- Tailoring Fluorene-based Oligomers for micron and sub-micron sized Photopatterning (2009)
- This thesis describes the work on tailor-made synthesis, characterization and application of well-defined fluorene oligomers for photoptatterning. Two types of fluorene oligomers are presented: pure fluorene oligomers and fluorene cooligomers incorporating various comonomers for adjusting the conductive properties towards electron and hole conduction. Since possible applications for these materials feature organic light emitting diodes and organic field effect transistors we focused on the preparation of well-defined and defect free oligomers and the preservation of their electro-optical properties during photopatterning. Further on the requirements for material synthesis are easy procedures and large quantities. Therefore we developed an approach, which produces large quantities combined with the adjustment of the desired properties in one single step. The synthetic strategy throughout the thesis comprises the addition of an endcapping species in aryl-aryl polymerization reactions. The tailormade endcapper fulfils three tasks at once, the control of the molecular weight, the introduction of polymerizable acrylate moieties and the avoidance of undesired endgroups. As aryl-aryl coupling methods the nickel catalyzed Yamamoto and palladium catalyzed Suzuki condensations were applied. The completeness of the aryl-aryl coupling and the endcapping was proven with Maldi-ToF mass spectrometry. With this approach the properties of the oligomers can be easily adjusted in view of the optimization of their photopolymerization behaviour. The first oligomer series deals with the effect of the molecular weight on the properties and photopatterning behaviour of pure fluorene oligomers. The molecular weights were controlled by the amount of functionalized endcapper, which carried the polymerizable acrylate groups. As coupling method the Yamamoto coupling was applied. The molecular weights defined the temperature range of the nematic mesophases. An increase of the average chain length leads to higher transition temperatures (e.g. clearing temperatures) and to better film forming properties. The photopolymerization is usually performed in the nematic state to achieve a sufficient mobility of the acrylates. The irradiation conditions had major consequences on the preservation of the characteristic electro-optical properties of the fluorenes, the harsher the conditions the higher was the probability to destroy the chemical structure by photooxidation. Further on since each chain only carries two acrylate functionalities attached to the endcappers the total number of acrylates is different for high molecular weight and low molecular weight mixtures. The lowest molecular weight mixture contains the most acrylates and shows the lowest transition temperature, which leads to the best micron sized photopatterns. The second generation of pure fluorene oligomers demonstrate how different contents of polymerizable groups affect the photopolymerization behaviour. Here the molecular weights were kept constant around 5000 g/mol by equal amounts of endcapper and the acrylate groups were introduced by the fluorene monomers. The Yamamoto coupling was used and upon cooligomerization with a non-acrylate fluorene monomer the acrylate content was changed from 10% to 100%. The photopolymerization times strongly depend on the acrylate content, the 100% acrylate oligomer could be photopatterned in 30 seconds, whereas the 80% and 60% mixtures needed 2-5 minutes. In the best case crosslinking is 20 times faster than found for the preceding series, which ensures the preservation of the electro-optical properties. With the highest acrylate content a photocrosslinking even at room temperature became possible. The third oligomer series describes the incorporation of various comonomers such as TPD and bithiophene via Yamamoto reaction. Taking the knowledge of the two preceding generations into account we exploited the acrylate monomer from series two and introduced 30% comonomer. This ensured a sufficient content of acrylates for a fast photopatterning and enough comonomer for a shift of the electronic properties. We found that the electronic structure of the comonomer strongly affected the behaviour in the Yamamoto reaction. The HOMO and LUMO energy values were shifted towards hole or electron conduction. The photopatterning conditions were similar as found for the corresponding pure oligofluorenes with a 60% acrylate content. 2-5 minutes exposure produced highly emissive micro patterns. Thus a change of the electro-optical properties does not affect the photopolymerization behaviour and vice versa. Since we found differences in the incorporation of comonomers during Yamamoto coupling we applied the Suzuki coupling, which ensures an alternately linkage of monomers. Here protective groups had to be used since the Pd-catalyst does not tolerate acrylate functionalities. We found that the comonomers were incorporated quantitatively, but the major difficulty proved to be the polymeranalogous reactions following the polycondensation. The energy values were shifted towards electron and hole conduction and the photopolymerization behaviour was similar to the Yamamoto oligomers. An exposure time of 2-5 minutes produced patterns with a maximum resolution of 700 nm. To conclude the endcapping strategy combined with the Yamamoto coupling is a most effective tool for the adjustment of properties within one single step. The acrylate content as well as the molecular weight can be precisely tuned, which allows a good control of the photopatterning properties. In some cases e.g. with electron withdrawing comonomers the Suzuki cross coupling is the method of choice.