Strength enhancement of nanostructured organogels through inclusion of phthalocyanine-containing complementary organogelator structures and in situ cross-linking by click chemistry

Article abstract of DOI:10.1002/chem.200800714

Stable photoactive organogels were successfully prepared by a two-step sequence involving: 1) formation of thermoreversible organogels by use of a combination of low-molecular-weight organogelators (LMOGs) and Zn ^II-phthalocyanine (Zn^II-Pc) moieties containing complementary organogelator structures, and 2) strength enhancement of the gels by in situ cross-linking with the aid of Cu^I-catalysed azidealkyne [3+2] cycloadditions (CuAACs). The optimum click reaction was carried out between a flexible C6 aliphatic diazide and a suitable diaIkyne (molar ratio 1:1) added in a low proportion relative to the organogelator system [LMOG+Zn^IIPc]. The diaIkyne unit was incorporated into a molecule resembling the LMOGs structure in such a way that it could also participate in the self-assembly of [LMOG+Zn^IIPc]. The significant compatibility of the multicomponent photoactive organogels towards this strengthening through CuAACs allowed their sol-to-gel transition temperatures (T_gel) to be enhanced by up to 15°C. The T_gel values estimated by the inverse flow method were in good agreement with the values obtained by differential scanning calorimetry (DSC). Rheological measurements confirmed the viscoelastic, rigid, and brittle natures of all Pc-containing gels. Transmission and scanning electron microscopy (TEM, SEM) and atomic force microscopy (AFM) revealed the fibrilar nature of the gels and the morphological changes upon cross-linking by CuAAC. Emission of a red luminescence from the dry nanoscale fibrous structure - due to the self-assembly of the Pc-containing compounds in the organogel fibres - was directly observed by confocal laser scanning microscopy (CLSM). The optical properties were studied by UV/Vis and fluorescence spectroscopy. Fluorescence, Fourier-transform infrared (FTIR) and circular dichroism (Cd) measurements were also carried out to complete the physicochemical characterization of selected gels. As a proof of concept, two different organogelators (cholesterol- and diamide-based LMOGs) were successfully used to validate the general strategy.

Full text of DOI:10.1002/chem.200800714

FULL PAPER
DOI: 10.1002/chem.200800714
Strength Enhancement of Nanostructured Organogels through Inclusion of
Phthalocyanine-Containing Complementary Organogelator Structures and In
Situ Cross-Linking by Click Chemistry
David Díaz Díaz,*[b] Juan JosØ Cid,[a] Purificación Vµzquez,[a] and Tomµs Torres*[a]
Abstract: Stable photoactive organo-
gels were successfully prepared by a
two-step sequence involving: 1) forma-
tion of thermoreversible organogels by
use of a combination of low-molecular-
weight organogelators (LMOGs) and
ZnII–phthalocyanine (ZnII–Pc) moieties
containing complementary organogela-
tor structures, and 2) strength enhance-
ment of the gels by in situ cross-linking
with the aid of CuI-catalysed azide–
[LMOG+ZnIIPc]. The significant com-
patibility of the multicomponent pho-
toactive organogels towards this
strengthening through CuAACs al-
lowed their sol-to-gel transition tem-
peratures (Tgel) to be enhanced by up
to 158C. The Tgel values estimated by
the “inverse flow method” were in
good agreement with the values ob-
tained by differential scanning calorim-
etry (DSC). Rheological measurements
confirmed the viscoelastic, rigid, and
brittle natures of all Pc-containing gels.
Transmission and scanning electron mi-
croscopy (TEM, SEM) and atomic
force microscopy (AFM) revealed the
fibrilar nature of the gels and the mor-
phological changes upon cross-linking
by CuAAC. Emission of a red lumines-
cence from the dry nanoscale fibrous
structure—due to the self-assembly of
the Pc-containing compounds in the or-
ganogel fibres—was directly observed
by confocal laser scanning microscopy
(CLSM). The optical properties were
studied by UV/Vis and fluorescence
spectroscopy. Fluorescence, Fourier-
transform infrared (FTIR) and circular
dichroism (CD) measurements were
also carried out to complete the physi-
cochemical characterization of selected
gels. As a proof of concept, two differ-
ent organogelators (cholesterol- and di-
amide-based LMOGs) were successful-
ly used to validate the general strategy.
alkyne
[3+2]
cycloadditions
(CuAACs). The optimum click reac-
tion was carried out between a flexible
C6 aliphatic diazide and a suitable dia-
lkyne (molar ratio 1:1) added in a low
proportion relative to the organogela-
tor system [LMOG+ZnIIPc]. The dia-
lkyne unit was incorporated into a mol-
ecule resembling the LMOGs structure
in such a way that it could also partici-
Keywords:
organogels
click chemistry
· phthalocyanines
·
·
supramolecular chemistry
pate
in
the
self-assembly
of
Introduction
Although gelation of liquids through the presence of small
quantities of a second component has been known for sever-
al centuries,[1] gels have recently been reborn as a new field
of supramolecular chemistry[2] because they represent a
powerful approach to the development of new materials and
devices of nanoscale dimensions with tailored functionality
such as high strength or particular electronic or optical
properties.
[a] J. J. Cid, Prof. P. Vµzquez, Prof. T. Torres
Departmento de Química Orgµnica
Universidad Autónoma de Madrid
Cantoblanco, 28049 Madrid (Spain)
Fax : (+34)914-973-966
E-mail: tomas.torres@uam.es
[b] Dr. D. D. Díaz
Current address: The Dow Chemical Company
Dow Europe GmbH
There are two types of gels: chemical gels,[3] which are po-
lymer gels based on chemical bonding, and physical gels,[4]
which are based on physical attractions. Systems based on
both types of connections are also known.[5] In general, a gel
is a viscoelastic solid-like (soft) material—constituting an
elastic cross-linked network—and a solvent, which is the
Bachtobelstrasse 3, 8810 Horgen (Switzerland)
Fax : (+41)55-416-8701
E-mail: DDiaz-Diaz@dow.com
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800714: Detailed experimen-
tal procedures, characterization data, control experiments, additional
information, tables and figures.
