Gaussian Free-Energy Dependence of Electron-Transfer Rates in Iridium Complexes
Lucius S. Fox 1,
Mariusz Kozik 2,
Jay R. Winkler 2, and
Harry B. Gray 1
1 Arthur Amos Noyes Laboratory, California Institute of Technology, Pasadena, CA 91125
2 Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973
The kinetics of photoinduced electron-transfer (ET) reactions have been measured in a series of synthetic donor-acceptor complexes. The electron donors are singlet or triplet excited iridium(I) dimers (Ir2), and the acceptors are N-alkylpyridinium groups covalently bound to phosphinite ligands on the Ir2 core. Rate constants for excited-state ET range from 3.5 x 106 to 1.1 x 1011 per second, and thermal back ET (pyridinium radical to Ir2+) rates vary from 2.0 x 1010 to 6.7 x 107 per second. The variation of these rates with driving force is in remarkably good agreement with the Marcus theory prediction of a Gaussian free-energy dependence.
