Aggregation‐Induced Improvement of Catalytic Activity by Inner‐Aggregate Electronic Communication of Metal‐Fullerene‐Based Surfactants

Abstract

A paradigm for active constituents in (homogeneous) catalysis is that optimum performance requires maximum dispersion. Generally, aggregation results in a decline. This is a different case in supramolecular catalysis. A new concept based on surfactants equipped with functional heads is presented, which becomes a more active catalyst itself upon aggregation. The head group of the surfactants is composed of a diethylenetriamine-functionalized fullerene capable of coordinating to catalytically active metals like Co II . The improvement of catalytic properties upon aggregation is demonstrated via electrocatalytic water-splitting reaction as a model system. Detailed electrochemistry studies were performed at concentrations below and above the critical aggregation concentration (cac). While isolated surfactant molecules represent only moderately active catalysts, drastic improvement of efficiency in the hydrogen evolution (HER) as well as in the oxygen evolution reactions (OER) were detected, once vesicular structures have formed. Self-organization of the surfactants leads to an increase in turnover frequencies of up to 1300 % (HER). The strongly beneficial effect of aggregation arises from the favorable alignment of individual molecules, thus, facilitating intermolecular charge transfer processes in the vesicles.In many cases, aggregation is seen as a major drawback for catalytic reactions since upon aggregation the number of accessible reactive centers is reduced and a maximum of dispersion, especially for heterogeneous systems was the goal to achieve. Research perspective changed and systems have been developed in which aggregation comes hand in hand with beneficial effects. The field of self-assembled compounds, the supramolecular chemistry gives rise to supramolecular catalysis. In micellar catalysis, the formation of aggregates is used to perform reactions at their interface with compounds that would otherwise not be combinable. It is also possible to use solvents that would normally not be suitable for such reaction. These aggregates can be formed by supporting compounds that do not take place in the reaction themselves or by the catalyst molecule. [1][2][3] The phrase supramolecular catalysis is manly known from reactions with enzymatic systems which can catalyze reaction via weak interactions within their structural motif. Furthermore, this field describes the assembly of small-molecule organic catalysts. [4][5] This very assembly of the catalyst influences the reactants in a way that reactive moieties are exposed and activated, intermediates or transition states are stabilized or that the reactivity is enhanced by increasing the local concentration of the reactants. [6][7][8] In this work, we present a new concept of supramolecular catalysis that does not influence the reactant's properties but the catalyst itself. This is achieved by a catalytically active surfactant. Surfactants are molecular compounds with amphiphilic properties, which form well-defined aggregates. Depending on concentration, individual su...

Micellar (Photo-)Catalysis driven by IR-Active Semiconductor Surfactants

“…As we have seen in previous publications, [13,[17][18] the amphiphilic behavior of fullerenol-based surfactants is distinct from classical surfactants. Fig.…”

“…[16] FuDy-4,5 were characterized by standard techniques which have also been used for other fullerenol-surfactants in the past (see the experimental part given in the Supporting Information). [13][14]17] As will be shown later, FuDy-4 containing amino-stilbene is the most important compound of the current study. Therefore, its amphiphilic properties will be described in more detail.…”

Boosting Photocatalytic Metal-Free Hydrogen Production of Viologen-Based Covalent Organic Frameworks

“…Catalyst morphology prior to photocatalytic process reveals the presence of sponge-like aggregates of different sizes (Figure a). It is known that aggregate formation generally has a negative impact on photocatalytic processes . However, here it is believed that mesopores on the aggregates are effective during the photocatalytic process.…”

“…It is known that aggregate formation generally has a negative impact on photocatalytic processes. 34 However, here it is believed that mesopores on the aggregates are effective during the photocatalytic process. Furthermore, it is thought that large aggregates can allow electrons to become more delocalized, potentially enabling more efficient charge transfer and participation in redox reactions.…”

Photoactive surfactant semiconductors characterized by a dissociative identity disorder integrated into the membranes of living cells as trojan horses for on-demand and spatial regulation of oxidative stress.

“…The basis for the work presented herein is our previous results on a novel class of surfactants containing a fullerenol head group. (19)(20)(21)(22) This (hydrophilic) head is C60 which is hydroxylated only on one side in a Janus-type fashion with ≈ 20 x OH groups. Precisely five organic residues can be attached to the other hemisphere of the fullerene as the hydrophobic entity.…”