Oligo(p-phenylene-ethynylene)-derived super-π-gelators with tunable emission and self-assembled polymorphic structures

Article abstract of DOI:10.1002/asia.201200410

Linear π-conjugated oligomers are known to form organogels through noncovalent interactions. Herein, we report the effect of π-repeat units on the gelation and morphological properties of three different oligo(p-phenylene-ethynylene)s: OPE3, OPE5, and OPE

Full text of DOI:10.1002/asia.201200410

DOI: 10.1002/asia.201200410
Oligo(p-phenylene-ethynylene)-Derived Super-p-Gelators with Tunable
Emission and Self-Assembled Polymorphic Structures
Anesh Gopal,[a] Reji Varghese,[a, b] and Ayyappanpillai Ajayaghosh*[a]
Abstract: Linear p-conjugated oligo-
mers are known to form organogels
substrate interactions in these OPEs
are strongly influenced by the conjuga-
tion length of the molecules. Silicon
wafer suppresses substrate–molecule
interactions whereas a mica surface fa-
cilitates such interactions. At lower
concentrations, OPE3 formed vesicular
assemblies and OPE5 gave entangled
fibers, whereas OPE7 resulted in spiral
assemblies on a mica surface. At higher
concentrations, OPE3 and OPE5 re-
sulted in super-bundles of fibers and
flowerlike short-fiber agglomerates
when different conditions were applied.
The number of polymorphic structures
increases on increasing the conjugation
length, as seen in the case of OPE7
with n=5, which resulted in a variety
of exotic structures, the formation of
which could be controlled by varying
the substrate, concentration, and hu-
midity.
through
noncovalent
interactions.
Herein, we report the effect of p-
repeat units on the gelation and mor-
phological properties of three different
oligo(p-phenylene-ethynylene)s:
OPE3, OPE5, and OPE7. All of these
molecules form fluorescent gels in non-
polar solvents at low critical gel con-
centrations, thereby resulting in a blue
gel for OPE3, a green gel for OPE5,
and a greenish yellow gel for OPE7.
The molecule–molecule and molecule–
Keywords: conjugation
· fluores-
cence · gels · polymorphism · self-
assembly
Introduction
strate–molecule and molecule–molecule interactions. In this
way, it is possible to access a variety of polymorphic archi-
tectures, such as vesicles, fibers, flowers, spirals, elongated
sheets, and entangled super-bundles, as shown in Figure 1.
Fluorescent p-gels that are based on linear p-systems are
attracting ever-more attention owing to their potential appli-
cations in imaging and sensing.[8] Previously, we have report-
ed the self-assembly of an OPE that contained three aro-
matic rings (OPE3) into vesicular assemblies,[6a] thereby
Oligo(p-phenylene-ethynylene)s (OPEs) are an important
class of p-systems in view of their fluorescence and electron-
ic properties and are potential candidates in molecular and
supramolecular electronics.[1,2] For application in electronic
devices, the size- and shape-controlled self-assembly of p-
systems are of great importance.[3] Organogelation is a suita-
ble approach for the creation of nano- to microsized p-sys-
tems.[4] The gelation of poly(p-phenylene-ethynylene)s,
OPEs, and related systems is known;[5–7] however, there is
no systematic understanding of the effect of the length of p-
conjugation (or repeat units) on their self-assembly, gela-
tion, and morphology. Herein, we report how the number of
oligomeric repeat units and the associated p-length in the
OPEs (OPE3, OPE5, and OPE7) control the gelation and
morphology of the structures through the interplay of sub-
[a] A. Gopal, Dr. R. Varghese, Prof. A. Ajayaghosh
Photosciences and Photonics Group
Chemical Sciences and Technology Division
National institute for Interdisciplinary Science
and Technology (NIIST)
CSIR, Thiruvanthapuram-695019 (India)
Fax : (+91)0471-2491712, 249018
E-mail: ajayaghosh62@gmail.com
[b] Dr. R. Varghese
Current affiliation:
Indian Institute of Science Education and
Research, (IISER-TVM)
Thiruvanthapuram (India)
Figure 1. Schematic representation of the polymorphic structures that
were derived from the OPEs by subtly controlling the molecular interac-
tions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200410.
