Kinetic and thermodynamic evaluation of the reversible N-heterocyclic carbene-isothiocyanate coupling reaction: applications in latent catalysis.

Using stopped flow and other spectroscopic techniques, the thermodynamic parameters of the coupling reaction between 1,3-dimesitylimidazolylidene and phenyl isothiocyanate were determined (H(o) = -96.1 kJ mol(-1) and ΔS(o) = -39.6 J mol(-1) K(-1)). On the basis of these data which indicated that the reaction was reversible (K(eq) = 5.94 × 10(14) M(-1) at 25 °C; k(f) = 252 M(-1) s(-1); k(r) = 4.24 × 10(-13) s(-1)), the adduct formed from the two aforementioned coupling partners was used as a latent catalyst to facilitate the [2 + 2 + 2] cyclotrimerization of phenyl isocyanate and the polymerization of DL-lactide.

Evaluating the role of additive pKa on the proton conductivities of blended sulfonated poly(ether ether ketone) membranes.

Sulfonated poly(ether ether ketone) (sPEEK) membranes were blended with various functionalized imidazoles. The effect of the pKa of the added heterocycle on the dry proton conductivities of the blended membranes was evaluated over the temperature range of 40-150 degrees C. These membranes showed nonmonotonous conductivities with respect to temperature, as well as a clear correlation of peak temperature conductivity to the pKa of the heterocycle. We use a theoretical model based on the reaction equilibria between sPEEK's sulfonic acid groups and the basicity of the added heterocycles in order to better understand the mechanistic origins of the observed temperature-conductivity profiles.

Adapting N-heterocyclic carbene/azide coupling chemistry for polymer synthesis: enabling access to aromatic polytriazenes.

A click by any other name: Coupling bis(N-heterocyclic carbene)s with bis(azide)s afforded a novel class of conjugated polytriazenes. These polymers were rendered electrically conductive upon doping, and fluorene-containing variants exhibited luminescence. This adaptation of N-heterocyclic carbene (NHC)/azide coupling chemistry to polymer synthesis reveals the potential of NHCs as building blocks for accessing polymers having useful electronic properties.

Size-dependent spectroscopic properties of conjugated polymer nanoparticles.

This paper is focused on how the spectroscopic properties of conjugated polymers evolve in the size range between single polymer chains and the bulk material. The measurements used single-particle spectroscopy techniques and include both static and dynamic measurements. The main observation of this work is that the spectroscopic properties of MEH-PPV evolve rapidly as a function of nanoparticle size and achieve bulk-like properties for nanoparticles greater than 10 nm in size. Nanoparticles were assembled by a reprecipitation technique and characterized by fluorescence emission spectroscopy. The physical origin of the size-dependent spectroscopic properties is assigned to the distance dependence of four main processes: electronic energy transfer between blue and red sites, triplet-triplet annihilation, singlet exciton quenching by triplets, and singlet exciton quenching by hole polarons.

Colloidal crystal microarrays and two-dimensional superstructures: a versatile approach for patterned surface assembly.

A simple, fast, and robust approach to colloidal assembly on patterned surfaces was developed. The approach involves the rapid settling and dewetting of suspensions of spherical colloids on lithographically templated surfaces. Using this method, we can quickly and easily fabricate close-packed colloidal crystal microarrays of both silica and polystyrene spheres that range in size from 500 nm to 4.5 microm. The microarrays tend to induce the formation of monolayer colloidal crystals, which can be interconnected and removed from the templates as free-standing colloidal crystal slabs. The same approach can also be used to assemble two-dimensional colloidal crystal superlattices that can adopt a variety of structures. Graphite, kagome, body-centered cubic, open hexagonal, tetragonal, and linear chain structures can all be quickly accessed by adjusting the ratio of the sphere diameter to the template diameter.