Novel organogelators based on amine-derived hexaazatrinaphthylene

Article abstract of DOI:10.1039/c1ob05811h

Novel C₃-symmetrical heteroaromatic hexaazatrinaphthylene (HATNA) gelators symmetrically end-substituted with pendant aromatic and aliphatic amines were synthesized. Some of these π-conjugated structures induce self-assembly, forming fibers abl

Full text of DOI:10.1039/c1ob05811h

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Novel organogelators based on amine-derived hexaazatrinaphthylene†
Daniel Garc´ıa Vela´zquez,*a Alejandro Gonza´lez Orive,b Alberto Herna´ndez Creus,b Rafael Luquec and
a
´
Angel Gutie´rrez Ravelo*
Received 24th May 2011, Accepted 18th July 2011
DOI: 10.1039/c1ob05811h
Novel C3-symmetrical heteroaromatic hexaazatrinaphthy-
lene (HATNA) gelators symmetrically end-substituted with
pendant aromatic and aliphatic amines were synthesized.
Some of these p-conjugated structures induce self-assembly,
forming fibers able to gelate solvents of different polarity at
low wt% as demonstrated by spectroscopic and microscopic
techniques.
gel formation and stabilization of self assembled fibrilar networks
(SAFINs) of AL6 gelators,11 such as compounds 1 and 2, is p–p
stacking of the aromatic units and London interactions of the alkyl
and aromatic chains. Their one-directional circular arrangement
facilitates p–p stacking interaction along the C3 axis to form
nanostructured fibers, which act as an organogelator. Recently,
other authors have reported that a triphenylene-fused metal trigon
conjugate was able to gel organic solvents.12 So far, only a very
limited number of p-conjugated gelators have been reported to be
able to gelate a variety of solvents of different polarity. This may
lead to an improvement with respect to previous work published
in the literature.
The development of novel structures and materials based on the
gelation of solvents by low-molecular weight organic gelators
(LMWOGs) is a rapidly expanding area of interdisciplinary
research. However, a rational structural design of LMWOGs still
remains a major challenge.1–3 Despite the increasing number of
discovered gelation motifs, it is impossible to reliably (de novo)
predict the gelation capability of a molecule. In our research group
there is a high interest to find new gelation motifs from both
fundamental and practical standpoints.4
Taking advantage of the water solubility of compound G113
we devised a microwave-assisted methodology to synthesize com-
pounds 1–4 (Fig. 1) consisting of the uncatalysed direct mono-N-
alkylation of aromatic amines by alkyl halides in water.14
LMWOGs with p-conjugated molecular structures represent
remarkable examples of stacked molecular self-assemblies, which
create one-dimensional nanostructured fibers.5 Such nanostruc-
tured morphology has attracted increasing interest, particularly
for applications in nanomaterials and nanodevices.6 Mostly, a
hydrogen bonding function and/or huge planar molecular sur-
faces need to be incorporated into a gelator molecule to produce
fibrous gels.7,8 Molecular gelation systems are characterized by a
strong tendency of the small organic molecules to aggregate in
solution into one-dimensional fibrillar architectures. Formation
of these elongated structures is mainly governed by solvophilic–
solvophobic effects stabilized by unidirectional non-covalent in-
teractions such as hydrogen bonding and p–p stacking. A potential
model is based on the concept of AL gelators.9,10 These compounds
consist of a p-conjugated aromatic nucleus (A) connected to an
alkyl chain or aromatic group (L). The principal driving force for
Fig. 1 Synthesis of the C3-symmetrical HATNA derivates.
