Sugar-based Organogelators for Various Applications - Langmuir

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Sugar-based Organogelators for Various Applications Annamalai Prathap, and Kana M. Sureshan Langmuir, Just Accepted Manuscript • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Sugar-based Organogelators for Various Applications Annamalai Prathap and Kana M. Sureshan* School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Maruthamala (P.O), Vithura, Kerala – 695551. KEYWORDS. Organogel. Sugar gelators. Self-assembly. Supramolecular chemistry. Hydrogen bonding.

ABSTRACT. In this feature article, we discuss the design strategy, syntheses and the selfassembly of various sugar-based gelators to form organogels. We illustrate the use of organogels formed by these sugar-based gelators for various applications such as a) development of scratchfree, shatter-free soft-optical devices using oil-gels formed by mannitol-based gelators, b) marine oil-spill recovery using sugar-based phase selective organogelators c) preparation of semiconducting cotton cloths using a diyne functionalized sugar gelator, d) development of sugar arrays on glass slides using a polymerizable diyne functionalized sugar gelator for efficient lectin binding, e) development of sintering resistant hybrid CaO-silica material for the absorption of CO2, f) preparation of porous polystyrene-crown ether matrix for the selective alkali metal ions sequestration, g) preparation of porous polystyrene, structured silica and fluorescent gels using a

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library of sugar-based gelators and also the mechanism of gelation of some of these gelators have been discussed. We have also given our perspective towards exploring sugar-based gelators for advanced applications.

Introduction Gels, the 3D soft materials exhibiting the properties between liquid and solid are academically as well as industrially significant as they possess intriguing physical properties such as transparency, thermo-response, visco-elasticity, moldability.1,2 Gels result when hierarchically ordered 3D-microstructures entrap solvents thereby controlling their fluidity. Broadly, gels are classified into two categories namely, physical gels and chemical gels. Physical gels result out of self-assembly of low molecular weight organic molecules in appropriate solvents through weak intermolecular interactions such as hydrogen bonding, electrostatic interactions, van der Waals interaction, π-π interaction, etc. Based on the solvent that is congealed, physical gels are categorized further into hydrogels or aquagels (water) and organogels (organic solvents). The common scaffolds for organogelators are carbohydrates,3,4 peptides,5 lipids,6 ureas,7 steroids,8,9 dendrons10,11 oligoarenes,12-15 etc. Exploiting their properties, various applications of physical gels in interdisciplinary areas are being explored. Hydrogels have mainly found applications in biomedical areas such as tissue engineering and drug delivery.16,17 Application of organogels in the

field

of

medicine,18,19

oil-spill

recovery,20-23 pesticides,24

pollutants

removal,25

supramolecular electronics,26-29 and for developing functional materials such as soft-optics,30 CO2 sorbent,31 conducting cloths,32 chemical sensors,33 metal ion scavengers34 etc. have been demonstrated. The development of novel gelators, study of mechanism of gelation and exploration of new applications of gelators are timely area of research.


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When a solute is introduced in a solvent, different fates of the mixture are precipitation, dissolution, crystallization and gelation. When the solute-solute interactions are stronger, than the solute–solvent interactions, either precipitation or crystallization results. Stronger solutesolvent interaction than solute-solute interaction, leads to dissolution. When there is an optimal balance between solute-solute and solute-solvent interaction, gelation results. Amphiphilicity is often an important structural feature for a gelator molecule. There is however only subtle difference between gelation and crystallization.35 While crystallization results out of selfassembly in all dimensions of space with more or less similar speed of propagation, in case of gelation the propagation of self-assembly is much faster in 1 dimension than the other dimensions, leading to superstructures of high aspect ratio (e.g. fibers, ribbons etc.). Entanglement of such microstructures in 3D leads to porous fibrous networks within which the solvent gets trapped through capillary force.

