Picosecond time-resolved X-ray emission spectroscopy: ultrafast spin-state determination in an iron complex.

Molecular systems with switchable electronic states have fascinated scientists for decades, in part due to their potential as ultrasmall magnetic devices. Among them, transitionmetal-based spin-crossover (SCO) compounds exhibit strikingly different properties, both structural and electronic, between their isoelectronic low-spin (LS) and high-spin (HS) states. Previous research resulted in a rich knowledge of the involved electronic states and key factors that affect the spintransition conditions. At low temperature (T< 50 K) many different excitations, including visible light, hard and soft X rays, and even nuclear decay, can leave an Fe SCO system in a very long-lived HS state. 3] Studies of the temperature and ligand dependence of the lifetime of this HS state led to valuable insights into the electronic structure of SCO complexes. Nevertheless, these mostly indirect tools left many details about the switching process and the involved electronic states in ambiguity. Ultrafast laser techniques offer direct real-time insight into the elementary steps of photoinduced spin transitions. Applying X-ray absorption spectroscopy (XAS) in combination with laser pumping has proven to be very successful; for example, the structure of the putative HS T2 excited state of [Fe(bpy)3] 2+ (bpy = 2,2’-bipyridine) was directly determined by picosecond-resolved XAS at the Fe K-edge. A recent structural dynamics study exploiting X radiation in the femtosecond range delivered a surprisingly simple picture of the photoinduced spin-state switching of aqueous [Fe(bpy)3] : Following laser excitation into the singlet metal-toligand charge-transfer (MLCT) manifold, prompt (within about 30 fs) intersystem crossing to the MLCT states occurs, and backtransfer of the excited electron to the central atom (with a lifetime of 130 fs) goes straight into the HS T2 state, and thus all of the metal-centered ligand-field states that lie between are bypassed. However, to unravel the elementary steps in this spin transition one needs observe both the geometric and electronic changes in real time. While X-ray absorption and scattering experiments often emphasize the geometric structure, optical spectroscopic methods address a mixture of vibrational and electronic levels within the entire molecule. Element-sensitive detection of the spin state itself has not been achieved on the subnanosecond timescale. Application of a technique that can unambiguously identify the spin state in bulk media under ambient conditions is thus of great interest for distinguishing electronic from structural variations. Here we report on direct detection of the spin state of aqueous [Fe(bpy)3] 2+ molecules 60 ps after an ultrashort laser pulse excitation by time-resolved X-ray emission spectroscopy for the first time. Hereby, the laser induces a low-spin to high-spin conversion, which switches back to the LS state on a subnanosecond timescale. We detect the variation of the iron Ka line shape, which unambiguously determines the actual spin state. The interpretation of the time-resolved data is supported by theoretical crystal-field multiplet calculations and experimental Ka spectra of related HS and LS Fe reference compounds. 1s X-ray emission spectroscopy (XES) represents a direct probe of the spin state of 3d transition metals, due to the exchange interaction of the core hole with the 3d electrons in the final state. Hereby, both Ka and Kb lines have proved useful as sensitive probes of the spin state. The Kb emission line has been the typical choice for spin-state studies, since the number of unpaired electrons is reflected in the intensity of the Kb’ satellite peak, which is well separated from the main line due to the large 3p3d exchange interaction. However, it has been shown both theoretically and experimentally that the Ka lines provide similar information. 14] [*] Dr. G. Vank KFKI Research Institute for Particle and Nuclear Physics P.O.B. 49, 1525 Budapest (Hungary) E-mail: vanko@rmki.kfki.hu