Nonadiabatic dynamics of photoinduced proton-coupled electron transfer: comparison of explicit and implicit solvent simulations

Surface hopping modeling of charge and energy transfer in active environments

An active environment is any atomic or molecular system changing a chromophore's nonadiabatic dynamics compared to the isolated molecule. The action of the environment on the chromophore occurs by changing the potential energy landscape and triggering new energy and charge flows unavailable in the vacuum. Surface hopping is a mixed quantum-classical approach whose extreme flexibility has made it the primary platform for implementing novel methodologies to investigate the nonadiabatic dynamics of a chromophore in active environments. This Perspective paper surveys the latest developments in the field, focusing on charge and energy transfer processes.

Ultrafast excited state dynamics of pyridine N-oxide derivative in solution; femtosecond fluorescence up-conversion and theoretical calculations

In this study we have investigated 2-ethylamino-4-nitro-6-methyl pyridine N-oxide (2E6M) molecule that belongs to important group of Proton Coupled Electron Transfer (PCET) compounds where both the charge transfer (CT) and proton transfer processes in excited states may proceed. In this case, this is possible due to the donors and acceptors of electrons and protons in this system, as well as due to the presence of intramolecular {N-H… O [2,566(3) Å}, hydrogen bond.Using stationary and time-resolved spectroscopy, as well as quantum chemical calculations on the DFT and TD DFT B3LYP/6-31G (d,p) level of theory, a partial CT nature of the S₀ → S₁ transition in both tautomeric forms (N and T) has been revealed. Additionally, the excited state intramolecular proton transfer (ESIPT) process shown to be more favorable in apolar and weakly polar solvents than in strongly polar acetonitrile (E_N(S1) > E_T(S1). The displacement of charge from the amine group and the ring to the nitro group has been observed on the changing shapes of the HOMO and LUMO orbitals involved in this transition what further quantitatively allowed to realize the increase in the dipole moment of both forms in the electronic excited state. The calculations show that in two solvents with radically different polarity (heptane, acetonitrile), dipole moments of both forms are very similar [in acetonitrile u_N(S₁) and u_T(S₁) are 11.0 D and 11.5 D, respectively]. Hence, in polar media both forms can be stabilized in a comparable manner. This made it difficult for us to assign a single fluorescent band in acetonitrile to one of the tautomeric forms. However, it seems that due to application of time-resolved spectroscopy, this problem has been clarified. The TCSPC decay curve in acetonitrile with an ultrafast lifetime assigned to the (N) form, along with the femtosecond up-conversion signals that demonstrated only an ultrafast decay without any rise-time of a new excited (T) species, allowed us to conclude that in 2E6M in strongly polar solvent the ESIPT does not occur.The unique fluorescence band origins from the (N) form. In protic solvents, the significant kinetic isotopic effects have provided us with conclusive evidence for the presence of the solvent-assisted ESIPT process. Furthermore, it was noticed that the fluorescence lifetime in D₂O (100-120 fs) estimated from the up-conversion signals is about 40 times shorter relative to methanol. This may suggest that the sine qua non for the ESIPT process in 2E6M in protic solvents is the formation of a complex with a solvent molecule in the hydrogen bridge between the proton donor and proton acceptor, respectively.

Electron-Induced Proton Transfer

The pathway of activationless proton transfer induced by an electron-transfer reaction is studied theoretically. Long-range electron transfer produces highly nonequilibrium medium polarization that can drive proton transfer through an activationless transition during the process of thermalization, dynamically altering the screening of the electron-proton Coulomb interaction by the medium. The cross electron-proton reorganization energy is the main energy parameter of the theory, which exceeds in magnitude the proton-transfer reorganization energy roughly by the ratio of the electron-transfer to proton-transfer distance. This parameter, which can be either positive or negative, is related to the difference in pK_a values in two electron-transfer states. The relaxation time of the medium is on the (sub)picosecond time scale, which establishes the characteristic time for activationless proton transfer. Microscopic calculations predict substantial retardation of the collective relaxation dynamics compared to the continuum estimates due to the phenomenology analogous to de Gennes narrowing. Nonequilibrium medium configuration promoting proton transfer can be induced by either thermal or photoinduced charge transfer.

Quantum and semiclassical dynamical studies of nonadiabatic processes in solution: achievements and perspectives

We concisely review the main methodological approaches to model nonadiabatic dynamics in isotropic solutions and their applications. Three general classes of models are identified as the most used to include solvent effects in the simulations. The first model describes the solvent as a set of harmonic collective modes coupled to the solute degrees of freedom, and the second as a continuum, while the third explicitly includes solvent molecules in the calculations. The issues related to the use of these models in semiclassical and quantum dynamical simulations are discussed, as well as the main limitations and perspectives of each approach.

