Activating Multistep Charge-Transfer Processes in Fullerene-Subphthalocyanine-Ferrocene Molecular Hybrids as a Function of π-π Orbital Overlap

We have synthesized two different fullerene-subphthalocyanine-ferrocene conjugates. The molecules were designed so that the ferrocene unit is linked at the subphthalocyanine axial position through a phenoxy spacer while the C-60 is rigidly held close to the concave face of the macrocycle via a 3-fold C-3-symmetrical anchoring. The Bingel trisaddition reaction leading to the final products proceeded with very high regioselectivities and full diastereoselectivity. The only difference between both systems is the length of the triple tether employed, which finely regulates the regioselectivity of the trisaddition reaction and the distance between the subphthalocyanine and the C-60 complementary pi-pi surfaces. Thus, when the tether is connected to the subphthalocyanine through a direct C-C bond, a short pi-pi distance of 3.25-3.30 angstrom was calculated. In contrast, when it is connected through a slightly longer C-O-C bond, the distance increases to 3.5-3.6 angstrom. This pi-pi distance has a strong influence on the ground-state electronic interactions between the subphthalocyanine and the C-60, as determined from electronic absorption and cyclic voltannmetry measurements. In addition, fluorescence and time-resolved transient absorption experiments demonstrated that different mechanisms operate in the two systems after photoexcitation. Despite the similar HOMO-LUMO gaps, only when the two complementary pi-pi surfaces of the subphthalocyanine and the C-60 are held at a close distance, therefore showing a high degree of orbital overlap, is a multistep electron transfer process triggered, ultimately leading to the long-lived, spatially separated C-60 radical anion and ferrocenium radical cation pair. A full account of the synthesis, characterization, and studies of the ground- and excited-state electronic interactions occurring in these conjugates, as well as in their reference C-60-subphthalocyanine and subphthalocyanine-ferrocene dyads, is presented in this article.