Stepwise Functionalization of a Pillar[5]arene-Containing [2]Rotaxane with Pentafluorophenyl Ester Stoppers.

Detailed investigations into the stepwise bis-functionalization of a pillar[5]arene-containing rotaxane building block have been carried out. Upon a first stopper exchange, the pillar[5]arene moiety of the mono-acylated product is preferentially located close to its reactive pentafluorophenyl ester stopper, thus limiting the accessibility to the reactive carbonyl group by the nucleophilic reagents. Selective mono-functionalization is thus very efficient. Introduction of a second stopper is then possible to generate dissymmetrical rotaxanes with different amide stoppers. Moreover, when dethreading is possible upon the second acylation, the pillar[5]arene plays the role of a protecting group allowing the synthesis of dissymmetrical axles that are particularly difficult to prepare under statistical conditions. Finally, detailed conformation analysis of the rotaxanes revealed that the position of the pillar[5]arene moiety on its axle subunit is mainly governed by polar interactions in nonpolar organic solvents, whereas solvophobic effects play a major role in polar solvents.

Stepwise Functionalization of a Pillar[5]arene-Containing [2]Rotaxane with Pentafluorophenyl Ester Stoppers.

Invited for the cover of this issue are the group of Iwona and Jean-François Nierengarten from the University of Strasbourg (LIMA, UMR 7042, CNRS) and collaborators from the University of Carthage. The image depicts a sky of umbrellas floating in the air to illustrate the protective effect allowing the efficient mono-acylation of a symmetrical rotaxane building block with two pentafluorophenyl ester stoppers. Read the full text of the article at 10.1002/chem.2023003501.

Solution and Solvent-Free Stopper Exchange Reactions for the Preparation of Pillar[5]arene-containing [2] and [3]Rotaxanes.

Diamine reagents have been used to functionalize a [2]rotaxane building block bearing an activated pentafluorophenyl ester stopper. Upon a first acylation, an intermediate host-guest complex with a terminal amine function is obtained. Dissociation of the intermediate occurs in solution and acylation of the released axle generates a [2]rotaxane with an elongated axle subunit. In contrast, the corresponding [3]rotaxane can be obtained if the reaction conditions are appropriate to stabilize the inclusion complex of the mono-amine intermediate and the pillar[5]arene. This is the case when the stopper exchange is performed under mechanochemical solvent-free conditions. Alternatively, if the newly introduced terminal amide group is large enough to prevent the dissociation, the second acylation provides exclusively a [3]rotaxane. On the other hand, detailed conformational analysis has been also carried out by variable temperature NMR investigations. A complete understanding of the shuttling motions of the pillar[5]arene subunit along the axles of the rotaxanes reported therein has been achieved with the help of density functional theory calculations.

Solution and Solvent-Free Stopper Exchange Reactions for the Preparation of Pillar[5]arene-Containing [2] and [3]Rotaxanes.

Invited for the cover of this issue are the group of Iwona and Jean-François Nierengarten from the University of Strasbourg (LIMA, UMR 7042, CNRS) and collaborators from the University of Carthage and the IPHC (University of Strasbourg and CNRS, UMR 7178). The image illustrates the fast motions of a pillar[5]arene subunit along the axle of a rotaxane, reminiscent of those of a guitarist's hand along the neck allowing him to use random parts of a scale with certain sweet spots when improvising a solo. Read the full text of the article at 10.1002/chem.202304131.

Pentafluorophenyl Esters as Exchangeable Stoppers for the Construction of Photoactive [2]Rotaxanes.

Stable pillar[5]arene-containing [2]rotaxane building blocks with pentafluorophenyl ester stoppers have been efficiently prepared on a multi-gram scale. Reaction of these building blocks with various nucleophiles gave access to a wide range of [2]rotaxanes with amide, ester or thioester stoppers in good to excellent yields. The rotaxane structure is fully preserved during these chemical transformations. Actually, the addition-elimination mechanism at work during these transformations totally prevents the unthreading of the axle moiety of the mechanically interlocked system. The stopper exchange reactions were optimized both in solution and under mechanochemical solvent-free conditions. While amide formation is more efficient in solution, the solvent-free conditions are more powerful for the transesterification reactions. Starting from a fullerene-functionalized pillar[5]arene derivative, this new strategy gave easy access to a photoactive [2]rotaxane incorporating a C60 moiety and two Bodipy stoppers. Despite the absence of covalent connectivity between the Bodipy and the fullerene moieties in this photoactive molecular device, efficient through-space excited state interactions have been evidenced in this rotaxane.

