Radical-functionalised gel: A building-block strategy for magnetochiral assembly

Article abstract of DOI:10.1002/cplu.201200250

A chiral organogelating scaffold is functionalised with a stable organic radical pendant. The morphology and spectroscopic properties of the gels are investigated as a function of the organogelator concentration and temperature. The combination of AFM and ESR spectroscopy provides firm evidence that the architecture responsible for the formation of the macroscopic gel (i.e., the fibrillar structure) is maintained below the gelling concentration. This opens relevant perspectives in terms of the realisation of complex supramolecular chiral structures incorporating paramagnetic functions, and demonstrates that the ESR technique is suitable as a fast and direct check of the sample state (gel, solution, or small aggregates). A radically new gel: AFM and ESR spectroscopic studies are used to investigate a radical-functionalised gel and the gelation process (see graph). The analyses also provide evidence that the fibrillar structure is maintained below the gelling concentration. This opens relevant perspectives in terms of the realization of complex supramolecular chiral structures incorporating paramagnetic functions.

Full text of DOI:10.1002/cplu.201200250

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DOI: 10.1002/cplu.201200250
Radical-Functionalised Gel: A Building-Block Strategy for
Magnetochiral Assembly
Matteo Mannini,[a, b] Stefano Cicchi,[a, b] Debora Berti,[a, c] Andrea Caneschi,[a, b]
Alberto Brandi,[a, b] Luisa Lascialfari,*[a, b] and Lorenzo Sorace*[a, b]
A chiral organogelating scaffold is functionalised with a stable
organic radical pendant. The morphology and spectroscopic
properties of the gels are investigated as a function of the or-
ganogelator concentration and temperature. The combination
of AFM and ESR spectroscopy provides firm evidence that the
architecture responsible for the formation of the macroscopic
gel (i.e., the fibrillar structure) is maintained below the gelling
concentration. This opens relevant perspectives in terms of the
realisation of complex supramolecular chiral structures incor-
porating paramagnetic functions, and demonstrates that the
ESR technique is suitable as a fast and direct check of the
sample state (gel, solution, or small aggregates).
Introduction
Organogelators are discrete molecules that are able to self-as-
semble in solution, giving rise to supramolecular fibrillar struc-
tures.[1] The fibres tangle to form an intricate isotropic network
that hampers the free flow of the solvent. These organogels
are soft materials, that is, the self-assembled architectures are
formed through noncovalent interactions (hydrogen bonding,
p-stacking, hydrophobic or solvophobic interactions) between
organogelator molecules. The final result, on the macroscopic
scale, is a thermoreversible gel possessing properties that are
intermediate between those of liquids and solids, and which is
built up, on the nanoscale, by a monodimensional framework
with chiral features that can be exploited for the design of
supramolecular multifunctional materials.[1–3] This can be ach-
ieved through the synthesis of appropriate organogelators
containing molecular portions with the desired physical prop-
erties.[4–6]
allows the characterisation of the evolution of the supramolec-
ular architecture by means of Electron Spin Resonance (ESR)
spectroscopy. This technique is used widely to obtain informa-
tion about “soft structures” by using external spin probes.[7] On
the other hand, there are only a few examples in the literature
of gels with a paramagnetic centre covalently linked to the ge-
lating units,[8] and only in a few cases has this feature been ex-
ploited to improve understanding of the gelation process.[8d,e]
In a recent study, Kato et al. reported the example of a nitro-
xide gelator, in which the spins were claimed to be aligned in
flexible 1D nanofibres.[9]
Herein, a derivative of the 2,2,6,6-tetramethyl-1-piperidiny-
loxy (TEMPO) radical was linked to a gelating moiety. The
TEMPO radical was chosen because of its extreme stability,
owing to which it has been studied widely for biological and
diagnostic applications,[10] as well as for active redox units in
organic-based batteries,[11] and it is currently applied in organic
syntheses and industrial applications for common oxidation
processes.[12] Moreover, nitroxide radicals show interesting
magnetic properties ranging from ferromagnetic order at low
temperatures to their use as building blocks for low-dimen-
sional magnetic systems,[13] either coordinated to transition
metal ions or embedded in semicrystalline polymers.[14,15]
The obtained gels were characterised from a physicochemical
and morphological point of view by using a combination of
specific techniques including differential scanning calorimetry
(DSC), ESR spectroscopy, Circular Dichroism (CD) spectroscopy,
and Atomic Force Microscopy (AFM). This characterisation pro-
vided an overview of the macroscopic properties of the system
and a detailed insight into the thermal evolution of its nano-
scale features.
