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 center dot(H2O)(1-6), Cy center dot(MeOH)(1-3), and Cy center dot 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 center dot(H2O)(n) are 0.5 ps for n = 1 and similar to 0.2-0.25 ps for n = 2-6. For Cy center dot(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 center dot THF shows a decay similar to that of the keto-Cy monomer, whereas the enol-Cy center dot 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.