Ultrafast Proton Transport between a Hydroxy Acid and a Nitrogen Base along Solvent Bridges Governed by the Hydroxide/Methoxide Transfer Mechanism

Superior proton conductivity of amino acid-modified UiO-66-(COOH)2 embedded in chitosan: Mechanistic insights into the acid-base interactions

We focus on optimizing acid-base interactions and hydrogen bonding networks to achieve enhanced proton conductivity under low relative humidity (RH) and high temperature. By functionalizing UiO-66-(COOH)2 with glutamic acid (Glu) and lysine (Lys), we generate Glu-UiO-66-(COOH)2 and Lys-UiO-66-(COOH)2. These modified materials are subsequently incorporated into chitosan (CS) to produce the composites Glu-UiO-66-(COOH)2@CS and Lys-UiO-66-(COOH)2@CS. The successful incorporation of amino acids and cross-linking between -COOH and -NH2 groups in the composites, confirmed by FTIR and PXRD analyses, significantly enhances the structural integrity and durability by strengthening the network, and reducing polymer chain mobility. Proton conductivity assessments reveal that Lys-UiO-66-(COOH)2@CS-7 exhibits a remarkable conductivity of 0.022 S/cm at 100 % RH and 363 K, outperforming Glu-UiO-66-(COOH)2@CS-7. The lower activation energy (Ea) of 0.28 eV for Lys-UiO-66-(COOH)2@CS-7, compared to 0.336 eV for Glu-UiO-66-(COOH)2@CS-7, highlights the significant improvement in intramolecular acid-base interactions and hydrogen bonding. Furthermore, Lys-UiO-66-(COOH)2@CS-7 maintains notable proton conductivity at 43 % RH, with an Ea of 0.216 eV, demonstrating its efficacy in low-humidity conditions. These findings underscore the profound impact of amino acid modifications and cross-linking on proton conductivity by reinforcing acid-base interactions, hydrogen bonding networks, and proton transfer efficiency.

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.

Hetero-Diels-Alder Cycloaddition Reaction of Vinyl Ethers Enables Selective Fluorescent Labeling of Plasmalogens in Human Plasma Lipids

Plasmalogens (Pls) are vinyl ether-containing glycerophospholipids of broad biological interest. Their abnormal levels are associated with neurological disorders and cardiovascular diseases. The intricacy of analyzing Pls in lipid samples arises from the wide variety of other coexisting lipid species, which underscores the urgent need for a Pls-specific labeling reaction. To address this challenge, we report an efficient hetero-Diels-Alder cycloaddition reaction between nonterminal vinyl ethers of Pls and o-quinolinone quinone methide probes under mild conditions. On the basis of this mechanism, a selective fluorescent labeling method for Pls is developed. The application of this method permits the exclusive derivatization of Pls over other human plasma lipids. The process also imparts labeled Pls with distinct fluorescence emission and chromatographic retention properties. By integrating this method with high-performance liquid chromatography, we are able to identify individual chromatographic signatures of Pls from 10 different human plasma samples. This Pls signature analytical technique, empowered by the Pls-specific labeling reaction, is cost-effective and simple in terms of instrumentation, suggesting its promising potential for the early screening and diagnosis of diseases linked to Pls abnormalities.

Ultrafast Proton Transfer Pathways Mediated by Amphoteric Imidazole

Imidazole, being an amphoteric molecule, can act both as an acid and as a base. This property enables imidazole, as an essential building block, to effectively facilitate proton transport in high-temperature proton exchange membrane fuel cells and in proton channel transmembrane proteins, enabling those systems to exhibit high energy conversion yields and optimal biological function. We explore the amphoteric properties of imidazole by following the proton transfer exchange reaction dynamics with the bifunctional photoacid 7-hydroxyquinoline (7HQ). We show with ultrafast ultraviolet-mid-infrared pump-probe spectroscopy how for imidazole, in contrast to expectations based on textbook knowledge of acid-base reactivity, the preferential reaction pathway is that of an initial proton transfer from 7HQ to imidazole, and only at a later stage a transfer from imidazole to 7HQ, completing the 7HQ tautomerization reaction. An assessment of the molecular distribution functions and first-principles calculations of proton transfer reaction barriers reveal the underlying reasons for our observations.

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.