Benchmarking the Surface Hopping Method to Include Nuclear Quantum Effects

We have benchmarked the surface hopping method to capture nuclear quantum effects in the spin-Boson model in the deep tunneling regime. The thermal populations and the rate constants calculated using the surface hopping method are compared with those calculated using Boltzmann theory and Fermi's golden rule, respectively. Additionally, we have proposed a simple kinetic model that partially includes nuclear quantum effects within Marcus theory, and the results of the surface hopping method are analyzed under the framework of this simple kinetic model. A broad range of parameters are investigated to identify the regimes for the successes and failures of the surface hopping method. This work shows that with the accurate treatment of decoherence and velocity reversal, surface hopping can generally capture the nuclear quantum effects in the deep tunneling and weak diabatic coupling regime.

A solvent–solute cooperative mechanism for symmetry-breaking charge transfer

Random rotational fluctuation of solvents generates a large transient electric field that breaks the symmetry required for symmetry breaking charge transfer.

Understanding light-driven H2 evolution through the electronic tuning of aminopyridine cobalt complexes

Electronic effects provide a general mechanistic scenario for rationalizing photocatalytic water reduction activity with aminopyridine cobalt complexes.

A new family of cobalt complexes with the general formula [Co^II(OTf)₂(^Y,XPy^Metacn)] (1^R, ^Y,XPy^Metacn = 1-[(4-X-3,5-Y-2-pyridyl)methyl]-4,7-dimethyl-1,4,7-triazacyclononane, (X = CN (1^CN), CO₂Et (1^CO2Et), Cl (1^Cl), H (1^H), NMe₂ (1^NMe2)) where (Y = H, and X = OMe when Y = Me (1^DMM)) is reported. We found that the electronic tuning of the ^Y,XPy^Metacn ligand not only has an impact on the electronic and structural properties of the metal center, but also allows for a systematic water-reduction-catalytic control. In particular, the increase of the electron-withdrawing character of the pyridine moiety promotes a 20-fold enhancement of the catalytic outcome. By UV-Vis spectroscopy, luminescence quenching studies and Transient Absorption Spectroscopy (TAS), we have studied the direct reaction of the photogenerated [Ir^III(ppy)₂(bpy˙^–)] (PS_Ir) species to form the elusive Co^I intermediates. In particular, our attention is focused on the effect of the ligand architecture in this elemental step of the catalytic mechanism. Finally, kinetic isotopic experiments together with DFT calculations provide complementary information about the rate-determining step of the catalytic cycle.

Photofragmentation of Tetranitromethane: Spin-Unrestricted Time-Dependent Excited-State Molecular Dynamics

In this study, the photofragmentation dynamics of tetranitromethane (TNM) is explored by a spin-unrestricted time-dependent excited-state molecular dynamics (u-TDESMD) algorithm based on Rabi oscillations and principles similar to trajectory surface hopping, with a midintensity field approximation. The leading order process is represented by the molecule undergoing cyclic excitations and de-excitations. During excitation cycles, the nuclear kinetic energy is accumulated to overcome the dissociation barriers in the reactant and a sequence of intermediates. The dissociation pathway includes the ejection of NO₂ groups followed by the formation of NO and CO. The simulated mass spectra at the ab initio level, based on the bond length in possible fragments, are extracted from simulation trajectories. The recently developed methodology has the potential to model and monitor photoreactions with open-shell intermediates and radicals.

Proton-coupled electron transfer in the electrocatalysis of CO2 reduction: prediction of sequential vs. concerted pathways using DFT

Herein we investigate computationally in detail the mechanism of the formation of the carboxylate adduct during the electroreduction of CO₂ in water catalysed by cobalt porphyrin complexes. Specifically, we address qualitatively the competition between the concerted and sequential pathways for the proton-coupled electron transfer. We use a simple methodology for accurate computation of the pK_a of the neutral and anionic carboxylate intermediates, [CoP-COOH] and [CoP-COOH]^- (where CoP is a cobalt porphine complex), based on the isodesmic proton-exchange reaction scheme. The predicted values are used as in input for a theoretical model that describes the transition between the sequential and concerted pathways. The activation of the sequential pathway (ET-PT) that leads to the formation of the neutral [CoP-COOH] intermediate at pH ≈ 3.5 (pK_a[CoP-COOH] = 3.5 ± 0.4), as predicted by the calculations, is in good agreement with the drastic increase in the faradaic efficiency of the CO₂ reduction reaction towards CO at pH = 3 compared to pH = 1, as experimentally observed. This confirms the existence of the CO₂ anionic adduct [CoP-CO₂]^- as a viable intermediate at pH = 3 and its crucial role for the pH dependence of the faradaic efficiency for the CO₂ reduction. The analysis also shows that when the pH is significantly higher than the pK_a of the neutral carboxylate adduct, the CO₂ reduction has to go through an alternative pathway with the formation of the anionic carboxylate intermediate [CoP-COOH]^-. It is formed through a concerted proton-electron transfer step from the anionic CO₂ adduct [CoP-CO₂]^- when the pH is below ∼8.6 (pK_a[CoP-COOH]^- = 8.6 ± 0.4). At pH ≈ 8.6 and above, another decoupled ET-PT is predicted to take place, leading to the formation of a dianionic CO₂ adduct [CoP-CO₂]^2-.

