Search

The single‐molecule conductance of a series of BN‐acene‐like derivatives has been measured by using scanning tunneling break‐junction techniques. A strategic design of the target molecules has allowed us to include azaborine units in positions that unambiguously ensure electron transport through both heteroatoms, which is relevant for the development of customized BN‐doped nanographenes. We show that the conductance of the anthracene azaborine derivative is comparable to that of the pristine all‐carbon anthracene compound. Notably, this heteroatom substitution has also allowed us to perform similar measurements on the corresponding pentacene‐like compound, which is found to have a similar conductance, thus evidencing that B–N doping could also be used to stabilize and characterize larger acenes for molecular electronics applications. Our conclusions are supported by state‐of‐the‐art transport calculations., MICINN, European Research Council (ERC), Junta de Andalucía, Comunidad de Madrid, Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub

A direct, efficient and versatile strategy for the modulation of optoelectronic and magnetic properties of indeno[1,2-b]fluorene has been developed. 4-Substituted-2,6-dimethylphenyl acetylene groups placed in the apical carbon of the five-membered rings lead to redshifted absorption maxima (lmax rangin from 600–700 nm) and considerable narrowing of the HOMO–LUMO energy gap (down to 1.5 eV). Experimental and theoretical data show an increase in the diradical character (y) and a decrease of the singlet-triplet energy gap. Moreover, we have investigated the single-molecule conductance of the antiaromatic indeno[1,2-b]fluorene for the first time by including thiomethyl (-SMe) anchor groups on the phenylacetylene moiety. Conductance values one order of magnitude higher than those of a reference linear 3-ring para-phenylene ethylene have been found, despite the longer length of the S-to-S molecular junction. First principles transport calculations support this high conductance value., MICINN, ERDF, Junta de Andalucía-Consejería de Economía y Conocimiento, Comunidad de Madrid, Ministerio de Universidades, Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub

The development of bio-MOFs or MOF biocomposites through the combination of MOFs with biopolymers offers the possibility of expanding the potential applications of MOFs, making use of more environmentally benign processes and reagents and giving rise to a new generation of greener and more bio-oriented composite materials. Now, with the increasing use of MOFs for biotechnological applications, the development of new protocols and materials to obtain novel bio-MOFs compatible with biomedical or biotechnological uses is needed. Herein, and as a proof of concept, we have explored the possibility of using short-peptide supramolecular hydrogels as media to promote the growth of MOF particles, giving rise to a new family of bio-MOFs. Short-peptide supramolecular hydrogels are very versatile materials that have shown excellent in vitro and in vivo biomedical applications such as tissue engineering and drug delivery vehicles, among others. These peptides self-assemble by noncovalent interactions, and, as such, these hydrogels are easily reversible, being more biocompatible and biodegradable. These peptides can self-assemble by a multitude of stimuli, such as changes in pH, temperature, solvent, adding salts, enzymatic activity, and so forth. In this work, we have taken advantage of this ability to promote peptide self-assembly with some of the components required to form MOF particles, giving rise to more homogeneous and well-integrated composite materials. Hydrogel formation has been triggered using Zn salts, required to form ZIF-8, and formic acid, required to form MOF-808. Two different protocols for the in situ MOF growth have been developed. Finally, the MOF-808 composite hydrogel has been tested for the decontamination of water polluted with phosphate ions as well as for the catalytic degradation of toxic organophosphate methyl paraoxon in an unbuffered solution.

