Ultrafast Excited-State Proton Transfer to the Solvent Occurs on a

Article pubs.acs.org/JPCA

Ultrafast Excited-State Proton Transfer to the Solvent Occurs on a Hundred-Femtosecond Time-Scale Ron Simkovitch, Naama Karton-Lifshin, Shay Shomer, 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 ultrafast time-resolved techniques were used to study a newly synthesized photoacid, phenol-carboxyether dipicolinium cyanine dye, QCy9. We found that the excited-state proton transfer (ESPT) to water occurs at the remarkably short time of about 100 fs, kPT ≈ 1 × 1013 s−1, the fastest rate reported up to now. On the basis of the Förster-cycle, the pKa* value is estimated to be −8.5 ± 0.4. In previous studies, we reported the photoacidity of another superphotoacid, the QCy7 for which we found an ESPT rate constant of ∼1.25 × 1012 s−1, one-eighth that of the QCy9 compound. We found a kinetic isotope effect of the ESPT of about two.

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INTRODUCTION Photoacids are a class of organic aromatic molecules that are weak acids in their ground electronic state, but their acidity increases by many orders of magnitude in their first excited electronic state. Thus, photoexcitation to the excited state, by short UV−vis laser pulses, enables one to follow the photoprotolytic processes. In recent years, many investigations have been carried out on the excited-state intermolecular proton transfer (ESPT) from the acidic group of the excited photoacid to 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 since the radiative rate constant kr ≈ 108 s−1, the overall fluorescencedecay rate is 2 × 108 s−1. During the 1990s, dicyano-naphthol derivatives were synthesized in order to increase the photoacidity. It was found that their ESPT rates increased by about 3 orders of magnitude and that 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. The very strong photoacids with pKa* ≤ −2 are called superphotoacids.14−20 QCy7, which is a new Cy7like molecule,21,22 is a superphotoacid with pKa* ≈ −6. Scheme S1 in the Supporting Information shows its molecular structure. The hydroxyl group of the phenol in the center of the structure is the proton emitter. The two 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 532 nm) and from the deprotonated RO−* © 2013 American Chemical Society

(700 nm) species of QCy7. The decay of the time-resolved emission of ROH* in water is rapid, slightly shorter than 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 explain these observations as arising from ESPT to the solvent. The ESPT rate to water is about twice that of NM6HQ+, one of the strongest photoacids on record up to now,24,25 making the ESPT rate for QCy7 and its derivatives the fastest reported ESPT rate thus far. In the current study, we used both steady-state and timeresolved emission techniques to study the photoprotolytic reactions of a recently synthesized phenol cyanine dye whose molecular structure is shown in Scheme 1 and is named QCy9 throughout this article. Dye 1 is composed of a phenol latent donor and two identical acceptors based on a picolinium moiety (as seen in Scheme 1). Deprotonation of the phenolic form leads to formation of the conjugated base. The QCy9 molecule consists of a phenol to which three functional groups are attached: two cyanine dipicolinium groups at the ortho position to the phenol and a carboxy-ether group at the para position. All these groups are strong electron-withdrawing groups and thus significantly increase the photoacidity of the phenol, which is a rather weak photoacid. We found that QCy9 is a superphotoacid with pKa* ≈ −8.5; an even more remarkable finding is that it exhibits a very large ESPT rate constant, kPT ≈ 1 × 1013 s−1. This is the largest kPT value reported in the literature up to now. In the discussion section, we suggest that kPT ≈ 1013 s−1 may be the upper limit of the rate constant of ESPT to the solvent, and thus, the kPT value of QCy9 reaches that limit. Received: February 10, 2013 Revised: March 17, 2013 Published: April 3, 2013 3405

dx.doi.org/10.1021/jp4014724 | J. Phys. Chem. A 2013, 117, 3405−3413

The Journal of Physical Chemistry A

Article

Scheme 1. Deprotonation Pathway of Dye 1 to Form an Extended π-Electron Conjugation between the Two Picolinium Acceptors

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EXPERIMENTAL SECTION The fluorescence up-conversion technique was employed in this study to measure the time-resolved emission of QCy9 at room temperature. The laser used for the fluorescence upconversion was a cavity-dumped Ti:sapphire femtosecond laser (Mira, Coherent), which provides short, 150 fs, pulses at about 800 nm. The cavity dumper operated with a relatively low repetition rate of 800 kHz. The up-conversion system (FOG100, CDP, Russia) operated at 800 kHz. The samples were excited by pulses of ∼8 mW on average at the SHG frequency. The time response of the up-conversion system is evaluated by measuring the relatively strong Raman Stokes line of water shifted by 3600 cm−1. It was found that the full width at halfmaximum (fwhm) of the signal is 340 fs. Samples were placed in a rotating optical cell to avoid degradation. We found that, during our five-minute time-resolved measurements in a cell rotating at a frequency of 10 Hz, the degradation of the sample was marginal and had no effect on the signal’s decay profile. The QCy9 samples were excited in their protonated ROH form; Scheme 1. Experiments were carried out on samples in concentrations of about 1 mM or less. Reversible and Irreversible Photoprotolytic Cycles of the 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).

