Biphasic Tautomerization Dynamics of Excited 7-Hydroxyquinoline in

The cis tautomerizes via proton relay from the enol group to the imino group along a hydrogen-bonded water bridge, unusual in water but common in alco...
3 downloads 4 Views 292KB Size
Download PDF
J. Phys. Chem. B 2006, 110, 11997-12004

11997

Biphasic Tautomerization Dynamics of Excited 7-Hydroxyquinoline in Reverse Micelles Oh-Hoon Kwon,† Taeg Gyum Kim,‡ Young-Shin Lee, and Du-Jeon Jang* School of Chemistry, Seoul National UniVersity, NS60, Seoul 151-742, Korea ReceiVed: December 15, 2005; In Final Form: February 24, 2006

The excited-state tautomerization dynamics of 7-hydroxyquinoline in the water pools of reverse micelles has been investigated by monitoring time-resolved fluorescence spectra and kinetics as well as static absorption and emission spectra with a variation of water content and isotopic fractionation. The normal and the tautomeric species are found to reside preferentially in the bound- and the free-water regions of the micelles, respectively. The excited-state tautomerization of the normal species in the bound-water layers is suggested to occur via two different channels, depending on rotamers at the moment of excitation. The cis tautomerizes via proton relay from the enol group to the imino group along a hydrogen-bonded water bridge, unusual in water but common in alcohols, whereas the trans tautomerizes via the stepwise individual acid-base reactions of two prototropic groups as found in bulk water. Proton relay can take place because water in the pools has substantially reduced polarity and disrupted hydrogen-bond networks compared with bulk water.

1. Introduction

SCHEME 1: ESPT of 7HQ in Water

Proton-transfer plays a key role in a wide variety of biological and chemical phenomena.1-9 However, it is enormously complicated in water. In the gas phase, water itself has low proton affinity compared with other proton acceptors.10 The proton affinity of bulk water is greatly enhanced by the formation of extensive hydrogen (H)-bond networks. The size, the structure, and the motion of solvent clusters as well as the nature of prototropic groups determine the dynamics of proton transfer.6,11 The dynamics and the mechanism of excited-state proton transfer (ESPT) in water have been widely investigated with diverse photoacids.2,6,11-16 Aromatic molecules having both acidic and basic functional groups are interesting because they may serve as experimental models to study the proton relay of enzymes, ion channels, and spanning membrane proteins.8,9 In this regard, hydroxyquinolines and their derivatives, having two prototropic groups of enol and imine, are extensively explored.17-39 Aqueous 7-hydroxyquinoline (7HQ) as well as 3-hydroxyquinoline and 6-hydroxyquinoline (6HQ) is reported to undergo excited-state tautomerization stepwise via forming anionic intermediate species (Scheme 1).30-34 On the other hand, 7HQ in nonaqueous protic solvents is known to form a H-bond chain, along which proton relay can take place (Scheme 2).17-29 Solvent reorganization prior to intrinsic relay to form a cyclically H-bonded complex is crucial in the dynamics of proton relay.18 However, two different possible rotamers of trans-7HQ and cis7HQ in Figure 1 are reported to affect the ESPT dynamics of 7HQ as well.20,29 Reverse micelles dispersed in hydrocarbon solvents are interesting to explore because they can be considered as nanoreactors. Reverse micelles formed by surfactant molecules having polar headgroups pointing inward are good mimic systems of biologically confined water molecules.40-43 A * To whom correspondence should be addressed. E-mail: [email protected]. † Present address: Laboratory for Molecular Sciences, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125. ‡ Present address: Korea Research Institute of Standards and Science, Daejeon 305-600, Korea.

