Title

Molecular and dendritic encapsulation of viologens

Date of Award

2002

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Angel E. Kaifer, Committee Chair

Abstract

This dissertation tackles two important aspects of modern science: molecular recognition and supramolecular electrochemistry. Throughout this work, we investigated the effect of encapsulation on the spectroscopic and electrochemical properties of the complexed 4,4'-bipyridinium (viologen) guests. Initially, we encapsulated simple viologen derivatives inside the non-polar cavity of the cucurbit[7]uril (CB7) host. The formation of the resulting non-covalent CB7·viologen inclusion complexes was monitored by 1H NMR spectroscopy and mass spectrometry and their association constant values were determined by UV-vis spectroscopy. These results disclosed a rare observation of tight binding in aqueous media, and can be attributed to their high affinities (∼105 M -1) and the perfect structural fits between the host and guest pairs. The electrochemical behavior of the CB7·viologen inclusion complexes also exhibited unprecedented phenomena: (1) The included viologen guests undergo redox conversions without dissociating from the CB7 host. This is a very unique electrochemical behavior because all electron transfer processes in supramolecular host-guest pairs are always preceded by complex dissociation. (2) The stability drop on the CB7·viologen complexes upon one-electron reduction was ∼0.4 kcal/mol. This drop is very moderate considering that all supramolecular host-guest pairs are known to rapidly dissociate upon redox conversion of either component.We also attempted to encapsulate the viologen units by attaching these redox-active groups to the focal points of the unsymmetrical, Newkome-type dendrimer building blocks (up to the third generation). We performed cyclic voltammetric experiments on these viologen dendrimers in solvents of low (CH 2Cl2 and THF), intermediate (CH3CN) and high (DMSO) polarities. Their electrochemical properties revealed two unusual behaviors: (1) With the exception in DMSO, the organic shell of the dendrimers somehow thermodynamically stabilized the oxidized forms (dicationic and monocationic) over the reduced form (neutral) of viologens. This observation is very uncommon because virtually all other classes of dendrimers thermodynamically favor the reduced forms of the encapsulated redox-active unit. (2) The kinetics of electron transfer for the redox conversion of viologen remained fast---in CH3CN and DMSO---regardless of dendrimer growth. Again, this is also uncommon because dendrimer growth usually attenuates the heterogeneous rate of electron transfer of a redox-active group that is attached at the focal point of a dendritic mass. Overall, our electrochemical results in CH 2Cl2 and THF suggest that the viologen residue is partially encapsulated by the higher generation (2nd and 3rd) dendrimers.Lastly, we combined the host and guest components that were investigated in the two previous studies, i.e., encapsulation of the viologen residues with CB7 host at the focal point of the dendrimers. The 1H NMR, UV-vis and MALDI mass spectra of the resulting CB7·viologen dendrimer complexes (1st to 3rd generations) conclusively revealed the formation of the inclusion complexes. Remarkably, the binding affinities were only slightly affected by dendrimer growth, suggesting that the binding stabilities of the complexes significantly compensated any steric effects that were imposed by the high generation dendritic mass.

Keywords

Chemistry, Organic; Chemistry, Physical

Link to Full Text

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