Oligo(phenyleneethynylene) (OPE) molecules are a class of conjugated aromatic molecules with excellent self-organizing properties, that attracts a lot of attention for their application as "molecular wires". By functionalizing this molecular wire with an optical probe, such as porphyrin, a well-organized organic surface can be achieved with optical and redox properties that can be controlled. Such surfaces may find applications in optoelectronic devices, such as solar cells or sensors.
Porphyrin-functionalized oligo(phenylene-ethynylene) (P-OPE) molecules were used to prepare a self-assembled monolayer (SAM) on Au(111) substrates. The molecules have an acetyl-protected thiophenol binding group (R-S-COCH3). The SAMs were characterized by synchrotron-based high-resolution X-ray photoelectron spectroscopy (HRXPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and results were compared to those for SAMs of unfunctionalized OPE. The chemical composition and the molecule-substrate interaction were probed by HRXPS of the C1s, S2p and N1s core levels, while the structure and the orientation of SAM were studied by angular-resolved NEXAFS. The HRXPS results indicate that the P-OPE molecules bind to the Au surface through a thiolate bond similarly to the OPE SAM system. Two N 1s peaks were observed in the HRXPS spectra of the P-OPE SAM, corresponding to the two different nitrogen species in the free-base porphyrin functional group (protonated and unprotonated nitrogen). The thicknesses of the P-OPE and OPE SAMs were derived from the HRXPS data and estimated to be 2.9 and 1.8 nm, respectively. The NEXAFS spectra of the P-OPE SAM measured at the C1s edge exhibit a characteristic adsorption resonance of porphyrin. Combined HRXPS and NEXAFS results show orientational order and dense packing in the P-OPE SAM with inclined molecules, for which the average tilt angle of the molecular axis was estimated to be about 35° with respect to the surface normal. In comparison with the OPE system, which was found to form a high quality SAM with densely packed and highly oriented molecules with the average tilt angle of 30°, a large system such as porphyrin is likely to affect the orientation of the P-OPE SAM.
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