| Summary: | The self-assembly of materials to form well defined nanostructures has been shown to be beneficial to charge carrier mobility in organic electronic devices. This thesis aims to study the self-assembly and aggregation properties of novel dissymmetric diketopyrrolopyrrole derivatives. The synthesis, of both symmetric and dissymmetric diketopyrrolopyrrole derivatives, are described and the influence of the N-alkyl chain, incorporation of chiral lactate groups through Mitsunobu reactions and increase in conjugation using Suzuki couplings on the optical and electronic properties of these materials are reported.
A selection of the novel materials synthesised self-assemble into twisted macrostructures in the form of twisted crystals. The morphologies of these twisted crystals are studied using optical, scanning electron and atomic force microscopy revealing differences that arise as a result of the length of the N-alkyl chain. Symmetric chiral diketopyrrolopyrrole derivatives are shown to selectively twist in one direction dependent on the chirality of the appended lactate. While the achiral dissymmetric derivatives form left- and right-handed crystals with equal prevalence. Twist-pitch analyses were performed on the twisted crystals that formed to aid in the identification of the mechanism of twisting. Single crystal and powder X-ray analyses were performed to attempt to deduce the crystal structure of the twisted crystals of the dissymmetric derivatives. Preliminary photoconductivity measurements were carried on standard and modified OFET chips.
The ability of dissymmetric diketopyrrolopyrrole derivatives to self-assemble into well-ordered monolayers at the solution – solid-state interface was studied using scanning tunnelling microscopy (STM). The prochiral nature of the molecules resulted in enantiomeric domains of well-ordered lamellae that undergo Ostwald ripening over time. High concentrations of solutions result in multilayers that are not stable to probing by the STM tip and so decrease the domains in size over time.
Spatially resolved circular dichroism and Mueller matrix polarimetry at Diamond Light Source were utilised to explore the influence of casting technique on the chiral packing of thin films of chiral dissymmetric diketopyrrolopyrrole derivatives. Spin-cast films exhibited increased optical activity when compared to dropcast films but the polarisation properties of both were dominated by circular dichroism with small contributions from circular birefringence. Incorporation of chiral dissymmetric diketopyrrolopyrrole derivatives into blends of P3HT:PCBM influences the chiral packing of the film and circular dichroism imaging reveals domains of the chiral dissymmetric diketopyrrolopyrrole that cannot be easily identified using other techniques.
Overall, this thesis explores the self-assembly of novel diketopyrrolopyrrole derivatives under a variety of conditions. The ability of these novel compounds to self assemble, along with the advantageous optical and electrochemical properties provides a foundation to include these materials in optoelectronic devices.
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