ARTICLE pubs.acs.org/JPCA
Ultrafast Excited-State Intermolecular Proton Transfer of Cyanine Fluorochrome Dyes Naama Karton-Lifshin, Itay Presiado, Yuval Erez, Rinat Gepshtein, Doron Shabat, and Dan Huppert* Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel
bS Supporting Information ABSTRACT: Steady-state and time-resolved emission spectroscopy techniques were employed to study the excited-state proton transfer (ESPT) to water and D2O from QCy7, a recently synthesized near-infrared (NIR)-emissive dye with a fluorescence band maximum at 700 nm. We found that the ESPT rate constant, kPT, of QCy7 excited from its protonated form, ROH, is ∼1.5 1012 s1. This is the highest ever reported value in the literature thus far, and it is comparable to the reciprocal of the longest solvation dynamics time component in water, τS = 0.8 ps. We found a kinetic isotope effect (KIE) on the ESPT rate of ∼1.7. This value is lower than that of weaker photoacids, which usually have KIE value of ∼3, but comparable to the KIE on proton diffusion in water of ∼1.45, for which the average time of proton transfer between adjacent water molecules is similar to that of QCy7.
’ INTRODUCTION For many years,113 intermolecular excited-state proton transfer (ESPT) to a protic solvent or to a mild base in a liquid solution, and more recently in ice, has been widely researched. In the past decades we extensively studied the photoprotolytic cycles of reversible and irreversible photoacids. We used a proton transfer model that explains the reversibility or the irreversibility and accounts for the diffusion assisted geminate recombination of the transferred proton with the deprotonated form of the photoacid.7,14 In recent years, a new class of very strong photoacids was synthesized and their ESPT rate monitored.1520 The first superphotoacid from this new class was synthesized by Tolbert and co-workers3 on top of a 2-naphthol backbone. The most promising photoacid of this class is 5,8-dicyano-naphthol (DCN2).3 Cyano groups stabilize the excess electron of the deprotonated form of the photoacid, hence the increase in the photoacidity. The pKa* value of this photoacid is much lower than that of 2-naphthol (pKa* ≈ 2.5), and was estimated to be 4.5 on the basis of the F€ orster cycle. The ESPT rate of DCN2 was measured21 by the time-correlated-single-photon-counting (TCSPC) technique with limited time-resolution of ∼20 ps in methanol, ethanol and propanol over a wide range of temperatures as well as at room temperature. It was found in these experiments that the ESPT rate constant, kPT, and the reciprocal of the dielectric relaxation times of the neat solvents, 1/τD, have similar values and temperature dependence, though at high temperatures, kPT has a smaller value than 1/τD and a weaker temperature dependence. Also, it was concluded that at sufficiently low temperatures solvent dynamics controls the ESPT rate. Thus, even though the driving force at thermodynamic equilibrium for r 2011 American Chemical Society
the ESPT reaction is sizable due to pKa* value of 4.5, solvent reorganization around the newly formed electronically excitedstate configuration is the rate limiting step. Unfortunately, DCN2 is insoluble in water, in which proton transfer is estimated to take place within a few picoseconds. In more recent studies a new superphotoacid, N-methyl-6hydroxyquinolinium (NM6HQ+) iodide was extensively studied by the research groups of Topp22 and Ernsting.23,24 Topp and coworkers reported the relationship between the high degree of photoacidity of these molecules and electron transfer in excitedstates, by comparing the fluorescence up-conversion data of (NM6HQ+) and N-methyl-7-hydroxyquinolinium (NM7HQ+). Their results supported the mechanism of tautomerization of NM6HQ+ and NM7HQ+ in neutral water. ESPT, electron