Using scanning tunneling microscope break-junction experiments and a new first-principles approach to conductance calculations, we report and explain low-bias charge transport behavior of four types of metal-porphyrin-gold molecular junctions. A nonequilibrium Green's function approach based on self-energy corrected density functional theory and optimally tuned range-separated hybrid functionals is developed and used to understand experimental trends quantitatively. Importantly, due to the localized d states of the porphyrin molecules, hybrid functionals are essential for explaining measurements; standard semilocal functionals yield qualitatively incorrect results. Comparing directly with experiments, we show that the conductance can change by nearly a factor of 2 when different metal cations are used, counter to trends expected from gas-phase ionization energies which are relatively unchanged with the metal center. Our work explains the sensitivity of the porphyrin conductance with the metal center via a detailed and quantitative portrait of the interface electronic structure and provides a new framework for understanding transport quantitatively in complex junctions involving molecules with localized d states of relevance to light harvesting and energy conversion.

We have prepared self-assembled monolayers (SAMs) of 4-aminothiophenol (4-ATP) and 1-(4-mercaptophenyl)-2,6-diphenyl-4-(4-pyridyl)pyridinium tetrafluoroborate (MDPP) functionalized with iron phthalocyanine (FePc) and copper phthalocyanine (CuPc) adsorbed on gold (111) electrodes. The catalytic activity of these SAMs/MPc was examined for the reduction of O 2 in aqueous solutions and compared to that of bare gold and with gold coated directly with preadsorbed MPc molecules. Scanning tunneling microscopy (STM) studies confirm the functionalization of the 4-ATP by MPc. STM images reveal that iron phthalocyanine molecules are chemically anchored to 4-aminothiophenol organic monolayers, probably having an "umbrella" type orientation with regards to the surface. The electrocatalytic studies carried out with Au/4-ATP/FePc and Au/ MDPP/FePc electrodes show that the O 2 reduction takes place by the transfer of 4-electron to give water in contrast to a 2-electron transfer process observed for the bare gold. The modified electrode obtained by simple adsorption of FePc directly to the Au(111) surface still promotes the 4-electron reduction process, but it shows a lower activity than the electrodes involving SAMs with FePc molecules positioned at the outmost portion of the selfassembled monolayers. The activity of the electrodes increases as follow: Au < Au/FePc < Au/4-ATP/FePc < Au/MDPP/FePc with the highest activity when FePc molecules are more separated from the Au surface. In contrast, the less active CuPc shows almost the same activity in all three configurations. Theoretical calculations suggest the importance of the back-bonding into the adduct formation, showing the relevance of the supporting gold surface on the electron-transfer process mediated by anchoring ligands.

An efficient and inexpensive catalyst for the oxygen reduction reaction (ORR) is the key missing component for large-scale development of fuel cells.

We have been able to “tune” the electrocatalytic activity of iron phthalocyanine (FePc) and iron hexadodecachlorophthalocyanine (16­(Cl)­FePc) for the oxygen reduction reaction (ORR) by manipulating the “pull effect” of pyridinium molecules axially bounded to the phthalocyanine complexes (FePcs). These axial ligands play both the role of molecular anchors and also of molecular wires. The axial ligands also affect the reactivity of the Fe metal center in the phthalocyanine. The “pull effect” originates from the positive charge located on the pyridinium core. We have explored the influence of the core positions (Up or Down), in two structural pyridiniums isomers on the activity of FePc and 16­(Cl)­FePc for the ORR. Of all self-assembled catalysts tested, the highest catalytic activity was exhibited by the Au(111)/Up/FePc system. XPS measurements and DFT calculations showed that it is possible to tailor the FePc–N­(pyridiniums) Fe–O2 binding energies, by changing the core positions and affecting the “pull effect” of pyridiniums. This affects directly the catalytic activity of FePcs. The plot of activity as (log I)E versus the calculated Fe–O2 binding energies gives an activity volcano correlation, indicating that an optimum binding energy of O2 with the Fe center provides the highest activity.

The formation of self-assembly monolayers (SAMs) based on a gold substrate and a thiolate ligand as "anchor" fragment of metallophtalocyanine has been employed as strategy toward the obtention of modified electrodes. In this Article, the formation of SAM's involving iron and cobalt phtalocyanines anchored by 4-aminothiophenol (4-ATP) and 4-mercatopyridine (4-MP) to the Au(111) surface is explored by both experimental and theoretical studies for a better understanding of their bonding pattern and optical properties. The self-assembly metallophthalocyanines complexes on gold electrode exhibits an interesting charge donation from the 4-ATP or 4-MP toward both gold substrate and phtalocyanine, denoting an effective goldÀMPc interaction mediated by the titled anchor ligands. In addition, the optical properties of the self-assembled complexes supported on the gold electrode exhibit in conjuction with the well-described Q-band an interesting charge transfer from the Pc (π) toward the gold surface, as could be observed in the FePc-4MP-Au 26 assembly.

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