Photophysical and electron-transfer properties of mono- and multiple-functionalized fullerene derivatives

Mono- and multiple-functionalized C-60 derivatives were synthesized and studied for their photophysical properties. Electronic absorption spectra and absorptivities of the C-60 derivatives in solution were measured and compared. By recording the fluorescence spectra using a near-infrared-sensitive emission spectrometer, we quantitatively determined fluorescence quantum yields of the C-60 derivatives. For the mono-functionalized CBD derivatives, the compound with a [5,6]-open fulleroid addition pattern on the fullerene cage appeared to be considerably less fluorescent than those with a [6,6]-closed cage addition pattern. Despite the disturbance of the electronic structure via multiple additions to the fullerene cage, the multiple-functionalized C-60 derivatives exhibited no dramatic changes in fluorescence quantum yields in comparison with the mono-functionalized C-60 derivatives. The fluorescence lifetimes of the C-60 derivatives, obtained using the time-correlated single photon-counting technique, were all in the range of 1-3 ns. In addition, the dependencies of the fluorescence intensities and lifetimes of the C-60 derivatives on the concentration of the quencher N,N-diethylaniline (DEA) were evaluated. Apparently, upon photoexcitation, even the C-60 derivatives with a hexa-functionalized fullerene cage underwent significant electron-transfer interactions with the electron donor DEA, resulting in efficient fluorescence quenching. In a polar solvent, the contribution of static quenching also became important, The results for different derivatives and their molecular structural and mechanistic significance are discussed.