Perspektiven der Chemie - Stufen zur komplexen Materie

Triphenylamine[3]arenes: Streamlining Synthesis of a Versatile Macrocyclic Platform for Supramolecular Architectures and Functionalities

Triphenylamine[3]arenes (TPA[3]s), featuring [16]paracyclophane backbone with alternating carbon and nitrogen bridging atoms, were synthesized through a BF3 ⋅ Et2O-catalyzed cyclization reaction using triphenylamine derivatized monomers and paraformaldehyde. This molecular design yielded a series of TPA[3] macrocycles with high efficiency, with their facile derivatizations also successfully demonstrated. On account of the strong electron-donating properties of the TPA moieties, these TPA[3]s exhibit remarkable delayed fluorescence, and possess a significant affinity for iodine. Furthermore, their inherent three-fold symmetry rendered TPA[3]s as novel building blocks for the construction of extended frameworks and molecular cages. This advancement expands the versatility of discrete macrocycles into complex architectures, enhancing their applicability across a broad spectrum of applications.

Molecular Ratchets and Kinetic Asymmetry: Giving Chemistry Direction

Over the last two decades ratchet mechanisms have transformed the understanding and design of stochastic molecular systems-biological, chemical and physical-in a move away from the mechanical macroscopic analogies that dominated thinking regarding molecular dynamics in the 1990s and early 2000s (e.g. pistons, springs, etc), to the more scale-relevant concepts that underpin out-of-equilibrium research in the molecular sciences today. Ratcheting has established molecular nanotechnology as a research frontier for energy transduction and metabolism, and has enabled the reverse engineering of biomolecular machinery, delivering insights into how molecules 'walk' and track-based synthesisers operate, how the acceleration of chemical reactions enables energy to be transduced by catalysts (both motor proteins and synthetic catalysts), and how dynamic systems can be driven away from equilibrium through catalysis. The recognition of molecular ratchet mechanisms in biology, and their invention in synthetic systems, is proving significant in areas as diverse as supramolecular chemistry, systems chemistry, dynamic covalent chemistry, DNA nanotechnology, polymer and materials science, molecular biology, heterogeneous catalysis, endergonic synthesis, the origin of life, and many other branches of chemical science. Put simply, ratchet mechanisms give chemistry direction. Kinetic asymmetry, the key feature of ratcheting, is the dynamic counterpart of structural asymmetry (i.e. chirality). Given the ubiquity of ratchet mechanisms in endergonic chemical processes in biology, and their significance for behaviour and function from systems to synthesis, it is surely just as fundamentally important. This Review charts the recognition, invention and development of molecular ratchets, focussing particularly on the role for which they were originally envisaged in chemistry, as design elements for molecular machinery. Different kinetically asymmetric systems are compared, and the consequences of their dynamic behaviour discussed. These archetypal examples demonstrate how chemical systems can be driven inexorably away from equilibrium, rather than relax towards it.

Supramolecular Shish Kebabs: Higher Order Dimeric Structures from Ring-in-Rings Complexes with Conformational Adaptivity

Use of abiotic chemical systems for understanding higher order superstructures is challenging. Here we report a ring-in-ring(s) system comprising a hydrogen-bonded macrocycle and cyclobis(paraquat-o-phenylene) tetracation (o-Box) or cyclobis(paraquat-p-phenylene) tetracation (CBPQT⁴⁺ , p-Box) that assembles to construct discrete higher order structures with adaptive conformation. As indicated by mass spectrometry, computational modeling, NMR spectroscopy, and single-crystal X-ray diffraction analysis, this ring-in-ring(s) system features the box-directed aggregation of multiple macrocycles, leading to generation of several stable species such as H4G (1 a/o-Box) and H5G (1 a/o-Box). Remarkably, a dimeric shish-kebab-like ring-in-rings superstructure H7G2 (1 a/o-Box) or H8G2 (1 a/p-Box) is formed from the coaxial stacking of two ring-in-rings units. The formation of such unique dimeric superstructures is attributed to the large π-surface of this 2D planar macrocycle and the conformational variation of both host and guest.

