Electron spin resonance in Langmuir films of merocyanine dyes

Abstract Electron spin resonance measurements were performed on Langmuir films of merocyanine dyes with three different donor nuclei at room temperature and 77 K. Each system showed a single anisotropic resonance line with a g factor close to 2.00. The spin concentration and its change due to illumination were markedly dependent on the donor nuclei, the dye concentration and the heat treatment, which affect the existence and type of chromophore aggregation. The system with benzothiazole as a donor nucleus, for example, showed an exceptionally high and stable spin concentration (1 spin per several hundred molecules) at higher dye concentrations except at a certain concentration. The spin concentration, however, drastically decreased to about one-twentieth and the lineshape changed after the chromophore aggregation was destroyed by heating the sample at 90°C for 30 min. This suggests that the generation of the stable spin species in high concentration is closely related to chromophore aggregation in this system.