SCHEME 2: ESPT of 7HQ in Alcohols

distinguished feature of reverse micelles is their ability to make nonpolar solvents solubilize a large amount of water by encapsulating water pools in their inner polar cores.44 About 20 molecules of surfactant Aerosol-OT (sodium 1,4-bis-2ethylhexylsulfosuccinate, AOT) form a reverse micelle having a radius of 1.5 nm above the critical concentration of 1 mM in a hydrocarbon solvent.50 The gradual addition of water to an AOT solution forms microemulsions of nanometer-sized water droplets surrounded by AOT molecules. In n-heptane, the radii in nanometers of the water pools are about 0.15w0, where w0 is the molar ratio of water to AOT.45 Water molecules at interfacial peripheries are strongly bound to the ionic headgroups of surfactant molecules, whereas those at micellar cores are relatively free.31 Thus, water confined in reverse micelles is known to have characteristic properties that are distinctively different from those of bulk water. The structures of AOT reverse micelles45-49 and the solvation dynamics and the dielectric relaxation of water inside reverse micelles50-58 have also been studied with diverse spectroscopic methods and molecular-dynamics simulations. It is reported that water molecules inside reverse micelles are roughly classified into two types: “bound water” and “free water”.40-42 Bound water molecules in the interfacial peripheries of water pools are immobile because of strong binding to the polar headgroups of AOT molecules. On the other hand, free water molecules in

10.1021/jp0573184 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/25/2006

11998 J. Phys. Chem. B, Vol. 110, No. 24, 2006

Kwon et al.

Figure 2. Absorption spectra of 7HQ in water (black) and in AOT reverse micelles having w0 ) 0 (red), 8 (green), 24 (blue), and 48 (magenta).

Figure 1. (a) Structure of AOT; (b) schematic of a reverse micelle having the three distinctive regions of free water (1), bound water (2), and headgroups (3); (c) rotamer-dependent ESPT mechanism of 7HQ in the bound-water layer of an AOT reverse micelle.

the central areas of reverse micelles have been suggested to be mobile, almost like bulk-water molecules. Acid-base reactions in the reverse micelle have been investigated for a few photoacids.43,59-63 However, the protontransfer dynamics of biprototropic photoacids has been rarely revealed in aqueous reverse micelles.64,65 Thus, we report in this paper the solvation dynamics and the proton transfer of photoexcited 7HQ in water pools of AOT reverse micelles, investigated with time-resolved fluorescence spectroscopy. We have focused on the unusual properties of water molecules in water pools of AOT reverse micelles to correlate the dynamics of proton transfer with the dynamics of solvent motion. The 7HQ molecules in water pools anomalously show two different pathways of ESPT depending on rotamers at the moment of excitation (Figure 1). A cis-7HQ molecule goes through proton relay from the enolic group to the imino group unusually as in alcoholic solvents, whereas a trans-7HQ molecule transfers a proton to/from a water cluster at each prototropic group as known in bulk water. Proton relay can occur in water as found in alcohols owing to the strongly perturbed H-bonding properties of water molecules in the bound-water layers of reverse micelles. 2. Experimental Section 2.1. Materials. 7HQ (99%), purchased from Acros, was further purified via column chromatography and vacuum sublimation. AOT (>99%) was used as received from SigmaAldrich without any further purification. n-Heptane (>99%), purchased from Merck, was distilled once and stored over molecular sieves of 4 Å before use. 7HQ-dissolved AOT micelles were prepared by adding 22 µmol of 7HQ to a 100 mL solution of 0.09 M AOT in n-heptane. In our samples, practically no more than one 7HQ molecule can be present in an AOT micelle having 20 AOT molecules because [7HQ]/ [micelle] is 0.05. Then a requisite amount of triply deionized

