Electron spin dynamics in a pair interaction between radical and electronically-excited molecule as studied by a time-resolved ESR method

This review reveals a recent progress in time-resolved (TR-) ESR spectroscopic studies on quenchings of photoexcited state molecules by free radicals. We explain how the electron spin polarization determined by TR-ESR spectroscopy intrinsically contributes to understanding of excited state deactivation dynamics. Several types of electron spin polarization ever observed in various pair systems of excited molecule and free radical are introduced and a radical-triplet pair mechanism for creation of the electron spin polarizations is presented. Details of intermolecular potential and pair interaction of singlet oxygen molecule, O2(1Δg), and 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) radical are introduced as a well-studied pair system of excited molecule-free radical. It has been found that a large dynamic electron spin polarization is created in the O2(1Δg)-TEMPO encounter pair and a theoretical analysis on the experimentally-determined magnitude of electron spin polarization leads to the evaluation of intermolecular exchange interaction in the O2(1Δ3Σg)-TEMPO pair. A new method for O2(1Δg) lifetime measurement based on the TR-ESR detection of the large electron spin polarization is demonstrated and its advantageous points are discussed. Finally, ferro- and antiferro-magnetic interactions in radical-triplet encounter pairs are summarized on the basis of TR-ESR signal phase which represents a direction of electron spin polarization. A role of intermolecular exchange and charge transfer interactions of the radical-triplet pairs is discussed.