How Fast Can a Proton-Transfer Reaction Be beyond the Solvent-Control Limit?

In this article, we review the field of photoacids. The rate of excited-state proton transfer (ESPT) to solvent spans a wide range of time scales, from tens of nanoseconds for the weakest photoacids to short time scales of about 100 fs for the strongest photoacids synthesized so far. We divide the photoacid strength into four regimes. Regime I includes the weak photoacids 0 < p<i>K</i>_a* < 3. These photoacids can transfer a proton only to water or directly to a mild-base molecule in solution. The ESPT rate to other protic solvents, like methanol or ethanol, is too small in comparison with the radiative rate. The second regime includes stronger photoacids whose p<i>K</i>_a*’s range from −4 to 0. They are capable of transferring a proton to other protic solvents and not only to water. The third regime includes even stronger photoacids. Their p<i>K</i>_a* is ∼ –6, and the ESPT rate constant, <i>k</i>_PT, is limited by the orientational time of the solvent which is characterized by the average solvation correlation function ⟨<i>S</i>(<i>t</i>)⟩. The fourth regime sets a new time limit for the ESPT rate of the strongest photoacids synthesized so far. The <i>k</i>_PT value of such photoacids is 10¹³ s^–1, and τ_PT = 100 fs. We attribute this new time limit (beyond the solvent control) to intermolecular vibration between the two heavy atoms of the proton donor and the proton acceptor, which assist the ESPT by lowering the height and width of the potential barrier, thus enhancing the ESPT rate

Temperature Dependence of the Excited-State Proton-Transfer Reaction of Quinone-cyanine‑7

Steady-state and time-resolved fluorescence techniques were used to study the temperature dependence of the photoprotolytic process of quinone-cyanine-7 (QCy7), a very strong photoacid, in H₂O and D₂O ice, over a wide temperature range, 85–270 K. We found that the excited-state proton-transfer (ESPT) rate to the solvent decreases as the temperature is lowered with a very low activation energy of 10.5 ± 1 kJ/mol. The low activation energy is in accord with free-energy-correlation theories that predict correlation between Δ<i>G</i> of reaction and the activation energy. At very low temperatures (<i>T</i> < 150 K), we find that the emission band of the RO^–*, the deprotonated form of QCy7, is blue-shifted by ∼1000 cm^–1. We attributed this band to the RO^–*···H₃O⁺ ion pair that was suggested to be an intermediate in the photoprotolytic process but has not yet been identified spectroscopically

Effect of Acid on the Ultraviolet–Visible Absorption and Emission Properties of Curcumin

Steady-state and time-resolved emission techniques were employed to study the acid–base effects on the UV–vis spectrum of curcumin in several organic solvents. The fluorescence-decay rate of curcumin increases with increasing acid concentration in all of the solvents studied. In methanol and ethanol solutions containing about 1 M HCl, the short-wavelength fluorescence (λ < 560 nm) decreases by more than an order of magnitude. (The peak fluorescence intensity of curcumin in these solvents is at 540 nm.) At longer wavelengths (λ ≥ 560 nm) the fluorescence quenching is smaller by a factor of ∼3. A new fluorescence band with a peak at about 620 nm appears at an acid concentration of about 0.2 M in both methanol and ethanol. The 620 nm/530 nm band intensity ratio increases with an increase in the acid concentration. In trifluoroethanol and also in acetic acid in the presence of formic acid, the steady-state emission of curcumin shows an emission band at 620 nm. We attribute this new emission band in hydrogen-bond-donating solvents to a protonated curcumin ROH₂⁺ form. At high acid concentrations in acetic acid and in trifluoroethanol, the ground state of curcumin is also transformed to ROH₂⁺ which absorbs at longer wavelengths with a band peak at ∼530 nm compared to 420 nm in neutral-pH samples or 480 nm in basic solutions. In hydrogen-bond-accepting solvents such as dimethyl sulfoxide and also in methanol and ethanol, curcumin does not accept a proton to form the ground-state ROH₂<sup>+</sup

Excited-State Proton Transfer from Quinone-Cyanine 9 to Protic Polar-Solvent Mixtures

