Bifunctional Photoacids: Remote Protonation Affecting Chemical Reactivity

Observation of Solvent-Dependence in the Mechanism of Neutral-Catalyzed Isomerization of para-Aminobenzoic Acid Protomers

Proton-transfer reactions are commonplace during electrospray ionization (ESI) mass spectrometry experiments and are often responsible for imparting charge to analyte molecules. Multiple protonation-site isomers (protomers) can arise for polyfunctional molecules and these isomers can interconvert via solvent-mediated proton transfer reactions during various stages of the ESI process. Studying the populations and interconversion of protonation isomers provides key insight into the ESI process, ion-molecule interactions, and ion dissociation mechanisms. An archetype molecule to study protomer interconversion fundamentals in this context is para-aminobenzoic acid (pABA), where both the amino and carboxylic acid protomers are typically formed under ESI and the mechanisms for interconversion are still under refinement. Using ion-trap mass spectrometry reaction kinetics (2.5 mTorr, 300 K), this study examines gas-phase interconversion catalysis of pABA protomers by seven neutral species, which are commen solvents and additives used for ESI: water, formic acid, methanol, ethanol, propanol, ammonia, and acetonitrile. Three distinct reaction cases are reported: (i) formic acid, methanol, ethanol, propanol, and ammonia each catalyze the interconversion between the amino and carboxylic acid protomers via a n = 1 solvent-molecule vehicle mechanism; (ii) for water, however, a n = 6 adduct complex is detected and this suggests that the observed protomer interconversion occurs through a Grotthuss mechanism, in accord with literature reports; (iii) acetonitrile inhibits proton transfer by the formation of particularly stable n = 1 and 2 adduct complexes. The second-order rate constants for the protomer interconversion are observed to increase in the following order: H2O < HCO2H < MeOH < EtOH < PrOH < NH3. Potential energy schemes are reported for all neutral-catalyzed proton transfer reactions using the DSD-PBEP86-D3(BJ)/aug-cc-pVDZ level of theory. A central transition state, which connects the protonation site adducts, is shown to be the key rate-limiting step. The energy of this transition state is sensitive to the proton affinity of the neutral solvent, and this is supported by the correlation between the reaction rate and the solvent proton affinity.

Effect of cyano-addition on the photoacidity switch in 5-cyano-8-amino-2-naphthol

Cyano- or CN-additions are often utilized in the design of photoacids to enhance and/or enable excited state proton transfer (ESPT) from the protic site to aqueous and nonaqueous solvents. In diprotic photoacid 8-amino-2-naphthol (8N2OH), the protonation state of the amino group (NH3+/NH2) acts as an on-off switch for ESPT at the OH site in water. This study investigated whether the addition of CN in 5-cyano-8-amino-2-naphthol (5CN8) could override this switch and promote new ESPT pathways. Analysis of the steady-state and time-resolved emission data showed that in the presence of protonated NH3+, CN enhances OH photoacidity (vs. in 8N2OH) and activates the ESPT pathway at NH3+. Both protic sites, OH and NH3+, can also donate a proton to methanol upon excitation. In contrast, in the presence of deprotonated NH2, despite the addition of CN, ESPT is still not observed at the OH site for 5CN8. Thus, the addition of CN cannot override or negate the inhibiting effect of NH2 on OH photoacidity. Potential causes for this inhibition are discussed, including electronic and antiaromaticity effects of CN and NH2 substitution.

Dual Excited State Proton Transfer Pathways in the Bifunctional Photoacid 6-Amino-2-naphtol

This study investigates the photoacidity and excited state proton transfer (ESPT) pathways of a bifunctional molecule, 6-amino-2-naphthol (6N2OH), using absorption, steady-state fluorescence, time-resolved fluorescence, and theoretical calculations. 6N2OH attains four different prototropic forms in the excited state (cation, neutral, anion, or zwitterion) depending on pH of the solution. Interestingly, ESPT at the OH site of the molecule can be controlled by the protonation state of the amino substituent. Conversion of the electron donating NH2 group to the electron withdrawing NH3+ group brings about a reduction of more than 7 pKa units for the deprotonation of OH in the excited state. Further, the position of the NH2 substituent on the naphthalene framework is found to play an important role in dictating the ESPT pathways of aminonaphthols. Unlike most aminonaphthol derivatives that undergo ESPT only at the OH site, akin to substituted naphthols, 6N2OH undergoes ESPT at both OH and NH3+ sites, indicating its similarity to substituted naphthols and substituted naphthylamines. ESPT at the NH3+ site resulting in cation ↔ neutral equilibrium of 6N2OH in the excited state is well-corroborated by comparative studies with another reference photoacid, 6-amino-2-methoxynaphthalene (6N2M). Correlation of the acidity constants of 6N2OH with the σp parameters according to the Hammett model reveals that while 6N2OH can be treated either as naphthol or as naphthylamine in the ground state, the structure-function correlation cannot be extrapolated directly in the excited state, thus highlighting the rich and complex photophysics of bifunctional photoacids.

