Bimodal proton transfer in acid-base reactions in water

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.

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.

Ultrafast transient absorption and solvation of a super-photoacid in acetoneous environments

The phenomenon of photoacidity, i.e., an increase in acidity by several orders of magnitude upon electronic excitation, is frequently encountered in aromatic alcohols capable of transferring a proton to a suitable acceptor. A promising new class of neutral super-photoacids based on pyranine derivatives has been shown to exhibit pronounced solvatochromic effects. To disclose the underlying mechanisms contributing to excited-state proton transfer (ESPT) and the temporal characteristics of solvation and ESPT, we scrutinize the associated ultrafast dynamics of the strongest photoacid of this class, namely tris(1,1,1,3,3,3-hexafluoropropan-2-yl)8-hydroxypyrene-1,3,6-trisulfonate, in acetoneous environment, thereby finding experimental evidence for ESPT even under these adverse conditions for proton transfer. Juxtaposing results from time-correlated single-photon counting and femtosecond transient absorption measurements combined with a complete decomposition of all signal components, i.e., absorption of ground and excited states as well as stimulated emission, we disclose dynamics of solvation, rotational diffusion, and radiative relaxation processes in acetone and identify the relevant steps of ESPT along with the associated time scales.

Highly sensitive and selective colorimetric sensing of CO2 for biomedical applications

The concentration of carbon dioxide (CO2) in unhealthy people differs greatly from healthy people. High-precision CO2 detection with a quick response time is essential for many biomedical applications. A major focus of this research is on the detection of CO2, one of the most important health biomarkers. We investigated a low-cost, flexible, and reliable strategy by using dyes for colorimetric CO2 sensing in this study. The impacts of temperature, pH, reaction time, reusability, concentration, and dye selectivity were studied thoroughly. This study described real-time CO2 analysis. Using this multi-dye method, we got an average detection limit of 1.98 ppm for CO2, in the range of 50-120 ppm. A portable colorimetric instrument with a smartphone-assisted unit was constructed to determine the relative red/green/blue values for real-time and practical applications within 15 s of interaction and the readings are very similar to those of an optical fiber probe. Environmental and biological chemistry applications are likely to benefit greatly from this unique approach.

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.

The Dual Use of the Pyranine (HPTS) Fluorescent Probe: A Ground-State pH Indicator and an Excited-State Proton Transfer Probe

Molecular fluorescent probes are an essential experimental tool in many fields, ranging from biology to chemistry and materials science, to study the localization and other environmental properties surrounding the fluorescent probe. Thousands of different molecular fluorescent probes can be grouped into different families according to their photophysical properties. This Account focuses on a unique class of fluorescent probes that distinguishes itself from all other probes. This class is termed photoacids, which are molecules exhibiting a change in their acid-base transition between the ground and excited states, resulting in a large change in their pKa values between these two states, which is thermodynamically described using the Förster cycle. While there are many different photoacids, we focus only on pyranine, which is the most used photoacid, with pKa values of ∼7.4 and ∼0.4 for its ground and excited states, respectively. Such a difference between the pKa values is the basis for the dual use of the pyranine fluorescent probe. Furthermore, the protonated and deprotonated states of pyranine absorb and emit at different wavelengths, making it easy to focus on a specific state. Pyranine has been used for decades as a fluorescent pH indicator for physiological pH values, which is based on its acid-base equilibrium in the ground state. While the unique excited-state proton transfer (ESPT) properties of photoacids have been explored for more than a half-century, it is only recently that photoacids and especially pyranine have been used as fluorescent probes for the local environment of the probe, especially the hydration layer surrounding it and related proton diffusion properties. Such use of photoacids is based on their capability for ESPT from the photoacid to a nearby proton acceptor, which is usually, but not necessarily, water. In this Account, we detail the photophysical properties of pyranine, distinguishing between the processes in the ground state and the ones in the excited state. We further review the different utilization of pyranine for probing different properties of the environment. Our main perspective is on the emerging use of the ESPT process for deciphering the hydration layer around the probe and other parameters related to proton diffusion taking place while the molecule is in the excited state, focusing primarily on bio-related materials. Special attention is given to how to perform the experiments and, most importantly, how to interpret their results. We also briefly discuss the breadth of possibilities in making pyranine derivatives and the use of pyranine for controlling dynamic 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.

