Britz Alexander, Gawelda Wojciech, Assefa Tadesse A, Jamula Lindsey L, Yarranton Jonathan T, Galler Andreas, Khakhulin Dmitry, Diez Michael, Harder Manuel, Doumy Gilles, March Anne Marie, Bajnóczi Éva, Németh Zoltán, Pápai Mátyás, Rozsályi Emese, Sárosiné Szemes Dorottya, Cho Hana, Mukherjee Sriparna, Liu Chang, Kim Tae Kyu, Schoenlein Robert W, Southworth Stephen H, Young Linda, Jakubikova Elena, Huse Nils, Vankó György, Bressler Christian, McCusker James K
European XFEL , Holzkoppel 4 , 22869 Schenefeld , Germany.
The Hamburg Centre for Ultrafast Imaging , Luruper Chaussee 149 , 22761 Hamburg , Germany.
Inorg Chem. 2019 Jul 15;58(14):9341-9350. doi: 10.1021/acs.inorgchem.9b01063. Epub 2019 Jun 26.
We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)] (where dcpp is 2,6-(dicarboxypyridyl)pyridine). This compound exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temperature fluid solution, raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the T/T crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the T state as the lowest energy excited state of this compound. Geometric changes associated with the photoinduced ligand-field state conversion have also been monitored with extended X-ray absorption fine structure. The data show the typical average Fe-ligand bond length elongation of ∼0.18 Å for a T state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited T state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin A ground state structure. This study illustrates how the application of time-resolved X-ray techniques can provide insights into the electronic structures of molecules-in particular, transition metal complexes-that are difficult if not impossible to obtain by other means.
我们采用了一系列超快X射线光谱技术，旨在表征[Fe(dcpp)]（其中dcpp为2,6-(二羧基吡啶基)吡啶）的最低能量激发态。对于一种低自旋Fe(II)多吡啶配合物，该化合物在室温流体溶液中的激发态寿命异常短，仅为270皮秒，这引发了一个问题，即dcpp的配体场强度是否已将该体系推过T/T交叉点，并使后者稳定为最低能量激发态。Kα和Kβ X射线发射光谱已被用于明确确定长寿命激发态的五重态自旋多重性，从而确定T态为该化合物的最低能量激发态。与光诱导配体场态转换相关的几何变化也通过扩展X射线吸收精细结构进行了监测。数据显示，对于T态，典型的平均Fe-配体键长伸长约0.18 Å，并表明激发态T中金属中心周围初级配位球具有高各向异性，这与表征低自旋A基态结构的近乎完美的八面体对称性形成鲜明对比。这项研究说明了时间分辨X射线技术的应用如何能够深入了解分子——特别是过渡金属配合物——的电子结构，而这些结构用其他方法即使不是不可能，也很难获得。
