The effect of vicinal di-halo substituents on the organogelling properties of aromatic supramolecular gelators and their application as soft templates

Article abstract of DOI:10.1039/d0nj01440k

A pronounced effect of vicinal dihalogen substituents on the gelling properties of aromatic low molecular weight organogelators is reported. A new family of N,N′-(4,5-dihalogen-1,2-phenylene)dialkylamides with fluorine, chlorine, bromine and iodine was designed and synthesized. A systematic investigation of their organogelling ability, thermic stability, mechanical properties and self-assembled structure was performed to elucidate the effect that the vicinal di-halo substituents have on the organogels. It was found that the presence of two halogen atoms (X) has a determinant effect as the brominated compounds are generally the most efficient organogelators. In hydrocarbons, the gelling ability increased from fluorine to iodine following the halogen bond donor ability trend. SAXS results were in agreement with a fibrillar self-assembly where the halogens are located at the surface of the fibers. Multiple cooperative interactions are involved in the self-assembly of the gels: π-π stacking, hydrogen bonds and X?X contacts. Thus, this work provides a new strategy for the design of new gelators or to improve the efficiency of known organogelators by introducing two vicinal halogen substituents into the aromatic rings. An ethanolic gel was also successfully used as a template to prepare silica and titania nanotubes. Hence, such organogels are promising materials for future research and development.

Full text of DOI:10.1039/d0nj01440k

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DOI: 10.1039/D0NJ01440K.
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Effect of vicinal di-halo substituents on the organogelling
properties of aromatic supramolecular gelators and their
application as soft template
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
c
d
Andrea S. Mac Cormack, Verónica M. Busch, M. Laura Japas, Lisandro Giovanetti, Florencia Di
DOI: 10.1039/x0xx00000x
e
a
Salvo and Pablo H. Di Chenna*
A pronounced effect of vicinal dihalogen substituents on the gelling properties of aromatic low molecular weight
organogelators is reported. A new family of N,N'-(4,5-dihalogen-1,2-phenylene)dialkylamides with fluorine, chlorine,
bromine and iodine was designed and synthesized. A systematic investigation of their organogelling ability, thermic
stability, mechanical properties and self-assembled structure was performed to elucidate the effect that the vicinal di-halo
substituents have on the organogels. It was found that the presence of two halogen atoms (X) have a determinant effect
being the brominated compounds the most general efficient organogelators. In hydrocarbons the gelling ability increased
from fluorine to iodine following the halogen bond donor ability trend. SAXS experiments were in agreement with a
fibrillar self-assembly where the halogens are located at the surface of the fibers. Multiple cooperative interactions are
…
..
involved in the self-assembly of the gels: - stacking, hydrogen bonds as well as X X contacts. Thus, this work provides a
new strategy for the design of new gelators or to improve the efficiency of known organogelators by introducing two
vicinal halogens substituents into aromatic rings. An ethanolic gel was also successfully used as template to prepare silica
and titania nanotubes so such organogels are promising materials for future research and development.
materials and the sol-gel transition occurs at a characteristic
Introduction
temperature (T ) that depends on the gelator, its concentration
gel
3
and the liquid medium. In the last two decades the interest on
Molecular gels (also called physical gels) are a special kind of soft
materials that originate from the reversible self-assembly of low
molecular weight gelators (LMWGs) through non-covalent
LMWGs continuously increased due to their unique properties: if
the gelator molecule is sensitive to a specific physical or chemical
stimuli, the supramolecular gel will respond to the same stimuli and
in many cases the sol-gel transition can be triggered by one or more
1
,2
intermolecular interactions.
generate a three dimensional self-assembled fibrillar network
SAFIN) that traps the liquid. Molecular gels are heat responsive
These reversible interactions
4
physical (heat, light, sound, etc.) or chemical (pH, ions, redox, etc.)
(
5
external stimuli. Therefore, supramolecular gels have many
6
,7
potential hi-tech applications i.e. in optoelectronic devices,
8
,9
10
11,12
13
18
chemosensing,
catalysis,
drug delivery,
bioimaging,
1
4
15–17
pollutant removal, as reaction media,
D printing, etc. In this context of such interesting applications,
self-healing materials,
1
9
3
modulating the physical, mechanical and chemical properties of
molecular gels is a great challenge. Nowadays, even with the
knowledge gained on the self-assembly process and the molecular
characteristics that lead to the formation of supramolecular gels, it
is still not possible to design de novo a new molecular gelator and
2
0,21
predict its properties.
Thus, it is crucial to unravel the subtle
molecular features that control the self-assembly of LMWGs to be
able to design new molecular gels with specific functions and
characteristics.
It is known that the assembly of LMWGs can be promoted by
multiple non-covalent interactions, being the most usual hydrogen
bonding, - stacking, van der Waals interactions and solvophobic
2
2,23
forces. In the last years the halogen bond
has been added to the
list as a versatile, dynamic, highly directional and tuneable
supramolecular interaction that can play a key role in the self-
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assembly process of molecular gels.
molecular gelation triggered by an halogen bond was reported by abilities replacing the alkoxy moieties by amide groups, a known
Steed et al. After this seminal work many other examples of strong and highly directional supramolecular synthon that would
supramolecular gelation with direct evidence of halogen bond allow a more stable self-assembly through hydrogen bond, hence
The first clear example of a halogenated aromatic compounds, with potential organogelling
View Article Online
DOI: 10.1039/D0NJ01440K
2
6
2
7–29
control were reported.
