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Probing Ultrafast Stereochemical Dynamics by Femtosecond Electronic Circular Dichroism Spectroscopy

Periodic Reporting for period 1 - CHIRALSCOPY (Probing Ultrafast Stereochemical Dynamics by Femtosecond Electronic Circular Dichroism Spectroscopy)

Reporting period: 2019-11-01 to 2021-10-31

The development of time-resolved femtosecond broadband circular dichroism (CD) spectroscopy is a challenging but an important area of research. Measurements of CD signal requires very high detection sensitivity of the optical setups as CD is typically a very small signal (10000-100000 times weaker than absorption). Time resolved photoinduced CD variations are expected to be even smaller. Furthermore, broadband CD measurements are rendered more difficult by the polarization artifacts arising from the wavelength-dependent polarization sensitivity of optics employed in such optical setups.

Among optical techniques, CD spectroscopy stands out for its unique senstivity towards chirality and molecular structure. The development of time resolved broadband CD spectroscopy is thus very crucial as it would allow one to gain fundamental insights into the reaction mechanisms and pathways. Such capability would impact many fields in science including protein folding/misfolding related to diseases such as Alzheimer’s and Parkinson’s, and DNA photodamage which is the first step in the development of cancer.

The goal of the CHIRALSCOPY project was to develop a high-sensitivity broadband circular dichroism spectrometer with femtosecond time resolution to study chiroptical and magneto-optical photophysics of biomolecules and materials.
We have carried out three main tasks towards achieving the goals of CHIRALSCOPY;

(i) We have developed a balanced detection Fourier transform interferometric setup for the steady-state measurements of broadband circular dichroism (CD) and optical rotatory dispersion (ORD) spectra with high-sensitivity. Circular dichroism arises due to interaction of chiral molecules with polarized light and manifests as the difference in absorption between left and right circularly polarized light by chiral molecules. In contrast to the conventional approaches, our methodology measures rotation and retardation of light polarization by chiral molecules across all the wavelengths in the time domain. This allows highly sensitive measurements of CD and ORD spectra. This work has already been published as a peer-reviewed article in ACS Photonics.

(ii) We have subsequently extended this balanced detection scheme to develop a setup for time resolved measurements of transient CD and ORD. Such novel detection scheme allows us to separate the pump-probe signal from transient CD and transient ORD signals. This work is currently in progress.

(iii) Alongside the developmental works, we have employed a variant of the above setup for time-resolved Faraday rotation measurements in 2D layered perovskites. Following optical preparation of high-energy exciton states at cryogenic temperature, these measurements have revealed sub 100 fs formation of exciton-polaron states that drastically slows down the spin depolarization in perovskites. These results raise the tantalizing possibility of optical control of spin lifetime in such materials. This work is currently under revision in the journal Nature Communication.
Overall, we have made substantial strides in the last two years towards the goal of the CHIRALSCOPY project. We have built a balanced detection optical activity setup that simultaneously measures broadband circular dichroism (CD) and optical rotatory dispersion(ORD) spectra with high-senstivity and high-speed using an incoherent light source. Prior to our work, broadband optical activity measurements were demonstrated mostly using coherent laser sources. Thanks to our development, it will be now possible to measure broadband CD and ORD spectra outside the laser laboratory. Due to the high speed of our measurements, it is now possible to monitor the progress of fast chiral chemical reactions in real time.
We have also devised novel strategy to probe time resolved CD and ORD dynamics with femtosecond time resolution using the balanced detection schemes. which we are now employing for time-resolved experiments. This work is still in progress. Once accomplished it will open new doors to understand femtosecond chiroptical and magnetooptical dynamics in biomolecules and materials systems.

In 2D layered perovskites, we have demonstrated optical control of spin lifetimes. We found that at cryogenic temperature, the spin relaxation in layered perovskites drastically slows down when pumping high-energy exciton states because of the polaron formation. These results have raised the tantalizing possibility of optical control of spin lifetime in layered perovskites materials.

Our broadband CD/ORD spectropolarimeter promises to be superior to the ones currently the market, so that it has strong potential for commercial exploitation and may in the long run have an impact on healthcare.