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leading to weak gels that were surprisingly different to those
from an analogous oligo(p-phenylenevinylene) (OPV),
which formed supramolecular tapes and strong gels.[9] The
reason for the vesicle formation of OPE3 was ascribed to
the weak p-interactions in this molecule. Therefore, we an-
ticipated that increasing the conjugation length by varying
the number of repeat units in the oligomeric backbone may
strongly influence the self-assembly process, thereby leading
to the formation of structures with diverse morphologies.
With this notion in mind, we prepared molecules OPE5
(which contains five aromatic rings) and OPE7 (which con-
tains seven aromatic rings). A detailed investigation of these
molecules revealed distinct differences in their self-assembly,
gelation, and morphological properties.
Results and Discussion
Molecules OPE3, OPE5, and OPE7 were synthesized by
using Sonogashira–Hagihara cross-coupling reactions[10] as
the key step and were characterized by using 1H and
13C NMR spectroscopy and by mass spectrometry (MALDI-
TOF MS). The UV/Vis absorption and emission spectra of
OPE3, OPE5, and OPE7 (c=1ꢁ10À5 m) in CHCl3 and n-
decane at 258C showed the expected effect of conjugation
length, with a gradual red-shift in the absorption and emis-
sion maxima (Figure 2). In n-decane, these compounds exist
as aggregates, as evidenced from the red-shifted shoulder
bands in the absorption spectra, which were temperature
sensitive (see the Supporting Information, Figure S1). Ag-
gregates of OPE3 and OPE5 in n-decane (c=1ꢁ10À5 m) ex-
hibited emission maxima at 445 nm and 506 nm when excit-
ed at 419 nm and 464 nm, respectively (Figure 2c). Their
corresponding fluorescence quantum yields (Ff) were 0.54
Figure 2. a) Absorption spectra of solutions of the OPEs in n-decane (c=
1ꢁ10À5 m) and b) their corresponding melting curves, plotted as the ag-
gregated fractions (a) versus temperature (normalized data points were
taken from the temperature-dependent UV/Vis absorption spectra).
c) Emission spectra of the OPEs in n-decane gel (at the minimum gelator
concentration). d) Plots of the gel-melting temperature versus the con-
centration of the gelators. Photographs of the n-decane gels under
normal light (e) and under UV light (f).
tor concentration (CGC) of 4.5 mgmLÀ1 (0.6 wt.%) that
showed a Tgel value of 358C, whereas OPE5 afforded
a greenish fluorescent (lem =540 nm) gel at a CGC of
and 0.42
N
3.3 mgmLÀ1 (0.45 wt.%) with
Tgel =558C. Remarkably,
OPE7 could gelate in n-decane (greenish-yellow fluores-
cence) at a CGC of 2.5 mgmLÀ1 (0.34 wt.%). The corre-
sponding Tgel value was 608C, which was 258C higher than
that of OPE3. From these data, it is clear that OPE3,
OPE5, and OPE7 belong to the class of super-gelators.[12]
The gelation data and photophysical properties of these
compounds are shown in Table 1.
Ff =0.31
N
dard). The aggregates of OPE3 and OPE5 exhibited melt-
ing-transition temperatures of 358C (monitored at 410 nm)
and 458C (monitored at 465 nm), respectively, whereas the
aggregate of OPE7 exhibited a melting-transition tempera-
ture of 528C (monitored at 485 nm), thus indicating higher
thermal stability for the aggre-
Transmission electron microscopy (TEM) images of
OPE3, OPE5, and OPE7 from aged solutions in n-decane
gates of OPE7 (Figure 2b).
Tgel
[8C] [nm]
Absorption lmax Emission lmax
Fluorescence life-
time [ns]
Ff
Molecules OPE3, OPE5, and
OPE7 could form gels in a vari-
ety of nonpolar hydrocarbon
solvents at room temperature;
the gels were transparent,
stable under ambient conditions
(Figure 2e), and strongly fluo-
rescent when excited (Fig-
ure 2 f). For example, OPE3 af-
forded a blue-light-emitting gel
[nm]
[%]
35
55
60
383, 420
445, 462[a]
t1 =0.70[d]
t2 =5.33
t3 =1.73
t1 =0.49[e]
t2 =3.90
t3 =1.11
t1 =0.44[e]
t2 =4.72
t3 =1.12
54[a]
42[b]
31[c]
425, 465
438, 484
506, 533[b]
546, 586[c]
from n-decane at a critical gela- 440 nm.
