Excited State Dynamics of Cold Protonated Cytosine Tautomers: Characterization of Charge Transfer, Inter System Crossing and Internal Conversion Processes

S. SOORKIA; G. GRÉGOIRE; M. BROQUIER; C. JOUVET; G. A. PINO; C. DEDONDER-LARDEUX

Carlos Paz

Seminario; XVII ELAFOT; 2017

Charge transfer reactions are ubiquitous in chemicalreactivity and often viewed as ultrafast processes. For deoxyribonucleic acid (DNA),femtochemistry has undeniably revealed the primary stage of the deactivationdynamics of the locally excited state following electronic excitation whilelonger time scales could hardly be investigated within the experimental timewindow. We here demonstrate that the complete excited state dynamics can befollowed from a few femtoseconds up to milliseconds through an originalpump-probe photodissociation scheme applied to cryogenic ion spectroscopy withpicosecond and nanosecond pulse lasers. Excited state calculations at theRI-CC2 level were also performed in order to help the interpretation of theexperimental results.Two tautomers of protonated cytosine werespectroscopically observed: the enol tautomer, in which the H+ isbonded to the C=O group of Cyt and the keto tautomer in which the H+is bonded to N3 of cytosine.[1] The excited state dynamics shows that in bothtautormers the locally excited 1pp* state decays in the femtosecond range towardslong-lived charge transfer state and triplet state with lifetimes ranging frommicroseconds to milliseconds. A three-step mechanism (1pp* → 1CT → 3pp*) is proposed where internal conversionfrom each state can occur leading ultimately to fragmentation in the groundelectronic state (Figure 1).