The transfer of population to the triplet state may be observed, at very modest rates, at around 50-100 fs. Interestingly, in this early stage the population seems to be driven mostly to the higher excited T4 state, which however are never showing a substantial population, also due to the very fast internal conversion (IC) to the lower triplet states. At larger time scales the population of T1 becomes more important, especially after 500/600 fs. Interestingly the increase of the population of T1 happens at similar time scales as the sharp decrease of the S2 and a stabilization of S1 population. As a matter of fact, at around 600 fs T1 bypasses S2 and at the end of the SH dynamics (200 fs) it reaches a population of more than 20%. Note that the other triplet states never achieve significant population and that a marginal participation of the higher singlet state S3 can also be observed. This model is also confirmed by the analysis of the hops during the SH dynamics which points to a population flow from S1 to both S2 (as well as higher excited singlet states) and T4 (see ESI). Interestingly, while almost no transfer to the triplet population is observed from S2 a complex equilibrium involving singlet states up to S4 can be highlighted. On the other hand, after the initial population of T4 a very rapid interval conversion on the triplet manifold ensues leading to the sequential ultrafast population of T3, T2, and finally T1. On these bases we may also sketch out a kinetic model (see Figure ) which yields out a rate-limiting S1àT4 ISC characteristic time tISC of 3.74 ps. On the contrary the cascade of IC on the triplet manifold takes place in the sub ps-range with characteristic time tIC comprised between 100 and 200 fs. As far as the H-transfer is involved we analyse in Figure the distribution of the O-H distance d highlighted in Scheme 1 on the ensemble of the SH trajectories and its time evolution. While for all the initial conditions d peaks at around 1.8 Å, which is coherent with a classical intramolecular H-bond, even at the very first steps of the dynamic we may observe the branching of the distribution with the establishment of a cluster of trajectories presenting shorter distances of 0.9 Å, which are coherent with the formation of a novel covalent oxygen hydrogen bond. The flow of population between the two branches continues regularly along the SH population leading to an increase of the importance of the shorter distance structures which become dominant at around 600 ps. Interestingly, a continuous and bidirectional flux between the two populations can be observed all along the dynamic indicating the establishment of an equilibrium between the two tautomeric forms. Furthermore, in some cases we may observe a sudden increase of d which may reach up to 4 Å and show important oscillations. This fact can be associate to the activation of the out-of-plane rotation of the enol OH group. Finally, and as reported pictorially in Figure we may observe H-transfer both for trajectories leading to ISC and for trajectories remaining on the singlet state. Conversely, a significant breaking of the planarity can be observed for the T1 state as shown in Figure . At the 2 ps limit, around 75% of the trajectories have transferred the involved hydrogen, and the phenomenon appeared for both trajectories leading to ISC or continuing exploring the singlet manifold.