Enhanced Excited-State Proton Transfer via a Mixed WaterâMethanol

Article Cite This: J. Phys. Chem. A 2018, 122, 4704−4716

pubs.acs.org/JPCA

Enhanced Excited-State Proton Transfer via a Mixed Water−Methanol Molecular Bridge of 1‑Naphthol-5-Sulfonate in Methanol−Water Mixtures Oren Gajst,† Luís Pinto da Silva,‡,§ Joaquim C. G. Esteves da Silva,§,∥ and Dan Huppert*,† †

Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel Research Unit (CIQUP), Department of Chemistry and Biochemistry and §LACOMEPHI, GreenUP and ∥Chemistry Research Unit (CIQUP), Department of Geosciences, Environment and Territorial Planning, Faculty of Sciences of University of Porto, R. Campo Alegre 687, 4169-007 Porto, Portugal

‡

S Supporting Information *

ABSTRACT: We used steady-state and time-resolved fluorescence techniques to study the excited-state proton transfer (ESPT) and the nonradiative properties of two irreversible photoacids, 1-naphthol-4-sulfonate (1N4S) and 1-naphthol-5sulfonate (1N5S). We found that the ESPT rate constant of 1N4S in water is 2.2 × 1010 s−1, whereas in methanol, it is smaller by about 3 orders of magnitude and is not observed. The ESPT process of 1N5S competes with a major nonradiative process of equal rate and kPT of 2.2 × 1010 s−1. In methanol− water mixtures of χH2O = 0.2, the fluorescence lifetime of the ROH form of 1N5S is lower by a factor of 10 than that in pure methanol. In the steady-state fluorescence spectra of 1N5S in methanol−water mixtures, there are two iso-emissive points, one for χH2O < 0.2 and one for χH2O > 0.3. This large reduction in fluorescence intensity and the two iso-emissive points are explained by the existence of a mixed water−methanol bridge of about three molecules that connects the proton donor 1-OH with the 5-sulfonate in mixtures of χH2O < 0.2. The bridge enhances both the ESPT and the nonradiative processes. For 1N4S in methanol−water mixtures at χH2O ≈0.2, the reduction in the fluorescence lifetime is only by ∼30%, and only one iso-emissive point exists in the steady-state fluorescence spectra for 0 0.3, the iso-emissive point is at 465 nm.

Figure 2. Plot of steady-state fluorescence intensity of 1N5S ROH and RO− forms as a function of χH2O in methanol−water mixtures. (a) ROH fluorescence intensity; (b) RO− fluorescence intensity.

The steady-state ROH fluorescence intensity decreases with an increase in the water content of the mixture. We noted that there are two slopes to the plot. For water molar fractions of χH2O < 0.2 the slope is steep, whereas at χH2O > 0.3, the slope of the plot is smaller by a factor of about four. Figure 2b shows the steady-state fluorescence intensity of the RO− band as a function of χH2O on a semilogarithmic plot. At χH2O ≈ 0.2, the fluorescence intensity of the RO− band is about 10 times that at χH2O = 0.02. As we will show in this article, the plot of the ESPT rate constant of 1N4S photoacid in methanol−water mixtures has a convex slope, whereas for 1N5S, it has a concave shape. The reason for this is that in weak photoacids (pKa*>0), the ESPT process does not take place in methanol, whereas in methanol−water mixtures, it does. The ESPT rate constant, kPT, increases with an increase in χH2O of the methanol−water mixture. The steady-state-fluorescence spectrum of a photoacid in water and the methanol−water mixture depends on an approximate and simple equation

protonated form (ROH) with a band peak at 407 nm. In methanol−water mixtures of χH2O < 0.2, the intensity of the ROH signal decreases sharply when water is added to the mixture. In mixtures of χH2O > 0.2, the ROH fluorescence intensity shows a marked decrease when water is added. In the semilog plot of Figure 1b, two iso-emissive points are seen (shown by arrows), where the ROH and RO− bands cross. For χH2O < 0.2, the isoemissive point is at about 505 nm, and for χH2O > 0.3, the isoemissive point is blue-shifted to about 465 nm. The purpose of this article is to explain this peculiar behavior of 1-naphthol-5sulfonate. To do so, we also studied the spectroscopy of 1-naphthol-4-sulfonate (1N4S), in which the fluorescence intensity of the ROH form decreases slightly with χH2O in methanol−water mixtures. The fluorescence intensity of the deprotonated form (RO−) of 1N5S is much smaller than that of the ROH band at χH2O ≈ 0.8. The fluorescence intensity of the RO− band is lower by a factor of about 6.5 than that of the ROH band in neat methanol. For 1N4S, the fluorescence intensity of the RO− band is higher than that of the ROH band by a factor of about 1.5. The RO− band appears in methanol−water mixtures, because an intermolecular proton transfer to H2O occurs in the first excited singlet state. Figure 2a shows the fluorescence intensity of the ROH form of 1N5S as a function of χH2O (the intensities are taken from Figure 1).

