Photochemistry of Charge Transfer Excited States

该奖项由无机化学、生物无机化学和有机金属化学计划颁发，以支持对两种稀土络合物激发态的光化学研究，其中Re原子具有六个d电子。在第一种情况下，该络合物将包含一个双齿双亚胺配体和一个单齿配体，前者在激发态充当电子受体，后者在激发态充当电子供体，其缺电子形式是反应的。这些分子的光化学激发将产生配体-配体电荷转移(LLCT)激发态。这一阶段的研究目标包括：1)描述发生的光化学反应和鉴定通过LLCT激发产生的活性中间体，2)确定反应基团的重排和键断裂的动力学，3)利用在2)中获得的速率常数来确定结构和能量因素对LLCT激发态寿命的影响。在第二种情况下，我们将研究含有双齿双亚胺和烷基配体的Re(I)配合物中的sigma-to-pi反键激发过程。在这里，激发态中键断裂的结果将产生金属和碳基自由基。这项工作的一个主要目标是将Re-碳键的强度与激发态键的断裂率联系起来。光化学激发态中的电荷转移是生物光能采集系统的一个基本方面。如何在生物系统中充分稳定电荷转移状态，从而使生产性反应发生得比电荷复合更快，这是一个谜，继续刺激着令人兴奋的研究。生物系统的成功模拟可能意味着为技术目的更有效地利用太阳能；在这方面，确定诸如这里将要研究的那些系统中激发态的反应性可能是有用的。正在研究的化学体系也可能用作聚合反应的光化学活化催化剂。

This award, from the Inorganic, Bioinorganic and Organo- metallic Chemistry Program, is for the support of studies of the photochemistry of excited states of two types of rhenium complexes in which the rhenium atom has six d-electrons. In the first case the complex will contain a bidentate diimine ligand that serves as an electron acceptor in the excited state and a monodentate ligand that serves as an electron donor in the excited state, and which is reactive in its electron deficient form. Photochemical excitation of these molecules will generate a ligand-ligand charge transfer (LLCT) excited state. Objectives of this phase of the investigation include: 1) delineation of the photochemical reactions that occur andidentification of reactive intermediates produced via LLCT excitation, 2) determination of the kinetics of rearrangements and bond fragmentations of the reactive groups, and 3) utilization of the rate constants obtained in 2) to determine the effects of structural and energetic factors on the lifetime of the LLCT excited state. In the second case, sigma-to-pi antibonding excitation processes in rhenium(I) complexes that contain bidentate diimine and alkyl ligands will be examined. Here the results of bond scission in the excited state will produce metal- and carbon-based radicals. A primary objective of this work is to relate rhenium-carbon bond strength to the rate of excited state bond fragmentation. %%% Charge transfer in photochemically generated excited states is a fundamental aspect of biological light energy harvesting systems. How charge transfer states are sufficiently stabilized in biological systems so that productive reactions occur faster than charge recombination is a mystery that continues to stimulate exciting research. Successful emulation of biological systems could mean more efficient utilization of solar energy for technological purposes; determination of the reactivity of the excited state in systems such as those to be examined here may be useful in this regard. The chemical systems under study may also be useful as photochemically activated catalysts for polymerization reactions.