Adsorption characteristics of tripodal thiol-functionalized porphyrins on gold.

X-ray photoelectron and Fourier transform infrared spectroscopy studies are reported for self-assembled monolayers (SAMs) of two tripodal thiol-functionalized metalloporphyrins (Zn and Cu) and three benchmark tripods on gold substrates. The tripodal unit common to all five molecules is 1-(phenyl)-1,1,1-tris(4-mercaptomethylphenyl)methane (Tpd). Both porphyrins contain S-acetyl-protected thiols and are linked to the 4-position of the phenyl ring of Tpd via a phenylethyne group. The benchmark molecules include (1) two tripods containing a bromine atom at the 4-position of the apical phenyl ring, one a free thiol and the other its S-acetyl-protected analogue, and (2) a S-acetyl-protected tripod containing a phenylethyne unit at the 4-position of the apical phenyl group. Together, the spectroscopic studies reveal that none of the five tripodal molecules bond to the gold surface via all three sulfur atoms. Instead, the average number of bound thiols ranges from 1.5 to 2, with the porphyrinic molecules generally falling at the middle to upper end of the range and the smallest benchmark tripods falling at the lower end. Similar surface binding is found for the S-acetyl-protected and free benchmark tripods, indicating that the presence of the protecting group does not influence binding. Furthermore, the surface binding characteristics of the SAMs are not sensitive to deposition conditions such as solvent type, deposition time, or temperature of the solution.

Triple-decker sandwich compounds bearing compact triallyl tripods for molecular information storage applications.

The design of redox-active molecules that afford multistate operation and high charge density is essential for molecular information storage applications. Triple-decker sandwich compounds composed of two lanthanide metal ions and three porphyrinic ligands exhibit a large number of oxidation states within a relatively narrow electrochemical window. High charge density requires a small footprint upon tethering triple deckers to an electroactive surface. All triple deckers examined to date for information storage have been tethered via the terminal ligand and have exhibited large footprints (approximately 670 A2). Five new homonuclear (Eu or Ce) triple deckers have been prepared (via statistical or rational methods) to examine the effect of tether attachment site on molecular footprint. Three triple deckers are tethered via the terminal ligand (porphyrin) or central ligand (porphyrin or imidazophthalocyanine), whereas two triple deckers each bear two tethers, one at each terminal ligand. The tether is a compact triallyl tripod. Monolayers of the triple deckers on Si(100) were examined by electrochemical and FTIR techniques. Each triple decker exhibited the expected four resolved voltammetric waves, owing to formation of the mono-, di-, tri-, and tetracations. The electrochemical studies of surface coverage (gamma, obtained by integrating the voltammetric waves) reveal that coverages approaching 10(-10) mol cm(-2), corresponding to a molecular footprint of approximately 170 A2, are readily achieved for all five of the triple deckers. The surface coverage observed for the tripodal functionalized triple deckers is approximately 4-fold higher than that obtained for monopodal-functionalized triple deckers (carbon, oxygen, or sulfur anchor atoms) attached to either Si(100) or Au(111). The fact that similar, relatively high, surface coverages can be achieved regardless of the location (or number) of the tripodal tether indicates that the tripodal functionalization, rather than the location of the tether, is the primary determinant of the packing density.

Porphyrin dyads bearing carbon tethers for studies of high-density molecular charge storage on silicon surfaces.

Redox-active molecules that afford high charge density upon attachment to an electroactive surface are of interest for use in molecular-based information-storage applications. One strategy for increasing charge density is to covalently link a second redox center to the first in an architecture that uses the vertical dimension in essentially the same molecular footprint. Toward this end, a set of four new porphyrin dyads have been prepared and characterized. Each dyad consists of two zinc porphyrins, an intervening linker (p-phenylene or 4,4'-diphenylethyne), and a surface attachment group (ethynyl or triallyl group). The porphyrin dyads were attached to an electroactive Si(100) surface and interrogated via electrochemical and FTIR techniques. The charge density obtainable for the ethynyl-functionalized porphyrin dyads is approximately double that observed for an analogously functionalized monomer, whereas that for the triallyl-functionalized dyads is at most 40% larger. These results indicate that the molecular footprint of the former dyads is similar to that of a monomer while that of the latter dyads is larger. For both the ethynyl- and triallyl-functionalized porphyrin dyads, higher charge densities (smaller molecular footprints) are obtained for the molecules containing the 4,4'-diphenylethyne versus the p-phenylene linker. This feature is attributed to the enhanced torsional flexibility of the former linker compared with that of the latter, which affords better packed monolayers. The FTIR studies indicate that the adsorption geometry of all the dyads is qualitatively similar and similar to that of monomers. However, the dyads containing the 4,4'-diphenylethyne linker sit somewhat more upright on the surface than those containing the p-phenylene linker, generally consistent with the smaller molecular footprint for the former dyads. Collectively, the high surface charge density (34-58 muC.cm(-)(2)) of the porphyrin dyads makes these constructs viable candidates for molecular-information-storage applications.

