Development Of Fluorescent Probes For Bisulfite And Sulfide Anion

It is well-known that2-(2-hydroxy-3-methoxyphenyl)benzothiazole (HMBT) can undergo an excited-state intramolecular proton transfer (ESIPT) from the acidic (hydroxyl proton) to the basic site (aromatic nitrogen) when photoexcitation changes their charge density distribution. In the ground state, the most stable form of HMBT is usually the enol form. However, in the first excited singlet state, in which the hydroxyl group is much more acidic and the nitrogen much more basic than in the ground state, the most stable structure is usually the keto tautomer. Fluorescein, the common fluorescent dye, has long excitation wavelength and higher fluorescence quantum yield. However, upon attached to the α, β-unsaturated ketone moiety, it gives weak fluorescence emission because of the photoinduced electron transfer (PET) quenching of the fluorophore by the intramolecular carbon-carbon double bond. Based on modulation of the ESIPT process of HMBT through the hydroxyl group protection/deprotection reaction, benzothiazole-based probes were developed to detect HSO3-and S2-. Meanwhile, based on modulation of the PET process of fluorescein, a novel fluorescent probe for S2-was developed. This dissertation consists of four chapters summarized as follows:In chapter1, a general introduction to the research progress on the detection of bisulfite and sulfide ions was presented. Based on referring to the relative literatures, the objective of this dissertation was proposed.In chapter2, a benzothiazole-based ratiometric fluorescent probe for HSO3-was designed and synthesized. Once the hydroxy of the HMBT was protected by a levulinoyl group, the ESIPT process was switched off, and the probe shows only fluorescence emission maximum at372nm. Upon treatment with HSO3-, the protective group of probe was removed readily and ESIPT of the probe was switched on, which resulted in a decrease of the emission band at475nm and an increase of a new fluorescence peak around372nm. The fluorescent intensity ratio at475and372nm (I475/I372) increases linearly with bisulfite concentration in a certain range. Based on this mechanism, a ratiometric fluorescent probe for HSO3-was developed.In chapter3, a new benzothiazole derivative (5) has been developed for the selective detection of S2-in aqueous media. Compound5is a non-fluorescent compound. As the hydroxy of benzothiazole was protected, the ESIPT process was switched off. Upon mixing with S2-, the protective group of probe was removed and ESIPT of the probe was switched on, which resulted in an increase of the emission at478nm. Based on the above mechanism, a fluorogenic method for the determination of sulfide ions was developed. The new fluorescent probe shows an excellent selectivity for sulfide anion over other anions.In chapter4, a new fluorescein-based S2--selective fluorescent probe was designed and synthesized. The fluorescence of this probe is significantly enhanced due to the conjugate addition of S2-to the α, β-unsaturated carbonyl moiety, thus eliminating the photoinduced electron transfer (PET) quenching of the fluorophore by the intramolecular carbon-carbon double bond. Based on this mechanism, a fluorogenic method for the determination of S2-was developed.