Two new Ru(II)Pt(II) dimers, [Ru(bpy)2(μ-L2)PtCl2]2+ (5) and [Ru(bpy)2(μ-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(μ-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 (E1/2 = −1.23 V) is nearly consistent with that of 4 (E1/2 = −1.20 V) but is more positive than that of 6 (E1/2 = −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 π* 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 H2-evolving catalyst, while 6 shows no activity at all. However, 6 was found to drive photochemical H2 evolution in the presence of both EDTA and methyl viologen (N,N′-dimethyl-4,4′-bipyridinium, MV2+), indicating that the 3MLCT excited state of the Ru(bpy)2(phen)2+ moiety is once oxidatively quenched by MV2+ to give MV+˙ and then hydrogen evolution from water by MV+˙ 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 (ΔG°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.
合成并表征了两种新的 Ru(II)Pt(II) 二聚体,即 [Ru(bpy)2(μ-
L2)PtC
L2]2+ (5)和 [Ru(bpy)2(μ-L3)PtC
L2]2+ (6),并评估了它们的电
化学和光谱特性及其光氢化活性;
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)。5 和 6 的结构与我们课题组开发的第一个光氢进化分子装置活性模型 [Ru(bpy)2(μ-L1)PtC
L2]2+ (4) (L1 = 4′-([1、10]phenanthrolin-5-ylcarbaMOyl)-[2,2′]bipyridinyl-4-carboxylic acid),除了与 Pt(II) 结合的 bpy 分子 4 位上的取代基不同(4 为 -COOH;5为-COOEt;6为-
CH3)。电
化学研究表明,5 的首次还原电位(E1/2 = -1.23 V)与 4 的首次还原电位(E1/2 = -1.20 V)基本一致,但比 6 的首次还原电位(E1/2 = -1.39 V)更正,而 6 的首次还原电位与结合到 Pt(II)上的 bpy 分子的还原有关,这与引入抽电子基团后 bpy 的首次还原呈阳极移动的一般趋势一致。对 5 和 6 的密度泛函理论(DFT)计算也表明,在所有 Ru(II)Pt(II) 二聚体中,最低未占据分子轨道(LUMO)对应于与 Pt(II) 结合的 bpy 分子的 π* 轨道,与 4 和 5 相比,6 的 LUMO 能级不稳定,这与电
化学研究的结果一致。研究还考察了 4-6 在牺牲电子供体(
乙二胺四乙酸)存在的条件下驱动
水的光
化学氢演化。研究发现,5 作为
氢气进化催化剂具有活性,而 6 则完全没有活性。然而,研究发现 6 在
EDTA 和甲基
紫草素(N,N′-二甲基-4,4′-联
吡啶鎓,MV2+)的存在下都能驱动光
化学氢气进化,这表明 Ru(bpy)2(phen)2+ 分子的 3MLCT 激发态一旦被 MV2+ 氧化淬灭,就会产生 MV+˙,然后 MV+˙ 从
水中进化
氢气的过程是一个暗反应。发射衰减和瞬态吸收光谱还表明,在活性 Ru(II)Pt(II) 二聚体 4 和 5 中,分子内电子转移(IET)被加速,而在非活性 Ru(II)Pt(II) 二聚体 6 中则没有实现这种加速。IET 过程的驱动力(ΔG°ET)估计为:4 为 -0.16 eV,5 为 -0.09 eV,6 为 0.03 eV,这表明 6 中的 IET 过程是上坡的。结论是,利用这些基于 Ru(II)Pt(II)的分子器件驱动
水的光
化学 H2 演化需要高效的 IET。