Role of Solvent Dynamics in Photoinduced Proton-Coupled Electron

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Letter

Role of Solvent Dynamics in Photoinduced Proton-Coupled Electron Transfer in a Phenol-Amine Complex in Solution Puja Goyal, and Sharon Hammes-Schiffer J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b01475 • Publication Date (Web): 14 Aug 2015 Downloaded from http://pubs.acs.org on August 19, 2015

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The Journal of Physical Chemistry Letters

Role of Solvent Dynamics in Photoinduced Proton-Coupled Electron Transfer in a Phenol-Amine Complex in Solution Puja Goyal and Sharon Hammes-Schiffer* Department of Chemistry, 600 South Mathews Avenue, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801 *corresponding author; e-mail: [email protected] Abstract: Photoinduced proton-coupled electron transfer (PCET) plays an essential role in a wide range of energy conversion processes. Previous experiments on a phenol-amine complex in solution provided evidence of an electron-proton transfer (EPT) excited state characterized by both intramolecular charge transfer and proton transfer from the phenol to the amine. Herein we analyze hundreds of surface hopping trajectories to investigate the role of solvent dynamics following photoexcitation to the EPT state. This solvent dynamics leads to a significant decrease in the energy gap between the ground and EPT states, thereby facilitating decay to the ground state, and generates an electrostatic environment conducive to proton transfer on the EPT state. In addition to solvent reorganization, the geometrical properties at the hydrogen-bonding interface must be suitable to allow proton transfer. These mechanistic insights elucidate the underlying fundamental physical principles of photoinduced PCET processes. TOC graphic:

Keywords: proton-coupled electron transfer, surface hopping, solvent dynamics, electrostatics, photoinduced processes, excited state decay 1

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Proton-coupled electron transfer (PCET) reactions1-4 occur widely in nature and play a vital role in photosynthesis, respiration, and a variety of other biological processes. Moreover, photoinduced PCET is essential for many solar energy conversion devices such as photoelectrochemical cells.5-8 Fundamental mechanistic insight into photoinduced PCET can be obtained through the study of relatively simple model systems.9-11 Recently photoinduced PCET in a hydrogen-bonded complex between p-nitrophenylphenol and t-butylamine (Figure 1) in 1,2dichloroethane was experimentally studied using transient absorption and coherent Raman spectroscopy.12 The experiments were interpreted as indicating the formation of an elongated NH bond upon vertical excitation from the ground electronic state to an excited electronic state described as an electron-proton transfer (EPT) state, although proton transfer (PT) from the phenolic oxygen atom to the amine nitrogen atom was not explicitly observed. Time-dependent density functional theory calculations with a polarizable continuum model for the solvent illustrated a change in electronic density at the hydrogen-bonding interface upon vertical excitation to the EPT state, which was also shown to have intramolecular charge transfer character.13 In addition, hybrid quantum mechanical/molecular mechanical nonadiabatic dynamics simulations14 were performed using a surface hopping approach.15-16 Explicit PT following photoexcitation to the EPT state was observed in these simulations, clarifying and augmenting the original interpretation of the experiments.14

Figure 1. The experimentally studied hydrogen-bonded complex between p-nitrophenylphenol and t-butylamine.12 The computational studies are carried out on a reduced system in which tbutylamine has been replaced with ammonia. In a previous study,14 the full and reduced systems were shown to exhibit very similar ground and excited state properties. Figure reproduced with permission from Ref. 14. Copyright 2015. 2


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In this Letter, we investigate the role of solvent dynamics in facilitating both PT on the EPT state and the decay from the EPT state to the ground state, as well as the apparent correlation between these two processes. For this purpose, we perform a comprehensive analysis of the solute and solvent dynamics for hundreds of molecular dynamics with quantum transitions (MDQT) surface hopping trajectories following excitation to the first excited electronic singlet state.14 Specifically, we examine changes in the solute geometry, particularly at the PT interface, as well as the PT barrier along the trajectories. We also investigate the time evolution of the solvent electrostatic potential and the solute-solvent electrostatic interaction energy, in conjunction with changes in the energy gap between the ground and excited electronic states. Analysis of the correlations among these solute and solvent properties, as well as the relation of these properties to PT in the excited state and decay to the ground state, provides fundamental insights into the mechanism of photoinduced PCET in this system. Such insights are expected to be applicable to other photoinduced PCET processes as well. The previous simulations of this phenol-amine system in solution provided direct evidence for PT on the EPT state.14 In the gas phase, the most thermodynamically stable geometry was found to correspond to the proton bonded to the phenolic O atom with a very high barrier for PT to the amine N atom for both the S0 and S1 states.17 In solution, however, free energy simulations illustrated that the most thermodynamically stable configuration for the S1 state corresponds to the proton bonded to the N atom, with a free energy barrier for PT from O to N of ~4 kcal/mol. These previous studies showed that PT from O to N is both kinetically and thermodynamically favorable on the S1 state but unfavorable on the S0 state. For 57% of MDQT trajectories photoexcited to the excited singlet state S1, which was characterized as the EPT state, forward PT from the O atom to the N atom was observed on the S1 state. Subsequently, most of these trajectories decayed to the

