Search results for "Transition temperature"
showing 10 items of 137 documents
Positronium as a probe in natural polymers: decomposition in starch
2009
Ortho-positronium (o-Ps) is used as a probe in positron annihilation lifetime spectroscopy (PALS) experiments, to characterise the behaviour of free volumes in natural starch samples, as a function of temperature (T). Up to about 540 K, the o-Ps intensity, I(3), remains constant at 26.2% while its lifetime, tau(3), is found to increase linearly. Both parameters undergo a decrease above this T, due to the onset of decomposition, which results in a shrinking of the sample pellets. The results indicate that the glass transition temperature should be above 501 K. Data from thermal gravimetry analysis (TGA) measurements are well described by supposing a first order process for the survival proba…
Pressure effect on temperature induced high-spin–low-spin phase transitions
2002
The effect of hydrostatic pressure on the transition temperature and the hysteresis widths of first-order spin crossover phase transitions is considered in the frame of the mean field theory and on the basis of the scope of recent pressure experiments. Relevant parameters for a qualitative description of the behaviour of spin transition compounds under pressure are derived and analysed.
Photochemical electrocyclisation of 3-vinylindoles to pyrido[2,3-a]-, pyrido[4,3-a]- and thieno[2,3-a]-carbazoles: Design, synthesis, DNA binding and…
2009
In the context of the design and synthesis of DNA ligands, some new hetarene annelated carbazoles were synthesized. As lead structure the intercalating tetracyclic systems pyrido[2,3-a]- and pyrido[4,3-a]-carbazoles and in one case a thieno[2,3-a]-carbazole were taken into account. A dialkyl amino amidic chain was introduced to the planar chromophoric system with the intent to generate minor groove binding properties. The cytotoxicity of some compounds was examined by the NCI antitumor screening. Furthermore, biophysical as well as biochemical studies were performed in order to get some information about the DNA-binding properties and inhibition of DNA related functional enzymes of this new…
Cooperativity in the Iron(II) Spin-Crossover Compound [Fe(ptz)6](PF6)2 under the Influence of External Pressure (ptz = 1-n-Propyltetrazole)
1997
The iron(II) spin-crossover compound [Fe(ptz)(6)](PF(6))(2) (ptz = 1-propyltetrazole) crystallizes in the triclinic space group Ponemacr;, with a = 10.6439(4) Å, b = 10.8685(4) Å, c = 11.7014(4) Å, alpha = 75.644(1) degrees, beta = 71.671(1) degrees, gamma = 60.815(1) degrees, and Z = 1. In [Fe(ptz)(6)](PF(6))(2), the thermal spin transition is extremely steep because of cooperative effects of elastic origin. The transition temperature at ambient pressure is 74(1) K. An external pressure of 1 kbar shifts the transition temperature to 102(1) K, corresponding to a stabilization of the low-spin state, which is smaller in volume. The volume difference between the high-spin and the low-spin stat…
Electronic Relaxation Phenomena Following 57Co(EC)57Fe Nuclear Decay in [MnII(terpy)2](ClO4)2·1/2H2O and in the Spin Crossover Complexes [CoII(terpy)…
2001
The valence states of the nucleogenic 57Fe arising from the nuclear disintegration of radioactive 57Co by electron capture decay, 57Co(EC)57Fe, have been studied by Mossbauer emission spectroscopy (MES) in the 57Co-labeled systems: [57Co/Co(terpy)2]Cl2·5H2O (1), [57Co/Co(terpy)2](ClO4)2·1/2H2O (2), and [57Co/Mn(terpy)2](ClO4)2· 1/2H2O (3) (terpy = 2,2‘:6‘,2‘ ‘-terpyridine). The compounds 1, 2, and 3 were labeled with ca. 1 mCi of 57Co and were used as the Mossbauer sources at variable temperatures between 300 K and ca. 4 K. [Fe(terpy)2]X2 is a diamagnetic low-spin (LS) complex, independent of the nature of the anion X, while [Co(terpy)2]X2 complexes show gradual spin transition as the temp…
Microphase separation in ternary ABC block copolymers: Ordering control in molten diblock AB copolymers by attaching a short strongly interacting C b…
1997
Some specific features of the microphase separation in molten ternary ABC triblock copolymers (in particular, modifications of the molten AB diblock copolymers by attaching a short third block C strongly incompatible with both A and B blocks) are studied rigorously within the framework of the weak segragation approach. It is shown that via such a modification one can control both the stability as to the microphase separation transition (i.e., the corresponding spinodal temparatures) and the smoothness of such a transition. Two modes of such modifications for every composition are studied in detail: (i) those resulting in the minimal spinodal temperatures and (ii) those that result in the s…
Studies of host-guest thin films of corona-poled betaine-type polar molecules by kelvin probe technique and atomic force microscopy
2004
In this work betaine-type molecules were investigated. As a result of the asymmetry of charge distribution, molecules possess in the ground state a considerable permanent dipole moment. The decay of surface potential of poled polymer films is dependent at least on two relaxation processes. The influence of glass transition of PMMA on thermal dependence of the surface potential is shown. The transition temperature, where no changes of the surface potential appeared, is related to glass transition temperature of the host-guest system. The topography of the film surface was obtained by AFM.
Charge Mobility and Dynamics in Spin-Crossover Nanoparticles Studied by Time-Resolved Microwave Conductivity
2018
We use the electrode-less time-resolved microwave conductivity (TRMC) technique to characterize spin-crossover (SCO) nanoparticles. We show that TRMC is a simple and accurate mean for simultaneously as-sessing the magnetic state of SCO compounds and charge transport information on the nanometre length scale. In the low-spin state from liquid nitrogen temperature up to 360 K the TRMC measurements present two well-defined regimes in the mobility and in the half-life times, possessing similar transition tempera-tures TR near 225 K. Below TR, an activation-less regime associated with short lifetimes of the charge carri-ers points at the presence of shallow-trap states. Above TR, these states ar…
Coordination isomerism in spin crossover (SCO) materials
2021
International audience; A new series of three spin crossover (SCO) Fe(II) complexes based on a cyanocarbanion and on the neutral quinolin-8-amine (aqin) ligands, [Fe(aqin)2(tcnsme)2] (1), [Fe(aqin)2(tcnset)2] (2), and [Fe(aqin)2(tcnspr)2] (3), has been studied. The three complexes display similar molecular structures consisting of discrete [Fe(aqin)2(tcnsR)2] complexes [R = Me (1), Et (2), and Pr (3)]. Infrared spectroscopy and magnetic studies, performed on the three complexes, revealed the presence of similar SCO behaviors which strongly differ by their transition temperatures [234 K (1) < 266 K (2) < 360 K (3)]. The increase of the transition temperatures when passing from 1 to 3 may be …
Short range charge/orbital ordering in La1−xSrxMn1−zBzO3 (B = Cu,Zn) manganites
2005
We have measured the reflectivity spectra of La1−x SrxMn1−zBzO3 (B = Cu, Zn; 0.17 x 0.30; 0 z 0.10) manganites over wide frequency (100–4000 cm −1 )a nd temperature (80–300 K) ranges. Besides the previously observed infrared active modes or mode pairs at about 160 cm −1 (external mode), 350 cm −1 (bond bending mode) and 590 cm −1 (bond stretching mode), we have clearly observed two additional phonon modes at about 645 and 720 cm −1 below the temperature T1 (T1 < TC), which coincides with the phase transition temperature when the system transforms from ferromagnetic metallic into a ferromagnetic insulator state. This transition is related to the formation of short range charge/orbitally orde…