The present work reports on a detailed discussion about the synthesis, characterization, and luminescence properties of three pairs of enantiopure 3D metal–organic frameworks (MOFs) with general formula {[Ln2(L/D-tart)3(H2O)2]·3H2O}n (3D_Ln-L/D, where Ln = Sm(III), Eu(III) or Gd(III), and L/D-tart = L- or D-tartrate), and ten pairs of enantiopure 2D coordination polymers (CPs) with general formula [Ln(L/D-Htart)2(OH)(H2O)2]n (2D_Ln-L/D, where Ln = Y(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III) or Yb(III), and L/D-Htart = hydrogen L- or D-tartrate) based on single-crystal X-ray structures. Enantiopure nature of the samples has been further corroborated by Root Mean Square Deviation (RMSD) as well as by circular dichroism (CD) spectra. Solid-state emission spectra of Eu(III), Tb(III), and Dy(III)-based compounds confirm the occurrence of ligand-to-metal charge transfers in view of the characteristic emissions for these lanthanide ions, and emission decay curves were also recorded to estimate the emission lifetimes for the reported compounds. A complete theoretical study was accomplished to better understand the energy transfers occurring in the Eu-based counterparts, which allows for explaining the different performances of 3D-MOFs and 2D-layered compounds. As inferred from the colorimetric diagrams, emission characteristics of Eu-based 2D CPs depend on the temperature, so their luminescent thermometry has been determined on the basis of a ratiometric analysis between the ligand-centered and Eu-centered emission. Finally, a detailed study of the polarized luminescence intensity emitted by the samples is also accomplished to support the occurrence of chiro-optical activity.

Self-assembled peptides and proteins have turned out to be excellent templates for the growth of inorganic minerals and can be used to emulate natural biomineralization processes. Doing this, researchers have developed complex sophisticated materials with properties, in some cases, similar to those found in nature. Of special interest is the development of scaffolds able to guide bone regeneration. The bone tissue comprises an organic matrix composed of aligned collagen fibers containing nanoapatite crystals oriented along the fiber direction. During bone mineralization, both processes, the self-assembly of collagen fibrils and mineralization occur simultaneously. Collagen fibers are able to control calcium phosphate nucleation and subsequent apatite crystal growth at a very limited range of collagen density and ionic concentration. In this study, we reproduced the simultaneity of both processes using an artificial peptide fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF) that has the ability to self-assemble in water after the addition of Ca2+ ions. Therefore, the peptide self-assembly process and the mineralization of apatite are Ca-demanding processes and occur simultaneously. The role of peptide and ionic concentrations has been investigated affording organic/inorganic hybrid hydrogels with different degrees of homogeneity and mineralization. Interestingly, at very low Ca2+ concentrations, we found that apatite nanocrystals are integrated into Fmoc-FF fibrils and oriented as in biologically mineralized collagen fibrils, the basic building blocks of bone.

Bone is a complex tissue in continuous process of remodeling that maintains the structural integrity of the skeleton and protects inner organs. Bone pathologies (i.e., osteoporosis) are expected to increase due to the changes in population demographics. According to WHO, the worldwide hip fracture is expected to rise to 6 million by 2050. Despite the considerable progress in conventional scaffold-based tissue engineering, the recreation of physiological complexity has remained a challenge. Self-assembled peptides and proteins have turned out to be excellent templates for the growth of inorganic minerals trying to emulate natural biomineralization processes. Herein the biomimetic mineralization of amphiphilic short-peptides was studied with the final aim of designing sophisticated inorganic/organic nanocomposites with tunable structure-function relationships. Concretely, we have developed a protocol in which the self-assembly of amphiphilic peptides and the formation of apatite nanocrystals occurs simultaneously by addition of calcium ions in such a way that the competition for Ca2+ ions modulates crystal growth in a more precise way, emulating bone mineralization. As far as we know, both processes have never been studied simultaneously for this peptide. We found that substequiometric calcium concentration provides nanocomposite materials in which nanoapatite crystals are perfectly embedded and oriented along the peptide fibers (Figure1: Influence of Ca2+ concentration on the mineralization of aromatic peptides), as observed in mineralized collagen fibrils, the basic building blocks of the complex bone structure.

[EN] On-surface synthesis, complementary to wet chemistry, has been demonstrated to be a valid approach for the synthesis of tailored graphenic nanostructures with atomic precision. Among the different existing strategies used to tune the optoelectronic and magnetic properties of these nanostructures, the introduction of non-hexagonal rings inducing out-of-plane distortions is a promising pathway that has been scarcely explored on surfaces. Here, we demonstrate that non-hexagonal rings, in the form of tropone (cycloheptatrienone) moieties, are thermally transformed into phenyl or cyclopentadienone moieties upon an unprecedented surface-mediated retro–Buchner-type reaction involving a decarbonylation or an intramolecular rearrangement of the CO unit, respectively.