These examples are extreme cases better gelators. Here we report the synthesis and systematic study
where strong halogen bond interactions were promoted using of the halogen effect (F, Cl, Br, I) and alkyl chain length effect on the
electron-deficient heavier halogen substituents, such as iodine in gelling
perfluorinated aromatic rings, and good halogen bond acceptor phenylene)dialkylamides analogues (Fig. 2).
atoms such as the basic nitrogen of pyridines. Nevertheless, in
recent years it was demonstrated that simple halogen substituents
properties
of
20
new
N,N'-(4,5-dihalo-1,2-
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in -gelators can have a pronounced positive effect on the gelling
ability and mechanical properties that can be tuned by varying the
3
0–34
halogen.
These effects have been associated with the fact that
3
1
halogen substituents can enhance - stacking, but also
participate in multiple weak interactions such as halogen····
Fig. 2 Chemical structures of the designed N,N'-(4,5-dihalogen-1,2-
phenylene)dialkylamides and their non-halogenated analogues.
30,35
(
X····), C-H····X hydrogen bonding, C-X····O halogen bonding
and
3
6
type I and II X····X contacts. Both types of halogen contacts have
been widely found in crystalline structures, but in solution and
soft materials, like molecular gels, their role has only been studied
3
7
To the best of our knowledge only one non-halogenated
organogelator bearing a 1,2-phenylenedialkylamide moiety has
been described and patented as a component of cosmetics and
3
6
indirectly. In solid state they are characterized by halogen-halogen
distances shorter than the sum of the van der Waals radii. Type II
contacts involve the interaction of the electrophilic region of one
halogen atom with the nucleophilic region of the other, while type I
contacts are likely to minimize repulsion by interfacing neutral
regions of their electrostatic potential surfaces. Thus, only type II
contacts are considered to be halogen bonds. As can be concluded
from these precedents, halogenated organic compounds are of
great interest for the rational and systematic design of
supramolecular functional materials, in particular molecular gels.
In 1999 Clavier et al. reported the gelling ability of a series of
simple substituted dialkoxy-benzene compounds. Among them,
three 1,2-dibromo-4,5-bis(alkoxy)benzene derivatives with octyl,
decyl (1) and hexadecyl groups were described as better gelators
than the non-brominated analogues concluding that the packing
ability of bromine atoms reinforce the strength of the network.
Nevertheless, their gelling ability is poor, they can only gel four
specific organic solvents at high concentrations (0.05M) and the
resultant gels have also a very low thermostability, in fact some of
45
pharmaceutics products. Our design was motivated by the
following hypothesis: a) that the presence of two amide groups on
the phenyl ring would allow, together with -stacking, the one
dimensional molecular self-assembly. b) The electronegative
character of halogen substituents would enhance the - stacking
interaction and c) The presence of two vicinal halogen atoms on the
phenyl core would improve the gelling property compared to the
non-halogenated compounds by complementary and cooperative
halogen-halogen contact interactions, already observed for this
3
8
40,46
3
9
moiety in solid state.
Experimental
Materials and methods
FT-IR spectra of solids were recorded in thin films using KBr
discs on a Scientific Nicolet iS50 FT-IR spectrophotometer,
values are given in cm . H and C NMR spectra were
recorded on Bruker spectrometers: AVANCE NEO 500 or
Fourier 300. Unless specified, NMR experiments were carried
-1
1
13
o
o
them can only gelate under 0 C (Tgel: -15 to 38 C). Years later,
40
3
out in deuterated chloroform (CDCl ). Chemical shifts (δ) are
Fonrouge et al. reported the crystal structure of one of the
gelators, 1,2-dibromo-4,5-bis(decyloxy)benzene (1), where type I
and II Br····Br contact interactions were observed (Fig. 1). These two
independent results allowed us to consider the hypothesis that
these Br····Br contacts present in the crystalline network could also
be involved in the self-assembly of the molecular gel.
reported in parts per million (ppm) with reference to
tetramethylsilane (TMS) or residual solvent peaks as internal
standards. The following abbreviations are used for the
proton spectra multiplicities: s: singlet, d: doublet, t: triplet, m:
multiplet. High resolution mass spectra (HRMS) were obtained
with a Bruker MicroTOF-Q II instrument using electrospray
ionization and a time of flight analyzer. When necessary,
organic solvents were routinely dried and/or distilled prior to
use and stored over molecular sieves under argon. Flash
column chromatography was carried out on Merck silica gel S
0
.040–0.063 mm. Organic extracts were dried over anhydrous
sodium sulfate (Na SO ). 4,5-Dibromobenzene-1,2-diamine
2
4
Fig. 1 Representation of type I and II contacts observed in the
crystal structure of 1,2-dibromo-4,5-bis(decyloxy)benzene (1).
47
was prepared as previously described in literature.
