DOI: 10.1002/slct.201800601
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Full Papers

z Electro, Physical & Theoretical Chemistry

Supramolecular Self-Assembly of Ionic Discotic Liquid
Crystalline Dimer with DNA at Interfaces
Samapika Mallik,[a] Alpana Nayak,*[a] Snehasis Daschakraborty,[b] Sandeep Kumar,[c] andKattera A. Suresh[d]Nanoarchitectonics through ionic self-assembly at interfaces is
an attractive approach to obtain advanced functional materials.
Here, a novel discotic dimer-DNA complex hybrid system has
been investigated at air-water and air-solid interfaces. The ionicdiscotic liquid crystalline dimer consisted of two triphenylene
cores linked via alkyl spacer with a imidazolium moiety. At airwater interface, the dimer formed a stable monolayer with areversible collapse. The surface manometry results suggested
that the condensed phase of the monolayer consisted ofmolecules arranged in an edge-on conformation. To understandthe folding behavior of the molecules, DFT calculations werecarried out which showed that the quantum chemically
optimized folded-form of the molecule was electronically morestable than its unfolded-form. Upon adding DNA to thesubphase, a complex monolayer was formed with enhanced
stability as was indicated by increased collapse pressure anddecreased limiting area. Importantly, this complexation enabled
an efficient and stable multilayer formation on silicon substrates with layers as many as 40. Since both DNA and discotic
dimer molecules share common properties of one-dimensionalcharge transport with compatible structures, this complex film
could serve as a model system for organic electronics.

Introduction

an interesting topic of research spanning from biology totechnology. Cui et al., about a decade ago, reported the firstsupramolecular complexation between DNA and asymmetrictriphenylene imidazolium salts in the bulk.[6] They attributed
the well-organized liquid-crystalline phases of DNA-triphenylene complexes to the 1:1 complexation between thephosphate group and the imidazolium salt. They also suggested potential applications of these functional materials inorganic microelectronics and gene transfection.[7]The most distinctive feature of DLCs is the p-p stacking
interaction between the aromatic cores, leading to intriguing
supramolecular architectures.[3] The more the number ofaromatic cores, the stronger will be the stacking interaction.
Consequently, DLC dimers that contain two aromatic coreslinked via a flexible (or rigid) spacer are expected to demonstrate a higher degree of order compared to their monomericanalogs.[8] Further, dimers represent ideal model compounds
for polymers and networks.[9] Their physical properties are
significantly different from the conventional low molar mass
DLCs because of restricted molecular motions.[10] One of thewidely studied systems in this regard is the HBC (hexa-perihexabenzocoronene) derivative.[11] While the HBC monomer
forms a well-defined columnar mesophase, the HBC dimer fails
to do so due to the intermolecular torsion. However, if two HBC
units are connected by a sufficiently long and flexible spacer, or
if seven HBC units are linked together forming a star-shape,
they show interesting hexagonal columnar mesophases.[12]There are also other examples of DLC oligomers in literature
which form various columnar mesophases and sometimes
exhibit very complex phase behavior exclusively due to the p-pinteraction.[13]So far, most of the studies on DLC oligomers are in thebulk. Studies on the organization of such molecules at the air-

Ionic discotic liquid crystals (DLCs) are fundamentally importantmodel systems for studying anisotropic ion-conduction
through intriguing supramolecular architectures.[1,2] Their longalkyl chains act as an insulating sheet for the ion conduction
through well-organized structures such as 1D columnar, 2D
smectic or 3D bicontinuous cubic.[3] These properties makeionic DLCs suitable for applications in batteries, photovoltaics,
electroluminescence and electrochemical devices.[4] Being ionicalso allow them to self-assemble through electrostatic couplingof structurally different building blocks. One such building
block could be a negatively charged naturally occurring
polyelectrolyte (i. e., DNA), which can self-assemble withpositively charged DLCs and provide a cost-effective approach
for large-scale production of supramolecular functional materials.[5] The DNA molecule is ubiquitous in nature and has been[a] S. Mallik, Dr. A. Nayak
Department of Physics
Indian Institute of Technology Patna
Patna 801103, India
E-mail: anayak@iitp.ac.in[b] Dr. S. Daschakraborty
Department of Chemistry
Indian Institute of Technology Patna
Patna 801103, India[c] Prof. S. Kumar
Raman Research Institute
Sadashivanagar, Bengaluru 560 080, India[d] Prof. K. A. Suresh
Centre for Nano and Soft Matter Sciences
Prof. U. R. Rao Road, Jalahalli, Bengaluru 560013, India
Supporting information for this article is available on the WWW underhttps://doi.org/10.1002/slct.201800601

