Immobilization of styrene-substituted 1,3,4-oxadiazoles into thermoreversible luminescent organogels and their unexpected photocatalyzed rearrangement

Article abstract of DOI:10.1002/chem.201202246

A series of styrene-substituted 1,3,4-oxadiazoles has been designed and investigated as new low-molecular-weight organogelators. The photophysical properties of the resulting thermoreversible organogels have been characterized by UV/Vis absorption and luminescence spectroscopies. Surprisingly, the gelation ability of the oxadiazoles depended on the presence of the styrene moiety as gelation of the investigated oxadiazoles did not take place in its absence. Gel formation was accompanied by a modification of the fluorescence of the organogelators in the supramolecular state. UV irradiation of the gels caused a rearrangement of the immobilized 1,3,4-oxadiazoles bearing a styrene moiety by a tandem [4+2] and [3+2] cascade reaction. Structure modification and color change of the gels were also evident upon irradiation. Copyright

Full text of DOI:10.1002/chem.201202246

FULL PAPER
DOI: 10.1002/chem.201202246
Immobilization of Styrene-Substituted 1,3,4-Oxadiazoles into
Thermoreversible Luminescent Organogels and Their Unexpected
Photocatalyzed Rearrangement
Frꢀdꢀric Dumur,*[a] Emmanuel Contal,[a] Guillaume Wantz,[b] Trang N. T. Phan,[a]
Denis Bertin,[a] and Didier Gigmes[a]
Abstract: A series of styrene-substitut-
ed 1,3,4-oxadiazoles has been designed
and investigated as new low-molecular-
weight organogelators. The photophysi-
cal properties of the resulting thermor-
eversible organogels have been charac-
terized by UV/Vis absorption and lu-
minescence spectroscopies. Surprising-
ly, the gelation ability of the oxadia-
zoles depended on the presence of the
styrene moiety as gelation of the inves-
tigated oxadiazoles did not take place
in its absence. Gel formation was ac-
companied by a modification of the flu-
orescence of the organogelators in the
supramolecular state. UV irradiation of
the gels caused a rearrangement of the
immobilized 1,3,4-oxadiazoles bearing
a styrene moiety by a tandem [4+2]
and [3+2] cascade reaction. Structure
modification and color change of the
gels were also evident upon irradiation.
Keywords: aggregation
rearrangement luminescence
organogel · oxadiazole · styrene
· cascade
·
·
Introduction
In this second approach, gelator molecules self-assemble
into nanoscale superstructures that further interlock into
three-dimensional networks trapping the solvent molecules
inside. The principal interest in this second type of gels
stems from the fact that their photophysical properties can
be finely modulated upon exposing them to external stimuli
such as heat, chemicals, or light.[5] Notably, when fluorescent
organogelators are employed, the supramolecular state in
organogels can significantly modify the emissive properties
of the gelator. In this context, 1,3,4-oxadiazoles constitute
an important class of heterocyclic compounds that have re-
ceived considerable attention due to their potential applica-
tions in medicinal chemistry[6] (antimicrobial, antifungal,
anti-inflammatory, and antihypertensive activities), as fluo-
rophores for metal-sensing applications,[7] and as n-type
charge carriers in organic light-emitting devices.[8] In recent
years, these fluorescent heterocycles have mostly been in-
vestigated as electron transporters owing to their electron
deficiency and good thermal stability.[9] To date, 1,3,4-oxa-
diazoles have only rarely been studied as organogelators,
and to the best of our knowledge only five examples of gela-
tors incorporating 1,3,4-oxadiazole derivatives in their scaf-
folds have been reported so far (Scheme 1).
