Ultrafast Excited-State Dynamics of Hydrogen-Bonded Cytosine Microsolvated Clusters with Protic and Aprotic Polar Solvents.

Microsolvation effects on the ultrafast excited-state 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 (H2O) and methanol (MeOH), and one aprotic solvent, tetrahydrofuran (THF), were investigated, and transients of Cy·(H2O)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 low-energy 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·(H2O) 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.

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.

Disentangling intrinsic ultrafast excited-state dynamics of cytosine tautomers.

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.