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Fingerprints of light-induced molecular transients: from quantum chemical models of ultrafast x-ray spectroscopy
Stockholm University, Faculty of Science, Department of Physics. 1990.ORCID iD: 0000-0003-1058-2588
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Absorption of sunlight generates renewable electricity and powers the growth of plants, but also causes severe damage both to synthetic materials and biological tissue. The wildly varying outcomes of these light-induced processes are ultimately determined by much slighter differences in their underlying reaction pathways, induced by the transient properties of short-lived and miniscule molecules; a powerful approach to their detection and characterization is offered by ultrafast x-ray spectroscopy, with identification of spectral fingerprints and further guidance from quantum chemical models.

This thesis contains the computational half of three experimentally joint projects that push the limits for detection of electronic, spin and structural dynamics of small molecular systems in solution. A wide selection of theoretical frameworks are combined to model various aspects of the measurements: from multi-configurational descriptions of non-adiabatic couplings in the photo-dynamics and multi-electron transitions in the x-ray spectroscopy, to affordable simulations of extensive aqueous solutions by density functional theory and classical mechanics.

Applied to experimental data, the presented quantum chemical results allowed in particular to: simultaneously identify molecular forms and electronic states of aqueous 2-thiopyridone, to determine a detailed pathway for its excited-state proton-transfer; characterize the charge-transfer state of aqueous ferricyanide, to extend well-known concepts from steady-state spectroscopy into the ultrafast domain; establish the newly implemented framework of multi-configurational Dyson orbitals, as a powerful tool for simulation of photoelectron spectroscopy.

A number of computational predictions are additionally presented for hitherto-unexplored experimental regions, which may help to guide and optimize future measurements.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2020. , p. 64
Keywords [en]
time-resolved x-ray spectroscopy, quantum chemistry, electronic structure, multi-configurational self-consistent field, density functional theory, molecular dynamics, Born-Oppenheimer dynamics, non-adiabatic dynamics, proton-transfer, charge-transfer, solvatization, Dyson orbital
National Category
Atom and Molecular Physics and Optics
Research subject
Theoretical Physics
Identifiers
URN: urn:nbn:se:su:diva-179758ISBN: 978-91-7911-052-9 (print)ISBN: 978-91-7911-053-6 (electronic)OAI: oai:DiVA.org:su-179758DiVA, id: diva2:1412962
Public defence
2020-04-22, FA32, Albanova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2015-03956Available from: 2020-03-30 Created: 2020-03-09 Last updated: 2020-05-22Bibliographically approved
List of papers
1. Ultrafast Independent N-H and N-C Bond Deformation Investigated with Resonant Inelastic X-Ray Scattering
Open this publication in new window or tab >>Ultrafast Independent N-H and N-C Bond Deformation Investigated with Resonant Inelastic X-Ray Scattering
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2017 (English)In: Angewandte Chemie International Edition, ISSN 1433-7851, E-ISSN 1521-3773, Vol. 56, no 22, p. 6088-6092Article in journal (Refereed) Published
Abstract [en]

The femtosecond excited-state dynamics following resonant photoexcitation enable the selective deformation of N-H and N-C chemical bonds in 2-thiopyridone in aqueous solution with optical or X-ray pulses. In combination with multiconfigurational quantum-chemical calculations, the orbital-specific electronic structure and its ultrafast dynamics accessed with resonant inelastic X-ray scattering at the N 1s level using synchrotron radiation and the soft X-ray free-electron laser LCLS provide direct evidence for this controlled photoinduced molecular deformation and its ultrashort time-scale.

