Temperature Dependence of the Excited-State Proton-Transfer

Publication Date (Web): April 18, 2013. Copyright .... Christian Spies , Shay Shomer , Björn Finkler , Dina Pines , Ehud Pines , Gregor Jung , Dan Hu...
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Temperature Dependence of the Excited-State Proton-Transfer Reaction of Quinone-cyanine‑7 Ron Simkovitch, Shay Shomer, Rinat Gepshtein, Doron Shabat, and Dan Huppert* Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel S Supporting Information *

ABSTRACT: 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 ΔG of reaction and the activation energy. At very low temperatures (T < 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.

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indolium ions stabilize the RO−* deprotonated form (the conjugate base). In a recent study,23 we measured the steady-state emission, excitation and absorption spectra, as well as the time-resolved emission properties of an excited QCy7 molecule. We found that QCy7 has dual emission bands when excited from its ground-state neutral form (the protonated form, ROH). The bands correspond to emission from the protonated ROH* (band maximum at 540 nm) and from the deprotonated RO−* (700 nm) species of QCy7. The decay of the time-resolved emission of ROH* in water is rapid, about 1 ps at room temperature. The time-resolved emission of RO−* has a distinctive rise time with about the same time constant as the decay time of ROH*. The decay time of RO−* in water is ∼130 ps, and its fluorescence quantum yield is roughly 10%. We interpret these optical observations as arising from ESPT to the solvent. The ESPT rate to water is about twice as that of NM6HQ+, one of the strongest photoacids on record so far,24,25 making the ESPT rate for QCy7 and its derivatives the fastest reported ESPT rate thus far. In the current work we have studied further the photoprotolytic properties of the QCy7 molecule. We conducted a temperature dependence study of the photoprotolytic process of QCy7 in H2O and D2O ice over the temperature range of 270−85 K. We found that the ESPT rate constant, kPT, as well as the proton geminate recombination process, decrease as the temperature is lowered. The Arrhenius plot of ln(kPT) versus 1/ T shows that kPT decreases with a low activation energy of 10.5 ± 1 kJ/mol over the range of 265−150 K. This activation energy is about one-quarter that of the weaker photoacid, 8hydroxy-1,3,6-pyrenetrisulfonate (HPTS), which has pKa* ∼

INTRODUCTION Photoacids are a class of molecules that are weak acids in their ground electronic state but show increased acidity in their first excited electronic state. Thus, photoexcitation to the excited state, by short UV−vis laser pulses, allows us to follow the photoprotolytic processes. In recent years, investigations have been carried out on the excited-state intermolecular proton transfer from the acidic group of the excited photoacid to the a nearby proton-accepting group of a solvent molecule or a base in solution.1−13 Naphthol and its derivatives are a common example of photoacid molecules. 2-Naphthol is a weak photoacid with ground- and excited-state pKa values in water of 9 and 2.7, respectively. The excited-state proton-transfer (ESPT) rate constant, kPT, is 108 s−1, and the radiative rate constant, kr, is ≈108 s−1; thus, the overall fluorescence-decay rate is the sum of the two rates, 2 × 108 s−1. During the 1990s, three cyano-naphthol derivatives were found to have their ESPT rates increased by about 3 orders of magnitude and their pKa* values increased by 6 orders of magnitude with respect to 2-naphthol. The stronger photoacids can transfer a proton not only to water but also to alcohols, whereas weak and intermediate strength photoacids with pKa* ≥ 0 cannot. Photoacids with pKa* ≤ −2 are called superphotoacids.14−20 Quinone-cyanine-7 (QCy7), which is a new Cy7-like molecule,21,22 is a superphotoacid with pKa* ∼ −6. Scheme 1 shows its molecular structure. The hydroxyl group of the phenol in the center of the structure is the proton emitter. The Scheme 1. QCy7 Molecule

Received: December 31, 2012 Revised: April 17, 2013

© XXXX American Chemical Society

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1.4 and kPT ≈ 1010 s−1. Over the temperature range of 150−112 K the emission spectrum of RO−*, the deprotonated form of QCy7, is blue-shifted by ∼1000 cm−1. We explain this shift by the creation of an ion pair: RO−*···H3O+. Scheme 2 shows the

program, developed by Krissinel and Agmon.36 The steadystate emission spectrum at low temperatures was acquired by sample excitation. The sample was excited by 150 fs 385 nm laser pulses at 800 kHz and monitored by a miniature spectrometer (MS-420 CVI).

