The sign of the exchange interaction between triplet excited fullerene andnitroxide 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 b y covalent bonds in different geometries. It is shown that the sign of J de pends on the overlap between the orbital of nitroxide unpaired electron and the LUMO of fullerene, which is singly occupied in the excited triplet sta te. When the overlap does not vanish, a negative contribution to J arises f rom the admiring 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 exci ted quartet state Q*. The mixing of D* and Q* states lowers the energy of t he former spin state and gives antiferromagnetic coupling.