Molecular architecture: Construction of self-assembled organophosphonate duplexes and their electrochemical characterization

Self-assembled monolayers of phosphonates (SAMPs) of 11- hydroxyundecylphosphonic acid, 2,6-diphosphonoanthracene, 9,10-diphenyl-2,6- diphosphonoanthracene, and 10,10′-diphosphono-9,9′-bianthracene and a novel self-assembled organophosphonate duplex ensemble were synthesized on nanometer-thick SiO ₂-coated, highly doped silicon electrodes. The duplex ensemble was synthesized by first treating the SAMP prepared from an aromatic diphosphonic acid to form a titanium complex-terminated one; this was followed by addition of a second equivalent of the aromatic diphosphonic acid. SAMP homogeneity, roughness, and thickness were evaluated by AFM; SAMP film thickness and the structural contributions of each unit in the duplex were measured by X-ray reflection (XRR). The duplex was compared with the aliphatic and aromatic monolayer SAMPs to determine the effect of stacking on electrochemical properties; these were measured by impedance spectroscopy using aqueous electrolytes in the frequency range 20 Hz to 100 kHz, and data were analyzed using resistance-capacitance network based equivalent circuits. For the 11-hydroxyundecylphosphonate SAMP, C _SAMP = 2.6 ± 0.2 μF/cm ², consistent with its measured layer thickness (ca. 1.1 nm). For the anthracene-based SAMPs, C _SAMP = 6-10 μF/cm ², which is attributed primarily to a higher effective dielectric constant for the aromatic moieties (ε = 5-10) compared to the aliphatic one; impedance spectroscopy measured the additional capacitance of the second aromatic monolayer in the duplex (2ndSAMP) to be C _Ti/2ndSAMP = 6.8 ± 0.7 μF/cm ², in series with the first.