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

Abstract

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: <b>ZnPc-OPE-AuP</b>^<b>+</b> and <b>ZnPc-OPE-C</b>_<b>60</b>. 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. <i>Chem. Commun.</i> <b>2007</b>, 4629). The results for <b>ZnPc-OPE-AuP</b>^<b>+</b> 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 (<i>k</i>_PET = 1.0 × 10¹² s^–1). The charge-shifted state in <b>ZnPc-OPE-AuP</b>^<b>+</b> recombines with a relatively low rate (<i>k</i>_BET = 1.0 × 10⁹ s^–1). In contrast, the rate for charge transfer in the other dyad, <b>ZnPc-OPE-C</b>_<b>60</b>, is relatively slow (<i>k</i>_PET = 1.1 × 10⁹ s^–1), while the recombination is very fast (<i>k</i>_BET ≈ 5 × 10¹⁰ s^–1). TD-DFT calculations support the hypothesis that the long-lived charge-shifted state of <b>ZnPc-OPE-AuP</b>^<b>+</b> 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 (<i>k</i>_BET = 1.2 × 10¹⁰ s^–1), where the excess electron is instead delocalized over the porphyrin ring