Samuel Drouet

Turnover Numbers, Turnover Frequencies, and Overpotential in Molecular Catalysis of Electrochemical Reactions. Cyclic Voltammetry and Preparative-Scale Electrolysis

The search for efficient catalysts to face modern energy challenges requires evaluation and comparison through reliable methods. Catalytic current efficiencies may be the combination of many factors besides the intrinsic chemical properties of the catalyst. Defining turnover number and turnover frequency (TOF) as reflecting these intrinsic chemical properties, it is shown that catalysts are not characterized by their TOF and their overpotential (η) as separate parameters but rather that the parameters are linked together by a definite relationship. The log TOF-η relationship can often be linearized, giving rise to a Tafel law, which allows the characterization of the catalyst by the value of the TOF at zero overpotential (TOF(0)). Foot-of-the-wave analysis of the cyclic voltammetric catalytic responses allows the determination of the TOF, log TOF-η relationship, and TOF(0), regardless of the side-phenomena that interfere at high current densities, preventing the expected catalytic current plateau from being reached. Strategies for optimized preparative-scale electrolyses may then be devised on these bases. The validity of this methodology is established on theoretical grounds and checked experimentally with examples taken from the catalytic reduction of CO(2) by iron(0) porphyrins.

Proton-coupled electron transfer cleavage of heavy-atom bonds in electrocatalytic processes. Cleavage of a C-O bond in the catalyzed electrochemical reduction of CO2.

Most of the electrocatalytic processes of interest in the resolution of modern energy challenges are associated with proton transfer. In the cases where heavy atom bond cleavage occurs concomitantly, the question arises of the exact nature of its coupling with proton-electron transfer within the catalytic cycle. The cleavage of a C-O bond in the catalyzed electrochemical conversion of CO2 to CO offers the opportunity to address this question. Electrochemically generated iron(0) porphyrins are efficient, specific, and durable catalysts provided they are coupled with Lewis or Brönsted acids. The cocatalyst properties of four Brönsted acids of increasing strength, water, trifluoroethanol, phenol, and acetic acid, have been systematically investigated. Preparative-scale electrolyses showed that carbon monoxide is the only product of the catalytic reaction. Methodic application of a nondestructive technique, cyclic voltammetry, with catalyst and CO2 concentrations, as well as H/D isotope effect, as diagnostic parameters allowed the dissection of the reaction mechanism. It appears that the key step of the reaction sequence consists of an electron transfer from the catalyst concerted with the cleavage of a C-O bond and the transfer of one proton. This is the second example, and an intermolecular version of such a concerted proton-electron bond-breaking reaction after a similar electrochemical process involving the cleavage of O-O bonds has been identified. It is the first time that a proton-electron transfer concerted with bond breaking has been uncovered as the crucial step in a catalytic multistep reaction.

Platinum and palladium complexes of fluorenyl porphyrins as red phosphors for light-emitting devices

Platinum(II) and palladium(II) complexes of a porphyrin with fluorenyl groups linked directly at the meso-positions (TFP) have been synthesized and characterized. Their luminescence properties in solution have been studied under ambient conditions and at 77 K. Compared to the corresponding complexes of tetraphenylporphyrin TPP, the emission is red-shifted in both cases. Radiative rate constants are found to be enhanced by the fluorenyl substituents, although non-radiative decay rates are also increased in solution. Short-wavelength UV excitation leads to no detectable fluorenyl-based fluorescence, indicating that a very efficient transfer of energy to the porphyrin occurs. An OLED in which PtTFP is used as a red phosphor has been prepared and its electroluminescence performance assessed. High current “roll-off” is significantly delayed to higher currents compared to previously described OLEDs that make use of standard platinum porphyrins as emitters, an effect which can be attributed to the shorter radiative lifetime of PtTFP.

Enhanced two-photon absorption cross-sections of zinc(II) tetraphenylporphyrins peripherally substituted with d 6 -metal alkynyl complexes

The syntheses of new Zn(II) tetraphenylporphyrin (ZnTPP) derivatives functionalized with electron-rich d6-transition metal alkynyl complexes at their periphery are reported. Z-scan measurements reveal remarkably large effective two-photon absorption (TPA) cross-sections in the visible range for these compounds.