Thermally Stable Photomechanical Molecular Hinge: Sterically Hindered Stiff-Stilbene Photoswitch Mechanically Isomerizes

Molecular photoswitches are extensively used as molecular machines because of the small structures, simple motions, and advantages of light including high spatiotemporal resolution. Applications of photoswitches depend on the mechanical responses, in other words, whether they can generate motions against mechanical forces as actuators or can be activated and controlled by mechanical forces as mechanophores. Sterically hindered stiff stilbene (HSS) is a promising photoswitch offering large hinge-like motions in the E/Z isomerization, high thermal stability of the Z isomer, which is relatively unstable compared to the E isomer, with a half-life of ca. 1000 years at room temperature, and near-quantitative two-way photoisomerization. However, its mechanical response is entirely unexplored. Here, we elucidate the mechanochemical reactivity of HSS by incorporating one Z or E isomer into the center of polymer chains, ultrasonicating the polymer solutions, and stretching the polymer films to apply elongational forces to the embedded HSS. The present study demonstrated that HSS mechanically isomerizes only in the Z to E direction and reversibly isomerizes in combination with UV light, i.e., works as a photomechanical hinge. The photomechanically inducible but thermally irreversible hinge-like motions render HSS unique and promise unconventional applications differently from existing photoswitches, mechanophores, and hinges.

Synthetic Receptors Based on Abiotic Cyclo(pseudo)peptides

Work on the use of cyclic peptides or pseudopeptides as synthetic receptors started even before the field of supramolecular chemistry was firmly established. Research initially focused on the development of synthetic ionophores and involved the use of macrocycles with a repeating sequence of subunits along the ring to facilitate the correlation between structure, conformation, and binding properties. Later, nonnatural amino acids as building blocks were also considered. With growing research in this area, cyclopeptides and related macrocycles developed into an important and structurally diverse receptor family. This review provides an overview of these developments, starting from the early years. The presented systems are classified according to characteristic structural elements present along the ring. Wherever possible, structural aspects are correlated with binding properties to illustrate how natural or nonnatural amino acids affect binding properties.

Can 2-Pyridyl-1,2,3-triazole "Click" Ligands be Used to Develop Cu(I)/Cu(II) Molecular Switches?

Molecular switching processes are important in a range of areas including the development of molecular machines. While there are numerous organic switching systems available, there are far less examples that exploit inorganic materials. The most common inorganic switching system remains the copper(I)/copper(II) switch developed by Sauvage and co-workers over 20 years ago. Herein, we examine if bidentate 2-(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine (pytri) and tridentate 2,6-bis[(4-phenyl-1H-1,2,3-triazol-1-yl)methyl]pyridine (tripy) moieties can be used to replace the more commonly exploited polypyridyl ligands 2,2'-bypyridine (bpy)/1,10-phenanthroline (phen) and 2,2';6',2″-terpyridine (terpy) in a copper(I)/(II) switching system. Two new ditopic ligands that feature bidentate (pytri, L1 or bpytri, L2) and tridentate tripy metal binding pockets were synthesized and used to generate a family of heteroleptic copper(I) and copper(II) 6,6'-dimesityl-2,2'-bipyridine (diMesbpy) complexes. Additionally, we synthesized a series of model copper(I) and copper(II) diMesbpy complexes. A combination of techniques including nuclear magnetic resonance (NMR) and UV-vis spectroscopies, high-resolution electrospray ionization mass spectrometry, and X-ray crystallography was used to examine the behavior of the compounds. It was found that L1 and L2 formed [(diMesbpy)Cu(L1 or L2)]²⁺ complexes where the copper(II) diMesbpy unit was coordinated exclusively in the tridenate tripy binding site. However, when the ligands (L1 and L2) were complexed with copper(I) diMesbpy units, a complex mixture was obtained. NMR and MS data indicated that a 1:1 stoichiometry of [Cu(diMesbpy)]⁺ and either L1 or L2 generated three complexes in solution, the dimetallic [(diMesbpy)₂Cu₂(L1 or L2)]²⁺ and the monometallic [(diMesbpy)Cu(L1 or L2)]⁺ isomers where the [Cu(diMesbpy)]⁺ unit is coordinated to either the bidentate or tridentate tripy binding sites of the ditopic ligands. The dimetallic [(diMesbpy)₂Cu₂(L1 or L2)](PF₆)₂ complexes were structurally characterized using X-ray crystallography. Both complexes feature a [Cu(diMesbpy)]⁺ coordinated to the bidentate (pytri or bpytri) pocket of the ditopic ligands (L1 or L2), as expected. They also feature a second [Cu(diMesbpy)]⁺ coordinated to the nominally tridentate tripy binding site in a four-coordinate hypodentate κ²-fashion. Competition experiments with model complexes showed that the binding strength of the bidentate pytri is similar to that of the κ²-tripy ligand, leading to the lack of selectivity. The results suggest that the pytri/tripy and bpytri/tripy ligand pairs cannot be used as replacements for the more common bpy/phen-terpy partners due to the lack of selectivity in the copper(I) state.

