A Library of Low Molecular Weight Fluorescent Probes for the Detection of Cu(II) and Fe(III) Ions
Year: 2020
Type: dissertation
Abstract: This dissertation reports the synthesis and photophysical properties of a family of rhodamine dyes (compounds 3.9-3.13 and 4.6). The rhodamine dyes are prepared in two steps, and fully characterized by ESI-MS (Low and High resolution), X-Ray crystallography, NMR spectroscopy, and FT-IR spectroscopy. The coordination environment of the low molecular weight fluorescent probes (LMFPs) was systematically changed to investigate the thermodynamic behavior between the LMFPs and an array of metal ions (Cu2+, Fe2+, and Hg2+ ions) in protic and aprotic solvent systems. Upon coordinating to metal ions, the π-conjugation of the LMFPs changed, resulting in a transition from the colorless, spirolactam form of the rhodamine dyes to the colored, ring open forms. Ultimately, the goal was to prepare LMFPs that are soluble and functional in aqueous systems. An extensive photophysical study was carried out in pure organic solvents (MeOH, DMSO, CH3CN, and THF). Molecular probe 3.11a, which contains a tridentate binding motif was found to coordinate both Fe3+ and Cu2+ in each of the solvents with calculated binding constants as high as K = 7.1 x 108 M-2 for Fe3+ ions and K = 2.1 x 107 M-1 for Cu2+ ions. In contrast, the binding affinity for compound 3.10 could not be determined. The binding constants are significantly influenced by the counterions of the metal ions. The triflate ion was found to dissociate more readily than the other counterions that were analyzed and is less likely to form bridging species in solution. This is consistent with the calculated binding constants obtained. Moreover, oxygen containing solvents inhibited the formation of coordination compounds between the LMFPs and metal ions. Once we obtained an understanding of the coordination environments in organic solvents, attempts were made to investigate the optical properties in water. This proved to be challenging as Fe3+ hydrolysis inhibited the ability of the LMFPs to coordinate to the metal ion. The analysis of Cu2+ ions in aqueous environments was however possible. The calculated binding constant between compound 4.6 and Cu2+ in a 1:1 water-organic solvent system was found to be 1.6 × 108 M-2, showing that this LMFP could be used in aqueous environments. Due to the low solubility of Fe(OH)3 (s) in water (logKsp = 3.50 ± 0.20), these ions must be solubilized prior to coordination. In nature, Fe3+ ions are often bound to a class of organic molecules known as siderophores. Therefore, we incorporated the rhodamine dye into a siderophore motif. The artificial rhodamine siderophore, compound 5.13, was designed and its synthesis is reported.
Author: Ashley Johnson
Language: English
Subfield: Spectroscopy
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W3111987123