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major component. A gel is usually prepared by heating a
low-molecular-mass compound in an appropriate solvent
(water for hydrogels, some organic solvent for organogels)
and cooling the resulting isotropic supersaturated solution
to room temperature.[4] When the hot solution is cooled, the
molecules start to condense, and three situations are then
possible: 1) a highly ordered aggregation giving rise to crys-
tals (i.e., crystallization), 2) a random aggregation, resulting
in an amorphous precipitate, or 3) an aggregation process
intermediate between these two, yielding a gel. The solid-
like appearance of a gel is a result of the entrapment and
adhesion of the liquid in a solid 3D matrix of large surface
area. The formation of the solid matrix is a consequence of
cross-linking of the polymeric strands of (macro)molecules
through physical or chemical forces.
bottom-up fabrication can be used for the assembly of nano-
scale architectures. In the last decade, the enhancement of
the thermostabilities of these gels has became an important
task in macromolecular science and crucial for numerous
applications.
As a part of our extensive research program directed to-
wards the synthesis, applications and supramolecular aspects
of Pcs and their derivatives,[21] we became interested in the
noncovalent inclusion of Pcs in self-assembled polymeric
supramolecular networks for study of the reciprocal effects
both on the photophysical properties of the metallophthalo-
cyanines (MPcs) and on the physicochemical properties of
the 3D organogel networks. In this regard, large fibrous net-
works made of LMOGs have been shown to provide specific
molecular environments for incorporated photoactive dye
molecules, and therefore to improve the functionality of
these smart materials.[22] Our previous contribution de-
scribed the physicochemical characterization of mesogenic
Organogels based on low-molecular-mass organic gelators
(LMOGs) are formed through self-assembled fibrillar net-
works (SAFINs) through the operation of a combination of
weak noncovalent physical interactions such as H-bonding,
p–p stacking, dipole–dipole or donor–acceptor interactions,
metal coordination, solvophobic forces (hydrophobic forces
for hydrogels) and van der Waals interactions. This ends up
in arrangements of gel fibres typically of micrometer scale
lengths and nanometer scale diameters (ca. 10–100 nm).[6]
Afterwards, the entanglement of such microheterogeneous
fibrillar phases engenders complex nanoscale 3D networks,[7]
which immobilize large volume of organic liquids in their in-
terstices, typically through surface tension and capillary
forces.[8] This process prevents the flow of solvent under
gravity, and the mass appears like a solid. Organogels may
hence immobilize up to 105 liquid molecules per gelator and
increase the viscosity of the medium by a factor of 1010, with
the potential to respond to a variety of external stimuli.[9]
Since the organogel networks involve weak interactions,
they can be readily transformed into fluids (sols) by heating
and are generally thermoreversible. (Gels based on strong
chemical bonds, on the other hand, cannot be redissolved
and do not show thermoreversible gel–sol transitions.) Gela-
tors have been classified according to their driving forces for
molecular aggregation into two major categories: non-hy-
drogen-bond-based gelators and hydrogen-bond-based gela-
tors. Cholesterol derivatives[10] are typical examples of the
former group, whereas aliphatic amide and urea[10] deriva-
tives and saccharide-containing gelators[10] are the main rep-
resentatives of the latter. These “intelligent”[2] systems, de-
fining a metastable state between liquid and solid, have re-
ceived increasing attention over the last 10–15 years[4] be-
cause of their unique supramolecular architectures and po-
tential applications[11] as functional soft materials in the
fabrication of sensors,[12] liquid crystalline substances,[13] elec-
trophoretic and electrically conductive matrices,[14] biocata-
lytic vessels,[15] vehicles for drug delivery,[16] lubricating
greases[17] and oil spill recovery agents,[18] optoelectronic ma-
terials,[19] templates for cell growth or the growth of sol-gel
structures,[20] and in many other industrial fields such as cos-
metics, oils and foods.[1] Supramolecular gel-phase materials
based on LMOGs are a prime example of the way in which
octakis
(alkyloxy)-substituted ZnII–phthalocyanines noncova-
R
lently incorporated into an organogel and their remarkable
morphological effect on the nanoscaled fibres.[23] Novel,
unique fibrous organic–inorganic hybrid materials—1D ag-
gregates of inorganic materials based on electrostatic and/or
hydrogen-bonding interactions—have also been fabricated
recently.[24]
The overall objective of this work was the fabrication of
stable photoactive organogels by combining LMOGs with
Pc-containing complementary organogelator structures.
Results and Discussion
Organogels consisting of combinations of LMOGs and com-
plementary LMOG structures containing ZnII–Pc moieties:
5a-Cholestan-3b-yl N-(2-anthryl)carbamate (1, Figure 1) is a
renowned LMOG for both alkanes and alkanols at concen-
trations in the 0.5–2.0 wt% range,[25] but it does not gel aro-
matic solvents such as benzene, styrene or toluene, in which
precipitation occurs upon cooling of the isotropic solutions
to room temperature. Nevertheless, we found that stable
gels were obtained in various aromatic solvents when 1 and
the ZnII–Pc-containing complementary cholestanyl structure
3 were mixed in a molar ratio of 20:1. In order to demon-
strate the scope of this method for the preparation of photo-
active organogels, the strategy was also applied to the differ-
ent, well-known LMOG 2,[26] with the ZnII–Pc-containing
analogues 7–9.
Synthesis:[27] All synthesized compounds here were satisfac-
torily characterized by UV/Vis, FTIR, MALDI-TOF MS
and NMR. The synthesis of 1 was carried out by means of a
condensation reaction between 5a-cholestan-3b-yl chlorofor-
mate and the corresponding 2-aminoanthracene as previous-
ly reported.[25] The UV/Vis absorption of 1 in CH2Cl2 solu-
tion showed the expected bands corresponding to a weak
1
perturbed La transition at about 394 nm and a strong per-
1
turbed Bb transition at about 263 nm.[25]
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Nanostructured Organogels
FULL PAPER
oxidation of the corresponding
alcoholic precursors 16[21a] and
19, respectively (Scheme 2).
Compounds 16 and 19 were
first converted into the corre-
sponding aldehydes 17[21a] and
20 by Parikh–Doering and
IBX-mediated
oxidations,[30]
respectively, and were further
oxidized to the desired acids
with sodium chlorite and sulfa-
mic acid as a chloride scaveng-
er.[30] The alcohol precursors 16
and 19 were obtained by statis-
tical crossover condensation of
the appropriate phthalonitriles,
15[21f,29] and 18 respectively,
with 4-tert-butylphthalonitrile
(14) in the presence of Zn-
(AcO)2. The absorption spec-
tra of the investigated ZnII–Pc-
containing compounds in solu-
tion each exhibited a strong
band in the 650–680 nm region
(Q-band) and a Soret transi-
tion (B-band) in the 300–
400 nm region.[27]
Figure 1. Organogelators (1, 2) and novel ZnII–Pc-containing complementary organogelator structures (3–9)
used in this work.
