Nanopools Governing Proton Transfer in Diametrical Ways in the

Dec 16, 2008 - As we go on increasing the water content up to a certain value in the ground state whereas deprotonation is favored in the excited stat...
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J. Phys. Chem. B 2009, 113, 567–573

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Nanopools Governing Proton Transfer in Diametrical Ways in the Ground and Excited State Madhuri Mukhopadhyay,† Abhijit Mandal,†,§ Ramprasad Misra,† Debi Banerjee,‡ Shankar Prasad Bhattacharyya,† and Samaresh Mukherjee*,† Department of Physical Chemistry, Indian Association for the CultiVation of Science, JadaVpur, India, and Department of Physics, Shibpur Dinabandhu Institution (College), Shibpur, Howrah, India ReceiVed: May 9, 2008; ReVised Manuscript ReceiVed: September 20, 2008

We present here the effects of geometrically constrained environments on the proton transfer reaction of 4-methyl 2,6-diformyl phenol (MFOH) both in the ground and excited states by employing steady-state and time-resolved fluorescence spectroscopy having picosecond and femtosecond resolutions. The nanometersized water pools formed in the ternary microemulsion of n-heptane-aerosol OT-water promote reprotonation of the probe. As we go on increasing the water content up to a certain value in the ground state whereas deprotonation is favored in the excited state. The emission intensity has a complex behavior as the water content is changed in the system. The lower fluidity of confined water within the reverse micelle with respect to the normal bulk water alters the related dynamics of the H-bonded network. These observations are rationalized on the basis of altered ionic water activity in the confined surroundings, i.e., on dielectric constant, ionic mobility, pH, and the favorable orientation of dipoles in the medium. Our observations might be helpful to infer about the characteristics of nanoreactors, which often mimic many biological hydrophilic pockets. 1. Introduction Recently, phenomena in low scale, confined regions have drawn great interest in the fields of physics, chemistry, and medical science due to their potential ability to alter the activity largely from the massive scale reactions.1-4 Nanometer-sized water cavities and interfacial hydrophobic interactions are central contents in controlling the structure, function, and dynamics of many biological systems such as proteins, biomembranes, vesicles, and so forth.5,6 Reverse micelles formed by surfactant molecules in hydrocarbon solvents with their polar head groups pointing inward can be good candidates for mimicking such nanoreactors.7-9 The reverse micelle is nothing but nanometersized droplets of water or polar solvent, surrounded by a layer made of surfactant molecule and immersed in a nonpolar solvent. Though the ground- and excited-state proton transfer (PT) reactions have extensively been studied in the microheterogeneous environment of reverse micelles,9-14 the acidity of surfactant bound water, the particular nature of the confined water, and its influence on PT are not yet completely understood. The proton transfer reaction is nothing but an excited-state acid-base reaction. The measure of acidity or more precisely the pKa value of the proton transfer probe in the ground state is notably different than that in its electronic excited state. The ground- and excited-state pKa values of pyranine (trisodium 8-hydroxy-1,3,6-pyrene trisulfonate) have been measured by Valeur et al., and they have reported that pyranine shows a pKa of 7.2 and 0.5 in its ground and excited electronic states, respectively.15 In the present study, we focus our attention on the modification of PT of 4-methyl 2,6-diformyl phenol (MFOH)16-18 in * Corresponding author. Phone: +91-33-2473-4971. Fax: (91) 3 24732805. E-mail: [email protected]. † Indian Association for the Cultivation of Science. ‡ Shibpur Dinabandhu Institution (College). § Present address: Sol-Gel Division, Central Glass & Ceramic Research Institute, 196 Raja S.C. Mallick Road, Jadavpur, Kolkata-700032.

