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The organic semiconductor dibenzotetrathiafulvalene (DB-TTF) presents, at ambient conditions, four polymorphic modifications. In addition to the already known monoclinic structure (α) with Z= 2, we have resolved by single crystal X-ray diffraction (XRD) a second crystal structure (β), also monoclinic but with Z= 4. Two other polymorphic modifications (γ and δ) have been found by means of lattice phonon confocal Raman microscopy and XRD. In this paper we present a series of growth experiments, from vapour or from solution, in films and in the bulk phase, describing the variety of transformations which occur among the different phases and their mixing, also at a micrometric scale. Quasi harmonic lattice dynamics calculations satisfactorily reproduce the structures of the α and the β polymorphs and help the phonon assignment of polarized Raman spectra in single crystals. [ABSTRACT FROM AUTHOR]

Car−Parrinello molecular dynamics (CPMD) calculations are presented for a Na +(Phe) complex in aqueous solution and for various stable Na +(Phe) complexes and Na +(H 2O) nclusters in the gas phase (with up to six water molecules). The CPMD results are compared to available experimental and ab initio reference data, to DFT results obtained with various combinations of density functionals and basis sets, and to previous classical mechanics MD simulations. The agreement with the reference data in the gas phase validates the CPMD method, showing that it is a valid approach for studying these systems and that it describes correctly the competing Na +−Phe and Na +−H 2O interactions. Analysis of MD trajectories reveals that the Na +(Phe) complex in aqueous solution maintains a stable configuration in which the Na +cation hovers above the phenyl ring, at an average distance of 3.85 Å from the ring center, while remaining strongly bound to one of the carboxylic oxygens of Phe. Constrained MD simulations indicate that the free energy barrier opposing dissociation of the complex exceeds 5.5 kcal/mol. We thus confirm that “cation− π” interactions between alcali cations and the π ring, combined with other kinds of interactions, may allow aromatic amino acids to overcome the competition with water in binding a cation. [ABSTRACT FROM AUTHOR]

Using micro-Raman techniques to investigate crystal polymorphism is an efficient method, capable of monitoring physical modifications and phase inhomogeneities in crystal domains at the micrometre scale. In the presence of polymorphism, phase mixing is a common occurrence which becomes a crucial issue in structured organic materials tailored for applications in molecular electronics and photonics. A good phase homogeneity is, in fact, required for optimal and reproducible device performance. We tackle the problem of polymorphism in organic semiconductors by combining experimental and theoretical methods. Experimentally we have found that different crystalline polymorphs may be conveniently investigated using their Raman spectra in the region of the lattice phonons, whose frequencies probe intermolecular interactions and are very sensitive to differences in molecular packing. We propose lattice phonon confocal micro-Raman mapping as a fast and reliable diagnostic tool for in-situ characterization of the phase purity. The theoretical approach aims to predict crystal structures and possible coexistence of polymorphs by ranking them in energy and proving that the deepest calculated minima actually correspond to the experimental X-ray diffraction structures of bulk crystals. This combined spectroscopic and theoretical approach to the dynamical properties of a crystal lattice provides a unique body of information on crystal structure recognition of molecular crystals. [ABSTRACT FROM AUTHOR]

We present a strategy for comparing the global properties of competing potential models. By systematically sampling the potential energy surface of crystalline tetracene, we assess how the number, energy and structure of its minima are modified by switching on (or off) the Coulombic interactions. The increased complexity of the Coulombic potential leads to a more “rugged” potential energy surface with a larger number of minima, but the effect is not large. In fact, we find a subset of minima stable only in presence of the Coulombic interactions, a smaller subset stable only in their absence, and a large majority stable in both cases. Among these, there is a very good, but not perfect, correlation between the energies and the structures computed with and without the electrostatic interactions. Although electrostatic interactions play a role even in a rigid nonpolar molecule such as tetracene, they are not as crucial as often believed, because altering the electrostatic model (or switching it off completely) leads, in most cases, to equivalent results. [ABSTRACT FROM AUTHOR]

We have systematically sampled the potential energy surface of crystalline tetracene to identify its local minima. These minima represent all possible stable configurations and constitute the “inherent structures” of the system. The crystal is described in terms of rigid molecules with Coulombic and atom−atom interactions. Hundreds of distinct minima are identified, mostly belonging to the space groups P (triclinic) and P21/c(monoclinic), with a variety of structural arrangements. The deepest minimum corresponds to the high temperature−low pressure polymorph. This is the only polymorph with a completely described X-ray structure, which is satisfactorily described by the calculations. The next deep minimum is likely to correspond to the low temperature−high pressure polymorph, which has been experimentally identified but not yet fully described. [ABSTRACT FROM AUTHOR]