Aggregation induced emission switching and electrical properties of chain length dependent π-gels derived from phenylenedivinylene bis-pyridinium salts in alcohol-water mixtures

Article abstract of DOI:10.1039/c2jm35012b

Supramolecular π-gels were formed in a mixture of aliphatic alcohols and water for a series of chromophoric phenylenedivinylene bis-N-alkyl pyridinium salts (PPV) appended with terminal aliphatic hydrocarbon chains of different lengths. Gelation could be

Full text of DOI:10.1039/c2jm35012b

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PAPER
Aggregation induced emission switching and electrical properties of chain
length dependent p-gels derived from phenylenedivinylene bis-pyridinium salts
in alcohol–water mixtures†
a
Suman K. Samanta and Santanu Bhattacharya
ab
*
Received 27th July 2012, Accepted 7th September 2012
DOI: 10.1039/c2jm35012b
Supramolecular p-gels were formed in a mixture of aliphatic alcohols and water for a series of
chromophoric phenylenedivinylene bis-N-alkyl pyridinium salts (PPV) appended with terminal
aliphatic hydrocarbon chains of different lengths. Gelation could be controlled either by altering the
ratio of various alcohol–water mixtures or by changing the aliphatic chain length of the gelator. The
temperature- and the ratio-variation in the ethanol–water mixtures exhibited a tunable emission
behavior depending on the extent of aggregation which was promoted by aromatic p-stacking, van der
Waals and electrostatic interactions among the individual PPV units. Thus, a light-blue emission at
higher temperature (>40 ꢀC), a reddish-orange emission at low temperature (<20 ꢀC) and a white-light
ꢀ
emission at room temperature (25–30 C) were observed in solution. The gelators possessing longer
aliphatic chains exhibited a higher gel-melting temperature, increased viscoelasticity and shorter fiber
diameter based on a delicate hydrophobic/hydrophilic balance. A semiconducting nature of the
electrical conductivity was observed for the individual compounds and the magnitude of the current
increased with increasing width of the gel fibers upon decreasing the aliphatic chain length. A reversible
one-electron redox behavior was observed for the chromophore and the redox potential decreased with
the increase in the chain length. A diffusion-controlled redox behavior was observed for the gelators
with shorter aliphatic chains. However, the compounds with longer chains made the process diffusion-
limited.
mechanism and their structure-property relationship remain an
Introduction
important objective.
Low-molecular-mass gelators (LMMGs) are capable of immo-
bilizing the flow of liquids for the generation of functional soft-
materials.1 The solvents immobilized can be either organic or
aqueous or even a mixture of the two.2 Supramolecular associ-
ation among these LMMGs leads to physical gel formation,
which represents a macroscopic expression of their self-assembly
via optimization of various non-covalent interactions, such as
hydrogen bonds, p–p stacking, solvatophobic, dipole–dipole,
electrostatic, van der Waals forces etc.3 Self-association of these
LMMGs into three-dimensional aggregates provides a route for
the generation of nano- or micro-sized materials for the fabri-
cation of optoelectronic devices.4 Thus, rational design of gelator
molecules together with a proper understanding of the gelation
The self-association of LMMGs depends upon a variety of
factors, which includes, solvent, concentration, temperature, and
above all the molecular structure.1,2,5 A minimal structural
alteration such as the length of the flexible aliphatic part of a
gelator is often crucial for the manipulation of the gelation
induced properties depending on the subtle hydrophobic/
hydrophilic balance.6 Most of the gelators are reported to form a
gel in a single solvent entity. However, a mixture of solvents
could reflect tuning of the supramolecular organization of the
LMMGs.2 We have reported previously how the gelation by an
amino acid based LMMG in a mixture of two different miscible
hydrocarbons controls the self-assembly pattern.7 Therefore, an
optimum balance between the hydrophilic and hydrophobic
groups in the LMMG obtained either by varying the molecular
structure or changing the polarity of a solvent mixture could
control the gelation assisted properties.
aDepartment of Organic Chemistry, Indian Institute of Science, Bangalore
560012, India. E-mail: sb@orgchem.iisc.ernet.in; Fax: +91-80-23600529;
Tel: +91-80-22932664
bChemical Biology Unit, Jawaharlal Nehru Centre for Advanced Scientific
Research, Bangalore-560064, India
In the field of functional molecular assemblies, achieving
supramolecular control over the chromophore-linked molecular
systems remains a challenge. Therefore, the design of novel p-
conjugated electroluminescent organic materials is important as
they show potential applications in organic light emitting diodes
† Electronic supplementary information (ESI) available: Experimental
section, synthesis and characterization, supporting figures and tables.
See DOI: 10.1039/c2jm35012b
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(OLEDs),8 enhanced charge transport,9 light harvesting and
photonic devices etc.10 Furthermore, the gelation process has
been shown to be capable of providing novel chromophore
assemblies and as a consequence, gelators based on porphyrins,11
phthalocyanines,12 perylene,13 conjugated oligothiophenes14 and
other chromophoric moieties15 have indeed been developed. In
this context, the self-assembly of conjugated systems based on
oligo(p-phenylenevinylene)s has drawn considerable attention
especially for their excellent photophysical properties.16,17
Herein, we report a new class of chromophoric gelator based
on phenylenedivinylene bis-pyridinium (PPV) salts with a fixed
central PPV core and aliphatic chains of different lengths which
are connected to the pyridinium ends (1–7, Chart 1). These
compounds can immobilize a mixture of aliphatic alcohol and
water through varying propensities of van der Waals interactions
at the end functional groups. Such variations of the chain lengths
indeed influence their aggregation and gelation properties, such
as minimum gelator concentration, morphology, fiber diameter,
viscoelasticity, photophysical and electrical properties etc. A
combination of spectroscopic, microscopic, and diffraction
methods were used to probe the influence of the aliphatic chains
on the molecular gelation ability and chromophore induced
properties.
the desired product. All the new compounds 1–7 were adequately
1
characterized by FT-IR, H NMR, 13C NMR, mass spectrom-
etry and elemental analysis (see ESI†). An all-trans configuration
of the two vinyl moieties was confirmed from the coupling
constants in the 1H NMR spectrum in CD3OD (e.g. J ¼ ꢁ16 Hz
for 5, Fig. S1a†).
Gelation studies
A weighed amount of a particular compound was added to an
excess of the chosen solvent or the solvent mixture and the
resulting mixture was heated until the solid was completely dis-
solved in the closed vial. The clear solution so obtained was left
to cool in air at 25 ꢀC without any disturbance. Observations
with regard to gelation were recorded from time to time, and
each experiment was performed in duplicate. The state of the
materials was examined by the ‘‘stable-to-inversion of a test
tube’’ method.17 If a gel was formed, it was evaluated quantita-
tively by determining the minimum gelator concentration
(MGC) which is the minimum amount of the gelator required to
form a self-standing gel.
Gel melting temperature
The melting temperatures of the gels were investigated using the
dropping ball method. In this method, a steel ball (130 mg) was
placed on top of a 0.5 mL volume of gel in a closed glass vialꢂo1f
Experimental
8 mminnerdiameter. Thenthegelswereslowly heated(2ꢀCmin
)
Materials and general methods
in a water bath. The temperature at which the ball reaches the
All reagents, starting materials and solvents were obtained from
the best known commercial sources. Solvents were distilled and
dried prior to use. FT-IR studies were performed on a Perkin–
bottom of the vial is recorded as the gel melting temperature (Tgel).
1
Variable temperature H NMR
1
Elmer FT-IR Spectrum BX system. H and 13C NMR spectra
1H NMR spectra of gels of 4 and 5 were recorded on an AMX
400 MHz (Bruker) spectrometer using 60% CD3OD/D2O as the
solvent.
were recorded on a Bruker-400 Avance NMR spectrometer.
