Supergelation via purely aromatic π-π Driven self-assembly of pseudodiscotic oxadiazole mesogens

Article abstract of DOI:10.1021/ja500607d

A series of highly luminescent oxadiazole-based stilbene molecules (OXD4, OXD8, OXD10, and OXD12) exhibiting interesting enantiotropic liquid crystalline and gelation properties have been synthesized and characterized. The molecules possessing longer alkyl substituents, OXD10 and OXD12, possess a pseudodisc shape and are capable of behaving as supergelators in nonpolar solvents, forming self-standing gels with very high thermal and mechanical stability. Notably the self-assembly of these molecules, which do not possess any hydrogen-bonding motifs normally observed in most reported supergelators, is driven purely by π-stacking interactions of the constituent molecules. The d-spacing ratios estimated from XRD analysis of OXD derivatives possessing longer alkyl chains show that the molecules are arranged in a columnar fashion in the mesogens and the self-assembled nanofibers formed in the gelation process.

Full text of DOI:10.1021/ja500607d

Article
pubs.acs.org/JACS
Supergelation via Purely Aromatic π−π Driven Self-Assembly of
Pseudodiscotic Oxadiazole Mesogens
Aneesh P. Sivadas,† N. S. Saleesh Kumar,† Deepak D. Prabhu,† Shinto Varghese,† S. Krishna Prasad,‡,*
D. S. Shankar Rao,‡ and Suresh Das*,†
†Photosciences and Photonics Section, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science
and Technology (NIIST) and Network of Institutes for Solar Energy, Council of Scientific and Industrial Research (CSIR),
Trivandrum 695 019 India
‡Centre for Soft Matter Research, Jalahalli, Bangalore 560 013 India
S
* Supporting Information
ABSTRACT: A series of highly luminescent oxadiazole-based stilbene molecules (OXD4, OXD8, OXD10, and OXD12)
exhibiting interesting enantiotropic liquid crystalline and gelation properties have been synthesized and characterized. The
molecules possessing longer alkyl substituents, OXD10 and OXD12, possess a pseudodisc shape and are capable of behaving as
supergelators in nonpolar solvents, forming self-standing gels with very high thermal and mechanical stability. Notably the self-
assembly of these molecules, which do not possess any hydrogen-bonding motifs normally observed in most reported
supergelators, is driven purely by π-stacking interactions of the constituent molecules. The d-spacing ratios estimated from XRD
analysis of OXD derivatives possessing longer alkyl chains show that the molecules are arranged in a columnar fashion in the
mesogens and the self-assembled nanofibers formed in the gelation process.
presence of strong hydrogen bonding moieties such as amides,
polypeptides, and sugar.5 Molecules without such hydrogen
bonding units which can act as supergelators have hitherto not
been reported and hence development of such systems can
vastly enhance the scope of designing this class of materials and
INTRODUCTION
■
The spontaneous generation of ordered superstructures with
well-defined functional properties from molecules randomly
dispersed in a medium requires a molecular level intelligence
which will drive the self-assembly in a desired manner.1 A
critical understanding of the weak forces that drive self-
assembly is essential for providing structural features to
molecules which can impart them with such intelligence.
Molecules displaying liquid crystalline behavior (mesogens) in
bulk or in the solution phase are ideal candidates for
investigating such behavior because of their propensity to
form aggregates in an organized manner.2 A class of self-
assembled materials which has attracted special attention in
recent years is gels, in view of their potential use in a variety of
applications including organic photovoltaics, as templates for
inorganic nanostructures and in controlled release systems.3 Of
particular interest are supergelators which can lead to strong
self-standing gels, can be molded into any shape, and can
undergo self-healing when damaged.4 A common structural
feature among molecules which form supergelators is the
remains a major challenge.
Herein, we report on the supergelating properties of 1,3,4-
oxadiazole derivatives capable of self-assembling into columnar
architectures in both bulk and in solution (Chart 1). Based on
XRD and NMR of the aggregates we could show that the
capability of this class of molecules to form unusually strong
self-standing gels could be attributed to purely intermolecular
π−π interactions between the oxadiazole-based mesogenic
molecules.
Received: January 25, 2014
Published: March 14, 2014
© 2014 American Chemical Society
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data of the derivatives are provided in the Supporting
Information.
