Palladium-induced transformation of nematic liquid crystals to robust metallogel comprising self-assembled nanowires

Article abstract of DOI:10.1039/c9cc05517g

The formation of a nematic liquid-crystalline phase in the bulk along with gelation of a novel asymmetric bolaamphiphilic NDI scaffold has been demonstrated. Further, a discrete metal NDI complex that is capable of forming a robust metallogel through wrapping of the NDI core with the oxyethylene chains of the neighboring molecules has been synthesized.

Full text of DOI:10.1039/c9cc05517g

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Palladium-induced transformation of nematic
liquid crystals to robust metallogel comprising
self-assembled nanowires†
Cite this: Chem. Commun., 2019,
5, 12651
5
Received 17th July 2019,
Accepted 17th September 2019
ab
a
ab
Bappa Maiti,‡ Subham Bhattacharjee‡§ and Santanu Bhattacharya
*
DOI: 10.1039/c9cc05517g
rsc.li/chemcomm
The formation of a nematic liquid-crystalline phase in the bulk In general, two strategies could be exploited to design supra-
along with gelation of a novel asymmetric bolaamphiphilic NDI molecular metallogels: (i) in situ metal–ligand coordination driven
scaffold has been demonstrated. Further, a discrete metal NDI formation of supramolecular polymers which eventually trans-
complex that is capable of forming a robust metallogel through form into metallogels in certain solvents or (ii) construction of a
wrapping of the NDI core with the oxyethylene chains of the discrete metal–ligand low molecular mass complex as a mono-
neighboring molecules has been synthesized.
meric unit for the fabrication of supramolecular polymeric metal-
logels via multiple non-covalent interactions. The emerging area
Nature has shown astonishing capabilities of creating complex of metallogels has witnessed fascinating advancements in
supramolecular assemblies through weak inter-molecular inter- molecular recognition, in situ syntheses of metal nanoparticles,
actions that are capable of pursuing highly specific functions. catalysis, applications in magnetic resonance imaging, dispersion of
7
Inspired by nature, chemists have utilized such non-covalent carbon-based nanomaterials, and so on. Inorganic–organic hybrid
interactions as a tool to engineer complex supramolecules complexes are also potential candidates for developing multiferroic
8
including liquid crystals (LCs) and gelators. Self-assembled materials. In spite of the existence of a large body of data on
molecules with long-range orientational order often show metallogels, there is continuous interest in the design of novel
1
liquid crystalline properties. In thermotropic LCs, long-range metallogels to solve hitherto unexplored issues.
order could be achieved by slowly cooling from the isotropic
Naphthalenediimides (NDIs) are among the most promising
2
melt. However, if the long-range order is reached by dissolving n-type organic semiconductors, which hold strong interest in
the molecules in a solvent above a particular concentration, areas as diverse as solar cells, photovoltaic devices, light harvesting
3
9
they are called lyotropic LCs. Such long-range orientational systems, organic field effect transistors, liquid crystal displays, etc.
order in these materials could be utilized in the construction of On the other hand, NDI based self-assemblies possessing
4
molecular ferroic and LC memory devices. On the other hand, compelling nano-structures such as fibers, belts, ribbons, solid
the self-assembly of molecules in a specific solvent could also spheres, and vesicles have been studied extensively in the last
1
0
form supramolecular gels which have received extensive attention two decades to explore various possibilities. Although there
5
owing to their potential utilization in multiple areas.
have been several reports on NDI based supramolecular gels,
1
1
Since metal–ligand bond formation is as facile as the other metallogels comprising the NDI moiety are rare. Herein, we
non-covalent interactions, such interactions could be used to report design, synthesis, and characterization of a NDI based
fabricate complex superstructures, especially metallogels that can nematic liquid crystalline ligand which forms a discrete metal–
6
adopt smart functionalities similar to natural supramolecules.
ligand complex amenable to self-organization into a robust
metallogel.
The synthesis of an asymmetric bolaamphiphilic NDI derivative,
, a waxy solid, is given in Scheme 1 and also in the ESI.†
a
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012,
1
India. E-mail: sb23in@yahoo.com
b
Differential scanning calorimetry (DSC) of 1 in the bulk displayed
a weak endothermic transition at B136.6 1C followed by melting
at B147.7 1C, suggesting the probable existence of a narrow LC
phase in between these two temperatures (Fig. S1, ESI†). During
cooling from its isotropic melt state, a broad exotherm appeared
at B116.8 1C. Under a polarized optical microscope (POM), a
weakly birefringent LC phase appeared after 130 1C during
School of Applied & Interdisciplinary Sciences, Indian Association for the
Cultivation of Science, Jadavpur, Kolkata 700 032, India
†
Electronic supplementary information (ESI) available: Description of physical
measurements and instrumentation; synthetic procedure and characterization;
Tables S1 and S2 and Fig. S1–S12. See DOI: 10.1039/c9cc05517g
‡
§
Both authors contributed equally to this work.