Chemical shifts were reported in ppm downfield from the
internal standard, tetramethylsilane. Mass spectrometry of
individual compounds was performed on a MicroMass ESI-TOF
MS instrument. Elemental analysis was recorded on a Thermo
Finnigan EA FLASH 1112 SERIES instrument.
UV-vis and fluorescence spectroscopy
The UV-vis and fluorescence spectroscopy of the gelators in
solution were recorded on a Shimadzu model 2100 spectropho-
tometer and Hitachi F-4500 spectrofluorimeter, respectively,
both equipped with a temperature-controller bath. Fluorescence
spectroscopy of the gels was performed using the front-face
geometry in a Jobin Yvon Horiba spectrofluorimeter.
Synthesis
Syntheses of all the amphiphilic PPV derivatives 1–7 were easily
accomplished using two step procedures as summarized in
Scheme S1 (see ESI†). Aliphatic chains of desired length were
first attached to 4-picoline as a quaternary salt, which in turn
increased the reactivity of the –CH3 protons at the para-position.
A base promoted aldol-type condensation reaction of the
respective 4-picolinium salt with terephthalaldehyde gave rise to
Fluorescence and polarized optical microscopy
Diluted solutions of 2–7 (1 ꢃ 10ꢂ4 M) in 60% ethanol–water
were drop cast on pre-cleaned glass slides and left overnight to
air-dry in a dust free environment and finally evacuated. The
fluorescence microscopy images were taken on an Olympus IX-
71 microscope with a blue excitation range of 340–400 nm.
Polarized optical microscopy images were taken of the same
samples using polarized light microscopy (Olympus BX51) and
the optical textures were recorded using cross-polarizers.
Scanning electron microscopy
The gels in 60% ethanol–water were melted by heating and
carefully drop cast onto brass stubs and were allowed to
Chart 1 Molecular structures of the PPV based gelators used in the
present study.
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freeze-dry. The samples were then coated with gold vapor and
analyzed on a Quanta 200 SEM operated at 15 kV.
based on a PPV backbone that only differs in chain lengths at the
termini. Herein, we describe the facile synthesis of these mole-
cules and report physical characterizations of their aggregation
induced properties.
X-ray diffraction
The gel samples prepared in 60% ethanol–water mixtures were
individually placed carefully on a pre-cleaned glass slide and
were allowed to freeze-dry. This yielded a self-supported cast film
of the individual gel on which the measurements were performed
using a Bruker Advance D8 instrument. The X-ray beam
generated with rotating Cu anode and Ka beam wavelength of
Gelation studies
The gelation efficiency of 1–7 was checked towards different
aliphatic and aromatic hydrocarbons, aliphatic alcohols and in
aqueous medium. Attempted gelation in hydrocarbons such as n-
heptane or toluene failed due to the lack of solubility. The
gelation also failed in a number of solvents including DMF,
dioxane, ethyl acetate, DMSO, CHCl3, acetone, ethanol, and
water. However, gelation was observed in acetonitrile for
example when compound 5 was used. This implied that the
polarity of the solvent plays a specific role in deciding the
hydrophilic/hydrophobic balance and hence the resulting gela-
tion. This observation prompted us to check gelation in a
mixture of solvents. We selected two solvents i.e., ethanol, where
all the compounds remained in solution; and water, where the
compounds with longer aliphatic chains (3–7) showed poor
solubility. Interestingly, physical gelation was observed in the
mixture of ethanol and water in different proportions indicating
an optimum polarity has been achieved in the mixture (Fig. 1a).
Also, the water molecules in the mixture provide extra stabili-
zation through hydration of the quaternary pyridinium moieties.
This result prompted us to check the gelation further in ethylene
glycol by imagining HOCH2CH2OH as a 1 : 1 mixture of
CH3CH2OH and water together. Interestingly, ethylene glycol
was also able to gelate the compounds. Following from this
result we tried to increase the polarity of the EtOH solution of
gelator 5. When we added oxalic acid into the ‘sol’, it again
ꢀ
1.5418 A was directed towards the film edge and scanning was
carried out up to a 2q value of 50ꢀ. Data were analyzed and
interpreted using the Bragg equation.
Rheological studies
An Anton Paar 100 rheometer using a cone and plate geometry
(CP 25-2) was utilized. The gap distance between the cone and
the plate was fixed at 0.05 mm. The gel was scooped onto the
plate of the rheometer. A stress amplitude sweep experiment was
performed at a constant oscillation frequency of 1 Hz for the
strain range 0.001 to 550 at 20 ꢀC. The rheometer has a built-in
computer which converts the torque measurements into either G0
(the storage modulus) or G00 (the loss modulus) in oscillatory
shear experiments. The values are plotted in the log scale.
Cyclic voltammetry
Cyclic voltammetric measurements were carried out at room
temperature (25 ꢀC) on an EG&G PAR 253 VersaStat poten-
tiostat/galvanostat using a three-electrode configuration con-
sisting of a glassy carbon working, platinum wire auxiliary, and
saturated calomel reference (SCE) electrode. Each compound
was dissolved in DMSO (1 mM) and 100 mM TBAP (tetrabutyl
ammonium perchlorate) was used as the supporting electrolyte.
All solutions were deoxygenated by passing a stream of pre-
purified N2 gas into the solution for at least 10 min prior to
recording the voltamograms.
Thermogravimetric analysis (TGA)
Thermal stability of the compounds (2–7) was analysed by
thermogravimetric measurements carried out in the presence of
air with the heating rate fixed at 10 ꢀC minꢂ1. The samples were
heated up to 1000 ꢀC on a TA instrument model SDT Q600 V8.2
Build 100 machine.
Current(I)–voltage(V) measurements
The gels of 2–7 in 60% ethanol–water (2 mg mLꢂ1) were drop-cast
on separate gold sputtered glass plates having an electrode gap of
30 mm, and allowed to air-dry in a dust free environment overnight.
Finally the samples were further dried in vacuo and the I–V char-
acteristics of each sample were measured in a two-probe electrode
using a semiconductor parameter analyzer 4155C (Agilent).
Fig. 1 (a) Typical sol–gel phenomena of gelator 5 in 60% ethanol–water
mixture under normal light (i) a ‘sol’, (ii) a ‘gel’ and under 365 nm UV
light (iii) a ‘sol’, (iv) a ‘gel’. (b) Physical gelation efficacies of 4, 5 and 6 in
different ratios of EtOH in H2O. (c) Comparison of the gelation efficiency
of 2–6 in 60% ethanol–water mixture. (d) Physical gelation efficacy of 4, 5
and 6 in different aliphatic alcohols in the presence of 40% H2O and also
in ethylene glycol.
Results and discussion
One of the main aims of this study is to develop a structure-
property relationship for a closely related new family of gelators
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turned into a gel. Clearly, the stabilization of the repulsive
positive charge is key and maintaining an optimum hydrophobic/
hydrophilic balance leads to the physical gelation.
gelators at a particular concentration. For example, at 8 mg
mLꢂ1 the Tgelꢀ for 4 was 47 ꢀC. Atꢀthe same concentration, as
ꢀ
much as an 8 C increase for 5 (55 C) and another 15 C for 6
The gelation efficiency of 2–6 was estimated in terms of the
minimum gelator concentration (MGC) by increasing
the percentage of water in the ethanol–water mixture (Fig. 1b).
The MGC decreased progressively with the increasing percentage
of water up to 60% ethanol–water and beyond that the MGC
again increased (Table S1†). Further addition of water rendered
these compounds insoluble. Thus, an optimum ratio of 60%
ethanol–water showed a minimum value of MGC for 4, 5 and 6
and this was taken as the standard medium for further studies.
The gelation efficiency was compared for 1–7 in 60% ethanol–
water mixtures. In this medium, while compound 1 remained
soluble, compound 7 showed only partial/weak gelation, clearly
due to the mismatch in the hydrophilic/hydrophobic balance.
Stable gelation was observed for 2–6 (Fig. 1c) and the lowest
value of MGC was obtained for 5, which has two tetradecyl
chains attached to the central chromophore. Also a trend in the
MGC for gelation was observed where, with decreasing chain
length, the MGC increased (Table S2†).
