Dynamic electron polarization created by the radical-triplet pair mechanism: Application to the studies on excited state deactivation processes by free radicals

Abstract

The radical-triplet pair mechanism for chemically induced dynamic electron polarization (CIDEP) created in the quenching of excited state molecules by free radicals is explained on the basis of recent time-resolved electron spin resonance spectroscopic results and theoretical studies. The CIDEP of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) and galvinoxyl radicals exhibit various CIDEP patterns of net and mutliplet types and CIDEP phases of absorption and emission. The CIDEP patterns are described by the quartet-doublet state mixings within the radical-triplet encounter pairs. The mixings by the spin-dipolar and the hyperfine interactions are responsible for the net and the multiplet patterns, respectively. The factors controlling the CIDEP phases are the spin multiplicity of the excited state quenched by radicals and the sign of the intermolecular exchange interaction of the radical-triplet encounter pairs. In particular, the intermolecular charge transfer effect on the exchange interaction is discussed much in detail from the viewpoints of CIDEP magnitudes and phases. A CIDEP creation in the O2(1Δg)-TEMPO system is also introduced and is described by the radical-triplet pair mechanisms. Applications of this CIDEP used as a probe of O2(1Δg) in condensed phase are mentioned.