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.

Reliable experimental method for determination of photoacidity revealed by quantum chemical calculations

Photoacids are aromatic acids that exhibit significantly different acidities when they are electronically excited. Three experimental methods have been extensively used to determine the photoacidity, : fluorescence titration, the Förster cycle, and time-resolved experiments. However, the photoacidities determined by these experimental methods are not consistent. In this work, we used a theoretical method to evaluate the reliability of experimentally determined values. In particular, density functional theory (DFT) and time-dependent DFT calculations were used to obtain the changes in Gibbs free energy for acid dissociation reactions which are directly related to values. The Förster cycle, which is frequently used to experimentally determine the photoacidity due to its simplicity, yielded inconsistent results depending on how the transition energy was defined. We evaluated six empirical parameters extracted from the absorption and emission spectra of acidic and basic species of photoacids to adequately define the transition energy in the Förster cycle. And we found that the values obtained using the optical bandgap as the transition energy in the Förster cycle were in the best agreement with the results of quantum chemical calculations.

Prototropic forms of hydroxy derivatives of naphthoic acid within deep eutectic solvents

Deep eutectic solvents (DESs) are not only recognized as benign and inexpensive alternatives to ionic liquids, they offer a unique solvation milieu due to the varying H-bonding capabilities of their constituents. Proton-transfer involving a probe and its prototropic forms strongly depend on the H-bonding nature of the solubilizing media. The presence of prototropic forms of three probes, 1-hydroxy-2-naphthoic acid (1,2-HNA), 3-hydroxy-2-naphthoic acid (3,2-HNA), and 6-hydroxy-2-naphthoic acid (6,2-HNA) is investigated in two DESs, named ChCl:urea and ChCl:glycerol, constituted of H-bond acceptor choline chloride and different H-bond donors, urea and glycerol, respectively, in a 1 : 2 mole ratio under ambient conditions. While 1,2-HNA and 3,2-HNA exhibit an intramolecular H-bonding ability, 6,2-HNA does not. In contrast to common polar solvents, where the monoanionic emitting form of 1,2-HNA is also supported along with the neutral one, in both the DESs only the neutral emitting form exists. Addition of acid to the two DESs, respectively, fail to generate the monocationic form of the probe. Addition of a base to ChCl:urea results in the generation of the monoanionic form; even a very high strength of the base fails to generate the monoanionic emitting form in ChCl:glycerol. Relatively higher H-bond donating acidity of ChCl:glycerol results in added hydroxyl getting involved in H-bonding with alcohol functionalities of ChCl:glycerol leading to the absence of proton extraction to create the monoanionic form of the probe. Only the monoanionic emitting form of 3,2-HNA is present in ChCl:urea; in ChCl:glycerol, due to its higher H-bond donor acidity, the neutral emitting form is also detected. Addition of high strength of acid to ChCl:urea does result in formation of the neutral emitting form. Addition of an aqueous base results in the formation of the dianionic form of 3,2-HNA in ChCl:urea; however, in ChCl:glycerol, the added base fails to convert the neutral form of this probe to the monoanionic form as efficiently as that in ChCl:urea. The monoanionic (carboxylate) form of 6,2-HNA exits in ChCl:urea, whereas the neutral form is present in ChCl:glycerol due to its higher H-bond donating acidity. Addition of an acid can induce a shift in prototropic equilibrium towards the neutral form of 6,2-HNA in ChCl:urea; no change is observed in the behavior of this probe in ChCl:glycerol as the acid is added. Both the DESs support the dianionic form of 6,2-HNA in the presence of the base; the added base helps extract both -OH and -COOH protons of this probe. The H-bond donor component of the DES is clearly established to play a critical role in the prototropic behavior of the probe.

