Water-Mediated Excited State Proton Transfer of Pyranine-Acetate in Aqueous Solution: Vibrational Fingerprints from Ab Initio Molecular Dynamics

A dual experimental-theoretical perspective on ESPT photoacids and their challenges ahead

Photoacids undergo an increase in acidity upon electronic excitation, enabling excited-state proton transfer (ESPT) reactions. A multitude of compounds that allow ESPT has been identified and integrated in numerous applications, as is outlined by reviewing the rich history of photoacid research reaching back more than 90 years. In particular, achievements together with ambitions and challenges are highlighted from a combined experimental and theoretical perspective. Besides explicating the spectral signatures, transient ion-pair species, and electronic states involved in an ESPT, special emphasis is put on the diversity of methods used for studying photoacids as well as on the effects of the environment on the ESPT, illustrated in detail for 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) and the naphthols as examples of prototypical photoacids. The development of exceptionally acidic super-photoacids and magic photoacids is subsequently discussed, which opens the way to applications even in aprotic solvents and provides additional insight into the mechanisms underlying ESPT. In the overview of highlights from theory, a comprehensive picture of the scope of studies on HPTS is presented, along with the general conceptualization of the electronic structure of photoacids and approaches for the quantification of excited-state acidity. We conclude with a juxtaposition of established applications of photoacids together with potential open questions and prospective research directions.

Temperature Effects and Activation Barriers in Aqueous Proton-Uptake Reactions

Aqueous proton transfer reactions are fundamental in biology and chemistry, yet kinetics and mechanisms of strong base-weak acid reactions are not well understood. In this work, we present a temperature-dependent reaction kinetic study of the water-soluble photobase actinoquinol, in the presence and absence of succinimide, a weak acid reaction partner. We study the temperature dependence of the reaction and connect the observed dynamics to the reaction's thermodynamics. We find that actinoquinol reacts in associated complexes with water/succinimide, creating an intermediate complex that can undergo either dissociation to create products, or reverse proton transfer within the complex to recreate the initial reactants. We find that the intermediates' formation is energetically unfavorable with both reaction partners, which impacts the net reaction rates. We also find that the net reaction rate is additionally strongly influenced by the competition between the dissociation of the intermediates and their reverse reaction.

DFT and ab initio molecular dynamics simulation study of the infrared spectrum of the protic ionic liquid 2-hydroxyethylammonium formate

Protic ionic liquids (PILs) typically show a complex band shape in their infrared (IR) spectra in the high-frequency range due to the hydrogen stretching vibrations of functional groups forming rather strong hydrogen bonds (H-bonds). In the low-frequency range, the intermolecular stretching mode of the H-bond leaves a mark in the far-IR spectrum of PILs. In this study, the IR spectrum of the PIL 2-hydroxyethylammonium formate, [HOCH2CH2NH3][HCOO], is investigated in order to identify the different modes that contribute to the high-frequency band shape, i.e. the cation ν(NH), ν(OH), and ν(CH) modes, and the anion ν(CH) mode, as well as the intermolecular mode of the strongest H-bond in the far-IR spectrum. The assignment is validated by quantum chemistry calculations of clusters at the density functional theory (DFT) level for four ionic pairs and by ab initio molecular dynamics (AIMD) simulations of ten ionic pairs. There is good agreement between the vibrational frequencies obtained from DFT and AIMD simulations for both the high- and low-frequency ranges. Based on the calculations, the strong H-bond interaction between the cation -NH3 group and [HCOO]- gives a broad band envelope associated with the ν(NH) mode in the high-frequency range of the IR spectrum on which there are narrower peaks corresponding to the ν(OH) and ν(CH) modes. In the far-IR (FIR) spectrum, the anions' rattling motion gives a broad feature with a maximum at 160 cm-1, while the H-bond's intermolecular NH⋯O stretching mode appears as a peak at 255 cm-1.

Proton penetration mechanism and selective hydrogen isotope separation through two-dimensional biphenylene

Hydrogen isotope separation is of prime significance in various scientific and industrial applications. Nevertheless, the existing technologies are often expensive and energy demanding. Two-dimensional carbon materials are regarded as promising candidates for cost-effective separation of different hydrogen isotopes. Herein, based on theoretical calculations, we have systematically investigated the proton penetration mechanism and the associated isotope separation behavior through two-dimensional biphenylene, a novel graphene allotrope. The unique non-uniform rings with different sizes in the biphenylene layer resemble the topological defects of graphene, serving as proton transmission channels. We found that a proton can readily pass through biphenylene with a low energy barrier in some specific patterns. Furthermore, large kinetic isotope effect ratios for proton-deuteron (13.58) and proton-triton (53.10) were observed in an aqueous environment. We thus conclude that biphenylene would be a potential carbon material used for hydrogen isotope separation. This subtle exploitation of the natural structural specificity of biphenylene provides new insight into the search for materials for hydrogen isotope separation.

