Reversible Proton Transfer Research Articles

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.

Hydroxyl-assisted globally spontaneous dynamic oxygen migration on biphenylene

Dynamic covalent interfaces (DCIs) can autonomously adjust their internal structures and dynamically respond to changes in the external environment, thus holding broad application prospects in drug delivery, molecular detection, and interface catalysis. However, the dynamic properties of such DCIs greatly rely on the localized exchanges or formations of chemical bonds, seriously constraining the adaptive response efficiency. To transcend the limitations of the dynamic two-dimensional (2D) scales, employing the experimentally synthesized biphenylene monolayer as a prototype, we systematically study the hydroxyl-assisted globally spontaneous dynamic oxygen migration on biphenylene interface based on the first-principles calculations and machine-learning-based molecular dynamics (MLMD) simulations. The calculated results indicate that the stable dangling oxygen atoms on the biphenylene monolayer can attract the proton in the interfacial water, which in turn undergoes a water dissociation process and produces hydroxyl groups on the biphenylene interface. Assisted by the hydroxyl groups, the dangling oxygen atoms can achieve long-range migration, exhibiting notably low energy barriers (0.09 eV, 0.04 eV) due to the reversible proton transfer reactions. Therefore, the local dynamic covalent behaviors of oxygen atoms on the biphenylene interface are broken with the oxygen atoms migrating between the adjacent C4 rings. More importantly, the charge density difference and Bader charge analysis demonstrate that the presence of hydroxyl groups significantly enhance the stability of the dangling oxygen, effectively reducing energy barriers of oxygen migration compared to epoxy on the biphenylene interface. Furthermore, the MLMD trajectories visually show the process of realizing large-area DCIs on the biphenylene monolayer. Our work not only clarifies the oxygen-related dynamic behavior of the biphenylene interface, but also provides new ideas for the realization of large-area dynamic covalent materials (DCMs) on other carbon-based or 2D materials.

Mechanistic insights into multimetal synergistic and electronic effects in a hexanuclear iron catalyst with a [Fe3(μ3-O)(μ2-OH)]2 core for enhanced water oxidation.

Multinuclear molecular catalysts mimicking natural photosynthesis have been shown to facilitate water oxidation; however, such catalysts typically operate in organic solutions, require high overpotentials and have unclear catalytic mechanisms. Herein, a bio-inspired hexanuclear iron(III) complex I, Fe6(μ3-O)2(μ2-OH)2(bipyalk)2(OAc)8 (H2bipyalk = 2,2'-([2,2'-bipyridine]-6,6'-diyl)bis(propan-2-ol); OAc = acetate) with desirable water solubility and stability was designed and used for water oxidation. Our results showed that I has high efficiency for water oxidation via the water nucleophilic attack (WNA) pathway with an overpotential of only ca. 290 mV in a phosphate buffer of pH 2. Importantly, key high-oxidation-state metal-oxo intermediates formed during water oxidation were identified by in situ spectroelectrochemistry and oxygen atom transfer reactions. Theoretical calculations further supported the above identification. Reversible proton transfer and charge redistribution during water oxidation enhanced the electron and proton transfer ability and improved the reactivity of I. Here, we have shown the multimetal synergistic and electronic effects of catalysts in water oxidation reactions, which may contribute to the understanding and design of more advanced molecular catalysts.

The role of an intramolecular hydrogen bond in the redox properties of carboxylic acid naphthoquinones.

A bioinspired naphthoquinone model of the quinones in photosynthetic reaction centers but bearing an intramolecular hydrogen-bonded carboxylic acid has been synthesized and characterized electrochemically, spectroscopically, and computationally to provide mechanistic insight into the role of proton-coupled electron transfer (PCET) of quinone reduction in photosynthesis. The reduction potential of this construct is 370 mV more positive than the unsubstituted naphthoquinone. In addition to the reversible cyclic voltammetry, infrared spectroelectrochemistry confirms that the naphthoquinone/naphthoquinone radical anion couple is fully reversible. Calculated redox potentials agree with the experimental trends arising from the intramolecular hydrogen bond. Molecular electrostatic potentials illustrate the reversible proton transfer driving forces, and analysis of the computed vibrational spectra supports the possibility of a combination of electron transfer and PCET processes. The significance of PCET, reversibility, and redox potential management relevant to the design of artificial photosynthetic assemblies involving PCET processes is discussed.

