INTRAMOLECULAR ELECTRON-TRANSFER IN THE INVERTED REGION

In the series [(4,4'-(X)2-bpy)Re(I)(CO)3(py-PTZ)]+ (X = OCH3, CH3, H, C(O)NEt2, CO2Et) and [(bpz)Re(I)(CO)3(py-PTZ)]+ (bpz is 2,2'-bipyrazine), d-pi-(Re) --> pi*(4,4'-(X)2-bpy) excitation is followed by rapid, intramolecular electron transfer to give the redox-separated states, [(4,4'-(X)2-bpy.-)Re(I)(CO)3(py-PTZ.+)]+. They return to the ground states by intramolecular, back electron transfer, [(4,4'-(X)2-bpy.-)Re(I)(CO)3(py-PTZ.+)]+ [GRAPHICS] [(4,4'-(X)2-bpy)Re(CO)3(py-PTZ)]+. The back electron transfer reactions are highly exergonic, DELTA-G-degrees approximately -1.18 to -1.93 eV in 1,2-dichloroethane, and occur in the inverted region. As determined by transient absorbance measurements following laser flash photolysis, k(b) varies from 6.7 x 10(6) s-1 for X = OCH3 to 9.1 x 10(7) s-1 for the bpz complex. The energies of the related MLCT excited states, [(4,4'-(X)2-bpy.-)Re(II)-(CO)3(4-Etpy)]+* (4-Etpy is 4-ethylpyridine) fall in the range E0 = 1.77-2.31 eV in the same medium. In this series the variations in the rate constant for nonradiative decay with driving force, k(nr) = 2.6 x 10(5)-2.8 x 10(7) s-1, are in accord with the energy gap law. Nonradiative decay has been analyzed quantitatively based on the results of a Franck-Condon analysis of emission spectral profiles. For back electron transfer in the inverted region, k(b) decreases logarithmically with -DELTA-G-degrees as predicted by the energy gap law. The slope of the correlation between ln (k(b) x 1s) and -DELTA-G-degrees is approximately 1/2 that between ln (k(nr) x 1s) and E0. An analysis based on the energy gap law provides an explanation for the decrease. The implications of our findings for the design of long-lived, redox-separated states are presented.