J. Phys. Chem. B 2007, 111, 11157-11161
11157
Films of Novel Mesogenic Molecules at Air-Water and Air-Solid Interfaces
Alpana Nayak, K. A. Suresh,* Santanu Kumar Pal, and Sandeep Kumar
Raman Research Institute, SadashiVanagar, Bangalore - 560 080, India
ReceiVed: April 25, 2007; In Final Form: July 16, 2007
Discotic molecules are known to form highly anisotropic structures at the air-water (A-W) interface. We
have studied two novel ionic discotic mesogenic molecules, viz., pyridinium tethered with hexaalkoxytriphenylene with bromide counterion (Py-Tp) and imidazolium tethered with hexaalkoxytriphenylene with bromide
counterion (Im-Tp) at A-W and air-solid interfaces. The monolayer phases were investigated at the A-W
interface employing surface manometry and Brewster angle microscopy techniques. They indicate a uniform
monolayer phase which shows negligible hysteresis on expanding and compressing. Also, in both the systems
the collapsed state completely reverts to the monolayer state. These monolayer films transferred at different
surface pressures by Langmuir-Blodgett technique were studied by employing atomic force microscopy.
The topographies of these films transferred at the low and high surface pressure region of the isotherm indicate
a transformation of the monolayer from face-on to edge-on structure.
1. Introduction
The studies on discotic molecules at air-water (A-W) and
air-solid (A-S) interfaces have drawn lot of attention.1,2 The
thin films of discotic mesogens have technological applications
in the fabrication of gas sensors,3 field-effect transistors,4 and
photovoltaic diodes.5 As compared to conventional discotic
liquid crystals, ionic discogens differ significantly as they
combine the properties of liquid crystals and ionic liquids. This
makes them promising candidates to design anisotropic ionconductive materials because of the anisotropic structural
organization and the presence of ions as charge carriers.6 The
long alkyl chains of the mesogenic molecule act as an insulating
sheet for the ion-conductive channel. Yoshio et al. have reported
one-dimensional (1D) ion transport in self-organized columnar
ionic liquids.7
Discotic mesogens can self-assemble into 1D columnar
superstructures due to the overlapping of the Π-orbitals of
adjacent molecules and can arrange in a two-dimensional lattice
(2D) at the A-W interface, thereby forming stable Langmuir
monolayers.2,8 These thin films need to be transferred to solid
supports for practical applications, and the preparation by means
of the Langmuir-Blodgett (LB) technique is an efficient method
to obtain highly ordered layers over large areas. LB films of
discotic mesogens have been reported to have a surface
morphology which are periodically modulated by columns that
lie predominantly along the film deposition direction.1 Evidence
has been presented for square-lattice discotic structures in the
bilayer films obtained by LB technique.9 Ionov et al. have
experimentally established smectic-like structures normal to the
substrate, and the nematic-like in-plane arrangement of the
discotic molecules in the LB films, resembling a discotic
smectic-nematic phase.10 The possibilities to induce different
structural arrangement in the LB films of discotic molecules
allow us to undertake detailed studies of the intra- and interlayer
molecular interactions, and especially of the steric and Π-Π
interactions.10 These interactions also govern the structure and
* Corresponding author. E-mail: [email protected]. Telephone: +9180-28382924. Fax: +91-80-23610492.
properties of thin films of many biological and disk-shaped
aromatic molecules.11
Although discotic molecules of alkoxytriphenylene derivatives
have been reported to form stable Langmuir monolayers,12 there
have been no reports to date on ionic alkoxytriphenylene
derivative monolayers. In this contribution, we report our studies
on Langmuir and LB films of novel cationic discotic molecules,
that is, triphenylene-based pyridinium and imidazolium salts
with bromine as counterion (Py-Tp and Im-Tp).13 We find that
both the molecules exhibit a stable monolayer. The surface
pressure-area per molecule (π-Am) isotherm is completely
reversible. We find that the LB films of Py-Tp with two layers
showed patterns with regular rectangular voids elongated in the
film deposition direction, whereas the LB films of Im-Tp with
two layers exhibited irregular shaped voids throughout the film.
