Dynamics of chiral molecules

Figure 1: D- and L-lactic acid as an example of a chiral molecule. Likewise, our hands can be thought of as enantiomers that behave like image and mirror image.

Chirality is a fundamental concept that is omnipresent in natural science. In the field of molecules, for example, the existence of certain enantiomers (chiral molecules that can only be converted into each other by mirroring, see Fig. 1) is preferred. In order to observe the ultrafast dynamics of states and changes of chiral structures, methods sensitive to the corresponding chirality of the molecule have to be applied. By measuring the circular dichroism, which is defined as the difference in the absorption of left- and right-circularly polarized light, this information becomes accessible.

To achieve a time resolution on the relevant femtosecond time scale, ultrashort laser pulses are used and the technique of pump–probe spectroscopy is employed in the experiment. The generation of the required broadband probe pulses with opposite circular polarization are generated using a specially designed setup (Fig. 2) [1]. This setup differs significantly from other techniques known in the literature [2,3].

Figure 2: Experimental setup for measuring time-resolved circular dichroism.

A thesis in this project can cover many areas of work. These include, of course, working with lasers and optical components. In addition, depending on the topic of the thesis, there are also the following areas of work:

Literature research and identification of new chiral molecules that can be examined.
Fine adjustment of the existing setup, which requires a sure instinct.
Pulse shaping and pulse compression.
Extension of the setup in order to be able to perform multidimensional spectroscopy in the future.
Data evaluation with common programs such as MatLab, Glotaran und LabView.

Contact:

Karina Heilmeier, karina.heilmeier@uni-wuerzburg.de

Literature for overview:

[1] L. Ress et al., Chem. Sci. (2023)

[2] A. Trifonov et al., Rev. Sci. Instrum., 81, 043104 (2010)

[3] M. Scholz et al., J. Phys. Chem. Lett., 10, 5160 (2019)

Completed theses

The topic of your thesis in our working group is always assigned based on the current research work in the laboratory. Therefore, it does not make sense to constantly post new open topics here. Just contact us! In order to get an impression of possible theses in this project, a selection of already completed theses is presented below.

Bachelor thesis

Characterization of a setup for the generation of circular laser pulses

This bachelor thesis consisted of characterizing and improving the new setup for measuring time-resolved circular dichroism. Various abberations and polarization states of a broadband laser beam were identified and optimized. For this purpose, the measurement data were analyzed in detail and the optics responsible for the aberrations were replaced. Figure 3 shows the two-dimensional beam profile around the focus of the laser beam before (left) and after the replacement of an optic (right). The profile improves from an elongated shape to a much rounder and smaller shape.

Figure 3: Beam profiles of the broadband laser beam around the focus position. The profiles on the left show an elongated beam profile. After replacing various optics, which contributed to the distortion of the beam profile, a significantly rounder beam profile could be generated. This can be seen on the right. In addition, the diameter of the beam is smaller and the astigmatism has been reduced.

Master thesis

Time-Resolved Circular Dichroism Spectroscopy of Squaraine Polymers

In this project, the ultrafast dynamics of a chiral squaraine polymer in different solvents were investigated. It has been shown that different polarities and solubility parameters of the three solvents result in different structures of the polymer. In acetone, for example, only a helical structure is formed, whereas in diethylformamide (DEF) a mixture of helical and linear strands is formed (see Fig. 4). The helix is oriented in only one direction and is therefore chiral. Using time-resolved absorption and circular dichroism spectroscopy, different dynamics of the excited states in the different solvents can be determined. Due to the special nature of the new setup for measuring the time-resolved circular dichroism, these dynamics can be examined and described more precisely than with conventional transient absorption spectroscopy.