1,721,023 research outputs found
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
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
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
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
Computing the structural and vibrational properties of polymorphic organic molecular crystals through van der Waals corrected density functional theory and the electronic properties of organic thin films through microelectrostatic calculations.
Full text linkThe present Thesis reports on the various research projects to which I have contributed during my PhD period, working with several research groups, and whose results have been communicated in a number of scientific publications.
The main focus of my research activity was to learn, test, exploit and extend the recently developed vdW-DFT (van der Waals corrected Density Functional Theory) methods for computing the structural, vibrational and electronic properties of ordered molecular crystals from first principles. A secondary, and more recent, research activity has been the analysis with microelectrostatic methods of Molecular Dynamics (MD) simulations of disordered molecular systems.
While only very unreliable methods based on empirical models were practically usable until a few years ago, accurate calculations of the crystal energy are now possible, thanks to very fast modern computers and to the excellent performance of the best vdW-DFT methods. Accurate energies are particularly important for describing organic molecular solids, since they often exhibit several alternative crystal structures (polymorphs), with very different packing arrangements but very small energy differences. Standard DFT methods do not describe the long-range electron correlations which give rise to the vdW interactions. Although weak, these interactions are extremely sensitive to the packing arrangement, and neglecting them used to be a problem. The calculations of reliable crystal structures and vibrational frequencies has been made possible only recently, thanks to development of some good representations of the vdW contribution to the energy (known as “vdW corrections”)
Programma del corso, con dispense on-line
No full textProgramma del corso. Dispense on-line, in PDF, HTML ed altri format
Toward a Reliable Description of the Lattice Vibrations in Organic Molecular Crystals: The Impact of van der Waals Interactions
Full text linkThis
work assesses
the reliability of different van der Waals (vdW)
methods to describe lattice vibrations of molecular crystals in the
framework of density functional theory (DFT). To accomplish this task,
calculated and experimental lattice phonon Raman spectra of a pool
of organic molecular crystals are compared. We show that the many-body
dispersion (MBD@rsSCS) van der Waals method of Ambrosetti et al. and
the pairwise method of Grimme et al. (D3-BJ) outperform the other
tested approaches (i.e., the D2 method of Grimme, the TS method of
Tkatchenko and Scheffler, and the nonlocal functional vdW-DF-optPBE
of Klimeš et al.). For the worse-performing approaches the
results could not even be fixed by the introduction of scaling parameters,
as commonly used for high-energy intramolecular vibrations. Interestingly,
when using the experimentally determined unit cell parameters, DFT
calculations using the PBE functional without corrections for long-range
vdW interactions provide spectra of similar accuracy as the MBD@rsSCS
and D3-BJ simulations
Structure, Stoichiometry, and Charge Transfer in Cocrystals of Perylene with TCNQ-Fx
Full text linkSemiconductor 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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