[EN] Glycosidases are phylogenetically widely distributed enzymes that are crucial for the cleavage of glycosidic bonds. Here, we present the exceptional properties of a putative ancestor of bacterial and eukaryotic family-1 glycosidases. The ancestral protein shares the TIM-barrel fold with its modern descendants but displays large regions with greatly enhanced conformational flexibility. Yet, the barrel core remains comparatively rigid and the ancestral glycosidase activity is stable, with an optimum temperature within the experimental range for thermophilic family-1 glycosidases. None of the ∼5500 reported crystallographic structures of ∼1400 modern glycosidases show a bound porphyrin. Remarkably, the ancestral glycosidase binds heme tightly and stoichiometrically at a well-defined buried site. Heme binding rigidifies this TIM-barrel and allosterically enhances catalysis. Our work demonstrates the capability of ancestral protein reconstructions to reveal valuable but unexpected biomolecular features when sampling distant sequence space. The potential of the ancestral glycosidase as a scaffold for custom catalysis and biosensor engineering is discussed.

Thiele (Th) and tetrabenzo-Chichibabin (TBC) derivatives with terminal dibenzocycloheptatriene (DBHept) units were prepared. A clear correlation between their electronic and molecular structures was stablished. Insights into their closed- or open-shell ground states were gained, where particular contribution of the heptagonal carbocycles as end-groups was proved. Remarkably, a thermally accessible triplet diradical configuration was confirmed for the DBHept-TBC compound.

New N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)-ethane-1,2-diamine derivatives bearing covalently linked OH and (CO)NH groups have been synthesized. The coordination of those pendant hydroxyl/amide groups to a Fe(ii) metal center is demonstrated both in solution, even in the presence of chloride as the counterion, and in solid state, by means of X-ray diffraction crystal structures. As a result of this coordination, the experimental bond dissociation free energies (BDFE) of O-H and (CO)N-H bonds are remarkably diminished down to 76.0 and 80.5 kcal mol respectively, which is also in agreement with DFT-based theoretical calculations. These BDFE values are in the range of commonly used hydrogen-atom donor reagents. The strategy presented here allows an unequivocal evaluation of the influence of metal coordination in X-H bond weakening in organic solvents which could be easily extended to other metal centers.

Lysozyme crystals grown in Fmoc-CF (Cys-Phe) hydrogels, unlike those grown in agarose, give rise to composite crystals that have an enhanced resistance against degradation caused by an intense exposure to X-ray irradiation. Fmoc-CF dipeptide shows a clear protection of the most sensitive groups (disulfide bonds and methionines) of the protein. The protection mediated by cysteine is exerted only in its gel state since cysteine in solution has an adverse effect. Probably, the reactive thiol groups of cysteine being locked within the rigid peptide fibers minimize cross-reactions with the proteins favoring the formation of protein crystals. Once located inside the protein crystal, the long peptide fibers are able to protect the protein against radiation damage. ©

Novel reinforced cross-linked lysozyme crystals containing homogeneous dispersions of single-walled carbon nanotubes bundles (SWCNTs) are produced and characterized. The incorporation of SWCNTs inside lysozyme crystals gives rise to reinforced composite materials with tunable mechanical strength and electronic conductivity, while preserving the crystal quality and morphology. These reinforced crystals show increased catalytic activity at higher temperatures, being active even above the denaturation temperature. The electron transport through the crystals is linked to the content and distribution of SWCNT bundles inside the crystals. The electron conduction through the crystals is isotropic and very efficient, presenting high conductivity values up to 600 nS at very low (0.05 wt%) SWCNT concentration. To obtain these crystals, a new protocol based on the in situ crystallization of lysozyme in composite SWCNT–peptide hydrogels is developed. These peptide hydrogels are able to homogeneously disperse bundles of hydrophobic SWCNTs allowing first, the crystallization of the enzyme lysozyme and second, transferring the new properties of the inorganic component to the crystals. Taken together, these composite crystals represent an example of the versatility of proteins as biological substrates in the generation of novel functional materials, opening the door to use them in catalysis and bioelectronics at macroscale.

