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.

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.

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 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.

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.

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.

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.

Manipulating and Monitoring On-Surface Biological Reactions by Light-Triggered Local pH Alterations.

Significant research efforts have been dedicated to the integration of biological species with electronic elements to yield smart bioelectronic devices. The integration of DNA, proteins, and whole living cells and tissues with electronic devices has been developed into numerous intriguing applications. In particular, the quantitative detection of biological species and monitoring of biological processes are both critical to numerous areas of medical and life sciences. Nevertheless, most current approaches merely focus on the "monitoring" of chemical processes taking place on the sensing surfaces, and little efforts have been invested in the conception of sensitive devices that can simultaneously "control" and "monitor" chemical and biological reactions by the application of on-surface reversible stimuli. Here, we demonstrate the light-controlled fine modulation of surface pH by the use of photoactive molecularly modified nanomaterials. Through the use of nanowire-based FET devices, we showed the capability of modulating the on-surface pH, by intensity-controlled light stimulus. This allowed us simultaneously and locally to control and monitor pH-sensitive biological reactions on the nanodevices surfaces, such as the local activation and inhibition of proteolytic enzymatic processes, as well as dissociation of antigen-antibody binding interactions. The demonstrated capability of locally modulating the on-surface effective pH, by a light stimuli, may be further applied in the local control of on-surface DNA hybridization/dehybridization processes, activation or inhibition of living cells processes, local switching of cellular function, local photoactivation of neuronal networks with single cell resolution and so forth.

Excited-state proton transfer of weak photoacids adsorbed on biomaterials: proton transfer to glucosamine of chitosan.

UV-vis steady-state and time-resolved techniques were employed to study the excited-state proton-transfer process from two weak photoacids positioned next to the surface of chitosan and cellulose. Both chitosan and cellulose are linear polysaccharides; chitosan is composed mainly of d-glucosamine units. In order to overcome the problem of the high basicity of the glucosamine, we chose 2-naphthol (pKa* ≈ 2.7) and 2-naphthol-6-sulfonate (pKa* ≈ 1.7) as the proton emitters because of their ground state pKa (≈9). Next to the 1:1 cellulose:water weight ratio, the ESPT rate of these photoacids is comparable to that of bulk water. We found that the ESPT rate of 2-naphthol (2NP) and 2-naphthol-6-sulfonate (2N6S) next to chitosan in water (1:1) weight ratio samples is higher than in bulk water by a factor of about 5 and 2, respectively. We also found an efficient ESPT process that takes place from these photoacids in the methanol environment next to the chitosan scaffold, whereas ESPT is not observed in methanolic bulk solutions of these photoacids. We therefore conclude that ESPT occurs from these photoacids to the d-glucosamine units that make up chitosan.

Excited-state proton transfer of weak photoacids adsorbed on biomaterials: 8-hydroxy-1,3,6-pyrenetrisulfonate on chitin and cellulose.

Time-resolved and steady-state florescence measurements were used to study the photoprotolytic process of an adsorbed photoacid on cellulose and chitin. For that purpose we used the 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS) photoacid which transfers a proton to water with a time constant of 100 ps, but is incapable of doing so in methanol or ethanol. We found that both biopolymers accept a proton from the electronically excited acidic ROH form of HPTS. The excited-state proton-transfer (ESPT) rate of HPTS adsorbed on chitin is greater than that on cellulose by a factor of 5. The ESPT on chitin also occurs in the presence of methanol or ethanol, but at a slower rate. The transferred protons can recombine efficiently with the conjugate excited base, the RO(-) form of HPTS.

Fast photoinduced reactions in the condensed phase are nonexponential.

Time-resolved measurements of photoinduced reactions reveal that many ultrafast reactions in the femto- to picosecond time scale are nonexponential. In this article we provide several examples of reactions that exhibit a nonexponential rate. We explain the wide range of the nonexponential reaction by the lack of time separation between τ(s), the characteristic fast equilibration time of the population in the reactant potential well, and the longer time τ(e), the characteristic time to cross the energy barrier between the reactant and the product.

Solvent dependence of excited-state proton transfer from pyranine-derived photoacids.

