TIME-RESOLVED X-RAY DIFFRACTION STUDIES OF PHOTOINDUCED SPIN TRANSITION IN M

University Of Chicago, Chicago IL

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Abstract

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. We propose to investigate the out-of-equilibrium switching dynamics during the photoinduced spin conversion from low-spin (LS) to high-spin (HS) states in two different spin-crossover solids (weakly and highly cooperative crystals respectively). The photo-switching is triggered by a femtosecond laser flash. Preliminary time-resolved x-ray diffraction and optical studies on a weakly cooperative system have shown that the dynamics span from sub-picosecond local photo-switching followed by volume expansion (nanosecond) and thermal switching (microsecond). We want to perform a detailed analysis of this multi-scale switching process in particular by - checking the temperature dependence of photoinduced spin-crossover in [(TPA)Fe(III)(TCC)]PF6 and it's derivative observed to be maximum around the crossover temperature: the thermal microsecond switching should be strongly dependent of temperature with a maximum around the thermal equilibrium crossover temperature T1/2 whereas the sub-ps non-thermal switching should not shown such a dependency. - investigating the (Fe(phen)2NCS2) crystal which reveal cooperatif effects such as thermal hysteresis loop in which elastic interactions are expected to enhace the conversion of molecules from low-spin to high-spin states. In this case elastic interactions may increase the converted fraction of molecules from low-spin to high-spin statesespecialy on the ns time-scale where volume expansion takes place. Time-resolved x-ray diffraction makes it possible to track the structural signatures of this photoswitching process for a better understanding of the multi-scale out-of-equilibrium dynamics. As the spin-crossover molecules switch from low spin to high spin states leading to a change in magnetic and optical properties the electronic redistribution of the electrons on the d orbitals induces a large variation (0.2 A) of the Fe-ligand bonds. For obtaining this structural information complete data collections for solving and refining the corresponding structures are necessary. We propose to investigate the complete time course of the out-of-equilibrium dynamics from 100 ps to 1 ms. By solving the average cristallographic structures at any delay we will track not only the intra-molecular reorganization but also inter-molecular changes volume expansion as well as heating propagation through Debye-Waller factors in particular. Such investigations are of fundamental interest for a better understanding of the photoinduced molecular switching in the solid state. The structural information is crucial for establishing the physical foundations on how to direct macroscopic photo-switching in materials. A key feature is that dynamics follows a complex pathway from molecular to material scales through a sequence of processes. Not only is the pathway indirect the nature of dynamical process along the pathway depends on time scale. This dictates which kind of degrees of freedom is involved in the subsequent dynamics or kinetics and which ones are frozen or statistically averaged. By comparing different compound we will have a better understanding of what is universal in these processes.