Long-lifetime and asymmetric singlet oxygen photoluminescence from aqueous fullerene suspensions

The photoexcited aqueous fullerene (C60) suspension was shown to exhibit an asymmetric photoluminescence (PL) spectrum, which, different from the symmetric spectrum observed previously in C60 solutions or suspensions, still stems from the characteristic phosphorescence of singlet oxygen (O2(a(1)Δ)) owing to its dependence on oxygen concentration. In contrast to the microsecond-level lifetime of O2(a(1)Δ) in water solutions, that in our C60 suspensions was measured at room temperature to be relatively long, about 2-3 ms, which is ~1000 times longer than the value reported by Bilski et al. The physical mechanism for the asymmetric O2(a(1)Δ) PL from C60 suspensions was studied in depth, indicating that it in fact originates from O2 molecules trapped in the C60 lattice within the suspended C60 aggregates (nC60). This mechanism, which can explain well our above results, was further validated by the nC60's high-resolution transmission electron microscopy (HRTEM) images with lattice fringes and the experimental temperature dependence of O2(a(1)Δ) lifetimes in nC60 suspensions. Our findings suggest that the bulk-phase O2(a(1)Δ) in aqueous nC60 suspensions results from the diffusion of the O2(a(1)Δ) generated within the interior of nC60 aggregates.