Doctoral Defense in Chemistry - Benedikt Orri Birgisson

Doctoral candidate:
Benedikt Orri Birgisson

Title of thesis:
Calculations of photoinduced energy relaxation and atomic rearrangement of molecules

Opponents:
Dr. Raffaela Cabriolu, Associate Professor at the Department of Physics, NTNU – Norwegian University of Science and Technology Dr. Jakob Schiøtz, Professor at the Department of Physics, Technical University of Denmark

Advisor:
Dr. Hannes Jónsson, Professor at the Faculty of Physical Sciences , University of Iceland

Other members of the doctoral committee:
Dr. Gianluca Levi, Post-doctoral Researcher at the Science Institute of the University of Iceland
Dr. Elvar Örn Jónson, Research Associate at the Science Institute of the University of Iceland
Dr. Asmus Ougaard Dohn, Research Engineer at the Department of Pysics, The Technical University of Denmark

Chair of Ceremony:
Dr. Birgir Hrafnkelsson, Professor and Head of the Faculty of Physical Sciences, University of Iceland

Abstract:
The excitation of molecules by the absorption of photons has wide applicability, for instance in sensitizers and photocatalysts for systems driven by energy carried by light. A key issue is how the energy is then distributed and how the atomic structure rearranges after the photoexcitation, as such processes can critically affect the outcome and efficiency of light energy conversion. The pathways of energy and atomic structure relaxation of three widely studied molecules, two binuclear photocatalytic transition metal complexes and one organic photoactive compound, are investigated using density functional theory (Density Functional Theory (DFT)) calculations. The vibrational decoherence and energy dissipation channels of the photoexcited [Pt2(P2O5H2)4]4− ( PtPOP) molecule in solution are studied using multiscale molecular dynamics simulations where the Excited State ( ES ) of the complex is calculated with a time-independent density functional approach. It is found that the energy delivered to the Pt-Pt pinching mode upon photoexcitation is released to the solvent through the ligand atoms rather than directly from the Pt atoms. Differences in energy relaxation channels in water and acetonitrile are explained in terms of the differences in solvation structure and interaction of the solvent molecules with the platinum atoms. The Ground State (GS) potential energy surface of the [Ir2(dimen)4]2+ (dimen = para-diisocyanomenthane) ( IrDimen) complex is explored to assist the interpretation of ultrafast experiments of the dynamics of conversion between two GS conformers induced by light absorption. A range of density functional approximations is assessed, finding that hybrid exchange-correlation functionals with added dispersion interactions are needed to reproduce the experimental observations. The minimum energy path between the two conformers is computed and a population analysis performed to support the experimentally observed depletion and restoration of the nearly equal GS populations. Lastly, the potential energy surface of the N,N’-dimethylpiperazine (DMP ) cation, a diamine, is investigated with various density functional approximations. This has become an important test case for the balance between localized and delocalized charge density and has recently raised some controversy. The original hybrid functional of Becke as well as some double hybrid functionals produce a minimum on the energy surface corresponding to a localized electronic state where the charge sits mainly on one of the N-atoms, in addition to the global, delocalized state minimum with the charge evenly shared by the two N-atoms. But, commonly used functionals such as PBE and PBB0, as well as recent neural network trained functionals do not produce the localized state. It is also found that explicit Perdew- Zunger self-interaction correction produces a localized state, while downscaled correction by a half does not. The calculated energy surfaces for the DMP cation thus provide valuable information about the delicate balance between localized and delocalized charge distribution in the various density functional approximations.

About the doctoral candidate:
Benedikt Orri Birgisson born 1988. He finished a BS-degree in chemistry from the University of Iceland 2014 and MS-degree in chemistry from the same school in 2017. After the MS-degree he worked as a research assistant at the University of Iceland from 2018-2019. Benedikt started his PhD at the University of Iceland in 2019