The "Franck-Condon" (FC) excited state is the first state created when a molecule absorbs a visible photon. Here we report Stark and visible absorption spectroscopies that interrogate the FC state of rigorously diamagnetic [M(bpy)3]2+ complexes, where bpy is 2,2'-bipyridine and M = Fe, Ru, and Os. Direct singlet-to-triplet metal-to-ligand charge transfer (MLCT) transitions are evident in the 550-750 nm region of the absorbance spectrum of [Os(bpy)3]2+, yet are poorly resolved or absent for [Ru(bpy)3]2+ and [Fe(bpy)3]2+. In the presence of a strong 0.4-0.8 MV/cm electric field, well-resolved transitions are observed for all the complexes in this same spectral region. In particular, an electroabsorption feature at 633 nm (15,800 cm-1) provides compelling evidence for the direct population of a high spin [Fe(bpy)3]2+* MLCT excited state. Group theoretical considerations and Liptay analysis of the Stark spectra revealed dramatic light-induced dipole moment changes that ranged from Δμ ⃑ = 3-9 D with the triplet transitions consistently showing shorter charge transfer distances. The finding that the spin of the initially populated FC excited state differs from that of the ground state, even with a relatively light first row transition metal, is relevant to emerging applications in energy up-conversion, dye-sensitization, spintronics, photo-redox catalysis, and organic light emitting diodes (OLEDs).

中文翻译：

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过渡金属多吡啶基配合物中自旋变化伴随光吸收的明显光谱证据

“弗兰克-康登”(FC) 激发态是分子吸收可见光子时产生的第一个状态。在这里，我们报告了 Stark 和可见光吸收光谱，它们询问了严格抗磁性 [M(bpy)3]2+ 复合物的 FC 状态，其中 bpy 是 2,2'-联吡啶，M = Fe、Ru 和 Os。在 [Os(bpy)3]2+ 的吸收光谱的 550-750 nm 区域，直接单线态到三线态金属-配体电荷转移 (MLCT) 跃迁很明显，但 [Ru (bpy)3]2+ 和 [Fe(bpy)3]2+。在存在 0.4-0.8 MV/cm 强电场的情况下，在同一光谱区域内的所有配合物都观察到了很好分辨的跃迁。特别是，633 nm (15,800 cm-1) 处的电吸收特征为高自旋 [Fe(bpy)3]2+* MLCT 激发态的直接群体提供了令人信服的证据。Stark 光谱的组理论考虑和 Liptay 分析揭示了剧烈的光诱导偶极矩变化，范围从 Δμ⃑ = 3-9 D，三重态跃迁始终显示较短的电荷转移距离。即使第一排过渡金属相对较轻，初始填充的 FC 激发态的自旋与基态的自旋不同，这一发现与能量上转换、染料敏化、自旋电子学、光氧化还原等新兴应用有关催化和有机发光二极管 (OLED)。