CASPT2//CASSCF studies on mechanistic photophysics of 3-hydroxyflavone

3-Hydroxyflavone (3-HF) as the backbone of flavonols has been extensively studied on the dual fluorescence and excited-state intramolecular proton transfer (ESIPT) after photoexcitation. However, its detailed excited-state relaxation mechanism is still unclear. Herein, the minimum-energy structures, conical intersections and singlet–triplet crossing points of 3-HF in the 1ππ∗, 1nπ∗, 3ππ∗, 3nπ∗, and S0 states in the gas phase were first optimized by the CASPT2//CASSCF method. Then, the minimum-energy proton transfer (MEPT) and excited-state decay (ESD) pathways were calculated as well. Based on the calculation results, we proposed several possible ESD pathways from the initially populated bright S2(ππ*) state. The major channel is singlet-involved and stretch-torsion coupled ESIPT pathway. 3-HF in normal (N) form first undergoes efficient 1ππ* ESIPT with a small barrier of 2.2 kcal/mol to yield the 1ππ* proton-transferred tautomer (T). Subsequently, the T tautomer approaches the nearby accessible S1/S0 conical intersection and then occurs internal conversion (IC) to fall back to the S0 state. Finally, the S0 species quickly transfers into the N conformer via the reverse ground state intramolecular proton transfer (GSIPT) to realize the ground-state recovery. In addition, the minor ESD pathways are involved less efficient intersystem crossing (ISC) processes. The present work could provide insights into understanding the photophysics of flavonols and their derivatives.

Reversible Intramolecular Proton Transfer in Curcumin Crystals and Nonlinear Size Correlation

Intramolecular proton transfer is more difficult for curcumin in the solid state than in the solution. In this study, a reversible transformation of keto-enol to the β-diketone form of curcumin was achieved by nonchemical methods. The obstacle to intramolecular proton transfer was shown to be the intermolecular hydrogen bond at the keto-enol linkage. When the intermolecular hydrogen bond in the keto-enol site of the curcumin molecule is broken, reversible proton transfer will occur more easily. The energy fluctuations and vibrational shifts in the functional group spectra were captured during the proton transfer and conformational rearrangement with increasing temperature (from 170 to 183 °C) after desolvation. The maximum energy change in the proton transfer process of curcumin occurs in the 200 μm crystal (extreme value point), influenced by the combination of the specific surface area of the crystal itself and the degree of thermal homogeneity. This paper presents an environmentally friendly and efficient method to regulate the physicochemical properties of solid organic materials.

Weak, Broken, but Working-Intramolecular Hydrogen Bond in 2,2'-bipyridine.

From an academic and practical point of view, it is desirable to be able to assess the possibility of the proton exchange of a given molecular system just by knowing the positions of the proton acceptor and the proton donor. This study addresses the difference between intramolecular hydrogen bonds in 2,2'-bipyridinium and 1,10-phenanthrolinium. Solid-state 15N NMR and model calculations show that these hydrogen bonds are weak; their energies are 25 kJ/mol and 15 kJ/mol, respectively. Neither these hydrogen bonds nor N-H stretches can be responsible for the fast reversible proton transfer observed for 2,2'-bipyridinium in a polar solvent down to 115 K. This process must have been caused by an external force, which was a fluctuating electric field present in the solution. However, these hydrogen bonds are the grain that tips the scales precisely because they are an integral part of a large system of interactions, including both intramolecular interactions and environmental influence.

Cations impact radical reaction dynamics in concentrated multicomponent aqueous solutions.