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Oligo(p-phenylene-ethynylene)-Derived Super-p-Gelators
hibited a spiral morphology (Figure 4e).[13] Such substrate-
dependent self-assembly has already been reported in the
case of OPVs and other similar systems.[14] The spiral self-as-
sembly of p-systems is rare and interesting; it indicates an
epitaxy-driven process that is associated with the strong in-
teractions between OPE7 and the mica surface. Freshly
cleaved mica surfaces are known to facilitate the epitaxial
self-assembly of electron-rich molecules, such as tetrathiaful-
valenes and thiophenes.[15] The individual ribbons of the spi-
rals had an average width of approximately 60 nm and
lengths of a few micrometers. Surprisingly, a constant height
of about 3.4–4.1 nm was observed for the spirals (Figure 4 f).
Detailed analysis of different regions of the images clearly
showed a uniform height over a broad area (see the Sup-
porting Information, Figure S3), which is unusual for self-as-
sembled p-systems. Thus, OPE7 exhibits a relatively more-
disciplined self-assembly on a mica surface when compared
to that of OPE5.
In the case of OPE5, the observed heights (3.4, 6.8, 10.2,
and 13.6 nm) matched the heights of the mono-, bis-, tris-,
and quadruple layers of the self-assembly. Such an arrange-
ment had earlier been proposed by Rabe and co-workers
for poly(p-phenylene-ethynylene)s (PPEs) based on AFM
analysis.[16] However, in the case of OPE7, the substrate–
molecule interactions and the molecule–molecule p-interac-
tions overcome the weak van der Waals interactions, there-
by suppressing the multilayer lamellar assembly and result-
ing in nanospirals of uniform height. To confirm the role of
freshly cleaved mica in the self-assembly process, we per-
formed AFM analysis of samples that were prepared on an
aged mica surface.[17] Interestingly, spiral formation is not
observed in this case (see the Supporting Information, Fig-
ure S4). On silicon wafer, OPE5 and OPE7 tend to mini-
mize the surface interactions, thereby resulting in particles.
However, on freshly cleaved mica surfaces, substrate–mole-
cule interactions may also become strengthened along with
the molecule–molecule interactions. In the case of OPE7,
strong substrate–molecule interactions lead to the formation
of nanospirals of uniform height. These interactions become
predominant during the direct drop-casting of dilute solu-
tion of the gelator followed by slow evaporation.
Figure 3. TEM morphologies of a) OPE3 (see reference [7]), b) OPE5,
and c) OPE7, which were obtained from solutions of the OPEs in n-
decane that were drop-cast onto carbon-coated copper grids at room
temperature (c=1ꢁ10À5 m). The TEM images were obtained without
staining.
(after 4 h at 258C, c=1ꢁ10À5 m) are shown in Figure 3.
Whilst OPE3 formed vesicular assemblies (Figure 3a),
OPE5 resulted in short fibers (Figure 3b) with an average
width of approximately 50 nm and lengths of typically a few
micrometers. TEM images of OPE7 obtained under the
same experimental conditions revealed the formation of rel-
atively long fibers of several micrometers in length that had
an average width of 30 nm (Figure 3c). These data indicated
that, in dilute solutions, OPE3 did not result in extended
self-assembly, whereas OPE5 and OPE7 were capable of
forming elongated fibers. In view of this observation, we de-
cided to monitor how these molecules self-assembled on dif-
ferent substrates and at different concentrations under am-
bient conditions.
Atomic force microscopy (AFM) images of OPE5 (1ꢁ
10À5 m in n-decane,) on silicon wafer and on freshly cleaved
mica are shown in Figure 4a and Figure 4b, respectively. On
silicon wafer, nanoparticles were present, whereas, interest-
ingly, on the mica surface, entangled fibers were formed.
Detailed section analysis revealed that the width of these
micrometer-length fibers varied from 60–300 nm with signifi-
cant height variations of 3.4–13.6 nm (Figure 4c). The AFM
images of OPE7 (1ꢁ10À5 m in n-decane) on silicon surfaces
exhibited both particles and fiber morphologies (Figure 4d).