Steady-state and time-resolved techniques were employed to study the excited-state proton-transfer (ESPT) rate of two newly synthesized 8-hydroxy-1,3,6-pyrenetrisulfonate (pyranine, HPTS) derived photoacids in three protic solvents, water, methanol and ethanol. The ESPT rate constant k(PT) of tris(1,1,1,3,3,3-hexafluoropropan-2-yl)-8-hydroxypyrene-1,3,6-trisulfonate, 1a, whose pK(a)* ~ -4, in water, methanol and ethanol is 3 × 10(11) s(-1), 8 × 10(9) s(-1) and 5 × 10(9) s(-1) respectively. (8-Hydroxy-N1,N3,N6-tris(2-hydroxyethyl)-N1,N3,N6-trimethylpyrene-1,3,6 trisulfonamide, 1b) is a weaker acid than 1a but still a strong photoacid with pK(a)* ~ -1 and the ESPT rate in water, methanol and ethanol is 7 × 10(10) s(-1), 4 × 10(8) s(-1) and 2 × 10(8) s(-1). We qualitatively explain our kinetic results by a Marcus-like free-energy correlation which was found to have a general form suitable for describing proton transfer reactions in both the proton-adiabatic and the proton-non-adiabatic limits.

Insight into the structure and the mechanism of the slow proton transfer in the GFP double mutant T203V/S205A.

Mutations near the fluorescing chromophore of the green fluorescent protein (GFP) have direct effects on the absorption and emission spectra. Some mutants have significant band shifts and most of the mutants exhibit a loss of fluorescence intensity. In this study we continue our investigation of the factors controlling the excited state proton transfer (PT) process of GFP, in particular to study the effects of modifications to the key side chain Ser205 in wt-GFP, proposed to participate in the proton wire. To this aim we combined mutagenesis, X-ray crystallography, steady-state spectroscopy, time-resolved emission spectroscopy and all-atom explicit molecular dynamics (MD) simulations to study the double mutant T203V/S205A. Our results show that while in the previously described GFP double mutant T203V/S205V the PT process does not occur, in the T203V/S205A mutant the PT process does occur, but with a 350 times slower rate than in wild-type GFP (wt-GFP). Furthermore, the kinetic isotope effect in the GFP double mutant T203V/S205A is twice smaller than in the wt-GFP and in the GFP single mutant S205V, which forms a novel PT pathway. On the other hand, the crystal structure of GFP T203V/S205A does not reveal a viable proton transfer pathway. To explain PT in GFP T203V/S205A, we argue on the basis of the MD simulations for an alternative, novel proton-wire pathway which involves the phenol group of the chromophore and water molecules infrequently entering from the bulk. This alternative pathway may explain the dramatically slow PT in the GFP double mutant T203V/S205A compared to wt-GFP.

Optical spectroscopy of molecular-rotor molecules adsorbed on cellulose.

Steady-state and time-resolved emission techniques were used to study the fluorescence properties of two molecular rotors, thioflavin-T and auramine-O adsorbed on cellulose powder. Molecular rotors are known for their weak fluorescence intensity and short fluorescence lifetime when dissolved in liquids of low viscosity. We found that these molecular-rotor molecules when adsorbed on cellulose exhibit a rather strong steady-state fluorescence spectrum as well as long emission lifetime. We explain these results by the inhibition of segmental intramolecular rotation when these molecules are adsorbed on cellulose.

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 k(PT) ∼ 10(13) 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.

Acid effect on photobase properties of curcumin.

Femtosecond UV-vis pump-probe spectroscopy was employed to study the acid effect on curcumin in the excited state. Curcumin in solutions of weak acids was found to be a photobase forming a protonated curcumin within a few tens of picoseconds from the time of excitation. The excited-state protonation reaction is also observed in the steady-state emission spectrum as a new red emission band with a maximum at 620 nm in the presence of weak acids. The transient pump-probe spectrum consists of four spectral bands, two emission bands, and two absorption bands. We assign a transient absorption band at ∼600 nm and an emission band at ∼540 nm to the neutral ROH form of curcumin. An absorption band at ∼500 nm and an emission band at 620 nm are assigned to the protonated ROH2(+) form of curcumin.

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(+)