Exciton Dynamics in Synthetic Multi-Chromophoric Model Systems

Investigating the excitonic properties of synthetic multichromophoric model systems can give insights into the behaviour of larger and more intricate structures, such as the photosynthetic complexes found in autotrophs or materials with applications in the area of organic photovoltaics (OPV). The unique properites of excitons depend critically on the electronic excited states of these systems, which present non-local character, and have short lifetimes. Hence, in order to characterise their dynamics it is helpful to employ laser spectroscopic techniques with ultrafast time resolution. Among these, the most widespread is broadband femtosecond transient absorption (fsTA), a two-pulse technique which has the drawback of being intrinsically ambiguous on the excitation frequency. A way to overcome this disadvantage is presented by two-dimensional electronic spectroscopy (2D-ES). In 2D-ES, the introduction of a third pulse allows the recovery of spectra in which excitation and detection frequencies are correlated on a two-dimensional surface. 2D-ES and fsTA have been used in a complementary fashion throughout this thesis in order to investigate photophysical processes in a range of different synthetic multichromophoric model systems. Experiments have been performed on a series of covalently-bound perylene bisimide (PBI) J-dimers. Here 2D-ES allowed us to identify a one- to two-exciton state transition in the strongly coupled dimer, which vanishes in the monomer or when the coupling is weakened. Such a transition is purely electronic in character, as confirmed by the calculated spectra, and its energy allowed us to estimate the excitonic coupling strength. We further report fsTA and 2D-ES studies of a subphthalocyanine-Zn porphyrin (SubPc-O-ZnTPP) heterodimer. fsTA allowed us to characterise the excitation energy transfer (EET) between the SubPc and ZnTPP moieties, which is well reproduced by the Forster model, while 2D-ES was used to observe sub-ps spectral diffusion, which is shown to be too fast to influence the incoherent EET. These studies were extended to larger systems. fsTA has been performed on a range of fully-conjugated porphyrin nanorings. fsTA and transient anisotropy on six-membered rings, with or without an inner template, revealed structural dynamics in the ground and in the excited states of the untemplated structure, which do not disrupt the exciton delocalisation. Finally, fsTA at increasing pump fluences allowed us to study exciton-exciton annihilation (EEA) dynamics in nanorings made up of 10, 20, 30 and 40 porphyrin units. Experiments confirmed that the exciton size is approximately 20 repeating units, and comparison with a one-dimensional diffusion model allowed estimation of the exciton diffusion coefficients, which decrease as the ring size increases, a result assigned to the increased static disorder experienced in larger structures