completely noninteracting media, in bulk polar protic media, and finally in restricted polar media. The spheroid reverse micellar aggregate is formed by adding a calculated amount of AOT (sodium bis(2 ethylhexyl) sulfosuccinate) and water in n-heptane.19-21 The almost dehydrated reverse micelle (w ) 0.2) exhibits a nearly rigid structure.22 The size (rw) of the “water pools” depends on the water:surfactant molar ratio23 (wn ) [H2O]:[S]). The rw value increases from 4 Å (when w ) 2) to 16 Å (when w ) 16), depending on the water content in the reverse micelle. All respective rw values are shown in Table 1. The water molecules at the peripheries of such water pools are strongly held by the polar head groups of the surfactants and are thus “bound”, while those at the center of the pool are relatively “free”.24 Previously, many efforts have been devoted to rationalize those by several studies of solvation.25-27 Moreover, as the water content inside the reverse micelle increases, an increasing number of completely hydrated AOT ion pairs are expected to achieve a mobility sufficient to reorient independently from the microaggregates. Newly, some interesting observation has been reported27 by Jang et al. where they have found that the proton transfer rate of 7-azaindole is anomalous, being faster in methanol nanopools than the bulk solvent. Mukherjee et al.11 has proposed that the essential requirements for the ESPT is microheterogeneous ternary media comprised of water and a hydrophobic phase separated by a negatively charged interface. Several attempts are found to characterize the effects of a nanoconfined system on proton transfer processes; e.g., Thompson used a mixed quantum classical molecular dynamics (MD) simulation to explain the transfer of a phenolic proton to the amino group within a hydrophobic spherical cavity.28 There it has been reported that the proton transfer is more facile when the probe is away from the hydrophobic cavity wall. In this work, we have compared PT reactions in different types of nanoreactors by varying the nature of the nanopool, i.e., comparing the proton transfer reaction in water, methanol,

10.1021/jp804103h CCC: $40.75  2009 American Chemical Society Published on Web 12/16/2008

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TABLE 1: Lifetime Components of MFOH in n-Heptane, in the Absence and Presence of Water in AOT Reverse Micellesa λexc ) 375 nm medium

τ1 (ps)

n-heptane (λmon ) 535 nm) n-heptane + AOT (λmon ) 460 nm) n-heptane + AOT (λmon ) 535 nm) n-heptane + AOT + w2 (rw ) 4 Å) (λmon ) 495 nm) n-heptane + AOT + w2 (λmon ) 520 nm) n-heptane + AOT + w4 (rw ) 8 Å) (λmon ) 495 nm) n-heptane + AOT + w4 (λmon ) 520 nm) n-heptane + AOT + w16 (rw ) 16 Å) (λmon ) 495 nm) n-heptane + AOT + w16 (λmon ) 530 nm) water (λmon ) 530 nm)

15 (35) 17 (24) 16 (11) 855 (34)

205 206 236 200

a

λexc ) 422 nm τ3 (ps)

τ2 (ps)

τ3 (ps)

(65) (26) (77) (26)

2000 (50) 2160 (12) 4426 (40)

173 (31)

2140 (69)

1133 (63)

4647 (37)

791 (23)

178 (21)

5067 (56) 994 (24)

4569 (76)

531 (8)

187 (10)

4946 (87) 799 (8) 771 (2)

4728 (92) 4820 (98)

397 (5)

τ2 (ps)

4740

The percentage contributions of the corresponding lifetimes are indicated in parentheses. The χ2 values range from 0.99 to 1.20.