On the other hand, the ZnII–
Pc dimers 7 and 8 were readily
obtained by DCC-coupling be-
LMOG 2, first described by Hanabusa and co-workers,[26]
was easily synthesized by DCC coupling between (1R,2R)-
(À)-1,2-diaminocyclohexane and undec-10-ynoic acid.
The photoactive building blocks 3–6 were obtained in
one-step fashion and in modest yields through DCC-assisted
coupling between the appropriate ZnII–Pc acid derivatives
12 or 13 and the corresponding 3b-cholest-5-en-3-yl N-(2-
aminoalkyl)carbamates[28] (10 or 11; Scheme 1).
tween (1R,2R)-(À)-1,2-diaminocyclohexane[31] (21) and the
appropriate ZnII–Pc acids 12 and 13, respectively
(Scheme 3). In contrast, ZnII–Pc dimer 9 was synthesized by
palladium-catalysed Sonogashira[32] coupling between dia-
lkyne 22[33] and unsymmetrical iodo-substituted ZnII–Pc
23[34] (Scheme 3).
Organogels made of LMOGs 1 and 2 and the complemen-
tary organogelator structures 3–9, containing ZnII–Pc moi-
eties: Even though many ALS compounds—containing aro-
matic (A), linking (L) and steroidal groups (S)—have been
reported as effective organogelators,[35] ZnII–Pc 3–6 were
The ZnII–Pc acid derivatives 12[21f,29] and 13 thus emerge
as powerful Pc-based scaffolds for DCC-mediated coupling
reactions in the construction of novel photoactive materials.
Acids 12 and 13 were each synthesized similarly, by two-step
Scheme 1. Synthesis of ZnII–Pc-containing complementary organogelator structures 3–6.
Chem. Eur. J. 2008, 14, 9261 – 9273
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T. Torres, D. D. Díaz et al.
Scheme 2. Synthesis of ZnII–Pc acid derivatives 12 and 13.
Scheme 3. Synthesis of ZnII–Pc dimers 7–9.
unable to form organogels after dissolution in hot solvents
with subsequent cooling to room temperature. Cooling
below room temperature (58C) did not afford organogels
either, despite the facts that 1) these compounds could in
principle facilitate the formation of 1D superstructures, be-
cause of their propensities to form hydrogen-bonded and p-
stacked structures,[36] and 2) they are structurally similar to
ZnII–porphyrin-appended cholesterol derivatives, which
were able to form transparent gels in several aromatic sol-
vents such as toluene, benzene and p-xylene.[28] However,
when 1 and the ZnII–Pc-containing complementary choles-
tanyl structure 3 were mixed in toluene at an optimum[27]
molar ratio of 20:1, a stable dark green organogel was ob-
Figure 2. Digital photographs. a) Organogelator 1 in toluene solution at
room temperature. b) Effect of the inclusion of the ZnII–Pc-containing
complementary organogelator structure 3 ([1]=12.12 mgmLÀ1, 20.0 mm,
[3]=2.52 mgmLÀ1, 2.0 mm) after a heating–cooling cycle allowing the for-
mation of a stable organogel. c) Photoactive organogel made of [1+3]
under irradiation at 356 nm. d) Solution obtained by means of the ther-
mally induced phase transition of the gel made from [1+3].
ol-based LMOGs,[25] one of the possible configurations for
the aggregates are outlined (Figure 3), which combine the
described self-assembly pattern of the cholesterol-based
LMOG 1 with the integration of the complementary LMOG
structures 3–6, containing ZnII–Pc moieties, which would
allow stabilizing p interactions between aromatic residues
through the gel network.
An interesting, albeit weak, influence of the separation
distance between the Pc moiety and the 3b-cholest-5-en-3-yl
skeleton on gel stability was found when 3 was replaced by
its higher analogue 5. Thus, photoactive organogels made of
tained within minutes after
a
heating–cooling cycle
(Figure 2). The existence of the gel state was verified by the
absence of flow upon turning the test vial upside-down and
was further confirmed by rheological experiments (see
Table 1, entry 1). Unfortunately, this gel remained homoge-
nous only for 17 days when stored in the dark in sealed
glass vials at 293 K, after which precipitation was observed.
The use of higher concentrations of 3 caused the collapse of
the gel within the first hour, whereas the use of lower con-
centrations did not afford any stable gel upon cooling. An
analogous situation was also observed in other solvents such
as acetonitrile, 1,2-dichloroethane, o-, m- or p-xylene, ben-
zene, chlorobenzene, or ethylbenzene.[27] Although the use
of a mixture of MPcs isomers should affect the gel proper-
ties,[38] we did not address this issue because of serious diffi-
culties in the separation of the pure isomers.
[1+5] or [1+6] (molar ratio [1]/[5 or 6]=20:1) were found
A
to remain stable for around three weeks. The mechanical
strengths of these gels (Table 1, entries 3 and 4) proved to
be similar to those of the gels made of [1+3] or [1+4]
(molar ratio [1]/[3 or 4]=20:1; Table 1, entries 1 and 2). No
A
odd–even effect on the gel stability of the aliphatic segment
(n=2, 3) between the two NHCO functions was observed
on comparing 3 with 4 or 5 with 6.
At this point we decided to investigate whether this stabi-
lizing effect was strictly dependent on the organogelator
structure. To this end, we chose the LMOG 2, which self-as-
sembles spontaneously into ordered fibrous nano-aggregates
at low concentrations in a variety of organic solvents.[26] The
Appropriate control experiments[27] showed that both cho-
lesterol and Pc units were necessary for the observed stabili-
zation phenomenon. Based on previous studies of cholester-
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Nanostructured Organogels
FULL PAPER
Table 1. Properties of organogels based on 1 or 2 in toluene in sealed glass vials, with the effects of the incorporation of compounds 3–9.[a]
[f]
Entry
Compound[b]
Molar ratio
MGC[c] [gdmÀ3
]
Dt[d] [d]
lAbs[e] [nm]
Tgel [8C]
DH[i] [JgÀ1
]
G’[j] [”103 Pa]
G’’[k] [”103 Pa]
DSC[g]
IFM[h]
1
2
3
4
5
6
C
20:1
20:1
20:1
20:1
22:1
22:1
14.64
14.63
14.78
14.81
15.28
15.46
17
19
23
28
31
36
672
672
672
672
671
671
41
45
48
51
57
49
40
43
47
49
56
51
0.47
0.70
0.44
1.03
0.91
1.07
14.53Æ0.05
15.32Æ0.09
16.89Æ1.03
16.73Æ1.24
17.38Æ1.76
17.58Æ1.12
7.61Æ0.04
8.27Æ0.06
9.35Æ0.05
9.12Æ0.12
10.13Æ0.13
10.02Æ0.13
[a] The gels were prepared with 2 mL of solvent. The state of each sample was determined by visual inspection after the sample had been allowed to
stand for 2 h at room temperature, although gelation in each case occurred within 10 min. The intensity of the characteristic greenish colour and the
transparency of the photoactive organogels depended both on the solvent and on the organogelator system (for an example, see inset of Figure 3). Nei-
ther a values nor relaxation times were obtained. [b] Components of the hybrid organogels. [c] MGC is defined here as the minimum concentration of
the organogelator system at which gelation was observed to restrict the flow of the medium. [d] Temporal stability defined as the time after gel formation
at which phase separation of the sample was macroscopically observed. The experimental error was 5–12 h. [e] Absorption wavelength maximum of the
photoactive organogels. [f] Sol-to-gel transition temperature (average of three independent measurements). [g] Tgel determined by DSC, which corre-
sponds to the maximum of the observed endothermic peak. [h] Tgel determined by IFM in a sealed vial, allowing measurements above the boiling point
of the solvents. The estimated error for the Tgel values obtained by DSC and the “inverse flow method” were Æ2 and Æ18C, respectively. [i] Enthalpic
change. [j] G’=average storage modulus. [k] G’’=average loss modulus.
