Design, synthesis and photophysics of dipyrrolequinoxaline receptor loaded fluorescent conjugated polymer chemosensors

This dissertation explores the systematic design, synthesis, characterization and application of fluorescent conjugated polymers as "turn-off" anion chemosensors. We have described recent synthetic efforts to prepare monomeric and polymeric chemosensors based on a pendant and a non pendant dipyrrolequinoxaline (DPQ) receptor unit on a fluorescent conjugated polymer backbone for fluorescence anion sensing capabilities. Conjugated polymers have been known to exhibit enhanced sensitivity due to energy migration along the polymer backbone. Based on this concept, two structurally different model monomeric DPQ chemosensors were designed and synthesized in chapter 2 of this dissertation. Their structural system was utilized in synthesis of a series of new fluorescent conjugated polymers with varying percentage loading of dipyrrolequinoxaline (DPQ) in chapter 3 and 4. The polymer backbones were incorporated with pendant (PDPQ-PPETE) and non-pendant (NDPQ) dipyrrolequinoxaline receptors. They were fully characterized by spectroscopic methods, Gel Permeation Chromatography (GPC) and evaluated as fluorescent chemosensors in solution for the detection of fluoride and cyanide anions.;The monomeric and polymeric chemosensors exhibited very bright fluorescence emission that was observed to quench in the presence anions, specifically fluoride and cyanide. The fluorescent quantum yield and lifetime of the chemosensors were measured at room temperature in tetrahydrofuran. Emission lifetime provide information on how long an excited species stays in the excited state before it returns to the ground state. The emission decay of the monomers and polymers were best fitted to a single exponential and biexponential function respectively. The excited states exhibited short emission lifetimes that are typical of singlet state emission a characteristic of strongly allowed transitions. The data obtained form fluorescence emission quenching experiment was treated quantitatively by Stern-Volmer analysis. The Stern-Volmer plot allowed the determination of the mechanism of quenching in our chemosensor systems. There was no change in the lifetime of the sensors when they were titrated with anions an indication that the mechanism of quenching was static. The polymers displayed the highest sensitivity towards anions, specifically fluoride and cyanide. The acidic pyrrole N-H protons serve as the receptor unit in each of the chemosensor and evidence of deprotonation of the N-H proton was observed in the NMR titration of the monomeric sensors. The measured fluorescence lifetime for the polymer series synthesized in chapter 4 decreased in value with decreasing percent loading of DPQ receptors, as the fluorescence quantum yield increased. The deviation from the expected trend can be attributed to stronger interaction between the excited state and the ground state wave functions in the lower percent loaded polymers resulting in shorter fluorescence lifetimes.;In chapter 5 a molybdenum-riboflavin (RF-Mo) complex has been synthesized. A powder XRD analysis showed that RF-Mo has an orthorhombic cubic system. The RF-Mo complex can be used as a model to envisage an in vitro interaction where Mo(V), necessary for the function of some important enzymes in human body is bound to the isoalloxazine moiety of RF, an essential nutrient. The complex was also subjected to various spectroscopic and photophysical studies. Fluorescence emission quenching in the presence of HCl was observed and the data was treated by Stern-Volmer analysis.