Stimulus-Responsive Supramolecular Nanostructures

Abstract

Owing to their intriguing C3-symmetry and three amide groups that can participate in hydrogen bonding, benzene-1,3,5-tricarboxamide (BTC) derivatives have been actively studied as supramolecular scaffolds for the generation of columnar structures. The strong three-fold intermolecular hydrogen bonding between amide groups and the intermolecular interactions caused by π-π stacking between central benzene rings are the driving forces for the formation of one dimensional nanostructures. In this dissertation research, novel stimulus-responsive BTC derivatives are described. Specifically, we have introduced hydrophilic benzocrown ether moieties to the core BTC structure. Three benzocrown ether-containing BTC derivatives 17-19 were found to display lowest critical solution temperature (LCST) behavior. Considering the fact that most of nonpolymeric organic molecules tend to display increased solubility at higher temperatures, the temperature dependent reversible solubility change observed with the crown ether containing BTC derivatives described in this work are very remarkable. Among the substances tested, the benzo-18-crown-6 substituted BTC 19 was found to display the most reliable LCST behavior. In addition, the observation of enhanced fluorescence in their aggregate states indicates that the mobility of benzocrown ether substituted molecules is reduced when water is expelled from the flexible crown ether moieties. It is believed that the LCST behavior of BTC derivatives investigated in this study is a result of the removal of water molecules from crown ether moieties at elevated temperatures. At room temperature, the removal of water from BTC occurs slowly and leads to the formation of long nanofibers. This dissertation study also has led to the development of BTC-derived photoresponsive supramolecules. Thus, the supramolecularly assembled BTC derivative 20 containing photoresponsive azobenzene moieties displayed a photoinduced reversible phase transition (fiber-to-melting state). Patterend images were readily obtained by a photomasked UV irradiation of the BTC 20 supramolecules. Solvent- and ion-induced morphological transformation of BTC 20-derived supramolecuels is also probed. Interestingly, the BTC 20 shows morphological diversity ranging from fibers and gels to spheres depending on the fabrication conditions employed for supramolecular assembly. In addition and quite surprisingly, self-assembly of the azobenzene derivative 20 in aqueous THF results in the formation of hollow spherical structures. In order to investigate the effect of functional groups (amide, azo, phenyl groups) in BTC 20-derived supramolecules, structural analogues 21-24 were investigated. A suramolecular system having bis(dipeptides) were prepared to probe specific molecular interactions. Thus, a bis(D-Ala-D-Ala)-containing biphenyl derivative was found to form fluorescent microfibers when subjected to self-assembly conditions. Incubation of the microfibers with vancomycin results in the disappearance of the microfibers along with a significant decrease in the fluorescence intensity, an observation which forms the basis for the rational design of chemosensors from an AIEE-displaying molecule. Considering the fact that the majority of the AIEE-based sensor systems reported to date have been designed to probe mostly nonspecific interactions (eg. binding of dye molecules to hydrophobic pockets in proteins or electrostatic macromolecule-dye aggregate formation), the results described in the dissertation study should be a very important finding in the AIEE-based chemosensors. The results of stimulus-responsive supramolecules described in this dissertation research should be important contributions to ever increasing functional supramolecules.