| The gelation phenomenon exhibited by low molecular mass gelators (LMMGs) has received increasing attention because of the growing number of applications of molecular gels and their role in learning how to control self-assembly. LMMGs can self-assemble into fibers, rods, ribbons, or other aggregates of different morphologies in suitable liquids through hydrogen bonding, π-π stacking, London dispersion forces, and other types of van der Waals interactions, leading to the formation of three-dimensional networks which are needed to form a gel. The weakness of these interactions, compared with covalent bonds, makes these gels intrinsically more likely to be stimulus-responsive than polymer-based gels. Stimuli including heat, light, ultrasound, shearing stress, and so on, have been demonstrated to affect the sol-gel transitions of LMMG-liquid mixtures.With the rapid development of molecular gels, there is a growing interest in functional gels, in particular, fluorescent gels. This is because remarkable variation in the profile of the fluorescence spectrum of a fluorescent gel may accompany a gel-to-sol or sol-to-gel phase transition process, which may provide crucial information at a molecular level about the structure changes during the phase transition process. LMMGs can be classified into LMHGs (low-molecular mass hydrogelators) and LMOGs (low-molecular mass organogelators) according to the solvents gelated. A large number of LMHGs and LMOGs have been created during the last two decades. However, only a few of them function as both LMHGs and LMOGs, which are called ambidextrous gelators. Carbohydrates provide a rich library of water-soluble, chiral building blocks that have been used successfully in the design of both LMOGs and LMHGs. The sugar-based fluorescent LMMGs bearing both fluorescent groups and the carbohydrate moieties have attracted growing attention due to their unique properties, and have also been applied in some fields. However, the research work in this field is still in its infancy. A great effort is still needed urgently to understand the correlation between the gelation and the structures of the LMMGs and the liquids. According to the literature and as a continuation of our efforts in the search of new LMMGs with superior properties, we chose polycyclic aromatic hydrocarbons, α,ω-diaminoalkanes and gluconic acid, as key components to design and prepare purposely a series of sugar-based fluorescent low-molecular mass compounds. Their gelation properties in water and many organic solvents were studied systematically. The LMMGs containing shorter spacers were hydrogelators, those with somewhat longer spacers were ambidextrous, and the ones with the longest spacers were organogelators only. And we also got some supergelators and some thixotropic gels. Several LMMGs showed rich self-assembly behaviors in the mixtures of tetrahydrofuran (THF) and water. A variety of techniques have been applied to analyze the dependence of the self-assembly processes on the structures. The thesis is composed of the following three parts.In the first part, five novel glucose-based naphthalene derivatives with linkers containing α,ω-diaminoalkanes (Nn, where n, the number of methylene units separating the amino groups,0,2,3,4, and6) were designed and prepared. The gelation test revealed the following points:(1) within the30solvents tested, NO gelates water only;(2) in contrast, N2gels not only water, but also11of the organic solvents tested, a typical ambidextrous gelator;(3) N3, N4, and N6, however, gelate organic solvents only, and the numbers of solvents gelled are11,11and13, respectively. Interestingly, N6is a super gelator to acetonitrile, of which the critical gelation concentration is only0.07%, w/v. Clearly, five Nn are effective low-molecular mass gelators, and show transitions from a low-molecular mass hydrogelator to an ambidextrous gelator and then to low-molecular mass organogelators with a slight increase in the length of the spacers. The morphology, microstructure and molecular aggregation of the system strongly depend on the transition, as revealed by SEM, contact angle, energy dispersive X-ray spectroscopy, and XRD measurements. More interestingly, an aggregation-induced enhanced emission was observed along with gelation. Furthermore, the system appeared as a supramolecular chiroptical switch in the sol-gel process that is the chirality disappeared when the gel was heated to solution, whereas it reappeared when cooled to a gel.In the second part, a series of glucono-appended1-pyrenesulfonyl derivatives containing α,ω-diaminoalkane spacers (Pn, n=2,3,4,6,7and8) have been prepared. The gelation behaviors have been examined in30liquids, and the properties of their gels and the gelation mechanisms have been investigated using a variety of techniques. Possible reasons are discussed regarding why the Pn are better gelators than the corresponding naphthyl analogues (Nn) which had been investigated previously. Careful analyses of correlations between the structures of these molecules and their gels have provided important insights into the factors responsible for one-dimensional aggregation of small molecules containing both lipophilic and hydrophilic parts. P2and P3are ambidextrous gelators, and P4-P8gelate some organic liquids which are protic and aprotic, but not water. In at least one of the liquids examined, P3, P4, P6, P7, and P8form gels at less than1w/v%concentrations, and some of the gels in1-decanol are thixotropic. Analyses of the gelation abilities using Hansen solubility parameters yield both qualitative and quantitative insights into the role of liquid-gelator interactions. For example, the critical gelation concentrations increase generally with increasing polar and hydrogen bonding interactions between the gelators and their liquid components. As revealed by FT-IR,1H NMR, UV-vis, and fluorescence spectra, hydrogen-bonding between glucono units and π-π stacking between pyrenyl groups are important in the formation and maintenance of the gel networks.In the third part, the self-assembly behaviors of Pn in v:v THF:water mixtures is examined at room temperature. The Pn at2w/v%concentration do not dissolve in either THF or water at room temperature. However, they can be dissolved in some THF:water mixtures, and they form gels spontaneously in other compositions at room temperature without heating and cooling. The self-assembly of the Pn in the liquid mixtures has been investigated using a variety of techniques. The particle sizes of the Pn in their solutions/sols, critical gelation concentrations, microstructures, thermal and mechanical stabilities of the gels, and molecular packing modes of Pn molecules in their gel networks are found to be very dependent on the composition of the liquid mixtures. Correlations between the self-assembly behavior of the Pn and the polarity of the liquid mixtures, as probed by Ej(30) and Hansen solubility parameters, yield both qualitative and quantitative insights into why self-assembly of the Pn can or cannot be achieved in different liquid compositions. As revealed by UV-vis and fluorescence spectroscopy studies, π-π stacking of the pyrenyl groups occurs as part of the aggregation process. Correlations between the rheological properties of the gels and the Hansen solubility parameters of the Pn and the solvent mixtures indicate that hydrogen-bonding interactions are a major contributor to the mechanical stability. Overall, the results of this study offer a new strategy to investigate the balance between dissolution and aggregation of molecular of the gelators. To the best of our knowledge, this is the first example of the spontaneous formation of molecular gels without heating by placing gelators in mixtures of liquids in which they are insoluble in the neat components. |
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