The Carbonyl⋅⋅⋅Tellurazole Chalcogen Bond as a Molecular Recognition Unit: From Model Studies to Supramolecular Organic Frameworks

In the last years, chalcogen bonding, the noncovalent interaction involving chalcogen centers, has emerged as interesting alternative to the ubiquitous hydrogen bonding in many research areas. Here, we could show by means of high-level quantum chemical calculations that the carbonyl⋅⋅⋅tellurazole chalcogen bond is at least as strong as conventional hydrogen bonds. Using the carbonyl⋅⋅⋅tellurazole binding motif, we were able to design complex supramolecular networks in solid phase starting from tellurazole-substituted cyclic peptides. X-ray analyses reveal that the rigid structure of the cyclic peptides is caused by hydrogen bonds, whereas the supramolecular network is held together by chalcogen bonding. The type of the supramolecular network depends on peptide used; both linear wires and a honeycomb-like supramolecular organic framework (SOF) were observed. The unique structure of the SOF shows two channels filled with different types of solvent mixtures that are either locked or freely movable.

Photo-controlled chirality transfer and FRET effects based on pseudo[3]rotaxane

The use of light to regulate the chirality of supramolecular assemblies in a non-invasive manner remains a challenge. Herein, we report a novel photochromic pseudo[3]rotaxane based on a (R/S)-2,2'-binaphthyl secondary ammonium salt guest (2) and anthracene-bridged bis(dibenzo-24-crown-8) (1), which features a chirality transfer and fluorescence resonance energy transfer (FRET) from 2 to 1. Benefiting from the photo-oxidation of anthracene, the induced circular dichroism (ICD) signals of (R/S)-2@1 can be switched off/on by irradiation with 365 nm UV light and heating. This noncovalent supramolecular assembly strategy provides us with unique opportunities to design and construct further smart photo-responsive chiral molecular switches.

Heteroaryl Rings in Peptide Macrocycles

This Review is devoted to the chemistry of macrocyclic peptides having heterocyclic fragments in their structure. These motifs are present in many natural products and synthetic macrocycles designed against a particular biochemical target. Thiazole and oxazole are particularly common constituents of naturally occurring macrocyclic peptide molecules. This frequency of occurrence is because the thiazole and oxazole rings originate from cysteine, serine, and threonine residues. Whereas other heteroaryl groups are found less frequently, they offer many insightful lessons that range from conformational control to receptor/ligand interactions. Many options to develop new and improved technologies to prepare natural products have appeared in recent years, and the synthetic community has been pursuing synthetic macrocycles that have no precedent in nature. This Review attempts to summarize progress in this area.

Heteroleptic copper phenanthroline complexes in motion: From stand-alone devices to multi-component machinery

Two and a half decades of copper phenanthroline-based switches, devices and machines have illustrated the rich dynamic nature of these metal complexes. With an emphasis on the metal-ligand dissociation as the rate-determining step the present review summarizes not only spectacular examples of machinery, but also highlights rate data collected during a variety of investigations. Copper-ligand exchange reactions are mostly triggered by redox processes, addition of metal ions or addition of ligands. While the rate data spread over >8 orders of magnitude, individual effects of solvent, steric bulk, flexibility, σ-basicity and the trajectory (intra- vs. intermolecular dissociation) have large impact. Unfortunately, in many cases the exact mechanism in the rate-determining step (nucleophile-induced vs. monomolecular metal-ligand dissociation) has not been determined, suggesting to invest further efforts in the physical (in)organic chemistry of such coordination-driven systems.