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ground state S0, followed by back PT from the N atom to the O atom. Optimization of the S1/S0 conical intersection in solution led to a solute geometry in which the proton was bonded to N instead of O, suggesting that PT from O to N on the S1 state provides a pathway for decay from S1 to S0. The occurrence of PT on the S1 state, however, does not significantly influence the overall rate of decay from S1 to S0 (Figure S1). The S1 and S2 excited electronic states are both intramolecular charge transfer (ICT) states,1214

and the solvent responds strongly to the change in solute electronic charge distribution upon

photoexcitation to either state. The present study focuses on the dynamics following photoexcitation to the S1 state, which was characterized computationally as an EPT state because the minimum of the free energy profile along the PT coordinate corresponds to bonding of the proton to N rather than O. Previously state-averaged complete active space self-consistent field (SA-CASSCF)18 as well as time-dependent density functional theory (TDDFT) calculations predicted the S1 state to have a gas phase dipole moment ~10-13 D higher than that of the S0 state.14 Upon photoexcitation to S1, the solvent responds to the increase in the magnitude of the solute dipole moment, which is associated with a positive charge at the phenol and amine groups and a negative charge at the nitro group, by reorienting the solvent dipoles such that the solvent electrostatic potential decreases at the phenol-amine interface and increases at the nitro group (Table S1 and Figure S3). Moreover, the larger solute dipole moment, combined with this solvent reorganization, leads to a stronger solute-solvent electrostatic interaction energy on the S1 state than on the S0 state (Table S3 and Figure S4). Note that the solvent response may be somewhat underestimated because of the non-polarizable force field used to describe the solvent.19 However, the trends in the solvent response are very clear from the calculations and are expected to be independent of the specific force field.

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The response of the solvent to the instantaneous electronic charge redistribution upon photoexcitation to the S1 state and to PT on this state was directly observed in the MDQT trajectories. Figure 2 depicts the solvent response following photoexcitation to the S1 state for trajectories that exhibit PT from O to N on S1 followed by decay from S1 to S0. Figure 2a illustrates that prior to vertical excitation from S0 to S1, the probability distribution of the solvent electrostatic potential at N is approximately centered at the equilibrium value on S0 (0.29 V). After photoexcitation to S1 but prior to PT from O to N, the solvent electrostatic potential at N decreases, reflecting the solvent response to the electronic charge distribution of the S1 state. Most trajectories exhibit PT from O to N before reaching the value of 1.73 V, which corresponds to the equilibrium potential on S1 with the proton bonded to O. Following PT, the solvent electrostatic potential at N decreases further, but most of the trajectories decay to S0 before reaching the value of 2.59 V, which corresponds to the equilibrium potential on S1 with the proton bonded to N. The solutesolvent electrostatic interaction energy exhibits qualitatively similar behavior, as illustrated in Figure 2b. The solvent response following photoexcitation to the S1 state for trajectories that do not exhibit PT from O to N is qualitatively similar to the response for trajectories that exhibit PT. Figure S5 illustrates that the average solvent electrostatic potential at the amine N decreases with time at the same rate for trajectories that exhibit PT as those that do not exhibit PT on the S1 state. Moreover, Figure S6 shows that the probability distribution of the lowest value of the solvent electrostatic potential at the amine N atom sampled on the S1 state is centered at the equilibrium value on the S1 state, 1.73 V, for trajectories that do not exhibit PT. This probability distribution is similar to that observed prior to PT for trajectories that exhibit PT, as depicted in Figure 2a, although the distribution has shifted to slightly more negative values in Figure S6 because of the 5



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greater average time that has elapsed. Finally, as mentioned above, the overall rate of population decay from S1 to S0 is also similar for trajectories with or without PT (Figure S1).

