Electronic structure, spectra, and magnetic circular dichroism of cyclohexa-, cyclohepta-, and cyclooctapyrrole

Three recently obtained expanded porphyrins represent nice examples of compounds for which the electronic and spectral properties can be predicted from symmetry considerations alone. Perimeter-model-based theoretical analysis of the electronic structure of doubly protonated cyclo[6], cyclo[7], and cyclo[8]pyrrole leads to the anticipation of qualitatively the same electronic absorption and magnetic circular dichroism patterns for all three compounds. These predictions are fully confirmed by experiments, as well as DFT and INDO/S calculations. Due to a characteristic pattern of frontier molecular orbitals, a degenerate HOMO and a strongly split LUMO pair, the three cyclopyrroles show comparable absorption intensity in the Q and Soret regions. Magnetic circular dichroism spectra reveal both A and B Faraday terms, of which the signs and magnitudes are in remarkably good agreement with theoretical expectations. The values of the magnetic moments of the two lowest degenerate excited states have also been obtained.     

Anion binding studies of fluorinated expanded calixpyrroles

The anion binding properties of fluorinated calix[n]pyrroles (n = 4-6) in aprotic solvents (acetonitrile and DMSO) and modified reaction conditions allowing for the synthesis and isolation of the hitherto missing dodecafluorocalix[6]pyrrole from the condensation of 3,4-difluoro-1H-pyrrole and acetone are described. In acetonitrile solution containing 2% water, the association constants for the 1:1 binding interaction between octafluorocalix[4]pyrrole and chloride anion obtained with isothermal titration calorimetry (ITC) and (1)H NMR titration methods were found to match reasonably well. As compared to its nonfluorinated congener, octafluorocalix[4]pyrrole was found to display enhanced binding affinities for several representative anions in pure acetonitrile as judged from ITC analyses. Similar analyses of the fluorinated calix[n]pyrroles revealed an increase in the relative affinity for bromide over chloride with increasing macrocycle size, as manifest in a decrease in the binding ratio K(a(Cl))/K(a(Br)). Anion binding studies in the solid state, involving single-crystal X-ray diffraction analyses of the chloride and acetate anion complexes of octafluorocalix[4]pyrrole and decafluorocalix[5]pyrrole, respectively, confirmed the expected hydrogen bond interactions between the pyrrolic NH protons and the bound anions.     

Synthesis and photophysics of a porphyrin-fullerene dyad assembled through Watson-Crick hydrogen bonding

A novel porphyrin-fullerene dyad assembled through Watson-Crick hydrogen bonds is described; this system undergoes photoinduced electron transfer upon irradiation with visible light to produce a charge separated state that is substantially longer lived than that of previous dyads of this type.     

Anion transfer at a micro-water/1,2-dichloroethane interface facilitated by beta-octafluoro-meso-octamethylcalix[4]pyrrole

The facilitated transfer of four hydrophilic anions, i.e., Cl-, Br-, NO2-, and CH3CO2-, at the micro-water/1,2-dichloroethane interface supported at the tip of a micropipet has been observed successfully using beta-octafluoro-meso-octamethylcalix[4]pyrrole 2 as the receptor. We have also shown for the first time that the dynamics of this process can be studied by micropipet voltammetry. The standard kinetic rate constants (kdegrees) for facilitated anion transfer at such an interface were determined to be (2.11 +/- 0.90) x 10(-2) and (0.75 +/- 0.50) x 10(-2) cm/s in the case of Cl- and CH3CO2-, respectively. These values are much smaller than those associated with the facilitated transfer of analogous alkali metal ions. This difference is thought to reflect a number of underlying factors, including the higher hydration of anions as compared to similar sized cations. Studies such as these are expected to be useful in understanding the mechanism of anion transport at soft interfaces and for the design of yet-improved anion receptors and carriers.     

Poly(methyl methacrylate)s with pendant calixpyrroles: polymeric extractants for halide anion salts

Poly(methyl methacrylate)s containing pendant octamethylcalix[4]pyrrole subunits were prepared and demonstrated to be capable of extracting tetrabutylammonium chloride and fluoride salts from aqueous media.     

Binuclear organometallic ruthenium complexes of a Schiff base expanded porphyrin

The synthesis of binuclear organometallic ruthenium complexes of an expanded porphyrin-type macrocycle is reported; pyrrolic hydrogen bonding donors were found to interact with ancillary ligands in the primary coordination sphere and to stabilize coordinated dioxygen in an eta(2)-fashion.     

Electrochemically driven intramolecular ring closure of a linear hexapyrrole

[24]hexaphyrin(1.0.1.0.0.0) was electrochemically synthesized in good yield starting from a linear hexapyrrolic precursor. This straightforward approach provides an efficient "green" alternative to the reported chemical synthesis, which requires the use of trifluoroacetic acid as the solvent and toxic chromium(VI) salts as the oxidant.