Monitoring ultrafast intramolecular proton transfer processes in an unsymmetric β-diketone

Abstract

We report the experimental determination of the intramolecular enol-enol tautomerization rate of an unsymmetric β-diketone, benzoylacetone, with femtosecond transient absorption in the ultraviolet. Initially, there is an equilibrium of two possible enolic structures in solution, which is disturbed upon UV excitation by exciting a disproportionate fraction of one enolic form. Comparison to symmetric β-diketones, acetylacetone and dibenzoylmethane, suggests that ground-state proton transfer gives rise to additional dynamics in benzoylacetone due to the dissimilarity of the two enolic forms. In the excited state of the molecules, the intramolecular H-bond is initially broken, followed by photochemical processes towards rotamer structures. Our studies therefore disclose intramolecular proton transfer among electronic ground as well as excited states of benzoylacetone. Considering the importance of β-diketones as a common model of enol-enol tautomerization and their resemblance to enzymatic enolates, the present study provides valuable information on the ultrafast mechanism of intramolecular proton transfer processes.

Broadband ultrafast photoprotection by oxybenzone across the UVB and UVC spectral regions

“…Given previous condensed phase observations, triplet state formation is one possible assignment, 12 although recent ab initio electronic structure calculations of likely triplet photoproducts coupled with transient vibrational absorption studies suggest such triplet states are negligible. Guided by recent ab initio calculations and experimental studies of similar systems, 8,[36][37][38]40 we suggest the photoproduct to be a trans keto tautomer formed from the extended rotation of the C-C bond of the vibrationally hot keto tautomer on the S 0 PES. Guided by recent ab initio calculations and experimental studies of similar systems, 8,[36][37][38]40 we suggest the photoproduct to be a trans keto tautomer formed from the extended rotation of the C-C bond of the vibrationally hot keto tautomer on the S 0 PES.…”

“…We note that ESHT may occur before IC or indeed concomitantly. 34,[36][37][38] Thus we suggest that, following IC and formation of vibrationally hot S 0 keto molecules, GSHT allows for keto → enol tautomerisation; the enol tautomer subsequently cools via VET with the surrounding solvent, which we therefore attribute to the lifetime, τ 2 . 4A) given that IC from higher electronic states will take longer to relax to the S 1 enol-OB tautomer before ESHT occurs.…”

Multiple-State Nonadiabatic Dynamics Simulation of Photoisomerization of Acetylacetone with the Direct ab Initio QTMF Approach

“…It is a difficult task to distinguish experimentally whether the rotational isomerization proceeds in the S 2 or S 1 state, which is probably the reason why a considerable difference in the S 2 time constants was reported in the previous experimental studies. 41,44,45 The S 0 , S 1 , and S 2 populations are plotted in Figure 6 Theory and Computation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Theory and Computation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 3...…”

“…29 In addition, they can be produced in the atmospheric oxidation of volatile organic compounds and their degradation can be initiated by photolysis. 30 As the simplest symmetric E-diketone, acetylacetone has been a subject of numerous theoretical [31][32][33][34][35][36][37][38] and experimental studies, 31,[39][40][41][42][43][44][45] which mainly focused on its structures in the ground state, the intramolecular proton transfer, and the Theory and Computation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 5 keto-enol tautomerism in the gas phase, 31-36,46-49 various solvents, 35 the isolated matrices, 31,49 and nano-confinement environments. 37 Although AcAc is a prototype of E-diketones, it exists mainly as the enolic form …”

“…[41][42][43][44][45] The second excited singlet state (S 2 ) of the AcAc enolic form is initially populated upon photoexcitation at ~265 nm, which has the 1 OEOE* character. Subsequently, there are multiple steps for the enolic form to deactivate from the S 2 state.…”