Quantum dynamics of the pi pi*/n pi* decay of the epigenetic nucleobase 1,5-dimethyl-cytosine in the gas phase

We study the ultrafast dynamics of 1,5-dimethyl-cytosine, a model for 5-methyl-cytidine, after photoexcitation to the first two bright pi pi* states, focusing on the possible population transfer to dark n pi* states. To that end we propagate the initial wave packets on the coupled potential energy surfaces of the seven lowest energy excited states modelled with a diabatic linear vibronic coupling (LVC) model, considering all the vibrational coordinates. Time-evolution is computed by the multilayer version of the multiconfigurational time dependent Hartree (ML-MCTDH) method. The LVC Hamiltonian is parametrized with time-dependent density functional theory (TD-DFT) calculations adopting PBE0 and CAM-B3LYP functionals, which provide a different energy gap between the lowest energy n pi* states and the spectroscopic pi pi* state. Population of the lowest pi pi* flows to a dark n pi* state which involves a lone pair (LP) of the carbonyl oxygen (n(O)pi*), but the extent of such transfer is much larger according to PBE0 than to CAM-B3LYP. Photoexcitation to the second bright state gives rise to much richer dynamics with an ultrafast (50 fs) complete decay to the lowest pi pi*, to n(O)pi* and to another n pi* in which the excited electron comes from the LP of the ring nitrogen. We perform a detailed analysis of the vibronic dynamics both in terms of normal modes and valence coordinates (bond lengths and angles). The comparison with the analogous dynamics in 1-methyl-cytosine, a model for cytidine, provides insights into the effect of methylation at carbon 5 on the electronic and nuclear dynamics.