O

OH

O

O

O HO

1

OH

O

O

2

O OH

O

Figure 1. Plausible mechanism of gelation for the gelators 1 & 2. (Reprinted with permission from ref (20); Copyright 2012 Royal Society of Chemistry)


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Carbohydrates are important class of gelators36-38 that use hydrogen bonding for their selfassembly especially in non-polar solvents. Pioneering works by Shinkai et al. established the structural features that are required for the self-assembly of glycopyranosides based gelators.4 These studies have revealed that amphiphilic sugar derivatives having vicinal diol functionality undergo 1D self-assembly in non-polar solvents through zig-zag H-bonding, leading to fibrillar structures resulting in gelation. In this feature article, our research efforts in designing various sugar-derived gelators and exploration of their applications in various fields are compiled. There are several important reviews dealing with various aspects of gels in general.39-43 Development of soft-optical devices Partial protection of hydroxyl groups of mannitol as ketals yielded amphiphilic diols 1 and 2. These diols were found to be efficient gelators for various non-polar solvents and oils. Several structural activity relations studies with different derivatives revealed that the vicinal diol functionality and ketal protecting groups are important structural features. Various spectroscopic studies suggested that the H-bonding is the main force responsible for gelation. A possible mechanism involves the self-assembly of gelators through H-bonding to form fibrils, followed by lateral assembly of fibrils to fibres, and entanglement of these fibers in 3D yielding a fibrous network (Figure 1). The fibrillar morphologies were established by SEM & AFM studies. The gels formed by these gelators were found to be transparent with high transmittance in the visible region. The critical gelation concentration (CGC) studies revealed that the gelators required were as low as 0.2 wt% to form stable gels. The oil gels of 0.2 wt% were stable enough for months without losing their transparency and strength. As the oils have refractive indices close to that of glass and as the gels formed by gelators 1 and 2 in oils have high transmittance, we have developed several soft-optical devices from oil gels of 1 and 2. Interestingly, the gels


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formed by 1 and 2 were found to show self-healing properties, which make them ideal soft material for scratch-free and shatter-free optical devices. Optical devices such as lenses, prism etc. made from oil gels of 1 and 2 were found to be surprisingly indistinguishable from normal glass-based optical devices (Figure 2). The possibility of doping the gel with many chromophore or other functional materials offer the possibility of developing smart optical devices.44 The crystal structure of the gelator 2 revealed the face-to-face hydrogen bonded dimers, which are further linked by H-bonding to form 1D molecular chains (Figure 3). This type of selfassembly allowed them to form a dynamic ion channels in lipid membrane which enabled the transport of chloride ions selectively.45

Figure 2. a) A gel prism made from pump oil gel of 2. b) The diffraction pattern observed using the gel prism. c) The gel prism mounted on a spectrometer table. d) The double convex gel lens made from the pump oil gel of 2 supported on a metallic ring. e) A planoconvex lens made from paraffin oil gel of 2. f) View of the structure of the gelators through their paraffin oil gels. g) A gel cube. h) Gel cylinder. i) Gel cone. (Reprinted with permission from ref (30); Copyright 2011 John Wiley and Sons). Marine oil-spill recovery


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Bhattacharya and co-workers proposed for the first time that the phase selective organogelators (PSOGs), gelators that can congeal oil phase selectively from a biphasic mixture of oil (hydrocarbon oils such as the mixture of aliphatic and aromatic solvents46) and water, have the potential to be used for marine oil-spill recovery.47 They demonstrated selective gelation of oil by heating and cooling a mixture of water and oil. Though this (heating and cooling) method was not practical for real application, this proof of concept paved the way for further research to explore PSOGs for marine oil-spill recovery. Later several researches circumvented this problem associated with heating-cooling method by using carrier solvent for the uniform application of gelators.48 However, the major drawbacks of these methods were in the use of a carrier solvent that get dissolved in aqueous phase. This raises problem in real life scenario as these carrier solvent may be harmful for the marine eco-system. In order to avoid the contamination by the carrier solvent, we have demonstrated that by using gelling solvent as the carrier solvent, both the oil and the carrier solvent can be congealed and removed out of the system. Model marine oil-spills with bi-component mixtures comprised of refined oil (pump oil or paraffin oil or diesel) and water were made separately and the oil-spill recovery was demonstrated by spraying the warm solution of the gelator 2 in the corresponding carrier solvent. The rigid gel pieces resulting upon the introduction of the gel solution could be scooped out easily (Figure 4). Interestingly, the diesel and pump oil gel pieces formed over the aqueous phase were strong enough at low concentration (1 wt%). The congealed oil could be recovered by simple distillations. However, as the carrier solvent itself can be congealed, the spray solution has to be maintained and sprayed as hot solution. Also the gelation prevents the solubility of gelator and this necessitates large volume of carrier solvent. This is a practical difficulty for marine oil-spill recovery.