Chart 1
Liquid crystalline properties of all the derivatives were
investigated in detail. Preliminary characterization of the
different phases was made by studying their optical polarizing
microscopic (OPM) textures and confirmed by differential
scanning calorimetric (DSC) analyses (Figure S1). All the
derivatives exhibited liquid crystalline behavior with the
exception of OXD1 and OXD6. While OXD4 exhibited a
smectic phase, the longer chain derivatives (OXD8, OXD10,
and OXD12) possessed columnar mesophases. For OXD4, the
smectic A (SmA) phase was seen over a range of 7 °C in the
heating cycle and a slightly larger range of 12 °C in the cooling
cycle. The presence of columnar organization for OXD8,
OXD10, and OXD12 was evident from the X-ray diffraction
(XRD) pattern (Figure 1). For OXD8, in the heating cycle, a
columnar phase (Col1) was observed at a temperature of 195
RESULTS AND DISCUSSION
■
The oxadiazole derivatives were synthesized by a multistep
process,6 and their structures were established by FTIR, 1H and
13C NMR, HRMS, and MALDI-TOF mass spectrometry.
Details of the synthetic procedures and spectral characterization
Figure 1. XRD scans for OXD series. The low angle region is shown in the main panel and the wide angle portion in the inset. In the inset, the thick
solid line represents the full fit, the individual contributions of which are shown as dashed lines. XRD pattern of (a) OXD4 at 242 °C; (b) OXD8 at
T = 200 °C; and (c) OXD10 at two different temperatures. The inset depicts the wide angle region of the diffraction pattern in (a) Colh and (b)
Colob phases. (d) XRD pattern of OXD12 at two different temperatures. The inset depicts the wide angle region of the diffraction pattern in (a) Colh
and (b) Colob phases.
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Table 1. X-ray Diffraction Data of the Mesophases of OXD10, Presenting the Measured and Calculated Spacing, Miller Indices,
a
and Lattice Dimensions
transition temperatures [°C] (ΔH [kJ mol−1])
packing motif dmeas [A°] dcalc [A°]
hk
lattice parameters
a = 47.6 Å at 200 °C
41.26
23.73
20.54
4.80 (d)
3.55 (d)
42.11
34.23
31.42
24.24
21.02
15.90
13.65
6.19
41.19
23.78
20.60
1 0
1 1
2 0
Colh
Cr1 108 (7.6) Col2 167 (21.3) Col1 233 (3.9) I
42.00
34.21
31.40
1 0
0 1
1 1
a = 43.9 Å b = 35.8 Å γ = 73.0° at 140 °C
I 232 (3.9) Col1 160 (0.64) Col2 152 (19.4) Cr2 106 (8.6) Cr1
21.00
15.94
14.00
6.28
2 0
−2 1
3 0
Colob
5 5
5.66
5.82
6 5
4.45
4.49
7 7
5.64
5.49
8 2
4.67
4.59
8 6
4.30 (d)
a
Cr = crystalline phase; Colh = columnar hexagonal mesophase; and Colob = oblique columnar mesophase.
°C which transformed to the isotropic phase at 220 °C (Table
S1). In the cooling cycle, the sample exhibited an isotropic to
Col1 transition at 218 °C and crystallization at 184 °C. In
contrast to OXD8, both OXD10 and OXD12 showed two
columnar OXD10 mesophases Col1 and Col2. For OXD10, the
Col2 and Col1 phases appeared at 108 and 167 °C, respectively,
in the heating cycle and at 160 and 233 °C in the cooling cycle.
A similar behavior was observed for OXD12. The thermal
phase transitions and their enthalpy values for the OXD series
are tabulated (Tables 1 and S1). Detailed identification of the
columnar phases for the different samples was carried out by
studying their temperature-dependent XRDs.
XRD measurements have been performed on all the four
compounds exhibiting liquid crystalline phases, and the results
of the measured and calculated spacing, miller indices and
lattice dimensions are summarized in Tables 1 and S1. Figure
1a shows the XRD profile of the mesophase of OXD4. Two
sharp peaks, one very strong and the other quite weak, are seen
at low angles, and a single diffuse peak is seen at wide angles.
The spacing of low angle peaks is in the ratio of 1:2. The d1/L
ratio of 0.84, where d1 is the spacing of the lowest angle peak
and L = 35.8 Å is the average length of the molecule, in its bow
form, as determined from a molecular model (Figure S2b),
indicates the phase to be a layered smectic A phase.
