Synthesis and Binding Studies of Carbazole-urea and Anthracene-imidazolium based Receptors for Anion and Cation Recognition
- Synthesis and Binding Studies of Carbazole-urea and Anthracene-imidazolium based Receptors for Anion and Cation Recognition
- Nisar Ahmed
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- Ions recognition chemistry, both for cations and anions, is an important front in supramolecular chemistry for many years. Despite many artificial receptors for anions and cations have been investigated, further works are still necessary in order to design well selective artificial receptors for a specific ions because ion binding is a key process in many chemical and biological processes. In this study, we synthesize carbazole-urea and anthracene-imidazolium based receptors for specific anion and cation recognition.
In chapter 2, a macrocyclic carbazole-urea compound, Cyclo-bis-(urea-3,6-dichlorocarbazole), has been synthesized which can be effectively utilized for sensing of fluoride/pyrophosphate by means of colorimetric/chromogenic and fluorogenic changes. The carbazole-urea receptor changes to a deprotonated form upon complexing with fluoride/pyrophosphate, and is used for a selective ratiometric sensing of Zn2+ by means of fluorescent changes and Cu2+ and Zn2+ by means of colorimetric changes. So far, no carbazole urea-based macrocycles are synthesized and reported for anion or cation sensing purpose. This macrocyclic fluorogenic and chromogenic ion receptor with dual sensing ability for anions and cations have superiority over simple anion or cation receptors. This phenomena is further supported by theoretical calculations.
In chapter 3, a water-soluble and fluorescent imidazolium-anthracene cyclophane effectively recognizes the biologically important GTP and I− over other anions in 100% aqueous solution of physiological pH 7.4. Fluorescence and 1H-NMR spectra and ab initio calculations demonstrate that emission arises from the formation of an excimer state and quenching occurs upon GTP/I− binding through (C−H)+•••A− hydrogen bond interactions.
In chapter 4, Cyclo-bis-(urea-3,6-dichlorocarbazole) forms a 1:2 complex with CH3CO2− and H2PO4− through hydrogen bonding with the two urea moieties, resulting in fluorescence enhancement via a combined photoinduced electron transfer (PET) and energy transfer mechanism. The binding mechanism involves a conformational change of the two urea receptors to a trans orientation after binding of the first anion, which facilitates the second interaction. Positive homotropic allostery is inferred from binding isotherms obtained from fluorescence and 1H–NMR titrations.
In chapter 5, A new water-soluble and fluorescent imidazolium-anthracene cyclophane effectively recognizes and differentiates the biologically important GTP via excimer formation (through π-π stacking interaction between guanine moiety of GTP and anthracene ring) and ATP via fluorescence enhancement (through the vertical interaction of the H atom at the C-2 position of the adenine base with the plane of the anthracene ring (π-H interaction) in 100% aqueous solution of physiological pH 7.4. Fluorescence and 1H-NMR spectra and ab initio calculations demonstrate that excimer formation and fluorescence enhancement occur upon GTP and ATP binding, respectively, through (C−H)+•••A− hydrogen bond interactions.
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