Ultrafast Proton Transfer of Three Novel Quinone Cyanine Photoacids

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Ultrafast Proton Transfer of Three Novel Quinone Cyanine Photoacids Itay Presiado, Naama Karton-Lifshin, 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 S Supporting Information *

ABSTRACT: Steady-state and time-resolved emission techniques were used to study the photoprotolytic properties of three recently synthesized strong quinone cyanine photoacids (QCy7 and its sulfonated derivatives). The rate coefficient of the excited-state proton transfer (ESPT), kPT, of the three dyes is roughly 1.5 × 1012 s−1, a high value that is comparable to the solvation dynamics rate of large polar organic molecules in H2O and D2O. It is twice as fast as the proton transfer rate between two adjacent water molecules in liquid water. We found that, as expected, two of the sulfonated photoacids geminately recombines with the proton at an elevated rate. The accelerated geminate recombination process of the sulfonated derivatives is different from a simple diffusion process of protons. The ESPT rate coefficient of these molecules is the highest recorded thus far.

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INTRODUCTION Photoacids are a class of organic molecules that are weak acids in their ground-state and much stronger acids in their first electronically excited-state. Hydroxyaryl compounds mostly exhibit this property. Excitation of these molecules with short laser pulses leads to an excited-state proton transfer (ESPT) process to the solvent. In the last 15 years, mode-locked Ti:Sapphire lasers were an efficient and stable excitation source capable of providing pulses of 100 fs or much shorter over a wide spectral range from the UV−visible to near-IR. These lasers in conjunction with a variety of optical techniques allow the monitoring of the ESPT process and other excited-state processes with a time resolution finer than 100 fs. Usually, upon electronic excitation of a dipolar organic molecule, solute− solvent interactions occur after redistribution of the electronic charge. The time-scale of solvation dynamics strongly depends on the solvent, and to a lesser extent, on the solute. Solute− water dynamics is relatively fast, spanning from a few tens to ∼800 fs. For many years,1−13 intermolecular ESPT to a protic solvent or to a mild base in a liquid solution, and more recently in ice, has been studied by many. In the past decades, we thoroughly examined the photoprotolytic cycle of photoacids. We used a model that explains the ESPT in the photoprotolytic cycle and accounts for the diffusion-assisted geminate recombination of the transferred proton with the deprotonated form of the photoacid.7,14 The recombination with the proton reforms the excited protonated RO*H form of the reversible photoacid (the asterisk denotes a molecule in an electronically excited-state) that can then undergo a second photoprotolytic cycle. These processes enlarge the RO*H population, whose decay at long times follows a power law, t−α. © 2012 American Chemical Society

In recent years, a new class of very strong photoacids with pKa* < −1 was synthesized and their ESPT rate monitored. These strong photoacids (termed superphotoacids) are able to transfer a proton not only to water but also to linear alcohols such as methanol and ethanol. The first superphotoacid from this new class was synthesized by Tolbert and co-workers3 on top of a 2-naphthol backbone. The strongest 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 estimated to be −4.5 on the basis of the Förster cycle. N-Methyl-6-hyroxyquinolinium (NM6HQ+) iodide was extensively studied by the research groups of Topp15 and Ernsting16,17 and more recently by Solntsev and co-workers.18 Topp and co-workers reported on the connection between the high degree of photoacidity of these molecules and electron transfer in excited-states. They showed that the hydroxyl group behaved much like a superphotoacid: the deprotonation rates for NM6HQ+ and N-methyl-7-hydroxyoquinolinium they found were (2 ps)−1 and (4.5 ps)−1, respectively. Cyanine dyes are widely employed as fluorescence labels for NIR imaging since they are compounds with large extinction coefficients and relatively high quantum yield. In order to generate a turn-on system for a cyanine molecule, a fluorescence resonance energy transfer (FRET) approach is usually applied. In such approach, the cyanine dye and a quencher are attached through a specific linker to obtain a quenched fluorophore. A linker, which is cleaved by a specific Received: April 29, 2012 Revised: June 4, 2012 Published: June 6, 2012 7353

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enzyme, separates the fluorophore from the quencher. When the linker is cleaved, it turns on the fluorophore’s fluorescence signal. An alternative simpler approach, to turn off and on a fluorophore, could be achieved by disrupting the pull−push conjugated π-electron system of the dye. Such concept was mostly demonstrated for dyes with UV−vis fluorescence. Technical difficulties concerned with molecular structure and synthesis hamper to implement this option with cyanine dyes. QCy7 is a new analogue molecule of Cy7, with a modular turnon mechanism19 (Scheme S1 in the Supporting Information). In a previous study,20 we measured the steady-state (timeintegrated) emission, excitation, and absorption spectra, as well as the time-resolved emission properties of an excited QCy7 molecule. We found that QCy7 has dual emission bands when excited from its ground-state neutral form (the protonated form, ROH). The bands correspond to emission from the protonated phenol RO*H (band maximum at 540 nm) and from the deprotonated phenolate RO*− (690 nm) species of QCy7. The decay of the time-resolved emission of the RO*H in water is fast, i.e., about 0.7 ps at room temperature. These optical observations indicate that QCy7 undergoes ESPT to the solvent. In the current work, we examine the photoacidic properties of three recently synthesized QCy7 derivatives. Scheme 1

is a much more significant difference between the sulfonated derivatives and the QCy7 molecule, both in geminate proton recombination rate and in the location of the RO*− band, which is red-shifted in comparison to that of QCy7. We attribute both effects to the influence of the negatively charged sulfonate groups in S-QCy7 and TS-QCy7.

