Design and Control of Fe and Co Spin Crossover Dynamics Using Tabletop Femtosecond M-edge XANES

在化学系化学结构，动力学和机制-A（CSDM-A）计划的支持下，伊利诺伊大学厄巴纳-香槟分校的Josh Vura-Weis和Greg Girolami博士正在研究含铁和钴分子的光诱导过程。吸光分子或发色团是某些太阳能电池和某些药物合成中的关键成分，但通常含有昂贵的钌或铱金属原子。虽然含钌和铱的分子将它们从光中吸收的能量储存足够长的时间以引起化学反应，但具有廉价且地球上丰富的铁和钴原子的分子迅速将能量作为热量耗散。该项目将利用从极紫外吸收光谱学中获得的洞察力，更好地了解导致非常快的能量损失的过程，以便设计具有长寿命激发态的新发色团。这种分子将减少在广泛的太阳能转换过程中对昂贵贵金属的依赖。该项目将通过接待来自历史悠久的黑人学院和大学的圣埃尔莫布雷迪暑期学者以及主要来自本科大学的本科研究生来扩大对物理化学的参与。电子转移和自旋动力学之间的相互作用是含开壳过渡金属络合物的物理学和反应性的关键因素。这种系统的光激发引发了一系列的弛豫步骤与竞争率的振动弛豫，系间交叉，和分子内电子转移。在这个桌面工作中，飞秒M边X射线吸收光谱被用来测量在Fe（II）和Co（III）光敏剂的弛豫动力学。在极紫外能量范围内的金属3 p-3d跃迁是元素、氧化态、自旋态和配体场特异性的，并且基于激光的源提供30飞秒的时间分辨率。从这种技术获得的激发态势能面的见解被用来设计新的Fe（II）生色团与定制的振动模式，动力学陷阱长寿命的激发态。Co（III）发色团的新配体正在设计中，以提供在可见光区的金属到配体的电荷转移跃迁，这些发色团的激发态动力学研究使用瞬态吸收光谱和量子动力学计算。该提案中展示的新光谱方法有望应用于物理和无机化学中的广泛问题，从太阳能电池设计到太阳能电池设计。该项目还将为研究生和本科生提供合成和光谱学方面的高级培训。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, Drs. Josh Vura-Weis and Greg Girolami at the University of Illinois at Urbana-Champaign are studying light-induced processes in iron- and cobalt-containing molecules. Light-absorbing molecules or chromophores are key components in certain solar cells and in the synthesis of some pharmaceuticals, but usually contain expensive ruthenium or iridium metal atoms. While the ruthenium and iridium-containing molecules store the energy they absorb from light for a long enough time to induce a chemical reaction, molecules with cheap and earth-abundant iron and cobalt atoms rapidly dissipate that energy as heat. This project will use insight gained from extreme ultraviolet absorption spectroscopy to better understand the processes leading to very fast energy loss in order to design new chromophores with long-lived excited states. Such molecules would reduce dependence on expensive precious metals in a wide range of solar energy conversion processes. This project will broaden participation in physical chemistry by hosting St. Elmo Brady Summer Scholars from historically black colleges and universities, as well as undergraduate research students from primarily undergraduate universities.The interplay between electron transfer and spin dynamics is a key factor in the photophysics and reactivity of complexes containing open-shell transition metals. Photoexcitation of such systems initiates a cascade of relaxation steps with competing rates of vibrational relaxation, intersystem crossing, and intramolecular electron transfer. In this tabletop work, femtosecond M-edge X-ray absorption spectroscopy is used to measure the relaxation dynamics in Fe(II) and Co(III) photosensitizers. The metal 3p-3d transitions in the extreme ultraviolet energy range are element, oxidation state, spin state, and ligand-field specific, and the laser-based source provides 30-femtosecond time resolution. Insights about the excited-state potential energy surfaces gained from this technique are used to design new Fe(II) chromophores with tailored vibrational modes that kinetically trap long-lived excited states. New ligands for Co(III) chromophores are being designed to provide metal-to-ligand charge transfer transitions in the visible region, and the excited-state dynamics of these chromophores are studied using transient absorption spectroscopy and quantum dynamics calculations. The new spectroscopic methods demonstrated in this proposal are expected to apply to a wide range of problems in physical and inorganic chemistry, from photocatalysis to solar cell design. The project also will provide advanced training in synthesis and spectroscopy for graduate and undergraduate students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.