Self-assembled nanotapes of oligo(p-phenylene vinylene)s: Sol-gel-controlled optical properties in fluorescent π-electronic gels

Article abstract of DOI:10.1002/chem.200401178

A rational approach to the design of supramolecular organogels of all-trans oligo(p-phenylene vinylene) (OPV) derivatives, a class of well-known organic semiconductor precursors, is reported. Self-assembly of these molecules induced gelation of hydrocarbo

Full text of DOI:10.1002/chem.200401178

FULL PAPER
Self-Assembled Nanotapes of Oligo(p-phenylene vinylene)s: Sol–Gel-
Controlled Optical Properties in Fluorescent p-Electronic Gels
[
a]
Subi J. George and Ayyappanpillai Ajayaghosh*
Abstract: A rational approach to the
design of supramolecular organogels of
all-trans oligo(p-phenylene vinylene)
packing of the molecules, facilitated by
cooperative hydrogen bonding, p stack-
ing, and van der Waals interactions be-
tween the OPV units. Gelation of
OPVs induced remarkable changes in
the absorption and emission properties,
which indicated strong electronic inter-
action in the aggregated chromophores.
Comparison of the absorption and
emission spectra in the gel form and in
the solid film indicated a similar chro-
mophore organization in both phases.
The presence of self-assembled aggre-
gates of OPVs was confirmed by sol-
vent-
and
temperature-dependent
(
OPV) derivatives, a class of well-
changes in the absorption and emission
properties, and by selective excitation
experiments. This is the first detailed
report of the gelation-induced forma-
tion of OPV nanotapes, assisted by
weak, nondirectional hydrogen-bond-
ing motifs and p–p stacking. These
findings may provide opportunities for
the design of a new class of functional
soft materials and nanoarchitectures,
based on p-conjugated organic semi-
conductor-type molecules, thereby ena-
bling the manipulation of their optical
properties.
known organic semiconductor precur-
sors, is reported. Self-assembly of these
molecules induced gelation of hydro-
carbon solvents at low concentrations
(
<1 mm), resulting in high aspect ratio
nanostructures. Electron microscopy
and atomic force microscopy (AFM)
studies revealed twisted and entangled
supramolecular tapes of an average of
50–200 nm in width, 12–20 nm in thick-
Keywords:
oligo(p-phenylene
organogels
nanostructures
vinylene)s
self-assembly
·
·
·
ness, and several micrometers in
length. The hierarchical growth of the
entangled tapes and the consequent ge-
lation is attributed to the lamellar-type
·
supramolecular chemistry
Introduction
self-assembly, leading to a variety of novel supramolecular
architectures with reversible functional properties. Nature
can control the architecture and function of supramolecular
assemblies, such as the DNA double helix, the collagen
triple helix, ion channels, and photosynthetic reaction cen-
ters, and this has been the major source of inspiration for
scientists to mimic natural systems with the help of synthetic
The control of the self-assembly of synthetic molecules in
the creation of nanosized architectures, using the principles
of supramolecular chemistry, is a topic of considerable im-
portance. The cooperative effect of noncovalent forces,
such as hydrogen bonding, p stacking, dipolar and van der
Waals interactions, is the driving force behind molecular
[1]
[2–6]
molecules.
In the domain of functional molecular assem-
blies and nanoarchitectures, the supramolecular control of
chromophore-linked molecular systems is a challenge, par-
ticularly in the fabrication of nanoscale devices, as chromo-
phore orientation has tremendous influence on optoelec-
[
a] S. J. George, Dr. A. Ajayaghosh
Photosciences and Photonics Division
Regional Research Laboratory, Trivandrum 695019 (India)
Fax : (+91)471-251-5306
[7]
tronic properties. Because organic p-conjugated systems
play a crucial role in supramolecular devices due to their in-
teresting optical and electronic properties, the modulation
of their optical properties by intermolecular interactions is
E-mail: aajayaghosh@rediffmail.com
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author: General experi-
mental procedures, synthesis, and characterization data of OPVs,
binary phase diagrams obtained by plotting Tgel vs. concentration,
DSC thermograms, OPM picture of OPV1a and SEM pictures of
OPV5 and OPV7 self-assemblies, time-resolved fluorescence decay
profiles of OPV1a, and a table containing photophysical properties
of the OPVs under investigation.
[3,8]
of great significance.
In this context, the supramolecular
chemistry of phenylene vinylenes has attracted considerable
[9]
attention and is the focus of much research.
