  We propose a model for the photo-deactivation mechanism for DNA based upon
accurate quantum chemical and molecular dynamical evaluations of model
Watson/Crick nucleoside pairs and stacked pairs. Our results corroborate recent
ultrafast experimental studies on DNA oligonucleotides and suggest that
following photo-excitation to a local $\pi-\pi^*$ state, the excitation is
rapidly delocalized over several (3-4) bases on an ultrafast time-scale.
However, this delocalized state is unstable with respect to the motions of the
protons involved in hydrogen-bonding between Watson/Crick pairs and rapidly
re-localizes to a charge-transfer state on a longer time-scale ranging from 10
to 100 ps. This state, too, is unstable and relaxes via a conical intersection
with the ground state near the geometry of the enol- and imino-tautomeric form.
We suggest that this internal deactivation mechanism is responsible for the
intrinsic photostability of DNA.
