Atomistic modelling of entropy driven phase transitions between different crystal modifications in polymers: the case of poly(3-alkylthiophenes).

Polymorphism and related solid-state phase transitions affect the structure and morphology and hence the properties of materials, but they are not-so-well understood. Atomistic computational methods can provide molecular-level insights, but they have rarely proven successful for transitions between polymorphic forms of crystalline polymers. In this work, we report atomistic molecular dynamics (MD) simulations of poly(3-alkylthiophenes) (P3ATs), widely used organic semiconductors to explore the experimentally observed, entropy-driven transition from form II to more common form I type polymorphs, or, more precisely, to form I mesophases.

Nucleophilicity and electrophilicity of the C(sp)-H bond: methane and ethane binary complexes with iodine.

The occurrence of stable van der Waals complexes of small saturated hydrocarbons with molecular iodine is assessed in order to investigate the ability of sp-hybridized carbon atoms to act as either electron donors or electron acceptors depending on the ligand orientation. Systematic ab initio potential energy surface exploration of methane-I and ethane-I model dimers was followed by thorough characterization. Despite modest evidence of whole-adduct polarization, the resulting interactions feature a dominant dispersive character.

Insights into the formation of chiral second sphere coordination complexes with aromatic tris amines: combined single crystal X-ray crystallography and molecular modeling analyses.

A family of isostructural, chiral supramolecular networks have been obtained in the solid state by exploiting second sphere coordination interactions in the self-assembly of achiral tris amines and with tetrahalometallate and halide ions. Quantum-Mechanical (QM) calculations specific for solid phases provided additional insights into the intramolecular and packing interactions which determine chirality, pointing to a direct effect of the methyl groups of the central benzene ring.

Structure-Photoluminescence Correlation for Two Crystalline Polymorphs of a Thiophene-Phenylene Co-Oligomer with Bulky Terminal Substituents.

Two crystal polymorphs of a thiophene-phenylene hexamer with bulky terminal substituents are characterized by different molecular conformations and parallel versus herringbone packing. Irrespective of their similar emissive spectra and common H-aggregate features, evidenced by crystal structure analysis and confirmed by solid-phase and excited-state first-principles calculations, their luminescence is relatively high and, for one form, nearly double than that for the other. Interaromatic packing energy contributions are established by quantum chemical calculations and can be compared quantitatively as the same species in different crystal environments is examined.

Pyrrolidinium-based ionic liquids doped with lithium salts: how does Li(+) coordination affect its diffusivity?

We present the characterization of LiX-doped room-temperature ionic liquids (ILs) based on the N-butyl-N-methyl pyrrolidinium (PYR14) cation with two fluorinated anions: (trifluoromethanesulfonyl)-(nonafluorobutanesulfonyl)imide (X═IM14) and bis(pentafluoroethanesulfonyl)imide (X═BETI). The new data are also compared with previous results on PYR14TFSI (bis(trifluoromethanesulfonyl)imide). Their local organization has been investigated via NMR nuclear Overhauser effect (NOE) experiments for {(1)H-(19)F} and {(1)H-(7)Li} that give us details on PYR14(+)/X(-) and PYR14(+)/Li(+) contacts.

Solvent-free phenyl-C61-butyric acid methyl ester (PCBM) from clathrates: insights for organic photovoltaics from crystal structures and molecular dynamics.

The first solvent-free crystal structure of PCBM, an organic semiconductor widely used in solvent-free nanocrystalline films in plastic solar cells, is reported and its relevance to structure-property relationships discussed. The PCBM structure, obtained from o-dichlorobenzene solvates by solvent abstraction, was solved using powder diffraction, demonstrating this possibility for functionalized fullerenes.

A solid state density functional study of crystalline thiophene-based oligomers and polymers.

We present the results of a molecular modeling study of several thiophene-based oligomers and polymers by solid state density functional theory (DFT) calculations. In particular, we consider two polymers for which limit-ordered crystal structures have been reported by our group, on the basis of powder X-ray data analysis: poly(3-(S)-2-methylbutylthiophene) (P3MBT) and forms I' and II of poly(3-butylthiophene) (P3BT). The calculations employing the functional by Perdew and Wang (PWC) in conjunction with the double-ζ polarized numerical basis set (DNP) demonstrate that a standard quantum mechanical (QM) approach for solid phases can provide a balanced description of the overall structure and energetics of thiophene-based oligomers and polymers maintaining a moderate computational cost.

Molecular modeling of crystalline alkylthiophene oligomers and polymers.

We present the results of a thorough molecular modeling study of several alkylthiophene-based oligomers and polymers. In particular, we consider two polymers whose limit-ordered crystal structures have been recently reported by our group, on the basis of powder X-ray data analysis: poly(3-(S)-2-methylbutylthiophene) (P3MBT) and form I' of poly(3-butylthiophene) (P3BT). We first describe the development of a series general purpose force fields for the simulation of these and related systems.

Synthesis, structure and conformation of partially-modified retro- and retro-inverso psi[NHCH(CF3)]Gly peptides.

Partially modified retro- (PMR) and retro-inverso (PMRI) psi[NHCH(CF(3))]Gly peptides, a conceptually new class of peptidomimetics, have been synthesized in wide structural diversity and variable length by aza-Michael reaction of enantiomerically pure alpha-amino esters and peptides with enantiomerically and geometrically pure N-4,4,4-trifluorocrotonoyl-oxazolidin-2-ones. The factors underlying the observed moderate to good diastereocontrol have been investigated. The conformations of model PMR-psi[NHCH(CF(3))]Gly tripeptides have been studied in solution by (1)H NMR spectroscopy supported by MD calculations, as well as in the solid-state by X-ray diffraction.