Ultraviolet (UV) photolysis of nitrite ions (NO2-) in aqueous solutions produces a suite of radicals, viz., NO·, O-, ·OH, and ·NO2. The O- and NO· radicals are initially formed from the dissociation of photoexcited NO2-. The O- radical undergoes reversible proton transfer with water to generate ·OH. Both ·OH and O- oxidize the NO2- to ·NO2 radicals. The reactions of ·OH occur at solution diffusion limits, which are influenced by the nature of the dissolved cations and anions. Here, we systematically varied the alkali metal cation, spanning the range from strongly to weakly hydrating ions, and measured the production of NO·, ·OH, and ·NO2 radicals during UV photolysis of alkaline nitrite solutions using electron paramagnetic resonance spectroscopy with nitromethane spin trapping. Comparing the data for the different alkali cations revealed that the nature of the cation had a significant effect on production of all three radical species. Radical production was inhibited in solutions with high charge density cations, e.g., lithium, and promoted in solutions containing low charge density cations, e.g., cesium. Through complementary investigations with multinuclear single pulse direct excitation nuclear magnetic resonance (NMR) spectroscopy and pulsed field gradient NMR diffusometry, cation-controlled solution structures and extent of NO2- solvation were determined to alter the initial yields of ·NO and ·OH radicals as well as alter the reactivity of NO2- toward ·OH, impacting the production of ·NO2. The implications of these results for the retrieval and processing of low-water, highly alkaline solutions that comprise legacy radioactive waste are discussed.

Unraveling the dependence of proton transfer on solvent polarity in ion pairs of carbamates and dithiocarbamates with nitrogen‐based counterions

AbstractSmall, but important, differences in the structure–property relationships between ionomers composed of amidinium or imidazolinium groups with alkylcarbamate or alkyldithiocarbamate counterions have been examined experimentally by us previously. To unravel the sources of these differences, DFT calculations are conducted here for ion‐pair complexes (IPs) of these systems and their corresponding uncharged base and acid components (NPs). Calculations include IPs and NPs in which the amidine/amidinium and imidazoline/imidazolinium groups are anchored to a dimethylsiloxane pentamer. A surprising dependence of proton transfer on the dielectric constant (ε) of the medium is found for the systems: Whereas interconversion between the NPs and IPs is strongly dependent on medium dielectric in systems with an alkylcarbamate, none of the initial IP forms with an alkyldithiocarbamate transforms to an NP. Although the calculations do not include individual solvent–solute molecular interactions, they do probe how the components sense their bulk environments. The lack of detectable reversible proton transfer from the amidinium or imidazolinium cations to the dithiocarbamates is consistent with the “principle of proton affinity/pKa equivalence”: Because the acidity of dithiocarbamates is higher than that of carbamates, the gap between their proton affinities (ΔPA) is decreased, favoring stronger electrostatic‐based H‐bonds in the ion‐pair complexes of the dithiocarbamates. The differences can also be estimated from the Deuri–Phukan nucleophilicity index scale, which is based on DFT calculations and suggests a strong dependence of proton transfer on the nucleophilicity of the base and the dielectric constant of the solvent. Predictions from these calculations on some important experimental systems are mentioned.

Compatibilization of Polymer Blends by Ionic Bonding

Chemically dissimilar polymers are rarely miscible due to a specific entropy of mixing that scales as 1/N, where N is the degree of polymerization. As a consequence, the compatibilization of immiscible polymer blends presents a major challenge to plastics recycling efforts. We demonstrate that when the chain ends of two highly immiscible polymers are functionalized with reacting acid and base groups, ionic junctions form in situ by proton transfer, leading to the stabilization of interfaces and the suppression of macroscopic phase separation. Due to the dynamic nature of proton transfer, the microstructure can be tuned via ionic bond energy (h) and segregation strength (χN). In this work, a library of ionically end-functionalized polystyrene (PS) and polydimethylsiloxane (PDMS) is used to demonstrate the importance of end-group acidity/basicity and molecular weight on compatibilization. Well-ordered lamellae result from blending polymers with strong acid and base units, while less ordered, more swollen microphases occur in systems with weak acid and base end groups or larger molar mass. Heating the ionic blends results in reverse proton transfer and destruction of the close-contact ion pairs. As a result, domains are swollen with the resulting unbonded homopolymers, which in sufficient amounts, leads to macrophase separation. Moreover, external salt ions screen the electrostatic attractions and disturb ionic compatibilization. The self-consistent field theory (SCFT) of this system reveals that the ionic interactions can be tuned to create a broad range of morphologies, from microphases to macrophases.

