163 research outputs found
Polymorphism, phonon dynamics and carrier-phonon coupling in pentacene
The crystal structure and phonon dynamics of pentacene is computed with the
Quasi Harmonic Lattice Dynamics (QHLD) method, based on atom-atom potential. We
show that two crystalline phases of pentacene exist, rather similar in
thermodynamic stability and in molecular density. The two phases can be easily
distinguished by Raman spectroscopy in the 10-100 cm-1 spectral region. We have
not found any temperature induced phase transition, whereas a sluggish phase
change to the denser phase is induced by pressure. The bandwidths of the two
phases are slightly different. The charge carrier coupling to low-frequency
phonons is calculated.Comment: 6 pages, 3 figures. Presented at ICFPAM-
Bulk and Surface-Mediated Polymorphs of Bio-Inspired Dyes Organic Semiconductors: The Role of Lattice Phonons in their Investigation
AbstractRaman spectroscopy of organic molecular materials in the low‐wavenumber region gives access to lattice vibrational modes and to the wealth of information on solid state properties that these can provide. In the field of organic electronics a useful application concerns the discrimination of the crystalline forms i. e. polymorphism of the semiconductor. The capability of characterizing and identifying the polymorphs of a compound is in fact the prerequisite for an exhaustive study of the charge transport characteristics which arise from the relationship between molecular, electronic, and crystal structures. Thus, the need is felt of a non‐invasive, non‐destructive tool such as Raman, which probes the crystal phase by detecting the lattice modes which are sensitive even to subtle variations of the packing. Here we review the contribution of the technique to the study of organic pigments displaying promising semiconducting properties and characterized by polymorphism both in their bulk and thin film phases
Solution equilibrium between two structures of Perylene-F <sub>2</sub> TCNQ charge transfer co-crystals
We report on the solution growth of the two known structures of Perylene-F(2)TCNQ charge transfer complexes. The transformation accompanied by a marked morphological change from needle 1:1 to platelet 3:2 crystal structure is observed in the mother liquor. Lattice phonon Raman spectroscopy is used for an easy structure identification of the different morphologies before and after the process. X-ray and lattice phonons spectra of reference samples obtained by physical vapor transport is used to identify the two complexes. A fully spectroscopic analysis of the intramolecular Raman modes is presented to estimate the degree of ionicity, which is found to agree with the value previously reported
Phonons and structures of tetracene polymorphs at low temperature and high pressure
Crystals of tetracene have been studied by means of lattice phonon Raman
spectroscopy as a function of temperature and pressure. Two different phases
(polymorphs I and II) have been obtained, depending on sample preparation and
history. Polymorph I is the most frequently grown phase, stable at ambient
conditions. A pressure induced phase transition, observed above 1 GPa, leads to
polymorph II, which is also obtained at temperatures below 140 K. Polymorph II
can also be maintained at ambient conditions.
We have calculated the crystallographic structures and phonon frequencies as
a function of temperature, starting from the configurations of the energy
minima found by exploring the potential energy surface of crystalline
tetracene. The spectra calculated for the first and second deepest minima match
satisfactorily those measured for polymorphs I and II, respectively. All
published x-ray structures, once assigned to the appropriate polymorph, are
also reproduced.Comment: 8 pages, 5 figures, RevTeX4, update after referees report
(Perylene)3-(TCNQF1)2: Yet Another Member in the Series of Perylene–TCNQFx Polymorphic Charge Transfer Crystals
The 3:2 Charge Transfer (CT) co-crystal (Perylene)3(TCNQF1)2 is grown by the Physical Vapor Transport (PVT) method, and characterized structurally and spectroscopically. Infrared analysis of the charge sensitive modes reveals a low degree of charge transfer (less than 0.1) between donor and acceptor molecules. The crystal is isostructural to the other 3:2 CT crystals formed by Perylene with TCNQF2 and TCNQF4, whereas such stoichiometry and packing is not known for the CT crystals with non-fluorinated TCNQ. The analysis of the isostructural family of 3:2 Perylene–TCNQFx (x = 1,2,4) co-crystal put in evidence the role of weak F…HC bonding in stabilizing this type of structure</jats:p
Engineering plastic phase transitions via solid solutions: the case of “reordering frustration” in ionic plastic crystals of hydroxyquinuclidinium salts
A family of salts of R-(+)-(3)-hydroxyquinuclidinium [QH]+, with SO42−, BPh4−, BF4− and PF6− counter-anions, have been prepared by the metathesis of [QH]Cl and metal salts of the corresponding anions. Solid solutions of formula [QH](PF6)x(BF4)1−x for x = 0.9, 0.8, 0.7 have also been obtained. The crystalline materials have been investigated by a combination of solid-state techniques, including variable temperature XRD, thermal analyses, multinuclear (11B, 13C, 15N, 19F, and 31P) solid-state NMR spectroscopy, variable temperature wideline 19F T1 relaxation measurements, and micro-Raman spectroscopy to investigate their thermal stability and phase transition behaviors. It has been shown that the salts [QH]PF6 and [QH]BF4 undergo an order–disorder solid–solid phase transition to plastic phases, whereas [QH]2SO4·H2O and [QH]BPh4 do not display any plastic phase transition. Doping [QH]BF4 into the [QH]PF6 lattice up to 30% results in the formation of a solid solution that is plastic in an expanded thermal range, thanks to a phenomenon that we describe here for the first time as “reordering frustration”
BEDT-TTF organic superconductors: the entangled role of phonons
We calculate the lattice phonons and the electron-phonon coupling of the
organic superconductor \kappa-(BEDT-TTF)_2 I_3, reproducing all available
experimental data connected to phonon dynamics. Low-frequency intra-molecular
vibrations are strongly mixed to lattice phonons. Both acoustic and optical
phonons are appreciably coupled to electrons through the modulation of the
hopping integrals (e-LP coupling). By comparing the results relevant to
superconducting \kappa- and \beta-(BEDT-TTF)_2 I_3, we show that
electron-phonon coupling is fundamental to the pairing mechanism. Both e-LP and
electron-molecular vibration (e-MV) coupling are essential to reproduce the
critical temperatures. The e-LP coupling is stronger, but e-MV is instrumental
to increase the average phonon frequency.Comment: 4 pages, including 4 figures. Published version, with Ref. 17
corrected after publicatio
Donor Simvastatin Treatment Is Safe and Might Improve Outcomes After Liver Transplantation: A Randomized Clinical Trial.
