Gaussian Free-Energy Dependence of Electron-Transfer Rates in Iridium Complexes

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 (Ir$_2$), and the acceptors are $N$-alkylpyridinium groups covalently bound to phosphinite ligands on the Ir$_2$ core. Rate constants for excited-state ET range from 3.5 × 10$^6$ to 1.1 × 10$^{11}$ per second, and thermal back ET (pyridinium radical to Ir$_2^+$) rates vary from 2.0 × 10$^{10}$ to 6.7 × 10$^7$ 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.