Kingsuk Mahata

Reversible phase transitions in self-assembled monolayers at the liquid-solid interface: temperature-controlled opening and closing of nanopores.

We present a variable-temperature study of monolayer self-assembly at the liquid-solid interface. By means of in situ scanning tunneling microscopy (STM), reversible phase transitions from a nanoporous low-temperature phase to a more densely packed high-temperature phase are observed. The occurrence of the phase transition and the respective transition temperature were found to depend on the type of solvent and solute concentration. Estimates of the entropic cost and enthalpic gain upon monolayer self-assembly suggest that coadsorption of solvent molecules within the cavities of the nanoporous structure renders this polymorph thermodynamically stable at low temperatures. At elevated temperatures, however, desorption of these relatively weakly bound solvent molecules destabilizes the nanoporous polymorph, and the densely packed polymorph becomes thermodynamically favored. Interestingly, the structural phase transition provides external control over the monolayer morphology and, for the system under discussion, results in an effective opening and closing of supramolecular nanopores in a two-dimensional molecular monolayer.

From 2-Fold Completive to Integrative Self-Sorting: A Five-Component Supramolecular Trapezoid

The amalgamation of two incomplete self-sorting processes into a process that makes quantitative use of all members of the library is described by 2-fold completive self-sorting. Toward this goal, individual metal-ligand binding scenarios were optimized for high thermodynamic stability and best selectivity, by screening a variety of factors, such as steric and electronic effects, pi-pi interactions, and metal-ion specifics. Using optimized, heteroleptic metal-ligand binding motifs, a library of four different ligands (1, 2, 3, 4) and two different metal ions (Zn(2+), Cu(+)) was set up to assess 2-fold completive self-sorting. Out of 20 different combinations, the self-sorting library ended up with only two metal-ligand complexes in basically quantitative yield. To demonstrate the value of 2-fold completive self-sorting for the formation of nanostructures, the optimized, highly selective binding motifs were implemented into three polyfunctional ligands. Their integrative self-sorting in the presence of Zn(2+) and Cu(+) led to the clean formation of the supramolecular trapezoid T, a simple but still unknown supramolecular architecture. The dynamic trapezoid T consists of three different ligands with four different donor-acceptor interactions. Its structure was established by (1)H NMR spectroscopy, electrospray ionization mass spectroscopy, and differential pulse voltammetry (DPV) and by exclusion of alternative structures.

From an Eight-Component Self-Sorting Algorithm to a Trisheterometallic Scalene Triangle

Using motifs from 3-fold completive self-sorting in an eight-component library, we report on the design and fabrication of a fully dynamic trisheterometallic scalene triangle, a demanding supramolecular structure that complements the so far known triangular structures.

Diversity and complexity through reversible multiple orthogonal interactions in multicomponent assemblies.

