Investigation of the Nonradiative Photoprocesses of Unnatural DNA Base: 7-(2-Thienyl)-imidazo[4,5-<i>b</i>]pyridine (Ds)─A Computational Study

7-(2-Thienyl)-imidazo[4,5-<i>b</i>]pyridine (Ds) is an unnatural nucleic acid that forms a 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 [Ghosh, P. <i>J. Phys. Chem. A</i> 2021, 125, 5556-5561], we investigated the nonradiative photoprocesses of the unnatural DNA base Pa, and also there are some studies on its stability and reactivity in the ground state. But, to consider it as 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 ultraviolet (UV) radiation. Therefore, in this study, the excited-state photoprocesses of Ds on UV irradiation and its nonradiative decay channels have been investigated using state-of-the-art multireference methods, and this investigation finally leads the molecule to access the minimum energy crossing point (MECP) via a downhill pathway.