Optical and magnetic resonance studies of ground and excited spin states on some open-shell orgnic radicals

Study of the electronic excited spin states on open-shell organic radicals are an important research field of organic magnetism. We report the optical and the magnetic resonance study of the electronic excited spin states of naphthalene nitroxide radical (1), pyrene nitronyl nitroxide radical (2), p yrene thioaminyl radical (3). The fluorescence and phosphorescence spectra of 1 and 2 in EPA glass are measured. Both compounds give weak phosphoresce nce which indicates the existence of metastable excited quartet (S=3/2) sta te. Doublet like time-resolved ESR signals are observed for these stable ra dicals. The decay time of the dynamic electron polarization in their excite d state is ms order. This finding indicates that the observed signals arise s from the radical-triplet pair originating the magnetic exchange interacti on and the polarization transfer between the triplet excited states of the condensed polynuclear aromatic groups and the dangling stable radicals. The g value and the polarization change of the doublet like signal were also o bserved for 3, showing a possibility that the signal comes from the middle Kramers doublet of the resulting quartet excited spin state.