Investigation of the non-radiative photo-processes of unnatural DNA base: 7-(2-thienyl)-imidazo[4,5-b]pyridine (Ds) - A computational study

7-(2-thienyl)-imidazo[4,5-b]pyridine (Ds) is an unnatural nucleic acid which forms stable pair with Pyrrole-2-carbaldehyde (Pa) in DNA. This Ds-Pa pair gets stabilized via van der Waals interaction and shape fitting. In our previous study,\cite{ghosh2021radiationless} we investigated the non-radiative photo-processes of unnatural DNA base Pa and also there are some studies on its stability and reactivity in the ground state. But to form a good unnatural base pair, one has to understand its stability not only in the ground state but also in the excited states after absorbing UV radiation. Therefore, in this study, the excited state photo-processes of Ds on UV irradiation and it's non-radiative decay channels have been investigated. It is shown using state of the art multi reference methods and this investigation finally leads the molecule to access minimum energy crossing point (MECP) via a downhill pathway.