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SYNTHESIS OF THE FLUORESCENT DYES QCy7, sulfo-QCy7, and tetrasulfo-QCy7 were synthesized as reported before19 through a two-step procedure as shown in Scheme 2. Commercially available dialdehyde 1 was condensed with 2 equiv of the corresponding indolium derivative to give the respective ester 2, 3, and 4. The acetate group was removed by using potassium carbonate in methanol to afford the QCy7 derivatives. Further elaboration on the synthesis of the dyes is given in the Supporting Information of this study.

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EXPERIMENTAL SECTION The fluorescence up-conversion technique was employed in this study to measure the time-resolved emission of the three QCy7 dyes at room temperature. The laser used for the fluorescence up-conversion was a cavity dumped Ti:Sapphire femtosecond laser, Mira, Coherent, which provides short, 120 fs, pulses at around 800 nm. The cavity dumper operated with a relatively low repetition rate of 800 kHz. The up-conversion system (FOG-100, CDP, Russia) operated at 800 kHz. The samples were excited by pulses of ∼8 mW on average at the SHG frequency. The time-response of the up-conversion system is evaluated by measuring the relatively strong Raman Stokes line of water shifted by 3600 cm−1. It was found that the full width at half-maximum (fwhm) of the signal is 280 fs. Samples were placed in a rotating optical cell to avoid degradation.

Scheme 1

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REVERSIBLE AND IRREVERSIBLE PHOTOPROTOLYTIC CYCLE OF PHOTOACIDS A more detailed description of ESPT followed by diffusionassisted proton geminate recombination is given elsewhere.7,14 Excitation of the protonated form of a photoacid, RO*H, in a solution at pH values lower than the photoacid’s ground-state pKa generates within ∼10−13 s a vibrationally relaxed, electronically excited ROH molecule (denoted by RO*H) that initiates a photoprotolytic cycle. Excited state proton transfer to a nearby solvent molecule with an intrinsic rate coefficient, kPT, leads to the formation of the contact ion pair (CIP) RO*−···H+ reversible (adiabatic) recombination with a rate coefficient ka that reforms the excited acid, RO*H. Back protonation may also proceed by an irreversible (nonadiabatic) pathway, involving fluorescence quenching of the RO*− by a proton with a rate coefficient kq forming the ground-state ROH. 2-Naphthol and its derivatives are known to reform the RO*H by the diffusion assisted proton geminate recombination process, whereas 1-naphthol and its derivatives are irreversible photoacids where reprotonation forms the ground state protonated photoacid ROH(g) and thus exhibits large fluorescence quenching of the deprotonated form, RO*−, in acidic aqueous solutions. Assuming spherical symmetry, the separation of an ion pair from the contact sphere radius, a, to infinity is quantitatively described by the transient numerical solution of the DebyeSmoluchowski equation (DSE).7 In addition, the fluorescence lifetimes of all excited species are considered. The amplitude of

shows the molecular structures of the three cyanine dyes, whose photoacidic properties we studied in H2O and D2O solutions. In addition to QCy7, we also chose to study sulfo-QCy7 (SQCy7) and tetrasulfo-QCy7 (TS-QCy7). We found that, in terms of photoacidity, all three molecules have similar strength with pKa* values of around −6, determined using the Förster cycle model. The ESPT rate coefficients, kPT, of the sulfonated derivatives are slightly higher even than that of QCy7, the fastest reported to date. The kPT values for QCy7, S-QCy7, and TS-QCy7 were found to be 1.5 × 1012, 1.8 × 1012, and 2.2 × 1012 s−1, respectively. The two new photoacids are rich with negative charges (the sulfonate groups) and thus strongly affects the proton−molecule Coulomb interaction. We found that all three compounds undergo reversible geminate recombination with a proton. This process is manifested as a long-time fluorescence tail in the time-resolved emission signal of the phenol form in all three of these photoacids. Here, there 7354

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Scheme 2. Synthesis of QCy7, Sulfo-QCy7, and Tetrasulfo-QCy7

the RO*H long-time fluorescence tail depends on the intrinsic rate coefficients, ka and kPT, on the proton diffusion coefficient, DH+, and the electric potential between the RO*− and the proton. The long-time fluorescence tail of the RO*H form’s emission decay scales with t−d/2, where d is the dimension of the diffusion space. The proton diffuses in a spherically symmetric diffusion space in which the power law decay is −3/ 2. 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. For large and charged molecules like the QCy7, S-QCy7 (doubly charged by two sulfonated groups), and TS-QCy7 (four sulfonates) molecules, a spherically symmetric Coulomb cage is not a good approximation. The diffusion-assisted geminate recombination of the RO*− with the proton could therefore only be qualitatively described using the spherically symmetric Debye−Smoluchowski equation (DSE). We further elaborate on these issues.

