New coumarin-[60]fullerene dyads connected by an alkynyl linkage: Synthesis and fluorescence studies. Evidence for efficient singlet–singlet energy transfer

• New coumarin-C60 dyads with alkyne rigid spacer.
• Dyads with very fast (<2 ps) and efficient singlet–singlet energy transfer.
• Competitive fast electron transfer enhanced in polar solvents.

Two new coumarin-[60]fullerene dyads, in which an alkyne group covalently links C60 to coumarin, are synthesized via 1,3-dipolar cycloaddition and spectroscopically characterized. Their photophysical properties in apolar (toluene) and polar (THF and benzonitrile) solvents are studied at room temperature, revealing the nature and polarity dependence of the excited-state interactions between the coumarin and C60 moieties. In both dyads and in all solvents, a strong quenching of the coumarin emission by C60 was observed. It mainly results from a fast and efficient singlet–singlet resonance energy transfer from the coumarin moiety to the C60 moiety, but an electron transfer contribution, enhanced in polar solvents, also exists. In toluene, the fluorescence emission of the fullerene moiety is increased by the nonradiative energy transfer process, which occurs mainly by a dipole–dipole (FRET) mechanism. In polar solvents (THF and benzonitrile), fast electron transfer from ground-state coumarin to excited fullerene is significant, leading to a partial or complete quenching of the fullerene emission, depending on the dyad.