Twisting of Alkynes towards a Carbon Double Helix

The carbon allotrope exhibiting only one-dimensional sp-hybridized carbon atoms is called carbyne. However, its existence is very controversial. Studies on model compounds for carbyne revealed that many oligoalkynes show not a straight, but a bent structure of the carbon chain. Here, we question whether it would also be possible to obtain a more complex structure from carbyne, such as a dimeric double helix. Based on quantum chemical calculations, we show that only a small energetic expense is needed for the formation of a double helix starting from oligoalkyne chains. In some cases, the double helix-like conformation is more stable than the corresponding conformation with a parallel arrangement of the acetylene chains. Furthermore, model systems were synthesized in which two diphenyl oligoalkyne chains are fixed and twisted by a chiral imidazole-containing clamp. A structural investigation of these model systems was performed based on UV and CD spectroscopy and quantum chemical calculations. The observed twisting in these model systems can be regarded as the first small step towards an imaginable carbon double helix.

Switching Process Consisting of Three Isomeric States of an Azobenzene Unit

Azobenzene and its derivatives are among the most commonly used switching units in organic chemistry. The switching process consists of two states, in which the trans isomer has a stretched and the cis isomer a compact form. Here, we have designed a system in which all isomeric states of an azobenzene moiety (trans → cis-(M) → cis-(P)) are passed step by step. The first step involves a change in the distance between the benzene units, which is common for azobenzene derivatives. In the second step an inversion of the helicity (M→P) of the cis azobenzene unit takes place. The third step leads back to the stretched trans isomer. This switching cycle is achieved by coupling the azobenzene unit with two chiral clamps and with a further azobenzene switching unit.

Bio-inspired Herringbone Foldamers: Strategy for Changing the Structure of Helices

Cyclic oligomers of azole peptides were isolated from a multitude of marine organisms and were used for a large number of molecular machines. As shown previously, oligomers derived from achiral imidazole amino acids fold into canonical helices. Here we show that a minor change, the introduction of a methyl group in the δ position, results in a significant change in the secondary structure of the corresponding oligomers. Instead of a canonical helix, a noncanonical herringbone helix is formed. In the latter, the slope along the helix changes its sign at least twice per turn. This strategy allows a remarkable change of the secondary structure via a small modification. By means of enantiomerically pure amino acids, we were able to control, for the first time, both the helicity of the helix and the form of the herringbone. The investigation of the underlying herringbone basic element and its folding to a noncanonical helix were conducted by NMR and CD spectroscopy, as well as by X-ray crystallography and quantum chemical calculations.

Planar-to-Axial Chirality Transfer in the Polymerization of Phenylacetylenes

A pair of enantiomerically pure planar chiral phenylacetylenes, R- and S-2'-ethynyl-1,10-dioxa[10]-paracyclophane, were prepared and polymerized under the catalysis of Rh(nbd)BPh₄ and MoCl₅, respectively. The resultant polymers had high cis-structure contents and took dominant cis-transoid helical conformations with an excess screw sense as revealed by ¹H NMR, Raman, polarimetry, circular dichroism spectroscopy, and computational simulation, manifesting the effective guidance of the planar chirality of monomers to the growth of the polymer main chains. The rigid ansa-structure of monomer unit made the helical structure of polymer backbone stable toward grinding and thermal treatments. The stereoselective interactions between these chiral polymers and the enantiomers of racemic ethynyl-1,10-dioxa[10]-paracyclophane and cobalt(III) acetylacetonate were observed. This work demonstrated the first planar-to-axial chirality transfer in the polymerization of acetylenes and offered a new strategy to prepare chiral materials based on optically active helical polymers.

A ferrocene based switchable molecular folding ruler

A bis(ferrocene) three tiered molecular folding ruler can be induced to undergo a large scale extension and contraction process using either chemical or better electrochemical methods.

Conformational changes and chiroptical switching of enantiopure bis-helicenic terpyridine upon Zn(2+) binding

The molecular conformation of a bis-helicenic terpyridine system is strongly modified upon binding to Zn(ii) ion, a process that is accompanied by large changes in the optical and chiroptical properties. This system affords a new type of helicene-based chiroptical switching.

