Generation of twisted nanowires with achiral organic amphiphilic copper complexes.

Drying under solvent atmosphere (DUSA) was investigated as an experimental technique to generate self-assembled nanowires and needles from solutions of organic molecules under controlled conditions. Experimental observations of twisted nanowires are reported. These twisted nanowires were obtained by drying of solutions of achiral molecules under solvent controlled atmospheres: achiral, amphiphilic copper complexes were dissolved in an achiral solvent and these solutions were dried under controlled conditions. Two structurally related copper complexes were investigated. Microscopic investigations of the resulting nanowires revealed, and scanning electron microscopy confirmed: self-assembled twisted ribbons could be selectively obtained from one of these compounds. This behavior could be explained by comparing the ratio of the size of the head group and the overall length of the molecules. The occurrence of chiral filaments and chiral phases of nanosegregated filaments are rare in achiral compounds. The occurance of such twisted filaments is thought to be related to symmetry-breaking during a phase transition from liquid crystalline or lyotropic liquid crystalline phases to a nanosegregated phase. In the reported experiments, the concentration of a solution was gradually increased until crystallization occurred. The results clearly show how DUSA can be applied to investigate the capability of achiral substances to yield twisted filaments. Moreover, the investigated compounds had high-enough charge carrier mobilities, such that the twisted filaments obtained are candidates for self-assembled, intrinsically coiled nano-inductivities.

Ultrasmall magnetic field-effect and sign reversal in transistors based on donor/acceptor systems.

We present magnetoresistive organic field-effect transistors featuring ultrasmall magnetic field-effects as well as a sign reversal. The employed material systems are coevaporated thin films with different compositions consisting of the electron donor 2,2',7,7'-tetrakis-(N,N-di-p-methylphenylamino)-9,9'-spirobifluorene (Spiro-TTB) and the electron acceptor 1,4,5,8,9,12-hexaazatriphenylene hexacarbonitrile (HAT-CN). Intermolecular charge transfer between Spiro-TTB and HAT-CN results in a high intrinsic charge carrier density in the coevaporated films. This enhances the probability of bipolaron formation, which is the process responsible for magnetoresistance effects in our system. Thereby even ultrasmall magnetic fields as low as 0.7 mT can influence the resistance of the charge transport channel. Moreover, the magnetoresistance is drastically influenced by the drain voltage, resulting in a sign reversal. An average B0 value of ≈2.1 mT is obtained for all mixing compositions, indicating that only one specific quasiparticle is responsible for the magnetoresistance effects. All magnetoresistance effects can be thoroughly clarified within the framework of the bipolaron model.

Photoinduced sign change of the magnetoresistance in field-effect transistors based on a bipolar molecular glass.

The application of a novel bipolar molecular glass in field-effect transistors leads to devices with photoinduced magnetoresistance (MR) sign change. In darkness a low external magnetic field increases the resistance (positive MR up to +0.1%), while a magnetic-field induced resistance decrease (negative MR up to -6.5%) can be achieved under illumination.

Electronic properties of spiro compounds for organic electronics.

The electronic properties of p-type, n-type, and ambipolar spiro materials have been investigated using a combination of photoemission spectroscopy, electron energy-loss spectroscopy, and density functional based calculations. Our results provide insight into the occupied density of states as well as the electronic excitation spectra. Comparison of experimental and theoretical data allows the identification of the orbitals responsible for charge transport and optical properties.

Ultralong single organic semiconducting nano/microwires based on spiro-substituted perylenetetracarboxylic diimide.

An ultralong single organic semiconducting nano/microwire (NMW) is difficult to obtain. Here we show that this NMW can easily be prepared by using drying under solvent atmosphere method. This technique is not only unique, but also very compatible with our active material, spiro-substituted perylenetetracarboxylic diimide. A single NMW with a length of up to ∼5.5 mm and an aspect ratio of ∼9200 can be obtained. Finally, we succeeded to measure the electrical resistivity of a single NMW with values between 1 × 10(2) and 1 × 10(4) Ω m and the growth direction can be controlled as well by using a prestructured substrate.

Bipolar redox behaviour, field-effect mobility and transistor switching of the low-molecular azo glass AZOPD.

We present electrochemical and spectroelectrochemical data for the bipolar azo compound N,N'-diphenyl-N,N'-bis[4-(phenylazo)phenyl]-4,4'diaminobiphenyl (AZOPD) demonstrating reversible bipolar redox behaviour with a bandgap of 2.1 eV. The reduced species formed upon two-electron transfer can be described as bis(radical anion) as was confirmed by comparison with a reference compound with only one azo chromophore. Hole and electron transport behaviour in amorphous films was demonstrated by the fabrication of organic field-effect transistors using gold and magnesium contacts, respectively. The transistors are sensitive to light due to E-Z photoisomerization.