Helical Bilayer Nanographenes: Impact of the Helicene Length on the Structural, Electrochemical, Photophysical, and Chiroptical Properties.

Helical bilayer nanographenes (HBNGs) are chiral π-extended aromatic compounds consisting of two π-π stacked hexabenzocoronenes (HBCs) joined by a helicene, thus resembling van der Waals layered 2D materials. Herein, we compare [9]HBNG, [10]HBNG, and [11]HBNG helical bilayers endowed with [9], [10], and [11]helicenes embedded in their structure, respectively. Interestingly, the helicene length defines the overlapping degree between the two HBCs (number of benzene rings involved in π-π interactions between the two layers), being 26, 14, and 10 benzene rings, respectively, according to the X-ray analysis. Unexpectedly, the electrochemical study shows that the lesser π-extended system [9]HBNG shows the strongest electron donor character, in part by interlayer exchange resonance, and more red-shifted values of emission. Furthermore, [9]HBNG also shows exceptional chiroptical properties with the biggest values of gabs and glum (3.6 × 10-2) when compared to [10]HBNG and [11]HBNG owing to the fine alignment in the configuration of [9]HBNG between its electric and magnetic dipole transition moments. Furthermore, spectroelectrochemical studies as well as the fluorescence spectroscopy support the aforementioned experimental findings, thus confirming the strong impact of the helicene length on the properties of this new family of bilayer nanographenes.

Development of an α'-hydroxy enone for the aminocatalytic asymmetric formal conjugate addition of aldehydes to acrylates, vinyl ketones and acrolein.

Aminocatalytic asymmetric conjugate addition of aldehydes to Michael acceptors is a well established C-C bond forming methodology. However, various acrylic-type acceptors, including acrylic acid derivatives and acrolein, remain reluctant. Here we demonstrate that the internal H-bonding self-activation in α'-hydroxy enones allows them to react smoothly with enolizable aldehydes using commercially available aminocatalysts to afford adducts in good yields and high enantioselectivity. Straightforward conversion of the ketol moiety of these adducts into aldehyde, ketone and carboxylic acid functionalities offers an indirect, unified entry to products derived from acrolein, alkyl-vinyl ketones and acrylates, respectively.

Electronic Control of the Scholl Reaction: Selective Synthesis of Spiro vs Helical Nanographenes.

Scholl oxidation has become an essential reaction in the bottom-up synthesis of molecular nanographenes. Herein, we describe a Scholl reaction controlled by the electronic effects on the starting substrate (1 a, b). Anthracene-based polyphenylenes lead to spironanographenes under Scholl conditions. In contrast, an electron-deficient anthracene substrate affords a helically arranged molecular nanographene formed by two orthogonal dibenzo[fg,ij]phenanthro-[9,10,1,2,3-pqrst]pentaphene (DBPP) moieties linked through an octafluoroanthracene core. Density Functional Theory (DFT) calculations predict that electronic effects control either the first formation of spirocycles and subsequent Scholl reaction to form spironanographene 2, or the expected dehydrogenation reaction leading solely to the helical nanographene 3. The crystal structures of four of the new spiro compounds (syn 2, syn 9, anti 9 and syn 10) were solved by single crystal X-ray diffraction. The photophysical properties of the new molecular nanographene 3 reveal a remarkable dual fluorescent emission.

Tetrahedraphene: A Csp3 -centered 3D Molecular Nanographene Showing Aggregation-Induced Emission.

The bottom-up synthesis of 3D tetrakis(hexa-peri-hexabenzocoronenyl)methane, "tetrahedraphene", is reported. This molecular nanographene constituted by four hexa-peri-hexabenzocoronene (HBC) units attached to a central sp3 carbon atom, shows a highly symmetric arrangement of the HBC units disposed in the apex of a tetrahedron. The X-ray crystal structure reveals a tetrahedral symmetry of the molecule and the packing in the crystal is achieved mostly by CH⋅⋅⋅π interactions since the interstitial solvent molecules prevent the π⋅⋅⋅π interactions. In solution, tetrahedraphene shows the same electrochemical and photophysical properties as the hexa-t Bu-substituted HBC (t Bu-HBC) molecule. However, upon water addition, it undergoes a fluorescence change in solution and in the precipitated solid, showing an aggregation induced emission (AIE) process, probably derived from the restriction in the rotation and/or vibration of the HBCs. Time-Dependent Density Functional Theory (TDDFT) calculations reveal that upon aggregation, the high energy region of the emission band decreases in intensity, whereas the intensity of the red edge emission signal increases and presents a smoother decay, compared to the non-aggregated molecule. All in all, the excellent correlation between our simulations and the experimental findings allows explaining the colour change observed in the different solutions upon increasing the water fraction.

