Excitonic coupling and delocalized states in the excited state dynamics of nucleotide oligomers.

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Function of many different materials is determined by the interaction between their individual building blocks. Excitonic coupling between adjacent chromophores is key to understanding and development of molecular photonic and electronic devices. It controls energy and charge transfer between individual monomers. In the frame of this project, a new experimental method will be developed that combines time-resolved photoelectron spectroscopy of liquid samples with circular dichroism and thereby provides direct experimental access to the excitonic coupling of chiral molecules. This method will be applied to single-stranded DNA/RNA building blocks in aqueous solution to investigate the role of delocalized states (in particular of excitonic states) for the photostability of those molecules. While the photophysics of individual DNA bases is reasonably well understood, the photophysics of DNA (single- or double-stranded) still raises many questions. What is the mechanism behind the extraordinary photostability of DNA and RNA and is therefore basis for the development of life on earth - in particular, when the ozon layer has not yet developed? Why do photoinduced lesions develop more often at specific sites in DNA/RNA than at others? Why is RNA more photostable than DNA? These are the central questions which are investigated in the frame of this project. Special emphasis is on the central role of delocalized states for the photophysics of those molecules. We will investigate DNA/RNA oligomers of different chain lengths. Both, the electronic coupling between the DNA/RNA bases as well as the lifetime of the optically populated Frenkel excitons are directly extractable from the experimental data. Frenkel excitons decay under formation of charge-transfer complexes, i. e. charge separation. Time-resolved photoelectron spectroscopy will investigate the electronic structure of those charge-transfer states, their spatial extent and their temporal evolution. Base-sequence dependent results allow conclusions on the key parameters in the photorelaxation. The role of delocalized states as well as the influence of certain structural parameters will be investigated, in detail.