Monomeric adenine decay dynamics influenced by the DNA environment

We report on-the-fly surface-hopping dynamics simulations of single adenine embedded in solvated DNA oligomers, (dA)10 and (dA)10·(dT)10. Both model systems are found to decay from the S1 to the S0 state via distinct monomeric channels, on account of the strong hydrogen-bonding interactions between the Watson-Crick pair in the double-stranded oligomer. Surprisingly, the decay times (several picoseconds) for the current models are 10 times longer than those of adenine in the gas or aqueous phase, while matching one of the time constants observed experimentally. We discuss possible reasons for these longer decay times, including steric hindrance in the DNA strands, electronic effects of the environment, and the presence of other local excited-state minima. We present optimized geometries and relative energies for representative S0 and S1 minima as well as conical intersections related to the hopping events. We have also computed steady-state and time-dependent fluorescence spectra that may help understand the experimental observations.