Stable Radical Species in Langmuir-Blodgett Films of Merocyanine Dyes Studied by Electron Spin Resonance

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Abstract
ESR measurements were performed on Langmuir-Blodgctt (LB) films of merocyanine dyes, diluted with arachidic acid. Stable signal due to two radical species was detected and the spectra showed clear anisotropy both for the in-plane and out-of-plane of the film. Measurements at X- and Q-bands revealed that the in-plane structure of the spectra arises from the hyperfine splitting of a nitrogen nucleus of the dye molecule. Based on this fact, the anisotropic line shape was analyzed by a computer simulation and the angle between the preferred direction of p π orbital axis of the nitrogen atom and the dipping direction of the film was determined to be 60° to 65° in the film plane. A possible correlation between the obtained angle and the structure of the dye aggregates is discussed in relation to the intermolecular charge transfer model for the origin of the radicals.