Excited-state Proton Transfer Rate Research Articles

Could Chemical Reaction at the Molecular Level Show Distinction between Two Liquid-Water States? Study of the Excited-State Water-Catalyzed Proton Transfer Reaction Provides a Clue.

The two liquid-water states, which lead to some anomalies when temperature crosses over 50 ± 10 °C at the atmospheric pressure, have been continuously catching popular attention. In this study, using the excited-state proton transfer (ESPT) catalyzed by water molecules as a prototypical reaction, we demonstrate that the kinetics of ESPT indeed is influenced by the two liquid-water states. In the water-catalyzed ESPT of 3-cyano-7-azaindole (3CAI), a repetitive and comprehensive temperature-dependent study of ESPT in H2O from 0 to 90 °C shows anomalous behavior. The plot of the logarithm of ESPT rate constant as a function of inverse of absolute temperature deviates from a straight line. The convex-Arrhenius behavior manifests the activation free energy for water-assisted ESPT being dependent on temperature and hence the liquid water structure. To simplify the discussion, the plot is well fitted by using two straight lines that are crossed over in the vicinity of 40 °C. The free energy difference between water-solvated 3CAI and the 1:1 H2O:3CAI complex is deduced to be 2.29 ± 0.04 and 1.96 ± 0.04 kcal·mol-1 in the regions of 0-40 and 40-90 °C water, respectively, which also results in different frequency factors, i.e., the proton transfer/tunneling rates of (5.83 ± 0.36) × 1010 and (3.48 ± 0.27) × 1010 s-1, respectively. In a qualitative manner, the results are then rationalized by the different types of H-bonding configuration as proposed for two liquid-water phases, rendering experimental evidence to support the different water phases in ambient temperatures at 1 bar.

Excited state proton transfer in reverse micelles: Effect of temperature and a possible interplay with solvation

Excited state proton transfer (ESPT) is a fundamental process of immense biophysical interest and considering the heterogeneity existing in real biological environments we investigate the process in a bio-mimicking reverse micellar (RM) systems. We herein report a detailed study on the ESPT process of a photo-acid d-luciferin at different temperatures in RMs composed of: anionic AOT, cationic DDAB, and neutral Igepal-520 using steady state and time resolved fluorescence measurements. We found that with increasing temperature both solvation as well as the ESPT rate accelerate, however, the extent of the increase is RM specific, and they even not complement each other. Our study clearly identifies the pivotal role of solvation, specially in micro-heterogeneous environments, to guide the ESPT process.

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 χwater < 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 χwater < 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 χwater = 0.14, the R*O- fluorescence band shifts noticeably to the blue region. For χwater > 0.23 the band resembles the free anion band observed in pure water. Concomitantly, the ESPT rate increases when χwater increases because the intermolecular ESPT to the solvent (bulk water) gradually prevails over the much slower intramolecular via the water-bridges ESPT process.

Ultrafast excited-state proton transfer dynamics in dihalogenated non-fluorescent and fluorescent GFP chromophores.

Green fluorescent protein (GFP) has enabled a myriad of bioimaging advances due to its photophysical and photochemical properties. To deepen the mechanistic understanding of such light-induced processes, novel derivatives of GFP chromophore p-HBDI were engineered by fluorination or bromination of the phenolic moiety into superphotoacids, which efficiently undergo excited-state proton transfer (ESPT) in aqueous solution within the short lifetime of the excited state, as opposed to p-HBDI where efficient ESPT is not observed. In addition, we tuned the excited-state lifetime from picoseconds to nanoseconds by conformational locking of the p-HBDI backbone, essentially transforming the nonfluorescent chromophores into highly fluorescent ones. The unlocked superphotoacids undergo a barrierless ESPT without much solvent activity, whereas the locked counterparts exhibit two distinct solvent-involved ESPT pathways. Comparative analysis of femtosecond transient absorption spectra of these unlocked and locked superphotoacids reveals that the ESPT rates adopt an "inverted" kinetic behavior as the thermodynamic driving force increases upon locking the backbone. Further experimental and theoretical investigations are expected to shed more light on the interplay between the modified electronic structure (mainly by dihalogenation) and nuclear motions (by conformational locking) of the functionalized GFP derivatives (e.g., fluorescence on and off).