Initiating Electron Transfer in Doubly Curved Nanographene Upon Supramolecular Complexation of C60.

The formation of supramolecular complexes between C60 and a molecular nanographene endowed with both positive and negative curvatures is described. The presence of a corannulene moiety and the saddle shape of the molecular nanographene allows the formation of complexes with 1:1, 1:2, and 2:1 stoichiometries. The association constants for the three possible supramolecular complexes were determined by 1 H NMR titration. Furthermore, the stability of the three complexes was calculated by theoretical methods that also predict the photoinduced electron transfer from the curved nanographene to the electron acceptor C60 . Time-resolved transient absorption measurements on the ns-time scale showed that the addition of C60 to NG-1 solutions and photo-exciting them at 460 nm leads to the solvent-dependent formation of new species, in particular the formation of the one-electron reduced form of C60 in benzonitrile was observed.

Site-Specific Reduction-Induced Hydrogenation of a Helical Bilayer Nanographene with K and Rb Metals: Electron Multiaddition and Selective Rb+ Complexation.

The chemical reduction of π-conjugated bilayer nanographene 1 (C138 H120 ) with K and Rb in the presence of 18-crown-6 affords [K+ (18-crown-6)(THF)2 ][{K+ (18-crown-6)}2 (THF)0.5 ][C138 H122 3- ] (2) and [Rb+ (18-crown-6)2 ][{Rb+ (18-crown-6)}2 (C138 H122 3- )] (3). Whereas K+ cations are fully solvent-separated from the trianionic core thus affording a "naked" 1.3 - anion, Rb+ cations are coordinated to the negatively charged layers of 1.3 - . According to DFT calculations, the localization of the first two electrons in the helicene moiety leads to an unprecedented site-specific hydrogenation process at the carbon atoms located on the edge of the helicene backbone. This uncommon reduction-induced site-specific hydrogenation provokes dramatic changes in the (electronic) structure of 1 as the helicene backbone becomes more compressed and twisted upon chemical reduction, which results in a clear slippage of the bilayers.

Helically Arranged Chiral Molecular Nanographenes.

A benchtop solution-phase synthesis of molecular nanographenes composed of two orthogonal dibenzo[fg,ij]phenanthro[9,10,1,2,3-pqrst]pentaphene (DBPP) moieties covalently connected through a tetrafluorobenzene ring is described. The helical arrangement of these three covalently linked molecular fragments leads to the existence of a chiral axis which gives rise to a racemic mixture, even with the molecular moieties being symmetrically substituted. X-ray diffraction studies show that both enantiomers cocrystallize in a single crystal, and the racemic mixture can be resolved by chiral HPLC. Asymmetric substitution in DBPP moieties affords a pair of diastereoisomers whose rotational isomerization has been studied by 1H NMR. Additionally, the electrochemical and photophysical properties derived from these new molecular nanographenes reveal an electroactive character and a significant fluorescent behavior.

Chiral Molecular Carbon Nanostructures.

Chirality is a fascinating property present in naturally occurring and artificial molecules and materials, observable as chiroptical behavior. The emerging area of carbon nanostructures has undergone tremendous development, with a wide variety of carbon nanoforms reported over the last two decades. However, despite interest in merging chirality and nanocarbons, this has been successfully achieved only in empty fullerenes, whereas in other kinds of fullerenes or carbon nanostructures such as carbon nanotubes, graphene, and graphene quantum dots (GQDs), to name the most popular systems, it is almost unknown. Therefore, controlling chirality in carbon nanostructures currently represents a major challenge for the chemical community. In this Account, we show our progress in the synthesis of chiral molecular carbon nanostructures, namely, metallofullerenes, endohedral fullerenes, GQDs, and curved molecular nanographenes, by using asymmetric catalysis and both top-down and bottom-up chemical approaches. Furthermore, we bring in a new family of lesser-known molecular chiral bilayer nanographenes, where chirality is introduced from the starting helicene moiety and a single enantiomer of the nanographene is synthesized. Some important landmarks in the development of chiral molecular carbon nanostructures shown in this Account are the application of synthesis-tailored, enantiomerically pure metallofullerenes as catalysts for hydrogen transfer reactions and the use of endohedral fullerenes to determine the effect of the incarcerated molecule in the carbon cage on the cis-trans stereoisomerization of optically active pendent moieties. Furthermore, the first top-down synthesis of chiral GQDs by functionalization with chiral alcohols is also presented. An emerging alternative to GQDs, when the desire for purity and atomistic control outweighs the cost of multistep synthesis, is the bottom-up approach, in which molecular nanographenes are formed in precise sizes and shapes and enantiomeric control is feasible. In this regard, a singular and amazing example is given by our synthesis of a single enantiomer of the first chiral bilayer nanographene, which formally represents a new family of molecular nanographenes with chirality controlled and maintained throughout their syntheses. The aforementioned synthetic chiral nanostructures represent groundbreaking nanocarbon systems where chirality is a further dimension of structural control, paving the way to a new scenario for carbon nanoforms in which chirality selection determines the properties of these novel carbon-based materials. Fine-tuning of such properties is envisioned to impact biomedical and materials science applications.

Enantioselective Addition of Alkynyl Ketones to Nitroolefins Assisted by Brønsted Base/H-Bonding Catalysis.

Various sets of enolizable alkynyl ketones (including methyl ynones with α-aryl, α-alkenyl, and α-alkoxy groups) were able to react smoothly with nitroolefins with the assistance of bifunctional Brønsted base/H-bond catalysts to provide adducts with two consecutive tertiary stereocenters in a highly diastereo- and enantioselective fashion. Further transformation of the obtained adducts into optically active acyclic and polycyclic molecules, including some with intricate carbon skeletons, was also demonstrated.

π-Extended Corannulene-Based Nanographenes: Selective Formation of Negative Curvature.

A geometrically selective bottom-up synthesis of curved nanographenes is described. The synthetic methodology used involves the extension of the π-system of positively curved corannulene by a [4+2] cycloaddition reaction followed by cyclodehydrogenation (Scholl oxidation). By selecting the conditions for the Scholl oxidation, the formation of a seven-membered ring that also confers negative curvature to the resulting nanographene can be activated, offering two topologically distinct, curved nanographenes from a common precursor. Additionally, the structure-property relationship in these new nanographenes is explored via theoretical, electrochemical, photophysical, Raman, and X-ray crystallographic studies.

α-Hydroxy Ketones as Masked Ester Donors in Brønsted Base Catalyzed Conjugate Additions to Nitroalkenes.

The catalyst-controlled enantioselective direct addition reaction of enolizable esters and related carboxylic acid derivatives to π electrophiles remains a difficult synthetic transformation. In this study, the suitability of α-hydroxy ketones as ester equivalents capable of being activated by bifunctional Brønsted base catalysts in the context of conjugate addition reactions to nitroolefins is demonstrated. The scope of the reaction, which affords the corresponding Michael adducts with very high stereoselectivity (diastereomeric ratio (d.r.) ≥95:5, up to 99 % enantiomeric excess (ee)), and its limitations are explored, as is the aftermath elaboration of adducts into densely functionalized enantioenriched products.

Asymmetric Assembly of All-Carbon Tertiary/Quaternary Nonadjacent Stereocenters through Organocatalytic Conjugate Addition of α-Cyanoacetates to a Methacrylate Equivalent.

An efficient, highly diastereo- and enantioselective assembly of acyclic carbonyl fragments possessing nonadjacent all-carbon tertiary/quaternary stereoarrays is reported based on a Brønsted base catalyzed Michael addition/α-protonation sequence involving α-cyanoacetates and 2,4-dimethyl-4-hydroxypenten-3-one as novel methacrylate equivalent.

Segmental fat-free and fat mass measurements by bioelectrical impedance analysis in 2,224 healthy Spanish women aged 18-85 years.

OBJECTIVES: This study provides updated data on body composition in adult Spanish women. METHODS: We considered data, including height and weight, from a survey conducted on a total of 4,013 adult women between 2009 and 2010. A subgroup of 2,224 women completed a bioelectrical body impedance analysis of body composition using a Tanita Body Composition Analyzer (Model no. BF-418). RESULTS: Total fat mass (FM) gradually increased between 18 and 74 years of age and decreased thereafter. FM increased in both legs between 65 and 74 years of age (5.69 ± 1.4 Kg and 5.66 ± 1.4 Kg for right and left legs, respectively) and decreased thereafter. FM in the right arm increased until 44 years of age (1.22 ± 2.6 Kg), decreased to 1.19 ± 0.5 Kg between 45 and 54 years of age, and increased to 1.54 ± 0.63 from 65 to 74 years of age. FM in the left arm increased constantly until it reached a peak of 1.63 ± 0.6 Kg between 65 and 74 years of age and decreased thereafter. FM increased in the trunk throughout life (peaks at 13.27±3.8 Kg) until subjects reached between 65 and 74 years of age. Fat free mass increased until 44 years of age (42.42 ± 4.17 Kg) and decreased thereafter. The prevalence of overweight/obesity significantly increased with age in the entire sample. CONCLUSIONS: Our results indicate that age-related increases in weight were at least partially due to increased adiposity.

Enantioselective construction of tetrasubstituted stereogenic carbons through Brønsted base catalyzed michael reactions: α'-hydroxy enones as key enoate equivalent.

Catalytic and asymmetric Michael reactions constitute very powerful tools for the construction of new C-C bonds in synthesis, but most of the reports claiming high selectivity are limited to some specific combinations of nucleophile/electrophile compound types, and only few successful methods deal with the generation of all-carbon quaternary stereocenters. A contribution to solve this gap is presented here based on chiral bifunctional Brønsted base (BB) catalysis and the use of α'-oxy enones as enabling Michael acceptors with ambivalent H-bond acceptor/donor character, a yet unreported design element for bidentate enoate equivalents. It is found that the Michael addition of a range of enolizable carbonyl compounds that have previously demonstrated challenging (i.e., α-substituted 2-oxindoles, cyanoesters, oxazolones, thiazolones, and azlactones) to α'-oxy enones can afford the corresponding tetrasubstituted carbon stereocenters in high diastereo- and enantioselectivity in the presence of standard BB catalysts. Experiments show that the α'-oxy ketone moiety plays a key role in the above realizations, as parallel reactions under identical conditions but using the parent α,ß-unsaturated ketones or esters instead proceed sluggish and/or with poor stereoselectivity. A series of trivial chemical manipulations of the ketol moiety in adducts can produce the corresponding carboxy, aldehyde, and ketone compounds under very mild conditions, giving access to a variety of enantioenriched densely functionalized building blocks containing a fully substituted carbon stereocenter. A computational investigation to rationalize the mode of substrate activation and the reaction stereochemistry is also provided, and the proposed models are compared with related systems in the literature.

Catalytic enantioselective quick route to aldol-tethered 1,6- and 1,7-enynes from ω-unsaturated aldehydes.

An effective asymmetric route to functionalized 1,6- and 1,7-enynes has been developed based on a direct cross-aldol reaction between ω-unsaturated aldehydes and propargylic aldehydes (α,ß-ynals) promoted by combined α,α-dialkylprolinol ether/Brønsted acid catalysis. This synergistic activation strategy is key to accessing the corresponding aldol adducts with high stereoselectivity, both enantio- and diastereoselectivity. The aldol reaction also proceeds well with propargylic ketones (α,ß-ynones) thus enabling a stereocontrolled access to the corresponding tertiary alcohols. The utility of these adducts, which are difficult to prepare through standard methodology, is demonstrated by their transformation into trisubstituted bicyclic enones using standard Pauson-Khand conditions.

Quantitative ultrasound measurements of the calcaneus and hand phalanges in elderly Spanish men: relationship with peripheral bone mineral density of the hand phalanges.

OBJECTIVES: The aims of this pilot study were to describe quantitative ultrasound (US) measurements and peripheral bone mineral density (BMD) of the hand phalanges on dual-energy x-ray absorptiometry and to examine the correlations between them in elderly Spanish men. METHODS: We studied 199 healthy men (mean age ± SD, 73.31 ± 5.10 years). The participants were not taking any medications, and they reported no diseases, including diseases that are associated with abnormalities in mineral metabolism. Phalangeal and calcaneal quantitative US measurements and phalangeal BMD measurements were performed in all participants. RESULTS: A bivariate correlation analysis showed no association between quantitative US assessments at the phalanges or the calcaneus (P = .409). After adjustment for potential confounders, the correlation between phalangeal BMD and phalangeal quantitative US measurements was r = 0.417 (P < .0001), and the correlation for calcaneal quantitative US was r = 0.26 (P = .001). Further adjustment by percentage of body fat increased quantitative US correlations with phalangeal BMD: r = 0.450 (P < .0001) at the phalanges; r = 0.291 (P = .001) at the calcaneus. CONCLUSIONS: There is a small correlation between quantitative US measurements at the calcaneus and phalangeal BMD that increases to a moderate level with quantitative US measurements at the phalanges in elderly Spanish men.

α-Hydroxy ketones as useful templates in asymmetric reactions.

Approaching the Nature's efficiency in controlling both reactivity and stereoselectivity of organic reactions by means of a catalyst species remains a formidable challenge for chemists to face. Despite impressive advances in the design of novel catalysts and activation modes, current catalytic and asymmetric methodologies rarely meet desirable standards of robustness, substrate scope, and selectivity altogether. One trick to improve catalyst behaviour is to identify adequate substrate template-catalyst combinations so that optimum performance of the reaction system could be achieved. During the last couple of years α-hydroxy ketones, and most particularly α'-hydroxy enones, have emerged as useful templates with applications in a number of metal-catalyzed as well as organocatalyzed C-C and C-X bond-forming stereoselective reactions. The first review of these accomplishments is presented here along with a brief historical introduction.

Organocatalytic Michael Addition of Unactivated α-Branched Nitroalkanes to Afford Optically Active Tertiary Nitrocompounds.

The direct, asymmetric conjugate addition of unactivated α-branched nitroalkanes is developed based on the combined use of chiral amine/ureidoaminal bifunctional catalysts and a tunable acrylate template to provide tertiary nitrocompounds in 55-80% isolated yields and high enantioselectivity (e.r. up to 96:4). Elaboration of the ketol moiety in thus obtained adducts allows a fast entry to not only carboxylic and aldehyde derivatives but also nitrile compounds and enantioenriched 5,5-disubstituted γ-lactams.

Curved Nanographenes: Multiple Emission, Thermally Activated Delayed Fluorescence, and Non-Radiative Decay.

The intriguing and rich photophysical properties of three curved nanographenes (CNG 6, 7, and 8) are investigated by time-resolved and temperature-dependent photoluminescence (PL) spectroscopy. CNG 7 and 8 exhibit dual fluorescence, as well as dual phosphorescence at low temperature in the main PL bands. In addition, hot bands are detected in fluorescence as well as phosphorescence, and, in the narrow temperature range of 100-140 K, thermally activated delayed fluorescence (TADF) with lifetimes on the millisecond time-scale is observed. These findings are rationalized by quantum-chemical simulations, which predict a single minimum of the S1 potential of CNG 6, but two S1 minima for CNG 7 and CNG 8, with considerable geometric reorganization between them, in agreement with the experimental findings. Additionally, a higher-lying S2 minimum close to S1 is optimized for the three CNG, from where emission is also possible due to thermal activation and, hence, non-Kasha behavior. The presence of higher-lying dark triplet states close to the S1 minima provides mechanistic evidence for the TADF phenomena observed. Non-radiative decay of the T1 state appears to be thermally activated with activation energies of roughly 100 meV and leads to disappearance of phosphorescence and TADF at T > 140 K.

Genome sequence of the methanotrophic poly-ß-hydroxybutyrate producer Methylocystis parvus OBBP.

Methylocystis parvus OBBP is an obligate methylotroph considered the type species of the genus Methylocystis. Two pmoCAB particulate methane monooxygenase operons and one additional singleton pmoC paralog were identified in the sequence. No evidence of genes encoding soluble methane monooxygenase was found. Comparison of M. parvus OBBP and Methylocystis sp. strain Rockwell (ATCC 49242) suggests that both species should be taxonomically classified in different genera.