Proton-Coupled Electron Transfer: Moving Together and Charging Forward

Proton-coupled electron transfer (PCET) is ubiquitous throughout chemistry and biology. This Perspective discusses recent advances and current challenges in the field of PCET, with an emphasis on the role of theory and computation. The fundamental theoretical concepts are summarized, and expressions for rate constants and kinetic isotope effects are provided. Computational methods for calculating reduction potentials and pKa's for molecular electrocatalysts, as well as insights into linear correlations and non-innocent ligands, are also described. In addition, computational methods for simulating the nonadiabatic dynamics of photoexcited PCET are discussed. Representative applications to PCET in solution, proteins, electrochemistry, and photoinduced processes are presented, highlighting the interplay between theoretical and experimental studies. The current challenges and suggested future directions are outlined for each type of application, concluding with an overall view to the future.

Excited State Dynamics of Ru10 Cluster Interfacing Anatase TiO2(101) Surface and Liquid Water

Charge transfer dynamics at the interface of supported metal nanocluster and liquid water by GGA+U calculations combined with density matrix formalism is considered. The Ru10 cluster introduces new states into the band gap of TiO2 surface, narrows the band gap of TiO2, and enhances the absorption strength. The H2O adsorption significantly enhances the intensity of photon absorption, which is due to the formation of Ti-O(water) and Ru-O(water) bonds at the interfaces. The Ru10 cluster promotes the dissociation of water, facilitates charge transfer, and increases the relaxation rates of holes and electrons. We expect that our results are helpful in understanding basic processes contributing to photoelectrochemical water splitting.

Theory of the transition from sequential to concerted electrochemical proton-electron transfer

A theory for the calculation of potential energy surfaces of electrochemical proton-coupled electron transfer is considered and parameterized on the basis of thermodynamic relations. The paper discusses the qualitatively different potential energy surfaces predicted by the theory, and their relation to the existence of sequential and concerted proton-electron transfer pathways. The concomitant activation energies for sequential and concerted PET are calculated. The applied overpotential may change the qualitative shape of the PES and therefore the mechanism of the proton-coupled electron transfer reaction.

Photoinduced proton-coupled electron transfer of hydrogen-bonded p-nitrophenylphenol-methylamine complex in solution

Proton-coupled electron transfer can occur through concerted (electron-proton transfer, EPT) or sequential mechanisms, but this distinction becomes less well-defined for photoinduced reactions. These issues have been examined with transient absorption experiments on a hydrogen-bonded complex consisting of p-nitrophenylphenol and t-butylamine. These experiments revealed two spectroscopically distinct states: the higher-energy excited state was interpreted to be a conventional intramolecular charge transfer (ICT) state within the p-nitrophenylphenol, whereas the lower-energy state was interpreted to be an ICT-EPT state, where photoexcitation resulted in both ICT and the shifting of electronic density corresponding to effective proton transfer from the phenol to the amine. In the present work, the singlet excited states of the hydrogen-bonded p-nitrophenylphenol-methylamine complex in 1,2-dichloroethane are studied with time-dependent density functional theory and higher-level ab initio methods. The calculations suggest that the ππ* state, which is the S(1) state at the Franck-Condon geometry, corresponds to the state denoted ICT-EPT in the experimental analysis, whereas the nπ* state, which is the S(2) state at this geometry, likely corresponds to the state denoted ICT in the experimental analysis. According to the calculations, the ππ* state has charge-transfer character, as well as a change in electronic density on the amine, with the minimum-energy structure corresponding to the proton bonded to the nitrogen acceptor, consistent with proton transfer. The nπ* state has little charge-transfer character, as well as negligible change in electronic density on the amine, with the minimum-energy structure corresponding to the proton bonded to the oxygen donor. The calculations also provide evidence of an avoided crossing between these two states located energetically close to the Franck-Condon point. These calculations provide the foundation for future nonadiabatic molecular dynamics studies of the relaxation process.