Langmuir-Blodgett films of pure long-chain nitroxides, their mixtures and mixtures with stearic acid studied by electron spin resonance

Abstract

In this paper we report information on the spreading isotherms and the electron spin resonance (ESR) spectra of mono- and multilayer films built up with long hydrocarbon chain nitroxides (namely doxyl stearic acid and hexadecanoyl- piperidine-oxyl radicals). These ultrathin molecular assemblies were prepared from (i) single components, (ii) nitroxide binary mixtures, and (iii) a diluted solution of the paramagnetic components in a diamagnetic (stearic acid) host. The spreading isotherms allowed us to establish that the presence of a five- or six-membered ring in the hydrocarbon chain rendered the film more expanded, and this effect depended on the substituent position in the chain. The ESR spectra of undiluted nitroxides were characterized by strong spin-spin (dipole-dipole and Heisenberg spin exchange) magnetic interactions whose extent also depended on the nitroxide type and mixture. The calculation of the parameters of motion governing the ESR lineshape allowed accurate evaluation both of the degree of order and of the chain flexibility, particularly for diamagnetically diluted systems.