The first report mentioning the use of 1,3,4-oxadiazole de-
rivatives as gelators described a C3-symmetric almost non-
fluorescent molecule, OX1, that formed a highly fluorescent
organogel at 0.1 wt% in chloroform through intermolecular
hydrogen bonding.[10] More recently, this approach was ex-
tended to the octupolar derivatives OX2 and OX3, which
bear an additional alkyl chain on the core of the discotic ge-
lator but do not contain a central hydrogen-bonding motif
as in the oxadiazole OX1.[11] The gelation processes of these
disk-shaped molecules are purely driven by p–p stacking in-
The development of low-molecular-weight linear p-conju-
gated molecules that can spontaneously self-organize into
well-defined assemblies is a significant goal in relation to or-
ganic electronic devices.[1] Control of the self-organization of
molecules at the nanoscale level constitutes a powerful ap-
proach for the design of new materials with tailored func-
tionalities. In this regard, gel formation that solely results
from van der Waals, p–p stacking, dipole–dipole, and hydro-
gen-bonding interactions between molecules constitutes a
simple and soft method for such self-assembly. Typically, the
driving force behind these molecular self-assemblies is the
cooperative effect of noncovalent forces. The functional
properties imparted to the generated supramolecular assem-
blies are reversible as these noncovalent interactions can
readily be weakened or even destroyed.[2] Over the years,
two different types of gelation have been developed, namely
chemical gelation,[3] which relies on the formation of chemi-
cal bonds and irreversibly constructs large polymers starting
from a small molecule, and physical gelation,[4] which relies
on the formation of intermolecular noncovalent interactions.
[a] Dr. F. Dumur, Dr. E. Contal, Dr. T. N. T. Phan, Prof. D. Bertin,
Dr. D. Gigmes
Aix-Marseille Universitꢀ, CNRS, Institut de Chimie Radicalaire
UMR 7273, 13397 Marseille Cedex (France)
Fax : (+33)2-91-28-87-58
E-mail: frederic.dumur@univ-amu.fr
[b] Dr. G. Wantz
Universitꢀ Bordeaux, IMS, UMR 5218, 33400 Talence (France)
and
CNRS, IMS, UMR 5218, 33400 Talence (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202246.
Chem. Eur. J. 2013, 19, 1373 – 1384
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bis(oxadiazole) OX4 (2,2’-
bis(3,4,5-trioctanoxyphenyl)-bi-
1,3,4-oxadiazole).
Gelation of OX4 was ob-
served at concentrations above
0.6 wt% in DMF, and gel for-
mation was monitored by lumi-
nescence as its fluorescence
was strongly blueshifted upon
aggregation.[12] The most fasci-
nating feature of this example
was undoubtedly the use of an
achiral fluorescent twin-ta-
pered oxadiazole that finally
formed chiral helical fibrous
organogels as a result of its
concentration- and solvent-de-
pendent helical structure. In-
terestingly, “chirality” within
individual molecules could be
transcribed to the final supra-
molecular assembly. As a final
and unique example, a phos-
phorescent gel was obtained by
combining a fluorescent oxa-
diazole with two phosphores-
cent platinum(II) terpyridyl
complexes.[13]
Although the resulting dica-
tionic dinuclear assembly OX5
did not contain conventional
organogel motifs, it formed or-
ganogels in acetonitrile or in a
mixture of acetonitrile and al-
cohols (methanol, ethanol,
propan-1-ol, propan-2-ol and
butan-2-ol). The gel-forming
ability of OX5 simply originat-
ed from intermolecular p–p in-
teractions between the square-
planar d8 platinum(II) terpyr-
idyl alkynyl moieties. However,
in none of these examples
Scheme 1. Examples of organogelators incorporating an oxadiazole moiety.
were the physical gels obtained
with the different oxadiazoles
OX1–OX5 chemically or pho-
teractions, and were found to occur in nonpolar solvents
such as toluene and decane. Interestingly, the spontaneous
columnar mesophase self-assembly of OX2 and OX3 in-
volved an evolution of form from spheres to fibrous gels
upon increasing the solution concentration, while the colum-
nar organization was retained. A high concentration of
these spheres resulted in the formation of interlocked fibrils,
which in turn led to gelation. In particular, the different
stages of the hierarchical self-assembly formation were
monitored by the changes induced in the emission spectra.