F F F −/ I kPT ≅ IRO ROH· τRO−

−1

(3)

where R = IFRO−/IFROH is the ratio of the steady-state fluorescence intensity of the RO− band to that of the ROH band, and τFRO− is the fluorescence lifetime of the RO− form of the photoacid. Since kPT increases as a function of χH2O, the ROH band fluorescence intensity decreases, whereas the RO− band intensity increases as a function of χH2O. 4706

DOI: 10.1021/acs.jpca.8b00957 J. Phys. Chem. A 2018, 122, 4704−4716

Article

The Journal of Physical Chemistry A

Figure 3. Time-resolved fluorescence of the ROH form of 1N5S measured at 420 nm in methanol, water, and methanol−water mixtures of up to about χH2O ≈ 0.7. (a) Linear scale; (b) semilog scale. Note the large changes in fluorescence lifetime of mixtures of χH2O < 0.2. The instrument response function (IRF) of the TCSPC system is also included in the figure.

The decrease in intensity of the ROH form of the 1N5S fluorescence band with an increase in χH2O in the methanol−water mixtures shows two slopes. We explain this phenomenon by the fact that when χH2O is small, a mixed water−methanol bridge of about three molecules connects the hydroxyl group at position 1 with the sulfonate at position 5. This bridge enables efficient transfer of a proton to the sulfonate but not to the bulk itself. The proton geminate recombination from the SO3H+RO− of the 1N5S forms the ground-state ROH(g) +

kPT

HO3SRO−* ⎯→ ⎯ −O3SROH(g)

Table 1. Fit Parameters for Three-Exponent Fit of the TCSPC Signal of the ROH Form of 1N5S in Different Methanol−Water Mixturesa

(4)

Therefore, the lifetime of the ROH band decreases much more than would be expected if the ESPT were taking place to the bulk water. The two iso-emissive points, at 505 nm for methanol-rich methanol−water mixtures (χH2O < 0.2) and at 465 nm for methanol−water mixtures of χH2O > 0.3, can be explained by the existence of a mixed water−methanol bridge that connects the 1-OH with the 5-sulfonate. Figure 3a,b shows, on linear and semilogarithmic scales, the time-resolved fluorescence of the ROH form of 1N5S, measured at 420 nm (ROH fluorescence maximum at ∼430 nm) in neat methanol, H2O, and various methanol−water mixtures. The signals were acquired by the time-correlated single-photoncounting (TCSPC) technique with a limited time response of 35 ps at the fwhm of the instrument response (IRF) shown also in the figure. The data analysis is performed by fitting the experimental data by a convolution of the IRF with a three-exponent fit function. The amplitudes and the exponential lifetimes are given in Table 1. The major time component has an amplitude higher than about 0.9 at χH2O ≤ 0.2. This major signal in the methanol−water mixtures was assigned to the ESPT process. In neat water, the major component of the fluorescence-decay lifetime of 1N5S is τ1 = 24 ps, and the fluorescence rate constant, kF, is therefore 4.16 × 1010 s−1. In neat methanol, the fluorescence lifetime is about 4.4 ns and is assigned to the fluorescence lifetime of the ROH form in the S1 state, where ESPT is not taking place. In mixtures of χH2O ≤ 0.2, as seen in Figures 2 and 3, the ESPT rate is lower than that of a mixture of higher water content. We assign the efficient ESPT rate at χH2O < 0.2 as arising (mainly) from a mixed water−methanol

a

χwater

a1

τ1 (ps)

a2

τ2 (ns)

a3

τ3 (ns)

1 0.66 0.62 0.56 0.49 0.39 0.24 0.22 0.20 0.18 0.16 0.14 0.11 0.09 0.06 0.03 0

0.86 0.75 0.74 0.75 0.77 0.77 0.79 0.92 0.92 0.92 0.91 0.91 0.89 0.91 0.85 0.56 1

22 75 77 95 110 165 310 320 370 430 510 640 800 1000 1250 1500 3600

0.043 0.13 0.16 0.16 0.16 0.16 0.17 0.077 0.075 0.075 0.091 0.091 0.11 0.086 0.15 0.44 -

0.17 0.180 0.35 0.38 0.44 0.55 0.67 1.25 1.25 1.3 1.5 1.5 2 4.5 3.2 3.0 -

0.097 0.12 0.098 0.09 0.067 0.067 0.040 0.0034 0.0035 0.0035 0.0034 0.0034 0.0037 -

4.4 3.7 4.7 4.7 4.7 4.7 4.5 4.9 4.9 4.9 4.9 4.9 4.5 -

The error in amplitudes is ±5%, and the error in time constants is ±5%.

bridge consisting of three molecules that connect the 1-OH with the 5-sulfonate. The forward reaction −

kPT

O3SROH* ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ +HO3SRO−* (via_bridge)

and the proton geminate recombination is an irreversible reaction that leads to the formation of the ground-state −O3SROH(g) −

kr

HO3SRO−* ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ −O3SROH(g) (via_bridge)

Figure 2 shows that the steady-state fluorescence intensity of the ROH* fluorescence band of the χH2O ≈ 0.2 mixtures is decreased by a factor of 10, whereas the fluorescence reduction of the ROH band in the methanol−water range of 0.2 < χH2O < 1, the reduction is only by a factor of 4. As we will show, the reduction of the fluorescence intensity of the ROH form of 1-naphthol-4-sulfonate in solutions of χH2O < 0.2 is only 30% of that in neat methanol. This led us to suggest that the ESPT of 1N5S in mixtures of χH2O < 0.2 proceeds by way of a mixed water−methanol bridge of three molecules. 4707


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Figure 4. Plot of the ESPT time constant, τPT, of 1N5S in methanol−water mixtures, as a function of the molar ratio of water of the solution. Shown on (a) linear scale and (b) semilogarithmic scale.

