Immobilization of molecular catalysts on electrode surfaces using host–guest interactions

Abstract

The strategy of anchoring molecular catalysts on electrode surfaces combines the high selectivity and activity of molecular systems with the practicality of heterogeneous systems. The stability of molecular catalysts is, however, far less than that of traditional heterogeneous electrocatalysts, and therefore a method to easily replace anchored molecular catalysts that have degraded could make such electrosynthetic systems more attractive. Here, we apply a non-covalent "click" chemistry approach to reversibly bind molecular electrocatalysts to electrode surfaces via host-guest complexation with surface-anchored cyclodextrins. The host-guest interaction is remarkably strong and allows the flow of electrons between the electrode and the guest catalyst. Electrosynthesis in both organic and aqueous media was demonstrated on metal oxide electrodes, with stability on the order of hours. The catalytic surfaces can be recycled by controlled release of the guest from the host cavities and readsorption of fresh guest. This strategy represents a new approach to practical molecular-based catalytic systems. 3Molecular electrocatalysts can exhibit surprisingly high activities and selectivities that are unmatched by most heterogeneous catalysts. 1,2 Therefore, the development of robust immobilization strategies for these molecular species on electrode surfaces is of great interest if these molecular catalytic activities and selectivities are to be transferred to more practical heterogeneous electrosynthetic systems, 3 which have already shown promising results for CO 2 reduction, water reduction and water oxidation in the context of storage of renewable energy. [4][5][6] Over the past decades, many immobilization strategies have been developed, 7 which can be categorized into covalent binding (achieved by adding anchoring groups such as carboxylate to the catalysts), 8 non-covalent binding (pi-stacking on carbon-based electrodes) 9,10 and polymerization-based binding. [11][12][13] Here, we report a new strategy for surface immobilization of molecular electrocatalysts, which relies on a non-covalent "click" chemistry approach to bind molecular species in welldefined sites on electrode surfaces. 14 The binding of electroactive molecular guests into molecular pockets by means of host-guest complex (HGC) formation has previously been studied by several groups, including those of Stoddart and Kaifer, 15 Reinhoudt 16,17 and Huskens. 18 Liu et al. reported the HGC-formation on gold surfaces with the C 60 monoanion as guest, which was found to be electrochemically stable over prolonged durations. 19 Light-induced electron transfer to and from dye molecules bound via the HGC approach was also demonstrated by Freitag and Galoppini. 20,21 The group of Sun demonstrated the use of HGC to improve electron transfer between a molecular catalyst and a dye molecule bound onto TiO 2 . 22 Among the diverse class of host molecules, cyclodextrins, cucurbiturils and calixarenes are the most studied for HGC formation on different surfaces. 23,24 For cy...

Adaptive insertion of the hydrophobic anchor into poly(ethylene glycol) host for programmable surface functionalization

“…Unlike βcyclodextrin, the addition of free PEG could not detach the loaded Py-PEG 4 -Os, which should be caused by the well hydration of PEG chains. After testing different reagents, we found that poly(sodium 4styrenesulfonate) (PSS) with hydrophobic phenyl group and hydrophilic sulfonate could 'pull' nearly all of the loaded Py-PEG 4 -Os into solution 9 . This should be attributed to the π-π interaction and electrostatic attraction between PSS and Py-PEG 4 -Os (Fig.…”

“…Thanks to the development of molecular recognition, biomolecular recognition and host-guest interaction have been used to load therapeutic proteins on polymeric particles, isolate plasma membrane proteins, and load molecular catalysts on the electrode, etc. [6][7][8][9][10] Although proved effective, the loaded functional modules can only be released by adding competitive host/guest molecules or through the photoisomerization of the guest molecule, which challenged the re-loading of functional modules, the recycling of host molecules-modi ed materials, and the synthesis of guest molecules 11,12 . Moreover, the host molecules, such as cyclodextrins, and cucurbiturils, have to be installed on the materials' surface in advance 13 .…”

Heterogenization of Molecular Water Oxidation Catalysts in Electrodes for (Photo)Electrochemical Water Oxidation

“…Electrosynthesis in both organic and aqueous media was demonstrated on metal oxide electrodes, with stability on the order of hours. Moreover, the authors showed that the catalytic surfaces could be recycled by controlled release of the guest from the host cavities and the reabsorption of fresh WOC guest by tunning the conditions [140]. This study showed the suitability of multidentate supramolecular hosts as the anchoring moieties for molecular WOC guests to generate stable and electrocatalytically active surface-bound host-guest complexes in both aqueous and organic solvents for electrocatalytic WO.…”

“…This demonstrated that hydrophobic interactions are suitable to in situ assemble and catalysts and chromophores on electrode surfaces. [72], and (d) bioinspired cyclodextrin and tpy derivative Ru and a Pt WOCs and simplified formation of the host-guest complex [140].…”

Catalysis in Single Crystalline Materials: From Discrete Molecules to Metal‐Organic Frameworks

“…Generally, the heterogeneous catalysts exist in two forms: restricted in insoluble nanoparticles [13][14][15][16] or immobilized (non)covalently onto the surfaces of different substates. [17,18] It has been proved that enormous of organic and inorganic reactions including water splitting, [19] hydrogenation of double bonds, [20,21] aryl-aryl coupling reactions [22] and even controllable combustion reactions [23] could be catalyzed through heterogeneous catalytic approaches. Although the heterogeneous catalysts shows great potential in controlling catalytic cost, however, the two-dimensional surface catalysis mechanism leads to low usage efficiencies.…”