An Expedited Route to Optical and Electronic Properties at Finite Temperature via Unsupervised Learning

Electronic properties and absorption spectra are the grounds to investigate molecular electronic states and their interactions with the environment. Modeling and computations are required for the molecular understanding and design strategies of photo-active materials and sensors. However, the interpretation of such properties demands expensive computations and dealing with the interplay of electronic excited states with the conformational freedom of the chromophores in complex matrices (i.e., solvents, biomolecules, crystals) at finite temperature. Computational protocols combining time dependent density functional theory and ab initio molecular dynamics (MD) have become very powerful in this field, although they require still a large number of computations for a detailed reproduction of electronic properties, such as band shapes. Besides the ongoing research in more traditional computational chemistry fields, data analysis and machine learning methods have been increasingly employed as complementary approaches for efficient data exploration, prediction and model development, starting from the data resulting from MD simulations and electronic structure calculations. In this work, dataset reduction capabilities by unsupervised clustering techniques applied to MD trajectories are proposed and tested for the ab initio modeling of electronic absorption spectra of two challenging case studies: a non-covalent charge-transfer dimer and a ruthenium complex in solution at room temperature. The K-medoids clustering technique is applied and is proven to be able to reduce by ∼100 times the total cost of excited state calculations on an MD sampling with no loss in the accuracy and it also provides an easier understanding of the representative structures (medoids) to be analyzed on the molecular scale.

Observing Aqueous Proton-Uptake Reactions Triggered by Light

Proton-transfer reactions in water are essential to chemistry and biology. Earlier studies reported on aqueous proton-transfer mechanisms by observing light-triggered reactions of strong (photo)acids and weak bases. Similar studies on strong (photo)base-weak acid reactions would also be of interest because earlier theoretical works found evidence for mechanistic differences between aqueous H⁺ and OH^- transfer. In this work, we study the reaction of actinoquinol, a water-soluble strong photobase, with the water solvent and the weak acid succinimide. We find that in aqueous solutions containing succinimide, the proton-transfer reaction proceeds via two parallel and competing reaction channels. In the first channel, actinoquinol extracts a proton from water, after which the newly generated hydroxide ion is scavenged by succinimide. In the second channel, succinimide forms a hydrogen-bonded complex with actinoquinol and the proton is transferred directly. Interestingly, we do not observe proton conduction in water-separated actinoquinol-succinimide complexes, which makes the newly studied strong base-weak acid reaction essentially different from previously studied strong acid-weak base reactions.

pH Jumps in a Protic Ionic Liquid Proceed by Vehicular Proton Transport

The dynamics of excess protons in the protic ionic liquid (PIL) ethylammonium formate (EAF) have been investigated from femtoseconds to microseconds using visible pump mid-infrared probe spectroscopy. The pH jump following the visible photoexcitation of a photoacid (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt, HPTS) results in proton transfer to the formate of the EAF. The proton transfer predominantly (∼70%) occurs over picoseconds through a preformed hydrogen-bonded tight complex between HPTS and EAF. We investigate the longer-range and longer-time-scale proton-transport processes in the PIL by obtaining the ground-state conjugate base (RO^-) dynamics from the congested transient-infrared spectra. The spectral kinetics indicate that the protons diffuse only a few solvent shells from the parent photoacid before recombining with RO^-. A kinetic isotope effect of nearly unity (k_H/k_D ≈ 1) suggests vehicular transfer and the transport of excess protons in this PIL. Our findings provide comprehensive insight into the complete photoprotolytic cycle of excess protons in a PIL.

Theoretical Study on the Photoacidity of Hydroxypyrene Derivatives in DMSO Using ADC(2) and CC2

This work applies the thermodynamic Förster cycle to theoretically investigate the pK_a^*, i.e., excited-state pK_a values of pyranine-derived superphotoacids developed by Jung and co-workers. The latter photoacids are strong enough to transfer a proton to the aprotic solvent dimethyl sulfoxide (DMSO). The Förster cycle provides access to pK_a^* via the ground-state pK_a and the electronic excitation energies. We use the conductor-like screening model for real solvents (COSMO-RS) to compute the ground-state pK_a and the correlated wavefunction-based methods ADC(2) and CC2 with the continuum solvation model COSMO to calculate the pK_a change upon excitation. A comparison of the calculated UV/Vis absorption and fluorescence emission energies to the experimental results leads us to infer that this approach allows for a proper description of the electronic excitations. In particular, implicit solvation by means of the COSMO model appears to be sufficient for the treatment of these photoacids in DMSO. The calculations confirm the presumption that a charge redistribution from the hydroxy group to the aromatic ring and the electron-withdrawing substituents is the origin of photoacidity for these photoacids. Moreover, the calculations with the continuum solvation model predict that the pK_a jump upon excitation decreases with increasing solvent polarity, as rationalized based on the Förster cycle.

