Solution-phase counterion effects in supramolecular and mechanostereochemical systems

The self-assembly of molecular components into complex superstructures involves the subtle interplay of various noncovalent forces. Charged species are often utilised in self-assembly processes as a result of the favorable π-π, cation-π, electrostatic, and hydrogen bonding interactions that form between these species. Although the counterions associated with these charged species can exert significant effects on the synthesis, stability, and operation of superstructures in solution, rarely are the counterions considered, leading to misinterpretations and misunderstandings of the studied systems. In this tutorial review, we discuss a variety of solution-phase counterion effects, from the fundamental origins to innovative ways in which these effects are exploited for useful functions.     

A neutral redox-switchable [2]rotaxane

A limited range of redox-active, rotaxane-based, molecular switches exist, despite numerous potential applications for them as components of nanoscale devices. We have designed and synthesised a neutral, redox-active [2]rotaxane, which incorporates an electron-deficient pyromellitic diimide (PmI)-containing ring encircling two electron-rich recognition sites in the form of dioxynaphthalene (DNP) and tetrathiafulvalene (TTF) units positioned along the rod section of its dumbbell component. Molecular modeling using MacroModel guided the design of the mechanically interlocked molecular switch. The binding affinities in CH(2)Cl(2) at 298 K between the free ring and two electron-rich guests--one (K(a) = 5.8 × 10(2) M(-1)) containing a DNP unit and the other (K(a) = 6.3 × 10(3) M(-1)) containing a TTF unit--are strong: the one order of magnitude difference in their affinities favouring the TTF unit suggested to us the feasibility of integrating these three building blocks into a bistable [2]rotaxane switch. The [2]rotaxane was obtained in 34% yield by relying on neutral donor-acceptor templation and a double copper-catalysed azide-alkyne cycloaddition (CuAAC). Cyclic voltammetry (CV) and spectroelectrochemistry (SEC) were employed to stimulate and observe switching by this neutral bistable rotaxane in solution at 298 K, while (1)H NMR spectroscopy was enlisted to investigate switching upon chemical oxidation. The neutral [2]rotaxane is a chemically robust and functional switch with potential for applications in device settings.     

Stability and activity of carbon nanofiber-supported catalysts in the aqueous phase reforming of ethylene glycol

Nickel, cobalt, copper and platinum nanoparticles supported on carbon nano-fibers were evaluated with respect to their stability, catalytic activity and selectivity in the aqueous phase reforming of ethylene glycol (230 C, autogenous pressure, batch reactor). The initial surface-specific activities for ethylene glycol reforming were in a similar range but decreased in the order of Pt (15.5 h1 ) >Co(13.0 h1 ) >Ni(5.2 h1 ) while the Cu catalyst only showed low dehydrogenation activity. The hydrogen molar selectivity decreased in the order of Pt (53%)>Co(21%)>Ni (15%) as a result of the production of methane over the latter two catalysts. Over the Co catalyst acids were formed in the liquid phase while alcohols were formed over Ni and Pt. Due to the low pH of the reaction mixture, especially in the case of Co (as a result of the formed acids), significant cobalt leaching occurs which resulted in a rapid deactivation of this catalyst. Investigations of the spent catalysts with various techniques showed that metal particle growth is responsible for the deactivation of the Pt and Ni catalysts. In addition, coking might also contribute to the deactivation of the Ni catalyst.     

Complexation between Methyl Viologen (Paraquat) Bis(Hexafluorophosphate) and Dibenzo[24]Crown‐8 Revisited

Paraquat bis(hexafluorophosphate) undergoes stepwise dissociation in acetone. All three species—the neutral molecule, and the mono‐ and dications—are represented significantly under the experimental conditions typically used in host–guest binding studies. Paraquat forms at least four host–guest complexes with dibenzo[24]crown‐8. They are characterized by both 1:1 and 1:2 stoichiometries, and an overall charge of either zero (neutral molecule) or one (monocation). The monocationic 1:1 host–guest complex is the most abundant species under typical (0.5–20 mM ) experimental conditions. The presence of the dicationic 1:1 host–guest complex cannot be excluded on the basis of our experimental data, but neither is it unambiguously confirmed to be present. The two confirmed forms of paraquat that do undergo complexation—the neutral molecule and the monocation—exhibit approximately identical binding affinities toward dibenzo[24]crown‐8. Thus, the relative abundance of neutral, singly, and doubly charged pseudorotaxanes is identical to the relative abundance of neutral, singly, and doubly charged paraquat unbound with respect to the crown ether in acetone. In the specific case of paraquat/dibenzo[24]crown‐8, ion‐pairing does not contribute to host–guest complex formation, as has been suggested previously in the literature.     

Influence of blend miscibility on the proton conductivity and methanol permeability of polymer electrolyte blends

The influence of miscibility on the transport properties of polymer electrolyte blends composed of a proton conductor and an insulator was investigated. The proton‐conductive component in the blends was sulfonated poly(ether ketone ketone) (SPEKK), while the nonconductive component was either poly(ether imide) (PEI) or poly(ether sulfone) (PES). The phase behavior of PEI‐SPEKK blends was strongly influenced by the sulfonation level of the SPEKK. At low sulfonation levels (ion‐exchange capacity (IEC) = 0.8 meq/g), the blends were miscible, while at a slightly higher level (IEC = 1.1 meq/g), they were only partially miscible and for IEC ≥ 1.4 meq/g they were effectively immiscible over the entire composition range. The PES‐SPEKK blends were miscible over the entire range of SPEKK IEC considered in this study (0.8–2.2 meq/g). At high IEC (2.2 meq/g) and at low mass fractions of SPEKK (<0.5), the miscible blends (PES‐SPEKK) had higher proton conductivities and methanol permeabilities than the immiscible ones (PEI‐SPEKK). The opposite relationship was observed for high mass fractions of SPEKK (>0.5). This behavior was explained by the differences in morphology between these two blend systems. At low IEC of SPEKK (0.8 meq/g), where both PEI‐SPEKK and PES‐SPEKK blend systems exhibited miscibility, the transport properties were not significantly different. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2253–2266, 2006     

Structured polymer electrolyte blends based on sulfonated polyetherketoneketone (SPEKK) and a poly(ether imide) (PEI)

The performance and economics of proton-exchange membrane (PEM) fuel cells are highly dependent on the membranes used to separate the fuel and oxidant. While maintaining reasonable cost, the membrane must feature a number of desirable properties including high proton conductivity. Blends of polymers are one approach to tailoring PEM properties; however, blending to achieve mechanically and chemically robust membranes has generally resulted in reduced conductivity. The objective of this work was to demonstrate the use of field alignment of the proton-conducting domains to increase the conductivity in a polymer blend PEM. A blend of sulfonated poly(etherketoneketone) (SPEKK) and a polyether imide (PEI) was used to illustrate this method. Blends of SPEKK/PEI with a 3:7 mass ratio were aligned using electric field strengths varying from 0 to 30 V/mm and frequencies varying from 0 to 10 kHz. In general, the degree of alignment agreed with theoretical predictions for the alignment of drops or particles suspended in a fluid with a different dielectric constant, e.g., when the frequency of the applied ac field was increased, the threshold field for phase alignment increased and the diameter of the oriented columnar structures decreased. Alignment resulted in up to three orders of magnitude increase in conductivity at low humidity. By careful selection of temperature and residual solvent content, alignment was shown to be possible in the melt state, which is essential for an economic process for producing alignment-enhanced membranes.