Journal of Physical Chemistry A, Vol.108, No.4, 524-531, 2004
Quenching mechanism of excited coronene by a nitroxide radical studied by probing dynamic electron polarization
By use of the time-resolved electron spin resonance (TR-ESR) method, quenching mechanisms of S-1 and T-1 coronene by the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical were investigated by probing absorptive (abs) and emissive (em) chemically induced dynamic electron polarizations (CIDEP), which are generated by interactions of S-1 and T-1 coronene with TEMPO radicals, respectively. Absolute magnitudes of abs and em CIDEPs created on TEMPO radicals in the coronene-TEMPO system in benzene were determined as 1.8 and 2.2, respectively, in the units of Boltzmann polarization by utilizing Bloch and kinetic equations. This result is evaluated by a theoretical model that describes the magnitude of CIDEP created in the radical-triplet (RT) system, and it is clarified that exchange interaction of the T-1 coronene-TEMPO pair is much larger than the Zeeman energy and the em CIDEP is dominantly generated in the level-crossing regions. This means that quenching of T-1 coronene by TEMPO in benzene occurs through an electron-exchange interaction in an RT encounter complex. A ratio of em to abs CIDEP magnitudes was determined to be 1.2. The difference between the magnitudes of abs and em CIDEPs implies that the mean reaction distance of S, quenching is about 1-2 Angstrom longer than that of T-1 quenching. This result suggests that an S-1-T-1 enhanced intersystem crossing occurs through both charge transfer and exchange mechanisms while T-1 quenching occurs only through exchange mechanism.