dimethyl formamide (DMF), and acetonitrile (ACN) cores. We have discussed how the restricted polarity and pH of the media play a crucial role in controlling the PT in different electronic states. 2. Experimental Section 2.1. Materials and Solutions. MFOH was prepared in a similar way to that reported earlier,16,17 and the compound was recrystallized from methanol and dried before use. All of the solvents used are of spectroscopic grade (Aldrich or Merck), and were checked for residual fluorescence before use. Triply distilled water was used throughout. The concentration of MFOH was maintained at ∼1 × 10-6 to 5 × 10-5 mol dm-3. Since the fluorescence quenching by dissolved oxygen was not important, the fluorescence measurements were made with nondegassed solution. All of the experiments were performed at ambient temperature (23 °C). 2.2. Instruments. The room temperature absorption and emission spectra of the solutions were recorded on a Shimadzu UV-vis recording spectrophotometer, UV-2401 (PC) S220V, and Fluoro Max 3 (Jobin Yvon Horiba) fluorimeter, respectively. For all spectral mesurements, a 1 cm path length quartz cell was used. In the femtosecond upconversion setup (FOG 100, CDP),29,30 the sample was excited at 375 and 405 nm wavelength using the second harmonic of a mode-locked Ti-sapphire laser with an 80 MHz repetition rate (Tsunami, Spectra Physics), pumped by 5 W Millennia (Spectra Physics). The fundamental beam was doubled in frequency by using a nonlinear crystal (1 mm BBO, θ ) 25°, φ ) 90°). The fluorescence emitted from the sample was upconverted in a nonlinear crystal (0.5 mm BBO, θ ) 38°, φ ) 90°) using a gate pulse of the fundamental beam. The upconverted light is dispersed in a monochromator and detected using photon counting electronics. A cross-correlation function obtained using the Raman scattering from ethanol displayed a full width at half-maximum (fwhm) of 350 fs. The femtosecond fluorescence decays were fitted using a Gaussian shape for the exciting pulse. To fit the femtosecond data, the long decay components are needed to be known beforehand. These were detected using a picosecond setup in which the samples were excited at 375 and 440 nm light using a picosecond diode laser (IBH Nanoled07). The emission was collected at a magic angle polarization using a Hamamatsu MCP photomultiplier (R3809) based on the time correlated single photon counting (TCSPC) technique. The TCSPC setup consists of an Ortec 9327 discriminator and Fluoro Hub Single Photon Counting controller. The data were collected with a DAQ card as a multichannel analyzer. The typical fwhm of the system response using a liquid scatterer was about 86

Figure 1. Absorption spectra of MFOH in n-heptane (I) and water (II).

ps. The fluorescence decays were deconvoluted using data station v2.3 IBH DAS6 software. The long picosecond components were kept fixed while fitting the femtosecond data. 3. Results 3.1. Steady-State Measurements. The absorption spectra of MFOH in n-heptane and bulk water are shown in Figure 1, which shows a single band at 350 nm in n-heptane and two bands at 355 and 430 nm in water. As reported earlier,16,17 the 350 and 430 nm absorption peaks correspond to the normal form and anionic form of MFOH, respectively (Scheme 1). When dry AOT is added to the n-heptane solution of MFOH, a new peak is developed at 422 nm that is enhanced by the gradual addition of AOT (Figure 2a). Upon addition of water into the resultant solution, the intensity of the 422 nm peak decreases with a concomitant increase of the normal form (Figure 2b). Similar observations are obtained in methanol, however, to a lesser extent (Figure 2c), whereas no change is observed in the case of ACN and DMF (Figure 2d). The change in the absorption spectra with variation of pH, keeping the water content fixed at w2, and the variation of water content, keeping the pH constant at ∼8, are shown in parts a and b of Figure 3, respectively. The spectral changes reflect that even at slightly alkaline pH the enhancement of water content increases the normal form. The normal form also increases when the pH of the added water decreases with keeping the water content unchanged. The fluorescence spectra of MFOH in the excited state in nonpolar noninteracting media, e.g., in the n-heptane,

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SCHEME 1

show a large Stokes-shifted single peak at 535 nm (Figure 4), which is due to the excited-state intramolecular proton transfer (ESIPT) reaction (Scheme 1) as reported earlier.16 When the dry AOT is added to this solution and the sample is excited at 350 nm, a new peak is generated at 460 nm along with the 535 nm peak (Figure 4). On the other hand, when the system is excited with 422 nm light, it shows a single peak at 460 nm with large fluorescence intensity. The interesting point to note is that a single peak arises at 500 nm instead of the two when water is added to this solution and excited with 350 nm radiation. The further addition of water in the system enhances the emission intensity, keeping the Franck-Condon envelope and position of the emission band unchanged (Figure 5a). When the w value reaches 16, the peak intensity saturates. Whereas in the case of 422 nm excitation, the first addition of water (w2)

in the solution causes a large red shift of 460-500 nm. Further addition of water results in a shifting of the emission peak to 530 nm when the w value reaches 16 (Figure 5b). When w2 is kept constant with varied pH of the added water, the 422 nm excitation only results in a change in intensity rather than the expected red shift (Figure 6). 3.2. Time-Resolved Measurements. Table 1 displays the results of time-resolved measurements obtained on excitation at 375 and 422 nm with femtosecond and picosecond resolution. When the MFOH is excited with a 375 nm laser pulse by using a femtosecond upconversion system, it shows a biexponential decay profile, as shown in Figure 7a, when monitored at 535 nm. After the addition of AOT, the decay profile changes to a triexponential one. Figure 7b shows that the decay curve is less steep compared to the decay in Figure 7a, reflecting the decrease

