Leshchev Denis, Harlang Tobias C B, Fredin Lisa A, Khakhulin Dmitry, Liu Yizhu, Biasin Elisa, Laursen Mads G, Newby Gemma E, Haldrup Kristoffer, Nielsen Martin M, Wärnmark Kenneth, Sundström Villy, Persson Petter, Kjær Kasper S, Wulff Michael
European Synchrotron Radiation Facility , 71 Avenue des Martyrs , 38000 Grenoble , France . Email:
Department of Chemical Physics , Lund University , P. O. Box 12 4 , 22100 Lund , Sweden.
Chem Sci. 2017 Oct 31;9(2):405-414. doi: 10.1039/c7sc02815f. eCollection 2018 Jan 14.
Recent years have seen the development of new iron-centered N-heterocyclic carbene (NHC) complexes for solar energy applications. Compared to typical ligand systems, the NHC ligands provide Fe complexes with longer-lived metal-to-ligand charge transfer (MLCT) states. This increased lifetime is ascribed to strong ligand field splitting provided by the NHC ligands that raises the energy levels of the metal centered (MC) states and therefore reduces the deactivation efficiency of MLCT states. Among currently known NHC systems, [Fe(btbip)] (btbip = 2,6-bis(3--butyl-imidazol-1-ylidene)pyridine) is a unique complex as it exhibits a short-lived MC state with a lifetime on the scale of a few hundreds of picoseconds. Hence, this complex allows for a detailed investigation, using 100 ps X-ray pulses from a synchrotron, of strong ligand field effects on the intermediate MC state in an NHC complex. Here, we use time-resolved wide angle X-ray scattering (TRWAXS) aided by density functional theory (DFT) to investigate the molecular structure, energetics and lifetime of the high-energy MC state in the Fe-NHC complex [Fe(btbip)] after excitation to the MLCT manifold. We identify it as a 260 ps metal-centered quintet (MC) state, and we refine the molecular structure of the excited-state complex verifying the DFT results. Using information about the hydrodynamic state of the solvent, we also determine, for the first time, the energy of the MC state as 0.75 ± 0.15 eV. Our results demonstrate that due to the increased ligand field strength caused by NHC ligands, upon transition from the ground state to the MC state, the metal to ligand bonds extend by unusually large values: by 0.29 Å in the axial and 0.21 Å in the equatorial direction. These results imply that the transition in the photochemical properties from typical Fe complexes to novel NHC compounds is manifested not only in the destabilization of the MC states, but also in structural distortion of these states.
近年来，用于太阳能应用的新型铁中心氮杂环卡宾（NHC）配合物得到了发展。与典型的配体体系相比，NHC配体为铁配合物提供了寿命更长的金属到配体电荷转移（MLCT）态。这种寿命的增加归因于NHC配体提供的强配体场分裂，它提高了以金属为中心（MC）态的能级，从而降低了MLCT态的失活效率。在目前已知的NHC体系中，[Fe(btbip)]（btbip = 2,6-双(3-丁基-咪唑-1-亚基)吡啶）是一种独特的配合物，因为它表现出寿命在几百皮秒量级的短寿命MC态。因此，这种配合物使得利用来自同步加速器的100皮秒X射线脉冲，对NHC配合物中中间MC态的强配体场效应进行详细研究成为可能。在这里，我们使用时间分辨广角X射线散射（TRWAXS）并借助密度泛函理论（DFT），来研究Fe-NHC配合物[Fe(btbip)]在激发到MLCT多重态后高能MC态的分子结构、能量和寿命。我们将其确定为一个260皮秒的以金属为中心的五重态（MC），并完善了激发态配合物的分子结构，验证了DFT结果。利用关于溶剂流体动力学状态的信息，我们还首次确定了MC态的能量为0.75±0.15电子伏特。我们的结果表明，由于NHC配体导致的配体场强度增加，从基态跃迁到MC态时，金属与配体的键在轴向异常大幅延伸0.29埃，在赤道方向延伸0.21埃。这些结果意味着，从典型的铁配合物到新型NHC化合物，光化学性质的转变不仅体现在MC态的不稳定上，还体现在这些态的结构畸变上。