Macromolecular Information Transfer

Macromolecular information transfer can be defined as the process by which a coded monomer sequence is communicated from one macromolecule to another. In such a transfer process, the information sequence can be kept identical, transformed into a complementary sequence or even translated into a different molecular language. Such mechanisms are crucial in biology and take place in DNA→DNA replication, DNA→RNA transcription and RNA→protein translation. In fact, there would be no life on Earth without macromolecular information transfer. Mimicking such processes with synthetic macromolecules would also be of major scientific relevance because it would open up new avenues for technological applications (e.g. data storage and processing) but also for the creation of artificial life. In this important context, this minireview summarizes recent research about information transfer in synthetic oligomers and polymers. Medium- and long-term perspectives are also discussed.

Dual-Readout of the Mechanical Response of a Bis-acridinium [2]Rotaxane

A [2]rotaxane built around a multi-responsive bis-acridinium macrocycle has been synthesized. Structural investigation has confirmed the interlocked nature of the molecule, and MD simulations illuminated its conformational dynamics with atomic resolution. Both halochromic and redox-switching properties were explored to shed light on the mechanical response and electronic changes that occur in the bis-acridinium [2]rotaxane. The topology of the rotaxane led to different mechanical behaviors upon addition of hydroxide ions or reduction that were easily detected by UV/Vis spectroscopy and electrochemistry.

On the Supra-LUMO Interaction: Case Study of a Sudden Change of Electronic Structure as a Functional Emergence

The synergistic functioning of redox-active components that emerges from prototypical 2,2'-di(N-methylpyrid-4-ylium)-1,1'-biphenyl is described. Interestingly, even if a trans conformation of the native assembly is expected, due to electrostatic repulsion between cationic pyridinium units, we demonstrate that cis conformation is equally energy-stabilized on account of a peculiar LUMO (SupLUMO) that develops through space, encompassing the two pyridiniums in a single, made-in-one-piece, electronic entity (superelectrophoric behavior). This SupLUMO emergence, with the cis species as superelectrophore embodiment, originates in a sudden change of electronic structure. This finding is substantiated by insights from solid state (single-crystal X-ray diffraction) and solution (NOE NMR and UV-vis-NIR spectroelectrochemistry) studies, combined with electronic structure computations. Electrochemistry shows that electron transfers are so strongly correlated that two-electron reduction manifests itself as a single-step process with a large potential inversion consistent with inner creation of a carbon-carbon bond (digital simulation). Besides, absence of reductive formation of dimers is a further indication of a preferential intramolecular reactivity determined by the SupLUMO interaction (cis isomer pre-organization). The redox-gated covalent bond, serving as electron reservoir, was studied via atropisomerism of the reduction product (VT NMR study). The overall picture derived from this in-depth study of 2,2'-di(N-methylpyrid-4-ylium)-1,1'-biphenyl proves that trans and cis species are worth considered as intrinsically sharply different, that is, as doubly-electrophoric and singly-superelectrophoric switchable assemblies, beyond conformational isomerism. Most importantly, the through-space-mediated SupLUMO may come in complement of other weak interactions encountered in Supramolecular Chemistry as a tool for the design of electroactive architectures.

Beyond the Molecule: Intermolecular Forces from Gas Liquefaction to X-H⋅⋅⋅π Hydrogen Bonds

Interest toward molecule-molecule interactions developed during the first half of the 19th century with studies on gas liquefaction. In 1869, Andrews carried out the first accurate study on the effects of temperature and pressure on the behaviour of real gases, and in 1873, van der Waals formulated an equation capable of accounting for critical phenomena of each individual gas The nature of the intermolecular forces responsible for aggregation of gases was investigated in the early 20th century by Keesom and Debye (electrostatic interactions between dipoles) and by London (interaction between instantaneous dipoles, studied by quantum mechanics). The hydrogen bond theory, a particular case of dipolar interaction, originated in the 1920s in Berkeley in the institute directed by G. N. Lewis. Later it was made clear that hydrogen bonding is responsible for the aggregation of most biological systems, from proteins to nucleic acids. This Perspective describes the most significant steps through which the science of intermolecular interactions has progressed over the last two centuries, revisiting classical experiments and theoretical formulations, which led to the complex state of art of today.

