The role of intermolecular interactions in solid state fluorescent conjugated polymer chemosensors

A functionalized fluorescent conjugated polymer, tolylterpyridine poly(p-phenyleneethynylene-thienyleneethynylene (ttp-PPETE), was designed and synthesized to detect trace amounts of toxic transition metal pollutants in ground water. Photophysical studies in tetrahydrofuran (THF) successfully demonstrated this polymer as a selective and sensitive chemosensor for Ni(2+) and Co(2+) in aqueous solution. Solid state composites of these chemosensors have now been prepared which can be modified to provide for inexpensive and portable field based chemical detection. A solid composite of ttp-PPETE, blended with poly (methyl methacrylate) shows UV-vis absorption and fluorescence emission spectra which are red- shifted when compared to solution phase spectra, suggesting an increase in conjugation in the solid state. An additional absorption peak, not present in solution, is also observed in the solid state. The presence of this new peak provides evidence of interacting FCP chains in the solid state. Concentration dependent experiments were done on the solid composite showing red-shifted emission peaks accompanied by a significant reduction in the fluorescent quantum yield. These observations are consistent with the formation of aggregated polymer species in the solid state. Intermolecular interactions of this type can be manipulated in the design of sensitive and selective solid state fluorescent conjugated polymer sensors.