Development of ultrafast broadband UV-Vis circular dichroism spectroscopy for investigating the photoinduced dynamics of chiral molecular systems
Final Report Abstract
In the course of this project, the signal-to-noise ratio of the setup for ultrafast transient circular dichroism (TrCD) spectroscopy in the UV-Vis range established in the first funding period was improved by a factor of 10. This was achieved by introducing a new Pockels-cell setup based on a shorter deuterated KDP crystal, which minimized the dispersion of the 400 nm supercontinuum seed pulses substantially and consequently resulted in more stable circularly polarized broadband TrCD supercontinuum probe pulses with low shot-to-shot intensity fluctuations, a much larger spectral width and a better time resolution of about 100 fs. Using this considerably improved setup, a comprehensive understanding of the ultrafast TrCD dynamics was obtained for cholesteric polyfluorene copolymer arrangements showing either large intrinsic chirality or giant chiral induction. Chiral helicene-type additives based on the dibenzo-[c,h]acridine motif with different substituents were particularly efficient in inducing a strong CD response of up to 10000 mdeg in achiral polymers, such as F8BT. It was found that the TrCD signal of these copolymer films arose from the supramolecular response of the systems, and the TrCD signal monitored the recovery of the electronic ground state. This allowed for a quantification of singlet-singlet annihilation and charge-pair formation processes based on a detailed global kinetic analysis approach. The supramolecular origin of the strong chiroptical response of these copolymers was further supported by the characteristic dependence of the CD signal on the film thickness. A newly developed setup for CD and circularly polarized luminescence (CPL) microscopy provided detailed information regarding the chiral arrangements in the thin films with diffraction-limited resolution. Moreover, measurements employing a newly developed setup for transient CPL spectroscopy with 28 ps time resolution provided evidence for a static mechanism of CPL emission, where initially linearly polarized light from the chromophores was converted into left- or right-circularly polarized light upon traveling through the circularly birefringent cholesteric polymer layers.
Publications
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Ultrafast excited-state dynamics of thin films consisting of helicene-like molecules based on dibenzo[c,h]acridine. Molecular Physics, 119(17-18).
Morgenroth, Marius; Scholz, Mirko; Guy, Laure; Oum, Kawon & Lenzer, Thomas
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Hot paper: Mapping chirality in time and space, Bunsen-Magazin, 2, 37 (2022). Invited research news
T. Lenzer & K. Oum
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Mapping the broadband circular dichroism of copolymer films with supramolecular chirality in time and space. Nature Communications, 13(1).
Morgenroth, Marius; Scholz, Mirko; Cho, Min Ju; Choi, Dong Hoon; Oum, Kawon & Lenzer, Thomas
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Neue Einblicke mit spezieller Laser-Messapparatur (New insights using special laser-based measurement equipment), Forschungsnews Universität Siegen, 19.01.2022 (also featured online on “Chemie.de”, “Analytica-World.com”, “Innovations Report“, etc.)
T. Lenzer & K. Oum
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Räumliche und zeitliche Kartierung effizienter chiraler Induktion durch helikale Additive in dünnen Copolymerfilmen. Angewandte Chemie, 134(31).
Morgenroth, Marius; Scholz, Mirko; Guy, Laure; Oum, Kawon & Lenzer, Thomas
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Annealing temperature-dependent induced supramolecular chiroptical response of copolymer thin films studied by pump-modulated transient circular dichroism spectroscopy. Scientific Reports, 14(1).
Gust, Domenic; Scholz, Mirko; Schumacher, Vivien; Mulatier, Jean-Christophe; Pitrat, Delphine; Guy, Laure; Oum, Kawon & Lenzer, Thomas
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Electronic Chiroptical Properties of Supramolecular Polymer Assemblies, Bunsen-Magazin, 5, 134 (2024). Summary of Bunsentagung keynote speech
T. Lenzer & K. Oum
