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Tuning the Ultrafast Dynamics of Photoinduced Proton-Coupled Electron Transfer in Energy Conversion Processes Puja Goyal and Sharon Hammes-Schiffer* Department of Chemistry, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States S Supporting Information *
ABSTRACT: Photoinduced proton-coupled electron transfer (PCET) is essential for a wide range of energy conversion processes in chemical and biological systems. Understanding the underlying principles of photoinduced PCET at a level that allows tuning and control of the ultrafast dynamics is crucial for designing renewable and sustainable energy sources such as artificial photosynthesis devices and photoelectrochemical cells. This Perspective discusses fundamental aspects of photoinduced PCET, including the characterization of different types of excited electronic states, as well as the roles of solute and solvent dynamics, nonadiabatic transitions, proton delocalization, and vibrational relaxation. It also presents strategies for tuning and controlling the charge transfer dynamics and relaxation processes by altering the nature and positions of molecular substituents, the distance associated with electron transfer, the proton transfer interface, and the solvent properties. These insights, in conjunction with further studies, will play an important role in guiding the design of more effective energy conversion devices.
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identified by the presence of a thermally stable intermediate associated with ET or PT,5−9 the rigorous, well-defined characterization of ultrafast photoinduced PCET processes as sequential or concerted is not possible because of the inherently nonequilibrium nature of these processes. In particular, photoexcitation suddenly alters the electronic charge distribution so that the nuclei are no longer in equilibrium, and thermal equilibration of intermediates associated with a sequential mechanism typically does not occur on the ultrafast time scale of hundreds of femtoseconds or several picoseconds. Moreover, the electrons and transferring proton are often highly delocalized, particularly in the excited electronic and vibrational states, preventing the characterization of well-defined mechanisms in the conventional sense. The complexity of these processes presents a challenge for achieving a sufficient level of understanding that allows the tuning of chemical systems to control energy conversion processes in practical devices. This Perspective focuses predominantly on theoretical studies aimed at elucidating the fundamental principles of photoinduced PCET. Following a brief summary of theoretical concepts underlying thermal PCET reactions, which occur at equilibrium, and initial studies of the nonequilibrium dynamics of photoinduced PCET in simple model systems, a more detailed discussion of an experimentally studied system will highlight key
hotoinduced proton-coupled electron transfer (PCET) plays an important role in the energy conversion processes central to the function of many chemical and biological systems. For example, both natural and artificial photosynthesis utilize sunlight to create charge-separated states that can drive redox reactions leading to the synthesis of fuels. As in Photosystem II, the coupling of an electron transfer to a proton transfer often facilitates charge separation.1 This mechanism is also often an essential element of photoelectrochemical cells that catalyze water splitting, CO2 reduction, and H2 production. Thus, understanding the fundamental principles underlying photoinduced PCET is critical for designing renewable and sustainable energy sources.2−4
Photoinduced proton-coupled electron transfer (PCET) plays an important role in the energy conversion processes central to the function of many chemical and biological systems. Photoinduced PCET is distinct from photoinduced electron transfer (ET) or proton transfer (PT) in that photoexcitation induces both a significant change in the electronic charge distribution associated with ET and a PT reaction involving the breaking and forming of chemical bonds. In contrast to thermal PCET reactions, where a sequential mechanism may be © XXXX American Chemical Society
Received: December 22, 2016 Accepted: January 24, 2017 Published: January 24, 2017 512
DOI: 10.1021/acsenergylett.6b00723 ACS Energy Lett. 2017, 2, 512−519
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http://pubs.acs.org/journal/aelccp