transfer, and solvent relaxation processes in NM6HQ+ and NM7HQ+ were investigated in both acidic and basic media. They showed that the hydroxyl group behaved much like a superphotoacid: the deprotonation rates in NM6HQ+ and NM7HQ+ they found were (2 ps)1 and (4.5 ps)1, respectively, which makes them among the fastest observed in intermolecular proton transfer to water molecules. Such a very high photoacidity is explained by fast intramolecular electron transfer from the hydroxylate (O) group to the positively charged pyridinium ring as soon as the proton transfer occurs. After proton ejection, which is the rate-limiting step in acidic solution, rapid intramolecular electron transfer occurs in 690 nm, we used the following eq.: If ¼ f
Scheme 2
τ1 (fs)
2
∑ ai ð1 exp½ ðt=τi Þα Þgexpð t=τ0 Þ i¼1 i
2O τH ¼ 130ps 0 D2 O τ0 ¼ 230ps
of QCy7 in several acidic solutions of varying HCl concentrations. As seen in the figure, the intensity of the RO* band decreases, whereas that of the ROH* band increases the more acid is introduced into the solution. For 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS), a well-studied reversible photoacid14 with pKa* value of ∼1 and kPT value of 1010 s1, the acid effect on the steady-state spectrum is observable at acid concentrations much lower than for QCy7, since the forward and backward reaction rate coefficients, kPT and ka, are smaller by 2 orders of magnitude. Thus, we conclude that the QCy7 molecule is a reversible photoacid. We observed that the intensity of the ROH* band at acid concentrations higher than 5 M is 2.5 times weaker than that of the RO* band at pH > 2. This may be the result of a certain degree of irreversibility of the geminate recombination process with the proton leading to the formation of a ground-state ROH form (see Scheme 2). HPTS, a prototype of reversible photoacids, also exhibits some level of irreversibility at high acid concentrations.
acids in their first excited singlet state. Usually, the ground-state pKa values of photoacids are in the range of 5 9. Figure 2a shows the time-integrated (steady-state) emission spectrum of the QCy7 sample at a pH level of 4, and excited at 440 nm, which is the ROH absorption band’s maximum. The spectrum shown on a semilogarithmic scale consists of two emission bands: a weak band with a maximum at ∼540 nm and a stronger band with a maximum at ∼700 nm. When a neutral pH sample is excited at 570 nm, the absorption maximum of the RO form, then the emission spectrum consists of a single band with a maximum at ∼700 nm, nearly identical to the signal of the RO* band, achieved when the sample is excited at 440 nm. The absorption and emission results indicate that QCy7 is a photoacid, namely an ESPT to the solvent is taking place. Figure 2b shows the steady-state emission 87
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Scheme 3
Table 2. Multi-Stretched Exponential Fitting Parameters of the Fluorescence up-Conversion Signals in D2O wavelength a1
(nm) a
a
τ1 (fs) α1
a2
τ2 (ps)
α2
a3
τ3 (ps) α3
520 0.23 530a 0.15
300 0.8 0.64 300 0.8 0.71
1.1 1.1
0.8 0.8
0.13 0.14
7.0 7.0
0.6 0.6
540a 0.12
300 0.8 0.74
1.1
0.8
0.14
7.0
0.6
550a 0.05
300 0.8 0.80
1.1
0.8
0.15
7.0
0.6
560a 0.05
300 0.8 0.84
1.1
0.8
0.11
6.0
0.6
570a 0.05
300 0.8 0.78
1.1
0.8
0.17
5.5
0.7
710b 0.22
200 0.9 0.78
1.1
0.90
For decay analysis at λ < 600 nm, we used the following eq.: If ¼
b
2
∑ ai exp½ ðt=τi Þα i¼1 i
For rise-time signals at λ> 690 nm, we used the following eq.: If ¼ f
2
∑ ai ð1 exp½ ðt=τi Þα Þgexpð t=τ0 Þ i¼1 i
2O τH ¼ 130ps 0 D2 O τ0 ¼ 230ps
by a long decay time of ∼140 ps. The rise time of the signals at long wavelengths is attributed to the formation of the RO* form by the photoprotolytic reaction:
Figure 4. Fluorescence up-conversion of QCy7 in D2O, excited at 387 nm and measured at several wavelengths.
