The mechanism of excimer formation: an experimental and theoretical study on the pyrene dimer

Please always quote using this URN: urn:nbn:de:bvb:20-opus-159514
  • The understanding of excimer formation in organic materials is of fundamental importance, since excimers profoundly influence their functional performance in applications such as light-harvesting, photovoltaics or organic electronics. We present a joint experimental and theoretical study of the ultrafast dynamics of excimer formation in the pyrene dimer in a supersonic jet, which is the archetype of an excimer forming system. We perform simulations of the nonadiabatic photodynamics in the frame of TDDFT that reveal two distinct excimerThe understanding of excimer formation in organic materials is of fundamental importance, since excimers profoundly influence their functional performance in applications such as light-harvesting, photovoltaics or organic electronics. We present a joint experimental and theoretical study of the ultrafast dynamics of excimer formation in the pyrene dimer in a supersonic jet, which is the archetype of an excimer forming system. We perform simulations of the nonadiabatic photodynamics in the frame of TDDFT that reveal two distinct excimer formation pathways in the gas-phase dimer. The first pathway involves local excited state relaxation close to the initial Franck–Condon geometry that is characterized by a strong excitation of the stacking coordinate exhibiting damped oscillations with a period of 350 fs that persist for several picoseconds. The second excimer forming pathway involves large amplitude oscillations along the parallel shift coordinate with a period of ≈900 fs that after intramolecular vibrational energy redistribution leads to the formation of a perfectly stacked dimer. The electronic relaxation within the excitonic manifold is mediated by the presence of intermolecular conical intersections formed between fully delocalized excitonic states. Such conical intersections may generally arise in stacked π-conjugated aggregates due to the interplay between the long-range and short-range electronic coupling. The simulations are supported by picosecond photoionization experiments in a supersonic jet that provide a time-constant for the excimer formation of around 6–7 ps, in good agreement with theory. Finally, in order to explore how the crystal environment influences the excimer formation dynamics we perform large scale QM/MM nonadiabatic dynamics simulations on a pyrene crystal in the framework of the long-range corrected tight-binding TDDFT. In contrast to the isolated dimer, the excimer formation in the crystal follows a single reaction pathway in which the initially excited parallel slip motion is strongly damped by the interaction with the surrounding molecules leading to the slow excimer stabilization on a picosecond time scale.show moreshow less

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Metadaten
Author: Joscha Hoche, Hans-Christian Schmitt, Alexander Humeniuk, Ingo Fischer, Roland MitrićORCiD, Merle I. S. RöhrORCiD
URN:urn:nbn:de:bvb:20-opus-159514
Document Type:Journal article
Faculties:Fakultät für Chemie und Pharmazie / Institut für Physikalische und Theoretische Chemie
Language:English
Parent Title (English):Physical Chemistry Chemical Physics
Year of Completion:2017
Schriftenreihe:Physical Chemistry Chemical Physics, 2017,19, 25002-25015. DOI: 10.1039/C7CP03990E
Volume:19
Issue:36
Pagenumber:25002-25015
URL:http://dx.doi.org/10.1039/C7CP03990E
DOI:https://doi.org/10.1039/C7CP03990E
Dewey Decimal Classification:5 Naturwissenschaften und Mathematik / 54 Chemie / 541 Physikalische Chemie
Tag:exciton dynamics; pyrene dimer
PACS-Classification:30.00.00 ATOMIC AND MOLECULAR PHYSICS
Release Date:2018/03/23
Note:
Accepted version
Embargo Date:2018/08/30
EU-Project number / Contract (GA) number:646737
OpenAIRE:OpenAIRE
Licence (German):License LogoDeutsches Urheberrecht