Versatile self-adapting boronic acids for H-bond recognition: from discrete to polymeric supramolecules

By taking advantage of the peculiar dynamic covalent reactivity of boronic acids to form tetraboronate derivatives, interest has risen to use the aryl derivatives in materials science and supramolecular chemistry, nevertheless their ability to form H-bonded complexes has been only marginally touched. Herein we report the first solution and solid-state binding studies of first double H-bonded DD•AA-type complexes of a series of aromatic boronic acids that adopt a synsyn conformation with suitable complementary H-bonding acceptor partners. The first determination of the Ka in solution of ortho substituted boronic acids showed that 1:1 association is in the range between 300 and 6900 M-1. Crystallization of dimeric 1:1, trimeric 1:2 and 2:1 complexes enabled in depth examination of these complexes in the solid state, proving the selection of the –B(OH)2 syn-syn conformer through a pair of frontal H-bonds with the relevant AA partner. Non-ortho substituted boronic acids result in “flat” complexes. On the other hand, sterically demanding analogues bearing ortho-substituents strive to retain their recognition properties rotating the ArB(OH)2 moiety forming “T-shaped” complexes. Solid-state studies of a diboronic acid and a tetraazanaphthacene provided for the first time the formation of a supramolecular H-bonded polymeric ribbon. Given the conformational dynamicity of the –B(OH)2 functional group, it is expected that these findings will also open new possibilities in metal-free catalysis or organic crystal engineering, where double H-bonding donor boronic acids could act as suitable organocatalysts or templates developing functional materials with tailored organizational properties

Solvent-dependent moulding of porphyrin-based nanostructures: solid state, solution and on surface self-assembly

A novel porphyrin derivative 1∙Zn was synthesised in order to mimic the self-assembly properties of natural light-harvesting antennas and its self-assembly behaviour in solution and in solid state were studied by NMR and X-Ray spectroscopies. The self-assembly of this molecule was triggered in apolar solvents and studied in solution by UV-Vis spectroscopy, suggesting it is able to form slipped face-to-face aggregates, or J-aggregates. The nanoscopic and microscopic morphology of the aggregates was elucidated by atomic force microscopy, revealing the formation of extended two-dimensional structures

Renaissance of an old topic: from borazines to BN-doped nanographenes

Graphene is one of the leading materials in today's science, but the lack of a band gap limits its application to replace semiconductors in optoelectronic devices. To overcome this limitation, the replacement of C=C bonds by isostructural and isoelectronic bonds is emerging as an effective strategy to open a band gap in monoatomic graphene layers. First prepared by Stock and Pohland in 1926, borazine is the isoelectronic and isostructural inorganic analogue of benzene, where the C=C bonds are replaced by B–N couples. The strong polarity of the BN bonds widens the molecular HOMO–LUMO gap, imparting strong UV-emission/absorption and electrical insulating properties. These properties make borazine a valuable molecular scaffold to be inserted as doping units in graphitic-based carbon materials to tailor a relevant band gap. It is with this objective that we became interested in the development of new synthetic organic methodologies to gain access to functionalized borazine derivatives. In particular, we have described the synthesis of borazine derivatives that, featuring aryl substituents at the B-centers bearing ortho-functionalities, are exceptionally stable against hydrolysis. Building on these structural motifs, we prepared hybrid BN-doped polyphenylene nanostructures featuring controlled doping patterns, both as dosage and orientation. Finally, exploiting the Friedel-Craft electrophilic aromatic substitution, we could develop the first rational synthesis of the first soluble hexa-peri-hexabenzoborazinocoronene and measured its optoelectronic properties, showing a widening of its gap compared to its full-carbon congener

Borazino-doped polyphenylenes

The divergent synthesis of two series of borazino-doped polyphenylenes, in which one or more aryl units are replaced by borazine rings, is reported for the first time, taking advantage of the decarbonylative [4 + 2] Diels− Alder cycloaddition reaction between ethynyl and tetraphenylcyclopentadienone derivatives. Because of the possibility of functionalizing the borazine core with different groups on the aryl substituents at the N and B atoms of the borazino core, we have prepared borazino-doped polyphenylenes featuring different doping dosages and orientations. To achieve this, two molecular modules were prepared: a core and a branching unit. Depending on the chemical natures of the central aromatic module and the reactive group, each covalent combination of the modules yields one exclusive doping pattern. By means of this approach, three- and hexa-branched hybrid polyphenylenes featuring controlled orientations and dosages of the doping B3N3 rings have been prepared. Detailed photophysical investigations showed that as the doping dosage is increased, the strong luminescent signal is progressively reduced. This suggests that the presence of the B3N3 rings engages additional deactivation pathways, possibly involving excited states with an increasing charge-separated character that are restricted in the full-carbon analogues. Notably, a strong effect of the orientational doping on the fluorescence quantum yield was observed for those hybrid polyphenylene structures featuring low doping dosages. Finally, we showed that Cu-catalyzed 1,3-dipolar cycloaddition is also chemically compatible with the BN core, further endorsing the inorganic benzene as a versatile aromatic scaffold for engineering of molecular materials with tailored and exploitable optoelectronic properties

Combining high-resolution scanning tunnelling microscopy and first-principles simulations to identify halogen bonding