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).26,27 In addition, the fluorescence lifetimes of all excited species are considered, with 1/k0 = τ0 for the acid and 1/k′0 = τ′0 for the base. Generally, k′0 and k0 are much smaller than both the proton-reaction and the diffusion-controlled rate constants. The amplitude of the ROH* long-time fluorescence tail depends on the intrinsic rate constants, ka and kPT, on the proton-diffusion constant, DH+, and on the electrical potential between RO−* and the proton. The motion of the transferred proton in water close to the photoacid depends strongly on the electrical potential existing between it and the deprotonated form. The diffusion-assisted geminate recombination of the RO−* with the proton can be quantitatively described with the use of the DSE.

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RESULTS Steady-State Measurements. Figure 1 shows the excitation and emission spectra of the QCy9 dye (the structure of which is shown in Scheme 1) in water at pH = 3. Figure S1 (Supporting Information) shows the peak absorbance of the deprotonated form, the RO− as a function of the solution’s pH. The pKa of the dye in water is 4.3 ± 0.2.

Scheme 2. Photoprotolytic Cycle

Proton transfer, with an intrinsic rate constant kPT, leads to the formation of the contact ion-pair RO−*···H3O+, whereas 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.

Figure 1. Excitation and steady-state emission spectra of the QCy9 dye in water at pH = 3. Note the weak ROH band at 480 nm. 3406



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The excitation of the dye in its ROH protonated form leads to a dual emission band: a weak emission with a peak at 485 nm and a much stronger band with a peak at about 680 nm. The dualband emission arises since a photoprotolytic reaction occurs. In the first electronic singlet state of the ROH*, a proton is transferred to the solvent, and RO−* is generated within the excited-state lifetime of the ROH* form. The excited-state proton transfer rate can be estimated by the following relationship:1,32 kPT ≈

IRO− 1 IROH τRO−

(1)

where IROH and IRO− are the peak fluorescence intensities of the ROH and RO− forms, respectively, and τRO− is the emission lifetime of the RO− form. As we will show below, the timeresolved emission data provide the excited-state lifetime of the RO− deprotonated form of QCy9. The emission lifetime in water is ∼60 ps, and the emission intensity ratio (IRO−/IROH) is ∼80, and thus, the estimated ESPT rate constant is ∼1.3 × 1012 s−1. As we will now show, the actual ESPT rate is larger than this and can be accurately evaluated from the time-resolvedemission studies given below. Time-Resolved Emission. Figure 2a,b shows the linear and semilogarithmic scales, respectively, of the time-resolved fluorescence of QCy9 in water measured by the fluorescence up-conversion technique. The instrument response function of the optical apparatus is ∼300 fs at fwhm; this limits the accurate determination of the ultrafast fluorescence decay profile of the ROH form. As seen in the figures, the decay of the ROH signals measured at 10 nm intervals in the spectral range of 450−560 nm is almost independent of the wavelength examined. The signal can be divided into three time intervals: the short time interval, up to about 0.4 ps after the peak of the signal, the intermediate interval 0.4−2 ps, and the long-time fluorescence tail t ≥ 2 ps. In the geminate recombination model described in the previous section, we explain the photoprotolytic cycle that an excited photoacid molecule undergoes. The first step is the creation of the RO−*···H3O+ ion pair; the short-time component of 100 ± 20 fs is therefore the creation time of the ion-pair. We fit the time-resolved signal to three time components. Table 1 shows the fitting parameters. In a previous paper on the photoprotolytic process of the QCy7 photoacid, the short time component was assigned to the solvation dynamics (200 fs), whereas the intermediate time component (around 800 fs) was assigned to the proton transfer to the solvent. In QCy7, the amplitude of the ultrashort decay time of 200 fs is rather small (a < 0.2) and strongly depends on the monitoring wavelengths. At λ > 540 nm (where the fluorescence ROH peak is at 540 nm), it is too small to be determined. In general, a short time component that decreases as λ increases is indicative of a dynamic process occurring on the potential surface of the initially prepared excited state, such as vibrational relaxation and solute−solvent rearrangement. The short-time decay of the ROH form of QCy9 is 100 fs, and its amplitude has a value of about 0.88 at wavelengths between 480 and 560 nm. We attribute the short-time fluorescence component to the first step of ESPT to the solvent. The intermediate time component of about 500 fs and an amplitude that varies between 0.1 and 0.2 we attribute to the breakup of the quasi-equilibrium formed by proton transfer

Figure 2. Time-resolved fluorescence of QCy9 in water measured by the fluorescence up-conversion technique: (a) linear scale and (b) semilogarithmic scale. (c) Time-resolved fluorescence of QCy9 in the RO− form in water, measured by the fluorescence up-conversion technique. Note the signal rise arising from the proton transfer reaction.