kPT
þ ROH sf rs RO þ H
Figure 3 shows the fluorescence up-conversion signals of QCy7 in neutral aqueous solutions at several wavelengths. The fluorescence up-conversion signals detected in the range of 520 610 nm that are part of the ROH* band, consist of a major component with a fast decay, and followed by a minor component with a small amplitude and a longer decay time. The decay time of the major component depends rather weakly on the detection wavelength. In the spectral region of 520590 nm, the signal can be reasonably fitted by three stretched exponents with decay times of 200 fs, 0.65 and 4.5 ps. Table 1 provides the fitting parameters of the fluorescence up-conversion signal at several monitored wavelengths. The amplitude of the short time component of ∼200 fs decreases as the monitored wavelength increases. At 590 nm, its amplitude is nearly zero, whereas at 520 nm it is about 0.32. We assign this component to solvation dynamics, usually observed in ultrafast spectroscopy studies of time-resolved emission of large dipolar molecules in protic and polar solvents. The main component of the ROH* fluorescence decay has a decay time of 0.65 ( 0.07 ps and amplitude of 0.68 ( 0.05. This component is assigned to the ESPT process, and therefore we 12 1 2O concluded the rate constant to be kH s . The PT = 1.5 10 long-time component decays nonexponentially and could be reasonably fitted to a stretched exponent of the form: exp[(t/τ)0.6], where τ = 4.5 ps, and its relative amplitude is ∼0.12 of the total signal. We attribute this long-time fluorescence tail to the repopulation of the excited protonated form, ROH* by geminate recombination process with the proton, which we describe later on. The value of α is 0.6, which is relatively small, and thus the deviation from an exponential decay curve is rather large. This long-time decay component fits the signal from a few picoseconds up to about 100 ps. The fluorescence up-conversion signals measured at 690 nm and at longer wavelengths have a distinctive rise-time, followed
ð1Þ
Analysis of the signals reveals that the rise-time is 1.1 ps long, which is longer than the major decay component of the fluorescence up-conversion signal measured at the ROH* emission band in the spectral range of 520590 nm. The relatively long decay time of 140 ps is attributed to the fluorescence lifetime of the RO*. The radiative rate of QCy7 is estimated to be around 3 ns. The nonradiative rate, probably due to cis-trans isomerization, controls the emission lifetime of the RO* form. The structure of QCy7 is somewhat similar to that of transstilbene. QCy7 has an ethylene bridge connecting the phenol with the indolium. This kind of ethylene bridge exists also in trans-stilbene, which is known to undergo fast transcis isomerization in the first electronically excited-state, that leads to a fast decay to the ground-state. Hemicyanine (shown in Scheme 3) differs from QCy7 in that it lacks one of the latter’s indolium moieties. The position and shape of the absorption and emission spectra of protonated hemicyanine is similar to that of protonated QCy7. The fluorescence decay time in propanol is nearly wavelength-independent and its average value varies slightly around 7.5 ps. Such a short decay time indicates that a very efficient nonradiative process happens. In shorter alcohols, the emission lifetime is relatively short, whereas in pentanol, for example, the lifetime is 15 ps. In aliphatic alcohols, the lifetime qualitatively scales with the solvent’s viscosity. The short lifetime of the ROH* form of hemicyanine is therefore the result of the efficient excited-state transcis isomerization. In QCy7, this isomerization is rather slow, making the nonradiative rate slow in comparison. This is due to the second conjugated chain and indolium moiety that make this process rather cumbersome for the both the phenol and the phenolate. 88
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Figure 6. Fluorescence up-conversion signal of QCy7 in both H2O and D2O, measured at 700 nm (a) on a linear scale (b) on a semilogarithmic scale. Figure 5. Fluorescence up-conversion signals of QCy7 in H2O and D2O solutions detected at 550 nm and plotted (a) on a linear scale; (b) on a loglog scale with a synthetic biexponential plot for comparison.
