Nonadiabatic Dynamics of Photoinduced Proton-Coupled Electron Transfer in a Solvated Phenol–Amine Complex

Abstract

Photoinduced concerted electron-proton transfer (EPT), denoted photo-EPT, is important for a wide range of energy conversion processes. Transient absorption and Raman spectroscopy experiments on the hydrogen-bonded p-nitrophenylphenol-t-butylamine complex, solvated in 1,2-dichloroethane, suggested that this complex may undergo photo-EPT. The experiments probed two excited electronic states that were interpreted as an intramolecular charge transfer (ICT) state and an EPT state. Herein mixed quantum mechanical/molecular mechanical nonadiabatic surface hopping dynamics is used to investigate the relaxation pathways following photoexcitation. The potential energy surface is generated on the fly with a semiempirical floating occupation molecular orbital complete active space configuration interaction method for the solute molecule and a molecular mechanical force field for the explicit solvent molecules. The free energy curves along the proton transfer coordinate illustrate that proton transfer is thermodynamically and kinetically favorable on the lower-energy excited state but not on the higher-energy excited state, supporting the characterization of these states as EPT and ICT, respectively. The nonadiabatic dynamics simulations indicate that the population decays from the ICT state to the EPT state in ∼100 fs and from the EPT state to the ground state on the slower time scale of ∼1 ps, qualitatively consistent with the experimental measurements. For ∼54% of the trajectories, the proton transfers from the phenol to the amine in ∼400 fs on the EPT state and then transfers back to the phenol rapidly upon decay to the ground state. Thus, these calculations augment the original interpretation of the experimental data by providing evidence of proton transfer on the EPT state prior to decay to the ground state. The fundamental insights obtained from these simulations are also relevant to other photo-EPT processes.

Recent Advances and Perspectives on Nonadiabatic Mixed Quantum–Classical Dynamics

“…Working on model Hamiltonians, Hammes-Schiffer and co-workers have developed a TSH methodology to study proton-coupled electron transfer (PCET) reactions in diverse media, such as solutions and interfaces with semiconductors. 15,42,[109][110][111] Their approach stands out from conventional TSH applications, by the treatment of the transferring proton among the fast particles.…”

Role of Solvent Dynamics in Photoinduced Proton-Coupled Electron Transfer in a Phenol–Amine Complex in Solution

“…The computational details for the MDQT simulations are described elsewhere 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 14 the ground state with initial solvent velocities sampled from a Maxwell-Boltzmann distribution at 296 K. Adding zero point energy to the solute normal modes, especially the O-H stretch mode, is necessary to describe qualitatively correct dynamics in the simulations. Our previous work illustrated that this zero point energy does not leak significantly into the solvent degrees of freedom during the short time scale of the decay process, thus maintaining the temperature of the solvent bath during the relaxation.…”

Direct observation of light-driven, concerted electron–proton transfer

“…Subsequent theoretical analysis by Hammes-Schiffer and coworkers (19,20) supported this conclusion and the coexistence of distinct spectroscopic states, one a conventional ICT state, with excitation followed by proton transfer (photoET-PT), and a concerted photoEPT state.…”