Single-Molecule Observation of Redox Reactions Enabled by Rigid and Isolated Tripodal Molecules

Abstract

Tracking various chemical reactions, including electrochemical and photochemical reactions at the single-molecule level, is expected to yield a great deal of knowledge from both fundamental and applied aspects. In this study, we report on a methodology to track the electronic-state changes of redox reactions at the single-molecule level by using electrochemical scanning tunneling microscopy (EC-STM). EC-STM is powerful for single-molecule analysis of redox reactions, but previous studies have shown difficulties separating the structural and electronic contributions due to orientational changes during the redox reaction. Here, we visualize the electronic-state changes of a single ferrocene associated with redox reactions using EC-STM by synthesizing and fabricating a monolayer of structurally rigid tripodal molecules based on triptycene, which act as ideal anchors to preserve a constant distance between the electrode and the ferrocene moieties. This methodology paves the way for versatile single-molecule measurements of important phenomena at the solid−liquid interface, such as photochemistry and heterogeneous catalysis.

Triptycene-Based Self-Assembled Monolayer as a Template for Successive Click Reactions

“…1,8,13-Tris(S-acetylmercaptomethyl)-10ethynyltriptycene (EtTripSAc, Scheme 1) was synthesized according to the previously reported procedures. 16 All other reagents were purchased from Sigma-Aldrich. Au substrates were purchased from Georg Albert PVD-Beschichtungen (Silz, Germany).…”

“…One of such cases are triptycene-based molecules with thiol and carboxylate anchoring groups, capable to form densely packed and highly ordered SAMs on gold, silver, and oxide substrates. − These molecules feature tripodal bonding to the substrate, which is rarely the case among tripod-shaped adsorbates, − and can host either three tail groups at the tripodal para-positions of the individual phenyl blades or a single tail group at the bridgehead position of the entire scaffold. However, the synthesis of these molecules in each specific case represents a many-step procedure, so that it would be reasonable to look for complementary approaches to create specifically substituted triptycene-based molecular systems as a versatile scaffold for surface modifications.…”

“…In this context, in the present study, we demonstrate the possibility of an on-surface synthesis of substituted triptycene-based molecules, relying on a suitable basic module. As such module, we take 10-ethynyl-decorated 1,8,13-trimercaptomethyltriptycene (EtTripS; see Figure ), which can be obtained from EtTripSAc (Scheme ). The terminal ethynyl moiety of this molecule is useful for functionalization using cross-coupling reactions and can potentially participate in click reaction with azide-decorated functional groups. , Analogous reactions were demonstrated for a variety of template SAMs, mostly with azide-terminated molecules and ethynyl-decorated substituents − and also other way around, − similar to the present case.…”

“…As such module, we take 10-ethynyl-decorated 1,8,13-trimercaptomethyltriptycene (EtTripS; see Figure ), which can be obtained from EtTripSAc (Scheme ). The terminal ethynyl moiety of this molecule is useful for functionalization using cross-coupling reactions and can potentially participate in click reaction with azide-decorated functional groups. , Analogous reactions were demonstrated for a variety of template SAMs, mostly with azide-terminated molecules and ethynyl-decorated substituents − and also other way around, − similar to the present case. Here, as a representative test, we apply click reaction to the EtTripS template assembled on Au(111), a most frequently used substrate for the SAM fabrication, relevant as well as an electrical contact material in organic and molecular electronics.…”

“…In this context, in the present study, we demonstrate the possibility of an on-surface synthesis of substituted triptycenebased molecules, relying on a suitable basic module. As such module, we take 10-ethynyl-decorated 1,8,13-trimercaptomethyltriptycene (EtTripS; see Figure 1), which can be obtained from EtTripSAc 16 (Scheme 1). The terminal ethynyl moiety of this molecule is useful for functionalization using crosscoupling reactions 16 and can potentially participate in click reaction with azide-decorated functional groups.…”