Electron spin resonance in Langmuir-Blodgett films of a merocyanine dye and its isotope-substituted analogs

Stable radical species existing in mixed Langmuir-Blodgett films of a merocyanine dye and arachidic acid are studied through their hyperfine couplings using the isotope substitution technique. One of the species shows a nitrogen hyperfine structure possibly arising from one of the two nitrogen sites in the molecule. With the employment of two dye analogs in which either one of the ¹⁴N nuclei is replaced by ¹⁵N, the observed nitrogen site has been identified to be the one in benzothiazole. That is, the wave function of the radical is directly associated with the dye chromophore. The photo-induced spectra have shown a similar change as the dark spectra upon ¹⁵N-substitution, indicating that the dark and photo-induced signals arise from a common species. These facts together with the recently observed correlation between the anisotropic behaviour of ESR and optical J-band strongly suggest an intermolecular charge transfer in J-aggregates as the origin of radical generation.