Microhydration Effects on the Ultrafast Photodynamics of Cytosine: Evidences for a Possible Hydration-Site Dependence

Ultrafast excited‐state deactivation dynamics of small cytosine (Cy) and 1‐methylcytosine (1mCy) microhydrates, Cy⋅(H2O)1‐3 and 1mCy⋅(H2O)1,2, produced in a supersonic expansion have been studied by mass‐selected femtosecond pump–probe photoionization spectroscopy at about 267 nm excitation. The seeded supersonic expansion of Ar/H2O gas mixtures allowed an extensive structural relaxation of Cy and 1mCy microhydrates to low‐energy isomers. With the aid of electronic structure calculations, we assigned the observed ultrafast dynamics to the dominant microhydrate isomers of the amino‐keto tautomer of Cy and 1mCy. Excited‐state lifetimes of Cy⋅(H2O)1‐3 measured here are 0.2–0.5 ps. Comparisons of the Cy⋅H2O and 1mCy⋅H2O transients suggest that monohydration at the amino Watson–Crick site induces a substantially stronger effect than at the sugar‐edge site in accelerating excited‐state deactivation of Cy. Hydration‐site dependence: The ultrafast excited‐state deactivation dynamics of small cytosine microhydrates produced in a supersonic expansion was studied by mass‐selected femtosecond pump–probe photoionization spectroscopy. The results suggest that monohydration of cytosine at the amino Watson–Crick site induces a substantially stronger effect on the deaktivation than at the sugar‐edge site in accelerating the excited‐state deactivation.