(70 ꢀC) ꢀwere observed. At 14 mg mLꢂ1, the Tgel for 3 was 37 ꢀC,
ꢀ
and 52 C for 4 and 57 C for 5 were observed. Clearly, greater
van der Waals interactions were experienced for the thermally
induced transitions with increasing chain length, giving rise to a
19
progressively greater Tgel
.
1
Variable temperature H NMR study of the gels
1H NMR spectra of the gels in 60% CD3OD in D2O at different
temperatures provide an insight into the self-assembly process of
1
the thermoreversible gelation. As a gel (at 25 ꢀC), the H NMR
signals of 5 were broadened and almost quenched due to the
restriction of the molecular tumbling to produce zero-average
dipolar coupling (Fig. 3).20 The broadening and subsequent
disappearance of the proton signals suggest intervention of the
strong intermolecular interactions leading to the aggregation of
the PPV units and subsequent self-organization of the molecules
leading to gelation. On gradual increase of the temperature, as
the gel melts, an isotropic distribution of the molecules was
achieved in solution and their NMR signals appeared gradually,
indicating a disruption of the self-organization. The protons
which appeared broadened in the gel phase became well-resolved
The gelation efficiency was also checked in different aliphatic
alcohols that included MeOH, n-PrOH, i-PrOH, t-BuOH in 60%
alcohol–water mixtures for the gelators 4, 5 and 6. These
compounds were found to be more efficient gelators compared to
2 and 3. In each alcohol–water mixture, gelator 5 was found to be
more efficient compared to 4 and 6 (Fig. 1d). Efficient gelation
was observed in EtOH, i-PrOH and ethylene glycol by each of 4–
6 (Table S3†).
ꢀ
as the gel melted on increasing the temperature. Above 45 C,
there was a dramatic sharpening of all the resonance lines upon
complete gel melting.21
Similar behavior of line-broadening was also observed for 4 in
60% CD3OD in D2O (Fig. S1b†). This kind of observation has
been made for various other types of LMMG gel,22 which provide
evidence of the manifestation of self-assembly in the gel phase.
Gel-melting temperatures
The melting temperature (Tgel) of the gels of 3–6 in 60% ethanol–
water were measured using the dropping ball method (Fig. 2).18
Tgel increases progressively with increasing concentration of the
individual gelators. This indicates that with increasing concen-
tration of the gelator, the density of the gel assembly increases to
ensure participation of a greater number of gelator molecules per
unit volume in the thermally induced transitions.17a Above a
certain concentration, the Tgel reached a plateau. In the case of 6,
when the concentration reached 10 mg of compound in 1 mL
solvent, this quantity of compound could not be solubilized,
indicating a saturation was reached. The Tgel increased gradually
upon increasing the length of the hydrocarbon chain of the
Rheological properties
The mechanical stability and the flow behavior of gels were
measured by rheological studies.23 In an oscillatory frequency
Fig. 3 Variable temperature 1H NMR spectra of the gel of 5 (7 mg
mLꢂ1) in 60% CD3OD in D2O. The spectrum is assigned to the protons in
the molecular structure for convenience.
Fig. 2 Melting temperatures of different gelators (3, 4, 5, 6) with
increasing concentrations in a 60% ethanol–water mixture.
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sweep experiment, the storage (G0) and loss moduli (G00) were
measured as a function of angular frequency (u) at a fixed strain
(0.01%). The G0 and G00 of the gels of 3–6 in 60% ethanol–water
mixtures showed a plateau region over the entire angular
frequency range (1–100 rad sꢂ1) (Fig. S2a and b†). At a partic-
ular concentration (10 mg mLꢂ1), G0 showed greater values with
increasing length of the aliphatic chain i.e. 6 (1040 Pa) > 5 (610
Pa) > 4 (230 Pa). Increasing the concentration (20 mg mLꢂ1) of
the gelators showed a significant increase in the G0 for individual
gelators although the trend remained the same i.e. 5 (5600 Pa) > 4
(2950 Pa) > 3 (1270 Pa). In each case a greater G0 over G00 sug-
gested a substantial elastic response of the gels and the ratio of
G0/G00 was found to be 7.5 for 4, 9.7 for 5 and 12.2 for 6 at 10 mg
mLꢂ1. Also ꢁ7-fold higher G0 than G00 was observed when the
excited at the near UV region (340–400 nm), the thin-films of 2–7
obtained from 60% ethanol–water mixtures showed yellowish-
orange emission and the presence of three-dimensional fiber
bundles was clearly visible (Fig. S3†). In the case of 2–5, fibers of
high aspect ratios were observed with diameters within the range
ꢁ4–5 mm. However, with increase in the hydrocarbon chain
length, the aspect ratio decreased and the fibrous nature started
to diminish. Fibers with lower aspect ratio were observed in the
case of compound 6 and even smaller fibers were seen for 7.
To discern the superstructures and the gel fiber morphologies,
scanning electron micrographs (SEM) of the freeze-dried gels
derived from the different gelators were recorded. The gels (2–7)
showed the presence of three-dimensional fibrillar networks with
uniform fiber diameters in all instances (Fig. 5). Progressively
decreasing fiber diameter and aspect ratio were observed with
increasing aliphatic chain length. The fiber diameters of the
xerogels ranged from 4–5 mm for 2, 2–3 mm for 3, 1–2 mm for 4, 1–
1.5 mm for 5, 700–800 nm for 6 and 300–400 nm for 7. Clearly, a
lateral association of the individual nano-fibers resulted in a
collation of themselves to form tape-like fibers for the shorter-
chain analogues. This means that the gel fibers probably consist
of hierarchical structures and growth of the monomeric entity
into the superstructure depends on the fine balance between the
aromatic and aliphatic parts.
concentration was raised to 20 mg mLꢂ1
.
In an oscillatory amplitude sweep experiment, the complex
modulus (G*) is composed of two components: (i) G0, repre-
senting the ability of the deformed material to restore its original
geometry, and (ii) G00, representing the tendency of a material to
flow under stress. For the gels, G0 is generally an order of
magnitude greater than G00, showing the dominant elastic
behavior of the system. An applied stress above which the gel
starts to flow (G00 > G0) is called the yield stress where the viscous-
liquid-like behavior dominates. Under applied stress the gels of
3–6 in 60% ethanol–water showed 6–7 times higher values of G0
over G00 indicating a dominating viscoelastic-solid-like behavior
(Fig. S2c and d†). When the gels succumbed to the applied stress,
they began to flow at 1.0 Pa for 4, 3.1 Pa for 5 and 7.2 Pa for 6 at
10 mg mLꢂ1. With increasing concentration the yield stress
values increased and became 5.7 Pa for 3, 11.1 Pa for 4 and 33.2
Pa for 5. Therefore, the viscoelastic-solid-like behavior (G0) and
the yield stress were increasingly dominant with increasing chain
length and also with increasing concentration of the gelators
(Fig. 4). Clearly, the aliphatic chains play a significant role in
modifying their mechanical properties.
X-ray diffraction studies (XRD)
X-ray diffraction studies were undertaken to acquire information
about the packing patterns of the gelator molecules in the
supramolecular self-assemblies. The diffraction pattern of the
xerogel of 2 obtained from 60% ethanol–water shows five peaks
corresponding to the d-values of 2.57 nm, 1.25 nm (which is in the
ratio of 1 : 1/2), 0.82 nm (1 : 1/3), 0.61 nm (1 : 1/4) and 0.49 nm
(1 : 1/5) (Fig. 6). This indicates a lamellar arrangement of the
aggregates24 of the xerogel of 2 with an interlayer spacing of
2.57 nm. The diffraction patterns of each of these xerogels
showed sharp Bragg’s reflection peaks ensuring highly ordered
lamellar organization was present in each case (Table 1). It may
be noted that with increasing hydrocarbon chain length, the
higher order Bragg’s reflections become weaker and eventually
they disappear. This indicates that the crystallinity decreases with
increasing aliphatic chain length of the gelators, which is in line
with the microscopic images. However, in each case the first
Morphological characterization of the gels
Microscopic studies were undertaken to observe the morphology
of the gel network. The luminescent nature of the gelator mole-
cules allowed us to observe the superstructures created in the self-
assemblies using fluorescence microscopy (FM).17b–d When
Fig. 4 A bar diagram showing the G0 and yield stress values of different
gels at different concentrations. A 60% ethanol–water mixture was used
in each experiment.
