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

在化学系化学结构、动力学和机理(CSDM-A)项目的支持下，伊利诺伊大学香槟分校的Josh Vura-Weis博士和Greg Girolami博士正在研究含铁和钴分子中的光诱导过程。光吸收分子或发色团是某些太阳能电池和某些药物合成的关键成分，但通常含有昂贵的Ru或Ir金属原子。虽然含Ru和Ir的分子会将它们从光中吸收的能量储存足够长的时间来引发化学反应，但含有廉价的地球上丰富的铁和钴原子的分子会迅速将这些能量作为热量消散。这个项目将利用从极端紫外吸收光谱中获得的洞察力来更好地理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的生色团。在广泛的太阳能转换过程中，这种分子将减少对昂贵贵金属的依赖。这个项目将通过接待来自历史上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生大学的本科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有开壳过渡金属的络合物光物理和反应性的关键因素。这类体系的光激发启动了一系列弛豫步骤，具有相互竞争的振动弛豫、系间交叉和分子内电子转移的速率。利用飞秒M边X射线吸收光谱测量了Fe(II)和Co(III)光敏剂的弛豫动力学。在极紫外能区的金属3p-3d跃迁是元素、氧化态、自旋态和配位场特有的，而基于激光的光源提供了30飞秒的时间分辨率。通过这项技术获得的激发态势能面的洞见被用来设计具有定制振动模式的新的Fe(II)发色团，该发色团可以动力学地捕获长寿命激发态。Co(III)发色团的新配体被设计用于在可见光区提供金属到配体的电荷转移，并用瞬时吸收光谱和量子动力学计算研究了这些发色团的激发态动力学。这项提案中展示的新光谱方法有望应用于从光催化到太阳能电池设计的物理化学和无机化学的广泛问题。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智力优势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.