BACKGROUND
The current curative approaches for ischemia/reperfusion injury on liver transplantation are still under debate for their safety and efficacy in patients with end-stage liver disease. We present the SIMVA statin donor treatment before Liver Transplants study.
METHODS
SIMVA statin donor treatment before Liver Transplants is a monocentric, double-blind, randomized, prospective tial aiming to compare the safety and efficacy of preoperative brain-dead donors' treatment with the intragastric administration of 80 mg of simvastatin on liver transplant recipient outcomes in a real-life setting. Primary aim was incidence of patient and graft survival at 90 and 180 d post-transplant; secondary end-points were severe complications.
RESULTS
The trial enrolled 58 adult patients (18-65 y old). The minimum follow-up was 6 mo. No patient or graft was lost at 90 or 180 d in the experimental group (n = 28), whereas patient/graft survival were 93.1% (P = 0.016) and 89.66% (P = 0.080) at 90 d and 86.21% (P = 0.041) and 86.2% (P = 0.041) at 180 d in the control group (n = 29). The percentage of patients with severe complications (Clavien-Dindo ≥IIIb) was higher in the control group, 55.2% versus 25.0% in the experimental group (P = 0.0307). The only significant difference in liver tests was a significantly higher gamma-glutamyl transferase and alkaline phosphatase at 15 d (P = 0.017), (P = 0.015) in the simvastatin group.
CONCLUSIONS
Donor simvastatin treatment is safe, and may significantly improve early graft and patient survival after liver transplantation, although further research is mandatory
Quasi Harmonic Lattice Dynamics and Molecular Dynamics calculations for the Lennard-Jones solids
We present Molecular Dynamics (MD), Quasi Harmonic Lattice Dynamics (QHLD)
and Energy Minimization (EM) calculations for the crystal structure of Ne, Ar,
Kr and Xe as a function of pressure and temperature. New Lennard-Jones (LJ)
parameters are obtained for Ne, Kr and Xe to reproduce the experimental
pressure dependence of the density. We employ a simple method which combines
results of QHLD and MD calculations to achieve densities in good agreement with
experiment from 0 K to melting. Melting is discussed in connection with
intrinsic instability of the solid as given by the QHLD approximation. (See
http://www.fci.unibo.it/~valle for related papers)Comment: 7 pages, 5 figures, REVte
Structure, stoichiometry, and charge transfer in cocrystals of perylene with TCNQ-F<sub>x</sub>
Semiconductor charge transfer (CT) cocrystals are an emerging class of molecular materials which combines the characteristics of the constituent molecules in order to tune physical properties. Cocrystals can exhibit polymorphism, but different stoichiometries of the donor-acceptor (DA) pair can also give different structures. In addition, the structures of the donor and acceptor as pristine compounds can influence the resulting cocrystal forms. We report a structural study on several CT cocrystals obtained by combining the polyaromatic hydrocarbon perylene with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its fluorinated derivatives having increasing electronegativity. This is achieved by varying the amount of fluorine substitution on the aromatic ring, with TCNQ-F2 and TCNQ-F4. We find structures with different stoichiometries. Namely, the system perylene:TCNQ-F0 is found with ratios 1:1 and 3:1, while the systems perylene:TCNQ-Fx (x = 2, 4) are found with ratios 1:1 and 3:2. We discuss the structures on the basis of the polymorphism of perylene as pure compound, and show that by a judicious choice of growth temperature the crystal structure can be in principle designed a priori. We also analyze the structural motifs taking into account the degree of charge transfer between the perylene donor and the TCNQ-Fx acceptors and the optical gap determined from infrared (IR) spectroscopy. This family of materials exhibits tunable optical gaps in the near-IR (NIR), promising applications in organic optoelectronics
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