An interesting feature of the noncovalent interactions in
certain organic molecules is their ability to entrap large vol-
Chem. Eur. J. 2005, 11, 3217 – 3227
DOI: 10.1002/chem.200401178
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umes of solvents within the supramolecular nano- and meso-
Results and Discussion
[10]
scopic structures to form gels in appropriate solvents. The
properties exhibited by this novel class of supramolecular
materials, with respect to the reversible self-assembly phe-
nomena, are striking in comparison to those exhibited by
the polymer-based organogels formed by permanent cova-
Design strategy: The bottom-up creation of three-dimen-
sional organic nanoarchitectures on the one hand, and gelat-
ion on the other hand, is a delicate balance between the
crystallization, precipitation, and solubility of noncovalently
interacting molecules in a suitable solvent. With this in
mind, a variety of tailor-made OPV derivatives were synthe-
sized, the structures of which are represented in Figure 1.
These molecules have many features that facilitate the for-
mation of a three-dimensional self-assembly, as depicted in
Figure 2a. The presence of the two hydroxymethyl end-
groups allows the molecules to self-assemble by weak, non-
directional 2-point hydrogen bonding, and the long hydro-
[11]
lent interactions. The recent and rapid growth in interest
in small-molecule-based organogelators is due to several po-
tential applications ranging from cosmetics, catalysis, and
controlled release to optoelectronics and related fields. Sev-
eral organic molecules are known to form organogels with
the help of hydrogen bonding, p stacking, dipole–dipole or
[
12–17]
van der Waals interactions.
However, chromophore-
[18]
based organogelators, particularly gels based on extended
[19]
p-conjugated systems,
are relatively few.
Although the self-assembly of oligo(p-phenyl-
ene vinylene)s (OPVs) is fairly well under-
stood, the design of supramolecular nanoarch-
itectures and their properties was not eluci-
dated until our preliminary studies on OPV
[20]
gels. Here, we describe details of the self-
assembly of OPVs into supramolecular nano-
tapes of high aspect ratio, and the consequent
gelation behavior that leads to the remarkable
Figure 2. a) Design features of OPVs. b) and c) OPV1a in decane under illumination before
and after gelation, respectively.
[21]
modulation of optical properties.
Figure 1. Library of the OPV derivatives under investigation.
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Self-Assembly of Fluorescent Organogels
FULL PAPER
carbon side-chains control the balance between solubility
and crystallization, as well as the packing of the molecules.
The rigid aromatic OPV backbone facilitates the p–p stack-
ing of the molecules. Cooperative interactions of these non-
covalent forces eventually lead to the formation of entan-
gled nanoscopic structures, which are able to hold a large
amount of the appropriate solvent molecules within the self-
assembly, thereby forming a gel.
(Scheme 2). All OPV derivatives under investigation were
characterized by spectral analyses. The all-trans configura-
tions of the OPVs were confirmed by the J values (16.5 Hz)
1
of the vinylic protons in their respective H NMR spectra.
Self-assembly and gelation: The dissolution of small
amounts of the newly synthesized OPV derivatives in a spe-
cific volume (1 mL) of different solvents, under heating and
cooling, resulted in the spontaneous gelation of the solvents,
indicating the formation of extended self-assemblies. The
gels obtained were transparent and stable, so that the glass
vial could be turned upside down without damaging the
structure (Figure 2c). Illumination of the OPV solutions
before gelation revealed strong greenish-blue fluorescence,
which changed upon gelation to greenish-yellow emission
(Figure 2b and c). The results of the gelation experiments
are presented in Table 1 and reveal that the bishydroxy com-
pounds OPV1a–c, which have long, linear hydrocarbon
chains, are efficient gelators of nonpolar hydrocarbon sol-
vents, such as hexane, decane, dodecane, cyclohexane, ben-
zene, and toluene. The critical gelator concentrations (CGC)
of OPV1a in dodecane, decane, and cyclohexane were 0.8,
Synthesis of OPVs: The bisalcohols OPV1a–e, OPV2, and
OPV3 were synthesized by means of the controlled Wittig
reaction of the appropriate bisaldehydes and bisphosphoni-
um salts, followed by reduction of the resulting OPV bisal-
[22]
dehydes with NaBH (Scheme 1). The controlled Wittig–
4
Horner reaction between the bisaldehyde (1) and the benzyl
phosphonate (2) afforded the monoaldehyde 3 in 35%
yield, which in turn was reduced to OPV4 by NaBH in
4
9
5% yield (Scheme 1). Synthesis of OPV7 was based on a
[23]
published procedure (90% yield). Preparation of the hy-
droxyl-protected derivatives OPV5 and OPV6 was accom-
plished by the alkylation of OPV1a with the corresponding
alkyl halides in 95% and 60% yields, respectively
Scheme 1. Synthesis of mono- and bishydroxy OPV derivatives.
