Gas-phase ultrafast excited-state dynamics of cytosine, 1-methylcytosine, and 5-fluorocytosine were investigated in molecular beams using femtosecond pump-probe photoionization spectroscopy to identify the intrinsic dynamics of the major cytosine tautomers. The results indicate that, upon photoexcitation in the first absorption band, the cytosine enol tautomer exhibits a significantly longer excited-state lifetime than its keto and imino counterparts. The initially excited states of the cytosine keto and imino tautomers decay with sub-picosecond dynamics for excitation wavelengths shorter than 300 nm, whereas that of the cytosine enol tautomer decays with time constants ranging from 3 to 45 ps for excitation between 260 and 285 nm.

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.

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.

Microsolvation effects on the ultrafast excitedstate deactivation dynamics of cytosine (Cy) were studied in hydrogen-bonded Cy clusters with protic and aprotic solvents using mass-resolved femtosecond pump−probe ionization spectroscopy. Two protic solvents, water (H 2 O) and methanol (MeOH), and one aprotic solvent, tetrahydrofuran (THF), were investigated, and transients of Cy•(H 2 O) 1−6 , Cy•(MeOH) 1−3 , and Cy•THF microsolvated clusters produced in supersonic expansions were measured. With the aid of electronic structure calculations, we assigned the observed dynamics to the lowenergy isomers of various Cy clusters and discussed the microsolvation effect on the excited-state deactivation dynamics. With the protic solvents only the microsolvated clusters of Cy keto tautomer were observed. The observed decay time constants of Cy•(H 2 O) n are 0.5 ps for n = 1 and ∼0.2−0.25 ps for n = 2−6. For Cy•(MeOH) n clusters, the decay time constant for n = 1 cluster is similar to that of the Cy monohydrate, but for n = 2 and 3 the decays are about a factor of 2 slower than the corresponding microhydrates. With the aprotic solvent, THF, hydrogen-bonded complexes of both keto and enol tautomers are present in the beam. The keto-Cy•THF shows a decay similar to that of the keto-Cy monomer, whereas the enol-Cy•THF exhibits a 2-fold slower decay than the enol-Cy monomer, suggesting an increase in the barrier to excited-state deactivation upon binding of one THF molecule to the enol form of Cy.

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