separation between the Pc moiety and the hydrogen bonds
region, were found to be crucial for the effective integration
of the Pc-containing analogues in the nanoscale-fibres of the
gels. Indeed, no organogel formation was observed for the
combination of 2 and the analogue 7, lacking the flexible ali-
phatic chains between the dye and the NHCO functions,
which most likely provide a suitable space for the groups to
interact and to organize their individual patterns of interac-
tion efficiently.
Along with insignificant contributions from linear dichro-
ism (no changes in the intensities of the signals were ob-
served when rotated by 908), the gels made of 2[26] exhibited
strong circular dichroism (CD) peaks for the amide units,
Figure 3. Plausible configuration for the aggregates made of LMOG
structures 1 and Pc-containing complementary LMOG structures 3–6.
due to the formation of chiral aggregates (molar ellipticity
values:
[q]201 =+4.20”104 cm2 dmolÀ1
;
[q]213 =À1.73”
105 cm2 dmolÀ1). However, the CD spectra of the photoac-
tive loose organogels ([2+8] or [2+9]) in acetonitrile at
208C showed no additional peaks due to the absorption of
the Pc units, and the CD signals disappeared when the iso-
tropic state was reached (49–578C). No further investigation
into the dependence of the CD spectra on cooling rate,
aging or thermal history was carried out.[35] Thus, as is also
the case with the analogous porphyrin system,[28] the posi-
tions of the chromophores appear to be barely affected by
the helical-like self-assembly of 2. In addition, highly chiral-
ly-oriented aggregates are very unlikely to exist in the solu-
tion state, because the CD pattern is not maintained at the
sol-to-gel transition temperatures (Tgel).
gelation mechanism of 2 differs remarkably from that of 1,
because the extended supramolecular structure of 2 is stabi-
lized by two cooperating noncovalent forces: the solvopho-
bic interaction of the long hydrocarbon chains and two hy-
drogen bonds between adjacent molecules with two equato-
rial amide NH and amide CO groups aligned antiparallel to
each other and perpendicularly to the cyclohexyl ring.[26] Al-
though no gel formation was observed upon cooling the iso-
tropic toluene solutions containing only the Pc-containing
analogues 7–9, the combination of 2 and analogues 8 or 9
(Table 1, entries 5–6) in an optimum molar ratio of 22:1
([2]=12.0 mgmLÀ1 =25.1 mm; [8]=3.28 mgmLÀ1 =1.79 mm;
[9]=3.46 mgmLÀ1 =1.79 mm) resulted in the formation of
photoactive organogels. These gels, in contrast to the gels
formed from 1 and 3–6, were found to remain stable for 31
and 36 days, respectively (Table 1, entries 5–6), after which
time partial crystallization was observed. Nevertheless, both
their thermostabilities and their mechanical strengths were
found to be slightly lower than those of the gel made only
of 2.[27] Once again, the preservation of the original organo-
gelator structure, and therefore the major interactions re-
sponsible for the gelation process, along with a considerable
Characterization of photoactive organogels: All gels were
characterized through a combination of rheological experi-
ments and different techniques including FTIR, differential
scanning calorimetry (DSC), electron microscopy and UV/
Vis and fluorescence spectroscopy.
Thermal analysis: The thermal properties of the gels were
examined by DSC and by determination of their Tgel values
(Table 1). The Tgel values—defined as the temperatures at
which the organogels are destroyed—of samples with similar
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T. Torres, D. D. Díaz et al.
thermal histories were first calculated by the “inverse flow
method” (IFM).[27] As Tgel determined by IFM depends on
factors such as aging, cooling rate, thermal history and
degree of hysteresis, among others,[25,38] the values were fur-
ther correlated with the endothermic transitions observed
by modulated DSC.[27] The Tgel values increased in an ap-
proximately linear manner as with the concentration of the
gelator system [LMOG+ZnIIPc], probably due to the for-
mation of more closely packed 3D-networks (Figure 4).
From the sol-to-gel phase diagram (Figure 4), the calculated
Morphologies: To gain visual insights into the microscopic
morphologies of the gels, we took transmission and scanning
electron micrographs (TEM, SEM), which proved to be
complementary, as well as atomic force micrographs (AFM)
(Figure 5).[27] Similarly to other ALS-based gelators,[36] all
the xerogels consist of entangled fibre-like aggregates with
numerous junction zones.[4b] For the gels made of [2+8] and
[2+9], most of the fibres presented relatively uniform diam-
eters of 180Æ30 nm (Figure 5A–J) and lengths on the
micron scale. The dimensions of the unit fibre were about
30 nm height”40 nm width (Figure 5L–M). The high aspect
ratios of the bundled fibres must arise from a strong aniso-
tropic growth process, indicating that the fibres have well-or-
dered molecular packing. No significant morphological dif-
ferences between the single-component (only 2; Figure 5A)
and the dual-component gels (Figure 5B–D) were observed
for these gels. Hence, the influence of added complementary
organogelator structures containing ZnII–Pc moieties is not
so great as to change the superstructure, at least at the
molar ratios indicated earlier. Nevertheless, remarkable
changes in the fibre morphologies were observed for speci-
mens prepared at different concentrations and solvents.[27]
Reductions of only 1 mol% (with respect to the optimum
values) in the concentrations of complementary organogela-
tor structures containing ZnII–Pc moieties, for instance, gen-
erated thinner and less knotted fibrillar aggregates (Figur-
e 5E,G–H), supporting their low stabilities towards mechan-
ical agitation. In addition, the more densely intertwined fi-
brous structure of the gel made of [2+8] in toluene (Fig-
ure 5A) or o-, m-, p-xylene (Figure 5J) in relation to the gel
in 1,2-dichloroethane (Figure 5I) seems to reflect the in-
creased thermal stability and intermolecular cohesiveness of
the former case with respect to the latter, as supported by
Tgel measurements (Tgel (xylene)=558C; Tgel (1,2-dichloro-
ethane)=448C).[27] Some pictures revealed an expected hel-
icity (anticlockwise) of fibres for the hybrid gels made of 2
(Figure 5F).[26] In addition, a red luminescence emitted by
the dry nanoscale fibrous structure (Figure 5K) could be di-
rectly observed by confocal laser scanning microscopy
(CLSM), and was probably due to the self-assembly of the
Pc-containing compounds in the organogel fibres. Similarly,
the gels made of 1 and 3–6 were found to consist of entan-
gled fibres with diameters of 90Æ40 nm (Figure 5N–P) and
lengths in the micron scale.
