Time-resolved Circular Dichroism Spectroscopy for Probing Chiral Dynamics

Abstract

Chirality can be understood as a property of an object that cannot be superimposed
on its mirror image, such as our left and right hands. It is ubiquitous in nature and can also be found in the photosynthesis process involving chlorosomes from green sulfur bacteria. Understanding the dynamics of chiral properties in chlorosomes, due to the excitons formed by the interaction between bacteriochlorophyll c from green sulfur bacteria, could lead to a breakthrough in designing systems or devices that mimic its functionality to provide clean and sustainable energy solutions. One way to achieve this is to build Time-Resolved Circular Dichroism (TRCD) Spectroscopy. This technique is based on the different absorption of left- and right-polarized probe pulses by the chiral material after being excited by the pump beam.

In this project, a TRCD is build by utilizing a motor-driven rotating quarter wave plate to modify the existing two-dimensional electronic spectroscopy (2DES) setup, to achieve alternating right- and left-circularly polarization of the probe pulses. Synchronization of the motor-driven rotating quarter wave plate to the 2DES system, optimization and test measurements of the TRCD setup were conducted. We demonstrate that the developed TRCD setup has sufficient capability to probe the chiral properties of an molecular antenna in photosynthetic bacteria, containing aggregates of bacteriochlorophyll c molecules. Thus, this work opens new avenues to explore transient dynamics in chiral materials and observe changes in chiral symmetry on a femtosecond timescale.

Popular Abstract

In nature, one can often find symmetry, particularly handedness symmetry referred to as chirality. Think of chirality as an object that does not match its mirror image, similar to how our left and right hands are mirror images but cannot be superimposed on each other. This concept extends to molecules, with amino acids being a prime example. Almost all of the 22 essential amino acids are left-handed, or left-enantiomers, and their mirror images are called enantiomers.

The necessity of understanding the presence of selective chirality in nature provides a motivation to explore mechanisms underlying chirality. Also a big part of chemistry, biochemistry and pharmaceutics rely on asymmetric synthesis (of selective enantiomers) and catalysis. New findings in these research fields would give essential understanding to have better efficacy of pharmaceutical synthesis with fewer side effects, and also could be used to understand the fundamental and intricate molecular mechanisms in the photosynthesis processes. These processes often involve chlorophyll-type molecules, which are for example building blocks of chlorosome, that has chiral properties due to its molecular structural geometry. The goal is to learn from the natural self-assembling and self-repairing photosynthetic systems how to efficiently convert sunlight to chemical energy and try to design systems or devices that mimic this functionality, to provide clean and sustainable energy for the future generations. To further this understanding, scientists utilize a method known as circular dichroism spectroscopy. Circular dichroism relies on the observation that chiral molecules interact differently with light that can twist—either to the right (clockwise) or to the left (counter-clockwise). When this twisted light passes through chiral molecules, light with different handedness is absorbed differently. This differential absorption helps scientists to study the unique properties of these molecules.

Typically, circular dichroism provides only a steady state information, similar to taking a photograph, which shows what these molecules look like in their ground state but not how they change over time when excited by light. To capture these rapid changes, especially those occurring in femtoseconds—a quadrillionth of a second—researchers are developing advanced techniques such as time-resolved circular dichroism spectroscopy. Such techniques will allow them to create movie-like sequences of molecular motion, revealing how excited chiral molecules dance and interact in time. Such detailed observation is pivotal for harnessing the secrets of nature’s processes and could lead to groundbreaking technologies in energy conversion and medicine. The main aim of this project was to build and test a novel TRCD spectroscopy setup by using a synchronously rotating quarter wave plate to provide an alternating right- and left-circular polarization of the probe pulse. The setup was tested on the photosynthetic antenna chlorosome featuring excitonic transitions sustained by the huge aggregates of bacteriochlorophyll molecules.

• author: Yahya, Ilyas Maulana
• supervisor: Donatas Zigmantas; Cord Arnold
• organization: Department of Physics; Atomic Physics
• course: PHYM03 20241
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• publication/series: Lund reports on atomic physics (LRAP)
• report number: LRAP 598
• language: English
• id: 9163613
• date added to LUP: 2024-06-17 14:02:39
• date last changed: 2024-06-17 14:02:39