Dynamic rheology. Rheological properties of organogels were
evaluated using a controlled shear rheometer (Paar Physica
MCR 300, Anton Paar GmbH, Austria) with a 30 mm diameter
In this context, and continuing with our research on the
development of more efficient organogelators
their self-assembly,
4
1,42
and the study of
4
3,44
we designed a family of vicinal di-
2
| J. Name., 2012, 00, 1-3
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parallel plates geometry and 1 mm of gap between the plates. 170 K for 5a and 293 K for 7 during data collectViieownA.rtTichleeOnfluinlel
Triplicates of discs (3 mL, 15 mM of each sample in ethanol) data collection was planned using CrysDAOlisI: 1P0r.1o03s9t/rDa0tNegJ0y14t4o0oKl
were evaluated. An amplitude sweep was performed with and data were reduced using CrysAlis Pro programme (Oxford
frequency control (1 Hz) in order to establish the viscoelasticity Diffraction/Agilent Technologies UK Ltd, Yarnton, England). A
linear range. Dynamic storage modulus (G´) and dynamic loss Gaussian method implemented in WinGX or a numerical
modulus (G´´) were evaluated by varying the frequency from model was used for the absorption correction. Using Olex2,
4
9
5
0
0.1 to 100 Hz with a 0.1% strain within the viscoelastic linear the structures were solved with the olex2.solve structure
o
ranges. The temperature was controlled at 25.0 ± 0.1 C with a solution program using Charge Flipping and refined with the
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Peltier plate. A solvent trap was used and the sample borders ShelXL refinement package using Least Squares minimization.
were covered with Vaseline in order to protect solvent Non-hydrogen atoms were anisotropically refined and
evaporation. For the sol-gel temperature determination G' was hydrogen atoms were mostly included at geometrically
recorded as a function of temperature by heating the sample calculated positions with thermal parameters derived from the
at a rate of 1 C s . The rheological sol-gel temperature (Tg) parent atoms. Hydrogen atoms attached to groups suitable to
was defined as the point where the G´ was half way between form hydrogen bonds were first located on Fourier maps,
o
-1
4
8
G´ for the solution and G´ for the gel.
fixed, and the isotropic displacement parameters were given
depending on the parent atoms. Idealised methyl group was
. Aerogels were refined as rotating moiety: C1(H1A,H1B,H1C) for 5a and
Aerogels preparation with supercritical CO
prepared from the self-assembled supramolecular gels by C18(H18A,H18B,H18C) for 7. Structural information is detailed
drying with supercritical CO
pressure cylindrical quartz cell (internal volume ca. 4 mL) for 5a and 7, respectively.
mounted on a metal frame in a way to allow rocking the cell, a
high-pressure syringe (Teledyne ISCO 100DM, capacity 100 Template polymerization of TEOS. Gelator 5f (15 mg) was
mL), high-pressure valves (High Pressure Equipment Co.) and a dissolved by heating and shaking in a mixture of ethanol (0.9
pressure gauge. The quartz cell was connected, on opposite mL) and TEOS (0.1 mL) with addition of benzylamine (5
sides and through high-pressure valves, to the syringe and water (5 L) as catalysts. The solution was cooled to room
2
†
2
. The set-up included a high- in ESI . CCDC Deposition Numbers are 1974220 and 1974435
L) and

vent. The quartz cell was placed in a thermostated water bath. temperature until gelation was observed and then left at room
Previous to the operation, the syringe was loaded with liquid temperature for 5 days. Subsequently, the template was
CO
2
from the tank by circulating cold water (ca 15°C) through dissolved in dichloromethane, the solid was centrifugated, and
its temperature control jacket. A gel sample contained on a washed once with dichloromethane. The silica was heated at


cylindrical glass tube (length 2.5 cm, diameter 5 mm) 200 C for 2 h and 600 C for 4 h in air.
containing 200 L of organogel was then transferred to the
quartz cell and immediately sealed, to avoid solvent Template polymerization of Ti(OiPr)
evaporation. Liquid CO was then injected into the sample dissolved in ethanol (0.95 mL) by heating and shaking, then
compartment at a flow rate of ca. 100 mL (NPT)·min , while Ti(OiPr)
the pressure was kept constant at 8 MPa. The temperature of temperature until gelation was observed. The flask was left at
the sample was maintained at ambient value (ca. 20°C) during room temperature semi-closed, in order to allow the moisture
the solvent-extraction procedure. Once no solvent was visually to diffuse and slowly initiate the polymerization. After 7 days
detected in the cell (absence of refractive index waves while the template was dissolved in dichloromethane, the solid was
rocking) and in the downstream (no liquid condensation in the centrifugated, and washed once with dichloromethane. The

4
. Gelator 5f (15 mg) was
2
-
1
4
(50 L) was added. The solution was cooled to room


collecting tube), the temperature of the sample was raised to titania was heated at 200 C for 2 h and 600 C for 4 h in air.
5°C and its pressure slowly reduced, always under
supercritical conditions. The drying process took around 3 h.
3
Small angle X-ray scattering (SAXS). SAXS measurements were
performed using a XEUSS 1.0 from XENOCS equipment located
Scanning electron microscopy (SEM). SEM pictures of the at the Instituto de Investigaciones Fisicoquímicas Teóricas y
xerogels, aerogels and inorganic nanoparticles were taken on a Aplicadas (INIFTA-CONICET-UNLP). This setup is equipped with
Carl Zeiss NTS SUPRA 40FEG scanning electron microscope a 2D photon counting pixel X-ray detector Pilatus 100k
(EHT = 3.00 kV). A small portion of the solid sample was (DECTRIS, Switzerland). The scattering intensity, I(q), was
attached to the holder by using a conductive adhesive carbon recorded in the range of the momentum transfer 0.25< q <6.7
tape. Prior to examination the xerogels were coated with a nm , where q = 4
thin layer of gold.