Figure 5. Time-resolved fluorescence (TRF) of 1N5S measured at 520 nm, near the maximum of the RO− form. (a) Linear scale TRF of RO− in methanol−water mixtures. (b) Semilogarithmic scale of the TRF of the RO− form in methanol−water mixtures. (c) Linear scale of the TRF of the RO− form in neat methanol and in neat water. (d) Semilogarithmic scale of the RO− TRF in neat methanol and neat water.

RO− Time-Resolved Fluorescence. Figure 5a,b shows, on linear and semilogarithmic scales, the time-resolved fluorescence (TRF) of the deprotonated form (RO−) of the 1N5S photoacid in methanol−water mixtures. Figure 5c,d shows the same in neat methanol and neat water, measured by the TCSPC technique. The signals were acquired at 520 nm, close to the maximum of the RO− form fluorescence band at 530 nm. At a low molar ratio of water, the signals have a significant rise time (see Figure 5a), since the RO− signal arises from an ESPT process where we excite the ROH form by a short laser pulse of about 120 fs. As the water content in the methanol−water mixture increases, the shape of the signal has a faster rise time, and at long times (see Figure 5b), the signal has a concave shape. For reversible

Figure 4 shows, on linear and semilogarithmic scales, the ESPT time constant, τPT, of 1N5S in methanol−water mixtures. We see that the curve of τPT versus χH2O has approximately two slopes. At χH2O < 0.2, the slope is large, whereas at χH2O > 0.2, it is smaller. This observation led us to propose two different mechanisms for the ESPT rate. At low water content (χH2O < 0.2), ESPT of 1N5S takes place by way of a mixed solvent bridge from 1-OH to the 5-sulfonate. The back reaction via the bridge leads to an efficient irreversible geminate-recombination process to form − O3SROH(g). At χH2O > 0.2, the main mechanism is an ESPT process to bulk-water molecules near the 1-OH group. The proton recombination is an irreversible process that forms ROH(g). 4708


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Figure 6. Steady-state fluorescence spectra of 1N4S in methanol−water mixtures in the range of 0 < χH2O < 0.7. Note the small decrease of the ROH signal in the range of χH2O < 0.2. (a) Linear scale; (b) semilogarithmic scale.

photoacids like HPTS, the long-time decay of the RO− signal is exponential, since the proton geminate-recombination process occurs in the excited state and leads to reformation of the excitedstate protonated form, ROH*. In an irreversible proton geminate-recombination process, recombination forms the ground-state ROH(g), and the RO− fluorescence decreases. This irreversible geminate-recombination process leads to a concave shape of the TRF signal of the RO− form. The TRF signal of the 1N5S RO− form behaves as expected of an irreversible photoacid, whereas the ROH TRF signal in methanol−water mixtures is quenched by water. The intensity of the steady-state fluorescence spectrum is lower by a factor of more than 10 when χH2O ≈ 0.2, and the ROH fluorescence lifetime falls from ∼4 ns in methanol to about 0.35 ns at χH2O ≈ 0.2. In 1N4S, the ROH lifetime decreases at χH2O ≈ 0.2 to about 0.75, the intensity of neat methanol. Figure 5c,d shows, on linear and semilog scales, the TRF signal of 1N5S measured at 520 nm (the maximum of the RO− fluorescence band) in neat methanol and in neat H2O. The signal in methanol shows exponential decay with the same lifetime as that of the ROH signal measured at 420 nm (the maximum of the ROH fluorescence band). The signal in neat water shows an almost exponential decay of τ = 2 ns, followed by a long exponential decay whose amplitude is 0.01 of the short-time component with a lifetime of about 10 ns. The long-time component may arise from a contaminant in the 1N5S sample or from a photochemical process that occurs by sample excitation by a ∼280 nm excitation pulse. 1-Naphthol-4-sulfonate. Figure 6a,b shows, on linear and semilogarithmic scales, the steady-state fluorescence spectra of 1N4S in methanol and methanol−water mixtures. In neat methanol at neutral pH, the 1N4S is in the groundstate ROH form. Excitation at 320 nm (near the absorption peak of the ROH form at 325 nm) leads to a single-band spectrum, that of the ROH with maximum fluorescence at ∼360 nm. When small amounts of water, not exceeding χH2O = 0.2, are added to the methanol, the ROH band intensity decreases slightly, and a band of the RO− with a much lower intensity appears with a maximum at 417 nm. This change in the 1N4S spectrum is assigned to the ESPT process, in which the proton transfer is to water in the bulk. At χH2O ≈ 0.2, the ROH band decreases to ∼0.70 of its intensity in neat methanol, and this decrease is rather small. In 1N5S, the ROH fluorescence band intensity drops by

