טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentAbraham Yael
SubjectDesign, Preparation and Characterization of New Receptors
for Anions
DepartmentDepartment of Chemistry
Supervisor Professor Yoav Eichen
Full Thesis textFull thesis text - English Version


Abstract

The main aim of this research is to present various different approaches in the design of new anion receptors and their subsequent characterization. Despite the fact that the first anion receptor and the first cation receptor were both reported in the late 1960s, the research in the field of anion coordination straggled far behind that of cation for various reasons. Only in the last three decades the interest in the development of efficient and specific anion binding molecules has risen with the increasing realization of their importance in biology and chemistry.

Most of the anion receptors published were shown to incorporate groups capable of binding anions either through electrostatic interaction or through hydrogen bonds. Lewis acids have also been implemented to a far lesser extent.


First, we discuss new anion receptors based upon a novel, previously unused, anion binding moiety; the imidazole ring. We strived to characterize their ability to bind anions as a function of different conformations and solvents.

v      Tetraimidazole aryl-based (benzene and pyridine) anion receptors have been prepared. Both of the receptors have been found to bind anions with moderate efficiency, while the pyridine-based receptor has shown a surprising ability to bind anions also in protic solvents.

v      A calix[2]benzimidazole molecule, that has been prepared, shows a significant increase in its emission intensity following anion recognition. The unbound molecule was found to go through excited- state intramolecular proton transfer (ESIPT), which provides the molecule with a non-radiative relaxtion pathway. The binding of the anion stabilizes a tautomer in which the ESIPT pathway is blocked, hence the increase in the emission.


Next, we discuss tripodal thiourea-based receptors that display solvent- dependent anion binding model and an effect of differing aromatic side rings on the anion binding ability.

Lastly, X-ray crystal structure determination of complexes of calix[6]pyrrole with various TBA salts point to the existence of the previously-unexplored interactions of anion with electron rich aromatic p-systems.

An additional subject that is discussed is the effect that anions have on their receptor. As hydrogen bonds can be viewed as incipient proton transfer, the question remains as to when it is actually a neat proton transfer. Fluoride anion shows evidence to be capable of abstracting a hydrogen atom from the receptor and the resulting species interaction with the host has been investigated.