Photodriven Deprotonation of Alcohols by a Quinoline Photobase

Sep 12, 2018 - Control of proton transfer is relevant to many areas in chemistry, particularly in catalysis where the kinetics of (de)protonation reac...
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Cite This: J. Phys. Chem. A 2018, 122, 7931−7940

Photodriven Deprotonation of Alcohols by a Quinoline Photobase Jonathan Ryan Hunt and Jahan M. Dawlaty* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States

J. Phys. Chem. A 2018.122:7931-7940. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/08/18. For personal use only.

S Supporting Information *

ABSTRACT: Control of proton transfer is relevant to many areas in chemistry, particularly in catalysis where the kinetics of (de)protonation reactions are often rate limiting. Photobases, which are molecules with enhanced basicity in the excited state, allow for control of proton transfer with light and have the potential to be used as functional units in catalytic systems. Alcohols are the feedstock in many catalytic reactions, where their deprotonation or dehydrogenation is often important. We report that the photobase 5-methoxyquinoline can deprotonate a series of alcohols upon excitation by light. We measure both the thermodynamic limits and the relevant kinetics of this process. A series of alcohols and water spanning the pKa range of 12.5−16.5 were used as the proton donors. First, we show evidence from absorption and emission spectroscopy that photoexcited 5-methoxyquinoline deprotonates all donors more acidic than methanol and fails to deprotonate donors that are more basic. Interestingly, in methanol a quasiequilibrium between the protonated and unprotonated forms of the photobase is established in the excited state, suggesting that the excited state pKa of the photobase is near the pKa of methanol (15.5). Second, using ultrafast transient absorption spectroscopy, we find that the time constants for excited state proton transfer range from a few picoseconds to tens of picoseconds, with faster speeds for the more acidic donors. Such a correlation between the thermodynamic drive and kinetics suggests that the same mechanism is responsible for proton transfer throughout the series. These results are necessary fundamental steps for applying photobases in potential applications such as deprotonation of alcohols for catalytic and synthetic purposes, optical regulation of pH, and transfer of protons in redox reactions.

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INTRODUCTION Controlling proton transfer with light is relevant to a wide range of chemical scenarios. Optical control over proton transfer has proven useful in rapidly initiating pH-dependent phenomena and therefore in understanding their kinetics. Prime examples are protein folding,1,2 acid-catalyzed reactions,3 and pH-dependent enzymatic activity.4,5 Optically initiating irreversible acid catalyzed reactions is common in photolithography, which is important in nanoscale engineering.6,7 It is shown that synthetic organic reactions can be catalyzed by optically generated protons.8,9 Working along the analogy between electronic and protonic effects, changing the bulk protonic conductivity of polymer electrolytes with light,10 and generation of a protonic potential difference with light11 have been demonstrated. Many redox reactions require transfer of several protons, including redox reactions relevant to chemical energy storage and use like CO2 reduction and methanol oxidation. Optical control over proton transfer is therefore an important goal throughout chemistry. Molecules that change their pKa in the electronic excited state allow for coupling light with chemistry involving protons. Photoacids are molecules that become more acidic upon absorption of light. Prime examples of photoacids are naphthols12−14 and pyrenols,15−17 which often have enhanced acidity in the excited state by more than 6 pKa units. Photoacids are the enabling elements in most of the applications listed in the previous paragraph. They have been © 2018 American Chemical Society