Dendritic-Like Molecules Built on a Pillar[5]arene Core as Hole Transporting Materials for Perovskite Solar Cells.

Multi-branched molecules have recently demonstrated interesting behaviour as charge-transporting materials within the fields of perovskite solar cells (PSCs). For this reason, extended triarylamine dendrons have been grafted onto a pillar[5]arene core to generate dendrimer-like compounds, which have been used as hole-transporting materials (HTMs) for PSCs. The performances of the solar cells containing these novel compounds have been extensively investigated. Interestingly, a positive dendritic effect has been evidenced as the hole transporting properties are improved when going from the first to the second-generation compound. The stability of the devices based on the best performing pillar[5]arene material has been also evaluated in a high-throughput ageing setup for 500 h at high temperature. When compared to reference devices prepared from spiro-OMeTAD, the behaviour is similar. An analysis of the economic advantages arising from the use of the pillar[5]arene-based material revealed however that our pillar[5]arene-based material is cheaper than the reference.

Dendritic-Like Molecules Built on a Pillar[5]arene Core as Hole Transporting Materials for Perovskite Solar Cells.

Invited for the cover of this issue are the groups of S. Seki (Kyoto), G. Reginato (Sesto Fiorentino), J.-F. Nierengarten (Strasbourg), A. Abate (Berlin) and J. L. Delgado (San Sebastian). The image depicts an artistic view of a dendrimer-like hole transporting material at work in a perovskite solar cell. Read the full text of the article at 10.1002/chem.202101110.

Grafting Dendrons onto Pillar[5]Arene Scaffolds.

With their ten peripheral substituents, pillar[5]arenes are attractive compact scaffolds for the construction of nanomaterials with a controlled number of functional groups distributed around the macrocyclic core. This review paper is focused on the functionalization of pillar[5]arene derivatives with small dendrons to generate dendrimer-like nanomaterials and bioactive compounds. Examples include non-viral gene vectors, bioactive glycoclusters, and liquid-crystalline materials.

Preparation of Pillar[5]arene-Based [2]Rotaxanes by a Stopper-Exchange Strategy.

A pillar[5]arene-containing rotaxane building block bearing exchangeable stoppers has been prepared in multigram scale quantities with high yields from the reaction of 2,4-dinitrophenol (DNP) with the inclusion complex resulting from the association of dodecanedioyl chloride with 1,4-diethoxypillar[5]arene. Stopper exchange reactions have been achieved by treatment of the resulting DNP diester with various amines through an addition-elimination mechanism preventing the unthreading of the axle component during the reaction and thus preserving the [2]rotaxane structures. The resulting diamide [2]rotaxane derivatives have thus been obtained in good to excellent yields. Importantly, [2]rotaxanes difficult or impossible to prepare by direct introduction of the two stoppers in a single synthetic step are now easily available.

A Rotaxane Scaffold Bearing Multiple Redox Centers: Synthesis, Surface Modification and Electrochemical Properties.

A rotaxane scaffold incorporating two dithiolane anchoring units for the modification of gold surfaces has been functionalized with multiple copies of a redox unit, namely ferrocene. Surface modification has been first assessed at the single molecule level by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) imaging, while tip enhanced Raman spectroscopy (TERS) provided the local vibrational signature of the ferrocenyl subunits of the rotaxanes grafted onto the gold surface. Finally, oxidation of the redox moieties within a rotaxane scaffold grafted onto gold microelectrodes has been investigated by ultrafast cyclic voltammetry. Intramolecular electron hopping is indeed extremely fast in this system. Moreover, the kinetics of charge injection depends on the molecular coverage due to the influence of intermolecular contacts on molecular motions.

A Rotaxane Scaffold for the Construction of Multiporphyrinic Light-Harvesting Devices.

A sophisticated photoactive molecular device has been prepared by combining recent concepts for the preparation of multifunctional nanomolecules (click chemistry on multifunctional scaffolds) with supramolecular chemistry (self-assembly to prepare rotaxanes). Specifically, a clickable [2]rotaxane scaffold incorporating a free-base porphyrin stopper has been prepared and functionalized with ten peripheral Zn(II)-porphyrin moieties. Electrochemical investigations of the final compound revealed a peculiar behavior resulting from the intramolecular coordination of the Zn(II) porphyrin moieties to 1,2,3-triazole units. Finally, steady state investigations of the compound combining Zn(II) and free-base porphyrin moieties have shown that this compound is a light-harvesting device capable of channeling the light energy from the peripheral Zn(II)-porphyrin subunits to the core by singlet-singlet energy transfer.

