Longer Acenes: Synthesis, Interface Formation and Thin Film Stability

• Applicants: Professor Dr. Holger Bettinger; Professor Dr. Heiko Peisert
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation ; Inorganic Molecular Chemistry - Synthesis and Characterisation ; Theoretical Chemistry: Molecules, Materials, Surfaces
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 570171322

1) Wider research context. Acenes are polycyclic aromatic hydrocarbons build of linearly fused benzene rings. Shorter acenes with lengths up to pentacene have been used as active materials in optoelectronic devices and have been extensively studied over the last decades. Going towards longer acenes one observes a strong decrease of the HOMO-LUMO gap and an increase in charge carrier mobilities, which qualifies them as organic semiconductors with a high potential for applications. But, their reactivity causes difficulties for synthesis and only recently discovered new synthesis routes have made them available for scientific studies. A possible approach to stabilize the molecular structure, while preserving their excellent electronic properties will be the introduction of fluorine substituents. 2) Objectives. The aim is a systematic understanding of the growth, geometric and electronic structure of the longer acenes from individual molecules to application relevant film thicknesses. To counter the enhanced reactivity in films, new acene derivatives will be synthesized and studied likewise. The experiments will be accompanied by ab-initio density functional theory(DFT). 3) Methods. Hexacene and diheptacene precursors have already successfully been synthesized by the Bettinger group. Their fluorinated counterparts are experimentally unknown. To achieve these novel compounds the synthesis will employ decarbonylation of the corresponding mono-CO bridge precursors. The surface science studies include state of the art methods such as scanning tunneling microscopy and X-ray photoemission spectroscopy. A particular focus will lie on photoemission orbital tomography, based on angle-resolved photoemission spectroscopy, which allows to assign orbitals, identify charge transfer states and gives information on the molecular adsorption geometry and interface chemistry. Ab-initio DFT will be performed to analyze the electronic structure of the frontier molecular orbitals by simulating orbital-projected density of states, scanning tunneling microscopy images and photoemission angular distributions. 4) Level of originality. Longer acenes only recently became available to the scientific community and still little is known regarding their molecular and interface/charge transfer properties and their stability in thin films. Their decreasing molecular band gap, approaching a lo-gap semiconductor, makes them particularly interesting for studies of low dimensional systems. To counter balance their increased reactivity, while keeping their electronic properties, selective fluorine substituents are planned, and will be synthesized. 5) Primary researchers involved. The consortium consists of surface scientists, experts in molecular synthesis and acene chemistry and theoretical solid state physicist. The team has a proven track record of cooperation.

• DFG Programme: Research Grants
• International Connection: Austria
• Cooperation Partners: Professor Dr. Georg Koller; Professor Peter Puschnig, Ph.D.