Photoacid-induced aqueous acid-base reactions probed by femtosecond infrared spectroscopy

Acid-base reactions involving an excited photoacid have typically been investigated at high base concentrations, but the mechanisms at low base concentrations require clarification. Herein, the dynamics of acid-base reactions induced by an excited photoacid, pyranine (DA), were investigated in the presence of azide ion (N₃^-) in D₂O solution using femtosecond infrared spectroscopy. Specifically, the spectral characteristics of four species (DA, electronically excited DA (DA*), the conjugate base of DA* (A*^-), and the conjugate base of DA (A^-)) were probed in the spectral region of 1400-1670 cm^-1 in the time range of 1 ps-1 μs. This broad timescale encompassed all the acid-base reactions initiated by photoexcitation at 400 nm; thus, reactions related to both DA* and A^- could be probed. Furthermore, changes in the populations of N₃^- and DN₃ were monitored using the absorption bands at 2042 and 2133 cm^-1, respectively. Following excitation, approximately half of DA* relaxed to DA with a time constant of 0.44 ± 0.04 ns. The remainder underwent an acid-base reaction to produce A*^-, which relaxed to A^- with a time constant of 3.9 ± 0.3 ns. The acid-base reaction proceeded via two paths, namely, proton exchange with the added base or simple deuteron release to D₂O (protolysis). Notably, all the acid-base reactions were well described by the rate constant at the steady-state limit. Thus, although the acid-base reactions at low base concentrations (< 0.1 M) were diffusion controlled, they could be described using a simple rate equation.

Information bounds in determining the 3D orientation of a single emitter or scatterer using point-detector-based division-of-amplitude polarimetry

Determining the 3D orientation of a single molecule or particle, encoded in its polar and azimuthal angles, is of interest for a variety of fields, being relevant to a range of questions in elementary chemical reactivity, biomolecular motors, and nanorheology. A popular experimental method, known as division-of-amplitude polarimetry, for determining the real-time orientation of a single particle is to split the emitted/scattered light into multiple polarizations and to measure the light intensity using point detectors at these polarizations during a time interval Δt. Here, we derive the Cramér-Rao lower bounds for this method from the perspective of information theory in the cases of utilizing a chromophore or a scattering particle as a 3D orientation probe. Such Cramér-Rao lower bounds are new for using this experimental method to measure the full 3D orientation in both the scattering case and the fluorescence case. These results show that, for a scatterer, the information content of one photon is 1.16 deg^-2 in the polar and 58.71 deg^-2 in the azimuthal angles, respectively. For a chromophore, the information content of one photon is 2.54 deg^-2 in the polar and 80.29 deg^-2 in the azimuthal angles. In addition, the Cramér-Rao lower bound scales with the square root of the total signal photons. To determine orientation to an uncertainty of one degree requires 7.40 × 10⁴ and 2.34 × 10³ photons for the polar and the azimuthal angles, respectively, for fluorescence, whereas it takes 1.62 × 10⁵ and 3.20 × 10³ photons for scattering. This work provides experimentalists new guidelines by which future experiments can be designed and interpreted.

Transient absorption spectroscopy of the electron transfer step in the photochemically activated polymerizations of N-ethylcarbazole and 9-phenylcarbazole