Scheme 2. Synthesis of the hydroxyl-protected OPVs, OPV5–6.
Chem. Eur. J. 2005, 11, 3217 – 3227
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S. J. George and A. Ajayaghosh
[
a]
Table 1. Critical gelator concentrations (CGC) [mm] of OPV1a–e, OPV2, and OPV3 in different solvents.
groups, could gelate solvents
such as dodecane and decane
only at high concentrations. In
the case of OPV4 with one ter-
minal hydroxymethyl group,
gel formation was observed in
nonpolar solvents, though it
was not as efficient as in the
case of OPV1a and OPV1b.
However, OPV7, without any
functional end-groups, could
not gelate any of the solvents
investigated. Instead, aggregate
formation was observed above
Gelator
Dodecane
Decane
Cyclohexane
Hexane
Toluene
Benzene
Chloroform
OPV1a
0.8
0.9
1.1
1.7
2.8
3.0
5.6
(
1.0
s, tr)
(s, tr)
1.1
(s, tr)
4.8 (PG)
(s, tr)
1.4
(s, tr)
6.3
(s, tr)
2.1
(s, tr)
I
(s, tr)
4.2
(s, tr)
4.1
(th, o)
17.3
(th, o)
S
OPV1b
OPV1c
OPV1d
(s, tr)
(s, tr)
12.6
(s, tr)
15.9
(s, tr)
S
(s, tr)
11.6
(s, tr)
16.0
(s, tr)
S
4.5 (PG)
(
s, tr)
10.6
s, tr)
I
I
I
S
(
OPV1e
OPV2
S
S
S
S
S
PS
PS
15.4
11.5
(s, o)
I
23.0
(s, o)
PS
PS
PS
(s, o)
OPV3
PS
PS
PS
PS
S
[
a] CGC is the minimum concentration required for the formation of a stable gel at room temperature. In pa-
renthesis; s=stable, tr=transparent, th=thixotropic, o=opaque. S=soluble (>25 mgmL ), I=insoluble,
PS=poor solubility, PG=partial gelation at room temperature.
À1
À5
a concentration of 1ꢁ10 m, as
indicated by the shoulder ab-
sorption band at 470 nm in do-
0
.9, and 1.1 mm, respectively. This means that OPV1a can
decane, but no solvent molecules could be trapped. These
studies indicate that an optimum balance of hydrogen bond-
ing, p stacking, and van der Waals interactions is crucial in
the process of self-assembly and gelation.
entrap approximately 10000 molecules of dodecane per ge-
lator molecule and can be considered a supergelator.
[24]
The gelation ability of OPV1d, which carries hexyloxy
side chains, is relatively poor (the CGC is 10 mm in cyclo-
hexane), whereas OPV1e, which has branched 2-ethylhex-
yloxy side chains, gave a homogeneous solution in all of the
solvents investigated. The nature of the solvents has consid-
erable influence on the gelation behavior of OPV1a–d
Thermotropic behavior: The thermotropic behavior of the
gels formed by OPV1a–c and OPV4–7 was investigated by
using the dropping ball method and differential scanning
calorimetry (DSC) to study the impact of the structure of
the OPVs and the nature of the solvents on gel stability (see
Supporting Information). In the case of OPV1a–c, a regular
(
Table 1). For example, the CGC of OPV1a in chloroform
is 5.6 mm and the gel formed is unstable upon shaking (thix-
otropic). However, in toluene, OPV1a forms a reasonably
stable gel with a CGC of 2.8 mm. The number of the hydro-
carbon side chains present on the conjugated backbone sig-
nificantly influences gelation behavior (Table 1). For exam-
ple, the CGC of OPV2 in cyclohexane is 15.6 mm resulting
in a turbid gel, whereas OPV3 does not gelate any of the
solvents investigated.