Herein we describe a distorted ribbon-shaped nanographene exhibiting unprecedented combination of optical properties in graphene-related materials, namely upconversion based on two-photon absorption (TPA-UC) together with circularly polarized luminescence (CPL). The compound is a graphene molecule of ca. 2 nm length and 1 nm width with edge defects that promote the distortion of the otherwise planar lattice. The edge defects are an aromatic saddle-shaped ketone unit and a [5]carbohelicene moiety. This system is shown to combine two-photon absorption and circularly polarized luminescence and a remarkably long emission lifetime of 21.5 ns. The [5]helicene is responsible for the chiroptical activity while the push–pull geometry and the extended network of sp2 carbons are factors favoring the nonlinear absorption. Electronic structure theoretical calculations support the interpretation of the results.

Here, we report the synthesis and structural characterization of novel iron nanoparticles (FeNPs)-based short-peptide supramolecular hydrogels. These hybrid hydrogels composed of Fmoc-diphenylalanine (Fmoc-FF) peptide and FeNPs were prepared through the self-assembly of Fmoc-FF in a suspension containing FeNPs in the presence or absence of an external magnetic field. Optical images of these hydrogels revealed the formation of column-like aggregates of FeNPs when the gels were formed in the presence of a magnetic field. Moreover, the intricate structure derived from the interwoven nature of the fiber peptides with these FeNP column-like aggregates resulted in anisotropic materials, more rigid under shear forces applied perpendicularly to the direction of the aggregates, presenting under these conditions values of G0 (storage modulus) about 7 times those of the native hydrogel. To the best of our knowledge, this is the first example in which the mechanical properties of peptide hydrogels were strongly enhanced due to the presence of FeNPs. A theoretical model trying to explain this phenomenon is presented. Quite interesting CD, FTIR and synchrotron X-ray diffraction analyses indicated that the anti-parallel b-sheet arrangement of Fmoc-FF peptide was highly conserved in the hydrogels containing FeNPs. Moreover, FLCS measurements showed that the diffusion of a small solute through the hydrogel network was improved in hydrogels containing FeNPs, probably caused by the formation of preferential channels for diffusion. Taken together, our results provide a new method for the synthesis of novel hybrid Fmoc-FF-FeNPs anisotropic hydrogels with enhanced mechanical strength and water-like diffusion behavior, thus easing their application in drug delivery and tissue engineering

A new family of homochiral silver complexes based on carbophilic interactions with ortho-phenylene ethynylene (o-OPE) scaffolds containing up to two silver atoms are described. These compounds represent a unique class of complexes with chirality at the metal. Chiral induction is based on the inclusion of chiral sulfoxides, which allow efficient transfer of chirality to the helically folded o-OPE, leading to circularly polarized luminescence (CPL)- and vibrational circular dichroism (VCD)-active compounds. In the presence of silver(I) cations, carbophilic interactions dominate, which promote helical structures with a defined helicity. This is one of the very scarce examples of the use of such interactions in the attractive field of abiotic foldamers. The switching event has been extensively studied by using different chiroptical techniques, including circular dichroism, CPL, and VCD, and represents one of the few CPL switches described in the literature.

Highly distorted polycyclic aromatic hydrocarbons (PAHs) are predicted to be attractive goals in nanoscience owing to the new properties they can exhibit. We have shown that a variety of functionalized distorted heptagon-containing nanographenes can be easily prepared from simple building blocks by a sequence of Co-catalyzed cyclotrimerization and cyclodehydrogenation reactions. The versatility of this strategy allows easy subsequent enlargement of these nanostructures by Ni-catalyzed cross-coupling and final cyclodehydrogenation reactions. Soluble extended distorted nanographenes 1 and 2 containing heptagon and an edge-shared pentagon-heptagon combination have been synthesized. High distortion of the polycyclic backbone of 2 caused by non-hexagonal rings and a helicene moiety was confirmed by X-ray crystallography. Experimental data reveal promising optical and electronic properties for distorted PAHs with long fluorescence lifetimes (up to 14.5 ns) and low band gaps (down to 2.27 eV). This straightforward and versatile synthetic strategy, the observed long fluorescence lifetimes and the small optical and electrochemical band gaps for the presented compounds may promote the future implementation of distorted graphene molecules in electronic devices.