red-shifted with respect to that of the QCy7, whereas the RO*H band positions are about the same. The difference in the band positions can be explained by the Förster cycle as arising from the greater photoacidity of the sulfo-QCy7 (S-QCy7). Naphthol and naphthol sulfonate are well studied photoacids. The excited-state pKa of these photoacids marked as pKa* depends on the number of sulfonate groups and their positions. In general, greater photoacidity is achieved by stabilizing the negatively charged hydroxyl oxygen of the RO*− form with electron withdrawing functional groups substituting the aromatic system. The pKa* value of 2-naphthol (2N) is 2.7, whereas that of 2-naphthol-6-sulfonate (2N6S) and 2-naphthol6,8-disulfonate (2N68DS) are ∼1.5 and 0.4, respectively. The RO*H and RO*− bands of 2N sulfonate derivatives are red-shifted with respect to the positions of those bands of the 2N spectrum. The pKa* values could be estimated from the Förster cycle. With the Förster cycle calculation,21,22 we can estimate the change in acidity upon excitation of the photoacid. 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 relationship between band positions and change in acidity

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RESULTS AND DISCUSSION Results. Figure 1a shows the absorbance and steady-state (time-integrated) emission of QCy7 and the two sulfo-QCy7 compounds shown in Scheme 1. All three compounds are mild acids in the ground-state, as they have a pKa value of about 4.5. When the three compounds are excited from their protonated form, the spectrum consists of two structureless emission bands. The peak maximum of the band at short wavelengths is at ∼540 nm, and it is attributed to the protonated and excited phenol moiety of the molecule, RO*H. The near-IR band at 690−725 nm is attributed to the deprotonated phenolate, RO*−. The RO*− emission band of the sulfo-compounds is

ΔpK *a = C Δν

(1)

where C is a factor of universal coefficients C=

NAh = 2.09 × 10−3 cm−1 ln(10)RT

(2)

where Δν is the difference between the positions of the RO*H and RO*− bands given in wavenumber units. The RO*H and RO*− emission bands of QCy7 have peaks at 532 (18 800 7355

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Figure 1. Absorbance spectrum of QCy7 and the steady-state emission spectra of QCy7, S-QCy7, and TS-QCy7 in aqueous solution at pH ≈ 3.5: (a) linear scale; (b) semilogarithmic scale. Figure 2. Time-resolved emission of QCy sulfonate derivatives measured at several wavelengths by fluorescence up-conversion technique: (a) S-QCy7; (b) TS-QCy7.

cm−1) and 692 nm (14 450 cm−1), respectively. This leads us directly to a ΔpKa* value of −9.2. If we were to use the difference between the absorption bands instead (22 750 cm−1 and 17 300 cm−1 for the ROH and RO−, respectively), we would obtain a 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 a value of −10.2. Since the ground-state pKa is ∼4.5, the estimated pKa* value is ∼−5.7. This value is roughly the same as that of HCl, which is a strong mineral acid. The positions of the RO*H bands of S-QCy7 and TS-QCy7 are at 537 nm (18 621 cm−1) and 542 nm (18 450 cm−1), respectively, and their RO*− bands are positioned at 700 nm (14 285 cm−1) and 715 nm (13 985 cm−1) respectively. This leads to ΔpKa* values of −9.1 and −9.35 for S-QCy7 and TSQCy7, respectively. As can be seen, the calculated photoacidity of the three cyanine dyes is quite similar. The reason for this is the exceptionally large contribution made by the resonant structures (seen in Scheme S1 in the Supporting Information) to the stability of the negatively charged phenolate moiety. Figure 2 shows the time-resolved fluorescence up-conversion signals of S-QCy7 and TS-QCy7 and Figure S1, Supporting Information, shows that of QCy7 measured at several wavelengths in the spectral range of 500−730 nm. The three compounds were excited by short 120 fs laser pulses at 395 nm from their ground-state ROH form. At short wavelengths,

corresponding to the RO*H band position, the signals exhibit a bimodal decay pattern. The short-time component could be fitted by a biexponential function. Similar measurements were carried out in D2O. Figure S1a,b in the Supporting Information shows the time-resolved emission of QCy7 in D2O measured between 520 to 720 nm. We fit the data with stretched exponents for convenience. The shorter decay component of 100 fs was about the same for the three cyanines, whereas the longer decay component of each molecule was different from the other two. The amplitude of the short time component becomes larger as the measured wavelength becomes shorter. At 500 nm, its relative amplitude is about the same as that of the longer time component, and it becomes smaller as the monitored wavelength becomes longer. Above 580 nm, the amplitude of the short-time component is smaller than 0.1, and virtually undetectable. We attribute this component to solvation dynamics and rearrangement of the phenol−water hydrogen bond complex occurring prior to the ESPT process. The decay time of the intermediate time components of tetrasulfo-QCy7, sulfo-QCy7, and QCy7 fluorescence are 450, 550, and 650 fs, respectively. We attribute this component to the ESPT process, whose rate depends on the strength of the photoacid, as it can vary by several orders of magnitude from weak photoacids pKa* > 1 to superphotoacids pKa* < −1. 7356