is much longer than that from the H2O sample. In Figure 6b the signals shown in part a are plotted on a semilogarithmic scale and on a wider time window. As seen in the figure, the decay of the RO* signal from the D2O sample is longer than that from the H2O sample. The rise-time of the RO* band in D2O is about 1.8 times longer than in H2O. The long-time fluorescence decay of the RO* band in D2O is 230 ps, whereas in H2O the long-time decay is 1.8 times shorter, i.e., 130 ps. Reversible and Irreversible Photoprotolytic Cycle of Photoacids. Excitation of a solution at pH values lower than the ground-state pKa of photoacids generates a vibrationally relaxed, electronically excited ROH molecule (denoted by ROH*) that initiates a photoprotolytic cycle (Scheme 2). A more detailed description of ESPT followed by diffusion-assisted proton geminate recombination is given elsewhere.7,14 Proton dissociation, with an intrinsic rate constant kPT, leads to the formation of the contact ion pair RO* 3 3 3 H+, 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 large fluorescence quenching of the deprotonated form, RO*, in acidic aqueous solutions. Separation of an ion pair from the contact radius, a, to infinity is described by the transient numerical solution of the DebyeSmoluchowski equation (DSE).7
Figure 4 shows the fluorescence up-conversion of QCy7 in D2O, excited at 387 nm and measured at several wavelengths. A comparison of the QCy7 signals in H2O and D2O reveals that there is a kinetic isotope effect (KIE), since the decay time of the ROH* emission band in D2O is slower than in H2O, and the formation of the RO* band, i.e., the rise-time, is also longer in D2O than in the corresponding H2O sample. Analysis of the decay profile of the ROH* band in D2O shows that the major component has a decay time of 1.1 ( 0.1 ps. The rise-time of the RO* band is 2 ps, and the long-time radiative decay is 230 ps. Table 2 provides the fitting parameters of the fluorescence upconversion signals of QCy7 in D2O. Figure 5a shows a comparison of the fluorescence up-conversion signals of QCy7 in H2O and D2O at 550 nm, which is close to the ROH* emission band’s maximum. The fits of both the H2O and D2O signals are the solid curves shown in the figure, and they are the result of a convolution of a multistretched exponential function with the IRF. The decay of the ROH* band in D2O is ∼1.7 times slower than in H2O. Figure 6a shows the fluorescence up-conversion signal of QCy7 in both H2O and D2O, measured at 700 nm, which is near the maximum of the RO* band. The fits of the RO* band are the solid lines. The fitting parameters of both the ROH* and RO* signals are given in Tables 1 and 2. The rise-time of the signal from the D2O sample 89
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In addition, the fluorescence lifetimes of all excited species are considered, with 1/k0 = τ0 for the acid, and 1/k0 0 = τ0 0 for the base. Generally, k0 0 and k0 are much smaller than both the proton reaction and the diffusion-controlled rates. 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 the electric potential between the RO* and the proton. The motion of the transferred proton in water close to the photoacid depends strongly on the electric potential existing between it and the deprotonated form. The diffusion-assisted geminate recombination of the RO* with the proton could quantitatively be described using the Debye-Smoluchowski equation (DSE). The QCy7 molecule is much larger than phenol, which is the actual ESPT site. It is only at very long distances (of more than twice the distance between the nitrogen atoms of the two indolium moieties in QCy7, at the very least) that a spherically symmetric Coulomb approximation, taking into account the molecule as a charged symmetrical sphere with radius a, may be applied to solve the DSE. We therefore, could not have used the spherically symmetric DSE, we routinely use for other photoprotolytic processes in order to fit the time-resolved emission of QCy7. The estimate for the ESPT time constant is given in the results of the fluorescence up-conversion data analysis in table 1. Solvation dynamics of excited large polar molecules in water has a short-time component of less than 50 fs and a long-time component of 0.8 ps. Thus, ESPT is solvent-controlled in H2O, but is slower still in D2O. The longitudinal relaxation time, given by the following: ε∞ τL ¼ τD ð2Þ εs