Coordination-Driven Selective Formation of D2 Symmetric Octanuclear Organometallic Cages

The coordination-driven self-assembly of organometallic half-sandwich iridium(III)- and rhodium(III)-based building blocks with asymmetric ambidentate pyridyl-carboxylate ligands is described. Despite the potential for obtaining a statistical mixture of multiple products, D₂ symmetric octanuclear cages were formed selectively by taking advantage of the electronic effects emanating from the two types of chelating sites - (O,O') and (N,N') - on the tetranuclear building blocks. The metal sources and the lengths of bridging ligands influence the selectivity of the self-assembly. Experimental observations, supported by computational studies, suggest that the D₂ symmetric cages are the thermodynamically favored products. Overall, the results underline the importance of electronic effects on the selectivity of coordination-driven self-assembly, and demonstrate that asymmetric ambidentate ligands can be used to control the design of discrete supramolecular coordination complexes.

A Switchable Catalyst Duo for Acyl Transfer Proximity Catalysis and Regulation of Substrate Selectivity

Enzymes are encoded with a gamut of information to catalyze a highly selective transformation by selecting the proper reactants from an intricate mixture of constituents. Mimicking biological machinery, two switchable catalysts with differently sized cavities and allosteric control are conceived that allow complementary size-selective acyl transfer in an on/off manner by modulating the effective local concentration of the substrates. Selective activation of one of two catalysts in a mixture of reactants of similar reactivity enabled upregulation of the desired product.

Cell-Selective siRNA Delivery Using Glycosylated Dynamic Covalent Polymers Self-Assembled In Situ by RNA Templating

Dynamic covalent libraries enable exploring complex chemical systems from which bioactive assemblies can adaptively emerge through template effects. In this work, we studied dynamic covalent libraries made of complementary bifunctional cationic peptides, yielding a diversity of species from macrocycles to polymers. Although polymers are typically expressed only at high concentration, we found that siRNA acts as a template in the formation of dynamic covalent polymers at low concentration in a process guided by electrostatic binding. Using a glycosylated building block, we were able to show that this templated polymerization further translates into the multivalent presentation of carbohydrate ligands, which subsequently promotes cell uptake and even cell-selective siRNA delivery.

Self-Assembly of Pseudo[1]rotaxanes by Palladium(II)/Platinum(II)-Directed Integrative Social Self-Sorting: Is the Metal Required?

New results are presented on the multicomponent supramolecular synthesis of pseudo[1]rotaxanes, achieved by designing pairs of structurally matching N-monoalkyl-4,4'-bipyridinium/2,7-diazapyrenium-based ligands having complementary π-donor/acceptor features, and intended to self-assemble into the targeted supramolecules by following integrative self-sorting processes. In all the studied cases, it was found that the envisioned species, characterized by NMR spectroscopy and MS spectrometry, arise as the main products of the self-assembly in aqueous media by using palladium(II)/platinum(II) metal centers as the guiding force. Crucially, we have also found that by improving the π-donor/acceptor properties of the matching pairs of ligands (L₄ and L₅ ), the integrative self-sorting processes prevail even in the absence of metallic ions to afford the heterodimeric species with an association constant being 756±43 M^-1 .

In Situ Mass Spectrometric and Kinetic Investigations of Soai's Asymmetric Autocatalysis

Chemical reactions that lead to a spontaneous symmetry breaking or amplification of the enantiomeric excess are of fundamental interest in explaining the formation of a homochiral world. An outstanding example is Soai's asymmetric autocatalysis, in which small enantiomeric excesses of the added product alcohol are amplified in the reaction of diisopropylzinc and pyrimidine-5-carbaldehydes. The exact mechanism is still in dispute due to complex reaction equilibria and elusive intermediates. In situ high-resolution mass spectrometric measurements, detailed kinetic analyses and doping with in situ reacting reaction mixtures show the transient formation of hemiacetal complexes, which can establish an autocatalytic cycle. We propose a mechanism that explains the autocatalytic amplification involving these hemiacetal complexes. Comprehensive kinetic experiments and modelling of the hemiacetal formation and the Soai reaction allow the precise prediction of the reaction progress, the enantiomeric excess as well as the enantiomeric excess dependent time shift in the induction period. Experimental structural data give insights into the privileged properties of the pyrimidyl units and the formation of diastereomeric structures leading to an efficient amplification of even minimal enantiomeric excesses, respectively.