from the ion-pair to a neighboring water molecule or by geminate recombination to reform the ROH* (see Scheme 2). When the proton is further removed by one water molecule, the Coulomb potential decreases to about the value of the thermal energy, and proton hopping through the bulk water molecules is less influenced by the presence of RO−. The hopping time of a proton in water is calculated from the high 3407



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measured at long wavelengths λ > 660 nm depends on solvent viscosity. The lifetime in water is about 100 ps, whereas in propanol, with viscosity η = 2.5 cp, it is around 300 ps. We therefore think that the cis−trans isomerization is leading to a nonradiative decay of both ROH and RO− and that it occurs on a much longer time-scale. It does not affect the fast decay timecomponents of QCy9, especially not the 100 fs component we attribute to the PT transfer rate. Kinetic Isotope Effect. Figures S2a and S2b (Supporting Information) show the time-resolved emission of QCy9 in D2O, measured by the fluorescence up-conversion technique at 10 nm intervals over a wide spectral range on both linear and semilogarithmic scales. Figures 3a and S3a (Supporting Information) show the fluorescence up-conversion signals of the ROH form of QCy9 in both H2O and D2O at several wavelengths within the ROH band.

Table 1. Fitting Parameters of up-Conversion Fluorescence of QCy9 in H2O λ (nm)

a1

τ1 (fs)

a2

τ2 (fs)

a3

τ3 (ps)

a4

τ4 (ps)

460 480 490 500 510 520 540 560

0.88 0.88 0.89 0.87 0.88 0.88 0.84 0.78

100 100 100 100 100 120 100 100

0.088 0.088 0.089 0.090 0.098 0.097 0.13 0.18

500 500 500 500 500 400 450 500

0.032 0.032 0.021 0.040 0.022 0.023 0.023 0.023

7 7 7 6 7 6 6 6

0.007 0.017

180 150

proton mobility and is also calculated from NMR bandwidth measurements. The hopping time derived from the mobility is 1.5 ps, and this value is close to the QCy9 intermediate time component. This correspondence is a strong justification of our assignment of the intermediate time component to the proton transfer from the ion-pair. The third time component shows extended nonexponential decay. It has a very small amplitude of ∼0.01, and the decay can be approximately fitted to a t−α power law. We attribute this time component to the diffusion-assisted proton geminate recombination to reform the excited state ROH* and subsequent proton transfer to the solvent. We further discuss the complex decay profile of the fluorescence up-conversion signals of the ROH form of the QCy9 photoacid in the following sections. At long wavelengths (λ ≥ 680 nm), the time-resolved fluorescence signal of QCy9 in H2O shows a fast rise followed by a long, nearly exponential, decay of about 60 ps. The fluorescence up-conversion signals measured at 660, 680, and 700 nm are shown in Figure 2c. We fit the data of the RO− form measured at 700 nm for which the overlap between the ROH and RO− bands is minimized. We estimate that the amplitude of the ROH signal at 700 nm is aROH = 0.25 ± 0.1 and that of the RO− is aRO− = 0.75 ± 0.1. The fit then contains the following equation − y700nm(t) = aROHIROH (t) + aRO−IRO (t) f f ROH The fit at 540 nm is taken for If (t). For the best fit of the signal measured at 700 nm, we find two time components for the rise a1 = 0.7 ± 0.1 and τ1 = 100 ± 20 fs and a2 = 0.3 ± 0.1 and τ2 = 500 ± 200 fs. As seen in the figure, the signal rise time has two components: a fast rise time component with large amplitude and a time constant of ∼100 fs, and a second with smaller amplitude and a time constant of 500 fs. These time components are attributed to proton transfer and the formation of the intermediate product, the ion-pair, and the subsequent hopping of the proton to the bulk water. The ion-pair fraction is probably very small since it is an intermediate product. The position of the emission band of RO− when the proton is on the water molecule nearest to the hydroxyl group is only slightly shifted compared to the emission bandwidth of the free RO−, and therefore, it is, as yet, not unequivocally identified in ESPT measurements. The short-lived ion-pair (∼500 fs) emission is masked by the overwhelmingly large contribution of the free RO−* emission with a lifetime of about 60 ps. A plausible contribution to fast decay components of both the ROH and RO− forms of QCy9 and QCy7 photoacids may arise from cis−trans isomerization of the conjugated system.23 We found that in QCy7 the emission lifetime of the RO−

Figure 3. Comparisons between the decay of the ROH signal of QCy9 in H2O and D2O. (a) Short wavelength ROH emission. (b) Long wavelength RO− emission.

