Energy-level alignment at strongly coupled organic-metal interfaces.

Energy-level alignment at organic-metal interfaces plays a crucial role for the performance of organic electronic devices. However, reliable models to predict energetics at strongly coupled interfaces are still lacking. We elucidate contact formation of 1,2,5,6,9,10-coronenehexone (COHON) to the (1 1 1)-surfaces of coinage metals by means of ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy, the x-ray standing wave technique, and density functional theory calculations. While for low COHON thicknesses, the work-functions of the systems vary considerably, for thicker organic films Fermi-level pinning leads to identical work functions of 5.2 eV for all COHON-covered metals irrespective of the pristine substrate work function and the interfacial interaction strength.

Saddle shaped hexaaryl[a,c,fg,j,l,op]tetracenes from 4,5,9,10-tetrafunctionalized pyrenes.

A new K-region functionalized pyrene is presented which was used as a building block for the straightforward synthesis of hexaaryl[a,c,fg,j,l,op]tetracene via fourfold Stille coupling and subsequent cyclodehydrogenation. Electronic properties and crystal structures are provided and reveal a saddle conformation for the curved hexaarylated tetracenes.

Asymmetric pyrene derivatives for organic field-effect transistors.

For the synthesis of an ortho-dithienylpyrene, a K-region bromination of pyrene was developed which enabled the first reported, non-statistical asymmetric functionalization of pyrene at the 4, 5, 9 and 10 positions. Crystal structures, optical and electronic properties and FET characteristics have been investigated.

Surface-assisted cyclodehydrogenation provides a synthetic route towards easily processable and chemically tailored nanographenes.

Atomically thin sheets of sp(2)-hybridized carbon--graphene--have enormous potential for applications in future electronic devices. Particularly promising are nanostructured (sub)units of graphene, the electronic properties of which can be tuned by changing the spatial extent or the specific edge termination of the carbon network. Processability and precise tailoring of graphene-derived structures are, however, still major obstacles in developing applications; both bottom-up and top-down routes are presently under investigation in attempts to overcome this limitation. Here, we propose a surface chemical route that allows for the atomically precise fabrication of tailored nanographenes from polyphenylene precursors. The cyclodehydrogenation of a prototypical polyphenylene on Cu(111) is studied using scanning tunnelling microscopy and density functional theory. We find that the thermally induced cyclodehydrogenation proceeds via several intermediate steps, two of which can be stabilized on the surface, yielding unprecedented insight into a dehydrogenative intramolecular aryl-aryl coupling reaction.

Complex interplay and hierarchy of interactions in two-dimensional supramolecular assemblies.

In order to address the interplay of hydrogen bonding, dipolar interactions, and metal coordination, we have investigated the two-dimensional mono- and bicomponent self-assembly of three closely related diaminotriazine-based molecular building blocks and a complementary perylenetetracarboxylic diimide by means of scanning tunneling microscopy. The simplest molecular species, bis-diaminotriazine-benzene, only interacts via hydrogen bonds and forms a unique supramolecular pattern on the Au(111) surface. For the two related molecular species, which exhibit in addition to hydrogen bonding also dipolar interactions and metal coordination, the number of distinct supramolecular structures increases dramatically with the number of possible interaction channels. Deposition together with the complementary perylene species, however, always results in a single well-defined supramolecular arrangement of molecules. A detailed analysis of the observed mono- and bicomponent assemblies allows shedding light on the hierarchy of the competing interactions, with important implications for the fabrication of surface-supported supramolecular networks by design.

A polymer with a benzo[2,1-b;3,4-b']dithiophene moiety for photovoltaic applications.

The characterization of a benzo[2,1-b;3,4-b']dithiophene containing conjugated polymer (PBTT) is demonstrated, with regard to its photovoltaic performance. X-ray diffraction measurements reveal that the thermal treatment results in an increased crystallinity within the PBTT:[70]PCBM network and subsequent spatial rearrangement in the film. Upon stepwise annealing, the PBTT-based bulk-heterojunction solar cells show an overall conversion efficiency of 2.7% under 1 sun light illumination. The photovoltaic devices based on PBTT show a high efficiency, maintained over one month. All these aspects suggest that the use of self-organizable materials is an efficient approach for high-performance photovoltaic applications.

Porous graphenes: two-dimensional polymer synthesis with atomic precision.

We demonstrate, by surface-assisted coupling of specifically designed molecular building blocks, the fabrication of regular two-dimensional polyphenylene networks with single-atom wide pores and sub-nanometer periodicity.

"Soft" metallic contact to isolated C60 molecules.

C60 adsorbed on a monolayer of hexaazatriphenylene-hexanitrile (HATCN) on Ag(111) is investigated by ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling microscopy. UPS and quantum-mechanical modeling show that HATCN chemisorbed on Ag(111) displays metallic character. This metallic molecular layer decouples C60 electronically from the Ag substrate and simultaneously acts both as template for the stable adsorption of isolated C60 molecules at room temperature and as "soft" metallic contact for subsequently deposited molecules.