Ralph Rieger

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.

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.

Rational optimization of benzo[2,1-b;3,4-b']dithiophene-containing polymers for organic field-effect transistors.

Adv. Mater. 2010, 22, 83–86 2010 WILEY-VCH Verlag Gmb Thin-film transistors based on organic semiconductors have attracted a lot of attention in academic and industrial research in recent years. First commercial applications are about to be launched into new products which cannot be realized with conventional, expensive, and energy-demanding silicon-based electronics. Technologies using organic materials promise large area devices, which are light weight, flexible, and come at low cost. Experts predict a huge demand for devices such as radio frequency identification tags (RFIDs) for single item tagging or other integrated circuits. The search for new materials which meet the requirements for these applications is, therefore, of great importance. These crucial requirements are (i) good solution processability especially on flexible polymer substrates, (ii) short annealing times at low temperatures to enable high-speed processing such as mass printing, (iii) high operational stability at ambient conditions to avoid the necessity of costly barrier layers, (iv) high charge-carrier mobilities, and (v) a facile and economically feasible synthesis which can be scaled up for production. Semiconducting polymers show a microcrystalline or amorphous superstructure simplifying the fabrication of homogeneous films over large area, which makes this class very promising. The charge-carrier mobilities are usually not as high as for small molecules but still sufficient for most applications. Thiophene-containing polymers have proven to be most effective. This paper presents the incorporation of benzo[2,1-b; 3,4-b0]dithiophene into a polythiophene as active material in a field-effect transistor (FET). The structure motif of a 2,5 connected thiophene, which is found, for example, in the well-known poly-3-hexylthiophene (P3HT), is modified by a benzo annellation. This reduces the ionization potential of the polymer and is thus expected to improve air stability under operation. The extended p system should, furthermore, help in forming highly ordered films which are needed to transport charge-carriers efficiently. The first approach toward a polymer-containing benzo[2,1-b;3,4-b0]dithiophene is the iron(III) chloride mediated oxidative polymerization of benzodithiophene having solubilizing alkyl chains attached to the benzo unit. In this way polymer P1 (Scheme 1) is obtained with a molecular weight of Mn1⁄4 18 kgmol 1 as determined by size exclusion chromatography in 1,2,4-trichlorobenzene at 135 8C in reference to polystyrene as standard. Good solubility in dichlorobenzene enables the polymer to be tested as the active material in an FET. The polymer is spin cast from 1,2-dichlorobenzene solution onto a silicon wafer and dried at 100 8C for 5min. Finally, top-contact gold electrodes are deposited by thermal evaporation. Good device characteristics can be recorded, showing low charge-carrier mobility in the 10 4 cm V 1 s 1 region. The organization in the thin film is probed with X-ray reflection diffraction revealing a low organization of the macromolecules. Even better insight into the organization behavior is gained by X-ray scattering of an extruded polymer fiber. In addition to the low degree of order, a p–p distance of 0.43 nm is detected, a large value in comparison to highperformance polymers reported in the literature and unfavorable for high mobility. It seems that polymer P1 is too stiff to obtain high order; more structural flexibility along the backbone is needed. For this reason, additional thiophene units bearing alkyl chains are incorporated into the backbone yielding polymer P2, a general concept known in the literature. By optimization of the polymerization conditions, a comparable molecular weight as for polymer P1 could be obtained (Mn1⁄4 22 kg mol ), minimizing field-effect mobility differences due to different polymer chain lengths. FETs are prepared following exactly the same procedure as for P1 to ensure maximum comparability. Average charge-carrier mobility in the low 10 3 cm V 1 s 1 region is measured for polymer P2. In fact, higher order and decreased p–p distance (0.39 nm) in comparison to polymer P1 are determined by fiber X-ray scattering. Obviously, the gain in backbone flexibility proves to be favorable, but the electronic performance is still not satisfactory. As polymer P2 possesses high solubility in organic solvents, fewer alkyl chains seemed possible to still guarantee the necessary solubility for processing. Therefore, polymer P3 is synthesized having no alkyl chains at the additional thiophene units to avoid ‘‘overalkylation’’ which may hinder close contact of adjacent polymer chains. Since using dodecyl chains as for polymer P1 renders an insoluble polymer, longer hexadecyl

