EPR studies on a binitroxide fullerene derivative in the ground triplet and first photoexcited quintet state

A fulleropyrrolidine with two covalently linked nitroxides has been studied by rw-EPR in its ground state and by TR-EPR and spin-echo detected EPR in the photoexcited state after pulsed laser excitation The electron exchange interaction between the two unpaired electrons in the ground state is obtai ned from the analysis of the EPR spectra in the dark. The EPR signal of pho toexcited samples consists of the superposition of the Q((+/-2)) <----> Q(( +/-1)) and of the Q((+/-1)) <----> Q((i)), transitions of the excited quint et state and of the signal due to the triplet state of the ground state bir adical. The assignment to the quintet excited stale was made by measuring t he frequency of the echo detected transient nutation, Fourier transformatio n of dir transient spectra along the microwave pulse length allowed us to d isentangle the quintet transitions and to obtain precise measurements of th e zero-field splitting parameters. The observed polarizations in the transi ent spectra are examined on the basis of selective intersystem crossing due to spin-orbit interaction and quintet-singlet mixing caused by the electro n dipolar interaction.