Polarization portraits of lightharvesting antennas: from single molecule spectroscopy to imaging

Abstract: Multichromophoric systems are very important in photosynthesis and any device that uses solar energy for its operation. This is because multichromophoric light-harvesting antennas are responsible for the absorption of light and the efficient transfer of the absorbed energy toward distinct places where it is to be used or stored. Over the last 10 years polarization sensitive single molecule methods have been extensively used to study the chromophore organisation and excitation energy transfer processes in lightharvesting antennas. In general, these methods probe in separate experiments the fluorescence excitation and emission polarization properties of the sample. This approach unfortunately averages out meaningful correlations between the polarization properties of the chromophores preferentially absorbing light, and the polarization state of the emitted fluorescence. Therefore, in 2009 an alternative method was proposed to detect these correlations called two dimensional polarization imaging. This is done by measuring a two dimensional function that describes the fluorescence intensity and polarization of a single object as a function of the electric field´s direction of the linearly polarized excitation light. However, in spite of the development of the technique, the main challenge still was to extract the excitation energy transfer information from the data. In this thesis we report the further understanding of the theoretical and experimental challenges developed for two dimension polarization imaging. Our development made possible the quantitative characterization of the excitation energy transfer efficiency of individual light-harvesting antennas, such as the LH2 complex and conjugated polymers, through a model based on a single funnel approximation. This method can be used to assess the “quality” of an artificial light harvesting antenna before trying it in a device. Further, we showed that our methodology is not only beneficial for studying of single molecules, but also can be used as a fluorescence imaging microscopy where parameters related to energy transfer and the chromophore organisation serve as imaging contrast. Two dimensional polarization imaging in combination with the single funnel approximation was successfully used to study thin films of a solar cell material, and is being tested on cell cultures and histological samples. The energy transfer sensitivity of our imaging technique opens exciting applications in life sciences for the study of biologically relevant systems, such as the aggregation of proteins involved in the causes of various diseases.