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water interface are comparatively rare. Some reports on
Langmuir monolayer of a star-shaped DLC oligomer thatexhibited a conformation with the peripheral triphenylene
subunits sitting perpendicular to, and the central core sitting
parallel to the air-water interface, are known.[14] A few reports
on Langmuir-Blodgett films of DLC dimer, trimer and polymer
molecules comprising of chemically connected donor andacceptor subunits are also available.[15–17] Particularly, theimidazolium-based DLCs are technologically important because
their supramolecular assemblies find applications as heat
carriers in solar thermal energy generators and as electrolytes
for batteries and capacitors.[18] Although some imidazoliumbased DLCs are known,[19] their dimers and especially, their selfassembly with DNA at interfaces have not yet been studied.
Here, first, the supramolecular assembly of an ionic DLC dimer,
consisting of two triphenylene cores linked via alkyl spacer witha imidazolium moiety (TpImTp), has been studied at air-waterand air-solid interfaces. Next, the ionic self-assembly of TpImTpmonolayer and DNA, resulting from non-covalent and electrostatic interactions, have been investigated. For a comparison,
the monomer analog of TpImTp has also been investigated.
Using DFT calculations, the energetically stable structuralconformations of TpImTp dimer have been revealed.
An important aspect of the TpImTp-DNA system is theunique matching in their structures and properties enabling
good packing and advanced functions. In the structure of adouble-stranded DNA, the distance separating adjacent planesof hydrogen-bonded base pairs is 0.34 nm,[20] which is close to
the p-p stacking distance between two adjacent discotic coresin the columnar mesophase.[21] As a consequence, DLCs withinterlayer distance of 0.34 nm have been regarded as chemically synthesized version of DNA-mimic.[22] On the other hand,DNAs are also known to exhibit liquid crystalline phases.[23]Regarding electrical properties, the effective coupling between
the p-stack of base pairs is responsible for long-range chargetransport in DNA.[24] The sugars and phosphates at the
periphery of base pairs provide insulation, similar to the longalkyl chains at the periphery of DLCs that provide insulation for
one-dimensional charge transport along the column axis. Thus,
both DNA and DLCs act as molecular wires. Therefore, complexing DNAs with DLCs through ionic self-assembly at interfaces isa novel approach to develop unique nanoarchitectured materials for organic electronics without the need of elaborate
covalent chemistry or sophisticated nanotechnology tools.

Results and Discussion
The material TpImTp in the bulk exhibits the following liquid
crystalline phase sequence:[21] Solid (S)-columnar phase (Col);
848C, Col-isotropic (I); 958C. On cooling, the columnar mesophase appears at 928C and remains stable down to room
temperature. The monomer analog of this material,[25] namely,
imidazolium tethered with hexaalkoxytriphenylene (ImTp), in
the bulk also exhibits similar phase sequence: S-Col; 678C, Col-I;
1018C. On cooling, the columnar mesophase appears at 988C
with the mesophase solidifying at 388C.