The polymerization of photoexcited N-ethylcarbazole (N-EC) in the presence of an electron acceptor begins with an electron transfer (ET) step to generate a radical cation of N-EC (N-EC˙+). Here, the production of N-EC˙+ is studied on picosecond to nanosecond timescales after N-EC photoexcitation at a wavelength λex = 345 nm using transient electronic and vibrational absorption spectroscopy. The kinetics and mechanisms of ET to diphenyliodonium hexafluorophosphate (Ph2I+PF6-) or para-alkylated variants are examined in dichloromethane (DCM) and acetonitrile (ACN) solutions. The generation of N-EC˙+ is well described by a diffusional kinetic model based on Smoluchowski theory: with Ph2I+PF6-, the derived bimolecular rate coefficient for ET is kET = (1.8 ± 0.5) × 1010 M-1 s-1 in DCM, which is consistent with diffusion-limited kinetics. This ET occurs from the first excited singlet (S1) state of N-EC, in competition with intersystem crossing to populate the triplet (T1) state, from which ET may also arise. A faster component of the ET reaction suggests pre-formation of a ground-state complex between N-EC and the electron acceptor. In ACN, the contribution from pre-reaction complexes is smaller, and the derived ET rate coefficient is kET = (1.0 ± 0.3) × 1010 M-1 s-1. Corresponding measurements for solutions of photoexcited 9-phenylcarbazole (9-PC) and Ph2I+PF6- give kET = (5 ± 1) × 109 M-1 s-1 in DCM. Structural modifications of the electron acceptor to increase its steric bulk reduce the magnitude of kET: methyl and t-butyl additions to the para positions of the phenyl rings (para Me2Ph2I+PF6- and t-butyl-Ph2I+PF6-) respectively give kET = (1.2 ± 0.3) × 1010 M-1 s-1 and kET = (5.4 ± 1.5) × 109 M-1 s-1 for reaction with photoexcited N-EC in DCM. These reductions in kET are attributed to slower rates of diffusion or to steric constraints in the ET reaction.

Steady-State Spectroscopy to Single Out the Contact Ion Pair in Excited-State Proton Transfer

Despite the outstanding relevance of proton transfer reactions, investigations of the solvent dependence on the elementary step are scarce. We present here a probe system of a pyrene-based photoacid and a phosphine oxide, which forms stable hydrogen-bonded complexes in aprotic solvents of a broad polarity range. By using a photoacid, an excited-state proton transfer (ESPT) along the hydrogen bond can be triggered by a photon and observed via fluorescence spectroscopy. Two emission bands could be identified and assigned to the complexed photoacid (CPX) and the hydrogen-bonded ion pair (HBIP) by a solvatochromism analysis based on the Lippert-Mataga model. The latter indicates that the difference in the change of the permanent dipole moment of the two species upon excitation is ∼3 D. This implies a displacement of the acidic hydrogen by ∼65 pm, which is in quantitative agreement with a change of the hydrogen bond configuration from O-H···O to ^-O···H-O⁺.

Quinoline Photobasicity: Investigation within Water-Soluble Light-Responsive Copolymers

Quinoline photobases exhibit a distinctly higher pKa in their electronically excited state than in the ground state, thereby enabling light-controlled proton transfer reactions, for example, in molecular catalysis. The absorption of UV light translates to a pKa jump of approximately 10 units, as established for small-molecule photobases. This contribution presents the first synthesis of quinoline-based polymeric photobases prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization. The integration of quinolines as photobase chromophores within copolymers offers new possibilities for light-triggered proton transfer in nanostructured materials, that is, in nanoparticles, at surfaces, membranes and interfaces. To exploit the light-triggered reactivity of photobases within such materials, we first investigated how the ground- and excited-state properties of the quinoline unit changes upon polymer integration. To address this matter, we combined absorption and emission spectroscopy with time-resolved transient-absorption studies to reveal photoinduced proton-transfer dynamics in various solvents. The results yield important insights into the thermodynamic and kinetic properties of these polymeric quinoline photobases.

Proton Transfer Dynamics in the Aprotic Proton Accepting Solvent 1-Methylimidazole

The dynamics of proton transfer to the aprotic solvent 1-methylimidazole (MeIm, proton acceptor) from the photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) was investigated using fast fluorescence measurements. The closely related molecule, 8-methoxypyrene-1,3,6-trisulfonic acid trisodium salt (MPTS), which is not a photoacid, was also studied for comparison. Following optical excitation, the wavelength-dependent population dynamics of HPTS in MeIm resulting from the deprotonation process were collected over the entire fluorescence emission window. Analysis of the time-dependent fluorescence spectra revealed four distinct fluorescence bands that appear and decay on different time scales. We label these four states as protonated (P), associated I (A_I), associated II (A_II), and deprotonated (D). We find that the simple kinetic scheme of P → A_I → A_II → D is not consistent with the data. Instead, the kinetic scheme that describes the data has P decaying into A_I, which mainly goes on to deprotonation (D), but A_I can also feed into A_II. A_II can return to A_I or decay to the ground state, but does not deprotonate within experimental error. Quantum chemistry and excited state QM/MM Born-Oppenheimer molecular dynamics simulations indicate that A_I and A_II are two H-bonding conformations of MeIm to the HPTS hydroxyl, axial, and equatorial, respectively.

