Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 20, Issue 10, Pages 6877-6890Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c7cp08235e
Keywords
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Funding
- European Research Council Advanced Grant STRATUS [291198]
- French Agence National de la Recherche [FEMTO-2DNA, ANR-15-CE29-0010]
- ENS-Lyon [900/S81/BS81-14, MI-LOURD-FR15]
- CNR/CNRS Progetto Bilaterale
- Chaire D'Alembert program of University Paris-Saclay [2016-10751]
- Agence Nationale de la Recherche (ANR) [ANR-15-CE29-0010] Funding Source: Agence Nationale de la Recherche (ANR)
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The most relevant 'dark' electronic excited states in DNA/RNA pyrimidine nucleosides are mapped in water employing hybrid MS-CASPT2/MM optimisations with explicit solvation and including the sugar. Conical intersections (CIs) between initially accessed bright (1)pi pi* and the lowest energy dark (1)n pi* excited states, involving the lone pair localised on the oxygen and/or nitrogen atoms are characterised. They are found in the vicinities of the Franck-Condon (FC) region and are shown to facilitate non-adiabatic population transfer. The excited state population of the (1)n(O)pi* state, localised in the carbonyl moiety on all pyrimidine nucleosides, is predicted to rapidly evolve to its minimum, displaying non-negligible potential energy barriers along its non-radiative decay, and accounting for the ps signal registered in pump-probe experiments as well as for an efficient population of the triplet state. Cytidine displays an additional (1)n(N)pi* state localised in the N3 atom and that leads to its excited state minimum displaying large potential energy barriers in the pathway connecting to the CI with the ground state. Sugar-to-base hydrogen/proton transfer processes are assessed in solution for the first time, displaying a sizable barrier along its decay and thus being competitive with other slow decay channels in the ps and ns timescales. A unified deactivation scheme for the long-lived channels of pyrimidine nucleosides is delivered, where the (1)n(O)pi* state is found to mediate the long-lived decay in the singlet manifold and act as the doorway for triplet population and thus accounting for the recorded phosphorescence and, more generally, for the transient/photoelectron spectral signals registered up to the ns timescale.
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