Elizabeth Karnas

Ion-Mediated Electron Transfer in a Supramolecular Donor-Acceptor Ensemble

Charging Back and Forth Ion binding by proteins can exert a major influence on electron transfer events in biological systems. Park et al. (p. 1324) discovered an analogous phenomenon in a simpler synthetic system. Specifically, a certain flexible molecule, known as a calix[4]pyrrole derivative, adopts a conical conformation upon binding anions, such as chloride or bromide, and this in turn leads to electron transfer to a guest acceptor that drifts into the cone. Addition of a cation that fitted more snugly into the conical cavity resulted in a reversal of the electron transfer reaction. The whole process was mapped out by spectroscopic and crystallographic characterization of the intermediates and products. Electron transfers in a weakly bound molecular complex are driven forward by anions and backward by cations. Ion binding often mediates electron transfer in biological systems as a cofactor strategy, either as a promoter or as an inhibitor. However, it has rarely, if ever, been exploited for that purpose in synthetic host-guest assemblies. We report here that strong binding of specific anions (chloride, bromide, and methylsulfate but not tetrafluoroborate or hexafluorophosphate) to a tetrathiafulvalene calix[4]pyrrole (TTF-C4P) donor enforces a host conformation that favors electron transfer to a bisimidazolium quinone (BIQ2+) guest acceptor. In contrast, the addition of a tetraethylammonium cation, which binds more effectively than the BIQ2+ guest in the TTF-C4P cavity, leads to back electron transfer, restoring the initial oxidation states of the donor and acceptor pair. The products of these processes were characterized via spectroscopy and x-ray crystallography.

Octafluorocalix[4]pyrrole: a chloride/bicarbonate antiport agent

meso-Octamethyloctafluorocalixpyrrole, a simple tetrapyrrolic macrocycle, has been shown to function as both a chloride/nitrate and a chloride/bicarbonate antiport agent for lipid bilayer transmembrane anion transport. This is the first example of a synthetic macrocyclic pyrrole-based receptor capable of transmembrane bicarbonate transport.

Stopped-flow kinetic analysis of the interaction of cyclo[8]pyrrole with anions.

The on and off rates corresponding to the binding of two test anions (acetate, AcO(-), and dihydrogen phosphate, H(2)PO(4)(-), studied as their tetrabutylammonium salts) to diprotonated cyclo[8]pyrrole have been determined in CH(3)CN using stopped-flow analyses carried out at various temperatures. For dihydrogen phosphate, this afforded the activation enthalpies and entropies associated with both off and on processes. The different dynamic behavior seen for these test anions underscores the utility of kinetic analyses as a possible new tool for the advanced characterization of anion receptors.

Charge Transfer in Sapphyrin−Fullerene Hybrids Employing Dendritic Ensembles†

A new approach to creating noncovalent charge transfer ensembles is described. It is based on two components that are linked through anion-receptor interactions. The first component is sapphyrin, a pentapyrrolic expanded porphyrin, which is capable of carboxylate anion recognition and more importantly can act as a photodonor when irradiated in the presence of a suitable electron acceptor. The second component is the electron acceptor and consists of one of two different C(60) fullerene cores functionalized with multiple carboxylate anion groups arranged in a dendritic fashion. Depending on the specific choice of the fullerene carboxylate anion dendrimer employed in ensemble construction, 1:1 or 1:2 complexes are formed when the C(60) cores are titrated with sapphyrin. The resulting noncovalent arrays undergo sapphyrin-to-fullerene electron transfer when irradiated with 387 nm light. This gives rise to charge separated states with lifetimes of ca. 470 and 600 ps in the case of the 1:1 and 1:2 sapphyrin-fullerene ensembles, respectively.

Perrhenate and pertechnetate anion recognition properties of cyclo[8]pyrrole

Cyclo[8]pyrrole, an octapyrrolic expanded porphyrin with no meso-linking atoms, was found to interact with a series of anions in the solid state and in a chloroform solution. The anion selectivities have been determined relative to a structurally characterised salt, H2cyclo[8]pyrrole2+·I− ·I3 − , via counter anion exchange. Although cyclo[8]pyrrole demonstrates a general selectivity for sulfate, it interacts well with both the pertechnetate and perrhenate anions. Moreover, it has been shown to act as a phase-transfer catalyst facilitating the extraction of pertechnetate from an aqueous to an organic phase in the presence of sulfate.

Disproportionation of Dipyrrolylquinoxaline Radical Anions via the Internal Protons of the Pyrrole Moieties

Disproportionation of dipyrrolylquinoxaline radical anions occurs via hydrogen atom transfer from the pyrrole moiety to the quinoxaline moiety to produce monodeprotonated dipyrrolylquinoxaline anions and monohydrodipyrrolylquinoxaline anions. In contrast, simple quinoxaline radical anions without pyrrole moieties are stable, and disproportionation occurs only in the presence of external protons.

Electroreduction and acid-base properties of dipyrrolylquinoxalines.

The electroreduction and acid-base properties of dipyrrolylquinoxalines of the form H(2)DPQ, H(2)DPQ(NO(2)), and H(2)DPQ(NO(2))(2) were investigated in benzonitrile (PhCN) containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). This study focuses on elucidating the complete electrochemistry, spectroelectrochemistry, and acid-base properties of H(2)DPQ(NO(2))(n) (n = 0, 1, or 2) in PhCN before and after the addition of trifluoroacetic acid (TFA), tetra-n-butylammonium hydroxide (TBAOH), tetra-n-butylammonium fluoride (TBAF), or tetra-n-butylammonium acetate (TBAOAc) to solution. Electrochemical and spectroelectrochemical data provide support for the formation of a monodeprotonated anion after disproportionation of a dipyrrolylquinoxaline radical anion produced initially. The generated monoanion is then further reduced in two reversible one-electron-transfer steps at more negative potentials in the case of H(2)DPQ(NO(2)) and H(2)DPQ(NO(2))(2). Electrochemically monitored titrations of H(2)DPQ(NO(2))(n) with OH(-), F(-), or OAc(-) (in the form of TBA(+)X(-) salts) give rise to the same monodeprotonated H(2)DPQ(NO(2))(n) produced during electroreduction in PhCN. This latter anion can then be reduced in two additional one-electron-transfer steps in the case of H(2)DPQ(NO(2)) and H(2)DPQ(NO(2))(2). Spectroscopically monitored titrations of H(2)DPQ(NO(2))(n) with X(-) show a 1:2 stoichiometry and provide evidence for the production of both [H(2)DPQ(NO(2))(n)](-) and XHX(-). The spectroscopically measured equilibrium constants range from log β(2) = 5.3 for the reaction of H(2)DPQ with TBAOAc to log β(2) = 8.8 for the reaction of H(2)DPQ(NO(2))(2) with TBAOH. These results are consistent with a combined deprotonation and anion binding process. Equilibrium constants for the addition of one H(+) to each quinoxaline nitrogen of H(2)DPQ, H(2)DPQ(NO(2)), and H(2)DPQ(NO(2))(2) in PhCN containing 0.1 M TBAP were also determined via electrochemical and spectroscopic means; this gave rise to log β(2) values ranging from 0.7 to 4.6, depending upon the number of nitro substituents present on the H(2)DPQ core. The redox behavior of the H(2)DPQ(NO(2))(n) compounds of the present study were further analyzed through comparisons with simple quinoxalines that lack the two linked pyrrole groups, i.e., Q(NO(2))(n) where n = 0, 1, or 2. It is concluded that the pyrrolic substituents play a critical role in regulating the electrochemical and spectroscopic features of DPQs.