Department of Chemistry, Michigan State University 578 South Shaw Lane, East Lansing, Michigan 48824, United States.
J Am Chem Soc. 2021 Jun 2;143(21):8086-8098. doi: 10.1021/jacs.1c02451. Epub 2021 May 20.
The development of chromophores based on earth-abundant transition metals whose photophysical properties are dominated by their charge-transfer excited states has inspired considerable research over the past decade. One challenge associated with this effort is satisfying the dual requirements of a strong ligand field and chemical tunability of the compound's absorptive cross-section. Herein we explore one possible approach using a heteroleptic compositional motif that combines both of these attributes into a single compound. With the parent complex [Fe(phen)] (; where phen is 1,10-phenanthroline) as the starting material, replacement of one of the phen ligands for two cyanides to obtain Fe(phen)(CN) () allows for conversion to [Fe(phen)(CHN)] (), a six-coordinate Fe(II) complex whose coordination sphere consists of two chelating polypyridyl ligands and one bidentate carbene-based donor. Ground-state absorption spectra of all three compounds exhibit A → MLCT transition(s) associated with the phen ligands that are relatively insensitive to the identity of the third counterligand(s). Ultrafast time-resolved electronic absorption measurements revealed lifetimes for the MLCT excited states of compounds and of 180 ± 30 and 250 ± 90 fs, respectively, values that are typical for iron(II)-based polypyridyl complexes. The corresponding kinetics for compound were substantially slower at 7.4 ± 0.9 ps; the spectral evolution associated with these dynamics confirms that these kinetics are tracking the MLCT excited state and, more importantly, are coupled to ground-state recovery from this excited state. The data are interpreted in terms of a modulation of the relative energies of the MLCT and ligand-field states across the series, leading to a systematic destabilization of metal-localized ligand-field excited states such that the low-energy portions of the charge-transfer and ligand-field manifolds are at or near an energetic inversion point in compound . We believe these results illustrate the potential for a modular, orthogonal approach to chromophore design in which part of the coordination sphere can be targeted for light absorption while another can be used to tune electronic-state energetics.
基于丰富的过渡金属开发的发色团，其光物理性质主要由其电荷转移激发态决定，在过去十年中引起了相当多的研究。这项工作的一个挑战是满足强配体场和化合物吸收截面化学可调性的双重要求。在此，我们探索了一种可能的方法，使用一种杂配位组成基序将这两个属性结合到单个化合物中。以母体配合物 [Fe(phen)] (; 其中 phen 是 1,10-菲咯啉) 为起始原料，用两个氰化物取代一个 phen 配体得到 Fe(phen)(CN) (), 可以转化为 [Fe(phen)(CHN)] (), 这是一个六配位的 Fe(II) 配合物，其配位球由两个螯合多吡啶配体和一个双齿卡宾基供体组成。所有三种化合物的基态吸收光谱都表现出与 phen 配体相关的 A → MLCT 跃迁(s)，这些跃迁对第三抗衡配体的身份相对不敏感。超快时间分辨电子吸收测量显示，化合物 和 的 MLCT 激发态的寿命分别为 180 ± 30 和 250 ± 90 fs，这是铁(II)基多吡啶配合物的典型寿命。化合物 的相应动力学速度要慢得多，为 7.4 ± 0.9 ps；这些动力学的光谱演化证实，这些动力学跟踪 MLCT 激发态，更重要的是，与从该激发态恢复到基态有关。这些数据可以根据 MLCT 和配体场态之间的相对能量的调制来解释，这导致金属局域配体场激发态的系统失稳，使得电荷转移和配体场谱的低能部分在化合物 中处于或接近能量反转点。我们认为这些结果说明了一种模块化、正交的发色团设计方法的潜力，其中配位球的一部分可以针对光吸收，而另一部分可以用于调整电子态的能量。