water was added to the AOT solution to control w0. 2H2O (isotopic purity g 99.9%, Sigma-Aldrich) was added to the AOT solution to study the 2H transfer of 72HQ. All our samples were transparent and monodisperse throughout the experiments. 2.2. Measurements. Absorption spectra were obtained by using a UV-vis spectrophotometer (Scinco, S-2040). Fluorescence spectra were obtained by using a fluorimeter that was home-built with a 75 W Xe lamp (Acton Research, XS432), 0.15 and 0.30 m monochromators (Acton Research, Spectrapro150 and 300, respectively), and a photomultiplier tube (Acton Research, PD438). A mode-locked Nd:YAG laser (Quantel, YG701) and a 10 ps streak camera (Hamamatsu, C2830) attached with a CCD detector (Princeton Instruments, RTE128H) were employed for time-resolved fluorescence measurements at room temperature. Samples were excited with 315 nm pulses generated through a Raman shifter filled with methane at 10 atm and pumped by the 266 nm pulses of the laser. Emission wavelengths were selected by combining band-pass and cutoff filters to measure kinetics or by using a 0.15 m monochromator (Acton Research, Spectrapro-150) to record spectra. Fluorescence decays were collected at every 5 or 10 nm from 340 to 610 nm to construct time-resolved fluorescence spectra. Fluorescence kinetic constants were extracted by fitting profiles to computer-simulated exponential curves convoluted with instrumental response functions. The fluorescence spectra reported here were not corrected for the wavelength-dependent variation of detector sensitivity. 3. Results 3.1. The Location of 7HQ in AOT Reverse Micelles. The lowest absorption bands of aqueous 7HQ prototropic equilibrium species are spectrally well distinguishable (Figure 2). The absorption bands of aqueous 7HQ solutions with the maxima at 328 and 400 nm are due to the lowest electronic transitions of the normal (N) and the proton-translocated tautomeric species (T), respectively.30,33 On the other hand, the spectrum of 7HQ in water-free (w0 ) 0) AOT reverse micelles shows only the absorption band of N at 331 nm. The N absorption is spectrally comparable to that in alcohols.18-20 Of note is that the binding constants of hydroxyquinolines to AOT micelles in n-heptane are reported to be 860 M-1.38 With the addition of water, T absorption increases gradually, whereas N absorption decreases concomitantly. However, the absorption spectrum of 7HQ in the water pools of AOT reverse micelles even at w0 ) 48 is hugely different from that in bulk water. Equilibrium between

Excited-State Proton Transfer in Reverse Micelles

Figure 3. Maximum-normalized emission spectra, excited at 330 nm, of 7HQ in water (∞) and in AOT reverse micelles at indicated w0.

N and T is obviously shifted toward N in AOT reverse micelles to become similar to that in alcoholic media,23b,25 indicating that the micropolarity of water near 7HQ is substantially low compared with that in bulk water. The spectrum of N absorption and the fractional equilibrium concentration of T indicate that the environment of 7HQ in nanoconfined water is very close to that in alcohols in polarity. The characteristic properties of the water pool and the residential environment of 7HQ are further revealed with w0dependent emission spectra (Figure 3). In the absence of water (w0 ) 0), the emission spectrum of 7HQ shows only an N* band at 370 nm. With the size increase of water pools in reverse micelles, the photoexcitation of N yields a new emission band growing around 530 nm at the expense of N* emission. The green emission is unambiguously originated from T* and it is bathochromically shifted by 20 nm in comparison with that in bulk water. Thus, it is rather similar spectrally to T* emission in alcohols. T at S0 has a zwitterionic structure, whereas T* at S1 has a ketonic structure.30,66 The electronic transition of T to the first-excited singlet state induces an intramolecular charge transfer to produce T* having a reduced dipole moment.31,37 This gives negative solvatochromism to T* fluorescence. This suggests that the polarity of the environment of N*-transformed T* is very similar to that of alcohols. According to two-state models, a water pool confined within a reverse-micellar sphere can be divided into the bound-water layer and the free-water core beside the interfacial polar headgroup layer of the micelle.40-42 Water molecules located in the core region of the reverse micelle have been reported to behave like bulk-water molecules while molecules trapped in the bound layer are suggested to have alcoholic properties in polarity and H bonding. Moreover, N* emission hardly changes spectrally with further increase of w0 once it shifts to the red with the initial addition of water to a water-free AOT solution. This designates that N molecules transforming into T* upon excitation experience a unique environment, where the polarity of water is similar to that of an alcohol. Considering this along with the absorption spectra of Figure 2, we suggest that the site of solubilization for N is primarily the bound-water layer of an AOT reverse micelle. 2-Naphthol is found to reside in the “headgroup regions” of reverse micelles, whereas its anion is found to reside in the bound layers.62,63 Moreover, a multiply charged anionic species dissolves in the free-water cores because of electrostatic repulsion by the anionic headgroup of the surfactant.43,59-61 Τwo polar groups of enol and imine are considered to permit the N molecule of 7HQ to reside at the bound layer near the interface of an AOT micelle. This is in accordance with the observation that the wavelength of the N*emission maximum shifts from 370 nm at absence of water to

J. Phys. Chem. B, Vol. 110, No. 24, 2006 11999

Figure 4. Emission spectra of 7HQ in AOT reverse micelles at w0 ) 36 with excitation at 330 (solid) and 410 nm (dotted).

