Theoretical Study Of Fluorescent Chemosensors For Fluoride Anion

Molecular recognition is one of the important topics in supermolecular chemistry, which belongs to the three basic functions of supermolecular systems, i.e. molecular recognition, molecular transport and chemical reaction. The central task is designing and synthesizing chemosensor, which refers to a molecule that can selectively sense and signal the presence of a matter or energy. As compared with cation species, chemosensing for anion is more challenging, thus less developed. As the smallest anion, fluoride has unique chemical properties and its recognition and detection are of growing interest because of its important role in dental caries and the treatment of osteoporosis in biology, medicine, food, and environmental sciences. In recent several decades, various kinds of novel chemosensors for fluoride have been developed, which show us great performance for the fluoride recognition. However, due to the fact that fluoride, as the most electronegative atom, displays a high hydration enthalpy, most the applications have been restricted to organic aprotic solvents. It is necessary for us to make great efforts to extend the study into protic solvents, especially water.In the present dissertation, we take two representative types of fluorescent chemosensor for fluoride (H-bond and Lewis acid) as models and investigate the host-guest interactions between chemosensor and fluoride together with colorimetric and fluorescent signaling mechanisms in detail by quantum chemical calculation methods. During the work, we apply counterpoise method for the first time in the gradient optimizations and second-derivative frequency calculations of halide H-bond complexes to obtain more accurate geometries under basis set superposition error (BSSE) correction, which produced better results. Moreover, the average charge difference density (CDD) is firstly used to characterize the nature of electronic transition in systems with doubly degenerated lowest-lying excited states.For the H-bond-typed fluoride chemosensor model (4-benzoylamido-N-butyl-1,8- naphthalimide, 1), strong H-bond interaction between the amido hydrogen and fluoride induces the deprotonation of the host sensor, while for chloride and bromide anions it is not strong enough for deprotonation, which leads to the selectively chemosensing for fluoride. Through frontier molecular orbital (FMO) and CDD analysis, the intramolecular charge transfer (ICT) transition of the characteristic absorption and fluorescence can be found. The experimentally observed color and fluorescence changes before and after fluoride sensing can be ascribed to the promoted electron donating ability of the amide group and the strengthenedπ-conjugated molecular structure.For Lewis acid-typed fluoride chemosensor, we choose the novel organic borane (trianthrylborane, TAB) as an example and systematically investigate the details of fluoride chemosensing. For one thing, the coordination interaction between TAB sensor and fluoride anion is clearly shown. For the other, the 3D real space charge density analysis method has been developed because we have used the average CDD of two degenerated excited states to elucidate the nature of electronic transitions in TAB and its fluoridized TABF–, which vividly describes the color signaling. According to the available reports, TAB is simply working in organic aprotic solvents. However, as an important fluoride binding site, Lewis acidic boron binds fluoride forming strong coordination bond interaction, which is much more stronger than that of H-bond. As a result, the host chemosensor if appropriately designed may be expected to effectively compete with the protic solvents to lessen the probability of solvent interference, thus realize the selective chemosensing of fluoride in protic (water) environment.