well-known for many decades and their thermodynamics and kinetics are reasonably well-studied.18−23 Despite the large body of literature on photoacids and their proven utility, their basic counterparts have been studied and utilized only minimally. Photobases are molecules that become more basic upon light absorption. Examples of photobases are quinolines,24−29 acridines,30−33 3-styrylpyridine,34 xanthone,35 and curcumin.36 Because photobases are understudied, they are consequently underutilized. Like photoacids, which have been shown to have wide applications, we envision photobases as useful tools in controlling chemical systems that involve proton transfer. For example, photobasic moieties appropriately incorporated in a catalyst could speed up reactions that are rate limited by proton removal from the catalytic site. An appealing application of photobases is optical control over the chemistry of alcohols. Transformation of alcohols is important from a synthetic perspective since alcohols serve as the feedstock for a wide range of reactions. Furthermore, alcohols are important as a source of chemical energy, as in direct alcohol fuel cells.37 Deprotonation, often concurrent with oxidation, is required to activate an alcohol38,39 and this necessitates using strong bases or oxidative chemistry near electrodes. We propose that it is reasonable to use light and an Received: June 27, 2018 Revised: September 12, 2018 Published: September 12, 2018 7931

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aqueous solvents24−29 or intramolecular proton transfer facilitated by protic solvent molecules.40−43 This work demonstrates direct optical deprotonation of alcohols by 5methoxyquinoline. In one previous study curcumin was used as a photobase for direct deprotonation of a low pKa alcohol.36 Our work is a systematic study of the thermodynamics and kinetics of alcohol deprotonation. We hope that this work will encourage the use of photobases as functional elements in chemical systems that require photomediated proton transfer, and particularly in those that require alcohol deprotonation.

appropriate photobase to achieve deprotonation. As we will show in this work, photobases have the capability of using light to deprotonate alcohols and therefore have the potential to serve as functional units in a wide range of applications involving alcohol chemistry. Recently, we reported systematic studies on a series of 5-Rquinoline photobases. We investigated their thermodynamic drives for proton removal in the excited state (pKa*) using Förster cycle analysis,28 and we investigated their corresponding proton transfer kinetics using transient absorption spectroscopy.29 In brief, thermodynamic analysis pointed to a large enhancement of basicity (ΔpKa = pKa* − pKa) of up to 10.6 units upon light absorption. Förster cycle analysis showed that the excited state drive for proton transfer is a function of the electron withdrawing power of the substituent. Kinetic studies pointed to proton transfer times of several tens of picoseconds in some members of the family, and it revealed that the triplet states of some of the molecules contributed to the kinetics of excited state proton transfer. These studies suggested that the excited state dynamics of one of the members, 5-methoxyquinoline (Figure 1), was relatively

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EXPERIMENTAL SECTION Materials. 5-Methoxyquinoline was purchased from Combi-Blocks. All solvents, including deuterated solvents, were purchased from Sigma-Aldrich. All compounds were used without further purification. Steady State Spectroscopy. Absorption spectra were obtained using a Cary 50 UV−vis spectrophotometer. Emission spectra were collected on a Jobin-Yvon Fluoromax 3 fluorometer. 5-Methoxyquinoline solutions of ∼2 × 10−5 M were used to obtain the steady state spectra in this paper. Measurements were made in a 1 cm fused quartz cuvette. Transient Absorption. Pump pulses were generated by pumping an OPA (OPerA Solo, Coherent) with the output of a 1 kHz Ti:sapphire amplifier (Legend Elite HE+, Coherent). UV pump pulses were generated by two successive doubling stages of the OPA output. A white light continuum probe was prepared by focusing the 800 nm Ti:sapphire output onto a 3 mm thick rotating CaF2 window after the seed pulse was modulated at 500 Hz using an optical chopper. The polarization of the 800 nm Ti:sapphire output was rotated to magic angle with respect to the pump prior to white light generation. The pump beam was modulated at 250 Hz. A balanced detection scheme was employed to eliminate noise due to fluctuations in the probe spectrum. The probe arm of the apparatus was split with a 50/50 beamsplitter into two arms: sample and reference. The beam in the sample arm was focused into the sample and overlapped with the focused pump beam. The reference arm was also focused onto the sample but in a location that was not pumped. The probe beams were detected using a 320 mm focal length spectrometer with 150 g/mm gratings (Horiba iHR320) and a 1340 × 100 CCD array (Princeton Instruments Pixis). Both probe beams were detected on the CCD by displacing their focal planes into the spectrometer. The sample and reference beams were captured by the top half and bottom half of the CCD, respectively. The signal resulting from the sample beam contains the transient absorption and fluctuations due to the instability of the probe. The signal resulting from the reference beam only contains the fluctuations. The reported transient absorption signals were calculated by subtracting the reference signal from the sample signal. The focal spot diameters for the pump and probe were 140 and 180 μm, respectively. Cross correlations of the pump and probe were collected using the nonresonant response of each pure solvent. The time resolution of each experiment was determined by either the cross correlation (∼300 fs) or the temporal step size, depending on which was longer. Solutions of ∼3 mM 5methoxyquinoline were prepared in each of the solvents studied via transient absorption. The samples were flowed through a fused quartz flow cell with a 0.5 mm path length. 5Methoxyquinoline was pumped at ∼315 nm, near its (π,π*)1 absorption maximum, in all solvents. Pulse energies of 50−100

