Fluorescent chemosensors based on energy migration in conjugated polymers: The molecular wire approach to increased sensitivity

We demonstrate herein how conjugated polymers (molecular wires) can be used to interconnect (wire in series) receptors to produce fluorescent chemosensory systems with sensitivity enhancements over single receptor analogues. The enhancement mechanism in the polyreceptor materials is based on an energy migration scheme in which excitations, diffuse along the polymer backbone. Analyte binding produces trapping sites for the excitations which results in greatly attenuated emission intensity. Three different cyclophane-based receptor systems that bind paraquat were investigated. These systems are quenched by paraquat binding, and the quenching enhancements relative to a monomeric model compound were used to determine the efficiency of energy migration. Two polymers with related poly(phenyleneethynylene) structures were investigated, and the all-para system was found to exhibit more facile energy migration than the more electronically localized analogue that contained meta linkages. The para polyreceptor system was found to display a 65-fold enhancement in sensitivity to paraquat as compared to a model monoreceptor fluorescent chemosensor. However, we have determined that delocalization alone is not sufficient to produce facile energy migration, and the more delocalized polythiophenes appear to be less effective at energy migration than the para poly(phenyleneethynylene) material. Paraquat-induced fluorescent quenching studies on homologous polymers that lacked the cyclophane receptors were also performed. These results indicate that diffusive quenching by paraquat is enhanced by energy migration.