The development of light-harvesting antenna molecules for visible photons is one of the important subjects in the basic issue of solar-energy conversion in natural photosynthesis and also to their potential applications to molecular electronics. [1] Recently, we reported light-harvesting dendritic zinc porphyrin arrays which can efficiently capture visible photons and channel the excitation energy to the focal core. [2,3] This observation prompted us to integrate such dendritic dye molecules as light-harvesting antennas into electron-transfer relay systems. Here we report the molecular design and photoinduced electron-transfer properties of a series of fullerene-terminated dendritic zinc-porphyrin arrays nP Zn -C 60 (Scheme 1). [4] Covalently linked porphyrin-C 60 dyads (P-C 60 ) have attracted a great deal of attention as novel photoinduced electron-transfer units, [5][6][7] in which the chargeseparated (CS) state (PC + -C CÀ 60 ), which results from electron transfer from the porphyrin moiety to C 60 , can take advantage of the low reorganization energy of C 60 . Thus, the present study featuring C 60 -terminated multiporphyrin arrays nP Zn -C 60 can make use of the well-established photochemistry of covalently linked porphyrin-C 60 dyads. Herein, we highlight a unique effect of a large dendritic array (7P Zn ) on the lifetime of the CS state.
The arrays nP Zn -C 60 (n = 1, 3, and 7) were synthesized from hydroxy-terminated zinc-porphyrin dendrons (nP Zn -OH) and a carboxylic acid appended C 60 derivative (C 60 -CO 2 H), by using azodicarboxylic acid diethyl ester as a condensation agent, and unambiguously characterized by 1 H NMR and UV/Vis spectroscopies, as well as with MALDI-TOF-MS spectrometry. [8] For example, a solution of 7P Zn -C 60 in benzonitrile (PhCN) showed an intense Soret absorption band at 417 nm arising from the zinc-porphyrin units and a weak C 60 absorption band at 700 nm. [8] Comparison of this spectral profile with those of 7P Zn -OH and C 60 -CO 2 H as reference compounds showed only a slight broadening of the