Femtosecond Intersystem
Crossing in the DNA Nucleobase
Cytosine
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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