Interestingly, on a freshly cleaved mica surface, OPE7 ex-
The influence of substrate–molecule interactions can be
minimized by performing the self-assembly process in solu-
tions at higher concentrations of the gelator with a subse-
quent transfer of the self-assembled structures onto the sub-
strate. For this purpose, we allowed the self-assembly of
OPE7 in n-decane (c=1ꢁ10À5 m) by aging the solution for
4 hours and then transferring it onto silicon- and mica surfa-
ces. AFM analysis of these specimens showed elongated
fibers in both cases (Figure 5), the morphologies of which
were significantly different from those that were obtained
by direct solution-casting of the dilute solutions. These ex-
periments strongly suggest that the morphology that is
formed by the direct drop-casting of p-systems onto differ-
ent substrates from dilute solutions may be significantly dif-
ferent to the actual morphology and that any interpretation
Figure 4. AFM images of OPE5 (n-decane, c=1ꢁ10À5 m) on silicon wafer
(a) and mica surfaces (b) and OPE7 on silicon wafer (d) and mica surfa-
ces (e). c,f) Zoomed-in images of OPE5 and OPE7 with their corre-
sponding cross-section analysis.
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from magnified images (Figure 6, insets). From these
images, it is also clear that, when a hot solution is drop cast,
spontaneous self-assembly occurs under the influence of the
substrates. From the AFM and SEM analyses it is clear that
OPEs do not interact with the silicon surface and, hence,
minimize the molecule–substrate interactions, thus resulting
in spherical agglomerates of the short fibers or petals. Inter-
estingly, SEM analysis performed after the molecules are al-
lowed to form gels in the appropriate solvents under sonica-
tion, followed by the transfer of the assemblies on silicon
substrates, showed entangled supramolecular tapes of sever-
al micrometers in length (Figure 6d–f), which is typical of
a gel morphology. This observation indicates that, in this
case, the substrate has no role in controlling the morpholo-
gy. When the same solutions are kept for longer amounts of
time, stable gels are formed, the SEM images of which re-
vealed the presence of super-fibers. Careful observation of
these fibers indicates that they are formed from wrinkled
2D sheets (Figure 6d–f, insets).
An interesting feature of the self-assembly of OPE7 is the
formation of “ambient-adaptable” polymorphic structures.
For example, when a hot solution of OPE7 (2.5 mgmLÀ1) is
allowed to undergo fast evaporation of the solvent in less
than 30% humidity on a silicon surface, flowerlike struc-
tures are observed (Figure 7a). However, when a hot solu-
tion is evaporated at an extremely slow rate in less than
30% humidity, followed by keeping for 3 days, elongated
multi-layered sheets are obtained (Figure 7b). Interestingly,
when the hot solution is evaporated at a fast rate at above
80% humidity, micron-sized fused spherical particles are
formed (Figure 7c). On the other hand, upon slow evapora-
tion of the same solution in above 80% humidity, wrinkled
2D sheets in the shape of flowers that contain micron-sized
cavities are formed (Figure 7d). Such polymorphic self-as-
semblies allow the access of
Figure 5. AFM image of OPE7 from aged solutions in n-decane (c=1ꢁ
10À4 m) on silicon wafer (a) and mica surfaces (b). The samples were pre-
pared by drop-casting aged solutions (after 4 h at room temperature) on
their respective freshly prepared surfaces.
of the electronic properties of the materials based on such
morphological features must be treated with caution.
Because distinct morphological variation was observed by
TEM and AFM analysis at lower concentrations in the solu-
tion state and on different substrates, respectively, we were
keen to identify the morphologies of the structures at higher
concentrations and in the gel state. Therefore, we performed
scanning electron microscopy (SEM) analysis under differ-
ent conditions, which resulted in interesting morphological
variation. For example, the drop-casting of hot solutions of
OPE3 under dry conditions (<30% humidity, from
a 4.5 mgmLÀ1 solution) resulted in micrometer-sized spheri-
cal agglomeration of curved short fibers (Figure 6a). Drop-
casting of hot solutions of OPE5 (3.3 mgmLÀ1 in n-decane)
and OPE7 (2.5 mgmLÀ1 in n-decane) on silicon wafer result-
ed in the formation of spherical assemblies that were
formed from thin flower petal-like structures (Figure 6b,c).
