Evidence for Inefficient Chain-to-Chain Coupling in Electron Tunneling through Liquid Alkanethiol Monolayer Films on Mercury
Evidence for Inefficient Chain-to-Chain Coupling in Electron Tunneling through Liquid Alkanethiol Monolayer Films on Mercury
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汞上液体烷硫醇单层膜电子隧道中低效链对链耦合的证据
DOI：
10.1021/ja9601191
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发表时间：
1996
影响因子：
15
通讯作者：
M. Majda
中科院分区：
文献类型：
--
作者：
K. Słowiński;R. Chamberlain;and Renata Bilewicz;M. Majda
距离依赖性和中间介质效应继续吸引大量科学家的注意力，作为电子转移动力学领域中尚未解决的关键问题。1 -3在本通讯中，我们报告了描述两种类型的可变厚度的烷醇单分子膜的长程电子转移动力学的电化学结果。我们的数据表明，几何距离衰减常数涉及链到链隧穿途径获得的液体单层是约5倍小于通过链衰减常数的特点，为有序的烷基硫醇单层。利用以下两种实验方法研究了电子隧穿中的中间介质效应，取得了许多新的进展。在第一种方法中，合成的供体/受体对被控制长度和结构的刚性分子间隔基团分开。4 -6在第二种方法中，研究涉及氧化还原蛋白，其中电子受体（或供体）附着在蛋白质外围的特定位点上。3，7，8此外，也已经成功地探索了将分子以特定距离拴系到电极表面的电化学测量。9在相关研究中，最近使用烷硫醇的自组装单层作为金电极上的阻挡膜的电子隧穿研究特别成功。10 -15下面描述的实验依赖于组装在汞上的烷硫醇单层。汞电极的使用使我们首次能够以两种不同的方式改变单层的厚度。第一种方法涉及通常形成具有不同长度烷基链的硫醇的有序单层。16第二种方法仅为Kublik型悬挂汞滴电极（HMDE）所固有。17单层厚度的变化通过使用测微螺杆小心地膨胀汞滴来实现，该螺杆控制HMDE毛细管尖端挤出的汞的体积。文献中充分记录了金上烷硫醇单层的形成结构和性质。18 -21类似的程序导致汞上有序硫醇单层的形成。22，23事实上，我们的结果表明，链长为9至18个碳原子的烷硫醇的不可渗透单层基本上是在HMDE上瞬间形成的。24如图1所示，微分电容（C）测量的分析25给出了1/C对烷基链碳原子数的线性图。这与双电层的亥姆霍兹模型一致，在该模型中，其电容由可变厚度（d）12、26的烃膜支配，并由C）oA/d（1）表示。
Distance dependence and the intervening medium effects continue to pin the attention of a large number of scientists as the key unresolved problems in the area of electron transfer kinetics.1-3 In this communication, we report electrochemical results describing long-range electron transfer kinetics across two types of alkanethiol monolayer films of variable thickness. Our data show that the geometric distance decay constant involving a chain-to-chain tunneling pathway obtained for liquid monolayers is approximately 5 times smaller than the throughchain decay constant characteristic for ordered alkylthiol monolayers. Many recent advances in the studies of intervening medium effects in electron tunneling have been made using the following two experimental approaches. In the first, synthetic donor/ acceptor pairs are separated by rigid molecular spacer groups of controlled length and structure.4-6 In the second, investigations involve redox proteins in which an electron acceptor (or donor) is attached to a specific site on the periphery of a protein.3,7,8 In addition, electrochemical measurements in which molecules are tethered to the electrode surface at a specific distance have also been successfully explored.9 In related research, recent studies of electron tunneling using selfassembled monolayers of alkanthiols as barrier films on gold electrodes have been particularly successful.10-15 The experiments described below rely on alkanethiol monolayers assembled on mercury. The use of Hg electrodes allows us, for the first time, to vary the thickness of the monolayers in two different ways. The first involves the usual formation of ordered monolayers of thiols with alkyl chains of different length.16 The second method is intrinsic only to Kublik type hanging mercury drop electrodes (HMDE).17 Variation of the monolayer thickness is accomplished by careful expansion of the mercury drop using a micrometric screw that controls the volume of mercury extruded at the tip of the HMDE capillary. Formation structure and properties of alkanethiol monolayers on gold are well documented in the literature.18-21 Similar procedures lead to the formation of ordered thiol monolayers on mercury.22,23 Indeed, our results suggest that impermeable monolayers of alkanethiols with chain length from 9 to 18 carbon atoms are formed essentially instantaneously on HMDE.24 As shown in Figure 1, analysis of the differential capacitance (C) measurements25 gave linear plots of 1/C vs number of the alkyl chain carbon atoms. This is consistent with the Helmholtz model of the double layer in which its capacitance is dominated by the hydrocarbon film of variable thickness (d)12,26 and is expressed by C ) oA/d (1)