Our ongoing research is aimed at exploiting the fibrillar ar-
chitecture of an organogelator to build an ordered, chiral,
supramolecular structure bearing functional magnetic units. As
a first step toward this goal, we have developed, and present
here, an organic-radical-functionalised organogelator. In this
system, the functional unit provides an internal probe that
[a] Dr. M. Mannini, Dr. S. Cicchi, Dr. D. Berti, Prof. Dr. A. Caneschi,
Prof. A. Brandi, Dr. L. Lascialfari, Dr. L. Sorace
Ugo Schiff Chemistry Department, Florence University
Via della Lastruccia 3-13, 50019 Sesto Fiorentino (Italy)
Fax: (+39)0554573531
E-mail: luisa.lascialfari@gmail.com
lorenzo.sorace@unifi.it
[b] Dr. M. Mannini, Dr. S. Cicchi, Prof. Dr. A. Caneschi, Prof. A. Brandi,
Dr. L. Lascialfari, Dr. L. Sorace
INSTM Consortium, UdR Firenze, Via G. Giusti 9, 50121 Firenze (Italy)
[c] Dr. D. Berti
CSGI Consortium, UdR Firenze
Via della Lastruccia 3, 50019 Sesto Fiorentino (Italy)
Results and Discussion
Despite the huge number of organogelator molecules de-
scribed so far, compounds that can be used as general gelling
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201200250.
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moieties (i.e., molecules able to form gels regardless of the
molecular pendant they are linked to) are rare.[16] As a conse-
quence, the most widely employed approach for obtaining
functional organogels is the production of a special organoge-
lator for each application.
Table 1. Gelling properties of compound 3 in some common organic sol-
vents.
Solvent
Range
[mgmLꢀ1
Gelation time
[min]
Aspect[a] Dielectric
]
constant
e
To overcome this problem, we recently proposed a triazine
derivative, compound 1 (Figure 1), as a “general gelator” that
n-hexane
cyclohexane
toluene
diisopropyl
ether
10.0
50
U
C
C
C
1.89
2.02
2.43
4.04
10.0–5.0
10.0–2. 5
10.0–5.0
10–20
5–30
5–15
isopropanol
acetonitrile
10.0
10.0
30
O
P
18.62
36.00
[a] U=unstable towards inversion; C=clear gel; O=opaque gel; P=pre-
cipitate.
minimum concentration and time required to obtain stable
gels, in agreement with previously reported characterisations
of other organogelator systems derived from 1 or similar.[17,23]
A rational explanation for solvent preferences in gel forma-
tion is not trivial, as the gelation process itself is a delicate bal-
ance between dissolution and precipitation. Because the self-
assembly of organogelator 1 is driven by hydrogen bonding
and interactions between the long aliphatic chains, the build-
up of the gel is favoured in solvents with low dielectric con-
stants,[24] which are poor hydrogen-bonding competitors and
do not efficiently solvate the aliphatic chains. Indeed, whereas
an unstable gel was obtained in n-hexane, also with a high or-
ganogelator concentration, much lower gelation thresholds
were observed in cyclohexane, toluene, and diisopropyl ether.
In particular, toluene turned out to be the most effective gel-
ling solvent, yielding stable gels with critical concentrations
down to 2.5 mgmLꢀ1 (1.8 mm). Finally, as the solvent polarity
was increased further (isopropanol), the gelling ability became
less effective again, and vanished completely for acetonitrile.
