DOCSLIB.ORG

Excited-State Dynamics of Small Organic Molecules Studied by Time-Resolved Photoelectron Spectroscopy

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Excited-State Dynamics of Small Organic Molecules Studied by Time-Resolved Photoelectron Spectroscopy Excited-state dynamics of small organic molecules studied by time-resolved photoelectron spectroscopy T i n g G e n g Excited -state dynamics of small organic molecules studied by time- resolved photoelectron spectroscopy Ting Geng ©Ting Geng, Stockholm University 2017 ISBN print 978-91-7649-758-6 ISBN PDF 978-91-7649-759-3 Printed by Universitetsservice US-AB, Stockholm 2017 Distributor: Department of Physics, Stockholm university Abstract Ultra-violet and visible light induced processes in small organic molecules play very important roles in many fields, e.g., enviromental 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 (-OC nH2n+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. Key words: time-resolved photoelectron spectroscopy, excited-state dynamics, organic molecules Stockholm 2017 This thesis is dedicated to my mom List of Papers The following papers, referred to in the text by their Roman numerals, are included in this thesis PAPER I: Cyclohexadiene revisited: a time-resolved photoelectron spectroscopy and ab initio study. O. Schalk, T. Geng , T. Thompson, N. Baluyot, R. D. Thomas, E. Tapavicza, and T. Hansson, Journal of Physical Chemistry A , 120 , 2320 (2016). PAPER II: Excited state dynamics of acrylonitrile: substituent effects at conical intersections interrogated via time-resolved photoelectron spectroscopy and ab initio simulation. R. J. MacDonell, O. Schalk, T. Geng , R. D. Thomas, R. Feifel, T. Hansson, and M. S. Schuurman, Journal of Chemical Physics , 145 , 114306 (2016). PAPER III: Influence of alkoxy groups on the photoinduced dynamics of organic molecules exemplified on alkyl vinyl ethers. O. Schalk, M. Stenrup, T. Geng , R. Lindh, R. D. Thomas, R. Feifel, and T. Hansson, Journal of Phyiscal Chemistry A , 119 , 11105 (2015). PAPER IV: Substituent effects on the relaxation dynamics of furan, furfural and β-furfural: A combined theoretical and experimental approach. S. Oesterling, O. Schalk, T. Geng , R. D. Thomas, T. Hansson, and R. de Vivie-Riedle, Phys. Chem. Chem. Phys , 19 , 2025 (2017). PAPER V: Dynamics in higher lying excited states: Valence to Rydberg transitions in the relaxation paths of pyrrole and methylated derivatives. T. Geng , O. Schalk, S. P. Neville, T. Hansson, and R. D. Thomas, submitted to Journal of Chemical Physics, January 2017 . PAPER VI: Time-resolved photoelectron spectroscopy studies on pyrazole and its derivates. T. Geng , I. F. Galván, O. Schalk, R. Lindh, T. Hansson, and R. D. Thomas, in manuscript . Reprints were made with permission from the publishers. Author’s contribution My contributions to the papers are as follows: Paper I: Cyclohexadiene revisited: a time-resolved photoelectron spectroscopy and ab initio study. • Participated in setting up the optics. • Participated in conducting the experiments. • Participated in the discussion of the results and the analysis presented in the paper. PAPER II: Excited state dynamics of acrylonitrile: substituent effects at conical intersections interrogated via time-resolved photoelectron spectroscopy and ab initio simulation. • Participated in setting up the optics. • Participated in conducting the experiments. • Participated in the discussion of the results and the analysis presented in the paper. PAPER III: Influence of alkoxy groups on the photoinduced dynamics of organic molecules exemplified on alkyl vinyl ethers. • Participated in setting up the optics. • Participated in conducting the experiments. • Wrote the first draft of the article and prepared the figures for the paper. PAPER IV : Substituent effects on the relaxation dynamics of furan, furfural and β-furfural: A combined theoretical and experimental approach. • Participated in setting up the optics. • Participated in conducting the experiments. • Analysed the spectra and prepared the figures for the paper. PAPER V: Dynamics in higher lying excited states: Valence to Rydberg transitions in the relaxation paths of pyrrole and methylated derivatives. • Participated in setting up the optics • Participated in conducting the experiments. • Analysed the data and prepared the figures for the paper. • Wrote the first draft of the article. PAPER VI: Time-resolved photoelectron spectroscopy studies on pyrazole and several methylated derivatives • Participated in setting up the optics • Participated in conducting the experiments. • Analysed the data and prepared the figures for the paper. • Wrote the first draft of the article. Contents Abstract V List of Papers VII Author’s contribution IX Abbreviations XIII 1. Introduction ............................................................................................... 1 2. Photoinduced reactions and concepts ..................................................... 6 2.1 Electronic transition .............................................................................. 7 2.2 Franck-Condon region ........................................................................ 10 2.3 Propagation ......................................................................................... 11 2.4 Conical intersection, internal conversion and intersystem crossing ... 12 3. Excited state dynamics of molecules...................................................... 15 3.1 Excited state dynamics of ethylene ..................................................... 15 3.2 Excited state dynamics of polyenes .................................................... 17 3.2.1 Linear polyenes ............................................................................ 17 3.2.2 Cyclo-polyenes ............................................................................ 21 3.3 Excited state dynamics of molecules with heteroatoms ...................... 27 3.3.1 Acrolein........................................................................................ 27 3.3.2 Heterocycles ................................................................................ 29 4. Experimental apparatus ......................................................................... 34 4.1 Laser system ....................................................................................... 35 4.2 Optical setup- two colour experiment ................................................. 36 4.3 Interaction region and magnetic bottle spectrometer .......................... 38 5. Time-resolved photoelectron spectroscopy and data analysis ............ 41 5.1 Time-resolved photoelectron spectroscopy ........................................ 41 5.2 Time-resolved photoelectron spectrum (TRPES) and data analysis ... 42 6. Discussion of the attached papers .......................................................... 49 6.1 Dynamics of polyenes – cyclohexadiene ............................................ 50 6.2.1 Ethylene molecules with a cyano group - acrylonitrile ................ 53 6.2.2 Ethylene molecules with alkoxy group- alkyl vinyl ethers .......... 56 6.3 Dynamics of cyclopolyenes with heteroatoms .................................... 59 6.3.1 Dynamics of cyclopolyenes with oxygen - furan ......................... 59 6.3.2 Dynamics of cyclopolyenes with nitrogen - pyrrole .................... 62 6.3.2 Dynamics of cyclopolyenes with two nitrogen - pyrazole ........... 66 7. Conclusion and outlook .......................................................................... 69 Sammanfattning LXXII Acknowledgements LXXV
Load more