-
1

/

sin(
and = 0.15419 nm is the weighted average of the X-ray
wavelength of the Cu-K 12 emission lines. All measurements
were carried out using liquid and gel samples contained in
), being 2 the scattering angle

Single Crystal X-ray diffraction (XRD) measurements
Single crystals of 5a and 7 were obtained after slow capillary tubes Borokapillaren: outside diameter 1.5 mm, wall
evaporation of the solvent at RT, using acetone. A suitable thickness 0.01 mm, length 80 mm WJM-Glas/Müller GmbH,
crystal for each compound was selected and mounted on a Germany. Reported data were corrected for the parasitic
loop and measured with an Oxford Diffraction Gemini Eos scattering and processed using standard procedures. Fitting
diffractometer with Mo Kα (λ = 0.71 Å) radiation and kept at
This journal is © The Royal Society of Chemistry 20xx
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experiments were performed using SasView software version Gelation ability
Journal Name
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View Article Online
DOI: 10.1039/D0NJ01440K
5
2‡
5.0.0.
The gelling ability of all compounds was tested using the inverted
tube method on 34 solvents covering the entire range of polarities,
from hydrocarbons to water. The non-halogenated and fluorinated
compounds were found to have a similar poor gelling ability. They
Results and discussion
Synthesis
were insoluble in the majority of the solvents (Table S1, ESI ). The
†
The non-halogenated di-amides 2 were prepared by reaction of best gelators were compounds 2f and 3f bearing the longer alkyl
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commercial o-phenylenediamine with the corresponding acid chain (stearylamides) being able to gel only 3 and 4 solvents
5
3
chloride following a standard procedure. Compounds 3-6 were respectively. The gelling ability improved for the chlorinated
synthesized following simple electrophilic aromatic substitution analogues 4b-f (Table 1 and Table S2), gelator 4d bearing
steps, followed by acylation of the corresponding 4,5-dihalogen-1,2- myristylamides was the gelator with the optimal chain length
phenylenediamine with acid chlorides. All new compounds were gelling 16 solvents with a wide spectrum of polarities, including a
fully characterized (see ESI†). 4,5-Difluoro-1,2-phenylenediamine hydrocarbon (n-heptane) and polar solvents like ketones and some
and 4,5-dichloro-1,2-phenylenediamine were prepared from o- alcohols. Nevertheless, as for the non-halogenated and fluorinated
difluorobenzene and o-dichlorobenzene using the methodology analogues, the gels were highly opaque probably due to low
5
4
described in literature. 4,5-Dibromo-1,2-phenylenediamine was solubility. The brominated derivatives 5, direct analogues of the
39,40
prepared from o-phenylenediamine using the methodology reported gelator 1,
were then studied. The compound with the
47
described in literature. 4,5-Diiodo-1,2-phenylenediamine was shorter alkyl chain (5a) could not gelate any of the tested solvent
prepared from commercial o-dinitrobenzene as described in while the following analogue of the series (5b) formed gels with
55
literature.
some non-protic polar solvents.
Table 1. Comparative gelation ability and critical concentration for gelation (CCG) of halogenated compounds with F (3), Cl (4), Br (5) and I
a
(
6) bearing laurylamides (c) and myristylamides (d) in representative solvents. Complete list can be found at ESI†.
entry
Solvent
n-Hexane
3c
P
P
P
P
P
P
P
P
P
S
P
P
P
P
P
P
P
P
P
P
P
P
4c
P
5c
P
6c
4d
P
5d
P
6d
1
2
3
4
5
6
7
8
9
OG (6.27)
OG (4.60) OG (83.6) OG (9.71)
G (4.91)
G (21.3)
OG (21.3)
OG (10.7)
P
n-Heptane
OG (92.5)
OG (22.7)
P
n-Decane
Cyclohexane
Ethyl ether
P
P
P
P
OG (13.8)
OG (13.8)
I
OG (23.0)
G (13.8)
G (13.8)
P
G (69.0)
P
G (13.8)
G (5.75)
P
G (11.5)
G (17.3)
G (11.0)
G (69.0)
G (23.0)
P
G (7.67)
G (23.0)
G (13.8)
G (11.5)
P
P
P
P
P
P
OG (8.81)
G (6.34)
G (2.27)
G (3.60)
G (15.6)
OG (52.9)
S
G (8.09)
G (5.20)
G (4.85)
G (7.28)
Isopropyl ether
1,2-Dichloroethane
Dichloromethane
P
OG (30.8)
OG (9.15)
G (4.91)
P
G (12.8)
P
P
P
S
OG (41.8) G (9.10)
CCl
4
P
P
OG (29.1)
G (16.2)
10
11
12
13
14
15
16
17
18
19
20
21
22
Chloroform
Acetonitrile
Ethyl acetate
Ethanol
Isopropanol
1-Propanol
1-Butanol
1-Pentanol
Cyclohexanol
1-Octanol
1-Dodecanol
Acetone
OG (1.85)
G (0.32)
G (3.30)
G (3.17)
G (19.8)
OG (7.93)
G (19.8)
OG (10.6)
G (26.4)
G(39.6)
G(79.3)
G (6.34)
G (7.93)
G (19.8)
G (7.93)
OG (13,2)
OG (15.9)
G (22.7)
G (79.3)
OG (41.8) G (0.29)
P
P
G (4.85)
G (4.41)
G (3.64)
G (5.43)
OG (8.01)
OG (16.0)
G (21.3)
G (32.0)
G (5.82)
P
OG (32.0)
OG (64.0)
G (8.01)
G (10.7)
G (12.8)
G (4.91)
P
OG (18.5)
OG (46.3)
G (20.9)
G (83.6)
OG (83.6) G (8.09)
G (83.6)
G (83.6)
G (83.6)
OG (83.6) G (14.6)
OG (83.6) G (58.2)
OG (27.9) OG (9.71)
OG (83.6) OG (11.2)
P
-
G (18.2)
G (10.4)
OG (72.8)
G (92.5)
-
-
-
P
G (30.8)
G (92.5)
P
Methylethylketone
Dimethoxyethane
Triethylamine
DMSO
23
24
25
26
P
P
P
G (58.2)
OG (83.6) G (7.28)
OG (24.6)
-
OG (8.63) OG (41.8) OG (4.85)
Dioxane
Toluene
Pyridine
P
P
P
G (92.5)
P
P
S
OG (83.6) G (6.62)
G (5.34)
2
2
7
8
P
P
S
G (12.1)
G (72.8)
P
G (92.5)
G (16.0)
[
a] The values in parentheses are CCG. The molecular weight of the halogens varies widely, in order to be able to compare the gelling
ability based on the CCG the values are expressed in mM. Inverted tube test code G: transparent gel, OG: opaque gel, I: Insoluble, S:
soluble; P: soluble after heating but precipitate upon cooling.
