The Wheeler Research Group - Eastern Illinois University

Research

Wheeler research group Word Cloud, based on publications and presentation titles through 2020

Research Areas: Organic Supramolecular Chemistry, Molecular Recognition, Nanomaterials, Solid-State Organic Photochemistry, and Crystal Engineering.

Structural Boundaries of Molecular Shape Based Molecular Recognition

Recent efforts in Prof. Wheeler's research group have developed a strategy to investigate the relationship between molecular shape and molecular recognition. We have found that an understanding can be effectively achieved by mapping the shape space of quasiracemic fractional crystallization using structurally simple chiral diarylamides. Cocrystallization of pairs of quasienantiomers were pursued using the Kofler contact fusion method. As shown below a utility of this technique draws attention to the formation of new crystalline phases during the heating cycle as indicated by the emergence of melting regions (eutectic regions) in the viewing area. In the case of quasiracemic mixtures, distinguishing between the various modes of crystallization (i.e., quasiracemic
compounds, conglomerates, and solid solutions) is possible given the unique thermal signature of each phase.

The collection of diarylamides selected for this initial investigation differed incrementally in size and shape that offered clear advantages for exploring the molecular recognition profiles that originate from topological features. Thermally processing all possible sets provided a comprehensive view of the shape space of this structural family. For example, the sets of H/F, Cl/Br, and Br/CF₃ quasienantiomeric pairs showed the formation of quasiracemic compounds, while the H/CN and F/Cl systems showed only conglomerate formation. These results and others were recently highlighted in a CrystEngComm and ChemCommun articles.

Figure
showing a snap-shot of a virtual melting
point diagram of quasienantiomers.

Asymmetric [2+2] Photodimerization Reactions in Molecular Crystals

Prof. Wheeler's research group have developed smart molecules that self-assemble in crystals capable of unique chemical processes. Because of the component handedness and engineered crystal organization, these materials align with predetermined asymmetric patterns that undergo remarkable reactions when exposed to UV light. Unlike traditional chemical reactions that require solvents, these processes are contained in solvent-free crystal environments. The fixed position of molecules and handedness of crystal packing provide a powerful tool for controlling chemical reactivity. The result is next generation materials equipped for high-yielding highly-selective chemical processes – all contained within crystals no larger than the head of pin as the reaction vessel.

The strategy of this program exploits sulfonamidecinnamic acid frameworks that form robust supramolecular dimers due to the features of molecular topology and directionality of hydrogen bonds. Several recent studies show these dimeric motifs persist regardless of the use of racemic, quasiracemic, and homochiral single-component building-blocks. Each of these systems undergoes topochemically controlled single-crystal-to-single-crystal (SCSC) photodimerization reactions in high yield (61–100% conversion). The current study presents several interesting opportunities to explore the supramolecular and photoreactive boundaries of this sulfonamidecinnamic acid framework.

This program extends the current crystal reaction technology to include the formation of robust molecular dimers that undergo asymmetric light-initiated reactions. Noted progress in this area has been realized by a team of researchers - undergraduates, a master’s student, and a high school teacher – all actively engaged in the design and development of a wide range of photoactive materials.

Quasiracemic Materials

Our recent interest in understanding the recognition profile of molecular topology makes use of the quasiracemate approach for organizing supramolecular arrays. It is generally accepted that quasiracemates are related to ‘true' racemates by an isosteric change in one of the components. When crystallized, they form supramolecular patterns with approximate inversion relationships that, in all known cases to date, mimic the structures of the corresponding racemate(s). One important advantage of this strategy is that it promotes the use of topological features rather than intermolecular electrostatic forces such as hydrogen bonds. In doing so, quasiracemic materials are not limited to specific chemical classes, and thus their construction may proceed using a wide variety of chemical frameworks and functions.

Our crystallographic investigations of quasiracemates have demonstrated that the deliberate use of isosteric components that differ in handedness consistently form supramolecular motifs that mimic the centrosymmetric alignment of racemic compounds. These systems include a variety of Some of our previously studies of quasiracemic systems are shown below and highlight the use of isosteric components

In summary, this collection of racemic and quasiracemic structures underscores the importance of molecular shape to the construction of supramolecular assemblies with near centrosymmetric arrangements. The deliberate use of phenoxypropionic acids that vary in Cl, Br, and CH₃ functions provided a logical entry point to isosteric components that were assessed for quasiracemate behavior. The structures of both racemic and quasiracemic systems contain molecular dimers organized by carboxylic acid head-to-head interactions. In the case of each quasiracemate, construction of these discrete motifs occurs from pairs of isosteric components that closely mimic the centrosymmetric local environment observed for the racemates. Further assembly of these heterodimers generates two possible crystalline phases with space group P2₁ and Z' = 2 or 8. The structural themes of isostructurality (both local and crystal structure patterns) and concomitant polymorphism in these studies emphasize the inherent importance of molecular topology as a contributor to molecular recognition processes.