First Observation of the Photoexcited Quintet State in Fullerene Linked with Two Nitroxide Radicals

Abstract

The bisadduct of fullerene (1) having two nitroxide radicals at the trans-3 position was newly synthesized. The ground and photoexcited states of 1 were studied by continuous-wave and pulsed two-dimensional electron paramagnetic resonance spectroscopy. An exchange interaction between the radicals was evaluated to be much smaller than an isotropic hyperfine splitting, showing a doublet (S = 1/2) character in the ground state. It was found from a 2D nutation spectrum that a photoexcited state has a quintet character (S = 2) which is generated via interactions between photoexcited triplet fullerene (S = 1) and the two nitroxide radicals (S = 1/2).