Free Energy Profiles of Proton Transfer Reactions: Density Functional Benchmark from Biased Ab Initio Dynamics

By coupling an enhanced sampling algorithm with an orbital-localized variant of Car-Parrinello molecular dynamics, the so-called atomic centered density matrix propagation model, we reconstruct the free energy profiles along reaction pathways using different density functional approximations (DFAs) ranging from locals to hybrids. In particular, we compare the computed free energy barrier height of proton transfer (PT) reactions to those obtained by a more traditional static approach, based on the intrinsic reaction coordinate (IRC), for two case systems, namely malonaldehyde and formic acid dimer. The obtained results show that both the IRC profiles and the potentials of mean force, derived from biased dynamic trajectories, are very sensitive to the density functional approximation applied. More precisely, we observe that, with the notable exception of M06-L, local density functionals always strongly underestimate the reaction barrier heights. More generally, we find that also the shape of the free energy profile is very sensitive to the density functional choice, thus highlighting the effect, often neglected, that the choice of DFA has also in the case of dynamics simulations.

Computational elucidations on the role of vibrations in energy transfer processes of photosynthetic complexes

Coupling between pigment excitations and nuclear movements in photosynthetic complexes is known to modulate the excitation energy transfer (EET) efficiencies. Toward providing microscopic information, researchers often apply simulation techniques and investigate how vibrations are involved in EET processes. Here, reports on such roles of nuclear movements are discussed from a theory perspective. While vibrations naturally present random thermal fluctuations that can affect energy transferring characteristics, they can also be intertwined with exciton structures and create more specific non-adiabatic energy transfer pathways. For reliable simulations, a bath model that accurately mimics a given molecular system is required. Methods for obtaining such a model in combination with quantum chemical electronic structure calculations and molecular dynamics trajectory simulations are discussed. Various quantum dynamics simulation tools that can handle pigment-to-pigment energy transfers together with their vibrational characters are also touched on. Behaviors of molecular vibrations often deviate from ideality, especially when all-atom details are included, which practically forces us to treat them classically. We conclude this perspective by considering some recent reports that suggest that classical descriptions of bath effects with all-atom details may still produce valuable information for analyzing sophisticated contributions by vibrations to EET processes.

Proton Transfer from a Photoacid to a Water Wire: First Principles Simulations and Fast Fluorescence Spectroscopy

Proton transfer reactions are ubiquitous in chemistry, especially in aqueous solutions. We investigate photoinduced proton transfer between the photoacid 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) and water using fast fluorescence spectroscopy and ab initio molecular dynamics simulations. Photoexcitation causes rapid proton release from the HPTS hydroxyl. Previous experiments on HPTS/water described the progress from photoexcitation to proton diffusion using kinetic equations with two time constants. The shortest time constant has been interpreted as protonated and photoexcited HPTS evolving into an "associated" state, where the proton is "shared" between the HPTS hydroxyl and an originally hydrogen bonded water. The longer time constant has been interpreted as indicating evolution to a "solvent separated" state where the shared proton undergoes long distance diffusion. In this work, we refine the previous experimental results using very pure HPTS. We then use excited state ab initio molecular dynamics to elucidate the detailed molecular mechanism of aqueous excited state proton transfer in HPTS. We find that the initial excitation results in rapid rearrangement of water, forming a strong hydrogen bonded network (a "water wire") around HPTS. HPTS then deprotonates in ≤3 ps, resulting in a proton that migrates back and forth along the wire before localizing on a single water molecule. We find a near linear relationship between the emission wavelength and proton-HPTS distance over the simulated time scale, suggesting that the emission wavelength can be used as a ruler for the proton distance. Our simulations reveal that the "associated" state corresponds to a water wire with a mobile proton and that the diffusion of the proton away from this water wire (to a generalized "solvent-separated" state) corresponds to the longest experimental time constant.

Structural Origin and Vibrational Fingerprints of the Ultrafast Excited State Proton Transfer of the Pyranine-Acetate Complex in Aqueous Solution

The excited state proton transfer (ESPT) reaction from the photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS or pyranine) to an acetate molecule has been investigated in explicit aqueous solution via excited state ab initio molecular dynamics simulations based on hybrid quantum/molecular mechanics (QM/MM) potentials. In all the trajectories, the direct proton transfer has been observed in the excited state within 1 ps. We find that the initial structural configuration extracted from the ground state distribution strongly affects the ESPT kinetics. Indeed, the relative orientation of the proton donor-acceptor pair and the presence of a water molecule hydrogen bonded to the phenolic acid group of the pyranine are the key factors to facilitate the ESPT. Furthermore, we analyze the vibrational fingerprints of the ESPT reaction, reproducing the blue shift of the acetate CO stretching (CO_ac), from 1666 to 1763 cm^-1 testifying the transformation of acetate to acetic acid. Finally, our findings suggest that the acetate CC stretching (CC_ac) is also sensitive to the progress of the ESPT reaction. The CC_ac stretching is indeed ruled by the two vibrational modes (928 and 1426 cm^-1), that in the excited state are alternately activated when the proton is shared or bound to the donor/acceptor, respectively.