Previous studies have shown that CD spectroscopy is
a useful tool for obtaining information about the supramolec-
ular architecture of organogel fibres and the chiral environ-
ment of the chromophores belonging to the organogela-
tor.[25,28b] Indeed, the features of the CD spectra are strictly cor-
related to the chirality and the organogelator aggregation pro-
cess.[26] The CD spectra of gels of compound 3 in various sol-
vents, which accounted for the presence of chiral
supramolecular structures, are shown in Figure 2 (note that
measurements in toluene were not possible because of detec-
tor saturation). Although the excitonic bands have the same
origin at around 270 nm in all the spectra, different trends are
found, with different shapes and different positions of the
maxima and minima. The CD spectra recorded in cyclohexane
and diisopropyl ether, two solvents with similar gelation be-
haviours, show similar spectral features, with a blueshift of the
positions of the positive and negative bands (from 235 to
223 nm for the positive one and from 217 to 201 nm for the
negative one).
Figure 1. Top: Synthesis of compound 3. a) dry pyridine, methane sulfonyl
chloride (MsCl), 4 h, 08C!RT, 98%; b) dry N,N-dimethylformamide, NaN3,
90 min, 808C, 37%; c) THF/H2O (3:1), CuI, sodium ascorbate, 22 h 408C, 31%.
Bottom: Gelation experiment for a 10.0 mgmLꢀ1 gel of 3 in toluene.
can be functionalised with preservation of its gelling proper-
ties.[17] Its terminal triple bond allows the straightforward syn-
thesis of triazole-substituted molecules through a copper-cata-
lysed formal cycloaddition (CuAAC) reaction.[18]
This “click reaction”[19] is known to be particularly effective,
because it works under mild reaction conditions, and gives
high yields and regioselectivity. Moreover, it exhibits a high tol-
erance toward a wide range of functional and protecting
groups, because the CuAAC reaction protocol involves only
the alkyne and azide functionalities even if the reactants are
complex molecules.[20] To exploit this strategy, we first synthes-
ised the 4-azido-TEMPO derivative 2, starting from the com-
mercially available TEMPO radical 4.[21] Subsequently, the
CuAAC reaction of compound 1 with 2 afforded the desired or-
ganogelator 3 (Figure 1). Details of the synthetic procedures
are reported in the Experimental Section.
Macroscopic characterisation
The gelling properties of the TEMPO-based organogelator 3
were evaluated in different solvents using the standard “test-
tube inversion” method (Figure 1, bottom).[22] The results, sum-
marised in Table 1, show that compound 3 exhibits good gel-
ling ability, together with a notable solvent dependence of the
On the other hand, a completely different spectrum is ob-
served for the gel obtained in isopropanol, the most polar sol-
vent and strongest hydrogen-bond competitor among those
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Entangled fibres with a distinct handedness, similar to those
reported for other organogelators and as indicated by CD
data,[28] can be identified clearly in the gel (Figure 3). A higher-
resolution scan allows measurement of the characteristic di-
mensions of a single fibre, that is, the diameter (10 nm) and
the helical pitch (60–80 nm). The right-handedness of the heli-
ces is evident, as expected for the absolute (s,s) configuration
of the organogelator.[28b] AFM images of the 1 mgmLꢀ1 solution
show the presence of smaller aggregates (fibres ranging from
2 to 5 nm in diameter), which are not sufficiently elongated to
promote gel formation (Figure 4). Right-handed helices are ob-
served even in this case, with a helical pitch of the same order
of magnitude as that of the fibres detected in the gel (Fig-
ure 4B).
The data obtained from the morphological analysis of the
fibres confirm that the structure of the gelling moiety present
in compound 3 is responsible for the self-assembly and tunes
the shape of the supramolecular structure, whereas the pend-
ant moiety, apparently, does not affect the self-assembly pro-
cess.[17]
Figure 2. CD spectra of 10.0 mgmLꢀ1 (7.2ꢁ10ꢀ3 m) gels of compound 3 in
different solvents at 298 K. Inset: UV absorption spectra of the same systems
in diluted solutions (3.7ꢁ10ꢀ4 m). The legend of the inset reports the wave-
length corresponding to the absorption maxima.