Recommended publications
  • Photoexcitation at Titania Surfaces
    Photoexcitation at Titania Surfaces Daniel Thomas Payne Thesis submitted for the Degree of Doctor of Philosophy of the University College London. Supervised by Prof. Geoff Thornton 2017 I, Daniel Thomas Payne, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. 2 Abstract This thesis employs energy- & time-resolved photoemission spectroscopy to ex- amine two surfaces of titanium dioxide (TiO2) single crystals. Both surfaces in- vestigated, the rutile TiO2(110) and anatase TiO2(101) surfaces, are pertinent to future energy research. Resonant two-photon photoexcitation at the reduced rutile TiO2(110) surface is found to involve the Ti 3d band gap states and energy levels of t2g or eg symmetry in the conduction band. The unoccupied state is determined to be centred ∼2.7 eV above the Fermi level (EF ), with a lifetime of less than 15 fs. Adsorption of bridging hydroxyls (OHb) is shown to enhance the intensity of the observed resonance. The signal intensity reaches a maximum under a monolayer of water, which is attributed to partial dissociation of adsorbed water. Electron bombardment of the anatase TiO2(101) surface is shown to increase the reduction state of the surface in ultra-violet photoemission spectra, which is attributed to the formation of oxygen vacancies. However, these point defects are known to migrate towards the bulk at temperatures above 200 K, restoring a thermally-equilibrated concentration. Bombardment also results in the appear- ance of new features in the valence band spectral region, which we associate with the 3σ and 1π molecular orbitals of OHb.
    [Show full text]
  • Photoelectrochemical Process
    Photoelectrochemical process Photoelectrochemical processes are processes in photoelectrochemistry; they usually involve transforming light into other forms of energy. These processes apply to photochemistry, optically pumped lasers, sensitized solar cells, luminescence, and photochromism. Electron excitation Electron excitation is the movement of an electron to a higher energy state. This can either be done by photoexcitation (PE), where the original electron absorbs the photon and gains all the photon's energy or by electrical excitation (EE), where the original electron absorbs the energy of another, energetic electron. Within a semiconductor crystal lattice, thermal excitation is a process where lattice vibrations provide enough energy to move electrons to a higher energy band. When an excited electron falls back to a lower energy state again, it is called electron relaxation. This can be done by radiation of a photon or giving the energy to a third spectator particle as well. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state. The excited state, in general, is in relation to the ground state, where the excited state is at a higher energy level than the ground state. Photoexcitation Photoexcitation is the mechanism of electron excitation by photon absorption, when the energy of the photon is too low to cause photoionization. The absorption of the photon takes place in accordance with Planck's quantum theory. Photoexcitation plays role in photoisomerization. Photoexcitation is exploited in dye-sensitized solar cells, photochemistry, luminescence, optically pumped lasers, and in some photochromic applications. Photoisomerization In chemistry, photoisomerization is molecular behavior in which structural change between isomers is caused by photoexcitation.
    [Show full text]
  • Enantioselective Organocatalytic Alkylation of Aldehydes and Enals Driven by the Direct Photoexcitation of Enamines † † † † † ‡ Mattia Silvi, Elena Arceo, Igor D
    View metadata, citation and similar papers at core.ac.uk brought to you by CORE This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. provided by ZENODO Communication pubs.acs.org/JACS Enantioselective Organocatalytic Alkylation of Aldehydes and Enals Driven by the Direct Photoexcitation of Enamines † † † † † ‡ Mattia Silvi, Elena Arceo, Igor D. Jurberg, Carlo Cassani, and Paolo Melchiorre*, , † ICIQ−Institute of Chemical Research of Catalonia, Av. Països Catalans 16, 43007 Tarragona, Spain ‡ ICREA−Catalan Institution for Research and Advanced Studies, Pg. Lluís Companys 23, 08010 Barcelona, Spain *S Supporting Information ABSTRACT: Disclosed herein is a photo-organocatalytic enantioselective α- and γ-alkylation of aldehydes and enals, respectively, with bromomalonates. The chemistry uses a commercially available aminocatalyst and occurs under illumination by a fluorescent light bulb in the absence of any external photoredox catalyst. Mechanistic investiga- tions reveal the previously hidden ability of transiently generated enamines to directly reach an electronically excited state upon light absorption while successively triggering the formation of reactive radical species from the organic halides. At the same time, the ground state chiral enamines provide effective stereochemical induction for the enantioselective alkylation process. Figure 1. Mechanisms that enamines can use to drive the photochemical generation of radicals: (a) by inducing ground state EDA complex formation; (b) acting as a photosensitizer upon direct photoexcitation. espite their synthetic potential, it is challenging to develop The enamine radical cation, resulting from the SET, is not shown. The D enantioselective catalytic variants of visible light-driven gray circle represents the chiral organocatalyst scaffold.
    [Show full text]