Fluorescence Anion Sensing Based On Hydrogen Bonding And Proton Transfer

Anions play a fundamental role in a wide range of chemical and biological processes, and agreat deal of attention has recently been focused on the selective sensing of anions by meansof synthetic receptors. In particular, molecules containing polarized N-H fragments behave asH-bond donors toward anions and are widely used as receptors for recognition and sensingpurposes. At present, different kinds of fluorescent signaling mechanisms including internalcharge transfer (ICT), photoinduced electron transfer (PET), metal-to-ligand charge transfer(MLCT) and excimer/exciplex formation have been utilized in anion fluorescent sensordesign.In order to selectively differentiate between biological interest anionic substrates of similarbasicity and surface charge density, such as F^-, CH₃COO^- and H₂PO₄^- based on H-bondreceptor contained fluorescent sensors, the following endeavors have been made in this thesis:New anion fluorescence chemosensors based on PET mechanism have been developed. Bycombining the naphthalimide fluorophore and diamide or disulfonamide receptors, "Y"structure based PET anion sensors 2a and 2b are synthesized. ¹H-NMR titration spectraindicate that sensors form hydrogen bond complexes with fluoride in the ground states. In theexcited states, the anions can PET quench the fluorescence of sensors.New anion fluorescence chemosensors based on tuning the proton transfer have beendeveloped. By changing the electron properties of substituents on the phenyl para position,phenyl-1H-anthra[1, 2-d]imidazole-6, 11-dione (3a) and its derivatives (3b and 3c) have beeninvestigated as new colorimetric and ratiometric fluorescent chemosensors for anions. In theground states, a two-step process has been observed. In the excited states, the excited-stateintermolecular proton transfer makes a contribution to the deprotonation. A ratiometricfluorescence change is observed for the 3a and F^- interactions. The excellent selectivity of 3afor F^- is attributed to the fitness in the acidity of its NH-group.New anion fluorescence chemosensors based on inhibition of the excited-stateintramolecular proton transfer (ESIPT) have been developed. By introducing the receptorgroups with different hydrogen-bond donor properties on phenyl-benzoxazole,phenyl-benzothiazole and phenyl-benzimidazole fluorophores, new series of ESIPT sensors4a, 4b, and 5a-5d are developed. Detail investigations are conducted on the photophysics ofthe sensors in different solvents and the ESIPT inhibition mechanisms of the different anions. Further investigations are given on the factors such as the solvent effects and the number ofhydrogen bond donors of the sensors that affect the ESIPT inhibition by anions.