Solvent effect on electron and proton transfer in the excited state of a hydrogen bonded phenol-imidazole complex

Density functional theory calculations have been carried out for the ground state (S₀) and the first excited state (S₁) of the H-bonded phenol and imidazole complex as a model system for the active site of photosystem II. Potential energy surfaces (PES) of S₀ and S ₁ along the proton transfer coordinate were obtained. Based on the relative stability and small energy barrier for proton transfer, it was found that proton transfer could take place in the excited state both in vacuum and in water. As confirmed by the charge distribution, the proton transfer is determined to be coupled with electron transfer (PCET) in vacuum, while not in water. Such phenomenon should originate from the solvent effect stabilizing π* state with a large dipole moment, which results in a different structure of the product.