Vertical Excitation Energies and Lifetimes of the Two Lowest Singlet Excited States of Cytosine, 5-Aza-cytosine, and the Triazine Family: Quantum Mechanics-Molecular Mechanics Studies
Abstract
A swarm of semi-classical quantum mechanics/molecular mechanics molecular-dynamics simulations where OM2/MNDO is combined with the Gromacs program for consideration of explicit water is performed, solving the time-dependent Schro''dinger equation in each step of the trajectories together with the TullATIN SMALL LETTER Y WITH ACUTEs fewest switches algorithm. Within this stochastic treatment, time dependent probabilities of t h e three lowest electronic states are determined.
The fact that nucleobases are quickly deactivated is confirmed in the cytosine case where our best lifetime estimation is tau 1=0.82 ps for the model with 100 water molecules with the SPCE force field and a time step of 0.1 fs. Lifetimes of the remaining molecules are visibly longer: 5-azacytosine, 2,4-diamino-1 ,3 ,5-triazine (DT) , and 2,4,6-triamino-1 ,3 ,5-triazine (TT) molecules have an S1 -> S0 de-excitation time of slightly above 10 ps.
The lifetimes of the triazine f a m i l y increases with the increasing number of exocyclic amino groups, that is, s-triazine < 2-amino-1,3,5-triazine < DT < TT. This can be explained by a higher mobility of the carbon-bonded hydrogen atoms in comparison with heavier amino groups since their movement is slowed down due to a substantially higher mass than hydrogen atoms, which can easier reach the out-of-plane positions required in the conical intersection structures.
Moreo v e r , bulkier NH2 ligands suffer due to greater friction caused by the surrounding water environment. These mechanical aspects caused a change in the explored lifetime dependences in comparison with our previous gas-phase study.