A Compact All-Carbon Tripodal Tether Affords High Coverage of Porphyrins on Silicon Surfaces

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 Å) in the monolayer is only ∼50% larger than the minimum achievable, indicating high surface coverage. IR spectroscopy indicates that the bands due to the ν(CC) (1638 cm^-1) and γ(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 (α) of the porphyrin ring with respect to the surface normal is ∼46°, 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 (t_1/2) for the first oxidation state of the Zn-1 monolayers increase from 10 to 50 s at low surface coverage (1−5 × 10^-11 mol·cm^-2) to near 200 s at saturation coverage (∼2 × 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.