Figure 1. Central concept of this work. The photobase 5methoxyquinoline (Q) removes a proton from a hydrogen bonded proton donor upon excitation by light. Both the limits of the thermodynamic drive ΔG and the rate k of proton capture are investigated.

straightforward. The excited state proton transfer of 5methoxyquinoline followed a simple singlet state population transfer model. That the molecule emitted with high quantum yield from both protonated and unprotonated excited state forms made it easy to study using steady state spectroscopy. In our previous work the excited state pKa of this molecule was estimated to be pKa* = 15.1, a 10.2 unit increase from its ground state pKa. Alcohols typically have pKa values in this range. For the above reasons, we chose 5-methoxyquinoline to study deprotonation of alcohols here. As will be shown in this work, 5-methoxyquinoline behaves in conformity with its estimated pKa* value and can deprotonate a series of alcohols with pKa values lower than that of methanol (pKa = 15.5). We report the kinetics of deprotonation of the alcohols by the photobase using ultrafast transient absorption spectroscopy. We will show that there is a free energy relationship (FER) between the pKa value of the proton donor and the proton transfer rates. Previous works on excited state proton capture by quinolines reported in the literature have studied deprotonation of 7932

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dichloroethanol, water, methanol, ethanol, 2-propanol, and tert-butanol. While the absorption spectra are nearly identical (with slight solvatochromic shifts), the emission spectra show large variations. In general, two emission regions are identified: one with a Stokes shift of nearly 100 nm, and the other with a larger Stokes shift of nearly 200 nm. Our previous work28 has unequivocally assigned emission with small and large Stokes shifts as arising from the unprotonated and protonated forms of 5-methoxyquinoline, respectively. Earlier works by several groups25,34,35 have shown similar emission behavior in other photobases. In brief, these assignments are based on measuring the emission spectra of 5-methoxyquinoline in aprotic solvents to obtain emission from the unprotonated species, and in acidic aqueous solutions with pH below the ground state pKa value of 5-methoxyquinoline to obtain emission from the protonated species. Representative spectra may be seen in the Supporting Information. It is consistently found that the unprotonated form of 5-methoxyquinoline emits in the blue region with a peak near 410 nm while the protonated form emits in the green region with a peak near 510 nm. These signatures in the emission spectra hold the key for identifying whether the protonated or unprotonated form of 5methoxyquinoline is responsible for emission in a given solvent, and therefore whether excited state proton transfer occurs in that solvent. Armed with this tool, we examine the data presented in Figure 2. We clearly identify that for proton donors with pKa values below that of methanol, our molecule emits from the protonated form. Since the ground state pKa of the conjugate acid form of 5-methoxyquinoline is ∼5, which is significantly lower than the pKa values of all the solvents studied here, there are practically no protonated molecules in the ground state. Emission from the protonated form therefore implies that the molecule captures a proton from the solvent in the excited state and later emits from the protonated form. The figure also shows that when the pKa of the donor is above that of methanol, the photobase emits in the unprotonated spectral region. This obervation implies that excited 5-methoxyquinoline does not deprotonate the donors with pKa values above that of methanol. Interestingly, when the photobase is presented with methanol, emission from both protonated and unprotonated forms is observed, implying that a fraction of the excited state population exists in protonated form while the rest are unprotonated. As will be discussed in more detail below, this observation suggests that the pKa of methanol (15.5) is close to that of the excited state pKa* of 5methoxyquinoline. This is consistent with our earlier analysis of the Förster thermodynamic cycle for 5-methoxyquinoline28 in which we had estimated its pK*a to be near 15. Kinetics of Proton Transfer. In this section, we report the measured kinetics of proton transfer for the same proton donors discussed above and show that the speed of proton transfer is faster when the donor is more acidic. The pKa of the donor is directly related to the thermodynamic drive of the excited state proton transfer reaction. In chemical kinetics, if the thermodynamic drive for the reaction is increased within the same family of reactions, the speed of the reaction is expected to increase as well provided the underlying mechanism remains the same.44 Such a relation between thermodynamics and kinetics is known as a free-energy relation (FER). The goal of this section is to investigate such a relation for excited state proton transfer in 5-methoxyquinoline and to