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Fig. 3 Thermal stability (Tgel) vs. concentration plots measured using the inverted tube method. a) non-halogenated gelator 2f in
DMSO and cyclohexane. b) Effect of the alkyl chain length for the brominated gelators 5b-f in ethanol c) Halogen effect (Cl, Br and I)
for the laurylamide derived gelators in ethanol.
The rest, with alkyl chains from 12 to 18 carbon atoms, turned thermal stability of the gels. Tgel were measured using the
out to be highly efficient gelators for a wide spectrum of inverted tube test on selected solvents. The Tgel of the non-
solvents (Table S3). The only solvents that could not be gelled halogenated and fluorinated gelators showed little
were water, acetic acid and methanol. Most of the gels were dependence with concentration, reaching
transparent, evidencing a good balance of solvophobicity and maximum Tgel near the CCG (Fig. 3a). The DMSO gel of gelator
solvophilicity. In some solvents, like acetonitrile (ACN), 2f showed the higher thermal stability with a constant Tgel of
compounds 5c-e can be considered supergelators, as they 85 C in the whole concentration range.
require a very low concentration to gel with CCG around 0.29- Gelators 4, 5 and 6 showed the typical behaviour of molecular
a
constant
o
†
0.67 mM. Comparing with the reported dibrominated gelator gels (Fig. 3b,c see ESI Fig. S1-5). As the concentration of
1, which in the same solvent required a concentration of gelator increased, the Tgel also increased with convergence to a
0.05M, it can be concluded that the presence of the amides plateau region were the Tgel had little dependence with the
(hydrogen bond donor and acceptor) enhances about 170 concentration. From the study it is clear that, for a given
manifold the gelling ability in ACN. In this case there is not a halogen and solvent, as the alkyl chain length increases (from
defined optimal chain length; gelators 5c-f have all a very good 10 to 18 carbon atoms) the thermal stability also increases in a
gelling ability in different solvents and have similar CCG values. systematic way (Fig. 3b, Fig. S3 and S4). On the other hand, the
This result suggests that the bromine atom has a strong alkyl chain length has, generally, a higher effect on the CCGs in
4
positive effect on the formation of the SAFIN and an alkyl non-polar solvents like hydrocarbons and CCl (Tables S2-S5).
chain of more than 12 carbon atoms does not influence the The halogen effect on the thermal stability was not as
gelling ability as observed for the chlorinated gelators. Finally, important (Fig 3c and Fig. S5). Comparison of the thermal
changing bromine for iodine caused a decrease in the gelling stability of chlorinated, brominated and iodinated gelators
ability, reflected in a smaller number of gelled solvents and, in with the same alkyl chain showed that the chlorinated gelators
general, higher CCG (Table 1 and Table S4). Gelators 6c and 6d are always the less thermostable followed, respectively, by the
had the best gelling scope, being compound 6d, with alkyl brominated and iodinated gelators. In particular the
chain of 14 carbon atoms, the best iodinated gelator. brominated and iodinated gelators with laurylamides have
Comparing the gelling ability of gelators bearing the same alkyl very similar thermal stability (Fig. 3c).
chain (Table 1 and S5-6) it is also notable that the gelling ability
for the first three halogens improves in the order F < Cl < Br, Dynamic Rheology
but this tendency is lost with iodine that has a lower gelling Rheology analyses were performed to assess the viscoelastic
ability than the brominated analogues, with some exceptions. nature and the mechanical strength of the gels. The mechanic
For example, in non-polar solvents such as hydrocarbons the spectra, storage (G´) and loss (G´´) moduli vs. frequency, of
gelling ability of laurylamides gelators follows the progressive different samples were measured. The effect of the halogen on
tendency expected for halogens F < Cl < Br < I. This can be the mechanic spectra of gels in ethanol is shown in Fig. 4a. All
attributed to the fact that the presence of the highly organogels showed the typical viscoelastic behavior of
polarizable iodine atoms considerable increases the solubility, supramolecular gels: G´ was greater than G´´ in at least 1 order
4
8
especially in polar solvents, and a higher concentration is of magnitude, with small frequency dependence. Comparing
needed to build up the SAFIN.
the mechanical spectra of the halogenated gelators with the
same alkyl chain a remarkable elastic strength difference can
only be attributed to the halogen substituents (Fig. 4a).
Thermal stability
All gels obtained were thermo-reversible, thus, they turned Gelator 5c (bearing bromine) rendered the stiffer organogel
into a fluid solution after heating and gelled again after cooling followed by the iodinated and the chlorinated gelators
to room temperature. Determination of the Tgel and its respectively. This result is in line with the gelling ability
dependence with concentration was performed in order to observed in the CCG, gelling scope and thermal stability. On
determine the halogen and alkyl chain length effect on the the other hand, the alkyl chain also has an important effect.
