Electron spin polarization by intramolecular triplet quenching of a nitroxyl radical labeled thioxanthonedioxide

Time-resolved electron spin resonance (TR-EPR) has been used to investigate the chemically induced dynamic electron polarization (CIDEP) generated by the interaction between a stable free radical (TEMPO) and the triplet state s of thioxanthonedioxide derivatives. The intensity of CIDEP for intermolec ular triplet quenching (thioxanthonedioxide methylester triplets with TEMPO ) and intramolecular triplet quenching were compared using a covalently lin ked TEMPO-thioxanthonedioxide molecule. It is shown that at equivalent conc entrations of TEMPO, the CIDEP is much larger in the case of intramolecular triplet quenching than for intermolecular quenching. The CIDEP intensity f or intermolecular triplet quenching is limited to a maximum TEMPO concentra tion of similar to 4 mM (at which point spin-spin interactions become domin ant) and reaches only 1/8 of the intensity for intramolecular quenching. La ser flash photolysis experiments at room temperature showed that the intram olecular triplet quenching is fast (tau(t) < 20 ns). This quenching reactio n is also dominant at low temperatures in organic glasses (77 K), which was demonstrated by low-temperature phosphorescence and EPR analysis.