Browsing by Author "Jones, Jason William"
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- Enhanced Architectural and Structural Regulation Using Controlled Free Radical Polymerization Techniques; Supramolecular Assemblies: Pseudorotaxanes and PolypseudorotaxanesJones, Jason William (Virginia Tech, 2001-02-16)Due in large part to the growth and development of reliable surface characterization techniques, as well as to advances in the physical and chemical techniques used to modify surfaces, the technology of surface modification has seen rapid expansion over the past two decades. A major thrust of this research is the growth of controlled/"living" polymeric brushes from the surface of various substrates, an advance that promises to be a facile and reproducible way of altering surface properties. A unique initiator bearing ATRP (atom transfer radical polymerization), cleavage, and condensation functionalities was prepared and attached to the hydrolyzed surface of silica gel. Preliminary results indicate that control of reversibly terminated grafts of varying degrees of polymerization with polydispersity indices approaching 1.5 can be readily achieved-significant findings in the quest to design desired surface characteristics. Important physical characteristics may also be altered by way of varying molecular topologies. In the second major research thrust, the use of self-assembly to construct such topologies in the form of pseudorotaxanes fashioned from diverse macrocycles with multifarious guest ions is discussed. While the underlying goal was to investigate and understand the mode of complexation based on such environmental factors as substituent affects and neighboring group influences, new insight was gained on the synthetic manipulation of cooperative events-events that freely occur in nature. The complexation behavior of several functionalized bis-(meta-phenylene)-32-crown-10 macrocycles with various paraquat guest moieties was. As expected, studies indicated that electron-donating substituents on the crown ether drive association, a likely result of increased p -p interactions among host and guest species. The association between a bicyclic macrocycle and dimethyl paraquat was also investigated. Not surprisingly, binding of paraquat by the bicyclic was much stronger than the binding found in analogous macrocycles. Lastly, the endgroup functionalization of poly(propyleneimine) dendrimers with two crown ether macrocycles was performed and the complexation with host-specific guests studied. Curiously, two extreme binding regimes were found: the larger 32-membered crown ether assembly displayed anti-cooperative behavior upon complexation with paraquat, while the smaller 24-membered macrocyclic system exhibited cooperative effects with 2o ammonium ions. These cooperative results are among the very first described for non-biological systems and hint at their potential use in developing highly efficient, synthetically designed supramolecular systems.
- Quantification of Supramolecular Complexes Involving Charged Species in Non-Aqueous Solvents: Theory and ApplicationJones, Jason William (Virginia Tech, 2004-05-13)We report for the first time a broad equilibrium model describing the complexation of ionic species in non-aqueous media that explicitly includes ion pairing for one of the components and that relies upon activities rather than molar concentrations. This model directly contradicts existing commonplace equilibrium treatments, which were shown to be incomplete, often invalid, and misleading. Experimental validation of our model was achieved through studies of pseudorotaxane formation between dibenzylammonium salts (DBAm-X) and dibenzo-24-crown-8 (DB24C8) in CDCl3:CD3CN (3:2). In that particular case, we showed that fluctuations in the apparent Ka,exp values as usually reported are attributable to ion pairing, with a dissociation constant Kipd, and that the constant Kassoc for pseudorotaxane complexation is independent of the counterion, a result of the complex existing in solution as a free cation. In accord with this model, we further described a straightforward and simple method to increase the extent of complexation by using either a ditopic cation and anion host, or adding to the charged host/guest solution a molecularly separate host capable of complexing the dissociated counterion. Also in accord with this model, we investigated the influence of the solvent¡¯s dielectric constant on Kipd and Kassoc. On the basis of competing condensation reactions between amines and ketones which were shown to occur within the timescale of host/guest recognition, we also challenged the commonly employed use of acetone in similar complexation studies involving 2o ammonium ions. Because a major goal of this work was to ultimately increase binding efficiency and selectivity, we explored new methods to drive complexation in related pseudorotaxane systems. We noted that addition of di- or tri-topic hydrogen bond accepting anions to solutions of bis(5-hydroxymethyl-1,3-phenylene)-32-crown-10 or bis(5-carboxy-1,3-phenylene)-32-crown-10 and paraquat di(hexafluorophosphate) served to significantly enhance host/guest interaction. The addition of Et4N+TFA- to an acetone solution of diacid crown and paraquat 2PF6 effectively boosted Ka,exp 40-fold, as estimated by 1H NMR studies. Similar increases in the apparent Ka,exp were observed upon the addition of n-Bu4N+OTs-. Evidenced by crystal structures, the increase in association resulted from chelation of the OH moieties of the crown by the di- or tri-topic anions, forming supramolecular bicyclic macrocycles (pseudocryptands) and stabilizing the complex in a cooperative manner. Significantly, Ka,exp of one of the pseudocryptands was shown to equal that determined in the corresponding cryptand complex.