Unusual Temperature Dependence of the Proton Transfer Rate from 8

Sep 21, 2009 - Unusual Temperature Dependence of the Proton Transfer Rate from 8-Hydroxy-1,3,6-pyrenetrisulfonate Photoacid to Methanol-Doped Ice...
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J. Phys. Chem. C 2009, 113, 17915–17926

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Unusual Temperature Dependence of the Proton Transfer Rate from 8-Hydroxy-1,3,6-pyrenetrisulfonate Photoacid to Methanol-Doped Ice Anna Uritski, Itay Presiado, Yuval Erez, Rinat Gepshtein, and Dan Huppert* Raymond and BeVerly Sackler Faculty of Exact Sciences, School of Chemistry, Tel AViV UniVersity, Tel AViV 69978, Israel ReceiVed: June 11, 2009; ReVised Manuscript ReceiVed: August 16, 2009

We studied the temperature dependence of the transfer rate of a proton from excited-state 8-hydroxy-1,3,6pyrenetrisulfonate (HPTS) to the solvent in methanol-doped ice over the wide temperature range of 80-268 K. Ice is a poor solvent, and therefore methanol is used as an amphiphilic cosolvent to dissolve the large HPTS molecule in ice. Time-resolved and time-integrated emission spectra are used to evaluate the rate of the proton transfer process, which decreases with a decrease in the temperature. We found that at temperatures below 173 K and at high methanol concentrations above 1% mole ratio the proton transfer rate is at least 10 times slower than the radiative rate, and consequently could not be detected clearly. The temperature dependence of the proton transfer rate constant, kPT, at T > 173 K cannot be described by a single mechanism of an activated process. We therefore used tunneling calculations to fit the values of ln(kPT) as a function of the temperature. Below 173 K and at low methanol concentrations, e1% mole ratio, we found that the proton transfer process indeed occurred. The process is independent of the temperature, and strongly depends on the methanol concentration. The kinetic isotope effect of H+/D+ is large at T < 173 K and strongly depends on the methanol concentration. Introduction The physical and chemical properties of ice have been studied for a long time.1-4 Ice exhibits a high static relative permittivity, comparable with that of liquid water. Two types of structural defects account for the ice’s electrical properties: (1) ion defects, which are the result of proton motion from one end of the bond to the other, thus creating a H3O+, OH- ion pair;5,6 (2) orientational Bjerrum defects,7,8 which are caused by the rotation of a water molecule to produce either a doubly protonated bond (D-defect) or a deprotonated bond (L-defect). For many years9-19 excited state intermolecular proton transfer (ESPT) to a solvent or to a base in a solution has been widely researched in the liquid phase. In the past decades we extensively studied the reversible photoprotolytic cycle of a photoacid. We used a proton transfer model that explains the reversibility and accounts for the diffusion assisted geminate recombination of the transferred proton with the deprotonated form of the photoacid.15,20,21 In aqueous solutions of several mild and strong photoacids, we found that the temperature dependence, displayed as an Arrhenius plot of the proton transfer rate constant, exhibited a convex shape. kPT of a commonly used 8-hydroxy1,3,6-trisulfonate (HPTS) photoacid at the high temperature range of T > 280 K gave an activation energy Ea of less than 5 kJ/mol. At lower temperatures, including the supercooled region 260-280 K, the activation energy was not constant and increased as the temperature decreased. At about 260 K, the plot ln(kPT) versus 1/T gave a slope of Ea ∼20 kJ/mol. In a previous work22 we extended the liquid phase studies on the photoprotolytic cycle of a photoacid to the ice phase. In ice, in the high-temperature range of 240-270 K, we found for the HPTS photoacid a nearly constant activation energy within an average value of about Ea ∼30 kJ/mol. Time-resolved * Corresponding author. E-mail: [email protected]. Phone: 972-36407012. Fax: 972-3-6407491.

