BN‐Patterning of Metallic Substrates through Metal Coordination of Decoupled Borazines

M. Schwarz, M. Garnica, F. Fasano, N. Demitri, D. Bonifazi, W. Auwärter
Chem. Eur. J. 2018, 24 (38), 9565-9571
DOI: 10.1002/chem.201800849

Abstract

We report on the synthesis of pyridine‐terminated borazine derivatives, their molecular self‐assembly as well as the electronic properties investigated on silver and copper surfaces by means of scanning tunneling microscopy and X‐ray photoelectron spectroscopy. The introduction of pyridine functionalities allows us to achieve distinct supramolecular architectures with control of the interdigitation of the molecules by surface templating. On silver surfaces, the borazine derivatives arrange in a dense‐packed hexagonal structure through van der Waals and H‐bonding interactions, whereas on Cu(111), the molecules undergo metal coordination. The porosity and coordination symmetry of the reticulated structure depends on the stoichiometric ratio between copper adatoms and the borazine ligands, permitting an unusual three‐fold coordinated Cu‐pyridyl network. Finally, spectroscopy measurements provide evidence that the borazine core is electronically decoupled from the metallic substrate. We thus integrate for the first time BNC‐containing molecular units into stable, metal‐coordination architectures on surfaces, opening pathways to patterned, BN‐doped sheets with specific functionalities, e.g., regarding the adsorption of polar guest gases.

Controlling the Functional Properties of Oligothiophene Crystalline Nano/Micro-Fibers via Tailoring of the Self-Assembling Molecular Precursors

F. Di Maria, M. Zangoli, M. Gazzano, E. Fabiano, D. Gentili, A. Zanelli, A. Fermi, G. Bergamini, D. Bonifazi, A. Perinot, M. Caironi, R. Mazzaro, V. Morandi, G. Gigli, A. Liscio, G. Barbarella
Adv. Funct. Mater., 2018, 28 (32), 1801946.
DOI: 10.1002/adfm.201801946

Abstract

Oligothiophenes are π-conjugated semiconducting and fluorescent molecules whose self-assembly properties are widely investigated for application in organic electronics, op-toelectronics, biophotonics and sensing. We report here an approach to the preparation of crystal-line oligothiophene nano/micro-fibers based on the use of a ‘sulfur overrich’ quaterthiophene building block, -T4S4-, containing in its covalent network all the information needed to promote the directional, π-π stacking driven, self-assembly of Y-T4S4-Y oligomers into fibers with hier-archical supramolecular arrangement from nano- to microscale. We show that when Y varies from unsubstituted thiophene to thiophene substituted with electron withdrawing groups, a wide redistribution of the molecular electronic charge takes place without substantially affecting the aggregation modalities of the oligomer. In this way a structurally comparable series of fibers is obtained having progressively varying optical properties, redox potentials, photoconductivity and type of prevailing charge carriers (from p- to n-type). A thorough characterization of the fibers based on SEM, CD, CV, X-ray diffraction, UV-vis and PL spectroscopies, photoconductivity and KPFM measurements is reported. With the aid of DFT calculations, combined with powder X-ray diffraction data, a model accounting for the growth of the fibers from molecular to nano- and microscale is proposed.

Kinked silicon nanowires-enabled interweaving electrode configuration for lithium-ion batteries

G. Sandu, M. Coulombier, V. Kumar, H. G. Kassa, I. Avram, R.Ye, A. Stopin, D. Bonifazi, J.-F.  Gohy, P. Leclère, X. Gonze, T. Pardoen, A. Vlad, S. Melinte
Sci. Rep. 2018, 8, 9794
DOI: 10.1038/s41598-018-28108-3

