Spectroscopic, microscopic and first rheological investigations in charge-transfer interaction induced organogels

Article abstract of DOI:10.1039/c0jm01192d

This article describes two-component charge-transfer interaction mediated organogels (CT-gels) derived from anthracene carboxamides obtained from 2-amino-2-hydroxymethyl-1,3-propanediol (TRIS), and 2,3-dialkoxyanthracenes as donors, with 2,4,7-trinitrofluorenone (TNF) as the common acceptor. We demonstrate the versatility of TNF as an electron acceptor in the formation of these gels. The effect of subtle changes in the donor structure on the gelation ability has been investigated by varying the alkyl chain length in the dialkoxyanthracene donors, and by varying the position of the TRIS substituent in the anthracene carboxamide donors. Distinct differences have been observed in the nature of the CT-gels based on these two kinds of anthracene donors. It has been reported in the literature that 2,3-dialkoxyanthracenes form gels on their own in various aliphatic hydrocarbons and alcohols for linear alkyl chains bearing at least 6 carbon atoms (C₆). In the present study, it is shown that the CT-complex of these molecules with TNF is able to gel many alcoholic and a few hydrocarbon solvents. Also, in the presence of TNF, the 2,3-dialkoxyanthracenes (C₄-C₅) which were non-gelators on their own at ambient temperatures, form CT-gels in a number of alcohols. The other series of gelators discussed, the anthracene carboxamides, require the mandatory presence of TNF to form gels. This donor-acceptor complex forms gels in various aliphatic alcohols. Interestingly, the formation of these CT-gels requires rapid cooling in most of the cases. Thermal stability studies with both types of CT-gels indicate an optimum stoichiometry of 1:1 between the donor and the acceptor. Dynamic rheological experiments reveal these gels as viscoelastic soft materials, with the mechanical strength of these gels depending on the amount of TNF present. This provides a means to tune the strength of the gel by varying the doping concentration of the acceptor.

Full text of DOI:10.1039/c0jm01192d

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Spectroscopic, microscopic and first rheological investigations in charge-
transfer interaction induced organogels†
a
a
b
b
b
Rajat K. Das,‡ Supratim Banerjee,‡ Guillaume Raffy, Andr ꢀe Del Guerzo, Jean-Pierre Desvergne
and Uday Maitra*a
Received 24th April 2010, Accepted 12th June 2010
DOI: 10.1039/c0jm01192d
This article describes two-component charge-transfer interaction mediated organogels (CT-gels)
derived from anthracene carboxamides obtained from 2-amino-2-hydroxymethyl-1,3-propanediol
(
TRIS), and 2,3-dialkoxyanthracenes as donors, with 2,4,7-trinitrofluorenone (TNF) as the common
acceptor. We demonstrate the versatility of TNF as an electron acceptor in the formation of these gels.
The effect of subtle changes in the donor structure on the gelation ability has been investigated by
varying the alkyl chain length in the dialkoxyanthracene donors, and by varying the position of the
TRIS substituent in the anthracene carboxamide donors. Distinct differences have been observed in the
nature of the CT-gels based on these two kinds of anthracene donors. It has been reported in the
literature that 2,3-dialkoxyanthracenes form gels on their own in various aliphatic hydrocarbons and
6
alcohols for linear alkyl chains bearing at least 6 carbon atoms (C ). In the present study, it is shown
that the CT-complex of these molecules with TNF is able to gel many alcoholic and a few hydrocarbon
solvents. Also, in the presence of TNF, the 2,3-dialkoxyanthracenes (C –C ) which were non-gelators
4
5
on their own at ambient temperatures, form CT-gels in a number of alcohols. The other series of
gelators discussed, the anthracene carboxamides, require the mandatory presence of TNF to form gels.
This donor–acceptor complex forms gels in various aliphatic alcohols. Interestingly, the formation of
these CT-gels requires rapid cooling in most of the cases. Thermal stability studies with both types of
CT-gels indicate an optimum stoichiometry of 1 : 1 between the donor and the acceptor. Dynamic
rheological experiments reveal these gels as viscoelastic soft materials, with the mechanical strength of
these gels depending on the amount of TNF present. This provides a means to tune the strength of the
gel by varying the doping concentration of the acceptor.
Introduction
interactions, which further self-assemble hierarchically to form
the gel nanostructure.
Low molecular mass organic compound based gels (LMOG)
1
have attracted much interest in recent years for their immense
The presence of two components in the gel provides scope for
structural tuning of the gel by altering the functionalities in the
individual components, thereby controlling the interactions in
the complex, and eventually modulating the gel strength, stability
and other properties. One of the first reports by Hanabusa et al.
utilized intermolecular hydrogen bonding interactions between
barbituric acid and pyrimidine to obtain two-component orga-
potential in the design of novel nanomaterials, which may find
2
use in optoelectronics, in sensing applications, as templates for
3
4
5
nanoparticles and various inorganic nanostructures. A small
amount of the gelator (typically <1 wt%) dissolved in an organic
solvent self-assembles through non-covalent interactions such as
van der Waals, H-bonding, p–p stacking, donor–acceptor
interactions etc. to form an entangled fibrous network, trapping
and immobilizing the bulk solvent (>99%). These supramolec-
ular gels have advantages over (covalent) polymeric gels since the
interactions involved are reversible, and hence the gel can be
reversibly broken and re-formed, usually by temperature control.
7
nogels. Shinkai and co-workers reported that the optical, thermal
properties and the morphology of the benzene gel of a thymidine-
based organogelator could be modulated in the presence of its
complementary polynucleotide in the gel medium through A-T
8
hydrogen bonding interactions. In 2001, Smith et al. reported
a dendritic two-component organogel with carboxylic acid-con-
9
taining dendritic peptides and alkyl diamines. An interesting
6
Of particular interest are two-component gels where the two
molecular units form a complex through self-complementary
example of a two-component organogelation has been reported by
2
Weiss and co-workers. They showed that bubbling CO through
a solution of primary and secondary aliphatic amines in organic
solvents led to reversible gelation due to the formation of
a
Department of Organic Chemistry, Indian Institute of Science, Bangalore,
60 012, Karnataka, India. E-mail: maitra@orgchem.iisc.ernet.in
5
b
10
Universit eꢀ Bordeaux 1, CNRS, Institut des Sciences Mol eꢀ culaires UMR
ammonium carbamates.
