Real-time probing of chirality during a chemical reaction

Real-time probing of chirality during a chemical reaction

Denitsa Baykusheva, Daniel Zindel, Vít Svoboda, Elias Bommeli, Manuel Ochsner, Andres Tehlar, and Hans Jakob Wörner

PNAS November 26, 2019 116 (48) 23923-23929; first published November 13, 2019;

Significance

Chiral molecules interact and react differently, depending on their handedness (left vs. right). This chiral recognition is the principle governing most biomolecular interactions, such as the activity of drugs or our perception of scents. In spite of this fundamental importance, a real-time (femtosecond) observation of chirality during a chemical reaction has remained out of reach in the gas phase. In the present work, we report this breakthrough with a seemingly unlikely technique: high-harmonic generation (HHG) in tailored intense near-infrared laser fields. Combining the transient-grating technique with HHG in counterrotating circularly polarized laser fields, we follow the temporal evolution of molecular chirality during a chemical reaction from the unexcited electronic ground state through the transition-state region to the final achiral products.

Abstract

Chiral molecules interact and react differently with other chiral objects, depending on their handedness. Therefore, it is essential to understand and ultimately control the evolution of molecular chirality during chemical reactions. Although highly sophisticated techniques for the controlled synthesis of chiral molecules have been developed, the observation of chirality on the natural femtosecond time scale of a chemical reaction has so far remained out of reach in the gas phase. Here, we demonstrate a general experimental technique, based on high-harmonic generation in tailored laser fields, and apply it to probe the time evolution of molecular chirality during the photodissociation of 2-iodobutane. These measurements show a change in sign and a pronounced increase in the magnitude of the chiral response over the first 100 fs, followed by its decay within less than 500 fs, revealing the photodissociation to achiral products. The observed time evolution is explained in terms of the variation of the electric and magnetic transition-dipole moments between the lowest electronic states of the cation as a function of the reaction coordinate. These results open the path to investigations of the chirality of molecular-reaction pathways, light-induced chirality in chemical processes, and the control of molecular chirality through tailored laser pulses.

The two enantiomers of a chiral molecule interact differently with chiral receptors and with chiral light. The former effect is the basis of chiral recognition, an essential mechanism of biomolecular function. The latter is the principle of optical techniques for detecting chirality. Although extensive control over molecular chirality has been achieved in enantio-selective synthesis of molecules (1⇓–3), chiral sensitivity has been lacking from all femtosecond time-resolved probes of chemical reactions in the gas phase demonstrated to date. The main origin of this shortcoming is the weakness of chiral light–matter interactions, which rely on magnetic-dipole, electric-quadrupole, and higher-order interactions. For this reason, circular dichroisms (CDs) in absorption spectroscopies are typically very weak effects in the range of 10−6 to 10−3 relative signal changes. Nevertheless, time-resolved CD (TRCD) techniques have gained increasing attention over the last 15 y (4) and nowadays cover the infrared (5, 6), visible (7), and ultraviolet (UV) (8⇓⇓–11) spectral regions. The promise of extending such measurements to the X-ray domain has recently been predicted (12⇓–14). Unfortunately, the inherent weakness of the optical CD effects and the related technical challenges have so far limited TRCD measurements to the condensed phase and to picosecond or longer time scales.

Full article at link:
https://www.pnas.org/content/116/48/23923