Figure 5. Time-resolved fluorescence spectra, normalized by the maxima of N* emission, of 7HQ in AOT reverse micelles at w0 ) 12. Emission was collected with excitation at 315 nm, and time delays in picoseconds between excitation and collection are given near spectra.

375 nm at presence of water. We infer that N resident in the interfacial region at w0 ) 0 migrates to the bound-water layer with the addition of water. On one side, Figure 4 shows that the ground-state equilibrium species of T having a zwitterionic character resides in the freewater core of the AOT micelle. The emission of photoexcited T* directly from T is shifted to the blue by 10 nm compared with that of N*-transformed T*. This indicates that the resident site of T is the free-water core whereas that of N is the boundwater layer. Note that fluorescence from photoexcited T* is in the middle (520 nm) of fluorescence from N*-tautomerized T* (530 nm) and that from T* in bulk water (510 nm). Thus, we suggest that while the micropolarity of the bound-water layer containing N is close to the polarity of an alcohol, that of the free-water core containing T is intermediate between the polarity values of water and an alcohol. 3.2. Solvation Dynamics. Both fluorescence bands of N* and T* are observable explicitly in the time-resolved fluorescence spectra of Figure 5 as well, showing the maxima at 375 and 530 nm, respectively. The rise of T* fluorescence with time increase confirms the previous suggestion with static spectra that N molecules undergo ESPT to form T* upon excitation. Figure 5 shows that the fluorescence maximum of N* shifts bathochromically as time progresses. Solvent molecules surrounding a 7HQ molecule in a reverse micelle reorganize to adjust to the new charge distribution of N*. Solvation kinetics occurring in the water pools of reverse micelles can be directly measured by monitoring the timedependent spectral shift of fluorescence. Solvation dynamics

12000 J. Phys. Chem. B, Vol. 110, No. 24, 2006

Kwon et al.

Figure 6. C(t) decays of 7HQ in AOT reverse micelles having w0 ) 12 of 1H2O (open) and 2H2O (closed). The solid lines denote the best fits to exponential decays.

has often been investigated by using the normalized spectralshift correlation function of C(t), defined as

C(t) )

V(t) - V(∞) V(0) - V(∞)

(1)

The V(t), V(0), and V(∞) of eq 1 are the frequencies of fluorescence at time delays of t, 0, and ∞, respectively, after photoexcitation. We have estimated ν(0) or V(t) by averaging the frequencies at the two-half-maxima of normal fluorescence. The value V(∞) was estimated by fitting experimental data to an exponential function.67 The properties of environmentally different water domains in an AOT reverse micelle, exerting on the diverse properties of molecules therein, depend on distances from the interface of the micelle. Thus, each spatial domain gives rise to a characteristic solvation time.68 Although a double-exponential decay fits C(t) well for most cases of reverse micelles,46-49 a single-exponential decay does as well if there is a small spread in the residence site of probe molecules.68 C(t) for 7HQ in the pools of 1H2O and 2H2O at w0 ) 12 decays single-exponentially with the time constants of 680 and 980 ps, respectively, showing a kinetic isotope effect (KIE) of 1.4 (Figure 6). The bound water of AOT reverse micelles near the hydrophilic interface shows a retarded solvation process because of orientational constraints. The KIE is close to the value (1.5) of 4-aminophthalimide in AOT reverse micelles at w0 ) 12 and the KIE (1.25) of the dielectric relaxation of water.64,68 The total Stokes shifts (ν(0) - ν(∞)) of 1H2O and 2H O are 210 cm-1 and 350 cm-1, respectively, at w ) 12. 2 0 They are very small compared with reported values (∼2000 cm-1) of other polar probe molecules.46-49 Thus, we infer that the fast solvation component of a substantial amount is missed in our work. We also note that our observed solvation times are comparable to the weight-averaged ESPT times of 7HQ (vide infra). Thus, we suggest that solvent motion monitored with our limited temporal resolution is the slow reorientation of solvent molecules. 3.3. Biphasic Fluorescence Kinetics. The N* fluorescence of 7HQ in the reverse micelles of w0 ) 12 shows a biexponential decay profile composed of 190 (57%) and 950 ps (43%) (Figure 7 and Table 1). T* fluorescence emitted with N excitation at 315 nm shows kinetics rising concomitantly with N* decay and decaying on the time scale of 2500 ps. This designates that N* undergoes ESPT to transform into T*. In the 2H2O pool of w0 ) 12, N* fluorescence decays in 290 (57%) and 1200 ps (43%) while T* fluorescence rises in 270 (70%) and 1200 (30%) ps