Present address: Department of Chemistry, Kazi Najrul University, Asansol-713340,
West Bengal, India.
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Fig. 2 (a) Phase-contrast optical microscopy image of 1 (gel) from EtOH.
The inset in panel a shows the photograph of the gel of 1 in EtOH. (b)
Powder XRD pattern of the freeze-dried gel of 1 from EtOH.
Scheme 1 Schematic illustrations of the synthesis of an asymmetric
bolaamphiphilic NDI derivative, 1, and its discrete metal complex 2.
(Table S1, ESI†). The critical aggregation concentration (CAC)
in EtOH measured from the concentration-dependent UV-Vis
spectra was around 1.2 mM (Fig. S5a and b, ESI†). Morpholo-
gical studies revealed the formation of fibrous networks with a
length in the range of several micrometers (Fig. 2a and Fig. S6,
ESI†). Powder X-ray diffraction (PXRD) studies of the freeze-
dried gel of 1 revealed the appearance of a strong first-order
Bragg reflection peak at 2y = 2.921 (B3 nm) along with the
repetitive reflections up to the sixth order (Fig. 2b). This result
indicates the existence of a highly ordered lamellar organization
of 1 with a p–p stacking distance of 0.38 nm in EtOH (Fig. 2b).
The layered distance of B3 nm in the lamellae matched well
with the estimated molecular length of 1 (B3 nm, Fig. S4, ESI†),
indicating the formation of a ‘monolayer’ assembly. Moreover,
the hydrazide units, which are located only on one side of the
bolaamphiphilic 1, have a strong tendency to form orthogonal
H-bonds among themselves. Parallel alignment of 1 in the
monolayer of the lamellae also allows the H-bonded hydrazide
units to stay on the same side. Based on these observations, a
probable molecular packing of 1 in the aggregates in EtOH is
proposed (Fig. S7, ESI†).
heating (Fig. S2, ESI†). The birefringence vanished above 140 1C,
which is in line with the observed DSC data. During cooling from
the isotropic melt, a highly birefringent LC phase appeared at
around 130 1C (Fig. 1a and Fig. S3, ESI†). It is important to note
that the corresponding transition was not observed in the DSC
thermogram, indicating the probable nematic nature of the LC
phase. A LC to crystalline phase transition was observed at
around 120 1C, which is in accord with the DSC data (Fig. 1a and
Fig. S2 and S3, ESI†).
12
Finally, variable temperature X-ray diffraction (XRD) studies
revealed that 1 in the bulk persisted in the crystalline state up
to B130 1C. The LC phase appeared above 130 1C and finally
melted at B150 1C as indicated by the absence of any XRD
signal at that temperature (Fig. 1b). The emergence of the first-
order reflection peak at B2.6 nm (2y = 3.41), which is relatively
smaller than the estimated molecular length of 1, and a broad
peak in the wide-angle regime confirmed the presence of a nematic
1
3
12b
LC phase (Fig. S4, ESI†). Similarly, XRD pattern characteristics of
the nematic phase also appeared during cooling from the isotropic
melt state before transforming into the crystalline state, thus
Since the pyridyl units are prone to form a coordination
complex with various metal ions, we further attempted to
synthesize a discrete metal complex of 1 by reacting it with
12b
confirming the enantiotropic nature of the LC phase.
The propensity of 1 to undergo self-aggregation in the solution
state was checked after investigating its self-assembly in the bulk. A
hot solution of 1 in EtOH was transformed into a robust gel upon
spontaneous cooling beyond a critical gelation concentration (CGC)
of 10.4 mM. A similar gelation propensity, however, was not
observed in other organic solvents like methanol, DCM, THF,
acetonitrile, methylcyclohexane, benzene, toluene and water
2 2
PdCl (PhCN) in a 2 : 1 molar ratio in dichloromethane (DCM).
To our surprise, we observed transformation of the above
mixture into a stable, light yellow colored gel in DCM after
completion of the reaction (Fig. 3a inset, CGC, [2] = 3.8 mM).
