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71.
Karolis Norvaiša Sophie Maguire Claire Donohoe Dr. John E. O'Brien Dr. Brendan Twamley Dr. Ligia C. Gomes-da-Silva Prof. Dr. Mathias O. Senge 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(4):e202103879
Inspired by the rigidified architecture of ‘picket-fence’ systems, we propose a strategy utilizing strain to impose intramolecular tension in already peripherally overcrowded structures leading to selective atropisomeric conversion. Employing this approach, tuneable shape-persistent porphyrin conformations were acquired exhibiting distinctive supramolecular nanostructures based on the orientation of the peripheral groups. The intrinsic assemblies driven by non-covalent bonding interactions form supramolecular polymers while encapsulating small molecules in parallel channels or solvent-accessible voids. The developed molecular strain engineering methodologies combined with synthetic approaches have allowed the introduction of the pivalate units creating a highly strained molecular skeleton. Changes in the absorption spectrum indicated the presence of severe steric repulsions between the peripheral groups which were confirmed by single crystal X-ray analysis. To release the steric strain introduced by the peripheral units, thermal equilibration strategies were used to selectively convert the most abundant atropisomer to the desirable minor one. 相似文献
72.
Maria J. Sanchis Ricardo Díaz‐Calleja Abel García‐Bernabé Luz Alegría Ligia Gargallo Deodato Radic 《Journal of Polymer Science.Polymer Physics》2008,46(2):109-120
Polymers consisting of poly(heterocyclic methacrylate)s are considered as potential materials for clinical applications such as drug delivery and cartilage repair. Much of the success of these systems has been attributed to the complex nature of their water sorption properties. Dielectric permittivity is very sensitive to water sorption. Dielectric relaxation spectroscopy studies have been carried out on two heterocyclic poly(methacrylate)s: poly(tetrahydrofurfuryl methacrylate) (PTHFMA) and poly(3‐methyl tetrahydrofurfuryl methacrylate) (P3MTHFMA). The isochrones representing the dielectric losses show in both cases high conductivity at low frequencies and high temperatures. In PTHFMA two conductive processes are observed, which can be associated to the existence of two types of water sorption. These effects have been analyzed and were removed from the dielectric spectra by using classical empirical equations. Both polymers show ostensible α‐relaxation centered in the vicinity of 350 K at 100 Hz. This relaxation was analyzed by means of the empirical Havriliak‐Negami equation. Reminiscent β‐relaxation could also exist. Both polymers present well defined γ and δ subglass absorptions at approximately 120 K, 160 K for PTHFMA and 125 K, 163 K for P3MTHFMA, at 100 Hz, associated to local intramolecular relaxations in side groups. These relaxations were analyzed using semiempirical symmetric model. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 109–120, 2008 相似文献
73.
Maria Cazacu Angelica Vlad Grigore Munteanu Anton Airinei 《Journal of polymer science. Part A, Polymer chemistry》2008,46(5):1862-1872
New polyazomethines have been synthesized by the reaction between 2,5‐dihydroxy‐1,4‐benzoquinone and siloxane diamines differing by the siloxane sequence length. A dimer has also been prepared as a model compound. The products were characterized by spectral (FTIR and 1H‐NMR) and elemental analyses, GPC, viscosity measurements, solubility tests, and transmission electron microscopy (TEM). The different properties have been investigated by adequate techniques: thermal (DSC and TGA), spectral (UV–vis and fluorescence spectroscopy), redox (Differential Pulse Voltammetry). pH‐sensitivity and metal complexing ability were also evaluated. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1862–1872, 2008 相似文献
74.
Photoluminescence attributed to excitons bound to neutral impurities has been observed from GaAs quantum wells in AlxGa1?xAs-GaAs heterostructures grown by molecular beam epitaxy. The quantum wells were either doped with [Be] ≈ 1017 cm-3 or Zn-diffused. At low temperatures both single and multiple quantum wells exhibited this extrinsic luminescence which is ascribed to the radiative recombination of the n=1 ground state heavy hole exciton E1h bound to a neutral acceptor Ao. The dissociation energy ED of the Ao-E1h complex is obtained directly from the measured separation of this extrinsic peak from the intrinsic E1h free exciton peak. For 46Å wide GaAs wells, ED=6.5meV and ED decreases with increasing well width. 相似文献
75.
When intermediate mass stars reach their last stages of evolution they show pronounced oscillations. This phenomenon happens when these stars reach the so-called asymptotic giant branch (AGB), which is a region of the Hertzsprung-Russell diagram located at about the same region of effective temperatures but at larger luminosities than those of regular giant stars. The period of these oscillations depends on the mass of the star. There is growing evidence that these oscillations are highly correlated with mass loss and that, as the mass loss increases, the pulsations become more chaotic. In this paper we study a simple oscillator which accounts for the observed properties of this kind of stars. This oscillator was first proposed and studied in Icke et al. [Astron. Astrophys. 258, 341 (1992)] and we extend their study to the region of more massive and luminous stars -the region of super-AGB stars. The oscillator consists of a periodic nonlinear perturbation of a linear Hamiltonian system. The formalism of dynamical systems theory has been used to explore the associated Poincare map for the range of parameters typical of those stars. We have studied and characterized the dynamical behavior of the oscillator as the parameters of the model are varied, leading us to explore a sequence of local and global bifurcations. Among these, a tripling bifurcation is remarkable, which allows us to show that the Poincare map is a nontwist area preserving map. Meandering curves, hierarchical-islands traps and sticky orbits also show up. We discuss the implications of the stickiness phenomenon in the evolution and stability of the super-AGB stars. (c) 2002 American Institute of Physics. 相似文献
76.