Figure 4. Onset sol-to-gel transition temperatures determined by IFM for
gels made at different concentrations of [1+3] in toluene, [2+9] in
CH3CN, and [2+9] in toluene. The differences in Tgel for the materials
prepared in CH3CN and toluene did not exceed 58C. Inset: digital photo-
graphs of the organogels. Collapse of the gels took place within 1 h at
concentrations higher than the maximum values outlined in the graphic.
thermodynamic parameters were found to be consistent
with the FTIR spectroscopic measurements and showed a
consistent compensation of the entropic change by the en-
thalpic contribution in the gelation process.[39] In addition,
all photoactive gels were found to be fully thermoreversible.
Rheological behaviour: Oscillatory rheological measure-
ments[27] confirmed the viscoelastic, rigid and brittle natures
of the gels (Table 1), which did not flow upon inversion of
the test tubes but did exhibit fully reversible sol-to-gel phase
transitions upon repeated heating–cooling cycles without the
gelation ability being affected. The storage moduli (G’)
were uniformly found to be of greater magnitude than the
corresponding loss moduli (G’’). At its Tgel, the elastic prop-
erties of each gel decreased strongly and the soft solid (G’
> G’’) was transformed into a viscoelastic liquid (G’’ > G’).
All the photoactive gels were stable over a wide frequency
range (0.1 to 100 radsÀ1), and dynamic strain sweep (DSS)
measurements showed that they break at less than 0.5%
strain. The material rigidity as indicated by G’ was found to
follow a relative uniform trend with respect to Tgel.[27]
FTIR experiments: In order to assist understanding of the
precise roles of the CO, NH and aromatic residues of the or-
ganogelator systems in the gelation process, temperature-
controlled FTIR studies were carried out.[27] The intensities
of the bands were found to be stronger in the gel samples,
indicating that the components interact more efficiently in
the gel phase. The results also demonstrated the necessity
for participation of hydrogen-bonded amides, van der Waals,
and aromatic interactions in different extensions during the
gelation phenomenon. Hydrogen bonding in the 2-based
hybrid gels thus uniformly shifted both carbonyl and NH
resonances to lower energy relative to the spectra recorded
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Figure 5. A) Negative TEM image of the xerogel made up of 2 (12 mgmLÀ1) in toluene. B) Negative TEM image of the xerogel made up of [2+8]
(Table 1, entry 5). C) Negative TEM image of the xerogel made up of [2+9] (Table 1, entry 6). D) Negative SEM image of the xerogel made up of [2+9]
(Table 1, entry 6). E) Negative TEM image of the xerogel made up of [2+8]. F) Negative TEM image showing the detail of the helicoidal fibres from
the xerogel made up of [2+8] (Table 1, entry 5). G) Negative TEM image of the xerogel made up of [2+8] (molar ratio: [2+8]=22:0.99). H) Negative
SEM image of the xerogel made up of [2+8] (molar ratio: [2+8]=22:0.99). I) Negative TEM image of the xerogel made up of [2+8] in 1,2-dichloro-
ethane (molar ratio: [2+8]=22:1). J) Negative TEM image of the xerogel made up of [2+8] in o-, m-, p-xylene (molar ratio: [2+8]=22:1). K) Emitted
red luminescence captured by CLSM of dry gel fibres made of [2+9] (Table 1, entry 6). L) Atomic force microscopy height image of the gel made of
[2+8] (Table 1, entry 5) showing the presence of parallel fibre bundles. M) 3D atomic force microscopy image of the fibre bundles from the gel made of
[2+8] (Table 1, entry 5). N) Negative TEM image of the xerogel made up of [1+3] (Table 1, entry 1). O) Negative TEM image of the xerogel made up of
[1+4] (Table 1, entry 2). P) Negative TEM image of the xerogel made up of [1+6] (Table 1, entry 4). The physicochemical properties of the specimens
are given in Table 1.
in the solid state: from 1636–1665 to 1628–1637 cmÀ1 for
amide I bands, from 1542–1558 to 1535–1544 cmÀ1 for ami-
de II bands, and from 3296–3334 to 3273–3295 cmÀ1 for NH
stretching bands. The same tendency was observed when the
concentrations of the organogelator systems were increased.
gen amide groups sharply decreased (homogeneous solu-
tions), whereas those from the free groups increased (amide
ca. 1660 cmÀ1). Bands in the region of the antisymmetric
À
and symmetric stretching vibrations of C H (ca. 2845–
2935 cmÀ1) were also observed for the gel states, thus indi-
cating significant electrostatic interactions between the alkyl
segments. In the case of 1-based hybrid gels, however,
À
In addition to signals routinely observed at about 1270 (Ar
O) and 1090 cmÀ1 (Ar-O-C), slight shifts of the aromatic C
À
À
H stretching frequencies to lower wavenumber (from ca.
