Extremely slow long-range electron transfer reactions across zeolite-solution interface
Extremely slow long-range electron transfer reactions across zeolite-solution interface
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DOI：
10.1021/ja015751v
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发表时间：
2001-11-14
影响因子：
15
通讯作者：
Imahori, H
中科院分区：
文献类型：
--
作者：
Fukuzumi, S;Yoshida, Y;Imahori, H
长程电子转移在合成系统和生物系统中与光合作用和新陈代谢的中心作用有关，已经得到了广泛的研究。电子转移速率常数随着给体和受体分子之间距离的增加而近似指数地下降，只要其他参数如电子转移的驱动力和重组能保持不变。因此，随着距离的增加，电子转移的时间尺度预计将从飞秒增加到几小时，甚至几天。然而，这种极慢的电子转移是史无前例的，因为以前任何其他关于固定距离电子转移反应的研究都涉及激发态，而且固有的短激发态寿命阻碍了对慢电子转移过程的研究。在连接的体系中，供体和受体分子之间的电子转移反应需要光激发来启动，因为如果电子转移发生在热的情况下，就不可能连接供体和受体分子。只有当一个电子供体或受体分子被包裹在一个大的惰性环境中时，才能实现缓慢的热电子转移，这种环境阻止了另一个分子的近距离接触。这项研究报告了这样一种系统，它利用Y型沸石超笼，其中封装了电子受体。在光诱导电荷分离体系中，分子筛超笼中生色离子的包裹被用来延缓分子筛-溶液界面的反电子转移。10-12然而，沸石-溶液界面上的电子转移速率尚未确定。在分子筛中加入一个不能穿透分子筛超笼的电子给体，可以通过分子筛-溶液界面启动远距离的热电子转移。在本系统中，观察到了极慢的电子转移，使得电子转移的完成需要几天时间，这与相应的溶液中的电子转移反应形成鲜明对比，溶液中的电子转移反应太快，即使使用停流技术也无法跟踪。在沸石溶液中的电子供体和分子筛内的电子受体之间，这种跨越沸石-溶液界面的极慢的电子转移过程为研究长距离的热电子转移提供了有价值的见解。金属络合物Fe(Bpy)32+(Bpy)2，2‘-联吡啶)通过瓶装合成法被装载到NaY沸石中(参见支持信息，S1，S2)，而Fe(Bpy)32+的12？尺寸使其能够牢固地安装在13？超笼内。由Fe(Bpy)32+的吸收测得Fe(Bpy)32+的负载量为5.4×10-5molg-1，相当于每10个超笼中有1.1个分子。一旦Fe(Bpy)32+加入到沸石中，就没有Fe(Bpy)32+进入溶液。因此，Fe(Bpy)32+不是放置在沸石表面，而是放置在超笼内。Fe(Bpy)32+的12？尺寸太大，无法从有7？宽度窗口的超级笼子中逃脱。Fe(Bpy)32+-Y型分子筛在氯气中氧化5min，得到相应的Fe(Bpy)33+-Y型分子筛。由于Fe(Bpy)32+在520 nm处的吸收消失，证实了氧化的完成。不同的电子给体在溶液中通过分子筛界面向分子筛内部的Fe(Bpy)3+转移电子的速率，监测了由于Fe(Bpy)3+引起的520 nm处吸光度的增加。添加…
Long-range electron transfer has been studied extensively in both synthetic and biological systems in relation to the central role in photosynthesis and metabolism. 1-8 It is now wellestablished that the electron transfer rate constant falls off approximately exponentially with increasing the distance between the donor and acceptor molecules, provided that the other parameters such as the driving force and reorganization energy of electron transfer remain the same. 9 Thus, the time scale of electron transfer is expected to increase from femtoseconds to hours and even to days with simply increasing the distance. However, such an extremely slow electron transfer is unprecedented because any other previous study on electron transfer reactions at fixed distances involves the excited state, and the inherent short excited-state lifetime has precluded the study on slow electron transfer processes. Photoexcitation is necessitated to start the electron transfer reactions between the donor and acceptor molecules in the linked systems, since it would be impossible to connect donor and acceptor molecules if the electron transfer occurred thermally. A slow thermal electron transfer would only be achieved if an electron donor or acceptor molecule is encapsulated in a large inert environment which prohibits the close access of the other molecule. This study reports such a system that utilizes the Y-type zeolite supercage in which an electron acceptor is encapsulated. Encapsulation of chromophore ions in the supercage of zeolite has so far been utilized to retard the back electron transfer across the zeolite-solution interface in photoinduced charge-separation systems. 10-12 However, the electron transfer rates across the zeolite-solution interface have yet to be determined. Addition of an electron donor which cannot penetrate into the zeolite supercage to the acceptor-encapsulated zeolite can start the thermal electron transfer at long distances through the zeolite-solution interface. In the present system, an extremely slow electron transfer such that the completion of electron transfer takes days is observed, in sharp contrast to the corresponding electron transfer reaction in solution which is too fast to be followed even by using a stopped-flow technique. Such extremely slow electron-transfer processes across the zeolite-solution interface between electron donors in solution and electron acceptors inside the zeolite provide valuable insight into thermal electron transfer at long distances. A metal complex, Fe (bpy) 3 2+(bpy) 2, 2′-bipyridine) was loaded into the NaY zeolite by the ship-in-bottle synthesis (see Supporting Information, S1, S2), and the 12 Å dimension of Fe (bpy) 3 2+ makes for a secure fit inside the 13 Å supercage. The loaded amount of Fe (bpy) 3 2+ was determined from the absorption due to Fe (bpy) 3 2+ as 5.4× 10-5 mol g-1, which corresponds to 1.1 molecules per 10 supercages. Once Fe (bpy) 3 2+ is incorporated into the zeolite, no Fe (bpy) 3 2+ comes out into solution. Thus, Fe (bpy) 3 2+ is not placed on the zeolite surface but inside the supercage. The 12 Å dimension of Fe (bpy) 3 2+ is too large to escape from the supercage which has the 7 Å width window. The Fe (bpy) 3 2+-zeolite Y was oxidized by exposing the sample under chlorine (1 atm) for 5 min to yield the corresponding Fe (bpy) 3 3+-zeolite Y. The completion of the oxidation was confirmed by the disappearance of the absorption at λ max) 520 nm due to Fe (bpy) 3 2+.Rates of electron transfer from various electron donors in solution to Fe (bpy) 3 3+ inside the zeolite across the zeolitesolution interface were followed by monitoring an increase in absorbance at 520 nm due to Fe (bpy) 3 2+. Addition …