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.

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.

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.

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 k nr of the excited protonated form of the photoacid is larger than that of the excited-state proton transfer (ESPT) to the solvent, k ESPT. 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.

Anomalous Rate of H+ and D+ Excited-State Proton Transfer (ESPT) in H2O/D2O Mixtures: Irreversible ESPT in 1-Naphthol-4-sulfonate.

We employed steady-state and time-resolved fluorescence techniques to study the rates of excited-state proton and deuteron transfer (ESPT) from an irreversible photoacid, 1-naphthol-4-sulfonate, to solvent mixtures of H2O and D2O. We found that the overall ESPT rate to the solvent mixture does not follow a linear relation with the H2O mole ratio. We used a chemical kinetic model to explain the deviation of the ESPT rate constant from linear behavior with H2O mole ratio. There are three water species in the H2O-D2O mixtures, H2O, D2O, and HOD. There are six rate constants of H+ and D+ transfers to the three species. When the H2O mole ratio before mixing is 0.5, HOD mole ratio in the mixture is 0.5. The ESPT rate to HOD is much smaller than that of H+ transfer to neat H2O and hence the concave shape of the plot of ESPT rate constants versus the H2O molar ratio of the mixtures.

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.

Irradiation by blue light in the presence of a photoacid confers changes to colony morphology of the plant pathogen Colletotrichum gloeosporioides.

We used the photoacid 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS) that converts blue photons to acidic protons in water, with an efficiency of close to 100%, and determined that this treatment conferred changes to colony morphology of the plant pathogen Colletotrichum gloeosporioides. The time elapsed until hyphal collapse is noticed depends on both the laser intensity in mW/cm2, and the concentration of HPTS in the Agar hydrogel. The time elapsed until hyphal collapse is noticed varies by only ±8% at HPTS concentrations of 500µM and at lower concentrations of HPTS the variance increases as the inverse of the concentration. We found that the effect on C. gloeosporioides was photoacid concentration and irradiation dose dependent. In the presence of 500µM of HPTS within the agar hydrogel-based medium, hyphae collapsed after 37±3.5min of irradiation at 405nm at an intensity of 25mW/cm2. We propose two mechanisms for such photo-alteration of C. gloeosporioides. One is based on the pH drop in the extracellular environment by the photo-protolytic process that the photoacid molecule undergoes. The second mechanism is based on an intracellular mechanism in which there is an uptake of HPTS into the interior of the fungus. We suggest that both mechanisms for photo-alteration which we found in this study may occur in plants during fungal infection.

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.

Photoprotolytic Processes of Lumazine.

Steady-state and time-resolved UV-vis spectroscopies were used to study the photoprotolytic properties of lumazine, which belongs to a class of biologically important compounds-the petridines. We found that in water an excited-state proton transfer occurs with a time constant of ∼70 ps and competes with a nonradiative rate of about the same value. The nonradiative rate of the protonated form of lumazine in polar and nonpolar solvents is large knr ≥ 1.5 × 1010s-1. The fluorescence properties indicate that in water, the ground-state neutral form of lumazine is already stable in two tautomeric forms. The fluorescence of the deprotonated form is quenched by protons in acidic solutions with a diffusion-controlled reaction rate. We conclude that the neutral form of lumazine is an irreversible mild photoacid.

Reversible Excited-State Proton Geminate Recombination: Revisited.

About three decades ago, Pines and Huppert found that the excited-state proton transfer to water from a photoacid (8-hydroxy-1,3,6-pyrene trisulfonate (HPTS)) is followed by an efficient diffusion-assisted reversible geminate-recombination of the proton. To model the reaction, Pines, Huppert, and Agmon used the Debye-Smoluchowski equation with boundary conditions appropriate for reversible contact reaction kinetics. This reaction model has been used successfully to quantitatively fit the experimental data of the time-resolved fluorescence of HPTS and several commonly used photoacids. A consequence of the reversibility of this reaction is an apparent long-time tail of the photoacid fluorescence signal, obeying (after lifetime correction) a t-3/2 power law asymptotics. Recently, Lawler and Fayer reported that in bulk water the observed power-law decay of the long-time fluorescence tail of HPTS is -1.1 rather than -1.5, as expected from the spherically symmetric diffusion model. In the current study, we reaffirm our previous reports of the power-law behavior of HPTS fluorescence. We also demonstrate that molecular-level complications such as the deviation from spherical symmetry, rotational dynamics, competitive proton binding to the sulfonate moieties of HPTS, distance-dependent diffusion coefficient, and the initial starting point of the proton can affect the observed kinetics only at intermediate times, but not at asymptotically long times. Theoretically, we analyze the rebinding kinetics in terms of the number of extrema of the logarithmic derivative, showing subtle effects on the direction of approach to the asymptotic line (whether from above or below), which also appears to be corroborated experimentally.

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.

Mutagenic induction of an ultra-fast water-chain proton wire.

Replacement of the hydroxyl group of a hydrophilic sidechain by an H atom in the proton wire of GFP induces formation of a water-chain proton wire. Surprisingly, this "non-native" water chain functions as a proton wire with response times within 10 ps of the wild type protein. This remarkable rate retention is understood as a natural consequence of the well-known Grotthuss mechanism of proton transfer in water.

Excited-State Proton Transfer and Formation of the Excited Tautomer of 3-Hydroxypyridine-Dipicolinium Cyanine Dye.

