Articles you may be interested inMolecular and electronic structure of electroactive self-assembled monolayers

The storage of multiple bits of information at the molecular level requires molecules with a large number of distinct oxidation states. Lanthanide triple-decker sandwich molecules employing porphyrins and phthalocyanines afford four cationic states and are very attractive for molecular information storage applications. Five triple-decker building blocks have been prepared of the type (phthalocyanine)Eu(phthalocyanine)Eu(porphyrin), each bearing one iodo, one ethyne, or one iodo and one ethyne group attached to the porphyrin unit. Two triple-decker building blocks with different oxidation potentials were derivatized with an S-acetylthiophenyl unit for attachment to an electroactive surface. To explore the preparation of arrays comprised of triple deckers, which may lead to the storage of a larger number of bits, two types of dyads of triple deckers were prepared. An ethyne-linked dyad of triple deckers bearing one S-acetylthiophenyl unit was prepared via repetitive Sonogashira couplings, and a butadiyne-linked dyad was prepared via a modified Glaser coupling. The triple deckers were characterized by absorption spectroscopy, laser-desorption mass spectrometry, and (1)H NMR spectroscopy. The thiol-derivatized triple deckers form self-assembled monolayers (SAMs) on gold via in situ cleavage of the thiol protecting group. The SAM of each array is electrochemically robust and exhibits three well-resolved, reversible oxidation waves. These electrochemical characteristics indicate that these types of molecules are well suited for storing multiple bits of information.

Self-assembled monolayers of 4-ferrocenylbenzyl alcohol attached to silicon provided the basis for electrolyte-molecule-silicon capacitors. Characterization by conventional capacitance and conductance techniques showed very high capacitance and conductance peaks near ∼0.6 V associated with charging and discharging of electrons into and from discrete levels in the monolayer owing to the presence of the redox-active ferrocenes. The reversible charge trapping of these molecules suggest their potential application in memory devices. Due to the molecular scalability and low-power operation, molecular-silicon hybrid devices may be strong candidates for next-generation electronic devices.

We present the rational design and synthesis of multiporphyrin arrays containing thiol-derivatized linkers for the purpose of multibit molecular information storage. Porphyrin dimers and trimers were synthesized by the Pd-mediated coupling of iodo-substituted and ethynyl-substituted porphyrin building blocks in 5-51% yields. Each porphyrin dimer bears one S-acetylthio group. The architecture of the trimers incorporates a trans-substituted porphyrin (central) bearing two S-acetylthio groups and two diphenylethyne-linked porphyrins (wings) in a trans geometry. The central porphyrin and the wing porphyrins bear distinct substituents and central metals, thereby affording different oxidation potentials. The S-acetylthio groups provide a means for attachment of the arrays to an electroactive surface. The dimers are designed for vertical orientation on an electroactive surface while the trimers are designed for horizontal orientation of the central porphyrin. Altogether seven different arrays were synthesized. Each array forms a self-assembled monolayer (SAM) on gold via in situ cleavage of the S-acetyl protecting group. The SAM of each array is electrochemically robust and exhibits multiple, reversible oxidation waves. In general, however, the trimeric arrays appear to form more highly ordered monolayers that exhibit sharper, better-defined redox features.