1
Thin layer chromatographic (TLC) as well as H-NMR investigations
Fig. 3 (a) Gradual evolution of metallogel 2 in DCM upon aging probed by
a transmittance method. The inset in panel a shows the photograph of the
metallogel 2 in DCM. (b) AFM image of the metallogel 2 ([2] = 0.3 mM) from
DCM.
Fig. 1 (a) Photograph of the nematic LC phase appeared during cooling
from the isotropic melt of 1 (in the bulk) under a POM. (b) Variable
temperature XRD studies of 1 (in the bulk).
1
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confirmed the quantitative formation of the discrete metal complex/ protons (H
i
, H
j
k
and H ) of the oxyethylene chains attached to the
gel (2) (Fig. S8, ESI†). A similar gelation propensity was also observed NDI core of the adjacent molecule of 2 (Fig. 4b and Fig. S10,
in CHCl (Table S2, ESI†). The kinetics of gel formation was found ESI†). Multiple through-space coupling signals among the proton
3
to depend strongly on the concentration of the 2 : 1 1/PdCl (PhCN)2 (H ) of the hydrazide unit and most of the protons of the
2
f
complex as well as on the nature of the solvent. The rate of gel oxyethylene chains indicate the existence of hydrogen-bonding
formation was faster in DCM than that in CHCl . Since the gel of 2 interactions among the hydrazide unit and oxygen atoms of the
3
is translucent in appearance, we probed the kinetics of gelation via a oxyethylene chains in the self-assembled networks. These results
transmittance method. Interestingly, the transmittance of the mix- strongly indicate that the NDI moieties in one of the discrete
ture in DCM started to decrease after B10 min and got saturated complex of 2 lie in close proximity with the oxyethylene chains
after 30 min, showing the maximum rate in between (Fig. 3a). UV/ that belong to the adjacent metal complex of 2. This structural
Vis spectroscopic studies at variable concentrations revealed that organization in 3D was further corroborated by the existence of
the relative intensity of the absorption bands arising from the NDI through-space coupling signals between the protons of the
core at 358 and 378 nm reversed in intensity upon increasing the oxyethylene chains of 2 and the protons of the gallate unit
concentration beyond 0.2 mM, indicating the onset of aggregation (H
Fig. S9a and b, ESI†). Morphological studies revealed the presence
of entangled nanowires with high aspect ratios which constituted from DCM showed the appearance of three broad Bragg reflections,
the metallogel (Fig. 3b and Fig. S9c, d, ESI†). indicating low crystallinity of the dried gel (Fig. 4a). The corres-
e
) belonging to the adjacent molecule of 2.
(
In addition, powder XRD patterns of the freeze-dried gel of 2
Rotating-frame Overhauser effect (ROESY) NMR spectro- ponding d-spacings of these peaks were 1.48, 0.72 and 0.51 nm with
scopic studies of the metallogel in CDCl were performed in an approximate ratio of 1 : 1/2 : 1/3, suggesting lamellar packing of
3
order to find probable molecular packing among the gelators, the gelators. However, a layered thickness of B1.5 nm of the
which revealed the presence of multiple through-space coupling lamellae obtained from the XRD pattern was much smaller relative
signals among the protons (H and H ) of the NDI core and the to the estimated molecular length of 2 (B5.7 nm, Fig. S11, ESI†),
a b
3
Fig. 4 (a) Powder XRD pattern of the dried gel of 2 from DCM. (b) Selected areas in the ROESY NMR spectrum (CDCl , 700 MHz, 10 mM, 293 K) of the gel
of 2. Cross-peak signals inside the red boxes are due to through space coupling. (c) A graphical representation of the molecular packing of 2 in the
metallogel matrix based on ROESY and XRD data.
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which suggests that the oxyethylene chains of one molecule
presumably wrap around the NDI core of the adjacent molecule
of 2 (Fig. 4c). The bending of the oxyethylene chains during the
wrapping may eventually reduce the effective length of 2.
Further, tilting of the molecular alignment in the lamellae may
account for the observed layered thickness in the gel. Therefore the
conclusion drawn from the ROESY NMR experiment was further
verified by XRD studies. On the other hand, unlike 1 in the bulk,
complex 2 did not display any LC behavior as it decomposed upon
heating at B220 1C under a POM (Fig. S12, ESI†).
In summary, we demonstrated the synthesis of a novel
asymmetric bolaamphiphilic NDI derivative, 1, capable of
showing a thermotropic nematic LC phase in the bulk. 1 also
formed a stable gel in EtOH. Further, we synthesized a novel
metal–ligand discrete complex (2) in nearly quantitative yield by
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Conflicts of interest
2
There are no conflicts to declare.
(
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