1,721,009 research outputs found
Spettroscopia Raman e polimorfismo: il ruolo dei fononi
No full textPer quanto la spettroscopia Raman sia una tecnica consolidata nello studio del polimorfismo, l'attenzione alla zona spettrale dei modi reticolari (fononi ottici) non ha ancora assunto il rilievo che merita la sua potenzialità di indagine. In questo articolo si illustra brevemente lo stato dell'arte al riguardo, con rilevanti applicazioni nel campo della tecnologia e della farmaceutica
Solid-state photodimerization of 9-methyl-anthracene
No full textRaman microscopy has been used to study the crystal-to-crystal photodimerization of 9-methyl-anthracene, analyzing the time evolution of both the molecular and the lattice phonon spectra during the transformation from the reactant to the product. This has made possible the concomitant detection of the chemical and crystal lattice changes in the same portion of the irradiated crystal. The topochemical mechanism of this reaction produces a fast process leading to a complete transformation of the monomer into the dimer. Copyright © 2016 John Wiley & Sons, Ltd
Structural Order and Thermal Behavior of Ph-BTBT-10 Monolayer Phases
Full text linkIR and Raman spectroscopy investigations are carried out on single crystals of the organic semiconductor 7-decyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) subjected to thermal cycling through the boundaries of its Smectic E (SmE) and Smectic A liquid crystal phases, gaining insight on their structural order. IR measurements in polarized light provide a precise description of both the orientational order of the molecule BTBT’s rigid cores and the conformational status of its substituents in the mesophases and shed light on the processes occurring at the phase transitions. The Raman mapping of the crystals obtained in the reverse SmE-to-crystal transition reveals the presence of a distinctive polycrystallinity pattern in the recovered samples, an effect which may contribute to the understanding of the variation of the charge carrier mobility of devices experiencing thermal annealing processes
Charge Transfer in Molecular Cocrystals: A Plane Wave vs Localized-Orbital View─Structural Information Obtained from Calculated Raman and IR Phonons
No full textThis article aims at laying the foundation for the development of electronic structure-property relationships in the field of charge-transfer (CT) organic semiconductors by studying the low-frequency phonon response of donor-acceptor (DA) compounds. In this article, it is shown and discussed how and why phonon frequencies and their delocalization depend on the interplay between single-molecule properties and crystal structures. These results are obtained by combining results from state-of-the-art quantum-mechanical calculations, carried out within both localized orbitals and a plane wave paradigm, with simple classical models. CT compounds obtained from perylene (Pery) as the donor and FnTCNQ (n = 0, 4) as the acceptors are considered
Simulated Raman spectra of four tetraphenylbutadiene polymorphs
No full textWe report a combined experimental and theoretical investigation on the Raman spectra of the polymorphs α, β, γ, and δ of 1,1,4,4-tetraphenyl-1,3-butadiene (TPB), in the region of the intramolecular modes. The interpretation of the polarized spectra is supported by ab-initio calculations for the isolated molecules and by lattice dynamics calculations for the crystals. The calculations reproduce the peculiar, and surprisingly large, differences among the spectra of the various polymorphs. The phenyl groups of 1,1,4,4-tetraphenyl-1,3-butadiene may arrange themselves around the butadiene skeleton in 2 stable conformers, which have either inversion (Ci) or 2-fold (C2) symmetry and therefore exhibit intramolecular vibrations with quite different Raman selection rules and spectra. The compound forms 4 crystalline polymorphs (α, β, γ, and δ) with different combinations of Ci and C2 conformers, and correspondingly different intramolecular spectra. The theoretical calculations provide a quantitative analysis of the various spectra
The impact of solid solution composition on kinetics and mechanism of [2 + 2] photodimerization of cinnamic acid derivatives
Full text linkWe propose the use of solid solutions for the modulation of the kinetic in [2+2] photocycloaddition. Solid solutions with general formula [1H]BrxCl1‒x (1 = 4-amino-cinnamic acid; 0 < x < 1) were prepared via mechanochemical methods, structurally characterized, and found to undergo a single-crystal-to-single-crystal [2+2] photodimerization. The effect of the solid solutions composition on the kinetics was successfully investigated via a combination of FTIR and Principal Component Analysis (PCA), and the results were compared with those obtained for the parent materials [1H]Cl and [1H]Br. All systems follow the same nearly first-order law, and the overall reaction kinetics is influenced by a subtle interplay of structural and electronic factors as well as other processes regulating the formation and lifetime of the reacting excimer
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
Electron-phonon coupling and mobility modeling in organic semiconductors: Method and application to two tetracene polymorphs
No full textWe have developed a first-principles method to calculate the electron-phonon coupling for specific modes and q points in the Brillouin zone for crystalline organic semiconductors. Using the obtained coupling strengths, we propose an approach to compute the temperature-dependent mobilities of electrons and holes.
To validate our treatment of the electronic structures and vibrational properties, we calculate the Raman spectra in the lattice-phonon region and compare them with experimental data. We then compare the computed mobilities with available data for single crystals of naphthalene, anthracene, and tetracene (bulk polymorph), finding good agreement within the experimental range, especially when accounting for possible charged impurities. Finally, we discuss the observed differences between tetracene polymorphs
Fast identification of rubrene polymorphs by lattice phonon Raman microscopy
No full textConfocal Raman microscopy in the lattice phonon region has been used to study the polymorphism of the organic semiconductor 5,6,11,12-tetraphenyl-tetracene (rubrene). Following literature guidelines, crystals of rubrene have been prepared using a number of solution growth and vapour deposition methods, obtaining samples of different morphologies which could be related to the various polymorphs of this compound. The technique has enabled us an easy and non invasive identification of the three known polymorphs and of their phase homogeneity with a lateral spatial resolution below 1 Î1⁄4m
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