No abstract

Room-temperature ionic liquids (RTILs) based on the N-butyl-N-methyl pyrrolidinium cation (PYR(14)(+)) combined with three different fluorinated anions have been prepared and characterized by NMR, conductivity, and rheological measurements. The anions are (trifluoromethanesulfonyl)(nonafluorobutanesulfonyl)imide (IM(14)(-)), bis(pentafluoroethanesulfonyl)imide (BETI(-)), and bis(trifluoromethanesulfonyl)imide (TFSI(-)). Intermolecular anion-cation nuclear Overhauser enhancements (NOEs) have been experimentally observed in all titled compounds. These findings indicate the formation of long-lived aggregates in the bulk liquids. The NOE patterns show marked selectivity and can be rationalized assuming that the perfluorinated moieties of the anions tend to adopt a preferential orientation with respect to the cations, with possible formation of mesoscopic fluorous domains. Self-diffusion coefficients D for the anion and the cation have been measured by DOSY NMR. Diffusion data show similar but not identical values for cation and anion, consistent with local ordering at the molecular level. The observed trend in diffusion coefficients, D(cation) > D(anion) for all compounds, is compatible with a higher degree of intermolecular organization of the anions. This nanoscale organization is connected to rather strong deviations of the experimental conductivities from those estimated from the ion diffusion coefficients through the Nernst-Einstein relationship. The measured viscosities and ion diffusion coefficients in PYR(14)IM(14) and in PYR(14)TFSI have similar temperature dependencies, leading to very close values of the activation energies for these processes. Ab initio density functional calculations on models of a PYR(14)TFSI ion pair lead to the identification of several local minima, whose structure and energy can be qualitatively related to the experimental NOE signals and activation energies.

An anthracene derivative,9 ,10-dicyanoanthracene, crystallizes as fluorescent needle-like single crystals that can be readily plastically bent in two directions.S patially resolved photoluminescence analysis revealed that this material has robust optoelectronic properties that are preserved upon extreme crystal deformation. The highly flexible crystals were successfully tested as efficient switchable optical waveguiding elements for both active and passive light transduction, and the mode of operation depends on the wavelength of the incident light. This prototypical dual-mode organic optical crystalline fiber brings mechanically compliant molecular organic crystals closer to applications as novel light-transducing media for wireless transfer of information in all-organic micro-optoelectronic devices.Unlike conduction of electrons through metal conductors, transduction of light is inherently impervious to interference with external electromagnetic fields,a nd this calls for new organic materials as light-weight, cost-effective and secure optical transducers of information. Thef avorable optical properties and long-range order of molecular crystals is increasingly being recognized as an ew platform for construction of metal-free,a ll-organic electronics and soft robotics.P oor processing ability and less-than-optimal mechanical properties of the organic crystals,and particularly their pronounced brittleness and fragility,h owever,a re usually taken as major impediments against their implementation in flexible devices,inwhich thin organic films have long been superior and the preferred phase of choice.T he recent advent of methodologies for controlled crystal growth has provided strategies for fairly good control over the habit, the aspect ratio,a nd mosaic spread of molecular crystals. [1] Moreover,t he burgeoning research into mechanical properties of molecular crystals has revealed that certain organic crystals can be extraordinarily mechanically compliant;t hey are endowed with atypical properties,s uch as elasticity and plasticity that are comparable to those of metallic conduc-

We report the synthesis, structural characterization, and features of the crystallization behavior of a highly regioregular head-to-tail poly(3-n-butylthiophene) (PBT) with average molecular weight M w ) 10.2 kDa. The thermal behavior and the X-ray diffraction (XRD) patterns of our native PBT samples allow a crystal polymorph (form I′) to be identified, never previously discussed in the literature but closely related to the more common but more disordered form I. The crystal structure of form I′ PBT has been determined and refined by Rietveld analysis of XRD patterns from polycrystalline samples and has been confirmed by molecular mechanics (MM) calculations adopting a thiophene-specific force field developed in our group. Using such a combined approach we are able to show that the studied polymorph, in analogy with poly-3-(S)-2-methylbutylthiophene (PMBT), is well described by a limit-ordered orthorhombic model in space group C222 1 with refined lattice parameters a ) 7.64(1) Å, b ) 7.75(1) Å (chain axis), and c ) 24.97(8) Å, yielding a calculated density of 1.24 g/cm 3 in good agreement with the experimental value. The refined structure, which presents stacking and layering similar to form I, is shown by MM to be a local potential energy minimum. We also find lower energy structures with looser stacking periodicity of the polythiophene chains, consistently with data on PBT polymorphism. Both the form I′ PBT and the PMBT crystal structures, the only two poly(3-alkylthiophenes) (P3ATs) structures for which detailed models are presently available, confirm features of the inverse comb model of P3ATs by Prosa et al. (Macromolecules, 1992, 25, 4364). Our models also suggest important new aspects: isodirectional arrangements are preferred over antiparallel ones within tightly packed polythiophene stacks. This feature is incompatible with chain-folding occurring between chains within individual stacks. On the contrary crystal growth with chain-folding between chains in adjacent layers is compatible with the refined structures and in principle possible, although plausibly slow, as it requires disruption of preexistent π-stacking interactions. The molecular and crystal models we devised allow a more detailed understanding of the reasons causing crystallization to be slow and limited in extent, especially for high molecular weight P3ATs.