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Gelator 5b (with the shorter alkyl chain) turned out to be the
weakest gel with the lowest modulus values in the frequency
range of 0.1-10 Hz (Fig. S6). From 10 Hz frequency dependence
of the elastic modulus arose and the fluid-like behavior started
to dominate at 18 Hz where a cross of both moduli evidenced
the predominance of viscous over elastic behavior. As the alkyl
chain length increased, both modulus (G´ and G´´) as well as
the viscoelasticity linear range also increased accounting for a
higher mechanical stability. Gelator 5f, with stearylamides, was
the most stable with the highest G´ and G´´ moduli with slight
dependence. The storage modulus (at 1 Hz) was evaluated as a
function of temperature in order to study the temperature
effect on the mechanical strength of the organogels with
different alkyl chain length (Fig. 4b) and halogens (Fig. 4c). The
thermal stability and Tgel values were then analysed based on
the G´ value decrease, which is indicative of the gradual
transformation from gel to sol. For all gels it could be observed
that the temperature at which the drop of the storage
modulus occurred coincided with the Tgel measured with the
inverted tube method at the same concentration (Fig. 3). For a
given halogen, for example bromine, as the alkyl chain length
increased both thermal stability and mechanical strength
increased as reflected by higher Tgel and G´ modulus (Fig. 4b).
In addition, the halogen effect can be revealed comparing
gelators with the same alkyl chain (Fig. 4c). Iodinated gelators
rendered the more thermostable gels with higher Tgel and a
slower decreasing slope. For the 12-carbon alkyl chain
derivatives, it can be observed that the chlorinated derivative
forms gels with lower Tgel and G´ values.
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Morphology
In order to get an insight into the microscopic morphologies of
the gels we dried gels from ethanol using the CO supercritical
2
drying process and the aerogels obtained were examined by
scanning electron microscopy (SEM). We chose this drying
method because supercritical conditions better preserve the
microscopic structure of supramolecular gels from collapsing
as it is usually observed using the typical vacuum drying
5
6,57
process.
In all cases a crossed-link fibrillar network was
Fig 4. Rheology: a) Storage (G) and loss (G”) moduli for
observed with fibers several micrometers length (Fig. 5). The
diameter of the fibers was dependent on the alkyl chain length
but independent on the halogen. Among the brominated
gelators the thinnest fiber was observed for 5b bearing the
shorter alkyl chain (caprilamides) showing also the less
entangled fibrillar network (Fig. 5a). As the alkyl chain
increased, the diameters also increased from 20 nm for 5c, to
chlorinated, brominated and iodinated gelators with the same
o
alkyl chain in ethanol, 25 C. b) alkyl chain length effect and c)
halogen effect on the thermal stability determinded by
rheologycal experiment (G` vs T at 1 Hz). Concentration 15 mM.
oxides nanomaterials (usually nanotubes) can be synthetized
following an in situ polymerization process from TEOS or other
44,58
metal alkoxides respectively.
This is interesting not only for
25 nm for 5d and 30 nm for 5f (Fig. 5) with the longest alkyl
the potential applications of the inorganic nanostructured
materials, but also because the in situ template polymerization
gives information about the morphology of the SAFIN in gel
phase. We tested the in situ polymerization of TEOS and
chain (stearylamides). The same trend was observed with the
other halogens (Fig. S7-S10). Comparing the images among
gelators with the same alkyl length, the brominated gelators
showed a more entangled SAFIN, in concordance with their
better gelling ability and higher stability.
4
Ti(OiPr) using as template organogels of compound 5f in
ethanol. In both cases hollow nanotubes were obtained with
an internal diameter of around 30 nm (Fig. 6). This is in
agreement with the dimensions of the fibers observed in the
aerogels, so we can conclude that the supercritical drying
conserved the SAFIN structure present in the organogel.
Organogel SAFIN as template
The SAFINs of molecular gels can be, sometimes, successfully
used as template for the preparation of nanostructured
2
inorganic materials. For example SiO and other inorganic
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Fig 5. Comparative Aerogel images (SEM) obtained from gels in ethanol of brominated gelators bearing different alkyl chain length:
5b (caprilamides), 5c (laurylamides), 5d (myristoylamides), 5f (stearylamides) and gelators bearing laurylamides with different
halogens: 4c (chlorinated), 5c (brominated) and 6c (iodinated) gelators. Mag. = 100.00 K X. Scale bar 200 nm.
involved. This demonstrates that as the concentration
increased, the molecular aggregation also increased based on
- stacking and hydrogen bond interactions.
Small angle X-ray scattering (SAXS). The structures of the self-
assembled systems present in the gels were also studied by
SAXS. This technique can be used to estimate on average the
shape and size of objects in the range on a few nm of size.
Since molecular gels are diluted systems it is not always
possible to obtain a useful dispersion curve. In the particular
case of bromine and iodine, the high density of these atoms
provided a high electronic density contrast compared to
solvent, that allow to obtain a good dispersion curve for the
used concentrations. We analyzed gels of the brominated and
iodinated gelators in different solvents and only obtained
useful data for gels of 5c in ethanol and 6c in 1-pentanol. The
dispersion curve as a function of the modulus of q obtained for
Fig. 6 SEM images of the nanotubes prepared following an in
situ polymerization process using an organogel of 5f in ethanol
as template: a) TiO and b) SiO . Internal diameters 30 nm.