Figure 2. Probability distributions of (a) the solvent electrostatic potential at the amine N atom and (b) the solute-solvent electrostatic interaction energy for MDQT trajectories in which forward PT from O to N occurs on the S1 state following photoexcitation. The width of the bins is indicated by small tic marks. The distributions just before PT on S1 use the last frame at which the OH distance is less than 1.15 Å before the N-H distance becomes less than 1.15 Å on the S1 state. The distributions just before decay from S1 to S0 are based on the subset of these trajectories in which forward PT is followed by S1 to S0 decay and subsequent back PT from N to O. The vertical dashed red line at 0.29 V (or 0.73 eV) represents the average equilibrium value on the S0 state when the proton is bonded to O, and the vertical dashed blue line at 1.73 V (or 3.37 eV) and green line at 2.59 V (or 4.52 eV) represent the average equilibrium value on the S1 state when the proton is bonded to O and N, respectively.

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Figure 3. Probability distributions of the S0/S1 energy gap for MDQT trajectories photoexcited to the S1 state. The width of the bins is indicated by small tic marks. In panel a, the blue and green distributions are analogous to those defined in the caption of Figure 2. This analysis implies that the solvent dynamics following photoexcitation is qualitatively similar for all trajectories. However, the trajectories that exhibit PT experience a further shift in the electrostatic potential at the amine N, as well as the solute-solvent electrostatic interaction energy, following PT, as illustrated in Figure 2. This additional solvent reorganization induced by PT leads to stabilization of the protonated amine and therefore further reduction of the S0/S1 energy gap. Figure 3 illustrates a decrease in the S0/S1 energy gap for all trajectories due predominantly to the solvent dynamics following photoexcitation, although solute dynamics could also influence the energy gap. Comparison of Figures 3a and 3b indicates that this decrease in the energy gap is 7



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greater for trajectories that exhibit PT on the S1 state, most likely due to the additional solvent reorganization following PT. As a result, the decay from S1 to S0 occurs at lower energy gap values for trajectories that exhibit PT on the S1 state than for those that do not exhibit PT. Interestingly, this difference in energy gaps does not significantly affect the rate of decay from S1 to S0 (Figure S1). To further elucidate the mechanism of photoinduced PCET, we analyzed the behavior of a representative trajectory that is photoexcited from S0 to S1 at time zero and subsequently exhibits PT from O to N on S1. As depicted in Figure 4, the solvent response begins immediately upon vertical excitation, as indicated by the decrease in both the electrostatic potential at the O and N atoms and the solute-solvent electrostatic interaction energy. This solvent dynamics corresponds predominantly to relaxation of the first solvation shell (Figure 5) and is accompanied by a decrease of the S0/S1 energy gap. During the initial 300 fs, the N-O distance remains relatively short (~2.7 Å), the O-H--N angle does not deviate significantly from linearity, and the barrier for PT from O to N decreases. At ~300 fs, the PT barrier is relatively low (~0.1 eV), and the proton transfers from O to N. Note that the O-H distance fluctuates about the equilibrium O-H bond length until the barrier to PT becomes low enough to allow PT, after which the N-H distance fluctuates about the equilibrium N-H bond length. In other words, following photoexcitation to the S1 state, the proton is trapped closer to the O until the solvent and solute relaxation dynamics lead to a lowering of the PT barrier to enable the proton to transfer to the more thermodynamically stable position closer to the N. After PT, the solvent electrostatic potentials and the solute-solvent electrostatic interaction energy decrease further, stabilizing the protonated amine and hence further reducing the S0/S1 energy gap. This additional decrease in the energy gap facilitates the decay from S1 to S0 at 550 fs, followed quickly by back PT from N to O because the proton is much more thermodynamically

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stable bonded to the O atom in the ground state.14 After the decay to the ground state, the electrostatic potentials and solute-solvent electrostatic interaction energy increase and eventually reach the equilibrium values calculated for S0 (Table S2).

Figure 4. Analysis of a representative MDQT trajectory that exhibits PT from O to N on the S1 state. The vertical dashed black line at 300 fs indicates the time when PT from O to N begins (panel a), and the vertical dotted black line at ~550 fs indicates the time at which S1 to S0 decay occurs, followed quickly by back PT from N to O (panel a). For the initial ~550 fs, the solvent electrostatic potentials at O and N and the solute-solvent electrostatic interaction energy decrease (panels b and c), accompanied by a decrease in the S0/S1 energy gap (panel d). During the initial ~300 fs, the barrier for PT from O to N decreases (panel e), while the N-O distance remains relatively short (panel f) and the O-H--N angle does not deviate significantly from linearity (panel g). After PT at ~300 fs, the solvent electrostatic potentials and interaction energy decrease further, stabilizing the protonated amine and hence further reducing the S0/S1 energy gap. After the decay to S0 at ~550 fs, the electrostatic potentials and interaction energy start increasing and reach their equilibrium values on S0.