Ferrocene-containing non-interlocked molecular machines

Ferrocene is chemically robust and readily functionalized which enables its facile incorporation into more complex molecular systems. This coupled with ferrocene's reversible redox properties and ability to function as a “molecular ball bearing” has led to the use of ferrocene as a component in wide range of non-interlocked synthetic molecular machine systems.

Conformational and Optical Characteristics of Unidirectionally Twisted Binaphthyl-Bipyridyl Cyclic Dyads

An axially chiral binaphthyl-bipyridyl cyclic dyad in which the two units are connected by short -CH2O- linkers was synthesized. Experimental and theoretical analyses indicate that the (R)-binaphthyl unit in the dyad induces (R)-chirality in the bipyridyl unit, both in the solid state and in solution. It is shown that vibrational circular dichroism (VCD) is useful to determine the twisting pattern of 2,2'-bipyridyl compounds. The dyad shows crystallization-induced emission enhancement (CIEE).

A heterometallic macrocycle as a redox-controlled molecular hinge

The ability to modify the structure of nanoscopic assemblies in a controlled fashion is an important prerequisite for the creation of functional supramolecular systems. Here, we describe a heterometallic Pt2Cu2-macrocycle which behaves as a molecular hinge. A square-planar Pt(ii) complex with pendent 2-formylpyridine groups was synthesized and structurally characterized. Condensation of the complex with benzylamine followed by reaction with Cu(MeCN)4BF4 resulted in the formation of a rectangular Pt2Cu2-macrocycle. Upon chemical oxidation of the Cu centers, the macrocycle folds up to adopt a butterfly-like geometry in which the Pt centers approach each other. This process can be reversed by chemical reduction.

Imidazole-peptide foldamers: parabolic dependence of the folding process on the water content of the solvent

Oligomers of azole peptides have been isolated from a multitude of marine organisms. Up to now, these azole-containing dipeptide-analogue oligomers have only been found as cyclic n-mers (mostly tri- and tetramers) in nature. Herein, we show that imidazole-containing pseudopeptides form helixlike secondary structures in different solvents. The screw sense of the helix can be determined by attaching a single chiral imidazole unit to the N terminus of the oligomer. Investigation by means of circular dichroism (CD) spectroscopy showed that the folding process of the helix depends on the water content of the solvent in a parabolic way. In a pure organic medium, the helix is stabilized by hydrogen bonds between the hydrogen atoms of the amide groups and the nitrogen atoms of the azole ring. In aqueous solution, the formation of the helix is driven by dispersion interactions. The formation of the helix is more pronounced in aqueous solution than in organic solvents.

Ion-mediated conformational switches

Molecular switches are ubiquitous in Nature and provide the basis of many forms of transport and signalling. Single synthetic molecules that change conformation, and thus function, reversibly in a stimulus-dependent manner are of great interest not only to chemists but society in general; myriad applications exist in storage, display, sensing and medicine. Here we describe recent developments in the area of ion-mediated switching.

Chemically and electrochemically induced expansion and contraction of a ferrocene rotor

A 2,2′-bipyridine-appended ferrocene rotor can be switched, upon treatment with [Cu(CH₃CN)₄](PF₆) and 6,6′-dimesityl-2,2′-bipyridine, from the stacked (syn) conformation to the unstacked (anti) conformation. The switching was completely reversible and could be triggered either chemically, or electrochemically.

Metal-ion-regulated miniature DNA-binding proteins based on GCN4 and non-native regulation sites

The design of artificial peptide dimers containing polypyridine switching domains, for which metal-ion coordination is shown to regulate DNA binding, is reported. Short peptides, based on the basic domain of the GCN4 transcription factor (GCN4bd), dimerised with either 2,2'-bipyridine (bipy(GCN4bd)2 ) or 2,2':6',2''-terpyridine (terpy(GCN4bd)2 ) linker units, undergo a conformational rearrangement on Cu(II) and Zn(II) coordination. Depending on the linker substitution pattern, this is proposed to alter the relative alignment of the two peptide moieties, and in turn regulate DNA binding. Circular dichroism and UV-visible spectroscopy reveal that Cu(II) and Zn(II) coordination promotes binding to DNA containing the CRE target site, but to a differing and opposite degree for the two linkers, and that the metal-ion affinity for terpy(GCN4bd)2 is enhanced in the presence of CRE DNA. Binding to DNA containing the shorter AP1 target site, which lacks a single nucleobase pair compared to CRE, as well as half-CRE, which contains only half of the CRE target site, was also investigated. Cu(II) and Zn(II) coordination to terpy(GCN4bd)2 promotes binding to AP1 DNA, and to a lesser extent half-CRE DNA. Whereas, bipy(GCN4bd)2 , for which interpeptide distances are largely independent of metal-ion coordination and less suitable for binding to these shorter sites, displays allosteric ineffective behaviour in these cases. These findings for the first time demonstrate that biomolecular recognition, and specifically sequence-selective DNA binding, can be controlled by metal-ion coordination to designed switching units, non-native regulation sites, in artificial biomolecules. We believe that in the future these could find a wide range of applications in biotechnology.

