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

这个子项目是利用资源的许多研究子项目之一。 由NIH/NCRR资助的中心拨款提供。对子项目的主要支持 子项目的首席调查员可能是由其他来源提供的， 包括美国国立卫生研究院的其他来源。为子项目列出的总成本可能 表示该子项目使用的中心基础设施的估计数量， 不是由NCRR赠款提供给次级项目或次级项目工作人员的直接资金。 我们研究了两种不同的自旋交叉固体(分别为弱合作晶体和高度合作晶体)中低自旋(LS)态到高自旋(HS)态的光致自旋转换过程中的非平衡开关动力学。 光开关是由飞秒激光闪光灯触发的。对弱合作系统的初步时间分辨X射线衍射和光学研究表明，其动力学跨度从亚皮秒的局域光开关，然后是体积膨胀(纳秒)和热开关(微秒)。我们想要对这种多尺度转换过程进行详细的分析，特别是通过 -检查[(TPA)Fe(III)(TCC)]PF6中光致自旋交叉的温度依赖关系，其导数在交叉温度附近最大：热微秒开关应强烈依赖于温度，最大值在热平衡交叉温度T1/2附近，而亚PS非热开关不应表现出这种依赖关系。 -研究(Fe(Phen)2NCS2)晶体，它揭示了合作效应，如热滞回环，其中弹性相互作用有望增强分子从低自旋态到高自旋态的转换。在这种情况下，弹性相互作用可能会增加分子从低自旋状态到高自旋状态的转换分数，特别是在发生体积膨胀的ns时间尺度上。 时间分辨的X射线衍射使得追踪这种光开关过程的结构特征成为可能，从而更好地理解多尺度非平衡动力学。由于自旋交叉分子从低自旋态转变为高自旋态，导致磁性和光学性质的变化，d轨道上电子的重新分布导致Fe-配体键的很大变化(0.2A)。为了获得这种结构信息，需要完整的数据集合来解决和提炼相应的结构。我们建议研究从100ps到1ms的非平衡动力学的完整时间过程。通过求解任意延迟的平均晶体结构，我们不仅可以跟踪分子内的重组，还可以跟踪分子间的变化，特别是通过德拜-沃勒因子的体积膨胀和加热传播。 这些研究对于更好地理解固体中的光诱导分子开关具有重要意义。结构信息对于建立如何指导材料中的宏观光开关的物理基础至关重要。一个关键特征是动力学遵循从分子到材料尺度的复杂路径，通过一系列过程。不仅路径是间接的，沿着路径的动态过程的性质取决于时间尺度。这决定了在随后的动力学或动力学中涉及哪种自由度，以及哪些自由度是冻结的或统计平均的。通过比较不同的化合物，我们将更好地了解这些过程中普遍存在的是什么。

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.