emission was employed to measure the photoprotolytic cycle of excited photoacid as a function of temperature in liquid water and in ice. As was found previously in the liquid phase, the proton is first transferred from the photoacid to a nearby water molecule. Subsequently, it diffuses in ice under the influence of the Coulomb potential between H+ and RO-* that enhances the geminate recombination. In a more recent article23 we studied the photoprotolytic cycle of four photoacids in ice, namely, HPTS, 2-naphthol-6,8-disulfonate (2N68DS), 2-naphthol-6sulfonate (2N6S) and 2-naphthol-8-sulfonate (2N8S). 2N68DS (pK* ∼0.7) is a stronger photoacid than HPTS (pK* ∼1.35) and transfers a proton to liquid water at about 40 ps, while 2N6S is a weak photoacid (pK ∼2) and its proton transfer process in liquid water is slow, i.e., τ ≈ 900 ps. In general, all four photoacid results provided similar information on the temperature dependence of kPT in ice. We found that the proton transfer rate constant kPT in ice strongly depends on the temperature. We observed three characteristic temperature regions for kPT. In the high-temperature region of 240-273 K, the proton transfer process strongly depends on the temperature. In the intermediate temperature range of 210-240 K, the Arrhenius plot of the rate constant, kPT, exhibited a relatively weaker temperature dependence. Below 180 K, we could not observe the RO-* band in the steady-state emission spectrum of HPTS. In the current study, we focused our attention on the role the methanol concentration plays in the proton transfer process over a wide range of temperatures from 80-268 K. We found that the proton transfer rate characteristics can be divided to two temperature ranges: the high temperature range of T > 173 K and the low temperature range of T < 173 K. In the high temperature range, kPT strongly depends on the temperature. When ln(kPT) is plotted as a function of 1/T (an Arrhenius plot), the plot assumes a concave shape, and we use tunneling models to evaluate the temperature dependence. We found that the proton transfer process also occurs at low temperatures, T
10 MΩ. Methanol of analytical grade was purchased from Fluka. All chemicals were used without further purification. The temperature of the irradiated sample was controlled by placing the sample in a liquid N2 cryostat with a thermal stability of approximately 1.5 K. Ice samples were prepared by first placing the cryogenic sample cell for about 20 min at a supercooled liquid temperature of about 260 K. The second step involved a relatively rapid cooling (5 min) to a temperature of about 240 K. Subsequently, the sample froze within a few minutes. To ensure ice equilibration prior to the time-resolved measurements, the sample temperature was kept for another 10 min at about 240 K.

Time-Resolved Emission. Figure 1 shows the time-resolved emission of three methanol-doped HPTS samples in neutral pH H2O at several temperatures in the range of 80-300 K, where in each temperature three different methanol concentrations were used. The samples are excited at 405 nm by 200 fs pulses at 500 kHz, and the signal of the ROH* form of HPTS is detected at 438 nm. At this wavelength the spectral overlap between the ROH* and the RO-* bands is small, i.e., 173 K the peak position occurs at ∼500 nm. We used the log-normal function32 to fit each individual vibronic band of the RO- band. The parameters for the fit of each vibronic band are the peak position νp, the relative height h, the bandwidth ∆ν and the asymmetry γ. Figure 5 shows the steady-state emission of HPTS in three methanol-d-doped D2O ice samples of 0.2, 0.5 and 1% mole ratio. At temperatures above 247 K the proton transfer rate constant, kPT, is larger than the radiative rate constant, krad, and the emission band’s intensity of RO-* is larger than that of ROH*. As in the H2O samples (Figure 4), in this temperature region the samples with a high doping level (0.5 and 1.0%) D2 O < have similar steady-state emission signals. In general, kPT H2O RO-* ROH* kPT at all temperatures, and thus the If /If ratio is smaller for the D2O samples. At low temperatures below 173 K, there are two distinguishable features: the ROH* band shows a vibration structure, and the RO-* emission band’s intensity is weak for the 0.2% mole ratio sample, and almost undetectable

