Theoretical methods for simulating the nonadiabatic dynamics of photoinduced proton-coupled electron transfer (PCET) have been developed. These methods were applied to a hydrogen-bonded p-nitrophenylphenol-t-butylamine complex solvated in 1,2-dichloroethane. Spectroscopic experiments suggested that this complex undergoes concerted electron-proton transfer (EPT) upon photoexcitation. Quantum mechanical/molecular mechanical nonadiabatic surface hopping dynamics simulations elucidated the relaxation pathways following photoexcitation of this complex. The potential energy surface was generated on-the-fly with a semiempirical multiconfigurational method for the solute and a molecular mechanical force field for the explicit solvent molecules. When the transferring hydrogen nucleus was treated quantum mechanically, the nonadiabatic dynamics was propagated on mixed electron-proton vibronic surfaces. The lowest two bright singlet excited states were characterized as electron-proton transfer (EPT) and intramolecular charge transfer (ICT), and the dynamics following photoexcitation to each state was investigated. The simulated relaxation times were qualitatively consistent with experimental measurements. The simulations illustrated that the proton transfers from the phenol to the amine on the EPT state and then transfers back to the phenol rapidly upon decay to the ground state. The role of the solvent dynamics in facilitating both proton transfer and decay to the ground state was also elucidated. Vibrational relaxation within the ground electronic state was found to contribute significantly to the overall relaxation time. The hydrogen/deuterium isotope effect was predicted to be negligible, indicating that the absence of an isotope effect does not imply the absence of proton transfer in these processes. The fundamental insights obtained from these simulations are relevant to other photoinduced PCET processes.

Nonadiabatic dynamics of photoinduced proton-coupled electron transfer processes

253rd National Meeting (2017)

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