Long-Lifetime and Asymmetric Singlet Oxygen Photoluminescence from Aqueous Fullerene Suspensions

The photoexcited aqueous fullerene (C-60) suspension was shown to exhibit an asymmetric photoluminescence (PL) spectrum, which, different from the symmetric spectrum observed previously in C-60 solutions or suspensions, still stems from the characteristic phosphorescence of singlet oxygen (O-2(a(1)Delta)) owing to its dependence on oxygen concentration. In contrast to the microsecond-level lifetime of O-2(a(1)Delta) in water solutions, that in our C-60 suspensions was measured at room temperature to be relatively long, about 2-3 ms, which is similar to 1000 times longer than the value reported by Bilski et al. The physical mechanism for the asymmetric O-2(a(1)Delta) PL from C-60 suspensions was studied in depth, indicating that it in fact originates from 02 molecules trapped in the C-60 lattice within the suspended C-60 aggregates (nC(60)). This mechanism, which can explain well our above results, was further validated by the nC(60)'s high-resolution transmission electron microscopy (HRTEM) images with lattice fringes and the experimental temperature dependence of O-2(a(1)Delta) lifetimes in nC(60) suspensions. Our findings suggest that the bulk-phase O-2(a(1)Delta) in aqueous nC(60) suspensions results from the diffusion of the O-2(a(1)Delta) generated within the interior of nC(60) aggregates.