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Figure 5. Time evolution of the average solvent electrostatic potential at (a) the amine N atom and (b) the nitro N atom for MDQT trajectories photoexcited to the S1 state. Data from trajectories that exhibit PT from O to N on the S1 state are included until PT begins. The first solvation shell is defined as all solvent molecules with any atom within 5 Å of at least one solute atom. This figure illustrates that the first solvation shell comprises the dominant contribution to the solvent relaxation. As discussed above, the solvent dynamics directly after photoexcitation is qualitatively similar for trajectories that exhibit PT and for those that do not exhibit PT. The distinction between these two types of trajectories can be understood in terms of differences in the solute rather than the solvent dynamics. Figure 6 depicts the time evolution of the average N-O distance and the average PT barrier on the S1 state following photoexcitation. For trajectories that exhibit PT on the S1 state, the average N-O distance remains relatively short (2.72.8 Å), and the average PT barrier decreases overall with time, although the barrier undergoes significant fluctuations. In contrast, for trajectories that do not exhibit PT on the S1 state, the average N-O distance gradually

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increases in time, leading to an overall increase in the average PT barrier, although other factors also influence the PT barrier.

Figure 6. Time evolution of the average PT barrier on the S1 state and the average N-O distance on the S1 state for MDQT trajectories photoexcited to the S1 state. Data from trajectories that exhibit PT from O to N on the S1 state are included until PT begins, while data from trajectories that do not exhibit PT on the S1 state are included until the lowest value of the solvent electrostatic potential at the amine N atom sampled on the S1 state is reached. Any frame in which the O-H--N angle is less than 100° is not considered because the calculation of the PT barrier along the NO axis is not meaningful for such a frame (~4% of all frames). To further compare these two types of trajectories, we calculated the probability distributions of the N-O distance, the O-H--N angle, and the PT barrier immediately prior to PT for trajectories that exhibit PT and when the solvent electrostatic potential at the N atom reaches its minimum value for trajectories that do not exhibit PT. A justification for this choice of time for analyzing the trajectories that do not exhibit PT is provided in the Supporting Information (Figures S11 and S12). Figure S7 indicates that PT occurs at relatively short N-O distances, mostly ~2.6 Å, and at nearly linear O-H--N angles of ~160°. The trajectories in which PT does not occur exhibit a much 11



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wider distribution of N-O distances and O-H--N angles when the solvent electrostatic potential at the N atom reaches its minimum value on the S1 state. In particular, the distribution of N-O distances is shifted to larger values, with some trajectories undergoing dissociation of the hydrogen bond between the phenol and the amine, and the distribution of O-H--N angles indicates sampling of some very small angles, which is caused by the O-H group on the phenol rotating away from the amine (Figure S8). The sampling of smaller O-H--N angles in trajectories that do not exhibit PT is also observed in the time evolution of the average O-H--N angle in these MDQT trajectories (Figure S9). This analysis indicates that the solvent dynamics following photoexcitation simultaneously decreases the energy gap and provides a favorable electrostatic environment for PT, but the geometry at the PT interface must also be suitable to allow PT to occur. Geometrical factors such as the N-O distance and the O-H--N angle play an important role in determining the PT barrier. The dihedral angle between the planes of the benzene rings was not found to be an important factor for PT on the S1 state (Figure S10). In addition to a suitable geometry at the PT interface, a thermal fluctuation is required to overcome the PT barrier on the S1 state before decay to the S0 state. After the initial solvent reorganization, there is a competition between decay to the S0 state and forward PT on the S1 state, although most trajectories exhibiting forward PT subsequently decay to the S0 state and exhibit back PT. In this Letter, we have elucidated the role of the solvent dynamics that occurs in response to the solute charge redistribution upon photoexcitation of a phenol-amine complex from the ground state to the S1 state. This solvent dynamics, which was found to correspond predominantly to relaxation of the first solvation shell, leads to a significant decrease in the energy gap between the S0 and S1 states and to an electrostatic environment conducive to PT from the O to the N atom on

