Photoinduced disruption of the strong intramolecular H-bond in the enol form of acetylacetone: Mechanisms of radiationless decay

We propose mechanisms which describe the photoinduced disruption of the intramolecular H-bond in the stable enol form of acetylacetone. The study was performed with the second-order approximate coupled cluster singles and doubles model CC2. The basis set aug-cc-pVDZ was applied. The excited-state reaction paths of the rotations around CC, CC and CO bonds were investigated using both coordinate-driven (relaxed scan) and linear interpolation in internal coordinates (LIIC) approaches. We show that the 1ππ∗ excited states of the rotations around the CC double bond relax in, a completely barrierless manner, to the ground state via S0-S1 conical intersections. This is the main relaxation channel of the excited states associated with the disruption of the strong H-bond in the enol form of acetylacetone. The CC and CO rotations did not show such mechanisms.

Energy-level diagram of acetylacetone.