Insights into the self-assembly of pi-conjugated systems

In Chapter 1 supramolecular polymerization of small molecules is introduced and exemplified. The self-assembly of these single component systems, for example by hydrogen bonding and p-p interactions, has reached a high level of understanding. However for the fabrication of complex architectures with specific properties, a perfect organization of multiple components in the aggregates needs to be achieved. In order to arrive at such complex architectures more insight into the specificities of the self- assembly of p-conjugated systems is a necessity, which is the subject of this thesis. A covalent strategy for the spatial organization of p-conjugated components by placement on a foldameric scaffold is discussed in Chapter 2. Although the synthesis of these foldamers was successful, the 30 synthetic steps make the synthesis of a specific foldamer a lengthy process while only achieving relatively small structures. It was found that placing the chromophores in a defined three dimensional environment hampered a uniform description of the charge transfer characteristics within the current theoretical models. Furthermore, the level of complexity in these synthetic systems pales in comparison with the examples found in Nature. A more promising approach could be the self-assembly of molecules. Supramolecular polymerization via the cooperative mechanism is generally observed for the examples discussed in this thesis. Cooperative mechanisms have a nucleation event preceding the formation of the selfassembled structures. The evidence presented in Chapter 3 showed that the nucleation of oligothiophenes can be either homogeneous or heterogeneous in nature, depending on sample purity. Especially when the supramolecular interactions are counterbalancing each other, heterogeneous nucleation is likely to be present. By changing the self-assembly protocol, e.g. altering the cooling rate and method, the outcome of the self-assembly was remarkably different. This showed that the oligothiophenes were able to self-assemble into multiple different structures thereby displaying the presence of a complex energy landscape in analogy to polymorphism in the crystallization of molecules and to protein folding. At higher concentration the internal structure of the oligothiophene assemblies was resolved by using a combination of small-angle X-ray scattering and linear birefringence on magnetically aligned assemblies as discussed in Chapter 4. Optical and chiroptical studies showed that the cooperative nature of the self-assembly at this higher concentration was maintained. Cylindrical assemblies were observed, where the thiophene molecules were radially organized and p-p stacking was in a tangential direction. Chapter 5 discusses the use of circular dichroism (CD) spectroscopy to monitor the coassembly of multiple components. For the achiral oligo(p-phenylene vinylene) ureidotriazine systems, artificial CD effects are observed for the self-assembled structures in dodecane that are a result of unwanted linear dichroic (LD) effects as a consequence of flow induced alignment in the cuvette. These LD artifacts interfere with the CD measurements. By using methylcyclohexane (MCH) the LD effects are absent and the observed CD effects can be considered as a result of exciton coupling between the chromophores. The influence of two preparation methods on the organization of mixed assemblies consisting of achiral and chiral OPV derivatives with different oligomeric length is studied with CD spectroscopy in MCH. The first preparation method allows the molecules to be fully mixed at high temperature, while subsequent cooling shows chiral amplification of the achiral longer oligomer by the shorter chiral oligomer. The study indicates the formation of enriched clusters of one of the two components either within one stack, or as separate stacks. The second method is based on the addition of the shorter oligomer at a temperature, where the assemblies of the achiral oligomer are already present and kinetically inert. In this case the chiral amplification is reduced, indicating the formation of more enriched clusters of compounds. However, to arrive at an unambiguous assignment of the internal structure of the assemblies, the CD data should be combined with complementary techniques. In Chapter 6 the influence of reduced dynamics on the self-assembly of p-conjugated star shaped molecules is discussed. Two different types of aggregates, A1 and A2, are distinguished for the selfassembly in MCH, where a transition from A1 to A2 occurs, thereby showing the increased thermodynamic stability of the A2 aggregate. The addition of a good solvent enhances the rate of the transition. SEC in these alkane solvents shows that the A1 state in the solvent mixture (4:1MCH/toluene) was far more dynamic than in pure MCH. Furthermore, it reveals an unusually high monomer content of 15 mol% in the solvent mixture, which is confirmed by 1H NMR. As expected, in pure MCH the monomer content is much lower and for that reason it could not be detected. In order to investigate the origin of the A1–A2 transition, the two enantiomers are mixed and chiral amplification in the form of the Majority Rules effect is shown to occur in MCH, while it is only weakly present in the solvent mixture. The presence of the other enantiomer significantly slows down the kinetics of the transition from A1 to A2, where the transition only occurred in the solvent mixture. Quite remarkably, chiral amplification is absent in the A2 state in the solvent mixture. The combined results suggest that the A1–A2 transition is related to a tightening of the internal structure, where the transition is facilitated by the presence of an enantiomerically pure cluster. In general the research described in this thesis reveals some important parameters that determine the self-assembly process and its outcome. Reflection on these parameters and their implication on the design and synthesis of complex multicomponent supramolecular architectures is given in an epilogue to this thesis.