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the S1 state. In addition to this solvent reorganization, geometrical properties of the PT interface, such as short N-O distances and nearly linear O-H--N angles, contribute toward lowering the PT barrier, and a suitable thermal fluctuation is required to surmount this barrier prior to decay to the ground state. When PT occurs on the S1 state, additional solvent relaxation stabilizes the protonated amine, thereby further reducing the S0/S1 energy gap and promoting decay from the S1 to the S0 state. However, the overall decay time from the S1 to the S0 state is similar for trajectories that exhibit PT and those that do not exhibit PT, suggesting that the majority of the solvent relaxation essential for this decay process occurs prior to PT. The insights gained from this study enhance our mechanistic understanding of the photoinduced PCET process in this hydrogenbonded phenol-amine complex. Furthermore, the underlying physical principles for this specific system are expected to be generally applicable to other photoinduced PCET systems.

Computational Methods The computational details for the MDQT simulations are described elsewhere14 and are only briefly summarized here. A semiempirical implementation of the floating occupation molecular orbital complete active space configuration interaction (FOMO-CASCI) method,20-21 with an active space comprised of six electrons in four orbitals, was used to calculate the electronic states of the phenol-amine complex. This complex was solvated in a sphere of 258 explicit 1,2dichloroethane molecules described by the general AMBER force field with soft restraints on the solvent molecules at the boundary of the sphere. The initial coordinates and velocities for the solute were sampled from quantum harmonic oscillator distributions corresponding to ground state normal modes in the gas phase, and the initial coordinates of the solvent were obtained from molecular dynamics equilibration of the solvent around each of these frozen solute geometries on

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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.14 Following instantaneous excitation to the S1 state, the MDQT surface hopping algorithm15-16 was implemented with the potential energy surface generated on-the-fly using the hybrid quantum mechanical/molecular mechanical semiempirical FOMO-CASCI approach.

Approximately 240 independent surface hopping trajectories were propagated and

subsequently analyzed. To monitor the solvent dynamics, the solvent electrostatic potential at a solute atomic site was calculated using the molecular mechanical force field charges on the solvent atoms, and the solute-solvent electrostatic interaction energy was calculated using the force field charges on the solvent atoms and the quantum mechanical Coulson charges on the solute atoms. The solvent electrostatic potential is lower at the N atom than at the O atom, and protonation of the N atom further lowers the solvent electrostatic potential at N by ~1 V, while the associated deprotonation of the O atom does not lead to a significant change in the solvent electrostatic potential at O (Table S2). Hence, the amine N atom was utilized as a probe for the solvent response in most of the analysis.

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ASSOCIATED CONTENT Supporting Information. Population decay of trajectories photoexcited to the S1 state; additional data illustrating solvent response to electron/proton transfer and the effect of solute geometry on the occurrence of proton transfer on the S1 state; data justifying the comparison of trajectories with and without proton transfer on the S1 state. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This material is based upon work supported by the Air Force Office of Scientific Research under AFOSR Award No. FA9550-14-1-0295. We are grateful to Todd Martinez and James Stewart for providing a modified MOPAC code that includes the semiempirical FOMO-CASCI QM/MM method. We also thank Dr. Alexander V. Soudackov for helpful discussions.

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be problematic due to limitations of this functional for charge transfer states. Moreover, this previous study did not investigate the nonadiabatic dynamics of this system. 18. Roos, B. O.; Taylor, P. R.; Siegbahn, P. E. M., A Complete Active Space SCF Method (CASSCF) Using a Density Matrix Formulated Super-CI Approach. Chem. Phys. 1980, 48, 157173. 19. Leontyev, I.; Stuchebrukhov, A., Accounting for Electronic Polarization in NonPolarizable Force Fields. Phys. Chem. Chem. Phys. 2011, 13, 2613-2626. 20. Granucci, G.; Persico, M.; Toniolo, A., Direct Semiclassical Simulation of Photochemical Processes with Semiempirical Wave Functions. J. Chem. Phys. 2001, 114, 10608-10615. 21. Toniolo, A.; Thompson, A. L.; Martinez, T. J., Excited State Direct Dynamics of Benzene with Reparameterized Multi-Reference Semiempirical Configuration Interaction Methods. Chem. Phys. 2004, 304, 133-145. 22. Gamiz-Hernandez, A. P.; Magomedov, A.; Hummer, G.; Kaila, V. R. I., Linear Energy Relationships in Ground State Proton Transfer and Excited State Proton-Coupled Electron Transfer. J. Phys. Chem. B 2015, 119, 2611-2619.

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Role of Solvent Dynamics in Photoinduced Proton-Coupled Electron

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