As seen in Figure 3a, the decay of the ROH signal of QCy9 in D2O is significantly slower than in H2O at all the wavelengths shown in the panels of the figure. The decay time of the short-time component is attributed to the proton/ deuteron transfer to form the RO − *···H 3 O + ion-pair intermediate. Table 2 provides the fitting parameters of the three time components fit for the fluorescence up-conversion signals of QCy9 in D2O. The decay time of the shortest time component of the signals at both 500 and 520 nm are 100 and 200 fs for H2O and D2O, 3408



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Table 2. Fitting Parameters of up-Conversion Fluorescence of QCy9 in D2O λ (nm)

a1

τ1 (fs)

a2

τ2 (fs)

a3

τ3 (ps)

α3

a4

τ4 (ps)

α4

460 480 520 540 560

0.87 0.87 0.83 0.82 0.76

160 180 200 220 220

0.09 0.09 0.14 0.14 0.19

900 900 900 900 900

0.04 0.04 0.027 0.029 0.027

12 12 12 12 12

0.75 0.75 0.75 0.7 0.6

0.003 0.011 0.023

200 200 280

0.7 0.7 0.7

respectively. We therefore determine that the kinetic isotope effect (KIE) on the proton transfer rate found for QCy9 is KIE ≈ 2. This value is somewhat larger than the KIE found for QCy7, (∼1.7). The KIE values for ESPT of both QCy7 and QCy9 are larger than the value found for proton mobility in H2O and D2O, namely, KIE ≈ 1.45, but much smaller than that found for weaker photoacids for which the KIE is about 3. The intermediate and long decay-time components of the ROH* decay signals of QCy9 in D2O are also longer than in H2O. The intermediate time component, τ, is approximately 3 ps in D2O, whereas in water it is 500 fs, and the long decay time is 120 ps in D2O, as compared to 60 ps in H2O. The intermediate decay component should also be longer in D2O since it is associated with proton mobility, and as mentioned before, the KIE of the mobility, as determined by electrochemical measurements, is ∼1.45. For many photoacids, the RO− lifetime depends on the isotopic substitution. QCy7 and naphthols exhibit a much longer lifetime of the RO− form in D2O than in H2O. We explain this qualitatively by the existence of a nonradiative process of the excited state of RO−* promoted by the strong hydrogen bonding of the water molecules to the oxygen anion. In D2O, the nonradiative rate is smaller than in H2O. Figures 3b and S3b (Supporting Information) show on a linear and semilogarithmic scales, respectively, the fluorescence up-conversion signals of both H2O and D2O samples measured at the long wavelengths of 680 and 700 nm. The major contribution of these signals is from the RO−, the deprotonated form of QCy9. The signal rise is not instantaneous (pulse limited) but shows a finite short rise time to about 0.75 of the maximum height of the H2O sample of 100 and 200 fs for D2O. The small contribution to the signal rise of about 0.25 is much longer in D2O than in H2O since the decay time is two times longer in D2O compared to H2O. Fitting of the Time-Resolved Emission. Figures 4 and S4 (Supporting Information) show, on linear and semilogarithmic scales, the experimental results along with the computer fit of the time-resolved emission of ROH measured near the emission-band maximum at 500 nm in both H2O and D2O. The fit is performed by the convolution of a threecomponent decay function with the fluorescence up-conversion system response of ∼300 fs fwhm of the nearly Gaussian shaped function. The best-fit function consists of two short exponential decays followed by a longer stretched exponential-decay component, exp[−(t/τ)α]. The fitting parameters are given in Tables 1 and 2 for H2O and D2O, respectively. As seen in the figures, the fit is rather good at short times (t ≤ 5 ps), but much less so at longer times. The mismatch at long times arises from the nonexponential origin of the long-time fluorescence tail. It arises from the diffusion-assisted proton geminate recombination (GR) (described in Scheme 2), which leads to an asymptotic fluorescence tail following a power-law, t−α, where α is d/2 for spherically symmetric diffusion (d is the diffusion-

Figure 4. Time-resolved emission of the ROH form of QCy9, in both H2O and D2O measured at 500 nm along with computer fits, see Tables 1 and 2.

space dimension). For a purely reversible GR process in three dimensions, α = 3/2. Förster-Cycle Calculation. With the Förster-cycle calculation,28,1 it is possible to estimate the change in acidity upon excitation of the molecule. This calculation is based on the position of the optical absorption or emission band of the protonated and deprotonated forms of a photoacid. The energy cycle leads to a simple relation between band positions and change in acidity:

ΔpK a* = C Δν

(2)

where C is a product of universal constants C=

NAh = 2.09 × 10−3 cm ln(10)RT

Δν is the difference, in wavenumber units, between the positions of the ROH* and RO−* absorption or emission bands. The ROH* and RO−* emission bands of QCy9 have peaks at 485 (20 600 cm−1) and 690 nm (14 490 cm−1), respectively. This leads directly to a ΔpKa* value of −12.8. The ground-state pKa is ∼4.3, and thus, the estimated pKa* value is ∼−8.5. This value is more negative than that of HCl, which is a strong mineral acid. For QCy7, we had previously found pKa* ≈ −6.1, which makes it a significantly weaker acid than QCy9. Comparison with QCy7. Figures 5 and S5 (Supporting Information) show the time-resolved emission measured at several wavelengths near the peak position by the fluorescence up-conversion technique of both QCy7 and QCy9 in H2O. There is a significant difference of ∼47 nm in the ROH emission peak position of the QCy7 and QCy9 photoacids that increases according to the Förster-cycle calculations of the absolute value of the pKa* of QCy9. Both compounds are considered superphotoacids since their pKa* ≤ −2. Super 3409



