Electron spin resonance characterization of Langmuir–Blodgett films containing functional molecules

The role of electron spin resonance (ESR) spectroscopy in the characterization of Langmuir–Blodgett (LB) films containing functional molecules is discussed. Unpaired electrons in LB films are associated with radical molecules produced by charge transfer, paramagnetic metallic ions such as Cu2+ and so on. ESR parameters such as g-value and hyperfine coupling provide information of local molecular orientation and electronic states. ESR can unambiguously determine the orientation distribution function of molecules through g-value anisotropy. The case of mixed LB films of a merocyanine dye and arachidic acid is discussed as a typical example. The analysis of the anisotropic ESR spectra of stable radicals existing in these films has revealed detailed in-plane orientation of radicals, which led to the discovery of flow orientation effect during LB dipping process. Light-induced ESR (LESR) is introduced to directly detect the dye radicals generated by photoinduced charge transfer. Observed LESR spectra, associated with a resolved 14N hyperfine structure, uniquely probe the orientation and electronic states of dye molecules forming the J-aggregates in highly ordered LB matrix. AFM observation of dye domains in these films is also discussed.