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.

A Fluorescent Visual Proton Donor and Photoacid Sterilant Based on Sulfonate-conjugated BODIPY

Increasing acidity is an effective method for bacterial inactivation by inhibiting the synthesis of intracellular proteins at low pH. Photo-driven proton release probe can be used for the measurement of proton in hydrophobic condition. To develop fluorescent proton donor, two boron dipyrromethene derivatives (BDP-S and BDP-S2) were characterized by spectroscopic methods. Irradiation of BDP-S by white LED light resulted in efficient generation of acidic species with changes of fluorescence emission. The linear relationship between the pH value and the fluorescence intensity of BDP-S was obtained, indicating that BDP-S is a fluorescent visual proton donor. Light-induced antibacterial results indicate that BDP-S can significantly inhibit the growth of E. coli. The results prove that BDP-S is a very promising photoacid sterilant.

Structure-Photochemical Function Relationships in the Photobasicity of Aromatic Heterocycles Containing Multiple Ring Nitrogen Atoms

Photobases are compounds that become more basic when promoted to an excited electronic state. Previous experimental and computational studies have demonstrated that several quinoline and quinoline-derived compounds are strong photobases (pK_a^* > 14). Moreover, the strength of photobasicity was shown to depend strongly on the identity and position of the substituent group(s), with the strongest photobases having multiple electron-donating substituents on a fused benzene ring as opposed to the ring containing the photobasic nitrogen atom. These electron-donating substituents build up electron density on one side of the molecule that shifts onto the nitrogen-containing ring in the electronic transition. This shift in electron density produces an increase in negative charge on the ring nitrogen atom responsible for the photobasicity. In this paper, we expand on our previous investigation to study the effect of an additional ring nitrogen atom on photobasicity in aromatic heterocycles. In particular, we consider how the thermodynamic driving force for excited-state protonation can be tuned by changing the relative placement of the ring nitrogen atoms and varying the position and number of electron-donating substituents. In the set of 112 molecules screened, we identified 42 strong photobases with generally comparable pK_a^* but lower vertical excitation energies than the quinoline derivatives with only a single ring nitrogen atom. We additionally explored photobasicity in substituted azaindole and carboline derivatives, identifying 76 strongly photobasic compounds with pK_a^* as large as 22.6 out of the 155 compounds that we considered. Overall, this work provides new insights into the design principles necessary to develop next-generation photocatalysts that employ photobasicity.

Changes in Optical Properties upon Dye-Clay Interaction: Experimental Evaluation and Applications

The development of hybrid materials with unique optical properties has been a challenge for the creation of high-performance composites. The improved photophysical and photochemical properties observed when fluorophores interact with clay minerals, as well as the accessibility and easy handling of such natural materials, make these nanocomposites attractive for designing novel optical hybrid materials. Here, we present a method of promoting this interaction by conjugating dyes with chitosan. The fluorescent properties of conjugated dye–montmorillonite (MMT) hybrids were similar to those of free dye–MMT hybrids. Moreover, we analyzed the relationship between the changes in optical properties of the dye interacting with clay and its structure and defined the physical and chemical mechanisms that take place upon dye–MMT interactions leading to the optical changes. Conjugation to chitosan additionally ensures stable adsorption on clay nanoplatelets due to the strong electrostatic interaction between chitosan and clay. This work thus provides a method to facilitate the design of solid-state hybrid nanomaterials relevant for potential applications in bioimaging, sensing and optical purposes.

Structure-Photochemical Function Relationships in Nitrogen-Containing Heterocyclic Aromatic Photobases Derived from Quinoline

Photobases are compounds that become strong bases after electronic excitation. Recent experimental studies have highlighted the photobasicity of the 5-R quinoline compounds, demonstrating a strong substituent dependence to the pK_a^*. In this paper, we describe our systematic study of how the thermodynamic driving force for photobasicity is tuned through substituents in four families of nitrogen-containing heterocyclic aromatics. We show that substituent position and identity both significantly impact the pK_a^*. We demonstrate that the substituent effects are additive and identify many disubstituted compounds with substantially greater photobasicity than the most photobasic 5-R quinoline compound identified previously. We show that the addition of a second fused benzene ring to quinoline, along with two electron-donating substituents, lowers the S₀ → S_PBS vertical excitation energy into the visible region while still maintaining a pK_a^* > 14. Overall, the structure-function relationships developed in this study provide new insights to guide the development of new photocatalysts that employ photobasicity.