for the 1% mole ratio sample. The 0.5% methanol-d-doped D2O sample has an RO-* band, which is three times less intense than the ROH* band. The RO-* main vibration band position at 21280 cm-1 at 88 K is only slightly blue-shifted with respect to the third band of the ROH* band at 20840 cm-1. Therefore, even an RO-* band with 30% of the intensity of the ROH* is almost undetectable in the spectrum. Figure 6 shows a comparison of the steady-state emission of HPTS in 0.2% methanol-doped H2O and D2O samples at several temperatures. At all temperatures there is a large difference in both the ROH* and the RO-* bands’ intensities of H2O and D2O. This difference is a result of a relatively large KIE on the proton transfer rate. In the liquid state, KIE ≈ 3, whereas in ice the KIE strongly depends on the temperature, the lower the temperature, the larger the KIE. The KIE also depends on the methanol doping level. At a high methanol concentration, such as 1.0% mole ratio, the ESPT rate in D2O samples at T < 173 K is much slower than the radiative and the nonradiative rates. Thus, the RO-* band is almost completely absent in the steady-

Proton Transfer from HPTS to Methanol-Doped Ice

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Figure 6. Comparison of the steady-state emission of HPTS in 0.2% methanol-doped H2O and D2O samples at several temperatures.

state spectra. Figures s4 and s5 in the Supporting Information clearly show that in samples with 0.5 and 1.0% mole ratio of methanol-d-doped D2O ice the proton transfer process is inefficient within the time window limited by the excited-state lifetime of ∼5 ns. Data Analysis and Processing. Figures s6 and s7 in the Supporting Information show the experimental data and computational fit of the steady-state spectrum of HPTS in H2O and D2O at 100 K. The HPTS in the D2O sample has a rather weak RO-* signal, whereas in the H2O spectrum the RO-* band intensity is very strong. This large difference in the RO-* band intensities of the H2O and D2O samples arises from the large difference in kPT, to wit, there is a large KIE on the proton transfer process at the low temperature region of T < 173 K. Figure 7a shows an Arrhenius plot of the proton transfer rate constant, ln kPT versus 1000/T, of three H2O samples doped with several methanol concentrations in the range of 0.2-1%. The value of kPT in the high temperature region (T > 200 K) weakly depends on the methanol concentration, whereas at low temperatures this dependence is strong. Figure 7b shows the same results as Figure 7a but on a reduced scale with temperatures above 160 K only. The Arrhenius plot at high temperatures is concave for all samples. The larger the methanol doping level, the larger the slope of the curve, which means a larger activation energy for the proton transfer process. When tunneling is also possible, the Arrhenius

plot is expected to deviate from linearity and the overall shape of the plot will be concave. At sufficiently low temperatures, we expect that tunneling will prevail, and that this will be a process that is temperature independent from any energetic state W, close to the bottom of the reactant’s potential energy well with a defined permeability (transmission coefficient). Consequently it should have a temperature independent rate constant, kPT (W). Figure 8a shows an Arrhenius plot of the rate constant of the deuteron transfer to ice, ln kDT vs 1000/T, for three D2O samples doped with different concentrations of methanol-d in the range of 0.2-1% mole ratio. As in the case of H2O, the plots show a non-Arrhenius behavior, and deviate strongly from linearity. The values of kDT at temperatures below 165 K indicate a temperature independence, whereas at temperatures above 165 K, the values of kDT indicate that there exists a strong temperature dependence, which explains the concave shape of the Arrhenius plot. Figure 8b shows the same Arrhenius plot as Figure 8a, but only at the high temperature region. The values of the rate constant, kDT, are almost independent of the doping level of methanol-d. The value of kDT is smaller than the corresponding value of kPT at any given temperature. The value of KIE is about 3 in the liquid state and somewhat larger in ice at 273 K. As the temperature decreases, KIE increases. Figure 9 shows an Arrhenius plot of kPT/kDT versus 1000/T at high temperatures of both H2O and D2O samples doped with

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Figure 7. (a) Arrhenius plot of the proton transfer rate constant, ln kPT versus 1000/T, of H2O samples doped with several methanol concentrations in the range of 0.2-1%. (b) The same results as shown in panel a but on a reduced temperature scale (only temperatures above 160 K).

