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Dynamics in higher lying excited states: Valence to Rydberg transitions in the relaxation paths of pyrrole and methylated derivatives
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physics.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The involvement of intermediate Rydberg states in the relaxation dynamics of small organic molecules which, after excitation to the valence manifold, also return to the valence manifold, is rarely observed. We report here that such a transiently populated Rydberg state may offer the possibility to modify the outcome of a photochemical reaction. In a time resolved photoelectron study on pyrrole and its methylated derivatives, N-methyl pyrrole and 2,5-dimethyl pyrrole, 6.2 eVphotons (200 nm) are used to excite these molecules into a bright ππ* state. In each case, a π3p-Rydberg state, either the B1(π3py) or the A2(π3pz) state, is populated within 20-50 fs after excitation. The wave packet then proceeds to the lower lyingA2(πσ*) state within a further 20 fs, at which point two competing reaction channels can be accessed: prompt NH (N-CH3) bond cleavage or ring puckering, the latter of which is predicted to require an additional 100 – 160 fs depending on the molecule.

National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-140471OAI: oai:DiVA.org:su-140471DiVA: diva2:1079421
Available from: 2017-03-08 Created: 2017-03-08 Last updated: 2017-03-08Bibliographically approved
In thesis
1. Excited-state dynamics of small organic molecules studied by time-resolved photoelectron spectroscopy
Open this publication in new window or tab >>Excited-state dynamics of small organic molecules studied by time-resolved photoelectron spectroscopy
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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 ethylene-like 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.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2017. 71 p.
Keyword
time-resolved photoelectron spectroscopy, excited-state dynamics, organic molecules
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-140482 (URN)978-91-7649-758-6 (ISBN)978-91-7649-759-3 (ISBN)
Public defence
2017-04-21, sal FA32, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 5: Manuscript. Paper 6: Manuscript.

Available from: 2017-03-29 Created: 2017-03-08 Last updated: 2017-03-29Bibliographically approved

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