Michael Klues

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.

Crystalline Packing in Pentacene-like Organic Semiconductors

Since optoelectronic properties of organic semiconductors (OSCs) are largely affected by the molecular packing in the solid phase, further advances of such materials require comprehensive structure–property interrelations beyond single molecule considerations. While single molecular electronic properties can be tailored by synthetic means and their electronic properties can be reliably predicted by quantum chemical calculations, crystal structure predictions of such van der Waals bond solids remain challenging. Here we analyze correlations between the molecular structure and the resulting packing motifs adopted in the crystalline phases of the prototypical OSC pentacene as well as various differently substituted but similarly shaped π-conjugated molecules. Based on a Hirshfeld surface analysis and related fingerprint plots, specific contact points and their distribution are identified which allows classification of different structural groups. Comparing the fingerprint plots with corresponding molecular properties such as electrostatic contour plots as well as quadrupole and polarizability tensors, which were calculated by density functional theory, allows rationalizing structure determining specific intermolecular interactions. Our analysis shows in particular that molecules with uniform electrostatic potential at their periphery favor a herringbone packing, while the highly electronegative substituents (O, N and F) enable the formation of H-bonds and prefer slip-stacking or criss-cross packing motifs. The present correlations might be useful guidelines for future strategies to synthesis new OSCs.

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.