Time-resolved optical magnetic-circular-dichroism study of ligand-driven light-induced spin-change complexes

在这个项目中，我们打算应用我们最近开发的时间分辨磁圆二色（MCD）技术来研究具有光磁效应的金属配合物-光诱导激发自旋态捕获（LIESST），在低于临界温度的情况下，人们可以通过辐照在低自旋（LS）和高自旋（HS）状态之间可逆地切换金属离子。该研究的目标化合物是已建立的自旋交叉（SCO）配合物Fe(II)phen2(NCS- <s:1>)2 （phen = 1,10-菲罗啉）。前人对相关LS- fe （II）配合物在室温溶液中的研究表明，激发后，LS化合物在亚皮秒的时间尺度内达到HS态。这一显著的结果表明，有机系统中有关自旋动力学（系统间交叉速率）的经典规则不需要适用于这种金属-有机化合物。精确的激发态动力学决定了磁开关的量子效率，从而决定了给定LIESST化合物的性能。在这里，我们提出在低温下（LIESST效应起作用的地方）对固态样品进行实验，使用顺磁性UV-vis MCD作为自旋态的敏感探针。我们将使用我们的飞秒光泵mcd探针系统（除了瞬态吸收测量外）确定HS态形成的时间尺度，并解决LS-HS转换过程中中间三重态的作用。我们还将进一步评估使用连续波MCD来表征辐照期间SCO与温度和LIESST效应的关系，并将结果与相应的SQUID磁强计测量结果进行比较。

In this project, we intend to apply our recently developed time-resolved magnetic circular dichroism (MCD) techniques to the study of metal complexes exhibiting the photomagnetic effect – light-induced excited-spin-state trapping (LIESST), where below a critical temperature one can reversibly switch the metal ion between a low-spin (LS) and high-spin (HS) state via irradiation. The target compound for the study is the well-established spin-crossover (SCO) complex Fe(II)phen2(NCS-)2 (phen = 1,10-phenanthroline). Previous investigations of related LS-Fe(II) complexes in room temperature solutions indicate that upon excitation, the LS compound reaches the HS state on a sub-picosecond time scale. This remarkable result demonstrates that the classical rules concerning spin dynamics (intersystem crossing rates) in organic systems need not apply in such metal-organic compounds. The precise excited-state dynamics dictates the quantum efficiency for magnetic switching and hence the performance of a given LIESST compound. Here we propose experiments for solid-state samples at low temperature (where the LIESST effect is operative), using paramagnetic UV-vis MCD as a sensitive probe of the spin state. We will determine the time scale for formation of the HS state using our femtosecond optical-pump MCD-probe system (in addition to transient absorption measurements), as well as addressing the role of intermediate triplet states during the LS-HS conversion. We will also further evaluate the use of continuous-wave MCD for characterising the SCO vs. temperature and LIESST effect during irradiation, and compare the results to corresponding SQUID magnetometry measurements.