For the first time, the pressure and temperature dependence of a chemical reaction at the solid/solution interface is studied by scanning tunneling microscopy (STM), and thermodynamic data are derived. In particular, the STM is used to study the reversible binding of O(2) with cobalt(II) octaethylporphyrin (CoOEP) supported on highly oriented pyrolytic graphite (HOPG) at the phenyloctane/CoOEP/HOPG interface. The adsorption is shown to follow the Langmuir isotherm with P(1/2)(298K) = 3200 Torr. Over the temperature range of 10-40 °C, it was found that ΔH(P) = -68 ± 10 kJ/mol and ΔS(P) = -297 ± 30 J/(mol K). The enthalpy and entropy changes are slightly larger than expected based on solution-phase reactions, and possible origins of these differences are discussed. The big surprise here is the presence of any O(2) binding at room temperature, since CoOEP is not expected to bind O(2) in fluid solution. The stability of the bound oxygen is attributed to charge donation from the graphite substrate to the cobalt, thereby stabilizing the polarized Co-O(2) bonding. We report the surface unit cell for CoOEP on HOPG in phenyloctane at 25 °C to be A = (1.46 ± 0.1)n nm, B = (1.36 ± 0.1)m nm, and α = 54 ± 3°, where n and m are unknown nonzero non-negative integers.

Nanorods produced from the sodium salt of tetrakis(4-sulfonatophenyl) porphyrin, dissolved in acidic aqueous solutions, were deposited onto Au(111) substrates and imaged by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). The AFM and STM images revealed individual rods with a diameters of 25-40 nm and lengths of hundreds of nanometers. Bundles of individual rods fashioned larger structures. We report for the first time high resolution STM images of TSPP on Au(111) which reveal that the rods are composed of disk-like building blocks approximately 6.0 nm in diameter. We speculate that the disks are formed by a circular J-aggregation of 14-16 overlapping electronically coupled porphyrin chromophores and that this circular porphyrin organization is driven by nonplanar distortions of the porphyrin diacid. The resonance Raman spectra of the solution phase aggregate and the surface-enhanced resonance Raman spectra of the aggregate on gold films were obtained at an excitation wavelength coincident with the exchange-narrowed J-band and found to be similar in peak frequencies and relative intensities. The UV-visible absorption spectrum of the solution phase aggregate was also found to be similar to that of the aggregate deposited on quartz. These comparisons confirm similar ground and excited electronic state structures of the excitonically coupled chromophores which comprise the aggregate in solution and on gold. Our results shed light on a number of previous experimental observations that could not be rationalized within the typical presumed staircase model of J-aggregation.

In this communication we provide the first UHV-STM images and STM-based current-voltage (I-V) and orbital mediated tunneling spectroscopy (OMTS) data on a self-assembled porphyrin nanostructure at the single structure level. 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. These nanorods are very highly rectifying.

We report polarized resonance Raman data of tetrakis(4-sulfonato)phenyl porphyrin (TSPP) aggregates in solution and deposited on Au(111) at wavelengths resonant with the red-shifted (J-band) and blue-shifted (H-band) components of the split Soret (B) band. We also report scanning tunneling microscopy (STM) images which reveal that the aggregate on Au(111) is a nanotube with a 2 nm wall thickness which tends to flatten on the substrate. Relative Raman intensities and their dependence on polarization of the incident and scattered light are found to vary greatly for H- and J-band excitation, revealing a much greater degree of coherence for the J-band, in agreement with the resonance light scattering spectrum. The J-band transition is found to have transition moment components both parallel and perpendicular to the long axis of the nanotube, consistent with a helical nanotube structure. The intensity increase of the Q-band on aggregation and the weak intensity of the H-band in both the absorption and the resonance light scattering spectra are explained by vibronic B-Q coupling, which is permitted in the lowered site symmetry of the aggregate. The resonance Raman data presented here provide insight into the molecular basis for the hierarchal structure of the aggregate.

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