A new visible light triggered Arrhenius photobase and its photo-induced reactions

Visible light triggered Arrhenius photobases are of potential use for excited state hydroxide ion dissociation (ESHID), photo-induced pOH jump experiments, and base-catalyzed reactions.

Halogen-Bond Assisted Photoinduced Electron Transfer

The formation of a halogen-bond (XB) complex in the excited state was recently reported with a quadrupolar acceptor–donor–acceptor dye in two iodine-based liquids (J. Phys. Chem. Lett.2017, 8, 3927–3932). The ultrafast decay of this excited complex to the ground state was ascribed to an electron transfer quenching by the XB donors. We examined the mechanism of this process by investigating the quenching dynamics of the dye in the S₁ state using the same two iodo-compounds diluted in inert solvents. The results were compared with those obtained with a non-halogenated electron acceptor, fumaronitrile. Whereas quenching by fumaronitrile was found to be diffusion controlled, that by the two XB compounds is slower, despite a larger driving force for electron transfer. A Smoluchowski–Collins–Kimball analysis of the excited-state population decays reveals that both the intrinsic quenching rate constant and the quenching radius are significantly smaller with the XB compounds. These results point to much stronger orientational constraint for quenching with the XB compounds, indicating that electron transfer occurs upon formation of the halogen bond.

A theoretical study on the size-dependence of ground-state proton transfer in phenol-ammonia clusters

Geometries and infrared (IR) spectra in the mid-IR region of phenol-(ammonia)_n (PhOH-(NH₃)_n) (n = 0-10) clusters have been studied using density functional theory (DFT) to investigate the critical number of solvent molecules necessary to promote ground-state proton transfer (GSPT). For n ≤ 8 clusters, the most stable isomer is a non-proton-transferred (non-PT) structure, and all isomers found within 1.5 kcal mol^-1 from it are also non-PT structures. For n = 9, the most stable isomer is also a non-PT structure; however, the second stable isomer is a PT structure, whose relative energy is within the experimental criterion of population (0.7 kcal mol^-1). For n = 10, the PT structure is the most stable one. We can therefore estimate that the critical size of GSPT is n = 9. This is confirmed by the fact that these calculated IR spectra are in good accordance with our previous experimental results of mid-IR spectra. It is demonstrated that characteristic changes of the ν_9a and ν₁₂ bands in the skeletal vibrational region provide clear information that the GSPT reaction has occurred. It was also found that the shortest distance between the π-ring and the solvent moiety is a good indicator of the PT reaction.

Ionic effects on the proton transfer mechanism in aqueous solutions

Proton dissociation (PD) reactions of weak acids and proton transfer (PT) processes in aqueous solutions are strongly influenced by ions.

Coherent intermolecular proton transfer in the acid–base reaction of excited state pyranine

The acidic proton in pyranine is transferred coherently to acetate through the stretching motion of the whole molecule.

A slowing down of proton motion from HPTS to water adsorbed on the MCM-41 surface

