Electron-spin polarizations generated from interactions between excited triplet porphyrins and stable radicals studied by time-resolved electron paramagnetic resonance

Time-resolved EPR studies were made on CIDEP mechanisms in the system of excited triplets (T) and doublets (D) using various kinds of porphyrins and nitroxide radicals. The CIDEPs were investigated by observing both signals of T and D and discussed in terms of an electron-spin polarization transfer (ESPT) and a radical-triplet pair mechanism (RTPM). It is demonstrated for the first time that ESPT is actually existent by comparing the triplet and doublet CIDEP signals. The ESPT process was found to be diffusion-controlled in toluene and 2-butanol, and an effective distance for ESPT was estimated as <11 Angstrom. For RTPM, an enhancement of CIDEP polarization was observed by axial ligation of radicals to zinc porphyrins. We also found novel CIDEP involving molecular oxygen, which is interpreted by RTPM between singlet molecular oxygen and stable radicals.