Chemical Doping by Fluorination and Its Impact on All Energy Levels of π-Conjugated Systems.

Halogenation of organic molecules causes chemical shifts of C1s core-level binding energies that are commonly used as fingerprints to identify chemical species. Here, we use synchrotron-based X-ray photoelectron spectroscopy and density functional theory calculations to unravel such chemical shifts by examining different partially fluorinated pentacene derivatives. Core-level shifts occur even for carbon atoms distant from the fluorination positions, yielding a continuous shift of about 1.8 eV with increasing degree of fluorination for pentacenes. Since also their LUMO energies shift markedly with the degree of fluorination of the acenes, core-level shifts result in a nearly constant excitation energy of the leading π* resonance as obtained in complementary recorded K-edge X-ray absorption spectra, hence demonstrating that local fluorination affects the entire π-system, including valence and core levels. Our results thus challenge the common picture of characteristic chemical core-level energies as fingerprint signatures of fluorinated π-conjugated molecules.

Regioselective Fluorination of Acenes: Tailoring of Molecular Electronic Levels and Solid-State Properties.

Optoelectronic properties of molecular solids are important for organic electronic devices and are largely determined by the adopted molecular packing motifs. In this study, we analyzed such structure-property relationships for the partially regioselective fluorinated tetracenes 1,2,12-trifluorotetracene, 1,2,10,12-tetrafluorotetracene and 1,2,9,10,11-pentafluorotetracene that were further compared with tetracene and perfluoro-tetracene. Quantum chemical DFT calculations in combination with optical absorption spectroscopy data show that the frontier orbital energies are lowered with the degree of fluorination, while their optical gap is barely affected. However, the crystal structure changes from a herringbone packing motif of tetracene towards a planar stacking motif of the fluorinated tetracene derivatives, which is accompanied by the formation of excimers and leads to strongly red-shifted photoluminescence with larger lifetimes.

Biphenylene network: A nonbenzenoid carbon allotrope.

The quest for planar sp2-hybridized carbon allotropes other than graphene, such as graphenylene and biphenylene networks, has stimulated substantial research efforts because of the materials' predicted mechanical, electronic, and transport properties. However, their syntheses remain challenging given the lack of reliable protocols for generating nonhexagonal rings during the in-plane tiling of carbon atoms. We report the bottom-up growth of an ultraflat biphenylene network with periodically arranged four-, six-, and eight-membered rings of sp2-hybridized carbon atoms through an on-surface interpolymer dehydrofluorination (HF-zipping) reaction. The characterization of this biphenylene network by scanning probe methods reveals that it is metallic rather than a dielectric. We expect the interpolymer HF-zipping method to complement the toolbox for the synthesis of other nonbenzenoid carbon allotropes.

Synthesis of 6,13-difluoropentacene.

6,13-Difluoropentacene was synthesized from 1,4-difluorobenzene. Friedel-Crafts annulation of the latter with phthalic anhydride and subsequent reduction of the anthraquinone gave 1,4-difluoroanthracene. After ortho-lithiation and reaction with phthalic anhydride a carboxylic acid was obtained whose Friedel-Crafts acylation and subsequent reductive removal of the oxygen-functionalities resulted in the formation of the target compound. The HOMO-LUMO gap of 6,13-difluoropentacene was determined via UV-vis spectroscopy and compared to other fluorinated pentacenes.

Unilaterally Fluorinated Acenes: Synthesis and Solid-State Properties.

The rapid development of organic electronics is closely related to the availability of molecular materials with specific electronic properties. Here, we introduce a novel synthetic route enabling a unilateral functionalization of acenes along their long side, which is demonstrated by the synthesis of 1,2,10,11,12,14-hexafluoropentacene (1) and the related 1,2,9,10,11-pentafluorotetracene (2). Quantum chemical DFT calculations in combination with optical and X-ray absorption spectroscopy data indicate that the single-molecule properties of 1 are a connecting link between the organic semiconductor model systems pentacene (PEN) and perfluoropentacene (PFP). In contrast, the crystal structure analysis reveals a different packing motif than for the parent molecules. This can be related to distinct F⋅⋅⋅H interactions identified in the corresponding Hirshfeld surface analysis and also affects solid-state properties such as the exciton binding energy and the sublimation enthalpy.