Catalytic-Type Excited-State N-H Proton-Transfer Reaction in 7-Aminoquinoline and Its Derivatives.

7-Aminoquinoline (7AQ) and various amino derivatives thereof (-NHR) have been strategically designed and synthesized to study their excited-state proton-transfer (ESPT) properties. Due to the large separation between the proton donor -NHR and the acceptor -N- site, ESPT in 7AQ derivatives, if available, should proceed under protic solvent catalysis. ESPT is found to be influenced by the acidity of -NHR and the basicity of the proton-acceptor -N- in the quinoline moiety. The latter is varied by the resonance effect at the quinoline -N- site induced by the -NHR substituent. For those 7AQ derivatives undergoing ESPT, increased quinoline basicity results in a faster rate of ESPT, implying that proton donation from methanol to the quinoline moiety may serve as a key step in the process. Our studies also indicate the existence of an equilibrium between cis and trans arrangements of -NHR in terms of its hydrogen-bond (H-bond) configuration with methanol, whereby only the cis-H-bonded form undergoes methanol-assisted ESPT. With one exception, the interconversion between cis and trans configurations is much faster than the rate of ESPT, yielding amino-type (normal form) and imine-type (proton-transfer tautomer) emissions with distinct relaxation dynamics.

Enhanced Excited-State Proton Transfer via a Mixed Methanol-Water Molecular Bridge of 1-Naphthol-3,6-disulfonate in Methanol-Water Mixtures.

Steady-state and time-resolved fluorescence techniques were used to study the excited-state proton transfer (ESPT) from an irreversible photoacid, 1-naphthol-3,6-disulfonate (1NP36DS), to methanol-water mixtures. We found that at χwater = 0.3 the ESPT rate constant is higher by a factor of 10 that in neat methanol. TD-DFT calculations show that a mixed molecular bridge of two methanol molecules and one water molecule enables the ESPT from the 1-OH to the 3-sulfonate. The RO-(S1) state is stable by -2.5 kcal/mol in comparison to the ROH(S1) state. We compare the ESPT rate constants of a reversible photoacid, 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS), in the same methanol-water mixtures. At χwater ≈ 0.3 the ESPT rate constant of HPTS increased by only 15%. We explain the large difference of the ESPT rate of 1NP36DS by the formation of a water bridge or a mixed methanol-water bridge from 1-OH to one of the sulfonates and the absence of such a bridge in HPTS. The water or mixed methanol-water bridge of 1NP36DS enhances the ESPT rate in methanol-water mixtures of low water mole ratio.

Different types of hydrogen-bonded complexes would accelerate or delay the excited state proton transfer in 3-hydroxyflavone

3-hydroxyflavone (3HF) has been found dual emission in polar and protic solvents for many years. The excited state proton transfer (ESPT) is the key to control the dual emissions of 3HF. We found the ESPT rate of 3HF in hexane is faster than in acetonitrile because of the lower ESPT energy barrier in hexane. Hence, it was found there are two ESPT rates in methanol, one is faster than in hexane, and the other is slower than in acetonitrile. Two solvent-solute hydrogen bonded complexes (I and II) response to the two ESPT rates. The energy barrier of ESPT was found the lowest (1.324 kcal/mol) in Complex I, corresponded to the shortest ESPT time constant (<0.09 ps). In Complex II, the ESPT time constant is the longest (21.97 ps) with the highest ESPT energy barrier (3.865 kcal/mol). Comparing the results of two complexes, we found that intermolecular hydrogen-bonds would accelerate or delay the ESPT of 3HF based on their different types.