2. Experimental Section
The materials Py-Tp and Im-Tp were synthesized in our
laboratory.13 The compounds were purified by repeated recrystallizations with diethyl ether and characterized by 1H NMR,
13C NMR, IR, UV spectroscopy, and elemental analysis which
indicated high purity (99%) of the materials. The thermotropic
liquid crystalline properties of these salts were investigated by
polarizing optical microscopy and differential scanning calorimetry. Small-angle X-ray diffraction (XRD) was carried out
on a powder sample using Cu KR (λ ) 0.154 nm) radiation
from a Rigaku ultrax 18 rotating anode generator (4 kW)
monochromated with a graphite crystal.
The surface manometry experiments were carried out using
a NIMA 611M trough. The subphase used was ultrapure
deionized water (pH 5.8) obtained from Millipore Milli-Q
system. The stock solution of 0.236 mM concentration was
prepared using chloroform (HPLC grade). After spreading, the
film was left for 20 min, allowing the solvent to evaporate. The
π-Am isotherms were obtained by symmetric compression of
the barriers with a constant compression rate of 0.103 nm2/
molecule/min. The surface pressure (π) was measured using
the standard Wilhelmy plate technique. The equilibrium spreading pressure (ESP) was measured at a constant area of 100 cm2.
10.1021/jp073196z CCC: $37.00 © 2007 American Chemical Society
Published on Web 08/31/2007
11158 J. Phys. Chem. B, Vol. 111, No. 38, 2007
Nayak et al.
High humidity of the order of 92 ( 5% was maintained during
the experiment to prevent any evaporation. To determine the
ESP value, we monitored the surface pressure with time until
its value reached a saturation. A Brewster angle microscope
(BAM), MiniBAM (NFT, Nanotech, Germany) was employed
to observe the films at the A-W interface. LB technique was
employed to transfer various layers of the films onto hydrophilic
and hydrophobic substrates at different target surface pressures
(πt) with a dipping speed of 2 mm/min. To obtain hydrophilic
surfaces, we treated polished silicon wafers for about 5 min in
hot piranha solution (mixture of concentrated H2SO4 and H2O2
in 3:1 ratio) which were then then rinsed in ultrapure deionized
water and later dried. For hydrophobic surfaces, freshly prepared
hydrophilic silicon substrates were dipped in hexamethyldisilazane (HMDS) for 12 h and then rinsed with HPLC grade
chloroform. Horizontal transfer technique was used to obtain
films on substrates at low π. The atomic force microscope
(AFM) studies on these LB films were performed using model
PicoPlus (Molecular Imaging). We used silicon tips with spring
constant of 21 N/m and resonance frequency 250 kHz. The AFM
images were procured using the AC mode in ambient condition.
UV-visible absorption spectroscopy was also carried out for
the LB films transferred on quartz plate. All of the experiments
were carried out at room temperature (25 ( 0.5 °C).
3. Results and Discussion
The material Py-Tp in the bulk exhibits the following phase
sequence: solid (S) - columnar phase (Col); 83.7 °C, Col isotropic (I); 95 °C. On cooling, the columnar mesophase
appeared at 92 °C and remained stable down to room temperature. The structure of the columnar mesophase obtained by
small angle XRD reveals a simple rectangular lattice of
parameter a ) 4.24 nm and b ) 3.64 nm. The material Im-Tp
in the bulk exhibits similar phase sequence: S - Col; 67 °C,
Col - I; 101 °C. The columnar phase has a simple rectangular
lattice with parameters a ) 5.76 nm and b ) 4.40 nm. On
cooling, the columnar mesophase appeared at 98 °C with the
mesophase solidifying at 38 °C.
