Ultrafast excited-state dynamics of gas-phase 5-methylcytosine tautomers

In this study, we investigated the gas-phase ultrafast excited-state deactivation dynamics of 5-methylcytosine (5mCy) tautomers excited between 257 and 300 nm in a free jet with femtosecond pump-probe photoionization spectroscopy. The results show that excited-state lifetimes of 5mCy amino-keto tautomer excited at 290-300 nm are similar to 3 ps, whereas those of 5mCy amino-enol tautomer excited at 257-290 nm range from 6 to 220 ps. These results indicate that, upon photoexcitation to the first (1)pi pi* state, 5mCy exhibits markedly slower deactivation dynamics than nonmethylated Cy, indicating that methylation at the C5 position increases the barrier to efficient excited-state deactivation pathways. Our analyses also suggest a surprising conclusion that the two rotamers of the 5mCy amino-enol tautomer exhibit markedly different (1)pi pi* state decay rates, implying that the N-C-O-H dihedral angle can have a notable effect on the excited-state deactivation dynamics.