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Scheme 2

RESULTS Steady-State Measurements. Parts a and b of Figure 1 show the normalized time-integrated (steady-state) emission spectra of QCy7 in H2O and D2O ice, respectively, over a wide range of temperatures, 86−247 K. In order to prevent the “guest” molecule (QCy7) from being expelled from the bulk of the ice during sample freezing, we added to the liquid water a small amount of a cosolvent of 0.5 mol % methanol. In previous studies of methanol-doped ice with organic photoacids as salts like naphthol sulfonates, we found that low concentrations of methanol prevent the aggregation of the photoacid molecule on the microcrystalline surface of the ice upon freezing of the sample.26−28 At high temperatures (T ≥ 185 K), the emission spectrum of QCy7 in ice is similar to that in water. The spectrum consists of a weak band with a peak at ∼560 nm assigned to the protonated form, ROH, and a strong band with a peak at ∼700 nm attributed to the deprotonated form, RO−. At high temperatures (T ≥ 200 K), the band shape and position of the RO− form is almost temperature-independent for both H2O and D2O samples. At lower temperatures, the band position depends strongly on temperature for both H2O and D2O samples. The RO− emission band shifts to the blue as the temperature is lowered. At T ≤ 150 K an abrupt change is noticed in the emission spectrum; the RO−* band shifts by about 1000 cm−1 to the blue. Over the temperature range of 115−150 K, the RO− band peak position is fixed at about 660 nm. We suggest that this unexpected emission band could be assigned to the ion pair RO−*···H3O+, the intermediate formed following the ESPT process. We further discuss this issue in the Discussion section. At temperatures below 115 K, the emission spectrum consists of only the ROH band with a maximum at ∼560 nm. The rate of the ESPT is slower than the radiative rate, and thus the ESPT process is not taking place at these low temperatures. Below 235 K, proton diffusion in neat ice decreases with an activation energy of 0.2−0.3 eV (17−28 kJ/mol) as the temperature is lowered.28−32 At T ≤ 150 K, the proton is transferred from the ROH* form of QCy7 to nearby water

physical and chemical photoprotolytic processes. The proposed intermediate of the photoprotolytic proton transfer to the solvent is the formation of this ion pair. To the best of our knowledge, the proposed RO−*···H3O+ ion-pair spectrum has never before been identified in studies carried out at room temperature, since the concentration of the ion pair is probably very small compared to the concentrations of ROH and RO−. In low-temperature ice, the ion-pair concentration is high since the next step (shown in the protolytic scheme) is too slow and the relatively high radiative decay rates of both ROH and RO− terminate the photoprotolytic event at the intermediate step.

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EXPERIMENTAL SECTION Measurements of time-correlated single-photon counting (TCSPC) were performed with the use of excitation from a cavity-dumped titanium:sapphire femtosecond laser (Mira, Coherent), which provides short, 150 fs pulses at approximately 800 nm. The second harmonic of the laser, operating over the spectral range of 380−420 nm, was used to excite the samples. The cavity dumper operated with a relatively low repetition rate of 800 kHz. The TCSPC detection system was based on a Hamamatsu 3809U photomultiplier and an Edinburgh Instruments TCC 900 computer module for TCSPC. The overall instrument response was approximately 40 ps (full width at half-maximum, fwhm) where the excitation pulse energy was reduced to about 10 pJ by neutral-density filters. The temperature of the irradiated sample was controlled by placing it in a liquid-nitrogen cryostat with a thermal stability of approximately 1.5 K. The fitting of the TCSPC data was carried out by the Spherical Symmetric Diffusion Problem (SSDP)

Figure 1. Steady-state emission spectra of QCy7 measured at several temperatures: (a) H2O; (b) D2O. B

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Figure 2. TCSPC signals of the ROH form of QCy7 measured at several temperatures; λ = 560 nm: (a) linear scale; (b) semilogarithmic scale. (c) SSDP fits for TCSPC signals of QCy7 in D2O; λ = 560 nm.