We report on the steady-state and femtosecond-nanosecond (fs-ns) behaviour of 8-hydroxypyrene-1,3,6-trisulfonate (pyranine, HPTS) and its interaction with mesoporous silica based materials (MCM-41) in both solid-state and dichloromethane (DCM) suspensions in the absence and presence of water. In the absence of water, HPTS forms aggregates which are characterized by a broad emission spectrum and multiexponential behavior (τsolid-state/DCM = 120 ps, 600 ps, 2.2 ns). Upon interaction with MCM41, the aggregate population is found to be lower, leading to the formation of adsorbed monomers. In the presence of water (1%), HPTS with and without MCM41 materials in DCM suspensions undergoes an excited-state intermolecular proton-transfer (ESPT) reaction in the protonated form (ROH*) producing a deprotonated species (RO(-)*). The long-time emission decays of the ROH* in different systems in the presence of water are multiexponential, and are analysed using the diffusion-assisted geminate recombination model. The obtained proton-transfer and recombination rate constants for HPTS and HPTS/MCM41 complexes in DCM suspensions in the presence of water are kPT = 13 ns(-1), krec = 7.5 Å ns(-1), and kPT = 5.4 ns(-1), krec = 2.2 Å ns(-1), respectively, The slowing down of both processes in the latter case is explained in terms of specific interactions of the dye and of the water molecules with the silica surface. The ultrafast dynamics (fs-regime) of the HPTS/MCM41 complexes in DCM suspensions, without and with water, shows two components which are assigned to intramolecular vibrational-energy relaxation (IVR) (∼120 fs vs. ∼0.8 ps), and vibrational relaxation/cooling (VC), and charge transfer (CT) processes (∼2 ps without water and ∼5 ps with water) of the adsorbed ROH*. Our results provide new knowledge on the interactions and the proton-transfer reaction dynamics of HPTS adsorbed on mesoporous materials.

Biexponential photon antibunching: recombination kinetics within the Förster-cycle in DMSO

Time-resolved experiments with pulsed-laser excitation are the standard approach to map the dynamic evolution of excited states, but ground-state kinetics remain hidden or require pump-dump-probe schemes. Here, we exploit the so-called photon antibunching, a purely quantum-optical effect related to single molecule detection to assess the rate constants for a chemical reaction in the electronic ground state. The measurement of the second-order correlation function g((2)), i.e. the evaluation of inter-photon arrival times, is applied to the reprotonation in a Förster-cycle. We find that the antibunching of three different photoacids in the aprotic solvent DMSO significantly differs from the behavior in water. The longer decay constant of the biexponential antibunching tl is linked to the bimolecular reprotonation kinetics of the fully separated ion-pair, independent of the acidic additives. The value of the corresponding bimolecular rate constant, kp = 4 × 10(9) M(-1) s(-1), indicates diffusion-controlled reprotonation. The analysis of tl also allows for the extraction of the separation yield of proton and the conjugated base after excitation and amounts to approximately 15%. The shorter time component ts is connected to the decay of the solvent-separated ion pair. The associated time constant for geminate reprotonation is approximately 3 ± 1 ns in agreement with independent tcspc experiments. These experiments verify that the transfer of quantum-optical experiments to problems in chemistry enables mechanistic conclusions which are hardly accessible by other methods.

How Acidic Is Carbonic Acid?

Carbonic, lactic, and pyruvic acids have been generated in aqueous solution by the transient protonation of their corresponding conjugate bases by a tailor-made photoacid, the 6-hydroxy-1-sulfonate pyrene sodium salt molecule. A particular goal is to establish the pK(a) of carbonic acid H2CO3. The on-contact proton transfer (PT) reaction rate from the optically excited photoacid to the carboxylic bases was derived, with unprecedented precision, from time-correlated single-photon-counting measurements of the fluorescence lifetime of the photoacid in the presence of the proton acceptors. The time-dependent diffusion-assisted PT rate was analyzed using the Szabo-Collins-Kimball equation with a radiation boundary condition. The on-contact PT rates were found to follow the acidity order of the carboxylic acids: the stronger was the acid, the slower was the PT reaction to its conjugate base. The pK(a) of carbonic acid was found to be 3.49 ± 0.05 using both the Marcus and Kiefer-Hynes free energy correlations. This establishes H2CO3 as being 0.37 pK(a) units stronger and about 1 pK(a) unit weaker, respectively, than the physiologically important lactic and pyruvic acids. The considerable acid strength of intact carbonic acid indicates that it is an important protonation agent under physiological conditions.

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.

