Self-assembly of partially fluorinated hexabenzocoronene derivatives in the solid state.

We report on the synthesis and structural characterization of novel, partially fluorinated hexabenzocoronene (HBC) derivatives. The fluorination of polycyclic aromatic hydrocarbons (PAHs) is a well-established method to enhance the stability of organic semiconductors (OSCs) and render them n-type. For HBC it has been observed that fluorination leads to a modification of the molecular packing motif from a herringbone arrangement to a parallel-packed motif. Here, we study whether this transformation of the molecular packing is also found for the partially fluorinated HBCs 2,5-difluoro-hexa-peri-hexabenzocoronene (F2HBC) and 2,5,8,11-tetrafluoro-peri-hexabenzocoronene (F4HBC). Combining powder diffraction and NEXAFS dichroism measurements, we reveal that indeed all partially fluorinated compounds adopt a parallel molecular packing, hence maximizing their intermolecular contact area. We identify fluorine-hydrogen bonds as the mediating driving force to specifically stabilize this molecular arrangement and direct self-assembly. Furthermore, we show that the relative orientation of the HBCs on the underlying surface can be precisely controlled by varying the substrate materials. Finally, the energetic states of the compounds are analyzed using photoelectron spectroscopy, optical spectroscopy and density functional theory to identify the effects of fluorination on these fundamental electronic characteristics.

Efficient Syntheses of Novel Fluoro-Substituted Pentacenes and Azapentacenes: Molecular and Solid-State Properties.

Non-symmetrical 6,13-disubstituted pentacenes bearing trifluoromethyl and aryl substituents have been synthesized starting from pentacenequinone. Diazapentacenes with a variety of fluorine substituents were prepared either via a Hartwig-Buchwald aryl amination route or by a SNAr strategy. As a result of a non-symmetric substitution pattern containing electron-donating substituents in combination with electron-accepting fluorine substituents, the synthesized compounds feature distinct molecular dipoles. All compounds are analyzed regarding their optoelectronic properties in solution with special focus on the frontier orbital energies as well as their molecular packing in the crystal structures. The analyses of isolated molecules are complemented by thin-film studies to examine their solid-state properties. A precise comparison between these and the molecular properties gave detailed insights into the exciton binding energies of these compounds, which are explained by means of a simple model considering the molecular packing and polarizabilities.

Analysis of the near-edge X-ray-absorption fine-structure of anthracene: a combined theoretical and experimental study.

The near-edge fine structure of the carbon K-edge absorption spectrum of anthracene was measured and theoretically analyzed by density functional theory calculations implemented in the StoBe code. It is demonstrated that the consideration of electronic relaxation of excited states around localized core holes yields a significant improvement of the calculated excitation energies and reproduces the experimentally observed fine structure well. The detailed analysis of excitation spectra calculated for each symmetry inequivalent excitation center allows in particular to examine the influence of chemical shifts and core hole effects on the excitation energies. Moreover, the visualization of final states explains the large variations in the oscillator strength of various transitions as well as the nature of Rydberg-states that exhibit a notable density of states below the ionization potentials.