Excited-State Proton Transfer and Proton Diffusion near Hydrophilic

Nov 6, 2013 - Time-resolved emission techniques were employed to study the reversible proton photoprotolytic properties of surface-attached 8-hydroxyp...
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Excited-State Proton Transfer and Proton Diffusion near Hydrophilic Surfaces Hagit Peretz Soroka, Ron Simkovitch, Alon Kosloff, Shay Shomer, Alexander Pevzner, Omer Tzang, Reuven Tirosh, Fernando Patolsky, 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: Time-resolved emission techniques were employed to study the reversible proton photoprotolytic properties of surface-attached 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) molecules to hydrophilic alumina and silica surfaces. We found that the excitedstate proton transfer rate of the surface-linked HPTS molecules, in H2O and D2O, is nearly the same as of HPTS in the bulk, while the corresponding recombination rate is significantly greater. Using the diffusion-assisted proton geminate-recombination model, we found that the best fit of the time-resolved fluorescence (TRF) signal is obtained by invoking a twodimensional diffusion space for the proton to recombine with the conjugated basic form, RO−*, of the surface-linked HPTS. However, we obtain an excellent fit by a three-dimensional diffusion space for diffusional HPTS in bulk water. These results indicate that the photoejected solvated protons are confined to the surface for long periods of time. We suggest two plausible mechanisms responsible for two-dimensional proton diffusion next to hydrophilic surfaces.

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INTRODUCTION Photoacids are aromatic organic molecules that display properties of weak acids in their ground electronic state, but of acidity greater by many orders of magnitude in their first excited electronic state. Thus, photoexcitation to the excited state, by short UV−vis laser pulses, enables one to follow their photoprotolytic processes. Excited-state intermolecular proton transfer (ESPT) from the acidic group of the excited photoacid to a nearby solvent molecule1−13 has been widely researched. Hydroxyaryls are a class of photoacids with a large range of photoacidity. 2-Naphthol is a rather weak photoacid with pKa ∼8.5 and pKa* ∼2.7 in water. The intermolecular excited-state proton transfer (ESPT) rate constant kPT is 108 s−1, whereas the excited-state radiative rate is larger, kr ≅ 2 × 108 s−1, and thus the PT efficiency is about 1/3. 1-Naphthol is a much stronger photoacid with pKa* ∼ 0.4 in water and kPT ∼ 3 × 1010 s−1. Once the proton is transferred to the surrounding water, forming a hydronium ion H3O+(H2O)n, it hops between the water molecules at a fast rate of 6 × 1011 s−1 (τPT ∼ 1.5 ps).7,14 The proton diffusion coefficient in water, ∼9.1 × 10−5 cm2/s, is quite large in comparison to the diffusion constant of hydrated Na+ ion, for which the diffusion constant is 7 times smaller. The deprotonated form of many photoacids, RO−*, is negatively charged, and thus the proton-to-RO−* geminate recombination probability is considerably large. The geminate recombination process is an important process and is easily detected by the time-resolved emission of the ROH* form of the photoacid. In reversible photoacids the geminate recombination process leads to an excited photoacid ROH* that can undergo a second proton dissociation, and the cycle of recombination and dissociation may go on until the radiative decay brings the photoacid to the inactive ground state. The geminate © 2013 American Chemical Society

recombination in irreversible photoacids, like 1-naphthol, leads to the formation of ROH(g) in its ground state and thus terminates the photocycle. The proton dissociation and the diffusion-assisted GR process modify the time-resolved emission of both the ROH* and RO−* forms. A more detailed and quantitative description of the photoprotolytic processes is given in the next section and in the references therein. 8-Hydroxypyrene-1,3,6-trisulfonate (HPTS) is a commonly used photoacid with a ground state pKa of ∼7.4, an excited state pKa* of ∼1.3, and a kPT = 1010 s−1 in H2O. The ROH emission is broad and reaches its maximum at 445 nm in water, whereas the RO− narrower emission band appears at 512 nm. The S0− S1 ROH band absorption peak is at ∼405 nm, and it also absorbs at shorter wavelengths down to about 330 nm. The RO−* form is highly negatively charged since it contains three sulfonate and one phenolate groups. The geminate recombination probability of HPTS is large due to the intrinsically large Coulombic potential, and therefore it is easily detected by the fluorescence long-time tail of the ROH emission created by the GR process. In a previous article, Patolsky and co-workers covalently linked HPTS through APTES linker molecules to silicon nanowires.15 They showed that upon photoexcitation of these devices they could detect a photoresponse due to the pH jump by proton transfer to the solvent by the surface-linked HPTS molecules. In these optically gated nanoelectrical devices, the photoactive molecular layer serves as the “gating agent”, which Received: September 1, 2013 Revised: November 6, 2013 Published: November 6, 2013 25786

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strongly on the electrical potential existing between it and the deprotonated form. The diffusion-assisted geminate recombination of the RO−* with the proton could be quantitatively described with the use of the numerical solution of the DSE under the initial and boundary conditions of the photoprotolytic process. In addition, the fluorescence lifetimes of all excited species are considered, with 1/k0 = τROH for the acid and 1/k′0 = τRO− for the conjugate base. Generally, k′0 and k0 are much smaller than both the proton-reaction and the diffusion-controlled rate constants. The amplitude of the longtime fluorescence tail of ROH* depends on the intrinsic rate constants, ka and kPT, on the proton-diffusion constant, DH+, and on the electrical potential between RO−* and the proton.