∼3 ps, it follows a power law, where the value of the scaling exponent α is 1.4 in H2O and 1.33 in D2O. For reversible photoacids, the diffusion-assisted geminate recombination with the proton and the subsequent second ESPT process strongly influence the ROH* and the RO* populations and hence the fluorescence decay as well. According to the geminate recombination (GR) model7 in a 3D diffusion space and assuming molecular spherical symmetry the time-dependence of the population size has the form of t 1.5 at long times. This was corroborated in a number of studies on reversible photoacids.14,32 The fit of the short and intermediate fluorescence decay of the ROH* band in H2O is given by 0.1 3 exp[t/200 fs] + 0.9 3 exp[t/350 fs]. In the results section, we analyze the ROH* signal using multistretched exponents, rather than the sophisticated GR model. The long-time decay component was fitted by a sretched exponent where τ = 4.5 ps and α = 0.6 (see Table 1). The plot shown in Figure 5b indicates that this long-time component approximately decays as a power law curve, as predicted by the GR model. A synthetic biexponential plot of this form is also displayed in the figure to demonstrate the large effect of the proton geminate recombination on the long-time fluorescence tail. The two time constants of 200 fs and 0.65 ps were derived from the best fit of the ROH* fluorescence decay signals of QCy7, and we attribute them to solvation dynamics and the ESPT process, respectively. In this study, we found that the ROH* decay of QCy7 follows a slightly more moderate power law than predicted by the model with a scaling exponent of 1.41 ( 0.15. This may arise from the large deviation from spherical symmetry of the electric potential. The fluorescence tail follows a power law of t d/2, where d is the dimension of the diffusion space. In the instance of two-dimensional reversible diffusion-influenced geminate recombination with the proton, the long-time decay follows a power law of t1. Another consequence of the inapplicability of the spherical symmetry is that the large recombination rate coefficient, ka, of QCy7 could not be quantitatively determined, since in the reversible proton geminate recombination model spherical symmetry is assumed in order to solve the DSE. An interesting point concerns the geminate recombination process of QCy7 with the proton that occurs after the step, in which proton is transferred to the solvent. From the loglog plot of the ROH* fluorescence decay shown in Figure 5b we find that already at around 1 ps the ROH* fluorescence decay deviates from exponentiality. We attribute this deviation to a geminate recombination of the RO* with the proton. The shift in the time range from 100 ps to 10 ns, found and discussed previously for HPTS in the 1988 paper14 to the much shorter time scale of QCy7, to wit 5 100 ps, seems to be a natural extension of the model. In short, the geminate recombination model is also attributable to shorter times, as displayed in Figure 5b. The nonexponential fluorescence decay of the ROH* in the first few picoseconds may also be explained as follows: for a solvent-controlled reaction the concept of an exponential reaction kinetics may fail and a nonexponential reaction may be more probable. If ESPT from QCy7 is nonexponential, then a part of the long-time fluorescence tail of the ROH* signal of QCy7 displayed in Figure 5b arises from the ESPT process rather from geminate recombination with the proton. Nevertheless, careful examination of the time-scale of solvation dynamics in water limits the nonexponential ESPT stage in the decay to about 2 ps, whereas in the D2O plot, one can follow the fluorescence tail up to 100 ps with reasonable signal-to-noise ratio.
