New insights into the photodynamics of acetylacetone: isomerization and fragmentation in low-temperature matrixes

Abstract

UV and IR photoreactivities of acetylacetone isolated at 4.3 K in four matrixes (N₂, Ne, Ar, Xe), pure and doped with O₂ are investigated, using either tunable UV and IR optical parametric oscillators, or a broad band mercury lamp. Samples are probed by UV and FTIR spectroscopies: electronic and vibrational transitions are assigned and irradiation kinetics are analyzed. Contrary to what is observed in the gas phase, stereoisomerization is the main reaction observed: UV irradiation breaks the strong H-bond of the stable enolic form of acetylacetone, leading to the observation of non-chelated forms. Isomerization among the different non-chelated forms as well as back-isomerization to the chelated form are also observed under UV irradiation. Similar reactions and reaction rates are observed for the four matrixes, indicating that the inter-system crossing to the T₁ state involved in the isomerization process is very fast, probably due to efficient coupling with phonons, in contrast with gas phase where inter-system crossing is rate-limiting. When matrixes are doped with O₂, dissociation of the non-chelated forms under UV irradiation is observed and fragments, in particular CO, are formed in large amounts. Dissociation through a Norrish type-I reaction is probably one of the reaction channels occurring during electronic relaxation: dissociation is hindered by the surrounding cage in the case of pure matrixes while fragments immediately react with O₂ in the case of doped matrixes. The differences between gas phase and cold solid medium photodynamics of acetylacetone are discussed.