Construction of segregated arrays of multiple donor and acceptor units using a dendritic scaffold: Remarkable dendrimer effects on photoinduced charge separation

Dendritic molecules appended with multiple zinc porphyrin units (DP _m, m [number of zinc porphyrin units] = 6, 12, and 24) trap bipyridine compounds carrying multiple fullerene units (Py₂P _n, n [number of C₆₀ units] = 1-3), affording coordination complexes DP_m⊃Py₂F_n having a photoactive layer consisting of spatially segregated donor and acceptor arrays on their surface. Complexes DP_m⊃Py₂F_n are stable enough (K [average binding affinity] = 1.1 × 10⁶-4.4 × 10⁶ M^-1 in CHCl₃ at 25 °C) to be isolated by gel permeation chromatography. UHV-STM microscopy enables clear visualization of a petal-like structure of DP₁₂⊃Py₂F₃. Photoexcitation of the zinc porphyrin units in DP_m⊃Py ₂F_n results in a zinc porphyrin-to-fullerene electron transfer to generate a charge separation. The charge-separation rate constant (k_CS) in CH₂Cl₂ at 20 °C increases from 0.26 × 10¹⁰ to 2.3 × 10¹⁰ s^-1 upon increment of m and n, whereas the charge-recombination rate constant (k _CR) remains almost unchanged at 4.5 × 10⁶-6.7 × 10⁶ s^-1. Consequently, DP₂₄⊃Py ₂F₃ furnishes the largest ratio of k_CS/k _CR (3400) among the family.