Comparative study of radiationless deactivation mechanisms in cytosine and 2,4-diaminopyrimidine

• Comparison of the radiationless deactivation processes via ring-puckered S1/S0 conical intersections.
• Comparison of the radiationless deactivation processes via S1/S0 conical intersections involving NH bond elongation.
• Reaction-path explorations and S1/S0 conical intersections for the phototautomerization process of 2,4-diaminopyrimidine via the photoinduced dissociation-association (PIDA) mechanism.

We present a comparative computational CASPT2 and CC2 study of the deactivation mechanisms of electronically excited cytosine (Cyt) and 2,4-diaminopyrimidine (DAPy). A number of S1/S0 conical intersections exhibiting NH bond elongation were optimized. These conical intersections are accessible from the Franck–Condon region along 1πσ* excited-state reaction paths. We also focus on the phototautomerism of DAPy and propose the photoinduced dissociation-association (PIDA) mechanism for this process. Supplementary experimental results on the UV irradiation of DAPy in acetonitrile solution show tautomerization to imino tautomers. By analyzing differences in the photophysics of the nucleobase Cyt and its analogue DAPy, this study provides new insight into the varying degrees of photostability between nucleobases and their close analogues.