Keywords
nitrogen, photochemistry, protonation, RIXS (resonant inelastic X-ray scattering), selective bond cleavage
National Category
Physical Sciences Chemical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-144820 (URN)10.1002/anie.201700239 (DOI)000401326300011 ()28374523 (PubMedID)
Available from: 2017-07-11 Created: 2017-07-11 Last updated: 2020-03-13Bibliographically approved
2. T-1 Population as the Driver of Excited-State Proton-Transfer in 2-Thiopyridone
Open this publication in new window or tab >>T-1 Population as the Driver of Excited-State Proton-Transfer in 2-Thiopyridone
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2019 (English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 25, no 7, p. 1733-1739Article in journal (Refereed) Published
Abstract [en]

Excited-state proton transfer (ESPT) is a fundamental process in biomolecular photochemistry, but its underlying mediators often evade direct observation. We identify a distinct pathway for ESPT in aqueous 2-thiopyridone, by employing transient N1s X-ray absorption spectroscopy and multi-configurational spectrum simulations. Photoexcitations to the singlet S-2 and S-4 states both relax promptly through intersystem crossing to the triplet T-1 state. The T-1 state, through its rapid population and near nanosecond lifetime, mediates nitrogen site deprotonation by ESPT in a secondary intersystem crossing to the S-0 potential energy surface. This conclusively establishes a dominant ESPT pathway for the system in aqueous solution, which is also compatible with previous measurements in acetonitrile. Thereby, the hitherto open questions of the pathway for ESPT in the compound, including its possible dependence on excitation wavelength and choice of solvent, are resolved.

Keywords
excited-state proton-transfer, intersystem crossing, nitrogen, photochemistry, X-ray absorption
National Category
Physical Sciences Chemical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-167674 (URN)10.1002/chem.201804166 (DOI)000459811300018 ()30452789 (PubMedID)
Available from: 2019-04-03 Created: 2019-04-03 Last updated: 2020-03-13Bibliographically approved
3. Interdependent Electronic Structure, Protonation, and Solvatization of Aqueous 2-Thiopyridone
Open this publication in new window or tab >>Interdependent Electronic Structure, Protonation, and Solvatization of Aqueous 2-Thiopyridone
2019 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 123, no 26, p. 5555-5567Article in journal (Refereed) Published
Abstract [en]

2-Thiopyridone (2-TP), a common model system for excited-state proton transfer, has been simulated in aqueous solution with ab initio molecular dynamics. The interplay of electronic structure, protonation, and solvatization is investigated by comparison of three differently protonated molecular forms and between the lowest singlet and triplet electronic states. An interdependence clearly manifests in the mixed-character T-1 state for the 2-TP form, systematic structural distortions of the 2-mercaptopyridine (2-MP) form, and photobase protolysis of the 2-TP- form, in the aqueous phase. In comparison, simplified continuum models for the solvatization are found to be significantly inaccurate for several of the species. To facilitate future computational studies, we therefore present a minimal representative solvatization complex for each stable form and electronic state. Our findings demonstrate the importance of explicit solvatization of the compound and sets the studies. stage for including it also in future studies.

National Category
Physical Sciences Chemical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-171766 (URN)10.1021/acs.jpcb.9b03084 (DOI)000474796300018 ()31244103 (PubMedID)
Available from: 2019-08-28 Created: 2019-08-28 Last updated: 2020-03-13Bibliographically approved
4. Ab initio simulations of complementary K-edges and solvatization effects for detection of proton transfer in aqueous 2-thiopyridone
Open this publication in new window or tab >>Ab initio simulations of complementary K-edges and solvatization effects for detection of proton transfer in aqueous 2-thiopyridone
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2019 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 151, no 11, article id 114117Article in journal (Refereed) Published
Abstract [en]

The nitrogen and sulfur K-edge X-ray absorption spectra of aqueous 2-thiopyridone, a model system for excited-state proton transfer in several recent time-resolved measurements, have been simulated from ab initio molecular dynamics. Spectral signatures of the local intra- and inter-molecular structure are identified and rationalized, which facilitates experimental interpretation and optimization. In particular, comparison of aqueous and gas phase spectrum simulations assesses the previously unquantified solvatization effects, where hydrogen bonding is found to yield solvatochromatic shifts up to nearly 1 eV of the main peak positions. Thereby, while each K-edge can still decisively determine the local protonation of its core-excited site, only their combined, complementary fingerprints allow separating all of the three relevant molecular forms, giving a complete picture of the proton transfer.