The fine features of these spherical structures were observed
a variety of aesthetic supra-
molecular
architectures
of
OPE7. In the cases of OPE3
and OPE5, such diverse struc-
tures with clear morphological
differences could not be ob-
served. Therefore, it is obvious
that the strong molecule–mole-
cule and molecule–substrate in-
teractions that are associated
with the electron-rich p-repeat
units of OPE7 play an impor-
tant role in the polymorphic
self-assembly process. Previous
reports on the formation of
such polymorphic self-assembly
processes strongly support our
findings.[18]
Figure 6. SEM images of the self-assembled OPEs (at the CGC) on silicon-wafer surfaces under different ex-
perimental conditions: a–c) The images were obtained when hot solutions of OPE3, OPE5, and OPE7, respec-
tively, were drop-cast under dry conditions (<30% humidity). d–f) Images of the OPE3, OPE5, and OPE7
gels, respectively, which were obtained after cooling to room temperature under normal atmospheric condi-
tions and sonication. Insets show images of the same gels after 2 days.
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Oligo(p-phenylene-ethynylene)-Derived Super-p-Gelators
Experimental Section
Materials
All starting materials and reagents were purchased from commercial sup-
pliers and used without further purification. The solvents were purified
and dried by using standard methods prior to use.
Synthesis and Characterization
The OPEs were synthesized through step-by-step Sonogashira–Hagihara
Pd-catalyzed cross-coupling reactions. The synthetic route is described in
Scheme 1. 1,4-Bis(dodecyloxy)2,5-diethynylbenzene(1),[6a] 2,5-bis(dodecy-
loxy)-4-iodobenzaldehyde (2),[21] ((2,5-bis(dodecyloxy)-4-iodophenyl)
ethynyl)trimethylsilane (3),[20] and OPE3[6a] were synthesized according
to literature procedures.
5,5’-(2,5-Bis(dodecyloxy)-1,4-phenylene)bis(ethyne-2,1-diyl)bis
bis(dodecyloxy)-2-ethynylbenzene) (4)
ACHTUNGTNER(NUNG 1,4-
Compound 1 (0.34 mmol), ((2,5-bis(dodecyloxy)-4-iodophenyl)ethynyl)-
trimethylsilane (3, 0.75 mmol), CuI (0.034 mmol), and [Pd(PPh3)2Cl2]
ACHTUNGTRENNUNG
Figure 7. SEM images of OPE7 (2.5 mgmLÀ1 in n-decane) on a silicon
wafer under different experimental conditions: a,b) The images were ob-
tained under dry conditions (<30% humidity) with fast and slow evapo-
ration of the solvents, respectively. c,d) The images were obtained under
wet conditions (>80% humidity) with fast and slow evaporation of the
solvents, respectively.
(0.034 mmol) were dissolved in THF/diisopropylamine (20 mL, 1:1), and
the mixture was stirred under an argon atmosphere at RT for 24 h. The
mixture was poured into MeOH (50 mL) and the formed precipitate was
filtered. The yellow precipitate was dissolved in a THF/MeOH mixture
(20 mL, 1:1) followed by the addition of excess K2CO3 and the reaction
mixture was stirred for 24 h. After removal of the solvent under reduced
pressure, the bis-ethynylene compound (4) was separated by column
chromatography on silica gel (n-hexane/CHCl3, 3:1). Yield: 60%; m.p.
1
91–938C; H NMR (300 MHz, CDCl3): d=6.92 (s, 2H), 6.91 (s, 2H), 6.90
Conclusions
(s, 2H), 3.93 (t, 12H), 3.26 (s, 2H), 1.76–1.74 (m, 12H), 1.49–1.43 (m,
24H), 1.17 (m, 84H), 0.81 ppm (t, 18H); 13C NMR (125 MHz, CDCl3,
TMS): d=14.23, 22.32, 25.16, 29.02, 29.05, 31.19, 69.46, 79.28, 82.82,
91.74, 112.23, 118.11, 146.05, 153.05 ppm; MS (MALDI-TOF): m/z calcd
for C162H262O10: 1433.3 [M+H]+; found: 1433.1.
In conclusion, we have revealed the influence of the length
of the oligomer in a linear p-system on molecule–molecule
and molecule–substrate interac-
tions, which, in turn, influence
the morphological features of
the structure. As the conjuga-
tion length is increased, the sta-
bility of the aggregates and the
gels increases with a significant
red-shift in the emission. More
importantly, this study provides
an insight into the surface and
“ambient-adaptable”
self-as-
sembly of extended p-systems,
thereby allowing access to a va-
riety of aesthetically appealing
super-structures. At this point,
the challenge to the scientific
community is how to precisely
engineer a certain structure of
an organic semiconductor out
of the available polymorphic
structures to, in principle, be
able to achieve a desired elec-
tronic property for a required
application.
