Jean-Michel Barbe

Molecular electrocatalysis for oxygen reduction by cobalt porphyrins adsorbed at liquid/liquid interfaces.

Molecular electrocatalysis for oxygen reduction at a polarized water/1,2-dichloroethane (DCE) interface was studied, involving aqueous protons, ferrocene (Fc) in DCE and amphiphilic cobalt porphyrin catalysts adsorbed at the interface. The catalyst, (2,8,13,17-tetraethyl-3,7,12,18-tetramethyl-5-p-amino-phenylporphyrin) cobalt(II) (CoAP), functions like conventional cobalt porphyrins, activating O(2) via coordination by the formation of a superoxide structure. Furthermore, due to the hydrophilic nature of the aminophenyl group, CoAP has a strong affinity for the water/DCE interface as evidenced by lipophilicity mapping calculations and surface tension measurements, facilitating the protonation of the CoAP-O(2) complex and its reduction by ferrocene. The reaction is electrocatalytic as its rate depends on the applied Galvani potential difference between the two phases.

Enhanced Electron‐Transfer Properties of Cofacial Porphyrin Dimers through π–π Interactions

pi-pi assisted: Photoinduced electron transfer from cofacial porphyrin dimers to electron acceptors is prominently accelerated, whereas the back electron transfer is decelerated, relative to the corresponding porphyrin monomer (see figure).The radical cation of zinc tetrapentylporphyrin is dimerized with an excess of the neutral counterpart to form the dimer radical cation in which the unpaired electron is delocalized over both porphyrin rings. The dimeric radical cation exhibits an NIR absorption spectrum characteristic of weak pi-bond formation between the porphyrin rings. When cofacial porphyrin dimers, linked by different spacers, are oxidized such pi-bond formation between the porphyrin rings is also recognized in cyclic voltammetry, and Vis/NIR and ESR spectroscopic measurements. The dynamics of photoinduced electron transfer from the triplet excited states of cofacial porphyrin dimers to a series of electron acceptors were investigated by using laser flash photolysis measurements and compared with the porphyrin monomer. The rates of photoinduced electron-transfer reactions of cofacial porphyrin dimers are prominently accelerated compared with the reference monomer. The driving-force dependence of the rate constants of photoinduced electron-transfer reactions was analyzed in light of the Marcus theory of electron transfer to afford the reorganization energies of electron transfer (lambda). The lambda values of cofacial porphyrin dimers are significantly smaller than those of the porphyrin monomer when compared at the same driving force of the photoinduced electron transfer. The lambda values increase linearly with an increase in the driving force of the photoinduced electron transfer. This is accompanied by an increase in the distance between electron donor and acceptor molecules, where the electron transfer occurs. The enhanced electron-transfer properties of cofacial porphyrin dimers, in relation with the important role of the special pair in the photosynthetic reaction center, result from the smaller reorganization energy (lambda) together with the larger driving force of the photoinduced electron transfer due to the pi-electron delocalization in the dimer radical cations.

Proton-Coupled Oxygen Reduction at Liquid−Liquid Interfaces Catalyzed by Cobalt Porphine

Cobalt porphine (CoP) dissolved in the organic phase of a biphasic system is used to catalyze O(2) reduction by an electron donor, ferrocene (Fc). Using voltammetry at the interface between two immiscible electrolyte solutions (ITIES), it is possible to drive this catalytic reduction at the interface as a function of the applied potential difference, where aqueous protons and organic electron donors combine to reduce O(2). The current signal observed corresponds to a proton-coupled electron transfer (PCET) reaction, as no current and no reaction can be observed in the absence of either the aqueous acid, CoP, Fc, or O(2).

Oxygen Reduction Catalyzed by a Fluorinated Tetraphenylporphyrin Free Base at Liquid/Liquid Interfaces

The diprotonated form of a fluorinated free base porphyrin, namely 5-(p-aminophenyl)-10,15,20-tris(pentafluorophenyl)porphyrin (H(2)FAP), can catalyze the reduction of oxygen by a weak electron donor, namely ferrocene (Fc). At a water/1,2-dichloroethane interface, the interfacial formation of H(4)FAP(2+) is observed by UV-vis spectroscopy and ion-transfer voltammetry, due to the double protonation of H(2)FAP at the imino nitrogen atoms in the tetrapyrrole ring. H(4)FAP(2+) is shown to bind oxygen, and the complex in the organic phase can easily be reduced by Fc to produce hydrogen peroxide as studied by two-phase reactions with the Galvani potential difference between the two phases being controlled by the partition of a common ion. Spectrophotometric measurements performed in 1,2-dichloroethane solutions clearly evidence that reduction of oxygen by Fc catalyzed by H(4)FAP(2+) only occurs in the presence of the tetrakis(pentafluorophenyl)borate (TB(-)) counteranion in the organic phase. Finally, ab initio computations support the catalytic activation of H(4)FAP(2+) on oxygen.

Slow and Fast Singlet Energy Transfers in BODIPY-gallium(III)corrole Dyads Linked by Flexible Chains

Red (no styryl), green (monostyryl), and blue (distyryl) BODIPY-gallium(III) (BODIPY = boron-dipyrromethene) corrole dyads have been prepared in high yields using click chemistry, and their photophysical properties are reported. An original and efficient control of the direction of the singlet energy transfers is reported, going either from BODIPY to the gallium-corrole units or from gallium-corroles to BODIPY, depending upon the nature of the substitution on BODIPY. In one case (green), both directions are possible. The mechanism for the energy transfers is interpreted by means of through-space Förster resonance energy transfer (FRET).