Steady-state and time-resolved emission techniques were used to study the excited-state proton-transfer (ESPT) process of quinone cyanine 9 (QCy9) in solvent mixtures. We found that the ESPT rate from QCy9 in water/methanol mixtures is independent of the mixture composition and the rate constant is <i>k</i>_PT ∼ 10¹³ s^–1. In ethanol/trifluoroethanol (TFE) mixtures the ESPT rate strongly depends on the solvent-mixture composition. We observe two ESPT rates rather than one over a wide range of solvent-mixture compositions. The average ESPT rate decreases as the mole fraction of TFE increases

Ultrafast Excited-State Proton Transfer to the Solvent Occurs on a Hundred-Femtosecond Time-Scale

Steady-state and ultrafast time-resolved techniques were used to study a newly synthesized photoacid, phenol-carboxyether dipicolinium cyanine dye, QCy9. We found that the excited-state proton transfer (ESPT) to water occurs at the remarkably short time of about 100 fs, <i>k</i>_PT ≈ 1 × 10¹³ s^–1, the fastest rate reported up to now. On the basis of the Förster-cycle, the p<i>K</i>_a* value is estimated to be −8.5 ± 0.4. In previous studies, we reported the photoacidity of another superphotoacid, the QCy7 for which we found an ESPT rate constant of ∼1.25 × 10¹² s^–1, one-eighth that of the QCy9 compound. We found a kinetic isotope effect of the ESPT of about two

Comprehensive Study of Ultrafast Excited-State Proton Transfer in Water and D₂O Providing the Missing RO^–···H⁺ Ion-Pair Fingerprint

Steady-state and time-resolved optical techniques were employed to study the photoprotolytic mechanism of a general photoacid. Previously, a general scheme was suggested that includes an intermediate product that, up until now, had not been clearly observed experimentally. For our study, we used quinone cyanine 7 (QCy7) and QCy9, the strongest photoacids synthesized so far, to look for the missing intermediate product of an excited-state proton transfer to the solvent. Low-temperature steady-state emission spectra of both QCy7 and QCy9 clearly show an emission band at <i>T</i> < 165 K in H₂O ice that could be assigned to ion-pair RO^–*···H₃O⁺, the missing intermediate. Room-temperature femtosecond pump–probe spectroscopy transient spectra at short times (<i>t</i> < 4 ps) also shows the existence of transient absorption and emission bands that we assigned to the RO^–*···H₃O⁺ ion pair. The intermediate dissociates on a time scale of 1 ps and about 1.5 ps in H₂O and D₂O samples, respectively

Excited-State Proton Transfer and Proton Diffusion near Hydrophilic Surfaces

Time-resolved emission techniques were employed to study the reversible proton photoprotolytic properties of surface-attached 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) molecules to hydrophilic alumina and silica surfaces. We found that the excited-state proton transfer rate of the surface-linked HPTS molecules, in H₂O and D₂O, is nearly the same as of HPTS in the bulk, while the corresponding recombination rate is significantly greater. Using the diffusion-assisted proton geminate-recombination model, we found that the best fit of the time-resolved fluorescence (TRF) signal is obtained by invoking a two-dimensional diffusion space for the proton to recombine with the conjugated basic form, RO^–*, of the surface-linked HPTS. However, we obtain an excellent fit by a three-dimensional diffusion space for diffusional HPTS in bulk water. These results indicate that the photoejected solvated protons are confined to the surface for long periods of time. We suggest two plausible mechanisms responsible for two-dimensional proton diffusion next to hydrophilic surfaces

Excited-State Proton Transfer of Firefly Dehydroluciferin

Steady-state and time-resolved emission techniques were used to study the protolytic processes in the excited state of dehydroluciferin, a nonbioluminescent product of the firefly enzyme luciferase. We found that the ESPT rate coefficient is only 1.1 × 10¹⁰ s^–1, whereas those of d-luciferin and oxyluciferin are 3.7 × 10¹⁰ and 2.1 × 10¹⁰ s^–1, respectively. We measured the ESPT rate in water–methanol mixtures, and we found that the rate decreases nonlinearly as the methanol content in the mixture increases. The deprotonated form of dehydroluciferin has a bimodal decay with short- and long-time decay components, as was previously found for both d-luciferin and oxyluciferin. In weakly acidic aqueous solutions, the deprotonated form’s emission is efficiently quenched. We attribute this observation to the ground-state protonation of the thiazole nitrogen, whose p<i>K</i>_a value is ∼3