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Surface manometry
The surface pressure (p) - area per molecule (Am) isotherms of
TpImTp monolayer with ultrapure deionized water subphaseand 108 M concentration of DNA in the subphase are shown in
Figure 1A. On pure water, the TpImTp monolayer shows single
phase with a limiting area (Ao) value of 1.97 nm2/molecule. The
monolayer collapses at an Am of 1.5 nm2/molecule with acollapse pressure of about 39 mN/m. The isotherm cycles
performed by expanding and compressing the monolayer film
at the air-water interface shows reversibility from the collapsed
state to the monolayer state with negligible hysteresis (seeSupporting Information, Figure S1). For ionic self-assembly with
DNA, TpImTp monolayer was formed on water containing DNA
at different concentrations. It was observed that, with theincrease in concentration of DNA in the subphase, the Ao valueof the isotherm decreases and the collapse pressure increases.
Beyond 108 M concentration of DNA, there was no further
change in the isotherm. The p - Am isotherm of TpImTp-DNA
complex monolayer with 108 M concentration of DNA in thesubphase exhibited a slope change around 1.6 nm2/molecule
indicating a phase transformation. The Ao value corresponding
to the gradual rise region was 2.3 nm2/molecule and steep rise
region was 1.56 nm2/molecule. The complex monolayer collapsed at an Am of 1.2 nm2/molecule with a collapse pressure of
47 mN/m. The increased collapse pressure and decreased Aovalue indicates that the presence of DNA in the subphasecondenses the TpImTp monolayer film. Unlike pure film, theisotherm cycles performed on the complex film showedirreversible collapse with large hysteresis.
Moreover, the compressional elastic modulus j E j wascalculated[17] from the isotherms of both the pure TpImTpmonolayer and the TpImTp-DNA complex monolayer using
Equation 1.

jEj ¼ Am

dp
dAm


ð1Þ

The j E j values are particularly useful in understanding thepacking of molecules in different phases of a monolayer film.
As shown in Figure 1C, the j E j value showed a maximum of
132.3 mN/m at Am of 1.56 nm2/molecule for the pure TpImTpmonolayer, whereas a maximum value of 163 mN/m at Am of
1.26 nm2/molecule for the TpImTp-DNA complex monolayer
with 108 M concentration of DNA in the subphase. In addition,
the curve showed a hump at 1.94 nm2/molecule with a value of
24 mN/m for the complex monolayer. On the basis of these j E jvalues, it is inferred that the pure TpImTp monolayer exhibits acondensed phase, whereas, the TpImTp-DNA complex monolayer undergoes a transformation from an expanded phase to acondensed phase.
Because the property of a dimer molecule could besignificantly different from that of its monomer analog, similar
surface manometry studies were carried out with the monomer
ImTp molecule for a comparison. Figure 1B presents the p - Am
isotherms of the pure monomer ImTp molecule and ImTp-DNA
complex monolayer with 108 M concentration of DNA in the7319

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Figure 1. (A, B) Surface pressure (p) - area per molecule (Am) isotherms of dimer TpImTp and monomer ImTp molecules with pure water subphase (dotted line)
and 108 M concentration of DNA in the subphase (solid line). (C,D) Variation of compressional elastic modulus (j E j) with Am for the isotherms of dimer TpImTp
and monomer ImTp with and without DNA in the subphase. (E, F) Chemical structures of TpImTp and ImTp molecules and their interaction with DNA.

subphase. (Notably, the 108 M concentration of DNA in thesubphase was the optimum condition for the formation of astable complex monolayer. Any further addition of DNA in thesubphase did not alter the monolayer properties at the airChemistrySelect 2018, 3, 7318 – 7326

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water interface. This was true for both the TpImTp and ImTp
molecules.) The Ao values were 1.40 and 1.11 nm2/molecule atthe steep-rise region and 3.4 and 1.73 nm2/molecule at thegradual-rise region for the pure and the complex monolayers,7320

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respectively. The isotherm cycles showed reversible collapse forthe pure monolayer but irreversible collapse for the complex
monolayer, similar to that of the dimer system (see Supporting
Information, Figure S1). The variation in j E j values with Am(Figure 1D) showed a maximum of 98 mN/m at 0.85 nm2/
molecule for the complex monolayer and 53.9 mN/m at
0.977 nm2/molecule for the pure monolayer. Additionally, the j
E j curves show humps with values of 24.3 mN/m at 1.4 nm2/
molecule and 12 mN/m at 2.7 nm2/molecule corresponding to
the gradual rise region of the isotherms of complex and puremonolayers, respectively. Therefore, unlike the dimer TpImTp
system, these j E j values indicate that both ImTp and ImTpDNA complex monolayers undergo a transformation from an
expanded phase to a condensed phase.
Brewster angle microscopy (BAM)
The TpImTp and the TpImTp-DNA complex monolayer films
were observed under BAM while isothermal compression.
Figure 2 shows the BAM images for the pure TpImTp mono-

