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.