Highly Altered State of Proton Transport in Acid Pools in Charged Reverse Micelles

Transport mechanisms of solvated protons of 1 M HCl acid pools, confined within reverse micelles (RMs) containing the negatively charged surfactant sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) or the positively charged cetyltrimethylammonium bromide (CTABr), are analyzed with reactive force field simulations to interpret dynamical signatures from TeraHertz absorption and dielectric relaxation spectroscopy. We find that the forward proton hopping events for NaAOT are further suppressed compared to a nonionic RM, while the Grotthuss mechanism ceases altogether for CTABr. We attribute the sluggish proton dynamics for both charged RMs as due to headgroup and counterion charges that expel hydronium and chloride ions from the interface and into the bulk interior, thereby increasing the pH of the acid pools relative to the nonionic RM. For charged NaAOT and CTABr RMs, the localization of hydronium near a counterion or conjugate base reduces the Eigen and Zundel configurations that enable forward hopping. Thus, localized oscillatory hopping dominates, an effect that is most extreme for CTABr in which the proton residence time increases dramatically such that even oscillatory hopping is slow.

Quantification of Excited-State Brønsted-Lowry Acidity of Weak Photoacids Using Steady-State Photoluminescence Spectroscopy and a Driving-Force-Dependent Kinetic Theory

Photoacids and photobases constitute a class of molecules that upon absorption of light undergoes a reversible change in acidity, i.e. pK_a. Knowledge of the excited-state pK_a value, pK_a*, is critical for predicting excited-state proton-transfer behavior. A reasonable approximation of pK_a* is possible using the Förster cycle analysis, but only when the ground-state pK_a is known. This poses a challenge for the study of weak photoacids (photobases) with less acidic (basic) excited states (pK_a* (pK_b*) > 7), because ground-state pK_a (pK_b) values are >14, making it difficult to quantify them accurately in water. Another method to determine pK_a* relies on acid-base titrations with photoluminescence detection and Henderson-Hasselbalch analysis. This method requires that the acid dissociation reaction involving the thermally equilibrated electronic excited state reaches chemical quasi-equilibrium, which does not occur for weak photoacids (photobases) due to slow rates of excited-state proton transfer. Herein, we report a method to overcome these limitations. We demonstrate that liquid water and aqueous hydroxide are unique proton-accepting quenchers of excited-state photoacids. We determine that Stern-Volmer quenching analysis is appropriate to extract rate constants for excited-state proton transfer in aqueous solutions from a weak photoacid, 5-aminonaphthalene-1-sulfonate, to a series of proton-accepting quenchers. Analysis of these data by Marcus-Cohen bond-energy-bond-order theory yields an accurate value for pK_a* of 5-aminonaphthalene-1-sulfonate. Our method is broadly accessible because it only requires readily available steady-state photoluminescence spectroscopy. Moreover, our results for weak photoacids are consistent with those from previous studies of strong photoacids, each showing the applicability of kinetic theories to interpret driving-force-dependent rate constants for proton-transfer reactions.

Electronic Structure Changes of an Aromatic Amine Photoacid along the Förster Cycle

Photoacids show a strong increase in acidity in the first electronic excited state, enabling real-time studies of proton transfer in acid-base reactions, proton transport in energy storage devices and biomolecular sensor protein systems. Several explanations have been proposed for what determines photoacidity, ranging from variations in solvation free energy to changes in electronic structure occurring along the four stages of the Förster cycle. Here we use picosecond nitrogen K-edge spectroscopy to monitor the electronic structure changes of the proton donating group in a protonated aromatic amine photoacid in solution upon photoexcitation and subsequent proton transfer dynamics. Probing core-to-valence transitions locally at the amine functional group and with orbital specificity, we clearly reveal pronounced electronic structure, dipole moment and energetic changes on the conjugate photobase side. This result paves the way for a detailed electronic structural characterization of the photoacidity phenomenon.

Solvent-Triggered Long-Range Proton Transport in 7-Hydroxyquinoline Using a Sulfonamide Transporter Group

The ability of long-range proton transport by substitution of 7-hydroxyquinoline at the eighth position with sulfonamide and sulfonylhydrazone rotor units to act as a crane-arm has been studied. Different proton transport pathways triggered by different stimuli have been established depending on the structure of the crane-arms. Solvent-driven proton switching from OH to the quinoline nitrogen (N_quin) site, facilitated by a sulfonamide transporter group in polar protic and aprotic solvents, has been confirmed by optical (absorption and fluorescence) and NMR spectroscopies as well as by single-crystal X-ray structure analysis. Photoinduced long-range proton transport to the N_quin site upon 340 nm UV light irradiation has been estimated in sulfonylhydrazone, which is not sensitive to solvent-driven switching. Both compounds have exhibited acid-triggered switching by trifluoroacetic acid due to the formation of a stable six-membered intramolecular hydrogen bonding interaction between the protonated N_quin and crane-arm. The structures of acid-switched form were confirmed by NMR spectroscopy and single-crystal X-ray structure analysis. The behavior of the compounds suggests a big step forward in the advanced proton pump-switching architecture because they cover three distinct driving forces in the switching process: solvent, light, and acid.