alters the surface potential of the devices, thus modulating their current flow. In recent articles by Pohl et al.,16,17 it was found that the 1D proton diffusion coefficient next to hydrophobic surfaces is similar to that of the bulk. Their result was unexpected, since previous indications showed that the proton surface diffusion is considerably smaller than the bulk diffusion. In the current study, we covalently linked a HPTS molecule derivative to alumina and silica hydrophilic surfaces through coupling to an APTES linker. Our motivation in the current work is to study the reversible photoprotolytic processes next to hydrophilic surfaces. Both proton transfer and geminate recombination processes are sensitive to the proton accepting medium. In previous studies we found that both the ESPT and the GR processes from HPTS are sensitive to the solvent, solvent mixtures, and ions in solution. The ESPT process does not take place in HPTS in alcohols and other hydrogen bonding solvents. The ESPT rate of HPTS decreases by more than 2 orders of magnitude in saturated MgCl2 aqueous solution. The proton diffusion in methanol is smaller by a factor of 2.5 than in H2O, and thus the GR rate is also modified. Here, we find that the ESPT rate of surface-linked HPTS, on both silica and alumina surfaces, is similar to that of HPTS in bulk H2O and D2O. However, the geminate recombination of surface-linked HPTS on these hydrophilic surfaces is largely modified, and the time-resolved fluorescence tail amplitude is considerably larger than in bulk water. Using the diffusion-assisted GR model to fit the time-resolved fluorescence signals of ROH* reveals that the proton diffusion space is two-dimensional, rather than threedimensional as that of bulk proton diffusion. From the analysis of the time-resolved emission of linked HPTS, we estimate that the proton diffusion coefficient next to the alumina or silica surfaces is about the same as in the bulk. Reversible and Irreversible Photoprotolytic Cycles of Photoacids. Excitation of a photoacid solution of pH lower than its ground-state pKa generates a vibrationally relaxed, electronically excited ROH molecule (denoted by ROH*) that initiates a photoprotolytic cycle (Scheme 1). Proton dissocia-

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EXPERIMENTAL SECTION Preparation of 8-Acetoxypyrene-1,3,6-trisulfonyl Chloride. Trisodium 1-hydroxypyrene-1,3,6-trisulfonate (20 g, 0.038 mol) was dissolved in a solution of NaOH (2.4 g, 0.06 mol) in water (30 mL) and then cooled to about 0 °C. Acetic anhydride (5 g, 4.8 mL, 0.48 mol) was added dropwise to the solution, and the reaction mixture was stirred for 2 h. Ethanol (20 mL) was added to complete a precipitation; the precipitate was collected by filtration, washed with ethanol (3 × 10 mL), and dried under reduced pressure for 24 h to obtain a yellow solid (17 g, 78.7% yield). A mixture of trisodium 8acetoxypyrene-1,3,6-trisulfonate (5 g, 0.0088 mol) and toluene (150 mL) was placed in 0.25 L round-bottomed flask, equipped with an automatic water separator (Dean−Stark trap) and a condenser. The mixture was heated under reflux for 2 h to dry the reaction mixture. Then it was cooled to 60 °C, and oxalyl chloride (6 mL) and DMF (2 drops) were added. The mixture was heated under reflux for 8 h, and a mixture of toluene and oxalyl chloride excess (30 mL) was distilled. A precipitate of sodium chloride was collected by filtration, and the solvent was removed from the filtrate under reduce pressure. A solid residue was dried in vacuum for 24 h to give the trichloride derivative (4 g, 81.5% yield). MS (m/e): [M] + 555.8 (C16H7Cl3O7S3). Deposition of Alumina Layers on Silicon Substrates. Alumina films, 50 and 200 nm, were deposited on silicon substrates by atomic layer deposition (ALD), using a Savannah 100 reactor (Cambridge NanoTech Inc.) at a growth temperature of 200 °C, at 10−2 Torr. We used trimethylaluminum [Al(CH3)3] (TMA) and high-purity water as the aluminum and oxygen sources, respectively. Nitrogen (99.9999%) was used as carrier and purging gas. Typical pulse lengths of both precursors were maintained at a predetermined value of 0.015 s, and the gap between pulses was maintained at 10 s. The growth rate is ∼2.1 nm per 10 cycles. Surface Chemical Modification of Silica and Alumina Surfaces with Sequential Layers of APTES and HPTS. Prior to the chemical modification, the silicon wafers were cleaned with acetone and isopropyl alcohol, followed by oxygen plasma treatment and dehydration of the surface for 1 h at 115 °C for effective chemical modification of the hydroxylterminated surface. Silicon wafers and silicon wafers covered with an alumina layer were first chemically modified with an APTES layer (Gelest) (Figure 1(1)). A solution of 1% (v/v) APTES in ethanol/H2O (95%/5%) at pH 5 was prepared and allowed to stand for 20 min before filtering through a 0.2 μm cutoff syringe filter. The wafers were then immersed in the silane/ethanol solution for 1 h, followed by a thorough rinse with isopropyl alcohol. Finally, the substrates were dried in a

Scheme 1. Photoprotolytic Cycle

tion, with an intrinsic rate constant kPT, leads to the formation of an ion-pair RO−*···H3O+ that subsequently forms an unpaired RO−* and a solvated proton, which diffuses into the bulk of the solvent. The proton and the RO−* may recombine via reversible (adiabatic) recombination with a rate constant ka and re-form 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 groundstate ROH. 1-Naphthol and its derivatives are known to exhibit considerable fluorescence quenching of the deprotonated form, RO−*, in acidic aqueous solutions. Removal of an ion pair from the contact radius, a, to infinity is described by the transient numerical solution of the Debye− Smoluchowski equation (DSE).18,19 The motion of the transferred proton in water near the photoacid depends 25787

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Figure 1. Preparation and binding of 8-acetoxypyrene-1,3,6-trisulfonyl chloride to silica and alumina surfaces. (a) Synthesis of the activated derivative of the photoactive molecule HPTS, 1-hydroxypyrene-1,3,6-trisulfonate. (b) Silica and alumina surfaces chemically modified by the activated HPTS derivative molecules. (1) Silica and alumina surfaces are first modified with 3-aminopropyltriethoxysilane (APTES) layer, followed by their reaction with 8-acetoxypyrene-1,3,6-trisulfonyl chloride (HPTS activated derivative) (2). (3) Acyl-protecting groups are removed with sodium acetate for the exposure of the phenol function group.