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DISCUSSION

Photoacidity of the QCy9 Compound. The photoacidity of phenol is rather small (pKa = 10 and pKa* = 3.6), but the photoacidity of the phenol increases significantly in both the QCy9 and QCy7 structures. The quinonal cyanine structure provides exceptionally strong electron-withdrawing (accepting) ions, namely, indolium or picolinium, which increase the stability of RO−*, the conjugate base of the ROH* acidic form, and thus lower the free energy of the proton transfer reaction. This is the main reason for the high photoacidity of both these compounds and, as a consequence, an extremely large ESPT rate constant approaching 1013 s−1 for QCy9. In addition to the superphotoacids based on the cyanine dye, N-methyl-6hydroxyquinolinium is also a superphotoacid with ESPT kPT ≈ 5 × 1011 s−1. This suggests that electron-withdrawing cations linked to the hydroxylaryl were an excellent choice for synthesizing a strong photoacid based on hydroxylaryl compounds. In a previous study on coumarin dipicolinium Cy photoacid, we found that the ESPT rate constant kPT = 1.4 × 1011 s−1. This is rather smaller than those for both QCy7 and QCy9 for which the values of kPT are 1.25 × 1012 s−1 and approximately 1 × 1013 s−1, respectively. The ESPT rate of 7hydroxy-4-methyl coumarin is much larger30 (kPT∼3 × 1010 s−1) than that of phenol (kPT < 108 s−1). If we assume that the higher acidity of coumarin-Cy photoacid arises from the cyanine-bridged dipicolinium units, then the ESPT rate constant of QCy9 should have increased by only a factor of 5. The carboxy-ether group in the para position to the hydroxyl of the current study is a well-known electron-withdrawing group with a large Hammet σ value of about 0.6. This group increases the photoacidity and the ESPT rate constant, but we estimate that the photoacidity will increase by only about two or 3 orders of magnitude over that of phenol. This estimate is based on previous studies 31 of monocyano-2-naphthol derivatives. In this study, we found an increase of about 2 orders of magnitude in both the ESPT rate and the pKa* values of 6-cyano-2-naphthol. We therefore suggest that additional contribution to the very high measured photoacidity of QCy9 may arise from the conjugated chains symmetrically linked to the phenol at the ortho position. In QCy7, the indolium ions are asymmetric with respect to the hydroxyl group since one indolium is connected at the meta and the second at the para position. ESPT-to-Solvent Rates: How Fast Can It Get? The ESPT process has been studied for more than half a century.32 In the early stages of the research, only steady-state-fluorescence methods were available, and therefore, these were used to estimate the proton transfer rate constant and the pKa*. In 1973, Ofran and Feitelson33 performed the first direct ESPT rate measurements using the time-resolved emission technique to study 2-naphthol in water. The ESPT rate constant they found was approximately 108 s−1. In the early ′90s, Pines et al. measured the fastest reported ESPT rate time for 1-naphthol34 and found kPT to be 3.5 × 1010 s−1. Two main parameters emerge from the long years of research: pKa* and the excitedstate proton transfer rate constant, kPT. Later, superphotoacids with pKa* ≤ −2 were studied by time-resolved techniques. In 2004, Topp and co-workers24 found a large value of kPT for Nmethyl-6-hydroxyquinolinium (5 × 1011 s−1). More recently,23,29 we measured the ESPT rate for QCy7, the molecular structure of which is seen in Scheme S1 in the Supporting Information. The emission peaks of the ROH and

Figure 5. Time-resolved emission of QCy7 and QCy9 in H2O measured at several wavelengths near the peak position by the fluorescence up-conversion technique, linear scale.