Figure 7. Fluorescence kinetic profiles with best-fitted curves (lines) of 7HQ in reverse micelles having w0 ) 12 of 1H2O (open) and 2H2O (closed), collected at short (top) and long (bottom) time windows. Samples were excited at 315 nm and monitored at 380 (blue) and g550 nm (red).

TABLE 1: Fluorescence Lifetimes of 7HQ in AOT Reverse Micelles of w0 ) 12 λex (nm)

λpr (nm)

315

380 g550

435 a

g550

water

rise time (ps)

decay time (ps)

1H O 2 2H O 2 1H O 2 2H O 2 1H O 2 2H O 2

instant instant 150(65%)a + 1000 270(70%)a + 1200 instant instant

190(57%)a + 950 290(57%)a + 1200 2500 6900 2300 5500

Intensity percentage of each component.

and decays in 6900 ps. The decay times of T* transformed from N* are noticeably longer than the respective values of 2.3 and 5.5 ns observed with the Franck-Condon excitation of T in the pools of 1H2O and 2H2O. This is because the ground-state equilibrium species of N and T are resident in the different sites of the bound-water layer and the free-water core, respectively (vide supra). The amplitude percentage and the time constant of the fast T*-rise component are larger and shorter, respectively, than those of the fast N*-decay component. These phenomenological differences are ascribed to the interference of T* FranckCondon excited from T to the emitting species of T* fluorescence. Hence, we have monitored N* fluorescence rather than T* fluorescence for the close examination of ESPT with the size variation of the water pool. The kinetic parameters of N* fluorescence measured with AOT water pools of various sizes are listed in Table 2. Upon increase of water content in reverse micelles, quite interesting features are recognized in the kinetics of N* fluorescence. On one hand, the decay times of the slow and the fast components decrease with the w0 increment. This is attributed to a viscosity decrease with a w0 increase, resulting

Excited-State Proton Transfer in Reverse Micelles

J. Phys. Chem. B, Vol. 110, No. 24, 2006 12001

TABLE 2: Fluorescence Decay Times, Excited at 315 nm and Monitored at 380 Nm, of 7HQ in AOT Reverse Micelles at Various w0 of 1H2O and 2H2O decay time (ps) w0

1H Oa 2

0 2 4 8 12 24 48

1100(40%) + 2900 560(52%) + 2000 280(55%) + 1100 190(57%) + 950 120(55%) + 650 110(54%) + 600

2H

3200

2O

b

1200(40%) + 2800 660(52%) + 2300 370(55%) + 1400 290(57%) + 1200 180(55%) + 1000 150(54%) + 820

a Values in parentheses indicate the initial intensity percentage. Decay times in 2H2O were fitted by fixing the initial intensity percentages to those at the same w0 of 1H2O.