Abstract

A tri-dimensional interweaving kinked silicon nanowires (k-SiNWs) assembly, with a Ni current collector co-integrated, is evaluated as electrode configuration for lithium ion batteries.  The large-scale fabrication of k-SiNWs is based on a procedure for continuous metal assisted chemical etching of Si, supported by a chemical peeling step that enables the reuse of the Si substrate. The kinks are triggered by a simple, repetitive etch-quench sequence in a HF and H2O2-based etchant. We find that the inter-locking frameworks of k-SiNWs and multi-walled carbon nanotubes exhibit beneficial mechanical properties with a foam-like behavior amplified by the kinks and a suitable porosity for a minimal electrode deformation upon Li insertion.  In addition, ionic liquid electrolyte systems associated with the integrated Ni current collector repress the detrimental effects related to the Si-Li alloying reaction, enabling high cycling stability with 80% capacity retention (1695 mAh/gSi) after 100 cycles.  Areal capacities of 2.42 mAh/cm2 (1276 mAh/gelectrode) can be achieved at the maximum evaluated thickness (corresponding to 1.3 mgSi/cm2).  This work emphasizes the versatility of the metal assisted chemical etching for the synthesis of advanced Si nanostructures for high performance lithium ion battery electrodes.

O‐doped zig‐zag molecular ribbons

A. Berenzin, N. Biot, T. Battisti, D. Bonifazi
Angew. Chem., Int. Ed. 2018, 57 (29), 8942-8946.
DOI: 10.1002/anie.201803282

Abstract

The synthesis of the first O‐doped molecular rhombic ribbon featuring two zig‐zag peripheries, has been achieved. This includes oxidative C‐C and C‐O bond formations that allowed the stepwise elongation and planarization of a hexaoxa‐congener of 2,7‐periacenoacene. X‐ray diffraction analysis corroborated the flat structure and the topology of the zig‐zag O‐doped edges. Photophysical and electrochemical investigations showed that the extension of the PXX into the molecular ribbon induces a noticeable shrinking of the molecular band gap devised by a dramatic rising of the HOMO energy level, a desirable property for p‐type organic semiconductors.

Magnetic shepherding of nanocatalysts through hierarchically-assembled Fe-filled CNTs hybrids

M. Melchionna, A. Beltram, A. Stopin, T. Montini, R. W. Lodge, A. N. Khlobystov, D. Bonifazi, M. Prato, P. Fornasiero
Appl. Catal., B 2018, 227, 356-365.
DOI: 10.1016/j.apcatb.2018.01.049

Abstract

Mechanically robust, chemically stable and electronically active carbon nanotubes (CNTs) are widely used as supports in catalysis. Synergistic effects between CNT and the active phase critically depend on the homogeneity of the carbon/inorganic interface, whose assembly is difficult to achieve without admixtures of free-standing inorganic matrix. Here we show that Fe-filled CNTs, employed as nanocatalyst supports, allow a facile preparation of highly pure and uniform CNT/nanocatalyst materials, by taking advantage of magnetic separation from poorly-defined components (e.g. aggregates of inorganic nanocatalysts). The higher homogeneity translates into higher catalytic activity in two industrially important processes: the photocatalytic hydrogen production and the water-gas shift reaction, WGSR (increase of ∼48% activity for the former and up to ∼45% for the latter as compared to catalysts isolated by standard filtration). In addition, the magnetic Fe core in the nanotubes enables effective separation and re-use of the nanocatalyst without loss of activity. This study demonstrates significant potential of magnetic CNTs as next generation of sustainable catalyst supports that can improve production of hydrogen and reduce the use of precious metals.

Customizing Photoredox Properties of PXX‐based Dyes through Energy Level Rigid Shifts of Frontier Molecular Orbitals

A. Sciutto, A. Fermi, A. Folli, T. Battisti, J. M. Beames, D. M. Murphy, D. Bonifazi
Chem. Eur. J. 2017, Just accepted.
DOI: 10.1002/chem.201705620

Abstract

Here we describe the synthesis of electron‐rich PXX derivatives in which the energy levels of the excited states have been rigidly shifted through the insertion of imide groups. This has allowed the development of a new series of oxygen‐doped photoredox‐active chromophores with improved oxidizing and reducing properties. Capitalizing on the dehalogenation of organic halides as a model reaction, we could investigate the photooxidative and photoreductive potential of these molecules in model chemical transformations. Depending on the substrate, solvent and dye the reaction mechanism can follow different paths. This prompted us to consider the first chemoselective transformation protocol, in which two different C−Br bonds could be chemoselectively reacted through the sequential photoactivation of two different colorants.