Reversible and Irreversible Photoprotolytic Cycles of Photoacids. Excitation of a solution at pH values lower than the ground-state pKa of photoacids generates a vibrationally relaxed, electronically excited ROH molecule (denoted by ROH*) that initiates a photoprotolytic cycle (Scheme 2). Proton dissociation, with an intrinsic rate constant kPT, leads to the formation of the contact ion pair RO−*···H3O+. The intermediate ion pair was not yet identified in experiments. In the current study we provide some evidence of its existence in ice at temperatures below 150 K. The transferred proton may recombine geminately to reform the excited-state ROH* or the ground state ROH(g). The reversible (adiabatic) recombination with a rate constant ka reforms the excited acid, ROH*. In general, back-protonation may also proceed by an irreversible (nonadiabatic) pathway, involving fluorescence quenching of the RO−* by a proton with a rate constant kq, forming the ground-state ROH. 1-Naphthol and its derivatives are known to exhibit considerable fluorescence quenching of the deprotonated form, RO−*, in acidic aqueous solutions. 2-Naphthol derivatives and 8-hydroxypyrene-1,3,6-trisulphate (HPTS) recombine reversibly. Separation of an ion pair from the contact radius, a, to infinity is described by the transient numerical solution of the Debye−Smoluchowski equation (DSE).34,35 In addition, the fluorescence lifetimes of all excited species are considered, with 1/k0 = τROH for the acid and 1/k′0 = τRO− for the conjugate base. Generally, k0′ and k0 are much smaller than both the proton reaction and the diffusion-controlled rate constants. The

molecules and cannot hop any further within the excited-state lifetime of the RO−* species, which is ∼3 ns at these temperatures as will be shown below. We base our interpretation on a previous study, in which a quasi-elastic neutron-scattering technique was used, where we found that the hopping time of a proton in ice at 150 K is rather slow, about 1 ns.32 We further refer to the ion-pair formation and identification in the Discussion section. Time-Resolved Emission. Parts a and b of Figure 2 show on a linear and semilogarithmic scale, respectively, the timeresolved emission of the ROH form of QCy7 in H2O ice measured at 560 nm over a wide range of temperatures, 148− 260 K. For the time-resolved emission in ice, we used the TCSPC technique with limited time resolution determined by the instrument response function (IRF) of ∼40 ps full width at half-maximum (fwhm). Figure 2b shows, on a semilogarithmic scale, that the decay of the emission signal is nonexponential and depends on the temperature. The higher the temperature, the shorter the average decay time. The data analysis of the emission signal is based on a wellstudied model describing the ESPT to the solvent and the diffusion-assisted geminate recombination of the proton to reform the excited ROH* form. Below, we show a brief description of the model. The model is described in detail in 1988 papers14,33 and more recently in a feature article by Agmon.7 C

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amplitude of the ROH* long-time fluorescence tail depends on the intrinsic rate constants, ka and kPT, on the proton diffusion constant, DH+, and the electrical potential between the RO−* and the proton. The fluorescence decay of both ROH and RO− forms depends on the time-dependent populations. SSDP is a computer program by Krissinel and Agmon36 that solves numerically the diffusion-influenced geminate recombination model described by Scheme 2. Figure 2c shows a fit to the signals of the time-resolved emission of the ROH of QCy7 in D2O ice at several temperatures by the model described above. As seen in the figure, the model fitting is rather good at all temperatures. The fitting parameters are the forward and reverse proton-transfer rate constants kPT and ka as well as the proton diffusion constant in D2O ice and the excited-state lifetimes of ROH and RO−. The reaction sphere radius is chosen to be a = 4 Å. In previous studies on various photoacids, we used larger values for a, in the range of 5.5−7 Å. The active site of the proton reaction is the phenol (see Scheme 1). The phenol is much smaller than the hydroxyaryl compounds we used before the naphthols and the hydroxy-pyren-trisulfonate (HTPS). The Coulombic attraction potential is given by the Debye radius RD. For water at 298 K with a dielectric constant of ∼78, the proton−RO− attraction provides RD = 7 Å. RD is given by

