Pyridinium-pentacyanoferrate complexes as novel iron photosensitizers: synthesis, photophysics, and function characterization towards light-driven water oxidation

取代含贵金属的光敏剂（PS），例如，具有铁PS的基准钌络合物对于低成本和可扩展的水裂解电池的发展一直是长期的兴趣。然而，由于激发态的寿命极短，铁PS在历史上对应用提出了重大挑战，这些激发态通常被认为在ps范围内。这些系统的相应的快速激发态衰减，一旦光激发到它们的金属-配体电荷转移（1 MLCT）状态，是由于能量低的金属中心（3，5 MC *）状态的可用性，这构成了激发态的有效失活通道。与将铁PS实际应用到光电化学电极中相关的另一个关键问题是设计适当的配体，其不仅可以增加铁PS的激发态寿命，而且还可以将其有效地连接到催化剂和半导体。在这方面，氰化物配体可以充当连接金属离子的短桥连配体。此外，它是一个强的供电子配体，可以使MC* 态不稳定，并阻碍相应络合物中激发态的快速失活。先前的研究表明，吡啶基氰铁酸盐配合物的数量增加的氰化物配体周围的Fe（II）网站有效地增加了MLCT的寿命，通过增加八面体分裂，因此，不稳定的MC* 状态。然而，氰铁酸盐光敏剂的研究一直局限于二，三，和四氰铁酸盐配合物与吡啶配体，和它们的利用在光催化应用尚未探索到现在。由于铁络合物的独特结构，吡啶铁-五氰基铁酸盐络合物具有解决上述所有问题的潜力。申请人的联合初步研究表明，使氰化物配体的数量最大化至5并利用阳离子有机配体是实现足够长的寿命以用于光催化水氧化过程的有效策略。在这个项目中，一系列的吡啶-pentacyanoferrate配合物与不同的电子受体吡啶和吡啶配体的溶剂依赖性的物理学将进行光谱研究，而我们分析的配合物的功能有关的能力，触发光驱动的水氧化时，结合共催化剂。

Replacing precious metal-containing photosensitizers (PSs), e.g., benchmark ruthenium complexes, with iron PSs, has been a long-standing interest for the advancement of low-cost and scalable water splitting cells. Iron PSs, however, have historically posed significant challenges for applications due to the extremely short-lived nature of the excited states, which were considered usually in the ps range. The corresponding rapid excited-state decay of these systems, once optically excited to their metal-to-ligand charge transfer (1MLCT) state, is due to the availability of energetically low-lying metal-centered (3,5MC*) states, which constitute efficient deactivation channels for the excited states. Another critical issue related to the practical implementation of an iron PS into a photoelectrochemical electrode is the design of a proper ligand, which cannot only increase the excited state lifetime of the iron PS but also connect it efficiently to a catalyst and the semiconductor. In this respect, the cyanide ligand can serve as a short bridging ligand connecting metal ions. Furthermore, it is a strong electron-donating ligand that can destabilize MC* states and hinder the rapid deactivation of the excited states in the corresponding complexes. Previous studies on pyridyl-cyanoferrate complexes indicate that increasing the number of cyanide ligands around the Fe(II) site effectively increases the MLCT lifetimes by increasing the octahedral splitting, thus, destabilizing the MC* states. However, studies on cyanoferrate photosensitizers have been limited to di-, tri-, and tetra-cyanoferrate complexes with pyridyl ligands, and their utilization in photocatalytic applications has not been explored until now. Pyridinium-pentacyanoferrate complexes hold the potential to address all the problems mentioned above due to the unique architecture of the iron complex. Joint preliminary studies of the applicants indicate that maximizing the number of cyanide ligands up to five and utilizing cationic organic ligands is an efficient strategy to achieve lifetimes long enough to be utilized in the photocatalytic water oxidation process. In this project, the solvent-dependent photophysics of a series of pyridinium-pentacyanoferrate complexes with varying electron acceptor pyridyl and pyridinium ligands will be investigated spectroscopically, while we analyze the complexes functionally concerning their ability to trigger light-driven water oxidation when combined with Co-catalysts.