Fig. 5 Scanning electron microscopy images of the freeze-dried samples
of (a) 2, (b) 3, (c) 4, (d) 5, (e) 6 and (f) 7.
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Fig. 6 X-ray diffraction patterns of the freeze-dried gels of 2–7 in 60%
Fig. 7 Proposed model depicting the probable self-assembly motif of 5
ethanol–water mixtures.
in the gel as evidenced from the XRD studies.
diffraction peak appeared more intense compared to the higher
order reflection peaks. Only the alternate peaks appeared for 3–5.
Depending upon the aliphatic chain length, the Bragg’s reflec-
tions also vary for how the peaks corresponding to the planes 1/3
(marked by ‘*’) and 1/5 (marked by ‘**’) appear in Fig. 6.
Interestingly another set of peaks appeared invariably in each
of the gelators. These can be assigned due to the presence of an
aromatic segment which is identical in every instance (Table 1).
These peaks provide valuable information about the arrange-
ment of the aromatic segment in the self-assembly. The p–p
stacking distance emerged at 0.39 nm, which is slightly above the
reported values25 and this could be due to the presence of two
positive charges at the end of the aromatic segment. The lengths
of the appended alkyl chains were obtained from the ground
state geometry of compounds 2–7 optimized through B3LYP/6-
31G* level computations (Fig. S4†). A model has been proposed
by considering the above mentioned results (Fig. 7).
The crystalline lamellar nanofibers were further examined
under a polarized optical microscope (POM) in order to confirm
the presence of birefringent textures. Thin films obtained from
solutions of the gelators in 60% ethanol–water mixtures indeed
showed the presence of birefringent textures in every instance.
This indicates an anisotropic growth of the three-dimensional
aggregates of the PPV molecules leading to the formation of
nanofibers (Fig. S6†).17 Also, the fiber diameters decreased
progressively with increase in the aliphatic chain length, which
agrees well with the FM and SEM studies. As observed under
XRD, the thin film obtained from EtOH is also crystalline in
nature which again showed the presence of birefringent texture
although the aspect ratio is smaller (Fig. S7†). The birefringent
textures were also evident in the gel of 5 in 60% ethanol–water
mixture.
XRD analysis reveals a periodic lamellar organization with the
repeat distances being slightly less than double the alkyl chain
length. This indicates that the hydrophobic chains are inter-
locked with each other so that they introduce strong van der
Waals interactions among the long chains. A J-type aggregation
in the aromatic segments may give rise to the d-spacing of
0.85 nm. The thickness of a single layer could be assigned to
0.41 nm (as obtained from the breadth of the optimized molec-
ular structure) and the distance between the successive layers is
0.79 nm. The XRD pattern of 5 obtained from only ethanol
revealed similar Bragg’s peaks, indicating a similar self-assembly
pattern remained in the thin film (Fig. S5†).
UV-vis and fluorescence spectroscopy
The chromophoric nature of this class of molecules allows
manifestation of interesting optical and photophysical proper-
ties.26 The absorption maxima (lmax) of each of 2–7 (1 ꢃ 10ꢂ5
M) in ethanol at 25 ꢀC appeared at 400 nm (S0 / S1) in
addition to a small band at 250 nm (S0 / S2) due to the
presence of the same chromophore unit (Fig. S8†). The lmax did
not shift when the absorption spectrum was recorded for
example with compound 5 in 60% ethanol–water where it
transformed into a physical gel. The emission maximum
appeared at 470 nm when excited at 380 nm for each of 2–7
(1 ꢃ 10ꢂ5 M) in ethanol.
The aggregation leading to the gelation in ethanol–water
mixture exhibited a drastic visual color change under normal
daylight as well as under a 365 nm UV lamp (Fig. 1a). The
color changed from intense light-blue to yellowish-orange
upon sol-to-gel transformation, as observed under the UV-
light. The self-assembly of the gelators clearly depends on the
polarity (the hydrophobic/hydrophilic balance) of the medium
as they formed a ‘sol’ in ethanol and an ‘aggregate’ in water.
Thus, by varying the ratio of ethanol–water it may be possible
to modulate their aggregation. Snapshots of the solution of 5
(1 ꢃ 10ꢂ5 M) in different ratios of ethanol–water mixtures
were viewed under a 365 nm UV lamp (Fig. 8a) and also under
normal daylight (Fig. S9†). The solution in ethanol alone
Table 1 Bragg’s reflection peaks (d-values)a obtained from XRD studies
for the xerogels of 2–7 in 60% ethanol–water mixture
1
1/2
1/3
1/4
1/5
Rb
2nd set of peaks
2
3
4
5
6
7
2.57 1.25 0.82 0.61 0.49 1.11 0.87 0.80 0.42 0.39
2.74
2.93
3.33
0.89
0.97
1.06
0.53 1.36 0.85 0.79 0.42 0.39
0.58 1.62 0.84 0.78 0.41 0.39
1.87 0.85 0.79 0.41 0.39
3.64 1.75 1.15
3.74 1.84 1.22 0.91
2.13 0.85 0.79 0.41 0.39
2.39
0.40 0.39
a
All the d-spacing values are expressed in nm. b R represents the length
of the hydrocarbon chain from the quaternary N.
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(100%) and up to 40% of ethanol–water mixture showed highly
fluorescent light-blue emission representing a ‘monomeric’
To imitate a thermoreversible sol–gel transition, a 35%
ethanol–water solution of 5 was subjected to temperature
variation. At higher temperature (50 ꢀC) the lmax appeared at
400 nm in addition to a small band at 250 nm which resembled
the monomeric absorption (Fig. 8c). When the temperature was
lowered, the intensity of the lmax decreased gradually and a red-
shifted band appeared at 480 nm. The generation of an aggre-
gated state was also accompanied by a gradual loss of the shape
and intensity of the 250 nm band along with a ꢁ10 nm red-shift
of the lmax which is suggestive of a possible J-type aggrega-
tion.28 Under fluorescence, at higher temperature (50 ꢀC) the
emission was observed at 470 nm due to the molecularly dis-
solved species (Fig. 8e). On decreasing the temperature the
intensity of the band decreases progressively with the gradual
generation of a red-shifted band at 575 nm indicating that an
aggregated state has been reached. The presence of clear iso-
sbestic points in the UV-vis spectra and an isoemissive point
(565 nm) in the fluorescence emission spectra indicated a
thermal equilibration between the aggregated and non-aggre-
gated states, which is approximately reversible irrespective of
whether the temperature was varied in the low-to-high or high-
to-low direction.
emission. But at the ratio of 30–0% ethanol–water,
a
yellowish-orange emission appeared indicating an aggregated
emission. This suggests that the aggregation of the gelator
molecules in the mixed solvent system is assisted by intermo-
lecular p–p stacking and van der Waals interactions. These
solutions were then analyzed using UV-vis and fluorescence
spectroscopies.
Under UV-vis, the lmax appeared at 400 nm when the ratio of
ethanol–water was between 100 and 50% (Fig. 8b). However, the
intensity of the lmax decreased when the ratio became 40%
although there was no significant shift in the lmax. Beyond this
ratio, the absorption patterns changed drastically. A new band
appeared at 370 nm followed by a broad band at 430 nm and a
hump-like shoulder band at 480 nm. Also the small band at
250 nm experienced a red-shift. Under fluorescence, the solutions
showed intense monomeric emission at 470 nm up to the
ethanol–water ratio of 50% (Fig. 8d). The intensity decreased at
40% ethanol–water without any change in the emission maxima
and beyond that a significantly hypochromic, red-shifted emis-
sion maximum at 575 nm was observed for 30–0% of ethanol–
water, indicating an aggregated emission.27 These studies indi-
cate that the monomeric species form self-assembled species
when the amount of ethanol is below 40% in the ethanol–water
mixture.
