Synthesis Of Perimidinium Based Receptors For Anion Recognition

Fluoride anion (F"), bearing a small ionic radius, high charge density, and high hydration enthalpy, displays special chemical and physical properties among the inorganic anions, such as its hard Lewis basic nature. It plays a fundamental role in a number of chemical and biological processes and is commonly found in daily life. For example, fluoride is used as additive in toothpaste to prevent dental caries and enamel demineralization, and it is also found in osteoporosis drugs and drinking water. Fluoride, as important chemicals, is also widely used in chemical fertilizers, pesticides, chemical industry, petrochemical industry and other industries. Due to its possible toxicity and pollution to the environment, the development of synthetic fluorescent sensors for its detection has attracted considerable attention in the field of modern supramolecular chemistry.The imidazolium analogues, perimidinium derivatives, appear as ideal candidates for sensors for F-, since:(1) like the imidazolium unit, the C(2)H of perimidinium is easily deprotonated by F- to form a carbene due to its inborn acidity, (2) the relatively high degree of π-conjugation of the aromatic rings could give rise to a visible photophysical property, and (3) more importantly, the anion-binding site directly embedded into the fluorophore should make the signaling response of the sensor more sensitive to both the anion interaction and also to the carbene formation. Based on these characteristics of perimidinium derivatives, we hypothesized that a receptor combining perimidinium unit with an appropriate optically responsive fragment should provide an original visible optical sensor for F-. The dissertation mainly consists of the following four chapter.Chapter 1:Preface. A concise introduction of fluoride anion and a review about the research status of fluoride ion recognition were formulated in this chapter.Chapter 2:Synthesis of monoperimidinium-based receptors 1 and 2 for Anion Recognition. In order to avoid the interference factors from complex chemical structure during characterization and the mechanism research, we synthetized two momoperimidinium-based chemodosisensors 1 and 2 with simple organic structure. The anion-binding performance of chemodosisensors 1 and 2 towards F- was evaluated by the changes in absorption and fluorescence spectra upon addition of the tetrabutylammonium (TBA) F- salt. The experimental results show that two monoperimidinium derivatives developed as efficient colorimetric and fluorescent chemodosisensors for F- in DMSO or more competitive media (DMSO containing 10% water). In the presence of F-, the yellow and non-fluorescent solution of 1/2 became colourless and exhibited strong blue fluorescence. Chemodosisensor 1 exhibited a lower detection limit, but the superior selectivity displayed by chemodosisensor 2 cannot be neglected. Based on the NMR titrations of chemodosisensors with TBAF and the separation of new products perimidone 8 and dihydroperimidine 9 after compound 7 gradually decomposed, a mechanism for the detection of F- with chemodosisensor 1/2 was proposed. The counterion Cl-/Br- of 1/2 is initially exchanged by F-, followed by deprotonation to form N-heterocyclic carbene 6, which subsequently reacts with water to generate the colourless and fluorescent carbinol 7, and finally the intermediate 7 undergoes a redox disproportionation to give the fluorescent dihydroperimidine 8 and non-fluorescent perimidone 9. From further verification experiment of NMR titration by AgF, instead of TBAF, we found carbine 6a finally result in its dimerization to form the olefin 11a. The free carbene 6a was also successfully trapped by elemental sulfur. All of these additional experiments unequivocally confirm the proposed sensing process between F- and the perimidinium-based chemodosisensor 1/2.Chapter 3:Synthesis of diperimidinium-based receptor 14 for Anion Recognition. We synthetized a novel diperimidinium-based chemodosisensor 14 with clamp structure to improve the binding ability of the anion. After adding F-, the yellow solution of 14 became colourless, but kept non-fluorescent. The chemodosisensor 14 can only colorimetric sensing F- by the naked eye. By comparing the difference of NMR between chemodosisensor 14 and chemodosisensor 14 in the presence of F", we found similar chemical change from chemodosisensor 14 to carbinol 16. Pyridine, as an electron rich group, impeded the process of photoinduced electron transfer (PET). So carbinol 16 is inhibited to generate fluorescence.