A C216-Nanographene Molecule with Defined Cavity as Extended Coronoid.

We describe the first coronoid nanographene C216-molecule. As an extended polycyclic aromatic hydrocarbon containing a defined cavity, our molecule can be seen as a model system to study the influence of holes on the physical and chemical properties of graphene. Along the pathway of an eight-step synthesis including Yamamoto-type cyclization followed by 6-fold Diels-Alder cycloaddition, C216 was obtained by oxidative cyclodehydrogenation in the final step. The defined molecular structure with a cavity was unambiguously validated by MALDI-TOF mass spectrometry and FTIR, Raman, and UV-vis absorption spectroscopy coupled with DFT simulations.

Photoconductive and supramolecularly engineered organic field-effect transistors based on fibres from donor-acceptor dyads.

We report on the formation of photoconductive self-assembled fibres by solvent induced precipitation of a HBC-PMI donor-acceptor dyad. Kelvin Probe Force Microscopy revealed that upon illumination with white light the surface potential of the fibres shifted to negative values due to a build-up of negative charge. When integrated in a field-effect transistor (FET) configuration, the devices can be turned 'on' much more efficiently using light than conventional bias triggered field-effect, suggesting that these structures could be used for the fabrication of light sensing devices. Such a double gating represents an important step towards bi-functional organic FETs, in which the current through the junction can be modulated both optically (by photoexcitation) and electrically (by gate control).

Enhanced mobility in P3HT-based OTFTs upon blending with a phenylene-thiophene-thiophene-phenylene small molecule.

An enhancement in charge transport capacity in a poly(3-hexylthiophene) (P3HT) semicrystalline film, up to field-effect mobilities approaching 0.1 cm(2) V(-1) s(-1), has been achieved by co-deposition with a small molecule, i.e. 5,5'-bis(4-n-hexylphenyl)-2,2'-bithiophene (dH-PTTP), forming highly ordered crystals bridging large polymeric domains.

Two-dimensional polymer formation on surfaces: insight into the roles of precursor mobility and reactivity.

We report on a combined scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) study on the surface-assisted assembly of the hexaiodo-substituted macrocycle cyclohexa-m-phenylene (CHP) toward covalently bonded polyphenylene networks on Cu(111), Au(111), and Ag(111) surfaces. STM and XPS indicate room temperature dehalogenation of CHP on either surface, leading to surface-stabilized CHP radicals (CHPRs) and coadsorbed iodine. Subsequent covalent intermolecular bond formation between CHPRs is thermally activated and is found to proceed at different temperatures on the three coinage metals. The resulting polyphenylene networks differ significantly in morphology on the three substrates: On Cu, the networks are dominated by "open" branched structures, on the Au surface a mixture of branched and small domains of compact network clusters are observed, and highly ordered and dense polyphenylene networks form on the Ag surface. Ab initio DFT calculations allow one to elucidate the diffusion and coupling mechanisms of CHPRs on the Cu(111) and Ag(111) surfaces. On Cu, the energy barrier for diffusion is significantly higher than the one for covalent intermolecular bond formation, whereas on Ag the reverse relation holds. By using a Monte Carlo simulation, we show that different balances between diffusion and intermolecular coupling determine the observed branched and compact polyphenylene networks on the Cu and Ag surface, respectively, demonstrating that the choice of the substrate plays a crucial role in the formation of two-dimensional polymers.

A conjugated thiophene-based rotaxane: synthesis, spectroscopy, and modeling.

A dithiophene rotaxane 1 subsetbeta-CD and its shape-persistent corresponding dumbbell 1 were synthesized and fully characterized. 2D NOESY experiments, supported by molecular dynamics calculations, revealed a very mobile macrocycle (beta-CD). Steady-state and time-resolved photoluminescence experiments in solution were employed to elucidate the excited-state dynamics for both systems and to explore the effect of cyclodextrin encapsulation. The photoluminescence (PL) spectrum of 1 subsetbeta-CD was found to be blueshifted with respect to the dumbbell 1 (2.81 and 2.78 eV, respectively). Additionally, in contrast to previous observations, neither PL spectra nor the decay kinetics of both threaded and unthreaded systems showed changes upon increasing the concentration or changing the polarity of the solutions, thereby providing evidence for a lack of tendency toward aggregation of the unthreaded backbone.

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.

A high-mobility electron-transporting polymer for printed transistors.