To investigate the role of hydrogen-bond-assisted p stack-
ing in the gelation process, we extended our studies to other
OPV derivatives (OPV4–7). The results are summarized in
Table 2. Interestingly, in the case of OPV5 and OPV6, gela-
tion occurred only at high concentrations, compared to the
case of OPV1a. The CGC of OPV5 is 3.3 mm in cyclohex-
ane and the resultant gel is thixotropic, which may be due to
the absence of hydrogen-bond donor groups that are neces-
sary for the positional locking of the molecules within the p-
stacked assembly. Compound OPV6, with the hexyloxy end-
increase in the melting temperature of the gel (T ) was ob-
gel
served as the concentration of the gelator molecules in-
creased. The increase in Tgel that accompanied an increase
in alkyl chain length demonstrates the enhanced stability of
the gels, which could be due to the solvation-assisted inter-
molecular packing of the long alkyl chains. Phase diagrams
of OPV1a gels from dodecane, cyclohexane, toluene, and
chloroform revealed that OPV1a forms strong gels in do-
decane and cyclohexane, even at very low concentrations.
This observation is in accordance with the gelation studies
presented in Table 1. A comparison of the phase diagrams
of the cyclohexane gels of OPV1a, OPV4, and OPV5 at dif-
ferent concentrations revealed remarkable stability of the
OPV1a gel, due to hydrogen bonding between the hydroxy-
methyl groups. For example, a cyclohexane gel of the me-
À1
thoxy derivative OPV5 (10 mgmL ) melts at 478C, which
is 158C lower than the melting temperature of the corre-
sponding OPV1a gel. Clearly,
the gel of the monohydroxy
derivative OPV4 is more
[
a]
Table 2. Critical gelator concentrations [mm] of OPV4–7 in different solvents.
Gelator
Dodecane
Decane
Cyclohexane
Hexane
Toluene
Benzene
Chloroform
S
stable than that of the hydrox-
yl-protected OPV5 gel, al-
though less stable than the
OPV1a gel.
In the cases of OPV1a–c,
the transition temperatures of
the heating exotherms and
cooling endotherms increase in
the order OPV1a>OPV1b>
OPV4
1.4
1.5
(s, tr)
1.8
(th, tr)
5.2
(th, tr)
PS
2.3
(s, tr)
3.3
(th, tr)
10.4
(th, o)
PS
4.6
(th, o)
4.4
(th, tr)
10.4
(th, o)
PS
4.6
(s, tr)
7.2
(th, tr)
S
4.2
(s, tr)
6.8
(th, tr)
S
(s, tr)
OPV5
OPV6
OPV7
1.6
16.5
PG
S
(th, tr)
4.9
(th, tr)
PS
PS
PS
PS
À1
[
a] In parenthesis; s=stable, tr=transparent, th=thixotropic, o=opaque. S=soluble (>25 mgmL ), PS=
poor solubility, PG=partial gelation at room temperature.
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Self-Assembly of Fluorescent Organogels
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OPV1c. In agreement with the dropping ball experiment,
DSC analysis confirmed increased stability of the gels in hy-
drocarbon solvents of greater chain length. The role of hy-
drogen bonding is also evident from the DSC thermograms,
in which OPV1a exhibited a melting temperature of 63.58C,
which is 12.58C and 148C higher than that of OPV4 and
OPV5, respectively.
Microscopy: The results of optical polarizing microscopy of
the OPV gels indicated the formation of strongly birefrin-
gent textures, characteristic of the mesoscopic alignment of
the chromophores during gelation (see Supporting Informa-
tion). Scanning electron microscopy (SEM) images of the
dried OPV1a gels from toluene and decane revealed entan-
gled networks of twisted supramolecular tapes formed by
the self-assembly (Figure 4a and b). Careful analysis re-
1
Variable temperature H NMR spectroscopy studies: The
1
H NMR spectra recorded at room temperature of OPV1a–
b in CDCl or [D ]benzene displayed distinctly solvent-de-
3
6
1
pendent features. For example, the H NMR spectrum of
OPV1a in CDCl (up to 4 mm) showed well-resolved reso-
3
nance signals in agreement with the structure of the mole-
cule, whereas in [D ]benzene (5 mm), none of the character-
6
istic resonance signals corresponding to the aromatic and vi-
nylic protons was seen. These observations indicate strong
intermolecular interactions due to the aggregation of the
OPV units in [D ]benzene. In such cases, the long correla-
6
1
tion time produces H NMR signals that are too broad and
weak to be distinguished. Changes in the resonance signals
of the aromatic and vinylic protons obtained within the tem-
perature range of 10–708C are shown in Figure 3a. As the
Figure 4. SEM images of OPV1a from a) toluene and b) decane. c) TEM
image of the OPV1a gel from toluene. Samples for TEM images were
prepared by drop casting the OPV solution from toluene or decane on
carbon-coated copper grids. The pictures were obtained without staining.