3075–3078 to 3069–3073 cmÀ1) were also observed at higher
concentrations, probably due to increasing p–p stacking in-
teractions. The gel-to-sol phase transitions occurred with in-
creasing temperature, and the IR bands arising from hydro-
stretching frequencies of N H bonds (carbamate groups) in
solution were found to be at about 3438–3460 cmÀ1, with C=
O stretching at about 1725–1735 cmÀ1, which were not very
different from those in the gel state. This is in good agree-
ment with the work of Weiss and co-workers,[25] in which it
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T. Torres, D. D. Díaz et al.
was shown that there is no clear evidence of H-bonding
2.2 ns, respectively) and low fluorescence quantum yields
(Ff =0.36 and 0.43, respectively). These values were close to
those observed for 3 or 8 in solution (t=1.3 and 1.5 ns, re-
spectively).[27] Interestingly, we also found that the fluores-
cence intensities of photoactive Pc-based references [for ex-
ample, (tBu)4ZnPc, (tBu)3ZnPcI, (tBu)3Pc-acetylene] in tolu-
ene solution or embedded in gels made of 1 or 2 (at the
same molar concentrations as those used for making the
hybrid gels) decreased on addition of methyl viologen
(MV2+) as electron transfer quenching reagent, and linear
Stern–Volmer plots due to intermolecular collisional
quenching were obtained.[27] In contrast, the fluorescence in-
tensities of toluene gels made of [1+3] or [2+8] remained
virtually intact while MV2+ concentration was increased up
to 1.5 mm.[27] In agreement with previous studies in our
group,[23] this difference in fluorescence quenching behav-
iour suggests the absolute suppression of collision between
ZnII–Pc moiety and MV2+ when incorporated into fibrous
assemblies.
À
from N H to other carbamate during the formation of the
gel made of 1. Hydrogen bonding here is thus subordinated
À
to other van der Waals forces (e.g. p–p and/or p···H N in-
teractions) in the promotion of gelation.
UV/Vis measurements: The cooperative natures of the multi-
ple noncovalent interactions were also supported by single
UV/Vis measurements,[27] which showed the typical Soret
(ca. 349 nm) and Q-bands (ca. 672 nm) for the ZnII–Pc-con-
taining gels, and confirmed their higher temporal stabilities
(ca. 2–5 weeks) in comparison with the common decomposi-
tion of ZnII–Pc solutions after exposure to light for several
days (colour change from blue-green to yellowish) due to
the efficient generation of singlet oxygen. We did not find
any noteworthy dependence of the spectral shapes and in-
tensities either on cooling rate or upon aging within the sta-
bility period of each gel. In the case of the 1-based hybrid
gels in toluene, the UV/Vis spectra at 293 K also each
showed a characteristic red-shifted band at about 420 nm to-
gether with an excitonic band at about 396 nm, characteris-
tic of aggregated anthryl-containing molecules in a gel state
group. The UV/Vis spectra showed almost no influence of
chain length on the positions of the Q-bands, which are de-
pendent on the aggregation state and the crystalline phases
present. Additional temperature-dependent UV/Vis absorp-
tion and fluorescence spectroscopy provided more insights
into the photochemical properties of the organic gels. Upon
cooling of the isotropic solutions, the spectral shape is slight-
ly broadened, and the Soret and Q bands are slightly blue-
shifted (<5 nm) relative to those in homogeneous solution.
According to the molecular exciton theory, both shifts can
point to stacking of ZnII–Pc molecules.[40] Close to the gel-
to-sol phase transitions, shoulder-like absorption patterns
were also observed around 349 nm in some spectra, proba-
bly due to molecules that were not yet taking part in the for-
mation of the 1D structure.
Strengthening of photoactive multicomponent organogels
through in situ cross-linking with the aid of CuI-catalysed
azide–alkyne [3+2] cycloadditions (CuAACs): The relative-
ly low thermal and temporal stabilities of the prepared Pc-
based photoactive organogels could jeopardize the potential
applications of such materials. For this reason, we were in-
terested in investigating strengthening of such materials
without loss of their main physicochemical properties. Sever-
al examples of in situ enhancement of organogel thermosta-
bility have been described; they include post-polymerization
of gel fibres made up of polymerizable LMOGs,[42] the use
of host–guest interactions,[43] the addition of polymers[44] and
the employment of metal ion coordination.[45] In addition,
CuAAC[46] has recently been used to modify the properties
of organic gels while retaining their overall structures and
thermoreversibilities.[33,47] CuAAC has emerged as the best
example of click chemistry,[48] characterized by extraordinary
reliability and functional group tolerance.[10] The above or-
ganic gels were thus prepared in the presence of tiny
amounts of mixtures consisting of the suitable complementa-
ry diacetylenes 24 and 25 (5 mol% with respect to 1 and 2,
respectively, in the hybrid gels) and a linear diazide such as
26 or 27 (Figure 6) in a diacetylene/diazide molar ratio of
1:1. In view of the objectives described above, the CuAAC
here would represent a much more challenging process than
that in previous studies,[34,49] since the cycloaddition reaction
would have to occur in an already multicomponent pre-
formed organic gel (e.g., [1+3] or [2+8]) in which the ap-
propriate “clickable” acetylene monomer (24 or 25) should
be at least partially associated with the gel fibres by virtue
of multiple noncovalent interactions.
Fluorescence measurements: The steady-state fluorescence
emission spectra of toluene gels made of [1+3] or [2+8] sys-
tems at 293 K were also closely consistent with those of 3 or
8, respectively, in homogeneous solutions, showing charac-
teristic Q-band emissions at about 672 nm. No additional
bands attributable to different aggregates were observed.
Similarly to the UV/Vis measurements, emission spectra
(position, shape and intensity) of the hybrid gels were found
to be independent of the cooling rate (slowly or rapidly
cooled gels[41]). In general, strong aggregates of MPcs tend
to exhibit blue shifts, and selection rules dictate that these
species are non-fluorescent. Nevertheless, another possible
geometry is the clamshell arrangement, in which the Pc mol-
ecules are constrained over one another but the rings are no
longer parallel and emission is not categorically excluded.
Although we have no conclusive data as yet, such an orien-
tation effect could occur in the cases of the gels made of
[1+3] or [2+8] in toluene (Table 1), which were character-
ized at 293 K by short fluorescence lifetimes (t=1.1 and
Synthesis: Enantiomerically pure dialkyne 25 was prepared
by DCC coupling of undec-10-ynoic acid and (1R,2R)-1,2-di-
aminocyclohexane.[34] Diazide 26 was prepared by nucleo-
philic substitution of the appropriate dichloride,[47] giving
spectroscopic data identical with those reported in the liter-
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Nanostructured Organogels
FULL PAPER
the catalyst. A further heating–
cooling cycle allowed the gel
state to be restored for physi-
cal characterization.
