ESTIMATION OF EXCITED-STATE REDOX POTENTIALS BY ELECTRON-TRANSFER QUENCHING - APPLICATION OF ELECTRON-TRANSFER THEORY TO EXCITED-STATE REDOX PROCESSES

Rate constants for electron-transfer quenching of Ru(bpy)32+* (bpy is 2, 2'-bipyridine) by a series of organic quenchers have been determined in acetonitrile (μ = 0.1 M) at 22 ± 2 °C. The reactions studied were based on three different series of structurally related quenchers having varying redox potentials. They include oxidative quenching both by a series of nitroaromatics (ArNO2) and by a series of bipyridinium ions (P2+) and reductive quenching by a series of aromatic amines (R2NAr). After corrections for diffusional effects, the quenching rate constant (kq') data fall into two classes both of which can be treated successfully using Marcus-Hush theory. For case 1, which includes the data for oxidative quenching by P2+ and reductive quenching by R2NAr, RT In kq’ varies as ∆G23/2 where | ∆G23|« λ/2. ∆G23 is the free energy change for electron-transfer quenching within an association complex between the quencher and excited state and λ is the vibrational contribution to the activation barrier to electron transfer. The experimental data are also consistent with Marcus-Hush theory over a more extended range in ∆G23 where the free energy dependence includes a quadratic term. For case II, which includes quenching by several of the nitroaromatics, RT In kq’ varies as ∆G23 and evidence is obtained from the remainder of the data for a transition in behavior from case 11 to case I. The microscopic distinction between the two cases lies in competitive electron transfer to give either groundor excited-state products following the electron-transfer quenching step. For case II, back-electron transfer (K32) to give the excited state, e.g., Ru(bpy)33+,ArNO2- → Ru(bpy)32+*,ArNO2, is more rapid than electron transfer to give the ground state (K30), e.g., Ru(bpy)33+, ArNO2- → Ru(bpy)3 2+,ArNO2. For case I, electron transfer to give the ground state is more rapid. The different behaviors are understandable using electron-transfer theory when account is taken of the fact that k30 is a radiationless decay rate constant, and the electron-transfer process involved occurs in the abnormal free-energy region where - ∆G23 > λ. An appropriate kinetic treatment of the quenching rate data allows estimates to be made of redox potentials for couples involving the excited state. Formal reduction potentials in CH3CN (μ = 0.1 M) at 22 ± 2 °C are E (RUB3 3+/2+*) = -0.81 ±; 0.07 V and E(RuB3 2+*/+) = +0.77 ± 0.07 V. Comparisons between ground-and excited-state potentials show that the oxidizing and reducing properties of the Ru(bpy3 2+ system are enhanced in the excited state by the excited-state energy, that the excited state is unstable with respect to disproportionation into Ru(bpy)3 + and Ru(bpy)33+, and that the excited state is thermodynamically capable of both oxidizing and reducing water at pH 7. A comparison between the estimated 0-0 energy of the excited state and the energy of emission suggests that there may be only slight differences in vibrational structure between the ground and excited states. © 1979, American Chemical Society. All rights reserved.