Dissertation Defense: Prashant Kumar Pandey
Candidate: Prashant Kumar Pandey
Advisor: K. Travis Holman, Ph.D.
Title: Revisiting the Molecular Recognition and Solid-State Properties of DimethoxypillarArene
Pillar[n]arenes, introduced by Ogoshi and co-workers in 2008, are among the recently found class of container molecules. Their unique name is derived from the tubular shape of these fascinating macrocycles. It is surprising that the first pillar[n]arene, dimethoxypillararene (DMP5), has so far been the least studied in the context of porosity and solid-state chemistry, considering that it is cheap and easily accessible. Indeed, next to nothing has been reported regarding the innate crystal/solid forms of DMP5. In the current research, set in motion by the quest for a guest-free porous (β) phase, DMP5 was shown to exist in three new polymorphic forms including meta-stable, close packed phase (ϒ), an empty crystalline non-porous phase (α) and an amorphous phase (a-DMP5). Both a-DMP5 and α-DMP5 phases transformed to β-CO2@DMP5 when pressurized with CO2 (30-35 bar) with a high kinetic barrier for CO2 release. β-CO2@DMP5 was found to retain the guest CO2 molecules for up to 90 days at room temperature, characteristic of a 0D porous material. Single crystals of gas clathrates, xCO2@DMP5 and xXe@DMP5, were obtained, opening avenues for Xe biosensing applications.
The solid-state properties of DMP5 have far reaching implications for its molecularrecognition behavior. Notably, a substantial majority of all previous guest binding studies of lipophilic pillararenes have been conducted in CDCl3 or a combination of CDCl3 and other small deuterated solvents. Yet, there is no experimental evidence to refute the possibility that chloroform can act as competitor guest for the pillararene cavity. It is evident from this study that CHCl3 can fit into the pillararene cavity acting as a guest itself and, therefore, compete with the intended guests when chosen as the solvent. In effect, the intrinsic affinity of DMP5 towards small guest molecules (e.g., n-pentane, n-hexane) and gas molecules previously perceived to be inherently low are shown to be significantly underestimated relative to their values in truly non-competing solvents. The understanding of true binding affinities of DMP5 towards apolar guest molecules also enabled the application of pillar[n]arenes towards the selective binding and separation of linear alkanes from branched alkanes and even more complex fuel mixtures.