A compact all-carbon tripodal tether affords high coverage of porphyrins on silicon surfaces.

[Structure: See text] Redox-active molecules designed to give high charge density on electroactive surfaces are essential for applications in molecular information storage. To achieve a small molecular footprint and thereby high surface charge density, a compound consisting of a triallyl tripod attached via a p-phenylene unit to a porphyrin (1) has been synthesized. The zinc chelate of 1 (Zn-1) was attached to Si(100). Electrochemical measurements indicate that the molecular footprint (75 A) in the monolayer is only approximately 50% larger than the minimum achievable, indicating high surface coverage. IR spectroscopy indicates that the bands due to the nu(C=C) (1638 cm(-1)) and gamma(CH) (915 cm(-1)) vibrations present in the solid sample (KBr pellet) are absent from the spectra of the monolayers of Zn-1, consistent with saturation of the double bond in each of the three legs of the tripod upon the hydrosilylation process accompanying attachment. Comparison of the relative intensities of the in-plane (998 cm(-1)) versus out-of-plane (797 cm(-1)) porphyrin modes indicates the average tilt angle (alpha) of the porphyrin ring with respect to the surface normal is approximately 46 degrees , a value also observed for analogous porphyrins tethered to Si(100) via monopodal carbon linkers. Accordingly, the higher packing densities afforded by the compact tripodal linker are not due to a more upright orientation on the surface. The charge-retention half-lives (t1/2) for the first oxidation state of the Zn-1 monolayers increase from 10 to 50 s at low surface coverage (1-5 x 10(-11) mol.cm(-2)) to near 200 s at saturation coverage (approximately 2 x 10(-10) mol.cm(-2)). Taken together, the high surface charge density (despite the lack of upright orientation) of the triallyl-tripodal porphyrin makes this construct a viable candidate for molecular information storage applications.

Diverse redox-active molecules bearing identical thiol-terminated tripodal tethers for studies of molecular information storage.

To examine the effects of molecular structure on charge storage in self-assembled monolayers (SAMs), a family of redox-active molecules has been prepared wherein each molecule bears a tether composed of a tripodal linker with three protected thiol groups for surface attachment. The redox-active molecules include ferrocene, zinc porphyrin, ferrocene-zinc porphyrin, magnesium phthalocyanine, and triple-decker lanthanide sandwich coordination compounds. The tripodal tether is based on a tris[4-(S-acetylthiomethyl)phenyl]-derivatized methane. Each redox-active unit is linked to the methane vertex by a 4,4'-diphenylethyne unit. The electrochemical characteristics of each compound were examined in solution and in SAMs on Au. Redox-kinetic measurements were also performed on the SAMs (with the exception of the magnesium phthalocyanine) to probe (1) the rate of electron transfer in the presence of an applied potential and (2) the rate of charge dissipation after the applied potential is disconnected. The electrochemical studies of the SAMs indicate that the tripodal tether provides a more robust anchor to the Au surface than does a tether with a single site of attachment. However, the electron-transfer and charge-dissipation characteristics of the two tethers are generally similar. These results suggest that the tripodal tether offers superior stability characteristics without sacrificing electrochemical performance.

Porphyrins bearing mono or tripodal benzylphosphonic acid tethers for attachment to oxide surfaces.

The ability to attach redox-active molecules to oxide surfaces in controlled architectures (distance, orientation, packing density) is essential for the design of a variety of molecular-based information storage devices. We describe the synthesis of a series of redox-active molecules wherein each molecule bears a benzylphosphonic acid tether. The redox-active molecules include zinc porphyrins, a cobalt porphyrin, and a ferrocene-zinc porphyrin. An analogous tripodal tether has been prepared that is based on a tris[4-(dihydroxyphosphorylmethyl)phenyl]-derivatized methane. A zinc porphyrin is linked to the methane vertex by a 1,4-phenylene unit. The tripodal systems are designed to improve monolayer stability and ensure vertical orientation of the redox-active porphyrin on the electroactive surface. For comparison purposes, a zinc porphyrin bearing a hexylphosphonic acid tether also has been prepared. The synthetic approaches for introduction of the phosphonic acid group include derivatization of a bromoalkyl porphyrin or use of a dimethyl or diethyl phosphonate substituted precursor in a porphyrin-forming reaction. The latter approach makes use of dipyrromethane building blocks bearing mono or tripodal dialkyl phosphonate groups. The zinc porphyrin-tripodal compound bearing benzylphosphonic acid legs tethered to a SiO(2) surface (grown on doped Si) was electrically well-behaved and exhibited characteristic porphyrin oxidation/reduction waves. Collectively, a variety of porphyrinic molecules can now be prepared with tethers of different length, composition, and structure (mono or tripodal) for studies of molecular-based information storage on oxide surfaces.