b

from the decreased charge density of the bound-water layer containing the N* species. On the other hand, the fast-decay times are much larger than the reported decay times (40 ps in 1H O and 83 ps in 2H O) in bulk water even at the observed 2 2 highest water content of w0 ) 48.30 This agrees with the fact that water activity in the bound-water layer of a reverse micelle is lower than that in bulk water.61 However, when 2-naphthol derivatives are present in free-water cores, their photoexcited proton-transfer dynamics is the same as that in bulk water.62 Thus, the lifetime decrease of N* fluorescence with w0 also supports the mentioned suggestion that N molecules reside in the bound-water layers. It is noteworthy that the amplitude percentages of the fast and the slow components at w0 g 8 remain almost invariant as 55% and 45%, respectively, with w0 increment. We consider that the fast and the slow decay times are the excited-state lifetimes of cis-7HQ and trans-7HQ, respectively. Semiempirical calculations and jet-cooled molecular-beam experiments have shown that the bare N species in the gas-phase exists in the rotamer of either cis or trans although the cis is lower by 0.71 kcal/mol in energy than the trans.25,26 The Gibbs-energy lowering of the cis, calculated with the initial intensity percentages of the fast and slow components, is 0.12 kcal/mol. This is smaller than the energy lowering in the gas phase because the cis is less favorable in H bonding than the trans in AOT water pools. The barrier energy of the internal rotation of an enolic group relative to an aromatic ring is substantially high,26 and it is expected to increase upon electronic excitation because of the increase of quinonoidal character on the hydroxyl moiety. Furthermore, H bonding between solvent water molecules and the enolic group will raise the barrier. The barrier of 3.5 kcal/ mol in the internal rotation of the enolic group in H-bonded phenol-water clusters increases largely indeed upon excitation to become 13.5 kcal/mol in S1.70 While the fast- and the slowdecay times of biphasic N* fluorescence in bulk alcohols have been attributed to the ESPT time of the cis and the relaxation time of the trans, respectively,20 those in a polymer matrix have been attributed to the internal-rotation time of the trans and the bridge-formation time of the cis, respectively, prior to proton relay.29 We infer that the cis and the trans rotamers of N* cannot rotate internally to transform into each other within their excitedstate lifetimes because of the high energy barrier. Thus, it is suggested that the fast tautomerization component is due to the proton relay of cis-N* whereas the slow tautomerization component is due to the acid-base reactions of trans-N* (vide infra). 4. Discussion 7HQ in neutral bulk water shows a single decay component of 40 ps resulting from excited-state tautomerization, indicating

Figure 8. Changes of krelay (circles) and kAB (squares) values with w0 of AOT reverse micelles having 1H2O (open) and 2H2O (closed). Diamonds indicate the deprotonation rate constants of N* in bulk water.

that both rotamers of 7HQ have equivalent photoacidity.33 Photoacids having a hydroxyl group go through acid-base reactions showing a single rate constant,2,6,11-16 although they can have different rotamers. The biphasic decay kinetics of 7HQ in the water pool has already been attributed to two different ESPT processes of trans-N* and cis-N* in the result section. The trans-N* can undergo tautomerization only via the individual acid-base reactions of the enolic and the imino groups with water molecules because it cannot undergo internal rotation to form cis-N* within its lifetime. However, cis-N* can undergo proton relay from the enolic group to the imino group along a H-bonded water chain as well as the acid-base reactions of the enolic and the imino groups with the water molecules. Assuming that tautomerization via individual acid-base reactions of the two prototropic groups can take place on the same time scale in both rotamers, we suggest the fast channel of ESPT in cis-N* is the proton-relay process along a H-bond water bridge. In addition, we can also infer that tautomerization via proton relay is much more facile than that via the acid-base reactions of cis-N* because the fast-decay time is 5 times shorter than the slow one at w0 ) 12. It is noteworthy that the cis-N* rotamer in the bound-water layers of the reverse micelles mainly goes through proton relay to tautomerize as widely observed in bulk alcohols although 7HQ in bulk water is known to undergo ESPT via sequential acid-base reactions. The total rate constants of cis-N* (kcis) and trans-N* (ktrans) are the inverses of the fast (τfast) and the slow fluorescence decay times (τslow) and consist of rate constants as eq 2 and eq 3, respectively.

kcis ) τfast-1 ) krelay + kAB + krxn

(2)

ktrans ) τslow-1 ) kAB + krxn

(3)