stream of nitrogen and baked at 115 °C. Afterward, the substrates were modified with 8-acetoxypyrene-1,3,6-trisulfonyl chloride for 1, 4, 8 and 16 h (Figure 1(2)). Lastly, the hydrolytic exposure of the phenol function group was followed by a through rinse with isopropyl alcohol (Figure 1(3)). QCM Measurements. A home-built QCM analyzer equipped with a Fluke 164T multifunction counter was used for the microgravimetric quartz-crystal-microbalance experiments. Quartz crystals (AT cut, 9 MHz) sandwiched between two Au electrodes (roughness factor ca. 3.5 with an area of 0.196 cm2) were used in microgravimetric experiments, after deposition of a thin silicon oxide layer of 5 nm by PECVD deposition. Quartz electrodes were cleaned with a piranha solution (70% H2SO4:30% H2O2) for 15 min, then rinsed thoroughly with DDW, and dried with a stream of argon before chemical modification steps. Measurements of Time-Correlated Single-Photon Counting (TCSPC). Measurements of time-correlated singlephoton counting (TCSPC) were performed with the use of excitation from a cavity-dumped titanium:sapphire femtosecond laser (Mira, Coherent), which provides short, 150 fs pulses at approximately 800 nm. The second harmonic of the laser, operating over the spectral range of 380−420 nm, was used to excite the samples. The cavity dumper operated with a

relatively low repetition rate of 800 kHz. The TCSPC detection system was based on a Hamamatsu 3809U photomultiplier and an Edinburgh Instruments TCC 900 computer module for TCSPC. The overall instrument response was approximately 40 ps (full width at half-maximum, fwhm). The fluorescence was collected by the front surface configuration for bulk samples of high concentrations (∼10−4 M) placed in 4 mm path length optical cell. The wafer samples were placed in a home-built optical cell. The water thickness in front of the wafer was about 1.6 mm. The wafer, single monolayer modified HPTS samples were irradiated at much stronger intensities of about 100 times than bulk samples, i.e., 0.5 nJ/pulse energy of SHG. The excitation pulse energy was reduced to about 5 pJ by neutraldensity filters. The fitting of the TCSPC data was carried out by the Spherical Symmetric Diffusion Problem (SSDP) program, developed by Krissinel and Agmon.20 Spectroscopy Characterization. A Ti-sapphire laser (Tsunami, Spectra-Physics) in a picosecond configuration with central wavelength of 800 nm and 80 MHz repetition rate was frequency doubled using a BBO nonlinear crystal to produce excitation at 400 nm. Raman measurements were performed by a home-built scanning Raman microscope, based on the Olympus BX41M-LED microscope. The scattered light 25788

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geminate-recombination probability of the transferred proton with the RO− species. Figure S1 shows the background and emission spectra of three samples under the same irradiation and signal recording conditions. The three samples represent the bare 50 nm alumina substrate immersed in buffered aqueous solution, an APTES-modified substrate, and HPTS-linked substrate. All three samples show a dark count of about 1000 counts. The APTES-covered sample shows a weak emission spectrum of about 200 counts in the spectral region of 420−600 nm. The emission intensity of the modified HPTS under the same conditions is about 4000 counts, and thus the APTES signal is 0.05 of that of the HPTS. Time-Resolved Emission of Alumina Samples. Figures 3a and 3b show the time-resolved emission of the ROH and

collected by a 0.7 NA objective (LCPLFLN50xLCD). A notch filter (Edmund 405 nm) attenuates the Rayleigh scattered light. Spectra were obtained by a spectrograph (Andor Shamrock model SR/303iA) and a cooled CCD camera (Andor IDUS model DU401A/BR/DD). A 150 lines/mm diffraction grating was used to cover the wavelengths 400−900 nm. Calibration was performed using a Hg/Ar calibration light (HG1, Ocean Optics). The laser spot on the sample had a Gaussian profile with a full width at half-maximum of 0.8 μm. The average power was set to 10 μW to avoid sample damage and minimize photobleaching.

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RESULTS Steady-State Emission. Figure 2 shows the timeintegrated (steady-state) fluorescence of 8-hydroxy-1,3,6-

Figure 2. Steady-state emission spectrum of bulk diffusional HPTS in H2O solution and surface-linked HPTS to alumina surface (see Experimental Section for details).

pyrene trisulfonate (HPTS) in bulk water, slightly acidified to pH ∼5.5, and HPTS covalently linked to 50 nm aluminacovered silicon immersed in buffered water at pH ∼6 (PBS buffer). The steady-state spectrum of HPTS in bulk water was measured by a fluorometer under normal conditions. The surface-linked HPTS sample was measured by an optical system consisting of a confocal microscope hooked to a mode locked Ti:sapphire laser for sample excitation, and the fluorescence detection is based on a spectrometer with an intensified CCD camera. The sample excitation was done by 400 nm 100 fs pulses, of 80 MHz, with an average power of about 10 μW. As seen in Figure 2, the spectrum consists of two emission bands. The ROH band with a maximum at ∼450 nm is much weaker than the RO− band at about 520 nm. The relative intensity of the protonated ROH state emission of the HPTS in bulk water is much smaller than that of the modified linked HPTS on the alumina surface. The RO− band shape and position of the linked HPTS differ from those of the bulk HPTS. The surfacelinked band peak appears at ∼525 nm, whereas the bulk sample RO− peak is at ∼512 nm. The width of the RO− band of the surface-linked sample is broader as well (78 nm fwhm versus 62 nm). As shown later on, the stronger intensity of the ROH emission band of the linked sample arises from the larger

Figure 3. ROH and RO− signals of HPTS on a 50 nm alumina substrate in H2O and D2O measured by the TCSPC technique: (a) linear scale; (b) semilogarithmic scale.

RO− forms of 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) covalently bound through the APTES linker (see Figure 1) on a 50 nm thick layer of alumina, deposited on a silicon wafer. The APTES linker length is about 6 Å, and the sample is placed in an optical cell filled with water (or D2O). The signals were acquired by the time-correlated single photon counting (TCSPC) technique, with a limited time resolution determined 25789

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Figure 4. HPTS on a 50 nm alumina wafer in H2O and D2O measured by the TCSPC technique: (a) ROH signal, linear scale; (b) ROH signal, semilogarithmic scale; (c) RO− signal, linear scale.