Excited-State Proton Transfer to H2O in Mixtures of CH3CN-H2O of a Superphotoacid, Chlorobenzoate Phenol Cyanine Picolinium (CBCyP).

Steady-state and time-resolved fluorescence techniques were employed to study a superphotoacid with a p Ka* of ∼-7, the chlorobenzoate phenol cyanine picolinium salt (CBCyP) in acetonitrile-water mixtures. We found that the time-resolved fluorescence is bimodal. The amplitude of the short-time component depends on χwater; the larger χwater, the greater the amplitude. We found that the excited-state proton-transfer (ESPT) rate constant, kPT, is ≥5 × 1012 s-1 in mixtures of χwater ≥ 0.08, whereas in neat water, kPT = 6 × 1012 s-1. The long-time component has a lifetime of 50 ps at χwater = 0.75. We attribute this time component to the CBCyP molecules that are not hydrogen-bonded to H2O clusters. The results suggest that the ESPT rate constant to water in acetonitrile-water mixtures depends only slightly on the water cluster size and structure surrounding the CBCyP molecule. We attribute the independence of the ESPT rate on the average water-cluster size to the large photoacidity of CBCyP. QM TD-DFT calculations found that in the excited-state the RO-(S1) species that is formed by the ESPT process is more stable than the ROH(S1) species by -5 kcal/mol when four water molecules accept the proton, and when six water molecules accept the proton, the RO-(S1) drops to -10 kcal/mol. The calculations show that energy stabilities are kept constant in implicit CH3CN-H2O solvent mixtures of dielectric constant of ε ≥ 45.

Excited-State Proton Transfer from the Photoacid 2-Naphthol-8-sulfonate to Acetonitrile/Water Mixtures.

Steady-state and time-resolved fluorescence techniques were used to study excited-state proton transfer (ESPT) to water of the reversible photoacid 2-naphthol-8-sulfonate (2N8S) in acetonitrile/water mixtures. In acetonitrile-rich mixtures, up to χwater ≤ 0.12, we found a slow ESPT process on the order of nanoseconds. At χwater ≈ 0.15, the RO- fluorescence band intensity is at the minimum, whereas at χwater ≈ 0.030, it is at the maximum. The steady-state fluorescence spectra of these mixtures show that the intensity of the RO- fluorescence band at χwater ≈ 0.030 is about 0.24 of that of the ROH band. We explain this unusual phenomenon by the presence of water clusters that exist in the acetonitrile-rich CH3CN/H2O mixtures. We propose that a water bridge forms between the 2-OH and 8-sulfonate by preferential solvation of 2N8S, and this enables the ESPT process between the two sites of the molecular structure of 2N8S. In mixtures of χwater ≥ 0.25, the ESPT process takes place to water clusters in the bulk mixture. The higher the χwater in the mixture, the greater the ESPT rate constant. In neat water, the rate constant is rather small, 4.5 × 109 s-1. TD-DFT calculations show that a single water molecule can bridge between 2-OH and 8-sulfonate in the excited state. The activation energy for the ESPT reaction is about 9 kcal/mol, and the RO-(S1) species is energetically above the ROH(S1) species by about 1.6 kcal/mol.

Modulation of the Excited-State Proton Transfer Rate of d-luciferin in Mixed Reverse Micellar Systems.

The excited-state proton transfer (ESPT) rate of photo-acids in a confined medium depends on several physical parameters of the immediate environment. We introduce a new parameter in the form of charge type at the interface of reverse micellar (RM) systems to modulate the ESPT rate. We investigate the ESPT reaction of d-luciferin in mixed RM systems composed of nonionic polyoxyethylene(5)nonylphenylether (Igepal CO-520) with cationic didodecyldimethylammonium bromide (DDAB) and anionic sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in cyclohexane (Cy) at different mole fractions of Ig (XIg) and fixed hydration. ESPT is feeble in AOT RM, whereas it is favorable in the other two RMs. Addition of Ig is observed to facilitate ESPT in AOT RM linearly, whereas in DDAB, it shows a synergistic effect. The various physical parameters of water in the mixed RM water pool have been investigated using dynamic light scattering, Fourier transform infrared, and time-resolved fluorescence spectroscopy measurements to underline the ESPT mechanism in these mixed RMs.

