Ultrafast Intersystem Crossing in Acetylacetone via Femtosecond X-ray Transient Absorption at the Carbon K-Edge

Abstract

Molecular triplet states constitute a crucial gateway in the photochemical reactions of organic molecules by serving as a reservoir for the excess electronic energy. Here, we report the remarkable sensitivity of soft X-ray transient absorption spectroscopy for following the intricate electronic structure changes accompanying the non-adiabatic transition of an excited molecule from the singlet to the triplet manifold. Core-level X-ray spectroscopy at the carbon-1s K-edge (284 eV) is applied to identify the role of the triplet state (T, ππ*) in the ultraviolet-induced photochemistry of pentane-2,4-dione (acetylacetone, AcAc). The excited-state dynamics initiated at 266 nm (ππ*, S) is investigated with element- and site-specificity using broadband soft X-ray pulses produced by high harmonic generation, in combination with time-dependent density functional theory calculations of the X-ray spectra for the excited electronic singlet and triplet states. The evolution of the core-to-valence resonances at the carbon K-edge establishes an ultrafast population of the T state (ππ*) in AcAc via intersystem crossing on a 1.5 ± 0.2 ps time scale.

Spin adapted implementation of EOM-CCSD for triplet excited states: Probing intersystem crossings of acetylacetone at the carbon and oxygen K-edges

“…As an illustrative application, we compute the spectral signatures of the intersystem crossings in acetylacetone, for which recent experimental and theoretical (MOM-TD-DFT) results are available. 6 The photoionization dynamics of acetylacetone has also been the subject of a recent experimental and theoretical effort 26 using UV-pump/XUV-probe FEL at Elettra. 26 Finally, we predict the oxygen K-edge spectrum, for which experimental data are currently not available.…”

“…For instance, a UV-Vis pump and an X-ray probe may be used to investigate the dynamics of valence excited states. [3][4][5][6] These experimental advances imply a growing need for reliable and computationally efficient tools to simulate and interpret experimental spectra. 1,7 Over the last five years, a number of computational approaches have been developed to simulate transient state X-ray absorption spectra (TS-XAS) of singlet excited states.…”

“…1,7 Over the last five years, a number of computational approaches have been developed to simulate transient state X-ray absorption spectra (TS-XAS) of singlet excited states. 5,6,[8][9][10][11] Applications include the investigation of ultrafast internal conversion in organic molecules. Particularly relevant for the present work is the recent development of coupled cluster approaches to near-edge timeresolved X-ray absorption spectroscopy (TR-XAS).…”

Non-equilibrium x-ray spectroscopy using direct quantum dynamics

“…With the arrival of femtosecond X-ray sources such as X-ray free-electron lasers (XFELs) [1][2][3][4] and high harmonic generation (HHG), [5][6][7][8][9] it is becoming increasingly possible to use these techniques to probe timeresolved structural changes on the fundamental time-scales of molecular nuclear motion, i.e., femtoseconds. [9][10][11][12][13][14][15] The high brilliance of these X-ray sources is also making it increasingly possible to perform second-order techniques, such as resonant X-ray emission spectroscopy (RXES), also referred to as resonant inelastic X-ray scattering (RIXS). This can overcome the inherently low resolution of traditional X-ray absorption and emission spectroscopy, which is associated with the very short (<10 fs) lifetime of the core-hole state leading to large lifetime broadening.…”

Picosecond timescale tracking of pentacene triplet excitons with chemical sensitivity

“…However, the application of such powerful experimental techniques to complex architectures with more than one chromophore does not allow the disentanglement of the contribution from different excitation sites, due to the lack of chemical sensitivity. Recently, time resolved X-ray spectroscopies have been applied to the study of photoexcited states in molecules either in solution or in gas phase 18,19 . Such techniques offer chemical selectivity since they probe excitations of localized core electrons to unoccupied valence states.…”