Frontier orbital engineering of photo-hydrogen-evolving molecular devices: a clear relationship between the H2-evolving activity and the energy level of the LUMO
Frontier orbital engineering of photo-hydrogen-evolving molecular devices: a clear relationship between the H2-evolving activity and the energy level of the LUMO
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DOI：
10.1039/c0dt00077a
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发表时间：
2010-01-01
影响因子：
4
通讯作者：
Sakai, Ken
中科院分区：
文献类型：
--
作者：
Masaoka, Shigeyuki;Mukawa, Yuichiro;Sakai, Ken
合成并表征了两种新的 Ru(II)Pt(II) 二聚体 [Ru(bpy)(2)(mu-L2)PtCl2](2+) (5) 和 [Ru(bpy)(2)(mu-L3)PtCl2](2+) (6)，并评估了它们的电化学和光谱性质以及光析氢活性（bpy = 2,2'-联吡啶；L2 = 4'-[1,10]菲咯啉-5-基氨基甲酰基)-[2,2']联吡啶基-4-甲酸乙酯； L3 = 4'-甲基-[2,2']联吡啶基-4-羧酸[1,10]菲咯啉-5-基酰胺）。 5和6的结构与我们课题组开发的第一个光析氢分子器件活性模型[Ru(bpy)(2)(mu-L1)PtCl2](2+) (4) (L1 = 4'-([1,10]菲咯啉-5-基氨基甲酰基)-[2,2']联吡啶基-4-羧酸)的结构基本相同，只是结构不同。与 Pt(II) 结合的 bpy 部分的 4 位取代基（-COOH 为 4；-COOEt 为 5；-CH3 为 6）。电化学研究表明，5 (E-1/2 = -1.23 V) 的第一次还原电位几乎与 4 (E-1/2 = -1.20 V) 一致，但比 6 (E-1/2 = -1.39 V) 更正，其中第一次还原与与 Pt(II) 结合的 bpy 部分的还原有关，这与 bpy 的第一次还原在引入吸电子基团。 5和6的密度泛函理论（DFT）计算还表明，对于所有Ru（II）Pt（II）二聚体，最低未占分子轨道（LUMO）对应于与Pt（II）结合的bpy部分的pi*轨道，并且与4和5相比，6的LUMO能级不稳定，与电化学研究的结果一致。研究了在牺牲电子供体 (EDTA) 存在下由 4-6 驱动的水的光化学析氢。发现 5 作为 H-2 放出催化剂具有活性，而 6 则完全没有活性。然而，发现 6 在 EDTA 和甲基紫精（N,N'-二甲基-4,4'-联吡啶鎓，MV2+）存在下驱动光化学 H-2 演化，表明 Ru(bpy)(2)(phen)(2+) 部分的 (MLCT)-M-3 激发态一旦被 MV2+ 氧化猝灭，得到 MV+中心点，然后通过 MV+中心点从水中析出氢气。作为暗反应进行。发射衰减和瞬态吸收光谱还表明，活性 Ru(II)Pt(II) 二聚体 4 和 5 中的分子内电子转移 (IET) 加速，而非活性 Ru(II) Pt(II) 二聚体 6 则没有实现这种加速。IET 过程的驱动力（Delta G 度 (ET)）估计为 4 的 -0.16 eV、5 的 -0.09 eV 和 0.03 eV对于6，表明6中的IET过程是艰难的。结论是，需要高效的 IET 来驱动这些基于 Ru(II) Pt(II) 的分子装置从水中光化学析出 H2。
Two new Ru(II)Pt(II) dimers, [Ru(bpy)(2)(mu-L2)PtCl2](2+) (5) and [Ru(bpy)(2)(mu-L3)PtCl2](2+) (6), were synthesized and characterized, and their electrochemical and spectroscopic properties together with their photo-hydrogen-evolving activities were evaluated (bpy = 2,2'-bypridine; L2 = 4'-[1,10]phenanthrolin-5-ylcarbamoyl)-[2,2']bipyridinyl-4-carboxylic acid ethyl ester; L3 = 4'-methyl-[2,2']bipyridinyl-4-carboxylic acid [1,10]phenanthrolin-5-ylamide). The structures of 5 and 6 are basically identical with that of the first active model of a photo-hydrogen-evolving molecular device developed in our group, [Ru(bpy)(2)(mu-L1)PtCl2](2+) (4) (L1 = 4'-([1,10]phenanthrolin-5-ylcarbamoyl)-[2,2']bipyridinyl-4-carboxylic acid), except for the difference in the substituent group at the 4-position of the bpy moiety bound to Pt(II) (-COOH for 4; -COOEt for 5; -CH3 for 6). Electrochemical studies revealed that the first reduction potential of 5 (E-1/2 = -1.23 V) is nearly consistent with that of 4 (E-1/2 = -1.20 V) but is more positive than that of 6 (E-1/2 = -1.39 V), where the first reduction is associated with the reduction of the bpy moiety bound to Pt(II), consistent with a general tendency that the first reduction of bpy shows an anodic shift upon introduction of electron-withdrawing group. Density functional theory (DFT) calculations for 5 and 6 also show that the lowest unoccupied molecular orbital (LUMO) corresponds to the pi* orbital of the bpy moiety bound to Pt(II) for all the Ru(II) Pt(II) dimers, and the energy level of the LUMO of 6 is destabilized compared with those of 4 and 5, consistent with the results of the electrochemical studies. The photochemical hydrogen evolution from water driven by 4-6 in the presence a sacrificial electron donor (EDTA) was investigated. 5 was found to be active as an H-2-evolving catalyst, while 6 shows no activity at all. However, 6 was found to drive photochemical H-2 evolution in the presence of both EDTA and methyl viologen (N,N'-dimethyl-4,4'-bipyridinium, MV2+), indicating that the (MLCT)-M-3 excited state of the Ru(bpy)(2)(phen)(2+) moiety is once oxidatively quenched by MV2+ to give MV+center dot and then hydrogen evolution from water by MV+center dot proceeds as a dark reaction. Emission decays and transient absorption spectra also show that the intramolecular electron transfer (IET) is accelerated in the active Ru(II)Pt(II) dimers 4 and 5, while such acceleration is not realized for the inactive Ru(II) Pt(II) dimer 6. The driving forces (Delta G degrees(ET)) for the IET processes are estimated to be -0.16 eV for 4, -0.09 eV for 5 and 0.03 eV for 6, indicating that the IET process in 6 is uphill. It is concluded that efficient IET is required to drive the photochemical H2 evolution from water with these Ru(II) Pt(II)-based molecular devices.