Enhanced Excited-State Proton Transfer via a Mixed Water-Methanol Molecular Bridge of 1-Naphthol-5-Sulfonate in Methanol-Water Mixtures.

We used steady-state and time-resolved fluorescence techniques to study the excited-state proton transfer (ESPT) and the nonradiative properties of two irreversible photoacids, 1-naphthol-4-sulfonate (1N4S) and 1-naphthol-5-sulfonate (1N5S). We found that the ESPT rate constant of 1N4S in water is 2.2 × 1010 s-1, whereas in methanol, it is smaller by about 3 orders of magnitude and is not observed. The ESPT process of 1N5S competes with a major nonradiative process of equal rate and kPT of 2.2 × 1010 s-1. In methanol-water mixtures of χH2O = 0.2, the fluorescence lifetime of the ROH form of 1N5S is lower by a factor of 10 than that in pure methanol. In the steady-state fluorescence spectra of 1N5S in methanol-water mixtures, there are two iso-emissive points, one for χH2O < 0.2 and one for χH2O > 0.3. This large reduction in fluorescence intensity and the two iso-emissive points are explained by the existence of a mixed water-methanol bridge of about three molecules that connects the proton donor 1-OH with the 5-sulfonate in mixtures of χH2O < 0.2. The bridge enhances both the ESPT and the nonradiative processes. For 1N4S in methanol-water mixtures at χH2O ≈0.2, the reduction in the fluorescence lifetime is only by ∼30%, and only one iso-emissive point exists in the steady-state fluorescence spectra for 0 <χH2O < 1. TD-DFT computations show that a mixed bridge of one water molecule and two methanol molecules that connects the 1-OH with 5-sulfonate is more stable by 7.7 kcal/mol than the 1-OH reactant in the S1 state, and the barrier is only 8.0 kcal/mol. The nonradiative channel is because the S2 dark state is about 4.6 kcal/mol higher than the S1 state.

Excited-State Proton Transfer of Phenol Cyanine Picolinium Photoacid.

Steady-state and time-resolved fluorescence techniques as well as quantum-mechanical calculations were used to study the photophysics and photochemistry of a newly synthesized photoacid—the phenol cyanine picolinium salt. We found that the nonradiative rate constant knr of the excited protonated form of the photoacid is larger than that of the excited-state proton transfer (ESPT) to the solvent, kESPT. We estimate that the quantum efficiency of the ESPT process is about 0.16. The nonradiative process is explained by a partial trans–cis isomerization reaction, which leads to the formation of a “dark” excited state that can cross to the ground state by nonadiabatic coupling. Moreover, the ESPT process is coupled to the photo-isomerization reaction, as this latter reaction enhances the photoacidity of the studied compound, as a result of photoinduced charge transfer. To prevent trans–cis isomerization of the cyanine bridge, we conducted experiments of PCyP adsorbed on cellulose in the presence of water. We found that the steady-state fluorescence intensity increased by about a factor of 50 and the lifetime of the ROH band increased by the same factor. The fluorescence intensity of the RO– band with respect to that of the ROH band was the same as in aqueous solution. This explains why inhibiting the photo-isomerization reaction by adsorbing the PCyP on cellulose does not lead to a higher ESPT rate.

