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

Two new coumarin-[60]fullerene dyads, in which an alkyne group covalently links C-60 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 Cm moieties. In both dyads and in all solvents, a strong quenching of the coumarin emission by C-60 was observed. It mainly results from a fast and efficient singlet-singlet resonance energy transfer from the coumarin moiety to the C-60 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 fifflerene emission, depending on the dyad. (C) 2014 Elsevier Ltd. All rights reserved.