Stimuli-responsive gold–polyoxometalate multielectron transformers

Abstract

Gold and atomically precise polyoxometalates (POMs) are widely used materials with electron reservoir redox properties. The combination of optically excited gold nanoparticles, gold surface/tip electrodes, or molecular gold(I) complexes with {V^V₆} Lindqvist-, {MnMo^VI₆} Anderson-Evans-, and {PW^VI₁₂} Keggin-type POM structures leads to interfacial multielectron transformations with far-reaching implications for electronics, photonics, biomedical diagnostics, and catalysis.

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Gold and its features

Although gold (Au) is a chemically “inert” noble metal, it is often the first choice when testing different ideas and concepts for preparing responsive materials and interfaces for application-oriented basic research. Gold is a conductor and is resistant to air and water. Gold surfaces can be easily modified by a variety of structurally exposed functionalities through, e.g., covalent (e.g., Au–SR) and coordinative (e.g., Au⋯NH₂R) interactions.¹ These characteristics allow materials scientists to efficiently process and use gold as a bottom electrode and/or as a sharp metal tip in scanning tunneling microscopy experiments, as nanoparticles with one-step chemically modified surfaces, or as metalorganic synthons in the synthesis of heterometallic ensembles under a range of conditions.

Gold in compounds has two main oxidation states (+I and +III), can be easily converted into gold(0) nanoparticles (AuNPs) of different sizes and morphologies (e.g., nanorods and nanoclusters) and, thus, exhibit different optical and electronic properties. AuNPs can be obtained by a conventional citrate reduction method or solid-state dewetting with Ar⁺ bombardment to control the oxidation state of gold.² When AuNPs are capped with electron acceptor agents (or so-called electron sponges), these engineered gold–sample interfaces can lead to plasmon-induced charge transfer triggered by an external stimulus such as light irradiation.³

Gold–polyoxometalate interfaces

Polyoxometalates (POMs) are atomically precise metal–oxygen clusters and are outstanding examples of electron sponges that exhibit element-selective redox activity in solution.⁴ Their assembly and use as anionic protecting ligands on AuNPs have been described in several studies, with particular emphasis on catalytic applications.^5–7

The latest study showed⁸ that attachment of H₃PW₁₂O₄₀ to the surface of AuNPs under UV light leads to the transfer and storage of hot electrons in these Kegging-type POM structures (Fig. 1A). Such hybrid ensembles with donor–acceptor interactions hold great potential for the development of experimental prototype devices with the underlying charge separation, recombination dynamics, and redox switching mechanisms. Their wavelength-selective photon-electron control could be used for future practical applications in osmotic energy conversion, photocatalysis, optical multiplexed bioassays, and opto-electronic transducers.

Fig. 1 (A) Optically activated AuNPs (35 nm) act as electron donors for POM acids H₃PW₁₂O₄₀ ({PW₁₂}), which behave as electron acceptors in POM-based plasmonic electron sponge membranes. (B) A single-POM junction [MnMo₆O₁₈{(OCH₂)₃CC₅H₄N}₂]³⁻ in the four-electrode configuration immersed in the ionic liquid BMIM-OTf. 1 is the POM junction. WE is a working electrode (Au substrate), CE is a counter electrode (Pt), and RE is a reference electrode (Pt). (C) Anderson-Evans-type POMs [MnMo₆O₁₈{(OCH₂)₃CNH₂}₂]³⁻ ({MnMo₆}) bis-functionalized with monovalent Au₃(carbene)₃, Au₂(carbene)₂, and Au(carbene) moieties. Color code: Au = yellow; Mo = green; Mn = brown; O = red; N = blue; C = gray and light blue. (D) STM image of a sample obtained by drop-casting of CH₂Cl₂ solution of a Lindqvist-type hexavanadate ({V₆}) bis-functionalized with monovalent Au(phosphine) moieties and coordinative S-termini on the Au(111) surface. The heterometallic dianion was synthesized from a [V₆O₁₃{(OCH₂)₃CCH₂N₃}₂]²⁻ precursor ({V₆}) via Cu(I)-catalyzed alkyne–azide cycloaddition. For (A)–(D) H atoms and counterions are omitted for clarity. Reproduced/adapted from ref. 8 (A, copyright Springer Nature), ref. 9 (B, copyright Wiley), ref. 10 (C, copyright Wiley), and ref. 14 (D, copyright RSC).