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aspects of photoinduced PCET. In particular, the roles of solute and solvent dynamics, as well as vibrational relaxation, will be illustrated for this case study. Strategies to tune PCET systems to control the charge transfer (CT) dynamics and relaxation processes will also be discussed. Finally, a comparison of the concepts revealed from this case study to those observed in previous studies of photoexcited ET and PT, as well as another type of photoinduced PCET, will place this work in the context of the CT field. A complete discussion of all related work is beyond the scope of this relatively focused Perspective. Over the past two decades, a general theory for thermal PCET reactions has been developed.6,7,9−11 This theory is built upon the foundation of Marcus theory for ET reactions12 but is distinguished by the quantum mechanical (QM) treatment of the transferring proton13 as well as the electrons. In this PCET theory, the free energy surfaces associated with the electron− proton vibronic states are functions of collective solvent coordinates, and an equilibrium solvent fluctuation may induce the electron and proton to tunnel between localized electron− proton vibronic states (i.e., to tunnel simultaneously from their donors to their acceptors). Analytical rate constant expressions for nonadiabatic PCET reactions have been derived in various well-defined regimes.6,7,11,14 Applications of this theory to experimentally studied systems have assisted in the interpretation of large hydrogen/deuterium kinetic isotope effects of ∼80 or even ∼700 in terms of hydrogen tunneling between localized vibronic states.15 Because the derivations assume that the system is at equilibrium prior to the PCET reaction, these rate constant expressions are not suitable to describe the ultrafast dynamics of photoinduced PCET, which is inherently a nonequilibrium process. In particular, photoexcitation of a solvated molecule to a state associated with ET or PCET significantly alters the charge distribution of the molecule, and the solute and solvent nuclei are not in equilibrium with this new charge distribution. Understanding the relaxation of the system to equilibrium following photoexcitation requires methods for simulating nonadiabatic dynamics. A variety of methods are available to simulate the nonadiabatic dynamics following photoexcitation of a molecule in solution or in a protein environment. Surface hopping is among the most popular approaches because of its simplicity and computational efficiency.16,17 In surface hopping, the nuclei move on a single adiabatic potential energy surface except for instantaneous transitions that are incorporated according to an algorithm based on integration of the time-dependent Schrödinger equation. To simulate photoexcitation, the system is equilibrated on the ground state potential energy surface and then is instantaneously switched to an excited state potential energy surface. An ensemble of surface hopping trajectories starting on the excited state surface provides information on the populations of the different electronic states as a function of time. The surface hopping method was used to study photoinduced PCET in twodimensional model systems and illustrated a wide range of mechanisms for relaxation to the ground state.18,19 Photoinduced PCET in the hydrogen-bonded complex consisting of p-nitrophenylphenol and t-butylamine, as depicted in Figure 1, has been studied experimentally and theoretically.1,21−25 Transient absorption experiments1,21 implicated two different mechanisms, which were qualitatively designated sequential and concerted by interpretation in terms of the states shown in Figure 1. The intramolecular charge transfer (ICT) state corresponds to ET across the phenol molecule, while the electron−proton transfer (EPT) state corresponds to this ET as
Figure 1. Hydrogen-bonded complex consisting of p-nitrophenylphenol and t-butylamine studied experimentally, along with the bonding scheme of the electronic states of interest and a schematic depiction of the photoexcitation and relaxation processes. According to this scheme, vertical excitation to the ICT state is characterized by ET across the phenol with the proton remaining covalently bonded to O, while vertical excitation to the EPT state is characterized by ET accompanied by a shift in electronic charge density from the O−H bond to the N−H bond, resulting in an elongated N−H bond. PT occurs on the EPT state, either after population decay from the ICT to the EPT state or after vertical excitation directly to the EPT state. Adapted with permission from ref 20.
