Energy transfer dynamics in novel macrocyclic polymers: A comparative study of depolarization and exciton annihilation using ultrafast time resolved spectroscopy

It has been predicted that by the end of the century the global demand for energy will triple. Currently fossil fuels, which are an infinite source of energy, supply the majority of energy consumed globally. It has therefore been a priority for several years to find alternative energy sources. Solar energy is one such source. It is one of the most under utilized natural energy resources today. Photosynthesis is a natural biological process that utilizes the sun's energy to power vital mechanisms in plants and some species of bacteria. Uncovering and replicating the molecular level mechanism of natural photosynthesis will lead to new platforms for artificial molecular solar devices.;The dynamics and spectroscopy of a novel molecular architecture have been characterized that mimics bacterial light harvesting using a 2-D macrocyclic homopolymer. This synthetic structure has chromophore molecules pendent to a polymer backbone that are arranged in a similar cyclic topology to the biological design. The technique of ultrafast spectroscopy is used to probe the dynamics in these novel polymer systems. The results indicate the pathways and timescales for energy flow in the polymer systems.;We have characterized two independent techniques (depolarization and annihilation) that allow us to look at energy transfer dynamics over large length scales within these novel macrocyclic systems. Depolarization measurements are sensitive to the local environment of the polymer (e.g. angles between transition dipole moments) and therefore give details on the local architecture of the macrocycle. Annihilation measurements, however are sensitive to migration of two excitations over the larger chromophore array and thus provides evidence on connectivity and how energy transfer scales with the polymer size. Both techniques provide complementary evidence of the occurrence of energy transfer and the specific rates at which they occur within these macrocyclic polymer systems as well as how these time scales compare to those of natural photosynthetic systems.;The effect of excimer formation on energy transfer in the macrocyclic polymers have also been studied using time resolved fluorescence measurements. It is found that macrocycles display different excimer photophysics from their linear analogs. Further, it has been shown that excimer formation in fluorene based macrocycles was substantially reduced compared to naphthalene based macrocycles.