  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 $^1\pi\pi^{*}$ (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
$^1n\pi^{*}$ (S$_1$) and $^3\pi\pi^{*}$ (T$_1$) states on a $1.6\pm0.4$ 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.
