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This work scrutinizes the relaxation mechanism of 2-oxopurine. Contrary to its ancestor, purine, which is a UVC chromophore, 2-oxopurine shows a red-shifted absorption spectrum centered in the UVA region. In 2-oxopurine, relaxation along the ππ* spectroscopic state directs the population from the Franck–Condon (FC) region towards a minimum, which acts as a crossroad for the further decay of the system either to triplet states or, alternatively, to the ground state through a C 6 -puckered S 1 /S 0 funnel. A comparison of the optical properties and excited state potential energy surfaces of purine, 2-oxopurine, 2-aminopurine, 6-oxopurine and adenine, allows establishing how the position and nature of substituent tune the photophysics of purine. For this series, we conclude that both C 2 and C 6 substitution redshift the absorption spectrum of purine, with 2-oxo substitution exhibiting the largest shift. An important exception is the canonical nucleobase adenine, which presents a blue shifted absorption spectrum. The topography of purine's ππ* potential energy surface experiences major changes when functionalized at the C 6 position. In particular, the disappearance of the minimum along the ππ* potential energy surface efficiently funnels the excited state population from the FC region to the ground state and increases the photostability of 6-aminopurine (adenine) and 6-oxopurine (hypoxanthine) nucleobases.