1,721,003 research outputs found
GAMMA-ISOTACTIC POLYPROPYLENE - AN MAS C-13 NMR-STUDY OF A CRYSTALLINE POLYMER WITH NONPARALLEL CHAINS
No full textPorous Supramolecular Architectures: Ultra-fast Molecular Rotors and Dynamics Control by Chemical Stimuli
No full textA challenging issue is the dynamics of nanoporous solids after the insertion of molecular rotors in their building blocks and promises access to the control of rotary motion by chemical and physical stimuli.[1] The combination of porosity with ultra-fast rotor dynamics was discovered in molecular crystals, covalent organic frameworks and MOFs by 2H spin-echo NMR spectroscopy and T1 relaxation times.[2-5] The rotors, as fast as 1011 Hz at 150 K, are exposed to the crystalline channels, which absorb CO2 and I2 from the gas phase, even at low pressures. Interestingly, the rotor dynamics can be switched on and off by vapor absorption/desorption, showing a remarkable change of material dynamics, which, in turn, produces a modulated physical response. Novel mesoporous organosiloxane frameworks allowed us to realize periodic architectures of fast molecular rotors on which C-F dipoles are mounted.[6] These dipolar rotors showed not only rapid dynamics (109 Hz at 325 K) in solid-state NMR experiments, but also a dielectric response typical of a fast dipole reorientation. Moreover, crystals with permanent porosity were exploited in an unusual way to decorate crystal surfaces with regular arrays of dipolar rotors. The inserted molecules carry alkyl chains which are included as guests into the channel-ends.[7] The rotors stay at the surface due to a bulky molecular stopper which prevents the rotors from entering the channels. In a final example, flexible molecular crystals were fabricated by a series of shape-persistent azobenzene tetramers that form porous molecular crystals in their trans configuration. The efficient trans→cis photoisomerization of the azobenzene units converts the crystals into a non-porous phase but crystallinity and porosity are restored upon Z→E isomerization promoted by visible light irradiation or heating. We demonstrated that the photoisomerization enables reversible on/off switching of optical properties as well as the capture of CO2 from the gas phase.[8]
We would like to thank Cariplo Foundation/Lombardy Region/INSTM Consortium.
References
1. A. Comotti, S. Bracco, P. Sozzani Acc. Chem. Res. 2016, 49, 1701.
2. S. Bracco, et al Chem. Eur. J. 2017, 23, 11210.
3. A. Comotti, et al J. Am. Chem. Soc. 2014, 136, 618.
4. A. Comotti, et al Angew. Chem. Int. Ed. 2014, 53, 1043.
5. S. Bracco, et al Chem. Comm. 2017, 53, 7776.
6. S. Bracco, et al Angew. Chem. Int Ed. 2015, 54, 4773.
7. L. Kobr, et al J. Am. Chem. Soc. 2012, 134, 10122.
8. M. Baroncini et al. Nature Chem. 2015, 7, 634
Novel Cellulosic Ethers with Low Degrees of Substitution - II. Magic Angle Spinning NMR Study
No full textMACROMOL. CHEM. PHYS
Dynamics and Flexibility in Gas-absorptive Porous Materials
No full textThe enormous interest manifested in recent years for porous materials has generated efficient systems for adsorbing gases of great interest for energy and the environment, such as CO2, CH4 and H2. Our approach was to design porosity in combination with switchable dynamics and flexibility for gaining control over gas capture and selectivity. This approach was made possible by fabricating rotor-on-axel molecular struts and tetrahedral building blocks.
Rotor-on-axel molecular struts. Ultra-fast molecular rotors were realized in porous solids by engineering crystalline frameworks (molecular crystals, MOFs and mesoporous organosilicas) containing rod-like linkers as amphidynamic elements.1-5 The porous frameworks promise access to the control of rotary motion by chemical and physical stimuli. We prepared rotors as fast as 1011 Hz (in the regime of conventional liquids) in properly designed porous crystals. Rotor dynamics was successfully hampered by the diffusion of gases or vapors. In turn, the on/off switching of dynamics produces modulated physical responses when C-F dipoles were mounted on the rotors.6 Direct evidence of host-guest relationships and interactions at the molecular level were established by 2D solid-state NMR and modelled by ab initio calculations.
Tetrahedral building-blocks. In the search for flexible molecular crystals endowed with porosity, we achieved the fabrication of expandable crystalline prototypal structures, which allow for the absorption of gases, without modifying the crystal architecture.7 The design brings together highly symmetrical tetrahedral elements to construct swellable porous adamantoid frameworks. The flexibility of the porous crystals manifests itself in response to stimuli of selected gases: the contact with CO2, Xe and hexane triggers the enlargement of channel cross-section and capacity. The accomodation of CO2 and Xe in the channel chambers was revealed by synchrotron-light XRD, combined with Molecular Dynamics and DFT calculations. Xenon dynamics, exploring various cavity orientations in the crystal, were gathered by 129Xe NMR chemical shift anisotropy profiles, which encode the shape and orientation of each visited cavity. Jump rate and activation energy experienced by exploring Xe atoms were uniquely established.
Moreover, photo-responsive molecular crystals were fabricated by a series of tetrahedral azobenzene tetramers that form porous molecular crystals in their trans configuration.8 The efficient trans-to-cis photoisomerization of the azobenzene units converts the crystals into a non-porous phase but crystallinity and porosity are restored upon cis-to-trans reverse isomerization promoted by visible light or heat. We demonstrated that the photo-isomerization enables reversible on/off switching of optical properties as well as CO2 capture from the gas phase. Acknowldegements to PRIN 2016 2016-NAZ-0104
(1) A. Comotti, A. et al. Acc. Chem. Res. 49 (2016) 1701-1710.
(2) S. Bracco, et al. Chem. Eur. J. 23 (2017) 11210.
(3) A. Comotti, et al. J. Am. Chem. Soc. 136 (2014) 618.
(4) A. Comotti, et al. Angew. Chem. Int. Ed. 53 (2014) 1043.
(5) S. Bracco, et al. Chem. Comm. 53 (2017) 7776.
(6) S. Bracco, et al. Angew. Chem. Int Ed. 54 (2015) 4773.
(7) I. Bassanetti, et al. J. Mater. Chem. A 6 (2018) 14231.
(8) M. Baroncini, et al. Nature Chem. 7 (2015) 634
NMR studies, molecular characterization, and degradation behaviour of poly(amido amine)s.-1-Poly(amido amine) deriving from the polyaddition of 2-methylpiperazine to 1,4-bis(acryloyl) piperazine
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
