Semiquantitative Submolecular Barrier Height Measurements of 4-Aza-8-fluorotryptanthrin Monolayers on HOPG: Orbital-Mediated Tunneling

摘要

4-Aza-8-fluoroindolo[2,1-b]quinazoline-6,12-dione (4-aza-8-fluorotryptanthrin) was imaged via scanning tunneling microscopy and barrier height tunneling spectroscopy at the graphite-solution interface. Submolecular resolution was achieved for the apparent barrier height (phi(ap)) images and revealed that phi(ap) displays significant variation over the area of the molecule. Topographic effects and electrostatic potential effects were found to be insufficient to explain the behavior of the measured phi(ap) variations. Specifically, where barrier height images were collected over regions of large molecular orbital (MO) wave function amplitudes, the measured phi(ap) did not agree with either affecting phenomenon. The authors attribute the disparity to the fact that electron tunneling is orbital-mediated for these systems. Thus, the phi(ap) is dominated by the spatial disposition of the mediating MO in locations of large wave function amplitude. This hypothesis is corroborated by excellent agreement with a computational treatment that takes into account only the spatial disposition of the MO wave function. It is possible that these measurements could be used to predict molecular behaviors, especially when a redox event or electron coordination is involved.