Development of ultrafast broadband UV-Vis circular dichroism spectroscopy for investigating the photoinduced dynamics of chiral molecular systems

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Transient circular dichroism (TrCD) spectroscopy is a promising method for tracking the transient evolution of photoexcited chiral systems. This is made possible by monitoring the difference in the photoinduced changes of absorption between left and right circularly polarized light. In the first funding period of this project, we established a setup for ultrafast TrCD spectroscopy covering the wavelength range 260-700 nm with a time resolution of 170-200 fs. It was successfully applied to obtain new information regarding the photoinduced dynamics and the coherent spectroscopy of chiral polymer thin films. In this renewal proposal, we would like to improve the sensitivity of the existing UV-Vis TrCD setup considerably by employing two new technical developments: (1) an advanced Pockels-cell-based design and (2) an alternative design involving a photoelastic modulator. Both approaches will improve the time resolution of the setup to ca. 90 fs and the present signal-to-noise ratio by at least a factor of ten. These new setups for TrCD spectroscopy will then be applied to elucidate the dynamics of three classes of chiral systems: (1) the electronic and structural dynamics of photoexcited cholesteric fluorene- and carbazole-based copolymer arrangements showing either large intrinsic chirality or giant chiral induction, (2) the transient CD response of helicene-type molecules and thin films in the excited electronic state and (3) the dynamics of chiral perovskite-inspired semiconductors upon photoexcitation to the conduction band. Measurements employing ultrafast broadband UV-Vis-NIR transient absorption will complement the studies using TrCD spectroscopy. The experiments will be analyzed by sophisticated global kinetic analysis techniques and calculations based on density functional theory. We are convinced that these advanced setups for ultrafast transient CD spectroscopy will be applicable to probing the structural and electronic dynamics of various chiral systems.