Table 2 (second and third columns) shows the data obtained
from DSC analysis performed on two toluene gels with differ-
ent concentrations of 3. These data indicate the presence of
a thermal hysteresis in gel formation, with two different tem-
perature values for the gel–sol and sol–gel transitions.
promoting gelation. We tentatively ascribed this behaviour to
a different supramolecular arrangement of the organogelator
molecules within the gel fibre. It is clear that the chromophore
experiences a different chemical
environment along the gel fibre
depending on the solvent used.
This fact confirms the influence
of the solvent on the formation
and modulation of the supra-
molecular assembly as a charac-
teristic of organogel-based sys-
tems.[27]
Because the toluene gels were
revealed to be the most stable
and homogeneous, we decided
to focus our attention on their
physicochemical
characterisa-
tion. In particular, although cy-
clohexane provides the best CD
signal, we note that its freezing
temperature is 279.5 K, so the
temperature range for the exis-
tence of the gel is much more
restricted compared to that in
toluene; as a consequence, it
provides less information on the
gelation process.
The presence of the fibrillar
network that determines the gel
structure can be visualised
through an AFM study, per-
formed on both concentrated
and dilute systems (3.0 mgmLꢀ1
gel and 1 mgmLꢀ1 solution).
Figure 3. AFM topography images of a 3.0 mgmLꢀ1 toluene gel of 3 (A, B) and relative profiles (I, II, III).
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Figure 5. ESR spectra recorded during the cooling cycle for a 7.5 mgmLꢀ1
gel of 3 in toluene. For the sake of legibility, only a few selected spectra
among those recorded are shown. The inset shows the ESR spectrum at
270 K.
Figure 4. A) AFM topography image of a gel of 3 (1.0 mgmLꢀ1) in toluene;
B) profile of the evidenced fibre showing the periodical pattern arising from
the helical pitch.
270 K (inset in Figure 5) there is a clear superposition of two
distinct contributions: a broader, single band, suggesting the
low mobility of closely neighbouring radicals with relevant di-
polar broadening, and a sharper, three-component spectrum
indicating the presence of radicals with a persistent remarkable
mobility. This spectral behaviour clearly reflects the progressive
loss of degrees of freedom in the organogelator and its pend-
ant radical moiety during the gelation process, leading to the
growth of supramolecular structures through self-assembly.
The presence of signals attributed to radicals with high mobili-
ty suggests the persistence of relatively unencumbered regions
in the proximity of the radical function. This implies that
a small percentage of the organogelator molecules is not in-
volved in the formation of the supramolecular structures, and
is free to exchange with molecules already included in the
fibres, or simply remains dissolved in the solvent portion en-
trapped by the gel.[31] As a further test of this hypothesis, we
recorded spectra at lower temperatures (Figure S2), which
showed that the three-line structure disappears only at the
freezing point of toluene (178 K). Finally, no evidence of a half-
field transition was obtained, even with low-temperature
measurements, confirming that exchange interactions between
the radicals are very weak. Thus, at variance with the recent
report by Kato et al., no conclusion can be drawn about the
relative arrangement of molecules inside the fibrils.[9]
Table 2. Comparison of transition temperatures obtained with ESR and
DSC of gels of 3 in toluene at two different concentrations.
Concentration
DSC measurements
ESR measurements
Tsolꢀgel [K]
Tgelꢀsol [K]
Tsolꢀgel [K]
Tgelꢀsol [K]
7.5 mgmLꢀ1
3.0 mgmLꢀ1
291[a]
286[b]
280.5[a]
<281[b]
309[a]
305[b]
307.5[a]
303[b]
294
–
288
–
297.5
–
294
–
Temperature gradients: [a] 1 Kminꢀ1, [b] 5 Kminꢀ1
.
Nanoscale structural and functional characterisation
The presence of a radical probe on our organogelator makes
ESR spectroscopy a useful tool for following the formation of
the supramolecular structures. The ESR response of the nitro-
xide radical is well known to be largely dependent on the flu-
idity of the environment,[7,29] so this technique can be used to
monitor the gelation process and its reversibility, as well as its
dependence on the concentration of the organogelator and
the temperature.
