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

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 1ππ* (S2) 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 1nπ* (S1) and 3ππ* (T1) states on a 1.6 ± 0.4 ps time scale, which
is similar to the 1.5 ps time scale earlier observed for acetylacetone
[J. Am. Chem. Soc.2017, 139, 16576−16583, DOI: 10.1021/jacs.7b07532]. 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.