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The fluorescence up-conversion signal of the RO*H band measured in the range 520−620 nm also exhibits a long decaytime tail, whose relative amplitude is 5−20% of the total signal. The decay of the fluorescence tail can be reasonably fitted to a stretched exponent function, exp[−(t/τ)α], where α = 0.55 ± 0.05, and also, as we further discuss in detail, to a power law decay, t−α. The lifetime is relatively short for QCy7, i.e., τ ≈ 4 ps, and relatively long, i.e., τ = 7.5 and 11.5 ps, for S-QCy7 and TS-QCy7, respectively. We attribute this long-time nonexponential decay component to a reversible geminate recombination with a proton to reform the RO*H. This process is explained by our model on ESPT to the solvent. The proton is first transferred to a water molecule to form H3O+ hydronium ion. The proton can recombine to reform the RO*H or be further transferred to other water molecules. This process is reflected in the value of the proton diffusion coefficient in water, DH+ ≃ 10−4 cm2/s. The proton diffusion is influenced by the electrical field of the RO*− form (the excited conjugate base). The field does not have spherical symmetry, thus it depends on both the angle and distance of the proton with respect to the molecular axis of QCy7. The total charge of the RO*− of TS-QCy7 is −3, where 4 negative charges are positioned on the sulfonate groups and 2 positive charges on the heterocyclic nitrogen atoms. The effective negative charge on the hydroxyl oxygen is rather small since it is smeared on the two indolium moieties (see Scheme S1 in the Supporting Information). We will further discuss the properties of the longtime fluorescence tail. Figures 3 and S2, Supporting Information, show the fluorescence up-conversion signals of the QCy7 and its derivatives in H2O and D2O measured at 540 nm, where the RO*H peak of the emission band is located. As seen in the figure, the kinetic isotope effect (KIE) on the ESPT rate coefficient of QCy7 is about 1.7 and of the two sulfonate molecules is somewhat smaller, i.e., 1.4 ± 0.1, which is close to the KIE on the proton diffusion in water. For many weaker photoacids with pKa* > 0, we found a KIE of 3.0 ± 0.2, which is over twice as strong as that of the sulfonate QCy7 derivatives. The KIE decreases as the strength of the photoacid increases. 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. The KIE on proton mobility and diffusion in water is 1.45, close to √2. The fundamental time-step associated with proton mobility and diffusion in water (DH+ ≃ 10−4 cm2/s) in a proton-hopping model is ∼1.5 ps,23 which is longer than the ESPT from QCy7 to water. Thus, the PT between water molecules is a slower process than between excited QCy7 and water. (1) Main Findings. The steady-state emission spectra of the three superphotoacids, QCy7, S-QCy7, and TSQCy7, upon excitation from their ground-state phenol, ROH, exhibit a dual emission band. We attribute the band with the peak at 540 nm to the RO*H and the near-IR band of QCy7, S-QCy7, and TS-QCy7, positioned at 695, 705, and 715 nm, respectively, to the deprotonated phenolate, RO*−. (2) The fluorescence up-conversion RO*H signals measured in the spectral range of 500−620 nm decay fast, whereas the signals of the RO*− emission band at the near-IR region rise and then decay for a relatively long time.

Figure 3. Comparison of the time-resolved emission of the ROH form measured at 540 nm of QCy7 sulfonate derivatives in both H2O and D2O: (a) S-QCy7; (b) TS-QCy7.

These signals’ decay profiles at short and long wavelengths in conjunction with the dually peaked steadystate emission spectra strongly indicate that ESPT to the solvent is taking place. (3) Fitting of the RO*H emission band to a multiexponent function reveals short, intermediate, and long decay components. The short time component is ∼100 fs long, which we attribute to solvation dynamics occurring before the ESPT process. The properties of the intermediate and long decay components vary with the QCy7 compound. The intermediate decay times of TSQCy7, S-QCy7, and QCy7 are 450, 550, and 650 fs, respectively. We attribute these components to the ESPT process to the solvent. The ESPT rate coefficients, kPT, of the three dyes are three to four times larger than that of NM6HQ+. (4) The rise-time of the RO*− up-conversion signal of the three molecules compliment the decay of the RO*H signals, thus confirming the existence of the photoprotolytic process in the excited-state. (5) The KIE on kPT for QCy7 is 1.7, and for its sulfonated analogues, its value is 1.4 ± 0.1. This value is similar to the KIE on the proton diffusion in water but, at the same time, much smaller than the KIE measured on the kPT of much weaker photoacids (pKa* > 0), which was ∼3. 7357

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Figure 4. Steady-state emission spectra of S-QCy7 in several acidic solutions. Note that the RO*− band at 700 nm at 10 M aqueous solution of HCl still exists.

Origin of the Large Photoacidity of QCy7 Photoacids. Phenol is a rather weak photoacid (pKa* = 3.6).24 The QCy7 molecule, which has a phenol moiety, has a particularly large charge transfer in its deprotonated form because of the two quinoid resonance structures shown in Scheme S1 in the Supporting Information. We attribute the strong photoacidity of QCy7 and its sulfonated derivations to the stability the unique structure of the deprotonated form grants it. The experimental results show unequivocally that QCy7 is a super photoacid, from which ESPT occurs even before the solvent reorganization and solvation processes are fully completed. Weller25 was the first to propose that intramolecular charge transfer (ICT) by π electron cloud redistribution takes place upon excitation of an aromatic photoacid’s RO*H form, and this phenomenon may lead to enhanced photoacidity in the excited-state. Agmon et al.26 performed QM calculations on 2naphthol and its cyano derivatives on both of their protonated and deprotonated forms. Their calculations did not consider, however, the important solvent effect, and therefore, their conclusions have only a qualitative value. They found that, upon excitation, an ICT occurs, that the electronic charge density on the oxygen atom is smaller in the S1 state, and that this drop in charge density is much larger in the RO*− form than in the RO*H form. Moreover, their calculations showed that the carbon atoms located at positions 5 and 8 on the aromatic distal ring are negatively charged in the S1 state of the RO*− form. This explains the larger photoacidity of 5,8dicyano-2-naphthol (pKa* = −4.5) and 5-cyano-2naphthol (pKa* = −2) compared to 6-cyano-2-naphthol (pKa* = 0.6). The fact that the length of the C−O bond shortens with the stronger the photoacid in both the ground and excited-states offers an additional support for the correlation between ICT and photoacidity. The new QCy7 photoacids can transfer a proton in their excited-state from the phenol functional group. A distinctive change of the π-electrons system leads to generation of a cyanine dye and formation of a phenolate active donor, which can now donate a pair of π-electrons to either one of the conjugated acceptors. The ICT state generates a resonance species of a new donor−acceptor pair with a π-electron pattern. The donor capability of the phenolate species can be masked