vealed that the twisted tapes of the toluene gel are approxi-
mately 50–100 nm in width and several micrometers in
length. However, in the case of the decane gel, the fibers
are more dense, entangled, and twisted. The relatively large
size of the gel structures in this case indicates that the inter-
actions between individual tapes in decane are much stron-
ger than those in toluene, thereby leading to the formation
of large, twisted tapes. Thus, the gelation efficiency and the
stability of the OPV gels are in agreement with the observed
morphology. The difference in the gelation behaviors of
OPV1a, OPV5, and OPV7, which is attributed to the effect
of the hydrogen-bonding group in the hierarchical growth of
the self-assembly, is clear from the different morphologies
observed by SEM analysis (see Supporting Information).
The transmission electron micrograph (TEM) of OPV1a
from a dilute toluene solution provides more information on
the morphology of the self-assembled textures (Figure 4c),
and shows the presence of isolated and randomly twisted
tapes of 50–100 nm in diameter and several micrometers in
length. Both right- and left-handed twists are visible. The
presence of several thin fibers of 10–20 nm in diameter
could also be seen in addition to the large, twisted struc-
tures.
1
Figure 3. Temperature-dependent H NMR spectra of OPV1a in [D
6
]ben-
zene (5 mm); a) for the aromatic and vinylic protons, b) for the aliphatic
protons.
temperature increases, the resonance signals corresponding
to the aromatic protons appeared gradually at d=7.49, 7.37,
and 7.05 ppm, along with the vinylic protons at d=
8
.05 ppm. The corresponding changes to the aliphatic pro-
tons can be seen in Figure 3b. The benzylic and ÀOCH pro-
2
tons, which appeared as broad, unresolved peaks at d=4.79
and 3.65–3.9 ppm, respectively, became well resolved when
the temperature was increased to 508C. Further heating pro-
duced no significant change, except for a small, upfield shift
of vinylic protons from 8.05 to 7.95 ppm.
The atomic force microscopy (AFM) image of the
OPV1a gel in toluene (Figure 5) is in full support of the
tape-like morphology of the self-assembly, as observed by
SEM and TEM analyses. It is also clear that the fiber bun-
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S. J. George and A. Ajayaghosh
fiber in Figure 5d, indicate the lamellar organization of the
individual OPV units at the molecular level. The detailed
cross-sectional analysis revealed that the thickness of the
tapes, which are several micrometers in length, is in the
range 12–20 nm and the widths are 50–200 nm. Notably, the
large width and thickness of the tapes is an indication of the
three-dimensional growth of the self-assembly, in contrast to
the formation of one-dimensional assemblies of other re-
[21]
ported systems.
X-ray diffraction studies: The xerogels obtained from
OPV1b exhibited well-resolved X-ray diffraction patterns
that were characteristic of the long-range ordering of the
molecules (see Supporting Information). A strong diffrac-
tion signal corresponding to a d-spacing of 23.2 ꢂ, which is
close to the calculated molecular length (21.2 ꢂ) of OPV1b,
could be seen, along with several higher-order reflection
peaks in the small-angle region. A prominent reflection
characteristic of a typical p–p stacking distance was ob-
served in the wide-angle region at 3.8 ꢂ. The intense diffrac-
tion peaks of OPV1a, b, and d at the short-angle region cor-
responded to d-spacings of 41.7 ꢂ, 35.2 ꢂ, and 16.5 ꢂ, re-
spectively. These distances match the calculated width of the
respective molecules with extended side chains and indicate
the lamellar packing distances, which should vary with the
length of the side chains, as seen by comparison of the ob-
served d-spacing (Supporting Information). Based on this
data, it appears that the xerogel of OPV1b is formed by la-
mellar-type packing through hydrogen bonding, p stacking,
and van der Waals interactions, as shown in Figure 6. In the
lamellar packing, OPV1b adopts a planar structure, in
which the aryl units are coplanar and the alkyl side chains
are laterally extended, with a complete stretching of the side
chains within the same plane as the conjugated backbone.