Unveiling the Eigen-Weller ion pair from the excited state proton transfer kinetics of 3-chloro-4-methyl-7-hydroxycoumarin

The prototropic reactions of the first excited singlet state of 3-chloro-4-methylumbelliferone (3Cl4MU), in dioxane:water mixtures (Dx:H2O), were revisited using ps-time-resolved fluorescence techniques. The data response to the dielectric constant of the mixtures revealed the presence of an additional fourth kinetic species, kinetically coupled to the neutral (N*), the tautomeric (T*), and anionic (A(-)*) forms of 3Cl4MU, which is assigned to the elusive geminate (A(-)*···H(+)) ion pair. From the data analysis, all rate constants of the prototropic and diffusion processes involved were separately evaluated. The results showed that, whenever the geminate ionic pair is not kinetically detected, the evaluated values for deprotonation and protonation rate constants can substantially deviate from the real ones, depending on the efficiencies of pair recombination and dissociation. Finally, the results provide convincing kinetic evidence for the Eigen-Weller mechanism (intermediacy of the geminate ionic pair) in a quasi-aqueous medium, which to our knowledge had not yet been given.

Excited state structural events of a dual-emission fluorescent protein biosensor for Ca²⁺ imaging studied by femtosecond stimulated Raman spectroscopy

Fluorescent proteins (FPs) are luminescent biomolecules that emit characteristic hues upon irradiation. A group of calmodulin (CaM)-green FP (GFP) chimeras have been previously engineered to enable the optical detection of calcium ions (Ca(2+)). We investigate one of these genetically encoded Ca(2+) biosensors for optical imaging (GECOs), GEM-GECO1, which fluoresces green without Ca(2+) but blue with Ca(2+), using femtosecond stimulated Raman spectroscopy (FSRS). The time-resolved FSRS data (<800 cm(-1)) reveal that initial structural evolution following 400 nm photoexcitation involves small-scale coherent proton motions on both ends of the chromophore two-ring system with a <250 fs time constant. Upon Ca(2+) binding, the chromophore adopts a more twisted conformation in the protein pocket with increased hydrophobicity, which inhibits excited-state proton transfer (ESPT) by effectively trapping the protonated chromophore in S1. Both the chromophore photoacidity and local environment form the ultrafast structural dynamics basis for the dual-emission properties of GEM-GECO1. Its photochemical transformations along multidimensional reaction coordinates are evinced by distinct stages of FSRS spectral evolution, particularly related to the ∼460 and 504 cm(-1) modes. The direct observation of lower frequency modes provides crucial information about the nuclear motions preceding ESPT, which enriches our understanding of photochemistry and enables the rational design of new biosensors.

A Novel Real-time Carbon Dioxide Analyzer for Health and Environmental Applications

To be able to detect carbon dioxide (CO₂) with high accuracy and fast response time is critical for many health and environmental applications. We report on a pocket-sized CO₂ sensor for real-time analysis of end-tidal CO_2, and environmental CO₂. The sensor shows fast and reversible response to CO₂ over a wide concentration range, covering the needs of both environmental and health applications. It is also immune to the presence of various interfering gases in ambient or expired air. Furthermore, the sensor has been used for real-time breath analysis, and the results are in good agreement with those from a commercial CO₂ detector.

Competition between excited state proton and OH− transport via a short water wire: solvent effects open the gate

We investigate the acid–base proton exchange reaction in a microsolvated bifunctional chromophore by means of quantum chemical calculations.

Early time excited-state structural evolution of pyranine in methanol revealed by femtosecond stimulated Raman spectroscopy