nJ were used. Samples were pumped at several different powers to check that the signals reported were linear with respect to pump power. In methanol and methanol-d, spectra were also taken after addition of ∼6 mM HCl in order to obtain the spectrum of the acid form.

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RESULTS Thermodynamics of Proton Transfer. Figure 2 shows the absorption and emission spectra of 5-methoxyquinoline when dissolved in a series of proton donors with pKa values spanning the range 12.5−16.5. These proton donors include, in order of increasing pKa, 2,2,2-trifluoroethanol, 2,2-

Figure 2. Absorption (dashed line) and emission (solid line) spectra of 5-methoxyquinoline in a series of proton donors (structures and pKa values shown to the right). The photobase has a ground state pKa of ∼5 and cannot deprotonate any of the donors in the ground state. Therefore, the absorption spectra of the photobase in these donors show little change. The emission spectra, however, behave very differently. For donors more acidic than methanol, the molecule emits from the protonated form (red shade), indicating successful deprotonation of the donor in the excited state. For donors more basic than methanol, the molecule emits from the unprotonated form (blue shade), indicating no excited state deprotonation of the donor. Assignment of the emission regions to unprotonated and protonated forms is discussed in the text. Interestingly, the photobase emits from both protonated and unprotonated forms when dissolved in methanol, indicating that the pK*a value of the photobase is close to the pKa of methanol. 7933

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Figure 3. (A) TA data for 5-methoxyquinoline in 2,2,2-trifluoroethanol (TFE). The development of a stimulated emission feature at ∼550 nm is indicative of the growth of protonated photobase population. TFE data are shown as representative TA data for proton donors with pKa values below that of methanol. TA for the other low pKa proton donors may be found in the Supporting Information. (B) TA data for 5-methoxyquinoline in 2-propanol. The observed spectral changes are due to relaxation of the solvent. Note that the spectral development is qualitatively different than that for TFE. Namely, no stimulated emission feature develops around 550 nm, indicating no formation of the protonated form of the photobase. 2Propanol data are shown as representative TA data for proton donors with pKa values above that of methanol. TA data for the other high pKa proton donors may be found in the Supporting Information.