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.

Adduct under Field-A Qualitative Approach to Account for Solvent Effect on Hydrogen Bonding

The location of a mobile proton in acid-base complexes in aprotic solvents can be predicted using a simplified Adduct under Field (AuF) approach, where solute–solvent effects on the geometry of hydrogen bond are simulated using a fictitious external electric field. The parameters of the field have been estimated using experimental data on acid-base complexes in CDF₃/CDClF₂. With some limitations, they can be applied to the chemically similar CHCl₃ and CH₂Cl₂. The obtained data indicate that the solute–solvent effects are critically important regardless of the type of complexes. The temperature dependences of the strength and fluctuation rate of the field explain the behavior of experimentally measured parameters.

Insight into 6-aminopenicillanic acid structure and study of the quantum mechanical calculations of the acid–base site on γ-Fe2O3@SiO2 core–shell nanocomposites and as efficient catalysts in multicomponent reactions

Magnetic 6-APA/γ-Fe₂O₃@Sio₂ nanocomposites have been developed by exploiting the potential of the acid–base bifunctional system to study the quantum mechanistic calculations.

Divergent excited state proton transfer reactions of bifunctional photoacids 1-ammonium-2-naphthol and 3-ammonium-2-naphthol in water and methanol

This paper highlights the challenge of predicting the excited state proton transfer (ESPT) reactions of small organic compounds with multiple proton transfer sites. Aminonaphthols, naphthalene compounds with both hydroxyl and amino substituents, can be viewed as a combination of two monoprotic photoacids, naphthol and naphthylammonium. Here, the ESPT reactions of 3-ammonium-2-naphthol (3N2OH) and 1-ammonium-2-naphthol (1N2OH) were studied in water and methanol using a combination of steady-state and time-correlated single-photon counting emission spectroscopy. For 3N2OH, ESPT was observed at the OH site in water but at neither of the sites in methanol; for 1N2OH, ESPT was observed at both the OH and NH₃⁺ sites in water but only at the NH₃⁺ site in methanol. Evidence of ESPT at the NH₃⁺ site is limited for aminonaphthols. The divergent dynamics of 3N2OH and 1N2OH in water and methanol are discussed; dependent on the substitution and solvent, the ESPT reactions were analysed within the frameworks of reference photoacids 2-naphthol and 1-naphthylammonium. The application of crown ether and salt to control the release of select protons in non-aqueous media is also discussed.

Effect of Photoacid Strength on Fluorescence Modulation of 2-Naphthol Derivatives inside β-Cyclodextrin Cavity: Insights from Fluorescence, Isothermal Calorimetry, and Molecular Dynamics Simulations

Fluorescence response of a photoacid inside a confined environment often differs markedly from the bulk response. Is there any correlation between the extent of fluorescence modulation and the strength of the photoacid? Here, we used three photoacids: 2-naphthol (2OH, pK_a* = 3.3), 6-sulfonate-2-naphthol (6SO₃-2OH, pK_a* = 3.06), and 6-cyano-2-naphthol (6CN-2OH, pK_a* = 0.6) with remarkably different excited-state acidities to investigate fluorescence modulation inside the nanocavity of β-cyclodextrin (β-CD). Interestingly, we found strong fluorescence modulation for 2OH and 6SO₃-2OH but almost none for 6CN-2OH. Isothermal calorimetry measurements showed that all three fluorophores form 1:1 inclusion complex with comparable binding constants (285, 420, and 580 M^-1 for 2OH, 6SO₃-2OH, and 6CN-2OH, respectively). Molecular dynamics simulation further revealed that binding modes are quite similar, and the distribution of water molecules around the proton-donating hydroxyl group of the photoacids are also comparable. Consequently, the difference in the fluorescence response should be accounted solely to the difference in the photoacidity strengths.

