TR-EPR of single and double spin labelled C60 derivatives:: observation of quartet and quintet excited states in solution

Time-resolved EPR spectra recorded after pulsed laser photoexcitation of two fullerene nitroxide radicals 1a and 1b and two fullerene nitroxide biradicals 2a and 2b in solution have been investigated at room temperature. The TR-EPR spectra of all the radicals are interpreted as arising from spin-polarized ground and excited states, the latter being obtained by the spin coupling of the nitroxide unpaired spins with the fullerene excited triplet state. The TR-EPR spectra of the monoradicals are consistent with strong (1a) or weak (1b) spin coupling, giving rise to pure quartet/doublet excited states (1a) or mixed states (1b). In the latter, the spectral simulation allowed the determination of the magnitude of the exchange coupling constant J(TR) between the triplet and the radical. The spin polarization of the excited and ground states is attributed to CIDEP effect induced by the radical triplet pair mechanism (RTPM). The sign of JTR is inferred from the sign of the spin polarization. In the case of biradicals 2a and 2b, the TR-EPR spectra are assigned, on the basis of hyperfine pattern and g-values, to quintet states obtained by spin coupling of the fullerene triplet photoexcited state with the two radicals. The spin polarization of the ground state is attributed to the triplet-triplet annihilation process and the sign of the polarization allows the assignment of the radical-radical exchange constant JRR.