fluorescence tail, which in bulk samples decays as a power law of t−d/2, where d is the dimension of the diffusion space. This fluorescence tail arises from the reversible proton geminate-recombination that takes place in the excited state and repopulates the ROH* form, which can then undergo a second ESPT cycle. HPTS is a unique photoacid, in which the GR process takes place efficiently since the RO− is strongly negatively charged, Z = 4e−. The Coulomb attraction between the negatively charged RO− and the positive charged hydronium is large and defined by the Debye radius RD

by the instrument response function, about 40 ps full width at half-maximum (fwhm). HPTS in bulk water is a mild photoacid with a pKa of ∼7.4 and pKa* ∼ 1.3. Electronic excitation of the protonated form, designated as ROH, increases the acidity of HPTS by about 6 orders of magnitude, and as a consequence the hydroxyl proton is transferred to a nearby water molecule. The ESPT rate constant of HPTS in bulk water is kPT = 1010 s−1, and thus the fluorescence decay time constant of the ROH form is about τPT = 100 ps. Both the ROH and RO− forms of HPTS have large oscillator strengths and weak nonradiative rates; thus, the fluorescence measurement is an excellent tool to follow the photoprotolytic processes that HPTS undergoes in the first electronic singlet state. The rate of the ESPT process of photoacids in D2O is distinguishably slower than in H2O. For weak to intermediate photoacids, with pKa* > 0, the kinetic isotope effect (KIE) is rather large, and for HPTS in bulk water, it was found to be 3.1 ± 0.1. Figure 3 also shows the ROH and RO− TCSPC signals in D2O of the surface-linked HPTS on alumina surfaces. As clearly seen, the ROH fluorescence decay rate in D2O is indeed much slower than the decay rate of the same sample immersed in H2O. The RO− TCSPC signals are measured at 525 nm, while the RO− emission band peak is at 512 nm. The RO− signal in both H2O and D2O shows a significant rise followed by an exponential decay with a lifetime of 5.4 ns and 5.6 ns for H2O and D2O samples, respectively. The RO− rise time is approximately the same as the ROH fluorescence decay time. This fact is the basis of the photoprotolytic model of the photoacid dissociation process. The ROH form of a photoacid’s time-resolved fluorescence exhibits a unique long-time

RD =

Ze 2 εkBT

(1)

where Z is the RO− charge in electronic units, ε is the dielectric constant of the medium (78 for both H2O and D2O at room temperature), kB is the Boltzmann factor, and T is the temperature. Assuming that the molecule charge distribution can be approximated by a point charge, then at the distance of RD from the RO− molecule center, the Coulombic attraction energy is equal to the thermal energy kBT. For the RO− of HPTS, RD in water is 28 Å and the hopping proton in the bulk can hardly escape the Coulomb cage, and the GR process occurs at a large probability to re-form the ROH*. Figures 4a and 4b show the ROH TCSPC signals, in H2O and D2O, of the surface-linked HPTS alumina surface as well as the ROH signals of diffusional HPTS in bulk H2O and D2O samples. The surface-linked HPTS alumina surface’s fluorescence decay rate of the ROH fast component is somewhat faster than the decay rate of both HPTS in the bulk H2O and D2O samples. 25790

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model20 to fit the ROH signals shown in Figure 2. As will be shown, the larger amplitude of the long-time fluorescence tail of the surface-confined HPTS ROH form may arise from a reduced proton diffusion space of only two dimensions compared to that of the bulk sample for which the proton diffusion space is three-dimensional. The GR model calculation of the ROH fluorescence tail amplitude, and shape, indeed shows a significant increase in the fluorescence tail amplitude of a two-dimensional diffusion space, as can be seen in Figure 2, when comparing the ROH signals of the surface-linked photoacid with these measured for bulk photoacids. Figure 4c shows the RO− signals measured at 525 nm of the same samples for which the ROH signals are shown in Figures 4a and 4b. The RO−* signals obtained from bulk HPTS and from surface-linked HPTS alumina surfaces, both measured in H2O, are quite similar. Silica Samples. Figures 5a and 5b show the TCSPC signals of the ROH and RO− time-resolved emission, in H2O and D2O, of surface-confined HPTS molecules covalently linked to a 200 nm silica layer on a silicon wafer. The ROH* decay rate in the D2O sample is smaller than that of the samples immersed in H2O. Both H2O and D2O ROH signal decays are nonexponential, with a distinctively large long-time fluorescence tail. The signals shown in this figure are similar to the signals of surface-linked HPTS covalently linked to 50 nm alumina layer, as shown in Figures 3 and 4. Figures 6a and 6b show the time-resolved fluorescence on a linear and semilogarithmic scale, respectively, for the ROH form of HPTS in bulk H2O and D2O, as well as for the surfacelinked HPTS immersed in both H2O and D2O. The initial fast decay rate of the ROH signals, in both H2O and D2O, are about the same for the bulk as well as the surface-linked HPTS molecules. We therefore conclude that the ESPT rate is the same for proton transfer to H2O or D2O in the bulk as well as at a short distance of about 6 Å from a silica surface. The long-time fluorescence tail of the ROH signals significantly differ between the surface-linked and bulk HPTS samples. The amplitude and shape of the tail in the surfacelinked HPTS on silica are much larger and decay much slower than the bulk HPTS signals in both H2O and D2O samples. An explanation of this result may originate from the difference in the dimension of the proton diffusion space of the two samples. The proton that is transferred next to the silica surface is hopping in a two-dimensional space, and therefore the geminate recombination probability increases, and hence the fluorescence tail amplitude is much larger. We address this issue in the next section using reversible GR model fit in which we change the diffusion space from three-dimensional to twodimensional. Figure 7 shows the TCSPC time-resolved fluorescence of the RO− form of HPTS in bulk H2O and D2O as well as that of the surface-linked HPTS silica surface. The signals’ time dependences are quite similar but not identical. All signals show a rise with a time constant that is similar to the ROH initial decay rate. This is a natural outcome of the formation of the RO−* form of the ROH* by the ESPT process. The signals decay exponentially with a radiative decay time around 5.5 ns, in both surface-linked and bulk HPTS.