To understand chemical reactivity of molecules in condensed phase in real time, a structural dynamics technique capable of monitoring molecular conformational motions on their intrinsic time scales, typically on femtoseconds to picoseconds, is needed. We have studied a strong photoacid pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, HPTS, pK(a)* ≈ 0) in pure methanol and observed that excited-state proton transfer (ESPT) is absent, in sharp contrast with our previous work on HPTS in aqueous solutions wherein ESPT prevails following photoexcitation. Two transient vibrational marker bands at ~1477 (1454) and 1532 (1528) cm(-1) appear in CH3OH (CD3OD), respectively, rising within the instrument response time of ~140 fs and decaying with 390-470 (490-1400) fs and ~200 ps time constants in CH3OH (CD3OD). We attribute the mode onset to small-scale coherent proton motion along the pre-existing H-bonding chain between HPTS and methanol, and the two decay stages to the low-frequency skeletal motion-modulated Franck-Condon relaxation within ~1 ps and subsequent rotational diffusion of H-bonding partners in solution before fluorescence. The early time kinetic isotope effect (KIE) of ~3 upon methanol deuteration argues active proton motions particularly within the first few picoseconds when coherent skeletal motions are underdamped. Pronounced quantum beats are observed for high-frequency modes consisting of strong phenolic COH rocking (1532 cm(-1)) or H-out-of-plane wagging motions (952 cm(-1)) due to anharmonic coupling to coherent low-frequency modes impulsively excited at ca. 96, 120, and 168 cm(-1). The vivid illustration of atomic motions of HPTS in varying H-bonding geometry with neighboring methanol molecules unravels the multidimensional energy relaxation pathways immediately following photoexcitation, and provides compelling evidence that, in lieu of ESPT, the photoacidity of HPTS promptly activates characteristic low-frequency skeletal motions to search phase space mainly concerning the phenolic end and to efficiently dissipate vibrational energy via skeletal deformation and proton shuttling motions within the intermediate, relatively confined excited-state HPTS-methanol complex on a solvent-dependent dynamic potential energy surface.

Accelerating slow excited state proton transfer

Visible light excitation of the ligand-bridged assembly [(bpy)(2)Ru(a)(II)(L)Ru(b)(II)(bpy)(OH(2))(4+)] (bpy is 2,2'-bipyridine; L is the bridging ligand, 4-phen-tpy) results in emission from the lowest energy, bridge-based metal-to-ligand charge transfer excited state (L(-•))Ru(b)(III)-OH(2) with an excited-state lifetime of 13 ± 1 ns. Near-diffusion-controlled quenching of the emission occurs with added HPO(4)(2-) and partial quenching by added acetate anion (OAc(-)) in buffered solutions with pH control. A Stern-Volmer analysis of quenching by OAc(-) gave a quenching rate constant of k(q) = 4.1 × 10(8) M(-1) • s(-1) and an estimated pK(a)* value of ~5 ± 1 for the [(bpy)(2)Ru(a)(II)(L(•-))Ru(b)(III)(bpy)(OH(2))(4+)]* excited state. Following proton loss and rapid excited-state decay to give [(bpy)(2)Ru(a)(II)(L)Ru(b)(II)(bpy)(OH)(3+)] in a H(2)PO(4)(-)/HPO(4)(2-) buffer, back proton transfer occurs from H(2)PO(4)(-) to give [(bpy)(2)Ru(a)(II)(L)Ru(b)(bpy)(OH(2))(4+)] with k(PT,2) = 4.4 × 10(8) M(-1) • s(-1). From the intercept of a plot of k(obs) vs. [H(2)PO(4)(-)], k = 2.1 × 10(6) s(-1) for reprotonation by water providing a dramatic illustration of kinetically limiting, slow proton transfer for acids and bases with pK(a) values intermediate between pK(a)(H(3)O(+)) = -1.74 and pK(a)(H(2)O) = 15.7.

Time-resolved mid-IR spectroscopy of (bio)chemical reactions in solution utilizing a new generation of continuous-flow micro-mixers

A specially designed micro-mixer made of silicon, calcium fluoride, and silicone with an optical transmission path of 8 μm has been used for mid-IR spectroscopy monitoring of mixing-induced chemical reactions in the low millisecond time regime. The basic principle of the proposed continuous-flow technique is to mix two liquids introduced in two times two alternatingly stacked layers through diffusion at the entrance of a 200 μm wide, 1 cm long micro-fluidic channel also serving as measurement area. By using this special, dedicated arrangement, diffusion lengths and hence the mixing times can be significantly shortened and the overall performance improved in comparison to previous systems and alternative methods. Measurements were carried out in transmission mode using an Fourier transform infrared (FTIR) microscope, recording spectra with spot sizes of 180 × 100 μm(2) each at defined spots along this channel. Each of these spots corresponds to a specific reaction time: moving the measurement spot towards the entry yields shorter reaction times, moving it towards the channel's end gives longer reaction times. This principle is generic in nature and provides a solution for accurate, chemically induced triggering of reactions requiring the mixing of two liquid reagents or reagent solutions. A typical experiment thus yields up to 85 time-coded data points, covering a time span from 1 to 80 ms at a total reagent consumption of only about 125 μL. Using the fast neutralization reaction of acetic acid with sodium hydroxide as a model, the time required for 90% mixing was determined to be around 4 ms. Additionally, first experiments on ubiquitin changing its secondary structure from native to "A-state" were carried out, illustrating the potential for time-resolved measurements of proteins in aqueous solutions.

