢀray, q is the
scattering vector (q = 4πSinθ/ λ), θ is the maximum of the reflection and ꢂq is the broadening
in q at half of the maximum intensity. The ꢂq is calculated from the fitting of the diffraction
peaks by Lorentzian profiles. Ratio of the ξ and corresponding dꢀspacings gives a realization
of correlation length in terms of correlated units of length scale d. The calculated correlation
lengths along the a and b axis of the rectangular lattice i.e. corresponding to (40) and (04)
peaks are 124.27 Å and 111.84 Å respectively. The corresponding number of correlated
columns are about 6 and 7, respectively. The calculated coreꢀcore correlation length
corresponding to hc peak along the column (c) axis is 23.23 Å which is equivalent to about 6
correlated cores (Table 1).
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As mentioned earlier, the diffraction patterns of compounds 7b and 7a are very similar to
7c and explained similarly (ESI Table S1, S2). Interestingly, the lattice parameter ‘a’ was
found to be less than 7c in 7b and even lesser than 7b in compound 7a which is obvious
because spacer length in 7a is shorter than 7b which is further smaller than 7c. However, the
lattice parameters, b and c remain constant in compounds 7a and 7b. The correlation lengths
corresponding to (40) and (04) peaks were found to decrease in 7b and 7a from its values in
7c. The corresponding values are 101.67 Å (5 correlated columns) and 93.20 Å (about 6
correlated columns), respectively in case of compound 7b and 78.76 Å (about 4 correlated
columns) and 69.03 Å (about 4 correlated columns) for compound 7a. Increase in correlation
length along a and b direction from 7a to 7c attributes to the stabilization of the rectangular
phase with increasing spacer length. Although, the coreꢀcore correlation length remains the
same in compound 7aꢀc.
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Figure 5. A schematic of unit cell of (a) columnar rectangular (Colr) phase and (b) columnar
hexagonal (Colh) phase. Rectangular and hexagonal unit cell exhibit the lamellar of the
column layers. The lamellar periodicities are a/4 and b/4 along the direction of a and b,
respectively of the Colr phase. The lamellar periodicity is a/2 along both the direction of Colh
phase.
Charge-transfer complexes
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CT complexes of synthesized compounds 7 were prepared with TNF in the molar proportions
of 1:1, 1:2, 1:3 and 1:4 [oligomer: TNF] respectively (see experimental section for details).
Interestingly, all the complexes displayed enantiotropic LC phase behaviour at room
temperature which was confirmed from characteristic birefringent textures with shearability
of the compound at room temperature. On cooling from the isotropic phase, these complexes
displayed focal conic textures typical of a columnar mesophase (Figure 6, ESI Figure S11).
The precise transition temperatures and associated enthalpy changes for all the complexes are
listed in Table 2 (see ESI, Figure S12). It was found that the doping resulted in mesophase
induction for the complexes of compound 7a which was a glassy columnar in its pure state.
Also, clearing temperatures of all the complexes increased drastically as compared to that of
the undoped compounds.
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Figure 6. Optical photomicrographs of the doped compounds (a) 7a: TNF [1:1] at 30.4 °C,
(b) 7a: TNF [1:2] at 121.3 °C, (c) 7b: TNF [1:1] at 45.6 °C, (d) 7b: TNF [1:2] at 65.2 °C, (e)
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7c: TNF [1:1] at 54 °C and (f) 7c: TNF [1:2] at 100.2 °C as obtained on cooling from
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isotropic through a polarized optical microscope with crossed polarizers (scale bar = 20 ꢁm).
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To confirm the mesophases and also for quantitative comparison, we have carried out
XRD studies on these complexes. Surprisingly, the mesophase switched from rectangular in
case of pure compounds to hexagonal phase in case of CT complexes as evident from XRD
studies (vide infra). 1:1 complexes displayed an intermediate phase between columnar
rectangular and hexagonal states and were completely transformed into columnar hexagonal
(Colh) in case of 1:2 complexes. Moreover, 1:2 CT complexes of all the compounds were
found to be most stable as their clearing temperatures were highest compared to other
complexes (Table 2).
