Organic Molecular Layer with High Electrochemical Bistability: Synthesis, Structure, and Properties of a Dynamic Redox System with Lipoate Units for Binding to Au(111)

Abstract

Biphenyl-2,2'-diylbis(10-methyl-9-methyleneacridan)-type electron donor 1,w hich has two tethered cyclic disulfideu nits at the 6,6'-positions, was designeda nd synthesized as the first member of ad ynamic redox (dyrex) system that can form molecular layerso naA u(111)e lectrode. Upon the two-electron (2 e) oxidation of 1,t he persistentd icationic dye 2 2 + was generated with the formationofanew CÀCbond, whichisr eversibly cleaved upon 2e reduction to regenerate 1 (dyrex behavior). Similar dyrex interconversion occurs in the molecular layer of 1 on gold. The chemical identities of 1/Au and electrochemically generated 2 2 + /Au wereu nambiguously determined by in situ IR spectroscopy in the attenuated totalr eflection mode. In situ scanning tunneling microscopy (STM) was conducted under electrochemical conditions to examine the surfaces tructure of 1 adsorbed on aA u(111)e lectrode. Although no longrange-ordered morphology was found in the STM image of 1, an in situ STM study of the potential-induced dyrex reaction of 1 to 2 2 + showed that the grained spots in the image became slightly brighter.There has been growing interesti nt he use of molecular layers (e.g.,s elf-assembled monolayers)o na ne lectrode surface with respectt ot heir potential application in molecular devices such as switches, data storage devices, and sensors. [2] An electrochemically bistable redox speciesb ound to as urface would be advantageous for the realization of am olecular-based data storagee lement( unimolecular memory), [2] since mutuale lectron exchange between neighboring molecules with different redox states would be suppressed. Unfortunately,t his situation is difficult to attain based on conventional organic redox systems, because the redox potential in the neutral state is identical to that in the reverse process for the corresponding ionic state.Some mechanicallyi nterlocked molecules (MIMs) exhibit bistability through rearrangement of the subunits depending on the redox state. [3] In addition to MIMs, dynamic redox (dyrex) systems [2a, 4] have been shown to exhibit electrochemical bistability as well as facile two-electron( 2e)t ransfer at nearly the same potential, whichr esults from drastic structural changes as well as formation/cleavage of ac ovalentb ond upon redox reactions. Thus, dyrex systems with af unctional group for binding to as urface( e.g.,d isulfide on Au) are promising candidates for the formation of molecular layersa saprototypeo fu nimolecular memory. [2a, 5] The 1,2-dithiolane ring of a-lipoic acid is often used as as urface-modifying pendant, as demonstrated by severalM IM derivatives containing at etrathiafulvalene unit. [6] Therefore, we designed ad yrex donor to which wasa ttached two units of lipoate at the 2,2'-positions of ab iphenyl (BP) framework that can be converted into ad ihydrophenanthrene (DHP) derivative through CÀCb ond formationu pon2et ransfer.B P-DHP-based dyrex systems can be categorized into two types: [4c] an exotype pair (EX-EX 2 + )a nd an endo-type pair (EN-EN 2 + ) (Sch...