It turned out that the substitution pattern around the hexaaza-
trinaphthylene (HATNA) core is crucial for the gelation abilities
of the formed complexes. Compounds 1 and 2 containing the
saturated-chain alkyl and benzyl terminal groups proved to be ex-
cellent gelators for a variety of customary solvents as summarized
in Table 1 (see ESI†). In fact, they act as “supergelators”15 with
a critical gelation concentration of <1 wt% for dimethylsulfoxide
(DMSO), toluene and benzene. Nevertheless, derivates 3 and 4,
possessing double and triple bonds, respectively, did not form
organogels. The observed gels (Fig. 2E) were found to be robust,
translucent and stable for months. It should be noted that gels
have been obtained by a classical heating–cooling process. The gel
formation of 1 and 2 was determined by a ‘stable to inversion of the
aInstituto Universitario de Bio-Orga´nica “Antonio Gonza´lez”, Universidad de
La Laguna and Instituto Canario de Investigacio´n del Ca´ncer, C/Astrof´ısico
Francisco Sa´nchez, 2, 38206, La Laguna, Tenerife, Spain. E-mail: agravelo@
ull.es; Fax: (+34) 922 318571; Tel: (+34) 922 318576
bDepartamento de Qu´ımica F´ısica, Facultad de Qu´ımica, Universidad de La
Laguna, Campus de Anchieta s/n, 38206, Spain. E-mail: ahcreus@ull.es
cDepartamento de Qu´ımica Quimica, Universidad de Cordoba Campus de
Rabanles Edificio Marie Curie (C-3) Ctra Nnal lV, Km396, E-14014,
Cordoba, Spain
† Electronic supplementary information (ESI) available: Synthetic proce-
dures, complete experimental details, spectroscopy, microscopy pictures,
and additional data. See DOI: 10.1039/c1ob05811h
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Table 1 Physical data for gels of 1 and 2 in various organic solvents
methylene moieties linked to nitrogen atoms in the gelation
process.13 This conclusion is strongly supported by the analysis of
temperature-dependent 1H NMR spectra of gels from compound
1. In DMSO-d6 at room temperature the experiments display
broadened signals of the HATNA moiety and slightly less broad-
ened resonance absorptions for the methylene moieties (see ESI†).
All these signals sharpen significantly at increasing temperatures,
indicating a partial deaggregation. In addition, hydrogens of the
central core suffer an important change in their multiplicity and
shifting. However, this temperature dependence is interestingly
reversible. Upon cooling, the originally observed spectra were
again obtained.
The thermal properties of the gels were examined by differential
scanning calorimetry (DSC) and determination of their gel melting
temperatures (Tgel) were in good agreement with a sharp transition
peak in the DSC thermograms. Tgel values also increased with
the concentration of the LMWOG 1 (see ESI†). In addition,
the gels made from compounds 1 and 2 were found to be fully
thermoreversible, thus raising the temperature above the Tgel
induced the gel–sol phase transition, and the solutions changed
back to the organogels upon cooling to room temperature. The
heating–cooling cycle could in principle be repeated many times
without affecting the gelation ability of the compounds.
(further solvents and details are given in the ESI†)
Compound 1 (green gels)
Solvent
cgc (wt%)a
Phaseb
T
gel (◦C)c
Dimethyl sulfoxide
Dimethyl formamide
Acetone
Toluene
n-Hexane
0.36
1.10
1.40
0.50
1.78
G
G
G
G
G
119
133
51
80
70
Compound 2 (red gels)
Solvent
cgc (wt%)a
Phaseb
T
gel (◦C)c
Benzene
Toluene
0.50
1.10
G
G
58
107
a The cgc is the critical gelation concentration at which gelation was
observed to restrict the flow of the medium. b Abbreviations: G = stable gel
(>1 month). c Gel–sol transition temperature determined by the dropping
ball method.
In order to understand the precise roles of the side-chains
and NH- residues of compound 1 in the gelation process, FT-
IR studies were carried out (see ESI†). Results pointed out very
minor differences between the solid, the solution and the gel phase.
In the latter, stretch vibration bands for NH were observed at
ca. n = 3276 cm-1. These results may indicate that the gelator
molecules experience intermolecular hydrogen bonding through
the amine groups in the nanofibers, which could be one of the key
driving forces in promoting the efficient organization of compound
1 into aggregates and fibers.
