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...

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.

We present a general approach for calculating the magic-angle condition for time-resolved spectroscopy of isotropic samples. Allowing for arbitrary static polarizations and propagation directions of each pulse enables us to retrieve not only the known magic-angle conditions for linear and circular polarization, but also analogous conditions for anisotropy-free spectroscopy using elliptically polarized laser pulses. The results are exemplified on transient absorption and transferred to coherent two-dimensional spectroscopy. Furthermore we derive for transient absorption spectroscopy the relation between the measurable anisotropy and the molecular structure, i.e., the angle between the pumped and probed transition dipole moments (TDMs). The impact of multiple spectrally overlapping signals is considered and discussed on the example of a molecule with a two-fold degenerate TDM.

We present polarimetry, i.e. the detection of optical rotation of light polarization, in a configuration suitable for femtosecond spectroscopy. The polarimeter is based on common-path optical heterodyne interferometry and provides fast and highly sensitive detection of rotatory power. Femtosecond pump and polarimeter probe beams are integrated into a recently developed accumulative technique that further enhances sensitivity with respect to single-pulse methods. The high speed of the polarimeter affords optical rotation detection during the pump-pulse illumination period of a few seconds. We illustrate the concept on the photodissociation of the enantiomers of methyl p-tolyl sulfoxide. The sensitivity of rotatory detection, i.e. the minimum rotation angle that can be measured, is determined experimentally including all noise sources to be 0.10 milli-degrees for a measurement time of only one second and an interaction length of 250 µm. The suitability of the presented setup for femtosecond studies is demonstrated in a non-resonant two-photon photodissociation experiment. 3682-3692 (1991). 9. S. Milder, S. Bjorling, I. Kuntz, and D. Kliger, "Time-resolved circular dichroism and absorption studies of the photolysis reaction of (carbonmonoxy)myoglobin," Biophys. J. 53, 659-664 (1988