3.1. Surface Manometry. The π-Am isotherm for Py-Tp
(Figure 1a) and Im-Tp (Figure 1b) show a gradual rise in π up
to 9 mN/m followed by a sharp rise. Both the isotherms exhibit
a collapse pressure of about 44 mN/m. The limiting area per
molecule (Ao) values obtained by extrapolation to zero surface
pressure were 1.15 and 1.40 nm2/molecule at the steep increasing
region and 3.1 and 3.4 nm2/molecule at the gradual increasing
region respectively. Figure 2 shows molecular structures with
approximate dimensions according to standard bond lengths and
angles. The computed surface area of the aromatic core is equal
to 0.785 nm2. The surface area occupied by the molecule
increases to 4.5 nm2 if the extended peripheral hydrocarbon tails
are included. The experimentally observed Ao values for the
first transformation suggest an expanded phase, whereas for the
second transformation, the observed Ao values suggest a
condensed phase.
The rigidity of the films were calculated from the compressional elastic modulus, |E|, given by
|E| ) (Am)dπ/dAm
(1)
The |E| value characterizes the nature of a monolayer’s stable
phases. Specifically, a maximum value of the |E| in the range
of 12.5 mN/m to 50 mN/m is characteristic of liquid expanded
phase, whereas a value between 100 and 250 mN/m is
characteristic of the liquid condensed phase.14 The variation of
Figure 1. Surface pressure (π)-area per molecule (Am) isotherms of
(a) pyridinium-based discotic (Py-Tp) and (b) imidazolium-based
discotic (Im-Tp) molecules.
Figure 2. Molecular structure of Py-Tp and Im-Tp molecules with
approximate dimensions according to standard bond lengths and angles.
The triphenylene disc area shown in dashed circle is about 0.785 nm2.
|E| with Am for Py-Tp and Im-Tp are shown in Figure 3. The
|E| value showed a maximum of 83 mN/m at Am of 0.871 nm2/
molecule for Py-Tp. The maximum value of |E| for Im-Tp was
53.9 mN/m at Am of 0.977 nm2/molecule. The maximum |E|
value attained by Py-Tp is much higher than that attained by
Im-Tp monolayer. This indicates that the packing of the
molecules in the Py-Tp monolayer to be better than that of ImTp monolayer. This may be attributed to the steric hindrance
caused by an extra methyl group attached to the imidazolium
nitrogen atom. We find a sharp change in the values of |E|
indicating a phase transition at an Am of 1.6 nm2/molecule for
Py-Tp molecule and at an Am of 1.8 nm2/molecule for Im-Tp
molecule. The phase below these Am has much higher value of
|E| compared to the phase above these Am values. The |E| values
calculated as a function of Am (Figure 3) indicate that in both
Mesogenic Molecules at A-W and A-S Interfaces
Figure 3. Variation of compressional elastic modulus (|E|) with area
per molecule (Am) for Py-Tp and Im-Tp molecules.
J. Phys. Chem. B, Vol. 111, No. 38, 2007 11159
Figure 6. Brewster angle microscope images of Py-Tp. (a) Condensed
phase, (b) coexistence of 3D crystals and condensed phase corresponding to the collapsed state. The scale bar in each image represents 500
µm.
Figure 4. Equilibrium spreading pressure of Py-Tp and Im-Tp
molecules.
Figure 7. AFM topography of Im-Tp monolayer transferred onto
hydrophilic silicon substrates at (a) πt ) 5 mN/m, (b) πt ) 35 mN/m.
The respective height profiles corresponding to the white line on the
images are shown below. All of the images are shown without any
image processing except flattening.
Figure 5. The π-Am isotherm cycles of Py-Tp showing reversibility
from the collapsed state to the monolayer state with negligible
hysteresis.
Py-Tp and Im-Tp, the monolayer undergoes a phase transition
from an expanded phase to a condensed phase.
The ESPs of Py-Tp and Im-TP molecules were found to be
around 9 mN/m (Figure 4). The time scale for the molecules to
elute from the crystallites was of the order of a few seconds.
An interesting feature of the present two systems is that the
monolayer on expanding and compressing shows negligible
hysteresis. Also, in both the systems, the collapsed state
completely reverts back to its monolayer state (Figure 5). Such
a system with a reversible collapse may act as an ideal model
system to understand several disc-like biological systems for
example porphyrins, vitamins which play a significant role in
the living systems.
