The mechanism of QED-driven radiative ET can be understood via eq 2. When an electron migrates from a donor moiety to an acceptor moiety, its motion interacts with infinite onephoton states [related to ρph(ωph)] through light-molecule coupling [corresponding to F(ωph), note that F(ωph) is not the strength for the light-matter coupling.], leading to the occurrence of radiative ET. In other words, radiative ET processes are driven by the combination of light-molecule coupling strength and photonic DOS [i.e., F(ωph) × ρph(ωph)], which corresponds to the radiation spectrum due to ET processes. In addition, according to quantum mechanics, the ET transition in a molecule relies on initial and final vibrational DOS together with their wavefunction overlap, and this transition can be described by vibrational transition DOS ρvib(ωph). On the basis of these concepts, we propose 'electron transfer overlap (ET overlap)', i.e., the spectral overlap of ET radiation spectrum F(ωph) × ρph(ωph) (the red curve) and vibrational transition DOS ρvib(ωph) (the blue curve) in Figure . A similar concept was proposed in Förster energy transfer theory, and the theory can be formulated based on QED. Equation 2 enables the quantitative calculation of the QEDdriven radiative ET rate and the qualitative characterization of ET overlap. For example, in free space, photonic DOS corresponds to ρph(ωph) = ωph 2 /π 2 c 3 (which is well-known as a factor in blackbody radiation ), where c is the speed of light. Next, the light-matter coupling factor in free space corresponds to F(ωph) = |μDA,D+A-| 2 ℏωph /6ε0, where μDA,D+A-is the molecular ET transition dipole and ε0 is the vacuum permittivity. Here, |μDA,D+A-| 2 ℏωph/2ε0 results from the light-matter interaction based on the electric field operator from QED, and a factor 1/3 results from the average dipole orientation. F(ωph) is different from Coulomb electronic coupling due to its frequency dependence. Furthermore, in the low-vibrational-frequency limit, vibrational transition DOS can be expressed as ρvib(ωph) = (4πkBTEr) -1/2 exp{-(ΔE + ℏωph + Er) 2 /4kBTEr}, where T is the temperature, Er is the vibrational reorganization energy, and ΔE denotes the electronic energy gap. The form of ρvib(ωph) resembles that of the Franck-Condon weighted DOS in previous ET theories; that is, the effect of vibrational Franck-Condon overlaps is included in ρvib(ωph). On the basis of this analysis, we can obtain the characteristics of ET overlap from F(ωph) × ρph(ωph) ∝ ωph 3 (the red curve in Figure ) and ρvib(ωph) ∝ exp{-(ΔE+ℏωph+Er) 2 /4kBTEr} (the blue curve in Figure ), which reveals that the radiative ET rate in free space can be enhanced by the increase of effective energy gap |ΔE+Er|. The main advantages of considering infinite one-photon states (infinite photonic modes) are summarized as follows. First, from the aspect of motivation, since it is nontrivial to control the experimental condition of single-photon mode (such as cavities) during chemical reactions, we would like to investigate whether it is possible to harness QED effects to control chemical reactions in the absence of cavities, where the light-matter interaction includes infinite photon modes. Second, from the aspect of experiments, a parameter-free theory is useful for excluding possible ET mechanism and verify QED-driven ET rates in experiments. By incorporating infinite photonic modes in our theory, we can eliminate effective volume (the free parameter in the single-mode theory) and develop a parameterfree QED-driven ET. We would like to highlight that the radiative ET process described by eq 2 is different from atomic spontaneous emission for the following reasons. First, kQED is associated with the molecular ET transition dipole μDA,D+A-, which depicts the electrodynamic effect caused by the migration of charge transfer. Second, vibrational transition DOS ρvib(ωph) is absent from atomic spontaneous emission due to the lack of vibrational degrees of freedom in atomic systems. Previous studies also showed that when QED effects emerge, molecular vibrations can play a significant role, e.g., Franck-Condon-Rabi oscillation and suppression of Rabi splitting in emission power spectra . QED Region of ET Rates. To clearly illustrate the influence of QED effects on ET reactions, we demonstrate the ΔE dependence of the total ET rate (kET, green dashed line) and separate its contribution into two components including the Marcus ET rate (kMarcus, black solid line) and the radiative ET rate (kQED, red solid line), as presented in Figure and 2b. According to Marcus theory, the ET mechanism can be divided into two regions: the Marcus normal region (ΔE > -Er) and the Marcus inverted region (ΔE < -Er). To determine the region in which the QED effects emerge, we define the QED region based on the intersection of kMarcus and kQED and divide the Marcus inverted region into two regions: the inverted region (yellow area) and QED region (blue area). The intersection of the abscissa ΔEM-Q characterizes the energy for the ET-mechanism transition from the inverted region to the QED region. In the normal region and the inverted region (ΔE > ΔEM-Q), the total ET rate is in accordance with Marcus theory (kET ≈ kMarcus) because electrostatic effects dominate the ET process. By contrast, in the QED region (ΔE < ΔEM-Q), the total ET rate is in accordance with the QED-driven radiative ET process (kET ≈ kQED) since electrodynamic effects dominate the ET process.