A computational study of ultrafast acid dissociation and acid-base neutralization reactions. I. The model

Ultrafast, time-resolved investigations of acid-base neutralization reactions have recently been performed using systems containing the photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) and various Bronsted bases. Two conflicting neutralization mechanisms have been formulated by Mohammed et al. [Science 310, 83 (2005)] and Siwick et al. [J. Am. Chem. Soc. 129, 13412 (2007)] for the same acid-base system. Herein an ab initio molecular dynamics based computational model is formulated, which is able to investigate the validity of the proposed mechanisms in the general context of ground-state acid-base neutralization reactions. Our approach consists of using 2,4,6-tricyanophenol (exp. pKa congruent with 1) as a model for excited-state HPTS( *) (pKa congruent with 1.4) and carboxylate ions for the accepting base. We employ our recently proposed dipole-field/quantum mechanics (QM) treatment [P. Maurer and R. Iftimie, J. Chem. Phys. 132, 074112 (2010)] of the proton donor and acceptor molecules. This approach allows one to tune the free energy of neutralization to any desired value as well as model initial nonequilibrium hydration effects caused by a sudden increase in acidity, making it possible to achieve a more realistic comparison with experimental data than could be obtained via a full-QM treatment of the entire system. It is demonstrated that the dipole-field/QM model reproduces correctly key properties of the 2,4,6-tricyanophenol acid molecule including gas-phase proton dissociation energies and dipole moments, and condensed-phase hydration structure and pKa values.

Simultaneous pH and temperature measurements using pyranine as a molecular probe

Steep variations in concentration and temperature frequently occur in small fluid compartments such as those found in cells or microfluidic devices. A quantitative characterization of concentration and temperature gradients is therefore required before these systems can be fully understood. Although different spatially resolved fluorescence methods have been developed to measure either the temperature or the concentration of ions such as proton or calcium, often concentration measurements depend on temperature and vice versa. Here, we describe a method allowing simultaneous measurement of pH and temperature. This method is based on the detection of the blinking of the fluorescent pH indicator pyranine, a process due to its alternating between a basic form and an acidic form. Fluorescence correlation spectroscopy allows measuring both the protonation and deprotonation rates of pyranine, and each pair of rates can be uniquely related to a pair of pH and temperature values. We show, however, that the relationship between rates, pH and temperature, is very sensitive to the presence of other acid-base molecules in solution. We also show that it is influenced by the overall ionic strength of the solution, in a manner that depends on buffer composition.

Distance-dependent proton transfer along water wires connecting acid-base pairs

We report time-resolved mid-IR kinetics for the ultrafast acid-base reaction between photoexcited 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS), and acetate at three concentrations (0.5, 1.0, and 2.0 M) and three temperatures (5, 30, and 65 degrees C) in liquid D(2)O. The observed proton-transfer kinetics agree quantitatively, over all times (200 fs-500 ps), with an extended Smoluchowski model which includes distance-dependent reactivity in the form of a Gaussian rate function, k(r). This distance dependence contrasts with the exponential k(r) that is typically observed for electron-transfer reactions. The width of k(r) is essentially the only parameter varied in fitting the proton-transfer kinetics at each concentration and temperature. We find that k(r) likely represents the rate of concerted (multi)proton hopping across "proton wires" of different length r that connect acid-base pairs in solution. The concerted nature of the proton transfer is supported by the fact that k(r) shows a steeper dependence on r at higher temperatures.