Supramolecular Rings as Building Blocks for Stimuli-Responsive Materials

Stimuli-responsive polymers are of great interest due to their ability to translate changing environmental conditions into responses in defined materials. One possibility to impart such behavior is the incorporation of optically active molecules into a polymer host. Here, we describe how sensor molecules that consist of a pi-extended benzothiadiazole emitter and a naphthalene diimide quencher can be exploited in this context. The two optically active entities were connected via different spacers and, thanks to attractive intramolecular interactions between them, the new sensor molecules assembled into cyclic structures in which the fluorescence was quenched by up to 43% when compared to solutions of the individual dyes. Detailed spectroscopic investigations of the sensor molecules in solution show that the extent of donor/acceptor interactions is influenced by various factors, including solvent polarity and ion concentration. The new sensor molecule was covalently incorporated into a polyurethane; the investigation of the optical characteristics in both the solid and solvent-swollen states indicates that a stimulus-induced formation of associated dye pairs is possible in polymeric materials. Indeed, a solvatochromic quenching effect similar to the behavior in solution was observed for solvent-swollen polymer samples, leading to an effective change of the green emission color of the dye to a yellow color.