Switching the ESIPT and TICT process of DP-HPPI via intermolecular hydrogen bonding

Intermolecular hydrogen bonding prominently figures in numerous photophysical and photochemical processes. It is worth discussing whether it can act as a switch to effectively convert excited-state intramolecular proton transfer (ESIPT) and twisted intramolecular charge transfer (TICT) process. In this paper, the effect of intermolecular hydrogen bonding on the ESIPT and TICT process of DP-HPPI in DCM and ACN solvent has been explored by the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The analytical results show that DP-HPPI experiences the ESIPT process via intramolecular hydrogen bonding in DCM. However, the ESIPT process is prohibited due to the involvement of intermolecular hydrogen bonding in ACN, triggering the TICT process. From the study results of photophysical properties in DP-HPPI, we reveal that DP-HPPI undergoes reversible ESIPT in DCM. Moreover, we elucidate the detailed proton transfer pathway and the advantage of reverse proton transfer over forward proton transfer. Notably, an unobstructed TICT process occurs in the S1 state after light irradiation of DP-HPPI in ACN. Our work successfully closes the ESIPT process and initiates the TICT process via intermolecular hydrogen bonding, and provides new insights into the design and synthesis of efficient optical devices.

Fine Tuning the Energetics of 2-(2′-Hydroxyphenyl)oxazoles to Obtain Highly Efficient Organic White-Light-Emitting Devices

By using the strategy of fine-tuning the energetics to explore single-molecule white materials with reversible excited-state intramolecular proton transfer (ESIPT), compound 4′-(di-p-tolylamino)-3-(5-(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)oxazol-2-yl)-5-methyl-[1,1′-biphenyl]-4-ol (2a) has been synthesized and then fabricated as organic white-light-emitting devices. The rhodium-catalyzed C–H activation reaction offers an effective method to construct the white-emitting molecules. The 2a-based devices display white electroluminescence with white region CIE coordinates, which is the first reported organic white-light-emitting device (WOLED) based on the single-molecule 2-(2′-hydroxyphenyl)oxazoles with a reversible ESIPT equilibrium. The external quantum efficiency (EQE) of the white-light device reached 6.3%, achieving the highest EQE in the single-molecular cold WOLEDs using an ESIPT material as the emitter. Furthermore, the hybridized local charge transfer (HLCT)-state feature of 2a has been confirmed by a fluorescence test and density functional theory (DFT) calculations. The result shows the first example of single-molecule cold WOLEDs with HLCT material.

Theoretical study the mechanism of ESIPT process for pyridine-hydrazone-substituted naphthalimide receptor 4a-E

The mechanism of excited-state intramolecular proton transfer in (E)-N'-(2-butyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)picolinohydrazonoyl cyanide (4a-E) was studied using time-dependent density functional theory. The calculative results promulgate that the proton transfer of the 4a-E molecule in S1 state can realize, producing red fluorescence. The change of intramolecular hydrogen bonds, infrared spectra, and non-covalent interactions upon photoexcited process can influence hydrogen bond interaction of the 4a-E molecule, and then effectively assist proton transfer process in S1 state. The structured potential energy curves were put out by varying N H bond lengths, which show 4a-E molecule can undergo intramolecular proton transfer in the S1 state, overcoming the energy barrier (3.76 kcal/mol). Additionally, 4a-E is required to cross the energy barrier of 2.91 kcal /mol for the reverse proton transfer, helping 4a-E recover to the stable. We hope this study can initiate the synthesis and exploit new-type molecules.