Precise Control Over Kinetics of Molecular Assembly: Production of Particles with Tunable Sizes and Crystalline Forms

It has been long-pursued but remains a challenge to precisely manipulate the molecular assembly process to obtain desired functional structures. Reported here is the control over the assembly of solute molecules, by a programmed recrystallization of solvent crystal grains, to form micro/nanoparticles with tunable sizes and crystalline forms. A quantitative correlation between the protocol of recrystallization temperature and the assembly kinetics results in precise control over the size of assembled particles, ranging from single-atom catalysts, pure drug nanoparticles, to sub-millimeter organic-semiconductor single crystals. The extensive regulation of the assembly rates leads to the unique and powerful capability of tuning the stacking of molecules, involving the formation of single crystals of notoriously crystallization-resistant molecules and amorphous structures of molecules with a very high propensity to crystallize, which endows it with wide-ranging applications.

Triple Self-Sorting in Constitutional Dynamic Networks: Parallel Generation of Imine-Based CuI , FeII , and ZnII Complexes

Three imine-based metal complexes, having no overlap in terms of their compositions, have been simultaneously generated from the self-sorting of a constitutional dynamic library (CDL) containing three amines, three aldehydes, and three metal salts. The hierarchical ordering of the stability of the three metal complexes assembled and the leveraging of the antagonistic and agonistic relationships existing between the constituents within the constitutional dynamic network corresponding to the CDL were pivotal in achieving the sorting. Examination of the process by NMR spectroscopy showed that the self-sorting of the FeII and ZnII complexes depended on an interplay between the thermodynamic driving forces and a kinetic trap involved in their assembly. These results also exemplify the concept of "simplexity"-the fact that the output of a self-assembling system may be simplified by increasing its initial compositional complexity-as the two complexes could self-sort only in the presence of the third pair of organic components, those of the CuI complex.

Molecular Springs: Integration of Complex Dynamic Architectures into Functional Devices

Molecular/supramolecular springs are artificial nanoscale objects possessing well-defined structures and tunable physicochemical properties. Like a macroscopic spring, supramolecular springs are capable of switching their nanoscale conformation as a response to external stimuli by undergoing mechanical spring-like motions. This dynamic action offers intriguing opportunities for engineering molecular nanomachines by translating the stimuli-responsive nanoscopic motions into macroscopic work. These nanoscopic objects are reversible dynamic multifunctional architectures which can express a variety of novel properties and behave as adaptive nanoscopic systems. In this Minireview, we focus on the design and structure-property relationships of supramolecular springs and their (self-)assembly as a prerequisite towards the generation of novel dynamic materials featuring controlled movements to be readily integrated into macroscopic devices for applications in sensing, robotics, and the internet of things.

Anion-Coordination-Assisted Assembly of Supramolecular Charge-Transfer Complexes Based on Tris(urea) Ligands

Charge-transfer (CT) complexes, formed by noncovalent bonding between electron-rich (donor, D) and electron-deficient (acceptor, A) molecules (or moieties) have attracted considerable attention due to their fascinating structures and potential applications. Herein, we demonstrate that anion coordination is a promising strategy to promote CT complex formation between anion-binding, electron-rich tris(urea) donor ligands (D) and electron-deficient viologen cation acceptors (A), which form co-crystals featuring infinite ⋅⋅⋅DADA⋅⋅⋅ or discrete (circular DADA or three-decker DAD) π-stacking interactions. These CT complexes were studied by X-ray diffraction, UV/Vis spectroscopy, electric conductivity measurements, charge displacement curve (CDC) calculations, and DFT computations.

