Bimolecular Electron Transfers That Follow a Sandros-Boltzmann Dependence on Free Energy

摘要

Rate constants (it) for exergonic and endergonic electron-transfer reactions of equilibrating radical cations (A~(·+) + B (≈ )A+ B~(·+)) in acetonitrile could be fit well by a simple Sandros—Boltzmann (SB) function of the reaction free energy (AG) having a plateau with a limiting rate constant K_(lim) in the exergonic region, followed, near the thermoneutral point, by a steep drop in log kvs △G with a slope of 1/RT. Similar behavior was observed for another charge shift reaction, the electron-transfer quenching of excited pyrylium cations (P~(+*)) by neutral donors (P~(+*+D→ P~· +D~(·+)). In this case, SB dependence was observed when the logarithm of the quenching constant (log k_q) was plotted vs AG + s, where the shift term, s, equals +0.08 eV and AG is the free energy change for the net reaction (E_(redox)- E(excit)). The shift term is attributed to partial desolvation of the radical cation in the product encounter pair (P~·/D~(·+)), which raises its free energy relative to the free species.+ Remarkably, electron-transfer quenching of neutral reactants (A~* + D→A~(·-)+ D~(·+)) usine excited cvanoaromatic accentors and aromatic hvdrocarbon donors was also found to follow an SB dependence of log K_q on AG, with a positive 5, +0.06 eV. This positive shift contrasts with the long-accepted prediction of a negative value, -0.06 eV, for the free energy of an A~*+D→A~(·-)+D~(·+) encounter pair relative to the free radical ions. That prediction incorporated only a Coulombic stabilization of the A~(·-)/D~(·+) encounter pair relative to the free radical ions. In contrast, the results presented here show that the positive value of s indicates a decrease in solvent stabilization of the A~(·-)/D~(·+) encounter pair, which outweighs Coulombic stabilization in acetonitrile. These quenching reactions are proposed to proceed via rapidly interconverting encounter pairs with an exciplex as intermediate, A~*/D(≡)exciplex(≡)A~(·-)/D~(·+). Weak exciplex fluorescence was observed in each case. For several reactions in the endergonic region, rate constants for the reversible formation and decay of the exciplexes were determined using time-correlated single-photon counting. The quenching constants derived from the transient kinetics agreed well with those from the conventional Stern—Volmer plots. For excited-state electron-transfer processes, caution is required in correlating quenching constants vs reaction free energies when AG exceeds ~+0.1 eV. Beyond this point, additional exciplex deactivation pathways—fluorescence, intersystem crossing, and nonradiative decay—are likely to dominate, resulting in a change in mechanism.