Direct Observation of Ultrafast Proton Rocking in the BLUF Domain

AbstractWe present direct observation of ultrafast proton rocking in the central motif of a BLUF domain protein scaffold. The mutant design has taken consideration of modulating the proton‐coupled electron transfer (PCET) driving forces by replacing Tyr in the original motif with Trp, in order to remove the interference of a competing electron transfer pathway. Using femtosecond pump–probe spectroscopy and detailed kinetics analysis, we resolved an electron‐transfer‐coupled Grotthuss‐type forward and reverse proton rocking along the FMN–Gln–Trp proton relay chain. The rates of forward and reverse proton transfer are determined to be very close, namely 51 ps vs. 52 ps. The kinetic isotope effect (KIE) constants associated with the forward and reverse proton transfer are 3.9 and 5.3, respectively. The observation of ultrafast proton rocking is not only a crucial step towards revealing the nature of proton relay in the BLUF domain, but also provides a new paradigm of proton transfer in proteins for theoretical investigations.

Direct Observation of Ultrafast Proton Rocking in the BLUF Domain

We present direct observation of ultrafast proton rocking in the central motif of a BLUF domain protein scaffold. The mutant design has taken consideration of modulating the proton-coupled electron transfer (PCET) driving forces by replacing Tyr in the original motif with Trp, in order to remove the interference of a competing electron transfer pathway. Using femtosecond pump-probe spectroscopy and detailed kinetics analysis, we resolved an electron-transfer-coupled Grotthuss-type forward and reverse proton rocking along the FMN-Gln-Trp proton relay chain. The rates of forward and reverse proton transfer are determined to be very close, namely 51 ps vs. 52 ps. The kinetic isotope effect (KIE) constants associated with the forward and reverse proton transfer are 3.9 and 5.3, respectively. The observation of ultrafast proton rocking is not only a crucial step towards revealing the nature of proton relay in the BLUF domain, but also provides a new paradigm of proton transfer in proteins for theoretical investigations.

Quantum mechanics/molecular mechanics studies on the mechanistic photophysics of sunscreen oxybenzone in methanol solution.

Herein, we have employed the QM(CASPT2//CASSCF)/MM method to explore the photophysical and photochemical mechanism of oxybenzone (OB) in methanol solution. Based on the optimized minima, conical intersections and crossing points, and minimum-energy reaction paths related to excited-state intramolecular proton transfer (ESIPT) and excited-state decay paths in the 1ππ*, 1nπ*, 3ππ*, 3nπ*, and S0 states, we have identified several feasible excited-state relaxation pathways for the initially populated S2(1ππ*) state to decay to the initial enol isomer' S0 state. The major one is the singlet-mediated and stretch-torsion coupled ESIPT pathway, in which the system first undergoes an essentially barrierless 1ππ* ESIPT process to generate the 1ππ* keto species, and finally realizes its ground state recovery through the subsequent carbonyl stretch-torsion facilitating S1 → S0 internal conversion (IC) and the reverse ground-state intramolecular proton transfer (GSIPT) process. The minor ones are related to intersystem crossing (ISC) processes. At the S2(1ππ*) minimum, an S2(1ππ*)/S1(1nπ*)/T2(3nπ*) three-state intersection region helps the S2 system branch into the T1 state through a S2 → S1 → T1 or S2 → T2 → T1 process. Once it has reached the T1 state, the system may relax to the S0 state via direct ISC or via subsequent nearly barrierless 3ππ* ESIPT to yield the T1 keto tautomer and ISC. The resultant S0 keto species significantly undergoes reverse GSIPT and only a small fraction yields the trans-keto form that relaxes back more slowly. However, due to small spin-orbit couplings at T1/S0 crossing points, the ISC to S0 state occurs very slowly. The present work rationalizes not only the ultrafast excited-state decay dynamics of OB but also its phosphorescence emission at low temperature.