Figure 2. Brewster angle microscope images of the TpImTp monolayer at airwater interface with deionized water subphase. (A) Uniform bright intensity
indicating a condensed phase at Am = 1.50 nm2/molecule, (B) 3D domains
developing over condensed phase indicating the onset of collapse state at
Am = 1.20 nm2/molecule, and (C) collapsed state at Am = 0.70 nm2/molecule.
The scale bar in each image represents 500 mm.

layer. The film exhibited a uniform phase from nearly zerosurface pressure. The brightness gradually increased upon
compressing the uniform condensed phase (Figure 2A) whichtransformed to 3D crystals at the collapse state (Figure 2B and
2C). On expansion, these crystalline domains disappeared andthe system reverted back to the uniform intensity region
indicating a completely reversible monolayer state. Figure 3
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Figure 3. Brewster angle microscope images of the TpImTp-DNA complex
monolayer at air-water interface with 108 M concentration of DNA in thesubphase. (A) Coexistence of gas phase and expanded phase at
Am = 2.4 nm2/molecule, (B) expanded phase at Am = 1.75 nm2/molecule, (C)coexistence of expanded and condensed phase at Am = 1.4 nm2/molecule, (D)condensed phase at Am = 1.1 nm2/molecule, and (E) fully collapsed state at
Am = 0.7 nm2/molecule. The scale bar in each image represents 500 mm.

shows the BAM images for the TpImTp-DNA complex monolayer. At large Am, the complex film exhibited a coexistence ofgas and expanded phase (Figure 3A). This transformed to a
uniform expanded phase upon compression (Figure 3B). At an
Am of about 1.4 nm2/molecule, a condensed phase developed
over the expanded phase (Figure 3C and 3D). Further compression led to thread-like 3D domains which appeared at thecollapsed state (Figure 3E). On expanding the film, thesethread-like domains remained without change and the film did
not revert back to the monolayer state indicating irreversibility.
For a comparison, the monomer ImTp and ImTp-DNA
complex monolayer films were observed under BAM during
compression. Unlike dimer TpImTp system, the monomer ImTp
showed a coexistence of expanded phase and condensed
phase at large Am. Upon compression, this transformed to a
uniform condensed phase and then to collapsed state showing
3D domains (see Supporting Information, Figure S2). Upon
expansion, these 3D domains reverted back to the uniform
monolayer state indicating reversible collapse similar to that ofthe dimer TpImTp system. However, the ImTp-DNA complex
monolayer showed irreversibility similar to the dimer TpImTpDNA complex system. Thus, the BAM observations support thesurface manometry results and confirm the existence of distinct
monolayer phases of the monomer and dimer systems.
Langmuir-Blodgett (LB) film deposition
Several layers of both the dimer TpImTp and the monomer
ImTp films were transferred onto silicon substrates by Langmuir-Blodgett (LB) technique at a target surface pressure (pt) of
35 mN/m (condensed phase). The efficiency of film deposition7321

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Figure 4. Transfer ratio as a function of number of layers of LB film deposition for (A) pure TpImTp monolayer and (B) TpImTp-DNA complex monolayer onhydrophilic and hydrophobic silicon substrates, respectively. The films were transferred at a target surface pressure of 35 mN/m and a dipping speed of 2 mm/
min. It can be clearly seen that the transfer ratio is close to 1 indicative of almost perfect deposition for the complex system, whereas the pure system suffersfrom alternate adsorption and desorption over successive layers of deposition.