Gold–POM interfaces with multielectron transformation processes were also observed⁹ under electrochemical control by incorporating a tris(tetrabutylammonium) salt of the pyridyl-terminated Anderson-Evans-type POM [MnMo₆O₁₈{(OCH₂)₃CC₅H₄N}₂]³⁻ as a single-molecule junction in a four-electrode cell configuration (Fig. 1B). Electrochemical scanning tunneling microscopy (EC-STM) measurements were performed in the ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIM-OTf). The results showed that this redox-active POM exhibits the behavior of a three-state electrochemical transistor.

Chemical post-functionalization of the related amine-terminated POM [MnMo₆O₁₈{(OCH₂)₃CNH₂}₂]³⁻ with N-heterocyclic carbene (NHC) gold(I) complexes resulted in the formation of molecular triads,¹⁰ in which the POM acceptor is covalently sandwiched by low-valent Au₃(NHC)₃, Au₂(NHC)₂, or Au(NHC) donor moieties (Fig. 1C). The synergistic effect between gold and POM also plays a key role here. The triads showed excellent performance in the catalytic conversion of H₂O₂. Moreover, these can be viewed as molecular models of the POM-based EC-STM break junction, albeit direct comparison is indeed difficult due to the different type of gold and the different binding situations in these gold–POM heterostructures (Au⋯NH₄C₅-POM in the single-molecule junctions vs. (NHC)₃Au₃–N-POM, (NHC)₂Au₂–NH-POM, and (NHC)Au–NH₂-POM in the individual molecules).

It is known from the chemistry of low-valent metalates that their highly reduced nature can be of great importance for the activation of inert bonds and for catalytically relevant multielectron transformations.¹¹ However, data on the covalent derivatization of POM-based inorganic–organic hybrids with organogold(I) moieties and their application on metal surfaces are still scarce. This contrasts with the extensive data on intermetallic compounds consisting of negatively charged POM structures counter-balanced with mono- or polynuclear gold(I) complexes acting as cations.¹² Discrete POMs such as, e.g., [NaAu₄Pd₈O₈(AsO₄)₈]¹¹⁻ with incorporated gold(III) centers were also reported.¹³ The combination of POM electron sponges and highly reduced organometallic compounds or even AuNPs may be critical for the development of memory devices with extremely high data capacity.

It has recently been demonstrated that organogold(I) substituents introduced at the periphery of the trisalkoxo ligands of the Lindqvist-type hexavanadate precursors such as, e.g., [V₆O₁₃{(OCH₂)₃CCH₂N₃}₂]²⁻ affect the electrical properties of these four-state {V₆}POM switches in new {Au(I)–V₆–Au(I)} POMs on the Au(111) surface (Fig. 1D).¹⁴ The enhanced multielectron transformations observed at the “gold//{Au(I)–V₆–Au(I)}-POM//vacuum–Pt-tip” interface are due to the efficient Au(I)-mediated charge redistribution across molecular linkages. This resulted in more than four distinct charge states of the POM under the influence of applied bias voltages and electron tunneling through vacuum. The described stimulus-responsive behavior is significantly different from the situation when Lindqvist-type hexavanadate structures bear non-metallic ligand substituents. In that case, no effect on the resistance steps in the current–voltage profiles was observed.¹⁵

Concluding remarks

The solution-processable POMs at gold interfaces prove to be promising redox-active hybrid materials with multielectron transforming character, which should be further explored using correlative microscopies at the nanoscale and electrical contact engineering at the micron-scale. For example, electrochemical STM and scanning electrochemical cell microscopy could be used to study their electrochemical activity under industry-relevant conditions. These methods should accelerate progress in the use of responsive gold–POM heterostructures,¹⁶ particularly in (liquid electrolyte) battery and computer data storage technologies.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

The author declares no competing interests.

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (DFG) through the SPP 2262 program MemrisTec (Memristive Devices Toward Smart Technical Systems). Project number 536022773 (2DPOMristor).

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