This feature is corroborated by the OPM textures which
indicated sharp fan-shaped features (Figure S1a,b). The diffuse
maximum in the wide angle region needs a special mention. A
proper peak profiling of the data required two Lorentzian
expressions, a feature consistent with the behavior of the other
compounds studied here. This is however not a common
feature in smectic phases but observed normally only in cases
where the bulkiness of the two terminal chains are substantially
different.7 Since the terminal chains on either side of the central
core are identical in the present case, the reason for such a
feature must be different; this aspect is discussed later.
1:√3:√4:√7, which can be identified to be arising from a
hexagonal lattice of a columnar structure and indexed as the
(10), (11), (20), and (21) reflections, with a lattice dimension
of a = 45.4 Å (Table S1). The wide angle region exhibited a
broad maximum that can be resolved into two diffuse peaks
with spacing of ∼5.0 and 3.8 Å. The former represents the
separation between the molten alkyl chains of the molecules
along the stacking direction of a column and is observed as a
rule in all fluid columnar structures. The second diffuse peak,
observed in molecules of disc-like shape, arises due to the
possibility of closer stacking of the aromatic cores than the
peripheral chains. Thus, such an observation in the present case
of nondiscoid molecules is somewhat surprising.
As mentioned earlier OXD10 and OXD12 exhibited two
mesophases. The high-temperature phase denoted as Col1,
displayed an XRD pattern which is identical to that shown by
the columnar phase of OXD8. The inverse of the spacing for
three low angle peaks being in the ratio of 1:√3:√4 and
indexed as the (10), (11), and (20) reflections from a
hexagonal columnar structure with a lattice dimension of a =
47.6 and 48.8 Å for OXD10 and OXD12, respectively (Tables 1
and S1). The low-temperature Col2 phase of both the materials
presented a dramatic change in the X-ray pattern, with
numerous sharp reflections spread over the entire angular
range covered, indicating a well-ordered structure (Figure 1c,d).
The presence of the two diffuse reflections at ∼5 and 3.8 Å
confirms the fluid nature of the phase. Even a cursory
inspection of the first three low-angle sharp reflections ruled
out, as expected, the hexagonal lattice.
Detailed indexed analysis, aided by the presence of a number
of reflections, brings out the tilted nature of the columnar
stacking, with the lattice parameters of a = 43.9 Å, b = 35.8 Å,
and γ = 73° and a = 45.5 Å, b = 42.9 Å, and γ = 78.3° for
OXD10 and OXD12 compounds, respectively. The reduction
in the lattice parameters from the Col1 to Col2 phase is in
conformity with the oblique nature of the Col2 phase. In the
hexagonal phase (Col1) of both the materials the lattice
dimension comes out to be about 0.95 times the molecular
diameter as determined from the molecular model (Figure S2).
The XRD profiles for OXD8, OXD10, and OXD12 exhibited
multiple low angle peaks whose spacing ratios do not conform
to layer ordering. For OXD8 (Figure 1b) the inverse of the
spacing for four low angle peaks was in the ratio of
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For OXD10 and OXD12 the chains are long enough to give it a
pseudodisc appearance. Thus the possible explanation for the
presence of the second diffuse peak is that the central
oxadiazole units must be strongly interacting with aromatic
rings of other molecules (π−π stacking) in the smectic as well
as the columnar phases.
TEM image (Figure 3a) of OXD10 at a concentration of 2 ×
10−5 M in toluene clearly indicated formation of intertwined
The OXD series showed a strong tendency to form
aggregates in solutions with the higher homologues forming
stable gels in organic solvents such as n-decane, hexadecane,
cyclohexane, hexane, n-butanol, and toluene (Table S2). It was
observed that for OXD8 to OXD12 the critical gelation
concentration was very low; for example, in n-decane, the CGC
for OXD10 was 0.35 wt %. Molecules capable of gelating
solvents at concentrations lower than 1 wt % are generally
classified as super gelators.8 Figure 2 depicts various character-
Figure 3. (a) TEM and (b) AFM images obtained for OXD10.
Samples were prepared by drop casting from toluene solution at a
concentration of 2 × 10−5 M. The inset depicts the height profile of
corresponding AFM image. XRD pattern of (c) OXD10 and (d)
OXD8 in their xerogel state. The inset depicts corresponding OPM
textures obtained for OXD10 and OXD8.
fibers with each fiber possessing a diameter ranging from 15 to
30 nm. AFM analysis was also carried out to confirm this
morphology. A film of OXD10 was prepared by drop casting 2
× 10−5 M solution in toluene to a mica sheet, evaporated, and
analyzed (Figure 3b). The height profile of fibers obtained by
AFM image was very much supportive for TEM images. The
AFM and TEM images obtained are further supported by SEM
and fluorescence microscopic studies.