Clarification of the Oxidation State of Cobalt Corroles in Heterogeneous and Homogeneous Catalytic Reduction of Dioxygen

Co(III) corroles were investigated as efficient catalysts for the reduction of dioxygen in the presence of perchloric acid in both heterogeneous and homogeneous systems. The investigated compounds are (5,10,15-tris(pentafluorophenyl)corrole)cobalt (TPFCor)Co, (10-pentafluorophenyl-5,15-dimesitylcorrole)cobalt (F 5PhMes 2Cor)Co, and (5,10,15-trismesitylcorrole)cobalt (Mes 3Cor)Co, all of which contain bulky substituents at the three meso positions of the corrole macrocycle. Cyclic voltammetry and rotating ring-disk electrode voltammetry were used to examine the catalytic activity of the compounds when adsorbed on the surface of a graphite electrode in the presence of 1.0 M perchloric acid, and this data is compared to results for the homogeneous catalytic reduction of O 2 in benzonitrile containing 10 (-2) M HClO 4. The corroles were also investigated as to their redox properties in nonaqueous media. A reversible one-electron oxidation occurs at E 1/2 values between 0.42 and 0.89 V versus SCE depending upon the solvent and number of fluorine substituents on the compounds, and this is followed by a second reversible one-electron abstraction at E 1/2 = 0.86 to 1.18 V in CH 2Cl 2, THF, or PhCN. Two reductions of each corrole are also observed in the three solvents. A linear relationship is observed between E 1/2 for oxidation or reduction and the number of electron-withdrawing fluorine groups on the compounds, and the magnitude of the substituent effect is compared to what is observed in the case of tetraphenylporphyrins containing meso -substituted C 6F 5 substituents. The electrochemically generated forms of the corrole can exist with Co(I), Co(II), or Co(IV) central metal ions, and the site of the electron-transfer in each oxidation or reduction of the initial Co(III) complex was examined by UV-vis spectroelectrochemistry. ESR characterization was also used to characterize singly oxidized (F 5PhMes 2Cor)Co, which is unambiguously assigned as a Co(III) radical cation rather than the expected Co(IV) corrole with an unoxidized macrocyclic ring.

Synthesis and photodynamics of fluorescent blue BODIPY-porphyrin tweezers linked by triazole rings.

Novel zinc porphyrin tweezers in which two zinc porphyrins were connected with π-conjugated boron dipyrromethenes (BDP meso-Por(2) and BDP β-Por(2)) through triazole rings were synthesized to investigate the photoinduced energy transfer and electron transfer. The UV-vis spectrum of BDP β-Por(2) which has less bulky substituents than BDP meso-Por(2) exhibits splitting of the Soret band as a result of the interaction between porphyrins of BDP β-Por(2) in the excited state. Such interaction between porphyrins of both BDP β-Por(2) and BDP meso-Por(2) is dominant at room temperature, while the coordination of the nitrogen atoms of the triazole rings to the zinc ions of the porphyrins occurs at low temperature. The conformational change of the BDP-porphyrin composites was confirmed by the changes in UV-vis and fluorescence spectra depending on temperature. Photodynamics of BDP meso-Por(2) and BDP β-Por(2) has also been investigated by laser flash photolysis. Efficient singlet-singlet energy transfer from the ZnP to the π-conjugated BDP moiety of both BDP meso-Por(2) and BDP β-Por(2) occurred in opposite direction as compared to energy transfer from conventional BDP to ZnP due to the π-conjugation in nonpolar toluene. In polar benzonitrile, however, additional electron transfer occurred along with energy transfer.

B,B-Diporphyrinbenzyloxy-BODIPY Dyes: Synthesis and Antenna Effect

B,B-Diporphyrinbenzyloxy-BODIPY derivatives have been prepared in high yields, and the photophysical properties are reported. Singlet energy transfers from BODIPY to the porphyrin units have been analyzed.

Metallocorroles as sensing components for gas sensors: remarkable affinity and selectivity of cobalt(III) corroles for CO vs. O2 and N2

Most commercially available CO detectors are based upon metal oxides or electrochemical cell technologies. None of these approaches use the selective adsorption of CO gas on a molecular complex. Conversely, cobalt(III) corroles can bind small gaseous molecules allowing them for an application as sensing components for gas detectors. Here we describe the ability of cobalt corroles to selectively coordinate carbon monoxide vs. dinitrogen and dioxygen. The coordination properties were determined in the solid state and the adsorption characteristics were compared to those of the reference compound (To-PivPP)Fe(1,2-Me2Im), known for its remarkable CO binding properties. The adsorption data evidence that the selectivity, affinity and capacity of the cobalt(III) corroles for CO are larger than those of the porphyrin complex. However, from a chemical point of view, the selectivity of cobalt(III) corroles for CO vs. O2 is infinite since these derivatives do not bind O2 while (To-PivPP)Fe(1,2-Me2Im) does with an M value (PO2(1/2)/PCO(1/2)) equal to 51. In this manuscript we also show that the affinity of cobalt(III) corroles for CO is closely related to the Lewis acid character of the central cobalt(III) ion and therefore to the nature of the substituents at the periphery of the corrole macroring.

Metalloporphyrins as sensing material for quartz-crystal microbalance nitroaromatics sensors

Five octaethylporphyrins (OEP) and tetraphenylporphyrins (TPP): (OEP)InCl, (OEP)MnCl, (OEP)GaCl, (TPP)Pd, and (TPP)RhI have been deposited as sensitive coating onto quartz-crystal microbalances. The sensitivities of the resulting sensors have been measured with respect to 2,4-dinitrotrifluoromethoxybenzene vapors. When exposed to the nitroaromatic compound, a large and significative response is recorded for every tested porphyrin, the detection process being slightly reversible. Along with a good sensitivity, the sensors exhibit an excellent selectivity when common solvents are used as interfering vapors. Among all the studied derivatives, (OEP)MnCl appears as the most sensitive and selective coating.