Energy transfer in oligofluorene-C60 and C60-oligofluorene-C60 donor-acceptor conjugates

Two complementary series of C-60-(Fl)(n) and C-60-(Fl)(n)-C-60 (Fl = 9,9-dihexylfluorene-2,7-diyl; n = 1-5) derivatives with terminal N-methylfulleropyrrolidine units have been synthesized from CHO-(Fl)(n) and CHO-(Fl)(n)-CHO precursors via 1,3-dipolar cycloaddition reaction of in situ generated azomethine ylides with an excess of C-60. In solution electrochemical experiments, these conjugates give rise to amphoteric redox behavior. Three consecutive quasireversible reduction waves have been observed at the expected potentials for the N-methylfulleropyrrolidine cores. For the C-60-(Fl)(n)-C-60 series, each reduction wave is a two-electron process with no observable interaction between the C-60 units. Two or, in some cases, three oxidation wavesmost of them irreversibleare ascribed to the oligofluorene system. These waves are cathodically shifted with an increasing number of fluorene units and anodically shifted by the conjugated terminal aldehyde units, compared to the N-methylfulleropyrrolidine termini. Steady-state and time-resolved photolytic techniques show that an efficient transduction of singlet excited-state energy transfer prevails from the photoexcited oligofluorene to the energy accepting fullerene.