The temperature dependence of the ESR spectra is reported
in Figure 5, which permits us to follow the assembly process
directly. At 318 K, well above the Tgelꢀsol value derived from the
DSC data, the ESR spectrum is typical for a nitroxide in the fast
(isotropic) motion regime with weak exchange among the cen-
tres. For this spectrum, the dynamic and exchange parameters
could be determined by a fit of the spectrum (see Supporting
Information, Figure S1), and best-fit values of tcorr =2.1 10ꢀ10 s
and Jexc =0.2 MHz were obtained.[30] On cooling, a progressive
lowering of the peak-to-peak signal intensity is evident, to-
gether with a linewidth broadening, which is clearly caused by
the slowing down of the motion related to the increased vis-
cosity of the system. Furthermore, below 285 K, and down to
On the other hand, we could use the temperature depend-
ence of the ESR spectra to obtain independent information
about the gelling temperature, by plotting the peak-to-peak
intensity of the first ESR line as a function of the temperature
in the heating/cooling cycles (Figure 6).
This approach provides an indication that on both cooling
and warming, the transition occurs smoothly and in a wide
temperature range (about 20 K), involving a thermal hysteresis.
This is in line with the results of calorimetric measurements
(Table 2) and points to the cooperative character of the sol–gel
transition. This behaviour can be attributed directly to the
good reproducibility of the gelation process (i.e., the possibility
of finding the same characteristic features with different exper-
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Figure 6. Values of maximum intensity of the first component in the ESR
spectra of a 7.5 mgmLꢀ1 gel of 3 in toluene as a function of temperature.
Points a and b show the beginning of the transition observed by ESR, and
c and d are the critical temperatures obtained by differentiating the IESR
versus T curve.
Figure 7. Variation with time of the first ESR component intensity at 295 K
(*) and at 315 K (r) for a 7.5 mgmLꢀ1 gel of 3 in toluene. The best fits are
reported. The black arrow indicates the observed gelling time (600 s) re-
quired for the classical gelation experiment (Figure 1, bottom).
imental methods) and to the high-fidelity nature of the supra-
molecular interactions.
A more detailed comparative analysis of the DSC and ESR re-
sults reveals that the transition temperatures observed with
DSC are in good agreement with the beginning of the transi-
tion observed by ESR (points a and b in Figure 6); on the other
hand, they are quite different from the critical temperatures
obtained by differentiating the curve of IESR versus T (see Fig-
ure S3 and points c and d in Figure 6). In particular, the ESR
spectra recorded at room temperature on cooling, that is, con-
ditions under which a gel is observed on the macroscopic
scale, are dominated by the fast motion component, with only
a weak contribution from the broad component. More general-
ly, the analysis of the temperature dependence of the ESR
spectra shows that a large proportion of the nitroxides are still
free to move, even under conditions at which a macroscopic
gel is formed, resulting in a hysteresis that is narrower than
that obtained by DSC. This is further confirmation of the dy-
namic character of the gel-formation process, so that the defi-
nition of the transition temperature depends on the specific
timescale of the technique by which it is investigated.[22,32]
As ESR turned out to be a powerful tool for the “real-time”
determination of the physical state of the gel sample, kinetic
experiments were performed to follow the gel formation in an
experiment that reproduces the standard procedure for gel
preparation. A boiling solution of 3 contained in an ESR tube
was introduced into the resonant cavity thermalised at room
temperature (295 K), and ESR spectra were acquired as a func-
tion of time. Figure 7 shows the variation of the normalised
ESR peak-to-peak signal intensity. The superposition of two
processes with different decay rates can be recognised clearly.
