CIDEP of fullerene C-60 biradical bisadducts by intramolecular triplet-triplet quenching: a novel spin polarization mechanism for biradicals

[60]fullerene bisadducts containing the C-2 nu-symmetrical 2,2,5,5-tetramet hylpyrroiidine-1-oxyl moiety have been synthesized. Their EPR spectrum is t ypical of nitroxide biradicals with electron exchange interaction J of the same order of the nitrogen hyperfine coupling. From the EPR spectrum the ab solute value of J was obtained, while the sign was measured by ENDOR. When the derivatives are photoexcited strong spin polarization is observed with A/E/A/E or E/A/E/A patterns, according to a positive or a negative sign of J. This behavior is accounted for by an intramolecular triplet-triplet inte raction. The analogy with the polarization of transient spin correlated rad ical pairs is pointed out. (C) 2000 Elsevier Science B.V. All rights reserv ed.