Uritski et al.

Figure 8. (a) Arrhenius plot of the rate constant of the deuteron transfer to ice, ln kPT vs 1000/T, for several D2O samples doped with different concentrations of methanol-d. (b) The same Arrhenius plot as shown in panel a, but only in the high temperature region.

0.5% mole ratio of methanol and methanol-d respectively. As seen in the previous two figures, at high temperatures the slopes of both plots are similar. However, in the intermediate temperature range the D2O plot continues to fall far below the H2O plot until the gap between them stabilizes around 4 orders of magnitude in the low temperature range. Figure s8 in the Supporting Information shows the calculated slope of the plot in Figure 8, which provides the effective activation energy of kPT for H2O samples as a function of 1/T. The activation energies decrease with the temperature, and this indicates that tunneling prevails at low temperatures. Figure s9 in the Supporting Information shows the KIE on kPT as a function of the temperature. There is a visible inverse relationship between these two parameters, namely that the KIE grows as the temperature descends. Discussion In this study we explore in depth the temperature dependence of the excited-state proton transfer rate from HPTS to its surroundings in methanol-doped H2O and D2O ice. For that purpose, we use the time-correlated single photon counting technique to measure the time-resolved emission of HPTS, a well-studied photoacid, as well as its steady-state (timeintegrated) emission spectra. Ice is known to be an ineffective solvent, and therefore we add a small concentration of methanol to prevent aggregation of HPTS molecules at the grain boundaries of the polycrystalline ice sample. However, even

Figure 9. Arrhenius plot of kPT and kDT versus 1000/T at high temperatures of both H2O and D2O samples doped with 0.5% mole ratio of methanol and methanol-d, respectively.

this small concentration of methanol complicates the study of the photoprotolytic properties of a guest molecule (HPTS in this study) in the well-defined crystal structure of the host (Ih ice).

Proton Transfer from HPTS to Methanol-Doped Ice The main findings of this study are as follows: 1. We observe two types of behavior of the proton transfer constant, kPT in two temperature regions: a. In the high temperature region, T > 173 K, the values of kPT strongly depend on the temperature, but are far less dependent on the methanol doping level in the concentration range of 0.05-1% mole ratio. b. In the low temperature region, T < 173 K, the values of kPT are almost independent of the temperature. The experimental data even indicate a small inverse temperature dependence, where the proton transfer rate actually increases as the temperature is lowered. In this temperature region, kPT strongly depends on the methanol concentration: the larger the methanol concentration, the smaller the kPT values. 2. The KIE is fairly large, and depends on the temperature of the ice sample. In ice at 273 K KIE ≈ 3.5, and at ∼190 K KIE ≈ 7. 3. In the low temperature region, T < 173 K, and at low methanol concentrations the isotope effect on kPT is very large, i.e., KIE ≈ 20, and it is independent of the temperature. 4. In the high temperature region, T > 173 K, the isotope effect depends only mildly on the methanol doping level of the ice sample. 5. The position and width of the emission spectrum of the deprotonated form of the HPTS photoacid, RO-*, depend on the temperature. In the high temperature region, T > 173 K, the structureless broad band shifts to the blue when the temperature decreases. In the low temperature region, T < 173 K, the position of the RO-* band blue-shifts abruptly by 800 cm-1. In addition, the vibration band’s width shrinks, and a vibrational structure with a spacing of ∼1250 cm-1 is seen. 6. The relative intensity of the emission spectrum of the ROH* band in the high temperature region, T > 220 K, is small in comparison to the RO-* band, since kPT . krad (eq 1). The spectrum shows a blurry vibrational structure. The spectral width shrinks as the temperature decreases, and the band shifts to the blue. In the low temperature region, T < 173 K, the bandwidth of the vibrational band further decreases, and the vibrational sub-bands are clearly seen. At about 80 K, the individual vibrations have bandwidths of 1150 cm-1 and the interspacing between them is 1200 cm-1. The higher the methanol concentration, the smaller the bandwidth of the ROH* band of HPTS. 7. For the analysis of the ROH* time-resolved emission signal, we use the reversible diffusion-assisted geminate proton recombination model.15,20,21,27 The transferred proton, which has a finite recombination probability, diffuses in ice. The ROH* population is larger than expected when this recombination process does not take place. As a consequence of the repopulation of the ROH*, the time-integrated signal of the ROH* provides a larger average decay-time of 〈τ〉 ) 220 ps than the actual decay time due to the proton transfer process, τPT ) 100 ps. The GR model predicts a nearly exponential fast decay at short-times, followed by a nonexponential long-time fluorescence tail. The short-time signal’s fast decay mainly depends on kPT. A nonexpontial nonradiative decay, which relates neither to the proton transfer nor to the proton geminate recombination, is observed in the time-resolved emission signals of the ROH* band of HPTS in ice (but not in the liquid), especially at temperatures below 220 K, where kPT = krad. The nonexponential, nonradiative process (which is not related to the proton transfer process), makes a contribution to the time-resolved emission signal at short times and adds to the signal’s fast components. This erroneously led us to the conclusion that the proton transfer rate constant is large, when in fact it is much