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Figure 3. Single crystal structure of 2 showing (A) face-to-face and interlayer hydrogen bonding; top (B) and side views (C) of face-to face aggregation of ladder like structures. (Reprinted with permission from ref (45); Copyright 2014. American Chemical Society) An ideal solution to circumvent this problem could be the use of gelator without any carrier solvents. However, most of the gelator, when sprayed as a powder congeals only a small layer of oil surrounding to gelator solid particles, thereby restricting the core of the solid gelator from gelation. This surface gelation insulates most of the gelator in the core. An ideal PSOG should be able to dissolve and disperse easily when applied as solid powder and be able to contribute towards gelation completely. Among several gelators tested, thioglycosides based PSOGs (3-5) were found to be efficient when applied in the powder form. Interestingly, they were found to disperse uniformly in crude oil and all refined oil when applied as a powder (Figure 5). One of the requirements for easy distribution is high oleophilicity. The S – alkyl group might be responsible for the improved oleophilicity of gelators and thus in the easy solubility and dispersion. After our report, Zeng and co-workers have reported the use of solvent-wet PSOG for room temperature oil-spill recovery.21


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Figure 4. (a) Biphasic mixture of diesel and water. (b) Introduction of a gelator solution into the biphasic mixture. (c) The solidified oil phase. (d) Removal of congealed oil phase. (e) Isolated gel. (f) Recovery of diesel by distillation. (g) A 3 wt% gel holding the weight of a litre of a dilute CuSO4 solution. (h) The gel after removing the CuSO4 solution. (i) A metal coin (1 Euro) on the surface of a 1.5 wt% diesel gel on the surface of water. (j) The gel disc formed on the surface of water being taken with hand. (Reprinted with permission from ref (20); Copyright 2012 Royal Society of Chemistry)Though the above method of congealing and collecting the congealed oil works in oil contained in small area (flask or tub), it poses practical difficulty for the recovery of the real marine oil spill spread over large surface area due to the fragile nature of the semi-solid gelly material.


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Figure 5. A) Spreading of gelator 3 powder (1 wt%, wt/v) over a layer of crude oil on water. B) Gelation of the crude-oil layer. C) The crude oil gel was removed with a spatula. D) The removed crude-oil gel. (Reprinted with permission from ref (23); Copyright 2016. John Wiley and Sons).

Figure 6. Scheme of proposal for the eco-friendly marine oil-spill recovery. (Reprinted with permission from ref (22); Copyright 2017. John Wiley and Sons).Hence it is necessary to contain the congealed oil in an enclosure (sorbent) which can be collected easily. We planned to impregnate the PSOG in an oleophilic porous matrix so that the oil will be sucked into the matrix as soon as it gets in touch with the oil-water mixture and would be congealed by the PSOG inside the matrix. Such gels formed in the matrix could be easily collected and recovered. In order to test this hypothesis, we impregnated the naturally abundant fibrous cellulose pulp with the PSOG 2. This rendered the inherent hydrophilic cellulose pulp temporarily hydrophobic due to the masking of surface exposed hydroxyl groups by the hydrophobic PSOG 2. The gelator adsorbed cellulose pulp (GACP) was found to be efficient in absorbing variety of crude oils and other refined oils and congealing them in the matrix at room temperature (Figure 6). Importantly,