Figure 4. TA data for 5-methoxyquinoline in methanol. The stimulated emission near 550 nm results in decay of the TA signal, similar to that seen for TFE in Figure 3A. However, in contrast to TFE, it stops before going to completion and no negative TA signal is observed. This observation suggests that an equilibrium population of protonated and unprotonated forms of the photobase has been generated. This interpretation is supported by the kinetic modeling shown later.

therefore suggest that a common mechanism underlies proton transfer from the studied proton donors. We performed transient absorption (TA) studies of 5methoxyquinoline dissolved in the four proton donors that can

be deprotonated according to the emission spectra shown above. These proton donors are methanol, water, 2,2dichloroethanol (DCE), and 2,2,2-trifluoroethanol (TFE). The TA spectra of unprotonated and protonated 57934

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When the underlying mechanism in a family of reactions remains the same, there is often a relation, known as the free energy relation (FER), between the thermodynamic drives ΔG and the activation energy barriers ΔG‡ of the reactions.44 In Arrhenius-type kinetics, the activation energy barriers ΔG‡ scale linearly with the natural logarithm of the rates ln(k) of the corresponding reactions. Therefore, an FER implies a relation between ln(k) and the thermodynamic drive. Using proton donor pKa as a proxy for the ΔG of the proton transfer reaction, we plot ln(k) for each reaction (extracted from the kinetic models discussed in the next section) as a function of the proton donor pKa to investigate the FER (Figure 5). An

methoxyquinoline in water have been identified in our previous work29 and have been reproduced in the Supporting Information for reference. Additional confirmation of the TA spectra of the unprotonated and protonated forms of 5methoxyquinoline is achieved in this work through TA in solvents incapable of being deprotonated (such as ethanol and 2-propanol) and in alcohol solvents acidified with HCl, respectively. These identifications were used to guide our analysis of the proton transfer kinetics and to, as described later, find suitable basis spectra for kinetic modeling. In all solvents with pKa values lower than that of methanol, proton capture by the excited photobase goes to completion or near completion. The spectra at early times match the spectrum of the unprotonated form, while the spectra at long times match the spectrum of the protonated form. TA data for 5-methoxyquinoline in TFE is shown in Figure 3A as representative TA data for the proton donors with pKa values lower than that of methanol. TA of 5-methoxyquinoline was also performed in ethanol and 2-propanol to confirm that no proton capture occurred in these solvents. The data for 2propanol are shown in Figure 3B as representative of the proton donors with pKa values higher than that of methanol. In this data, the only observed kinetics are those associated with solvent relaxation of the unprotonated form. No changes resembling a population transfer from unprotonated to protonated forms were observed. This is, once again, in conformity with our steady state spectra and the observation that no net proton transfer occurs for proton donors with pKa values above that of methanol during their excited state lifetime. For 5-methoxyquinoline in methanol a transient absorption spectrum that was a linear combination of the protonated and unprotonated forms was observed in the limit of long time (Figure 4). This will be shown more explicitly later in the kinetic modeling section. It seems that the photoexcited 5methoxyquinoline molecules establish an excited state equilibrium between the protonated and unprotonated forms (Q* + ROH ⇌ QH*+ + RO−) at early times (tens of picoseconds). They then emit on longer time scales in a ratio related to the above equilibrium, as indicated by the dual emission observed in the steady state measurement. The observation of a quasi-equilibrium in the excited state implies that the rates of photobase proton capture and photobase proton release in methanol are the same once the observed equilibrium ratio of protonation states is reached. This further implies that the thermodynamic drive for excited state proton transfer in methanol is not very large. This is consistent with the hypothesis that the pKa of methanol is near the pK*a of the photobase. This argument and its implications will be addressed in the Discussion. The fact that the reaction in solvents with pKa lower than methanol reaches completion does not exclude the possibility of a dynamic equilibrium between the protonated and unprotonated forms with equal rates of forward and backward proton transfer. It only implies that the equilibrium is highly skewed in favor of the protonated photobase, which is consistent with the higher pKa* of the photobase compared to the solvent pKa. The only difference between the behavior of the photobase in methanol and in the lower pKa solvents, then, is that in methanol the equilibrium is not as highly skewed and significant populations of protonated and unprotonated forms exist after equilibrium is established.