Raman Interference of ROH TCSPC Measurements in H2O. When exciting the sample by a mode-locked laser at 387 nm, the water −OH stretching Raman Stokes line is scattered at ∼440 nm. The water Raman scattering coincides with the ROH emission. The TCSPC response time to Raman scattering is the same as the instrument response function, which in our case has ∼40 ps fwhm. This water Raman scattering signal is also collected by the TCSPC system when measuring the ROH fluorescence signal at 450 nm. The superimposed signal containing both fluorescence and Raman contributions as seen in Figures 3 and 4. The overall decay time is shorter than the actual ROH* fluorescence decay. The OD stretch of D2O is at around 2580 cm−1, and in our case the Stokes Raman line is at 425 nm. The ROH measurements of D2O samples at 440 nm, or 450 nm, are almost free from the Raman scattering interference. An important finding in the current study is that the amplitude of the long-time fluorescence tail attributed to the GR process is much larger for the surface-linked HPTS samples than for the bulk HPTS in both H2O and D2O. This is clearly seen in the semilogarithmic plot shown in Figure 5b. In the next section we will use computational tools based on the reversible diffusion-assisted geminate recombination

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DISCUSSION Time-Resolved Emission Fitting by the GR Model. The photoprotolytic processes of a photoacid in bulk liquid are quantitatively described by the diffusion-assisted geminate

−

Figure 5. ROH and RO signals of HPTS on a 200 nm silica layer, in H2O and D2O, measured by the TCSPC technique: (a) linear scale; (b) semilogarithmic scale. 25791

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Figure 6. HPTS on a 200 nm silica substrate in H2O and D2O measured by the TCSPC technique: (a) ROH signal, linear scale; (b) ROH signal, semilogarithmic scale.

effective GR rate. The asymptotic long-time fluorescence tail amplitude and the time dependence of the ROH form of a reversible photoacid are given by22 IfROH(t ) ∼

πa 2ka exp[−V (a)] 2kPT(πDt )d /2

(2)

where a is the reaction sphere radius, kPT and ka are the intrinsic ESPT and GR rates occurring on the reaction sphere, −V(a) = RD/a is the Coulomb potential in thermal energy units (kBT) at the reaction sphere, r = a, and d is the diffusion space dimension. This expression predicts a nonexponential fluorescence decay that fits a power law of t‑d/2. Control Experiments and Data Processing. Figures S2− S5 show the TCSPC signals of a dry bare alumina sample (Figure S2), a sample of alumina with a covalently bonded monolayer of APTES, the linker between the alumina surface and the HPTS molecule (Figure S3), a wet sample of alumina (Figure S4), and a wet sample of alumina with an APTES monolayer (Figure S5), all measured at 440 and 525 nm, that coincide with the HPTS ROH and RO− emission bands position. The bare dry alumina sample signal intensity is rather small at both measured wavelengths. The signals were measured under the same illumination and fluorescence collection conditions as that of the HPTS data shown in Figures 3−7. The signal intensity per unit time of the bare alumina samples is smaller by a factor of ∼50 than the HPTS data. Figure S3 shows the TCSPC signals of a sample of alumina with only APTES coverage. The signal intensity is about 5 times larger than that of the bare alumina alone; this contribution could not be ignored in the data analysis of the covalently linked HPTS samples. The emission decay time of the TCSPC signal measured at 440 nm of APTES on alumina is about 850 ps. Prior to the HPTS data fitting by the SSDP program, we subtract from the HPTS data measured at 440 nm an exponential decay function of τ = 850 ps with the appropriate intensity. Figure S4 shows the TCSPC signals of alumina samples immersed in H2O, measured at both 440 and 525 nm. The 440 nm signal shows an intense short-time component, τ < 10 ps, followed by a weak signal. We attribute the short-time component to a Stokes Raman signal arising from the OH stretch of H2O at ∼3600 cm−1. When we measure the bare alumina sample in D2O the Raman short-time component is detected at about ∼425 nm and not at 440 nm. The Stokes Raman signal frequency of the OD stretch of D2O is at about

Figure 7. TCSPC time-resolved fluorescence of the RO− form of HPTS, in bulk H2O and D2O, and of the surface-linked HPTS on a 200 nm silica substrate in H2O and D2O.

recombination model.21,22 The model is briefly described in the previous section. After the ESPT process, the probability of finding the RO−···H3O+ ion-pair at a distance r, at time t, is given by the Debye−Smoluchowski equation (DSE).21 The SSDP program of Krissnel and Agmon20 combines the photochemical steps of proton dissociation and recombination with the spherical symmetric DSE. The model approximates the photoacid molecule by a sphere of radius a. Both excited state reactions occur at the sphere’s surface r = a. Several parameters affect the ROH* population as a function of time and hence the unique ROH emission time dependence. At short times the TCSPC signal is mostly affected by the proton dissociation rate constant kPT. At longer times the proton geminate recombination process repopulates the ROH* and the TCSPC signal deviates from an exponential decay. The GR process dynamics depend on several parameters. The RO−··· H3O+ Coulomb potential in kBT units is given by V(r) = −RD/ r, where RD is the Debye radius is determined by the medium’s dielectric constant and the RO− charge. The intrinsic rate constant of the GR ka determines the GR rate at r = a, but the effective GR rate depends also on the mutual diffusion constant D, since the probability of finding a proton at r = a is affected by both D and V(r). The larger the value of ka, the faster the GR rate is. The smaller the diffusion constant the larger is the 25792

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2580 cm−1. In all of our measurements shown in this article we used the second harmonic of a Ti-sapphire laser at 387 nm for sample excitation. We indeed notice that the ROH TCSPC signals of wet HPTS covalently bonded on silica and alumina surfaces, and measured at 440 nm, exhibit a short time component of τ < 10 ps. In D2O HPTS sample, the short-time component at 440 nm is not observed. In summary, H2O Raman signal interferes with the fluorescence measurements of the ROH form of HPTS covalently bonded to alumina and silica surfaces. In future experiments we plan to modify the measuring cell in order to reduce the water layer thickness in which the wafer is immersed to minimize the water Raman signal. In the current experiments the water layer thickness was about 1 mm, and this unfortunately led to the intense Raman scattering. Figure S6 and Figure 8 show on linear and semilogarithmic scales the experimental ROH time-resolved emission signal in both H2O and D2O of covalently linked HPTS on a 50 nm alumina layer, along with the computer fit using the SSDP program. The figure also shows the TCSPC signal of the ROH form of HPTS in bulk water and D2O along with the computer fits using the reversible GR model. As seen in these figures, the