The diffraction patterns of OPV5 carrying hexadecyl side
Figure 5. AFM images displaying the a) amplitude and b) height of
OPV1a from toluene under different magnifications. c) Image of an iso-
lated OPVIa gel fiber and d) a cross-sectional profile of the supramolec-
ular tape displayed in (c). Samples were prepared by drop casting the
OPV solution onto a freshly cleaved, muscovite mica surface.
dles observed in the AFM images are built up from thinner
fibers of 10–20 nm in width. The width of the smallest fiber
bundle that can be distinguished is 50–70 nm. Several isolat-
ed fibers could be observed by AFM, whereby a dilute so-
lution of OPV1a was deposited by drop casting. The mor-
phology of such a single fiber is shown in Figure 5c. The
array of dark and bright areas of approximately 2 nm in
width, as seen in the cross-sectional profile of the OPV1a
Figure 6. Schematic view of the lamellar packing of OPV1b in the gel state. a) Structure of OPV1b, b) side view of the lamellar packing, and c) top view
of the molecular model.
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Self-Assembly of Fluorescent Organogels
FULL PAPER
chains, in which the hydrogen bonding of the two hydroxy-
methyl groups was blocked, displayed relatively broad re-
flections. The absence of higher-order reflections and the
presence of broad reflections in the wide-angle region sug-
gest a low degree of ordering in this case. This data is in
agreement with the gelation behavior of OPV1a and OPV5,
as indicated in Tables 1 and 2.
1
Based on the results obtained by H NMR spectroscopy,
X-ray diffraction, and microscopy, it is clear that the supra-
molecular tapes of the OPV units, formed by the coopera-
tive hydrogen bonding, p stacking, and van der Waals inter-
actions, are responsible for gelation. The weak, nondirec-
tional hydrogen-bonding interaction of the hydroxymethyl
end-groups enables the formation of linear, hydrogen-
bonded assemblies (Figure 6), which form supramolecular
layers by lamellar packing. The layered assemblies are rein-
forced by p stacking and van der Waals interactions, leading
to the formation of three-dimensional, supramolecular
tapes. Hydrogen bonding may also facilitate the reversible,
noncovalent cross-linking between layers to produce a net-
work assembly. The extended growth of these assemblies re-
sults in twisting of the tapes to form elongated networks.
This facilitates the immobilization of large volumes of sol-
vents within and between the networks, resulting in gelati-
on.
À5
Figure 7. Absorption (a) and emission (b) spectra of OPV1a (1ꢁ10 m)
Self-assembly-induced modulation of optical properties: The
most important feature of the gelation of OPVs is the alter-
ation observed in the optical and photophysical properties
in chloroform (–··–··–), cyclohexane (g), dodecane at 258C (c), do-
decane at 658C (d), and in the solid state (b) as a coated film (lex
380 nm). Inset shows the temperature-dependent visual change in the
=
(
see Supporting Information). The absorption and emission
emission colors of the molecularly dissolved (left) and self-assembled
(
right) molecules in dodecane.
spectra of OPV1a under different experimental conditions
are represented in Figure 7a and b. In chloroform (1ꢁ
À5
1
4
(
0
m) the absorption maximum of OPV1a is observed at
spectrum matches that in chloroform, except for a slight
red-shift. The thermoreversible visual color change in the
emission of OPV1a in dodecane is shown in the inset of Fig-
ure 7b. Similar to the absorption changes, the emission of
the OPV1a film corresponded to that in dodecane. These
observations reveal that in chloroform, OPV1a prefers to
be in the molecularly dissolved, monomeric form, whereas
in dodecane or in film form, it exists as a self-assembled spe-
cies. However, in cyclohexane at room temperature, OPV1a
molecules are present in more than one co-existing species,
the composition of which varies as a function of concentra-
tion and temperature. The resemblance of the absorption
and emission spectra of the OPV1a gel in dodecane at 258C
with those of the drop-cast film indicates that the chromo-
phore packing in the two cases is nearly identical.
Variable temperature UV/Vis and fluorescence spectra of
OPV1a in dodecane revealed a transition, as the tempera-
ture was increased from 15 to 658C, from the self-assembled
species to the molecularly dissolved species (Figure 8). In
the UV/Vis spectra, an increase in the intensity of the ab-
sorption maximum at 404 nm was observed upon increasing
the temperature. This is accompanied by a concomitant de-
crease in the intensity of the shoulder band at 467 nm
through an isosbestic point at 428 nm (Figure 8a). Similarly,
07 nm. In cyclohexane (lmax =401 nm) and dodecane
lmax =404 nm) the absorption maxima are slightly blue-
shifted and the intensity of the p–p* band is decreased, with
the formation of a red-shifted shoulder band at 467 nm.