5
255, Nanostructures Organiques, 351 cours de la Lib eꢀ ration, 33405
Talence c eꢀ dex, France
Donor–acceptor interaction mediated formation of gels
constitute a special class of two-component gels where a complex
is formed through partial charge transfer between the two
components – a relatively electron rich molecule (donor) and an
electron deficient molecule (acceptor). The gel formation is
†
Electronic supplementary information (ESI) available: Synthesis and
characterization of all the new compounds and experimental details.
See DOI: 10.1039/c0jm01192d
‡
Equal contribution to the work.
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 7227–7235 | 7227

View Article Online
driven by this charge transfer interaction, and the complex
generally absorbs in the visible region (in a relatively non-polar
solvent), leading to the intensification of the color upon forming
the gel. Such donor–acceptor promoted organogelation may
stabilize the organization of the functionalities of its molecular
components which otherwise may not individually be able to
Lambda 35 spectrophotometers. IR and fluorescence spectra
were recorded on JASCO-70 FT-IR and Perkin Elmer LS-50 B
spectrophotometers,
respectively.
Dynamic
rheological
measurements were done on the gels on an AR 1000 rheometer
(TA instruments) using a plate-plate geometry. HRMS was
recorded on Micromass Q-TOF. Microanalyses were done on
a Carlo Erba Strumentazione CHNS Analyser-model 1106 and
Flash EA 1112. Transmission electron micrographs of the
xerogels were obtained using a TECNAI T-20 electron micro-
scope operating at 160 kV. Scanning electron micrographs were
recorded on a FEI-Quanta 200 electron microscope. Melting
points were recorded on a Buchi Melting Point B-540 apparatus.
The confocal fluorescence microscopy (CFM) measurements
were performed on a Picoquant Microtime 200 using SPAD
detectors (MPD) and a Ti-Sa laser chain from Coherent.
The synthesis, characterization and the details of the experi-
ments are given in the electronic supplementary information
(ESI).†
11,12
form stable supramolecular assemblies.
The CT-interaction
may often lead to stronger gels relative to the gels derived from
one of the components and modulate the optical properties of the
13
resultant self-assembly. This has the potential to expand the
scope of the applications of these CT-gels as functional materials
in fields as diverse as thermochromics and optoelectronics.
Besides, the donor–acceptor charge transfer interaction is
a demonstration of specific molecular recognition, which may
14
enable these gels to act as sensors. The first example of donor–
acceptor interaction promoted organogelation was reported by
11,12
us,
in which a variety of pyrene derivatives were found to
form strongly colored gels in organic solvents in the presence of
an equivalent amount of 2,4,7-trinitrofluorenone (TNF). Since
our initial work, there have been quite a few other reports on
donor–acceptor interaction promoted gel formation in organic
Gelation properties
13
solvents.
The tris-substituted derivatives (1–6)
The electron acceptor, TNF (Chart 3), used in our earlier
studies, is known in the literature to form strong charge transfer
Among the tris derivatives with 2–4 benzene rings, the naph-
thalene derivatives 1 and 2 did not form CT complexes with
TNF, and they did not gel any of the solvents tested (data not
shown). The pyrene derivative 6 formed a red CT complex with
TNF in aliphatic alcohols, but no gelation was observed. Only
compounds 3–5 were found to be gelators.
15
complexes with three, four and five membered aromatic rings.
In the present work, we demonstrate that the charge transfer
interaction of TNF with anthracenes can be utilized to access
a variety of anthracene-based organogelators, which can gel
a range of organic solvents. We discuss here two distinct classes
of charge transfer organogels derived from (a) anthracene TRIS-
carboxamides (3–5, Chart 1), and (b) a series of 2,3-dialkoxy-
anthracenes (7–11, Chart 2) in the presence of TNF as the
common acceptor. We had earlier reported the use of 2-amino-2-
hydroxymethyl-1,3-propanediol (TRIS) group for modulating
Common to all these gelators was the observation that none of
them was able to form gels in the absence of TNF. All the three
anthracene derivatives formed charge-transfer gels in aliphatic
alcohols in the presence of varying amounts of TNF, typically at
concentrations below 1 wt%. Depending on the position of the
TRIS carboxamide on the anthracene backbone, distinct differ-
ences in the gelation behavior of the compounds 3–5 were
observed. While compound 3 formed gels in aliphatic alcohols
ranging from 1-octanol to 1-dodecanol both by ‘normal cooling’
16
the solubility profiles of bile acid derivatives, which were potent
hydrogelators. Extending the concept to organogelators, the
design of the first series of gelators involved attaching TRIS with
anthracene through an amide linkage. The amide functionality is
known to participate in directional H-bonding, and the
hydroxyls of TRIS are expected to improve the solubility of these
molecules in protic solvents. The second series, 2,3-dialkoxyan-
(
the hot sols were allowed to cool under ambient conditions) and
ꢀ
‘rapid cooling’ (hot sol at 120 C was dipped in an ice-water bath
for 1-octanol gels; for 1-decanol and 1-undecanol gels, the hot sol
ꢀ
17
was dipped in a water bath maintained at 15 C; for 1-dodecanol
ꢀ
thracenes, has already been reported as non-hydrogen bonding
gelators of a variety of aliphatic hydrocarbons and alcohols on
their own.
gels, a water bath maintained at ꢁ25 C was used), compound 4
Interestingly, the present study revealed that the gelation
properties of these molecules can be significantly modulated, and
in some cases enhanced, in the presence of TNF.