SSDP fit of the HPTS bulk samples in both H2O and D2O are good at all times, whereas the fit at short times of the ROH fluorescence of the covalently linked HPTS monolayer on alumina surface is only good for the D2O samples. The H2O ROH TCSPC signal also contains the H2O Raman signal, and thus the fit is rather bad at short times (see figure inset for short times showing the water Raman contribution to the signal). Table 1 provides the SSDP fitting parameters of the experimental TCSPC results shown in Figure 8. Two key conclusions can be drawn from comparing the fitting parameters of bulk and surface-linked HPTS experiments. The proton transfer rate from the hydroxyl group of HPTS to the solvent is about the same for both bulk and surface-linked HPTS, in both H2O and D2O samples. This finding is not trivial and needs further consideration. Hydroxypyrene (no sulfonates) is a very weak photoacid. The ESPT rate in H2O is much slower than the radiative time, and thus the ESPT rate constant is estimated to be kPT ∼ 107 s−1, whereas HPTS with three sulfonates has a kPT ∼ 1010 s−1, 3 orders of magnitude larger. Another example for the large sensitivity of a photoacid’s acidity with functional electron withdrawing groups is 5,8dicyano-2-naphthol (DCN2). It is a strong photoacid23 with a pKa* ≅ −4, whereas 2-naphthol is a weak photoacid1 with a pKa* ≅ 2.7. The surface-linked HPTS is linked to the alumina, or silica, surface by covalently bonding one of its sulfonate groups to a surface-confined APTES linker. This should in principle change the photoacidity properties, since the electron-withdrawing group on the aromatic rings determines both the pK*a and kPT. The HPTS derivative displays acidic properties similar to HPTS since the sulfonate group is not exchanged, but now bound to the amino group of APTES. Although the negative charge of the original sulfonate disappears when binding to the APTES amino group, still the sulfoamino newly formed group is a strong electron-withdrawing group, as per the case of sulfonate groups in regular HPTS. The experimental results shown in Figure 8, and the fitting parameters in Table 1, clearly show that the ESPT rate in D2O of HPTS linked to the surface is nearly the same as that of diffusional HPTS in bulk. Another extremely important outcome of the current study is that the water next to the alumina surface, or silica surface, accepts the proton with about the same affinity as bulk water. Previous studies on photoacids, in either water−alcohol mixtures or in aqueous solutions of high concentrations of LiClO4 or MgCl2, have shown that the ESPT rate is very sensitive to the proton accepting environment. HPTS does not transfer a proton to methanol or other alcohols within the excited-state lifetime. It is estimated that kPT decreases by at least 3.5 orders of magnitude in methanol. When the molar ratio of water is χH2O ≅ 0.5 in water−methanol mixtures, the kPT of HPTS decreases by a factor of 2, and when it reaches χH2O ≅ 0.25, the kPT is 10 times smaller than in neat H2O. A similar reduction in kPT of HPTS and other photoacids is found in aqueous salt solutions of Li+ and Mg2+. These ion radii are rather small, ∼0.7 Å, and the ion−water dipole interaction is large; thus, each ion links electrostatically many water molecules in its solvation shell. At about 3 M aqueous solution of these salts the ESPT rate of HPTS is reduced by more than 2 orders of magnitude. Another example of ESPT rate dependence is the ESPT rate of HPTS in reverse micelles, which decreases when the water droplet radius is smaller than 15 Å. The smaller the water droplet radius, the smaller the ESPT rate gets.

Figure 8. ROH time-resolved emission signals of covalently linked modified HPTS on a 50 nm alumina layer on a silicon wafer and TCSPC signals of the ROH form of HPTS along with computer fits using the GR model (a) in H2O and (b) in D2O on a semilogarithmic scale. 25793

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Table 1. SSDP Fit Parameters HPTS sample 50 nm alumina wafer (H2O) 200 nm alumina wafer (D2O) bulk H2O bulk D2O

dimension 2 2 3 3

RD (Å) 21 21 28 28

a (Å) 6.8 6.8 6.8 6.8

kPT (ns−1)

ka (Å/ns)

12 4.5 10 3.5

7 4 6 4.2

D (cm2/s) 9 6 9 6

× × × ×

τf−1 (ns−1)

−5

10 10−5 10−5 10−5

0.165 0.165 0.175

similar to that of bulk. Their result was unexpected since previous findings indicated that proton surface diffusion is much smaller than its bulk counterpart. Our current study on proton diffusion next to hydrophilic surfaces also hints that on short times, about 5 ns (the radiative decay time of a photoacid), the proton diffusion constant value is nearly the same as that of the bulk. From our fits, the dimension of the diffusion space found for the long time fluorescence tail of the ROH form, for a proton released next to alumina and silica surfaces, is close to two. This result may indicate that the released proton does not leave the surface region. Two plausible reasons for surface confinement of proton diffusion in two dimensions as observed are the following: (i) The silica or alumina surfaces are covered by APTES linker molecules, in addition to modified HPTS molecules which are connected through the APTES linker to the surface. Using a quartz crystal microbalance (QCM) technique, we determined that the APTES coverage is 2.3 × 1014 molecules/ cm2, whereas that of HPTS is only 1.1 × 1013 molecules/cm2. This rather dense coverage of APTES may serve as a proton scavenger to form RNH+, followed by a fast APTES proton release. The APTES surface coverage act as a local pH buffer that may also alter proton diffusion near the surface. The buffer capacity is probably low since we were able to change the HPTS equilibrium between the ROH and RO− forms of HPTS (pKa ∼ 7.4 in bulk water) by adding a small amount of HCl acid. We estimate that the pKa of surface linked APTES is around 5.5 ± 0.5. The pKa of APTES in aqueous solution is around 9, but the pKa of APTES covalently bonded to the surface drops by several orders of magnitude.24 The pH of the HPTS linked to the surface experiments is at a slightly acidic solution with a pH of ∼6.5 since we excite the ROH form of HPTS in aqueous solution, and the HPTS pKa is ∼7.4 ± 0.2. In case that the pKa of the covalently linked APTES is 5.5, most of the APTES molecules are in their neutral RN form at pH ∼6.5. We wish now to portray a plausible scenario of events and processes that lead to the two-dimensional proton diffusion next to an APTES covered alumina or silica surfaces. Photoexcitation of a ROH form of modified HPTS molecules linked to the surface leads to an ESPT process, which in turn leads to a pH jump next to the silica or alumina surface. The released protons are captured by APTES molecules.

