Fullerene C60-perylene-3,4:9,10-bis(dicarboximide) light-harvesting dyads: spacer-length and bay-substituent effects on intramolecular singlet and triplet energy transfer

Novel covalent fullerene C(60)-perylene-3,4:9,10-bis(dicarboximide) (C(60)-PDI) dyads (1-4) were synthesized and characterized. Their electrochemical and photophysical properties were investigated. Electrochemical studies show that the reduction potential of PDI can be tuned relative to C(60) by molecular engineering through altering the substituents on the PDI bay region. It was demonstrated using steady-state and time-resolved spectroscopy that a quantitative, photoinduced energy transfer takes place from the PDI moiety, acting as a light-harvesting antenna, to the C(60) unit, playing the role of energy acceptor. The bay-substitution (tetrachloro [1 and 2] or tetra-tert-butylphenoxy [3 and 4]) of the PDI antenna and the linkage length (C(2) [1 and 3] or C(5) [2 and 4]) to the C(60) acceptor are important parameters in the kinetics of energy transfer. Femtosecond transient absorption spectroscopy indicates singlet-singlet energy-transfer times (from the PDI to the C(60) unit) of 0.4 and 5 ps (1), 4.5 and 27 ps (2), 0.8 and 12 ps (3), and 7 and 50 ps (4), these values being ascribed to two different conformers for each C(60)-PDI system. Subsequent triplet-triplet energy-transfer times (from the C(60) unit to the PDI) are slower and in the order of 0.8 ns (1), 6.2 ns (2), 2.7 ns (3), and 9 ns (4). Nanosecond transient absorption spectroscopy of final PDI triplet states show a marked influence of the bay substitution (tetrachloro- or tetra-tert-butylphenoxy), and triplet-state lifetimes (10-20 micros) and the PDI triplet quantum yields (0.75-0.52) were estimated. The spectroscopy showed no substantial solvent effect upon comparing toluene (non-polar) to benzonitrile (polar), indicating that no electron transfer is occurring in these systems.