NMR spectroscopy con-
firmed the presence of triazole
moieties in the cross-linked
gels, whereas FTIR spectrosco-
py showed that the cross-link-
ing reaction had consumed
most of the azide groups.[27]
When no copper catalyst was
employed, the azide peak re-
mained unchanged, and when
Figure 6. Cross-linkers 24–27 used to amplify the thermostabilities of the photoactive organogels through
CuAAC reactions. Control experiments and additional trials with an expanded collection of polyvalent azides
are described in the Supporting Information.[43]
ature. Dialkyne 24 was synthesized by DCC coupling be-
tween 3,5-bis(prop-2-ynyloxy)benzoic acid (28)[50] and 3b-
cholest-5-en-3-yl N-(2-aminoethyl)carbamate (10, n=2;[28]
Scheme 4).
an approximately 2:1 ratio of azide to alkyne was used, the
IR spectrum still displayed an azide resonance, which could
potentially be functionalized after cross-linking, providing
another means of tailoring the properties of these materials.
Characterization of cross-
linked photoactive organogels:
The resulting cross-linked pho-
toactive organogels were char-
acterized similarly to their pre-
cursor gels.
Thermal and mechanical analy-
sis: Interestingly, all cross-
linked organogels displayed
Scheme 4. Synthesis of diacetylene-containing complementary organogelator structure 24.
thermoreversibility,
together
with better stability over time
(>3 months) than their precur-
Cross-linking of preformed organogels through CuAAC: In
order to test the compatibilities of the CuAACs with the en-
tangled supramolecular gel networks, we chose the hybrid
LMOG systems [1+3] and [2+8] as models. The nanoscaled-
fibres were prepared in the presence of low concentrations
of the alkyne-containing “clickable” compounds 24 or 25,
together with the diazides 26 or 27 (Figure 6). CH3CN/2,6-
lutidine (1:0.01, v/v, 0.1m in diacetylene) was used as a sol-
vent system. With a cautious level of click connectivity, the
multicomponent system was subsequently subjected to
CuAAC, permitting the strengthening of the photoactive or-
ganogels while preserving their photochemical properties.
The use of an optimized gelator system/diacetylene/diazide
molar ratio of 20:1:1 for [1+3]/24/26, [1+3]/24/27, [2+8]/25/
26 and [2+8]/25/27 afforded the strongest materials. The use
of larger amounts of cross-linkers ended either in partially
gelled materials or in macroscopically phase-separated ma-
terials. CuI catalyst was introduced by layering a stock solu-
tion of CuI in CH3CN (1.5m) on top of the pre-set organo-
gel,[50] and allowing this to diffuse into the material for 10
days to complete the reaction, as indicated by preliminary
GC-MS experiments. The resulting materials were then
broken up by vigorous stirring and exposed to oxygen for
3 h to ensure quenching of the click reaction by oxidation of
sors under identical storage conditions. The enhanced stabil-
ities of the gels after CuAAC were simultaneously accompa-
nied by enhancements both of thermoresistance and the me-
chanical strength, as demonstrated by Tgel determination
and oscillatory shear measurements (Table 2). The Tgel
values were thus enhanced by up to 158C after CuAAC,
whereas the storage moduli (G’) were uniformly found to be
one order of magnitude greater than those of the precursor
gels and their viscoelastic natures were retained (G’’ > G’).
In general, relatively uniform trends between Tgel and mate-
rial rigidity (G’) were observed upon cross-linking. Stress–
strain curves consistently showed the transformation of
hard, brittle materials (high elastic modulus, small elonga-
tion) into soft, tough materials (low elastic modulus, large
elongation) as temperature increased. Appropriate control
experiments[27] demonstrated the successful modification of
the photoactive organogels only when the appropriate cata-
lyst and clickable monomers were used.
Morphologies: TEM pictures revealed significant morpho-
logical differences in the stabilized xerogels (Figure 7B–L)
in relation to the non-cross-linked materials (Figure 7A).
Patches of agglomerated fibres with non-uniform diameters
(20–180 nm) and lengths in the micron scale were consis-
tently found after CuAAC (Figure 7B–E). The denser and
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T. Torres, D. D. Díaz et al.
Table 2. Physical data for organogels after CuAAC cross-linking under N2.[a]
in diameter. One of these im-
pressive architectures resem-
bles the tentacles of the sea
anemone (Figure 7I–J), where-
as the other is made up of ag-
glomerates of tubular struc-
tures of about 1 mm in length,
0.5 mm in width and 0.4 mm in
diameter, greatly resembling a
bunch of silk cocoons (Fig-
ure 7K,L). As these aggregates
were not observed in all cross-
linked samples, we presume
that the less stable fibrils of
[c]
[d]
Entry Components
Molar ratio Dt[b] [d] lAbs [nm] Tgel [8C] G’[e] [”104 Pa] G’’[f] [”104 Pa]
1
2
3
4
[1+3+24+26] 18.1:1.8:1:1
94
673
673
677
677
56
59
71
73
30.27Æ1.32
33.02Æ2.03
37.51Æ1.78
39.44Æ1.52
9.23Æ0.28
11.16Æ1.14
12.31Æ1.03
12.31Æ1.03
[1+3+24+27] 18.1:1.8:1:1
[2+8+25+26] 18.7:1.3:1:1
[2+8+25+27] 18.7:1.3:1:1
103
121
118
[a] Each reaction was performed in degassed CH3CN (2.0 mL) and 2,6-lutidine (0.02 mL). Note: the Tgel
values of the materials prepared in CH3CN did not overly differ from those in toluene (Æ58C), and the
former were chosen for testing the cross-linking because CH3CN in combination with 2,6-lutidine has been
found to be a good solvent system for CuAAC. [b] Temporal stability defined as the time after gel formation
at which phase separation of the sample is macroscopically observed. The experimental error was 5–12 h.
[c] Absorption wavelength maxima of the photoactive modified organogels. [d] Sol-to-gel transition tempera-
ture determined by IFM in sealed vials, allowing measurements above the boiling point of the solvents.
[e] G’=average storage modulus. [f] G’’=average loss modulus.
intertwined fibrous morphologies of the xerogels after cross-
linking, in relation to the straight-fibre networks of non-
cross-linked gels, could be related to the increased thermal
stability and intermolecular cohesiveness after CuAAC, as
determined by Tgel measurements (Table 2). Less homogene-
ous networks characterized by shorter fibres were observed
for phase-separated materials obtained at higher concentra-
tions of the clickable monomers (Figure 7F–G). Very inter-
estingly, the observation of the cross-linked xerogels under
SEM (Figure 7H–L) revealed the occasional presence of
two new morphological moieties forming sheet-like struc-
tures, combined with thick entangled strands of about 5 mm
the cross-linked gels undergo some morphological changes
during the freeze-drying treatment and aggregate into the
tentacle- and cocoon-like forms.