RD =

ze 2 4πε0εkBT

Table 2. SSDP Fitting Parameters of the QCy7 ROH Form Decay in D2Oa,b T [K] 160 173 185 197 210 222 235 247 263

148 160 173 185 197 210 222 235 247

1.4 2 3 7 11 18 25 27 33

3 3 3 6.5 8.5 9 10 12 15

D [cm2 s−1] 0.04 0.07 0.15 0.27 0.45 0.6 0.85 1.5 2.0

× × × × × × × × ×

−5

10 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−5

7 7 7 8 9 7.5 10 11 7

0.8 1.5 3.0 5.0 7.0 0.6 1.6 1.0 3.0

× × × × × × × × ×

−6

10 10−6 10−6 10−6 10−6 10−5 10−5 10−5 10−5

[1/τRO−] [ns−1]c 0.25 0.26 0.26 0.3 0.3 0.3 0.3 0.35 0.45

kPT

H 2O + ROH* ⎯→ ⎯ RO−* + H3O+

As a result of a large overlap between the ROH and RO− emission bands, the signal measured at 720 nm contains contributions from both the ROH and RO− forms. From the steady-state emission shown in the figure, we estimate that 12.5% of the signal at high temperatures (T ≥ 185 K) arises from ROH emission. The ROH emission is seen as a rapid rise of the signal at very short times, limited by the IRF response time (see Figure 3). The long exponential decay arises from the emission lifetime of the RO−* form. It is dominated by a large nonradiative rate constant knr(T) that depends on the temperature. Tables S1 and S2 in the Supporting Information provide the average decay time of the ROH and RO− forms of QCy7, respectively, in ice at several temperatures. The decay time of RO− increases by a factor of more than 6 between 260 and 148 K. ESPT in D2O Ice. Parts a and b of Figure 4 show the timeresolved emission of QCy7 ROH and RO− bands in methanold-doped D2O ice at several temperatures between 88 and 263 K. The ROH signal was measured at 560 nm, near the peak of the ROH emission band, whereas the RO− emission was monitored at 720 nm at a slightly lower energy than the peak position in order to reduce the overlap with the ROH band. The decay rate of the ROH emission band in D2O ice is slightly slower than that measured in H2O ice. The fluorescence decay of the ROH band in both H2O and D2O could be fitted with the use of the ESPT geminate recombination model. This model accounts for the nonexponential long fluorescence tail. At long times, this fluorescence tail obeys a power-law decay of t−d/2 where d is the diffusion-space dimension. As the temperature is lowered, the decay rate decreases. At T ≤ 124 K, the decay rate of ROH is nearly exponential with an average decay time of ∼3 ns at 88 K. At this low temperature, the ESPT rate in D2O ice is smaller than the radiative rate and the proton/deuteron transfer efficiency is rather small since most of the excited ROH molecules decay radiatively or nonradiatively to the ground electronic state.

(1)

ka [109 Å s−1]

2.2 3.6 5 8 10 12 19 25 31

D [cm2 s−1]

At short times, the signals exhibit a rise, and at longer times the signal decay is nearly exponential. Both the rise time and the decay times of the RO− emission of QCy7 depend strongly on temperature. The higher the temperature, the longer the rise and decay times. The rise of the signal is attributed to the formation of the RO− form of QCy7 from ROH by the photoprotolytic process:

Table 1. SSDP Fitting Parameters of the QCy7 ROH Form Decay in H2Oa,b kPT [109 s−1]

ka [109 Å s−1]

a Reaction sphere radius a = 4 Å. bDebye radius RD = 7 Å. cRadiative rate of RO−.

where z is the charge of RO− in electron units, e is the electronic charge, ε is the dielectric constant, kB is the Boltzmann constant, T is the temperature on the Kelvin scale, and ε0 is the electric permittivity. RD(T) depends formally on the temperature. The value of ε(T) in ice increases from about 100 at 273 K to about 200 at T ∼ 175 K.29 Since ε(T) increases as T decreases, RD depends only slightly on T in the temperature range of 100−270 K. We used the SSDP of Krissinel and Agmon36 to fit the experimental data. Tables 1 and 2 provide the fitting parameters for H2O and D2O, respectively.

