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 states of thioxanthonedioxide derivatives. The intensity of CIDEP for intermolecular triplet quenching (thioxanthonedioxide methylester triplets with TEMPO) and intramolecular triplet quenching were compared using a covalently linked TEMPO-thioxanthonedioxide molecule. It is shown that at equivalent concentrations of TEMPO, the CIDEP is much larger in the case of intramolecular triplet quenching than for intermolecular quenching. The CIDEP intensity for intermolecular triplet quenching is limited to a maximum TEMPO concentration of similar to 4 mM (at which point spin-spin interactions become dominant) and reaches only 1/8 of the intensity for intramolecular quenching. Laser flash photolysis experiments at room temperature showed that the intramolecular triplet quenching is fast (tau(t) < 20 ns). This quenching reaction is also dominant at low temperatures in organic glasses (77 K), which was demonstrated by low-temperature phosphorescence and EPR analysis.