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17923 smaller than the signal slope initially measured. At temperatures above 220 K, kPT in H2O ice is much larger than knr, and the interference of the nonradiative process in the determination of kPT from the time-resolved emission analysis is relatively small. Therefore, at temperatures above 220 K we used the kPT values extracted from the GR model fits of the time-resolved emission data. For temperatures below 220 K, we used the steady-state emission spectra to deduce the fluorescence emission intensities -* ROH* /If , and its relation to kPT (eq s1 in the Supporting ratio, IRO f Information). High Temperature Ice. The combined analysis of the timeresolved and time-integrated (steady-state) emissions of HPTS in methanol-doped H2O and D2O ice at temperatures above 173 K provides the excited-state intermolecular proton transfer rate constant, kPT. Figures 7b and 8b provide the Arrhenius plots of ln kPT versus 1/T, describing the temperature dependence of the data in the high temperature region. The non-Arrhenius plots indicate that tunneling prevails at low temperatures, while at high temperatures an over the barrier reaction prevails. To fit the plots of Figures 7b and 8b we used two tunneling models, which describe the temperature dependence of H2O and D2O samples. The first one, which is presented in R. Bell’s book,33 is based on tunneling calculations of a proton transfer reaction in a one-dimensional potential surface. This reaction occurs between two symmetrical (equipotenial) wells, separated by a substantially large barrier, whose shape is an inverted parabola. The origin of the x coordinate of the one-dimensional potential surface is at the top of the barrier so that the potential energy is defined by

1 V(x) ) Ax2 2

(2)

The properties of the parabolic barrier are expressed in terms of the so-called imaginary frequency, iν†, where ν† is the oscillation frequency of a particle of mass m in a parabolic potential well having the same curvature as in eq 2:

ν† )

1 2π

 mA

(3)

The transmission coefficient (permeability), G, is defined as the ratio between the transmitted and reflected beams of the particle flux from the left potential well toward the barrier, and is given by

G ) {1 + exp(V0 - W)/hν†}-1

(4)

where W is the energy of the transmitted particle and V0 is the barrier height from the bottom of the well. Under regular conditions the energy distribution of a reactant ensemble will be a Boltzmann distribution. The reaction rate over and under the barrier is given by (eq 3.2 in ref 33)

J)

J0 kBT

∫0∞ G(W) exp[-W/kBT] dW

(5)

where J0 is the total flux of particles striking the left-hand side of the barrier, and J is the rate at which the particles appear on the right-hand side of the barrier.

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Figure 10. Arrhenius plot of kPT, presenting experimental data (dots) and a calculated model fit (solid line), which takes into account both over and under the barrier reaction paths (see in the text).

