Title

Solvent effects and redox control on host-guest binding phenomena

Date of Award

1998

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

First Committee Member

Angel E. Kaifer, Committee Chair

Abstract

In the first chapter the binding constant between the cyclophane host cyclobis(paraquat-p-phenylene) and the guests indole and catechol was found to exhibit strong solvent dependence. With both guests, the measured free energy of complexation correlated linearly with the solvent Z- and $E\sb{\rm T}$(30) values, which were proven to be more suitable measurements of solvent polarity. The results obtained were consistent with the concept of enthalpically-driven hydrophobic effect for inclusion type complexation by cyclophane receptors, which is a result of (i) strong cohesive interactions between polar solvent molecules, and (ii) less favorable solvent-solute interactions between highly polarizable apolar hydrocarbon surfaces and the less polarizable molecules of the polar solvent. Charge-transfer interactions between the $\pi$-electron rich guest and the $\pi$-electron deficient cavity of the cyclophane were found to play a minor role in the stabilization of the complex, while solvophobic interactions were shown to act as the governing binding force.The complexation properties of the hydrophilic viologens 4,4$\sp\prime$-bipyridinium-$N,N\sp\prime$- di-(2-(2-(2-ethoxy)ethoxy)ethanol), 4,4$\sp\prime$-bipyridinium-$N,N\sp\prime$-di-(carboxyhexane), 4,4$\sp\prime$-bipyridinium-$N,N\sp\prime$-di-(propylsulfonate) with the hosts $\beta$-cyclodextrin ($\beta$-CD) and heptakis-(2,6-O-dimethyl)-$\beta$-cyclodextrin (DM-$\beta$-CD) were investigated in the chapter II using electrochemical and digital simulation techniques. The hydrophilic substituents of these viologens allowed their full electrochemical characterization without problems associated with the precipitation of reduced viologen species. Detailed analysis of the voltammetric results using digital simulation techniques revealed that the oxidized forms of the guests did not interact appreciably with either CD host, the 2-electron reduced guests formed extremely stable inclusion complexes, with association constants in the range $\rm10\sp3{-}10\sp4\ M\sp{-1},$ while the cation radical forms exhibited intermediate binding affinities $\rm({\sim}10\sp2\ M\sp{-1}).$ In all cases, DM-$\beta$-CD was found to form more stable complexes than unmodified $\beta$-CD.The synthesis and characterization of water soluble electroactive 4,4$\sp\prime$-bipyridinium derivatives was described in chapter III. The complexation properties of these compounds in aqueous solution were studied electrochemically. All four $\beta$-CD derivatives were found to exhibit a novel phenomenon of concentration dependent and electrochemically driven intermolecular self-complexation, where their reduced species of form "head-to-head" type intermolecular dimer complexes in aqueous solution. These complexes are destabilized in the presence of a competing water soluble guest (1-adamantane carboxylic acid or 1-adamantane tetramethylammonium chloride) or a host (DM-$\beta$-CD).

Keywords

Chemistry, Analytical; Chemistry, Physical

Link to Full Text

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