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this strategy allowed recovery the congealed oil simply by squeezing the sorbent-gel pieces at room temperature. In general PSOGs have been demonstrated to be promising agents for combating marine oil-spill recovery. Still, they are either to be hybridized with suitable sorbents or to be applied using a carrier solvent for their introduction. Development of semiconducting fabrics Natural fibers and the fabrics derived from them are electric insulators and they build-up static charge, which can harm humans and properties.49 There is a great interest in producing semiconducting fabrics having applications as antistatic applications,50 electromagnetic interference shielding material,51 and flexible electronics.52 Inhomogeneous physical blend consisting of conducting materials (carbon or metals) and fabrics are less attractive due to the abrasion or leach out of these dopants during handling.53,54 In order to address the drawbacks, it is essential to develop methods to generate conducting fabrics where the conducting material is uniformly distributed and are held together strongly on fibers. Polydiacetylene (PDA) which can be prepared by topochemical polymerization of organized diacetylenes55 are appealing semiconductive material for developing semiconducting fabrics. For this, it is necessary to arrange the diacetylene derived monomers in an orientation suitable for its topochemical polymerization and design the monomer such that the polymer adheres to the fiber surface. Based on previous reports on self-assembly and gelation of glycopyranosides,56-58 we designed the sugar based gelator 6.


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O O HO

O O HO

O S HO

Me

3

O O HO

O

S HO

4

O

SC H 6 13 HO

5

C6H13 O O

O O HO

O O

O O

HO

OH

OH

6

7

Chart 1. Chemical structures of compounds 3-7. It was anticipated, upon gelation in medium containing cotton fabrics that the monomers not only adhere to the fabrics through H-bonding but also self-assemble with properly aligned diacetylene moieties for the photopolymerization (Figure 7). As envisioned, when cotton fabrics dipped in a gelator solution was irradiated with light of wavelength 254 nm, the self-assembled diacetylene underwent polymerization yielding semiconducting fabrics. After polymerization, the benzylidene protecting groups could be unmasked by mild acidic hydrolysis and this increased the H-bonding interactions between the cotton and the PDA. The polymer coated cotton fibers were found to be semiconducting in nature. Interestingly, cotton cloths fabricated with the semiconducting PDA fibers showed tunable surface resistivity, making them attractive for anti-electrostatic and static dissipative applications (Figure 8). Development of sugar arrays for high affinity lectin binding Sugar–protein binding interactions form the basis of several cellular recognition events such as cell–cell adhesion, cell–cell communication, fertilization, antigen–antibody interactions, inflammation, infection, cancer metastasis, etc.59 But, the binding affinity of a single sugar unit with protein is very weak and in order to evade this, biological system uses multivalent glycoconjugates for effective interaction.60 Developing synthetic multivalent glycoconjugates61


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is an active area of scientific research. Such synthetic glycoconjugates are not only important tools to study the effect of multivalency but also important to develop biosensors based on sugar–protein binding.62 For instance, easy-to-process multivalent synthetic glycoconjugates that can be immobilized on surfaces are essential for the development of diagnostic tools.63 We reasoned that a sugar-derived gelator containing diacetylene motif upon gelation aligns its diyne moieties coaxially and would undergo topochemical polymerization on photoirradiation to provide PDA with pendant carbohydrate units. Such a solution processable gelator would be appropriate for in situ synthesis and immobilization of multivalent glycoconjugates on untreated surfaces. Organogelators having a 4,6-O-benzylidene glycopyranoside skeleton are known to undergo 1D hydrogen-bonded self-assembly leading to fibrils with a stacked arrangement of the anomeric substituent (Figure 7A and B).4 Hence, we designed the gelator 7 which in its gel align its diyne motifs in an orientation suitable for light-induced topochemical polymerization reaction to yield the multivalent glycocluster (glyco-PDA polymer, Figure 9). When a glass slide dipped in a gel was irradiated, PDA was formed in situ, with surface-exposed galactose units. These surfaceimmobilized galactosyl polymers was demonstrated to exhibit 1000 fold enhanced binding with various galactose binding lectins compared to the monomeric galactose molecules.64


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Figure 7. A) Structure of methyl 4,6-O-benzylidene-b-d-glucopyranoside (1β). B) Crystal packing of 1β showing hydrogen-bonded 1D assembly. C) View of hydrogen-bonded 1D assembly along the direction of hydrogen bonding showing the stacking of methyl groups. D) Chemical structure of photo-polymerizable organogelator 5. E) Representation of the molecular packing arrangement in gel state and subsequent topochemical polymerization of monomeric gelator 6. (Reprinted with permission from ref (32); Copyright 2015. John Wiley and Sons).

