CHLORIN-BASED SUPRAMOLECULAR ASSEMBLIES FOR ARTIFICIAL PHOTOSYNTHESIS

Photosynthetic reaction center models consisting of zinc 9-desoxo-pyro chlorophyllide a primary electron donors, ZC, that are directly bonded at their 3-position to the 5-position of a 8,12,18-tetraethyl-3,7,13, 17-tetramethylporphyrin, ZP, which is in turn bonded at its 15-positio n to 2-triptycenenaphthoquinone, 2-triptycenebenzoquinone, 1-triptycen ebenzoquinone, or 4-phenyl),N'-(n-octyl)-1,4,5,8-naphthalenediimide, 1 , 2, 3, and 4, respectively, were prepared. Steric hindrance between a djacent substituents positions the pi system of the chlorophyll perpen dicular to that of the porphyrin. In turn, the pi system of the chloro phyll is held about 60 degrees to that of the O-O axis of the quinones in 1 and 2, parallel to the quinone O-O axis in 3, and parallel to th e N-N axis in the diimide acceptor in 4. The resulting structures plac e the ZC donors in 1, 2, 3 and 4 at fixed center-to-center distances o f 20, 18, 14, and 21 Angstrom from the accepters, respectively. Photoe xcitation of 1-4 in 2-methyltetrahydrofuran glass at 77K results in a single observable electron transfer reaction: (1)ZC-ZP-X --> ZC(+)-ZP -X(-), where X = benzoquinone (BQ), naphthoquinone (NQ) or naphthalene diimide (NI), that occurs with tau = 4.5, 3.3, 2.0, and 2.0 ps for 1, 2, 3, and 4, respectively. The final ZC(+)-ZP-X(-) radical pairs live for 12.7, 8.4, 2.5, and 10 ms at 77K in 1, 2, 3, and 4, respectively, and exhibit spin-polarized EPR spectra characteristic of spin-correlat ed radical pairs. The EPR spectra of 1-4 can be simulated using the di stances and orientations of the radicals relative to one another deter mined from the molecular structures of 1-4. These long-lived, spin-pol arized radical ion pairs closely mimic the bacteriochlorophyll cation - quinone anion radical pair produced in photosynthetic reaction cente rs and provide a useful tool for studying the interaction of the surro unding medium with the charge separated radical ion pair.