Theoretical Studies on the Excited-State Decay Mechanism of Homomenthyl Salicylate in a Gas Phase and an Acetonitrile Solution

Here, we employ the CASPT2//CASSCF and QM(CASPT2//CASSCF)/MM approaches to explore the photochemical mechanism of homomenthyl salicylate (HMS) in vacuum and an acetonitrile solution. The results show that in both cases, the excited-state relaxation mainly involves a spectroscopically "bright" S1(1ππ*) state and the lower-lying T1 and T2 states. In the major relaxation pathway, the photoexcited S1 keto system first undergoes an essentially barrierless excited-state intramolecular proton transfer (ESIPT) to generate the S1 enol minimum, near which a favorable S1/S0 conical intersection decays the system to the S0 state followed by a reverse ground-state intramolecular proton transfer (GSIPT) to repopulate the initial S0 keto species. In the minor one, an S1/T2/T1 three-state intersection in the keto region makes the T1 state populated via direct and T2-mediated intersystem crossing (ISC) processes. In the T1 state, an ESIPT occurs, which is followed by ISC near a T1/S0 crossing point in the enol region to the S0 state and finally back to the S0 keto species. In addition, a T1/S0 crossing point near the T1 keto minimum can also help the system decay to the S0 keto species. However, small spin-orbit couplings between T1 and S0 at these T1/S0 crossing points make ISC to the S0 state very slow and make the system trapped in the T1 state for a while. The present work rationalizes not only the ultrafast excited-state decay dynamics of HMS but also its low quantum yield of phosphorescence at 77 K.

Near-IR two-photon absorption photostabilizers for polymers intensified by molecular assembly in mixed organic solvents/H2O

This study reported on a variety of target phenanthridine dyes carrying hydroxy groups that were employed as near-IR two-photon absorption (TPA) photostabilizers for polymers. It was demonstrated that the target phenanthridine dyes underwent reversible excited state proton transfer (ESPT) in strong polar protic and aprotic solvents, such as ethanol, which was more remarkable under near-IR two-photon excitation. It was determined that the target dyes processed regular molecular stacking in mixed organic solvents/H2O solvents (such as ethanol/H2O, 90% water volume). The orderly molecular aggregation of the target phenanthridine dyes intensified the reversible ESPT emission under one-photon and near-IR two-photon excitation, respectively. Hence, the absorbed energy was consumed by the reversible ESPT process of the target dyes. As a consequence, the target dyes were evaluated as near-IR TPA photostabilizers for polymers. In contrast, the reference phenanthridine dyes lacking of hydroxy groups did not exhibit ESPT or photostability under light irradiation.

Benzothiazole‐based chemosensor: a quick dip into its anion sensing mechanism

AbstractDensity functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) methods have been applied to decipher the F− anion sensing mechanism of a fluorescent probe N‐(2‐(benzothiazole‐2‐yl)phenyl)‐4‐methoxybenzamide (4). The amidic proton (N–H) of the selected probe may exhibit an intramolecular hydrogen bond (IMHB) with the nitrogen atom of benzothiazole moiety. The weak IMHB of N24–H26‐‐‐N12 in the excited state favors reverse proton transfer process in 4. In the ground state of 4, the F− anion interacts via F−‐‐‐H–N bond, which immediately deprotonates the amidic proton, leading to the distortion of the geometry. Subsequent to deprotonation, the significant intramolecular charge transfer (ICT) character was observed in 4. On account of the significant ICT process, the redshift in absorption and emission spectra was observed. These outcomes were employed to delineate the sensing mechanism of molecule 4.

Exploring the Role of H‐Bonding in Organic Electrochemistry – From Supramolecular Applications to Mechanistic Investigations

H-bonds can exert a substantial impact on the course of organic electrode reactions due to their ability to stabilize charged intermediates and products formed during these reactions, as well as facilitate proton-coupled electron transfer (PCET) reactions. This has fundamental implications for the mechanism of organic electrode reactions, but also practical impact in supramolecular chemistry and potentially synthetic electrochemistry. My group's main focus has been on the supramolecular applications, using electron transfer to alter the strength of H-bonds to create highly redox-responsive H-bond dimers. Initially we sought to avoid proton transfer because we feared that would lead to irreversible electrochemistry. However, inevitably proton transfer did show up, but, to our surprise, did not lead to irreversible electrochemistry. To explain this, we developed a new mechanism, the "wedge scheme", that shows how H-bonding can facilitate reversible electron and proton transfer. This insight recently led us to a new PCET-based design strategy for the creation of our most highly redox-responsive H-bond dimers yet.