Figure 8. A) Preparation of conducting cloth. Photographs of conducting cloth (B) and conducting fiber (C) prepared by in situ synthesis of gelator 6 on cotton fabrics/fiber. D) Conducting fiber prepared by in situ synthesis of benzylidine deprotected compound 6 on cotton


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fibers. (Reprinted with permission from ref (32); Copyright 2015. John Wiley and Sons).

Figure 9. Schematic showing the preparation of multivalent glycoconjugate for lectin binding (Reprinted with permission from ref (64); Copyright 2016. Royal society of chemistry). CaOSiO2 hybrid material for CO2 absorption Concerns regarding the increased levels of atmospheric CO2 and its effect on global warming65 and use of CO2 as the cheap raw material for the production of useful chemicals,66,67 energy68,69 etc. highlight the importance of developing efficient ways of capturing and storage of CO2. Quicklime has been thought to be the primary sorbent as it is economically cheap, recyclable and efficient in capturing and storing CO2 through a simple reversible carbonation reaction of CaO (CaO ↔ CaCO3).70 Multiple cycles of carbonation cycles leads to the phenomena called sintering by which CaO particles aggregate and loses its crystallinity and results in reduced surface area.71,72 Due to sintering, CaO crystals/particles at the inner core of the sorbent become inaccessible for carbonation reaction as CaCO3 layer formed at the surface prevents the diffusion of CO2 gas into the inner core. To minimize the effect of sintering, surface area-to-volume ratio of active CaO sorbent must be increased.73 Though the use of blend of CaO and inert matrices (e.g. silica) was attempted,74 such physical blends resulted in phase separation after a few cycles


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thereby reduction in the carbonation efficiency due to sintering.75,76 In order to prevent the phase separation and thus reduce the sintering effect, we envisioned using structured silica as the matrix. We planned to fabricate CaO nanocrystals over the silica microtubes as CaO nanocrystals can be grown on both inner and outer surfaces. Shinkai et al. have shown that micro-structured silica of different shapes can be prepared by polymerization of TEOS around gelator-microstructures templates in the oragnogel followed by the removal of the gelator by calcination.77-82 We have prepared gel of simple, cheap and easyto-prepare gelator 1,2;5,6-di-O-isopropylidene mannitol 1 (Figure 10) in TEOS. The TEOS upon polymerization formed silica tubes around the gel fiber templates and by washing off the gelator, hollow silica tubes could be obtained. CaCO3 nanocrystals were grown on the surface of these silica tubes, which up on calcination gave nanocrystals of CaO on silica-microtubes (CaOnc@SiO2µt). This sorbent was efficient in minimizing the sintering effect and was stable for several cycles of carbonation-decarbonation without losing their structural integrity.

Figure 10. Schematic proposal for (a) large-scale preparation of hollow silica microtubes using recyclable organogel-template; (b) use of silica microtubes as a platform for growing CaO nanocrystals and their use as sintering-free sorbent for multi-cycle calcium looping. (Reprinted


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with permission from ref (31); Copyright 2015. Royal society of chemistry). Porous matrix for alkali metal ions sequestration Crown ethers known for complexing metal ions are versatile synthetic compounds.83 Simple crown ethers are not useful in extracting metal ions from aqueous solutions as they are highly hydrophilic in nature and completely soluble in water. Though hydrophobic crown ether derivatives have been employed in deionization by liquid (water) −liquid (organic solvent) biphasic extraction method, they are not useful in practical scenario.84 Moreover, as the solid crown ether derivatives are prone to be dispersed in aqueous solutions, they cannot be used as solids. Though polymers of CE-derived monomers have been synthesized,85,86 the efficiency of such polymers in extracting metal ions was poor due to the inaccessibility of interior metalbinding sites in solid matrix. To address this, we prepared a porous polymer containing crown ethers in the pores. We polymerized styrene co-gel formed by two different gelators namely a sugar-based sacrificial gelator and a crown ether gelator, which would undergo self-assembly through different noncovalent interactions. Washing off of the sacrificial gelator, after the polymerization, yielded porous covalent-noncovalent hybrid polymer consisting of polystyrene with self-assembled crown ether in the pores (Figure 11). The porous morphology of the matrix would permit metal ions to access interior crown ether sites. This porous polystyrene was found to be efficient in extracting the alkali metal ions from the aqueous solutions for several cycles. As an example, the matrix with 18-crown-6-ether gelator could extract 75%, 59% and 88% of Li+, Na+ and K+ ions respectively.