Scanning tunnelling microscopy (STM) is commonly used to identify on-surface molecular self-assembled structures. However, its limited ability to reveal only the overall shape of molecules and their relative positions is not always enough to fully solve a supramolecular structure. Here, we analyse the assembly of a brominated polycyclic aromatic molecule on Au(111) and demonstrate that standard STM measurements cannot conclusively establish the nature of the intermolecular interactions. By performing high-resolution STM with a CO-functionalised tip, we clearly identify the location of rings and halogen atoms, determining that halogen bonding governs the assemblies. This is supported by density functional theory calculations that predict a stronger interaction energy for halogen rather than hydrogen bonding and by an electron density topology analysis that identifies characteristic features of halogen bonding. A similar approach should be able to solve many complex 2D supramolecular structures, and we predict its increasing use in molecular nanoscience at surfaces

Magnetically-Active Carbon Nanotubes at Work

Endohedral and exohedral assembly of magnetic nanoparticles (MNPs) and carbon nanotubes (CNTs) recently gave birth to a large body of new hybrid nanomaterials (MNPs-CNTs) featuring properties that are otherwise not in reach with only the graphitic or metallic cores themselves. These materials feature enhanced magnetically guided motions (rotation and translation), magnetic saturation and coercivity, large surface area, and thermal stability. By guiding the reader through the most significant examples in this Concept paper, we describe how researchers in the field engineered and exploited the synergistic combination of these two types of nanoparticles in a large variety of current and potential applications, such as magnetic fluid hyperthermia therapeutics and in magnetic resonance imaging to name a few

Chalcogen-bond driven molecular recognition at work

Out of the supramolecular toolbox, Secondary Bonding Interactions (SBIs) have attracted in the last decades the attention of the chemical community as novel non-covalent interactions of choice for a large number of chemical systems. Amongst all SBIs, halogen-bonding (XBIs) and chalcogen-bonding (EBIs) interactions are certainly the most important. However, the use of EBIs have received marginal consideration if compared to that of XBIs. By sieving the most significant examples, this review focuses on the theoretical and experimental studies carried out with EBIs in functional systems. In a systematic way the reader is guided through the most recent and representative examples in which chemists have rationally designed molecular modules that, through EBIs, trigger the initiation of chemical reactions, molecular recognition events in solutions and at the solid state to produce self-assembled and self-organised functional materials at different length scales. The study and understanding of the fundamental geometrical and physical parameters ruling EBIs is at its infancy, and it still needs to establish those principles to rationally design and program synthons that, undergoing molecular recognition through EBIs, allow the development of new tailored materials for applications in the field of optoelectronic, sensing, catalysis, and drug discovery

O-Annulation leading to five-, six-, and seven-membered cyclic diaryl ethers involving C-H cleavage

Cyclic diaryl ethers are present in multiple natural compounds, organic pollutants as well as in π-conjugated organic molecular materials. This short review aims at overviewing the main synthetic advances in the O-annulation methods for preparing five-, six-, and seven-membered rings through C–H cleavage

Concurring Chalcogen- and halogen-bonding interactions in supramolecular polymers for crystal engineering applications

The engineering of crystalline molecular solids through the simultaneous combination of distinctive non‐covalent interactions is an important field of research as it could allow chemist to prepare materials depicting multiresponsive properties. It is in this contest that, pushed by our will to expand the chemical space of chalcogen‐bonding interactions that, in this work we put forward the concept for which chalcogen‐ and halogen‐bonding interactions can be used simultaneously to engineer multicomponent co‐crystals. Through the rational design of crystallizable molecules, we prepared chalcogenazolo pyridine scaffold (CGP) modules that, bearing either a halogen‐bond acceptor or donor at the 2‐position can interact with suitable complementary molecular modules, undergoing formation of supramolecular polymers at the solid state. The recognition reliability of the CGP moiety to form chalcogen‐bonded dimers allow the formation heteromolecular supramolecular polymers through halogen‐bonding interactions as confirmed by single‐crystal X‐ray diffraction analysis

Structural properties of highly doped borazino polyphenylenes obtained through condensation reaction

Here we describe the synthesis and spectroscopic and structural characterization of various borazine-doped polyphenylenes displaying high doping dosages (16–18%). Capitalizing on the condensation reaction approach, the desired products were formed using a mixture of p-phenylendiamine and aniline with BCl3, followed by the addition of an aryl lithium derivative. The use of mesityl lithium (MesLi) yields strained multiborazine derivatives, which proved to be unstable in the presence of moisture. However, when xylyl lithium (XylLi) was used, chemically stable multiborazines were obtained, with oligomers showing molecular weight up to 104, corresponding to 16–18 monomer units. While the dimer, trimer, and tetramer could be isolated as pure products and their structure characterized by mass and NMR analysis, higher oligomers could only be isolated as mixtures of B-hydroxy-substituted derivatives and characterized by gel permeation chromatography. The structures of the dimer and trimer derivatives were confirmed by X-ray analysis, which nicely showed the presence of the two and three borazine rings spaced by one and two 1,4-aryl bridges, respectively. Notably, the trimer forms a porous crystalline clathrate. The peripheral xylyl and phenyl moieties of each molecule intramolecularly embrace each other through C–H and π–π stacking interactions. Steady-state UV–vis absorption characterization suggested that the molecules are UV absorbers, with the extinction coefficient linearly scaling with the degree of oligomerization. On the other hand, low-emission quantum yields were obtained for all derivatives (<7%), suggesting that high BN-doping dosages dramatically affect the emission properties of the doped polyphenylenes