Johannes A. A. W. Elemans

Molecular and Supramolecular Networks on Surfaces: From Two-Dimensional Crystal Engineering to Reactivity

The invention of the scanning tunneling microscope has led to the visualization of molecules in real space on atomically flat conductive substrates. This has boosted research into supramolecular chemistry on surfaces. In this Review, we highlight recent developments in the design and functionality of supramolecular surface patterns, with special attention paid to those networks which are chiral or contain a high degree of porosity as well as to the reactivity, which is one of the most important recent developments in supramolecular surface chemistry.

Mastering molecular matter. Supramolecular architectures by hierarchical self-assembly

Since the serendipitous event that led to the first synthesis of a molecule by the hands of Man in 1826, the creation of molecular matter depended for 150 years on linking together molecules from other molecular building blocks with the help of strong covalent bonds. The advent of supramolecular chemistry in the last decades of the 20th century has provided chemists with a wealth of new possibilities to synthesize molecular structures and materials that are held together by relatively weak, non-covalent interactions, such as hydrogen bonding, π–π stacking, electrostatic and van der Waals interactions. Using nature as a source of inspiration, the creation of even more complex supramolecular architectures has recently become possible by applying the concept of hierarchical self-assembly, i.e. the non-covalent organization of molecules and macromolecules which takes places over distinct multiple levels, in which the assembly processes gradually decrease in strength. This review will focus on some recent discoveries in the field of spontaneous hierarchical organization of synthetic amphiphiles, disk-like molecules and concave building blocks into well-defined nano-sized assemblies.

Macroscopic Hierarchical Surface Patterning of Porphyrin Trimers via Self-Assembly and Dewetting

The use of bottom-up approaches to construct patterned surfaces for technological applications is appealing, but to date is applicable to only relatively small areas (∼10 square micrometers). We constructed highly periodic patterns at macroscopic length scales, in the range of square millimeters, by combining self-assembly of disk-like porphyrin dyes with physical dewetting phenomena. The patterns consisted of equidistant 5-nanometer-wide lines spaced 0.5 to 1 micrometers apart, forming single porphyrin stacks containing millions of molecules, and were formed spontaneously upon drop-casting a solution of the molecules onto a mica surface. On glass, thicker lines are formed, which can be used to align liquid crystals in large domains of square millimeter size.

Real-time single-molecule imaging of oxidation catalysis at a liquid-solid interface

Many chemical reactions are catalysed by metal complexes, and insight into their mechanisms is essential for the design of future catalysts. A variety of conventional spectroscopic techniques are available for the study of reaction mechanisms at the ensemble level, and, only recently, fluorescence microscopy techniques have been applied to monitor single chemical reactions carried out on crystal faces and by enzymes. With scanning tunnelling microscopy (STM) it has become possible to obtain, during chemical reactions, spatial information at the atomic level. The majority of these STM studies have been carried out under ultrahigh vacuum, far removed from conditions encountered in laboratory processes. Here we report the single-molecule imaging of oxidation catalysis by monitoring, with STM, individual manganese porphyrin catalysts, in real time, at a liquid-solid interface. It is found that the oxygen atoms from an O2 molecule are bound to adjacent porphyrin catalysts on the surface before their incorporation into an alkene substrate.

Mechanism of threading a polymer through a macrocyclic ring.

The translocation of biopolymers through pores and channels plays a fundamental role in numerous biological processes. We describe here the mechanism of the threading of a series of polymer chains through a synthetic macrocycle, which mimics these natural processes. The threading of polymers involves a kinetically favorable “entron” effect, which is associated with the initial filling of the cavity by the end of the polymer. A preassociation between the outside of the macrocycle and the polymer induces a process in which the polymer end loops back into the cavity of the macrocycle. This looping mechanism results in accelerated threading rates and unidirectional motion and is reminiscent of the protein translocation through membrane pores.

Detection of different oxidation states of individual manganese porphyrins during their reaction with oxygen at a solid/liquid interface.

Manganese porphyrins have been extensively investigated as model systems for the natural enzyme cytochrome P450 and as synthetic oxidation catalysts. Here, we report single-molecule studies of the multistep reaction of manganese porphyrins with molecular oxygen at a solid/liquid interface, using a scanning tunnelling microscope (STM) under environmental control. The high lateral resolution of the STM, in combination with its sensitivity to subtle differences in the electronic properties of molecules, allowed the detection of at least four distinct reaction species. Real-space and real-time imaging of reaction dynamics enabled the observation of active sites, immobile on the experimental timescale. Conversions between the different species could be tuned by the composition of the atmosphere (argon, air or oxygen) and the surface bias voltage. By means of extensive comparison of the results to those obtained by analogous solution-based chemistry, we assigned the observed species to the starting compound, reaction intermediates and products.

Extremely Strong Self-Assembly of a Bimetallic Salen Complex Visualized at the Single-Molecule Level

A bis-Zn(salphen) structure shows extremely strong self-assembly both in solution as well as at the solid-liquid interface as evidenced by scanning tunneling microscopy, competitive UV-vis and fluorescence titrations, dynamic light scattering, and transmission electron microscopy. Density functional theory analysis on the Zn(2) complex rationalizes the very high stability of the self-assembled structures provoked by unusual oligomeric (Zn-O)(n) coordination motifs within the assembly. This coordination mode is strikingly different when compared with mononuclear Zn(salphen) analogues that form dimeric structures having a typical Zn(2)O(2) central unit. The high stability of the multinuclear structure therefore holds great promise for the development of stable self-assembled monolayers with potential for new opto-electronic materials.

Structure and function revealed with submolecular resolution at the liquid–solid interface

The liquid–solid interface is a unique medium to support the self-assembly of molecules into surface-confined networks. Non-covalent interactions are key in forming these two-dimensional (2D) architectures, and a deep understanding is crucial for successful 2D crystal engineering. Scanning tunnelling microscopy is the tool of choice to reveal the structure and function of these patterns with subnanometre resolution. A recent success is the formation of 2D nanoporous molecular patterns and their host–guest chemistry. However, this is not the only functionality addressed by this review. Surface-confined molecular architectures at the liquid–solid interface are also relevant in the field of molecular electronics. Furthermore, inducing and probing chemical reactivity at the single-molecule level at the liquid–solid interface might turn out to be one of the most exciting developments.

Processive enzyme mimic: kinetics and thermodynamics of the threading and sliding process

The kinetics and thermodynamics of the treading and dethreading process of polymers through the cavity of a synthetic toroidal host is investigated by studying its complexation with a series of end-functionalized polymers of different lengths containing an end group that is selectively recognized by the host. The system is designed in such a way that complexation is only observed if the host has traveled all of the way across the complete polymer. Detailed kinetic investigations using fluorescence spectroscopy have revealed that the barrier for this process is length dependent and most likely related to the stretching of the polymer. Moreover, the results indicate that our previously reported processive enzyme mimic most likely operates by randomly sliding along its macromolecular substrate.

A Bifunctional Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions in Water

Abstract Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H+/4 e− process, while oxygen can be fully reduced to water by a 4 e−/4 H+ process or partially reduced by fewer electrons to reactive oxygen species such as H2O2 and O2 −. We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.