Figure 10 shows the data fit (solid line) as shown on Figure 7b by using eq 5. The parameters of the fit are ν† ) 150 cm-1 and V0 ) 35 kJ/mol. The fit is quite satisfactory in the intermediate temperature region of 210-248 K but much less so at both low and high temperatures. At temperatures below 210 K, the experimental values of kPT are somewhat higher than the calculated ones. A plausible explanation for this discrepancy is the observation that, at temperatures below 173 K, a new channel for the proton transfer process prevails, and its proton transfer reaction rate constant, kPT, is very large in comparison with kPT in the range of 173-210 K. In the intermediate low temperature region 183 < T < 210 K, both the regular high temperature process and the low temperature T < 173 K process coexist. Therefore the unusually high kPT values found at T < 173 K interfere with the determination of the “regular” proton transfer rate constants of kPT at 173-210 K. The end result is that the experimental data are larger in this low temperature range than expected from tunneling calculations. Figure s10 in the Supporting Information also shows a fit of the experimental results to a model of a tunneling process assisted by intermolecular vibrations, which was proposed by Trakhtenberg and co-workers34,35 and by Kuznetsov and co-workers.36,37 The model emphasizes the importance of the relative distance between the two oxygen atoms (the ROH* oxygen and the H2O oxygen) in determining the proton tunneling rate constant. The intermolecular vibration of the two oxygens modifies the potential surface of the tunneling problem. A small change in the O-O distance, due to the intermolecular vibration, changes both the barrier’s height and width. These two parameters basically determine the permeability of the proton under the barrier, while the barrier’s height also modifies the over the barrier reactant flux. In the Supporting Information we present a short summary of this theory. We use equation s4 to fit the data shown in Figure s10. The parameters of the fit are the intermolecular frequency, Ω0, the tunneling rate at an equilibrium position of the ROH* · · · OH2 intermolecular distance, R0, J(R0), the derivative with respect to the distance of J(R), ∂J/∂R and the amplitude of the intermolecular vibration, δO · · · O. At low temperatures, T < 200 K, the curve of the fit underestimates the values of the experimental results of kPT. We explain the mismatch between the fit and the experimental results as arising from the contribution of the additional proton transfer channel operative at low temperatures, as we explained already above. Low Temperature Ice. As shown in Figures 7-9 at T < 173 K, kPT is nearly temperature-independent in the measured temperature region of 80-173 K. It also strongly depends on