Scheme 1. Synthesis of the OPEs. Reagents and conditions: a) [PdACHTNUGTRNEG(UN PPh3)2Cl2] (5 mol%), CuI (5 mol%), diiso-
propylamine/THF (1:1 v/v), 308C, 24 h; b) NaBH4, MeOH/CH2Cl2 (1:2 v/v), 308C, 2 h; c) K2CO3, THF/MeOH
(2:1 v/v), 308C, 6 h.
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5,5’-(2,5-Bis(dodecyloxy)-1,4-phenylene) bis(ethyne-2,1-diyl)bis(2-
(dode-cyloxy)-4-ethynylphenyl)ethynyl)-1,4-bis(dodecyloxy)benzene) (5)
N
using Rhodamine 6G (in EtOH, Ff =0.94 at 228C) and Rhodamine 101
(in EtOH, Ff =1 at 258C) as standards.[22] Atomic force microscopy
(AFM) images were recorded under ambient conditions by using a Digital
Instrument Multimode Nanoscope IV operated in the tapping-mode
regime. Microfabricated silicon cantilever tips (MPP-11100–10) with a res-
onance frequency of 299 kHz and a spring constant of 20–80 NmÀ1 were
used. The scan rate was varied from 0.5 to 1.5 Hz. To rule out the possi-
bility of any artifacts, we carried out blank experiments after evaporation
of the neat solvents on the substrates. AFM section analysis was done
offline. TEM was performed on a JEOL-JEM0310 microscope with an
accelerating voltage of 80 kV. TEM images were obtained without stain-
ing. SEM images were obtained on on a Zeiss EVO 18 cryo Special Edn
SEM equipped with a variable-pressure detector working at 20–30 kV.
The prepared SEM samples were sputter-coated (Au/Pd) before imaging.
A mixture of compound 4 (0.167 mmol), ((2,5-bis(dodecyloxy)-4-iodo-
phenyl)ethynyl)trimethylsilane (3) (0.368 mmol), CuI (0.032 mmol), and
[PdACHTUNGTRENNUNG(PPh3)2Cl2] (0.017 mmol) in diisopropylamine (10 mL) and THF
(10 mL) was stirred for 24 h at RT under an argon atmosphere. The reac-
tion mixture was then precipitated with MeOH. The yellow precipitate
was dissolved in a THF/MeOH mixture (20 mL, 1:1), excess K2CO3 was
added, and the mixture was stirred for 24 h. After removal of the solvent
under reduced pressure, the bis-ethynylene compound that was formed
(5) was purified by column chromatography on silica gel (n-hexane/
CHCl3, 3:1). Yield 20%; m.p. 103–1058C; 1H NMR (300 MHz, CDCl3):
d=7.00 (s, 2H), 6.98 (s, 2H), 6.97 (s, 6H), 4.02–4.0 (t, 20H), 3.33 (s, 2H),
1.83 (m, 20H), 1.55–1.50 (m, 30H), 1.24 (m, 150H), 0.86 ppm (t, 30H);
13C NMR (125 MHz, CDCl3, TMS): d=15.12, 21.62, 25.65, 29.01, 29.95,
30.25, 30.65, 30.88, 31.69, 60.56, 67.88, 90.74, 111.78, 112.03, 120.98,
122.36, 127.78, 150.05, 156.69 ppm; MS (MALDI-TOF): m/z calcd for
C162H262O10: 2370.81 [M+H]+; found: 2370.12.
Gel Characterization
The gelation studies were carried out according to literature proce-
dures.[19] In a typical procedure, a known amount of the required OPE
was added to the solvent (1 mL) in a glass vial and the mixture was
heated to dissolve the gelator. After cooling to RT, the vessel was turned
upside down to verify the gel formation. The reversibility of the gelation
process was confirmed by repeated heating and cooling. The critical gela-
tor concentration (CGC) was the minimum amount of the gelator that
was required for the formation of a gel at RT (228C). The gel-melting
OPE5
Compound 4 (50 mg, 0.035 mmol), 4-iodo-2,5-bis(dodecyloxy)benzalde-
hyde (0.08 mmol) (2), [PdACHTUNGTRNEUGN(PPh3)2Cl2] (10 mol%), and CuI (10 mol%)
were dissolved in a degassed mixture of diisopropylamine (10 mL) and
THF (10 mL). The mixture was stirred under an argon atmosphere at
408C for 24 h. After cooling to RT, the mixture was added dropwise to
vigorously stirring MeOH. The pale-yellow solid of bis-aldehyde (30 mg)
that was obtained was dried and redissolved in a mixture of MeOH
(10 mL) and CH2Cl2 (20 mL) and was reduced into its corresponding al-
cohol with NaBH4 (4 equiv) at RT. The mixture was washed with water
and extracted with CH2Cl2. The product was precipitated with MeOH.