The krelay, kAB, and krxn of eq 2 and eq 3 are the rate constants of proton relay, acid-base reactions, and relaxation, respectively. The krxn is assumed to be the reciprocal of the N*-decay time at w0 ) 0. The variation of krelay and kAB with w0 is presented in Figure 8. Two water molecules are presumed to participate in the formation of a bridging H-bonded water chain. Small KIEs of krelay imply that the rate determination of tautomerization via proton relay for cis-N* is solvent reorganization. Isotopedependent intrinsic proton motions are assumed to be very fast. The reported KIE of ESPT is as low as 1.2 in bulk methanol.19 The initial ESPT step of 7HQ in bulk alcohols has already been proposed to involve thermally activated solvent reorganization

12002 J. Phys. Chem. B, Vol. 110, No. 24, 2006

Figure 9. krelay KIE (circles) and microviscosity values (squares) versus w0 at the interface of aqueous AOT micelles. The viscosity values determined in 1H2O reverse micelles were taken from ref 72. We note that the viscosity of 2H2O is higher by 24% at room temperature than that of 1H2O.73

to achieve a proper geometry for proton relay. Interactions between the charges of headgroups and the dipoles of water molecules at the interfaces of reverse micelles are expected to decrease the H-bond network of 7HQ with water molecules in the bound-water layer, as reported with the vibrational spectroscopic results of AOT reverse micelles.45 Water in AOT reverse micelles, having a relative dielectric constant of 3040,40-42 has been suggested to be alcohol-like in polarity. The formation of a cyclically H-bonded optimum structure is prerequisite to the intrinsic proton relay of 7HQ.18 It is not sensitive to hydrogen isotopes and results mostly from heavyatom motions requiring reorganization energy. If the motions of hydrogen atoms including tunneling solely limit the rate, the KIE is predicted to be neither dependent on solvent viscosity nor equal to unity. However, if solvent reorganization plays an important role in the overall dynamics of proton relay, the KIE depends on viscosity. Solvent reorganization, required to reach the cyclic configuration optimal for tunneling, becomes fast with the decrease of solvent viscosity. Thus, both tunneling contribution and the KIE tend to increase with the viscosity decrease of the medium.71 Figure 9 shows that the KIE of krelay tends to increase with w0 as the value of krelay changes as presented in Figure 8. Microviscosity at the vicinity of an AOT-micellar interface is reported to be very large at low w0 (40 cP at w0 ) 4).72 However, it decreases with a w0 increase, rapidly below w0 ) 10 and gradually above w0 ) 10 to reach 10 cP at w0 ) 50. Both the translational and the rotational motions of water are reported to be strongly suppressed in the vicinity of the micellar interface because of the interactions of water with charged headgroups and counterions.55 Thus, the mobility of water molecules increases with w0 because the density of charges interacting with water decreases subsequently. The overall rate of the protonrelay reaction of cis-N* at low w0 is mostly controlled by solvent reorganization owing to high viscosity. In the bound-water layers of reverse micelles having substantially smaller polarity ( ) 30-40) than bulk water ( ) 78), the barrier of deprotonation, the initial step of ESPT to the form anionic intermediate species, increases extensively. In contrast, the barrier of protonation to the imino group of the anionic intermediate to form the neutral keto species of T* decreases subsequently. The immediate decay of an intermediate prototropic species following its slow formation switches the acid-base mechanism of cis-N* tautomerization in bulk water to a proton-relay mechanism in the polarity-reduced bound-water

Kwon et al. layer of an AOT reverse micelle. In other words, the altered acid-base energetics of 7HQ prototropic groups and the reduced activity of water render cis-N to undergo excited-state tautomerization via proton relay upon absorption of a photon in the bound layer of the AOT water pool. The result is in line with the observation that the instability of a charged species in the hydrophobic cages of β-cyclodextrin causes the slow formation and the fast depletion of the anionic intermediate species in the ESPT process of 7HQ.33 The deprotonation rates of photoacids have also been reported to be slower by factors of 2 orders of magnitude in the bound-water layers of reverse micelles than in bulk water.61 Acid-base reactions in water are closely related to solvent motion and relaxation in general. The overall rate constant of kr is generally predicted as

kr ) Ws exp(-∆G‡a /(kBT))

