Ecole Polytechnique Fédérale de Lausanne, Laboratoire de Spectroscopie Ultrarapide, Faculté des Sciences de Base, ISIC, Lausanne CH-1015, Switzerland.
Nat Chem. 2015 Aug;7(8):629-33. doi: 10.1038/nchem.2305. Epub 2015 Jul 20.
It is known that excitation by visible light of the singlet metal-to-ligand charge-transfer ((1)MLCT) states of Fe(II) complexes leads to population of the lowest-lying high-spin quintet state ((5)T) with unity quantum yield. Here we investigate this so-called spin crossover (SCO) transition in aqueous iron(II)tris(bipyridine). We use pump-probe transient absorption spectroscopy with a high time resolution of <60 fs in the ultraviolet probe range, in which the (5)T state absorbs, and of <40 fs in the visible probe range, in which both the hot MLCT state and the (5)T state absorb. Our results show that the (5)T state is impulsively populated in less than 50 fs, which is the time we measured for the depopulation of the MCLT manifold. We propose that non-totally-symmetric modes mediate the process, possibly high-frequency modes of the bipyridine (bpy) ligand. These results show that even though the SCO process in Fe(II) complexes represents a strongly spin-forbidden (ΔS = 2) two-electron transition, spin flipping occurs at near subvibrational times and is intertwined with the electron and structural dynamics of the system.
已知，可见光激发 Fe(II) 配合物的单重态金属-配体电荷转移态 ((1)MLCT) 会导致最低的高自旋五重态 ((5)T) 以 unity quantum yield 占据。在此，我们研究了水溶液中 Fe(II) 三(联吡啶)的这种所谓的自旋交叉 (SCO) 跃迁。我们使用具有 <60 fs 的紫外探测范围内的高时间分辨率 (<60 fs) 和 <40 fs 的可见探测范围内的泵浦-探测瞬态吸收光谱，其中 (5)T 态吸收，而 hot MLCT 态和 (5)T 态都吸收。我们的结果表明，(5)T 态在不到 50 fs 的时间内被瞬间占据，这是我们测量 MLCT 态简并态去占据的时间。我们提出非完全对称模式介导了这个过程，可能是联吡啶 (bpy) 配体的高频模式。这些结果表明，尽管 Fe(II) 配合物中的 SCO 过程代表强烈的自旋禁阻 (ΔS = 2) 两电子跃迁，但自旋翻转发生在近亚振动时间内，与体系的电子和结构动力学交织在一起。
