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 (N(2), Ne, Ar, Xe), pure and doped with O(2) 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 T(1) 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 O(2), 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 O(2) in the case of doped matrixes. The differences between gas phase and cold solid medium photodynamics of acetylacetone are discussed.

Acetylacetone in hydrogen solids: IR signatures of the enol and keto tautomers and UV induced tautomerization

“…Table 1 reports the observed frequencies in hydrogen and Ne matrices, compared with calculations. Our results also compare well with IR spectra in classical Ar, Xe and N 2 matrices, which all show a similar signature for the chelated enol form [6,16]. Bandwidths are found to be very different from one mode to another in all matrices, with a similar behaviour in 'quantum' and 'classical' solids.…”

“…In all matrices, the vibrational bands of CH 2 stretching modes (2900-3020 cm À1 ) are indeed weak and that of OH stretch is not visible. The spectra in hydrogen matrices (Figure 2) look quite similar to those in traditional cryogenic matrices [5,6]. We also performed experiments in Ne in order to compare directly with previous experimental results in matrices.…”

“…conformers without internal H-bond) [5]. The photodynamics of the processes involved was detailed in a recent publication [6]. On the other hand, Nagashima et al observed an increase of the keto form upon UV irradiation in Ar matrices doped with AcAc [7].…”

“…The temperature dependence of the keto/enol equilibrium was also investigated in the gas phase [4]. Studies in cryogenic matrices showed a large amount of enol form as in the gas phase at room temperature [5][6][7].…”

“…IR and UV spectra of both tautomers were measured in different environments such as the vapor phase, neat liquid, organic solvents or rare gas matrices [1,[4][5][6][7][8][9]. An electronic excitation of AcAc by UV irradiation can be expected to induce tautomerization, and cryogenic matrices are ideal environments to monitor such a process.…”