Hydrogen Bonding as a Supramolecular Tool for Robust OFET Devices

In the present study, we demonstrated the effect of hydrogen bonding in the semiconducting behaviour of a small molecule used in organic field-effect transistors (OFETs). For this study, the highly soluble dumbbell-shaped molecule, Boc-TATDPP based on a Boc-protected thiophene-diketopyrrolopyrrole (DPP) and triazatruxene (TAT) moieties was used. The two Boc groups of the molecule were removed by annealing at 200 °C, which created a strong hydrogen-bonded network of NH-TATDPP supported by additional π-π stacking. These were characterised by thermogravimetric analysis (TGA), UV/Vis and IR spectroscopy, XRD and high-resolution (HR)-TEM measurements. FETs were fabricated with the semiconducting channel made of Boc-TATDPP and NH-TATDPP separately. It is worth mentioning that the Boc-TATDPP film can be cast from solution and then annealed to get the other systems with NH-TATDPP. More importantly, NH-TATDPP showed significantly higher hole mobilities compared to Boc-TATDPP. Interestingly, the high hole mobility in the case of NH-TATDPP was unaffected upon blending with [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁ BM). Thus, this robust hydrogen-bonded supramolecular network is likely to be useful in designing efficient and stable organic optoelectronic devices.

Supramolecular Organization in Confined Nanospaces

Empty spaces are abhorred by nature, which immediately rushes in to fill the void. Humans have learnt pretty well how to make ordered empty nanocontainers, and to get useful products out of them. When such an order is imparted to molecules, new properties may appear, often yielding advanced applications. This review illustrates how the organized void space inherently present in various materials: zeolites, clathrates, mesoporous silica/organosilica, and metal organic frameworks (MOF), for example, can be exploited to create confined, organized, and self-assembled supramolecular structures of low dimensionality. Features of the confining matrices relevant to organization are presented with special focus on molecular-level aspects. Selected examples of confined supramolecular assemblies - from small molecules to quantum dots or luminescent species - are aimed to show the complexity and potential of this approach. Natural confinement (minerals) and hyperconfinement (high pressure) provide further opportunities to understand and master the atomistic-level interactions governing supramolecular organization under nanospace restrictions.

Integrative Self-Sorting: One-Pot Synthesis of a Hetero[4]rotaxane from a Daisy-Chain-Containing Hetero[4]pseudorotaxane

The structural complexity of mechanically interlocked molecules are very attractive to chemists owing to the challenges they present. In this article, novel mechanically interlocked molecules with a daisy-chain-containing hetero[4]rotaxane motif were efficiently synthesized. In addition, a novel integrative self-sorting strategy is demonstrated, involving an ABB-type (A for host, dibenzo-24-crown-8 (DB24C8), and B for guest, ammonium salt sites) monomer and a macrocycle host, benzo-21-crown-7 (B21C7), in which the assembled species in hydrogen-bonding-supported solvent only includes a novel daisy-chain-containing hetero[4]pseudorotaxane. The found self-sorting process involves the integrative recognition between B21C7 macrocycles and carefully designed components simultaneously containing two types of secondary ammonium ions and a host molecule, DB24C8 crown ether. The self-sorting strategy is integrative to undertake self-recognition behavior to form one single species of pseudorotaxane compared with the previous report. This self-sorting system can be used for the efficient one-pot synthesis of a daisy-chain-containing hetero[4]rotaxane in a good yield. The structure of hetero[4]rotaxane was confirmed by ¹ H NMR spectroscopy and high-resolution electrospray ionization (HR-ESI) mass spectrometry.

Quantifying Guest Exchange in Supramolecular Systems

The ability to accurately determine and quantitatively evaluate kinetic phenomena associated with supramolecular assemblies, in real time, is key to a better understanding of their defined architectures and diverse functionalities. Therefore, analytical tools that can precisely assess a wide range of exchange rates within such systems are of considerable importance. This study demonstrates the ability to use an NMR approach based on saturation transfer for the determination of rates of guest exchange from molecular capsules. By using cavitands that assemble into distinct dimeric assemblies, we show that this approach, which we term guest exchange saturation transfer (GEST), allows the use of a conventional NMR setup to study and quantitatively assess a wide range of exchange rates, from 35 to more than 5000 s^-1 .