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https://hdl.handle.net/2142/22634
Description
Title
Electron transfer in cyanine borate ion pairs
Author(s)
Murphy, Sean Timothy
Issue Date
1994
Doctoral Committee Chair(s)
Schuster, Gary B.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Language
eng
Abstract
The ions chosen for our electron transfer studies are cyanine dye acceptors and tetraarylborate donors (see below for representative examples). The two ions are dissolved in a nonpolar solvent to assure a contact ion pair structure. The cyanine is then irradiated, and this induces the electron transfer process. The electron transfer can be monitored both with laser spectroscopy and by fluorescence. In order to study this system, it was necessary to learn more about both the dyes and the borates.(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI)
Some of the cyanine dyes were previously known and the others were made by analogous syntheses. A variety of molecular properties were measured for our set of cyanine dyes including the excited state lifetime, the singlet energy and the reduction potentials. The photophysics of the cyanines was also examined, and a unique solvent dependence was observed for the excited state relaxation. The effect was examined under high pressure as well as at atmospheric pressure.
The tetraarylborates used as donors have been synthesized previously for a variety of reasons. In general, the synthesis of these compounds is simple and quick. The most important property needed for our electron transfer studies is the oxidation potential, but this is not available through standard electrochemical means. This led to the development of a kinetic method by which we have obtained precise measures of the oxidation potential.
We found that the cyanine and the borate are not appropriately modeled as spheres, and the actual structure of the ion pair had to be determined. By using semi-empirical calculations, examining the NMR spectra and testing the effect of the borate on the cyanine relaxation processes, a consistent structure for the ion pair was obtained. We have defined the structure as a penetrated ion pair. It is a subclass of contact ion pairs where one of the ions penetrates within the hypothetical sphere containing the other. It is a useful term because it alerts the reader that the ions may affect each other in ways not typically observed for spherical ions.
By knowing the combined cyanine and borate molecular properties along with the ion pair structure, we were able to control the electron transfer rate. The free energy change was made as exergonic as possible for these ions and the effect on the electron transfer rate was observed. Unfortunately, the rates only reach a maximum. However, by reaching the top in the Marcus plot, we have determined both the maximum rate and the reorganization energy for cyanine borates. Also, an attempt at finding the distance dependence was made but failed. While the electron transfer studies were not completely successful, these studies provide valuable information about electron transfer in ion pairs.
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