Comparison of Electron-Transfer and Charge-Retention Characteristics of Porphyrin-Containing Self-Assembled Monolayers Designed for Molecular Information Storage

Abstract

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 (k0) for oxidation (in the presence of applied potential) and the rate of charge dissipation after the applied potential is disconnected (characterized by a charge-retention half-life (t1/2)). 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 k0 values for all the porphyrin SAMs are in the range of 104−105 s-1. The k0 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 k0 values are paralleled in the t1/2 values, that is, porphyrin SAMs that exhibit relatively faster electron-transfer rates also exhibit faster charge-dissipation rates (shorter t1/2 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 k0 and t1/2 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.