The derivation of eq 3 and the related asymptotic analysis are presented in SI Section 2. Equation 3 clearly reveals that kQED is associated with the effective energy gap |ΔE + Er| (not the energy gap ΔE) because radiative ET processes must undergo molecular vibrational reorganization. That is, a large vibrational reorganization energy Er inhibits the radiative ET rate. The energy gap power law not only highlights the difference between molecular radiative ET and atomic spontaneous emission but can also be employed to identify QED effects on ET in experiments. The characteristic energy ΔEM-Q is a key factor for the observation of QED effects on ET, which is significantly influenced by Coulomb electronic coupling VET. If Coulomb electronic coupling is strong (such as VET = 0.1 eV in Figure ), ΔEM-Q approaches the visible region. As a result, in most photoinduced ET reactions with large VET, the observed ΔE dependence of ET rates is usually consistent with Marcus theory because the QED region occurs only for an extremely negative ΔE. Therefore, to experimentally probe the QED region in such molecular systems, highly exothermic ET reactions and time-resolved measurements with a wide temporal dynamic range are required. By contrast, when Coulomb electronic coupling is weak (such as VET = 0.01 meV in Figure ), the rate enhancement becomes more pronounced in the Marcus inverted region. Our result reveals that the cavity-free QED effect may be important in biological systems due to their weak-coupling condition. Comparison Between Theory and Experiment. The available experimental results validate the existence of the QED-driven radiative ET. To provide more insight into these results, we revisited previous experimental studies. We chose three representative molecular systems, namely estrone derivatives, Ru complexes on TiO2 thin films, and Ru-modified cytochrome c (RuL2 (X)(His33)cytochrome c), and compared the observed ΔE dependence of ET rates with the predictions according to the QED-ET theory, as presented in Figure . These three systems were studied in solution. For simplicity, we consider the solution to be a non-absorbing, non-dispersive, and homogeneous dielectric environment that can be described with dielectric permittivity εr. The detailed calculation of the modified ET rates is presented in SI Section 3. The QED-ET theory requires only five parameters: VET, Er, T, εr, and |μDA,D+A-|, which can be obtained from experiments or theoretical calculations. That is, the QED-ET theory has no free parameters. To compare the QED-ET theory with existing experiments, the Gibbs free energy of the systems must be estimated. On the basis of the literature, it is reasonable to assume that the experimental Gibbs free energy approximates the theoretical energy gap, i.e., ΔG ≈ ΔE. As a result, all required parameters can be obtained; a detailed description of the parameters used to produce Figure are listed in SI Section 4.