Intersystem Crossing Dynamics in Thio-Substituted Pyrimidine Nucleobases Studied by Time-resolved Photoionization Techniques

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In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Prof. Ullrich of the University of Georgia is using laser spectroscopy to study the response of modified nucleobases to ultraviolet (UV) irradiation. Nucleobases are molecular components of DNA, and thus form the building blocks of our genetic coding material. Naturally occurring nucleobases are known to protect themselves against UV radiation through processes within the nucleobase molecule that dissipate potentially harmful UV energy into heat. However, the details of these processes are not well understood. Prof. Ullrich is uncovering these details by making slight changes to the nucleobase structure, and observing changes in how UV light is dissipated. In some cases, for example when an oxygen atom is replaced by a sulfur atom, the nucleobase has difficulty dissipating the energy from the UV light. This is to say it remains "excited", and therefore vulnerable to damage. While such a condition may not be desirable for normal strand of DNA, it may be beneficial in cancer treatments, where UV or other light energy may be used to specifically attack cancer cells.In this project time-resolved photoelectron spectroscopy is applied to the study of pyrimidine thiobases to unravel the mechanistic details governing their unique ultrafast intersystem crossing dynamics. Supplemented by ab-initio calculations and dynamics simulations intricate specifics of the excited state potential energy surfaces such as couplings between the lowest excited states, potential energy barriers, and accessibility of conical intersections are revealed. Particular focus is on the role of low-lying singlet states of n-pi character, given their widespread emergence as the key doorway for accessing the triplet manifold. The project trains undergraduate and graduate students as well as summer interns who participate in the research activities and outcomes of the research are broadly disseminated to the public through science show performances.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.