The decay channels of singlet excited adenine uracil monophosphate (ApU) in water are studied with CASPT2//CASSCF:MM potential energy calculations and simulation of the 2D-UV spectroscopic fingerprints with the aim of elucidating the role of the different electronic states of the stacked conformer in the excited state dynamics. The adenine 1La state can decay without a barrier to a conical intersection with the ground state. In contrast, the adenine 1Lb and uracil S(U) states have minima that are separated from the intersections by sizeable barriers. Depending on the backbone conformation, the CT state can undergo inter-base hydrogen transfer and decay to the ground state through a conical intersection, or it can yield a long-lived minimum stabilized by a hydrogen bond between the two ribose rings. This suggests that the 1Lb, S(U) and CT states of the stacked conformer may all contribute to the experimental lifetimes of 18 and 240ps. We have also simulated the time evolution of the 2D-UV spectra and provide the specific fingerprint of each species in a recommended probe window between 25000 and 38000cm-1 in which decongested, clearly distinguishable spectra can be obtained. This is expected to allow the mechanistic scenarios to be discerned in the near future with the help of the corresponding experiments. Our results reveal the complexity of the photophysics of the relatively small ApU system, and the potential of 2D-UV spectroscopy to disentangle the photophysics of multichromophoric systems

We acknowledge the participation of the late Luis Serrano- Andrés in the first stage of this work. L.B. and A.V. acknowledge financial support from the Spanish Ministry of Economy (CTQ2011-26573, CTQ2015-69363-P) and the Generalitat de Catalunya (2014SGR-1202) and the use of computer time at the Consorci de Serveis Universitaris de Catalunya. Q.L. acknowledges the National Natural Science Foundation of China (21303007). M.G. acknowledges support by the European Research Council Advanced Grant STRATUS (ERC-2011-AdG No.291198) and the Agence Nationale de la Recherche for the 2015 project FEMTO-2DNA (Spectroscopie 2DUV: un nouvel outil pour l’étude de biomolécules, ANR-15-CE29-0010). D.R.-S. acknowledges support from the Generalitat Valenciana (GV2015-057) and the Spanish Ministry of Economy (project CTQ2014-58624-P and grant JCI-2012-13431). S.M gratefully acknowledges the support of the Chemical Sciences, Geosciences, and Biosciences division, office of Basic Energy Sciences, U.S. Department of Energy, and the National Science Foundation (grant CHE-1361516)