Photoinduced Electron Transfer to C₆₀ across Extended 3- and 11-Bond Hydrocarbon Bridges:  Creation of a Long-Lived Charge-Separated State

Two new, rigid donor−bridge−C60(acceptor) dyads are presented. In one system (C60[3]TMPD) a 3-σ-bond bridge separates the fullerene from a powerful tetraalkyl-p-phenylenediamine donor; in the other (C60[11]DMA) the bridge comprises an extended array of 11 bonds, while the donor unit is a dimethylaniline group. Photoexcitation of the 3-bond system induces fast (kcs ≥ 1.6 × 1010 s-1) and virtually complete intramolecular charge separation, irrespective of solvent polarity. It is concluded that this charge separation occurs under nearly “optimal” conditions. Charge recombination, however, is also very fast, preventing the detection of the charge-separated state on a nanosecond time scale. For the 11-bond system, photoinduced charge separation only occurs in polar solvents, reaching kcs = 5.5 × 109 s-1 in benzonitrile, which still implies a charge separation yield of ∼90%. Interestingly, charge recombination is now slowed down considerably, thereby allowing easy detection of the “giant dipolar” charge-separated state of C60[11]DMA with a lifetime of ca. 0.25 μs. The experimental results, together with semiempirical MO calculations, indicate that the special symmetry properties of the fullerene π-system may cause it to enter into very strong electronic coupling with the hydrocarbon bridge to allow fast photoinduced charge separation, while at the same time the electronic coupling relevant for charge recombination remains small.