The structures of the dry gels were generally investigated by
XRD,14 and the XRD pattern obtained for OXD12 xerogel film
showed more reflections, indicating almost crystalline nature of
the film. This could be attributed to strong interaction between
the neighboring molecules (Table S3). The XRD patterns
obtained are shown in Figure 3c,d for OXD10 and OXD8,
respectively. The OXD10 xerogel showed reflection peaks at
42.70, 30.91, 22.08, 14.89, 9.02, 7.58, 6.48, 5.68, 4.99, 4.46, and
3.34, and the reciprocal d-spacing ratios follow the order 1:1/
√2:1/2:1/√7 which are clearly indicative of a columnar
arrangement of the molecules within the nanofibers.
In the case of OXD8, the reflection peaks obtained are 37.94,
27.75, 13.89, 9.26, 6.83, 5.54, 4.38, and 3.93, and the large
number of reflection in the low angle region indicates the
formation of a columnar arrangement in the xerogel state. The
columnar arrangement in the nanofibers was also confirmed by
the birefringent textures15 obtained by OPM investigations. A
scheme representing the formation of supramolecular aggre-
gates in the higher homologue of OXD series is expressed in
Scheme 1.
The elastic response of OXD10 gel was estimated using
rheological measurements.16 The storage modulus (G′) and
loss modulus (G″) were determined as a function of applied
stress at constant angular frequency for a 1 × 10−2 M solution
of OXD10 in n-decane. The results of a strain−sweep
experiment at a temperature of 27 °C with an angular
frequency of 10 Hz, shown in Figure 4a, indicate the value of
G′ being significantly higher compared to G″ pointing to the
dominant elastic nature of the gel. The dynamic moduli, G′ and
Figure 2. Photographs of OXD12 gel in n-decane (1 × 10−2 M) (a)
freshly prepared and (b) after a time interval of 12 h. Photographs of
OXD10 gel (1 × 10−2 M) at room temperature under (c) normal light
and (d) UV (λex = 365 nm) illumination. (e) SEM image of OXD10
xerogel. (f) Plots of Tgel versus concentration of the OXD gelators in n-
decane (dropping ball method; wt of ball = 46.55 mg).
istics of the gel formed by OXD10 and OXD12 in n-decane. An
unusual phase transition was observed for OXD12 gel with
time. On letting the gel stand for a period of 12 h, a significant
amount of the trapped solvent was squeezed out of the gel as a
result of shrinkage of the gel into a small cylindrical mass
indicating a secondary level of self-organization. The shrunken
gel was highly stable and self-supporting (Figure 2a,b).
The thermal stability of the gels increased slightly with
increasing concentration (Figure 2f); with the gels formed from
OXD10 showing unusually higher thermal stability compared
to earlier reported systems.9The super gelating capacity of
OXD10 gel in n-decane was also evident from the formation of
self-standing gels. Reports of such self-standing gels in the
literature are rare. Apart from metal−organic hybrids10 and
polymers,11 earlier reported low molecular weight gelators
capable of forming such self-standing gels normally possess
strong hydrogen bonding units such as polypeptides12 and
sugars.13 OXD10 forms a unique example in which the
molecule does not possess strong hydrogen-bonding units.
Because of the unique stability of the gel it could be cast into
any shape (Figure 2c,d). The SEM micrographs of the dry gels
(Figure 2e) of OXD10 obtained from n-decane exhibited
typical images of gel networks consisting of entangled fibers.
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Pa at an angular frequency of 10 Hz for 300s. In the recovery
process, the storage modulus G′ and loss modulus G″ was
monitored as a function of time for a low shear stress of 1 Pa at
the same angular frequency (10 Hz). This experiment was
repeated six times and the results are summarized in Figure 4c.
From the Figure it is very clear that the gel recovered
immediately after removing the applied stress.