Figure 2. (a) Absorption spectra of MFOH in n-heptane in the presence of AOT, with the concentration ranging from 0 M (0), 0.025 M (1), 0.050 M (2), 0.075 M (3), to 0.1 M (4). Absorption spectra of MFOH in 0.1 M AOT reverse micelles in n-heptane (b) with the water content at different wn values, (c) with content of methanol, and (d) ACN, respectively, at different wn values, w0, w2, w4, w6.

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Figure 3. Absorption spectra of MFOH in 0.1 M AOT reverse micelles in n-heptane having a nanopool at different pH with a w2 values (a) and with different water contents for fixed pH (b), respectively.

Figure 4. Emission spectra of MFOH in n-heptane (0) and in the presence of AOT. The range of [AOT] is from 0 M (0), 0.025 M (1), 0.050 M (2), 0.075 M (3), to 0.1 M (4). λexc ) 350 nm.

of the fast component (15 ps) contribution from 35 to 11%. Table 1 shows a new component arising of 2000 ps lifetime of small amplitude (12%) by the addition of AOT. On the other hand, the contribution of this newly formed species (2000 ps)

Mukhopadhyay et al.

Figure 5. Fluorescence spectra of MFOH in n-heptane and 0.1 M AOT microemulsions by varying water content at different wn values: (a) λexc ) 350 nm; (b) λexc ) 422 nm.

Figure 6. Fluorescence spectra of MFOH in n-heptane and 0.1 M AOT microemulsions, with water content at fixed w2 values but with different pH values. λexc ) 422 nm.

is significantly large (50%) when decay is monitored at 460 nm (Figure 8, I). AOT compartmentalizes the system into the bulk n-heptane solution and AOT microemulsion. The addition of water produces a nanopool where a long lifetime component of 4000 ps (τ3) is developed, as shown in the decay profile, while the τ2 component remains unchanged, when monitored

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J. Phys. Chem. B, Vol. 113, No. 2, 2009 571 content in the reverse micelle, the magnitude of τ1 (900 ps) decreases with a concomitant decrease in amplitude, as shown in parentheses, and that of the longest component increases significantly both in magnitude and contribution. When the water content reaches w16, the contribution of these two components (τ2, τ3) becomes similar to that obtained in the bulk water. The decay profiles for 422 nm excitation are biexponential in nature and similar to the results obtained at 375 nm excitation (Figure 8). 4. Discussion

Figure 7. Femtosecond transient of MFOH in (a) n-heptane (λexc ) 375 nm, λmon ) 535 nm) and (b) in 0.1 M AOT reverse micelles in n-heptane (λexc ) 375 nm, λmon ) 535 nm).

Figure 8. Typical decay profile of MFOH (λexc ) 375 nm) (I) in n-heptane and 0.1 M AOT (λmon ) 460 nm) and (II and III) in n-heptane and 0.1 M AOT with water contents w2 and w16, respectively (λmon ) 495 nm). Global analysis of the decay and the lamp profile is also shown.

at 495 nm. Instead of the 17 and 2000 ps components, a new component is obtained of 900 ps (τ1). The contribution of which can be found by taking the average of the former two components. It has been noticed that, with the increase of water

