Ultrafast X-ray Spectroscopy of Intersystem Crossing in Hexafluoroacetylacetone: Chromophore Photophysics and Spectral Changes in the Face of Electron Withdrawing Groups

Abstract

Intersystem crossings between singlet and triplet states represent a crucial relaxation pathway in photochemical processes. Herein, we probe the intersystem crossing in hexafluoro-acetylacetone with ultrafast X-ray transient absorption spectroscopy at the carbon K-edge. We observe the excited state dynamics following excitation with 266 nm UV light to the ${}^1\pi {\pi }^{\ast }$ (S${}_2$) state with element and site-specificity using a broadband soft X-ray pulse produced by high harmonic generation. These results are compared to X-ray spectra computed from orbital optimized density functional theory methods. It is found that the electron withdrawing fluorine atoms decongest the X-ray absorption spectrum by enhancing separation between features originating from different carbon atoms. This facilitates the elucidation of structural and electronic dynamics at the chromophore. The evolution of the core-to-valence resonances at the carbon K-edge reveals an ultrafast population transfer between the ${}^{1}n{\pi }^{\ast }$ (S${}_1$) and ${}^3\pi {\pi }^{\ast }$ (T${}_1$) states on a $1.6\pm 0.4$ ps timescale, which is similar to the 1.5 ps timescale earlier observed for acetylacetone [J. Am. Chem. Soc. 139, 16576 (2017)]. It therefore appears that terminal fluorination has little influence on the intersystem crossing rate of the acetylacetone chromophore. In addition, the significant role of hydrogen-bond opened and twisted rotational isomers is elucidated in the excited state dynamics by comparison of the experimental transient X-ray spectra with theory.

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Cited by in corpus (2)

• Generalization of one-center non orthogonal configuration interaction singles to open shell singlet reference states: Theory and application to valence-core pump-probe states in acetylacetone
• Measurement of Coherent Vibrational Dynamics with X-ray Transient Absorption Spectroscopy Simultaneously at the Carbon K- and Chlorine L${}_{2,3}$- Edges