In this framework, the time dependence of the ESR signal was
fitted in the two regions using two different exponential decay
functions (Table S1). In the first time lapse, the decay follows
a monoexponential behaviour with a time constant t=(187ꢁ
17) s, whereas in the second one, a double exponential func-
tion was used with two different time constants: t1 =(219ꢁ
8) s and t2 =(5270ꢁ496) s. Since t ꢂ t1 we may conclude that
there is one kinetic process that proceeds during the whole ex-
periment with essentially the same rate constant (probably re-
flecting the progressive cooling of the sample), and a second
one that becomes important only after a given time (i.e., after
the sample reaches the gelling temperature) and is character-
ized by a long time constant, t2 (gelation process and ageing).
Kinetic experiments were also repeated at the extremes of
the temperature range studied in the hysteresis experiment
(270 and 315 K, respectively). The spectra recorded at 270 K
were completely time independent, and were characterised by
the same features as those observed at the same temperature
for the “static” experiment, indicating that the gel was com-
pletely formed during the time needed to thermalise the
sample at this temperature. On the other hand, at 315 K
(Figure 7) the peak-to-peak intensity showed a monotonic de-
crease with time, which could be reproduced by a double ex-
ponential decay (t1 =(90ꢁ15) s and t2 =(1973ꢁ21) s), indicat-
ing that even at this temperature, two processes (cooling and
gelation) influence the ESR response. We ascribe the different
behaviour observed at 315 K compared to that at 295 K to
a lower aggregation efficiency, which is caused by the temper-
ature being higher than the gel transition temperature. This is
confirmed by the fact that the ESR signal intensity of the
narrow line decreases only by about 35% during the kinetic
experiment run at 315 K, whereas during the 295 K kinetic ex-
periment the decrease is greater than 65%, following the slow-
down of the radical motion owing to gel formation. These con-
siderations are reinforced by noting that the larger variation in
intensity occurs in a time comparable with the gelation time
(Figure 7, black arrow).
The 315 K kinetic investigation suggests that the formation
of supramolecular paramagnetic assemblies also occurs when
it is not visible on a macroscopic scale. ESR spectroscopy can
then be used to study diluted systems down to the limit of iso-
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lated nanofibres. To probe this, we studied a 1.0 mgmLꢀ1 solu-
tion of 3 following the same characterisation strategy as used
for the higher-concentration gels. Figure 8 shows the spectra
acquired as a function of temperature. Compared to the
higher-concentration spectra, the contribution of the broad
thanks to the specific sensitivity of the ESR technique to the
dynamics of the organic radical pendant. The combination of
the chiral nature with the presence of radical centres opens
relevant perspectives for the realisation of complex, supra-
molecular, magnetochiral systems.[33] In this direction, we are
currently investigating similar gels functionalised by nitronyl-
nitroxide radicals for their relevant roles in molecular magnet-
ism.[34,35] In perspective, the use of a high magnetic field to
align the gel fibres thanks to their shape anisotropy may pro-
vide a means of obtaining ordered arrays of magnetically
active gels, and thus, of improving their properties.[36]
Experimental Section
Materials
Reagents and solvents were purchased from Sigma–Aldrich and
Alfa–Aesar and used as received. Pyridine was distilled from KOH,
and N,N-dimethylformamide was dried on molecular sieves prior to
use. Rf values refer to silica thin-layer chromatography (TLC) on alu-
minium (0.25 mm thickness, Merck F254).
Instruments and procedures
All FTIR spectra were recorded on a Perkin Elmer FTIR 881 spec-
trometer. Absorption frequencies are expressed in wavenumbers n
(cmꢀ1), and the s, m, and w notations refer to strong, medium, and
weak signal intensities, respectively.
Figure 8. A) ESR spectra recorded during the cooling cycle for a 1 mgmLꢀ1
solution of 3 in toluene. For the sake of legibility, only a few spectra are re-
ported. The inset shows the ESR spectra at 275 K. B) Peak-to-peak intensity
of the first narrow line in the ESR spectra of a 1 mgmLꢀ1 solution of 3 as
a function of temperature.
MS spectra were recorded with a 70 eV electronic ionisation (EIMS)
technique, using a GCMS-QP5050A Gas Chromatograph Mass Spec-
trometer (Shimadzu), or with an electron spray ionisation (ESI-MS
or HR-ESI) technique (LCQ-Fleet Thermo Scientific).
