Supramolecular fluorene based materials

This thesis describes the use of noncovalent interactions in order to manipulate and control the self-assembly and morphology of electroactive fluorene-based materials. The supramolecular arrangement of p-conjugated polymers and oligomers can strongly influence their electronic and photophysical properties. Therefore, a detailed understanding of such organisation processes is essential for the optimisation of the performance of these materials as applied in optoelectronic devices. In order to achieve this goal, fluorene-based chromophores were equipped with various groups able to form specific supramolecular interactions. Their distinct optical properties were exploited to monitor the organisation processes of single- and multicomponent systems both in solution and in the solid state. Chapter 1 presents an overview covering the scientific literature published on organic fluorene-based semiconductors, their optoelectronic properties and their supramolecular chemistry. In Chapter 2, a set of three chiral fluorene (co)polymers and their syntheses are introduced. The phase behaviour of these materials in thin spin coated films was investigated. CD spectroscopy was used as the main instrument in this study and demonstrated to provide a sensitive tool to monitor structural rearrangements upon thermal treatment. At elevated temperatures, an irreversible transition to a thermotropic liquid crystalline phase with chiral organisation and high degrees of circular polarisation (gabs) was observed. When cooled below the transition temperatures, the resulting order was conserved by vitrification. The importance of several factors, most notably the average molecular weight, on the supramolecular organisation was demonstrated by the existence of optimal values for a maximal degree of chiral order. Finally, these materials were used for the construction of novel organic photovoltaic devices that were able to distinguish between right and left circularly polarised light. A distinct film thickness dependence of the difference in photocurrent under left and right polarised illumination (gSC) revealed that several competing charge generation mechanisms were active in these devices. Chapter 3 presents the use of recycling gel permeation chromatography (recGPC) as a novel high performance purification technique to obtain an ultrapure fluorene oligomer. This compound was used in a detailed study to elucidate the degradation mechanisms responsible for the widespread colour instabilities found in PF based light emitting devices. The extraordinarily high purity and monodispersity of the model oligomer allowed to apply mass spectrometric techniques for the identification of intermediate degradation products. Furthermore, all observed reaction pathways had to be inherent to the material itself, since the presence of impurities such as monoalkylated repeat units could be excluded. The combined evidence favoured a light-induced radical degradation mechanism involving carboxylic acid functionalities as the most prominent intermediates in the decomposition of the side chains. Quadruple hydrogen bonding of the ureidopyrimidinone (UPy) group is used in Chapter 4 to create red, green and blue emitting noncovalent polymers in solution and in thin films. Attached to oligofluorenes and other chromophores at both ends, these UPy groups selfassembled by the strong and directional but reversible interactions to form high virtual molecular weight aggregates. These supramolecular polymers could easily be processed from solution into smooth films by spin coating. This approach allowed the incorporation of fluorescent energy acceptors into the fluorene chains in an easy and controllable fashion while the advantageous processability was retained and no phase separation occurred. It was demonstrated that variation of the emission wavelength by mixing in different hydrogenbonded energy acceptors was possible in solution, in the solid state and in electroluminescent devices. Partial energy transfer was used in these systems to obtain white emissive supramolecular random copolymers, which were applied in solution processed white organic light emitting diodes. In Chapter 5, the synthesis of a new set of fluorene co-oligomers is reported with emission colours covering the entire visible range. It is demonstrated that subtle differences in the relative strength of several supramolecular interactions (p-p-stacking, single hydrogen bonding, local phase separation) dramatically change the self-assembly properties. For some compounds, this interplay of interactions resulted in precipitation, whereas in other cases, a hierarchical organisation process led to the formation of fluorescent organogels. The spectral properties of these gels suggested a similar organisation as in the solid state. Mixing several components in the appropriate ratio gave rise to white emitting organogels due to partial energy transfer. Finally, hydrophobic interactions are exploited in Chapter 6 to self-organise a set of bolaamphiphilic fluorene based oligomers into nanoparticles that emit light of all colours (RGB and white) in aqueous solution. For the vast majority of the compounds, the properties of these particles were independent of the supramolecular preparation method. By contrast, one exceptional example (with naphthalene as the comonomer) gave two types of aggregates with distinctly different shape, size and optical properties. In this case, applying the appropriate preparation methods allowed selective tuning of these properties. Once formed, the assemblies were stable for at least several days and did not transform into one another. Fluorescence energy transfer was used as a tool to study the stability and dynamics of mixed mesoscopic self-assembled objects. Within the time frame of several hours, no exchange of molecules between the particles was observed