Differing HOMO and LUMO Mediated Conduction in a Porphyrin Nanorod Benjamin A. Friesen, Bryan Wiggins, Jeanne L. McHale, Ursula Mazur*, and K. W. Hipps* Washington State University, Department of Chemistry and Materials Science and Engineering Program, PO Box 644630, Pullman, WA 99164-4630. JACS 2010, 132, 8554-8556; hipps@wsu.edu There has been considerable recent interest in the production of porphyrin self-assembled nanostructures. 1,2 Although porphyrin materials in general are known to be photoconductors, 3 photovoltaics, 4 and capable of light-induced charging, 5 there has been very little reported in terms of the conducting properties of their self-assembled nanostructures. To our knowledge, Schwab and co-workers 6 have provided the only electronic transport study of self-assembled porphyrin nanostructures to date. They reported on the photoconductivity of nanorods formed from highly acidic solutions of meso-tetrakis(4-sulfonato- phenyl)porphine. Schwab et al. 6 found that the photoconductivity grows over hundreds of seconds upon light exposure and decays slowly when the light is off. They proposed a qualitative model where the conduction occurred through the LUMOs of the molecules. They also reported that the rods were insulating over the voltage range studied (±0.5V) in the dark. In addition to the dearth of electronics studies of porphyrin nanostructures, there is also a conspicuous shortage of scanning tunneling microscopy, STM, work. Friesen et al. reported the only detailed STM images available to date [also on meso- tetrakis(4-sulfonatophenyl)porphine]. 2 Their work was performed in air. By contrasting STM and AFM images, they inferred that the nanorods were highly conducting at voltages above about +1.5 V relative to the substrate. In this communication we will provide the first UHV-STM images, STM-based current-voltage (I-V) curves, and orbital mediated tunneling spectroscopy 7 (OMTS) of a self-assembled porphyrin nanostructure at the level of a single nanorod or single molecule constituent. We will show that transverse conductivity over distances less than 10 nm can occur by barrier type tunneling, but that long distance conduction solely occurs through the LUMO band. The HCl adduct of the meso-tetrakis(4-sulfonato- phenyl)porphine HCl, H 2 (H 4 TSPP)2HCl was purchased from Porphyrin Products and used as supplied. Either 1x10 -6 M or 5x10 -6 M solutions were freshly prepared in 0.75 M HCl. These solutions were then placed on either HOPG or Au(111) surfaces, allowed to stand for 45 or 60 minutes, respectively. Then the substrates were spun dry. AFM and STM images in air showed rods that were about 4 nm high, 27 nm wide, and 0.3 to 2 m long. These sizes are consistent with those reported previously whether made using the sodium salt as starting material 2 or using the HCl adduct. 6 All samples were transferred to UHV and some were heated to 100C for 9 min before measurement. They were then analyzed either by STM or XPS. I-V and dI/dV spectra were collected in constant height mode. The tip was set at a desired position at a set-point of +1.6V and 15 pA, the feedback was turned off, and 64 I-V curve were collected at the selected point. dI/dV curves were numerically calculated from the I-V curves. I- V and dI/dV curves are averages of those 64 scans. Images were acquired in constant current mode at +1.6 V sample bias and 1 pA setpoint current. All data was acquired using an RHK variable substrate temperature UHV STM and RHK XPM Pro software. XPS samples were prepared with the same procedures as for the STM samples but they were not heated in UHV. 180-200 watts of achromatic radiation at energy 1253.6 eV (MgK  ) was used as XPS excitation sources. The analyzer was set for a spatial resolution of 120 m. The energy resolution was set to 1.0 eV for survey spectra, and to 0.15 eV for the higher resolution acquisitions of C1s, N1s, S2p, Cl2p, and Au4f 7/2 peaks. Binding energies were calibrated against the Au4f 7/2 peak taken to be located at BE = 84.3 eV and against the C1s peaks for HOPG samples (BE=284.5 eV). For unheated Au(111) samples XPS revealed that chlorine was present on the surface, both for H 2 (H 4 TSPP) in HCl and HCl alone. They also showed that no chlorine was present on the surface for heated Au(111) samples and no chlorine was detected at any time on HOPG. Our XPS studies indicated that all four nitrogen atoms were protonated. These results are consistent with previous assumptions that the neutral form of the acid, H 2 (H 4 TSPP), is the only chemical species in the nanorods. The inset in Figure 1 is the structure of that species. It is thought that the driving force for rod formation is electrostatic interaction between the negative sulfonate and the positive core of adjacent molecules. Sulfonates stack above and below the +2 core of a third molecule to create an extended structure. 1,2 FIGURE 1. Constant current UHV-STM image of a nanorod on Au(111) taken at a setpoint of 1.5 V bias and 5.0 pA.