photoacids are able to transfer a proton to protic solvents in general, and not only to water, which is the ultimate protonacceptor and proton-conductor solvent. As seen in both figures, the fluorescence decay rate of QCy9 ROH* is much larger than that of the QCy7 (shown in Scheme S1 in the Supporting Information). In previous studies,23,29 we found that the ESPT rate constant of QCy7 (kPT ≈ 1.25 × 1012 s−1) was the largest kPT reported up to then and made it the strongest photoacid. In the current study, we measured the photophysics and chemistry of QCy9 (shown in Scheme 1). We found that QCy9 is a much stronger photoacid than QCy7, and its ESPT rate constant (kPT ≈ 1 × 1013 s−1) is eight times that of QCy7. On the basis of the Förster-cycle method, the pKa* values of QCy7 and QCy9 are −6.1 and −8.5, respectively. The large difference in their pKa* values qualitatively explains the large difference in their kPT values. The value of kPT of acids in general and photoacids in particular depends on several factors. The pKa is directly related to the free energy of reaction, ΔG0, and the Förster-cycle provides a rough estimate of the pKa* values. Both the free energy of activation, ΔG‡, and the pre-exponential factor determine the kPT value of the ESPT. Free-energy correlations provide only a qualitative correlation between ΔG0 and ΔG‡. In a previous study on hydroxycoumarin dipicolinium Cy7, we found a much smaller ESPT rate, with kPT ≈ 1.4 × 1011 s−1, an order of magnitude smaller than that of QCy7. QCy7 contains two strong electron-accepting indolium ions, while the QCy9 has two picolinium ions, which are much weaker electron acceptors. We therefore conclude that there is an additional contribution to the higher photoacidity of QCy9. We address this issue in the discussion section. Main Findings. (1) QCy9 is a very strong photoacid. When excited in its ROH form, the steady-state emission consists of a weak band with a peak at 485 nm (ROH) and a much stronger band at 680 nm (RO−). (2) The acidity in the excited state, pKa*, is estimated by the Förster-cycle to be −8.5. (3) The rate constant, kPT, of the ESPT to the solvent, is deduced from the time-resolved emission of the ROH band. It was found that kPT ≈ 1 × 1013 s−1, the fastest rate reported up to now. (4) The kinetic isotope effect, kPT, is ∼2. (5) The value of kPT for QCy9 is about eight times that found previously for QCy7, which has a pKa* of −6 and kPT = 1.25 × 1012 s−1. 3410



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RO− forms of this molecule are at 532 and 700 nm, respectively. The pKa* was determined with the use of the Förster-cycle to be approximately −6. The ESPT rate constant was found to be 1.25 × 1012 s−1, i.e., the ESPT of QCy7 is more than twice as fast as that of NM6HQ+. In addition, with a value of τPT ≈ 800 fs, QCy7 is the first reported photoacid with τPT below 1 ps. In this report, we show that QCy9 (Scheme 1) has an even larger ESPT rate, a kPT of about 1 × 1013 s−1, eight times that of QCy7. The question arises if there is an upper limit to the rate of proton transfer. The Arrhenius equation shown below was derived more than a hundred years ago from empirical observations and is also widely used today: k = A ·exp[−Ea /RT ]

37 coefficient, κwc nm, for use in this equation, Borgis and Hynes used the general Landau−Zener (LZ) transmission coefficient, κnm adapted for the present problem. One obtains the nonadiabatic limit, and this leads to

κ nm =

1/2 ⎡⎛ ⎤ ‡ β ⎞ −β ΔGnm 2π ⎥ Cnm 2⎢⎜ ⎟ e ⎢⎣⎝ 4ESπ ⎠ ⎥⎦ ℏ

(4)

in which ΔG‡nm is the free energy of activation, Cnm is the coupling matrix element, Es is the reorganization energy, and β = 1/kBT. The authors37 also made a theoretical examination of the adiabatic limit, which leads to the rate expression. Under regular theoretical considerations, Cnm2 is large, the gap is large, the Landau−Zener transmission coefficient κnm ≈ 1, and the reaction rate proceeds on an adiabatic potential surface. For the adiabatic limit, they found that

(3)

where k is the reaction rate constant, A is the pre-exponential factor, and Ea is the reaction activation energy. After many years of research, the value of the pre-exponential factor A was chosen empirically to be ∼1013 s−1. The Arrhenius equation ignores any mechanistic considerations, such as whether one or more reactive intermediates are involved in the conversion of a reactant to a product.35 Transition-state theory (TST), developed in 1936, explains the two parameters associated with the Arrhenius law, the preexponential factor (A) and the activation energy (Ea). TST is used primarily to understand qualitatively how chemical reactions take place. TST has been less successful in its original goal of calculating absolute reaction-rate constants. In TST, it is assumed that the activated complex is in quasi-equilibrium with the reactants. The pre-exponential factor is given by the frequency (kBT/h) with which the activated complex is converted into products. For a barrierless reaction, the rate constant is approximately the value of the frequency factor (kBT/h). At room temperature, its value is 1013 s−1. Thus, the TST predicts that the largest rate constant for a chemical reaction taking place at room temperature is about 1013 s−1. The knowledge of the kinetics of many reactions that has been accumulated during the last 100 years indicates that the prefactor of the Arrhenius expression of rapid reactions is indeed about 1013 s−1. This value is reached by the current measurement of the ESPT-tosolvent rate constant of QCy9. More advanced rate theories are the quantum-mechanical adiabatic and nonadiabatic theories. More recent experimental results and theories on proton transfer reactions have revealed that tunneling is the dominant reaction mode for proton transfer, even at ambient temperatures. The theoretical development for the solution-phase proton transfer reaction was undertaken by Dogonadze, Kuznetzov, Ulstrup, and coworkers36 and then extended by Borgis and Hynes,37 Cukier,38 and Voth.39 These theories suggest that when a potential energy barrier is present in the proton-reaction coordinate, the reaction pathway involves tunneling through the barrier, as opposed to passage over it. Borgis and Hynes37 derived an expression for the rate constant, knm, for the transition between the Q-vibrational state, n, in the reactant to the Q-vibrational state, m, in the product. They wrote an expression for knm in a transition-state-theory form. In particular, knm can be expressed as the average one-way flux in the solvent coordinate, through the crossing point, Snm, of the free-energy curves for the n and m vibrational states, with inclusion of the transmission coefficient, κwc nm, giving the probability of successful curvecrossing. To find the appropriate nonadiabatic transmission