Table 1 Results of the gelation studies (rapid cooling) with compounds
3, 4 and 5 in the presence (1 : 1 mole ratio) and absence of TNF in
aliphatic alcohols, toluene and n-dodecane
a
Solvent
3
3/TNF
4
4/TNF
5
5/TNF
Experimental
methanol
1-propanol
1-butanol
1-pentanol
S
S
S
S
P
P
P
P
P
I
P
P
P
P
G
G
G
G
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
I
P
P
P
P
G
G
G
P
I
Materials
G
G
G
G
P
P
P
P
I
All starting materials were of commercial grade and purchased
from Aldrich or Fluka, and were used without further purification.
1
1
1
-octanol
-decanol
-undecanol
Characterization and instrumentation
1-dodecanol
toluene
n-dodecane
NMR spectra were recorded on a 300 MHz (JEOL Lambda-300)
I
I
I
and 400 MHz (Bruker) in DMSO-d
6
and CDCl
3
. Absorption
a
G: gel, P: precipitate, I: insoluble.
spectra were recorded on Shimadzu UV 2100 and Perkin-Elmer
7
228 | J. Mater. Chem., 2010, 20, 7227–7235
This journal is ª The Royal Society of Chemistry 2010

View Article Online
formed gels in alcohols (1-propanol to 1-octanol) only through
rapid cooling. Both the gels were deep red in color, showing the
CT band at ꢁ530 nm in the absorption spectra (vide infra).
However, the CT-gels derived from compound 5 (also formed
only through rapid cooling) were deep brown in color. These gels
showed CT band at ꢁ570 nm in the absorption spectra (vide
infra). Unlike the CT-gels from 3, which broke down over
a period of 2–10 h, the CT-gels from 4 and 5 were stable for
months, although they become translucent over time. All these
charge transfer gels can form at quite low concentrations of the
donor molecule, e.g., the critical gelation concentration for
compound 4 to form CT gel in 1-octanol was ꢁ0.16 wt %. The
results of the gelation experiments are summarized in Table 1.
this prompted us to investigate the behavior of the smaller chain
derivatives in the presence of TNF. 2,3-Di-n-(heptyloxy)an-
thracene (8) is a much weaker gelator than DDOA and with
6
chains shorter than C , they become non-gelators at ambient
temperatures. However, in the presence of TNF, the weaker
gelator 2,3-diheptyloxy (8) and the non-gelator 2,3-dibutyloxy-
anthracenes (9) were found to transform into efficient gelators of
alcohols and hydrocarbons. Subsequent studies revealed that the
1 3
alkoxy derivatives with C –C did not show any gelation even in
the presence of TNF. The results are summarized in Table 2.
Interestingly, this charge transfer interaction driven gelation
phenomenon was observed only with 2,3-dialkoxy anthracene
19
derivatives. Several other alkoxy derivatives of anthracene
shown in Chart 4, 9-n-decyloxy (12), 1,2-di-n-decyloxy (13), 1,5-
di-n-decyloxy (14) and 9,10-di-n-decyloxy (15), did not form gels
in the presence of TNF. Even the derivatives with 2,3-methyl-
enedioxy (16) and 2,3-ethylenedioxy (17) substituents also did
not form gels in any of the organic solvent tested. In all these
cases the charge transfer complexes either precipitated out or
remained in solution.
2
,3-Di-n-alkoxyanthracenes (7–11)
2
,3-Di-n-alkoxyanthracene derivatives (having 6–16 carbons on
the alkoxy chains) were reported to be good gelators of a variety
18
of organic solvents (alkanes, alcohols, amines and nitriles). The
derivatives with 10–12 carbon chains were found to be the most
efficient. But with decreasing chain length, the gelation ability
became weaker. This interesting chain length dependent gelation
ability of these derivatives made us investigate the modulation of
their gelation properties by TNF. We started with the didecyloxy
derivative 7 (DDOA) which is the most efficient and most
explored gelator in the series. When DDOA was heated in the
presence of TNF (1 : 1 mole ratio) in alcoholic solvents (except
MeOH), the hot sols initially assumed a light brown color and on
keeping at room temperature, they formed dark brown gels. The
intensification of the color suggested that DDOA was able to
form a charge transfer complex with TNF which participates in
the gelation process. This behavior was observed in a variety of
alcohols ranging from ethanol to 1-decanol, and in alkanes such
as n-hexane and n-dodecane. A few interesting points emerged
from these studies. Firstly, the gels of DDOA in branched
alcohols (isopropanol, 2-butanol) have been reported to be
Therefore, for the charge transfer interaction promoted gela-
tion, the following structural aspects in the anthracene moiety
are important: (1) the n-alkoxy substituents must be in the 2 and
17
weaker than in their linear analogues, but the CT-gels were
stronger in these branched alcohols. Secondly, in MeOH where
DDOA forms gels most efficiently (its critical gelation concen-
tration is 0.6 mM in MeOH), the brown coloration did not
appear. A yellowish gel was obtained at a much higher concen-
tration (probably DDOA formed a gel of its own and TNF was
just dispersed within the gel network). These results suggested
a modulation of the gelation property of DDOA by TNF and
Table 2 Results of the gelation studies with compounds 7, 8, 9 and 10 in
the presence (1 : 1) of TNF in aliphatic alcohols and n-dodecane
Chart 1
a
Solvent
7/TNF
8/TNF
9/TNF
10/TNF
1-propanol
isopropanol
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
—
P
—
—
P
P
P
—
P
1
2
-butanol
-butanol
t-butanol
1
1
1
-pentanol
-octanol
-decanol
n-dodecane
a
G: gel, P: precipitate, I: insoluble.
Chart 2
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 7227–7235 | 7229

View Article Online
that for both these two classes of anthracene derivatives, the
optimum stoichiometry is 1 : 1 (Fig. 1).
After determining the optimum stoichiometry for these CT
gels, the thermal stability profile of the CT gels in different
solvents and the variation of the Tgel against concentration in
a particular solvent (in which the thermal stability is highest) for
each derivative were evaluated maintaining a 1 : 1 mole ratio of
the two components.