about a factor of 10 when χH2O ≈ 0.2. The large decrease in the fluorescence intensity of 1N5S led us to propose that a mixed water−methanol bridge of three molecules connects the 1-OH group with the 5-sulfonate and that the ESPT process is much more efficient than that of 1N4S. At χH2O > 0.2, the intensity of the 1N4S spectrum shows a linear dependence on χH2O in the decrease of the ROH fluorescence intensity and the buildup of the intensity of the RO− fluorescence band. The fluorescence lifetime of the RO− band of 1N4S in neat water is rather long (τ = 14 ns). The intensity of the RO− fluorescence band in neat water is about 1.5 times that of the ROH band in methanol, whereas in 1N5S, the RO− intensity is 6.5 times smaller than that of the ROH form in neat methanol. This is a sign that knr is larger in 1N5S or the oscillator strength is smaller than that of 1N4S. Figure 7a,b shows the fluorescence intensity of the ROH and RO− forms of 1N4S (taken from the steady-state spectra of Figure 6) as a function of χH2O. The slope of the ROH band intensity as a function of χH2O has a convex shape, whereas that of 1N5S is concave. Figure 8a,b shows, on linear and semilogarithmic scales, the time-resolved fluorescence of the ROH form of 1N4S in neat methanol and in methanol−water mixtures. When small amounts of water are added to methanol, the fluorescence lifetime of the ROH form decreases slightly. The average lifetime of the ROH form measured at 350 nm in neat methanol is about 2.4 ns, whereas at χH2O ≈ 0.2, the lifetime decreases to about 1.8 ns. By contrast, in 1N5S, the lifetime of the ROH form decreases from τmethanol ≈ 4.0 to 0.33 ns when water is added to methanol to χH2O ≈ 0.2. This large drop in the lifetime of 1N5S shows that it has an ESPT mechanism different from that of 1N4S, which behaves as expected for an intermolecular excitedstate proton transfer to water in the bulk. As before, we propose that in 1N5S, a mixed water−methanol bridge is formed from 1-OH to 5-sulfonate, and the ESPT process is much more efficient. At χH2O > 0.2, the fluorescence lifetime of 1N4S decreases further with a linear dependence on χH2O. In neat H2O, the fluorescence lifetime is about 43 ps, and the ESPT rate, kPT, is ∼2.2 × 1010 s−1. The analysis of the time-correlated data acquired by the TCSPC technique for 1N4S is performed as for 1N5S by fitting the experimental data by a convolution of the IRF with a 4709


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Figure 7. Fluorescence intensity of the ROH (panel a) and RO− (panel b) forms of 1N4S as a function of χH2O.

Figure 8. Time-resolved fluorescence of the ROH form of 1N4S measured, at 350 nm, in methanol, water, and methanol−water mixtures of up to χH2O ≈ 0.7. (a) Linear scale; (b) semilog scale. The IRF of the TCSPC system is also shown in the figure.

The plot of τPT of 1N4S as a function of χH2O has a convex shape, whereas for 1N5S, the shape is concave with a very large slope at χH2O < 0.2, and at χH2O ≥ 0.3, it decreases. Time-Resolved Fluorescence (TRF) of RO−. Figure 10a,b shows, on linear and semilog scales, the time-resolved fluorescence of the RO− form of 1N4S measured at 435 nm (the band maximum of the RO− form) in methanol−water mixtures in the range of χH2O ≈ 0.2−0.6. The signal shows the expected concave shape of irreversible photoacid behavior. The concave shape is expected because of the proton geminate-recombination (GR) rate that leads to the ground-state ROH(g), and therefore, the population of the RO−* form is reduced by the GR process. Figure 10c shows the TRF of the RO− form of 1N4S measured at 435 nm in neat water and methanol. In methanol solution, the signal is weaker than that of water by a factor of about 20. This is because an ESPT process does not take place in methanol, whereas in water, it takes place with a high rate constant of 2.2 × 1010 s−1. The fluorescence lifetime in neat methanol is the same as that measured at 350 nm, near the band peak of the ROH form. In water, at long times, the lifetime is about 14 ns, much longer than that in methanol. Kinetic H+/D+ Isotope Effect in 1N5S and in 1N4S. Figure 11a,b shows the time-resolved fluorescence (TRF) of the ROH forms of 1N5S and 1N4S in H2O and D2O. Each panel shows the TRF of the photoacid ROH form in H2O and D2O. For weak photoacids, in which pKa* ≥ 0, for which 1N4S and 1N5S belong to the weak photoacids with pKa* ≈ 0, the kinetic isotope effect (KIE) is about 3. For stronger

three-exponent fit function. The amplitudes and the exponential lifetimes are given in Table 2. Table 2. Fit Parameters for Three-Exponent Fit of the TCSPC Signal of the ROH Form of 1N4S in Different Methanol−Water Mixturesa

a

χwater

a1

τ1 (ps)

a2

τ2 (ns)

a3

τ3 (ns)

0 0.027 0.053 0.078 0.101 0.123 0.144 0.164 0.184 0.202 0.22 0.36 0.46 0.53 0.58 0.63 0.66 0.69 1

0.94 0.94 0.94 0.94 0.94 0.94 0.94 0.94 0.94 0.94 0.94 0.94 0.94 0.93 0.93 0.94 0.93 0.94 0.96

1540 1520 1500 1480 1430 1400 1370 1300 1250 1200 1150 790 530 400 325 275 240 210 43

0.059 0.059 0.059 0.059 0.059 0.059 0.059 0.059 0.059 0.059 0.059 0.053 0.064 0.067 0.068 0.060 0.060 0.047 0.034

4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.0 3.5 3.0 2.5 2.2 1.6 1.2 0.88 0.7 0.22

0.0058 0.011 0.0047

2.0 2.0 1.8

The error in amplitudes is ±5%, and the error in time constants is ±5%.