National Category
Physical Sciences Chemical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-175063 (URN)10.1063/1.5109840 (DOI)000487317400033 ()31542028 (PubMedID)
Available from: 2019-11-05 Created: 2019-11-05 Last updated: 2020-03-13Bibliographically approved
5. Ultrafast dynamics of photo-excited 2-thiopyridone: Theoretical insights into triplet state population and proton transfer pathways
Open this publication in new window or tab >>Ultrafast dynamics of photo-excited 2-thiopyridone: Theoretical insights into triplet state population and proton transfer pathways
2020 (English)In: Structural Dynamics, E-ISSN 2329-7778, Vol. 7, no 2, article id 024101Article in journal (Refereed) Published
Abstract [en]

Ultrafast non-adiabatic dynamics of the small heteroaromatic compound 2-thiopyridone has been studied with surface hopping simulations based on multi-configurational quantum chemistry. Initial excitation of the bright S2 (π,π) state is found to promptly relax to S1 (n,π) through in-plane motion. The subsequent dynamics are oppositely driven by out-of-plane motion, which results both in complex population transfers among all of the available states and intersystem crossing predominantly through the ‘El-Sayed forbidden ’S1 (n,π) to T2 (n,π) channel, through significant mixing of electronic excitation characters. Despite this complexity, the femto- to picosecond triplet population, expected from several spectroscopic measurements, is well described as a simple exponential decay of the singlet state manifold. No proton transfer is found in the reported trajectories, but two mechanisms for its possible mediation in previously reported experiments are proposed based on the observed structural dynamics: (i) ultrafast intra-molecular transfer driven by the initially coherent in-plane motion and (ii) inter-molecular solvent-mediated transfer driven by the out-of-plane modes that dominate the later motion.

National Category
Atom and Molecular Physics and Optics
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-179764 (URN)10.1063/1.5143228 (DOI)000521264300001 ()
Funder
Swedish Research Council, 2015-03956Carl Tryggers foundation , CTS18:285Swedish National Infrastructure for Computing (SNIC), 2019-1-9
Available from: 2020-03-05 Created: 2020-03-05 Last updated: 2020-05-04Bibliographically approved
6. Ultrafast nonadiabatic dynamics probed by nitrogen K-edge absorption spectroscopy
Open this publication in new window or tab >>Ultrafast nonadiabatic dynamics probed by nitrogen K-edge absorption spectroscopy
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2020 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 22, no 5, p. 2667-2676Article in journal (Refereed) Published
Abstract [en]

The emergence of X-ray free electron lasers (X-FELs) has made it possible to probe structural dynamics on the femtosecond timescale. This extension of experimental capabilities also calls for a simultaneous development in theory to help interpret the underlying structure and dynamics encoded within the experimental observable. In the ultrafast regime this often requires a time-dependent theoretical treatment that describes nuclear dynamics beyond the Born–Oppenheimer approximation. In this work, we perform quantum dynamics simulations based upon time-evolving Gaussian basis functions (GBFs) and simulate the ultrafast X-ray Absorption Near-Edge Structure (XANES) spectra of photoexcited pyrazine including two strongly coupled electronically excited states and four normal mode degrees of freedom. Two methods to simulate the excited state XANES spectra are applied, the first is based upon the multi-configurational second order perturbation theory restricted active space (RASPT2) method and the second uses a combination of the maximum overlap method (MOM) and time-dependent density functional theory (TDDFT). We demonstrate that despite the simplicity of the MOM/TDDFT method, it captures several qualitative features of the RASPT2 simulations at much reduced computational effort. However, features such as the conical intersection are a particular exception as they require a multi-configurational treatment. For the nuclear dynamics, we demonstrate that even a small number of GBFs can provide reasonable description of the spectroscopic observable. This work provides perspectives for computationally efficient approaches important for addressing larger systems.

