Quantitative Transduction of Excited-State Energy inFluorophore-Heterofullerene Conjugates

A novel series of fluorophore-heterofullerene conjugates 10–14 — the fluorophores range from benzene, naphthalene, phenanthrene, and fluorene to pyrene — was synthesized. NMR spectroscopy and quantum mechanical calculations demonstrate that the flexibility of the acetyl-group linker opens the way for conformations with π–π stacking interactions between the chromophores. This leads to favorable electronic communication between the two subsystems and is reflected in a series of ground- and excited-state assays ranging from fluorescence to fast transient absorption measurements. It was found that a common deactivation process of the photoexcited fluorophores takes place, namely, a quanti-tative transduction of singlet excited state, yielding the hetero-fullerene singlet excited state. This reaction pathway leads to the long-lived and highly reactive fullerene triplet state, which forms, in a reaction with molecular oxygen, a cytotoxic oxygen species (i.e. singlet oxygen)