All the above examples indicate that the ESPT rate of a photoacid in general, and of HPTS in particular, should be sensitive to the distance between the surface-linked photoacid molecules, to both the presence of APTES neighbor molecules and also to the water properties next to the respective surface. Apart from proton exchange between different photoacid molecules, either between the donating OH groups or the other functionalities (sulfonate groups, etc.), it may also be that protons temporarily are located at OH groups of the aluminum oxide or silicon oxide surfaces. However, in this study we could not observe a distinct difference in the ESPT rate of HPTS in bulk water and of the linked HPTS molecules next to the alumina or silica surfaces. Perhaps the reason for this unexpected finding is that both surfaces are hydrophilic, and the water on these hydrophilic interfaces behaves as a proton acceptor in a similar way as in bulk water. Molecular dynamics simulations may provide further information on water properties next to these surfaces. The second main finding of this study concerns to the diffusion-assisted geminate recombination process. The amplitude of the intermediate and long-time fluorescence tail of the covalently linked HPTS ROH form are much larger than in bulk water. Equation 2 provides a quantitative expression for both the amplitude and time dependence of the ROH fluorescence at long times for a spherical symmetric environment. The linker length is only 6 Å whereas the proton diffusion in bulk water and at times of ∼5 ns (the radiative lifetime) permits most of the protons to reach distances in excess of 100 Å. The large amplitude and long effective decay time of the fluorescence tail could be a result of a change in one or many parameters that affect the geminate recombination. The simplest fitting parameter that has a profound effect on the fluorescence tail is the dimensionality of the diffusion space. We were able to get a reasonable fit of the ROH fluorescence decay using a reduced diffusion space dimension of 2 rather than 3, that of the bulk, whereas all the other parameters that affect GR are similar for HPTS in bulk H2O or D2O (Table 1 shows the fitting parameters). For a satisfactory fit of the TCSPC fluorescence of the ROH form of linked HPTS, we used a similar proton diffusion constant as that of bulk water, ∼9 × 10−5 cm2/s, or D2O, 6 × 10−5 cm2/s, as well as similar ESPT kPT values. For further assessment that indeed the ROH fluorescence tail decays at long times as a power-law it is accustomed to multiply the ROH signal by exp(t/τf) to account for its finite excited-state lifetime and plot it on a log−log plot. Such a plot is shown in Figures S7a and S7b in the Supporting Information for both H2O and D2O samples, respectively. As can be seen in the plots, the fluorescence tail indeed decays as a power law. The slope is −1 ± 0.2. The signal-to-noise ratio of the measurement is not large since we measure a 1/10 of a monolayer of HPTS. We already mentioned that the APTES fluorescence interferes with our measurements and thus reduces the quality of the slope determination. In recent articles by Pohl et al.,16,17 it was found that the proton diffusion coefficient next to a hydrophobic surface is

k⃗

RN + H+ ⇄ RNH+ k⃗

Both the forward and backward reaction rate constants are fast in comparison to the HPTS ROH and RO− lifetimes of about 5.4 ns. We observe experimentally a distinct geminate proton recombination (GR) by the RO− form of the linked HPTS molecules. RO− * + H+ → RO*H

If the APTES pKa is high, pKa ∼ 7, then the above suggested mechanism is inapplicable. The TCSPC ROH signals of HPTS clearly show a large amplitude of the long time fluorescence tail 25794

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that serves as a fingerprint to the GR process. The fit of the ROH TCSPC signal by the geminate recombination model is rather good for a two-dimensional diffusion space and deviates from the experimental data when a three-dimensional diffusion space model is used. The GR model asymptotic long time ROH fluorescence is given by eq 2. The time dependence of the ROH fluorescence at long times goes by a power law of t‑d/2 for d = 2 it is t−1 whereas for d = 3 it is t−3/2, where d is the diffusion space dimension. t−1 decays at a much slower rate than t−3/2, and this is indeed the case of the fluorescence tail of HPTS linked by APTES to the alumina or silica surfaces. Similar results, but with intermediate dimensionality d = 2.4, were obtained for water-soluble HPTS confined within ∼10 nm pores of oxidized porous silicon (PSi).25 Similarly, relatively long 1d lateral migration of hydronium ions tangential to hydrophobic membranes was obtained by theoretical analysis of Pohl15 results, made by Agmon and Gutman,26 yielding fast lateral proton diffusion, as for bulk water. However, the lateral confinement of this proton diffusion remains a mystery. Taking into account also our present results, it now becomes clear that this confinement is independent of the surface ionic, hydrophobic, or hydrophilic properties. Therefore, one may rather consider a reasonable role for the interfacial bulk water phase. (ii) The modified HPTS linked to the surface by APTES has two sulfonate groups and the counter positive ions are sodium ions. In water the sodium ions are in solution but strongly attracted to the negatively charged layer. The simplest approach to this problem is to use the mean-field Gouy−Chapman27−29 theory and calculate the average electrostatic potential from the one-dimensional Poisson−Boltzmann (PB) equation. This approach makes a number of simplifying assumptions, including assuming the charges are smeared uniformly over a planar surface, but it provides a surprisingly adequate description of many electrostatic phenomena. It was found by McLaughlin and co-workers30 that for three electronic charges the mean-field theory partially fails to provide a good approximation. They used the nonlinear Poisson−Boltzmann equation to calculate electrostatic potentials in the aqueous phase adjacent to model phospholipid bilayers. We applied the diffused double-layer model to the HPTS covered alumina or silica surfaces. The Gouy−Chapman theory provides the characteristic distance between the negatively and the positively diffused charged layers. The following equation gives the Gouy−Chapman distance:

lGC =

1 2πσlB

that restricts the emitted protons to diffuse in a narrow layer parallel to the surface. This proton diffusion layer explains the time-resolved emission results of the ROH form of the HPTS. We were able to fit the TCSPC signals of the HPTS linked molecules by the geminate recombination model, using a twodimensional diffusion rather than a three-dimensional model that is used to fit the emission of HPTS in bulk water. The electrical diffused double-layer model is very sensitive to electrical screening by addition of salt or other ions. If other ions are accumulated near the surface, the electrical field built by the double layer decreases or is ineffective. We also found that the proton transfer rate constant kPT as well as the intrinsic recombination rate ka of the linked HPTS and that of HPTS in bulk water have the similar values. Another outcome of the fit is that the proton diffusion constant next to linked HPTS alumina or silica surfaces is about the same as that of bulk water. This surprising result is also found by Pohl and co-workers.16,17 To summarize, the surface confinement of hydronium cations is maintained by a perpendicular electric field generated between the negatively charged HPTS surface and an extended positive space charge accumulation of two sodium ions released by each HPTS within the interfacial water region. Further experiments are carried out aiming at determining the scale of the perpendicular extension of the space charge and its possible modulation by external electrical gating.

