Design, Synthesis and Photophysics of Fluorescence "Turn-on" Conjugated Polymer Chemosensors
This dissertation explores the synthesis, characterization, and application of conjugated polymers as fluorescence “turn-on” chemosensors. A series of conjugated polymers using the poly[p-(phenyleneethynelene)-alt-(thienylene-ethylene)](PPETE) polymer backbone were prepared using N,N-diethylamino (dea) and N,N,N’-trimethylethylenediamino (tmeda) groups as receptors. The conjugated polymers were designed as fluorescence “turn-on” chemosensors based on a photoinduced electron transfer (PET) mechanism in which the polymer fluorescence is quenched in the absence of coordinating analytes. A chelation-enhanced fluorescence (CHEF) phenomenon results upon coordination of a cation to the redox active receptor as a result of termination of the fluorophore quenching process. The polymers were fully characterizated by NMR, FTIR, Gel Permeation Chromatography (GPC) and elemental analysis. Detailed photophysical studies of dea-PPETE and tmeda-PPETE demonstrated relatively weak emission at /Imax= 488 nm with quantum yields of 0.11 and 0.09. Room temperature emission studies show that tmeda-PPETE exhibited a fluorescence “turn-on” response in the presence of many cations at less than 500 nM concentrations. For example, Hg2+ in aqueous solution causes the fluorescence of tmeda-PPETE to increase by a factor of 2.7 at less than millimolar concentrations. This represents the first example of a conjugated polymer applied as a fluorescence “turn-on” chemosensor based on the PET mechanism. The competitive role of PET and energy migration is critical to sensor function. This was investigated by synthesizing a series of PPETE’s with different amino receptor loadings. Theoretical and experimental studies revealed that the limited sensitivity achieved in this system may be attributed to relatively slow energy migration (<109s-1) along the polymer backbone relative to the emissive lifetime (?10-10s). A highly selective and sensitive sensory system towards iron cations in solution was achieved by preparing a transition metal derivatized conjugated polymer based on Cu2+ quenching. By preloading Cu2+onto the receptor of tmeda-PPETE, fluorescence enhancements of 150 fold were achieved in the presence of 10 micromolar of iron cations . The combination of the conjugated polymer and metal cation created a selective sensor and offers a new paradigm for further sensor design.