  The gas phase relaxation dynamics of photoexcited 5-azacytosine has been
investigated by means of SHARC (surface-hopping including arbitrary couplings)
molecular dynamics, based on accurate multireference electronic structure
computations. Both singlet and triplet states were included in the simulations
in order to investigate the different internal conversion and intersystem
crossing pathways of this molecule. It was found that after excitation,
5-azacytosine undergoes ultrafast relaxation to the electronic ground state
with a time constant of about 1~picosecond. Two important conical intersections
have been identified as the funnel responsible for this deactivation mechanism.
The very low intersystem crossing yield of 5-azacytosine has been explained by
the size of the relevant spin-orbit coupling matrix elements, which are
significantly smaller than in related molecules like cytosine or 6-azauracil.
This difference is due to the fact that in 5-azacytosine the lowest singlet
state is of $n_\mathrm{N}\pi^*$ nature, whereas in cytosine and 6-azauracil it
is of $n_\mathrm{O}\pi^*$ character.

Chemical Physics