Chlorin-based supramolecular assemblies for artificial photosynthesis

Abstract

Photosynthetic reaction center models consisting of zinc 9-desoxo-pyrochlorophyllide a primary electron donors, ZC, that are directly bonded at their 3-position to the 5-position of a 2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin, ZP, which is in turn bonded at its 15-position to 2-triptycenenaphthoquinone, 2-triptycenebenzoquinone, 1-triptycenebenzo-quinone, or N-(4-phenyl),N′-(n-octyl)-l,4,5,8-naphthalenediimide, 1, 2, 3, and 4, respectively, were prepared. Steric hindrance between adjacent substituents positions the π system of the chlorophyll perpendicular to that of the porphyrin. In turn, the π system of the chlorophyll is held about 60° 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 the N-N axis in the diimide acceptor in 4. The resulting structures place the ZC donors in 1, 2, 3 and 4 at fixed center-to-center distances of 20, 18, 14, and 21 Å from the acceptors, respectively. Photoexcitation 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 naphthalenediimide (NI), that occurs with τ = 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-correlated radical pairs. The EPR spectra of 1–4 can be simulated using the distances and orientations of the radicals relative to one another determined from the molecular structures of 1–4. These long-lived, spin-polarized radical ion pairs closely mimic the bacteriochlorophyll cation – quinone anion radical pair produced in photosynthetic reaction centers and provide a useful tool for studying the interaction of the surrounding medium with the charge separated radical ion pair.