Excited State Dynamics of protonated DNA/RNA bases: All roads lead to the triplet state

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

Tokyo

Simposio; WRHI International symposium on Advanced Laser Spectroscopy for soft molecular systems; 2018

Charge transfer reactions are ubiquitous in chemical reactivity and often viewed as ultrafast processes. For DNA, femtochemistry has undeniably revealed the primary stage of the deactivation dynamics of the locally excited state following electronic excitation. We here demonstrate that the full time scaleexcited state dynamics can be followed up to milliseconds through an original pump−probe photodissociation scheme applied to cryogenic ion spectroscopy. Protonated cytosine was chosen as a benchmark system in which the locally excited 1ππ* state decays in the femtosecond range toward long-lived chargetransfer and triplet states with lifetimes ranging from microsecondsto milliseconds, respectively. A three-step mechanism (1ππ* → 1CT → 3ππ*) is proposed where internal conversion from each state can occur leading ultimately to fragmentation in the ground electronic state.Studies on protonated tymine and uracile also show several short lived states and a long lived excited state lifetime, indicating the the final step in the deactivation of the locally excited state is the intersystem crossing to the 3ππ* state.