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Table 2. Thermal behaviour of the charge transfer complexes of compounds 7 with TNF.a, b, c
Complexes
Heating Scan
Cooling Scan
I 67 Col
7a:TNF [1:1]
Col 80 (1.28) I
7a:TNF [1:2]
7a:TNF [1:3]
7a:TNF [1:4]
7b:TNF [1:1]
7b:TNF [1:2]
7b:TNF [1:3]
7b:TNF [1:4]
7c:TNF [1:1]
7c:TNF [1:2]
Colh 135.7 (3.40) I
Colh 134.5 (3.15) I
Colh 118.9 (1.26) I
Col 100 I
I 125.2 (6.16) Colh
I 121.9 (4.98) Colh
I 111.32 (2.20) Colh
I 83 Col
Colh 136.9 (0.41) I
Colh 132.6 (0.68) I
Colh 130 I
I 109 (1.28) Colh
I 117Colh
I 119 Colh
Col 100 I
I 70 Col
Colh 121.6(2.23) I
I 103.4 (3.32) Colh
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7c:TNF [1:3]
7c:TNF [1:4]
Colh 118.3(1.64) I
Colh 91.98(0.51) I
I 99.63 (2.35) Colh
I 86 (0.55) Colh
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[a] Phase transition temperatures in °C and transition enthalpies in kJ molꢀ1 (in parentheses).
[b] Phase assignments: Col = Columnar, Colh = Columnar hexagonal, I = isotropic.
[c]For some of the complexes, transitions were not observed in DSC, however it was
confirmed from POM & XRD studies.
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We present here a detailed analysis on CT complexes of compound 7c. The CT complexes
of compounds 7a & 7b show similar behaviour to that of 7c. Addition of TNF to compound
7c leads to a stable Colh phase. However, for the molar ratio (7c:TNF) 1:1 it exhibits a mixed
phase which transformed into a stable Colh phase for the molar ratio 1:2 and is retained for
ratios 1:3 and 1:4 (Figure 7). The observation of two peaks in the small angle region with dꢀ
spacing in ratios of 1: 1/√3 confirms a 2D hexagonal phase and hc peak in the wideꢀangle
region indicates the columnar nature of the structure. Thus, the addition of TNF to compound
7c leads to changes in the structure of assembly and Colr phase is transformed into Colh
phase. Inꢀorder to get the most possible indexing of the hexagonal phase, the unit cell is
derived from the model of the rectangular lattice as shown in Figure 5b. The unit cell exhibits
the lamellar ordering of the column layers along the two directions of hexagonal lattice with
the periodicity a/2 which suggest that the allowed reflection would be h, k = 2n. Hence, the
indexes of the first two peaks of small angle region are (20) and (22) respectively. The lattice
parameter, a is calculated by using the relation a = 4 d20/√3. The calculated value of the
lattice parameter, a is 43.42 Å for the complex 7c: TNF = 1:2 and hc peak gives the value of
other lattice parameter i.e. c = 3.74 Å. Further, one broad peak, ha of spacing 4.85 Å was also
found in wide angle region which is mainly originated due to alkyl chainꢀchain correlation
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(Figure 8, Table 3). The lattice parameter, a (columnꢀcolumn separation) as well as c (coreꢀ
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core separation) is found to decrease with increasing molar ratio of TNF (Table 4).
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30
1:4
a
b
1:3
1:2
1:1
1:0
25
20
15
10
5
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103
102
0
1:4
1:0
1:2
7c:TNF
1:3
1:1
0.4
0.8
q (Åꢀ1
1.2
1.6
)
Figure 7. (a) Xꢀray diffraction pattern of compound [7c: TNF] = 1:0 to 1:4 which describes
the changeover of columnar rectangular (Colr) phase into columnar hexagonal (Colh) phase
on increasing the TNF concentration. (b) Variation of tilt angle (of the TP cores in column
with respect column axis) with increasing concentration of TNF along with error bar.
Figure 8. (a) Xꢀray diffraction pattern of compound 7c: TNF = 1: 2 with indexing on the
columnar hexagonal (Colh) phase. (b) Corresponding 2D diffraction pattern in the mesophase.
Table 3. The observed and calculated dꢀspacings and planes, correlation length of the
diffraction peaks of the columnar hexagonal (Colh) phase observed in compound 7c: TNF =
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1:2 with the lattice parameters: a = 43.42 Å and c = 3.74 Å. dexp is the observed dꢀspacing.
ꢍ
ꢍ
ꢈ
dcal is the calculated dꢀspacing according to formula:
phase. ξ is the correlation length.
ꢍ = ꢑꢒ (ꢎ ꢓꢎ ꢏꢓꢏ
)
for hexagonal
ꢍ
ꢉ
ꢂ
ꢊꢋꢌ
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dꢀspacing
exp (Å)
dꢀspacing
dcal (Å)
Planes (hkl)
ξ
ξ/ dexp
11.89
d
2 0
2 2
ha
18.80
10.78
4.85
18.80
10.86
223.68
hc
3.74
43.01
11.57
The calculated columnꢀcolumn correlation length corresponding to (20) peak is ~224 Å for
complex 7c: TNF = 1:2 which is equivalent to about 12 correlated columns. The calculated
coreꢀcore correlation length was found to be approximately ~43 Å, corresponding to about 11
correlated cores which is double of the value as observed in the pure compound 7c. The
number of correlated cores remains same with increasing molar ratio of TNF (Table 4).
