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Title: Intermolecular interactions in π-conjugated molecules : optical probes of chain conformation
Author: Clark, Jenny
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2007
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A study of interactions between molecules of the same material was performed on a model polymer system. Regio-regular poly(3-hexylthiophene) (P3HT) self-organises into two-dimensional π-stacked lamellar structures. Although the chains pack at a distance of only 3.8Å, the resonant coulomb interaction between nearest-neighbour molecules is small, owing to large average conjugation lengths within the lamellae. When the resonant coulomb (or excitonic) interaction is smaller than the vibrational relaxation energy, the excitation is localised on a single molecule and the coupling is termed weak. Here we demonstrate that the polymer absorption and emission spectra can be comprehensively explained using a weakly interacting H-aggregate model. Using an analytic form of the model we can estimate the excitonic interaction energy from the ratio of the 0-0 and 0-1 absorbance peaks. It was found to vary between 5-30meV. We use this powerfully simple tool to determine how film micromorphology depends on processing. We find that films spun from low boiling point solvents show a decrease in crystalline quality, an increase in excitonic coupling and more amorphous regions, than films spun from high boiling point solvents. We correlate these findings with field-effect transistor characteristics to produce a model of film micro-morphology. We also demonstrate preliminary results that show that due to the aggregate nature of the primary neutral excitation, photo-induced charge generation in P3HT is due to exciton-exciton annihilation. Finally, we study the intermolecular interactions at an interface between two different molecules (a hole transporting polymer, TFB (Poly(9,9-dioctylfluorene –co-N-(4-butylphenyl)diphenylamine)) and an electron-transporting disoctic, HATNA-SC12 (Hexa-azatrinaphthylene)). Depending on the energetics of the interface between two materials, the inter-molecular state can be stabilised by either coulomb interactions (as in the case of single material interfaces) or charge-transfer interactions. In this case, we find that the charge-transfer interactions dominate and an excited-state complex, or exciplex, is formed. The exciplex is generated efficiently in light-emitting diodes, producing a pure red emission. In polymer/polymer blends efficient bulk exciton emission can occur due to endothermic transfer from the exciplex to the bulk exciton. In this system, however the exciplex is stable at the interface, acting as an energy bottleneck with inefficient transfer to bulk exciton states. Furthermore, the yield of charge separation is low.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available