either by a proton or by a specific protecting group. Such protected phenol can be used as a molecular probe for detection or imaging of a specific analyte, which has the chemical reactivity for removing the protecting group. Reversible Photoacidity of QCy7 and Its Derivatives. Reversible geminate recombination is a process in which the proton that was transferred to the solution returns to the molecule and reforms the protonated excited RO*H form. The excited protonated molecule can then undergo a second proton transfer process. Consequently, within the excited-state lifetime, a quasi-equilibrium is maintained between the RO*−, RO*H, and H3O+ species. Geminate recombination of a proton with an irreversible photoacid leads to a protonated ground-state form of the photoacid, ROH(g). Distinguishing the reversible from the irreversible photoacid is done by measuring its steady-state fluorescence in the presence of excess protons in the solution. When a photoacid is titrated with a strong acid, the RO*H and RO*− emission bands’ relative and absolute intensities depend on the acid concentration.27 If the photoacid titrated is reversible, the intensity of the RO*H will increase and that of the RO*− will decrease, as the titration progresses. At low acid concentrations, pH > 3.5, the diffusion-controlled recombination rate is too slow for detection by the fluorescence measurements due to the shortness of both the ROH and RO− excited-state lifetimes (ns). Therefore, only at pH levels below 3 can an acid effect on the dual band steady-state emission spectrum be detected. Irreversible photoacids exhibit a strong reduction in the intensity of the RO*− emission band and a minor impact on the RO*H band during titration. Figure 4 shows the steady-state emission of S-QCy7 in several acidic solutions of varying HCl concentrations. As seen in the figures, the intensity of the RO*− band decreases, whereas that of the RO*H band increases with the more acid introduced into the solution. The excited-state lifetime of QCy7 and the two derivatives is short, only a few hundred picoseconds, because of the photoconversion to spiro compounds. For 8-hydroxy-1,3,6pyrenetrisulfonate (HPTS), a well-studied reversible photoacid with pKa* value of ∼1 and kPT value of 1010 s−1, 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 7358

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RO*H repopulation, the fluorescence decay follows a power law of t−d/2, where d is the dimension of the proton diffusion space. In the past, we have seen that, in bulk solutions, RO*H molecules decay with a long-time tail obeying the t−d/2 rule. In the solution’s bulk d = 3 with t−1.5. Recently, we measured28 the decay rate of the RO*H form of HPTS in a confined volume. When HPTS molecules were introduced into oxidized porous silicon, we found that the fluorescence tail of the RO*H decay scales with a power law, whose exponent was 1.2 ± 0.1 instead of the expected 1.5 naturally occurring in homogeneous solutions. We therefore concluded that the narrow pores in the porous silicon whose average radius was 10 nm had a dimensionality lower than 3. The geminate recombination process of photoacids with protons deviates from the power law in 3-D space such as a bulk solution because of geometric constraints prevailing inside confined volumes. Figure 5a−c shows the RO*H fluorescence decay of the three homologue QCy7 molecules on linear, semilogarithmic, and log−log scales. In the latter, a power law can be readily found if the plot descends in a straight line. As seen in the figure, the RO*H fluorescence does not decay exponentially but rather follows a power law given a reasonable approximation. We added an auxiliary line to the figure that describes a biexponential decay with a 100 fs component, whose amplitude is 0.3, and a 650 fs component, with an amplitude of 0.7. Data analysis of the QCy7 fluorescence decay shows that these are the values of the two fast components in its RO*H decay measured at 560 nm. The scaling exponents of QCy7, S-QCy7, and TS-QCy7 are −1.45, −0.89, and −0.82, respectively. Moreover, the amplitude of the long-time component is much larger in the S-QCy7 and TS-QCy7 decay profiles than in that of QCy7. S-QCy7 and TSQCy7 both have two propylsulfonate functional groups close to the phenol, from which the proton is supposed to dissociate and also recombine. These groups limit the access of the proton to this exit/entrance route. The sulfonate groups at the tip of the propyl groups draw the proton while also organizing the water molecules in their vicinity. The net result of this interference is the enlargement of the amplitude of the fluorescence tail and a dramatic reduction in the slope of the power law decay. The drop in the value of the scaling exponent from 1.45 to 0.89 and 0.82 indicates that there is a momentous change in the dimensionality of the space in which the proton moves to and from the molecule. Under the assumption of spherical symmetry the scaling exponent is given by d/2. This means that the effective dimension around S-QCy7 and TSQCy7 is fractal and equals 1.78 and 1.74, respectively. The propylsulfonate groups can rotate in a conical fashion about the dihedral bond. This rotation also contributes to the determination of the scaling exponent. Another plausible explanation is that a large fraction of the protons emitted from the phenol recombines first with the sulfonate headgroup and then, on a slower time scale, is released to the bulk water, and since the distance between the sulfonate and the oxygen of the phenolate is close, the proton then recombines efficiently to the phenolate to reform the RO*H.

