Oliver Henze

Surface-controlled self-assembly of chiral sexithiophenes

We report on the self-assembly of two enantiomeric sexithiophenes in solution and on surfaces. Circular dichromism of aggregated sexithiophenes and drop-cast films reveals, as expected, mirror image spectra for both enantiomers. The aggregation in thin deposits from sexithiophenes molecularly dispersed in a solution on different types of substrates was investigated by atomic force microscopy (AFM). On graphite, one-dimensional objects (nanowires) are formed while on mica platelets are generated. Remarkably, we found that both enantiomers form left-handed helices on silicon. This observation depends on the hydrophilicity of the silicon. Furthermore, the achiral sexithiophene did not form helical aggregates suggesting that the stereocenter is required to obtain chirality in the fibers.

Side‐Chain Degradation of Ultrapure π‐Conjugated Oligomers: Implications for Organic Electronics

The degrdn. of two defect-free pi-conjugated oligomers and the participation of their solubilizing side chains in the process are studied in unprecedented detail. The detected intermediate products reveal a mechanism of successive shortening of alkyl and oligo(ethylene glycol) substituents. Eventually, these processes cause chem. modifications in the conjugated backbone, which strongly influence the electronic properties of the materials.

Ultrathin organic films for field effect transistors

Organic field effect transistors FET have been fabricated with active semiconducting organic thin films that are only a few monolayers thick. The motivation of this study has been to establish a direct correlation between transistor performance and the polymer microstructure in the ultrathin accumulation layer of the transistor. Monolayer thick films of a block copolymer and several model oligomers consisting of a rigid conjugated sexithiophene (6T) block and a flexible polyethyleneoxide (PEO) block have been deposited onto the surface of e.g. SiO2 gate dielectrics functionalized with a self-assembled monolayer. Block copolymer phase behavior and surface morphology has been studied as a function of chain length, solvent and film thickness. Operational transistors have been demonstrated with film thicknesses of only one or two monolayers. Typical device characteristics show a carrier mobility on the order of 10-2 - 10-3 cm2/Vs and ON-OFF current ratio of 103 - 105. Film microstructure, orientation of micro-crystallites and film thickness have been studied by atomic force microscopy (AFM), UV-Vis absorption spectroscopy and X-ray diffraction.

Anharmonic Magnetic Deformation of Self-Assembled Molecular Nanocapsules

High magnetic fields were used to deform spherical nanocapsules, self-assembled from bolaamphiphilic sexithiophene molecules. At low fields the deformation--measured through linear birefringence-scales quadratically with the capsule radius and with the magnetic field strength. These data confirm a long standing theoretical prediction [W. Helfrich, Phys. Lett. A 43, 409 (1973)10.1016/0375-9601(73)90396-4], and permit the determination of the bending rigidity of the capsules as (2.6+/-0.8) x 10(-21) J. At high fields, an enhanced rigidity is found which cannot be explained within the Helfrich model. We propose a complete form of the free energy functional that accounts for this behavior, and allows discussion of the formation and stability of nanocapsules in solution.

Oligoethyleneoxide functionalised sexithiophene organic field effect transistors

Thin films of oligoethyleneoxide functionalised sexithiophene have been prepared utilising the self organising properties of the material. The polarity difference of the constituents induces preferential order in thin films of the material which enhances transistor operation. Devices are fabricated with the material as the organic semiconductor and the thin film transistor (TFT) characteristics can be related to the structure and organisation of the thin film.

Molecular organization of cylindrical sexithiophene aggregates measured by X-ray scattering and magnetic alignment.

We have determined the internal organization of elongated sexithiophene aggregates in solution by combining small-angle X-ray scattering and magnetic birefringence experiments. The different aggregate axes can be probed independently by performing the experiments on magnetically aligned aggregates. We have found multiwalled cylindrical aggregates consisting of radially oriented sexithiophene molecules with pi-pi-stacking in the tangential direction, a structure that is considerably different from those previously found in other solvents. The aggregate morphology of this semiconducting material can thus be tuned by using different solvents, which offers the attractive perspective to steer chemical self-assembly toward nanostructures with desired functionalities, especially in combination with the alignment in a magnetic field.

The synthesis and characterisation of amphiphilic α,α-linked sexithiophenes with substituents at the terminal α-positions

A series of polydisperse amphiphilic α,α-linked sexithiophenes with an oligoethylene oxide substituent at one terminal α-position and a hydrogen atom or a hexyl group at the other have been synthesised via Stille and Suzuki cross coupling reactions. The products have been characterised by a combination of 1H and 13C NMR spectroscopy and detailed MALDI-TOF mass spectrometry.

The synthesis of amphiphilic sexithiophene–poly(ethylene oxide) segmented copolymers

Sexithiophene–poly(ethylene oxide) segmented copolymers with ethylene oxide blocks with Mn = 1000 and Mn = 600 and with alternating hydrophobic and hydrophilic blocks sandwiching the sexithiophene blocks have been synthesised via Stille and Suzuki cross coupling reactions. All polymers showed limited solubility; the soluble low molecular weight fractions have been characterised by a combination of 1H NMR spectroscopy and GPC.

Magnetic Field Alignment of Liquid Crystalline Pentathiophene

Liquid crystalline α,α′-disubstituted pentathiophene was aligned using high magnetic fields (>5 T) resulting in macroscopically oriented samples. The pentathiophene crystallites were found to be aligned perpendicular to the magnetic field direction, as is evidenced by microscopy images. The samples are stable at room temperature and will be used for further investigations such as the determination of the (anisotropic) mobility and the polarization of the light emission.

One and Two-Dimensional Semiconducting Nanostructures Self-Assembly of Conjugated Oligomers

The properties of org. electronic materials in the solid-state are detd. not only by those of individual mols. but also by those of ensembles of mols. The ability to control the architectures of these ensembles is thus essential for optimizing the properties of conjugated materials for use in electronic devices (light emitting diodes, field effect transistors, solar cells, ...) and is primordial for potential technol. applications in nanoelectronics. Here, we report on the observation by at. force microscopy (AFM) of 1D and 2D nanoscale architectures obtained in the solid-state from solns. of molecularly-dissolved conjugated block copolymers or oligomers, and demonstrate that the conjugated mols. can organize onto a surface over lengthscales from nanometers to several microns, forming semiconducting fibrils by p-stacking processes