CD measurements were performed with a Jasco J-715 spectropo-
larimeter equipped with a PS-150 J power supply and a thermostat-
ic bath. The sample holder consisted of a thermostable quartz cell
with a path length of 0.01 cm. CD spectra were obtained as an
average of 10 scans, repeated after rotation of the cell to minimise
linear dichroism contributions. All samples were prepared through
the same procedure, because the measurements can be affected
by differences in the dynamics of gel formation.[28]
band in the lowest-temperature spectra is less pronounced, in
agreement with the higher dilution of paramagnetic centres[30]
and the reduced aggregation of the fibrillar structure. Howev-
er, by presenting the intensity of the first narrow line as a func-
tion of the temperature, a transition can also be observed in
this case at temperatures lower than in massive gels and with
a thermal hysteresis as well (Figure 8B). This is a very interest-
ing result which clearly confirms the occurrence of an aggrega-
tion process similar to that observed in massive gels, and re-
sults in the formation of fibrillar structures as demonstrated by
AFM studies (Figure 4).
DSC measurements were performed with a TA Instrument DSC
Q2000 apparatus. Steel pans were used. All reported results refer
to scans with a heating/cooling gradient of 1 or 58Cminꢀ1
.
ESR measurements were performed with an X-band Bruker Elexsys
E500 spectrometer equipped with an ER 4131VT liquid nitrogen
flow cryostat for variable-temperature studies and an ER4122SHQE
cavity. Unless otherwise stated, samples were prepared by transfer-
ring a boiling solution of 3 in an ESR tube, which was subsequently
sealed and allowed to cool to room temperature. When the gel
was formed, the sample was inserted in the cavity of the ESR spec-
trometer and heated to 318 K for 10 min (highest temperature al-
lowed in the cavity); then, the ESR signal was measured as a func-
tion of temperature.
Conclusion
Here, we have reported the design, synthesis and physico-
chemical characterisation of a radical-functionalised organoge-
lator. By using a combination of spectroscopic and morpholog-
ical characterisation tools, we have studied the gel formation
thoroughly, demonstrating the intactness of the paramagnetic
function and the chiral nature of the entangled fibres consti-
tuting the gel.
Samples for AFM investigations were prepared by dropping 50 mL
of the fluids on a freshly cleaved mica slide (Dumico, Rotterdam,
The Netherlands) and then spinning directly for 60 s at 3000 rpm
with a spin-coater (KW 4 A Chemat Technology) to reduce and ho-
mogenise the thickness of the deposit. The obtained samples were
dried under a nitrogen flux and mounted on the sample holder of
The investigation of the diluted system provided evidence
of the persistence of isolated 1D chiral supramolecular struc-
tures and the dynamic nature of the gel-formation process,
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the AFM (P47-PRO, NT-MDT co. Zelenograd, Moscow, Russia). Sili-
con cantilevers NSC36 (MikroMasch, Tallinn, Estonia) with a resonat-
ing frequency of 155 kHz and a spring constant of 1.75 Nmꢀ1 were
used. For the prevention of deformation or damage of the exam-
ined soft samples, measurements were performed in semicontact
mode (tapping mode). All the images were processed using the
WSXM 4.0 Nanotec software.[37]
thane and diethylether. The product was a pale yellow solid
(110 mg) obtained in 31% yield (52% conversion). Rf =0.45
(CH2Cl2/(CH3CH2)2O 2:1); m.p. 162–1648C; [a]D21 =ꢀ4.4 (c=1 in
CH2Cl2); IR (KBr): n=3332 (m, vs(NꢀH)), 3070 (s, vas(CꢀH)), 2923 (s,
vas(CꢀH)), 2851 (m, vas(CꢀH)), 1695 (s, vas(C=O)), 1585 (m), 1527 (m),
1498 (m), 1411 (s), 1364 (s), 1260 (m), 1143 cmꢀ1 (s). HRMS (ESI): m/
z
calcd for C75H136N13O10 +H+: 1380.06059 [M+H+]; found:
1380.05806; calcd for C75H136N13O10+Na+: 1402.04253 [M+Na+];
found: 1402.03968.