T [K]

kPT [109 s−1]

[1/τRO−] [ns−1]c 0.3 0.3 0.35 0.35 0.45 0.50 0.55 0.7 0.8

a Reaction sphere radius a = 4 Å. bDebye radius RD = 7 Å. cRadiative rate of RO−.

Parts a and b of Figure 3 show, on a linear and semilogarithmic scale, respectively, the TCSPC time-correlated emission signals monitored at 720 nm of the RO− form of QCy7 in ice measured over a wide range of temperatures, 148− 260 K. D

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Figure 3. TCSPC time-correlated emission signals monitored at 720 nm of the RO− form of QCy7 in ice measured over a wide range of temperatures, 148−260 K: (a) linear scale; (b) semilogarithmic scale.

Figure 4. Time-resolved emission of QCy7 in methanol-d-doped D2O ice at several temperatures over the temperature range of 88−263 K: (a) ROH measured at λ = 560 nm on a semilogarithmic scale; (b) RO− measured at λ = 720 nm.

Figure 4b shows at early times the fluorescence signal rise of the RO− form followed by an exponential decay. Both the signal rise and decay strongly depend on the temperature. The signal rise time is attributed to the ESPT rate, whereas the long exponential decay depends on both the radiative and nonradiative decay rates of the RO− form. Both the ESPT rate and the nonradiative rates depend on the temperature as is evident from Figures 2−4. The nonradiative process of the RO− form of QCy7 in the solid ice matrixes does not involve rotation or other skeletal motion of the conjugated chain. Many cyanine dyes exhibit a large nonradiative rate that depends on the solvent viscosity. Supporting Information Tables S3 and S4 provide the average times of the QCy7 ROH and RO− bands, respectively, in D2O ice. Kinetic Isotope Effect. Figure 5a and Supporting Information Figure S4 show, on linear and semilogarithmic scales, respectively, the fluorescence signals of the ROH form of QCy7 in H2O and D2O ice at four temperatures. Each panel shows the TCSPC signals of both the H2O and D2O samples. As seen in the figures, the decay of the fluorescence signals of the ROH form in D2O is slower than in H2O at short and long times. At short times, the decay depends mainly on the ESPT rate, whereas at long times, it depends on proton geminate recombination. The long-time fluorescence of the ROH form is given asymptotically by

−

* /(4πDt )d /2 exp[−t /τfRO ] IfROH ∼ [keq

(2)

The proton diffusion constant in D2O ice is smaller than in − H2O, and also the excited-state lifetime of the RO−, τRO , is f longer in D2O. These two parameters in eq 2 lead to the longer decay time of the fluorescence tail of RO− in D2O ice. Figure 5b shows the fluorescence decay curves of the RO− form of QCy7 in both H2O and D2O ice at four temperatures. As seen in the figure the fluorescence decay of the RO− form in D2O ice is slower than in H2O. Similar results are observed for many other photoacids. The fluorescence decay of the RO− form of 2-naphthol and 2-naphthol sulfonate derivatives in D2O in both liquid and ice shows longer lifetimes than that in H2O. This phenomenon has not yet been systematically studied and explained in the literature. Nonradiative theories of the isolated molecules predict a mild isotopic effect due to the change in the vibration frequencies of a molecule upon deuteration. This is not the case for the RO− form which lacks the deuteron. Thus, hydrogen bonding with the solvent’s deuteron at the phenol hydroxyl may be the reason for the kinetic isotope effect (KIE) on the nonradiative decay of the RO− form. Main Findings. 1. QCy7 is a strong photoacid and transfers a proton to methanol-doped ice at high rate at both high and low temperatures. E

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Figure 5. Fluorescence signals of the ROH form of QCy7 in H2O and D2O ice at four temperatures: (a) linear scale; (b) semilogarithmic scale.