In order to further ascertain the supramolecular organization
of the reported gels, transmission electron microscopy (TEM),
scanning electron microscopy (SEM) and atomic force microscopy
(AFM) experiments were conducted to obtain images of dried
gels from compounds 1 and 2 in dimethylsulfoxide and toluene,
respectively.
TEM (Fig. 2A and 2B) and SEM (Fig. 2C and 2D) images
revealed that a fibrous network, typical of LMWOG gels, was
formed. Nevertheless, the observed networks significantly differed
in their shape and dimensions. Thus, twisted cylindrical fibers of
several micrometres long with diameters of 150–250 nm wide and
75–150 nm long were obtained in DMSO for compound 1 (Fig.
2A and C). On the other hand, compound 2 in toluene exhibited
fibrous lamellar architectures (Fig. 2B and D). These ribbons
resulted in considerably shorter but evidently thicker nanofibers
(300–600 nm wide), in comparison to those obtained for com-
pound 1. Fig. 2F shows a loosely entangled higher magnification
region of 1-gel in which well defined fibers are clearly depicted.
Tapping mode AFM imaging of 1 and 2 (Fig. 3) was utilised as
a direct method to examine the three-dimensional gel structure.
From these measurements, it can be concluded that a noticeable
change in the morphology of the assembly may take place when
the saturated alkyl chains of 1 were replaced by aromatic rings in
2. The aggregation of 1 thus formed elongated twisted ropes (Fig.
3A) consisting of intertwined fibers with diameters in the range of
50–80 nm which self-assemble to generate such helical ropes.
Fig. 2 TEM images of xerogels of 1 (A) and 2 (B) in DMSO and toluene
respectively. SEM images of 1 (C, F) and 2 (D). Image E shows digital
photographs of the gels 1 (right) and 2 (left) in DMSO (0.36 wt%) and
toluene (1.10 wt%) respectively.
test-tube’ method and tested for 20 solvents (see ESI†). Gelation
using as little as 0.36 wt% of 1 in dimethylsulfoxide was observed
within 15 min. All gels displayed high stability over time when
stored in sealed glass vials, as no changes in both appearance and
melting temperatures, were detected in several weeks.
Formation of amides (HATNA-NHCOR) instead of amines
(HATNA-NHCH2R) results in a complete loss of the gelating
ability, which clearly indicates the important role of the a-
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Fig. 4 Fluorescent emission of (A) compound 1 in DMF (lex = 360 nm)
and (B) compound 2 in DMF (lex = 360 nm) at 25 ◦C.
and provided more information on the stacking pattern of such
compounds. No remarkable changes in the profiles obtained for
compounds 1 and 2 were observed apart from an important shift
of the bands at lower wavelengths for compound 2 (Fig. 4B). The
fluorescence of compound 2 under UV light (365 nm) in benzene
at high dilution (see ESI†, page S29) was also indicative of the
remarkable properties of these molecules.
The aggregation of compounds 1 and 2 was also studied by
UV–vis spectroscopy as shown in Fig. 5 for compound 1. Firstly,
concentration-dependent UV–vis spectra of the compound 1 in
solution and gel state were recorded. As shown in Fig. 5A, the
p–p* transition bands of 1 in the dilute DMF solution appeared
at 380 nm, which shifted to 362 nm in the gel state. The blue shift
of the UV absorption indicated the formation of the p–p stacking
by hexaazatrinaphthylene cores during the gelation, which were
forced to adopt a face-to-face mode.
Fig. 3 AFM images of xerogels of 1 (0.36 wt%) and 2 (1.10 wt%) in
DMSO and toluene, respectively. 2.25 ¥ 2.25 mm2 3D image of 1 (A) and
4.00 ¥ 4.00 mm2 3D image of 2 (B). (C), 2.25 ¥ 2.25 mm2 contrast phase
image of the gel 2, shows the striations inside the ribbons.
