PHOTOINDUCED ELECTRON-TRANSFER TO C-60 ACROSS EXTENDED 3-BOND AND 11-BOND HYDROCARBON BRIDGES - CREATION OF A LONG-LIVED CHARGE-SEPARATED STATE

Two new, rigid donor-bridge-C-60(acceptor) dyads are presented. In one system (C-60[3]TMPD) a 3-sigma-bond bridge separates the fullerene fr om a powerful tetraalkyl-p-phenylenediamine donor; in the other (C-60[ 11]DMA) the bridge comprises an extended array of 11 bonds, while the donor unit is a dimethylaniline group. Photoexcitation of the 3-bond s ystem induces fast (k(cs) greater than or equal to 1.6 x 10(10) s(-1)) and virtually complete intramolecular charge separation, irrespective of solvent polarity. It is concluded that this charge separation occu rs under nearly ''optimal'' conditions. Charge recombination, however, is also very fast, preventing the detection of the charge-separated s tate on a nanosecond time scale. For the Ii-bond system, photoinduced charge separation only occurs in polar solvents, reaching k(cs) = 5.5 x 10(9) s(-1) in benzonitrile, which still implies a charge separation yield of similar to 90%. Interestingly, charge recombination is now s lowed down considerably, thereby allowing easy detection of the ''gian t dipolar'' charge-separated state of C-60[11]DMA with a lifetime of c a. 0.25 mu s. The experimental results, together with semiempirical MO calculations, indicate that the special symmetry properties of the fu llerene pi-system may cause it to enter into very strong electronic co upling with the hydrocarbon bridge to allow fast photoinduced charge s eparation, while at the same time the electronic coupling relevant for charge recombination remains small.