FTIR experiments: FTIR spectroscopy showed comparable
evidence for the participation of hydrogen-bonded amides
and van der Waals and aromatic interactions in the gelled
states of both non-cross-linked and cross-linked materials,
indicating that the greater part of the original noncovalent
pattern is still retained upon stabilization.
Figure 7. A) Negative TEM image of the xerogel made up of [2+8] (molar ratio 22:1) in CH3CN/2,6-lutidine 1:0.01, 0.1m). B)–E): Negative TEM images
of the cross-linked xerogels (Table 2, entries 1, 2, 3, 4, respectively). F) and G): Negative TEM images of the xerogels made of [1+3+24+26] (molar
ratio 18.1:1.8:3:3) and [2+8+25+27] (molar ratio 18.7:1.3:3:3), respectively. H) Negative SEM image of the cross-linked xerogel (Table 2, entry 3) show-
ing the presence of new structural moieties. I)–L) Zoom-in on H), showing the sea anemone-like structures (I–J) and the silk cocoon-like structures (K–
L).
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Nanostructured Organogels
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UV/Vis and fluorescence measurements: Slight bathochromic
shifts in the positions both of the Soret bands and of the Q-
bands of the Pc moieties were observed in the UV/Vis spec-
tra after CuAAC. This phenomenon could be explained by
the cross-linking process, which can ultimately promote the
agglomeration of the Pcs moieties. The fluorescence intensi-
ties of cross-linked gels remained virtually unchanged while
the quenching (MV2+) concentrations were increased up to
1.5 mm. These foregoing results thus show only a small influ-
ence of the cross-linking process on the optical properties of
the gels. First-round experiments essentially showed no im-
provement in fluorescence lifetimes.
Figure 8 provides an overview of the procedure developed
for the preparation of standard functional organogels and
their further stabilization, if necessary, by in situ cross-link-
ing through CuAAC. System 4 represents a standard orga-
nogel formed upon cooling of the isotropic solution of the
appropriate LMOG (system 2). If further strength enhance-
ment of the material is required, a well-defined concentra-
tion of a suitable diacetylene-containing complementary
LMOG structure, linear diazide and CuI catalyst can be
swollen into the organogel network or premixed with the
LMOG prior to gel formation (system 3) in the heating–
cooling process. The subsequent cross-linking reaction
should enhance the thermostability of the organogel while
preserving its thermoreversibility (system 5).[33] On the other
hand, the organogel 4 may be prepared in the presence of
the appropriate Pc-containing complementary organogelator
structures to afford a multicomponent photoactive viscoelas-
tic material (system 6). Also in this case, suitably designed
clickable monomers (diacetylene-containing complementary
LMOG structure and linear diazide) and the CuI catalyst
can be incorporated into the gel network (system 7) in order
to achieve further cross-linking to generate a new photoac-
tive organogel with enhanced thermostability and mechani-
cal properties.
Interestingly, all attempts to incorporate the [1,3,5]-tria-
zole-based polymeric structure (system 9), made by CuAAC
of the appropriate monomers in solution, into the photoac-
tive organogels failed to form stable gels, due to evident and
fast phase-separation processes. The explanation was found
to lie in the structural differences between the preformed
Figure 8. Schematic illustration of the synthesis of standard organogels and Pc-based photoactive organogels, and their further stabilization by in situ
cross-linking achieved through CuAAC.
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T. Torres, D. D. Díaz et al.
Cajal contract. We are also indebted to the following researchers for per-
mitting some of the characterization and spectroscopic measurements to
be performed in their laboratories: Prof. R. Zentel and his group (Uni-
versity of Mainz), Prof. M. MenØndez (CSIC-Madrid), Prof. J. Catalµn
(UAM), and Prof. J. J. M. Tellado (IUBO-ULL).
polymer 9 and the cross-linked material (system 10) ob-
tained by in situ CuAAC of the clickable monomers, which
could be extracted by evaporation of the solvent system
under vacuum followed by a washing protocol with the re-
maining solid material. MALDI-TOF experiments indicated
that the system 10 was made up of a mixture of short oligo-
mers whereas the system 9, prepared in CH3CN solution,
proved to be a polydisperse polymer of much higher
MW.[27,49] In summary, a photoactive gel could in principle
be fabricated by the self-assembly process of a LMOG in
the presence of a small amount of a suitable complementary
organogelator structure containing a dye (e.g., MPc). The
amount of dye can be optimized in order to minimize the
negative effect on the thermostability of the organogel, and
in some cases to maximize a possible positive effect. Further
thermal/mechanical stabilization can be achieved by
CuAAC-mediated cross-linking of a suitable complementary
diacetylene and a linear diazide previously incorporated
into the multicomponent gel network.
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Conclusion
In summary, photoactive organogels can be prepared
through the association of LMOGs with suitably designed
ZnII–Pc-containing complementary organogelator structures
through a subtle balance of noncovalent interactions operat-
ing cooperatively. The above findings consistently support
the view that these new multicomponent photoactive mate-
rials define a unique chemical system driven by two compet-
ing and antagonistic interactions: self-assembly versus phase
separation. As a whole, this class of hybrid organogels, in
which cross-linked materials are generated by swelling the
appropriate monomers and activators into preformed supra-
molecular assembled networks, can be considered to consist
of thermoplastic semi-interpenetrating supramolecular poly-
mer networks prepared by a sequential process.[51] There-
fore, these systems with enhanced mechanical properties
due to their peculiar entanglement behave as thermosets at
ambient temperatures, but flow at elevated temperatures.
The physicochemical characterization of these soft materials
shows that maintaining the original molecular network con-
structed in the gel phase is crucial for the improvement of
gel stability by post-modification. Studies directed towards
the use of this procedure for the integration of other func-
tional photoactive-based systems into both organo- and hy-
drogels, as well as the study of electron transfer reactions
between these organogels and electron acceptors, are under-
way in our laboratories.
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Acknowledgements
Financial support from the Ministerio de Ciencia
y Tecnología
(CTQ2008-00418/BQU, Consolider-Ingenio 2010 CSD2007-00010 Nano-
ciencia Molecular, ESF-MEC MAT2006-28180-E, SOHYDS), and the
Comunidad de Madrid MADRISOLAR, S-0505/PPQ/0225) is gratefully
acknowledged. D.D.D. also thanks the Spanish MEC for a Ramón y
9272
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Chem. Eur. J. 2008, 14, 9261 – 9273

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Received: April 15, 2008
Revised: June 25, 2008
Published online: August 26, 2008
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9273