Chapter 1: Supramolecular gels and their applications Gels are viscoelastic materials composed of a solid-like three dimensional fibrillar network that is embedded in a liquid. Supramolecular gels belong to a class of gels which are derived from low molecular weight compounds (typically < 1000). A variety of non-covalent interactions like H-bonding, π-π stacking, donor-acceptor, metal coordination, solvophobic and van der Waals interactions are involved in the formation of the self-assembled fibrous networks (SAFIN’s) in these gels. These non-covalent interactions are weak in nature and as a result, these gels can be reverted back to sol by heating and this process is reversible. These gels are further classified as hydrogels, organogels and aero/xerogels depending on the medium they encompass. Although low molecular weight gelators were known in the early part of the 20th century, it is only in the last two decades that this field has generated widespread interest among scientists. In the 90s, the investigations on these kinds of gels mainly focused on designing new gelator molecules. However, during the last decade, the research interest in this field has shifted more towards designing functional gels. Such gels Scheme 1. Various applications of functional supramolecular gels have been extensively utilized in the templated synthesis of inorganic nanomaterials, in making hybrid materials, as synthetic light harvesting systems, as sensors, in the field of biomaterials such as drug delivery, screening of enzyme inhibitors and tissue engineering and also in the field of organic optoelectronics. In this chapter a few selected examples from each of these fields are highlighted. Chapter 2: Charge transfer induced organogels from 2,3dialkoxyanthracenes and 2,4,7-trinitrofluorenone 2,3-Di-n-alkoxyanthracenes formed charge transfer (CT) interaction promoted organogels in the presence of electron acceptor 2,4,7-trinitrofluorenone (TNF). These dialkoxyanthracences (in the absence of TNF) have been reported previously to form gels in a variety of organic solvents. The gelation property was found to be dependent on the chain length and the derivatives with C6-C16 chains were found to be gelators. On the other hand derivatives with C5-C1 chains were found to be non-gelators. It was found that TNF not only modulated the gelation property of the efficient organogelators, it also transformed the weak and non-gelators into efficient gelators. This charge transfer induced gelation was observed for the derivatives with C10-C4 chains in alcoholic and hydrocarbon solvents whereas the shorter chain derivatives C3-C1 did not form gels. Several other alkoxy and dialkoxy derivatives with substituents in other positions did not show gelation in the presence of TNF. These results suggested that two structural aspects are necessary for these derivatives to form CT gels- the alkoxy chain length and the position of the alkoxy substituents. The thermal stability of all these gels was found to be maximum with a 1:1 stoichiometry of the donor and the acceptor. The common observation, the intensification of color in going from the sol to the gel phase, supported the crucial role of the charge transfer interaction behind the formation of these gels. The rheological characterization of the gels demonstrated that they Figure 1. Chemical structures of 2,3-dialkoxyanthracenes and TNF (middle) and a fluorescence confocal microscopy image (left) and a photograph (right) of DDOA-TNF gel. behaved like viscoelastic soft solids. Chapter 3: A new class of perfluorinated derivatives of bile acids: Synthesis and gelation properties A new class of bile acid based gelators was designed by connecting the side chains of the facially amphiphilic bile acid with perfluoroalkyl chains of different lengths through two different ester linkages-–O-(CO)-and –(CO)-OCH2-. All these three structural aspects i.e. the bile acid moiety, the fluoroalkyl chain length and the spacer were found to influence the gelation properties of the derivatives. Depending on them, there was a variation in terms of the nature of the solvent gelated, the CGCs, the mechanical properties of the gels, etc among the derivatives. The deoxycholic and lithocholic derivatives with the spacer –O-(CO)-formed gels in aromatic hydrocarbons and also in DMSO depending on the fluoroalkyl chain length. The