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 ~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 1ππ* 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 1ππ* state decay rates, implying that the N‐C‐O‐H dihedral angle can have a notable effect on the excited‐state deactivation dynamics. Gas‐phase excited‐state dynamics of 5‐methylcytosine was studied using femtosecond pump‐probe photoionization spectroscopy. The results indicate that the first 1ππ* state lifetimes of 5‐methylcytosine are longer than those observed for cytosine. Our analyses suggest that the two amino‐enol rotamers exhibit markedly different 1ππ* state lifetimes, implying that the N‐C‐O‐H dihedral angle can have an effect on the excited‐state dynamics.