MFOH is a well-studied intramolecular excited-state proton transfer probe. In nonpolar noninteracting media, it undergoes ESIPT to form a tautomer, whereas in water it shows intermolecular excited-state proton transfer, resulting in the formation of anion (Scheme 1). In the present study, with femtosecond resolution, a biexponential decay profile is obtained when the tautomeric form in n-heptane is monitored at 535 nm (Figure 7a, Table 1). The 205 ps component can be attributed to the tautomeric form, whereas the very short 15 ps component is due to the normal form. In the ground state, an aqueous solution of MFOH undergoes partial deprotonation, resulting in equilibrium between the normal form and the anion, and is concluded through the existence of two peaks at 350 and 430 nm. According to the previous observation in the nanosecond time scale,17,18 the longest component of ∼5 ns is due to the anionic species in the excited state and the shorter component that arises may be due to the normal form. However, from fluorimetric measurement, the fluorescence from the normal form has not been detected in aqueous media in the presence of high anionic emission. It is also supported by the fact that when excitation is done at 375 nm the contribution of the shorter component is greater than the corresponding 422 nm excitation. When AOT is added to the n-heptane solution of MFOH, the appearance of a new peak at 422 nm in addition to the 350 nm peak in the absorption spectra can be attributed to the formation of a blueshifted H-bonded anionic ion pair (Scheme 2) due to abstraction of the acidic proton of MFOH by the negatively charged sulfonate groups (SO3-). The formation of such a type of ion pair has been reported by Cohen et al.31 This proposal is also supported by the time-resolved observation that the tautomeric species contribution is 77% (τ2 ) 236 ps) when monitored at 535 nm, whereas it changes to only 26% when monitored at 460 nm. Among the triexponential decay components, the newly formed 2000 ps component is due to the H-bonded anionic species which is in agreement with the previously obtained lifetime value of the H-bonded species in acetone.17 The tautmeric form resides mainly in the bulk and the H-bonded anionic form generated at the AOT n-heptane interface (Scheme 2). The 8 nm blue shift with respect to the anion in bulk water (at 430 nm) is expected to be due to the lack of free solvation of the negatively charged anion in the vicinity of the sulfonate group of AOT. As it is quite established24 that with the increase of water content the amount of free water in the reverse micelle nanopool increases, it was expected that a larger water pool will favor anion formation via solvation. However, interestingly, contrary to the expectation, the H-boded anionic ion pair decreases in the ground state with simultaneous formation of the normal form by the enlargement of the water pool (Figure 2b). On the other hand, in the excited state, the scenario is completely different. In the bulk water, both the anion and the normal form of MFOH coexist, indicating the comparable pKa of MFOH and water in the ground state. Surprisingly, within the nanopool, the pKa of the confined water decreases as it acts

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SCHEME 2

SCHEME 3

as a proton donor to the MFOH anion. A similar type of observation was reported by Cohen et al.31 in the case of proton transfer reaction of 2-napthol-6,8-disulfonate (2N68DS). There they have reported the equilibrium constant of proton transfer reaction at the interface (pKa) values of the water into the reverse micelle while varying the water content of the system. They have shown that, for w5, the pKa value of water is 1.5. There is a constant increase in the pKa with the increase of the water content into the reverse micelle. The pKa value reaches a maximum at w14, and a slight decrease in pKa is observed when the water content is further increased into the reverse micellar system. Hence, it can be said that, in the absence of water in the nanopool, the negatively charged sulfonate group of AOT abstracts the acidic proton of MFOH, but in the presence of confined water, the sulfonate group abstracts the proton of the water rather than that of MFOH (Scheme 3), as confined water has a lower pH than bulk water.31 The gradual decrease of the 422 nm peak with the increase of water content can also be explained by the solvation of the ion pair by water. Hence, in the ground state with the increase of water content in the solution, the H-bonded anionic species decreases with the concomitant increase of the normal form (Figure 2b). In our earlier work, we have shown that the anionic species of MFOH is more stabilized by water than the normal form through the solvation. Thus, apparently, it was expected that the water will favor the MFOH anion, but due to the low dielectric characteristics of the confined water (ε ) 38)23 and the low pKa value, i.e., high dissociating constant of water compared to the bulk, it fails to provide the sufficient driving force for intermolecular proton transfer; rather, it acts as a proton donor; even the water used is slightly alkaline. On the other hand, in the excited state, the modified electronic distribution of MFOH makes it more acidic than confined water, resulting in intermolecular proton transfer from MFOH to water. The electronic redistribution of MFOH is highly excitation dependent, and thus, when excited at 350 nm in the bulk n-heptane, ESIPT occurs, resulting in the formation of the tautomeric form in the bulk. However, the H-bonded ion pair and a very little amount of the normal form coexist with the anion at the interface and nanopool, resulting in broad spectra at 495 nm. This coexistence of these four different species is also supported by the time-resolved mea-

