Highly efficient singlet-singlet energy transfer in light-harvesting [60,70]fullerene-4-amino-1,8-naphthalimide dyads

New C60 and C70 fullerene dyads formed with 4-amino-1,8-naphthalimide chromophores have been prepared by the Bingel cyclopropanation reaction. The resulting monoadducts were investigated with respect to their fluorescence properties (quantum yields and lifetimes) to unravel the role of the charge-transfer naphthalimide chromophore as a light-absorbing antenna and excited-singlet-state sensitizer of fullerenes. The underlying intramolecular singlet-singlet energy transfer (EnT) process was fully characterized and found to proceed quantitatively (Φ(EnT)≈1) for all dyads. Thus, these conjugates are of considerable interest for applications in which fullerene excited states have to be created and photonic energy loss should be minimized. In polar solvents (tetrahydrofuran and benzonitrile), fluorescence quenching of the fullerene by electron transfer from the ground-state aminonaphthalimide was postulated as an additional path.