Structural and Electronic Properties of Columnar Supramolecular Assemblies Formed from Ionic Metal-Free Phthalocyanine on Au(111)

Highly ordered assemblies prepared from tetra (4-sulfonatophenyl) phthalocyanine (TSPc) by employing very acidic aqueous solutions were deposited onto Au(111) substrates and studied in UHV using X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and orbital mediated tunneling spectroscopy (OMTS). XPS of the TSPc aggregates shows that the ratio of protonated to unprotonated nitrogens does not change with decreasing solution pH. STM images of TSPc deposited from pH <1 solutions reveal ordered branched web-like assemblies hundreds of nanometers in length, generally 2 nm tall and having variable widths. High-resolution UHV-STM images of TSPc aggregates on Au(111) reveal detailed coherent columnar architecture with the phthalocyanine macrocycles orientated parallel to the substrate surface. OMTS was used to identify high-energy occupied orbitals, the LUMO of the TSPc aggregates, and the results are contrasted with the same molecular states in unsubstituted metalated phthalocyanines (MPc’s). The positions of the filled and the empty states of the TSPc are comparable to those of other unsubstituted MPc’s, indicating that the electronegative sulfonate substituents have minimal effect on the electronic properties of the macrocycle in this aggregated state on Au. The HOMO–LUMO separation of the TSPc is slightly >2 eV, a value consistent with the literature assignments for the Pc ring band gap.