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

[60]fullerene bisadducts containing the C2ν-symmetrical 2,2,5,5-tetramethylpyrrolidine-1-oxyl moiety have been synthesized. Their EPR spectrum is typical of nitroxide biradicals with electron exchange interaction J of the same order of the nitrogen hyperfine coupling. From the EPR spectrum the absolute 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 interaction. The analogy with the polarization of transient spin correlated radical pairs is pointed out.