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

The attachment of redox-active molecules such as porphyrins to an electroactive surface provides an attractive approach for electrically addressable molecular-based information storage. Porphyrins are readily attached to a gold surface via thiol linkers. The rate of electron transfer between the electroactive surface and the porphyrin is one of the key factors that dictates suitability for molecular-based memory storage. This rate depends on the type and length of the linker connecting the thiol unit to the porphyrin. We have developed different routes for the preparation of thiol-derivatized porphyrins with eight different linkers. Two sets of linkers explore the effects of linker length and conjugation, with one set comprising phenylethyne units and one set comprising alkyl units. One electron-deficient linker has four fluorine atoms attached directly to a thiophenyl unit. To facilitate the synthesis of the porphyrins, convenient routes have been developed to a wide range of aldehydes possessing a protected S-acetylthio group. An efficient synthesis of 1-(S-acetylthio)-4-iodobenzene also has been developed. A set of porphyrins, each bearing one S-acetyl-derivatized linker at one meso position and mesityl moieties at the three remaining meso positions, has been synthesized. Altogether seven new aldehydes, eight free base porphyrins and eight zinc porphyrins have been prepared. The zinc porphyrins bearing the different linkers all form self-assembled monolayers (SAMs) on gold via in situ cleavage of the S-acetyl protecting group. The SAM of each porphyrin is electrochemically robust and exhibits two reversible oxidation waves.

The redox kinetics for a variety of porphyrin-containing self-assembled monolayers (SAMs) on Au are reported. The measurements probe both the rate of electron-transfer (k 0 ) for oxidation (in the presence of applied potential) and the rate of charge dissipation after the applied potential is disconnected (characterized by a chargeretention half-life (t 1/2 )). The porphyrins include (1) monomeric Zn complexes that contain phenylmethylene linkers wherein the number of methylene spacers varies from 0 to 3, (2) monomeric Zn complexes that contain different ethynylphenyl-derived linkers, and (3) a triple-decker lanthanide sandwich complex with a phenylethynylphenyl linker. The k 0 values for all the porphyrin SAMs are in the range of 10 4 -10 5 s -1 . The k 0 values for the monomeric ethynylphenyl-linked porphyrin SAMs are generally faster than those for the monomeric phenylmethylene-linked SAMs. The rates for the latter SAMs decrease as the number of methylene spacers increases. The rates for the triple-decker SAM are generally slower than those for the monomers. The trends observed in the k 0 values are paralleled in the t 1/2 values, that is, porphyrin SAMs that exhibit relatively faster electron-transfer rates also exhibit faster charge-dissipation rates (shorter t 1/2 values). However, the charge-dissipation rates (no applied potential) are approximately 6 orders of magnitude slower than the electron-transfer rates (applied potential). Both the k 0 and t 1/2 values for the porphyrin SAMs are sensitive to the surface coverage of the molecules. The rates for both processes decrease as the monolayers become more densely packed. This behavior is attributed to exclusion of solvent/counterions and space-charge effects. The effect of surface coverage on rates can overshadow differences that result from differences in linker type/ length. Collectively, the studies help to delineate the molecular design features that could be manipulated to control the redox processes in porphyrin SAMs. The understanding of these processes is essential for the successful implementation of molecules as the active media in information-storage elements.