2 2
This also indicates that the negatively charged silicates and
titanates species formed during the process were able to
interact with the SAFIN allowing the polymerization to take
place at the surface of the fibers that could act as template.
5
c and 6c are shown in Fig. 7. In order to give an estimation of
the shape and size of the nano-objects a simple model of a
core-shell cylinder was used that is implemented in the
Investigation on the Self-assembly mode
59
1
Variable concentration H NMR spectroscopy. To get an
insight into the supramolecular forces involved in the self-
assembly we studied the H NMR variation with concentration
52‡
SASView code.
1
Different core/shell relative densities were fixed during the
fitting routine and, in both systems, the SAXS data modeling is
compatible with a cylindrical object with a low density core,
and a thin high density shell (Fig. 7a and 7b). Therefore, from
these results it can be deduced that the halogen atoms are
located in the shell of the cylinder. The short length of the
cylinders (around 5.5 Å) indicated that the system may be
better described as a set of stacked discs shaped objects. Thus,
(
Fig. S11). As the brominated derivatives resulted to be the
best gelators we carried out the study above and below the
CCG of gelator 5f in CDCl (0.71 %wt/v). As the concentration
3
increased from 0.5 to 1 %wt/v, the sign of the aromatic
protons showed a gradual upfield shift from 7.65 to 7.59 ppm,
this shielding effect is indicative of intermolecular
interactions. Furthermore, the NH signal showed a downfield
shift from 8.16 to 8.28 ppm. Typically the presence of
hydrogen bond interactions induces charge transfer and
polarization leading to a de-shielding effect of the proton
-
59
a stacked discs cylinder model implemented in the SASView
code was used in order to test this idea. This model considers
repetitive stacking of a core and a separation layer of uniform
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densities with disc shaped objects. For the core disc density we
considered an intermediate density between the densities
obtained in the core-shell cylinder model. The complete
parameters of the fitting routines for are shown in the ESI†. A
good fitting agreement obtained with both cylindrical models
encouraged us to describe the systems as being formed by the
stacking of core-shell discs (Fig. 7c) with dimensions for the
core diameter of around 40.0 and 44.2 Å for the brominated
and iodinated gelators with shells 2.0 Å thick and an overall
diameter of 44.0 and 48.2 Å. These dimensions are in
accordance with the length of an elongated molecule of the
gelators. Fixing the solvent density at 0.77, the relative
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densities or the core and shell were estimated to be
1.55 for the brominated gelator 5c and
the iodinated gelator (Fig. 7).
shell core
 /
 /core = 2.91 for
shell
=
To prove that the SAXS dispersion observed were generated by
the SAFIN, variable temperature SAXS experiments were
performed in both gels between 21-70 C (Fig. S13). As the
temperature increased, the SAXS dispersion curves gradually
loosed the form factor that completely disappeared after
reaching the Tgel accounting for the molecular disassembly of
o
†
the SAFIN (Fig. S13, ESI ). Based on these results we propose
the self-assembly mode depicted in Fig. 7c where the discs are
formed by gelator molecules with their non-polar alkyl chain
pointing to the core and the halogen atoms at the exterior rim
of the discs, stacked each other through hydrogen bond and
-

interactions. The cylinders, then, would be able to interact
with other cylinders by halogen-halogen contacts to generate
the cross-linked SAFIN that leads to the organogel.
X-Ray Diffraction Study. Elucidating in detail the self-assembly
mode involved in gelation at the molecular level is difficult. In
some cases gelators have a similar mode of packing in crystals
and gel state and this can be used to gain information on the
assembly by comparing the powder X ray diffraction (PXRD)
patterns of crystals and xerogels. If they match, the crystal
structure can be used to understand the molecular assembly in
the gel. However we could not use this strategy; all attempts
to obtain a single crystal of any of the gelators failed, either
because of their high gelling capacity (it was not possible to
find a crystallization solvent) or the crystals obtained were too
small or did not diffracted at all. Nevertheless, in order to get
an insight on the molecular interactions and halogen contacts
that may be involved in the microstructure of the gels of this
family of gelators, we have synthetized an analogue of gelator
3
6
Fig 7. SAXS for a gels of compounds 5c in ethanol (a) and 6c in 1-
pentanol (b) and the fittings obtained with the core-shell
cylinder model. Inset figures show the dimensions and relative
densities obtained from the fitting in each case. Based on these
results a self-assembly model with the halogens atoms located
at the surface of the fiber can be proposed (c). For the stacked
disc cylinder fitting see ESI†.
4c bearing only one alkylamide moiety (compound 7, Fig. 8).
Compound 7 could not gel any solvent but, did give place to
crystals easily obtained from acetone and thus, its structure
was solved using Single Crystal X-ray Diffraction studies (for
details see ESI ). The crystal packing of compound 7 is
sustained by N–H···O=C H-bonds between the amide moieties
with a distance dO1H1 = 2.11 Å, resulting in an infinite chain
along crystallographic axis c (Fig. 7, ESI ). In this case a halogen
contact between Cl1 substituents of consecutive molecules
with a dCl1Cl1 = 3.516 Å and
†
observed; based on this geometry, it can be classified as Type I
halogen contact. This interaction involves two molecules and is
extended alternatively along the structure and intercalated
with the N–H···O=C H-bond chains.
