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Synthesis of amphiphilic azo-anion-radical complexes of chromium(III) and the development of ultrathin redox-active surfaces by the Langmuir-Schaefer technique.

Neutral tris-chelated chromium complex [Cr(L(a))(3)] (1a), and its surfactant derivatives [Cr(L(b))(3)] (1b), [Cr(L(c))(3)] (1c), and [Cr(L(d))(3)] (1d) (where L(a)=2-(4'-methoxyphenylazo)pyridine, L(b)=2-(4'-butyloxyphenylazo)pyridine, L(c =2-(4'-octyloxyphenylazo)pyridine, and L(d)=2-(4'-dodecyloxyphenylazo)pyridine) were synthesized. The molecular structure of compound 1a, determined by X-ray diffraction, showed that the local geometry around the metal center is a distorted octahedral with meridional coordination of the ligands. The structural parameters, spectroscopic data, and density functional theory (DFT) calculations on representative complex 1a suggest that ligand L(a) is predominantly an azo-anion-radical-type, and so the complex can be represented as [Cr(III)(L(a.-))(3)]. An assessment of their physicochemical and surface properties was performed with the aim of using these triple-tailed metallosurfactants as precursors for redox-responsive films. The surface-pressure-molecular-area isotherm measurement for compound 1d shows that the complex forms a stable Langmuir film at the air/water interface. The monolayer and multilayers were successfully transferred onto the quartz substrate and the platinum working electrode at a surface pressure of 10 mN m(-1) by the Langmuir-Schaefer (LS) technique. The LS films were studied by UV/Vis spectrometry, infrared spectroscopy, field-emission scanning electron microscopy, and atomic force microscopy. A good linear relationship between the absorbance at 370 nm and the thickness of the layers against the number of deposited layers indicated the uniformity and reproducibility of this transfer process. Voltammograms for platinum-surface-bound LS film of compound 1d showed that the redox response owing to the first oxidation is stable and reproducible after many cycles (>300 cycles). Spectroscopic studies and electrochemical measurements of compound 1d on the LS films revealed that these complexes are potential candidates for molecular devices.