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

Abstract

Galvinoxyl layers on Au͑111͒ have been studied by scanning tunnelling microscopy ͑STM͒, electron paramagnetic resonance ͑EPR͒, and cyclic voltammetry ͑CV͒. We observe two phases: configuration I having a molecular density of 1.57± 0.16ϫ 10 −10 mol/ cm 2 and a rectangular lattice ͑15 Å by 7 Å͒ observed at room temperature and down to 140 K; configuration II with a slightly smaller molecular density of 1.37± 0.14 ϫ 10 −10 mol/ cm 2 and oblique cells ͑22.5 Å by 5.4 Å͒ 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 Å. 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."

Formation of dense nitroxide radical layers on the Au(111) substrate for ESN-STM measurement

“…This is because the early results presented by Michel [20] were not confirmed in the later studies reported by Pong et al [19]. It has been also shown that galvinoxyl (2,6-di-tert-butyl-a-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy) molecules deposited on the Au(1 1 1) substrate form two different self-assembled structural configurations and retain their paramagnetic properties [21]. The most recent studies showed that the self-assembled monolayer of family of nitronyl nitroxide radicals (NitRs) can be obtained by immersing of the Au(1 1 1) substrate in dichloromethane solution of NitRs.…”

Electrochemistry of redox-active self-assembled monolayers

“…The study of surface-bound radicals is directed towards the development of organic ferromagnets that show long range magnetic order in the crystalline phase. TEMPO and galvinoxyl are highly stable radicals that persist at room temperature and have been studied using SAMs [190,204–206]. A number of excellent reviews cover the area of PCET in greater detail [66,67].…”

“…Galvinoxyl layers on Au(111) have been studied by STM, EPR, and CV [206]. In the STM, two phases are observed with different molecular densities.…”

“…A one-electron oxidation with corresponding loss of a proton was determined from oxidation potential vs. pH curves and is similar to the behavior of the free radical in solution. This indicates there is no dramatic change of the electronic properties of the radical upon adsorption [206]. …”

The study of paramagnetic self‐assembled monolayers formed by nitroxide radical (TEMPO)3O3P on HOPG

“…There are a number of works reporting the possibility of keeping paramagnetic properties by nitroxides after the deposition on well-defined surfaces. [24][25][26] The first part of EPR studies was devoted to investigations of powdered (TEMPO) 3 O 3 P compound. In Fig.…”