Figure 5. Comparison of the kinetics and thermodynamics of the excited state proton transfer of 5-methoxyquinoline. The pKa of the donor tunes the thermodynamic drive for proton transfer. The rate of proton transfer shows an overall increase with increasing thermodynamic drive. These observations indicate that a free energy relation (FER) exists for these proton transfer reactions.

overall trend presents itself: more acidic proton donors tend to have shorter proton transfer time scales. Water, being the only nonalcohol in the series, is likely to behave differently than the other proton donors. It is possible that the kinetics in water is more rapid compared to that in the alcohols because of its ability to form two hydrogen bonds. The importance of these observations will be discussed later, as will the legitimacy of using proton donor pKa as a proxy for the ΔG of the proton transfer reaction. It is common practice in the excited state proton transfer literature to correlate proton transfer rates in water with the respective thermodynamic drive using a Marcus free energy relation.14,27 Correlating proton transfer rates in different solvents is more complicated, since certain Marcus free energy parameters are a function of proton donor. Comparison of our proton transfer rate in water and its thermodynamic drive (ΔpKa = 14 − 15.1 = −1.1, where 14 is the pKa of water and 15.1 is the pKa* of the photobase from Förster cycle analysis) with Marcus free energy relations that already exist in the literature27 shows that our water proton transfer data are consistent with these relations. This confirms the plausibility of the proton transfer time scales presented here, as inferred from the model discussed below. Proton capture dynamics were also studied in deuterated forms of water and methanol to establish whether a kinetic isotope effect (KIE) existed. In heavy water, a deuteron capture time of 22 ± 5 ps was extracted, indicating a KIE of 0.9 ± 0.3. A KIE below 1 is unlikely, so we can conclude that the KIE is 1 within error. In heavy methanol, a deuteron capture time of 133 ± 30 ps was extracted, indicating a KIE of 2.5 ± 0.7. These observations, and their implications for the 7935

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Figure 6. (a) Basis spectra used in modeling the TA data of 5-methoxyquinoline in TFE. The earliest resolved time is taken as the B* (excited base) spectrum, since the population of the A* (excited acid) species should be small near time zero. We use a spectrum taken long after proton transfer is completed as the A* spectrum. (b) Spectral slices of the TA data for 5-methoxyquinoline in TFE. (c) Fit constructed using the model discussed in the paper. Only one adjustable parameter is used, the proton transfer time kp, along with the basis spectra presented in (a). The fitting equation and other details may be found in the Supporting Information.