Upon heating the same solutions to 658C, the shoulder band
at 467 nm disappeared and the original intensity of the p–p*
transition was regained, as in the case of chloroform, except
for a slight red-shift of the absorption maxima. This latter
effect could be due to the change in the solvent polarity. In-
terestingly, the absorption spectrum of a film of OPV1a
closely resembled the spectrum corresponding to the dodec-
ane solution. In chloroform, OPV1a showed an intense blue
emission with maxima of around 463 and 490 nm (F =0.73),
f
whereas in dodecane the emission was completely shifted to-
wards the long-wavelength region (green emission), with
maxima of around 537 and 567 nm. However, in cyclohex-
ane at room temperature, a broad emission with several
maxima at 454, 476, 527, and 561 nm (F =0.28) was ob-
f
tained, the relative intensities of which are dependent upon
temperature and concentration (Figure 7b). As the concen-
tration increases, the solution becomes gel-like and the
emission changes to greenish-yellow, probably due to the
scattering of light (Figure 2c). Upon heating the cyclohexane
or dodecane solution, the emission changes to blue and the
in the fluorescence spectra (l =380 nm) recorded as the
ex
Chem. Eur. J. 2005, 11, 3217 – 3227
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3223

S. J. George and A. Ajayaghosh
À5
Figure 8. Temperature-dependent absorption and emission changes of OPV1a in dodecane (1ꢁ10 m). a) Absorption, b) emission on excitation at lex
80 nm, and c) emission on excitation at lex =470 nm.
=
3
temperature is increased from 20–658C, the intensity of the
long-wavelength maxima at 537 and 567 nm decreases with
the simultaneous increase in intensity of the emission bands
at 452 nm and 459 nm (Figure 8b). These observations indi-
cate the existence of self-assembled species at room temper-
ature and nonassembled species at elevated temperatures.
The gradual decrease in emission from the self-assembled
species as the temperature was increased could be observed
more clearly if the species were selectively excited at
470 nm (Figure 8b and c).
The temperature- and concentration-dependent shifts in
the absorption and emission spectra are attributed to the
self-assembled species and not to the formation of excimers.
This is clear from the excitation and emission spectra of
OPV1a in dodecane and cyclohexane (Figure 9). The first
argument against excimer formation is the solvent- and tem-
perature-dependent growth of the long-wavelength absorp-
tion shoulder in the UV/Vis spectrum, indicating ground-
state interactions between molecules. Secondly, the excita-
tion spectrum of OPV1a in dodecane at room temperature
(
monitored at 520 nm) matched the absorption spectrum
having a maximum at 400 nm with a shoulder at 470 nm
Figure 9a). Interestingly, the excitation spectrum (dashed
(
line) at higher temperatures (monitored at 520 nm) in do-
decane did not show the shoulder band at 470 nm and
matched the absorption spectrum. In addition, the excitation
spectra of OPV1a collected at 450 and 620 nm matched the
absorption spectra of the free and self-assembled OPV1a
molecules, respectively (inset of Figure 9a). The presence of
the self-assembled molecules is also justified by the forma-
tion of a red-shifted emission spectrum (lem =527 and
À5
Figure 9. a) Excitation spectra (lem =520 nm) of OPV1a (1ꢁ10 m) in
dodecane at 208C (c) and at 608C (g). Inset shows the excitation
spectra in cyclohexane, collected at 450 (g) and 620 nm (c).
b) Emission spectra of OPV1a in cyclohexane at an excitation wave-
length of 380 (c) and 470 nm (b).