†
C3Cl1Cl1 = 150.20° (Fig. 8) is
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Fig. 9 Relative contributions of intermolecular contacts to the
Hirshfeld surface. Compound 7 (top), compound 5a (middle)
and gelator 1 (bottom). Van der Waals (H···H), hydrogen bond
(
O···H) and halogen contacts (X···X).
crystal structure of compounds 5a, 7 and 1 were performed
see ESI ) in order to have a tool to compare the predominant
short contacts in each system determining the relative
contribution of intermolecular contacts to the Hirshfeld
surface area (Fig. 9). Based on the reported crystal structure of
†
(
40
39
compound 1, a very poor gelator, the preponderant
interactions are Van der Waals forces (H···H) associated with
the alkyl chains followed by X···H; C···H and X···X contacts
respectively. Compound 5a with two shorter alkyl chains of 7 C
atoms, has a smaller Van der Waals surface contact with a
higher hydrogen bond surface related with the presence of
two amide functions and, as expected, X···X contacts are
absent. For compound 7 with only one amide and a 12 C alkyl
chain, the Van der Waals contacts have a very important
contribution. So, for long-chain compounds 1 and 7, the
packing of the aliphatic chains seems to be the main driving
force for the crystal structure and in both, halogen contacts
are present and also contribute to the stability of the
structure. For the shorter chain length compound, the highly
and strong hydrogen bond interaction has a predominant
contribution to the assembly and the weaker halogen contacts
do not contribute significantly to the packing. Thus, the
presence of one or two long alkyl chain (at least 12 C) offers a
higher non polar surface than two short alkyl chains and would
allow the formation of halogen contacts. This analysis
extrapolated to the gel SAFIN highlights the importance of the
long alkyl chain length, in line with the self-assembly proposed
based on SAXS experiments. As compound 7 cannot gel any
solvent these results suggest that the two amide functions are
essential to provide a good intermolecular hydrogen bond
interaction, together with a high non polar surface given by
long alkyl chains of at least 12 C atoms.
Fig 8. Crystal structure of compound 7 determined by single
crystal XRD using ORTEP diagram with ellipsoids at 50%
displaced along crystallographic bc plane. Colour code: H, light
grey; C, grey; N, light blue; O, red; Cl, green. Hydrogen bonds and
halogen contacts are represented with dotted lines.
The other Chlorine atom experiments non-conventional H-
bonds involving the hydrogens of the aromatic ring and one of
the methylenes of the alkyl chains. In brief, for compound 7
with only one long alkylamide, type I chlorine contacts are
stabilizing the crystal structure. Single crystals of compound 5a
were also easily obtained from acetone (see ESI ). The crystal
packing is also sustained by N–H···O=C H-bonds between
amide moieties with a distance dO1H1 = 2.012 Å (see ESI ). The
†
†
resulting arrangement is given by the interaction between the
carbonyl of one branch and the NH of the adjacent molecule,
and vice versa. The result is an infinite chain extended along
the crystallographic axis c. As consequence of this main
interaction other short contacts are also developed such as
non-conventional C-H···O hydrogen bond between the
aromatic ring and a carbonyl. Based on the crystal structure of
the reported gelator 1, it was expected the development of
halogen bonds concerning the bromide substituents. In 5a
bromine atoms of one molecule face the end of the amide
branches of another one and consequently, no short X···X
contacts are possible. Based on these crystal structures a
Hirshfeld surface analysis has been performed for compounds
7, 5a and the reported gelator 1, by using Crystal Explorer17
Conclusions
We have designed and synthetized a new family of efficient
and versatile low molecular weight gelators based on N,N'-
4,5-dihalo-1,2-phenylene)dialkylamides bearing fluorine,
chlorine, bromine and iodine and different alkyl chain lengths.
The pronounced effect of the vicinal di-halogen moiety was
6
0
(
program. The Hirshfeld surface analysis is a valuable method
for the analysis of intermolecular contacts that offers a whole-
6
1
of-the-molecule approach. The 2D fingerprint plots of the
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studied and evidenced by their gelling ability, thermal stability to project “Nanopymes” (EuropeAid/132184/D/SUP/AR-
and mechanical properties. Considering the general gelling Contract31-896).
ability, reflected in the critical concentration for gelation (CCG)
and the variety of solvents gelled, brominated gelators are the
most versatile and powerful with an outstanding ability to gel
View Article Online
DOI: 10.1039/D0NJ01440K
Notes and references
3
1 of the 34 solvents tested with exception of water, acetic ‡ This work benefited from the use of the SasView application,
acid and methanol. In some solvents like acetonitrile they have originally developed under NSF award DMR-0520547. SasView
contains code developed with funding from the European
a supergelling ability with minimal concentration for gelation
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Union’s Horizon 2020 research and innovation programme
under the SINE2020 project, grant agreement No 654000.
as low as 0.29 mM (0.02 %wt/v). These brominated gels show
a higher crossed linked SAFIN that is directly related to their
higher stability. Specifically in hydrocarbons the gelling ability
increased from fluorine to iodine following the halogen bond
donor ability trend (F<Cl<Br<I). In the rest of the solvents
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(
(
polar liquids) bromine has the optimal balance of solubility
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iodine this balanced is broken in more polar solvents probably
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1
the driving forces for the 1D self-assembly. SAXS experiments
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8
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-
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Thus, this work provides a novel strategy to design new
-
gelators or to improve and tune the efficiency of known low
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We acknowledge Universidad de Buenos Aires (grant
22
200201500100121BA),
CONICET
(grant
PIP
23
24
11220170100736CO) and Agencia Nacional de Promoción
Científica y Tecnológica (grant PICT2016-621) for financial
support. SAXS experiments at INIFTA were performed thanks
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Vicinal di-halo substituents have a determinant effect on the supramolecular self-assembly and
properties of aromatic physical gelators with application as soft templates
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