Perfluorinated Acenes: Crystalline Phases, Polymorph-Selective Growth, and Optoelectronic Properties

The emerging field of organic electronics triggers the search for new molecular semiconductors and fluorophores. Since optoelectronic properties of molecular solids also depend crucially on molecular packing motifs, it is not sufficient to only consider single-molecule properties, but also to gain precise knowledge of the crystalline phases to understand structure-property relationships. Here, we analyze and compare structural and optoelectronic properties of perfluorinated acenes ranging from perfluoronaphthalene (PFN) via perfluoroanthracene (PFA) and perfluorotetracene (PFT) to perfluoropentacene (PFP), while previous studies, due to the lack of availability of PFA and PFT, were limited to PFN and PFP, which exhibit rather different crystal structures. Applying various crystallization techniques such as gradient sublimation, solution growth, and liquid-assisted organic molecular beam deposition, we identified different crystalline phases that allow closing this structural gap. A comparison of all known phases reveals clear trends in the molecular packing motifs, which are rationalized by corresponding Hirshfeld analyses. Using UV/vis and X-ray absorption spectroscopy, also optoelectronic solid state properties of the various compounds were analyzed, and from a comparison with the corresponding solution spectra, the exciton binding energies are estimated. In addition, we demonstrate that like PFP PFT also exhibits a pi-stacked polymorph in films grown on graphene, where molecules are flat lying and adopt a slip-stacked packing. This interface-mediated phase is found to be stable only for PFP and PFT, while it is unstable for the smaller PFA. Finally, we demonstrate for the case of PFT that the different packing motifs occurring in the various phases have a strong influence on the photoluminescence spectra.

Automated summary

The search for new molecular semiconductors and fluorophores in the field of organic electronics requires understanding the relationship between molecular packing motifs and optoelectronic properties. This study compared the structural and optoelectronic properties of perfluorinated acenes, revealing different crystalline phases with distinct molecular packing motifs, and demonstrated the influence of these packing motifs on photoluminescence spectra.