Mechanically induced intramolecular electron transfer in a mixed-valence molecular shuttle.
The kinetics and thermodynamics of intramolecular electron transfer (IET) can be subjected to redox control in a bistable [2]rotaxane comprised of a dumbbell component containing an electron-rich 1,5-dioxynaphthalene (DNP) unit and an electron-poor phenylene-bridged bipyridinium (P-BIPY(2+)) unit and a cyclobis (paraquat-p-phenylene) (CBPQT(4+)) ring component. The [2]rotaxane exists in the ground-state co-conformation (GSCC) wherein the CBPQT(4+) ring encircles the DNP unit. Reduction of the CBPQT(4+) leads to the CBPQT(2(•+)) diradical dication while the P-BIPY(2+) unit is reduced to its P-BIPY(•+) radical cation. A radical-state co-conformation (RSCC) results from movement of the CBPQT(2(•+)) ring along the dumbbell to surround the P-BIPY(•+) unit. This shuttling event induces IET to occur between the pyridinium redox centers of the P-BIPY(•+) unit, a property which is absent between these redox centers in the free dumbbell and in the 1:1 complex formed between the CBPQT(2(•+)) ring and the radical cation of methyl-phenylene-viologen (MPV(•+)). Using electron paramagnetic resonance (EPR) spectroscopy, the process of IET was investigated by monitoring the line broadening at varying temperatures and determining the rate constant (k(ET) = 1.33 x 10(7) s(-1)) and activation energy (ΔG(‡) = 1.01 kcal mol(-1)) for electron transfer. These values were compared to the corresponding values predicted, using the optical absorption spectra and Marcus-Hush theory.
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3113, USA.
Jonathan C
JC
Barnes
Albert C
AC
Fahrenbach
Scott M
SM
Dyar
Marco
M
Frasconi
Marc A
MA
Giesener
Zhixue
Z
Zhu
Zhichang
Z
Liu
Karel J
KJ
Hartlieb
Ranaan
R
Carmieli
Michael R
MR
Wasielewski
J Fraser
JF
Stoddart
D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport / Q000502:physiology; D007700:Kinetics; D008956:Models, Chemical; D010084:Oxidation-Reduction; D011487:Protein Conformation; D043862:Rotaxanes / Q000138:chemical synthesis* / Q000737:chemistry*; D019265:Spectroscopy, Near-Infrared; D013816:Thermodynamics
D043862:Rotaxanes