orders of magnitude. Thus, we conclude that the QCy7 and SQCy7 molecules are reversible photoacids. Long-Time Fluorescence Tail. The second, intermediate time-component of the multistretched exponential fit of the RO*H signal qualitatively provides the ESPT rate coefficients in H2O and D2O. The third, long-time component of the fit approximates the long-time fluorescence tail of the QCy7 compounds. This tail indicates that these superphotoacids are reversible. The major decay time component of QCy7 is 650 and 1100 fs in H2O and D2O respectively, and we attribute it to ESPT. The fitting parameters of the time-resolved emission of QCy7, S-QCy7, and TS-QCy7 in H2O to multistretched exponential analysis are given in Tables S1, S2 and S3 of the Supporting Information, respectively. The mathematical treatment of the reversible photoprotolytic cycle in neutral pH was given by Agmon, Pines, and Huppert,14 and it is based on a kinetic equation coupled with the Debye− Smoluchowski equation (DSE). This kinetic equation describes the photoacid’s dissociation by a proton transfer to water and proton geminate recombination. Also, it deals with the RO*− and H3O+ pair distribution function using the DSE. The potential in use has a spherical symmetry despite the asymmetric nature of the recombination process in all photoacids studied so far. In the spherically symmetric description of the problem, both proton transfer and recombination reactions take place on the surface of a reaction sphere with radius a. In our previous article20 on the timeresolved spectroscopy of QCy7, we reported that the fluorescence decay of the RO*H is not exponential but follows a power-law of t−1.45 at relatively short times between 2 ps to ∼100 ps. In previous studies we measured the proton dissociation rate of weaker photoacids for which the ESPT rate is slow, and thus, we could examine only the long-time fluorescence tail on much longer time scales ranging from 50 ps to 10 ns.7,14 The long-time fluorescence tail of QCy7 and its two sulfonated derivatives could in principle be fitted by the model using the proper initial and boundary conditions. Unfortunately, we have at our disposal only a spherically symmetric DSE program, whose distance coordinate between the proton and the RO*− center is the only variable, and therefore, a problem in 3-D diffusion space becomes onedimensional. The QCy7 molecule in its protonated forms, ROH and RO*H, has one positive charge on each of the indolium rings, while the ground and excited deprotonated forms, RO− and RO*−, have a total of one positive charge. The size of the photoacid molecule (see Scheme S1, Supporting Information) is much larger than the 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. According to the geminate recombination model, a reversible process in which the back protonation leaves the molecule in the excited ROH form, the RO*H population does not decay exponentially with the coefficient k PT throughout the experimental time window, but it rather decays exponentially with kPT, only at the beginning, and afterward because of the

HO3SRO *− ⇄ O3−SRO * H ⇄ O3−SRO *− + H+

CIP As the First Step in ESPT. The existence of the CIP species was first suggested by Eigen29 in the early 1960s as an intermediate step in the dissociation process of an acid: HA ⇄ H+····A− ⇄ H+ + A−. We, therefore, suggest that a considerable 7359

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including HPTS, and they concluded that the short 3 ps component arises from slow charge rearrangement rather than formation of a CIP. We wish to stress how important it is to study this class of photoacids, due to it having the highest ESPT rate in record thus far. In this study, we compare the spectroscopic properties of the three photoacids shown in Scheme 1 belonging to the QCy7 family of molecules, characterized by a quinoid shape. All three molecules have nearly the same level of photoacidity; they transfer a proton to the solvent at an amazing rate of about 1.5 × 1012 s−1. At first, the proton is probably transferred to an H2O molecule hydrogen bonded to the hydroxyl group at the central phenol, thus creating a CIP. In the second step, the proton is transferred to an adjacent water molecule, completing the transfer, and thus creating a noncontact RO*− and H3O+ ion pair. From that point onward, proton transfer between water molecules is a diffusion-driven process with a diffusion coefficient of DH+ ≃ 10−4 cm2/s. Proton diffusion in water occurs via proton transfer between neighboring water molecules and not H3O+ migration in solution. Quantum mechanical computer simulations were carried out by two primary methods. An accurate though timeconsuming method is the Car−Parrinello method,31 while a less accurate but time-saving method based on empirical VBE parameters is used mostly by Voth et al.32 and other groups. A diffusional proton transfer in which there is no directionality at times much longer than a transfer process of a single proton can be described as a random walk, where the relevant parameters are the average distance hopping step, ⟨L⟩, and average time for a single hop between two neighboring molecules, τhop. Since the average distance between oxygen atoms of two neighboring water molecules is ∼2.7 Å, ⟨L⟩ can be taken to be this value. The relationship between the diffusion coefficient and average distance of a single step in three dimensions is given by L2 = 6Dt

