The sign of the exchange interaction between triplet excited fullerene and nitroxide free radicals

The sign of the exchange interaction J in a series of radical triplet pairs (RTPs), formed by a nitroxide free radical and a triplet excited fullerene, has been determined from the spin polarization of time-resolved electron paramagnetic resonance spectra. Radical and fullerene are linked together by covalent bonds in different geometries. It is shown that the sign of J depends on the overlap between the orbital of nitroxide unpaired electron and the LUMO of fullerene, which is singly occupied in the excited triplet state. When the overlap does not vanish, a negative contribution to J arises from the admixing of a charge transfer structure in the wave function of the excited doublet state D of the RTP, which does not take place in the excited quartet state Q. The mixing of D* and Q' states lowers the energy of the former spin state and gives antiferromagnetic coupling.