Long-Range Electron Transfer in Zinc-Phthalocyanine-Oligo(Phenylene-ethynylene)-Based Donor-Bridge-Acceptor Dyads

In the context of long-range electron transfer for solar energy conversion, we present the synthesis, photophysical, and computational characterization of two new zinc(II) phthalocyanine oligophenylene-ethynylene based donor-bride-acceptor dyads: ZnPc-OPE-AuP+ and ZnPc-OPE-C-60. A gold(III) porphyrin and a fullerene has been used as electron accepting moieties, and the results have been compared to a previously reported dyad with a tin(IV) dichloride porphyrin as the electron acceptor (Fortage et al. Chem. Commun. 2007, 4629). The results for ZnPc-OPE-AuP+ indicate a remarkably strong electronic coupling over a distance of more than 3 nm. The electronic coupling is manifested in both the absorption spectrum and an ultrafast rate for photoinduced electron transfer (k(PET) = 1.0 x 10(12) s(-1)). The charge-shifted state in ZnPc-OPE-AuP+ recombines with a relatively low rate (k(BET) = 1.0 X 10(9) s(-1)). In contrast, the rate for charge transfer in the other dyad, ZnPc-OPE-C-60, is relatively slow (k(PET) = 1.1 X 109 s(-1)), while the recombination is very fast (k(BET) approximate to 5 x 10(10) s(-1)). TD-DFT calculations support the hypothesis that the long-lived charge-shifted state of ZnPc-OPE-AuP+ is due to relaxation of the reduced gold porphyrin from a porphyrin ring based reduction to a gold centered reduction. This is in contrast to the faster recombination in the tin(IV) porphyrin based system (k(BET) = 1.2 X 10(10) s(-1)), where the excess electron is instead delocalized over the porphyrin ring.