was estimated from the transfer ratio (t) which was calculatedby the ratio of the area of the monolayer removed to the areaof the substrate coated. A t of unity is indicative of good
deposition. The t data of the LB films of the pure TpImTp with15 layers on hydrophilic and hydrophobic silicon substrates areshown in Figure 4A. For the hydrophilic substrate, an alternate
desorption and adsorption after the first two layers ofdeposition was observed. For hydrophobic substrate, thetransfer was efficient only for the first few layers. Thus, for thepure TpImTp dimer film, multilayer formation was not very
efficient. However, when the TpImTp film was complexed with
DNA (108 M concentration in the subphase), the transfer
efficiency increased remarkably as shown in Figure 4B. On
hydrophilic substrate, the t value was close to 1 for every
upstroke and was slightly less (ranging between 0.9 to 0.7) for
every downstroke of film deposition. Interestingly, on hydrophobic substrate, the t value was close to 1 for both the up
and down strokes and continued with the same efficiency forseveral tens of layers. Furthermore, the t data collected for themonomer ImTp system also showed deposition only for the
first few layers and multilayers could not be formed. Although
the ImTp-DNA complex film formed multilayers,they were notas efficient as that of the dimer TpImTp-DNA complex film.
The observed differences in deposition may be attributed
to the molecule-molecule and the molecule-substrate interactions together with the effect of flow patterns of thesubphase near the meniscus region during the up and down
strokes.[26] The deposition of charged monolayer involves
complicated physical and chemical processes controlling theefficiency of film deposition. Both the dimer TpImTp and themonomer ImTp have cationic imidazolium polar head groupwith Br as counterion. When these molecules form monolayer
at the air-water interface, the small Br counterions dissolve
into the subphase due to the dissociation of the ionic groups.
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The presence of these Br counterions in the subphase and thepositively charged monolayer at the surface form an electric
double layer at the air-water interface. These Br counterions
might not be sufficient enough to compensate the charge ofthe monolayer during deposition. It is quite possible thatconcentration polarization would have developed leading tomeniscus instability, thereby disrupting the multilayer formation. On contrary, addition of small amount of DNA (~ 108 M
conc.) in the subphase suppresses the concentration polarization effect, thereby facilitating stable multilayer formation.
This is evident from the high transfer efficiency (t ~ 1) observed
for the deposition of the complex films. It is worth mentioningthat van der Waals forces also play significant role in theadhesion of monolayers, but for the charged monolayer
deposition, its contribution is much smaller than the double
layer repulsive contribution.[27]Atomic force microscopy (AFM)
To understand the organization of molecules at different phasesof the isotherm, AFM imaging of monolayer and multilayer
films transferred at different pt were carried out for both thepure and complex films. Topography images of the pure
TpImTp LB-film with different layers transferred onto silicon
substrates at a pt of 35 mN/m are shown in Figure 5. The filmwith single layer on a hydrophilic substrate exhibited a uniform
surface and was scratched with AFM tip to reveal the actual
thickness of about 2 nm (Figure 5A). The films with 2, 4 and 8
layers on hydrophobic substrates revealed thicknesses of about
4, 8 and 16 nm respectively (Figure 5B, 5C and 5D). The
roughness and irregularities in film morphology may be
attributed to factors like reorganization of molecules in the film
during the transfer process, natural dewetting of the film,

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Figure 5. AFM topography images of pure TpImTp LB films transferred onto
silicon substrates in the condensed phase at a pt of 35 mN/m with (A) 1 layer,(B) 2 layers, (C) 4 layers, and (D) 8 layers. The films are scratched with the
AFM tip to find the actual height. The respective height profiles corresponding to the lines drawn on the images are shown below.

evaporation of entrapped water within the layers and varying
transfer efficiency for different layers of deposition.
For the TpImTp-DNA complex film, the images of single
layer transferred at pt values of 10, 35 and 55 mN/m ontohydrophilic silicon substrates are shown in Figure 6A, 6B and 6c
respectively. At 10 mN/m, the film was not compact andshowed a height of about 0.5 nm corresponding to the
expanded phase. At 35 mN/m, the film was compact
corresponding to the condensed phase, and was scratched toreveal a thickness of about 2.4 nm. The film transferred at 55
mN/m showed thread-like domains of the collapsed state,similar to those observed in the BAM images. Further, TpImTpDNA complex film with 2, 4 and 8 layers were transferred ontohydrophobic silicon substrates as shown in Figure 6D, 6E and
6F, respectively. The line profiles drawn on the images giveinformation about the roughness of the film but not the actual
film thickness. Since all of these films were compact, they werescratched with AFM tip which revealed the actual thicknessesof slightly greater than 4, 8 and 16 nm, respectively, due to thepresence of DNA. Additional images of scratched films areshown in the Supporting Information (Figure S3).
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Figure 6. AFM topography images of TpImTp-DNA complex LB film with 1
layer in the (A) expanded phase (pt = 10 mN/m), (B) condensed phase(pt = 35 mN/m), and (C) collapsed state (pt = 55 mN/m). Multilayer films
transferred in the condensed phase at a pt = 35 mN/m with (D) 2 layers, (E) 4
layers and (F) 8 layers. The respective height profiles of the film surface
corresponding to the line drawn are shown below each images.