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Figure 11. Proposal for the preparation of porous metal scavenging hybrid polymer. (Reprinted with permission from ref (34); Copyright 2018. American Chemical Society) Development of multi-functional gelator library for various applications Though various applications of gelators have been demonstrated, usually a particular gelator is only useful for a particular application. This one-gelator-one-use format poses practical difficulty in synthesizing gelators for various applications. To address this drawback, we have synthesized a library of gelators from a common gelator core using click chemistry.87 Thus, the gelator 4,6O-benzylidene -D-galactopyranosyl azide (8) was reacted with various alkynes in presence of Cu(I) catalyst to obtain a library of eleven new gelators (Figure 12). We have demonstrated different applications of some of these member gelators.


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Figure 12. Click-chemistry approach to a library of gelator and their applications. (Reprinted with permission from ref (84); Copyright 2015. John Wiley and Sons).

O O

O O O N3

HO

OH

HO

O O

N

HO

N O N

N O N

N

OH

N

OH

OH

10

9

8

O O

HO

N O N

O O HO

O N 3 OH

12

11

O O O HO

HO O

13

Chart 2. Chemical structures of compounds 8-13. Preparation of structured silica Polymerizing TEOS gel of 9 yielded corrugated rods of silica having approximate diameters of 200-300 nm. Organogel-templated synthesis of silica rod of such unique morphology has not been reported previously. Preparation of porous plastic


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Porous polymers find applications in many areas such as gas storage, drug encapsulation,88 catalysis, filtration/separation membranes, and energy-storage materials.89-91 Templating is the simplest and most-efficient method for making porous polymers.92 Organogels formed by polymerizable organic solvent offer the possibility of obtaining porous polymer material upon removal of the 3D nanofibers formed by the gelator. We prepared styrene gel of 10 and the gel was polymerized to polystyrene. Removal of the gelator by washing produced porous polystyrene. As the thickness of the gel fibers can be tuned by adjusting the concentration, the pore size can be tuned. Such porous polystyrene are in high demand in the field of catalysis, biomedical field, composite materials, hydrophobic coating, etc. Development of fluorescent gel Fluorescent gels have been extensively studied and shown to be useful in organic electronics,12 photovoltaics93 and sensors.33 In order to show versatility of our method of gel-library synthesis, we made a fluorescent gel by introducing anthracene moiety to the gelator core. Similar to other known fluorescent gelators, the gel formed by 11, showed red-shifted absorption compared to the monomeric absorption in solution in a non-gelling solvent. While the wavelength of maximum absorption in isotropic solution was 364 nm, it got red-shifted to 372 nm upon gelation. Also the monomer emission (CH2Cl2 solution) at 454 nm got quenched upon gelation (in benzene gel) and a new emission arising from aggregated species (excimer) appeared at max = 519 nm.94 Further, evidences for gelation-induced fluorescence were obtained from concentration-dependent and temperature-dependent fluorescence experiments. Gels for topochemical reaction We have demonstrated the topochemical reaction of two identical sugar-based gelators having complementary reactive motifs (i.e. azide and alkyne) 12 and 13 in their xerogel state.56 Both the


