A window of opportunity: A transparent graphene film has been obtained by a new bottom‐up chemical approach involving the thermal reaction of synthetic nanographene molecules of giant polycyclic aromatic hydrocarbons which are cross‐linked with each other and further fused into larger graphene sheets. Such graphene films have been applied as window electrodes in organic solar cells (see picture).

The effects of the core size, the side chain length, and the number of substituents on the supramolecular organization of polycyclic aromatic hydrocarbons have been investigated by 2D wide-angle X-ray scattering experiments performed on oriented filaments prepared by extrusion. The aromatic core size of the compounds varied between the hexa-peri-hexabenzocoronene core consisting of 42 carbon atoms and the enlarged aromatic core of 132 carbon atoms, whereas the length of the side chains extended up to 20 carbon atoms per chain. It has been observed that the lateral packing characterized by the lattice constant of the 2D intercolumnar hexagonal arrangement increases with the molecular masses of the core, forming the columnar stacks, and of the side chains which fill the core periphery. A model describing the relationship between the morphological and molecular parameters for the columnar hexagonal arrangements of the discotics is suggested and indeed proven by comparison with the experimental results.

We report herein an unexpected circular dichroism (CD) effect observed for a dilute solution of an achiral oligo(p-phenylene vinylene) derivative, A-OPV3 (Scheme 1) in dodecane (c = 5 10 À5 m). This effect likely originates from macroscopic phenomena during the self-assembly process of A-OPV3 into fibers after slow cooling from 363 K to 293 K at a rate of 60 K h À1 (Figure 1 a). To our surprise, the CD response increased in intensity after shaking the fully transparent, nonviscous solution. To shed light on this observation, we studied the self-assembly of A-OPV3 in more detail. Upon cooling the A-OPV3 solution from 363 to 293 K, a typical blue shift of the absorption maximum to l = 407 nm with a shoulder at l = 475 nm is observed. This finding indicates that at high temperature the molecules are molecularly dissolved, while at low temperature they are self-assembled into fibers (Figure 1 b).[1] However, at low temperatures, a small response in the linear dichroism (LD) spectrum is observed, which increased slightly after shaking (Figure 1 c). LD and CD are absent at higher temperatures. The atomic force microscopy (AFM) images of dried drop-cast solutions of these selfassembled systems on mica showed large bundles of long fibers (Figure 1 d).The three spectra (UV/Vis, CD, and LD) are almost identical in shape (Figure 1 a-c). However, when monitoring the spectra during the temperature-induced self-assembly process (absorption and CD intensity at l = 475 nm and the LD intensity at l = 400 nm), the absorbance changes first, while the LD and CD signals arise together at lower temperatures.[2] Because A-OPV3 molecules form fibrous aggregates with a length that increases upon lowering the temperature, this behavior indicates that a certain fiber length is needed to induce the LD and CD effects. Hence, the increase in the LD is most likely associated with the (partial) alignment of the A-OPV3 fibers in solution arising from convective flow, which is caused by temperature differences in the cuvette.[3] Additional measurements show that the alignment is less significant when a smaller cuvette (0.1 vs. 1 cm) is used, [2] that is, when the flow pattern is altered. The flow pattern induced by shaking might enhance the alignment

A magnetic field has been utilized for producing highly oriented films of a substituted hexabenzocoronene (HBC). Optical microscopy studies revealed large area HBC monodomains that covered the entire film, while wide-angle X-ray measurements showed that the HBC molecules are aligned with their planes along the applied field. On the basis of this method, solution-processed field-effect transistors (FET) have been constructed with charge carrier mobilities of up to 10(-3) cm2/V.s, which are significantly enhanced with respect to the unaligned material. Exceptionally high mobility anisotropies of 25-75 for current flow parallel and perpendicular to the alignment direction have been measured as a function of the channel length. Atomic force microscopy performed on the FET structures reveals fibril superstructures that are oriented perpendicularly to the magnetic field direction, consisting of molecular columns with a slippage angle of 40 degrees between the molecules. For channel lengths larger than 2.5 mum, the fibrils are smaller than the electrode spacing, which adversely affects the device performance.

The optical absorption and charge transport properties of a series of discotic molecules consisting of peripherally alkyl-substituted polycyclic aromatic cores have been investigated for core sizes, n, of 24, 42, 60, 78, 96, and 132 carbon atoms. In dilute solution, the wavelength maximum of the first absorption band increases linearly with n according to lambda(max) = 280 + 2n and the spectral features become increasingly broadened. The two smallest core compounds display a slight red-shift and increased spectral broadening in spin-coated films. For derivatives with n = 24, 42, 60, and 96, the one-dimensional, intracolumnar charge mobility, Sigma mu(1D), was determined using the pulse-radiolysis time-resolved microwave conductivity technique. For the compounds which were crystalline solids at room temperature, Sigma mu(1D) lay within the range 0.4-1.0 cm(2)/Vs. In the discotic mesophases at ca. 100 degrees C, Sigma mu(1D) was somewhat lower and varied from 0.08 to 0.38 cm(2)/Vs. The mobility values in both phases are considerably larger than the maximum values found previously for discotic triphenylene derivatives. However, the recently proposed trend toward increasing mobility with increasing core size is not substantiated by the results on the present series of increasingly large aromatic core compounds.

Intended for use in high performance applications where electrical conductivity is required, we developed a CNT-TPU composite. Such a composite can be prepared by melt processing (extrusion) on an industrial scale. Due to the known hazard upon inhalation of CNTs, we assessed three degradation scenarios that may lead to the release of CNTs from the composite: normal use, machining and outdoor weathering. Unexpectedly, we find that the relative softness of the material actually enhances the embedding of CNTs also in its degradation fragments. A release of free CNTs was not detected under any condition using several detection methods. However, since machining may induce a high acute dose of human exposure, we assessed the cytotoxicity potential of released fragments in the in vitro model of precision-cut lung slices, and found no additional toxicity due to the presence of CNTs. At very low rates over years, weathering degrades the polymer matrix as expected for polyurethanes, thus exposing a network of entangled CNTs. In a preliminary risk assessment, we conclude that this material is safe for humans in professional and consumer use.

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The thermal properties and self‐organization of hexa‐peri‐hexabenzocoronene (HBC) derivatives with dove‐tailed alkyl chains of various lengths have been investigated using polarized optical microscopy and wide‐angle X‐ray scattering. It is shown that the size‐related increase of steric interactions among the peripheral side chains substituted to the aromatic core leads to a dramatically lowered isotropization temperature, allowing thermal processing at practical temperatures. Additionally, the introduction of ether linkages within the side chains enhances the affinity of the discotic molecules towards polar surfaces, resulting in homeotropic self‐assembly when the compounds are processed from the isotropic state between two surfaces and, for the first time, as a thin film on a single surface. It is established that the degree of homeotropic order is influenced by the phase behavior, the supramolecular order in the bulk, and the surface affinity of the corresponding derivatives. These results are important for the design of photovoltaic cells based on HBC derivatives.