Steady-state and time-resolved fluorescence techniques and theoretical calculations were employed to study the photoprotolytic properties of a newly synthesized photoacid 3-hydroxypyridine-dipicolinium cyanine (HPPC) dye. This dye is similar to quinone cyanine 9, which we have previously studied and is the strongest photoacid currently synthesized. In this compound, we found that several proton transfer phenomena occur after excitation. We found that the excited-state proton transfer (ESPT) rate in water is ultrafast with kPT ≈ 1.5 × 10(12) s(-1). In methanol and ethanol the rate is slower by about 5 and 6 times, respectively. The fluorescence spectrum of HPPC in water consists of three bands with maxima at 520, 600, and 665 nm, whereas in monols and other protic solvents the fluorescence spectrum consists only of two emission bands at 530 and ∼700 nm. We assign the emission bands of HPPC at 520 nm to the protonated form and the 700 nm band in monols and 665 nm in water to the deprotonated form. The 600 nm band that is the most intense band in the fluorescence spectrum of HPPC in water we assign to the tautomeric form in which the proton is attached to the pyridine's nitrogen atom. On the basis of density functional calculations, we suggest that in water the proton transfer process to the pyridine's nitrogen atom occurs in a stepwise manner via a two water molecule bridge.

Noncovalent Interactions with Proteins Modify the Physicochemical Properties of a Molecular Switch.

It is reported that spiropyran-a widely investigated molecular photoswitch-can be stabilized in aqueous environments in the presence of a variety of proteins, including human serum albumin, insulin fibrils, lysozyme, and glucose oxidase. The optical properties of the complexed photoswitch are protein dependent, with human serum albumin providing the spiropyran with emission features previously observed for a photoswitch confined in media of high viscosity. Despite being bound to the protein molecules, spiropyran can undergo a ring-opening reaction upon exposure to UV light. This photoisomerization process can affect the properties of the proteins: here, it is shown that the electrical conduction through human serum albumin to which the spiropyran is bound increases following the ring-opening reaction.

Photoprotolytic Processes of Umbelliferone and Proposed Function in Resistance to Fungal Infection.

The photoprotolytic processes of 7-hydroxy-coumarin (Umb) were investigated by steady-state and time-resolved-fluorescence techniques. We found that the Umb compound is a photoacid with pK(a)* ≈ 0.4 and a rate constant of the excited-state proton transfer (ESPT) to water of 2 × 10(10) s(-1). Umb is also a photobase and accepts an excess proton in solution and also directly from weak acids like acetic acid. When Umb is adsorbed on cellulose it also functions as a photoacid and a photobase. Hydroxycoumarins are known to accumulate next to fungal-, bacterial-, and viral-infected regions in the leaves and stems of plants in general and also in trees. We propose that these compounds when irradiated by sunlight UV, combat the fungi or bacteria by excited-state proton-transfer reactions. These photoprotolytic reactions provide a universal resistance mechanism to infections in plants.

Excited-State Proton Transfer in Resveratrol and Proposed Mechanism for Plant Resistance to Fungal Infection.

Steady-state and time-resolved fluorescence techniques were employed to study the photophysics and photochemistry of trans-resveratrol. trans-Resveratrol is found in large quantities in fungi-infected grapevine-leaf tissue and plays a direct role in the resistance to plant disease. We found that trans-resveratrol in liquid solution undergoes a trans-cis isomerization process in the excited state at a rate that depends partially on the solvent viscosity, as was found in previous studies on trans-stilbene. The hydroxyl groups of the phenol moieties in resveratrol are weak photoacids. In water and methanol solutions containing weak bases such as acetate, a proton is transferred to the base within the lifetime of the excited state. When resveratrol is adsorbed on cellulose (also a component of the plant's cell wall), the cis-trans process is slow and the lifetime of the excited state increases from several tens of picoseconds in ethanol to about 1.5 ns. Excited-state proton transfer occurs when resveratrol is adsorbed on cellulose and acetate ions are in close proximity to the phenol moieties. We propose that proton transfer from excited resveratrol to the fungus acid-sensing chemoreceptor is one of the plant's resistance mechanisms to fungal infection.

Excited-State Proton Transfer of Weak Photoacids Adsorbed on Biomaterials: Proton Transfer on Starch.

Steady-state and time-resolved fluorescence techniques were employed to study the excited-state proton transfer (ESPT) from a photoacid adsorbed on starch to a nearby water molecule. Starch is composed of ∼30% amylose and ∼70% amylopectin. We found that the ESPT rate of adsorbed 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS) on starch arises from two time constants of 300 ps and ∼3 ns. We explain these results by assigning the two different ESPT rates to HPTS adsorbed on amylose and on amylopectin. When adsorbed on amylose, the ESPT rate is ∼3 × 10(9 )s(-1), whereas on amylopectin, it is only ∼3 × 10(8) s(-1).

Excited-State Intramolecular Proton Transfer of the Natural Product Quercetin.

Intramolecular proton-transfer dynamics in the lowest excited state (ESIHT) were studied in the natural product quercetin. We found that in all seven solvents used in this study, the ESIHT rate is ultrafast. We estimate that the ESIHT rate is about 70 fs or less. We found that in deuterated protic solvents, such as methanol-d or ethanol-d, the ESIHT rate is slower and the proton-transfer time constant is about 110 fs. The tautomeric form fluorescence quantum yield of quercetin is very low, of the order of the normal form.