Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution

作者：Zhang Wenkai; Kjaer Kasper S^*; Alonso Mori Roberto; Bergmann Uwe; Chollet Matthieu; Fredin Lisa A; Hadt Ryan G; Hartsock Robert W; Harlang Tobias; Kroll Thomas; Kubicek Katharina; Lemke Henrik T; Liang Huiyang W; Liu Yizhu; Nielsen Martin M; Persson Petter; Robinson Joseph S; Solomon Edward I; Sun Zheng; Sokaras Dimosthenis; van Driel Tim B; Weng Tsu Chien; Zhu Diling; Warnmark Kenneth; Sundstromb Villy; Gaffney Kelly J^*

来源：Chemical Science, 2017, 8(1): 515-523.

DOI：10.1039/c6sc03070j

Developing light-harvesting and photocatalytic molecules made with iron could provide a cost effective, scalable, and environmentally benign path for solar energy conversion. To date these developments have been limited by the sub-picosecond metal-to-ligand charge transfer (MLCT) electronic excited state lifetime of iron based complexes due to spin crossover - the extremely fast intersystem crossing and internal conversion to high spin metal-centered excited states. We revitalize a 30 year old synthetic strategy for extending the MLCT excited state lifetimes of iron complexes by making mixed ligand iron complexes with four cyanide (CN-) ligands and one 2,2'-bipyridine (bpy) ligand. This enables MLCT excited state and metal-centered excited state energies to be manipulated with partial independence and provides a path to suppressing spin crossover. We have combined X-ray Free-Electron Laser (XFEL) K beta hard X-ray fluorescence spectroscopy with femtosecond time-resolved UV-visible absorption spectroscopy to characterize the electronic excited state dynamics initiated by MLCT excitation of [Fe(CN)(4)(bpy)](2-). The two experimental techniques are highly complementary; the time-resolved UV-visible measurement probes allowed electronic transitions between valence states making it sensitive to ligand-centered electronic states such as MLCT states, whereas the Kb fluorescence spectroscopy provides a sensitive measure of changes in the Fe spin state characteristic of metal-centered excited states. We conclude that the MLCT excited state of [Fe(CN)(4)(bpy)](2-) decays with roughly a 20 ps lifetime without undergoing spin crossover, exceeding the MLCT excited state lifetime of [Fe(2,2'-bipyridine)(3)](2+) by more than two orders of magnitude.

• 出版日期2017