Heteroleptic Copper(I) Complexes Prepared from Phenanthroline and Bis-Phosphine Ligands: Rationalization of the Photophysical and Electrochemical Properties.

The electronic and structural properties of ten heteroleptic [Cu(NN)(PP)]+ complexes have been investigated. NN indicates 1,10-phenanthroline (phen) or 4,7-diphenyl-1,10-phenanthroline (Bphen); each of these ligands is combined with five PP bis-phosphine chelators, i.e., bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp), 1,2-bis(diphenylphosphino)benzene (dppb), and bis[(2-diphenylphosphino)phenyl] ether (POP). All complexes are mononuclear, apart from those based on dppm, which are dinuclear. Experimental data-also taken from the literature and including electrochemical properties, X-ray crystal structures, UV-vis absorption spectra in CH2Cl2, luminescence spectra and lifetimes in solution, in PMMA, and as powders-have been rationalized with the support of density functional theory calculations. Temperature dependent studies (78-358 K) have been performed for selected complexes to assess thermally activated delayed fluorescence. The main findings are (i) dependence of the ground-state geometry on the crystallization conditions, with the same complex often yielding different crystal structures; (ii) simple model compounds with imposed C2 v symmetry ([Cu(phen)(PX3)2]+; X = H or CH3) are capable of modeling structural parameters as a function of the P-Cu-P bite angle, which plays a key role in dictating the overall structure of [Cu(NN)(PP)]+ complexes; (iii) as the P-Cu-P angle increases, the energy of the metal-to-ligand charge transfer absorption bands linearly increases; (iv) the former correlation does not hold for emission spectra, which are red-shifted for the weaker luminophores; (v) the larger the number of intramolecular π-interactions within the complex in the ground state, the higher the luminescence quantum yield, underpinning a geometry locking effect that limits the structural flattening of the excited state. This work provides a general framework to rationalize the structure-property relationships of [Cu(NN)(PP)]+, a class of compounds of increasing relevance for electroluminescent devices, photoredox catalysis, and solar-to-fuels conversion, which so far have been investigated in an unsystematic fashion, eluding a comprehensive understanding.

Coordination-Driven Folding in Multi-ZnII -Porphyrin Arrays Constructed on a Pillar[5]arene Scaffold.

Pillar[5]arene derivatives bearing peripheral porphyrin subunits have been efficiently prepared from a deca-azide pillar[5]arene building block (17) and ZnII -porphyrin derivatives bearing a terminal alkyne function (9 and 16). For the resulting deca-ZnII -porphyrin arrays (18 and 20), variable temperature NMR studies revealed an intramolecular complexation of the peripheral ZnII -porphyrin moieties by 1,2,3-triazole subunits. As a result, the molecules adopt a folded conformation. This was further confirmed by UV/Vis spectroscopy and cyclic voltammetry. In addition, we have also demonstrated that the coordination-driven unfolding of 18 and 20 can be controlled by an external chemical stimulus. Specifically, addition of an imidazole derivative (22) to solution of 18 or 20 breaks the intramolecular coordination at the origin of the folding. The resulting molecular motions triggered by the addition of the imidazole ligand mimic the blooming of a flower.

Conjugated Porphyrin Dimers: Cooperative Effects and Electronic Communication in Supramolecular Ensembles with C60.

Two new conjugated porphyrin-based systems (dimers 3 and 4) endowed with suitable crown ethers have been synthesized as receptors for a fullerene-ammonium salt derivative (1). Association constants in solution have been determined by UV-vis titration experiments in CH2Cl2 at room temperature. The designed hosts are able to associate up to two fullerene-based guest molecules and present association constants as high as ∼5 × 108 M-1. Calculation of the allosteric cooperative factor α for supramolecular complexes [3·12] and [4·12] showed a negative cooperative effect in both cases. The interactions accounting for the formation of the associates are based, first, on the complementary ammonium-crown ether interaction and, second, on the π-π interactions between the porphyrin rings and the C60 moieties. Theoretical calculations have evidenced a significant decrease of the electron density in the porphyrin dimers 3 and 4 upon complexation of the first C60 molecule, in good agreement with the negative cooperativity found in these systems. This negative effect is partially compensated by the stabilizing C60-C60 interactions that take place in the more stable syn-disposition of [4·12].