3.2. Brewster Angle Microscopy. The BAM images show
that the brightness increases gradually upon compression from
nearly zero surface pressure (Figure 6a). This transforms to 3D
crystals (Figure 6b) at the collapsed state. On expansion, these
crystalline domains disappeared, and the system reverted back
to the uniform intensity region indicating a monolayer state.
This observation confirms the reversibility of the film.
Some alkoxytriphenylene derivatives have been reported12 to
form stable Langmuir monolayers with Ao value in the range
0.7-1.0 nm2 but the Py-Tp and Im-Tp systems exhibit
Figure 8. Schematic representation of (a) lying down and (b) standing
up conformation at the A-W interface.
comparatively higher Ao values. This may be because of the
presence of ionic groups, pyridinium and imidazolium, which
tend to expand the films due to the direct electrostatic repulsion.15 On spreading the ionic discotic molecules, the small Brcounterions dissolve into the subphase due to the dissociation
of the ionic groups. The presence of these Br- counterions and
the charged insoluble film form an ionic double layer at the
surface. The rigid aromatic rings which form the core of the
discotics contain delocalized Π electrons. At low area per
molecule, these cores overlap due to the strong Π-Π stacking
interaction between the cores, thereby forming a 2D analogue
of the columnar phase.
3.3. Atomic Force Microscopy. The AFM topography of
the Im-Tp monolayer transferred at expanded phase (5 mN/m)
by horizontal method on a hydrophilic silicon substrate (Figure
7a) shows the height of the film to be about 0.7 nm with
reference to the substrate. This corresponds to the thickness of
the molecules lying parallel to the A-S interface. When the
monolayer was transferred at the condensed phase (35 mN/m)
11160 J. Phys. Chem. B, Vol. 111, No. 38, 2007
Nayak et al.
Figure 9. AFM topography of LB films of Py-Tp with two layers on hydrophobic silicon substrate at (a) πt ) 30 mN/m, (b) πt ) 35 mN/m, and
(c) πt ) 42 mN/m. The uniformly distributed rectangular voids at πt ) 35 mN/m are shown at scan ranges (d) 2 × 2 µm2 and (e) 1 × 1 µm2. The
respective height profiles corresponding to the white line on the images are shown below. All the images are shown without any image processing
except flattening.
by LB technique (Figure 7b), the topography shows a uniform
and homogeneous film with a height of about 2 nm with
reference to the substrate. This corresponds to the thickness of
the molecules lying normal to the A-S interface.
The Π-Am isotherms and the calculated |E| values indicate
a phase transformation from an expanded phase to a condensed
phase. The Ao values obtained in the expanded phase were 3.1
and 3.4 nm2/molecule and in the condensed phase were 1.15
and 1.40 nm2/molecule for Py-Tp and Im-Tp, respectively. Also,
the AFM topography images showed a height of about 0.7 nm
for the film transferred at expanded phase and a height of about
2 nm for the film transferred at condensed phase. Comparing
all of these results with the molecular dimensions (Figure 2),
we infer the molecules to have a face-on structure at the
expanded phase and an edge-on structure at the condensed
phase. This also indicates that the structure of the Langmuir
films was maintained throughout the transfer process from the
A-W interface to the A-S interface.
On the basis of these results, we schematically represent the
face-on (lying down) and edge-on (standing up) configuration
at the A-W interface as shown in Figure 8. In face-on
configuration, the core lies flat on the water surface and the
hydrocarbon tails (partially interdigited) extend away from the
interface. Such a configuration may be preferred if the core of
the molecule is capable of hydrogen bonding. It also maximizes
the configuration space available to the aliphatic tails. In the
edge-on configuration the core lies normal to the interface with
the polar end submerged in the water and bottom-most tails
extending roughly parallel and slightly away from the interface.
