Ultrafast Chiral Spectroscopy for Stereocontrolled Photochemistry

Chiral molecules exist in two forms, called enantiomers, which are mirror images of each other but non-superimposable. Even though enantiomers share most chemical and physical properties, they may differ greatly in their (bio-)chemical activities, which turns chirality into a key design feature for (bio-)chemical function. In this spirit, the incorporation of chiral structures into photoactive molecular systems has emerged as a powerful strategy to control their photochemical functions: uni-directional molecular motors, chiral photocatalysts, and chiral metal nano-structures permit new levels of stereocontrol over mechanical motion, energy transfer, and electric charge-carriers on the nanoscale, with applications ranging from the construction of nanoscale machines to the light-driven synthesis of enantiopure compounds. However, the direct characterization and optimization of the chiral photoexcited states that drive these processes has remained a formidable challenge, due to a lack of analytical techniques with chiral sensitivity and sufficient time resolution to capture the excited state dynamics – especially in the native solution phase of many photochemical processes. To address this gap, I will build on previous breakthrough studies to deliver a new laser-based chiral spectroscopy technique with femtosecond time resolution and ultra-broad spectral detection from the visible to the far ultraviolet (UV) regime. These capabilities will be unique world-wide and open the path to resolve the stereocontrol mechanisms of chiral photoactive systems that have thus far remained inaccessible. For the chiral photochemistry community, this can be transformative: the direct analysis of excited state chirality promises design strategies that have previously been difficult to pursue. This project will establish the conceptual foundations for these strategies by focusing on the three processes that are the fundamental building blocks of most photoactive systems: photoisomerization, excitation energy transfer, and electric charge carrier dynamics. In a joint effort with experts in chiral synthesis, the project will study their stereocontrol via tailor-made chiral compounds, as summarized in the project goals:

Goal 1: Ultrafast chiral spectroscopy. This project will deliver an ultrafast laser spectroscopy setup with unprecedented time resolution and chiral sensitivity. Via novel photonic technology it will cover the entire deep-UV window (190-300 nm) for the first time, which encodes the chiral structures of many (bio-)chemical systems.

Goal 2: Stereocontrol of mechanical motion in molecular motors. Tracking the motion of synthetic molecular machines has remained a challenge, as the dynamics may cover multiple time and length scales. This project will capture the kinetics of chiral molecular motors in real-time, thereby informing future design improvements.

Goal 3: Stereocontrol of energy transfer in luminescent complexes. Circularly polarized luminescence (CPL) from chiral organo(-metallic) materials promises applications in mobile phone screens and bio-sensing. This project will determine the mechanisms of record-breaking CPL complexes to develop rational design guidelines.

Goal 4: Sterecontrol of charge-carrier dynamics in gold nanoparticles. Chiral metal nanoparticles hold tremendous promise as photosensitizers for enantioselective photochemistry. This project will determine the coupling of their chiral structural morphology to the underlying electric charge-carrier dynamics on opposite ends of the size scale: in large plasmonic nanohelices (>30 nm) and in molecule-like gold nanoclusters (<3 nm).

Impact: This project will establish ultrafast chiral spectroscopy as a new analytical tool for photochemistry and advance current capabilities for stereocontrolling photochemical processes in a wide range of molecular systems. It will thus have a strong impact on multiple disciplines developing photoactive materials, from enantioselective photocatalysis to nano-material science. Additionally, this project addresses two high-impact challenges in ultrafast spectroscopy: (1) expanding it to the far-UV spectral regime, and (2) developing a new method that combines high time and structural resolution in liquids.