Investigation of\nthe Nonradiative Photoprocesses of\nUnnatural 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\n(Ds) is an\nunnatural nucleic acid that forms a stable pair with pyrrole-2-carbaldehyde\n(Pa) in DNA. This Ds–Pa pair gets stabilized via van der Waals\ninteraction and shape fitting. In our previous study [Ghosh, P.J. Phys. Chem. A 2021, 125, 5556–5561], we\ninvestigated the nonradiative photoprocesses of the unnatural DNA\nbase Pa, and also there are some studies on its stability and reactivity\nin the ground state. But, to consider it as a good unnatural base\npair, one has to understand its stability not only in the ground state\nbut also in the excited states after absorbing ultraviolet (UV) radiation.\nTherefore, in this study, the excited-state photoprocesses of Ds on\nUV irradiation and its nonradiative decay channels have been investigated\nusing state-of-the-art multireference methods, and this investigation\nfinally leads the molecule to access the minimum energy crossing point\n(MECP) via a downhill pathway.