Optical and Magnetic Resonance Studies of Ground and Excited Spin States on Some Open-Shell Orgnic Radicals

Abstract

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), pyrene thioaminyl radical (3). The fluorescence and phosphorescence spectra of 1 and 2 in EPA glass are measured. Both compounds give weak phosphorescence which indicates the existence of metastable excited quartet (S=3/2) state. Doublet like time-resolved ESR signals are observed for these stable radicals. The decay time of the dynamic electron polarization in their excited state is ms order. This finding indicates that the observed signals arises from the radical-triplet pair originating the magnetic exchange interaction 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 observed for 3, showing a possibility that the signal comes from the middle Kramers doublet of the resulting quartet excited spin state.

Key Words:

• Excited State
• Open-Shell Organic Radical
• Time-Resolved ESR Radical-Triplet Pair Mechanism
• Quartet Spin State