“Life” in its complexity is not possible without selforganization processes that themselves rely on weak interactions, such as hydrogen bonding, salt bridges, ion–dipole interactions, and metal–ligand coordination, but also on reversible covalent bonds. Indeed, in countless fascinating instances Nature avails itself in an ingenious and efficient manner of the noncovalent interplay of covalently prefabricated building blocks to realize diverse complex biological functions. An outstanding example is the ribosome, a multiprotein and multi-RNA complex conceived to translate the mRNA into proteins. To achieve a similarly high emergence of complex functionality from artificial chemical systems through the interplay of various substructures, one will have to make decisive progress in the development of the thermodynamically controlled synthesis of equilibrating multicomponent assemblies. Unfortunately, so far there are no quantifiable systematics to describe the complexity level of supramolecular systems in regard to their structural and functional diversity and possibly their associated emergence. However, we may approximate the level of complexity of a structure by the number of its possible permutations based on the number of different components and interactions as well as on the total number of pieces. Such analysis swiftly indicates that the realization of spectacular supramolecular aggregates in the last couple of years, such as Stang*s dodecahedron from 50 pieces or Fujita*s palladium sphere do not necessarily reflect a mature state of the area. Nearly all systems, even if they are based on multiple components such as Drain*s nonakis-porphyrin, so far rely solely on the utilization of just one kind of reversible interaction. High complexity in current supramolecular and equilibrating systems is thus mainly the result of a very large number of reversible interactions of the same kind. A quintessence of the above analysis is that our tool kit for the construction of multicomponent assemblies needs further development and refinement of reversible orthogonal interactions. Particular attention should be devoted to assure compatibility of all interactions. Analogous to the situation for dynamic combinatorial libraries (DCL), it is imperative to realize that thermodynamic control implies a minimization of the energy Gtotal of the total system. At this juncture, all possible aggregates are linked together in a complex network and influence the total energy in an intricate manner. Thus, it is desirable, aside from the development of novel multicomponent systems, to understand their dynamics and thermodynamics as a function of all determining factors. Actually, those systems appear to be most attractive that use orthogonal interactions for new structural motifs, and not for conceptionally simpler host–guest systems or pending molecular groups, except when this entails the introduction of novel functions. Some examples, though numerically limited, already describe supramolecular structures that are built on more than two components (n 3) as well as on two different weak interactions and reversible bonds. Provisionally, it should suffice to categorize these by the operating interactions: two different metal–ligand interactions, metal–ligand coordination + reversible covalent bond, metal–ligand coordination + ion–dipole interaction, metal–ligand coordination + hydrogen bonding, metal–ligand coordination + p–p stacking, reversible covalent bond + ion–dipole interaction, and two different reversible covalent bonds. Supramolecular multicomponent (n 4) self-assemblies, built on more than two different reversible interactions, are still remarkable exceptions, in particular if they form in a quantitative manner. Every additional orthogonal interaction will furthermore open up a multitude of new chances. Using three orthogonal metal–ligand interactions, we realized the quantitative construction of a discrete porphyrin stack (Scheme 1), reminiscent of a superstructure, independent of the sequence of addition of the four components. Most impressive is, however, the work by Stoddart et al. on a Solomon*s knot (Scheme 2), because the extraordinary [*] Prof. Dr. M. Schmittel, K. Mahata Organische Chemie I and Center for Microand Nanochemistry and Engineering Universit/t Siegen Adolf-Reichwein-Strasse 2, 57068 Siegen (Germany) Fax: (+49)271-740-3270 E-mail: schmittel@chemie.uni-siegen.de

Multicomponent Assembly of Heterometallic Isosceles Triangles

The multicomponent synthesis and solution-state characterization of three supramolecular bis-heterometallic isosceles triangles are elaborated. The triangular assemblies are isosceles both geometrically and chemically; they comprise multiple ligands, metals, and binding motifs. Variation of the length of one side of the triangle by changing the number of phenyl spacers n = 0, 1, and 2 influences the redox potential of the opposing copper(I) center, allowing translation of the nanomechanical changes into electronically readable values.

Incorporation Dynamics of Molecular Guests into Two-Dimensional Supramolecular Host Networks at the Liquid–Solid Interface

The objective of this work is to study both the dynamics and mechanisms of guest incorporation into the pores of 2D supramolecular host networks at the liquid-solid interface. This was accomplished by adding molecular guests to prefabricated self-assembled porous monolayers and the simultaneous acquisition of scanning tunneling microscopy (STM) topographs. The incorporation of the same guest molecule (coronene) into two different host networks was compared, where the pores of the networks either featured a perfect geometric match with the guest (for trimesic acid host networks) or were substantially larger than the guest species (for benzenetribenzoic acid host networks). Even the moderate temporal resolution of standard STM experiments in combination with a novel injection system was sufficient to reveal clear differences in the incorporation dynamics in the two different host networks. Further experiments were aimed at identifying a possible solvent influence. The interpretation of the results is aided by molecular mechanics (MM) and molecular dynamics (MD) simulations.

Impact of the level of complexity in self-sorting: Fabrication of a supramolecular scalene triangle

Summary The impact of the level of complexity in self-sorting was elaborated through the fabrication of various scalene triangles. It turned out that the self-sorting system with a higher level of complexity was far superior to less complex sorting algorithms.