Interestingly, no thread-like structures were observed in themultilayered films of the TpImTp-DNA complex unlike thoseobserved in a previous study of a discotic PyTp-DNA complex
system.[28] Even in the case of monomer ImTp-DNA complex
film with 20 layers, no such thread-like features were seen.
However, the presence of DNA was confirmed by Fourier
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transform infrared spectroscopy on these films which showedcharacteristic absorption bands of DNA molecules. DNA does
play an important role to obtain almost perfect transfer ratio
for the formation of stable multilayers on a substrate.
Apparently, the amount of DNA molecules themselves getting
transferred to the substrate depends on the cationic polar headgroup of the discotic amphiphile. In the previous study of
PyTp-DNA system,[28] the polar head group was a pyridinium
moiety. For a pyridine moiety, the replacement of a CH in thebenzene ring by more electronegative nitrogen atom induces adipole moment of 2.2 D, denoting a shift of electron densityfrom the ring towards the nitrogen atom.[29] (Benzene molecule
which is symmetrical has zero dipole moment.) For ImTp and
TpImTp molecules, the polar head group is a imidazolium
moiety. In an imidazole moiety, the replacement of CH byelectronegative nitrogen atom at two positions, induces adipole moment of 3.61 D.[30] The valence bond (resonance)
description indicates that both the pyridinium ring and theimidazolium ring have delocalized positive charge.[31] Although
DNA complexation is primarily electrostatic in nature, experimental observation indicates that the pyridinium polar headgroup is more effective in transferring DNA than the imidazolium group. Understanding the reason behind this is open for
future investigations.
On the basis of surface manometry results and AFM images,
the conformation of the TpImTp dimer molecule in themonolayer film can be suggested. From molecular dimensions,
the TpImTp dimer molecule on a surface has Ao values of
8.6 nm2/molecule and 1.46 nm2/molecule for the face-on andedge-on conformations, respectively (Figure 7A and 7B). Experimentally, both BAM and isotherm showed a single condensed
phase for the TpImTp monolayer with a corresponding Aovalue of 1.97 nm2/molecule. The AFM image of TpImTpmonolayer film transferred at the condensed phase (35 mN/m)showed a height of 2 nm. Comparing the film height and theAo value with the molecular dimensions, this condensed phaseappears to be composed of molecules arranged in an edge-on
conformation. Due to the electrostatic repulsion between themolecules, the Ao value obtained from the surface manometry
measurements was larger than the theoretically estimatedvalue.
Since the physical properties of discotics that are essential
for device applications can be tailored, the alignment ofmolecules in thin films has been an important topic of researchover the past several decades.[32–34] Depending upon themolecule-molecule and molecule-substrate interactions, discotics are known to exhibit interesting molecular conformations on a substrate. Perova et al. observed the anchoringtransition (similar to the local Fredericks transition) in discoticliquid crystals from the edge-on to the face-on (side-on)alignment for a number of substrates both untreated andcoated with polymer.[34] Using infrared dichroic ratio, they also
showed explicitly that the cores of triphenylene align face-on(i. e., side-on) to the Si-substrates.[32] They inferred from theseobservations that the condition of the minimum surface energy
is fulfilled if the director (axis of the column) has a planar
orientation (i. e. edge-on alignment) and this type of alignment
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Figure 7. Schematic diagram of the possible conformations of TpImTp
molecules on a surface. (A) Face-on. (B) Edge-on. (C) Folded- and unfoldedforms of edge-on conformation. The Ao values are estimated based on themolecular dimensions and configurations. (D) Two quantum chemically
optimized conformers of the molecule: (I) the folded-form and (II) the
unfolded-form. The H atoms are not shown for clarity.

