Applied Analytical Techniques to Study Solution Behaviour of TIPS-pentacene and 1,6-bis(pyren-1-ylthio)hexane

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Abstract

Aggregation and cluster formation in solution is often found for molecules with extensive planar structures. This can affect properties ranging from photochemical stability, device structure and electronic properties in organic electronic materials to pipeline clogging and flow rheology in petrochemical extraction and production. Some analytical techniques can be sensitive to aggregation and photostability issues. In this thesis, a range of methods are applied to two scenarios: aggregation and stability of solutions and blends of bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) and in pyrene-based models for asphaltene systems. A common organic electronic material, pentacene has a very short half-life in solution with respect to photo-degradation. This behaviour can be attenuated markedly by derivatisation at the 6- and 13- positions, producing 6,13-bis(triisopropylsilylethynyl)pentacene, making this a more appropriate molecule for realistic devices. TIPS-pentacene does still decay photochemically in solution and the process has been shown to be concentration-dependent with greater stability at higher concentrations: it has been suggested that this may be due to the formation of aggregates. In this thesis, new measurements of the kinetics of TIPS-pentacene photo-degradation were obtained by 1H NMR spectroscopy and compared with more conventional UV-visible spectroscopic methods and fluorescence measurements. Diffusion-ordered (DOSY) NMR spectroscopy measurements were also made on TIPS-pentacene solutions and polymer blends, although this did not provide any convincing evidence for aggregation. However, the application of isothermal titration calorimetry did show effects which may be due to some aggregate dissociation at higher concentration. From this it is concluded that the timescale of the TIPS-pentacene self-aggregation or polymer interaction is likely to be much shorter than that of the NMR experiment.The NMR methods applied to TIPS-pentacene were also used to study the solvent- and concentration-dependent behaviour of 1,6-bis(pyren-1-ylthio)hexane (BPH), a potential model for asphaltenes that is simpler to study. 1H NMR measurements proved difficult because of the effects of solvent mixtures on the NMR methods. Generally, the DOSY NMR results obtained do not provide firm evidence for aggregation. Since these molecules are very likely to aggregate, this further suggests that aggregation and disaggregation may occur faster than the NMR time scale.In both cases, the results presented here show that all of these methods are to some extent sensitive to aggregation effects, but that care is needed in specific applications to choose methods which are (i) sufficiently sensitive at appropriate concentrations; (ii) not perturbed by concentration dependent phenomena; and (iii) not limited by timescales of the measurement.

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