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

许多不同材料的功能是由它们各自组成部分之间的相互作用决定的。相邻发色团之间的激子耦合是理解和发展分子光子和电子器件的关键。它控制单个单体之间的能量和电荷转移。在本项目框架内，将开发一种新的实验方法，将液体样品的时间分辨光电子能谱与圆二色性相结合，从而为手性分子的激子耦合提供直接的实验途径。该方法将应用于水溶液中的单链DNA/RNA构建块，以研究离域状态（特别是激子状态）对这些分子光稳定性的作用。虽然对单个DNA碱基的光物理学已经相当了解，但DNA（单链或双链）的光物理学仍然提出了许多问题。DNA和RNA具有非凡的光稳定性，因此是地球上生命发展的基础，特别是在臭氧层尚未形成的情况下，其背后的机制是什么？为什么光致病变更常发生在DNA/RNA的特定部位而不是其他部位？为什么RNA比DNA更耐光？这些都是本项目框架内研究的核心问题。特别强调了离域态在这些分子的光物理中的中心作用。我们将研究不同链长的DNA/RNA低聚物。DNA/RNA碱基之间的电子耦合以及光学填充的Frenkel激子的寿命都可以直接从实验数据中提取出来。弗伦克尔激子在电荷转移络合物的形成下衰变，即电荷分离。时间分辨光电子能谱将研究这些电荷转移态的电子结构，它们的空间范围和时间演化。碱基序列相关的结果允许光松弛的关键参数的结论。我们将详细研究离域状态的作用以及某些结构参数的影响。

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.