Exciton dynamics in π-conjugated polymer systems

Thesis

Experimentally, it is observed that upon photoexcitation, conjugated polymers undergo sub-picosecond coupled exciton-phonon dynamics, in which the nuclei distort to stabilise the new electronic state. Previously, such relaxation dynamics of highly excited electronic states of poly(p-phenylenevinylene) have been modelled using Ehrenfest dynamics. Rather than the system relaxing onto a single emissive chromophore as physically expected, the dynamics were seen to bifurcate into a quantum superposition of chromophore states.

In this thesis, the dynamics are instead modelled using a quantum mechanical treatment for the nuclei. In accordance with previous work, the excited electronic states of a polymer are described by a single Frenkel exciton and exciton-phonon coupling is explicitly included using the Frenkel-Holstein model. The time evolution of the system is then described by a Lindblad master equation, which includes damping of the internal nuclear degrees of freedom by the environment.

Firstly, the associated equations of motion are solved using the quantum jump trajectory and time evolving block decimation techniques. Within such a framework, we find that the unphysical bifurcation behaviour is corrected for and the dynamics reproduce the ≲50 fs time scale observed in experiment. Processes such as exciton-polaron formation, exciton decoherence and exciton density localisation are also present within the model dynamics.

Secondly, the steady state solution associated with the dynamics is calculated using a Lindblad perturbation theory. Within this approach, the limited functional space (LFS) method is used to obtain numerical results for the steady state. These results confirm that our model dynamics do proceed into a 'chromophore like' steady state, with associated short-ranged exciton-phonon, exciton coherence and exciton localisation lengths.

Finally, we introduce two techniques designed to efficiently describe the time evolution of 'classical' torsional modes. These modes are thought to give rise to the ~100 fs time scale observed in experiments, which is currently absent from our model.

Authors: Mannouch, J

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford