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.

Site- and orientation-selective anchoring of a prototypical molecular building block.

The controlled anchoring of molecular building blocks on appropriate templates is a major prerequisite for the rational design and fabrication of supramolecular architectures on surfaces. We report on a particularly selective adsorption process of hexa-peri-hexabenzocoronene on Au(111), which leads to well-controlled adsorption position and orientation of the polycyclic aromatic hydrocarbons. Scanning tunneling microscopy reveals selective adsorption on monatomic steps in the fcc stacking regions with a specific orientation of 18 degrees between the molecular axis and the step normal. Ab initio calculations for various adsorption sites reveal the lowest total energy for adsorption on a kink site. Energy considerations and the excellent agreement between experimental and simulated images show that adsorption on kink sites is responsible for the specific adsorption angle.

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.

Suppressing aggregation in a large polycyclic aromatic hydrocarbon.

With the approach presented herein, a large aromatic pi-system is synthesized, which shows extraordinarily high solubility and an effective suppression of aggregation. This was due to a distortion of the aromatic core by bulky tert-butyl groups and the solubilizing effects of alkyl chains in the corona of the aromatic core. Therefore not only the processing and cleaning of the materials with standard laboratory techniques became possible, but moreover the first structure-rich UV/vis and a resolved (1)H NMR spectra for an aromatic system two times larger than hexa-peri-hexabenzocoronene were recorded. The bulk properties in an extruded fiber as well as on the surface showed a columnar self-assembly including a phase in which a homeotropic alignment on a substrate was observed, which turns the material into an interesting candidate for future applications in electronic devices.

Influence of alkyl substituents on the solution- and surface-organization of hexa-peri-hexabenzocoronenes.

Three hexa-peri-hexabenzocoronenes (HBCs) with branched, bulky alkyl substituents of different lengths in the periphery of the aromatic core have been synthesized to tune the self-association properties in solution. 1H NMR and photophysical measurements were used to probe the solution organization in comparison to the known hexa-dodecyl-substituted HBC in different solvent systems. Thermodynamic parameters for the self-association in solution, obtained by curve fitting of the concentration- and temperature-dependent NMR data using van't Hoff analysis, indicated that the self-association is an enthalpically driven process that is entropically disfavored. Photoluminescence and NMR results were both employed to determine the critical concentration where no self-association for different compounds occurred. The interactions between the molecules could be controlled by varying the nonsolvent content in the solvent mixtures, supporting the model of solvophobic effects. The spatial demand of the solubilizing side chains modulated the self-association in solution. This behavior was translated into the solution casting process, where the kinetic in addition to the thermodynamic parameters played an essential role for structure formation. The study illuminates the relationship between the solution association of HBCs and the morphology, when processed on a surface. These results are essential for the application of these materials in devices.

Charge recombination via intercolumnar electron tunneling through the lipid-like mantle of discotic hexa-alkyl-hexa-peri-hexabenzocoronenes.

The recombination of the mobile charge carriers formed in pulse-ionized hexa-alkyl-substituted hexa-peri-hexabenzocoronenes occurs mainly via intercolumnar electron tunneling through the intervening hydrocarbon mantle. This is evidenced as a dramatic increase in the time scale of the decay of the radiation-induced conductivity from a few hundred nanoseconds to close to a millisecond as the peripheral alkyl substituents increase in size from 8 to 24 carbon atoms with corresponding disk diameters, D, from 23.4 to 36.6 A. The decay kinetics are a function only of the total number of peripheral carbon atoms with no evidence for specific effects of chain branching. The 1/e decay time, tau(e), increases exponentially with D according to tau(e) = tau(e)(0) exp(betaD) with tau(e)(0) = 48 fs and beta = 0.63 A(-1). Taking into account the tilted columnar configuration of the molecules in the solid phase leads to a beta value of ca. 0.8 A(-1) for the distance dependence of intercolumnar electron tunneling. In contrast to the orders of magnitude changes in the time scale for intercolumnar charge recombination, the intracolumnar charge hopping times vary by only a factor of 4, between 40 and 160 fs, with no systematic dependence on the nature of the alkyl substituents. On the basis of the results, the time scale estimated for electron tunneling across a 40 A thick lipid membrane is estimated to be close to 1 ms.

Exceptionally long-range self-assembly of hexa-peri-hexabenzocoronene with dove-tailed alkyl substituents.

The substitution of a hexa-peri-hexabenzocoronene by bulky, space-demanding, 2-decyltetradecyl side chains proved to be an effective procedure to influence the thermal and self-aggregation behavior. The extremely large steric requirement of the introduced side chains modulated the aggregation and resulted in a dramatic lowering of the isotropization temperature and a higher solubility. As an additional consequence of the exceptional, long-range self-aggregation of the discotic molecule, spherulite formation was observed during crystallization by polarized light microscopy. Macroscopic self-assembly was induced by zone crystallization of the material, leading to aligned columnar superstructures as revealed by 2D-WAXS experiments. These extraordinary properties have not yet been reported for a discotic columnar material, making this compound very promising for application in electronic devices.