Scheme 1. Schematic Representation for the Hierarchical
Formation of the Self-Standing Gel of OXD10
Temperature-dependent 1H NMR studies of OXD10 in
solution were conducted in order to understand the nature of
the strong intermolecular aromatic π−π stacking leading to the
formation of the strongly self-assembled systems.18 Temper-
1
ature-dependent H NMR experiments of 1.1 mg/mL solution
of OXD10 in toluene, showed slight downfield shifts of the
resonance signals for the aromatic protons on the central
benzyl ring with decrease in temperature from 70 to 20 °C. As
shown in Figure 5, the aromatic central benzyl ring proton
1
Figure 5. Temperature-dependent H NMR spectra of 1.1 × 10−3
M
OXD10 in toluene.
signal of OXD10 at 8.02 (s, J = 8.61 Hz, Ha) at 70 °C
undergoes a downfield shift to 8.056 (s, J = 8.61 Hz, Ha; (Δδ =
0.036)) finally merging with the Hb doublet upon cooling to 20
°C. On cooling the solution self-aggregation can take place;
NMR studies indicate that as a result of interaction between the
neighboring molecules in the aggregates, there is a decrease in
electron density around the central benzene ring. This can
happen when the central benzene ring of the OXD10 molecule
interacts with the electron-deficient oxadiazole moiety of the
neighboring molecule in the self-assembled system. Such a
donor−acceptor type interaction between the neighboring
molecules could be the source of strong π−π stacking observed
in the self-assembled materials.
The absorption and emission properties of the OXD
derivatives were studied in solvents of varying polarity (Table
S4). The photophysical properties of the derivatives were
nearly identical, with a strong absorption band centered around
370 nm which was relatively insensitive to solvent polarity and
a broad emission band which underwent a red-shift with
increasing solvent polarity (Figure S5). The fluorecence
quantum yields were very high (0.5−0.8) in the various
solvents studied. The fluorescence quantum yield of the
OXD10 gel in n-decane was estimated as 0.39 using an
integrated sphere.
Figure 4. Dynamic moduli, G′ (slanted, open triangle) and G″
(slanted, solid triangle), vs strain on double logarithmic scale for a gel
of OXD10 (1 × 10−2 M) in n-decane at 27 °C with (a) stress = 0.1−
100 Pa, angular frequency, ω = 10 Hz and (b) stress = 1 Pa. (c)
Thixotropic nature of OXD10 (1 × 10−2 M) in n-decane at 27 °C.
(Deformation: stress: 0.1 to 100 Pa, time: 300 s, angular frequency, ω
= 10 Hz; Recovery: Stress: 1 Pa, time: 300 s; angular frequency, ω =
10 Hz.) (c, inset) Six continuous cycles of measurement for OXD10
gels (1 × 10−2 M) to prove that gel is thixotropic.
G″ remained unchanged until a particular stress value of 57 Pa.
Above this yield stress (σy) value, G′ and G″ decreased abruptly
leading to collapse of gel. Figure 4b shows G′ and G″ as a
function of angular frequency (ω) at a temperature of 27 °C in
the linear viscoelastic regime of Figure 4a, i.e., at a stress value
of 1 Pa and the dynamic moduli are invariant over a wide range
of strain, indicating a strong solid-like nature of the gel under
large deformations.
The thixotropic nature of OXD10 gel was also studied by
monitoring the recovery of the destroyed gel. For this process,
the gel sample was examined by two continuous processes, i.e.
deformation and recovery.17In the deformation step, the gel
was subjected to a continuously varying stress from 0.1 to 100
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Since the OXD derivatives showed a strong tendency to form
aggregates in nonpolar solvents, their absorption and emission
properties as a function of concentration in n-decane were
studied (Figure 6a). A relative increase in intensity of the
Figure 7. Contour map of the observed fluorescence intensity as a
function of the fluorescence excitation and emission wavelength for (a)
5 × 10−7 M and (b) 1 × 10−5 M of OXD10 in n-decane.
clearly seen that the emission arises predominantly from
excitation of the H-type aggregates and monomeric species.
Although H aggregates are generally known to be non-
luminescent, there have been a few reports on fluorescent H
aggregates in the literature.20−23 As described above, the
pseudodisc-shaped chromophores of OXD10 organize in a
columnar fashion within the fibers due to strong π−π
interactions which is conducive to formation of H-type
aggregates. The strong intermolecular interactions between
the neighboring molecules within the aggregates could lead to
rigidization of the chromophores leading to suppression of the
nonradiative decay channels.21 The unusual red-shifted
emission from the H-type aggregates observed in these systems
may be attributed to an H-to-J coupling as reported earlier for
some merocyanine dyes22 and a few other systems.23 Such an
H-to-J coupling was also clearly observable in the concen-
tration-dependent absorption, emission, and matrix scan
experiments of the lower homologue, OXD4 (Figure S6).
