Untersuchung der strukturellen und optoelektronischen Eigenschaften fluorierter organischer Halbleiter
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Philipps-Universität Marburg
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The rapid development of organic electronics and the multitude of its applications, be it light emitting diodes, transistors or sensors, leads to a continuous increase in the demands on the underlying molecular materials. While versatile chemical functionalizations are possible nowadays and the molecular properties can be predicted well by theoretical calculations, controlling solid-state properties by molecular design is not fully possible, as these solid-state properties are governed by the packing motif. The packing motif in turn is determined by intermolecular interactions. The theoretical description of these intermolecular interactions and the prediction of molecular packing is only possible for simple systems, raising the need to experimentally correlate molecular design and the realized structure.
In this work, the intermolecular interactions of the well-studied polycyclic aromatic hydrocarbons (PAHs) are specifically modified by heteroatom substitution in order to investigate the effects on the packing motif and thus the solid-state properties. Fluorination is the chosen substitution, as fluorine is the atom with the highest electronegativity and thus the non-polar carbon-hydrogen bonds are replaced by polar carbon-fluorine bonds. This changes the electron density in the PAHs and thus also the molecular electrostatic potential (MEP). Consequently, the intermolecular electrostatic interactions can be specifically modified by regioselective fluorination.
The incorporation of local, strongly polar bonds not only affects the molecular electrostatic potential, but also the molecular optoelectronic properties. The electron withdrawing character of the fluorine atoms leads to a reduction in the electron density inside the PAH and thus induces more positively charged carbon atoms. As a result, the core electrons are more strongly bound, which is visible in a strong chemical shift in the X-ray photoelectron spectra. The π-electrons also experience a stronger Coulomb interaction with the carbon atoms, which leads to a lowering of the frontier orbital energy levels. Using optical absorption and near-edge X-ray absorption fine structure spectroscopy, it can be shown that the core and frontier orbital energy levels are lowered equally.
Indeed, the altered electrostatic potential of the fluorinated molecules influences the intermolecular interactions and thereby changes the packing motif. Thus, in the case of acenes, partial fluorination leads to the breaking of the typical herringbone packing motif and the appearance of planarly stacked molecules. Crystal growth of polar acenes in a polar environment enables the formation of a dipole-parallel arrangement of the acenes, as the dipole moments of the polar solvent molecules compensate for the dipole moments of the polar acenes during growth. In contrast, crystal growth in the absence of polar molecules, for example in non-polar toluene or by vacuum sublimation, leads to the crystallization of polar acenes in the more stable dipole-antiparallel packing.
The different solid-state packing of tetracene, which crystallizes in the herringbone packing motif, and fluorinated tetracenes, which exhibit planarly stacked molecules in the solid state, suggest the spectroscopic characterization. The investigation of the solid-state photoluminescence shows that the fluorinated tetracenes exhibit a broadened and red-shifted luminescence compared to the original tetracene and the molecules in solution. This is attributed to excimer formation, i.e. a structural relaxation of the excited states, which can occur for planarly stacked molecules. This example shows impressively that fluorination changes the solid-state packing by modifying the electrostatic potential and thus determines the optoelectronic solid-state properties, such as photoluminescence.
Considering that electrostatics significantly influence the packing motif, an analysis of the packing motifs of known PAHs and their functionalized counterparts is carried out. The observed crystal structures are classified into four basic packing motifs and these in turn are correlated with the molecular electrostatics. Based on these considerations, partially fluorinated tetracenes are designed, which have the same degree of fluorination, but for which different packing motifs and thus different optoelectronic solid-state properties are expected. Hence, this work paves the way to specifically modify solid-state properties by varying the fluorination pattern.
In general, the knowledge gained in this work expands the understanding of the molecular properties and intermolecular interactions of organic semiconductors. This knowledge allows the optoelectronic properties in the solid state to be specifically tailored by the molecular design. For example, the energy levels, which can be adjusted via the degree of fluorination, influence the injection barriers and charge transport in organic electronic devices. The molecular orientation, which is given by the fluorination pattern, determines the direction of the emitted light in light-emitting diodes or the anisotropic charge transport in organic transistors. Hence this work provides novel strategies to control molecular properties and solid-state packing and thus to improve organic components such as sensors, transistors or light-emitting diodes.
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Created: 2024Issued: 2024-08-20Updated: 2024-08-20
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Fachbereich Physik
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Philipps-Universität Marburg
Language
ger
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DoctoralThesis
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530
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Bischof, Daniel (0000-0001-5117-7947): Untersuchung der strukturellen und optoelektronischen Eigenschaften fluorierter organischer Halbleiter. : Philipps-Universität Marburg 2024-08-20. DOI: https://doi.org/10.17192/z2024.0230.
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