Leventis, Anastasia; (2019) Understanding the Fundamental Properties of Conjugated Materials for Organic Electronics. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The effects of heteroatomic substitution, conjugation length and aggregation on the absorptive and emissive properties of conjugated organic materials was explored. Heteroatomic substitution was first investigated with the synthesis of two nitrogencontaining polymeric derivatives of benzothiadiazole (BT): a pyridylthiadiazole (PyT)-based polymer, PTTPy; and the novel pyridazine thiadiazole (PzT)-based polymer, PTTPz. It was discovered that PzT was a much stronger acceptor in comparison to the more commonly used BT and PyT moieties, and that per electronegative nitrogen atom that is substituted into the acceptor, an effective HOMO stabilisation energy of ~0.1 eV can be achieved. The second part of this thesis introduces the renowned chromophore diketopyrrolopyrrole (DPP) and was used to study how chain length, polydispersity and defects effect the absorption ability of conjugated materials. A series of welldefined DPP-based conjugated oligomers were synthesised (from n=1-6 repeat units long) via iterative Suzuki-Miyaura cross-coupling, as well as their polymeric counterpart. The extinction coefficients were measured however their unusual trends were difficult to rationalise. Significant jumps in the red-shift from pentamer to hexamer were tentatively credited to the oligomers self-aggregating, however the exact nature of which (inter- or intra-molecularly) was inconclusive. Thus, to understand the unusual red-shift, aggregation had to be controlled. Encapsulating alkyl chains that prevent polymer backbones from !–! stacking were therefore incorporated onto a phenyl-DPP derivative and gave rise to extremely high fluorescence quantum yields in both solution (>70%) and thin film (>20%). Their absorption and emission spectra showed clearer, more defined features compared to their naked counterparts, demonstrating the suppression of both inter and intramolecular aggregation. The macrocyclic shield resulted in a dramatically increased backbone co-linearity and gave rise to structurally ordered, defect-free polymer domains, as evidenced by STM, allowing DPP to be used, for the first time, as a emissive material in the solid state. Finally, using the optimised reaction conditions for phenyl-DPP, the synthesis of the corresponding encapsulated polymer based on thienyl-DPP was also attempted.

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