F Borcan CM Soica S Ganta MM Amiji CA Dehelean O Cerga MF Munteanu 《Chemistry Central journal》2012,6(1):87
ABSTRACT: BACKGROUND: Polymers have been considered as important materials in fabrication of microstructures for various medical purposes including drug delivery. This study evaluates polyurethane as material for hollow microstructures preparation. RESULTS: Polyurethane microstructures were obtained by interfacial polyaddition combined with spontaneous emulsification and present slightly acid pH values. Scanning electron microscopy revealed the existence of irregular shapes and agglomerated microstructures. The material is heat resistant up to 280 [DEGREE SIGN]C. Good results were recorded on murine skin tests in case of polyurethane microstructures based on isophorone diisocyanate. Mesenchymal stem cells viability presents good results for the same sample after 48 hours based on the Alamar Blue test. CONCLUSIONS: The research revealed the reduced noxiousness of this type of microstructures and consequently the possibility of their use for therapeutic purposes. 相似文献
77.
E. M. F. Curado J. P. Gazeau Ligia M. C. S. Rodrigues 《Journal of statistical physics》2012,146(2):264-280
We examine a generalization of the binomial distribution associated with a strictly increasing sequence of numbers and we
prove its Poisson-like limit. Such generalizations might be found in quantum optics with imperfect detection. We discuss under
which conditions this distribution can have a probabilistic interpretation. 相似文献
78.
Tanţa Spătaru Takeshi Kondo Crina Anastasescu Ioan Balint Petre Osiceanu Cornel Munteanu Nicolae Spătaru Akira Fujishima 《Journal of Solid State Electrochemistry》2017,21(4):1007-1014
An innovative composite was obtained by a straightforward sol-gel procedure, involving boron-doped diamond powder (BDDP) incorporation into a SiO2 veil (SiO2V) matrix. Composite-coated glassy carbon plates were used as substrate for Pt electrochemical deposition, and the electrodes thus obtained (Pt/BDDP–SiO2V) were compared on a relative basis with those prepared in the absence of the silica matrix (Pt/BDDP). SEM measurements have shown that a BDDP substrate promotes Pt cluster formation, whereas on BDDP–SiO2V, particles are much smaller (ca. 45 nm to ca. 140 nm). The activity for CH3OH oxidation was checked by cyclic voltammetry, and it was found that at Pt/BDDP–SiO2V, the main anodic peak is shifted with ca. 0.35 V toward lower potentials, indicating a considerable improvement in the overall process kinetics. Stripping experiments together with long-term polarization measurements demonstrated that when deposited on the BDDP–SiO2V support, Pt particles are less susceptible to CO poisoning and this behavior was tentatively ascribed to the presence of a higher relative surface concentration of more stable, oxidized platinum species, as evidenced by XPS. 相似文献
79.
It iswell-known that the positionvector function is themost basic geometric object for a surface immersed in the three dimensional Euclidean space $\mathbb{E}^3 $ . In 2001, B.-Y. Chen defined constant ratio hypersurfaces in Euclidean n-spaces. Independently, in 2010, by using another approach in dimension 3, the second author classified constant slope surfaces. In this paper, we extend this concept in order to study surfaces with the property that the tangential component of the position vector is a principal direction on the surface. 相似文献
80.
Ligia Todan Cristian Andronescu Dumitru Mircea Vuluga Daniela C. Culita Maria Zaharescu 《Journal of Thermal Analysis and Calorimetry》2013,114(1):91-99
Phosphosilicate glasses are of great interest in important fields, such as optical active systems, energy generating systems, humidity sensors, and as materials for biomedical applications. Many studies were accomplished to establish the influence of different reaction parameters on the evolution and final structure of sol–gel prepared phosphosilicate gels. In the present work, we studied the thermal behavior of the silicophosphate gels obtained starting with different phosphorous precursors, the influence of these precursors on the composition and structure of the resultant gels, and their evolution with thermal treatment. By Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and thermodifferential analysis (DTA/TG), and Differential Scanning calorimetry coupled with Mass Spectrometry (DSC-MS), it was established that the type of the precursors essentially influences the composition and structure and consequently the thermal behavior of the obtained gels. In the case of triethylphosphate precursor, all used methods of investigation have shown that the ester is trapped in the silica matrix and it is eliminated during the thermal treatment. Triethylphosphite partially hydrolizes and reacts with the silica network during post-preparation thermal treatment. Only in the case of H3PO4, an interaction with TEOS takes place and leads to Si–O–P bond formation. By thermal treatment, the gels with different composition and structure lead to materials with different properties. 相似文献