The Surface Diels-Alder Reaction of Tetracene and Pentacene Crystals; Blurring the Lines Between Solid State, Surface, and Synthetic Chemistry

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In this project, funded by the Chemical Structure, Dynamic & Mechanism B Program of the Chemistry Division, Professor Jacob W. Ciszek of the Department of Chemistry and Biochemistry at Loyola University Chicago studies the surface chemistry of the prototypical organic semiconductors: tetracene and pentacene. Because these materials bear little resemblance to classical surfaces, the rules governing their reactivity require entirely new reaction mechanisms which are examined herein. The results from these experiments allow us to improve the performance of these materials in next generation devices including the light emitting diodes and transistors found in small displays (e.g. smartphone screens). This grant also supports their Emerging Scientists Workshop, which immerses high school students in experiments found in host laboratories. This heavily subscribed program is also evaluated as part of the grant, and is expanded as a pilot at DePaul University.More specific to the science, this project looks at three areas, which highlight differences between chemistry at traditional inorganic substrates and the surfaces of molecular crystals. The first two center on the fact that geometric considerations (based off molecular orientation in the crystal) appears to be the primary determinant of reactivity. In part one, the idea of a surface cavity, where surrounding molecules envelop the reactive loci, is developed to explain the reactivity differences between the faces of tetracene and pentacene crystals. Coverage is assessed over time via X-ray photoelectron spectroscopy. Part two examines how the reaction cavity is also responsible for inhibition of the most reactive bonds in the semiconductor. This manifests itself in different regioisomers formed on the various surfaces of tetracene and pentacene crystals. Both substrate and adsorbate parameters (electronic, steric, and phase) are examined for their impact on regioisomer formation. The final portion of the project examines how adsorbate behavior is impacted by the very low energy surface of the crystal. With surface energies less than a tenth of metallic surfaces, the coverage and residence time of the adsorbates are expected to be much lower on pentacene and tetracene. Quartz crystal microbalance studies in conjunction with infrared reflection absorption spectroscopy allow reaction kinetics to be corroborated with adsorbate coverage for further mechanistic refinement.