High-Field Transient Electron Paramagnetic Resonance of Excited Triplet States

The zero-field splitting of the excited triplet states of organic molecules often is of the order of 1 GHz or less, and transient EPR at X-band is generally sufficient for the determination of the zero-field splitting and kinetic parameters in these type of molecules. However, information on the g-factor and g-anisotropy cannot be obtained at conventional EPR frequencies, and interpretations of the data in terms of electronic structure are mostly limited to symmetry considerations. On the other hand information of the g-anisotropy provides additional clues with respect to electronic structure, while a direct comparison with radical-ion forms of the molecules becomes possible. Experimental data of transient EPR at 240 GHz will be presented for a variety of methano-fullerenes and pyrrolidino-fullerenes, either embedded in a polymer of in partially oriented liquid crystals. It will be shown that transient EPR at these very high frequencies can accurately determine the g-anisotropy and in some cases the orientation of the g-tensor with respect to the ZFS tensor. The information gained will be discussed in relation to the electronic structure of the lowest excited triplet state of these molecules.