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The quenching of the emitting excited states of Cr(bpy)33+, Ru(bpy)32+, and Ir(Me2phen)2Cl2+ (bpy, 2,2'-bipyridine; Me2phen, 5,6-dimethyl-1,10-phenanthroline) by some 30 amines or methoxybenzenes having variable oxidation potentials has been studied in acetonitrile solution. Flash photolysis experiments showed that the quenching process takes place with formation of the one-electron oxidation product of the quencher. The bimolecular quenching constants obtained from the Stern-Volmer constants and the excited-state lifetimes have been found to be related to the free-energy change of the electron transfer process. The plots of log kq vs. E1/2(Q/Q+) show a region of linear increase at high E1/2 values and reach a plateau at low E1/2- With aromatic amines and methoxybenzenes as quenchers, the comparison between theoretical curves14,16 and experimental plots indicates that the kinetically estimated value of the excited-state reduction potential is in reasonable agreement with that expected on spectroscopic grounds. The results also indicate that the intrinsic barrier for the excited-state selfexchange reaction is comparable with that of the ground-state self-exchange reaction. For Cr(bpy)33+ and Ir(Me2phen)2Cl2+, no evidence of kq decrease with AG is present even for free-energy changes which are two to three times larger than that expected for the onset of the Marcus “inverted” region. The difference between the electron transfer quenching properties of aromatic and aliphatic amines is also briefly discussed. © 1978, American Chemical Society. All rights reserved.
BALLARDINI et al. (Wed,) studied this question.