AD = kPT

⎛ ωs ⎞ ‡ ⎜ ⎟ exp( − β ΔG AD) ⎝ 2π ⎠

(5)

ΔG‡AD

ΔG‡NA

For the symmetric case, ΔG = 0 and = − C0 . Because ΔG‡NA was found, experimentally, to be small, ΔG‡AD is also very small. The pre-exponential factor, ωs/2π, is the solvent-response frequency. Although they estimated that ωs/ 2π ≈ 1013 s−1, this brings us back to the initial question of what is the upper limit of the proton transfer reaction rate and is it indeed in the vicinity of kPT = 1013 s−1? A plausible answer to this question is given below. The intermolecular vibration frequency of the two relatively heavy oxygen atoms involved in the PT reaction, that of the hydroxyl group of the photoacid and that of the hydrogen-bonded water molecule, is about 200 cm−1. The period of such an oscillation is 100 fs. When the two oxygen atoms oscillate, they modify the potential barrier through which the proton tunnels. When the oxygen−oxygen distance is small, both the width and the height of the barrier are reduced and the tunneling rate increases exponentially. This may be the reason for the maximum rate constant of about 1013 s−1 for intermolecular proton transfer. Previous studies of excited-state intramolecular proton transfer (ESIPT) between a proton donor and an adjacent acceptor in heterocyclic structures reveal that these reactions are ultrafast. The reported ESIPT rate constants for such reactions in nonprotic liquids are kPT ≥ 1013 s−1. These large ESIPT kPT values indicate that proton transfer rates between donor and acceptor can exceed the value found in the current study for QCy9, namely, 1 × 1013 s−1. High values of kPT are obtained when solvent motions are not directly involved in the proton transfer mechanism.

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SUMMARY AND CONCLUSIONS Steady-state and time-resolved emission techniques were employed to study the photoprotolytic properties of a recently synthesized phenol-cyanine photoacid (shown in Scheme 1 and symbolized as QCy9 throughout the article). Using the Förstercycle, we found that QCy9 has a pKa* of ∼−8.5, which is the lowest reported value for a photoacid based on a hydroxyl group as proton emitter. The ESPT rate of QCy9 was measured in H2O and D2O, and the fluorescence up-conversion technique was used to monitor the time-resolved emission at 10 or 20 nm intervals in the spectral range covering both the ROH and RO− emission bands. Analysis of the ROH time-resolved emission signal revealed that kPT ≈ 1 × 1013 s−1 in H2O. This is the largest kPT value reported up to now. Chemists have used 3411



