Hiruma Yusuke, Yoshikawa Kai, Haga Masa-Aki
Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan.
Faraday Discuss. 2019 Feb 18;213(0):99-113. doi: 10.1039/c8fd00098k.
A new type of memristor inspired by bio-membranes is presented, based on the proton movement resulting from proton-coupled electron transfer (PCET) processes in dinuclear Ru complexes, whereby a two-terminal device based on said Ru complexes and a proton-conducting polymer was constructed as a proof-of-concept. Two ITO electrodes were modified separately with dinuclear Ru complexes that bear tetraphosphonic acid linkers at both ends and a 2,6,2',6'-tetrakis(benzimidazol-2-yl)-4,4'-bipyridine (RuNH-OH) or 1,3,1',3'-tetrakis(benzimidazol-2-yl)-5,5'-biphenyl (RuCH-OH) bridging ligand, and both ITO electrodes exhibit PCET processes with different Ru(ii/iii) redox potentials and pKa values. Poly(4-vinylpyridine) (P4VP; pKa = 4-5), a proton-conducting polymer, was sandwiched between the two modified ITO electrodes to construct a two-terminal device of the type ITO|(RuNH-OH)3|P4VP|(RuCH-OH)3|ITO. Initially, the oxidation state of the metal centers in RuNH-OH and RuCH-OH is Ru(ii) and Ru(iii), respectively. Upon applying a bias voltage between the two ITO electrodes, the high and low current states switch at approximately ±1.10 V due to Ru(ii/iii) redox reactions. At the RuNH-OH|P4VP and RuCH-OH|P4VP interfaces, a proton is released from Ru(ii)NH-OH and subsequently captured by Ru(iii)CH-OH through the hydrogen-bonding interaction with the P4VP polymer, which is driven by the changes in the pKa values of the Ru complexes from 4.1-8.8 [Ru(ii)NH-OH] to <3.8 [Ru(iii)NH-OH] and from <8.4 [Ru(ii)CH-OH] to 5.2-9.8 [Ru(iii)CH-OH] under these conditions. The redox reactions on the modified Ru films create a large proton gradient between the two electrodes, enhancing the proton conductivity through the P4VP layer (pKa = 4-5). When the applied bias potential was inverted, the pKa gradient returned to the original state and the current decreased. Such a proton-conductivity enhancement is relevant to the transport of protons by proton gradients in bio-membranes. Therefore, the present protonic coordination-network films containing metal complexes that exhibit PCET should open new avenues for the design of a new type of memristor devices mimicking the function of synapses.
本文介绍了一种受生物膜启发的新型忆阻器，其基于双核钌配合物中质子耦合电子转移（PCET）过程产生的质子运动，据此构建了一种基于上述钌配合物和质子传导聚合物的两终端器件作为概念验证。两个氧化铟锡（ITO）电极分别用两端带有四膦酸连接基的双核钌配合物以及桥连配体2,6,2',6'-四（苯并咪唑-2-基）-4,4'-联吡啶（RuNH-OH）或1,3,1',3'-四（苯并咪唑-2-基）-5,5'-联苯（RuCH-OH）进行修饰，且两个ITO电极均表现出具有不同Ru（ii/iii）氧化还原电位和pKa值的PCET过程。将质子传导聚合物聚（4-乙烯基吡啶）（P4VP；pKa = 4 - 5）夹在两个修饰后的ITO电极之间，构建出ITO|(RuNH-OH)3|P4VP|(RuCH-OH)3|ITO类型的两终端器件。最初，RuNH-OH和RuCH-OH中金属中心的氧化态分别为Ru（ii）和Ru（iii）。在两个ITO电极之间施加偏置电压时，由于Ru（ii/iii）氧化还原反应，高电流状态和低电流状态在约±1.10 V时切换。在RuNH-OH|P4VP和RuCH-OH|P4VP界面处，质子从Ru（ii）NH-OH释放，随后通过与P4VP聚合物的氢键相互作用被Ru（iii）CH-OH捕获，这是由Ru配合物的pKa值在这些条件下从4.1 - 8.8 [Ru（ii）NH-OH]变为<3.8 [Ru（iii）NH-OH]以及从<8.4 [Ru（ii）CH-OH]变为5.2 - 9.8 [Ru（iii）CH-OH]所驱动的。修饰后的钌膜上的氧化还原反应在两个电极之间产生了较大的质子梯度，增强了通过P4VP层（pKa = 4 - 5）的质子传导性。当施加的偏置电位反转时，pKa梯度恢复到原始状态且电流减小。这种质子传导性的增强与生物膜中质子梯度对质子的传输相关。因此，目前含有表现出PCET的金属配合物的质子配位网络薄膜应为设计模仿突触功能的新型忆阻器器件开辟新途径。