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gelators were designed based on their parent gelator skeleton reported by Shinkai et al.4 Both the gelators 12 and 13 formed gels in toluene individually. They also formed strong gel in toluene, when they are mixed in 1:1 ratio, via their co-assembly. The co-assembly places the azide and alkyne of adjacent molecules at proximity. A xerogel made from this gel, upon mild heating underwent thermal topochemical azide-alkyne cycloaddition (TAAC) reaction. This is the first report of topochemical azide-alkyne reaction in a xerogel medium. Understanding Gelation vs Crystallization As we have already discussed, the mechanism of gelation differs from that of crystallization with respect to the nature of the molecular self-assembly. A phase separated crystal results out of a solution when the self-assembly occurs in 3D fashion whereas homogeneous gels result if the molecules self-assemble in 1D. We have developed a myo-inositol (a carbasugar) based organogelator with 1,3 diaxial diol motifs capable of congealing aliphatic hydrocarbons. We found that the gelator congeals petrol in the absence of water and is crystallized out when the critical concentration of adventitious water is present in petrol (i.e. if the ratio of gelator to water exceeds one, then crystallization occurs whereas if it is lesser than one gelation happens). The diol groups facilitate the 1D assembly through zig-zag H-bonding interaction as in the case of glycopyranosides. It has been demonstrated from the single crystal X-ray diffraction analysis that the fibrils formed by the 1D hydrogen bonding interactions are further connected laterally through the adventitious water present in the system. These results give useful insight into how the minute amount of water can decide the fate of gelation. Conclusions and perspective Low molecular weight organogelators possess immense potentials to be explored as advanced functional materials due to their unique properties such as thermoreversibility, viscoelasticity,


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etc., resulting out of their self-assembly. Though many categories of molecular systems have been adopted as scaffolds for gelators, cheap and abundant carbohydrates constitute an important class as it offers multiple handles for functionalization. Exploiting the carbohydrates such as mannitol, glycopyranosides, etc., we have developed various gelators. Based on the unique physical and chemical behavior of the individual building blocks, we explored them for various applications in areas such as oil-spill recovery, CO2 absorption, soft-optical devices, semiconducting fabrics, lectin binding, and alkali metal ions sequestration and also in preparing other functional materials such as porous plastics, structured silica and fluorescent gels. Though limited, these examples of demonstrations suggest that sugar-based gelators hold promising potentials to be explored further for advanced applications. The availability of many stereocentres in carbohydrates can be exploited for developing chirooptical materials for photonic applications. We hope, this article would elicit further research interests along this line.

AUTHOR INFORMATION Corresponding Author [email protected] ACKNOWLEDGMENT KMS thanks the Department of Science and Technology, Govt. of India for a SwarnaJayanti Fellowship. REFERENCES (1) Terech, P.; Weiss, R. G. Low Molecular Mass Gelators of Organic Liquids and the Properties of their Gels. Chem. Rev. 1997, 97, 3133-3160.


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of

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Content

(TOC)


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Langmuir

Dr. Kana M. Sureshan (left in the picture) obtained his Ph. D. in Organic Chemistry from the National Chemical Laboratory, Pune in 2002. He carried out his postdoctoral research as a JSPS postdoctoral fellow at Ehime University, Japan (2002-2004), as a Research Officer at Dept. of Pharmacy and Pharmacology at University of Bath, U. K. (2004-2006) and as an Alexander von Humboldt fellow at Max Planck Institute for Molecular Physiology at Dortmund, Germany (2006-2008). He joined Indian Institute of Science Education and Research Thiruvananthapuram in April 2009 as an Assistant Professor. Since 2014, he is an Associate Professor. His research interests lie in the area of supramolecular chemistry, crystal engineering and topochemical reactions. He is the recipient of Ramanujan fellowship, Swarnajayanti fellowship, Young Scientist award of YIM-Boston, Bronze medal from Chemical research Society of India (CRSI) and a Medal from Materials Research Society of India (MRSI). He has also won an innocentive challenge award for designing the shortest and economical route to the tuberculosis drug, PA824. He is a Fellow of the Royal Society of Chemistry, elected under the leader of the field category. Annamalai Prathap obtained his M. Sc. degree from Anna University, Chennai in 2008 and started working in the R & D center of Shasun Pharmaceuticals Ltd. Chennai. In 2011 he joined Dr. Kana M. Sureshan’s group at IISER Thiruvananthapuram as a project assistant. Currently he has completed his Ph. D (2014-2019) in the area of low molecular weight organogelators and their environmental applications. He is interested in materials research based on molecular selfassembly and supramolecular chemistry.


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Sugar-based Organogelators for Various Applications - Langmuir

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