Strong Changes in the Dielectric Functions of Cytosine Upon Molecular Modification

The DNA base cytosine and its variant 1-allylcytosine were deposited as thick layers onto silicon substrates using organic molecular beam deposition. The dielectric functions of these materials were measured in the vacuum ultraviolet range from 4.0to9.5eV using synchrotron radiation. In the experiments, the slight modification of the molecules results in very pronounced differences in the optical response. This was unexpected in the light of time dependent density functional theory calculation predicting a very similar optical response for both molecules. We attribute the substantial change in the optical response to stronger intermolecular interaction upon the molecular modification.