561 nm) of OPV1a in cyclohexane, following the selective
excitation at the shoulder band of 470 nm, in contrast to the
broad spectrum observed on excitation at 380 nm (Fig-
ure 9b). The time-resolved fluorescence analysis of OPV1a
in chloroform revealed monoexponential decay with an ex-
cited state lifetime of 1.64 ns (100%) at room temperature,
whereas in cyclohexane, biexponential decay with lifetimes
of 2.09 (57.85%) and 0.82 ns (42.2%) was obtained (see
Supporting Information). In cyclohexane at 508C, monoex-
ponential decay with a lifetime of 1.58 ns was observed, irre-
spective of the monitoring wavelength. These studies sup-
port the strong aggregation of OPVs in nonpolar solvents,
which is concentration-, polarity-, and temperature-depend-
ent. Notably, the extent of the aggregation-induced changes
in the optical properties of OPV1a–e differs from observa-
[9b–e]
tions in previous reports of hydrogen-bonded OPVs,
in-
dicating that the electronic interaction between the OPV
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Chem. Eur. J. 2005, 11, 3217 – 3227

Self-Assembly of Fluorescent Organogels
FULL PAPER
units in the present case is much stronger, leading eventually
to gel formation.
in concentration (Figure 10b). These stability responses ob-
tained from the sigmoidal temperature transition curves of
the OPV self-assembly are characteristic of the cooperativi-
ty of the different noncovalent interactions during the self-
assembly process. Figure 10c reveals that the stability of the
self-assemblies increases as the length of the alkyl chains in-
creases, with a maximum stability exhibited by OPV1a with
hexadecyl chains. Thus, the change in absorption and emis-
sion properties associated with the self-assembly is a useful
tool to probe the sol–gel phenomena of OPVs. The efficien-
cy of aggregation and the stability of the aggregates of
OPVs, as determined by the change in optical properties,
are in agreement with their gelation behavior, as shown in
Tables 1 and 2.
The stability of the OPV self-assembly and the resultant
gel under different experimental conditions could be deter-
mined from plots of aggregate fraction (a) against tempera-
ture, which are obtained by monitoring the temperature-de-
pendent changes in the UV/Vis absorption spectra at
470 nm (Figure 10). As expected, the transition temperature
of the OPV1a self-assembly increases upon changing the
solvent from toluene to cyclohexane to dodecane (Fig-
ure 10a). The stability of the self-assembled molecules in-
creases by 10–158C with each order of magnitude increase
Conclusion
We have illustrated a rational approach to the design of
supramolecular nanotapes of OPVs that form a novel class
of fluorescent organogels. Interestingly, although the OPVs
described here are functionalized by one of the weakest,
nondirectional hydrogen-bonding motifs, they form strong
organogels when tailored with long hydrocarbon side chains.
Detailed morphological studies revealed that gelation is due
to the hierarchical three-dimensional self-assembly of OPVs
to form supramolecular tapes of high aspect ratio, assisted
by p–p interactions and lamellar packing. The interesting
feature of this system is the gelation-induced changes to the
optical properties, particularly to the emission behavior,
which is useful in probing the self-assembly and the sol–gel
phenomena. The extent of modulation of the optical proper-
ties in three-dimensional self-assembly is remarkable when
compared to that observed for one-dimensional aggregates,
indicating strong electronic interaction of the chromophores
within the self-assembled tapes. The three-dimensional
growth of nanostructures, the gelation, and the consequent
reversible changes in the optical properties assign these
OPV organogels to a novel class of functional materials with
tunable properties. This study should provide opportunities
for the development of nanostructured advanced materials
from the gelation of a variety of linear, p-conjugated sys-
tems and organic semiconductors, which may find applica-
tions in the emerging field of supramolecular electronics.
Acknowledgements
We are grateful to Prof. E. W. Meijer, Prof. R. A. J. Janssen, and Dr.
A. P. H. J. Schenning of the Laboratory of Macromolecular and Organic
Chemistry, Technology University of Eindhoven for their help and fruit-
ful discussions. We thank Pascal Jonkheim and Jochaim de Loos of the
Technology University of Eindhoven for the AFM and TEM measure-
ments, respectively. The help of Mr. Gurusamy with XRD and Mr. Prab-
hakar Rao with SEM is gratefully acknowledged. This work was support-
ed by the Department of Science and Technology (DST), Government of
India, New Delhi and the CSIR Task Force program (CMM 010). S.J.G.
thanks the CSIR for a research fellowship. This is manuscript No. RRL-
PPD-194.
Figure 10. Stability of the OPV self-assemblies obtained from plots of ag-
gregate fraction (a) versus temperature. a) OPV1a in different solvents,
b) OPV1a at different concentrations, and c) OPV1a, b, and d in dodec-
ane (1ꢁ10 m). The data points were obtained from the variable temper-
ature absorption spectral changes at 470 nm.
À5
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FULL PAPER
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Published online: February 25, 2005
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www.chemeurj.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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