title: Femtosecond Intersystem Crossing in the DNA Nucleobase Cytosine.
abstract: Ab initio molecular dynamics including nonadiabatic and spin-orbit couplings on equal footing is used to unravel the deactivation of cytosine after UV light absorption. Intersystem crossing (ISC) is found to compete directly with internal conversion in tens of femtoseconds, thus making cytosine the organic compound with the fastest triplet population calculated so far. It is found that close degeneracy between singlet and triplet states can more than compensate for very small spin-orbit couplings, leading to efficient ISC. The femtosecond nature of the ISC process highlights its importance in photochemistry and challenges the conventional view that large singlet-triplet couplings are required for an efficient population flow into triplet states. These findings are important to understand DNA photostability and the photochemistry and dynamics of organic molecules in general.
authors: †Institute of Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany.
authors: Martin
authors: M
authors: Richter
authors: ‡Institute of Theoretical Chemistry, University of Vienna, Währinger Strasse 17, 1090 Vienna, Austria.
authors: Philipp
authors: P
authors: Marquetand
authors: ¶Departamento de Química Física I, Universidad Complutense, 28040 Madrid, Spain.
authors: Jesús
authors: J
authors: González-Vázquez
authors: ¶Departamento de Química Física I, Universidad Complutense, 28040 Madrid, Spain.
authors: Ignacio
authors: I
authors: Sola
authors: ‡Institute of Theoretical Chemistry, University of Vienna, Währinger Strasse 17, 1090 Vienna, Austria.
authors: Leticia
authors: L
authors: González
keywords: DNA photostability; conical intersection; excited-state dynamics; intersystem crossing; spin−orbit coupling