Chart 3
CT-gels derived from 3–5
All the gels showed high thermal stability in aliphatic alcohols,
ꢀ
with Tgel above 60 C (Fig. 2). For 3 and 4, the thermal stability
of the CT gels was generally higher with increasing the chain
length of the alcohol. For example, for the gels of 4/TNF, Tgel
followed the order 1-octanol > 1-pentanol ꢁ 1-propanol. The 3/
TNF gels showed similar trend 1-dodecanol ꢁ 1-decanol > 1-
octanol. However this trend is reversed for 5/TNF gels: 1-unde-
canol ꢁ 1-decanol < 1-octanol. A plot of T as a function of the
gel
CT gelator concentration (donor and acceptor present in 1 : 1
mole ratio) for all the three systems showed a linear increase in
thermal stability with concentration (Fig. 3), which is a general
observation for many of the gels derived from the self-assembly
Chart 4
3
positions, and (2) n-alkoxy substituents with alkyl chains > C
3
20
are required.
of small-size organic molecules. It is observed that 4/TNF gels
in 1-octanol have higher gel melting temperature compared to
Thermal stability of gels
The thermal stability of the gels was determined by inverted test
tube method. Sealed tubes containing gels were kept upside down
in a thermostated water bath and the temperature of the water
ꢀ
ꢂ1
bath was raised slowly (ꢁ2 C min ) until the gel fell under
gravity.
Optimum stoichiometry of the donor and the acceptor
As we have observed previously in the case of the pyrene derived
12
CT gels, the maximum thermal stability was obtained when the
components were present in a 1 : 1 mole ratio. To determine the
optimum stoichiometry for these anthracene derived CT gels,
T
gel was determined by varying the concentration of TNF at
Fig. 2 Bar diagram showing the gel melting temperatures of the charge
transfer gels of 3/TNF (black), 4/TNF (blue) and 5/TNF (red) in different
aliphatic alcohols. The concentration of each component in the gel was
a constant concentration of the donor. The thermal stability
reached a maximum at ꢁ1 : 1 mole ratio of the donor and
acceptor. At higher acceptor to donor ratios, the gel melting
point again started to decrease (the extent depends on the specific
gelling systems). It is possible that the excess TNF (acceptor)
precipitates out in the gel matrix, compromising the strength of
the gels. The general conclusion drawn from these experiments is
1
5.4 mM (for 3 and 4) and 14.7 mM (for 5).
Fig. 3 Variation of Tgel with the gelator concentration for the 1-decanol
gels of 3/TNF (1 : 1) (blue triangle), 1-octanol gels of 4/TNF (1 : 1) (black
square) and 5/TNF (1 : 1) (red circle). The linear fits to the corresponding
data points are also shown.
Fig. 1 Variation of the gel melting temperature of (a) 1-octanol gel of 5/
TNF (maintaining a fixed concentration of 15.4 mM of 5) and (b) 2-
BuOH gel of 7/TNF (maintaining a fixed concentration of 5.1 mM of 7).
7
230 | J. Mater. Chem., 2010, 20, 7227–7235
This journal is ª The Royal Society of Chemistry 2010

View Article Online
5
/TNF gels. The data for 3/TNF correspond to the gels in 1-
the color when the gels form from the pale yellow/light brown
sols. All these gels showed intense charge transfer bands in the
visible region, which were absent in the sol. The 3/TNF and 4/
TNF gels (deep red, Fig. 6) show the CT band at ꢁ530 nm,
whereas the CT band appeared at ꢁ570 nm for the 5/TNF gels
decanol, because this CT complex forms stronger gel in 1-dec-
anol than in 1-octanol.
CT-gels from 7–9
(
deep brown). The gels of the charge transfer complexes based on
As mentioned previously, DDOA has been reported to form
2,3-dialkoxyanthracene donors were dark brown (Fig. 6) in
color, similar to the 5/TNF gels. However, they showed two
characteristic maxima (473 nm and 583 nm) in the absorption
spectra (Fig. 5a). The change in the intensity of these two bands
with temperature was monitored in variable temperature exper-
iments (Fig. 5b).
17
stronger gels in linear alcohols. But the CT-gels obtained in
branched alcohols, especially in isopropanol, were found to be
stronger. The same trend was observed for the diheptyloxy (8)
and dibutyloxy (9) derivatives. In terms of the thermal stability,
the charge transfer gels formed from these three anthracene
derivatives (didecyloxy, diheptyloxy and dibutyloxy) did not
show significant differences, except in 2-butanol (Fig. 4).
As seen from Fig. 5b, the intensity of both the bands decreased
gradually with an increase in the temperature and a similar
pattern was observed for didecyloxy 7 and dibutyloxy 9. This
dual spectrum might correspond to the occurrence of two
different CT complexes (of distinct geometry or stoichiometry);
further investigations beyond the scope of this study will be
necessary for understanding this. In any case, these observations
confirm the importance of the charge transfer interaction in the
self-assembly process.
The doping of these CT gels with the non-gelator alkoxy
derivatives of anthracene (maintaining the net concentration of
the donor as constant) led to some interesting observations.
When 2,3-di-alkoxy derivatives (dimethoxy 11 and dipropyloxy
1
0, both non-gelators) were incorporated replacing 50% of the
gelator derivative 7, the thermal stability of the resulting gels did
not vary significantly whereas the incorporation of structurally
dissimilar dialkoxy derivatives (14 and 15) led to a decrease in the
thermal stability as shown in Table 3. These results suggest
strong structural recognition at a molecular level in the forma-
tion of the supramolecular aggregates and the replacement of the
gelling agent by 14 or 15 within the supramolecular organization.
Variable temperature fluorescence studies
These charge transfer gels in general showed a significant
quenching of the anthracene emission in the gel state, compared
to the sol state, because of the strong charge transfer interaction
with TNF in the gel (Fig. 7a). As seen in absorption, the lowest-
lying excited state is indeed a charge transfer state, which is well-
known to be weakly or non-emissive. When the fluorescence
intensity was monitored in variable temperature experiments, the
initial increase of the donor emission was gradual with increasing
Variable temperature absorption spectral study of CT
gels
The gelation process in these two-component gels is driven by the
charge transfer interaction, as indicated by the intensification of
Fig. 5 (a) Charge transfer band for 7/TNF (1 : 1 mole ratio) gel in i-
PrOH (8.2 mM). (b) Temperature dependence of 8/TNF CT-band (1 : 1
mole ratio) gel (0.33%, 8.2 mM) in i-PrOH.