Printed electronics is a revolutionary technology aimed at unconventional electronic device manufacture on plastic foils, and will probably rely on polymeric semiconductors for organic thin-film transistor (OTFT) fabrication. In addition to having excellent charge-transport characteristics in ambient conditions, such materials must meet other key requirements, such as chemical stability, large solubility in common solvents, and inexpensive solution and/or low-temperature processing. Furthermore, compatibility of both p-channel (hole-transporting) and n-channel (electron-transporting) semiconductors with a single combination of gate dielectric and contact materials is highly desirable to enable powerful complementary circuit technologies, where p- and n-channel OTFTs operate in concert. Polymeric complementary circuits operating in ambient conditions are currently difficult to realize: although excellent p-channel polymers are widely available, the achievement of high-performance n-channel polymers is more challenging. Here we report a highly soluble ( approximately 60 g l(-1)) and printable n-channel polymer exhibiting unprecedented OTFT characteristics (electron mobilities up to approximately 0.45-0.85 cm(2) V(-1) s(-1)) under ambient conditions in combination with Au contacts and various polymeric dielectrics. Several top-gate OTFTs on plastic substrates were fabricated with the semiconductor-dielectric layers deposited by spin-coating as well as by gravure, flexographic and inkjet printing, demonstrating great processing versatility. Finally, all-printed polymeric complementary inverters (with gain 25-65) have been demonstrated.

Electron-deficient N-heteroaromatic linkers for the elaboration of large, soluble polycyclic aromatic hydrocarbons and their use in the synthesis of some very large transition metal complexes.

The selective oxidation of the perimeter of an extended polycyclic aromatic hydrocarbon (PAH), namely a six-fold tert-butylated tetrabenzo[bc,ef,hi,uv]ovalene, led to the formation of an alpha-diketone. The newly installed carbonyl centers allowed this building block to be converted into the largest known heteroatom-containing PAHs (up to 224 atoms in the aromatic core) by way of the quinoxaline ring condensation reaction. The tert-butyl substituents caused a distortion of the usually planar aromatic frameworks, which hampered the aggregation tendency of the extended aromatic pi-systems and led to extraordinarily high solubilities. All of the systems described here, even the giant phthalocyanine, could thus be purified using standard chromatographic techniques and characterized using typical spectroscopic methods. For the first time, fully resolved 1H NMR spectra of soluble, diamagnetic, 98- and 104-atom-containing aromatic systems are presented. The computed and experimental UV/vis spectra emphasize the dependence of the characteristic alpha-, p-, and beta-bands upon the size of the PAHs. It was also possible to obtain the largest known ligand to yet be complexed around a ruthenium center. A quadrupolar solvatochromic effect was observed when two donating PAH moieties were fused to an accepting quinoxaline center, in which case the photoluminescence spanned a range of about 80 nm. Electrochemical properties of the new nanographenes were investigated using cyclic voltammetry, and this showed quasi-reversible reductions.

Nanostructuring with a crosslinkable discotic material.

A high-yielding synthesis afforded a hexa-peri-hexabenzocoronene carrying acrylate units at the end of six attached alkyl spacers. The polymerization of these acrylate moieties could be initiated with thermal energy and through direct photoactivation without the addition of a photoinitiator. This allowed the organization of the liquid-crystalline material to be fixed in either the crystalline state or the mesophase, which preserved the organization in the respective phase. The use of a focused synchrotron beam permitted selected regions of a thin film to be rendered insoluble. After "developing" the film in this lithographic process by dissolving the soluble, unpolymerized material, defined nano-objects remained on the substrate. In addition, the pronounced aromatic pi stacking of the novel material allows an organization in mesoporous membranes that could be fixed by thermal crosslinking. After the removal of the inorganic template, mechanically stable nanotubes were obtained, which were characterized by different microscopy techniques.

Columnar mesophase formation of cyclohexa-m-phenylene-based macrocycles.

Two novel discotic macrocycles, substituted cyclohexa-m-phenylene (CHP) and cyclo-3,6-trisphenanthrylene (CTP), and the linear oligomer 3,3':6',3''-terphenanthrene (TP) as a model substance have been synthesized by repetitive cross-coupling reactions. To correlate the molecular design with the supramolecular architecture and the established macroscopic order, 2D wide-angle X-ray scattering experiments were performed on mechanically extruded filaments. At room temperature in their crystalline phases, all three compounds revealed columnar assemblies in which the macrocycles self-organized by pi-stacking interactions. The degree of macroscopic order was found to depend upon the planarity and stiffness of the aromatic core. The flexible CHP ring showed a poor macroscopic order of the columnar structures and a low isotropization temperature, whereas the more-planar, less-flexible CTP self-assembled into well-defined superstructures. The larger pi-stacking area and the more-pronounced intermolecular interactions for CTP led to the formation of a mesophase over a very large temperature range. The surprising columnar organization of the "open" TP system was explained by back-folding of the molecule into a ringlike structure.

Dendritic morphology in homeotropically aligned discotic films.

A hexa-alkylether-substituted hexa-peri-hexabenzocoronene, as a discotic liquid crystal, spontaneously homeotropically self-orients when cooled from the isotropic phase. During this process, a dendritic morphology is formed in which a significant optical difference is observed between the dendritic shapes and their periphery. However, local structural analysis using microfocus synchrotron radiation experiments demonstrate that there is an identical supramolecular order in both areas. Three-dimensional confocal surface measurements reveal that these dendritic structures result from a dewetting process, which occurs during solidification. Thereby, the contact area between the organic material and the surface is reduced. These results are important for the design of organic electronics, since the reduction of the contact area in an electronic device might inhibit the charge transport between the discotic semiconductor and the electrode.