for the discotic LC is normally preferred. Moreover, whether thisalignment has a homogeneous or heterogeneous character
depends on whether or not a substrate has one or more than
one easy direction of orientations. Microscopically, the contributions to the anchoring energy were attributed to the
topology, steric, polar and dispersive (caused by van der Waals)
interactions.
In the present work, the film is formed on the surface ofwater. In general, the anchoring of discotic molecules on water
surface could be face-on or edge-on depending on theavailability of surface area and anchoring sites (polar groups)
per molecule (Figures 7A and 7B). At larger surface area permolecule, they tend to align in a face-on conformation,
whereas at smaller surface area per molecule, they align in anedge-on conformation. The transition between these two
conformations occur reversibly during symmetric compression
and expansion of the monolayer as shown in the isotherm
cycle for the monomer molecule (Figure S1). However, thedimer molecules exhibit only edge-on conformation due to thestrong pi-pi interaction between their cores. Interestingly, these7324

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conformations were preserved when monolayer films weretransferred from water surface to solid substrate indicatinghigh stability. This was confirmed by the height information
extracted from the AFM topography images of the films
transferred on solid substrate. Similar observations were also
reported in our previous work.[35]DFT calculations
Further insight into the molecular conformation was sought
through DFT calculations. For discotic dimers, the edge-on
conformation could be of two types, namely, folded- andunfolded-form as shown in Figure 7C. Particularly, if the two
discotic cores of the dimer are connected by flexible spacer,
they tend to fold due to the strong p-p interactions.[36–38] In thepresent work, the TpImTp dimer molecule is symmetric and theexperimentally observed Ao values only indicate an edge-on
conformation without clarifying whether or not it is folded. Inorder to study the folding behavior of the molecule, firstprinciples calculations were carried out based on densityfunctional theory (DFT) at B3LYP/6-311G** level and basis set. It
is known that DFT does not handle the dispersion interaction
accurately.[39,40] The p-stacking interaction energy is generally
underestimated. Therefore, the structures resulted from theabove DFT calculation were re-optimized using a new functional B97-D developed by Grimme with D95(d,p) basis set.
Despite the fact that the above combination is less computationally expensive, it provides good optimized geometry withmore accurate dispersion interaction. This calculation scheme
has been called as DFTD hereafter. Figure 7D shows the
optimized structures of the folded (I) and the unfolded (II)forms of the molecule. These structures were obtained byunconstrained geometry optimizations starting from a variety
of extended and folded conformations. The coordinates of theatoms for the two forms are given in Table S1 and S2 of the
Supporting Information. Table 1 presents energy values and theTable 1. Energy values and structural data obtained from first-principles
calculations for comparison between the folded and unfolded forms of the
TpImTp molecule.
Parameters

MethodFoldedForm (I)

UnfoldedForm (II)Relative Energies(setting the energy of form II is zero)

DFT

12.6 kcal/
mol
40.0 kcal/
mol
5.4 Å
5.0 Å
4.2 Å
3.0 Å
4.248-5.0780.0000DFT-D
Distance between the two center ofmass of the aromatic segments
Perpendicular Distance between the
two aromatic planes
Dihedral angle between the highlighted atoms (See Figure 7)