Figure 9 shows, on linear and semilogarithmic scales, the value of τPT of the ROH form of 1N4S in methanol−water mixtures. 4710


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Figure 9. τPT of the ROH form of 1N4S (measured at 350 nm) in methanol−water mixtures shown on the (a) linear scale and (b) semilogarithmic scale.

Figure 10. Time-resolved fluorescence of the RO− form of 1N4S measured at 435 nm (the band maximum of the RO− form), in methanol−water mixtures in the range of χH2O ≈ 0.2−0.6, shown on the (a) linear scale and (b) semilogarithmic scale. Measurements made in neat methanol and neat H2O shown on the (c) linear scale and (d) semilogarithmic scale.

photoacids with pKa* < −2, the KIE is about 2. In the case of HO 1N5S, the fluorescence lifetimes in H2O and D2O are τF 2 = 24 ps D2O and τF = 38 ps. On the basis of the fluorescence lifetime of the ROH form decay time of the photoacid, the ratio of the fluorescencedecay rate constants, kHF 2O/kDF 2O, for 1N5S is only ∼1.6. For 1N4S, the KIE is about 2.85, and τHF 2O = 43 ps, and τDF 2O=122 ps. We propose, that in 1N5S, the nonradiative rate of ROH is about equal to that of the excited-state proton-transfer rate, and therefore, the KIE based on the fluorescence lifetime should be corrected to ∼3 as for all the weak photoacids with pKa* ≈ 0. k F = k rF + k nr + kPT

nonradiative and the ESPT rate constants kFr ≪ knr, kPT. The assumption is that the KIE of the two photoacids with pKa* ≈ 0 is about three. H 2O D2 O KIE = kPT /kPT = 3

(6)

For 1N5S, the fluorescence ROH lifetimes provide a value smaller than 3. +

+

H D (k nr + kPT )/(k nr + kPT ) = 1.6 +

(7)

+

We also assume that kHnr = kDnr , and therefore (from eqs 5−7 + + + and the assumptions), for 1N5S, kHnr ≈ kHPT = 2.2 × 1010 s−1, kDPT =+ 7.45 × 109 s−1, and KIE ≈ 3. For 1N4S, knr is about 0.06 × kHPT H+ and does not much affect the determination of k PT ,

(5)

where kF is the inverse of the fluorescence lifetime, τ−1 F , and the radiative rate constant krF is much smaller than the 4711


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Figure 11. Time-resolved fluorescence of the ROH form of each of the two photoacids 1N4S and 1N5S in H2O and D2O. (a) The signals of 1N5S photoacid. (b) The signals of the 1N4S photoacid. Note the small difference in the lifetimes of the ROH signal of 1N5S in H2O and D2O.

which is 2.2 × 1010 s−1. We conclude that the ESPT rate constants of the two photoacids are about the same. The large difference between 1N5S and 1N4S is the large nonradiative rate constant knr for 1N5S, which is about the same as the ESPT rate constant kPT ≈ 2.2 × 1010 s−1. We propose that in methanol−water mixtures of χH2O ≤ 0.2, a mixed water−methanol bridge of three molecules between 1-OH and the 5-SO3− enhances the values of kPT and knr by about a factor of 10 over those of 1N4S. Computational Results. First, we tested the hypothesis of the ESPT reaction for 1N5S occurring via a two-water bridge in methanol. The potential curves are present in Figure 12A, while the relevant structures can be found in Figure 13. The IRC calculations determined that while we can observe proton transfer from the hydroxyl group of 1N5S to the water bridge, the sulfonate group is not able to abstract it. In fact, the product of this reaction consists of an ion pairing between doubly anionic 1N5S and a hydronium cation. The hydronium cation is expected to interact with 1N5S via attractive electrostatic interactions with the deprotonated sulfonate and hydroxyl groups and by cation−π interactions. Nevertheless, this reaction appears to be unfavorable in either S0, S1, and S2 states. In the S0 state, the reaction is accompanied by a continuous increase in energy up to 20.7 kcal mol−1. This is not exactly the case for S1 and S2 states. An activation barrier of 9.8 and 14.4 kcal mol−1 (respectively) can be seen, followed by a decrease in energy up to the products of the reactions. However, these products are 4.4 and 12.0 kcal mol−1 less stable than the reactions, which indicates that these reactions are not favorable. Given this, we assessed the possibility of the ESPT occurring via a mixed-type bridge, composed of two methanol and one water molecules (Figure 14), giving the methanol-rich content of the studied samples. First of all, the IRC calculations indicated that 1N5S can both transfer a proton from the hydroxyl group to the bridge and abstract a proton from it to the sulfonate group. Nevertheless, this reaction does not appear to be favorable in the S0 state (Figure 12B). Besides an activation barrier of 17.6 kcal mol−1, the product complex is less stable than the reactant by 14.5 kcal mol−1. This is not the case for the S1 state (Figure 12B). The activation barrier is only 8.0 kcal mol−1, which is 9.6 kcal mol−1 lower than that in the S1 state, which demonstrates the photoacidity of S1 1N5S. Moreover, the product complex is 7.7 kcal mol−1 more stable than the reactant. These results help to explain why ESPT occurs in methanol-rich mixtures with low water content. In Figure 13C are presented the oscillator strengths for the S1 and S2 states as a function of intrinsic reaction coordinates for the

