Characterization of Langmuir—Blodgett films using electron spin resonance spectroscopy

Abstract

The role of electron spin resonance (ESR) spectroscopy in the characterization of Langmuir—Blodgett (LB) films is discussed. Early examples of the application of ESR spectroscopy in LB films are the studies of the structural and magnetic properties of fatty acid LB films containing metallic ions such as Cu 2+ and Mn 2+ and those of photoinduced electron-transfer processes in films containing donor and acceptor monolayers. In recent years, many varieties of molecules have been incorporated into LB films to develop films having new functions related to optical, electrical and other properties. LB films have also been employed as model systems for the study of the basic properties of molecular aggregates. As a result, there are increasing numbers of cases where the ESR technique has played a major role in elucidating the microscopic properties of these new LB films. Three cases are discussed here as illustrative examples. The first topic is the analysis of the in-plane anisotropy of the ESR spectra of the stable radical in merocyanine dye LB films, from which a flow orientation of the dye aggregates has been suggested as the origin of the in-plane anisotropy often observed in LB films. The hyperfine coupling due to a nitrogen in the dye molecule has been identified. The second case introduces the studies of conducting LB films composed of charge-transfer complexes. ESR can provide direct evidence for the formation of the conducting columnar structures of the constituent molecules through the temperature-dependent spin susceptibilities, which show the characteristics of low-dimensional metals or magnetic systems associated with structural disorder. In some systems, the molecular orientation and the magnetic interactions of quasi-one-dimensional spin chains have been deduced from the analysis of the spectral line shapes. The third case refers to LB films containing paramagnetic metallic ions, which still represent an important class of materials. The metallic ions act as well-localized probes in revealing the local orientation of molecules, the aggregation structure of molecules (such as dimer formation) and so on. Most of the research so far has been based on the standard continuous-wave ESR technique. A few works have been reported using the more advanced techniques of ESR, such as ENDOR (electron nuclear double resonance) and ESE (electron spin echo). These techniques, as well as others, are expected to extend the area of ESR application in the studies of LB films.