Ultrafast UV-Induced Photoisomerization of Intramolecularly H-Bonded Symmetric β-Diketones

Abstract

In photoinduced molecular reaction dynamics, the effects of electronic charge redistribution can lead to multiple pathways that are determined by the nature of the initial structures involved and the environment the molecule of interest is studied in. The β-diketones are a common example of this complexity. They show keto−enol tautomerism that is almost totally shifted toward the enolic form. However, compared to the gas phase, the photochemistry proceeds completely differently by virtue of the solvent environment for these compounds, which are used in commercial sunscreen agents due to a high absorption in the ultraviolet (UV) and fast deactivation processes. We disclose these dynamics by investigating three symmetrical β-diketones in various solvents. To observe these effects on an ultrafast time scale directly in the UV spectral region where the relevant electronic transitions take place, we have developed and employed femtosecond transient absorption with detection capability in the deep UV. Our studies confirm that electronic excitation of the chelated enol form does not lead to any ultrafast photochemistry other than proton transfer followed by rotamerization. The formation of the nonchelated conformers takes place on a picosecond time scale through a dark state, whereas the recovery to the stable chelated enol form is a comparably slow process. ■ INTRODUCTIONDerivatives of β-diketones are of great importance in diverse research fields by virtue of several remarkable chemical features that lead to a variety of applications. 1 For example, they are widely employed as chelating agents due to their binding affinity for transition metals, 1 or they are even contained in commercial sunscreen products 2−4 owing to the fast deactivation processes after ultraviolet (UV) irradiation. The most prominent property of β-diketones is their keto−enol tautomerism that is shifted almost totally toward the enolic form. 5−7 With structural similarities to relevant biomolecules and photochromic substances on the one hand, and the molecules' own versatility in combination with their structural simplicity on the other hand, small β-diketones are prototypical candidates for a systematic study of the photoinduced processes and the subsequent deactivation channels upon which the wide applicability of β-diketones is based.C o m p o u n d s l i k e β -d i k e t o n e s o f t y p e R−C(O)−CH 2 −C(O)−R, where R = H, CH 3 , have drawn continuous attention from chemists and physicists of different research areas because of the pronounced keto−enol tautomerism that is observable (and exploitable) in the gas 5,8 and the liquid phase 6,9 and even in isolated cryogenic matrices. 7,10 The reason for the stabilization of the enolic form is an intramolecular H-bond, coupled with a π-electronic delocalization over the O−C−C−C−O pseudocycle (see Scheme 1). The two simplest and smallest structures exhibiting the central six membered ring closed by the intramolecular Hbond in the chelated enol (CE) form are the compounds malonaldehyde (M...

Excited-state intramolecular proton transfer of 2-acetylindan-1,3-dione studied by ultrafast absorption and fluorescence spectroscopy

“…A detailed description of the utilized TA setup can be found elsewhere 27 . Briefly, a fraction of the fundamental beam from a commercial Ti:sapphire regenerative-amplifier laser system (Spitfire Pro, Spectra-Physics, center wavelength λ c  = 800 nm, pulse duration Δτ = 120 fs, repetition rate ν rep  = 1 kHz) was used to seed a commercial non-collinear optical parametric amplifier (TOPAS White, Light Conversion) which was frequency-doubled to generate the UV pump beam.…”

“…To directly observe ESIPT in the electronically excited state, time-resolved measurements with femto- to picosecond time resolution are required. The real-time dynamics of such a proton transfer in various ESIPT molecular systems have been explored in the gas phase, 6–10 techniques applied to ESIPT in solution comprise time-resolved fluorescence based on streak cameras, 11 time-correlated single-photon counting, 12–14 and upconversion, 12,15–17 as well as transient absorption (TA) with pump/probe combinations being UV/mid-IR, 2,16,18–20 UV/VIS, 12–14,16,19–29 and recently UV/UV 27,29 . Most of these ultrafast measurements have shown that ESIPT typically occurs on a subpicosecond time scale, with a key role for the low-frequency Raman-active modes modulating the H-bond distance between the proton donor and the acceptor atom 11,25,30–38 …”

Fate and implication of acetylacetone in photochemical processes for water treatment

“…b-diketones and their derivatives are of great importance in many fields by virtue of a number of interesting and specific properties originating from their dicarbonyl structures (UrbaniakDomagala, 2011;Verma et al, 2014). Acetylacetone (abbreviated as AA) is one of the simplest b-diketones.…”

“…With structural similarities to relevant biomolecules and photochromic substances, in combination with its structural simplicity, AA is a prototypical candidate for a systematic study of the photoinduced processes (Verma et al, 2014). The basic photophysical and photochemical processes of AA have been intensively investigated in gas phase (Zhou et al, 2008;Mayo and Takeshita, 1963;Yoon et al, 1999;Harrison and Lossing, 1959) and in isolated cryogenic matrices (Trivella et al, 2010).…”

Ultraviolet Absorption Spectrum of Malonaldehyde in Water Is Dominated by Solvent-Stabilized Conformations

“…Each cluster contains the four water molecules closest to the two oxygen atoms to explicitly include hydrogen bonding and first-solvation-shell-specific complexation; each cluster is surrounded by SMD continuum solvent. Figure 2 shows that the simulated ππ* absorption maximum of the CE conformer in water is slightly red-shifted to 262 nm (4.73 eV) with respect to that in the gas phase, in contrast to the distinct blue-shift observed 9 9 This raises the question: is the discrepancy due to inadequacies of the theory or have the experiments on this prototype molecule been misinterpreted? The conclusion of this letter will be that the latter answer is the correct one.…”