Nitronyl nitroxide and imino nitroxide mono- and biradicals in Langmuir and Langmuir-Blodgett films

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In an effort to prepare high-spin-ordered layers, several conjugated molecules bearing one nitronyl nitroxide radical (NN) or two imino nitroxide magnetically coupled radicals (IN) have been synthesized. The monoradical and the biradical molecules proved to be amphiphilic enough to form Langmuir films on a water surface. For the two monoradicals, the molecular area extrapolated at zero-surface pressure in the final part of the isotherms is nearly the same (A0 ∼ 0.55 nm2 molecule-1) and close to what may be expected for such molecular shapes. For the longest rigid-rod biradical, bis imino nitroxide pentamer, 5p(bisIN), the pressure-area isotherm shows a low compressibility and Brewster angle microscopy has revealed that the film is solidlike, the molecules having a tendency to aggregate, forming plates on the water surface. The bis imino nitroxide trimer, 3p(bisIN), has a larger final molecular area A0 ∼ 0.80 nm2 molecule-1, the molecules being, by then, oriented perpendicular to the surface as deduced from grazing incidence X-ray analysis (GIXA) and surface potential measurements. After deposition of monolayers on various hydrophilic substrates by the Langmuir-Blodgett (LB) technique, the biradical 3p(bisIN) happens to be the most interesting molecule since good transfer ratios (TR) are easily obtained and electron paramagnetic resonance (EPR) measurements indicate paramagnetic properties. In addition, atomic-force microscopy (AFM) imaging of the films shows defects in the form of elevated zones. The AFM reveals that these zones are partially made of molecules piling up, their heights being multiples of single-molecule lengths. These elevated domains occur in different shapes, either randomly distributed circular zones or aligned, coalesced domains lying along preferred directions. Y-type multilayers up to nine layers were also transferred on hydrophilic glass, with a somewhat poorer TR.

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