Figure 12. Energetic profile of the proton transfer reaction in methanol via a two-water bridge (A). Energetic profile of the proton transfer reaction in methanol via a mixed-type bridge (B). Oscillator strength of the S1 and S2 states during the proton transfer reaction via a mixed-type bridge (C). 4712


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Figure 13. Structures of the reactant (A), product (B), and transitionstate (C) complexes for the proton transfer reaction via a two-water bridge, of which the energetic profile can be found in Figure 12A. Figure 14. Structures of the reactant (A), product (B), and transitionstate (C) complexes for the proton transfer reaction via a mixed-type bridge, of which the energetic profile can be found in Figure 12B.

mixed-bridge-assisted ESPT. The oscillator strength of the S1 product (0.257) is lower than of the reactant complex (0.276), which can account partially for the nonradiative decay found in methanol-rich mixtures. However, the most important finding appears to be related with the S2 state. This is a “dark” state from the reactant complex up to IRC of ∼−1.10 amu1/2 bohr. Moreover, in this region of the potential energy surface, the energy difference between S1 and S2 is only of 4.6−5.8 kcal mol−1. These are small enough values to allow for nonadiabatic coupling between states (especially considering that here was used a single-reference method), and so, S1 → S2 transition is possible. It should be noted that while the S1 ESPT reaction is energetically favorable, the same reaction in the S2 state is not. The activation barrier is of 12.7 kcal mol−1, which is 4.7 kcal mol−1 higher than that for S1,

and the product complex is less stable than the reactant by 4.2 kcal mol−1. Thus, upon transition from S1 to S2 in the beginning of the ESPT reaction, it is more energetically favorable for the reacting S2 molecules to return to the reactant complex, which is a “dark” state to the contrary of the product complex (Figure 12C). Thus, these results provide an explanation for the nonradiative decay found in methanol-rich mixtures. The different photochemical pathways are presented schematically on Figure 15. Main Findings. 1. The steady-state fluorescence spectra of both photoacids in neat methanol show only the ROH band, and therefore, 4713


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Figure 15. Diagram explaining the photochemical processes involving 1N5S in low-water−water−methanol mixtures. Photoexcitation to the emissive S1 state triggers an ESPT reaction from the hydroxyl to the sulfonate group of 1N5S via a mixed-type bridge, followed by fluorescence from the ESPT product complex. Alternatively, internal conversion to the S2 state is made possible by nonadiabatic coupling with the S1 state. Given that the S2 ESPT reaction is not energetically feasible, the reacting S2 molecules should then return to the reactant complex, which is a “dark” state, and so, this would explain the nonradiative decay observed experimentally.

2.

3.

4.

5.

6.

7.

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There are two kinds of photoacid behavior. The substances that show them are termed reversible and irreversible photoacids. For reversible photoacids, after the excited-state proton transfer (ESPT) to the solvent takes place, there is a finite probability that the solvated proton will geminately recombine with the RO−* and reform the photoacid in the excited-state ROH*. The photocycle can repeat itself as long as the ROH* and the RO−* forms of the photoacid are in the excited state. A negatively charged group like a sulfonate enhances the geminate-recombination (GR) process, because it enhances the attractive Coulomb potential. The time-resolved fluorescence of the ROH form of reversible photoacids shows a fluorescence tail that depends on the Coulomb potential between the positive proton and the RO− species and on the proton diffusion coefficient. The GR process is assisted by the large proton diffusion coefficient (which is about 10 times that of the RO−). The GR model long-time fluorescence tail, when compensated by the radiative lifetime, is a power law of time.24−26