The gelation process was also investigated with the extended
tochemically converted into chemical gels due to the ab-
sence of reactive functional groups. If a polymerizable unit
is incorporated into the scaffold of the organogelator, the
supramolecular arrangement existing within the organogel
may be advantageously exploited during polymerization to
induce a specific arrangement of the polymer chains that
cannot be obtained by spin-coating of the corresponding
polymer. Such structuring of the polymers is indeed widely
reported to be a crucial parameter determining device per-
formance in numerous applications, including as photovolta-
ic cells[14–16] and organic light-emitting diodes (OLEDs).[17,18]
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Luminescent Organogels Based on Styrene-Substituted 1,3,4-Oxadiazoles
FULL PAPER
Notably, poly(oxadiazole)s are widely used as electron trans-
porters in OLEDs.
two and three steps, respectively, starting from 2-(4-methyl-
phenyl)-4,4-dimethyl-2-oxazoline 4. Lithiation of 4 with lith-
ium diisopropylamide (LDA) followed by nucleophilic sub-
stitution with 1-bromododecane furnished the key substrate
5 in 94% yield. Subsequent lithiation with butyllithium se-
lectively deprotonated the aromatic ring at the ortho-posi-
tion with respect to the oxazoline ring, and reaction with 1-
bromododecane afforded 6 in 71% yield. The acid function
was then regenerated in two steps, first by ring-opening of
the oxazoline under acidic conditions and secondly by hy-
drolysis of the remaining ester under basic conditions.
Acids 7 and 8 were obtained in yields of 82 and 67%, re-
spectively. The well-known standard procedure for the syn-
thesis of 1,3,4-oxadiazoles involving the cyclodehydration of
diacylhydrazines was then performed.[21] Acids 7–9 were first
converted into their corresponding esters 10–12. Reactions
of 10 and 12 with an excess of hydrazine monohydrate in
ethanol furnished the corresponding hydrazones 13 and 15,
respectively, but this standard route proved to be unsuitable
for converting 11 into the target hydrazone 14. As an alter-
native synthetic pathway, acids 7–9 were converted into
their corresponding acyl chlorides 16–18 by using neat thion-
yl chloride. Subsequent reaction, without purification of the
acid chlorides, with an excess of hydrazine monohydrate fur-
nished the respective hydrazones 13–15 in yields ranging
from 67 to 94%. Further treatment of hydrazones 13–15
with an equimolecular amount of methyl 4-(chlorocarbo-
nyl)benzoate (19) afforded hydrazides 20–22 in yields of 84–
96%. The dehydrative cyclization reaction forming the oxa-
diazole ring was then induced by heating the respective
diacyl hydrazides in anhydrous phosphorus oxychloride,[22]
whereupon the 1,3,4-oxadiazole products 23–25 were ob-
tained in high yields. Finally, hydrolysis of the esters 23–25
and esterification with 4-chloromethylstyrene furnished the
monomers 1–3 in good yields after purification by column
chromatography. The structures and purities of all of the
monomers and intermediates were thoroughly verified by
1H and 13C NMR and high-resolution mass spectrometry
(HRMS).
In the present work, an unprecedented formation of fluo-
rescent organogels from styrene-substituted oxadiazole gela-
tors in polar and apolar organic solvents is reported. Highly
luminescent organogels were obtained through p–p stacking
interactions of oxadiazoles only when these precursors bore
a styrene substituent. The gelation ability of the oxadiazoles
was found to depend on the presence of the styrene group,
which is not recognized as a usual gelation motif. Interest-
ingly, photoirradiation of the gels produced a remarkable
structural modification with a concomitant color change
from blue to yellow, whereas irradiation of a solution of the
corresponding monomer resulted in the formation of a poly-
mer that retained the blue emission of the oxadiazole.
Results and Discussion
Three 1,3,4-oxadiazoles were investigated as novel lumines-
cent organogelators and their structures are presented in
Scheme 2.
In particular, the substitution pattern of 2 could be veri-
fied by NMR spectroscopy. In the H NMR spectrum of 2
1
Scheme 2. Structure of the oxadiazoles investigated in this work.
(Figure 1b), two triplets located at d=2.65 and 3.10 ppm
confirmed the presence of two alkyl chains on the oxadia-
zole, whereas only one two-proton signal at d=2.69 ppm
could be observed in the spectrum of the mono-substituted
oxadiazole 1 (Figure 1a). The upfield triplet at d=3.10 ppm
could be confidently assigned to the CH2 group in the ortho
position with respect to the oxadiazole ring. This ortho-sub-
stitution on the benzene ring also gave three signals corre-
sponding to the aromatic protons: one doublet at d=
7.15 ppm overlapped with a singlet at d=7.17 ppm and a
second doublet at d=7.93 ppm (Figure 1d). In contrast, the
protons on the corresponding aromatic ring of 1 gave rise to
two doublets of 2H at d=7.35 and 8.05 ppm with a coupling
constant of 8.3 Hz (Figure 1c).
