Electron-spin resonance of stable radicals in Langmuir-Blodgett films of a merocyanine dye: Study of hyperfine coupling using a enriched15 dye

Two kinds of stable radical species exist in mixed Langmuir-Blodgett (LB) films of a merocyanine dye and arachidic acid. One of the species exhibits clear nitrogen hyperfine structure in ESR spectra, which was previously confirmed by measurements at X and Q bands. Use of the enriched15 dye derivative clearly identified the hyperfine coupling to arise from the nitrogen nucleus in benzothiazole, which belongs to the chromophore of the dye molecule; N-C"C-C"C-C"O. ESR spectra measured at both X and K bands in unsubstituted and substituted15 systems permits the detailed analysis of in-plane molecular orientation using the spectrum-simulation method incorporating three-dimensional freedom of orientation. The analysis of the dependence of the spectra on the substrate position yields the local orientation of the radical molecules, which shows a higher degree of orientation toward the edge of the substrate. This behavior coincides with that of the majority of molecules forming J aggregates, obtained from optical analysis, and is well described by a recent theory of the flow orientation of molecular domains caused during the dipping process of LB films. This coincidence between the orientation of the radicals and J aggregates provides new evidence that stable radicals are generated in the J aggregate. The mechanism of radical generation is discussed in view of our previously proposed model of an intermolecular charge transfer in J aggregates.