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, afterphotoexcitation to the first two brightpp* states, focusing on the possible population transfer to darknp* states. To that end we propagate the initial wave packets on the coupled potential energy surfacesof 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 ofthe multiconfigurational time dependent Hartree (ML-MCTDH) method. The LVC Hamiltonian isparametrized with time-dependent density functional theory (TD-DFT) calculations adopting PBE0 andCAM-B3LYP functionals, which provide a different energy gap between the lowest energy np* states andthe spectroscopicpp* state. Population of the lowestpp* flows to a dark np* state which involves a lonepair (LP) of the carbonyl oxygen (nOp*), but the extent of such transfer is much larger according to PBE0than to CAM-B3LYP. Photoexcitation to the second bright state gives rise to much richer dynamics withan ultrafast (50 fs) complete decay to the lowestpp*, to nOp* and to another np* in which the excitedelectron comes from the LP of the ring nitrogen. We perform a detailed analysis of the vibronicdynamics both in terms of normal modes and valence coordinates (bond lengths and angles). Thecomparison with the analogous dynamics in 1-methyl-cytosine, a model for cytidine, provides insightsinto the effect of methylation at carbon 5 on the electronic and nuclear dynamics.