The synthetic strategy for preparing oxadiazoles 1–3 is
outlined in Scheme 3. As the structures of the three mono-
mers vary only in the substitution pattern on one benzene
ring, an iterative parallel-synthesis approach was employed.
For the series of parallel syntheses, the ester-substituted aro-
matic moiety of the oxadiazole was kept constant and differ-
ent substituents on the second aromatic ring of the oxadia-
zole were surveyed. The presence of peripheral long-chain
alkyl groups is known to keep the “nanodomains” of the ge-
lator separated from one another and thus to favor self-or-
ganization.[19] For comparison with monomers 1 and 2, an
oxadiazole bearing a tert-butyl substituent was also synthe-
sized. The synthesis of monomer 3 has previously been re-
ported in the literature.[20] Acids 7 and 8 were prepared in
Oxadiazoles 1–3 and 23–25 were tested for their gelation
abilities in various polar, nonpolar, aromatic, and nonaro-
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F. Dumur et al.
Scheme 3. Synthesis of monomers 1–3.
matic solvents by the “stable-to-inversion-of-test-tube”
method. The gelation properties of all six compounds are
summarized in Table 1. Interestingly, whereas oxadiazoles
23–25 esterified with a methyl group did not form gels in
any of the investigated solvents, two of the oxadiazoles es-
terified with 4-chloromethylstyrene were found to form
white, almost transparent gels. Notably, styrene-appended
oxadiazole 1 was found to form stable gels in alkanes (pen-
tane, heptane, dodecane, cyclohexane), whereas oxadiazole
3 formed organogels in alkanes, alcohols (methanol, etha-
nol, butanol), and diethyl ether at gelation concentrations
between 0.83 and 4 mgmLÀ1 (Table 1). In contrast, oxadia-
zole 2, which is a structural analogue of 1 and 3, was unable
to form a gel in any of the tested solvents. Indeed, this oxa-
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Luminescent Organogels Based on Styrene-Substituted 1,3,4-Oxadiazoles
FULL PAPER
diazole proved to be highly soluble in most of the common
solvents, except for alcohols, acetone, and DMF, from which
it precipitated. It may be noted that organogelators 1 and 3
are end-capped with a styrene unit. However, despite the
presence of this polymerizable unit, gels could be formed
without polymerization of the monomer in high-boiling al-
kanes (nonane, decane, dodecane) by heating until the solid
gelator had completely dissolved and subsequently cooling
the solution. The absence of polymerization was established
by gel permeation chromatography (GPC).
As revealed by these results, the styrene moiety played an
important role in the formation of the organogels as no ge-
lation occurred without its presence on the oxadiazole
moiety. To the best of our knowledge, the only previous ex-
ample of organogels based on a styrene-substituted mole-
cule was reported in 2005, in which divinylbenzene served
as the solvent and bis(2-ethylhexyl) sodium sulfosuccinate
and 4-chlorophenol were used as mixed organogelators.[23]
Consequently, in our case, the gel-forming ability of oxadia-
zoles 1 and 3 could be confidently ascribed to p–p stacking
interactions arising from the presence of the additional aro-
matic ring of the styrene moiety as no gel forms without this
group. However, the presence of the styrene moiety on the
oxadiazole is not the only requirement. Planarity of the final
molecules is another factor of crucial importance as no gel
formed with oxadiazole 2. As a result of the severe steric
hindrance generated by the second alkyl chain and the tor-
sion induced by its presence in the ortho-position with re-
spect to the oxadiazole ring, the lack of planarity combined
with the steric hindrance prevented any p–p stacking inter-
actions.
