Dr. Sebastian Hammer
Experimental Physics VI
Julius-Maximilians University Würzburg
spricht zum Thema
Capturing Molecular Motions of Organic Semiconductors with Ultrafast Electron Diffraction Dynamics of Molecular Systems
The optical excitations in molecular semiconductors are dominated by Frenkel exciton-polarons, which are primarily localized on individual molecular units and strongly coupled to the vibrational and phononic system of the solid [1]. This coupling is one of the dominating factors that determines the fate of the excited state. For example, coupling with optical phonons can distort the local inter-molecular geometry, facilitating the formation of a delocalized, energetically more favorable excimer state [2,3] or can assist the triplet-pair separation in singlet-fission processes [4]. While these processes are extensively studied with optical pump-probe techniques, these experiments are mostly sensitive to the electronic system. The dynamics of nuclear system however can only be observed indirectly. In this talk, I will give an overview on how ultrafast electron diffraction (UED) can be used as to reveal the complex molecular movements in those systems. After a quick introduction on the necessary scattering theory and experimental challenges, I will show how structural modeling can extract the underlying geometric distortions and guide ab-initio quantum chemical calculations using polycrystalline zinc-phthalocyanine (ZnPc) as a case study [5].
[1] Hestand, N. J. and Spano, F. J. , Chem. Rev. 118 (2018), 7069-7163
[2] Hammer S. et al., Mater. Horiz. 10 (2023), 221-234
[3] Bialas, A. L. and Spano, F. J. , J. Chem. Phys. C 126 (2022), 4067-4081
[4] Seiler et al. , Sci. Adv. 7 (2021), eabg0869
[5] Hammer S. et al., PNAS 121 (2024), e241197512
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