Such a conformation allows the polar oxygen to be in contact
with water, while minimizing water contact with the hydrophobic alkyl tails.12 This maximizes the Π-Π interactions
between the conjugated cores with the molecules arranged into
columns parallel to the water surface, forming the 2D analogue
of a columnar phase. Josefowicz et al.1 have reported the edgeon columnar structure in the LB films of triphenylene-based
discotic molecules with the column alignment predominantly
along the film deposition direction. However, they did not report
face-on arrangement based on AFM.1 In our system, the
presence of highly polar pyridinium and imidazolium moieties
together with six oxygen atoms at the periphery of the
triphenylene core have enhanced the hydrophilicity of the
molecules, thereby facilitating the face-on configuration at large
Am values at the A-W interface.
Figure 9 shows AFM images of the LB films of Py-Tp with
two layers transferred onto hydrophobic silicon substrates at
various πt. For the films transferred at πt below 30 mN/m,
irregular-shaped domains with significantly low surface coverage
(45%) were observed (Figure 9a). However, the films transferred
at πt of 35 mN/m (Figure 9b) and 42 mN/m (Figure 9c) revealed
Mesogenic Molecules at A-W and A-S Interfaces
a much higher surface coverage (75%) with narrow rectangular
voids elongated in the dipping direction of the LB transfer
process. The height profile yields a value of about 4.0 ( 0.2
nm. This corresponds to the value of two layers of the molecules
in the edge-on configuration.
Panels d and e of Figure 9 show different scan ranges for the
pattern with narrow rectangular voids (width 80-100 nm, length
250-400 nm) uniformly distributed over the entire region. We
find consistency in the scaling of the void shapes and sizes with
different scan ranges. The corresponding phase images show
uniform polarity of the film, indicating good packing of the
discotic core as expected due to Π-Π stacking. We also find
that the number of voids decreased and the film became more
compact with increase in π. However, full coverage could not
be obtained although these films showed smooth top surfaces
(Figure 9). The topography image of the LB films of Im-Tp
with two layers on hydrophobic silicon substrates shows
increased roughness with irregular voids throughout the film.
This suggests that the molecular organization is sensitive to the
particular head group-substrate interaction.
With discotic mesogens, the effects of Π-Π and steric
interactions on the molecular organization of the LB films are
stronger as compared to the case of the classical rodlike
molecules. UV-visible absorption spectra taken for the LB film
with two layers of Py-Tp yield peaks at 258, 275, 305, and 344
nm. Similar absorption peaks were also observed for the ImTp films. These are particularly significant, indicating Π-Π
stacking interaction between the conjugated triphenylene cores.
We suggest that the molecular organization of the ionic
discotic mesogens is driven by the interaction between the
substrates and the polar head (pyridinium, imidazolium) groups,
the steric intermolecular interactions, and the strong Π-Π
stacking interaction between the triphenylene cores. In addition,
the molecular packing is also affected by natural dewetting of
the substrate while draining or evaporation of entrapped water
within the films.16,17
4. Conclusions
The novel mesogenic molecules exhibited stable Langmuir
monolayer which show negligible hysteresis on expanding and
compressing. Also, the collapsed state completely reverts to the
monolayer state. As compared to the monolayers of nonionic
triphenylene derivatives reported so far, these cationic discotics
showed higher limiting area per molecule due to the direct
J. Phys. Chem. B, Vol. 111, No. 38, 2007 11161
electrostatic repulsion between the molecules within the film.
We find that the molecules Py-Tp and Im-Tp were arranged
into columns in the bulk as seen by our X-ray studies, and twodimensional columns at A-W and A-S interfaces as seen by
our surface manometry and AFM studies. The presence of a
cationic moiety in the discotic molecule opens the scope for
studying electrostatic interaction between such a molecule and
a negatively charged species such as DNA at interfaces. Our
system of ionic discotic mesogens can act as a model to
understand several disk-shaped biological systems such as
porphyrins and vitamins.
Supporting Information Available: X-ray diffraction patterns obtained for Py-Tp and Im-Tp molecules in the columnar
phase at 85 °C; AFM topography images of LB films with two
layers of Im-Tp molecules; AFM phase images of LB films
with two layers of Py-Tp molecules. This material is available
free of charge via the Internet at http://pubs.acs.org.
References and Notes
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