2. The temperature dependence of the ESPT rate of QCy7 in H2O is much less than that of weak photoacids (low activation energies). 3. The nonradiative rate of the RO− form of QCy7 which is not related to the ESPT process also depends on the temperature in ice. The emission lifetime of the RO− varies from 0.5 ns at 260 K to 3 ns at 148 K. 4. In D2O ice the ESPT rate of the ROH of QCy7 is lower than in H2O ice. The KIE in ice is similar to what was found in water at room temperature. The KIE is ∼1.7 at all studied temperatures.

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5. The nonradiative decay rate of the RO− form of QCy7 in D2O ice is smaller than in H2O ice; at 148 K the RO− emission lifetime in D2O is 4.1 ns, whereas in H2O ice it is 3 ns.

DISCUSSION

Nonradiative Rate of the RO− Form. The fluorescence decay rate of the RO− form of QCy7 dissolved in both water and D2O at room temperature is rather large, much larger than expected for a pure radiative process. The lifetime of the RO− fluorescence is 130 and 230 ps in H2O and D2O, respectively.23 F

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In the current study, we measured the fluorescence lifetime of the RO− form of QCy7 at low temperatures in ice of both H2O and D2O. We assume that, in the ice, the QCy7 is in the bulk of a microcrystal a few micrometers or more in size.28 The two flexible conjugated bridges are not free to rotate in the bulk of the ice, whereas in the liquid state, the rotation time is determined by the solvent viscosity. The rotation of the cyanine chain around the double bond decreases the energy gap between the ground and excited electronic states. The nonradiative rate increases upon rotation around the double bond. In the nonadiabatic limit, this rate depends exponentially on the energy gap, which decreases upon rotation, and hence the rate increases. In the ice phase, rotation is prevented but the fluorescence lifetime of the RO− form is still short: ∼0.5 ns at 260 K. We obtain the values of knr(T) by the following procedure. We assume that at ∼80 K, the lowest temperature reached in this study, knr < kr and the fluorescence rate constant k at higher temperatures is given by k(T ) = k r + k nr(T )

ultrafast internal conversion. The EDPT process plays an essential role in fluorescence quenching of many molecular systems in protic solvents. We also measured the temperature dependence of the kPT of HPTS in liquid water and in ice. HPTS is a much weaker photoacid with pKa* of 1.4 and kPT of 1010 s−1 at room temperature.14,33 Parts a and b of Figure S1 in the Supporting Information show the time-resolved emission of the ROH* form of HPTS in methanol-doped ice at several temperatures in the range of 207−296 K. The decay rate of the TCSPC signals of the ROH* form of HPTS at any given temperature is much slower than that of QCy7, which is a far stronger photoacid. Supporting Information Figure S2 shows the TCSPC signals of the RO−* form of HPTS at several temperatures in the range of 207−296 K. The RO−* signals rise at short times and decay exponentially at longer times with a radiative lifetime of 5.4 ns, nearly independent of the temperature. The rise component of the signal is temperature-dependent and is attributed to the ESPT process. The decay of the ROH* form signal fits the rise time of the RO−* form. Supporting Information Figure S3 shows the steady-state emission of HPTS at several temperatures. The sample was excited at 400 nm, where the peak of the absorption band of the ROH* form is located. It is seen that, as the temperature is lowered, the intensity of the ROH* band increases and that of the RO−* decreases. In Figure 7 we plot log kPT as a function of 1/T for both HPTS and QCy7. The activation energy of HPTS is 40 ± 4 kJ/mol, while that of QCy7 is only 10.5 ± 1.0 kJ/mol. This large difference is in accord with free-energy correlations like bond energy/bond order (BEBO39) and electron transfer by Marcus.40,41 Both theories predict that the activation energy would depend on the free energy of the reaction. Spectroscopic Identification of the Ion Pair RO−*···H3O+ at Low Temperatures. Parts a and b of Figure 1 show the steady-state emission of QCy7 in H2O and D2O ice, respectively. Between 112 and 150 K, the emission spectrum consists of two emission bands, a broad band assigned to the ROH form with a peak at 560 nm and a weaker band at around 660 nm which we assign to the ion pair RO−*···H3O+ or, as was suggested by Stoyanov et al.,42 RO−*···H+···H2O. This peak is blue-shifted by about 1000 cm−1 from that of the RO− emission band (700 nm) at higher temperatures of T ≥ 197 K. Scheme 2 portrays the proposed mechanism of the photoprotolytic proton dissociation and recombination as well as the relaxation of the electronically excited photoacid and the conjugated base back to the ground state. We propose that the proton dissociation reaction intermediate is an ion pair consisting of the hydroxide (RO−*) and a hydronium ion (H3O+). This intermediate was suggested for acid−base dissociation recombination reactions by Eigen and De Mayer more than half a century ago.43 The following step involves dissociation by proton transfer between the hydronium ion formed and an adjunct water molecule. The reverse reaction leads to recombination and reformation of the ROH*. The population of the ion-pair transient strongly depends on the four rate constants shown in Scheme 2. On the basis of the recent knowledge42,44 about proton hydrates at high temperatures (>227 K), ion pairs of RO−*, if they exist, are large hydrates with H13O6+ cations. These large hydrates developed an RO− band at 700 nm since the proton is far removed from the RO−. The positive charge in these cations is delocalized significantly owing to high proton mobility as the