The crude product was further purified by column chromatography on
silica gel (n-hexane/CHCl3, 1:1). Yield: 60%; m.p. 140–1438C; 1H NMR
(300 MHz, CDCl3, TMS): d=6.93 (s, 2H), 6.93 (s, 6H), 6.82 (s, 2H), 4.61
temperature (Tgel
) in n-decane was measured by the dropping-ball
method.
Sample Preparation for Morphological Analysis
The samples were prepared by solution drop-casting at required concen-
trations and under different atmospheric conditions. The drying atmos-
phere was controlled by keeping the sample in a closed desiccator under
various pressures and humidity. AFM samples for imaging were prepared
on freshly cleaved mica surfaces or pre-cleaned silicon-wafer surfaces.
Samples for TEM analysis were prepared on carbon-coated copper grids
without staining. Samples for the SEM studies were prepared on a sili-
con-wafer substrate that was pasted above an aluminum stub by using
a conductive carbon tape.
(s, 4H), 3.97–3.88 (t, 20H), 2.25 (t, 2H,), 1.77ACTHNUGRTNEUNG(m, 20H), 1.48 (m, 30H),
1.17 (m, 150H), 0.81–0.79 ppm (t, 30H); 13C NMR (125 MHz, CDCl3,
TMS): d=14.93, 22.52, 25.25, 29.01, 29.95, 30.18, 31.69, 60.56, 67.88,
68.74, 84.34, 111.28, 112.03, 118.18, 122.36, 134.78, 146.05, 155.88 ppm;
MS (MALDI-TOF): m/z calcd for C160H266O12: 2382.82 [M+H]+; found:
2382.42.
OPE7
Acknowledgements
OPE7 was prepared by a coupling reaction between diethynylene com-
pound 5 (0.021 mmol) and 4-iodo-2,5-bis(dodecyloxy)benzaldehyde (2,
0.084 mmol) under the same experimental conditions as for OPE5. The
crude product was further purified by column chromatography on silica
gel (n-hexane/CHCl3, 3:1). Yield: 55%; m.p. 150–1538C; 1H NMR
(300 MHz, CDCl3): d=7.01 (s, 2H), 7.00 (s, 2H), 6.99 (s, 10H), 4.68 (s,
4H); 4.02–4.0 (t, 28H), 2.17 (s, 2H), 1.83 (m, 28H), 1.55–1.50 (m, 56H),
1.24 (m, 196H), 0.86 ppm (t, 42H); 13C NMR (125 MHz, CDCl3, TMS):
d=14.90, 22.62, 24.54, 25.65, 29.01, 29.85, 30.01, 30.65, 30.98, 32.52, 60.06,
65.06, 67.88, 70.01, 70.67, 79.74, 95.28, 112.03, 120.98, 130.36, 145.78,
A.A. is grateful to the Department of Atomic Energy for a DAE-SRC
Outstanding Researcher award. A.G. acknowledges the CSIR for a fel-
lowship. We thank Mr. C.K. Chandrakanth for SEM analysis. This report
is contribution No. NIIST-PPG-315 from the NIIST.
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Received: May 7, 2012
Published online: && &&, 0000
Chem. Asian J. 2012, 00, 0 – 0
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www.chemasianj.org
7
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FULL PAPER
Self-Assembly
Anesh Gopal, Reji Varghese,
Ayyappanpillai Ajayaghos-
h*
&&&& — &&&&
Oligo(p-phenylene-ethynylene)-
Derived Super-p-Gelators with
Tunable Emission and Self-Assembled
Polymorphic Structures
p in the sky: The p-conjugated repeat
units of oligo(p-phenylene-ethyny-
lene)-based super-gelators influenced
the molecule–molecule and molecule–
substrate interactions in self-assembly
processes. Silicon wafer suppressed
substrate–molecule interactions whilst
mica surfaces facilitated such interac-
tions. The formation of polymorphic
structures could be controlled by
changing the substrate, concentration,
and humidity.
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!