A novel 2,2 '-bipyridine[2]catenane and its ruthenium complex: Synthesis, structure, and intramolecular electron transfer - A model for the photosynthetic reaction center
Yz. Hu et al., A novel 2,2 '-bipyridine[2]catenane and its ruthenium complex: Synthesis, structure, and intramolecular electron transfer - A model for the photosynthetic reaction center, CHEM-EUR J, 5(4), 1999, pp. 1267-1277
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.