Fatima Juniva Jaramiillo Garcia

A systematic study of the influence of solvent and the size of C(3)-symmetric discotics on their supramolecular polymerization mechanism is presented. The cooperativity of the self-assembly of the reported compounds is directly related to their gelation ability. The two series of C(3)-symmetric discotics investigated herein are based on benzene-1,3,5-tricarboxamides (BTAs) and oligo(phenylene ethynylene)-based tricarboxamides (OPETAs) that are peripherally decorated with achiral (1a and 2a) or chiral N-(2-aminoethyl)-3,4,5-trialkoxybenzamide units (1b and 2b). The supramolecular polymerization of compounds 1a,b and 2a,b has been exhaustively investigated in a number of solvents and by using various techniques: variable-temperature circular dichroism (VT-CD) spectroscopy, concentration-dependent (1)H NMR spectroscopy, and isothermal titration calorimetry (ITC). The supramolecular polymerization mechanism of compounds 2 is highly cooperative in solvents such as methylcyclohexane and toluene and is isodesmic in CHCl(3). Unexpectedly, chiral compound 1b is practically CD-silent, in contrast with previously reported BTAs. ITC measurements in CHCl(3) demonstrated that the supramolecular polymerization of BTA 1a is isodesmic. These results confirm the strong influence of the π-surface of the central aromatic core of the studied discotic and the branched nature of the peripheral side chains on the supramolecular polymerization. The gelation ability of these organogelators is negated in CHCl(3), in which the supramolecular polymerization mechanism is isodesmic.

Triangular-shaped oligo(phenylene ethynylene) amphiphiles 1a and 1b decorated in their periphery with two- and four-branched hydrophilic triethyleneglycol dendron wedges, have been synthesized and their self-assembling properties in solution and onto surfaces investigated. The steric demand produced by the dendritic substituents induces a face-to-face rotated pi stacking of the aromatic moieties. Studies on the concentration and temperature dependence confirm this mechanism and provide binding constants of 1.2 x 10(5) and 1.7 x 10(5) M(-1) in acetonitrile for 1a and 1b, respectively. Dynamic and static light scattering measurements complement the study of the self-assembly in solution and demonstrate the formation of rod-like supramolecular structures in aqueous solution. The nanofibers formed in solution can be efficiently transferred onto surfaces. Thus, TEM images reveal the presence of strands of various thickness, with the most common being several micrometers long and with diameters of around 70 nm. Some of these nanofibers present folded edges that are indicative of their ribbon-like nature. Interestingly, compound 1b can also form thick filaments with a rope-like appearance, which points to a chiral arrangement of the fibers. AFM images under highly diluted conditions also reveal long fibers with height profiles that fit well with the molecular dimensions calculated for both amphiphiles. Finally, we have demonstrated the intercalation of the hydrophobic dye Disperse Orange 3 within the filaments and its subsequent release upon increasing the temperature.

The rectangular oligo(phenylene ethynylene) amphiphile 1 has been synthesized to investigate its self-assembling features in solution and onto surfaces. Concentration-dependent and variable-temperature NMR experiments firstly demonstrate the influence of the solvent in the stabilization of the non-covalent forces involved in the association of 1, namely, pi-pi stacking interactions between the aromatic fragments and van der Waals, hydrogen-bonding and/or solvophobic forces between the triethyleneglycol chains. This subtle balance of non-covalent interactions also conditions the thermodynamics of the self-assembly process and concentration-dependent UV/Vis investigations show a linear correlation between the polarity of the solvent and the K(a) values (K(a) approximately 5.2 x 10(5) M(-1) for CH3CN/H2O mixtures and 4.4 x 10(4) M(-1) for benzene). Moreover, these UV/Vis studies prove the organization of this compound following the indefinite self-association model. Microscopy techniques reveal that the morphology and dimensionality of the assemblies formed from 1 can be finely modulated. Although polar solvents yield hollow vesicles or toroidal 3D objects, depending upon concentration, the utilization of non-polar benzene results in the formation of unimolecular wires that can grow to form networks upon increasing concentration. These findings support the direct relationship existing between the self-assembling features of this amphiphile in solution and onto surfaces.

A detailed investigation of the self-assembly behaviour of C3-symmetrical tricarboxamides reveals that a larger π-conjugated core does not increase the stability of assemblies in an apolar solvent but makes the system more sensitive to destabilization by addition of a good solvent.