well as a shift of the electronic density from the O−H to the N− H covalent bond, thereby describing PT as well as ET. In the designated sequential mechanism, the system is photoexcited to the ICT state, followed by decay to the energetically lower EPT state and subsequently excited state PT (Figure 1). In the designated concerted mechanism, the system is photoexcited directly to the EPT state, followed by vibrational relaxation within this state. Note that the hydrogen nucleus itself does not move immediately upon photoexcitation to the EPT state because of the Franck−Condon principle, but the shift in the electronic density at the PT interface corresponds to the formation of an elongated N−H bond, which subsequently relaxes to equilibrium. The evidence for the elongated N−H bond was obtained from coherent Raman experiments.21 According to this interpretation, photoexcitation to the EPT state corresponds to a concerted PCET process in which the proton effectively transfers instantaneously upon photoexcitation through covalent bond rearrangement at the PT interface. However, the actual PT process involving movement of the proton from the oxygen to the nitrogen was not observed experimentally. Given these intriguing experiments, theoretical calculations on the p-nitrophenylphenol−ammonia complex solvated in 1,2dichloroethane were performed to determine whether the proton actually transfers on the EPT state and the time scale of this PT if it occurs.22 The calculations also provided insight into the roles of solute and solvent dynamics23 and vibrational relaxation,25 as well as a prediction of the isotope effect for this system.25 In these calculations, the excited state electronic potential energy surfaces were generated on-the-fly with a semiempirical implementation of the floating occupation molecular orbital complete active space configuration interaction (FOMO-CASCI) method.26,27 The semiempirical parameters were modified to reproduce key aspects of the potential energy surfaces for this system, namely, the optimized geometry on the ground state, vertical excitation energies at the Franck−Condon geometry, proton potential energy curves at different N−O distances, and potential energy scans along the N−O distance and the dihedral angle between the planes of the two benzene rings. A mixed quantum mechanical/molecular mechanical (QM/MM) approach was used,28 in which the solute was treated quantum mechanically with the FOMO-CASCI method 513
DOI: 10.1021/acsenergylett.6b00723 ACS Energy Lett. 2017, 2, 512−519
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and was immersed in a sphere of explicit 1,2-dichloroethane solvent molecules that were treated with a MM force field (Figure 2). This QM/MM approach utilizes an electrostatic
Figure 3. Potential of mean force (PMF) for PT between the hydroxyl O atom of p-nitrophenylphenol and the N atom of ammonia in the p-nitrophenylphenol−ammonia complex solvated in 1,2-dichloroethane for each of the three electronic states of interest. Negative and positive values of the reaction coordinate correspond to the proton being closer to the hydroxyl O atom of pnitrophenylphenol and the N atom of ammonia, respectively. Adapted with permission from ref 22. Figure 2. Hydrogen-bonded complex consisting of p-nitrophenylphenol and ammonia solvated in a 20 Å radius sphere of 1,2dichloroethane molecules.
embedding scheme that includes the interactions with the partial charges on the MM atoms in the calculation of the QM wave function. The energy and the forces on all QM and MM atoms are calculated on-the-fly during the molecular dynamics simulations. The three electronic states of interest were characterized in both the gas phase and solution.22 The S1 and S2 states both correspond to ππ* transitions but differ in the extent of CT. While excitation from the ground state to the S1 state leads to an electronic charge density shift from the phenolic end to the nitro end of p-nitrophenylphenol, excitation to the S2 state leads to a smaller degree of electronic charge density shift from the nitro group-bearing phenyl ring to the nitro group. In the gas phase, the PT potential energy curves along the O−N axis exhibit a deep minimum on the O side for the S0 (ground), S1 (EPT), and S2 (ICT) states, indicating that PT is highly unfavorable on all of these states. The free energy profiles in solution were obtained by calculating the potential of mean force (PMF) along the PT coordinate with QM/MM umbrella sampling simulations. As illustrated in Figure 3, the S0 and S2 states still exhibit a deep minimum on the O side, but the S1 state has a lower minimum on the N side, with a small barrier of ∼4 kcal/mol for PT from the O to the N. These free energy profiles suggest that the solvent significantly influences the nature of the S1 state and that PT to the N atom is thermodynamically favorable on this state. Connecting directly to the transient absorption experiments requires nonequilibrium dynamical simulations. For this purpose, the system was equilibrated on the ground state and photoexcited to either the EPT or the ICT state,22 as depicted schematically in Figure 1. Several hundred surface hopping trajectories were propagated on the electronic surfaces to simulate the relaxation to the ground state. Figure 4 depicts the population decay following photoexcitation to either the S1 or the S2 state. The time scale of the decay from the S1 to the S0 state is ∼0.9 ps, which is in qualitative agreement with the transient absorption time scale21 of ∼4.5 ps. The decay from the S2 to the S1 state occurs in ∼100 fs, which is consistent with the transient absorption time scale of