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(11) Mondal, S. K.; Sahu, K.; Sen, P.; Roy, D.; Ghosh, S.; Bhattacharyya, K. Excited State Proton Transfer of Pyranine in a γCyclodextrin Cavity. Chem. Phys. Lett. 2005, 412, 228−234. (12) Prasun, M. K.; Samanta, A. Evidence of Ground-State ProtonTransfer Reaction of 3-Hydroxyflavone in Neutral Alcoholic Solvents. J. Phys. Chem. A 2003, 107, 6334−6339. (13) Bhattacharya, B.; Samanta, A. Excited-State Proton-Transfer Dynamics of 7-Hydroxyquinoline in Room Temperature Ionic Liquids. J. Phys. Chem. B 2008, 112, 10101−10106. (14) Agmon, N.; Pines, E.; Huppert, D. Geminate Recombination in Proton-Transfer Reactions. II. Comparison of Diffusional and Kinetic Schemes. J. Chem. Phys. 1988, 88, 5631−5638. (15) Popov, A. V.; Gould, E. A.; Salvitti, M. A.; Hernandez, R.; Solntsev, K. M Diffusional Effects on the Reversible Excited-State Proton Transfer. From Experiments to Brownian Dynamics Simulations. Phys. Chem. Chem. Phys. 2011, 13, 14914−14927. (16) Solntsev, K. M.; Poizat, O.; Dong, J.; Rehault, J.; Lou, Y.; Burda, C.; Tolbert, L. M. Meta and Para Effects in the Ultrafast Excited-State Dynamics of the Green Fluorescent Protein Chromophores. J. Phys. Chem. B 2008, 112, 2700−2711. (17) Szczepanik, B.; Styrcz, S. Protolytic Dissociation of Cyanophenols in Ground and Excited States in Alcohol and Water Solutions. Spectrochim. Acta, Part A 2011, 79, 451−455. (18) Banerjee, D.; Mitra, S.; Mukherjee, S. Proton Transfer Reaction in 4-hydroxy-3-formyl Benzoic Acid in Protic Solvents at Room Temperature and 77 K and Some Theoretical Aspects. Spectrochim. Acta, Part A 2005, 61, 1271−1278. (19) Quina, F. H.; Moreira, P. F., Jr.; Vautier-Giongo, C.; Rettori, D.; Rodrigues, R. F.; Freitas, A. A.; Silva, P. F.; Mac-anita, A. L. Photochemistry of Anthocyanins and Their Biological Role in Plant Tissues. Pure Appl. Chem. 2009, 81, 1687−1694. (20) Presiado, I.; Erez, Y.; Huppert, D. Excited-State Intermolecular Proton Transfer of the Firefly’s Chromophore D-Luciferin. J. Phys. Chem. A 2010, 114, 8075−8082. (21) Karton-Lifshin, N.; Segal, E.; Omer, L.; Portnoy, M.; SatchiFainaro, R.; Shabat, D. A Unique Paradigm for a Turn-ON NearInfrared Cyanine-Based Probe: Noninvasive Intravital Optical Imaging of Hydrogen Peroxide. J. Am. Chem. Soc. 2011, 133, 10960−10965. (22) Karton-Lifshin, N.; Albertazzi, L.; Bendikov, M.; Baran, P. S.; Shabat, D. Donor−Two-Acceptor Dye Design: A Distinct Gateway to NIR Fluorescenc. J. Am. Chem. Soc. 2012, 134, 20412−20420. (23) Karton-Lifshin, N.; Presiado, I.; Erez, Y.; Gepshtein, R.; Shabat, D.; Huppert, D. Ultrafast Excited-State Intermolecular Proton Transfer of Cyanine Fluorochrome Dyes. J. Phys. Chem. A 2012, 116 (1), 85−92. (24) Kim, T. G.; Topp, M. R. Ultrafast Excited-State Deprotonation and Electron Transfer in Hydroxyquinoline Derivatives. J. Phys.Chem. A 2004, 108, 10060−10065. (25) Pérez-Lustres, J. L.; Kovalenko, S. A.; Mosquera, M.; Senyushkina, T. A.; Flasche, W.; Ernsting, N. P. Ultrafast Solvation of N-Methyl-6-quinolone Probes Local IR Spectrum. Angew. Chem., Int. Ed. 2005, 44, 5635−5639. (26) Debye, P. Reaction Rates in Ionic Solutions. Trans. Electrochem. Soc. 1942, 82, 265. (27) Collins, F. C.; Kimball, G. E. Diffusion-Controlled Reaction Rates. J. Colloid Sci. 1949, 4, 425−437. (28) Fö rster, T. Die pH-Abhängigkeit der Fluoreszenz von Naphthalinderivaten. Z. Electrochem. 1950, 54, 531−535. (29) Presiado, I.; Karton-Lifshin, N.; Erez, Y.; Gepshtein, R.; Shabat, D.; Huppert, D. Ultrafast Proton Transfer of Three Novel Quinone Cyanine Photoacids. J. Phys. Chem. A 2012, 116, 7353−7363. (30) Huppert, D.; Cohen, B. Excited State Proton-Transfer Reactions of Coumarin 4 in Protic Solvents. J. Phys. Chem. A 2001, 105, 7157− 7164. (31) Huppert, D.; Tolbert, L.; Linares-Samaniego, S. Ultrafast Excited-State Proton Transfer from Cyano-Substituted 2-Naphthols. J. Phys. Chem. A 1997, 101, 4602−4605. (32) Weller, A. Fast Reactions of Excited Molecules. Prog. React. Kinet. 1961, 1, 187−214.

the Arrhenius equation for many years to determine the rate constant of a chemical reaction. A value of 1013 s−1 was often used as the pre-exponential parameter in the Arrhenius equation. We suggest that a rate constant of 1013s−1 may be the upper limit of the ESPT-to-solvent reaction rate constant, and the value of kPT for QCy9 almost reaches that limit. We found that the kinetic isotope effect (KIE) is approximately two for the ESPT reaction. This value is larger than that for proton mobility in water (KIE = 1.45), and it is somewhat larger than the value for QCy7 (∼1.7). QCy7 has a similar molecular structure and also shows an ultrafast ESPT rate with a rate constant kPT = 1.25 × 1012 s−1. QCy compounds show the highest reported photoacidity and are therefore very useful for further studies on photoacidity in general and proton transfer kinetics and mechanism in particular.

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ASSOCIATED CONTENT

S Supporting Information *

Absorbance plots, up-conversion signals, and time-resolved emission. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(D.H.) E-mail: [email protected]. Phone: 972-36407012. 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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REFERENCES

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Ultrafast Excited-State Proton Transfer to the Solvent Occurs on a

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