STUDIA UBB CHEMIA, LXVI, 3, 2021 (p. 239-253) (RECOMMENDED CITATION) DOI:10.24193/subbchem.2021.3.15 THE FULL MAPPING OF LOW-LYING EXCITED STATE RELAXATION DYNAMIC PATHWAYS FOR ACETOPHENONE ATTILA BENDE a ABSTRACT. Several relaxation pathways of the low-lying electronic excited state dynamics for acetophenone including also the singlet-triplet intersystem crossings were studied. The multireference Hartree-Fock and second order perturbation theory methods together with def2-tzvp basis set were used to characterize the equilibrium geometries and the crossing points between different potential energy surfaces (PES) up to the third excited state level considering both the singlet and triplet spin states. The electronic deactivation pathways studies reveal that the acetophenone shows several possible deactivation channels but their occurring probability strongly depends on the local profile of the PES of the intermediate states. Keywords: acetophenone, conical intersection, intersystem crossing, spin- orbit coupling, deactivation pathway INTRODUCTION Acetophenone (C6H5C(=O)CH3, APN) is an aromatic ketone molecule with interesting photochemical and spectroscopic properties. Due to the electronic excitation of the π electrons of the carbonyl group or the benzene ring, electron charges can easily migrate between the carbonyl fragment and the benzene ring and change the electronic structure of fragments. In general, this π electron conjugation between the aromatic and carbonyl groups could influence the ordering of the n → π* and π →π* states, their photochemical reactivities, and the photo-dissociation mechanisms [1-3]. It was experimentally shown, that the gas phase absorption spectrum for APN in the region of 210–380 nm exhibits three broad peaks centered at 325, 273 and 230 nm, which are assigned to the electronic transitions from the ground state to S1, S2 and S3 excited states, respectively [4]. On a Molecular and Biomolecular Physics Department, National Institute for Research and Development of Isotopic and Molecular Technologies, Str. Donat 67-103, C.P. 700, Cluj Napoca, RO-400293, Romania, attila.bende@itim-cj.ro