Fig. 4 Bar diagram showing the gel melting temperatures of the charge
transfer gels of 7/TNF (black), 8/TNF (red) and 9/TNF (blue) in different
aliphatic alcohols. The concentration of each component was 10 mM for
all the gels.
Table 3 Comparison of Tgel of the CT gels (with TNF) of DDOA and
DDOA/non-gelator dialkoxy anthracene derivatives
Anthracene derivatives (conc in
mM)
ꢀ
TNF/mM
T
g
/ C
7
7
7
7
7
(10)
10
10
10
10
10
65
68
64
57
54
(5) + 11 (5)
(5) + 10 (5)
(5) + 14 (5)
(5) + 15 (5)
Fig. 6 1-Octanol gel of 4/TNF (1 : 1), 8 mM in both donor (4) and the
acceptor (TNF) (left) and i-PrOH gel of 7/TNF (1 : 1), 8 mM in both
donor (7) and acceptor (TNF) (right).
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 7227–7235 | 7231

View Article Online
Confocal fluorescence microscopy
Confocal fluorescence microscopy (CFM) images were taken for
both 7 and 7/TNF gels in isopropanol. This technique is non-
destructive, allows imaging of the solvated fibers and had already
been used for the study of 7 in the presence of traces of trini-
21
trobenzene. As shown by fluorescence spectroscopy, the emis-
sion of the 7/TNF gels is weaker as compared to 7. CFM allows
within an area of the gel (e.g. 20 ꢃ 20 mm) to determine whether
emission intensities are fluctuating. The image (Fig. 9a) shows that
the fluorescence stems exclusively from the fibers, and the local
variations of intensity are attributed to changes in the fiber sizes
ꢀ
Fig. 7 (a) Emission spectra of 9/TNF (1 : 1) in gel (25 C) and in sol (65
ꢀ
C) in i-PrOH (6.8 mM, lex ¼ 345 nm). (b) Comparison of variable
(
besides, some darker fibers are out of focus). Fig. 9b and 9c,
temperature emission spectra of 7/TNF and 8/TNF gels in i-PrOH (lex
¼
obtained in the 7/TNF gels with more intense laser excitation,
show comparable intensity fluctuations within the fibers as in 7.
Since no particularly bright fibers are observed, this indicates that
TNF is dispersed in all the fibers that have been imaged and
quenches their fluorescence. Moreover, comparable fiber network
morphologies were observed for 7 and 7/TNF at both 1 : 0.5 and
1 : 1 stoichiometries. The solvated fiber network is well preserved
and the widths of the fibers are similar in the presence or absence
of TNF (limited by optical diffraction, i.e. # 230 nm). Larger
fibers or bundles seen by electron microscopy could not be
observed by this technique, as they might be too weakly emissive
or not present in solvated organogels.
345 nm).
temperature. But when the temperature approached the melting
point of the system, the increase was more significant, though the
extent of increment varied from one system to another as illus-
trated in Fig. 7b. This provides a tool for probing the gel to sol
transition as a function of temperature.
Microstructures of the CT gels
Electron microscopy
The morphology of the xerogels of these charge transfer systems
was investigated by electron microscopy. An interconnected
fibrous network of the gel aggregates was observed (Fig. 8a) for
the charge transfer gels of 4/TNF. Gels of 8/TNF and 9/TNF in
solvents such as isopropanol and 1-pentanol also showed entan-
gled fibrous structure (Fig. 8b). However, in case of 7/TNF only
broken fibers could be observed (Fig. 8c). TEM investigation of
the xerogel obtained from i-PrOH gel of 8/TNF (1 : 1) showed that
the average diameter of the fibers was in the range of 50–80 nm
Rheology of CT gels
The CT gels derived from the two different classes of anthracene
derivatives were characterized by dynamic rheology. Gels are
viscoelastic materials and so they can both store and dissipate
0
energy (characterized by storage modulus G and loss modulus
0
0
G respectively). The three important parameters obtained from
0
dynamic rheology are elasticity (G ), fragility (s*) and stiffness
(
Fig. 8d).
Fig. 9 Fluorescence confocal microscope intensity images (20 ꢃ 20 mm)
of 8 mM organogels in i-PrOH (a) 7, (b) 7/TNF 1 : 0.5, (c) 7/TNF 1 : 1.
The widths of the solvated fibres is typically #230 nm (optical diffraction
limit) in a, b and c. lex 385 nm, lem > 405 nm.
Fig. 8 SEM images of the charge transfer gels of (a) 4/TNF in 1-butanol,
b) 9/TNF in 1-pentanol and (c) 7/TNF in i-PrOH. (d) TEM image for
a xerogel of 8/TNF (1 : 1) in i-PrOH.
(
7
232 | J. Mater. Chem., 2010, 20, 7227–7235
This journal is ª The Royal Society of Chemistry 2010

View Article Online
0
00
(
G /G ). All the rheology experiments were performed in 1-
Stoichiometry dependent mechanical strength
octanol or 1-decanol to minimize solvent evaporation during the
experiments.
We have discussed earlier that the thermal stability of these CT
gels is dependent on the stoichiometry. A similar trend is
expected in the mechanical strength of these CT gels, too. When
the dynamic frequency sweep experiment was performed with 8/
Dynamic frequency sweep experiments
0
TNF (1 : 0.5) gel in 1-octanol, the values of dynamic moduli (G ,
0
0
Dynamic frequency sweep experiments at a small value of
applied stress were performed for all the CT gels and they showed
G ) obtained were one order in magnitude lower than those
obtained from the 8/TNF gel (1 : 1) in 1-octanol. Similarly, the 4/
TNF (1 : 0.5) gel in 1-octanol showed ꢁ2.5 times reduced value
0
a consistently higher value for the storage modulus (G ) over the
0
0
0
loss modulus (G ), which is typical for viscoelastic soft solids
00
of G and the gel started to flow at considerably lower applied
0
(
Fig. 10). The ratio of (G /G ) is ꢁ18–24 for CT gels of 3, 4 and 5
stress (Fig. S4, ESI†). So, a possible way of modulating the
mechanical strength and the flow property of these CT gels is to
vary the ratio of the donor and the acceptor.
and 9–14 for the CT gels of 7, 8 and 9 (Table 4). It suggests that
the dissipation of energy is more for the CT gels of 2,3-dia-
lkoxyanthracenes via viscous mechanisms.