Compared to other typical low-molecular-weight organic
organogelators, the oxadiazoles 1 and 3 investigated here re-
quired similar concentrations to form gels. For instance, all
oxadiazoles previously evaluated as organogelators gelified
at concentrations between 0.05[10] and 3.5 wt%,[13] a range
that encompasses the concentrations required with 1 and 3,
supporting their satisfactory gelating properties. The critical
gelator concentration (CGC) of 1 in n-dodecane was deter-
mined to be 0.62 mgmLÀ1, which implies that 1 can entrap
approximately 1150 solvent molecules per molecule of gela-
tor. Although our gelators exhibited higher CGCs than
some of the oxadiazole-based examples reported in the liter-
ature, gel 3 proved to be relatively thermally stable, which
was advantageous since heating at temperatures higher than
968C was necessary to completely melt the gels in n-dodec-
ane and to obtain a clear and nonviscous solution. Thermor-
eversibility of the gel formation was also verified by repeat-
ed heating and cooling, thus providing evidence for the
physical character of the gel formation as the physical bonds
could be disrupted upon heating and reformed during cool-
ing.[4b] All of the organogels remained stable for months at
room temperature.
Figure 1. 1H NMR spectra (400 MHz) in CDCl3 at 258C of a) 1, b) 2, and
expanded aromatic regions of c) 1 and d) 2.
Table 1. Gelation properties of oxadiazoles 1–3 and 23–25.[a]
Solvent
1
23
2
24
3
25
chloroform
n-pentane
cyclohexane
n-heptane
n-dodecane
benzene
para-xylene
ethyl acetate
diethyl ether
THF
S
S
P
P
P
P
S
S
P
P
S
P
P
P
P
P
S
S
S
S
S
S
S
S
S
S
P
P
P
P
P
S
P
P
P
P
S
S
P
P
S
P
P
P
P
P
S
S
G (1.67)
G (1.25)
G (1)
G (0.62)
T
T
P
P
G (1.25)
G (0.83)
G (0.83)
G (1.67)
T
C
C
C
C
C
C
C
C
C
C
C
C
C
C
T
T
G (4)
T
G (2.50)
G (1.50)
G (2.85)
T
S
P
P
methanol
ethanol
n-butanol
acetone
G (0.71)
P
P
The thermal properties of gels 1 and 3 in n-dodecane
were investigated by differential scanning calorimetry
(DSC). Their thermograms are shown in Figure 2. The ther-
mal behavior of gel 1 was very different from that of gel 3.
DMF
T
[a] S=solution, P=precipitation, G=gelation, C=crystallization, T=
turbid. The values in parentheses denote the CGC values [mgmLÀ1] of
the gelators at 258C.
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F. Dumur et al.
To gain direct insight into
the nature of the self-assem-
bled microstructures, scanning
electron microscopy (SEM)
was used to observe the two
gels (see Figure 3 and further
images in the Supporting Infor-
mation). Dry gels were pre-
pared by freeze-drying the or-
ganogels from heptane. SEM
images of the dried gels re-
vealed that the two monomers
had self-assembled into two
well-defined structures of dis-
tinct morphology. As shown in
Figure 3a, the organogel made
of monomer 1 consisted of an
entanglement of fibers with
lengths of several micrometers,
generating an extended and
three-dimensional fibrillar net-
work that stabilized the gel
state. Figure 3a also clearly
shows the fibers to have very
uniform structures with rela-
tively smooth surfaces. On the
other hand, Figure 3b, which
shows an electron micrograph
of the gel prepared with mono-
mer 3, indicates that the
lengths of the fibers were sig-
nificantly reduced compared to
those obtained with 1 as only
fibers of length 2 mm were visi-
ble in a weakly interconnected
network. More surprising was
the roughness and the presence
of microstructures on the sur-
face of the fibers. These results
clearly show the profound
Figure 2. DSC thermograms of gels formed from a) 1 and b) 3.
effect of the substitution pat-
The DSC thermograms of gel 1 show an endothermic transi-
tion with a maximum at 318C in the heating process and a
sharp exothermic transition with a maximum at 178C in the
cooling process. In contrast, the DSC curves of gel 3 exhibit
very broad transitions in both the heating and cooling proc-
esses, with a maximum of the melting peak at 1078C and a
maximum of the crystallization peak at 708C. Repeated cy-
cling showed similar transition behavior. These results sug-
gest that the sol–gel transitions of gels 1 and 3 in n-dodec-
ane are thermodynamically reversible. We noted that the
thermal transitions of gel 1 occurred at temperatures lower
than those of gel 3. This difference can be attributed to the
presence of the C13H27 group in the structure of 1, which is
more flexible and mobile than the tert-butyl group in the
structure of 3.
tern of the monomers on the self-assembled microstructures.