ESPT does not occur in either 1-naphthol-5-sulfonate (1N5S) or 1-naphthol-4-sulfonate (1N4S) in neat methanol. In methanol−water mixtures, an ESPT process occurs in both 1N5S and 1N4S, and the RO− fluorescence band exists at about 535 nm for 1N5S and 430 nm for 1N4S. The time-resolved fluorescence of the ROH form of 1N5S in methanol−water mixtures of water molar ratios up to 0.2 shows a rapid decrease in the decay time with an increase in the water molar ratio. At about χwater = 0.2, the ROH form of 1N5S fluorescence steady-state intensity and the fluorescence lifetime versus χwater are lower by a factor of 10 than the intensity in neat methanol. For 1N4S, the fluorescence lifetime of the ROH form decreases in methanol−water mixtures much more slowly than it does for 1N5S. It has fallen by only 30% at the water molar ratio of χwater ≈ 0.2, whereas in 1N5S, it has decreased 10-fold. Two iso-emissive points exist in the steady-state spectra of 1N5S in methanol−water mixtures, one for methanol− water mixtures of χH2O < 0.2 and one for χH2O > 0.3. We explain the above experimental phenomena by the fact that the ROH form of 1N5S has efficient ESPT and nonradiative mechanisms: knr is about the same as kPT. In methanol−water mixtures of χwater < 0.2, knr is large, since the proton is transferred efficiently to the sulfonate position 5 and returns to the oxygen at position 1, and recombination forms the ground-state ROH(g). We propose a mixed water−methanol bridge of three molecules that supports proton transfer in samples of χwater < 0.2 from the 1-OH to the 5-SO3−. The bridge also supports the irreversible recombination of a proton with the RO−* to form the ground-state ROH(g).

* ·exp( −V (a)) ⎛t ⎞ Keq P(t ) = IFROH(t ) ·exp⎜ ⎟ ≈ ⎝ τF ⎠ (4πDt )3/2

(8)

where Keq* is the equilibrium constant of the ESPT process of the photoacid, exp(−V(a)) is the Coulomb attraction potential term at the reaction contact radius a, D is the diffusion coefficient of the proton, and IROH F (t) is the TCSPC fluorescence signal of the ROH form of the photoacid. In the case of irreversible photoacids, the GR process leads to the formation of the ground-state ROH(g). RO−* + H+ → ROH(g)

(9)

This process terminates the ESPT cycle. The RO−* population decays exponentially, but in addition, it also decreases with GR probability, which has a large slope at short times and decreases at longer times. In this research, we found that the ROH form of 1-naphthol-5sulfonate (1N5S) in methanol−water mixtures of low water content (up to χH2O ≈ 0.2) has both a large ESPT rate constant, kPT, and a large nonradiative rate constant, knr. We also conducted similar steady-state and time-resolvedfluorescence measurements on another irreversible photoacid, 1-naphthol-4-sulfonate (1N4S), in methanol−water mixtures. The results obtained for methanol−water mixtures of low water

DISCUSSION The main findings of this research show that both the steadystate and time-resolved fluorescence of the ROH form of 1N5S in methanol-rich mixturesup to χH2O ≈ 0.2exhibit unusually large ESPT and nonradiative rate constants compared to those of the 1N4S photoacid. 4714


Article

The Journal of Physical Chemistry A content (χH2O < 0.2) did not show strong dependence of knr and kPT on χH2O, as was observed for 1N5S. Kinetic Isotope Effect of 1N4S and 1N5S. We conducted an ESPT study of both photoacids in D2O.We found that the ratio of the fluorescence lifetime of the ROH form of 1N5S in H2O to that in D2O is rather small (kHF 2O/kDF 2O ≈ 1.6), whereas for 1N4S, it is about 2.85. 1N5S and 1N4S have pKa* values of about −0.7 and −0.1,18 respectively, and therefore are at the edge of the weak photoacid regime (0 < pKa* < 3.4).27 We found that for reversible weak photoacids, the ESPT process has a KIE of about 3.0 ± 0.2, and Pines et al. found KIE = 3.3 for 1N4S and KIE = 2.1 for 1N5S.18 For the irreversible photoacid 1N4S, we found KIE ≈ 2.85, and thus, we confirmed that the KIE is independent of the GR process, whether it is reversible or irreversible. The small kHF 2O/ kDF 2O ratio of 1N5S of 1.6 results from the fact that we did not take into account the nonradiative process that takes place in 1N5S in neat water and D2O and in methanol−water mixtures. The simple TRF measurement of the ROH form provides the fluorescence-decay rate of a photoacid, and usually, it provides the ESPT rate, since the radiative rate is much smaller than kPT, and the assumption is that knr in irreversible photoacids is smaller than kPT. The fluorescence rate constant kF of the ROH form of a photoacid depends on three decay components given in eq 5. The KIE for the ESPT process of 1N5S should be of the order of 3. We calculated knr for 1N5S from kHF 2O/kFD2O ≈ 1.6. On the basis of a KIE value of 3 for weak photoacids, we calculated that for 1N5S, kPT ≈ knr. Another interesting point is that the steadystate spectra of 1N5S in methanol−water mixtures show two isoemissive points, one at the low molar ratio of χH2O < 0.2 and one at χH2O > 0.3. This phenomenon also indicates that the spectra of water-rich mixtures differ from those of χH2O < 0.2 (see Figure 1). In 1N4S, only one iso-emissive point exists, at about 395 nm, for all methanol−water mixtures. We therefore propose in this study, that for 1N5S in lowwater-content methanol−water mixtures (χH2O < 0.2), knr and kPT are larger than they are for 1N4S and increase with increasing water content. We propose here that for 1N5S photoacids in methanol−water mixtures of low water content (χH2O < 0.2), a mixed water−methanol bridge of three molecules exists that connects the 1-OH and the negatively charged 5-sulfonate. This bridge is stable at low-water-content mixtures with methanol. At higher content of water (χH2O > 0.2), the bridge is less stable, and the usual ESPT process occurs mainly to bulk water. This bridge at χH2O < 0.2 shuttles the proton, first to the sulfonate, and the back reaction causes the larger GR and larger nonradiative rate processes. Similar results are also obtained in ethanol−water mixtures, but in methanol−water mixtures, the rate of change in the TRF lifetime of the ROH of 1N5S is greater.