The photoacidity of phenol chloro benzoate cyanine picolinium salt photoacid in alkanols

Abstract Steady-state and time-resolved fluorescence techniques were employed to study the excited-state proton transfer (ESPT) rate to methanol, ethanol and propanol of a new photoacid, the chloro benzoate phenol cyanine picolinium salt (CBCyP). We found that the ESPT rate constants for methanol, ethanol and propanol are about 3 × 1012 s−1, 2 × 1012 s−1 and 1.2 × 1012 s−1 respectively, whereas for water it is 6 × 1012 s−1. The photoacid pKa* in water is about pKa ∼ −7. The kinetic isotope effect as measured from the fluorescence decay rate of the protonated form is 1.5 and 1.25 for methanol and ethanol respectively, whereas in water it is 1.7. We suggest that a nonradiative process takes place and reduces the measured kinetic isotope effect in both methanol and ethanol.

Excited‐State Proton Transfer Reaction of Pyranine in Aqueous Sugar and Alcohol Solutions Investigated by Fluorescence Spectroscopy

The fluorescence emission behaviors of excited pyranine (PyOH*) and deprotonated pyranine (PyO−*) were studied to explore the effects of sugar or alcohol on the excited‐state proton transfer (ESPT) reaction of pyranine in aqueous solutions. The fluorescence intensity ratio, Q between the two emission bands of pyranine varies significantly with the type and concentration of sugar or alcohol in aqueous solution. The results clearly show that a higher Q value (more PyOH* and less PyO−*) reflects a lower ESPT rate due to less free water molecules for solvation of PyO−* and H+. Higher Q values were observed in sugar solutions while lower values in alcohol solutions. Larger alcohols yielded higher Q values and the values for polyols (ethylene glycol and glycerin) were between those of methanol and ethanol, revealing that the hydroxyl groups in polyols form intramolecular hydrogen‐bond. The location of the hydroxyl group was also found to affect the intensity ratio, Q. The conductivities of 0.01 M HCl in aqueous sugar and alcohol solutions are consistent with the Q values observed, and the inverse of Q show a quadratic relation with conductivity. The enthalpy and entropy changes for the deprotonation reaction of PyOH* in fructose and ethanol aqueous solutions were also determined, assuming that the deprotonation reaction was an equilibrated process. The higher ΔH in fructose solution reveal that deprotonation of PyOH* in sugar solution is energetically less favored than in ethanol solution because solvation of fructose requires more water molecules.

Anomalous H+ and D+ Excited-State Proton-Transfer Rate in H2O/D2O Mixtures.

We used the time-resolved fluorescence technique to measure the excited-state proton-transfer (ESPT) rates from 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS) to solvent mixtures of H2O and D2O. We found an anomalous deviation from linear mole-fraction behavior of the ESPT rate in H2O/D2O mixtures. We provide a chemical model based on equilibrium constant of the reaction H2O + D2O ↔ 2HOD and rate constants of the ESPT process of H and D transfers from HPTS to the mixed solvent. Anomalous deviation from linear mole-fraction behavior was previously found for H+/D+ conductance in these mixtures.

A fresh look into the time-resolved fluorescence of 8-hydroxy-1,3,6-pyrenetrisulfonate with the use of the fluorescence up-conversion technique

Abstract Steady-state and time-resolved fluorescence techniques were used to study the excited-state-proton-transfer (ESPT) process of 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS) in H 2 O and D 2 O. In this contribution we use the fluorescence up-conversion technique with a time resolution of ∼100 fs to monitor the short-time components of HPTS ROH (protonated) and RO − (deprotonated) signals. The ESPT rate constant, k PT , for HPTS in H 2 O and D 2 O is rather small 10 10 s −1 and 3.3 × 10 9 s −1 , respectively. In the time-resolved fluorescence signal of the deprotonated form we find a rise-component of 2.5 ps which we assign to slow charge rearrangement as was already suggested by Spry and Fayer [Spry, D. B.; Fayer, M. D. Charge Redistribution and Photoacidity: Neutral Versus Cationic Photoacids. J. Chem. Phys . 2008 , 128, 084508-1–084508-9]. Already in the time-window of 0.2–1.2 ns, the proton geminate recombination (GR) fluorescence tail of the ROH form decays as t −α where α ≈ 3/2, as predicted by the diffusion-assisted GR model, but for much longer times ( t > 5 ns). We also found that the rotation-relaxation time of the ROH form is about τ or = 80 ps in H 2 O, shorter than previously reported, whereas in methanol solution, with much lower viscosity, it is much larger – τ or = 190 ps. We explain this large difference of τ or by counter-ion association on all the three sulfonate groups of HPTS.