mechanical properties of the gels formed in DMSO were found to be dependent on the bile acid moiety and the fluoroalkyl chain length. In general, the deoxy analogues showed higher elasticity, stiffness and yield stress values for their gels than the litho derivatives. The perfluorinated derivatives having the spacer –(CO)-OCH2-showed gelation properties in organic-aqueous media and in DMSO. Interestingly, organogelation was observed in the deoxy and lithocholic derivatives from both spacer series whereas in the literature most of the bile acid based organogelators are derived from cholic acid. (b) (c) Figure 2. (a) Perfluorinated derivatives of bile acids, (b) photographs of a few DMSO gels and (c) TEM image of a xerogel of a deoxy derivative Chapter 4: Composite aerogels and organogels from 2,3didecyloxyanthracene and bile-perfluoro derivatives Aerogels are unique materials among solids. They have extremely low densities (up to 95% of their volume is air), large pores and high inner surface area. As a result aerogels have very interesting physical properties such as extremely low thermal conductivity, low sound velocity and high optical transparency. There are only a few reports of aerogel formation by low molecular weight gelators. We have investigated the aerogel formation ability of three long 7 chain perfluoroalkyl esters (two deoxycholic and one lithocholic derivative, chart 1) in supercritical CO2. A deoxy derivative, formed aerogel in sc-CO2. When mixed with DDOA (which has been reported previously to form good aerogels in sc-CO2), the perfluoro compound formed aerogels of better quality. The mixed aerogels were characterized by the presence of very large fibers in the micron range (as observed in the aerogel formed by only the fluoro derivative) as well as fibers of smaller size observed in pure DDOA aerogel. We also investigated the behavior of the composite systems in organic solvents. It was found that in DMSO, another deoxy derivative, Figure 3. SEM images of a mixed aerogel of DDOA-DC23C13F27 (left) and a mixed organogel (DMSO) of DDOA-DC23C11F23 (right). DC23C11F23 formed gels with higher thermal stability and improved mechanical properties compared to the native gels of the perfluoro compound or DDOA. Chapter 5: Hydrogels from lanthanide(III) cholates: Tunable, multiple color luminescence from hydrogels and xerogels In this chapter, facile hydrogel formation by several lanthanide cholates is reported. When sodium cholate was added to aqueous solutions of Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III) and Yb(III) and sonicated, the mixtures formed gels within a few seconds. The gels thus obtained were transparent/translucent and thermoirreversible. Rheological measurements showed that all of them could be classified as viscoelastic soft solids. A naphthalene derivative, 2,3-dihyroxynaphthalene was found to sensitize Tb(III) emission very efficiently in its cholate gel when doped in micromolar concentrations. The importance of the gel matrix behind sensitization of Tb(III) was demonstrated by the inefficiency of the same sensitizer DHN in an SDS micellar solution. In mixed gels of Tb(III)-Eu(III) doped with DHN, a energy transfer pathway was found to occur from the sensitized Tb(III) to Eu(III). By a simple tuning of the ratio of these two lanthanide ions, multiple color emissive gels could be made.The emissive properties of the hydrogels were retained in the xerogels and the suspensions of these xerogels in n-hexane were used for making luminescent coatings on glass surface. Figure 4. Tunable, multi-color luminescent hydrogels and xerogels of lanthanide cholates

Chapter 1. A Brief Overview of Low Molecular Mass Gels and their Applications This chapter deals with molecular gels derived from the self-assembly of small organic molecules (typical molecular weight < 2000 daltons), endowed with appropriate functions to promote anisotropic growth of supramolecular aggregates, by means of various non-covalent interactions like van der Waals, π-πstacking, H-bonding etc., leading to a self-assembled fibrillar network (SAFIN). Several representative examples from the literature (Chart 1) are discussed to demonstrate the structural diversity of the gelator molecules which form self-assembled organogels or hydrogels. Chart 1 Besides emphasizing on the diverse molecular structures of the