Biologically Active Heteroglycoclusters Constructed on a Pillar[5]arene-Containing [2]Rotaxane Scaffold.

A synthetic approach combining recent concepts for the preparation of multifunctional nanomolecules (click chemistry on multifunctional scaffolds) with supramolecular chemistry (self-assembly to prepare rotaxanes) gave easy access to a large variety of sophisticated [2]rotaxane heteroglycoclusters. Specifically, compounds combining galactose and fucose have been prepared to target the two bacterial lectins (LecA and LecB) from the opportunistic pathogen Pseudomonas aeruginosa.

Piling Up Pillar[5]arenes To Self-Assemble Nanotubes.

New liquid-crystalline pillar[5]arene derivatives have been prepared by grafting first-generation Percec-type poly(benzylether) dendrons onto the macrocyclic scaffold. The molecules adopt a disc-shaped structure perfectly suited for self-organization into a columnar liquid-crystalline phase. In this way, the pillar[5]arene cores are piled up, thus forming a nanotubular wire encased within a shell of peripheral dendrons. The capability of pillar[5]arenes to form inclusion complexes has been also exploited. Specifically, detailed binding studies have been carried out in solution with 1,6-dicyanohexane as the guest. Inclusion complexes have also been prepared in the solid state. Supramolecular organization into the Colh mesophase has been deduced from X-ray diffraction data and found to be similar to that observed within the crystal lattice of a model inclusion complex prepared from 1,4-dimethoxypillar[5]arene and 1,6-dicyanohexane.

Pillar[5]arene-Based Glycoclusters: Synthesis and Multivalent Binding to Pathogenic Bacterial Lectins.

The synthesis of pillar[5]arene-based glycoclusters has been readily achieved by CuAAC conjugations of azido- and alkyne-functionalized precursors. The lectin binding properties of the resulting glycosylated multivalent ligands have been studied by at least two complementary techniques to provide a good understanding. Three lectins were selected from bacterial pathogens based on their potential therapeutic applications as anti-adhesives, namely LecA and LecB from Pseudomonas aeruginosa and BambL from Burkholderia ambifaria. As a general trend, multivalency improved the binding to lectins and a higher affinity can be obtained by increasing to a certain limit the length of the spacer arm between the carbohydrate subunits and the central macrocyclic core.

From Pillar[n]arene Scaffolds for the Preparation of Nanomaterials to Pillar[5]arene-containing Rotaxanes.

Pillar[n]arenes are a new class of macrocycles that are efficiently prepared from readily available building blocks. In this particular field, our research teams became interested in the use of a pillar[5]arene core as a compact scaffold for the synthesis of nanomaterials with a controlled distribution of functional groups on both rims of the macrocyclic framework. Such compounds have found applications in biology as multivalent ligands for specific lectines or as polycationic compounds for gene delivery. Liquid-crystalline derivatives have been prepared by grafting mesogenic subunits on the pillar[5]arene core. On the other hand, we also became interested in the preparation of pillar[5]arene-containing [2]rotaxanes. In particular, we have shown that pillar[5] arene-based [2]rotaxanes can be obtained from the reaction of amine stoppers with pseudo-rotaxanes resulting from the association of a pillar[5]arene derivative with a diacyl chloride reagent. Finally, amphiphilic [2]rotaxanes have been prepared and incorporated in thin ordered films at the air-water interface.

Langmuir and Langmuir-Blodgett Films from Amphiphilic Pillar[5]arene-Containing [2]Rotaxanes.

Amphiphilic pillar[5]arene-containing [2]rotaxanes have been prepared and fully characterized. In the particular case of the [2]rotaxane incorporating a 1,4-diethoxypillar[5]arene subunit, the structure of the compound was confirmed by X-ray crystal structure analysis. Owing to a good hydrophilic/hydrophobic balance, stable Langmuir films have been obtained for these rotaxanes and the size of the peripheral alkyl chains on the pillar[5]arene subunit has a dramatic influence on the reversibility during compression-decompression cycles. Indeed, when these are small enough, molecular reorganization of the rotaxane by gliding motions are capable of preventing strong π-π interactions between neighboring macrocycles in the thin film.