Tunnelling conductance of vectorial porphyrin monolayers

A method for antipodal attachment of sulfur linkers to a nickel chelated porphyrin has been developed to facilitate robust chemisorption at gold electrodes. At sufficiently high molecular coverage ellipsometric, infrared and electroanalytical data are supportive of a close-to-vertical molecular orientation with respect to the substrate. Molecular conductance was assessed both within neat porphyrin monolayer films and on individual molecules isolated within an ordered saturated alkyl thiol (1-tetradecanethiol) self-assembled monolayer and then capped with chemisorbed gold nanoparticles. In the former, three distinct groups of current-voltage (I-V) curves were observed both by scanning tunnelling microscopy (STM) and conductive probe atomic force microscopy (CP-AFM), corresponding to the conductance of one, two, and three molecules, which were quantizable as integer multiples of a fundamental molecular conductance (1.37 +/- 0.25 nS). This value compares well with that obtained on isolated molecules and is significantly higher than the host matrix. In all cases I-V trends are well described by a non-resonant tunnelling model