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 (DPm, m [number of zinc porphyrin units] = 6, 12, and 24) trap bipyridine compounds carrying multiple fullerene units (Py2Fn, n [number of C60 units] = 1-3), affording coordination complexes DPm superset Py2Fn having a photoactive layer consisting of spatially segregated donor and acceptor arrays on their surface. Complexes DPm superset Py2Fn are stable enough (K [average binding affinity] = 1.1 x 10(6)-4.4 x 10(6) M(-1) in CHCl3 at 25 degrees C) to be isolated by gel permeation chromatography. UHV-STM microscopy enables clear visualization of a petal-like structure of DP12 superset Py2F3. Photoexcitation of the zinc porphyrin units in DPm superset Py2Fn results in a zinc porphyrin-to-fullerene electron transfer to generate a charge separation. The charge-separation rate constant (kCS) in CH2Cl2 at 20 degrees C increases from 0.26 x 10(10) to 2.3 x 10(10) s(-1) upon increment of m and n, whereas the charge-recombination rate constant (kCR) remains almost unchanged at 4.5 x 10(6)-6.7 x 10(6) s(-1). Consequently, DP24 supersetPy2F3 furnishes the largest ratio of kCS/kCR (3400) among the family.