Zhang, Wenkai3; Alonso-Mori, Roberto4; Bergmann, Uwe4; Bressler, Christian5; Chollet, Matthieu4; Galler, Andreas5; Gawelda, Wojciech5; Hadt, Ryan G.3; Hartsock, Robert W.3; Kroll, Thomas3; Kjær, Kasper Skov1; Kubicek, Katharina6; Lemke, Henrik Till7; Liang, Huiyang W.3; Meyer, Drew A.3; Nielsen, Martin Meedom1; Purser, Carola3; Robinson, Joseph S.4; Solomon, Edward I.4; Sun, Zheng3; Sokaras, Dimosthenis4; Brandt van Driel, Tim7; Vanko, Gyoergy9; Weng, Tsu-Chien4; Zhu, Diling4; Gaffney, Kelly J.3
1 Department of Physics, Technical University of Denmark2 Neutrons and X-rays for Materials Physics, Department of Physics, Technical University of Denmark3 Stanford University4 SLAC National Accelerator Laboratory5 European XFEL6 Deutsches Elektronen-Synchrotron7 Risø National Laboratory for Sustainable Energy, Technical University of Denmark8 Hungarian Academy of Sciences9 Hungarian Academy of Sciences
Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons(1-4). But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics(5) and the flux limitations of ultrafast X-ray sources(6,7). Such a situation exists for archetypal poly-pyridyl iron complexes, such as [Fe(2,2'-bipyridine)(3)](2+), where the excited-state charge and spin dynamics involved in the transition from a low-to a high-spin state (spin crossover) have long been a source of interest and controversy(6-15). Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2'-bipyridine)(3)](2+) on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.