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 cw-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 obtained from the analysis of the EPR spectra in the dark. The EPR signal of photoexcited samples consists of the superposition of the Q<INF>(</INF><INF>±</INF><INF>2)</INF>  Q<INF>(</INF><INF>±</INF><INF>1)</INF> and of the Q<INF>(</INF><INF>±</INF><INF>1)</INF>  Q<INF>(0)</INF> transitions of the excited quintet state and of the signal due to the triplet state of the ground state biradical. The assignment to the quintet excited state was made by measuring the frequency of the echo detected transient nutation. Fourier transformation of the transient spectra along the microwave pulse length allowed us to disentangle the quintet transitions and to obtain precise measurements of the zero-field splitting parameters. The observed polarizations in the transient spectra are examined on the basis of selective intersystem crossing due to spin−orbit interaction and quintet−singlet mixing caused by the electron dipolar interaction.