Theoretical Insights on the Excited State Deactivation Mechanism in Protonated Adenosine.

We present herein our computational exploration of the conformational landscape and photophysical properties of protonated adenosine (AdoH⁺). Several different protonated isomers and conformers have been considered and their relevant photophysical properties have been addressed. From our ab initio quantum computational results, an S₁/S₀ conical intersection (CI) has been located for all considered conformers, providing a significant route for the ultrafast deactivation mechanism of the S₁ excited state of AdoH⁺. Our results are also supported by nonadiabatic dynamics (NAD) simulation results indicating the S₁ excited state lifetime of 240-300 fs for the two most stable conformers of AdoH⁺ (i.e., the most stable syn- and anti-N₃ protonated tautomers), which is comparable with protonated adenine, reported in the literature. The results confirm the ultrafast deactivation mechanism as well as photostability in nucleosides in protonated form.