A Lewis acid-mediated conformational switch

An isonicotinamide-substituted diphenylacetylene undergoes conformational switching upon recognition of Lewis acids.

Artificial redox-driven directionally controlled switches as a basis for redox-driven molecular motors

SIGNIFICANCE

This review relates to artificial redox-driven molecular devices. The advantages of using very simple chemical building blocks for the bottom-up design of nanoleveled functional motors and the importance of the unidirectionality of a switching process for the development of redox-driven molecular motors are discussed. Furthermore, the crucial difference between artificial molecular switches and motors is explained.

RECENT ADVANCES

This review discusses few selected examples of redox-driven devices exhibiting partially complex-coupled movement sequences, which, however, due to the lack of an overall directionally controlled movement are not able to perform mechanical work on a molecular scale. Recent examples for redox-driven devices with at least one directionally controlled switching process as well as the proof for the unidirectionality of the switching process are presented.

CRITICAL ISSUES

The challenge in designing directionally controlled switches is the fact that during the switching process, a configuration (or conformation) must be changed reversibly. This crucial process can be a flip caused by the change of the coordination sphere of a metal ion, a rotation around a C-C single bond, or around a C-C double bond.

FUTURE DIRECTIONS

For future developments, we suggest designing artificial redox-based molecular motors in which the motion process of the presented directionally controlled switches are coupled with another switchable unit. The latter could also be switchable in a nondirected way.

Conformational Study of an Artificial Metal-Dependent Regulation Site for Use in Designer Proteins

This report describes the dimerisation of glutathione, and by extension, other cysteine-containing peptides or protein fragments, with a 5, 5'-disubstituted-2, 2'-bipyridine or 6, 6"-disubstituted-2, 2':6',2"-terpyridine unit. The resulting bipy-GS2 and terpy-GS2 were investigated as potential metal ion dependent switches in aqueous solution, and were found to predominantly adopt the transoïd conformation at physiological pH. Metal complexation with CuII and ZnII at this pH has been studied by UV/Vis, CD, NMR and ion-mobility mass spectrometry. ZnII titrations are consistent with the formation of a 1:1 ZnII:terpy-GS2 complex at pH 7.4, but bipy-GS2 was shown to form both 1:1 and 1:2 complexes with the former being predominant under dilute micromolar conditions. Formation constants for the resulting 1:1 complexes were determined to be log KM 6.86 (bipy-GS2 ) and 6.22 (terpy-GS2 ), consistent with a higher affinity for the unconstrained bipyridine, compared to the strained terpyridine. CuII coordination involves the initial formation of 1:1 complexes, followed by 1.5Cu:1bipy-GS2 and 2Cu:1terpy-GS2 complexes at micromolar concentrations. Binding constants for formation of the 1:1 complexes (log KM 12.5 (bipy-GS2 ); 8.04 and 7.14 (terpy-GS2 )) indicate a higher affinity for CuII than ZnII. Finally, ion-mobility MS studies detected the free ligands in their protonated form, and were consistent with the formation of two different Cu adducts with different conformations in the gas-phase. We illustrate that the bipyridine and terpyridine dimerisation units can behave like conformational switches in response to Cu/Zn complexation, and propose that in future these can be employed in synthetic biology with larger peptide or protein fragments, to control large scale folding and related biological function.

An azobenzene unit embedded in a cyclopeptide as a type-specific and spatially directed switch

By embedding an azobenzene unit into a chiral scaffold, switching of azobenzene from the trans-(P) isomer to the cis-(P) isomer and back was achieved (black arrows in picture). The embedding leads to a flipping process in which the phenyl rings can only move directly towards one another in the switching process.