From macro- to nanoscopic templating with nanographenes.

A hexa-peri-hexabenzocoronene carrying six long, branched alkyl chains has been processed in nano- and macroscopic pore templates. An extraordinary self-organization of this material was observed within macroscopically large glass capillaries after cooling from the isotropic phase. Thereby, the columnar structures were long-range aligned along the capillary axis over several centimeters. This behavior was explained by the pronounced directional self-assembly of the molecules, whereby the curvature effect of the capillary was negligible. The processing in nanoscopic pores of an inorganic membrane yielded an improved supramolecular organization. It was possible to remove the inorganic template and to obtain bundles and single nanorods. The templating over different dimensions opens a variety of potential applications.

Pyrene as chromophore and electrophore: encapsulation in a rigid polyphenylene shell.

Starting from the fourfold ethynyl-substituted chromophore 1,3,6,8-tetraethynylpyrene as core, a series of polyphenylene dendrimers was prepared in high yield by combining divergent and convergent growth methods. The fluorescence quantum yields (Q(f)>0.92) of the encapsulated pyrene chromophore were independent of the size of the polyphenylene shell. Fluorescence quenching studies and temperature-dependent fluorescence spectroscopy were performed to investigate the site isolation of the core. They indicate that a second-generation dendrimer layer is needed to efficiently shield the encapsulated pyrene and prevent aggregate formation. Alkali-metal reduction of the encapsulated pyrene core was carried out to afford the corresponding pyrene radical anions, for which hampered electron transfer to the core was observed with increasing dendrimer generation, which is further proof of the site isolation due to the polyphenylene shell. To improve film formation and solubility of the material, solubilizing alkyl chains were introduced on the periphery of the spherical particles. Furthermore, highly transparent films obtained by a simple drop-casting method showed blue emission mainly from the unaggregated species. The materials presented herein combine high quantum efficiency, good solubility, and improved film-forming properties, which make them possible candidates for several applications in electronic devices.

From armchair to zigzag peripheries in nanographenes.

Synthetic concepts toward the synthesis of large, not-fully benzenoid polycyclic aromatic hydrocarbons (PAHs), decorated with phase-forming and solubilizing n-dodecyl chains, are presented based on the intramolecular cyclodehydrogenation reaction of suitable oligophenylene precursors. The formal addition of successive C2 units into the armchair bays of the parent hexa-peri-hexabenzocoronene extends the aromatic system and leads to PAHs with a partial zigzag periphery. This variation of the nature of the periphery, symmetry, size, and shape has a distinct impact upon the electronic properties and the organization into columnar superstructures. Both computational and experimental UV/vis spectra, which are in good agreement, emphasize the dependence of the characteristic bands alpha, p, and beta upon the overall size and symmetry of the PAHs. While the number and the substitution patterns of attached n-dodecyl chains do not influence the electronic properties, the thermal behavior and supramolecular organization are strongly influenced, which has been elucidated with differential scanning calorimetry (DSC) and 2D wide-angle X-ray diffractometry (2D-WAXS) on mechanically aligned samples. This study provides valuable insight into the future design of semiconducting materials based on extended PAHs.

Self-assembly of extended polycyclic aromatic hydrocarbons on Cu111.

We present a low-temperature scanning tunneling microscopy study on the self-assembly of extended polycyclic aromatic hydrocarbons with different symmetries on the Cu111 surface. All molecules show a commensurate monolayer structure, with significant structural differences with respect to the unit cell of the molecular lattice and the orientational ordering. We find that the molecular lattice and the molecular orientation are largely dominated by molecule-substrate interactions, whereas molecule-molecule interactions determine the molecular packing density via steric repulsion. Moreover, we show that the structure of the monolayer is transferred to the second layer via molecule-molecule interaction.

Control of morphology in efficient photovoltaic diodes from discotic liquid crystals.

We report on the role of morphology in photovoltaic diodes with blend active layers composed of perylene tetracarboxdiimide (EPPTC) and hexabenzocoronene (HBC) derivatives as electron and hole acceptors. Controlled annealing of HBC:EPPTC films while in conformal contact with a flat elastomeric stamp improves photovoltaic response, leading to an external quantum efficiency of 29.5% at 460 nm and an open-circuit voltage of 0.70 V. The improved performance is attributed in part to larger crystalline domains following annealing. The elastomeric stamp restricts the top surface of the thin film during annealing, leading to low surface roughness, while also allowing for greater vertical stratification of the components in the bulk. Blended HBC:EPPTC films also exhibit an unique optical absorption feature near 590 nm, which we attribute to an altered crystalline packing of EPPTC in the presence of HBC. The significance of the local structure at molecular heterojunctions in blended active layer photovoltaic diodes is discussed.