Huang J, Mara M W, Stickrath A B, Kokhan O, Harpham M R, Haldrup K, Shelby M L, Zhang X, Ruppert R, Sauvage J-P, Chen L X
Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, USA.
Dalton Trans. 2014 Dec 21;43(47):17615-23. doi: 10.1039/c4dt02046d.
Photophysical and structural properties of a Cu(I) diimine complex with very strong steric hindrance, Cu(I)(dppS)2 (dppS = 2,9-diphenyl-1,10-phenanthroline disulfonic acid disodium salt), are investigated by optical and X-ray transient absorption (OTA and XTA) spectroscopy. The bulky phenylsulfonic acid groups at 2,9 positions of phenanthroline ligands force the ground state and the metal-to-ligand charge-transfer (MLCT) excited state to adopt a flattened pseudo-tetrahedral coordination geometry in which the solvent access to the copper center is completely blocked. We analyzed the MLCT state dynamics and structures as well as those of the charge separated state resulting from the interfacial electron injection from the MLCT state to TiO2 nanoparticles (NPs). The OTA results show the absence of the sub-picosecond component previously assigned as the time constant for flattening, while the two observed time constants are assigned to a relatively slow intersystem crossing (ISC) rate (∼13.8 ps) and a decay rate (100 ns) of the Cu(I)(dppS)2 MLCT state in water. These results correlate well with the XTA studies that resolved a flattened tetrahedral Cu(i) coordination geometry in the ground state. Probing the (3)MLCT state structure with XTA establishes that the (3)MLCT state has the same oxidation state as the copper center in Cu(II)(dppS)2 and the Cu-N distance is reduced by 0.06 Å compared to that of the ground state, accompanied by a rotation of phenyl rings located at 2,9 positions of phenanthroline. The structural dynamics of the photoinduced charge transfer process in the Cu(I)(dppS)2/TiO2 hybrid is also investigated, which suggests a more restricted environment for the complex upon binding to TiO2 NPs. Moreover, the Cu-N bond length of the oxidized state of Cu(I)(dppS)2 after electron injection to TiO2 NPs shortens by 0.05 Å compared to that in the ground state. The interpretation of these observed structural changes associated with excited and charge separated states will be discussed. These results not only set an example for applying XTA in capturing the intermediate structure of metal complex/semiconductor NP hybrids but also provide guidance for designing efficient Cu(I) diimine complexes with optimized structures for application in solar-to-electricity conversion.
通过光学和X射线瞬态吸收光谱（OTA和XTA）研究了具有非常强空间位阻的Cu(I)二亚胺配合物Cu(I)(dppS)2（dppS = 2,9 - 二苯基 - 1,10 - 菲咯啉二磺酸钠盐）的光物理和结构性质。菲咯啉配体2,9位上庞大的苯磺酸基团迫使基态和金属 - 配体电荷转移（MLCT）激发态采用扁平的伪四面体配位几何结构，其中溶剂与铜中心的接触完全被阻断。我们分析了MLCT态动力学和结构以及由MLCT态向TiO2纳米颗粒（NPs）的界面电子注入产生的电荷分离态的动力学和结构。OTA结果表明不存在先前被指定为扁平化时间常数的亚皮秒成分，而观察到的两个时间常数分别归因于相对较慢的系间窜越（ISC）速率（约13.8 ps）和Cu(I)(dppS)2在水中的MLCT态的衰减速率（100 ns）。这些结果与XTA研究结果很好地相关，XTA研究解析了基态下扁平的四面体Cu(I)配位几何结构。用XTA探测(3)MLCT态结构表明，(3)MLCT态与Cu(II)(dppS)2中的铜中心具有相同的氧化态，并且与基态相比，Cu - N距离缩短了0.06 Å，同时位于菲咯啉2,9位的苯环发生了旋转。还研究了Cu(I)(dppS)2/TiO2杂化物中光诱导电荷转移过程的结构动力学，这表明该配合物在与TiO2 NPs结合后所处的环境受到更多限制。此外，电子注入到TiO2 NPs后，Cu(I)(dppS)2氧化态的Cu - N键长度与基态相比缩短了0.05 Å。将讨论对这些与激发态和电荷分离态相关的观察到的结构变化的解释。这些结果不仅为应用XTA捕获金属配合物/半导体NP杂化物的中间结构树立了榜样，也为设计具有优化结构的高效Cu(I)二亚胺配合物以应用于太阳能到电能的转换提供了指导。