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SUMMARY AND CONCLUSIONS In the current work, we study the reversible photoprotolytic transient processes that a photoacid undergoes next to alumina and silica surfaces, immersed in both H2O and D2O. For this purpose we used a modified 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) photoacid whose bulk water pKa and pK*a prior to its modification is 7.4 and 1.3, respectively. For covalently bonding the HPTS to silica or alumina surfaces we replaced one sulfonate by an APTES linker whose length is ∼6 Å. We used the time-correlated single photon counting (TCSPC) technique to measure the time-resolved emission of the protonated ROH and deprotonated RO− forms of the modified linked HPTS in both H2O and in D2O. We found that the intermolecular excited-state proton transfer (ESPT) rate of the surface-linked HPTS, in both H2O and in D2O, is about the same as in bulk H2O and D2O. The ROH time-resolved fluorescence signal of a reversible photoacid exhibits a long time fluorescence tail since the proton geminate recombination (GR) repopulates the excited ROH* at longer times. In linked modified HPTS on alumina or silica surfaces the RO− is immobile and only the proton diffusion is taking place. The GR process is assisted in the bulk by the mutual diffusion of both RO− and the hydrated proton. The amplitude of the long-time TRF signal of the ROH form of modified HPTS linked to either alumina or silica surfaces is much larger than in HPTS in bulk. The fluorescence tail of the modified HPTS effective decay rate is much slower than that of HPTS in bulk. For data analysis and TCSPC signal fitting we used the spherical symmetric geminate recombination model that was developed for a reversible photoacid more than two decades ago. The fitting parameters include the Coulomb potential, the diffusion coefficient, the ESPT rate, and the intrinsic proton recombination rate to re-form the ROH*. We used the SSDP program (by Agmon and Krissnel20) to fit the experimental ROH TCSPC signal. The ESPT rate of surface-linked HPTS is nearly the same as in the bulk, for both H2O and D2O, independent of whether the surface is silica or alumina. The

(3)

where σ is the charge density per unit area and lB is the Bjerrum length:

lB =

e2 εkBT

(4)

where e is the electron charge, ε is the dielectric constant, kB is the Boltzmann factor, and T is the temperature. For water, ε = 78 at room temperature and lB is 7 Å. The surface density per cm2 of linked HPTS is ∼1.1 × 1013/cm2. Each HPTS molecule releases two sodium ions to the solution and therefore has two sulfonate moieties negatively charged. The calculated Gouy− Chapman length is ∼11.5 Å. This short length between the negative and positive layers provides a strong electrical field 25795

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geminate recombination long-time fluorescence amplitude of the surface-linked HPTS molecules is considerably larger and decays much slower than bulk values. We got a reasonable fit for the data using the SSDP program in a two-dimensional diffusion space rather than the three-dimension space used routinely for bulk experiment fittings. The proton diffusion coefficient obtained from the ROH fit of surface-linked HPTS was similar to the proton or deuteron in the bulk. The present results raise two simple suggestions about surface confinement of proton diffusion. The first one involves the generation of a perpendicular electric field, developed between a negatively charged surface and a positive space charge layer of sodium and/or hydronium ions within the water/substrate interface region.31 A second mechanism involves the active participation of the APTES molecules that are covalently bonded to the alumina and silica surfaces; their coverage is larger than 1014 molecules/cm2. The RN APTES form reacts with the ejected proton to form the RNH+ form and subsequently releases the proton within 5.4 ns, the HPTS excited state lifetime. This mechanism assures two-dimensional proton diffusion. Further experimental studies and simulations are needed to determine the mechanism that causes the enhanced proton geminate recombination of photoacids covalently bonded to surfaces, such as we found in the current study. We are not aware of a study with real results on the important issue of the number of solvent layers in which the proton moves to display a two-dimensional diffusion. Our results clearly indicate that the two-dimensional fast proton diffusion is maintained within a thin interfacial layer, where characteristic properties of the bulk water phase are demonstrated. Thus, we can set aside any significant interference of the much slower migration that is obviously related to proton transferring between fixed and soluble buffer moieties. But what might be the origin of this mysterious effect? We suggest that the photoprotolytic flux might generate a positive space charge that electrically drives the hydronium cations population within the thin layer toward the negatively charged surface containing the attached HPTS molecules. This surface enrichment of hydronium ions becomes a lateral source for the effective two-dimensional proton diffusion toward recombination with the photoinduced deprotonated negatively charged HPTS molecules. It should be noted that the most informative diffusion model, used to analyze our records, does not account for the initial fast rise profile of geminate recombination, which reach its peak value at about 100 ps. By our hypothesis, we plan to investigate the following questions: what might constitute the hypothetical electric potential gradient, its initial kinetics, and its orthogonal extension into the water phase?

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Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

We thank Professors H. Diamant, M. Gutman, N. Agmon, and Dr. Y. Erez for helpful discussions. This work was supported by grants from the James-Franck German-Israeli Program in LaserMatter Interaction and by the Israel Science Foundation. F.P. thanks the Legacy Foundation, ISF, for the partial support.

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ASSOCIATED CONTENT

S Supporting Information *

Steady-state and time-resolved emission of control measurments of alumina layer on a silicon wafer and APTES; steadystate and time-resolved emission measurments of covalently linked modified HPTS on alumina layer on a silicon wafer. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; phone 972-3-6407012; fax 972-3-6407491 (D.H.). 25796

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