DFT
DFT-D
DFT
DFT-D
DFT
DFT-D

0.000023.6 Å
23.5 Å
1.2 Å
0.6 Å
1248
148

structural data for comparison between these two optimized
structures. It is evident from Table 1 that while the DFT
calculation predicated the folded form of the molecule is
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electronically stable by ~ 12.62 kcal/mol over its unfolded form,
the DFTD calculation predicts much stronger folding interaction energy of ~ 40 kcal/mol. Also, the DFT calculation
predicts folded form (I) as a parallel-displaced conformer,
having p–p stacking interaction with certain inter-disk separation. The distance between the two center of mass of thearomatic segments were estimated to be 5.4 Å and 23.6 Å, andthe perpendicular distance between the two aromatic planes
were estimated to be 4.2 Å and 1.2 Å for the folded- and theunfolded-forms, respectively, from DFT calculations. However,the DFTD calculation predicts a smaller inter-disk separationof ~ 3.0 Å for the folded-form and 0.6 Å for the unfolded-form.
This large difference between the results obtained from the
DFTD and the DFT calculations is consistent with earlier
studies.[41,42] However, this difference may change slightly upon
considering bigger basis set, which is pretty difficult for thecurrent system.
This model is in good qualitative agreement with theexperimental results of film heights from AFM imaging andlimiting area from surface manometry corresponding to theedge-on configuration. Moreover, the fact that the folded form
is energetically more stable than the unfolded form is in linewith the previous reports in literature on similar systems. For
instance, modelling done by Boden et al. suggested that thesteric crowding in dimers makes conformations in which both
discotic cores are coplanar unlikely.[36] In another study, Bozek
et al. reported that folding allowed the dimers to assemble intoa columnar liquid crystal phase, despite their short linking
group.[37] Recently, it has also been shown for discotic dyads
that the folded-form is significantly more stable than theunfolded-form.[38]Thus, this study deals with the interfacial nanoarchitectureof a novel discotic dimer and its complexation with DNA. Thisapproach is suitable for a cost-effective and large-scale
production of technologically important supramolecular hybrid
films without the need of elaborate covalent chemistry orcomplicated nanotechnology tools. In literature, DNA-cationicsurfactant complexes are known as advanced materials withunique mechanical, electrical, optical, and biological properties
for specific applications.[43] In the present study, the compelling
rationale for the building blocks being examined, viz., DNA anddiscotic, is twofold: Firstly, both exhibit one-dimensional chargetransport. Secondly, they have compatible structures. Moreover,DNA being polyelectrolyte provides additional benefit of
improved deposition through LB technique. Such hybrid films
could be model system for organic electronics. Charge transport measurements, i. e., current as a function of applied field,
have already been carried out in a previous report on similarsystem.[44] Future research endeavour, using the present system,
is to explore organic atomic-switch based neuromorphic device
applications.[45,46] The discotic-DNA complex film, when sandwiched between an electrochemically active metal (Ag) and an
inert metal (Pt), is expected to show resistance switching due
to electrochemical metallization. One of the preliminary resultsof current-voltage (IV) switching measurements using conducting AFM is presented in the Supporting Information(Figure S4). A detailed study in this direction is underway.
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Conclusions
The ionic discotic liquid crystalline dimer (TpImTp) forms stable
Langmuir monolayer with reversible collapse. As compared toits monomer ImTp analog, the dimer TpImTp monolayer
exhibits much higher compressional elastic modulus, indicating
a better packing of molecules due to two aromatic cores. DFTD calculations show that the quantum chemically optimized
conformation with the folded-form of the dimer molecule iselectronically stable by 40 kcal/mol over its unfolded form,
which is consistent with the experimentally obtained limiting
area of the TpImTp molecules. Upon adding appropriate
amount of DNA in the subphase, the ionic self-assembly of the
TpImTp monolayer with DNA results in a decrease in limiting
area and an increase in collapse pressure indicating enhanced
stability. Interestingly, DNA complexation facilitates an efficient
multilayer formation of the TpImTp-DNA system on substrates,
indicating the potential for device applications. This work
highlights the fact that ionic self-assembly at interfaces is anefficient nanoarchitectonic approach for the designing ofadvanced functional materials.
Supporting Information Summary
The supporting information contains detailed experimental
section, results of isotherm cycles, additional BAM images,
additional AFM images and current - voltage spectroscopy
data, and tables of cartesian coordinates of atoms of the
optimized structures obtained in quantum chemical modelling.

Acknowledgment
Authors acknowledge Dr. S. K. Pal at IISER, Mohali, India, for thesynthesis of the discotic molecules during his PhD at RRI,
Bangalore.

Conflict of Interest
The authors declare no conflict of interest.
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Submitted: March 1, 2018
Accepted: June 25, 20187326

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