Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Excited-state dynamics of small organic molecules studied by time-resolved photoelectron spectroscopy
Stockholm University, Faculty of Science, Department of Physics.
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. , p. 71
Keywords [en]
time-resolved photoelectron spectroscopy, excited-state dynamics, organic molecules
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-140482ISBN: 978-91-7649-758-6 (print)ISBN: 978-91-7649-759-3 (electronic)OAI: oai:DiVA.org:su-140482DiVA, id: diva2:1079487
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
List of papers
1. Cyclohexadiene Revisited: A Time-Resolved Photoelectron Spectroscopy and ab Initio Study
Open this publication in new window or tab >>Cyclohexadiene Revisited: A Time-Resolved Photoelectron Spectroscopy and ab Initio Study
Show others...
2016 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 120, no 15, p. 2320-2329Article in journal (Refereed) Published
Abstract [en]

We have reinvestigated the excited state dynamics of cyclohexa-1,3-diene (CHD) with time-resolved photoelectron spectroscopy and fewest switches surface hopping molecular dynamics based on linear response time dependent density functional theory after excitation to the lowest lying pi pi* (1B) state. The combination of both theory and experiment revealed several new results: First, the dynamics progress on one single excited state surface. After an incubation time of 35 +/- 10 fs on the excited state, the dynamics proceed to the ground state in an additional 60 +/- 10 fs, either via a conrotatory ring-opening to hexatriene or back to the CHD ground state. Moreover, ring-opening predominantly occurs when the wavepacket crosses the region of strong nonadiabatic coupling with a positive velocity in the bond alternation coordinate. After 100 fs, trajectories remaining in the excited state must return to the CHD ground state. This extra time delay induces a revival of the photoelectron signal and is an experimental confirmation of the previously formulated model of two parallel reaction channels with distinct time constants. Finally, our simulations suggest that after the initially formed cis-Z-cis HT rotamer the trans-Z-trans isomer is formed, before the thermodynamical equilibrium of three possible rotamers is reached after 1 ps.

National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-130867 (URN)10.1021/acs.jpca.5b10928 (DOI)000374811400006 ()27018427 (PubMedID)
Available from: 2016-06-08 Created: 2016-06-07 Last updated: 2017-11-30Bibliographically approved
2. Excited state dynamics of acrylonitrile: Substituent effects at conical intersections interrogated via time-resolved photoelectron spectroscopy and ab initio simulation
Open this publication in new window or tab >>Excited state dynamics of acrylonitrile: Substituent effects at conical intersections interrogated via time-resolved photoelectron spectroscopy and ab initio simulation
Show others...
2016 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 145, no 11, article id 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.

National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-135174 (URN)10.1063/1.4962170 (DOI)000383959300018 ()
Available from: 2016-11-25 Created: 2016-11-01 Last updated: 2017-11-29Bibliographically approved
3. Influence of Alkoxy Groups on the Photoinduced Dynamics of Organic Molecules Exemplified on Alkyl Vinyl Ethers
Open this publication in new window or tab >>Influence of Alkoxy Groups on the Photoinduced Dynamics of Organic Molecules Exemplified on Alkyl Vinyl Ethers
Show others...
2015 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 119, no 45, p. 11105-11112Article in journal (Refereed) Published
Abstract [en]

A series of different alkyl vinyl ethers is investigated to decipher the possible reaction channels upon photoexcitation to the pi 3s-Rydberg and the pi pi*-valence state at 200 nm using time-resolved photoelectron spectroscopy and on-the-fly time-dependent density functional theory dynamics simulations. The results indicate two possible relaxation pathways: (1) a radiationless decay through the pi pi*-state back to the ground state via torsion of the C=C double bond, in accordance with the dynamics found in ethylene; and (2) a fast dissociation of the C-O bond between the alkyl and the vinoxy group in the pi sigma*-state. The latter state can be accessed only after excitation to the pi 3s-Rydberg state (quantum yield of similar to 50% according to the dynamics simulations). Additionally, the excited state barrier leading to formation of a vinyl radical was found to be too high to be crossed. These results indicate that the dynamics of ethers crucially depend on the excitation wavelength and that the pi sigma*-state constitutes an important competitive reaction channel that leads to dissociation of the molecules.

