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.

单重态与三重态之间的系间窜越（intersystem crossing）是光化学过程中至关重要的弛豫途径。本文利用基于高次谐波产生（high harmonic generation）的宽带软X射线脉冲，在碳K边（carbon K-edge）采用超快X射线瞬态吸收光谱，对六氟乙酰丙酮中的系间窜越过程开展了探究。我们通过266 nm紫外光将样品激发至1ππ*（S2）态，并借助该技术的元素与位点特异性，观测了激发态动力学过程。将实验结果与基于轨道优化密度泛函理论（orbital optimized density functional theory）计算得到的X射线光谱进行对比后发现：吸电子氟原子通过拉大不同碳原子对应的光谱特征之间的间距，简化了X射线吸收光谱，这有助于解析生色团（chromophore）的结构与电子动力学过程。碳K边处的芯态到价态共振演化揭示了1nπ*（S1）态与3ππ*（T1）态之间在1.6 ± 0.4皮秒的时间尺度上发生了超快布居转移，这与此前在乙酰丙酮中观测到的1.5皮秒时间尺度结果[J. Am. Chem. Soc. 2017, 139, 16576−16583, DOI: 10.1021/jacs.7b07532]高度相似。由此可见，末端氟化对乙酰丙酮生色团的系间窜越速率几乎无影响。此外，通过将实验瞬态X射线光谱与理论计算结果进行对比，我们阐明了氢键打开与扭曲的旋转异构体在激发态动力学中的重要作用。