Thesis defense: Excited-state dynamics of small organic
Friday 21 April 2017
to 17:00 at
Ting Geng (Stockholm University, Department of Physics)
Ultra-violet and visible light induced processes in small organic molecules play very important roles in many fields,
e.g., environmental sciences, biology, material development, chemistry, astrophysics and many others. Thus it is of great
importance to better understand the mechanisms behind these processes. To achieve this, a bottom-up approach is most
effective, where the photo-induced dynamics occurring in the simplest organic molecule (ethylene) are used as a starting
point. Simple substituents and functional groups are added in a controlled manner to ethylene, and changes in the dynamics
are investigated as a function of these modifications. In this manner, the dynamics occurring in more complex systems
can be explored from a known base.
In this thesis, the excited state dynamics of small organic molecules are studied by a combination of time-resolved
photoelectron spectroscopy and various computational methods in order to determine the basic rules necessary to help
understand and predict the dynamics of photo-induced processes.
The dynamics occurring in ethylene involve a double bond torsion on the ππ* excited state, followed by the decay to
the ground state coupled with pyramidalization and hydrogen migration. Several different routes of chemical modification
are used as the basis to probe these dynamics as the molecular complexity is increased. (i) When ethylene is modified by
the addition of an alkoxyl group (-OCnH2n+1), a new bond cleavage reaction is observed on the πσ* state. When modified
by a cyano (-CN) group, a significant change in the carbon atom involved in pyramidalization is observed. (ii) When
ethylene used to build up small cyclic polyenes, it is observed that the motifs of the ethylene dynamics persist, expressed
as ring puckering and ring opening. (iii) In small heteroaromatic systems, i.e., an aromatic ring containing an ethylenelike
sub-structure and one or two non-carbon atoms, the type of heteroatom (N: pyrrole, pyrazole O: furan) gives rise to
different bond cleavage and ring puckering channels. Furthermore, adding an aldehyde group (-C=O) onto furan, as a way
to lengthen the delocalised ring electron system, opens up additional reaction channels via a nπ* state.
The results presented here are used to build up a more complete picture of the dynamics that occur in small molecular
systems after they are excited by a visible or UV photon, and are used as a basis to motivate further investigations.