Uritski et al. the methanol concentration, and exhibits a large KIE, which is independent of the temperature. Figure s11 in the Supporting Information shows on a log-log scale the dependence of kPT on the methanol concentration in H2O ice. The value of the rate constant dramatically decreases with an increasing methanol concentration. The slope of the plot in Figure s11 in the Supporting Information is nearly -2.4, which means that kPT(T < 173 K) ∝ c-2.4. This strikingly strong dependence of HPTS on the methanol doping level at low temperatures was not observed in our previous studies on HPTS.30,31 In those studies we used higher concentrations of methanol, and therefore we were unable to clearly observe the proton transfer process below 173 K. In the current study, the methanol concentrations in use were between 0.1 and 1% mole ratio. The upper limit in the current experiment was the lowest concentration in our previous experiments. A plausible explanation for the large proton/deuteron KIE, the very strong dependence on the methanol concentration, and the temperature independence of the proton transfer rate at T < 173 K is as follows: The HPTS is capable of forming a proton wire consisting of hydrogen-bonded water molecules between the OH and the SO3- functional groups. The SO3- group in the 1 position of the 8-hydroxy-1,3,6-trisulfonate is most likely located at the receiving end of this proton wire. The proton wire is capable of transferring a proton via a concerted tunneling mechanism, and therefore its rate constant has no activation energy at temperatures below 173 K. The water molecule wire exhibits an isotope effect that is much larger than that of a single step proton transfer to a nearby water molecule. Gutman and co-workers38 found a remarkably fast proton transfer rate (kPT ∼1012 s-1) between proton donor and acceptor groups in the fluorescein molecule. The explanation they furnished for their results was that transient proton wires, consisting of two hydrogen-bonded water molecules or more, interconnect the donor and acceptor groups. Whenever a proper pathway was established, the proton transfer took place via a concerted mechanism. However, due to the rapid fluctuation of the water molecules, the rate limiting step in the reaction is the random appearance of a competent proton wire. Hynes and Laage et al.39,40 studied the resident time of water next to a solute by molecular dynamics simulations. They found that the current accepted methods of calculation are sensitive to the tolerance times value designed to account for barrier recrossing. They suggested an alternative method of determination of the resident time where consideration of the recrossing is not needed. The calculated resident times are in good agreement with the estimation from experimental data measured by ultrafast laser spectroscopy and NMR. In the solid state, as in the present case, in contrast to the liquid state, the system is rigid, and consequently the reaction can proceed without the need to wait for the formation of the wire by random molecular motion. Typical for the reaction reported by Gutman and co-workers is the exceptionally large KIE of about 50 on the rate constant, after replacing the water with D2O. Nibbering, Pines and co-workers12,13,41-43 found that electronically excited HPTS efficiently and directly transfers a proton to basic groups in aqueous solutions, such as acetates and chloro-, dichloro- and trichloroacetates. The fastest proton transfer rate is to an acetate anion, which is directly hydrogen-bonded to the OH group of HPTS (τPT e 100 fs). Furthermore, they found that the proton transfer rate to weaker bases, such as chloroacetate, depends on the basicity of the acceptor. Pines and Nibbering suggest using the term solvent switch. It includes several physically different mechanisms describing the mediation of the proton

Proton Transfer from HPTS to Methanol-Doped Ice transfer between acid and base in an aqueous solution by water molecules. They proposed that a short solvent switch is capable of increasing both the proton dissociation rate of the acid and the rate at which the proton spans the distance between acid and base. The water separating the acid and base constructing the switch may also mediate the transfer of proton/deuterons in a diffusional fashion. The water between the acid and the base operates by a long-range, nonspecific mechanism. In some cases the water molecules may also be aligned as wires, i.e., forming linear hydrogen-bonded chains that are temporarily connecting the acid and base, where within the lifetime of these water wires, which is usually less than 1 ps in bulk water, the proton propagates concertedly or sequentially from the acid to the base over large distances. The proton wires can be considered as specific long-range solvent switches having the ability of efficiently assisting point-to-point transfer of the proton over a relatively long distance in solution. Ice is a poor solvent, and therefore methanol is used as a cosolvent to prevent HPTS from aggregating in the grain boundaries since it is amphiphilic (contains both hydrophobic and hydrophilic groups). Preferential solvation increases the methanol presence near HPTS in specific positions.44 It is plausible that the much better dissolution of HPTS is achieved when CH3 groups point toward the aromatic rings, whereas the OH groups point toward the ice water molecules. In ∼1% mole ratio of methanol (∼0.5 M) the proton transfer process in the low temperature phase is very slow. This could be explained by the assumption that the methanol molecules largely interfere in the formation of the proton wire, which supposedly extends from the OH group to the sulfonate group. Our explanation to the strong kPT dependence on the methanol concentration (kPT(T < 173 K) ∝ c-2.4) is that the probability of replacing a water molecule with a methanol molecule in the local proton wire scales to the power law we found for the methanol concentration. Future experiments using solid-state NMR will hopefully clear the mystery of the efficient proton transfer rate of excited HPTS in the low temperature region below 173 K. We conducted a preliminary study on the temperature dependence of other photoacids in ice doped with low concentrations of methanol including the well-studied 2-naphthol-6,8disulfonate (2N68DS). We could not find an efficient proton transfer process at T < 173 K for 2N68DS. This fact indicates that the proton transfer process of HPTS at low temperatures is unique. This uniqueness probably arises from a local structure, rather than the bulk ice structure, that enables the proton transfer process to occur. An interesting result, which corroborates the fact that HPTS proton transfer at low temperatures is exceptional, is shown in Figure s12 in the Supporting Information, which shows the timeresolved emission of the ROH* of HPTS in ice doped with 0.1% mole ratio of methanol and very small concentrations of HCl, cHCl e 1 mM. A more detailed description of the effect is given in the Supporting Information. In ice at high temperatures, the ROH* signal of a 1 mM acid sample exhibits a nearly biexponential decay. The long-time exponential fluorescence tail of ROH*, seen in the panel of T ) 258 K, arises from the repopulation of the ROH* by the proton recombination reaction with an excess proton introduced in the sample by adding HCl to ice. At lower acid concentrations, cH+ e 2.5 × 10-4 M, the exponential longtime fluorescence tail is missing, but the ROH* decay becomes longer with increasing acid concentrations. In the low temperature region of T < 173 K we find that for acid samples with cH+ g 2.5 × 10-4 M the proton transfer