National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-124174 (URN)10.1021/acs.jpca.5b06592 (DOI)000364435300008 ()26490385 (PubMedID)
Available from: 2015-12-21 Created: 2015-12-15 Last updated: 2017-12-01Bibliographically approved
4. Substituent effects on the relaxation dynamics of furan, furfural and β-furfural: a combined theoretical and experimental approach
Open this publication in new window or tab >>Substituent effects on the relaxation dynamics of furan, furfural and β-furfural: a combined theoretical and experimental approach
Show others...
2017 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, p. 2025-2035Article in journal (Refereed) Published
Abstract [en]

For the series furan, furfural and β-furfural we investigated the effect of substituents and their positioning on the photoinduced relaxation dynamics in a combined theoretical and experimental approach. Using time resolved photoelectron spectroscopy with a high intensity probe pulse, we can, for the first time, follow the whole deactivation process of furan through a two photon probe signal. Using the extended 2-electron 2-orbital model [Nenov et al., J. Chem. Phys., 2011, 135, 034304] we explain the formation of one central conical intersection and predict the influence of the aldehyde group of the derivatives on its geometry. This, as well as the relaxation mechanisms from photoexcitation to the final outcome was investigated using a variety of theoretical methods. Complete active space self consistent field was used for on-the-fly calculations while complete active space perturbation theory and coupled cluster theory were used to accurately describe critical configurations. Experiment and theory show the relaxation dynamics of furfural and β-furfural to be slowed down, and together they disclose an additional deactivation pathway, which is attributed to the nO lonepair state introduced with the aldehyde group.

National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-140474 (URN)10.1039/C6CP06240G (DOI)000394426400036 ()
Available from: 2017-03-08 Created: 2017-03-08 Last updated: 2017-05-02Bibliographically approved
5. Dynamics in higher lying excited states: Valence to Rydberg transitions in the relaxation paths of pyrrole and methylated derivatives
Open this publication in new window or tab >>Dynamics in higher lying excited states: Valence to Rydberg transitions in the relaxation paths of pyrrole and methylated derivatives
Show others...
(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:nbn:se:su:diva-140471 (URN)
Available from: 2017-03-08 Created: 2017-03-08 Last updated: 2017-03-08Bibliographically approved
6. Time-resolved photoelectron spectroscopy studies on pyrazole and several methylated derivatives
Open this publication in new window or tab >>Time-resolved photoelectron spectroscopy studies on pyrazole and several methylated derivatives
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We present femtosecond time-resolved photoelectron spectra and ab initio studies on pyrazole andits methylated derivatives 1-, 3-, and 5-methylpyrazole. Excitation at 200 nm populates both the two lowest lying states, a 1ππ* state and a mixed 1πσ*/1π3s Rydberg state, from where three relaxation channels are observed: ring puckering, N-H bond cleavage, and ring opening via N-N bond breaking. The N-N bond breaking channel is the fastest process, occurring within one vibrational cycle of the Franc Condon active ring stretching mode. N-H bond cleavage is observed to be aminor channel, and occurs upon direct excitation to the mixed π3s/πσ* state. Finally, ring puckering occurs after a timescale of a few hundred fs because the molecules need time to find the gradient towards this conical intersection. However, this channel is accessed if the initially triggered processes are not successful. The quantum yields of the different channels were found to be very sensitive of the relative positioning of the excited states. In pyrazole and 5-methylpyrazole, N-Nbond cleavage dominates. In 1-, and 3-methylpyrazole, while the 1ππ* state drops in energy the dissociating 1πσ* valence state does not, and this leads to an increased barrier towards ring cleavage and a decreased the quantum yield for N-N bond cleavage. Upon excitation at 267 nm of 1- and 3-methylpyrazole, ring puckering is the only available pathway.

National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-140473 (URN)
Available from: 2017-03-08 Created: 2017-03-08 Last updated: 2017-03-08Bibliographically approved

Open Access in DiVA

Excited-state dynamics of small organic molecules studied by time-resolved photoelectron spectroscopy(2459 kB)168 downloads
File information
File name FULLTEXT01.pdfFile size 2459 kBChecksum SHA-512
5349a3d0fa7b88bb8e4f206bbb75a6fb9340d243e4cf0a22fe8c36f61de8d41cb40ed043aae61b861c656155aebd840f8549b192da436ab8b5790ef752029281
Type fulltextMimetype application/pdf

Search in DiVA

By author/editor
Geng, Ting
By organisation
Department of Physics
Atom and Molecular Physics and Optics

Search outside of DiVA

GoogleGoogle Scholar
Total: 168 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 1487 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf