Thiophene-based helicenes : eclectic scaffolds in organometallic chemistry

Helicenes are ortho-annulated polycyclic aromatic compounds, endowed with an inherently chiral \u3c0-conjugated system that are intensively studied in different areas of science.1 Among helicenes, thiophene-based helicenes are emerging as an intriguing and promising class of screw-shaped structures, thanks to the presence of the thiophene rings, which confer peculiar chemical, structural, and electronic features.2 For several years, we have been interested in the synthesis and functionalization of tetrathia[7]helicenes (7-TH, Figure 1), which are configurationally stable heterohelicenes, potentially very interesting for applications in optoelectronics,3 catalysis,4 and biology.5 In particular, transition metal-based 7-TH systems are an extremely appealing class of complexes, in which the coordination of metals with the \u3c0-helical ligand, bearing proper coordinating groups, provides unusual chiral architectures. Indeed, the selective functionalization of 2 and 13 positions of the 7-TH scaffold allows the introduction of a variety of substituents, including those with efficient coordinating ability (e.g. cyano, phosphane, phosphine oxide). In this communication we report our recent studies on the synthesis and characterization of 7-TH-based organometallic complexes, and their potential applications in optoelectronics and catalysis

Synthesis and characterization of a tetrathia[7]helicenebased rhenium(I) complex

Tetrathia[7]helicenes (7-TH), formed by thiophene and benzene rings ortho-fused in an alternating fashion, are emerging as one of the most popular class of chiral helical-shaped molecules, thanks to their peculiar electronic and chiroptical properties suitable for manifold applications in different areas of science.1 In particular, transition metal-based 7-TH systems are an extremely appealing class of complexes, in which the coordination of metals with the \uf070-helical ligand, bearing appropriate coordinating functionalities, provides original chiral architectures. Indeed, the effective functionalization of the \uf061-position(s) of the terminal thiophene ring(s) of the 7-TH scaffold allows the introduction of a variety of substituents, including those with efficient coordinating ability (e.g. cyano2, phosphane3, phosphine oxide4). For example, Rh(I)5 and Au(I)6 complexes based on 7-TH phosphanes have been successfully used in the homogenous transition metal catalysis. In our ongoing studies on 7-TH-based organometallic complexes, we have focused on a novel field of investigation concerning the development of rhenium-based polynuclear complexes containing 7-TH phosphine oxide ligands. In this communication, we describe the synthesis and the characterization of a novel dinuclear rhenium(I) complex (Figure 1), along with the elucidation of its tridimensional structure by single crystal X-ray diffraction studies

Visible and near-infrared observations of asteroid 2012 DA14 during its closest approach of February 15, 2013

Near-Earth asteroid 2012 DA14 made its closest approach on February 15, 2013, when it passed at a distance of 27,700 km from the Earth's surface. It was the first time an asteroid of moderate size was predicted to approach that close to the Earth, becoming bright enough to permit a detailed study from ground-based telescopes. Asteroid 2012 DA14 was poorly characterized before its closest approach. We acquired data using several telescopes on four Spanish observatories: the 10.4m Gran Telescopio Canarias (GTC) and the 3.6m Telescopio Nazionale Galileo (TNG), both in the El Roque de los Muchachos Observatory (ORM, La Palma); the 2.2m CAHA telescope, in the Calar Alto Observatory (Almeria); the f/3 0.77m telescope in the La Hita Observatory (Toledo); and the f/8 1.5m telescope in the Sierra Nevada Observatory (OSN, Granada). We obtained visible and near-infrared color photometry, visible spectra and time-series photometry. Visible spectra together with color photometry of 2012 DA14 show that it can be classified as an L-type asteroid, a rare spectral type with a composition similar to that of carbonaceous chondrites. The time-series photometry provides a rotational period of 8.95 +- 0.08 hours after the closest approach, and there are indications that the object suffered a spin-up during this event. The large amplitude of the light curve suggests that the object is very elongated and irregular, with an equivalent diameter of around 18m. We obtain an absolute magnitude of H_R = 24.5 +- 0.2, corresponding to H_V = 25.0 +- 0.2. The GTC photometry also gives H_V = 25.29 +- 0.14. Both values agree with the value listed at the Minor Planet Center shortly after discovery. From the absolute photometry, together with some constraints on size and shape, we compute a geometric albedo of p_V = 0.44 +- 0.20, which is slightly above the range of albedos known for L-type asteroids (0.082 - 0.405).Comment: 7 pages, 4 figures, 1 table. Accepted in A&A (June 17 2013

Transmission curves and effective refraction indices of MKO near infrared consortium filters at cryogenic temperatures

We report transmission measurements at cryogenic temperatures for 4 broad-band filters of the Mauna Kea Observatories (MKO) near-infrared filter set and 5 narrow-band filters. The spectral scans were collected using the multi-mode IR instrument of the TNG telescope (NICS) in which these filters are permanently mounted and commonly used for astronomical observations. We determined the transmission curves at a temperature of 78K and found no significant red-leak up to 2.6 microns, the data are available in electronic form on the TNG web page. We also estimated the variation of the wavelength response with the incidence angle and found it compatible with an effective refractive index of about 2

Visible spectroscopy of the new ESO Large Program on trans-Neptunian objects and Centaurs: final results

A second large programme (LP) for the physical studies of TNOs and Centaurs, started at ESO Cerro Paranal on October 2006 to obtain high-quality data, has recently been concluded. In this paper we present the spectra of these pristine bodies obtained in the visible range during the last two semesters of the LP. We investigate the spectral behaviour of the TNOs and Centaurs observed, and we analyse the spectral slopes distribution of the full data set coming from this LP and from the literature. We computed the spectral slope for each observed object, and searched for possible weak absorption features. A statistical analysis was performed on a total sample of 73 TNOs and Centaurs to look for possible correlations between dynamical classes, orbital parameters, and spectral gradient. We obtained new spectra for 28 bodies, 15 of which were observed for the first time. All the new presented spectra are featureless, including 2003 AZ84, for which a faint and broad absorption band possibly attributed to hydrated silicates on its surface has been reported. The data confirm a wide variety of spectral behaviours, with neutral--grey to very red gradients. An analysis of the spectral slopes available from this LP and in the literature for a total sample of 73 Centaurs and TNOs shows that there is a lack of very red objects in the classical population. We present the results of the statistical analysis of the spectral slope distribution versus orbital parameters. In particular, we confirm a strong anticorrelation between spectral slope and orbital inclination for the classical population. A strong correlation is also found between the spectral slope and orbital eccentricity for resonant TNOs, with objects having higher spectral slope values with increasing eccentricity.Comment: 11 pages, 9 figure

(65) Cybele: detection of small silicate grains, water-ice, and organics

Context. (65) Cybele is the most representative member of a population of primitive asteroids in the outer edge of the main belt, the Cybele asteroids. Recent dynamical models suggest that a significant fraction of them originated in the primordial transneptunian disk, so the study of the physical properties of these asteroids is potentially a useful test of these models. Aims. Our aim is to obtain information on the surface composition of this asteroid. In particular we want to obtain information on the composition and properties of the regolith and the possible presence of ices and organic materials. Methods. We present 2-4 mu m and 5-14 mu m spectroscopy of (65) Cybele obtained with the NASA IRTF telescope and Spitzer Space Telescope respectively. We compare the results with spectra of Trojan asteroids and asteroid (24) Themis. We analyze the 2-4 mu m spectrum using scattering models and we apply thermal models to the 5-14 mu m data. Results. The 2-4 mu m spectrum of (65) Cybele presents an absorption band centered at similar to 3.1 mu m and more weaker bands in the 3.2-3.6 mu m region, very similar to those observed in (24) Themis. No hydrated silicates are detected. From the spectrum in the 5-14 mu m region an effective diameter D = 290 +/- 5 km, a beaming paramete eta = 0.967 +/- 0.014, and a geometric visible albedo pV = 0.05 +/- 0.01 are derived using the NEATM thermal model. The emisivity spectrum in the 5-14 mu m range exhibits an emission plateau at about 9 to 12 mu m with an spectral contrast of similar to 5%. This emission is similar to that of Trojan asteroids and active comets and may be due to small silicate grains being imbedded in a relatively transparent matrix, or to a very under-dense (fairy-castle) surface structure. The lower amplitude of the silicate emission in Cybele\u27s spectrum with respect to that of Trojan asteroids could be attributed to larger dust particles and/or a slightly denser structure. Conclusions. The surface of (65) Cybele is covered by a fine anhydrous silicate grains mantle, with a small amount of water ice and complex organic solids. This is similar to comet surface where non-equilibrium phases coexist. The presence of water-ice and anhydrous silicates is indicative that hydration did not happened or is incomplete, suggesting that the temperatures were always sufficiently low

Photometry of cometary nuclei: Rotation rates, colours and a comparison with Kuiper Belt Objects

We present time-series data on Jupiter Family Comets (JFCs) 17P/Holmes, 47P/Ashbrook-Jackson and 137P/Shoemaker-Levy 2. In addition we also present results from `snap-shot' observations of comets 43P/Wolf-Harrington, 44P/Reinmuth 2, 103P/Hartley 2 and 104P/Kowal 2 taken during the same run. The comets were at heliocentric distances of between 3 and 7 AU at this time. We present measurements of size and activity levels for the snap-shot targets. The time-series data allow us to constrain rotation periods and shapes, and thus bulk densities. We also measure colour indices (V-R) and (R-I) and reliable radii for these comets. We compare all of our findings to date with similar results for other comets and Kuiper Belt Objects (KBOs). We find that the rotational properties of nuclei and KBOs are very similar, that there is evidence for a cut-off in bulk densities at ~ 0.6 g cm^{-3} in both populations, and the colours of the two populations show similar correlations. For JFCs there is no observational evidence for the optical colours being dependant on either position in the orbit or on orbital parameters.Comment: 15 pages, 19 figures, accepted for publication in MNRA

The nuclei of comets 7P/Pons-Winnecke, 14P/Wolf and 92P/Sanguin

Jupiter Family comets (JFCs) are short period comets which have recently entered the inner solar system, having previously orbited in the Kuiper Belt since the formation of the planets. We used two nights on the 3.6m New Technology Telescope (NTT) at the European Southern Observatory, to obtain VRI photometry of three JFCs; 7P/Pons-Winnecke, 14P/Wolf and 92P/Sanguin. These were observed to be stellar in appearance. We find mean effective radii of 2.24 \pm 0.02 km for 7P, 3.16 \pm 0.01 km for 14P and 2.08 \pm 0.01 km for 92P, assuming a geometric albedo of 0.04. From light-curves for each comet we find rotation periods of 7.53 \pm 0.10 and 6.22 \pm 0.05 hours for 14P and 92P respectively. 7P exhibits brightness variations which imply a rotation period of 6.8 \le P_rot \le 9.5 hours. Assuming the nuclei to be ellipsoidal the measured brightness variations imply minimum axial ratios a/b of 1.3 \pm 0.1 for 7P and 1.7 \pm 0.1 for both 14P and 92P. This in turn implies minimum densities of 0.23 \pm 0.08 g cm^{-3} for 7P, 0.32 \pm 0.02 g cm^{-3} for 14P and 0.49 \pm 0.06 g cm^{-3} for 92P. Finally, we measure colour indices of (V-R) = 0.40 \pm 0.05 and (R-I) = 0.41 \pm 0.06 for 7P/Pons-Winnecke, (V-R) = 0.57 \pm 0.07 and (R-I) = 0.51 \pm 0.06 for 14P/Wolf, and (V-R) = 0.54 \pm 0.04 and (R-I) = 0.54 \pm 0.04 for 92P/Sanguin.Comment: 12 pages, 14 figures, accepted for publication in A&

Spitzer observations of the asteroid-comet transition object and potential spacecraft target 107P (4015) Wilson-Harrington

Context. Near-Earth asteroid-comet transition object 107P/ (4015) Wilson-Harrington is a possible target of the joint European Space Agency (ESA) and Japanese Aerospace Exploration Agency (JAXA) Marco Polo sample return mission. Physical studies of this object are relevant to this mission, and also to understanding its asteroidal or cometary nature. Aims. Our aim is to obtain significant new constraints on the surface thermal properties of this object. Methods. We present mid-infrared photometry in two filters (16 and 22 microns) obtained with NASA's Spitzer Space Telescope on February 12, 2007, and results from the application of the Near Earth Asteroid Thermal Model (NEATM).We obtained high S/N in two mid-IR bands allowing accurate measurements of its thermal emission. Results. We obtain a well constrained beaming parameter (eta = 1.39 +/- 0.26) and obtain a diameter and geometric albedo of D = 3.46 +/- 0.32 km, and pV = 0.059 +/- 0.011. We also obtain similar results when we apply this best-fitting thermal model to single-band mid-IR photometry reported by Campins et al. (1995), Kraemer et al. (2005) and Reach et al. (2007). Conclusions. The albedo of 4015 Wilson-Harrington is low, consistent with those of comet nuclei and primitive C-, P-, D-type asteorids. We establish a rough lower limit for the thermal inertia of W-H of 60 Jm^-2s^(-0.5)K^-1 when it is at r=1AU, which is slightly over the limit of 30 Jm^-2s^(-0.5)K-1 derived by Groussin et al. (2009) for the thermal inertia of the nucleus of comet 22P/Kopff.Comment: 4 pages, 1 figure and 3 tables. Paper accepted for publicatio

The binary near-Earth asteroid (175706) 1996 FG3 - An observational constraint on its orbital evolution

Using our photometric observations taken between 1996 and 2013 and other published data, we derived properties of the binary near-Earth asteroid (175706) 1996 FG3 including new measurements constraining evolution of the mutual orbit with potential consequences for the entire binary asteroid population. We also refined previously determined values of parameters of both components, making 1996 FG3 one of the most well understood binary asteroid systems. We determined the orbital vector with a substantially greater accuracy than before and we also placed constraints on a stability of the orbit. Specifically, the ecliptic longitude and latitude of the orbital pole are 266{\deg} and -83{\deg}, respectively, with the mean radius of the uncertainty area of 4{\deg}, and the orbital period is 16.1508 +/- 0.0002 h (all quoted uncertainties correspond to 3sigma). We looked for a quadratic drift of the mean anomaly of the satellite and obtained a value of 0.04 +/- 0.20 deg/yr^2, i.e., consistent with zero. The drift is substantially lower than predicted by the pure binary YORP (BYORP) theory of McMahon and Scheeres (McMahon, J., Scheeres, D. [2010]. Icarus 209, 494-509) and it is consistent with the theory of an equilibrium between BYORP and tidal torques for synchronous binary asteroids as proposed by Jacobson and Scheeres (Jacobson, S.A., Scheeres, D. [2011]. ApJ Letters, 736, L19). Based on the assumption of equilibrium, we derived a ratio of the quality factor and tidal Love number of Q/k = 2.4 x 10^5 uncertain by a factor of five. We also derived a product of the rigidity and quality factor of mu Q = 1.3 x 10^7 Pa using the theory that assumes an elastic response of the asteroid material to the tidal forces. This very low value indicates that the primary of 1996 FG3 is a 'rubble pile', and it also calls for a re-thinking of the tidal energy dissipation in close asteroid binary systems.Comment: Many changes based on referees comment