The presence of the flexible tail on 1 led to the formation of
a strongly interconnected and extended network, in agree-
ment with the higher gelation ability seen with this mono-
mer.
The macroscopic organization of the gels was also investi-
gated by variable-temperature 1H NMR measurements as
NMR could provide a fundamental understanding of the
characteristics of molecular self-assembly. To this end, a gel
was prepared from 3 in deuterated cyclohexane and NMR
spectra were recorded upon varying the temperature from
25 to 658C. Surprisingly, a relatively well-defined NMR
spectrum could be obtained even at room temperature (see
Figure 4a), whereas a spectrum with relatively broad peaks
was expected as a result of the anisotropy of the gel. Upon
raising the temperature to 658C, the intensity of the peaks
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Luminescent Organogels Based on Styrene-Substituted 1,3,4-Oxadiazoles
FULL PAPER
Figure 3. SEM images obtained from gels made of a) 1 and b) 3 after
drying. Scale bar is 2 mm.
increased, as is clearly shown for the singlet of the tert-butyl
group at d=1.29 ppm. Similarly, a complete change in the
signal arising from the of the oxadiazole moiety was ob-
served and the initially broad multiplet at d=7.17–7.25 ppm
was simplified and shifted to d=7.20–7.35 ppm. Higher res-
olution of the spectrum upon heating can be confidently as-
signed to an improvement in the thermal motion in the or-
ganogel.
Figure 4. Variable-temperature 1H NMR spectra of the organogel made
of 3 (10 mgmLÀ1) in [D12]cyclohexane.
Based on previous reports, the observation of well-re-
solved H NMR signals in the gel state suggests the forma-
chemical polymerizations were conducted in heptane in a
Rayonet photochemical chamber by using light of wave-
length 350 nm (2.1 JcmÀ2), which corresponds to the maxi-
mum absorption wavelength of the two monomers. Surpris-
ingly, irradiation of the initially blue-emitting gels for 16 h
provided gels that were green-yellow in appearance (see
Figure 5). Interestingly, whereas the structure of the gel pre-
pared with 3 was retained, complete loss of the gel state was
observed with 1 and only a viscous solution was recovered
after irradiation. Modification of the emission color upon ir-
radiation cannot be attributed to a polymerization of the
monomer because poly(1,3,4-oxadiazole)s are known to be
blue-emitting polymers.[20a,25] The change in the emission
color can only be attributed to a modification of the conju-
gation of the emitting part of the compounds.
1
tion of a “wet gel” with gelator 3, in which solvent mole-
cules are incorporated into the gel fiber.[24] With organogela-
tor molecules being separated by the solvent molecules,
there is still sufficient thermal motion to provide a well-re-
1
solved H NMR spectrum. On the other hand, the NMR sig-
nals of a “dry gel” are broadened as a result of insufficient
thermal motion of the molecules and the isotropy of the gel.
Similar to those observed for gelator 3, temperature-de-
pendent NMR measurements indicated that gelator 1 also
formed a wet gel (see Figure in the Supporting Informa-
tion).
Gel polymerization of 1 and 3 was also considered, since
the two oxadiazoles bear polymerizable styrene end groups.
Only photopolymerization was investigated as classical
chemical polymerization techniques may require heating of
the gels beyond their sol–gel transition temperature. Photo-
Notably, electron-deficient 1,3,4-oxadiazoles have been
widely reported to take part in intermolecular inverse-elec-
tron-demand Diels–Alder reactions with electron-rich dieno-
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F. Dumur et al.
gelators were designed with a
diacetylenic group covalently
linked to an additional group
such as urea,[27] acrylic acid,[28]
oligoethylene oxide,[29] or an
aryl ring.[30] Depending on the
spacers introduced between
the second group and the buta-
diyne moiety, gel states were
either retained or lost upon
photopolymerization. In all
cases, photopolymerization of
colorless translucent organo-
gels produced polydiacetylene
chromophores that absorbed
strongly in the UV/Vis region.