When we looked at the variable temperature emission spectra
of 5 in 35% ethanol–water closely, we found that at room
ꢀ
temperature (ꢁ25–30 C) the spectra covered the whole visible
range of emission (ꢁ450–650 nm). This prompted us to observe
the solution under UV light (365 nm) at room temperature.
Interestingly, the solution showed a white-light emission under
this condition (Fig. 9). However, the same solution when cooled
(ꢁ10 ꢀC), showed a yellowish-orange emission and when heated
(ꢁ50 ꢀC), it showed a light-blue emission. We have recorded the
emission spectra of these three solutions after pre-adjusting their
emission colors visually under the UV lamp. The emission
spectra were consistent with the previous observations and
resembled that of a ‘monomeric’ emission (470 nm) at high
temperature, aggregated emission (575 nm) at low temperature
and a wide-range emission (ꢁ450–650 nm) at room temperature.
The CIE chromaticity diagram exhibited the coordinates for the
white-light emission (0.32, 0.39) which are quite close to pure
white-light emission (0.33, 0.33). The excitation spectra of these
three solutions resembled the spectral patterns as observed under
the variable temperature absorption spectra (Fig. S11†). There-
fore, it may be assumed that, at room temperature a certain
proportion of the monomeric and aggregated species exist
Fig. 8 (a) Snapshots showing solutions of 5 (1 ꢃ 10ꢂ5 M) in different
ethanol–water mixtures (% of EtOH) (A) 100%, (B) 90%, (C) 80%, (D)
70%, (E) 60%, (F) 50%, (G) 40%, (H) 30%, (I) 20%, (J) 10%, (K) 0%. (b)
UV-vis and (d) fluorescence emission spectra of 5 (1 ꢃ 10ꢂ5 M) at
different ratios of ethanol–water mixtures (lex ¼ 380 nm). Variable
temperature (c) UV-vis and (e) fluorescence spectra of 5 (2 ꢃ 10ꢂ5 M) in
35% ethanol–water mixture (lex ¼ 380 nm).
Fig. 9 Variable temperature fluorescence spectra of 5 (2 ꢃ 10ꢂ5 M) in
35% ethanol–water mixture (excited at 380 nm).
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Table 2 Cyclic voltammetry peak potentials (Ered, Eox) and redox
potentials (E) obtained from differential pulse voltammetry for 2–7a
together in the mixture which could cover the whole visible
emission spectral range. This depends upon the temperature,
ratio of ethanol–water in the mixture and most importantly the
E0 (V)
from DPV
concentration of the gelator. Thus, when a gel of 5 (10 mg mLꢂ1
)
Ered (V)
Eox (V)
DE (V)
Ipc/Ipa
E1/2 (V)
in 60% ethanol–water was heated and then cooled it did not show
white-light emission, instead it showed a direct transformation
from light-blue to yellowish-orange emission. This is due to the
direct and fast transformation from the monomeric to the
aggregated species as the concentration was very high
(Fig. S12†). Thus, white-light emission can be observed from a
single component depending on the aggregate fraction in the
mixture, unlike the other white-light emitting substances repor-
ted in the literature which generally contain more than one
chromophore.29
2
3
4
5
6
7
ꢂ0.853
ꢂ0.853
ꢂ0.853
ꢂ0.832
ꢂ0.813
ꢂ0.803
ꢂ0.787
ꢂ0.787
ꢂ0.721
ꢂ0.717
ꢂ0.705
ꢂ0.695
0.066
0.066
0.132
0.115
0.108
0.108
1.2
1.2
1.1
1.2
0.9
0.8
ꢂ0.820
ꢂ0.820
ꢂ0.787
ꢂ0.774
ꢂ0.759
ꢂ0.749
ꢂ0.804
ꢂ0.804
ꢂ0.806
ꢂ0.783
ꢂ0.771
ꢂ0.761
a
Conditions: scan speed 100 mV sꢂ1, temperature 25 ꢀC, supporting
electrolyte 100 mM TBAP, solvent DMSO, three electrode system:
SCE as reference, Pt wire counter and glassy carbon working electrode.
E1/2 is the half-wave potential ¼ (Ered + Eox)/2.
Cyclic voltammetry
electron exchange process.32 However, the DE value for the
compounds with longer aliphatic chain length departed from
the constant value (0.059 V) suggesting that probably structural
reorganization and aliphatic chain reorientation take place on
reduction.30 The half-wave potential (E1/2) can be obtained from
a voltammogram by calculating the average value of the anodic
and cathodic peaks. The subscript 1/2 indicates that the
potential is obtained approximately at the half-height of the
cathodic and anodic peaks and at these points the concentra-
tions of the reduced and oxidized species on the electrode are
equal.33 The redox potentials (E0) were also obtained from the
The structural features of such compounds prompted us to
examine their electronic redox behavior. Accordingly, the
cyclic voltamograms (CV) were recorded for a freshly prepared
solution of 2–7 (10ꢂ3 M) in DMSO at 100 mV sꢂ1 scan rate
and using a glassy carbon working electrode. The cyclic vol-
tammograms exhibited a cathodic peak (reduction) with a
directly associated anodic peak (oxidation) in the reverse scan
(Fig. 10a). In each case only one oxidation–reduction wave
was observed throughout the scan range +1 to ꢂ2
V
(Fig. S13a†). Corresponding reduction (Ered) and oxidation
(Eox) potentials were obtained from the CV plots (Table 2).
The potentials were found to be greater for the compounds
with shorter aliphatic chains (2 and 3). Interestingly, a sudden
jump in Eox was observed for 4 (with –C12H25 chain) and after
that a gradual decrease in the Ered and Eox was observed with
the increase in the aliphatic chain length (Fig. 10b). Clearly,
the electron transfer is strongly influenced by the length of the
alkyl chains.
differential
pulse
voltammetry
(DPV)
measurements
(Fig. S13b–g†). E1/2 decreases progressively with increasing
aliphatic chain length in the series and is consistent with the E0
values obtained from DPV.
The peak current (Ip) for compound 2 increases linearly as a
function of square root of the scan rates (v1/2) ranging from 25–
300 mV sꢂ1 indicating that the electrode reactions are diffu-
sion-controlled for the redox process (Fig. S14†).34 However, a
deviation from linearity in the Ipc vs. v1/2 is observed for 5, a
member with longer aliphatic chains, indicating that at suffi-
cient overpotential, the reaction is diffusion-limited.35 In both
the cases, compounds 2 and 5 display larger peak separations
at higher scan rates, which may be due to a relatively slow
electron transfer rate between the oxidized species and the
electrode.36 The plot of Ipc vs. v1/2 for 2 shows a linear fit
passing through y ¼ 0. Using the ‘slope’, the diffusion coeffi-
cient (D) for reduction can be calculated using the well known
Randles–Sevcik eqn (1).37
The ratio of the reduction (Ipc) and the oxidation (Ipa
)
currents (Ipc/Ipa) is close to 1, which indicates the electro-
chemical reversibility of the electron transfer process (Table 2).30
The Ered and Eox for 2 appeared at ꢂ0.853 and ꢂ0.787 V,
respectively, and show a sudden jump for compound 5 (Ered
ꢂ0.832 V and Eox ꢂ0.717 V) as observed clearly in Fig. 10a.
