K W Hipps - Scanning Tunneling Microscopy, Orbital-Mediated Tunneling Spectroscopy, and Ultraviolet Photoelectron Spectroscopy of Metal(II) Tetraphenylporphyrins Deposited from Vapor

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      Publication Details (including relevant citation   information):

      Scudiero, L., Barlow, Dan E., Mazur, Ursula, Hipps, K. W.   Journal of the American Chemical Society  2001 123 (17) 4073-4080

      Abstract: Thin films of vapor-deposited Ni(II)   and Co(II) complexes of tetraphenylporphyrin (NiTPP and CoTPP)   were studied supported on gold and embedded in Al−Al2O3−MTPP−Pb   tunnel diodes, where M = Ni or Co. Thin films deposited onto   polycrystalline gold were analyzed by ultraviolet photoelectron   spectroscopy (UPS) using He I radiation. Scanning tunneling   microscopy (STM) and orbital-mediated tunneling spectroscopy   (STM−OMTS) were performed on submonolayer films of CoTPP and   NiTPP supported on Au(111). Inelastic electron tunneling   spectroscopy (IETS) and OMTS were measured in conventional tunnel   diode structures. The highest occupied π molecular orbital of the   porphyrin ring was seen in both STM−OMTS and UPS at about 6.4 eV   below the vacuum level. The lowest unoccupied π* molecular   orbital of the porphyrin ring was observed by STM−OMTS and by   IETS−OMTS to be located near 3.4 eV below the vacuum level. The   OMTS spectra of CoTPP had a band near 5.2 eV (below the vacuum   level) that was attributed to transient oxidation of the central   Co(II) ion. That is, it is due to electron OMT via the   half-filled dz2 orbital present in Co(II) of CoTPP. The NiTPP   OMTS spectra show no such band, consistent with the known   difficulty of oxidation of the Ni(II) ion. The STM-based OMTS   allowed these two porphyrin complexes to be easily distinguished.   The present work is the first report of the observation of   STM−OMTS, tunnel junction OMTS, and UPS of the same compounds.   Scanning tunneling microscope-based orbital-mediated tunneling   provides more information than UPS or tunnel junction-based OMTS   and does so with molecular-scale resolution.

      Address (URL): http://dx.doi.org/10.1021/ja0100726