where ε∞ and εs are the high frequency and static dielectric constants respectively, is suggested in the literature to be a gauge for solvent dynamics controlling the rates of chemical reactions. Rips and Jortner26 derived an expression for an adiabatic electron transfer (ET) rate coefficient. The pre-exponential factor in this expression includes τL. In an activationless ET process, the solvent controls the ET rate and the rate coefficient is mainly determined by τL. Borgis and Hynes27 derived a similar expression for an adiabatic PT rate, which, however, does not specifically incorporate τL but rather a general term for the solvent motion that influences the PT reaction. The KIE on the ESPT rate constant of QCy7 is only 1.7. It is postulated that the KIE √on fast ESPT processes asymptotically approaches a value of 2 = 1.41 for ultrafast ESPT processes. Photoacids of mild strength with positive pKa* values have a larger KIE of about 3 in water, methanol and other simple aliphatic alcohols. Stronger photoacids that have negative pKa* values have KIE values lower than 3. This is in accord with the rule stipulating that the stronger the photoacid the lower the KIE on proton mobility and diffusion in water is KIE.2830 The √ 1.45, close to 2. The fundamental time-step associated with proton mobility and diffusion in water (DH+= 104 cm2/s) in a proton-hopping model is ∼1.5 ps,31 which is longer than the ESPT from QCy7 to water. Thus, the PT between adjacent water molecules is a slower process than between excited QCy7 and water. Geminate Recombination Process of QCy7 with a Proton. Figure 5b shows on loglog scales the fluorescence up-conversion ROH* signal of QCy7 measured at 540 nm, near the peak of this band, in both H2O and D2O. At times longer than 1 ps the fluorescence decay becomes nonexponential at times longer than 90
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Yet another explanation to the nonexponentiality of the photoacid dissociation dynamics can be found in Ando and Hynes’s simulation of HCl dissociation.33 The mechanism is found to involve the following: first, a nearly activationless motion in a solvent coordinate, which is adiabatically followed by the quantum proton rather than tunneling, to produce a contact ion pair Cl H3O+, which is stabilized by ∼7 kcal/mol; second, motion in the solvent with a small activation barrier, as a second adiabatic proton transfer produces a solvent-separated ion pair from the contact ion pair in a nearly thermoneutral process. Motion of a neighboring water molecule—to accommodate the change of the primary coordination number from 4 for H2O to 3 for H3O+ of a protonaccepting water molecule—is indicated as a key feature in the necessary solvent reorganizations. € rster Cycle Calculation. With the F€ Fo orster cycle calculation1,34 we can 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: ΔpKa ¼ C 3 Δν
that QCy7 does not degrade much under reasonable irradiation condition. Figure 2b shows the steady-state emission spectra of QCy7 in acidic solutions with HCl concentrations varying from 0.5 to 8 M. It shows the stability and reactivity of QCy7 in acidic solutions at HCl concentrations below 8 M. The results indicate that QCy7 in aqueous HCl solutions of up to 8 M is stable for at least one hour, which was the time needed to perform the SS measurements. The advantages of QCy7 as a superphotoacid lie in its high degree of solubility in water and large absorption and emission cross sections which enables accurate time-resolved measurements with high signal/noise. We are currently synthesizing new QCy7 analogs with much higher stability.
’ CONCLUSIONS In this study we focused our attention on studying the photoacid properties of a recently synthesized near-infrared (NIR)fluorescent dye, QCy7, (Scheme 1) emitting at 700 nm with a fluorescence quantum yield of about 0.1. Ground-state QCy7 has a pKa value of ∼4.5, while in its excited-state it is lower than that by more than 10 logarithmic units, and therefore it is a very strong photoacid. The absorption bands maxima of the protonated ROH and deprotonated RO forms are positioned at 430 and 578 nm respectively. When QCy7 is excited in its protonated ROH form the steady-state emission spectrum consists of two bands with maxima at 540 and 700 nm. We assign the weak band at 540 nm to the protonated ROH* form, and the strong band at 700 nm to the deprotonated form, RO*. We estimate the pKa* value of QCy7 from the difference between the energies of the ROH* and RO* absorption bands as well as from the energy difference of the emission bands. Since the ΔpKa* is more than 10 the pKa* value is therefore ∼6. Photoacids with pKa* value of 2 or less are termed superphotoacids. For this category of molecules, it is perfectly reasonable that the ESPT rate to water would be higher than 1011 s1, roughly the same as the fluorescence decay time of the protonated form, τPT e 10 ps. For QCy7 as for many other photoacids, the fluorescence decay of the ROH* is complex and it does not decay exponentially. It has a long fluorescence decay tail, which is indicative of a reversible geminate recombination with a proton (see Scheme 2) to reform the ROH* that can undergo a second protolytic cycle. We found that the decay time of the major time component of the fluorescence signal, whose amplitude is ∼0.72 (see table 1), is 0.65 ps in H2O and 1.1 ps in D2O (see Table 2). We assign this component to the ESPT from QCy7, making it the fastest reported ESPT to the solvent to date. There is a distinctive KIE on the ESPT rate of ∼3 for many photoacids. Superphotoacids have a KIE value lower than 3 and closer to ∼2,2830 whereas the KIE on proton mobility and diffusion in water is rather small, i.e., 1.45, a value that is close to the square root of the D/H mass ratio affecting the tunneling probability. We found that the KIE on the ESPT from QCy7 to be 1.7. As previously mentioned, this value is similar to the KIE on the proton diffusion in water.
