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Excited state dynamics of acrylonitrile: Substituent effects at conical intersections interrogated via time-resolved photoelectron spectroscopy and ab initio simulation
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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Number of Authors: 7
2016 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 145, no 11, 114306Article in journal (Refereed) Published
Abstract [en]

We report a joint experimental and theoretical study on the photoinitiated ultrafast dynamics of acrylonitrile (AN) and two methylated analogs: crotonitrile (CrN) and methacrylonitrile (MeAN). Time-resolved photoelectron spectroscopy (TRPES) and ab initio simulation are employed to discern the conical intersection mediated vibronic dynamics leading to relaxation to the ground electronic state. Each molecule is pumped with a femtosecond pulse at 200 nm and the ensuing wavepackets are probed by means of one and two photon ionization at 267 nm. The predominant vibrational motions involved in the de-excitation process, determined by ab initio trajectory simulations, are an initial twisting about the C=C axis followed by pyramidalization at a carbon atom. The decay of the time-resolved photoelectron signal for each molecule is characterized by exponential decay lifetimes for the passage back to the ground state of 60 +/- 10, 86 +/- 11, and 97 +/- 9 fs for AN, CrN, and MeAN, respectively. As these results show, the excited state dynamics are sensitive to the choice of methylation site and the explanation for the observed trend may be found in the trajectory simulations. Specifically, since the pyramidalization motion leading to the conical intersection with the ground state is accompanied by the development of a partial negative charge at the central atom of the pyramidal group, the electron donation of the cyano group ensures that this occurs exclusively at the medial carbon atom. In this way, the donated electron density from the cyano group directs the wavepacket to a particular region of the intersection seam. The excellent agreement between the experimental and simulated TRPES spectra, the latter determined by employing trajectory simulations, demonstrates that this mechanistic picture is consistent with the spectroscopic results.

Place, publisher, year, edition, pages
2016. Vol. 145, no 11, 114306
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-135174DOI: 10.1063/1.4962170ISI: 000383959300018OAI: oai:DiVA.org:su-135174DiVA: diva2:1049581
Available from: 2016-11-25 Created: 2016-11-01 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)
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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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