surements. The decay profile monitored at 495 nm consists of the 4000 ps component for the anionic species, the 200 ps component for the tautomeric species, and the 900 ps component which is basically the average of the 2000 ps H-bonded anionic species and the normal form 15 ps species. An interesting point to note is that, though in the ground state the increase of water content favors reprotonation, in the excited state, the increase of water content increases the anionic contribution, i.e., encourages the deprotonation. An interesting observation is found from the absorption spectra of the probe after keeping the water content fixed and varying the pH of the water added. The result is shown in Figure 3a. From there, it can easily be noted that, with the increase of the acidity of the water in the reverse micelle, the intensity of the 460 nm peak decreases notably (Figure 6). A similar result was obtained when we recorded the absorption spectra of the molecule in the water-AOT-heptane reverse micelle with varied water content of the system. The result of this experiment is depicted in Figure 2b. Figure 2b shows that the absorbance of the 430 nm peak decreases with the increase in the water content in the system. Thus, it is clear that there is a close similarity in both types of observations we got. Comparing these two results, one can easily infer that with the increase of the water content in the reverse micelle the H+ content increases in the system. This observation is similar to what was observed by Huppert et al.31 They have also reported that the dielectric constant (ε) at the center of the reverse micelle is increased with the increase of the water content. At w5, the dielectric constant is ε ) 60. It increases to 78 at w40 but is still below the dielectric constant of water (ε ) 80). When the system is excited at 422 nm and monitored at 530 nm, the anionic species predominantly coexist with the average contribution of H-bonded anionic species and the normal form. In the steady state on 422 nm excitation, the spectra show almost a 30 nm red shift to 530 nm by the gradual increase of the water content. This observation reflects that with the increase of the water content the anionic species get more stabilized by the loosely bound water in the core. Here, it is pertinent to mention that most of the solvation studies in the reverse micellar nanopool show that the contribution from the shortest component increases with the increase of water content, i.e., the free water

Nanopools Governing Proton Transfer in Diametrical Ways responsible for faster solvation. In the present study, the shorter component decreases as the free water increases. The reason is mainly the short component contributing species converts to the long-lived anionic species in the excited state with the gradual swelling of the nanopool. 5. Conclusion The present study reflects that depending on the nature of the nanopool the PT reaction gets significantly modified both in the ground and excited states. The preferential formation of the normal form over the anion in the ground state confirms the lower pH of the confined water than that of the bulk. The reverse observation in the excited state is due to the changed electronic distribution of MFOH. In the excited state, MFOH has a higher proton donating ability over the confined water. The size of the nanopool also plays a significant role in the solvation of the anionic species. Interestingly, contrary to most of the previous observations, the shorter lifetime component decreasing with the concomitant increase of free water can be explained on the basis of preferential solvation of the anionic species by the free water over the short-lived normal form. The reprotonation of MFOH anion in the ground state adds further support to the low static dielectric value of the confined water. Here, we propose that the pKa value of this bound water falls, enhancing its proton donating ability compared to the bulk water in the presence of a negatively charged headgroup ion (here provided by the surfactant), which is also in accordance with the observations observed by Huppert et al.31 for a different PT probe. The results also reflect that the effect of perturbation of the foreign ion on the confined water is significantly larger than that on the bulk water. Acknowledgment. We thank Prof. K. Nag for his help in synthesis of MFOH. R.M. and M.M. thank integrated PhD division, IACS and CSIR, respectively, for fellowships. A.M. and D.B. acknowledge UGC [project no. PSW-145/06-07 (ERO)] and DST [project no. SR/S1/PC-II/2003], respectively, for financial support. Thanks are due to Subrata Das, for picosecond lifetime measurements. Sincere thanks are due to Professor Kankan Bhattacharyya for his help with DST funded femtosecond measurement facility (project no. IR/I1/CF-01/ 2002). References and Notes (1) Wallraff, A.; Schuster, D. I.; Blais, A.; Frunzio, L.; Huang, R.-S.; Majer, J.; Kumar, S.; Girvin, S. M.; Schoelkofpt, R. J. Nature 2004, 431, 162.

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