Magnetic field and spin effects from sequential p-type and d-type triplet mechanisms

CIDEP signals of semireduced thionine radicals produced by reacting thionine triplets with aniline and halogenated anilines were measured by time resolved CW and pulsed FT EPR. For aniline as quencher, the polarization was emissive while for 4-Br- and 3-I-aniline a time dependent change in polarization from emissive to enhanced absorption was observed. For 4-I-aniline the signals were in enhanced absorption for all delay times. The time and concentration dependence of the signals was analysed in terms of a sequential double triplet mechanism: polarization of the thionine triplet due to selective population of the molecular triplet substates (classical 'p-type' triplet mechanism) and modification of this polarization by substate selective, heavy atom induced depopulation of triplet exciplexes (triplet contact radical pairs) formed as intermediates in the triplet quenching by electron transfer ('d-type' triplet mechanism). A quantitative theoretical treatment that combines the time-integrated solution of the stochastic Liouville equations for precursor triplet and triplet exciplex with the kinetic rate equation of the bimolecular quenching process is presented. The equations derived allow the extraction of two polarization enhancement factors, V d for the pure d-type and V pd for the combined p- and d-type triplet mechanism from the concentration dependence of the time dependent CIDEP signals. The CIDEP curves and the previously observed magnetic field and heavy atom effects on the free radical yield can be quantitatively simulated with a consistent set of kinetic parameters