Proton Traffic Jam: Effect of Nanoconfinement and Acid Concentration on Proton Hopping Mechanism

The properties of the water network in concentrated HCl acid pools in nanometer-sized reverse non-ionic micelles were probed with TeraHertz absorption, dielectric relaxation spectroscopy, and reactive force field simulations capable of describing proton hopping mechanisms. We identify that only at a critical micelle size of W0=9 do solvated proton complexes form in the water pool, accompanied by a change in mechanism from Grotthuss forward shuttling to one that favors local oscillatory hopping. This is due to a preference for H+ and Cl- ions to adsorb to the micelle interface, together with an acid concentration effect that causes a "traffic jam" in which the short-circuiting of the hydrogen-bonding motif of the hydronium ion decreases the forward hopping rate throughout the water interior even as the micelle size increases. These findings have implications for atmospheric chemistry, biochemical and biophysical environments, and energy materials, as transport of protons vital to these processes can be suppressed due to confinement, aggregation, and/or concentration.

Proton-Transfer Dynamics of Photoacidic Merocyanines in Aqueous Solution

Photoacids attract increasing scientific attention, as they are valuable tools to spatiotemporally control proton-release reactions and pH values of solutions. We present the first time-resolved spectroscopic study of the excited state and proton-release dynamics of prominent merocyanine representatives. Femtosecond transient absorption measurements of a pyridine merocyanine with two distinct protonation sites revealed dissimilar proton-release mechanisms: one site acts as a photoacid generator as its pKa value is modulated in the ground state after photoisomerization, while the other functions as an excited state photoacid which releases its proton within 1.1 ps. With a pKa drop of 8.7 units to -5.5 upon excitation, the latter phenolic site is regarded a super-photoacid. The 6-nitro derivative exhibits only a phenolic site with similar, yet slightly less photoacidic characteristics and both compounds transfer their proton to methanol and ethanol. In contrast, for the related 6,8-dinitro compound an intramolecular proton transfer to the ortho-nitro group is suggested that is involved in a rapid relaxation into the ground state.

Photoredox halogenation of quinolones: the dual role of halo-fluorescein dyes

An efficient C-3 halogenation of quinolin-4-ones is reported with halogenated fluorescein dyes which serve both as a halogen source and photocatalyst. This reaction shows broad substrate scope and gives good to excellent yields of C-3 brominated/iodinated quinolones with eosin Y/rose bengal in green light under ambient conditions. The mechanistic investigations suggest a radical pathway involving the oxidative dehalogenation of the dye in the presence of air.

Switching between Proton Vacancy and Excess Proton Transfer Pathways in the Reaction between 7-Hydroxyquinoline and Formate

Bifunctional or amphoteric photoacids simultaneously present donor (acidic) and acceptor (basic) properties making them useful tools to analyze proton transfer reactions. In protic solvents, the proton exchange between the acid and the base is controlled by the acidity or basicity strength and typically occurs on two different pathways known as protolysis and hydrolysis. We report here how the addition of a formate base will alter the relative importance of the possible reaction pathways of the bifunctional photoacid 7-hydroxyquinoline (7HQ), which has been recently understood to predominantly involve a hydroxide/methoxide transport mechanism between the basic proton-accepting quinoline nitrogen site toward the proton-donating OH group with a time constant of 360 ps in deuterated methanol (CD₃OD). We follow the reaction dynamics by probing the IR-active marker modes of the different charged forms of photoexcited 7HQ, and of formic acid (HCOOD) in CD₃OD solution. A comparison of the transient IR spectra as a function of formate concentration, and classical molecular dynamics simulations enables us to identify distinct contributions of “tight” (meaning “contact”) and “loose” (i.e., “solvent-separated”) 7HQ–formate reaction pairs in our data. Our results suggest that depending on the orientation of the OH group with respect to the quinoline aromatic ring system, the presence of the formate molecule in a proton relay pathway facilitates a net proton transfer from the proton-donating OH group of 7HQ-N* via the methanol/formate bridge toward the quinoline N site.