(3)

where kr and knr(T) are the radiative and nonradiative rate constants, respectively. kr is independent of T, and its value is deduced from the emission lifetime at ∼80 K. Figure 6 shows the nonradiative rate of the RO− form of QCy7 in H2O and D2O ice as a function of 1/T.

Figure 6. Nonradiative rate of the RO− form of QCy7 in H2O and D2O ice as a function of 1/T.

We find that, in the ice phase, the nonradiative rate constant knr decreases as the temperature is lowered and the activation energies of the nonradiative process in both H2O and D2O are similarabout 11 kJ/mol. As seen in the figure, knr in the liquid state is larger than in the ice phase at the high temperature of 260 K. The change in knr between the liquid and the solid is by a factor of 4, and further cooling of the ice lowers knr by an order of magnitude from ∼109 s−1 at 250 K down to 108 s−1 at 175 K. This large temperature dependence and the large isotope effect of about a factor of 2 on the value of knr, suggests that in the solid state, knr is associated with hydrogen bonding of QCy7 to the solvent. The ESPT process shows a similar activation energy and hydrogen bonding is unique to this process. Sobolewski and Domcke37,38 found in their computational study a common feature of the photochemistry of various systems that undergo electron-driven proton-transfer (EDPT) mechanism. Highly polar charge-transfer states of 1ππ*, 1nπ*, or 1πσ* character drive the proton transfer, which leads also to G

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Figure 7. Log kPT as a function of 1/T of both HPTS and QCy7.

ion-pair spectrum is clearly seen at these temperatures. At T < 112 K, the ESPT rate is much smaller than the kr + knr values for ROH and the efficiency of ESPT in forming the ion pair will diminish. In short, At T < 150 K we attribute the red emission band to the intermediate of the ESPT process, namely, the RO−*···H3O+ ion pair. In order to observe the ion-pair intermediate, the rate constants of generation and dissociation of the ion pair must be balanced in such a way that its population will be large in comparison to those of the ROH* and free RO− forms. The intermediate could be not a constant ion pair but in general an H+(solvent)x complex. For simplicity we used the term hydronium for all kinds of H+(solvent)x complexes.

