Berlin 2018 – wissenschaftliches Programm
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HL: Fachverband Halbleiterphysik
HL 54: Organic semiconductors
HL 54.10: Vortrag
Freitag, 16. März 2018, 12:00–12:15, EW 203
Ultrafast dynamics through a conical intersection in a donor-acceptor oligomer thin film — Ephraim Sommer1, •Antonietta De Sio1, Johannes Krantz2, Elena Mena-Osteritz2, Peter Bäuerle2, and Christoph Lienau1 — 1Institut für Physik, Universität Oldenburg, Germany — 2Institut für organische Chemie II und neue Materialien, Universität Ulm, Germany
Conical intersections (CoIns) arising from strong multimode vibronic coupling are a universal feature of molecular systems. They are predicted to play a key role for the ultrafast dynamics of ubiquitous processes such as charge transfer in donor-acceptor (D-A) systems. However, clear experimental signatures of nonadiabatic dynamics at CoIns are challenging to obtain. Here we use ultrafast two-dimensional electronic spectroscopy to track the initial dynamics in an A-D-A oligomer thin film used in efficient organic photovoltaics (OPV). Upon impulsive excitation, we observe a clear grid-like peak pattern suggesting coherent wavepacket motion in the excited state. After only 40 fs, this peak structure transforms into a broad, nearly featureless peak. The data show an increase of oscillation period and concurrent, abrupt amplitude drop of the optically excited wavepacket within 40 fs, followed by the appearance of a new wavepacket with different oscillation. These results cannot be described in a single mode vibronic coupling scenario, as recently reported for some polymers [1,2]. They instead show that the initial intramolecular charge separation in this OPV material is governed by strong multimode vibronic coupling via a CoIn. [1] Nature Commun. 7,13742(2016), [2] PCCP 19,18813(2017)