(3a)

and for an elementary step, the average time is therefore given by

τhop =

L2 6D

(3b) −4

For liquid water, DH ≃ 10 cm /s, and therefore, τhop = 1.5 ps. Proton transfer between QCy7 and water is roughly twice as fast as an elementary step of a proton between two water molecules. In addition to the fast ESPT rate as aforesaid, we also found a significant fluorescence tail in the decay profiles of all three molecules. This tail is best fitted to a power law function from very early times after excitation of the molecule, i.e., as early as 2 ps. In our previous study,20 we explained the presence of the fluorescence tail at very short times. We mentioned earlier that τhop = 1.5 ps in water. A proton found three water molecules away from the RO*− will arrive to the latter position, according to eq 3b, after about 13 ps. This means that a fluorescence tail appearing in the decay profile after only 2 ps requires that a proton approaches the RO*− from a closer source. By default, a large part of the tail stems from the geminate recombination with the proton when the molecule is still in a CIP state. Can the optical spectroscopy QCy7 molecules provide a certain proof of the existence of the CIP as an intermediary step in the dissociation of acids in general and photoacids in particular? The chemical equation for acid dissociation HA ⇄ +

Figure 5. Comparison of the time-resolved emission of the ROH form of measured QCy7 and the sulfonate derivatives at 540 nm: (a) linear scale; (b) semilogarithmic scale; (c) log−log scale. Note the long-time fluorescence tail; see text.

part of the deviation of the QCy7 RO*H form fluorescence decay from exponentiality arises from the geminate recombination of the CIP and not the recombination of RO*− with a relatively distant proton. A few years ago, we proposed30 a three-phase model explaining the dissociation of a photoacid in which the intermediate phase was a CIP. The transient absorption spectrum of HPTS has a distinct ∼3 ps component, whose peak is at ∼540 nm. This component is indicative of an intermediate step in the photoacid’s dissociation, which we named as CIP. However, Fayer and co-workers8 measured the transient absorption of a number of pyrene derivatives, 7360

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H+····A− ⇄ H+ + A− can be approximated by a kinetic equation describing consecutive reversible first-order chemical reactions as follows: k1

k3

k2

k4

A⇄B⇄C+D

(4)

where D denotes the diffusing proton, and k4 is a second order reaction rate constant. In the Supporting Information, we use an approximate analytic solution that solves a simpler reaction scheme where the back recombination reaction of C + D to reform B is replaced by an effective first order rate constant. We fit the decay curve of the RO*H, designated as A, and the emission at 710 nm, of the RO*−, designated as C. We assumed that the emission band of the intermediary species, the CIP, designated as B, will have a higher energy than C but lower than A. We also assumed that the position of B in the spectrum will be closer to the peak of the RO*−. Given these assumptions, we determined the peak of B at 15 000 cm−1, 1000 cm−1 higher than that of C. The solution to this kinetic scheme predicts that the RO*H will decay biexponentially. The fast component is affected by the ESPT rate coefficient kPT (k1) as well as by the geminate recombination rate ka (k2). B is formed with a reaction rate coefficient of kPT + ka and disappears at a rate largely governed by the sum of k3 and k4. Component C (RO*−) is formed at a rate of k3. The reaction rate coefficient of the diffusing proton in the solution with C, k4, is slower than the other reactions in Scheme S2 in the Supporting Information. The computed time-resolved spectra of QCy7 and S-QCy7 using the ABC model are given in Figures S3 and S4, respectively, in the Supporting Information. Figure 6 shows the constructed time-resolved emission spectra of S-QCy7 and TS-QCy7 at several times in the range of 50 fs to 5 ps. We constructed the spectra as follows. We assumed that, at 250 fs, the spectrum mainly consists of the RO*H emission and a smaller contribution from the RO*−. The shapes of the RO*H and RO*− emission bands at 100 fs are assumed to be roughly the same as those measured with the steady-state fluorometer. This assumption somewhat distorts the constructed spectra since it does not fully account for spectral shifts associated with solvation dynamics. Fortunately, the spectral shifts in the case of QCy7 are relatively small. This is shown in Figure 2, where the fluorescence up-conversion signals of the RO*H band (520−570 nm) are nearly identical. When the change in solvent reorganization energy is low, the time-dependent band shift is small, and the approximation is justified. The spectra shown in the figure were constructed from the time-resolved emission signals of QCy7 in water, and sampled in 10 nm intervals in the spectral range of 520−730 nm. As seen in the figure, the RO*H band shifts by ∼500 cm−1 within the first 400 fs after excitation, and its intensity diminishes with time. The time-resolved spectra of all three molecules have similar time dependence and spectral behavior. The intensity of the RO*H band decreases over time, whereas that of the RO*− band increases. The rates at which the RO*H decreases and the RO*− increases are similar. We therefore conclude that all three compounds undergo ESPT to the solvent at similar rates. We also constructed time-resolved spectra of QCy7 and its sulfonated derivatives under a different assumption, as previously described. We assumed that, after 1.5 ps, most of the molecules had undergone a photocycle, that the RO*H concentration is low, and that part of the CIP molecules had

Figure 6. Constructed time-resolved spectra of QCy7 and the sulfonated derivatives: (a) S-QCy7; (b) TS-QCy7.

