Galvinoxyl monolayers on Au(111) studied by STM, EPR, and cyclic voltammetry

Abstract

Galvinoxyl layers on Au(111) have been studied by scanning tunnelling microscopy (STM), electron paramagnetic resonance (EPR), and cyclic voltammetry (CV). We observe two phases: configuration I having a molecular density of 1.57 /- 0.16x10(-10) mol/cm(2) and a rectangular lattice (15 angstrom by 7 angstrom) observed at room temperature and down to 140 K; configuration II with a slightly smaller molecular density of 1.37 /- 0.14x10(-10) mol/cm(2) and oblique cells (22.5 angstrom by 5.4 angstrom) arranged alternatingly in stacks yielding a molecular layer with lower symmetry and comparatively large crystallographic unit cell. The latter is only observed upon cooling down to 40 K and subsequent annealing to room temperature. For both assemblies typical domain sizes in the range of 100 nm have been found. The EPR results confirm that the radical character is preserved upon adsorption and that the intermolecular distance is smaller than 11 angstrom. The interaction between the overlapping singly occupied spin orbitals is high, indicating no participation of the unpaired electron in the binding to the surface or laterally between neighboring radicals. The average surface concentration deduced from CV measurements is in excellent agreement with the surface coverages deduced from STM topographies. In aqueous electrolyte the adsorbate undergoes a one-electron oxidation with concomitant loss of a proton as determined from oxidation potential vs pH curves in a similar fashion as known for the free radical in solution. This indicates no dramatic change of the electronic properties of the radical upon adsorption. Structure models are proposed with molecules standing upright like ``bicycles in rows.''