Synchrotron radiation study of photo-induced spin-crossover transitions: Microscopic origin of nonlinear phase transition

In situ X-ray absorption spectroscopy (XAS) using synchrotron radiation was used to probe the local structure of photo-induced phase transition of Fe (II) spin-crossover complex. We demonstrate that the local structure of spin-converted metastable phase can be probed by XAS using a novel pixel array detector using less than 1 mg of specimen. We find that the arrangement of nearest neighbors, i.e., the FeN6 cluster does not change its symmetry upon the diamagnetic (S=0) ↔paramagnetic (S=2) transformation under laser light (532 nm) excitation below 50 K. The intermediate-range structure (next-nearest neighbor correlation) shows that ligand molecules in the photo-induced high spin phase is distorted. The results suggest that the essential difference between the photo-induced and thermally induced high spin phases is the absence of intermediate-range order in the former. We propose that quenched disorder (inhomogeneity) in the intermediate-range structure is the microscopic origin of nonlinear nature of photo-induced phase transition (crippled cooperativity). It is demonstrated that in situ XAS using synchrotron radiation and state-of-the-art X-ray detector can provide detailed information on the local structure of metastable states trapped at low temperature.