ð3Þ
where C is a factor of universal constants: C¼
NA h ¼ 2:09 103 cm1 lnð10Þ 3 RT
ð4Þ
Δν is the difference between the positions of the ROH* and RO* bands given in wavenumber units. The ROH* and RO* emission bands of QCy7 have peaks at 532 (18 800 cm1) and 700 nm (14 260 cm1) respectively. This leads us straightforward to ΔpKa* value of 9.9. If we were to use the difference between the absorption bands instead (22 750 cm1 and 17 300 cm1 for the ROH and RO, respectively), we would obtain a ΔpKa*ΔpKa* value of ∼ 11.3. The difference between these results arises from the contribution of the solvation energies of the ground and excited-states and the difficulty of determining the exact positions of the zero energies. Averaging the ΔpKa* values obtained by the two methods results in an average value of 10.6. Since the ground-state pKa is ∼4.5 the estimated pKa* value is ∼6.1. This value is roughly the same as that of HCl, which is a strong mineral acid. Chemical Stability and Photochemistry of QCy7. It is wellknown that dyes with related molecular structure undergo chemical and photochemical reactions to form a spiro derivative, which is incapable of ESPT. This reaction occurs faster under basic conditions, high temperature, and visible light. However, in the short time scale of the spectroscopic measurements QCy7 remains stable as indicated by its emitted fluorescence spectrum. Consequently, the intensity of the RO* emission band decreases with time. In a continuously heated aqueous solution the conversion time to a spiro compound is about 30 h. Nevertheless, we found that during our five-minute time-resolved measurements in a rotating cell at 10 Hz frequency, sample degradation is negligible, and does not affect the signal’s decay profile. In a recent study by Hobley et al.35 transient absorption and emission techniques were employed to study the reversible photoinduced ring-closing reaction of merocyanine to form spiro pyran. They found that the time constant of this reaction is 350 ps. We therefore, examined the photostability of QCy7 further. The data presented in the SI show
’ ASSOCIATED CONTENT
bS
Supporting Information. 1. Photostability of QCy7. This material is available free of charge via the Internet at http://pubs. acs.org.
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’ AUTHOR INFORMATION
(29) Nibbering, E. T. J.; Fidder, H.; Pines, E. Annu. Rev. Phys. Chem. 2005, 56, 337–367. (30) Pines, E. In Isotope Effects in Chemistry and Biology; Taylor & Francis: Boca Raton, 2006, 451464. (31) Agmon, N. Chem. Phys. Lett. 1995, 244, 456–462. (32) Solntsev, K. M.; Huppert, D.; Agmon, N.; Tolbert, L. M. J. Phys. Chem. A 2000, 104, 4658–4669. (33) Ando, K.; Hynes, J. T. J. Phys. Chem. B 1997, 101, 10464–10478. (34) F€orster, T. Z. Electrochem. 1950, 54, 531–535. (35) Hobley, J.; Pfeifer-Fukumura, U.; Bletz, M.; Asahi, T.; Masuhara, H.; Fukumura, H. J. Phys. Chem. A 2002, 106, 2265–2270.
Corresponding Author
*Phone: 972-3-6407012; fax: 972-3-6407491; e-mail: huppert@ tulip.tau.ac.il.
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dx.doi.org/10.1021/jp2095856 |J. Phys. Chem. A 2012, 116, 85–92