Experimental Conclusions. We employed steady-state and time-resolved fluorescence techniques to study the properties of 1-naphthol-4-sulfonate (1N4S) and 1-naphthol-5-sulfonate (1N5S) photoacids. The study includes the excited-state properties of the ROH and RO− forms of these photoacids in methanol and in H2O and D2O and in methanol−H2O mixtures. Pines et al. found that the photoacidity of 1N4S is pKa* ≈ −0.1.18 The excited-state proton transfer (ESPT) rate constant in H2O obtained by us is kPT ≈ 2.2 × 1010 s−1, whereas Pines found kPT ≈ 3.0 × 1010 s−1. In methanol, the ESPT rate constant is lower by about 3 orders of magnitude, and the ESPT process could not be observed, since the radiative rate is much larger than that for kPT. In methanol−water mixtures, the ESPT rate increases with increasing water content in the mixture. In the case of 1N5S, we found that knr for the ROH form (∼2 × 1010 s−1) is high in water and is equal to the ESPT rate constant. In methanol−water mixtures of χH2O < 0.2, the ESPT rate and the nonradiative rate are high for 1N5S and much lower for 1N4S. For 1N5S at χH2O = 0.2, the fluorescence lifetime is about 10% of that in neat methanol, and the intensity of the steady-state fluorescence spectrum is reduced by the same amount. By contrast, for 1N4S in χH2O = 0.2 methanol−water mixtures, the kPT and the steady-state fluorescence intensity were reduced by only 30%. In 1N5S steady-state spectra of methanol−water mixtures, there are two iso-emissive points. For mixtures of χH2O < 0.2, the point is at 505 nm, and for χH2O > 0.3, the second point is at ∼465 nm. For 1N4S steady-state spectra in methanol−water mixtures, only one iso-emissive point exists at about 395 nm for all methanol−water mixtures in the range of 0 < χH2O < 1. We therefore propose that in 1N5S in low-water−methanol− water mixtures that there is a mixed water−methanol bridge of about three molecules that bridges the proton donor site 1-OH with the 5-sulfonate and enhances both kPT and the nonradiative rate constant, knr, of the ROH form of the photoacid.

SUMMARY AND CONCLUSIONS Computation Calculation Conclusions. The DFT-based theoretical calculations allow us to explain the enhancement of kPT in low-water−methanol−water mixtures with the formation of a mixed-type bridge, composed by two methanol and one water molecules. This bridges the proton donor site 1-OH with the 5-sulfonate group and allows for ESPT in the S1 state. The enhancement of the nonradiative rate constant can be explained by internal conversion from the optically active S1 state to the “dark” S2 state, during the ESPT reaction.

Notes

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b00957. Fits of the ROH signals of 1N5S in methanol−water mixtures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Phone: 972-3-6407012; Fax: 972-3-6407491 (D.H.) ORCID

Luís Pinto da Silva: 0000-0002-5647-8455 Joaquim C. G. Esteves da Silva: 0000-0001-8478-3441 Dan Huppert: 0000-0002-0292-4106

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Israel Science Foundation 1587/16. This work was also made in the framework of the project Sustainable Advanced Materials (NORTE-010145-FEDER-000028), funded by FEDER through NORTE2020. Acknowledgment to project POCI-01-0145FEDER-006980 funded by FEDER through COMPETE2020 4715


Article


(21) Adamo, C.; Jacquemin, D. The calculations of excited-state properties with Time-Dependent Density Functional Theory. Chem. Soc. Rev. 2013, 42, 845−856. (22) Scalmani, G.; Frisch, M. J. Continuous surface charge polarizable continuum models of solvation. I. General formalism. J. Chem. Phys. 2010, 132, 114110. (23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (24) Pines, E.; Huppert, D.; Agmon, N. Geminate recombination in excited-state proton-transfer reactions: Numerical solution of the Debye−Smoluchowski equation with backreaction and comparison with experimental results. J. Chem. Phys. 1988, 88, 5620−5630. (25) 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. (26) Simkovitch, R.; Pines, D.; Agmon, N.; Pines, E.; Huppert, D. Reversible Excited-State Proton Geminate Recombination: Revisited. J. Phys. Chem. B 2016, 120, 12615−12632. (27) Simkovitch, R.; Shomer, S.; Gepshtein, R.; Huppert, D. How fast can a proton-transfer reaction be beyond the solvent-control limit? J. Phys. Chem. B 2015, 119, 2253−2262.

is also made. L. Pinto da Silva acknowledges the Post-Doc grant funded by NORTE-01-0145-FEDER-000028. The Laboratory for Computational Modeling of Environmental Pollutants− Human Interactions (LACOMEPHI) is acknowledged.

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