When we compare the CT gels of 7, 8 and 9 in 1-octanol, no
particular trend was observed on going from didecyloxy (7) to
dibutyloxy (9) derivative (Table 4). For example, 7/TNF gel
Dynamic stress sweep experiments
When the CT gels were subjected to an ascending stress ramp,
above a particular value of stress (s*, yield stress), the gels broke
down and flow occurred. From these dynamic stress sweep
experiments (performed at f ¼ 1.0 Hz, the stress sweep for 9/TNF
is shown in Fig. 11), it was found that 7/TNF gel was the most
fragile among the CT gels of 7, 8 and 9. It started flowing at
a relatively low stress of 42 Pa whereas the yield stress values for
0
showed the lowest value of elasticity (G ) whereas 8/TNF and 9/
TNF exhibited similar values. In terms of stiffness of the gel, 9/
TNF shows the lowest value but comparable values were
observed for 7/TNF and 8/TNF.
0
The 1-octanol gel of 4/TNF (1 : 1) showed higher G values
(
Table 4) compared to the 5/TNF gel in the frequency sweep
8
/TNF and 9/TNF (Table 4) and 7 (530 Pa) were significantly
0
experiment, indicating higher elasticity, which also correlates well
with their thermal stability (vide supra). However, although the 1-
octanol gel of 3/TNF (1 : 1) showed a significantly lower melting
00
higher. G and G of 7/TNF are also one order of magnitude
22
lower than in 7 without TNF (1 wt% in 1-octanol).
A
ꢀ
0
comparative study of the CT-gels of 3, 4 and 5 showed that the 3/
TNF gel has the highest yield stress (245 Pa). The 1-octanol gels
obtained from 4/TNF and 5/TNF showed comparable yield stress
values (Table 4). However, both these gels are more fragile than
the 3/TNF gel.
temperature (64 C), a higher value of G (8336 Pa) indicated
higher mechanical strength.
Variable temperature rheology
The gel-sol transition temperature can be estimated by using
rheology. It is done by following the dynamic moduli as a func-
tion of temperature at a fixed frequency and at a low enough
stress amplitude so that the perturbation caused to the system is
minimum. When this experiment was performed for a gel of 8/
TNF (1 : 1, 10 mM of each component, Fig. 12), the values of the
dynamic moduli initially decreased gradually and then at a point
the decrease became sharper which can be considered as the gel-
sol transition point. It was satisfying to note that the value of the
transition temperature obtained from this experiment was close
Fig. 10 Dynamic frequency sweep experiment (0.01 Hz to 10.0 Hz @
constant stress of 1.0 Pa) for 7/TNF (1 : 1) gel in 1-octanol (open circles
0
00
for G and closed circles for G ).
0
00
Table 4 Dynamic rheology of CT gels. The G and G values shown in
the table are obtained from frequency sweep experiments performed at
a fixed stress of 1.0 Pa for the CT gels (1 : 1) of 7, 8 and 9 (10 mM of
donor) and 6.0 Pa for the CT gels for 3, 4 and 5 (15.4 mM for 3 and 4, and
1
4.7 mM for 5). The values are at f ¼ 1.0 Hz. The s* values shown are
obtained from stress sweep experiments at a fixed frequency of 1.0 Hz.
Error ꢄ 10–15%)
(
0
00
0
00
System
G (Pa)
G
(Pa)
G /G
s* (Pa)
3
4
5
7
8
9
/TNF
/TNF
/TNF
/TNF
/TNF
/TNF
8300
4300
2900
900
4000
4200
500
180
150
70
275
450
17
24
20
13
14
9
245
150
170
42
295
140
Fig. 11 Dynamic stress sweep experiment (@1.0 Hz) for 9/TNF (1 : 1)
0
00
gel in 1-octanol (open squares for G and closed squares for G ).
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 7227–7235 | 7233

View Article Online
form comparable CT gels in aliphatic alcohols and in some
aliphatic hydrocarbons in the presence of TNF. Even the non-
gelator 2,3-dibutyloxyanthracene (9) forms CT gels in aliphatic
alcohols. The appearance of a CT-absorption band, the fluores-
cence quenching and the uniform emission intensity of the fibers
shown by confocal fluorescence microscopy indicate that this
enhanced gelation is linked to the homogeneous and tight asso-
ciation of TNF with the anthracene derivatives in the nanofibers.
In contrast, even in the presence of TNF no gels are formed with
3
C . This supports the hypothesis that the propensity of these
derivatives to produce fibrils and gels is connected to their crys-
talline packing into triads. TNF seems thus to emphasize the
predisposition of a molecule to gelate, perhaps without funda-
mentally altering the molecular packing. A more precise insight on
the association of the gelator and TNF require nevertheless a more
precise determination of the molecular structure. The thermal
stability of the CT gels from the three derivatives 7, 8 and 9 were
comparable, suggesting that the presence of TNF not only
modulates their normal gelation efficiencies but at the same time
also transforms them into gels of similar thermal strengths. The
mechanical strengths of these CT-gels are also similar, although 7
is somewhat weakened with the addition of TNF. Though the
Fig. 12 The dynamic moduli as function of temperature for 8/TNF
0
00
(
1 : 1) gel in 1-octanol (open circles for G and closed circles for G ) at f ¼
ꢀ
ꢂ1
1
.0 Hz, s ¼ 0.5 Pa and at a heating rate of 0.5 C min
.
to that obtained from the inverted test tube experiments (the
values obtained from these experiments differed by only a few
degrees).
