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 C60 unit, playing the role of energy acceptor. The baysubstitution (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, [2 and 4]) to the C-60 acceptor are important parameters in the kinetics of energy transfer. Ferntosecond 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 triplettriplet 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 tripletstate lifetimes (10-20 mu s) 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.