Small Molecule And Conjugated Polymer Fluorescent Chemosensors Based On Aromatic N-heterocycle

In recent years, fluorescent chemosensors have attracted much attention because of their simplicity, high sensitivity, high selectivity, and real-time detection. Many sensors and receptors have been reported to detect various analytes, such as metal ions, anions, and amino acids.8-hydroxy-2-methylquinoline is non-fluorescent owing to the efficient radiationless relaxation to the ground state by intra and intermolecular excited-state proton transfer, and it is a well-known building block for chelation enhanced fluorescent sensors for transition metals.2,2’-Biimidazole has excellent coordination ability and diverse coordination modes with metal ions, however, there are no reports on using2,2’-biimidazole to design and prepare fluorescent chemosensors. In this thesis, we designed and synthesized three new fluorescent chemosensors based on8-hydroxy-2-methylquinoline, and developed a new conjugated polymer sensing platform based on2,2’-biimidazole for detection of metal ions, anions, and amino acids. This dissertation includes five parts:1. A CHEF (chelation-enhanced fluorescence) based ratiometric fluorescent chemosensor with conjugated8-hydroxy-2-methylquinoline moiety has been successfully prepared. The sensor shows excellent selectivity to Cd2+avoiding the interference from other metal ions and the detection limit is down to nanomolar range concentration in aqueous solution. The sensor exhibits very fast response, and there is no time dependence effect in the sensing process. The sensor can be used in a wide pH range, which favours its application in physiological system and basic environment. Moreover, it can distinguish Cd2+from Zn2+with both fluorescence intensity and emission shift, which is based on the different coordination modes, and can be obviously observed by naked eyes.2. A colorimetric and ratiometric fluorescent chemosensor for fluoride anion has been preparedand investigated. It was found that upon interaction with fluoride ions, the sensor shows obvious changes of color from colorless to yellowish-green, meanwhile the blue fluorescence turns yellowish-green with enhanced intensity, which make it possible to detect the fluoride not only from the spectroscopic changes but also from the color changes. Since the sensor exhibits very fast response, almost with no time dependence in the sensing process, it provides a rapid detection of fluoride anion in THF and organic aqueous solution. The sensor possesses high sensitivity to fluoride with a detection limit below1.0μM, and excellent selectivity avoiding the interference from other anions. In addition, the facile synthesis of the sensor with low cost reagents makes the sensor readily accessible. Thus, the new "naked eyes" dual signaling chemosensor is indeed an exellent candidate for fluoride detection. To the best of our knowledge, this is the first report that the mechanism of a fluoride chemosensor is based on combination of desilylation reaction and excited state proton transfer from the desilylation product to fluoride.3. We have successfully designed and synthesized a novel bifunctional fluorescent chemosensor for detection of F" and Zn+with dual output modes, which is based on a combination of F-induced desilylation reaction and chelation-enhanced fluorescence effect (CHEF) caused by Zn2+. The sensor shows excellent selectivity for F" over other anions, and the deprotection product in situ exhibits high selectivity to Zn2+with a nanomolar range detection limit. The fluorescent sensing process can be obviously observed by naked eye under UV illumination, and the sensing properties of sensor to F-and Zn2+can be efficiently tuned by varying the solvents.4. Two novel conjugated polymers containing2,2’-biimidazole and carbazole moiety have been successfully designed and synthesized through Suzuki coupling reaction and their sensitivity to metal ions and amino acids have been demonstrated. Based on the unique interaction of Ag+with the polymers, the two conjugated polymer-Ag+complexes have been demonstrated to be potential ratiometric fluorescent sensors for detection of cysteine (Cys). Furthermore, upon interaction with Cys, the sensors exhibit obvious fluorescence intensity changes as well as wavelength shift. The sensors possess high sensitivity to Cys with nanomolar range detection limits, and excellent selectivity avoiding the interference from other amino acids. Since the sensors display good reversibility and very fast response, it provides a rapid and efficient detection of Cys. In addition, the facile preparation of the2,2’-biimidazole based monomers under mild conditions makes the sensors more accessible compared with the other N-heterocycle based conjugated polymers. Thus, the conjugated polymers can be used as a novel fluorescent sensing platform. To the best of our knowledge, this is the first report using a conjugated polymer-Ag+complex as a fluorescent sensor to selectively detect Cys.5. We have successfully designed and synthesized three novel conjugated polymers based on2,2’-biimidazole and alkoxybenzene through the Suzuki coupling reaction, and also investigated their ability to sense metal ions and anions. The fluorescence of the two polymers with hydrophilic ether side chains can be efficiently quenched by Cu2+ions through a photoinduced electron transfer process. Based on the unique interaction of Cu2+with the polymers, the two conjugated polymer-Cu2+complexes have been demonstrated to be potential "turn on" fluorescent sensors for detection of pyrophosphate anion (PPi). The sensors possess high sensitivity to PPi with the detection limit of about0.17ppm, and excellent selectivity avoiding the interference from other anions. And the linear detection range of PPi can be tuned conveniently by changing the amount of Cu2+ions. Since the sensors display very fast response (less than3minutes), this affords a rapid and efficient approach for the detection of PPi. The experiment results show that the fluorescence sensing properties of the polymers can be efficiently optimized through the reasonable structure modification. Moreover, the synthesis of2,2’-biimidazole-based conjugated polymers is more accessible under mild conditions compared with the other N-heterocycle based conjugated polymers. To the best of our knowledge, this is the first example utilizing the2,2’-biimidazole based conjugated polymers to detect PPi.