Figure 6. (a) Absorption and (b) emission spectra of OXD10 in n-
decane (λex = 360 nm) with increase in concentration from 5 × 10−7, 1
× 10−6, 2 × 10−6, 4 × 10−6, 6 × 10−6, 8 × 10−6, 1 × 10−5, and 2 × 10−5
M. (c) Absorption and (d) emission spectra of OXD10 in n-decane (c
= 1 × 10−5 M; λex = 360 nm) with increase in temperature from 25, 30,
35, 40, 45, 50, and 70 °C.
shorter wavelength band was observed with increase in
concentration, with a distinct shoulder band being observed
at 325 nm for concentrations ≥2 × 10−5 M indicating the
formation of H-type aggregates at higher concentrations.19 At
low concentrations (5 × 10−7 M) the emission showed
structured peaks in the 400−470 nm region characteristic of the
molecular emission. With increasing concentration the intensity
of this band decreased, and this was accompanied by formation
of a broad red-shifted emission band attributable to the
formation of aggregates (Figure 6b). In contrast the
concentration-dependent absorption and emission study of
the simple stilbenoid precursor 3e (Scheme S1) did not show
any evidence for aggregate formation (Figure S7).
The formation and break-up of the aggregates as a function
of temperature was also studied. Figure 6c,d shows the effect of
temperature on the absorption and emission spectra of 1 ×
10−5 M solution of OXD10 in n-decane. At 70 °C, a sharp
intense absorption peak was observed with a maximum
centered at 370 nm attributable to the monomer absorption
(Figure 6c). A decrease in temperature leads to a reduction in
the intensity and broadening of the absorption band. The
emission of 10−5 M solution of OXD10 in n-decane at 70 °C
showed intense structured peaks in the 400−470 nm region
which is characteristic of the molecular emission (Figure 6d),
and upon cooling, the intensity of these bands decreased and a
new broad red-shifted band centered at 510 nm attributable to
the aggregate was observed.
CONCLUSIONS
■
In summary, we report a series of 1,3,4-oxadiazole molecules
(OXD series, Chart 1) exhibiting columnar super structures
both in liquid crystalline and self-assembled nanofibers state. In
this series of molecules, OXD10 and OXD12, which possess
longer alkyl substituents, exhibited capabilities of forming self-
standing gels with very high thermal and mechanical stability in
nonpolar solvents. Such an unusually strong supergelating
property in molecules which do not possess any hydrogen-
bonding motifs and is driven purely by π-stacking interactions
of the constituent molecules has hitherto not been observed.
Based on temperature-dependent NMR and XRD studies of the
gel and the mesophase, the unusual stability of OXD10 gels
could be ascribed to the combined effect of aromatic π−π
stacking and the columnar arrangement of molecules in the gel
fibers.
EXPERIMENTAL SECTION
■
The solvents and the reagents were dried and purified by standard
methods prior to use. The solvents and the reagents were dried and
purified by standard methods prior to use. Melting points were
determined with a Mel-Temp-II melting point apparatus and are
uncorrected. FT-IR spectra were recorded on an IR Prestige-21
Shimadzu FT-IR spectrometer. 1H (300 and 500 MHz) and 13C NMR
(125 MHz) spectral analysis were performed on a Bruker Avance DPX
spectrometer with TMS as internal standard. The coupling constant
values (J = 16 Hz) of the olefinic protons in 1H NMR spectrum of all
derivatives confirmed the E stereochemistry of the derivatives. HRMS
analysis was obtained from a JEOL JM AX 505 HA instrument.
Electronic absorption spectra were recorded on a Shimadzu UV-3101
PC NIR scanning spectrophotometer, and emission spectra were
In order to explore the nature of the aggregates formed, a
matrix scan experiment of OXD10 at varying concentrations
was conducted (Figure 7a,b). Fluorescence spectra were
recorded at excitation wavelengths between 280 and 500 nm.