Interestingly, addition of TNF to compound 7c reduces the tilting angle of TP core with
respect to the columnar axis from 23.37 ± 3.69 degrees to 9.45 ± 9.45 degrees and helps to
relax the TP cores in columns leading to the transformation from Colr to Colh phase (Figure
7). Effect of introducing TNF in the compounds 7a and 7b is very similar to that of 7c and
can be explained similarly (ESI, Figure S13ꢀ15, Table S3 and S4).
A thorough literature survey revealed that there are two possible ways in which TNF can
be accommodated in the columnar assembly.30ꢀ37 One of them is side by side manner and the
other is in intercalated fashion. We believe that our study supports that TNF molecules are
more likely to be present in an intercalated fashion rather than side by side manner which is
based on the following facts: 1) Tilt angle of columns with respect to columnar axis in
compound 7 exhibiting Colr phase decreases on transformation to Colh phase on introducing
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TNF. Further, tilt angle decreases with increasing molar ratio of TNF. 2) The calculated
intercolumnar and intracolumnar correlation lengths in the Colh phase were found to be
approximately double of the values observed for Colr phase, confirming that the addition of
TNF into compound 7 significantly increases the ordering in the system. Further, the
calculated volume of the unit cell of Colr lattice is about 20,826 Å3 (for compound 7c) which
contains about two oligomers (Figure 5a). However, the calculated volume of the unit cell of
Colh lattice is about 6,106 Å3 (for compound 7c: TNF = 1:2) which accommodates nearly one
oligomeric unit (Figure 5b). Therefore, the available volume per molecule in the Colh phase is
significantly less in comparison to Colr phase which indicates that the TNF molecule will
most likely not arrange in the side by side fashion, due to lack of enough gap between column
to accommodate TNF molecules. Further, the probability of TNF molecules being present in
the fluid chain matrix can also be ignored because that will randomise the system and hence
reduce the correlation length which is in direct conflict with our experimental observations.
Hence, TNF molecules are most possibly arranged in the intercalated manner. Further, TNF
molecule can form a charge complexion with TP moiety, thus leading to an increased
ordering along the column axis which was indeed observed in our studies. Charge
complexion also relaxes the column and reduces the tilt angle which helps the columns to be
more symmetric and arrange on 2D hexagonal lattice with significant planar ordering as seen.
All these observations support an intercalated arrangement of TNF molecules.
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Table 4. Variation of different observed and calculated lattice parameters and corresponding
correlation lengths for compounds 7 and their mixtures with TNF.
7
8
9
a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
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38
39
40
41
42
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Compound/ Mixtures
7a:TNF
[1:1]
Mixed
7a:TNF
[1:2]
7a:TNF
[1:3]
7a:TNF
[1:4]
7a
Phase
Colr
(77.28, 65.20)
(78.76, 69.03)
(4.09, 4.23)
23.23
Colh
40.34
223.68
12.80
43.01
11.50
Colh
40.22
223.68
12.84
46.60
12.60
Colh
39.58
199.71
11.65
46.60
12.53
Lattice parameter: 2D (a, b) &
ColumnꢀColumn correlation
No. Of correlated columns
CoreꢀCore correlation length (Å)
No. Of correlated cores
ꢀꢀ
ꢀꢀ
29.43
7.68
5.23
b
Compound/ Mixtures
7b:TNF
[1:1]
Mixed
7b:TNF
[1:2]
Colh
7b:TNF
[1:3]
Colh
7b:TNF
[1:4]
Colh
7b
Phase
Colr
(80.26, 65.20)
(101.67, 93.20)
(5.06, 5.72)
23.23
Lattice parameter: 2D (a, b) &
ColumnꢀColumn correlation
No. Of correlated columns
CoreꢀCore correlation length (Å)
No. Of correlated cores
ꢀꢀ
ꢀꢀ
41.90
223.68
12.33
43.01
11.57
41.52
223.68
12.44
46.60
12.53
40.60
199.71
11.65
46.60
12.53
29.43
7.68
5.91
c
Compound/ Mixtures
7c:TNF
[1:1]
Mixed
7c:TNF
[1:2]
Colh
7c:TNF
[1:3]
Colh
7c:TNF
[1:4]
Colh
7c
Phase
Colr
(81.28, 65.20)
(124.27, 111.84)
(6.11, 6.86)
23.23
Lattice parameter: 2D (a, b) &
ColumnꢀColumn correlation
No. Of correlated columns
CoreꢀCore correlation length (Å)
No. Of correlated cores
ꢀꢀ
ꢀꢀ
21.71
223.68
11.89
43.01
11.57
20.95
223.68
12.33
46.60
12.53
21.05
199.71
10.95
46.60
12.53
26.62
7.68
5.91
Photophysical & Electrochemical studies
For the estimation of electrical band gap, electrochemical behaviour of compound 7c was
studied by cyclic voltammetry of 0.001 M solution in 0.1 M of tetrabutylammonium
perchlorate (TBAP) as a supporting electrolyte in dry dichloromethane (Figure 9).43 For these