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

Aqueous proton transport plays a key role in acid–base neutralization and energy transport through biological membranes and hydrogen fuel cells. Extensive experimental and theoretical studies have resulted in a highly detailed elucidation of one of the underlying microscopic mechanisms for aqueous excess proton transport, known as the von Grotthuss mechanism, involving different hydrated proton configurations with associated high fluxional structural dynamics. Hydroxide transport, with approximately 2-fold-lower bulk diffusion rates compared to those of excess protons, has received much less attention. We present femtosecond UV/IR pump–probe experiments and ab initio molecular dynamics simulations of different proton transport pathways of bifunctional photoacid 7-hydroxyquinoline (7HQ) in water/methanol mixtures. For 7HQ solvent-dependent photoacidity, free-energy–reactivity correlation behavior and quantum mechanics/molecular mechanics (QM/MM) trajectories point to a dominant OH^–/CH₃O^– transport pathway for all water/methanol mixing ratios investigated. Our joint ultrafast infrared spectroscopic and ab initio molecular dynamics study provides conclusive evidence for the hydrolysis/methanolysis acid–base neutralization pathway, as formulated by Manfred Eigen half a century ago. Our findings on the distinctly different acid–base reactivities for aromatic hydroxyl and aromatic nitrogen functionalities suggest the usefulness of further exploration of these free-energy–reactivity correlations as a function of solvent polarity. Ultimately the determination of solvent-dependent acidities will contribute to a better understanding of proton-transport mechanisms at weakly polar surfaces and near polar or ionic regions in transmembrane proton pump proteins or hydrogen fuel cell materials.

Divergent Hammett Plots of the Ground- and Excited-State Proton Transfer Reactions of 7-Substituted-2-Naphthol Compounds

The rational design of photoacids requires accessible predictive models of the electronic effect of functional groups on chemical templates of interest. Here, the effect of substituents on the photoacidity and excited-state proton transfer (PT) pathways of prototype 2-naphthol (2OH) at the symmetric C7 position was investigated through photochemical and computational studies of 7-amino-2-naphthol (7N2OH) and 7-methoxy-2-naphthol (7OMe2OH). Time-resolved emission experiments of 7N2OH revealed that the presence of an electron-withdrawing versus electron-donating group (EWG vs EDG, NH₃⁺ vs NH₂) led to a drastic decline in photoacidity: p K_a* = 1.1 ± 0.2 vs 9.6 ± 0.2. Time-dependent density functional theory calculations with explicit water molecules confirmed that the excited neutral state (x = NH₂) is greatly stabilized by water, with equation-of-motion coupled cluster singles and doubles calculations supporting potential mixing between the L_a and L_b states. Similar suppression of photoacidity, however, was not observed for 7OMe2OH with EDG OCH₃, p K_a* = 2.7 ± 0.1. Hammett plots of the ground- and excited-state PT reactions of substituted 7-x-2OH compounds (x = CN, NH₃⁺, H, CH₃, OCH₃, OH, and NH₂) vs Hammett parameters σ_p showed breaks in the linearity between the EDG and EWG regions: ρ ∼ 0 vs 1.14 and ρ* ∼ 0 vs 3.86. The divergent acidic behavior most likely arises from different mixing mechanisms of the lowest L_b state with the L_a and possible B_b states upon substitution of naphthalene in water.

Ultrafast proton transport in water-methanol mixtures

Femtosecond UV/IR pump-probe experiments and ab initio molecular dynamics simulations of 7-hydroxyquinoline in water-methanol mixtures demonstrate an unexpectedly dominant OH-/CH3O- transport pathway but consistent with a solvent-dependent photoacidity free energy-reactivity correlation behaviour.

pH switch for OH-photoacidity in 5-amino-2-naphthol and 8-amino-2-naphthol

pH switch for OH-photoacidity in 5-amino-2-naphthol and 8-amino-2-naphthol Switching of the amino protonation state turns on and off the OH-photoacidity.

Photoactive antimicrobial nanomaterials

Nanomaterials for killing pathogenic bacteria under light irradiation.

Monomolecular pyrenol-derivatives as multi-emissive probes for orthogonal reactivities

Chameleons in a test tube: up to four easily distinguishable emission colors result from conversion by two hydrolytic enzymes at opposite reaction sites.

Controlling reactivity by remote protonation of a basic side group in a bifunctional photoacid

Ultrafast reactivity-switch is achieved by remote-protonation caused by protons diffusing from acidic to basic side-groups of bifunctional photoacids.