Elisabetta Venuti

Significant progress in predicting the crystal structures of small organic molecules – a report on the fourth blind test

We report on the organization and outcome of the fourth blind test of crystal structure prediction, an international collaborative project organized to evaluate the present state in computational methods of predicting the crystal structures of small organic molecules. There were 14 research groups which took part, using a variety of methods to generate and rank the most likely crystal structures for four target systems: three single-component crystal structures and a 1:1 cocrystal. Participants were challenged to predict the crystal structures of the four systems, given only their molecular diagrams, while the recently determined but as-yet unpublished crystal structures were withheld by an independent referee. Three predictions were allowed for each system. The results demonstrate a dramatic improvement in rates of success over previous blind tests; in total, there were 13 successful predictions and, for each of the four targets, at least two groups correctly predicted the observed crystal structure. The successes include one participating group who correctly predicted all four crystal structures as their first ranked choice, albeit at a considerable computational expense. The results reflect important improvements in modelling methods and suggest that, at least for the small and fairly rigid types of molecules included in this blind test, such calculations can be constructively applied to help understand crystallization and polymorphism of organic molecules.

Towards crystal structure prediction of complex organic compounds – a report on the fifth blind test

The results of the fifth blind test of crystal structure prediction, which show important success with more challenging large and flexible molecules, are presented and discussed.

Epitaxial growth of π-stacked perfluoropentacene on graphene-coated quartz.

Chemical-vapor-deposited large-area graphene is employed as the coating of transparent substrates for the growth of the prototypical organic n-type semiconductor perfluoropentacene (PFP). The graphene coating is found to cause face-on growth of PFP in a yet unknown substrate-mediated polymorph, which is solved by combining grazing-incidence X-ray diffraction with theoretical structure modeling. In contrast to the otherwise common herringbone arrangement of PFP in single crystals and “standing” films, we report a π-stacked arrangement of coplanar molecules in “flat-lying” films, which exhibit an exceedingly low π-stacking distance of only 3.07 Å, giving rise to significant electronic band dispersion along the π-stacking direction, as evidenced by ultraviolet photoelectron spectroscopy. Our study underlines the high potential of graphene for use as a transparent electrode in (opto-)electronic applications, where optimized vertical transport through flat-lying conjugated organic molecules is desired.

Raman phonon spectra of pentacene polymorphs

We report for the first time lattice phonon Raman spectra of pentacene measured by means of a Raman microprobe technique. We experimentally prove the existence of two polymorphs, as expected from recent structural studies. A comparison with Quasi Harmonic Lattice Dynamics calculations, previously performed starting from the available X-ray data, help us in identifying the phase to which each crystal belongs.

HIGH DIMENSIONAL ANHARMONIC POTENTIAL ENERGY SURFACES : THE CASE OF METHANE

The overtone vibrational spectra of all Td symmetry isotopomers of methane have been analyzed simultaneously. A Hamiltonian expressed in internal curvilinear coordinates expanded to the fourth order has been employed, with a nine-dimensional basis of harmonic oscillator wave functions in symmetry coordinates. Near-resonant anharmonic interactions are treated to first order, while weaker interactions are handled as second order perturbations. A set of optimized Born–Oppenheimer force constants is obtained, which reproduces the observations up to 9500 cm−1 and shows an excellent agreement with the results of ab initio calculations.

Probing polymorphs of organic semiconductors by lattice phonon Raman microscopy

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.

Inherent structures of crystalline pentacene

Using a quasi-Monte Carlo scheme, we search the potential energy surface of crystalline pentacene to sample its local minima, which represent the “inherent” structures, i.e., the possible configurations of mechanical equilibrium. The system is described in terms of rigid molecules interacting through a standard atom–atom potential model. Several hundreds of distinct minima are encountered, with a surprising variety of structural arrangements. We find that deep minima are easily accessible because they exhibit a favorable energy distribution and their attraction basins tend to be wide. Thanks to these features of the potential surface, the localization the global minimum becomes entirely feasible, allowing reliable a priori predictions of the crystallographic structures. The results for pentacene are very satisfactory. In fact, the two deepest minima correspond to the structures of the two known experimental polymorphs, which are described correctly. Further polymorphs are also likely to exist.

Influence of intermolecular vibrations on the electronic coupling in organic semiconductors: the case of anthracene and perfluoropentacene.

We have performed classical molecular dynamics simulations and quantum-chemical calculations on molecular crystals of anthracene and perfluoropentacene. Our goal is to characterize the amplitudes of the room-temperature molecular displacements and the corresponding thermal fluctuations in electronic transfer integrals, which constitute a key parameter for charge transport in organic semiconductors. Our calculations show that the thermal fluctuations lead to Gaussian-like distributions of the transfer integrals centered around the values obtained for the equilibrium crystal geometry. The calculated distributions have been plugged into Monte-Carlo simulations of hopping transport, which show that lattice vibrations impact charge transport properties to various degrees depending on the actual crystal structure.

Pressure-induced phase transition in pentacene

We have recently studied two solid phases of bulk pentacene (polymorphs H and C) by means of lattice phonon Raman spectroscopy. The assignment, previously based on lattice dynamics calculations alone, is now verified by X-ray diffraction measurements, conclusively confirming the existence of both polymorphs. Furthermore, Raman phonon spectra indicate a pressure-induced phase transition where the polymorph C (lower density phase) transforms to the H form (higher density phase). The onset pressure for the phase transition is only 0.2 GPa. The phase change is irreversible.

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.