Light-induced rearrangement
Figure 5. Gels prepared with organogelators 1 and 3: a) before irradiation in a Rayonet photochemical cham-
ber at 350 nm for 16 h (left) and the same gel after irradiation (right); b) color change around the beam ob-
tained in a spectrophotometer cell on leaving the cuvette in the spectrophotometer.
philes.[26] Briefly, in a cascade mechanism, initial reaction of
the oxadiazole with an olefinic dienophile proceeds through
a [4+2] cycloadduct that loses N2 and provides a cyclic car-
bonyl ylide that reacts in a second step with the olefin in a
1,3-dipolar cycloaddition (see Scheme 4). Interestingly, these
of gels has also been accomplished by the metathesis of al-
kenes[31] and by the dimerization of anthracene.[32]
The effects of irradiation on the microstructures of the
two gels were examined by SEM (see Figure 6). Significant
modifications of the two structures were observed. In the
case of 1, the microstructure evolved towards a denser fi-
brous network with thinner fibers, which is consistent with
the lowered stability of the gel after irradiation. During
standing at room temperature, solvent was progressively
Scheme 4. Mechanism of the cascade reactions reported with electron-de-
ficient 1,3,4-oxadiazoles and electron-rich dienophiles.
transformations did not occur when the molecules were dis-
solved in organic solvents at the same concentration level
and then irradiated. The results indicate that due to the
presence of J-aggregates in the solid state, intermolecular
[4+2] cycloadditions and/or intermolecular cascade cycload-
dition reactions between the oxadiazole rings and styrene
moieties of different molecules could be photochemically in-
duced, these reactions being favored by the close proximity
of the reactive functionalities in the solid state.
A careful survey of the literature revealed that irreversi-
ble color changes of organogels and photopolymerizations
of gels have scarcely been reported to date. In most cases,
Figure 6. SEM images of the gels made of a) 1 and b) 3 after overnight ir-
radiation at 350 nm. Scale bar is 2 mm.
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Luminescent Organogels Based on Styrene-Substituted 1,3,4-Oxadiazoles
FULL PAPER
exuded and the unstable gel liquefied upon ageing. Con-
versely, in the case of monomer 3, the lengths of the fibers
increased and the resulting network consisted of well-sepa-
rated fibers tens of micrometers in length. The roughness of
the fibers also decreased as a result of their collapse upon ir-
radiation.
In order to characterize and identify the different prod-
ucts formed during irradiation, the yellow-emitting residue
was subjected to gel permeation chromatography. The gels
were dissolved in THF and the molecular-weight distribution
was analyzed. The obtained chromatogram clearly showed
that reaction of monomer 3 formed mainly dimers and trim-
ers (see Figure 7). According to the respective intensities, a
small proportion of the molecules also formed oligomers
with chains comprising between 4 and 14 monomers. These
results clearly indicated that a chemical reaction occurred
upon irradiation, producing new species that were responsi-
ble for the new emission color of the gel. Similar behavior
was seen with 1, irradiation of the gel mainly producing
dimers and trimers. The stabilities of the irradiated gels can
be ascribed to their contents of unreacted monomer, which
maintains the supramolecular architecture. Surprisingly,
upon irradiating solutions of 1 and 3 in chloroform at the
same concentrations as those used for the gel formation and
for the same time, the blue emissions of the two solutions
were maintained. On examining the products formed during
irradiation, dimers and trimers were again detected as the
main species. However, oligomers are formed by a com-
pletely different mechanism to that previously observed in
the gels. Since the blue emissions of the solutions are pre-
served, the formation of dimers and trimers can be confi-
dently assigned to photopolymerization of the monomers
and not to a photochemical rearrangement of the oxadia-
zoles (see Scheme 4), the oxadiazole core not being affected
during oligomer formation.
As a result of the simultaneous generation of numerous
radicals, the presence of oxygen that can also initiate the
formation of radicals, and the lack of stirring, there is little
or no propagation of the polymerization and a rapid recom-
bination of radicals is observed, resulting in the formation
of oligomers of low molecular weight. Final proof of poly-
merization was provided by NMR analyses (see the NMR
spectra in the Supporting Information). On examining the
NMR spectra of the two irradiated solutions, a significant
decrease in the intensity of the vinylic signals compared to
that of the parent monomers was apparent, confirming the
polymerization. The concomitant appearance of new signals
in the aliphatic region corresponding to the formation of
CH2-CH(R)- bonds between the styrene units was also ob-
served. No modifications of the oxadiazole signals were ob-
served, either by 1H or 13C NMR (see the spectra in the Sup-
porting Information).
