Fluorescent Sensors Based On Thioxanthone And Fluorene Derivatives

Owing to the operational simplicity, low cost, real time monitoring and high sensitivity, fluorescence chemosensors have become the dominant strategy for molecular recognition such as metal ions, anions, mercaptans, reactive oxygen species, amino acids and explosives. Over the years, fluorescent chemosensor systems and molecular structures, along with sensing mechanisms display the diversity of development. Some important performance parameters have been the research focus and difficulties, for instance, sensitivity, selectivity, response time, water solubility, complexity of synthesis and application in the near infrared region.Fluorescence chemosensor typically has a configuration of "signalling unit-spacer-binding site". In this thesis, we describe the rational design and synthesis of series of small molecular compounds which contain thioxanthone, fluorescein and fluorene groups serving as signalling unit. Through the method of symmetric and asymmetric side chain modifications, and also introduction of various receptor units, we regulated and controled the compounds’ optical properties such as absorption and fluorescence emission spectra, and we also detailedly studied their sensing abilities.In Chapter1, the concepts of supramolecular chemistry and molecular recognition are briefly introduced. Structures and the basic mechanisms of fluorescent chemosensor are mainly described, and the latest research progresses of fluorescent chemosensors for Hg2+are briefly reviewed. Then the research subject of this dissertation is proposed.In Chapter2, two thioxanthone-based fluorescent probes whose fluorescence properties exhibited intriguing dependence on solvent polarity were developed. In the "D-A-D" structural framing of BDPA-TXO and BTPA-TXO, the diphenylamine and triphenyl-amine groups were the donor part while the carbonyl group belonging to thioxanthone served as the acceptor part. Furthermore, the probes showed fluorescence quenching by addition of water in organic solvents. Consequently, they were found useful as fluorescence indicators for the qualitative and quantitative detection of low-level water in various solvent media. The detection limit of BDPA-TXO in dioxane reached to19ppm.In Chapter3, two thioxanthen-9-thione derivatives (DP-TXT and BDPA-TXT) were designed and synthesized based on Hg2+-induced desulfurization mechanism. Both of them can serve as naked-eye colorimetric probes for Hg2+:in the absence and presence of Hg2+, the solution color changed from orange to colorless, purple to yellow, respectively. DP-TXT in CH3CN-H2O (5:5, v/v) and BDPA-TXT in DMSO-H2O (9:1, v/v) showed significant fluorescent enhancement upon addition of Hg2+, and the detection limits for Hg2+of chemodosimeters DP-TXT and BDPA-TXT were determined to be21nM and75nM, respectively. Both of DP-TXT and BDPA-TXT exhibited specific selectivity for Hg2+over other examined metal ions except the little interference of Ag+for BDPA-TXT.In Chapter4, hydrazinecarbothioamide recognition group was introduced into thioxan-thone system, and two novel fluorogenic and chromogenic chemosensors X1and Y1for the dual-channel detection of Hg2+and F-were developed. Addition of Hg2+and F-resulted in significant changes in absorption spectra of X1and Y1in tetrahydrofuran solutions. F-leaded to a red shift of maximum absorption wavelength, and the solution color changed obviously from colorless to yellow. Furthermore, owing to the formation of a1:1complex between X1or Y1and Hg2+, as well as the deprotonation process of the thioamide protons induced by F-considerable fluorescent quenching was observed for X1and Y1after addition of Hg2+or F-In Chapter5, four novel Hg2+-selective fluorescent chemodosimeters featuring the thiourea moiety as the recognition unit were successfully designed by modifying the substituent groups of fluorene fluorophore, and the substitution of acetyl group with nitro group led to large bathochromic shifts both in absorption spectra and fluorescence spectra. They all exhibited specific sensitivity and selectivity not only for Hg2+but also CH3Hg+over other examined metal ions. The changes in their spectral properties are attributed to the transformation of thiourea unit to guanidine via Hg2+-induced and CH3Hg+-induced desulfurization reaction.In Chapter6, a fluorescein-based water-soluble chemosensor (F1) for Cu2+was designed and synthesized. It showed excellent absorbance and fluorescence response towards Cu2+, especially the addition of Cu2+induced significant fluorescence quenching. The recognition mechanism is the formation of a1:1complex between F1and Cu2+. The association constant and detection limit were determined to be3.8×105M-1and2.8×10-8M, respectively.Chapter7, conclusions.