A novel 2,2 '-bipyridine[2]catenane and its ruthenium complex: Synthesis, structure, and intramolecular electron transfer - A model for the photosynthetic reaction center

A novel [2]catenane 1 incorporating 2,2'-bipyridine and cyclobis-(paraquat- p-phenylene) (BXV4+) was synthesized by self-assembly. X-ray analysis and m olecular modeling revealed the structure of the ligand 1. The complexation of 1 within a ruthenium complex afforded a catenane-type artificial photosy nthesis assembly 2, in which the sensitizer (Ru2+ center) and the acceptor (BXV4+) are linked noncovalently. Molecular modeling indicated that the cat enane complex 2 has two main conformers with different sensitizer-acceptor distances; its macrocyclic polyether unit is more extended than that in 1. Dynamic H-1 NMR spectroscopy and electrochemical studies confirmed the pres ence of different conformers. Spectroscopic investigations showed effective photoinduced electron transfer between the noncovalently linked sensitizer and acceptor in the ([2]catenane)ruthenium(II) complex 2. The electron tra nsfer rate was estimated to be greater than or equal to 2.1 x 10(8) s(-1) i n H2O. Two almost linear decay processes were observed with respective life times of the charge-separated state of tau(CS1) = 242 +/- 25 ns (55 +/- 3%) and tau(CS2) = 517 +/- 44 ns (45 +/- 3%), corresponding to back electron t ransfer from the different conformers, with alternative positions of the vi ologen units, to the oxidized metal center in 2. The back electron transfer rates are remarkably slow because of the spatial separation of the photoge nerated redox products and the location of the back electron transfer in th e Marcus inverted region.