In contrast, the AFM images of 2 showed entangled lamellar
ribbons (Fig. 3B) with straight narrow striations ranging from
20–30 nm thick (Fig. 3C). It is worth noting that the striations
observed on the ribbons could be considered as individual fibers
that aggregate into a parallel columnar packing to form bundles.
Taking into account the microscopic analysis carried out, we can
suggest that the gel generation might arise from the self-assembly
of 1 and 2 molecules into 1D fibrillar architectures though p–
p interactions and further hydrogen bonding and van der Waals
forces between the side chains of these columnar structures to form
bundles of fibers. These interactions are somehow hinted by FT-IR
spectra (ESI†) as well as by the reactive points of the structure of
compounds 1 and 2. Subsequently, these bundles could aggregate
to give rise to a randomly connected network of long ropes (1) or
lamellar ribbons (2) as well.
Oscillatory rheological measurements confirmed the viscoelas-
tic, rigid and brittle nature of the obtained gels (Table 2). G¢ was
uniformly found to be of greater magnitude than G¢¢. The gels were
stable over a wide frequency range (0.1–100 rad s-1), and dynamic
strain sweep measurements showed that they break at less than
0.5% strain.
A spectroscopic characterisation of the gels was also performed
to complete all characterisation results. Fluorescence emission
spectra of compounds 1 and 2 in DMF are shown in Fig. 4
In conclusion, we have reported for the first time the gelation
ability of two HATNA compounds 1 and 2 as well as the formation
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Additionally, the present gels have a significant potential for
further development and a range of applications are envisaged
in the near future. Based on preliminary computational studies
on their HOMO–LUMO gap, these materials have promising
properties to act as n-type semiconductors. Several studies are
currently ongoing in our laboratories and in collaboration with
other groups in this regard.
Acknowledgements
Financial support by Ministerio de Educacio´n y Ciencia de
Espan˜a (SAF2006-06720) and Ministerio de Ciencia e Innovacio´n
(MCI, CTQ2008-06017/BQU) is gratefully acknowledged. D.G.V.
thanks Gobierno de Canarias for a predoctoral contract. RL
gratefully acknowledges Ministerio de Ciencia e Innovacion,
Gobierno de Espan˜a for the concession of a Ramon y Cajal
contract (ref. RYC2009-04199) and funding under project P10-
FQM-6711 from Consejeria de Ciencia e Innovacion, Junta de
Andalucia.
Notes and references
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Fig. 5 UV–vis spectra of compound 1; (A) in solution and gel-state
at different concentrations (10-5–10-3 M) and (B) in dilute solutions
(10-5 mM) of solvents with different polarity (DMF, DMSO, acetoni-
trile-ACN, and H2O) at 25 ◦C.
Table 2 Rheological data for gelsa of 1 and 2 in DMSO and toluene,
respectively
c
d
Compound
Solvent
cgcb (wt%)
G¢ (¥103 Pa)
G¢¢ (¥103 Pa)
1
2
DMSO
Toluene
0.50
1.20
10 0.6
70 1.3
1.0 0.80
7.2 0.90
a Gels◦(1 mL) were prepared upon warming the cool isotropic solutions
to 25 C. b Critical gelation concentration. c G¢ = average storage modulus.
d G¢¢ = average loss modulus.
of their stable organogels, which was studied by FT-IR, DSC,
TEM, SEM and AFM images. A variety of solvents can be
gelated, providing robust and translucent gels stable for months.
Furthermore, very subtle structural changes were shown to
potentially have a dramatic effect on supramolecular aggregation.
The results obtained from spectroscopic and microscopic
techniques reveal the occurrence of self-assembly and auto-
aggregation within the gel, induced by the terminal groups present
in the periphery of the HATNA core. With this surprising
discovery, studies are ongoing in our laboratories to design new
LMWOGs based on this core for fine-tuning the range of solvents
to be gelated and determine the influence of the terminal groups
in the self-assembly and in the gel formation.
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This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 6524–6527 | 6527
©