National Category
Atom and Molecular Physics and Optics
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-179755 (URN)10.1039/C9CP03019K (DOI)000517561500005 ()
Funder
Swedish Research Council, 2015-03956Swedish National Infrastructure for Computing (SNIC), SNIC2018-1-4
Available from: 2020-03-05 Created: 2020-03-05 Last updated: 2020-04-14Bibliographically approved
7. Fingerprints of electronic, spin and structural dynamics from resonant inelastic soft X-ray scattering in transient photo-chemical species
Open this publication in new window or tab >>Fingerprints of electronic, spin and structural dynamics from resonant inelastic soft X-ray scattering in transient photo-chemical species
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2018 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 10, p. 7243-7253Article in journal (Refereed) Published
Abstract [en]

We describe how inversion symmetry separation of electronic state manifolds in resonant inelastic soft X-ray scattering (RIXS) can be applied to probe excited-state dynamics with compelling selectivity. In a case study of Fe L-3-edge RIXS in the ferricyanide complex Fe(CN)(6)(3-), we demonstrate with multi-configurational restricted active space spectrum simulations how the information content of RIXS spectral fingerprints can be used to unambiguously separate species of different electronic configurations, spin multiplicities, and structures, with possible involvement in the decay dynamics of photo-excited ligand-to-metal charge-transfer. Specifically, we propose that this could be applied to confirm or reject the presence of a hitherto elusive transient Quartet species. Thus, RIXS offers a particular possibility to settle a recent controversy regarding the decay pathway, and we expect the technique to be similarly applicable in other model systems of photo-induced dynamics.

National Category
Physical Sciences Chemical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-156105 (URN)10.1039/c7cp08326b (DOI)000429286100052 ()29484313 (PubMedID)
Available from: 2018-05-11 Created: 2018-05-11 Last updated: 2020-03-13Bibliographically approved
8. Disentangling Transient Charge Density and Metal-Ligand Covalency in Photoexcited Ferricyanide with Femtosecond Resonant Inelastic Soft X-ray Scattering
Open this publication in new window or tab >>Disentangling Transient Charge Density and Metal-Ligand Covalency in Photoexcited Ferricyanide with Femtosecond Resonant Inelastic Soft X-ray Scattering
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2018 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 9, no 12, p. 3538-3543Article in journal (Refereed) Published
Abstract [en]

Soft X-ray spectroscopies are ideal probes of the local valence electronic structure of photocatalytically active metal sites. Here, we apply the selectivity of time resolved resonant inelastic X-ray scattering at the iron L-edge to the transient charge distribution of an optically excited charge-transfer state in aqueous ferricyanide. Through comparison to steady-state spectra and quantum chemical calculations, the coupled effects of valence-shell closing and ligand-hole creation are experimentally and theoretically disentangled and described in terms of orbital occupancy, metal-ligand covalency, and ligand field splitting, thereby extending established steady-state concepts to the excited-state domain. pi-Back-donation is found to be mainly determined by the metal site occupation, whereas the ligand hole instead influences sigma-donation. Our results demonstrate how ultrafast resonant inelastic X-ray scattering can help characterize local charge distributions around catalytic metal centers in short-lived charge-transfer excited states, as a step toward future rationalization and tailoring of photocatalytic capabilities of transition-metal complexes.

National Category
Physical Sciences Chemical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-158243 (URN)10.1021/acs.jpclett.8b01429 (DOI)000436382400057 ()29888918 (PubMedID)
Available from: 2018-08-14 Created: 2018-08-14 Last updated: 2020-03-13Bibliographically approved
9. Photoelectron shake-ups as a probe of molecular symmetry: 4d XPS analysis of I-3(-) in solution
Open this publication in new window or tab >>Photoelectron shake-ups as a probe of molecular symmetry: 4d XPS analysis of I-3(-) in solution
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2018 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 30, p. 19916-19921Article in journal (Refereed) Published
Abstract [en]

A combination of multi-configurational restricted active space calculations with a Dyson orbital formalism has been applied for accurate simulations of 4d photo-electron spectra of the I-3(-) molecular ion. The analysis based on the occupation numbers of natural orbitals allowed to predict and rationalize the spectral fingerprints of solvent-induced nuclear asymmetry. In particular, it demonstrates how the nuclear asymmetry directly causes an increase of shake-up intensity. The relative intensity of shake-up and main features of the I 4d XPS spectrum could therefore serve as a simplified experimental observable of structural asymmetry, complementary to changes in the shape of the main spectral features.

National Category
Physical Sciences Chemical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-160170 (URN)10.1039/c8cp02530d (DOI)000441583300018 ()30020286 (PubMedID)
Available from: 2018-09-17 Created: 2018-09-17 Last updated: 2020-03-13Bibliographically approved

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