8-(Pyridin-2-yl)quinolin-7-ol as a Platform for Conjugated Proton Cranes: A DFT Structural Design

Theoretical design of conjugated proton cranes, based on 7-hydroxyquinoline as a tautomeric sub-unit, has been attempted by using ground and excited state density functional theory (DFT) calculations in various environments. The proton crane action request existence of a single enol tautomer in ground state, which under excitation goes to the excited keto tautomer through a series of consecutive excited-state intramolecular proton transfer (ESIPT) steps with the participation of the crane sub-unit. A series of substituted pyridines was used as crane sub-units and the corresponding donor-acceptor interactions were evaluated. The results suggest that the introduction of strong electron donor substituents in the pyridine ring creates optimal conditions for 8-(pyridin-2-yl)quinolin-7-ols to act as proton cranes.

Steric effects in light-induced solvent proton abstraction

The significance of solvent structural factors in the excited-state proton transfer (ESPT) reactions of Schiff bases with alcohols is reported here. We use the super photobase FR0-SB and a series of primary, secondary, and tertiary alcohol solvents to illustrate the steric issues associated with solvent to photobase proton transfer. Steady-state and time-resolved fluorescence data show that ESPT occurs readily for primary alcohols, with a probability proportional to the relative -OH concentration. For secondary alcohols, ESPT is greatly diminished, consistent with the barrier heights obtained using quantum chemistry calculations. ESPT is not observed in the tertiary alcohol. We explain ESPT using a model involving an intermediate hydrogen-bonded complex where the proton is "shared" by the Schiff base and the alcohol. The formation of this complex depends on the ability of the alcohol solvent to achieve spatial proximity to and alignment with the FR0-SB* imine lone pair stabilized by the solvent environment.

Broadband fluorescence reveals mechanistic differences in excited-state proton transfer to protic and aprotic solvents

Excited-state proton transfer (ESPT) to solvent is often explained according to the two-step Eigen-Weller model including a contact ion pair (CIP*) as an intermediate, but general applicability of the model has not been thoroughly examined. Furthermore, examples of the spectral identification of CIP* are scarce. Here, we report on a detailed investigation of ESPT to protic (H₂O, D₂O, MeOH and EtOH) and aprotic (DMSO) solvents utilizing a broadband fluorescence technique with sub-200 fs time resolution. The time-resolved spectra are decomposed into contributions from the protonated and deprotonated species and a clear signature of CIP* is identified in DMSO and MeOH. Interestingly, the CIP* intermediate is not observable in aqueous environment although the dynamics in all solvents are multi-exponential. Global analysis based on the Eigen-Weller model is satisfactory in all solvents, but the marked mechanistic differences between aqueous and organic solvents cast doubt on the physical validity of the rate constants obtained.

Reversible intermolecular-coupled-intramolecular (RICI) proton transfer occurring on the reaction-radius a of 2-naphthol-6,8-disulfonate photoacid

Steady-state and time-resolved fluorescence techniques were employed to study the excited-state proton transfer (ESPT) from a reversibly dissociating photoacid, 2-naphthol-6,8-disulfonate (2N68DS). The reaction was carried out in water and in acetonitrile-water solutions. We find by carefully analyzing the geminate recombination dynamics of the photobase-proton pair that follows the ESPT reaction that there are two targets for the proton back-recombination reaction: the original O^- dissociation site and the SO₃ ^- side group at the 8 position which is closest to the proton OH dissociation site. This observation is corroborated in acetonitrile-water mixtures of χ_water < 0.14, where a slow intramolecular ESPT occurs on a time scale of about 1 ns between the OH group and the SO₃ ^- group via H-bonding water. The proton-transferred R^*O^- fluorescence band in mixtures of χ_water < 0.14 where only intramolecular ESPT occurs is red shifted by about 2000 cm^-1 from the free R^*O^- band in neat water. As the water content in the mixture increases above χ_water = 0.14, the R^*O^- fluorescence band shifts noticeably to the blue region. For χ_water > 0.23 the band resembles the free anion band observed in pure water. Concomitantly, the ESPT rate increases when χ_water increases because the intermolecular ESPT to the solvent (bulk water) gradually prevails over the much slower intramolecular via the water-bridges ESPT process.

Optimized effective potentials to increase the accuracy of approximate proton transfer energy calculations in the excited state

Many fundamental chemical reactions are triggered by electronic excitations. Here, we propose and benchmark a novel approximate first-principles molecular dynamics simulation idea for increasing the computational efficiency of density functional theory-based calculations of the excited states. We focus on obtaining proton transfer energy at the S₁ excited state through actual density functional theory calculations at the T₁ state with additional optimized effective potentials. The potentials are optimized as such to reproduce the excited-state energy surface obtained using time-dependent density functional theory, but can be generalized to other more accurate quantum chemical methods. We believe that the presented method is not only suitable for studies on excited-state proton transfer and ion mobility in general systems but can also be extended to investigate more involved processes, such as photo-induced isomerization.