Discussion
The present study on anthracene based charge transfer gels
illustrates the versatility of TNF as an acceptor molecule in the
gelation process. The charge transfer interaction is the driving
force for gelation, which is supported by the appearance of
a strong CT absorption band in the visible region. For the
anthracene carboxamide-based gelators 3–5, CT complex
formation is a prerequisite for the gelation to occur. The naph-
thalene carboxamides fail to show any gelation because the
formation of a CT complex with TNF is not favorable probably
due to p-surface mismatch. However, the pyrene-based carbox-
amide 6, although it forms CT complex with TNF, fails to form
gels. This indicates that CT complex formation may be an
essential condition for this class of gelators, but it is not the sole
determining factor. Moreover, most of the gels from anthracene
carboxamides form only by rapid cooling. Slow cooling results in
the precipitation of the CT complex. This observation suggests
that kinetics plays an important role in the gelation, and these
gels can be essentially regarded as metastable states. The position
of the TRIS substituent on the anthracene skeleton has
a profound effect on the stability of the gels. For example, the
gels derived from 3/TNF were stable only for a few hours,
whereas 4/TNF and 5/TNF gels were stable for weeks. Such
effects of the substituent position on gel stability is a fairly
common observation in many gels derived from low molecular
mass species, both single component and two component gels.
derivatives having C –C chains did not form gels (vide supra)
1 3
even in the presence of TNF, incorporation of these derivatives
into CT gels (replacing the same equivalent of a gelator derivative)
did not significantly affect the thermal strengths. In contrast, the
incorporation, or lack of incorporation of dialkoxy derivatives
having the substitutents in other positions of the anthracene
moiety decreases the thermal stability of the resultant gels. These
observations suggest that structural similarity between gelating
and non-gelating dialkoxy derivatives is required to preserve the
original organization of the CT gels.
The general trend observed in these two classes of anthracene
derivatives is that both the thermal stability and the mechanical
strength of these CT gels are dependent on the stoichiometry of
the donor and the acceptor. So, for these gels, a simple tuning of
these properties is possible by varying the ratio of the donor and
the acceptor.
Conclusions
We have presented two different classes of anthracene based
organogelators, anthracene carboxamides and 2,3-dialkoxyan-
thracenes, which showed efficient gelation in the presence of
acceptor TNF. The two types of charge transfer gels showed
distinct differences in their properties. Indeed, the anthracene
carboxamides were able to form gels only in the presence of TNF.
For the dialkoxy derivatives, the gelation property of the efficient
organogelator could be modulated by the addition of TNF, while
the weak and non-gelators could actually be transformed into
efficient gelators in its presence. Several other dialkoxy derivatives
having alkyl substituents not in the 2 and 3 positions did not show
gelation in the presence of TNF. These two points illustrate the
two major structural aspects necessary for these derivatives to
form CT gels- the alkyl chain length and the position of the alkyl
substituents. However, the anthracene monocarboxamide donors
were able to form CT gels irrespective of the position of the TRIS
substituent on the anthracene moiety. The thermal stability of all
these gels was found to be maximum with a 1 : 1 stoichiometry of
2
6
,3-Di-n-alkoxyanthracenes (C –C16) can form gels on their
own (in the absence of TNF), preferably in aliphatic alcohols
rather than in hydrocarbons, while with shorter chains (C
2
–C
5
)
no organogels are formed. However, compound 9 (C ) and C
4
5
were recently shown to form fibrillar nanoobjects as observed with
the gelling components of the series, i.e., C –C12. This propensity
6
to produce fibrillar aggregates was found to be connected to their
specific molecular packing in triads in the crystalline phase. In
contrast, the shortest chains, such as C and C , do not produce
2
3
23
any fibrillar aggregates and crystallize in a herringbone structure.
Interestingly, our studies show that the incorporation of TNF
significantly influences the gelling ability of some of these 2,3-
dialkoxyanthracenes. Thus, the three tested compounds 7, 8 and 9
7
234 | J. Mater. Chem., 2010, 20, 7227–7235
This journal is ª The Royal Society of Chemistry 2010

View Article Online
the donor and the acceptor. The common observation, the
intensification of color on going from the sol to the gel phase,
supports the crucial role of the charge transfer interaction behind
the formation of these gels. The rheological characterization of the
gels showed that they behave like viscoelastic soft solids, as has
been observed for many other single component supramolecular
gels. In summary, we have discussed a new class of charge transfer
organogels based on anthracene carboxamides and also presented
a detailed study on the modulation of the properties of 2,3-dia-
lkoxyanthracene based gels on doping with TNF, the common
electron acceptor. We believe that these studies will enable us and
others to expand our understanding and the scope of research in
the field of two component gels.
4 S. Bhat and U. Maitra, Chem. Mater., 2006, 18, 4224; S. Srinivasan,
S. S. Babu, V. K. Praveen and A. Ajayaghosh, Angew. Chem., Int.
Ed., 2008, 47, 5746; J. van Herrikhuyzen, S. J. George,
M. R. J. Vos, N. A. J. M. Sommerdjik, A. Ajayaghosh,
S. C. J. Meskers and A. P. H. J. Schenning, Angew. Chem., Int. Ed.,
2007, 46, 1825; A. Mantion, A. G. Guex, A. Foelske, L. Mirolo,
K. M. Fromm, M. Painsi and A. Taubert, Soft Matter, 2008, 4, 606.
5
Y. Ono, K. Nakashima, M. Sano, Y. Kanekiyo, K. Inoue, J. Hojo and
S. Shinkai, Chem. Commun., 1998, 1477; J. H. Jung, Y. Ono and
S. Shinkai, Angew. Chem., Int. Ed., 2000, 39, 1862; M. Llusar,
L. Pidol, C. Roux, J. L. Pozzo and C. Sanchez, Chem. Mater.,
2
002, 14, 5124; C. Zhan, J. Wang, J. Yuan, H. Gong, Y. Liu and
M. Liu, Langmuir, 2003, 19, 9440; P. Xue, R. Lu, D. Li, M. Jin,
C. Tan, C. Bao, Z. Wang and Y. Zhao, Langmuir, 2004, 20, 11234.
6 For a review, see: A. R. Hirst and D. K. Smith, Chem.–Eur. J., 2005,
1, 5496.
