Another fundamental, yet important, type of photochemical reaction is excited state charge transfer. The basic principle of this reaction is that a charged particle, often an electron, can transfer among molecules after the system is excited, often due to photoexcitation. This transfer is allowed by the presence of electronic coupling between so-called "donor" and "acceptor" states. Accompanying the transfer of charge, there is often a reorganization of the nuclei based on the new electric potential of the acceptor state. The free energy difference ∆G, donor-acceptor coupling strength V DA , and reorganization energy λ all play key roles in the rate of excited state charge transfer. Marcus theory 319-322 is one of the most commonly used descriptions of charge transfer in the weak donoracceptor coupling regime when V DA ≪ k B T , where k B is the Boltzmann constant and T is the temperature. The electron transfer rate constant k ET for the |D⟩ → |A⟩ transition described by Marcus theory 319-322 is