XFEL: New avenues for solution-phase chemistry
New avenues for solution-phase chemistry
Molecular complexes containing a transition metal ion, such as Iron, can be excited with visible light. Photoabsorption causes the electronic and magnetic properties of the entire molecular complex to undergo profound alterations. These changes can all be traced back to an ultrafast spin-flip at the metal ion center. These so-called spin-switching complexes are already used successfully in the crystalline phase, e.g. for information storage and display technologies. They are also used in solution-phase chemistry, e.g. as biomimetic guest-host signaling agents.
To date, the large majority of spin-switching complexes contain just a single metal ion. However, an increasing number of studies are now targeting the synthesis of molecular complexes containing several metal ions. For example, metallogrid complexes can coordinate up to four metal ions to form compact molecular squares. Several reports have already shown that such polynuclear systems are more robust and exhibit enhanced performances compared to their mononuclear counterparts. Scientists are therefore keen to understand the ultrafast response of metallogrid complexes upon photoexcitation. However, standard analytical techniques are not sensitive enough to explore their spin-switching in detail.
Now, for the first time, an international team of researchers have successfully studied the photoinduced spin-state switching in solvated metallogrid complexes containing 3 and 4 iron centers using femtosecond X-ray emission spectroscopy at the instrument for femtosecond X-ray experiments (FXE) at European XFEL. The scientists revealed that such metallogrids possess unexpected properties which make them particularly interesting for a relatively new field of science – spin-driven photochemistry in the solution phase.
“In general, polynuclear complexes are known to be very difficult to study due to the many competing processes that take place in large molecular assemblies after photoexcitation” explains lead author of the study and beamline scientist at DESY Maria Naumova. “An important step has been realized in the past for mononuclear complexes at XFEL sources by enabling the direct monitoring of spin dynamics with X-ray emission spectroscopy. Our work is now extending the use of this technique to tackle the specific challenges posed by polynuclear complexes, such as these Iron metallogrids.”
The proof of concept experiment took place at FXE soon after the user operation started in 2017. Owing to the very high temporal resolution of the X-ray pulses available at FXE, the scientists could follow the sequence of events that occur as the molecule absorbs light and undergoes spin-switching. The measurements demonstrated that the lifetime of the photoexcited state is 100 times longer than the ones reported for mononuclear complexes, and that most of the absorbed energy remains within the system. The existence of this so-called ‘hot’ state is what will establish metallogrid complexes as versatile reactants in novel photochemical schemes.
“We have identified a transient state where the molecule is very responsive” explains Sophie Canton, group leader at Extreme Light Infrastructure (ELI) in Hungary, and principal investigator of the study. “Now we know that these states can be populated, we need to further characterize them so that we can integrate them into chemical reactions. These are very exciting results, and we hope that the scientific community will now join us in exploiting these effects to develop efficient practical applications.”
“The FXE team is extremely pleased to see that, already at the very early stage of the user operation, the instrument was performing well and could yield significant results for the chemical community "said Dmitry Khakhulin, Acting group leader and Instrument scientist at the FXE instrument.
Sophie Canton and her colleagues already have plans for follow-up experiments based on the increased capabilities of the FXE instrument. Future measurements will exploit X-ray absorption spectroscopy and X-ray diffuse scattering techniques in order to build the first visualization of the structural rearrangements that take place right after photoabsorption. Future work will also focus on heterometallic grids containing two different types of atoms employed in fourth generation photovoltaics. Finally, this study is also setting the ground for upcoming investigations at the ELI-ALPS facility, on the sub-femtosecond time scale.
1) https://pubs.acs.org/doi/abs/10.1021/acs.jpclett.9b03883