also dissociated to RO*−. We measured the steady-state emission spectrum of QCy7 at a pH level of ∼4. The transient spectrum at 1.5 ps was fitted to both the RO*H and RO*− bands. By using the spectral log-normal function, we assumed for the fitting that ∼30% of the molecules were in RO*H form and 70% were in the RO*− form after 1.5 ps. Both frames in Figure 6 show that the RO*H band at ∼18 000 cm−1 decays, whereas the RO*− band at ∼14 000 cm−1 rises. Figure 6 indicates that, at short times (up to 1 ps), the RO*− band is located at 15 000 cm−1 rather than at 14 000 cm−1 at longer times (t > 1 ps). This time-dependent red band shift could be attributed to solvation dynamics or to formation of a contact ion-pair as a first step in the acid dissociation process. The simulations of the time-resolved spectra using the ABC model show a distinctive difference between the simulation and the time-resolved spectra based on actual experimental timeresolved emission data, shown in Figure 6. The ABC simulations shown in Figure S3, Supporting Information, clearly indicate the existence of an iso-emissive point at about 14 400 cm−1, whereas the experimental data failed to show such a point. A clear indication of transformation of the CIP to RO*− and a separated hydronium ion is an iso-emissive point. The quantitative analysis of the time-resolved spectra could not conform unequivocally the existence of a large fraction of the deprotonated RO*− form as contact ion-pair that emits as a 7361

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separate emission band with a maximum at 15 000 cm−1 about 1000 cm−1 blue-shifted from the steady-state RO*− emission band. We are currently studying the intricate work of verifying the fingerprint of the CIP as an intermediate species in the acid dissociation reaction. At this time, we are not yet certain that the time-resolved spectra in Figure 6 are an unequivocal example of a CIP spectrum. Further study is needed to certify this. Nonexponential Dynamics. The nonexponential fluorescence decay of the RO*H in the first few picoseconds may also be explained as follows: for a solvent-controlled reaction, nonexponential kinetics may be more probable than exponential reaction kinetics. If the ESPT from QCy7 is nonexponential, a part of the long-time nonexponential fluorescence tail of the RO*H signal of QCy7 displayed in Figure 5b arises from the ESPT process rather than 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. The nonexponentiality of the photoacid dissociation dynamics can be indirectly found in Ando and Hynes’s simulation of HCl dissociation.33 The mechanism involves two consecutive steps: first, a nearly activationless motion in a solvent coordinate, which is adiabatically followed by the proton rather than tunneling, to produce a contact ion pair Cl−−H3O+, which is stabilized by ∼7 kcal/mol. Solvent motion was found to be highly nonexponential in solvation experiments and IR spectroscopy, and since the ESPT process rate depends on the solvent rearrangement, it is also nonexponential. The second step involves motion in the solvent coordinate with a small activation barrier and a second adiabatic proton transfer that produces a solvent-separated ion pair from the CIP in a nearly thermo-neutral 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 proton-accepting water molecule is indicated as a key feature in the necessary solvent reorganizations.

proton’s motion during recombination with the phenolate. Reversible geminate recombination, assuming spherical symmetry, predicts a power law decay curve for the RO*H species fluorescence, t−d/2, where d is the dimension of the diffusion space. We found that the three decay curves of the three photoacids could reasonably be fitted by a power-law plot. The scaling exponents of the best fits were 1.45, 0.89, and 0.82 for QCy7, S-QCy7, and TS-QCy7, respectively. Namely, the power law decays suggest that the proton diffuses in a nearly 3-D space in a QCy7 sample, whereas, in the S-QCy7 and TS-QCy7 samples, the proton diffuses in space of effectively reduced dimensionality around the photoacids. The difference in values arises from interference from the propylsulfonate groups. The proton can recombine with the phenolate only if it approaches it from a certain direction, and this leads to the reduction in the dimensionality of the diffusion space.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic schemes and experimental procedures; data from QCy7 measurements; three-step ESPT model; multiexponential fitting parameters of QCy-7, S-QCy7, and TS-QCy7. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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

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ABBREVIATIONS USED ACN, acetonitrile; NaOAc, sodium acetate; MeOH, methanol; K2CO3, potassium acetate; Ac2O, acetic anhydride; AcOH, acetic acid; DMSO, dimethylsulfoxide

SUMMARY Time-resolved and steady-state spectroscopy techniques were employed to study the intermolecular ESPT process from recently synthesized quinone cyanine dyes that were also used for their near-IR fluorescence properties in imaging experiments.19 These three superphotoacids, QCy7, S-QCy7, and TSQCy7, shown in Scheme 1, have the same quinoid structure yet differ in their functional groups. Titration of QCy7 with HCl revealed that it is a reversible photoacid for which recombination with the proton leads to the reformation of the RO*H form, which may undergo a second photoprotolytic cycle. The ESPT rate from all three photoacids is ultrafast, as the ESPT rate coefficients are 1.5, 1.8, and 2.2 × 1012 s−1 for QCy7, S-QCy7, and TS-QCy7, respectively, the fastest reported values in the literature thus far. The amplitude of the long-time fluorescence tail increases with the number of sulfonates in the molecule. This observation is explained in terms of the attractive electric potential governing the proton’s diffusive motion near the phenolate moiety of both S-QCy7 and TS-QCy7. Moreover, the amplitude of the tail is not only influenced by the charges of the groups but also by steric considerations. The propylsulfonate groups located on both sides of the phenol inhibit the

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REFERENCES

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