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 (Py2F 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 ⊃Py2F 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⊃Py2F3 furnishes the largest ratio of k CS/k CR (3400) among the family.