studies, a 0.1 M solution of tetrabutylammonium hexafluorophosphate was used as
supporting electrolyte in dichloromethane. A single compartment cell fitted with Ag/AgNO3
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reference electrode, platinum wire counter electrode and glassy carbon working electrode was
used for experiment. The reference electrode was calibrated with the ferrocene/ferrocenium
(Fc/Fc+) redox couple (absolute energy level of 4.80 eV to vacuum, see ESI, sectionꢀ8). The
cyclic voltammogram was recorded with a scanning rate of 0.1 V/s. Compound 7c displayed
reversible oxidation and reduction curves as indicated from its anodic and cathodic peak
potential responses. The HOMO and LUMO energy levels were calculated using the formula:
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E
HOMO = ꢀ (Eoxd,onset – E1/2, Fc, Fc++ 4.8) eV
ELUMO = ꢀ (Ered,onset – E1/2, Fc, Fc+ + 4.8) eV
The band gap was estimated using the relation:
∆Eg = EHOMO ꢀ ELUMO
The HOMO and LUMO energy levels were estimated to be ꢀ5.82 and ꢀ2.62 eV, respectively.
The electrical band gap was calculated to be 3.20 eV. The optical band gap calculated from
the red edge of the absorption spectra for compounds 7 was around 2.30 eV which is much
lesser as compared to electrical band gap.
We did not observe any photoisomerization effects due to azobenzene core in these oligomers
even in very dilute solutions. This effect is in contrast to the earlier reported examples where
the azobenzene core was connected to relatively flexible units. e.g. azobenzene
tetracarboxylic core when connected to benzene or 1, 3, 4ꢀoxadiazole units through various
linkages, displayed trans to cis conversion on irradiation with UV light and subsequent
45
conversion from cis to trans on irradiation with visible light.44,
The absence of
photoisomerization effects in the present oligomers could be ascribed to the restricted rotation
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of the photoꢀresponsive azobenzene core as it is tethered to four bulkier triphenylene units
which provides more steric restriction as compared to the benzene and 1,3,4ꢀoxadiazole units.
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3.00x10-5
4.50x10-5
a
b
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3.00x10-5
1.50x10-5
0.00
1.50x10-5
0.00
-1.50x10-5
-3.00x10-5
-1.50x10-5
-3.00x10-5
-0.4
0.0
0.4
0.8
1.2
-1
0
1
Potential (V)
Potential (V)
Figure 9. (a) Cyclic voltammogram of ferrocene in anhydrous dichloromethane solution of
tetrabutylammonium hexafluorophosphate (0.1 M). (b) Cyclic voltammogram of compound
7c in anhydrous dichloromethane solution of tetrabutylammonium hexafluorophosphate (0.1
M) at a scanning rate of 0.5 mV/s.
CONCLUSIONS
Triphenyleneꢀbased oligomeric LCs in which TP units were connected to an azobenzene core
have been synthesized and characterized using well known characterization techniques. The
compounds with spacer length m = 10, 12 exhibited LC properties even at room temperature
as confirmed from POM and XRD studies. Compounds with spacer lengths of m = 8
displayed a columnar glassy state. Interestingly, the transition from Colr to Colh phase was
observed on introducing the TNF moiety into columnar selfꢀassemblies of these compounds.
These kind of oligomeric LCs generally possess intermediate properties between that of the
monomeric LCs and those of the polymeric LCs. In addition, the observation of mesophase at
room temperature and increased charge hopping arising from the donorꢀacceptor charge
transfer complexes makes them very useful for device application. We have also
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demonstrated the methodology to control selfꢀassembly of columnar mesophase which could
be advantageous for certain device applications.
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ASSOCIATED CONTENT
Supporting Information
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1H and 13C NMR, UVꢀvis and IR spectra of the synthesized oligomers, DSC thermogram, 2D
Xꢀray diffraction patterns and details of cyclic voltammetry. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Corresponding author: Tel.: +91ꢀ172ꢀ2240266; fax: +91ꢀ172ꢀ2240266; Eꢀmail:
skpal@iisermohali.ac.in and santanupal.20@gmail.com
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENT
We are thankful to NMR and SAXS/WAXS facility at IISER Mohali. Dr. SK Pal is grateful
for financial support from INSA (bearing sanction No. SP/YSP/124/2015/433). M. Gupta
acknowledges the receipt of a graduate fellowship from IISER Mohali. We thank Mr.
Vaibhav Pal for experimental support.
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