Programming Recognition Arrays through Double Chalcogen-Bonding Interactions

N. Biot, D. Bonifazi
Chem. Eur. J. 2017, Just accepted.
DOI: 10.1002/chem.201705428

Abstract

In this work, we have programmed and synthesized a recognition motif constructed around a chalcogenazolo-pyridine scaffold (CGP) that, through the formation of frontal double chalcogen-bonding interactions, associates into dimeric EX-type complexes. The reliability of the double chalcogen-bonding interaction has been shown at the solid-state by X-ray analysis, depicting the strongest recognition persistence for a Te-congener. The high recognition fidelity, chemical and thermal stability and easy derivatization at the 2-position makes CGP a convenient motif for constructing supramolecular architectures through programmed chalcogen-bonding interactions.

 

Toward fractioning of isomers through binding-induced acceleration of azobenzene switching

R. Vulcano, P. Pengo, S. Velari, J. Wouters, A. De Vita, P. Tecilla, D. Bonifazi
J. Am. Chem. Soc., 2017, Just accepted.
DOI: 10.1021/jacs.7b09568

Abstract

The E/Z isomerization process of a uracil-azobenzene derivative in which the nucleobase is conjugated to a phenyldiazene tail is studied in view of its ability to form triply-H-bonded complexes with a suitably complementary 2,6-diacetylamino-4-pyridine ligand. UV-Vis and 1H-NMR investigations of the photochemical and thermal isomerization kinetics show that the thermal ZE interconversion is four-fold accelerated upon formation of the H-bonded complex. DFT calculations show that the formation of triple H-bonds triggers a significant elongation of the N=N double bond, caused by an increase of its πg antibonding character. This results in a reduction of the N=N torsional barrier and thus in accelerated thermal ZE isomerization. Combined with light controlled EZ isomerization this enables controllable fractional tuning of the two configurational isomers.

 

Renaissance of an Old Topic: From Borazines to BN-doped Nanographenes

M. – L. Lorenzo-García, D. Bonifazi
CHIMIA Int. J. C., 2017, 71550-557.
DOI: 10.2533/chimia.2017.550

Abstract

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.

A twisted bay-substituted quaterrylene phosphorescing in the NIR spectral region

T. Miletić, A. Fermi, I. Papadakis, I. Orfanos, N. Karampitsos, A. Avramopoulos, N. Demitri, F.De Leo, S. J. A. Pope, M. G.  Papadopoulos, S. Couris, D. Bonifazi
Helv. Chim. Acta, 2017, Just accepted.
DOI: 10.1002/hlca.201700192

Abstract

The preparation of the first soluble quaterrylene derivative featuring peripheral tert-butyl substituents and sterically hindering, core-anchored triflate groups has been achieved. This involves a facile synthetic route based on an oxidative coupling of perylene precursors in the presence of H2O2 as oxidant. The steric hindrance between the OTf substituents at the central bay position of the quaterrylene board triggers a strong deformation of the central perylene planarity, which forces the quaterrylene platform to adopt a twisted geometry as shown by X-Ray analysis. Exceptionally, photophysical investigations show that the core-twisted quaterrylene phosphoresces in the NIR spectral region at 1716 nm. Moreover, third-order nonlinear optical (NLO) measurments on solutions and thin film containing the relevant molecule showed very large second hyperpolarizability values, as predicted by theoretical calculations at the CAM-B3LYP/6-31G** level of theory, making this material very appealing for photonic applications.