The optical properties of 1–3 were investigated by UV/Vis
absorption and photoluminescence spectroscopies. The ab-
sorption and emission spectra of 1–3 in chloroform are
shown in Figure 8. Compounds 1–3 show similar absorption
spectra with a strong band centered at 300 nm attributable
to p–p* transitions resulting from the conjugation. When
monomers 1–3 were excited at 300 nm, their emission peaks
were observed at 370, 368, and 376 nm, respectively. The sol-
ution-to-gel transition was accompanied by a redshift of the
emission maxima (370 and 387 nm, respectively). After over-
night irradiation in a Rayonet photochemical chamber
(lexc =350 nm), light-induced cycloaddition resulted in a sig-
nificant modification of the gel emission with the appear-
ance of distinct emission peaks at higher wavelength. The
peak of oxadiazole 1 appeared slightly redshifted (397 nm
instead of 387 nm). However, this apparent shift was due to
the overlap of numerous bands at wavelengths above
387 nm. Furthermore, a broad and intense peak with maxi-
mum emission centered at 496 nm was observed. This new
peak was indicative of photochemical conversion of mole-
cules into oligomers, these being responsible for the new
emission color of the material (see Figure 9a).
Figure 7. GPC traces of 1 (top) and 3 (bottom) in CHCl3 before irradia-
tion, after overnight irradiation of the solution at 350 nm, and after over-
night irradiation of the gel at 350 nm.
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F. Dumur et al.
The broadness of this new emission band may be attribut-
ed to the formation of the numerous oligomers during irra-
diation, as shown by GPC analysis. Interestingly, the solu-
tion-to-gel transition of 3 was not accompanied by a redshift
of the emission as the maximum emission wavelength re-
mained unchanged at 368 nm. Similarly to what was ob-
served with 1, irradiation of the gel prepared from 3 resulted
in the formation of new species, as is evident from Fig-
ure 9b. The emission spectrum of 3 was dominated by a
main band at 368 nm with a second peak centered at
496 nm. Interestingly, completely different behavior was ob-
served after irradiation of solutions of the monomers.
Whereas irradiation of the gels resulted in the appearance
of a new emission band in the yellow region, no new emis-
sion peaks could be detected after overnight irradiation of
solutions of the same concentration as those used for gel
formation. However, shifts of the emission peaks were seen,
with the emission maxima shifting from 368 to 394 and
385 nm for monomers 1 and 3, respectively. Broadening of
the two signals could be confi-
Figure 8. UV/Vis absorption and photoluminescence spectra of 1–3 in
chloroform (lexc =300 nm).
dently assigned to the forma-
tion of a wide range of oligom-
ers as a result of the radical
polymerization of 1 and 3, and
not to a rearrangement of the
two monomers.
Conclusion
Six oxadiazoles have been
evaluated as new luminescent
organogelators. Interestingly,
the presence of the styrene
moiety on the oxadiazoles was
required for gelation, provid-
ing evidence that the styrene
unit serves as a gelation auxili-
ary. Complete planarity of the
molecules was also necessary
for gelation, clearly demon-
strating that the driving force
for supramolecular aggregation
relies on p–p overlap. Notably,
the steric constraints imposed
by the additional alkyl chain
on 2 prevented any supramo-
lecular self-assembly. Finally,
investigation of the effects of
irradiation on solutions of the
monomers and on the two gels
generated contrasting results.
First, irradiation of the gels ob-
tained with 1 and 3 in alkanes
with light of wavelength
350 nm induced a photochemi-
cal reaction that transformed
Figure 9. Photoluminescence spectra in chloroform, in the gel state, before and after irradiation (lexc =300 nm)
of a) 1 and b) 3.
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Luminescent Organogels Based on Styrene-Substituted 1,3,4-Oxadiazoles
FULL PAPER
(Eds.: H. B. Bohidar, P. Dubin, Y. Osada), American Chemical Soci-
ety, Washington, DC, 2003.
the blue photoluminescent organogels into yellow ones. On
the other hand, irradiation of the two solutions under the
same conditions (concentration, reaction time) resulted in
photopolymerization of the monomers with the formation of
low-molecular-weight oligomers.
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Received: June 25, 2012
Revised: October 7, 2012
Published online: November 30, 2012
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