Watching Ultrafast Photo-Induced Non-Adiabatic Transformations in Molecular Switches and DNA Building Blocks

by Friedrich Temps (Institute of Physical Chemistry, Christian-Albrechts-University,Kiel)

Thursday, November 14, 2013 from to (Europe/Berlin)
at AER 19 ( Room 3.11 )
Photoswitchable organic molecules and biomolecules like the DNA bases form two sides of a medal regarding their elementary photochemical properties when it comes to photo¬reactivity and photostability. The former should ideally undergo very fast chemical reactions after excitation to ensure efficient conversion from one state to another, and these reactions should be photoreversible. The DNA and its building blocks on the other hand must be highly photostable to protect the encoded genomic information against UV-photolesions which can lead to mutations and carcinogenesis. Despite the entirely different outcomes, however, photoreactivity and photostability are intimately related, as the direction that a molecule takes after photoexcitation is decided on by the topography of the potential energy hypersurfaces and the ensuing molecular dynamics in the vicinity of the conical inter¬sections mediating the respective ultrafast non-adiabatic transformations. 
Femtosecond time-resolved pump-probe stroboscopy is a tremendously powerful technique for elucidating the ultrafast photodynamics in organic chromophores and biomolecules. The talk will highlight selected case studies. The first part will be concerned with the dynamics of photo¬chromic molecular switches (fulgides and azobenzene derivatives) with intra- and inter-molecular constraints which we have investigated in the last few years. The second part will focus on the ultrafast electronic deactivation pathways in DNA building blocks, including free bases in solution, hydrogen-bonded base pairs and two dinucleotides, where recent experiments illuminate the spectro-temporal evolutions of the elusive exciton and excimer states.