The experimental DE value (Ered ꢂ Eox) for 2 is 0.066 V which is
close to 0.059 V (theoretically expected for an electrochemically
reversible one-electron redox process).31 Using the DE value, the
number of electrons (n) involved in the redox process is calcu-
lated from the equation DE ¼ 0.059/n, which indicates a one
Ipc ¼ (2.687 ꢃ 105) n3/2 v1/2 D1/2 A C
(1)
Slope ¼ (2.687 ꢃ 105) n3/2 D1/2 A C
where, n ¼ the number of electrons appearing in the half-
reaction for the redox couple, A is the electrode area (0.2827
cm2) and C is the concentration of the electrolyte solution (10ꢂ6
mol cmꢂ3). Using the magnitude of slope (5.1909 ꢃ 10ꢂ5
A
s1/2
V
ꢂ1/2) and considering a one-electron transfer reaction, the
diffusion coefficient (D) of 2 is calculated to be 4.67 ꢃ 10ꢂ7
Fig. 10 Cyclic voltammogram of 1 mM solution of (a) 2 and 5 and (b)
comparison of 2–7 in DMSO at a scan rate 100 mV sꢂ1
.
cm2 sꢂ1
.
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Thermogravimetric analysis (TGA)
The decomposition behavior of 2–7 was elucidated using TGA.
A typical weight-loss profile shows a two-step decomposition
pathway for this class of molecules (Fig. S15†). A sharp
decomposition between 250 and 340 ꢀC with ꢁ60% mass loss
followed by another comparatively slower decomposition
ꢀ
between 340 and 600 C with more than 95% weight loss. The
derivative plots of the TGA curves show the peak of the mass loss
(the peak degradation temperature) between 299 and 330 ꢀC for
the first step followed by 481–535 ꢀC for the second step of
decomposition (Fig. S16†). The weight loss in the first step is due
to the degradation of the alkyl chains and the second step
corresponds to the loss of aromatic residue of the gelators.38 The
peak degradation temperatures are listed in Table 3. The
decomposition temperatures and hence the thermal stability
increased progressively with increasing aliphatic chain length
appended to the PPV core.
Fig. 11 Electrical conductivity studies showing I–V characteristics of 2–
7 in a two-probe electrode. Inset shows a magnified plot for easier
comparison.
Conclusions
Electrical conductivity (I–V) measurements
We have described the synthesis and aggregation properties
exhibited by a new family of PPV derivatives tailored with
different aliphatic chains on the periphery. With increasing
length of the aliphatic chain, their gelation ability increases.
However, the presence of excess hydrophobic moiety at the
periphery weakens the gelation ability. Spectroscopic studies
unravel a thermoreversible self-assembly leading to emission
switching depending on the temperature and a white-light
emission at room temperature. A high degree of crystallinity and
the presence of lamellar structures in the self-assembly of the
gelators were evident from the X-ray diffraction studies.
Microscopic studies suggest that the shorter alkyl chains favor
the self-assembly of PPVs leading to a greater aspect ratio of the
fibrillar networks. The electrical conductivity increases progres-
sively with larger fiber diameter i.e. with the shorter aliphatic
chains. Also, the redox reaction at the electrode surface depends
on the aliphatic chain length on the PPV core and the redox
potentials decrease with increasing aliphatic chain length.
However, their mechanical and thermal stabilities increase
gradually with the increase in the chain length. Clearly, the subtle
balance between the hydrophobic and hydrophilic groups in the
PPV backbone holds the key in tuning the gelation phenomenon
assisted with the modulation of the ethanol–water solvent
polarity. These systems therefore provide a convenient way for
the modulation of macroscopic properties of the resulting soft
materials. This study thus provides opportunities for the devel-
opment of nanostructured advanced materials from the gelation
of a variety of linear, p-conjugated systems and organic semi-
conductors, which may find applications in the emerging field of
supramolecular electronics.
Since the gelators 2–7 have p-electron conjugation in their
backbones and also carry two cationic charges, it would be
interesting to investigate their electrical conductivity across the
fibers created by the intermolecular stacking interactions
through the aromatic surface in the xerogel matrices. The I–V
measurements for the dried gels of 2–7 were measured in a two
probe electrode. In the low voltage region (ꢁ1–10 V) the curve
followed Ohm’s law (V/I ¼ R) (the linear region of the curve)
and the corresponding resistance (R) was calculated from the
I–V curve (Fig. 11).39 However, in the higher voltage region, the
I–V curves show deviation from the linearity which is charac-
teristic of a typical semiconducting nature of the gels.40 On
increasing the voltage, the current increased in a nonlinear
fashion. Notably, the device did not collapse even after applying
100 V, which showed the robustness of the system comprising
the nanoassembly. The current at voltages of 10 V, 50 V and
100 V and the calculated resistance at 10 V are recorded
(Table S4†). The magnitude of current increases with the
decrease of the length of the aliphatic hydrocarbon chains. It
has been observed from the microscopic studies that the fiber
diameter increases with the decrease in the aliphatic chain
length. Thus, with larger fiber diameter and higher aspect ratio,
the electrical conductivity increases. It is also possible that with
increasing chain length the weight fraction of the semicon-
ducting chromophore in the gelator decreases and hence the
conductivity decreases.
Table 3 Peak degradation temperatures of 2–7 in the two-step decom-
position profiles based on thermogravimetric analysis
Loss of aliphatic
parts (ꢀC)
Loss of aromatic
parts (ꢀC)
Notes and references
1 (a) P. Terech and R. G. Weiss, Chem. Rev., 1997, 97, 3133–3159; (b)
T. Ishi-I and S. Shinkai, Top. Curr. Chem., 2005, 258, 119–160; (c)
S. Bhattacharya and S. K. Samanta, Langmuir, 2009, 25, 8378–
8381; (d) S. Bhattacharya and S. N. G. Acharya, Chem. Mater.,
1999, 11, 3504–3511.
2
3
4
5
6
7
299
295
300
300
306
330
481
502
503
505
505
535
2 (a) I. Furman and R. G. Weiss, Langmuir, 1993, 9, 2084–2088; (b)
N. M. Sangeetha and U. Maitra, Chem. Soc. Rev., 2005, 34,
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 25277–25287 | 25285

View Article Online
821–836; (c) P. Terech, N. M. Sangeetha and U. Maitra, J. Phys.
Chem. B, 2006, 110, 15224–15233; (d) J. Cui, J. Zheng, W. Qiao and
X. Wan, J. Colloid Interface Sci., 2008, 326, 267–274; (e) G. Wang,
H. Yang, S. Cheuk and S. Coleman, Beilstein J. Org. Chem., 2011,
7, 234–242.
3 (a) M. Ikeda, M. Takeuchi and S. Shinkai, Chem. Commun., 2003,
1354–1355; (b) L. A. Estroff and A. D. Hamilton, Chem. Rev.,
2004, 104, 1201–1217; (c) J.-H. Ryu and M. Lee, J. Am. Chem.
Soc., 2005, 127, 14170–14171; (d) E. Lee, J.-K. Kim and M. Lee,
Angew. Chem., Int. Ed., 2008, 47, 6375–6378; (e) L. Chen, S. Revel,
K. Morris and D. J. Adams, Chem. Commun., 2010, 46, 4267–4269.
4 (a) T. Goodson III, W. Li, A. Gharavi and L. Yu, Adv. Mater., 1997,
9, 639–643; (b) F. Wurthner, Chem. Commun., 2004, 1564–1579; (c)
K. Sugiyasu, N. Fujita and S. Shinkai, Angew. Chem., Int. Ed.,
2004, 43, 1229–1233; (d) M. Llusar and C. Sanchez, Chem. Mater.,
2008, 20, 782–820; (e) K. Sugiyasu, S. Kawano, N. Fujita and
S. Shinkai, Chem. Mater., 2008, 20, 2863–2865.
15 (a) P. Mukhopadhyay, Y. Iwashita, M. Shirakawa, S.-i. Kawano,
N. Fujita and S. Shinkai, Angew. Chem., Int. Ed., 2006, 45, 1592–
1595; (b) T. Shu, J. Wu, M. Lu, L. Chen, T. Yi, F. Li and
C. Huanga, J. Mater. Chem., 2008, 18, 886–893; (c) J. W. Chung,
S.-J. Yoon, S.-J. Lim, B.-K. An and S. Y. Park, Angew. Chem., Int.