therefore be reasonably obtained from experimental observation of the TA spectra at early times. The transient spectrum of A* can be experimentally determined from TA spectra at long times in proton donors where protonation of 5-methoxyquinoline goes to completion. Otherwise, the transient spectrum of A* in a given solvent can be obtained through stoichiometric protonation of 5-methoxyquinoline using an external proton source, like HCl. The last method was used to obtain the transient spectrum of A* in methanol since protonation of 5methoxyquinoline does not go to completion in methanol. The transient spectra in alcohols with pKa values higher than methanol, where proton transfer does not occur, suggest that solvent relaxation in alcohols changes the shape of the B* spectrum on a time scale similar to the proton transfer rates observed for the proton donors. The effects of the solvent relaxation may be observed in the transient absorption spectra of 5-methoxyquinoline in 2-propanol in Figure 3B, where it manifests primarily as small blue shifts and magnitude changes of the spectral features. This behavior is qualitatively different from proton transfer for which a clear conversion of one spectrum to another is observed. For the three proton donors that are more acidic than methanol (TFE, DCE, water), we ignore the effects of solvent relaxation in our model. In the case of water, solvent relaxation should occur entirely on short time scales ( I transition of apomyoglobin induced by ultrafast pH jump. Biophys. J. 2000, 78, 405−415. (2) Causgrove, T. P.; Dyer, R. B. Nonequilibrium protein folding dynamics: laser-induced pH-jump studies of the helix−coil transition. Chem. Phys. 2006, 323, 2−10. (3) Dempsey, J. L.; Winkler, J. R.; Gray, H. B. Mechanism of H2 evolution from a photogenerated hydridocobaloxime. J. Am. Chem. Soc. 2010, 132, 16774−16776. (4) Kohse, S.; Neubauer, A.; Pazidis, A.; Lochbrunner, S.; Kragl, U. Photoswitching of enzyme activity by laser-induced pH-jump. J. Am. Chem. Soc. 2013, 135, 9407−9411. (5) Peretz-Soroka, H.; Pevzner, A.; Davidi, G.; Naddaka, V.; Kwiat, M.; Huppert, D.; Patolsky, F. Manipulating and monitoring on-surface biological reactions by light-triggered local pH alterations. Nano Lett. 2015, 15, 4758−4768. (6) O’Connor, N. A.; Berro, A. J.; Lancaster, J. R.; Gu, X.; Jockusch, S.; Nagai, T.; Ogata, T.; Lee, S.; Zimmerman, P.; Willson, C. G.; et al. Toward the design of a sequential two photon photoacid generator for double exposure photolithography. Chem. Mater. 2008, 20, 7374− 7376. (7) Gather, M. C.; Koehnen, A.; Falcou, A.; Becker, H.; Meerholz, K. Solution-processed full-color polymer organic light-emitting diode displays fabricated by direct photolithography. Adv. Funct. Mater. 2007, 17, 191−200. (8) Keitz, B. K.; Grubbs, R. H. A tandem approach to photoactivated olefin metathesis: combining a photoacid generator with an acid activated catalyst. J. Am. Chem. Soc. 2009, 131, 2038−2039. (9) Das, A.; Ayad, S.; Hanson, K. Enantioselective protonation of silyl enol ether using excited state proton transfer dyes. Org. Lett. 2016, 18, 5416−5419. (10) Haghighat, S.; Ostresh, S.; Dawlaty, J. M. Controlling proton conductivity with light: a scheme based on photoacid doping of materials. J. Phys. Chem. B 2016, 120, 1002−07. (11) White, W.; Sanborn, C. D.; Reiter, R. S.; Fabian, D. M.; Ardo, S. Observation of photovoltaic action from photoacid-modified nafion due to light-driven ion transport. J. Am. Chem. Soc. 2017, 139, 11726− 11733. (12) Gutman, M.; Huppert, D.; Pines, E. The pH jump: a rapid modulation of pH of aqueous solutions by a laser pulse. J. Am. Chem. Soc. 1981, 103, 3709−3713. (13) Agmon, N. Elementary steps in excited-state proton transfer. J. Phys. Chem. A 2005, 109, 13−35. (14) Prémont-Schwarz, M.; Barak, T.; Pines, D.; Nibbering, E. T. J.; Pines, E. Ultrafast excited-state proton-transfer reaction of 1naphthol-3,6-disulfonate and several 5-substituted 1-naphthol derivatives. J. Phys. Chem. B 2013, 117, 4594−4603. (15) Pines, E.; Huppert, D. pH jump: a relaxational approach. J. Phys. Chem. 1983, 87, 4471−4478. (16) Spry, D.; Goun, A.; Fayer, M. Deprotonation dynamics and stokes shift of pyranine (HPTS). J. Phys. Chem. A 2007, 111, 230− 237. (17) Spry, D. B.; Fayer, M. D. Charge redistribution and photoacidity: neutral versus cationic photoacids. J. Chem. Phys. 2008, 128, 084508. (18) Förster, T. Elektrolytische Dissoziation angeregter Molekle. Z. Elektroch. Angew. Phys. Chem. 1950, 54, 42−46. (19) Weller, A. Fast reactions of excited molecules. Prog. React. Kinet. Mech. 1961, 1, 187−214. (20) Tolbert, L. M.; Solntsev, K. M. Excited-state proton transfer: from constrained systems to super photoacids to superfast proton transfer. Acc. Chem. Res. 2002, 35, 19−27. 7939

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