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17925 process does not occur. The steady-state emission spectrum lacks the RO-* band, and the time-resolved emission lacks the RO-* band signal. The time-resolved emission of the ROH* band, measured at 438 nm, shows a nearly exponential decay with an average decay time of 3.3 ns, which is independent of the acid concentration above 2.5 × 10-4 M. We explain the acid effect on the proton transfer by the supposition that the local excess protons, which are located inside the Coulomb cage, recombine with the SO3- groups to form SO3H, and thus eliminate the local proton acceptors. The net result is the cessation of the proton transfer process in acidic samples. Summary We studied the temperature dependence of the proton transfer rate of electronically excited HPTS over a wide range of temperatures and methanol concentrations. We used timeresolved and steady-state emission techniques to determine kPT over a wide range of temperatures. The reversible geminate recombination model predicts an efficient diffusion-assisted proton recombination to re-form the ROH*, which in turn may undergo a second photoprotolytic cycle, and the overall effect is a nonexponential fluorescence decay of the ROH*. In the current study, we surprisingly found that there exists a nonexponential nonradiative decay process (unrelated to the photoprotolytic process) of the excited ROH* form of photoacids in the solid ice matrix. In the data analyses, we considered the interference of the nonexponential, nonradiative rate in the determination of the values of the proton transfer rate constant, kPT, at sufficiently low temperatures. We found that the nonexponential decay rate of the protonated ROH* form, due to the photoprotolytic cycle of the photoacid in ice at low temperatures, overlaps with the nonexponential, nonradiative decay, thus tremendously increasing the difficulty in determining the exact value of the proton transfer rate constant. We found that the proton transfer process behaves differently at high and low temperatures. At T > 173 K, the values of kPT are nearly independent of the methanol concentration. The temperature dependence of kPT is strong near the melting point (273 K). It decreases as the temperature drops, which results in a concave shape of the Arrhenius plot. We used two tunneling models to fit the concave shape of the Arrhenius plot in the high temperature region. In the low temperature region, T < 173 K, we found that kPT is nearly temperature independent. We also found a power law of c-2.4, which scales the strong dependence of kPT to the methanol concentration, i.e., the lower the methanol concentration, the larger the rate constant. For 0.1% mole ratio of methanol, kPT at T < 173 K is larger than the proton transfer rate constant at 258 K. On the other hand, for 1% mole ratio, kPT at T < 173 K is more than 2 orders of magnitude smaller than at 258 K. We explain the proton transfer at low temperatures in terms of a local and specific proton transfer from the OH group of the HPTS to one of its SO3- groups via a distinctive proton wire consisting of several water molecules. We also argue that the methanol molecules may disrupt the integrity of the proton wire, which makes this system highly dependent on the methanol concentration. The local proton transfer rate constant has a very large KIE, whose value is estimated to be over 20. Acknowledgment. We thank Professor H. Diamant for the helpful discussions. This work was supported by grants from the Israel Science Foundation and from the James-Franck German-Israeli Program in Laser-Matter Interaction.

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