Tracking Excited State Charge and Spin Dynamics in Iron Coordination Complexes

Solar energy applications could become vastly more wide spread, if it was possible to make them work without rare or expensive materials, such as ruthenium, and use cheap and abundant materials instead, for example iron.
Martin Meedom kontaktboksW. Zhang1, R. Alonso-Mori2, U. Bergmann2, C. Bressler3, M. Chollet2, A. Galler3, W. Gawelda3, R. G. Hadt4, R. W. Hartsock1,4, T. Kroll4, K. S. Kjær5,7, K. Kubiček6, H. T. Lemke2, H. W. Liang1,4, D. A. Meyer1,4, M. M. Nielsen7, C. Purser1, J. S. Robinson2, E. I. Solomon4,8, Z. Sun1, D. Sokaras8, T. B. van Driel7, G. Vankó9, T.-C. Weng8, D. Zhu2, K. J. Gaffney1* 

Nature: Published online 7 May 2014

A major obstacle for using earth abundant metals is that the electrons excited from the metal centers by sunlight return to the ground state much faster in iron than they do in ruthenium. This happens so fast, that solar energy devices are unable to use the excited electrons to generate an electrical current.  

With the help of an X-ray laser we were able to observe changes in excited, very short-lived transition states, which other methods miss. Such intermediate states are decisive for the subsequent course—and final outcome—of chemical reactions. Here we for the first time have been able to follow the photo-induced transformation of a molecular magnetic moment with mechanistic detail.  This is an important step towards atomic- and electronic-scale movies of chemical reactions, and provides a fundamental understanding that may enhance our ability to use earth abundant metals in solar energy applications.

Martin Meedom - Nature 070514

Læs også: 'Forskerne optager molekylær film af elektroners hurtige spring' Ingeniøren 7 May 2014

Understanding how molecular properties dictate the non-equilibrium dynamics of electronic excited states represents a critical step towards a better utilization of light triggered molecular phenomena. The multitude of excited states in transition metal complexes with distinct charge and spin distributions present unique opportunities and challenges for light driven devices.

Poly-pyridal iron complexes, such as [Fe(2,2’-bipyridine)3]2+, provide archetypical coordination complexes where the excited state charge and spin dynamics involved in spin crossover have long been a source of interest and controversy. Robust characterization of these dynamics has been impeded by the complexity of the phenomena, the indirect sensitivity of optical spectroscopy to spin dynamics, and the flux limitations of ultrafast x-ray sources. We present a mechanistic study of the spin crossover dynamics of [Fe(2,2’-bipyridine)3]2+ induced by metal to ligand charge transfer (MLCT) excitation. Our study utilizes femtosecond resolution x-ray fluorescence to track the charge and spin dynamics of photo-excited [Fe(2,2’-bipyridine)3]2+ and demonstrates the critical role of intermediate spin states in the spin crossover mechanism. 

Animation of the electronic configuration of [Fe(2,2’-bipyridine)3]2+ as a function of time as it is excited to the MLCT state and sequentially transitions into the Triplet and Quintet states. By György Vankó

The ability to resolve intermediate spin states and characterize electronic excited state dynamics with mechanistic detail should be applicable to a wide range of transition metal complexes and highlights the power of ultrafast x-ray lasers for the study of chemical reactivity. 

Affiliations: 
1PULSE Institute, SLAC National Accelerator Laboratory, Stanford University, Stanford, California 94305, United States. 
2LCLS, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States. 
3European XFEL, D-22761 Hamburg, Germany 
4Department of Chemistry, Stanford University, Stanford, California 94305, United States. 
5Centre for Molecular Movies, Niels Bohr Institute, University of Copenhagen, DK-2100, Copenhagen, Denmark 
6Max Planck Institute for Biophysical Chemistry, 37077, Göttingen, Germany. 
7Centre for Molecular Movies, Department of Physics, Technical University of Denmark, DK-2800, Lyngby, Denmark. 
8SSRL, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States. 
9Hungarian Academy of Science, Wigner Research Centre for Physics, H-1525 Budapest, Hungary