Ed., 2009, 48, 7030–7034; (d) R. Abbel, R. van der Weegen,
W. Pisula, M. Surin, P. Leclere, R. Lazzaroni, E. W. Meijer and
A. P. H. J. Schenning, Chem.–Eur. J., 2009, 15, 9737–9746; (e)
K. V. Rao, K. Jayaramulu, T. K. Maji and S. J. George, Angew.
Chem., Int. Ed., 2010, 49, 4218–4222.
16 (a) A. El-ghayoury, A. P. H. J. Schenning, P. A. van Hal, J. K. J. van
Duren, R. A. J. Janssen and E. W. Meijer, Angew. Chem., Int. Ed.,
2001, 40, 3660–3663; (b) A. Ajayaghosh and S. J. George, J. Am.
Chem. Soc., 2001, 123, 5148–5149; (c) A. P. H. J. Schenning,
J. V. Herrikhuyzen, P. Jonkheijm, Z. Chen, F. Wurthner and
E. W. Meijer, J. Am. Chem. Soc., 2002, 124, 10252–10253; (d)
F. J. M. Hoeben, M. Wolffs, J. Zhang, S. D. Feyter, P. Leclere,
A. P. H. J. Schenning and E. W. Meijer, Angew. Chem., Int. Ed.,
2004, 43, 1976–1979; (e) J. F. Hulvat, M. Sofos, K. Tajima and
S. I. Stupp, J. Am. Chem. Soc., 2005, 127, 366–372; (f) M. Mba,
A. Moretto, L. Armelao, M. Crisma, C. Toniolo and M. Maggini,
Chem.–Eur. J., 2011, 17, 2044–2047.
5 (a) K. Murata, M. Aoki, T. Suzuki, T. Harada, H. Kawabata,
T. Komri, F. Ohseto, K. Ueda and S. Shinkai, J. Am. Chem. Soc.,
1994, 116, 6664–6676; (b) K. J. C. van Bommel, A. Friggeri and
S. Shinkai, Angew. Chem., Int. Ed., 2003, 42, 980–999.
6 (a) T. Kunitake, Y. Okahata, M. Shimomura, S. Yasunami and
K. Takarabe, J. Am. Chem. Soc., 1981, 103, 5401–5413; (b)
Y. C. Lin, B. Kachar and R. G. Weiss, J. Am. Chem. Soc., 1989,
111, 5542–5551; (c) K. Hanabusa, Y. Maesaka, M. Kimura and
H. Shirai, Tetrahedron Lett., 1999, 40, 2385–2388; (d) G. Mieden-
Gundert, L. Klein, M. Fischer, F. Vogtle, K. Heuze, J. L. Pozzo,
M. Vallier and F. Fages, Angew. Chem., Int. Ed., 2001, 40, 3164–
3166; (e) H. Koshima, W. Matsusaka and H. Yu, J. Photochem.
Photobiol., A, 2003, 156, 83–90; (f) M. George, S. L. Snyder,
P. Terech, C. J. Glinka and R. G. Weiss, J. Am. Chem. Soc., 2003,
125, 10275–10283; (g) A. Pal, Y. K. Ghosh and S. Bhattacharya,
Tetrahedron, 2007, 63, 7334–7348; (h) M. Cote, T. Nicholls,
D. W. Knight, I. R. Morgan, P. G. A. Rogueda, S. M. King,
R. K. Heenan and P. C. Griffiths, Langmuir, 2009, 25, 8678–8684;
(i) A. Ghosh and J. Dey, Langmuir, 2009, 25, 8466–8472; (j)
B. G. Bag, S. K. Dinda, P. P. Dey, V. A. Mallia and R. G. Weiss,
Langmuir, 2009, 25, 8663–8671; (k) A. G. L. Olive, G. Raffy,
H. Allouchi, J. M. Leger, A. D. Guerzo and J. P. Desvergne,
Langmuir, 2009, 25, 8606–8614; (l) J. Cui, Y. Zheng, Z. Shen and
X. Wan, Langmuir, 2010, 26, 15508–15515.
7 S. Bhattacharya and A. Pal, J. Phys. Chem. B, 2008, 112, 4918–4927.
8 (a) M.-Y. Lai, C.-H. Chen, W.-S. Huang, J. T. Lin, T.-H. Ke,
L.-Y. Chen, M.-H. Tsai and C.-C. Wu, Angew. Chem., Int. Ed.,
2008, 47, 581–585; (b) A. Liedtke, M. O’Neill, A. Wertmoller,
S. P. Kitney and S. M. Kelly, Chem. Mater., 2008, 20, 3579–3586.
9 (a) A. J. Zucchero, J. Tolosa, L. M. Tolbert and U. H. F. Bunz,
Chem.–Eur. J., 2009, 15, 13075–13081; (b) S. M. Kim, J. H. Jang,
K. K. Kim, H. K. Park, J. J. Bae, W. J. Yu, I. H. Lee, G. Kim,
D. D. Loc, U. J. Kim, E.-H. Lee, H.-J. Shin, J.-Y. Choi and
Y. H. Lee, J. Am. Chem. Soc., 2009, 131, 327–331.
10 (a) T.-Q. Nguyen, J. Wu, V. Doan, B. J. Schwartz and S. H. Tolbert,
Science, 2000, 288, 652–656; (b) S. Hecht and J. M. J. Frechet, Angew.
Chem., Int. Ed., 2001, 40, 74–91; (c) K. Becker, J. M. Lupton,
J. Feldmann, S. Setayesh, A. C. Grimsdale and K. Mullen, J. Am.
Chem. Soc., 2006, 128, 680–681; (d) Y. Sun, N. C. Giebink,
H. Kanno, B. Ma, M. E. Thompson and S. R. Forrest, Nature,
2006, 440, 908–912.
11 (a) S.-i. Tamaru, M. Takeuchi, M. Sano and S. Shinkai, Angew.
Chem., Int. Ed., 2002, 41, 853–856; (b) M. Shirakawa,
S.-I. Kawano, N. Fujita, K. Sada and S. Shinkai, J. Org. Chem.,
2003, 68, 5037–5044; (c) H. Jintoku, T. Sagawa, T. Sawada,
M. Takafuji, H. Hachisako and H. Ihara, Tetrahedron Lett., 2008,
49, 3987–3990.
17 (a) S. K. Samanta, A. Pal and S. Bhattacharya, Langmuir, 2009, 25,
8567–8578; (b) S. K. Samanta, A. Pal, S. Bhattacharya and
C. N. R. Rao, J. Mater. Chem., 2010, 20, 6881–6890; (c)
S. K. Samanta, A. Gomathi, S. Bhattacharya and C. N. R. Rao,
Langmuir, 2010, 26, 12230–12236; (d) S. K. Samanta,
K. S. Subrahmanyam, S. Bhattacharya and C. N. R. Rao, Chem.–
Eur. J., 2012, 18, 2890–2901; (e) S. Bhattacharya and
S. K. Samanta, Chem.-Eur. J., 2012, DOI: 10.1002/chem.201201940.
18 (a) J. W. Chung, B.-K. An and S. Y. Park, Chem. Mater., 2008, 20,
6750–6755; (b) N. Mohmeyer, P. Wang, H.-W. Schmidt,
S. M. Zakeeruddin and M. Gratzel, J. Mater. Chem., 2004, 14,
1905–1909; (c) R. Schmidt, M. Schmutz, M. Michel, G. Decher and
P. J. Mesini, Langmuir, 2002, 18, 5668–5672.
19 (a) N. Zweep, A. Hopkinson, A. Meetsma, W. R. Browne,
B. L. Feringa and J. H. van Esch, Langmuir, 2009, 25, 8802–8809;
(b) Low Molecular Mass Gelators: Design, Self-Assembly, Function;
Topics in Current Chemistry, ed. F. Fages, Springer, New York,
2005, vol. 256.