New Phenol Benzoate Cyanine Picolinium Salt Photoacid Excited-State Proton Transfer.

Steady-state and time-resolved fluorescence techniques were employed to study the excited-state proton transfer (ESPT) to water and D2O of a new photoacid, phenol benzoate cyanine picolinium salt (BCyP). We found that the ground-state pKa is about 6.5, whereas the excited-state pKa* is about -4.5. The ESPT rate constant, kPT, to water is ∼0.5 × 1012s-1 (τPT ≈ 2 ps) and in D2O the rate is 0.33 × 1012 s-1. We determined that the BCyP photoacid belongs to the third regime of photoacids, the solvent-controlled regime.

Combined experimental and theoretical study of the photochemistry of 4- and 3-hydroxycoumarin

Abstract Steady-state and time-resolved optical spectroscopy techniques were employed to study the photophysics and photochemistry of 4- (4HC) and 3-hydroxycoumarin (3HC). We found that the p Ka of 4HC in aqueous solutions is about 7.5 and the p Ka* is only 4.5. We also found that the nonradiative rate of 4-hydroxycoumarin is high and the excited state proton transfer (ESPT) rate to water is rather slow and so an ESPT process is not observed. Quantum mechanical calculations confirm that the ESPT rate should be slow and the intersystem crossing rate S1 → T is large. An ESPT process was also not observed for 3HC, but it might still take place and not being observed as the fluorescence quantum yield of the anion is two orders of magnitude smaller than that of the protonated form. The nonradiative decay of protonated 3HC is explained with an H-atom abstraction mechanism, leading to the formation of a radical species.

Comparison of the Photoprotolytic Processes of Three 7-Hydroxycoumarins.

Steady-state and time-resolved fluorescence techniques, in addition to quantum mechanical calculations, were employed to study the excited-state proton transfer (ESPT) to solvent of three 7-hydroxycoumarin dyes. We found that for 7-hydroxycoumarins in water, the ESPT rate is high, about 2 × 1010 s-1, whereas in methanol the ESPT rate is much lower than that over the nonradiative lifetime of the excited singlet state; thus, the ESPT efficiency is very low.

Effect of neutral salts on the excited state proton transfer in the fluorescent probe anchored to the uncharged micelles

Abstract We study the salt effect on the excited state proton transfer (ESPT) in the fluorescent probe of 2-hydroxynaphthalene (dodecylo)-6-sulphonamide (NSDA) anchored on the nonionic Brij35 micelles. The results of spectrofluorimetric, electrophoretic and equilibrium dialysis measurements, allowed us to explain the variations in the ESPT rate constants in the presence of different neutral salts (NaCl, KCl, KSCN, NH 4 SCN) as resulting from anion accumulation at the particle surface. Similarly to electric double layer in the charged micelles, an ionic layer system is also formed around the nonionic micelles. The dimensions of anionic and cationic layers as well as those of the whole ionic envelope around the nonionic particles are estimated. Furthermore, the electric potential generated within the ionic layer system and acting on the proton dissociating from the probe is determined and interpreted. Different impact of Cl − and SCN − ions on photophysical parameters of the targeted system have been explained through a synergy effect between hydrophobicity and polarizability of anions adsorbing on the nonionic micelle surface.Taking into account the existing analogy between micelles and living cells, we can assume that these results will contribute to a better understanding of salt effect in such important biological phenomena as the cell adhesion to surfaces, thermal denaturation of proteins and the aggregation of erythrocytes.