Capturing ring opening in photoexcited enolic acetylacetone upon hydrogen bond dissociation by ultrafast electron diffraction

Photoinduced biological and chemical reactions are often based on key structural transformations of a molecule driven across multiple electronic states. Acetylacetone (AcAc) is a prototypical system for complex chemical pathways involving several conical intersections (CI) and singlet-triplet intersystem crossings (ISC) characterized by distinct geometries. In the gas phase, AcAc is predominantly in a planar ring-like enolic form stabilized by a strong intramolecular $\mathrm{O-H\cdots O}$ hydrogen bond. Following excitation into the \ce{S2} ($\pi\pi^*$) state at 266~nm, acetylacetone undergoes rapid internal conversion followed by intersystem crossing. Such relaxation pathways are associated with structural changes including ring opening, deplanarization and bond elongation. In this work, ultrafast electron diffraction (UED) at the SLAC MeV-UED setup is employed as a direct structural probe with a time resolution of 160 fs. Together with trajectory surface hopping simulations, analysis of the UED data provides a new perspective on the early time nuclear dynamics in acetylacetone. Specifically, AcAc is observed to undergo ring opening, deplanarization and bond elongation all within the first 700~fs after photoexcitation. The monitored dynamics is associated mainly with the nuclear motion on the \ce{S1} potential energy surface, formed after very rapid transfer from \ce{S2} to \ce{S1}, allowing AcAc to reach the conical intersection to intersystem crossing. Such timescales of nuclear motion are contrasted with the timescales of electronic transitions in AcAc that were previously characterized with spectroscopic methods, specifically internal conversion ($<$100~fs) and intersystem crossing ($\approx$ 1.5~ps).