Mixed self-assembled monolayers of Co-porphyrin and n-alkane phosphonates on gold

The present work focuses on the surface immobilization by self-assembly of pure and mixed Co-porphyrin (Co-Porph-PO3H2) and n-alkane phosphonic acids (n-CnH2n + 1PO3H2; n = 4, 5 and 10) from n-butanol solutions on gold substrates. The stability, amount, and packing of the phosphonic molecules attached to the Au (111) surface were investigated by electrochemical reductive desorption studies, and monolayersʹ thickness was estimated by ellipsometry. The morphological changes induced by the adsorption of n-decane phosphonic acid on gold were analysed by scanning tunnelling microscopy. The redox behavior of Co-Porph-PO3H2 SAMs was assessed in organic medium and compared that of Co-Porph-CO2CH3 precursor in solution, confirming the self-assembly of the metalloporphyrin molecules. With the purpose of reducing the electrostatic interactions between the porphyrin bulky terminal groups in the SAM, n-C5H11PO3H2 and n-C10H21PO3H2 were used to form mixed monolayers with Co-Porph-PO3H2 on gold. Intermediate electrochemical desorption potentials regarding those values of pure monolayers, as well as an increase of phosphonate surface density compared to that of Co-Porph-PO3H2 SAM, confirm the presence of two-component SAMs, which indicates that porphyrin moieties are diluted in the monolayer. The electrocatalytic activity of the immobilized molecules was demonstrated towards the reduction of molecular oxygen, in acidic medium.