Theory of optical transitions in pi-conjugated polymers

Conjugated Polymers have attracted a great deal of research interest in recent years due to their optoelectronic properties which makes them suitable for applications in organic light-emitting devices (OLEDs) and organic photovoltaics. Their properties are strongly dependent on the electron-electron and electron-nuclear interactions as well as the disorder which is present in almost all systems at finite temperatures. In this thesis the optical properties of electronically neutral conjugated polymers will be investigated. The results obtained are general and applicable to a wide range of parameters. In order to compare these to experiment the optical properties of poly( paraphenylene), poly(para-phenylene vinylene), and derivatives have been calculated. In these polymers the primary photoexcitations are Frenkel excitons which can be described by the Frenkel-Holstein Hamiltonian, which explicitly takes into account the exciton-nuclear coupling. Disorder can be introduced into this model both as diagonal and off-diagonal disorder within the Hamiltonian. First the optical transitions in ordered, linear conjugated polymers are investigated. It is found that the length of the polymer has a direct spectroscopic signature in the emission spectrum. When off-diagonal disorder is introduced the excitons localise on portions of the chain and the length of these portions, the conjugation length, then shows a clear emission signature. As such, a disordered polymer can be described theoretically as a chain of shorter segments, which define chromophores in a polymer context. Following from these calculations the role of conformation was investigated and effects were observed that greatly determine the optical properties of non-linear polymers. Most notable the Herzberg-Teller effect, which renders symmetrically forbidden transitions weakly allowed and greatly affects the absorption and emission spectra. The signatures observed in these spectra allow the determination of the (coarse grained) conformation of the polymer, something that has been difficult to measure directly.