1
K. Hanabusa, T. Miki, Y. Taguchi, K. Koyama and H. Shirai,
J. Chem. Soc., Chem. Commun., 1993, 1382.
7
Acknowledgements
8 K. Sugiyasu, M. Numata, N. Fujita, S. M. Park, Y. J. Yun,
B. H. Kimand and S. Shinkai, Chem. Commun., 2004, 1996.
9 K. S. Partridge, D. K. Smith, G. M. Dykes and P. T. McGrail, Chem.
Commun., 2001, 319.
This work was supported by the Indo-French Centre for the
Promotion of Advanced Scientific Research (IFCPAR) and the
Jawaharlal Nehru Centre for Advanced Scientific Research
1
1
0 M. George and R. G. Weiss, J. Am. Chem. Soc., 2001, 123, 10393.
1 U. Maitra, P. VijayKumar, N. Chandra, L. J. D’Souza,
M. D. Prasanna and A. R. Raju, Chem. Commun., 1999, 595.
2 P. Babu, N. M. Sangeetha, P. VijayKumar, U. Maitra, K. Rissanen
and A. R. Raju, Chem.–Eur. J., 2003, 9, 1922.
(
JNCASR). The Institute Nanoscience Initiative (INI) is
1
1
acknowledged for TEM and SEM facility. We also thank Pierre
Terech (CEA, Grenoble) for stimulating discussions. The Council
of Scientific and Industrial Research (CSIR), New Delhi is
thanked for the award of Research Fellowships to RKD and SB.
3 T. Ishi-I, R. Iguchi, E. Snip, M. Ikeda and S. Shinkai, Langmuir, 2001,
7, 5825; A. Friggeri, O. Gronwald, K. J. C. van Bommel, S. Shinkai
1
and D. N. Reinhoudt, J. Am. Chem. Soc., 2002, 124, 10754;
D. Rizkov, J. Gun, O. Lev, R. Sicsic and A. Melman, Langmuir,
2
005, 21, 12130; B. G. Bag, G. C. Maity and S. K. Dinda, Org.
Lett., 2006, 8, 5457; J. R. Moffat and D. K. Smith, Chem.
Commun., 2008, 2248.
Notes and references
1
Molecular Gels, Materials with Self-assembled Fibrillar Networks, ed.
R. G. Weiss and P. Terech, Springer, 2006; S. Banerjee, R. K. Das and
U. Maitra, J. Mater. Chem., 2009, 19, 6649.
14 P. Mukhopadhyay, Y. Iwashita, M. Shirakawa, S.-i. Kawano,
N. Fujita and S. Shinkai, Angew. Chem., Int. Ed., 2006, 45, 1592.
15 W. F. Newhall, J. Org. Chem., 1958, 23, 1274; G. L. K. Hunter and
W. B. Brogden, Anal. Chem., 1964, 36, 1122; G. H. Schenk,
P. W. Vance, J. Pietrandrea and C. Mojzis, Anal. Chem., 1965, 37,
372.
2
T. Kitahara, M. Shirakawa, S.-i. Kawano, U. Beginn, N. Fujita and
S. Shinkai, J. Am. Chem. Soc., 2005, 127, 14980; T. Katakabe,
T. Kaneko, M. Watanabe, T. Fukushima and T. Aida,
ꢀ
16 N. M. Sangeetha, R. Balasubramanian, U. Maitra, S. Ghosh and
A. R. Raju, Langmuir, 2002, 18, 7154.
J. Electrochem. Soc., 2005, 152, A1913; J. Puigmart ꢀı -Luis, A. P. del
Pino, E. Laukhina, J. Esquena, V. Laukhin, C. Rovira, J. Vidal-
Gancedo, A. G. Kanaras, R. J. Nichols, M. Brust and
D. B. Amabilino, Angew. Chem., Int. Ed., 2008, 47, 1861;
K. Sugiyasu, S.-i. Kawano, N. Fujita and S. Shinkai, Chem. Mater.,
17 T. Brotin, R. Uterm o€ hlen, F. Fages, H. Bouas-Laurant and
J.-P. Desvergne, J. Chem. Soc., Chem. Commun., 1991, 416.
18 J.-P. Desvergne, T. Brotin, D. Meerschaut, G. Clavier, F. Placin,
J.-L. Pozzo and H. Bouas-Laurent, New J. Chem., 2004, 28, 234.
19 These compounds are non-gelators even as single components.
20 D. J. Abdallah, L. Lu and R. G. Weiss, Chem. Mater., 1999, 11, 2907.
21 C. Giansante, A. G. L. Olive, C. Sch €a fer, G. Raffy and A. Del
Guerzo, Anal. Bioanal. Chem., 2010, 396, 125.
2008, 20, 2863.
3
K. Murata, M. Aoki, T. Nishi, A. Ikeda and S. Shinkai, J. Chem. Soc.,
Chem. Commun., 1991, 1715; J. L. Pozzo, G. M. Clavier and J.-
P. Desvergne, J. Mater. Chem., 1998, 8, 2575; M. Ayabe,
T. Kishida, N. Fujita, K. Sada and S. Shinkai, Org. Biomol. Chem.,
2
003, 1, 2744; J. J. D. de Jong, L. N. Lucas, R. M. Kellogg,
22 P. Terech, J.-P. Desvergne and H. Bouas-Laurent, J. Colloid Interface
Sci., 1995, 174, 258.
23 A. G. L. Olive, G. Raffy, H. Allouchi, J.-M. L ꢀe ger, A. Del Guerzo
and J.-P. Desvergne, Langmuir, 2009, 25, 8606.
J. H. van Esch and B. L. Feringa, Science, 2004, 304, 278;
P. Mukhopadhyay, Y. Iwashita, M. Shirakawa, S.-i. Kawano,
N. Fujita and S. Shinkai, Angew. Chem., Int. Ed., 2006, 45, 1592.
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 7227–7235 | 7235