Gelation experiments
A solution with a known concentration of organogelator was
sealed in a vial and then heated to reflux. The obtained solution
was then allowed to cool to room temperature. Gelation was de-
tected by the “test-tube inversion” method.
Acknowledgements
L.L. acknowledges Dr. Elena Michelucci of the Mass Spectrometry
Centre (CISM) of the Florence University Pharmacy Dept. for ESI
Analysis, and Lorenzo Calugi for his contribution to the synthetic
work. M.M., L.S., and A.C. acknowledge the European Research
Council through ERC-AdG “MolNanoMaS” (grant no. 267746),
and Ente Cassa Risparmio di Firenze for funding.
Syntheses
Compound 1: The synthesis of 1 has been described elsewere.[17]
4-Methanesulfonyl-2,2,6,6-tetramethyl-1-piperidinyloxy
radical
(5):[21] Methanesulfonylchoride (0.45 mL, 5.8 mmol) was added
dropwise to an ice-bath-cooled 1.4m solution of 4-hydroxy-2,2,6,6-
tetramethyl-1-piperidinyloxy radicals (4) (500 mg, 2.9 mmol) in dry
pyridine. The reaction mixture was kept in a nitrogen atmosphere
and stirred at room temperature for 4 h. Then, an ice-cooled satu-
rate solution of NaHCO3 (20 mL) was added to the reaction mix-
ture, which was then extracted with chloroform (4ꢁ15 mL). The or-
ganic extracts were washed with a saturate solution of NaHCO3
(2ꢁ15 mL), then with brine (1ꢁ15 mL), dried on anhydrous
sodium sulfate, and concentrated under vacuum. An orange solid
was obtained in 98% yield. Rf =0.20 (CHCl3); m.p. 948C; IR (KBr):
n=2980–2941 (m; vas(CꢀH)), 1475 (w), 1349 (s; vs(S=O)), 1245 (m)
1171 cmꢀ1 (s; vs(S=O)); MS (70 eV): m/z (%): 250 (7) [M+], 154 (10)
[M+-OSO2CH3], 139 (19) [M+-OSO2CH3-CH3], 124 (46) [M+-OSO2CH3-
2CH3], 109 (69) [M+-OSO2CH3-3CH3].
Keywords: chirality · ESR spectroscopy · gels · radicals · self-
assembly
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radical
(2):[21] Sodium azide (468 mg, 7.2 mmol) was added to a 1.2m solu-
tion (300 mg, 1.2 mmol) of 4-methanesulfonyl-2,2,6,6-tetramethyl-
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37% (88 mg, conversion 57%) of an orange solid. Rf =0.50 (CHCl3);
m.p. 688C; IR (KBr): n=2980–2940 (m, vas(CꢀH)), 2100 (s, v(N3)),
1466 (w), 1379 (w), 1362 (w, v(Cꢀ(CH3)2)), 1324 (m), 1235 (s), 1171
(m), 1092 cmꢀ1 (m); MS (70 eV): m/z (%): 197 (24) [M+], 154 (4) [M+
-N3], 139 (100) [M+-N3-CH3], 124 (25) [M+-N3-2CH3], 109 (18) [M+
-N3-3CH3].
Organogelator 3:[20] Azide
2 (29.4 mg, 0.149 mmol) and CuI
(10.2 mg, 0.054 mmol) were added to a 0.05m solution (160 mg,
0.135 mmol) of organogelator 1 in THF/H2O (3:1). The reaction mix-
ture was stirred at 408C under nitrogen and monitored with TLC.
After 7 h, because the reaction was not complete, sodium ascor-
bate (5.3 mg, 0.027 mmol) and an additional 1 mL of solvent mix-
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night. The reaction mixture was extracted with chloroform, washed
with brine, dried, concentrated, and purified through flash column
chromatography, eluting with a polarity gradient with dichlorome-
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Published online on December 3, 2012
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