As seen in Figure 7a, at low concentrations (5 × 10−7 M), the
emission arises from excitation of the monomer band in the
340−400 nm region. Figure 7b shows the recorded emission
intensities of 1 × 10−5 M OXD10. From the figure it can be
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recorded on a SPEX-Fluorolog FL-1039 spectrofluorimeter. Optical
absorption measurements were carried out using 1 or 10 mm cuvettes
with a thermistor directly attached to the wall of the cuvette holder for
controlling the temperature. Fluorescence quantum yields, with an
estimated reproducibility of 10%, were determined using the
standard procedure by comparison with 9,10-diphenyl anthracene in
cyclohexane (ΦF = 1) and 10-methylacridiniumtriflouro methane-
sulphonate in water (ΦF = 0.99) as fluorescence standards.24 Absolute
quantum yield in the gel state was measured using a calibrated
integrating sphere in a SPEX fluorolog FL-1039 fluorimeter.25Fluor-
Fluorescence lifetimes were measured using IBH (FluoroCube) time
correlated picoseconds single photon counting (TCSPC) system.
Solutions were excited with a pulsed diode laser (<100 ps pulse
duration) at a wavelength of 375 nm (NanoLED-11) with a repetition
rate of 1 MHz. The detection system consisted of a microchannel plate
photomultiplier (5000U-09B, Hamamatsu) with a 38.6 ps response
time coupled to a monochromator (5000 M) and TCSPC electronics
[data station hub including Hub-NL, NanoLED controller, and
preinstalled Fluorescence Measurement and Analysis Studio (FMAS)
Software]. The fluorescence lifetime values were obtained using DAS6
decay analysis software. For SEM measurements, samples were drop
cast and air-dried on flat surface of mica sheet and subjected to thin
gold coating using JEOL JFC-1200 fine coater. The probe was inserted
into JEOL JSM-5600 LV scanning electron microscope for taking
photographs. TEM measurements were carried out in JEOL 100 kV
HRTEM. The samples were prepared by drop casting 25 μL of 2 ×
10−5 M onto a carbon-coated copper grid, and solvent was allowed to
evaporate under vacuum. AFM images were recorded under ambient
conditions using NTEGRA Prima-NT-MDT, Russia, scanning probe
microscope operated in tapping mode. Microfabricated silicone
cantilever tips (NSG 20) with a resonance frequency of 260−630
kHz and a force constant of 20−80 N m1− were used. The tip
curvature radius was 10 nm. The scan rate was 1 Hz. The AFM
samples were prepared by drop casting 2 × 10−5 M solution onto a
mica sheet, and solvent was allowed to evaporate under vacuum.
Liquid crystalline phase transitions and optical anisotropy were
observed using a Leica DFC 490 polarized light optical microscope,
equipped with a Mettler Toledo FP82HT (temperature programmer)
heating and freezing stage. Differential scanning calorimetric experi-
ments were performed using a Perkin-Elmer Pyris 6 DSC instrument
in sealed aluminum pans under nitrogen flow, at a heat/cooling rate of
5 °C/min. The rheological investigations were carried out in Physica
Modular Compact (MCR 150) stress controlled rheometer from
Anton Paar with a cone-and-plate geometry (CP 50-1). XRD studies
of xerogel were carried out on samples coated in glass slide, and
temperature-dependent XRD studies at liquid crystalline phases were
carried out on samples filled in Lindemann capillaries and were held at
required temperatures using a Mettler hot stage and irradiated with
CuKα radiation (λ = 1.5418 Å). The apparatus essentially involved a
high-resolution X-ray powder diffractometer (PANalytical X’Pert
PRO) equipped with a high-resolution fast detector, PIXCEL.
ACKNOWLEDGMENTS
■
Dedicated to Professor C. N. R. Rao on the occasion of his 80th
birthday. This work is supported by Council of Scientific and
Industrial Research (CSIR) under the Project NWP 55. A.P.S.,
D.D.P., S.V., N.S.S.K., are grateful to CSIR for fellowships. This
is contribution no. NIIST-PPG-349. The authors thank Dr. J.
D. Sudha for the rheological studies and Dr. C. N.
Ramachandran, Assistant Professor, IIT Roorkee for theoretical
calculations.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Synthesis, mesomorphic properties, optimized structures of
OXD derivatives in bow and fully extended direction,
photophysical properties, temperature-dependent XRD data
in the mesophases of OXD derivatives, XRD data in xerogel
state, CGCs of OXD derivatives in various organic media, and
morphological investigation. This information is available free
of charge via the Internet at http://pubs.acs.org/
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AUTHOR INFORMATION
■
Corresponding Authors
sureshdas@niist.res.in
skprasad@csmr.res.in
Notes
The authors declare no competing financial interest.
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