Metalloporphyrin electropolymerization: electrochemical quartz crystal microgravimetric studies

The electrochemical quartz crystal microbalance (EQCM) study of several metalloprotoporphyrin (MPP) deposition (Ni(II)PP, Cu(II)PP, Co(II)PP, Zn(II)PP, Fe(III)PP) on gold surfaces from dichloromethane solution is reported. The QCM mass–time evolution during film deposition has shown for all porphyrins studied a monotonic film growth during the positive voltammetric scan which is consistent with the accepted vinyl cation electropolymerization (EP) mechanism [J. Am. Chem. Soc. 105 (1983) 5601]. Quartz crystal impedance at 10 MHz has shown that NiPP, CuPP, ZnPP and FePP films dissipate the acoustic wave much less, rendering acoustically thinner films than a CoPP film that behaves as a loose viscoelastic film. For the former acoustically thin films, gravimetry was possible; unlike CoPP films where the viscoelastic properties are dominant at 10 MHz. Comparison of the redox charge and QCM mass demonstrates that homogeneous chemical reactions coupled to the electroinitiation of vinyl cations should take place, and therefore the application of the Faraday Law to estimate the polymer mass from the oxidation charge is precluded. The optimal EP conditions to deposit single and mixed films are also described.