Gold work function reduction by 2.2 eV with an air-stable molecular donor layer

Ultraviolet photoelectron spectroscopy was used to investigate neutral methyl viologen (1,1′-dimethyl-1H,1′H-[4,4′]bipyridinylidene, MV0) deposited on Au(111). As a result of molecule-to-metal electron transfer, the work function of Au(111) was decreased from 5.50to3.30eV. The energy levels of electron transport layers deposited on top of modified Au surfaces were shifted to higher binding energies compared to layers on pristine Au, and the electron injection barrier was reduced by 0.80eV for tris(8-hydroxyquinoline)aluminum (Alq3) and by 0.65eV for C60. The air-stable donor MV0 can thus be used to facilitate electron injection into organic semiconductors even from high work function metals.

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.

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.

Electronic structure of large disc-type donors and acceptors

Searching for new pi-conjugated charge-transfer systems, the electronic structure of a new acceptor-donor pair derived from coronene (C(24)H(12)) was investigated by ultraviolet photoelectron spectroscopy (UPS). The acceptor coronene-hexaone (C(24)H(6)O(6), in the following abbreviated as COHON) and the donor hexamethoxycoronene (C(30)H(24)O(6), abbreviated as HMC) were adsorbed as pure and mixed phases on gold substrates. At low coverage, COHON adsorption leads to the appearance of a charge-transfer induced interface state 1.75 eV below the Fermi energy. At multilayer coverage the photoemission intensity of the interface state drops and the valence spectrum of neutral COHON appears. The sample work function decreases from 5.3 eV (clean Au) to 4.8 eV (monolayer) followed by an increase to 5.6 eV (multilayer). The formation of a significant interface dipole due to charge-transfer at the metal-organic interface is possibly accompanied by a change in molecular orientation. HMC on Au exhibits no interface state and the sample work function decreases monotonically to ca. 4.8 eV (multilayer). The UPS spectra of individual donor and acceptor multilayers show good agreement with density functional theory modeling. In donor/acceptor mixed films the photoemission signal of the donor (acceptor) shifts to higher (lower) binding energy. This trend is predicted by the calculation and is anticipated when charge is transferred from donor to acceptor. We propose that mixed films of COHON and HMC constitute a weak charge-transfer system.

Interlayer molecular diffusion and thermodynamic equilibrium in organic heterostructures on a metal electrode

In this paper, we investigate by photoemission the electronic properties of an organic/organic interface consisting in a strong electron acceptor hexaazatriphenylene-hexacarbonitrile (HATCN) deposited on a physisorbed N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD) monolayer on Ag(111). At the first HATCN deposition steps (∼1 monolayer), the sample work function increases by 1.05 eV and the hole injection barrier (HIB) in the pre-adsorbed α-NPD monolayer is lowered by 0.65 eV. This results from HATCN diffusion to the silver surface through the α-NPD monolayer. Furthermore, this HATCN monolayer is proposed to form a compact chemisorbed monolayer, with a different structural arrangement than that observed on pristine Ag(111). In a second step, the additional deposited HATCN start growing on top of the α-NPD layer, and Fermi-level pinning, associated with the formation of HATCN negative polarons, is identified at the HATCN/α-NPD interface. Finally, HATCN is deposited on a α-NPD multilay...

Solid-solid transfer of organic semiconductors for field-effect transistor fabrication

We present a simple yet potentially universally applicable method for the solid–solid transfer of organic materials under ambient conditions for the fabrication of organic field-effect transistors. Thermal annealing of sprinkled powders of organic semiconductors on gold patterned SiOx surfaces yielded functional transistors with some of the characteristics comparable to those of solution-processed devices.

Charge-Transfer Controlled Crystallization of a Model Oligomer for Donor-Acceptor-Polythiophenes

A model donor-acceptor oligomer consisting of benzodithiophene-diketone and thiophene has been investigated with regard to its molecular packing and opto-electronic properties. The crystal structure suggests that the packing is dominated by charge-transfer interactions between the electron-rich part of the molecule and the electron-poor part. A series of observations corroborate this assumption, among them are a charge-transfer band in the film absorption spectra and exceptionally low π-π distances. A detailed analysis of the energetic levels of the present system reveals that only the HOMO level of the acceptor is shifted by conjugation to the donor. The results can contribute to the development of improved materials for organic electronics.