gelators, applications of gel phase materials as functional nanostructures are also discussed (Scheme 1). Some of the aspects that have been elaborated in this context include the use of gels as reaction media, as sensors, in light harvesting, as biomaterials and in optoelectronic applications. Scheme 1 Chapter 2. Supramolecular Chirality in Organogels: Spectroscopic, Morphological and Rheological Investigations of Gels/Xerogels derived from Alkyl Pyrenyl Urethanes This chapter addresses the formation of chiral supramolecular structures in the organogels derived from chiral 1R (or 2R), and its mixture with its enantiomer (1S) and a series of achiral analogues (3-9) by extensive circular dichroism (CD) spectroscopic measurements (Chart 2). Morphological studies by atomic force microscopy (AFM) and scanning electron microscopy (SEM) were complemented by the measurements of their bulk properties by thermal stability and rheological studies. Specific molecular recognition events (1/3 vs 2/3) and solvent effects (isooctane vs dodecane) were found to be critical in the formation of the chiral aggregates. Computational studies were carried out to understand the interactions responsible for the formation of chiral superstructures. Chapter 3. Self-assembled Composite Organogels based on a Thermo-reversible Photoactive n-Acene Fibrillar Scaffold and Organic Ligand stabilized ZnO Nanoparticles Organic/inorganic composite organogels were obtained in n-BuOH by the self-assembly of 2,3-di-n-decyloxyanthracene (DDOA, Chart 3) in this solvent in the presence of ZnO nanoparticles (NPs) capped with different organic ligands (Chart 4). When ligands (oleic acid or 2,3-substituted anthracenic acid/oleic acid mixed shell) having structural similarity with the gelator molecule were used to cap the NPs, a homogeneous dispersion of the NPs in the gel matrix was obtained, as confirmed by microscopy (TEM and confocal fluorescence microscopy) experiments. The efficient integration of these NPs into the gel fibers resulted in a significant quenching (20-25%) of DDOA emission, even with extremely small loading of these NPs (~ 10-4mol% compared to DDOA) into the gel fibers. The mechanical properties (rheology were unaffected relative to the pristine DDOA organogel. However, the presence of the NPs lowered the critical gelation concentration and accelerated the gelation kinetics. Attempts to disperse these NPs (the ones without fluoro capping) on the aerogel fibers of DDOA by dissolving both DDOA and the NPs in supercritical (sc) CO2 were not successful (Fig. 1), since the NPs could not be dissolved in scCO2. Figure 1. (a) TEM images of DDOA aerogels obtained from scCO2, containing A23-NPs, scale bar 200 nm; (b) SEM image of DDOA aerogel obtained in the presence of OL-NPs, scale bar 10 µm. Chapter 4. Donor-Acdeptor Interaction Promoted Gelation Of Organic Fluids by Anthracene Carboxamides/2,4,7-Trinitrofluorenone Tris Carboxamides of anthracene were found to form charge-transfer driven organogels in a range of aliphatic alcohols in the presence of an equivalent of (electron-deficient) 2,4,7-trinitrofluorenone (TNF) (Chart 5). Intense color developed in the gel state during the sol to gel phase transition process (Fig 2) Besides, none of these carboxamides were able to form gel in the absence of TNF, suggesting the importance of charge-transfer interaction in the gel formation. Importantly, most of these gels formed only through rapid cooling of the hot solution, otherwise, leading to the precipitation of the CT complex from the solvent. This result indicated that the kinetics is very important for the formation of these gels. Optimum stoichiometry of the donor and acceptor was found to be 1:1. At this molar ratio of the donor and the acceptor, the gels not only showed the highest thermal stability (thermal gel melting experiments), they also displayed the highest values of the mechanical strength and the yield stress (rheology experiments). All the gels showed extensive quenching of the emission of the monomeric anthracenic donor. For the gels derived from the 2-substituted donor, a low energy emission at high wavelength indicated the formation of an emissive CT exciplex. X-ray powder diffraction studies of these xerogels revealed the presence of layered, fibrillar structures in the xerogel phase. (For structural formula pl see the abstract file)