distance between the negative and positive centers of charge is large. By decreasing the temperature to 160 K, the dynamic properties of H+ in H(aq)+ clusters slows down resulting in an increasing charge concentration on the center of the cluster and to give more tightly formed ion pairs. This is developed in the RO− form of QCy7 spectra as a gradual blue-shifting of the emission band to 680 nm. Then at T < 150 K an abrupt change in the spectrum is observed because the ion pair is transferred to a proton disolvate of L1−H+−L2, where L1 is RO−* anion and L2 is H2O with H+ shared more or less equally by both bases. With further decreasing of the temperature a second hoping in the spectra is observed because of a proton transfer to the anion: RO−H*···H2O. The intermediate state of L1−H+−L2 type species, exhibits a short strong hydrogen bond with two-well or a flat bottom hydrogen potential. The asymmetric proton disolvates, L1− H+−L2, compared with those of symmetric, L−H+−L, are much less stable and have a narrower region of existence. Formation of anion−H+−OH2 intermediates for acids is known in the literature.42 The hopping time of a proton in low-temperature ice was recently measured for HCl-doped ice by the quasi-elastic neutron-scattering technique (QENS).32 It was found that the hopping time is ∼1 ns at 150 K, whereas at 190 K it is rather short, 8 ps. The activation energy of the hopping in this temperature range is 17 ± 1 kJ/mol. The ESPT rate constant for ion-pair formation, kPT, at 150 K is about 2 × 109 s−1; this is twice that of the hopping rate in ice. If the forward and reverse reaction rates are of the same magnitude, then the ion-pair population should be quite large at short times of a few nanoseconds, limited by the excited-state lifetimes of both ROH* and RO−* forms of QCy7. The hopping rate at lower temperatures decreases further, as expected from the large activation energy of 17 kJ/mol. The forward reaction has a much smaller activation energy of 10.5 kJ/mol, and as a result, at such low temperatures, kPT decreases at a much slower rate as the temperature is lowered than is the case for the protonhopping rate between adjacent water molecules in ice. We expect that at lower temperatures, 110 < T < 150 K, the hopping time will be much longer than the lifetime of the excited state, and therefore the ion pair will not dissociate further. As seen in parts a and b of Figure 2, the RO−*···H3O+

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SUMMARY AND CONCLUSIONS In this work, we studied the photoprotolytic process of the superphotoacid QCy7 (pKa* ∼ −6) as a function of temperature in both H2O and D2O ice over a wide temperature range of 85−270 K. We found that the photoprotolytic process over this large temperature range can be divided to three temperature subregions. In the high-temperature region (150−270 K), the ESPT rate constant, kPT, decreases as the temperature is lowered, with a small activation energy, ΔG⧧, of 10.5 ± 1 kJ/mol. We compared this small ΔG⧧ with that of HPTS, a much weaker photoacid with a pKa* ≈ 1.4, where we found ΔG⧧ = 40 ± 4 kJ/mol. The large activation energy of HPTS limits the ESPT process to high temperatures (T ≥ 220 K), whereas in QCy7, the ESPT could also be observed at 112 K. Over the intermediate temperature range (112−150 K), the steady-state emission spectrum of RO−, the deprotonated form of QCy7 is blue-shifted by 1000 cm−1 and the band peak is at ∼660 nm, whereas at T > 160 K, its position is at 700 nm. We attribute this large and abrupt shift of the RO− emission band to the formation and stabilization of the intermediate acid dissociation product, the RO−*···H3O+ ion pair. Scheme 2 shows the suggested photoprotolytic proton dissociation and recombination processes that a photoacid undergoes in the first electronic excited state. The vibrationally relaxed ROH* photoacid transfers a proton to a hydrogen-bonded water molecule next to the hydroxyl group of the QCy7 phenol, and the intermediate product is the RO−*···H3O+ ion pair. This ion H

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pair has never before been identified spectroscopically in the literature, although the photoprotolytic scheme has been known for at least half a century. More research is needed to explore the spectroscopy of the excited-state RO−*···H3O+ ion pair. Over the lowest temperature range (85−112 K), the excited QCy7 is unable to transfer a proton to the solvent within the excited-state lifetime of the ROH* form, whose emission lifetime is 3 ns at these low temperatures.

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ASSOCIATED CONTENT

S Supporting Information *

Temperature dependence of ESPT from HPTS and QCy7 data analysis tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 972-3-6407012. Fax: 972-3-6407491. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants from the James-Franck German−Israeli Program in Laser−Matter Interaction and by the Israel Science Foundation.

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