20 (a) T. M. Duncan and C. Dybowski, Surf. Sci. Rep., 1981, 1, 157–250;
(b) D. C. Duncan and D. G. Whitten, Langmuir, 2000, 16, 6445–6452.
21 The 1H NMR signals due to the aliphatic –CH2– protons adjacent to
the quaternary N merged with the H-D-O signals of the solvent.
However, the signal can be observed for the spectra taken in only
CD3OD (Fig. S1a†).
22 (a) M. Tata, V. T. John, Y. Y. Waguespack and G. L. McPherson, J.
Am. Chem. Soc., 1997, 116, 9464–9470; (b) J. H. Jung, G. John,
M. Masuda, K. Yoshida, S. Shinkai and T. Shimizu, Langmuir,
2001, 17, 7229–7232; (c) H. Basit, A. Pal, S. Sen and
S. Bhattacharya, Chem.–Eur. J., 2008, 14, 6534–6545; (d) A. Pal,
H. Basit, S. Sen, V. K. Aswal and S. Bhattacharya, J. Mater.
Chem., 2009, 19, 4325–4334.
23 (a) H. A. Barnes, in A Handbook of Elementary Rheology, University
of Wales, Institute of Non-Newtonian Fluid Mechanics,
Aberystwyth, 2000, pp. 55–61; (b) F. M. Menger and K. L. Caran,
J. Am. Chem. Soc., 2000, 122, 11679–11691; (c) P. Terech,
D. Pasquier, V. Bordas and C. Rossat, Langmuir, 2000, 16, 4485–
4494.
24 (a) D. J. Abdallah, S. A. Sirchio and R. G. Weiss, Langmuir, 2000, 16,
7558–7561; (b) M. Caffrey, J. Hogan and A. S. Rudolph,
Biochemistry, 1991, 30, 2134–2146; (c) J. Kadam, C. F. Faul and
J. U. Scherf, Chem. Mater., 2004, 16, 3867–3871.
25 C.-X. Yuan, X.-T. Tao, Y. Ren, Y. Li, J.-X. Yang, W.-T. Yu,
L. Wang and M.-H. Jiang, J. Phys. Chem. C, 2007, 111, 12811–12816.
26 (a) B. Juskowiak, M. Ohba, M. Sato, S. Takenaka, M. Takagi and
H. Kondo, Bull. Chem. Soc. Jpn., 1999, 72, 265–277; (b) J. Tang,
C. Wang, Y. Wang, J. Sunb and B. Yang, J. Mater. Chem., 2001,
11, 1370–1373; (c) E. P. Petrov, F. Cichos, F. Wagner, St. Spange
and C. von Borczyskowski, Photochem. Photobiol., 2005, 81, 898–
907; (d) C.-X. Yuan, X.-T. Tao, Y. Ren, Y. Li, J.-X. Yang,
W.-T. Yu, L. Wang and M.-H. Jiang, J. Phys. Chem. C, 2007, 111,
12811–12816.
12 H. Engelkamp, S. Middlebeek and R. J. M. Nolte, Science, 1999, 284,
785–788.
13 (a) K. Sugiyasu, N. Fujita and S. Shinkai, Angew. Chem., Int. Ed.,
2004, 43, 1229–1233; (b) K. Sugiyasu, S.-i. Kawano, N. Fujita and
S. Shinkai, Chem. Mater., 2008, 20, 2863–2865.
14 (a) F. S. Schoonbeek, J. H. van Esch, B. Wegewijs, D. B. A. Rep,
M. P. de Haas, T. M. Klapwijk, R. M. Kellogg and B. L. Feringa,
Angew. Chem., Int. Ed., 1999, 38, 1393–1397; (b) P. Pratihar,
S. Ghosh, V. Stepanenko, S. Patwardhan, F. C. Grozema,
€
L. D. A. Siebbeles and F. Wurthner, Beilstein J. Org. Chem., 2010,
6, 1070–1078.
27 When the emission spectrum of the gel of 5 in 60% ethanol–water was
recorded under the front-face geometry, it showed excitation and
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emission maximum at 470 and 590 nm respectively. Therefore, the
emission maximum at 575 nm ensured the aggregated emission (see
Fig. S10†).
31 E. R. Brown and J. R. Sandifer, in Physical Methods in Chemistry.
Electrochemical Methods, ed. B. W. Rossiter and J.H. Hamilton,
Wiley, New York, 1986, vol. 4, ch. 4.
28 (a) S. Ghosh, X.-Q. Li, V. Stepanenko and F. Wurthner, Chem.–Eur.
J., 2008, 14, 11343–11357; (b) F. Wurthner, T. E. Kaiser and
C. R. Saha-Moller, Angew. Chem., Int. Ed., 2011, 50, 3376–3410; (c)
M. Shirakawa, S.-i. Kawano, N. Fujita, K. Sada and S. Shinkai, J.
Org. Chem., 2003, 68, 5037–5044; (d) H. Wu, L. Xue, Y. Shi,
Y. Chen and X. Li, Langmuir, 2011, 27, 3074–3082; (e) X.-Q. Li,
X. Zhang, S. Ghosh and F. Wurthner, Chem.–Eur. J., 2008, 14,
8074–8078.
29 (a) C. Vijayakumar, V. K. Praveen and A. Ajayaghosh, Adv. Mater.,
2009, 21, 2059–2063; (b) Y.-L. Liu, Y.-H. Wu and C.-Y. Hsu,
Nanotechnology, 2009, 20, 235704; (c) X. Wang, J. Yan, Y. Zhou
and J. Pei, J. Am. Chem. Soc., 2010, 132, 15872–15874; (d)
C. Vijayakumar, K. Sugiyasu and M. Takeuchi, Chem. Sci., 2011,
2, 291–294; (e) C. Giansante, G. Raffy, C. Schafer, H. Rahma,
M.-T. Kao, A. G. L. Olive and A. D. Guerzo, J. Am. Chem. Soc.,
2011, 133, 316–325; (f) M. R. Molla and S. Ghosh, Chem.–Eur. J.,
2012, 18, 1290–1294.
32 S. Petrovic, Chem. Educ., 2000, 5, 231–235.
33 N. G. Tsierkezos, J. Solution Chem., 2007, 36, 289–302.
34 X. H. Zhang and S. F. Wang, Sensors, 2003, 3, 61–68.
35 S. Ershad and J. Khodmarz, Int. J. Electrochem. Sci., 2010, 5, 1302–
1309.
36 (a) R. Nicholson, Anal. Chem., 1965, 37, 1351–1355; (b) C. N. Myer
and C. W. Allen, Inorg. Chem., 2002, 41, 60–66.
37 B. L. Funt and P. M. Hoang, J. Electrochem. Soc., 1984, 131, 2295–
2298.
38 (a) J. M. Cervantes-Uc, J. V. Cauich-Rodriguez, W. A. Herrera-Kao,
H. Vazquez-Torres and A. Marcos-Fernandez, Polym. Degrad. Stab.,
2008, 93, 1891–1899; (b) J. Liu, W. Yang, L. Tao, D. Li, C. Boyer and
T. P. Davis, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 425–
433.
39 Y. Yao, C. Liu and S. Fan, Nanotechnology, 2006, 17, 4374–4378.
40 (a) Q. Zheng, B. J. Jung, J. Sun and H. E. Katz, J. Am. Chem. Soc.,
2010, 132, 5394–5404; (b) S. Ahn, Y. Kim, S. Beak, S. Ishimoto,
H. Enozawa, E. Isomura, M. Hasegawa, M. Iyoda and Y. Park, J.
Mater. Chem., 2010, 20, 10817–10823.
30 S. Perez, C. Lopez, A. Caubet, A. Roig and E. Molins, J. Org. Chem.,
2005, 70, 4857–4860.
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