(4)

where Ws is the preexponential factor determined by the frequency of solvent relaxation and ∆G‡a is the free energy of activation depending on bonds involved in proton transfer and on solvent-reorganization energy.6,74-76 Equation 5, similar to eq 4, has also been suggested independently to describe the overall rate of a deprotonation reaction (kd).

kd ) τd-1 exp(-∆Gs/(kBT))

(5)

where τd is the collective dipole-correlation time and ∆Gs is the free energy of activation. While τd is about the same as the dielectric-relaxation time of the solvent (τD), ∆Gs results mainly from the solvent entropy of activation following the solvation of the proton.77 In the cases of very exothermic acid-base reactions, the rate constants are mainly controlled by solvent reorganization to approach the Ws of eq 4 so that the KIE of kr becomes close to Ws(1H)/Ws(2H). Because τD is very large (680 ps at w0 ) 12) in our case, solvent reorganization is very demanding for facile ESPT. The deprotonation time of 950 ps and the KIE of 1.42 at w0 ) 12 imply that the deprotonation rate of cis-N* is mainly controlled by solvent rearrangement. The KIEs of kAB obtained in our experimental range of w0 are 1.32 ( 0.13. In bulk water having τD of 8 ps, the deprotonation of N* occurs on the time scale of 40 ps with the KIE of 2.1.30 Note that the KIE of the deprotonation of pyranine is reported to decrease from 5.6 in bulk water to 3.2 in the water pools of reverse micelles at w0 ) 8.2.59 Therefore, it is inferred that the deprotonation process of trans-N* in the water pools of reverse micelles is in accordance with the common view of free-energy relationship in bimolecular proton-transfer reactions. Our results are considered to provide an experimental example of proton-relaying phenomena in biological systems.78-80 The proton translocation of an excited-state cis-7HQ molecule located in the vicinity of an AOT-micellar interface takes place preferentially via proton relay from the acidic enolic group to the distant basic imino group along a proton wire because water solvating 7HQ therein has reduced polarity and disrupted H-bond networks compared with bulk water. Acid-base reactions are inefficient owing to the low activity of water molecules in interfaces so that proton-transfer mechanisms are often switched to relatively efficient proton relay processes coupled to inductive electronic effects along a H-bonded water chain. We note that 7HQ complexed cyclically with water cannot undergo ESPT because of a very high barrier in gas phases.10 Considering our low KIEs with this, one might doubt our mechanism of proton relay for cis-N*. It has been reported that although 1:1 7-azaindole-water complexes cannot undergo

Excited-State Proton Transfer in Reverse Micelles ESPT in gas phases,81,82 they are reported to undergo ESPT in condensed phases.83 This also supports that cyclically complexed cis-N* with water can undergo proton relay in reverse micelles. Our unpublished data also show that cyclic 7HQ-(H2O)2 complexes in ether go through proton relay indeed. As described already, the KIEs are low because the rate determining step is solvent reorganization. Also note that we have suggested proton relay rather than concerted proton transfer for cis-N*. Proton relay can occur asymmetrically4,5 or concertedly. 5. Conclusions The normal and the tautomeric species of ground-state 7HQ are found to reside preferentially in the bound- and the freewater layers of AOT reverse micelles, respectively. The spectra and the kinetics of 7HQ in the water pools of reverse micelles are enormously different from those in bulk water. In particular, the excited-state tautomerization of the normal species is suggested to take place via two different channels, depending on rotamers at the moment of excitation, as summarized in Figure 1c. cis-N* tautomerizes via proton relay from the enol group to the imino group along the bridge of two H-bonded water molecules, unusual in water but common in alcohols, whereas trans-N* tautomerizes via the stepwise individual acidbase reactions of two prototropic groups as known in bulk water. Proton relay can take place for cis-N* in the bound water because water confined in the pools of reverse micelles, compared with bulk water, has greatly reduced polarity and substantially disrupted H-bond networks. Acknowledgment. The authors thank The Korea Research Foundation for Grant KRF-2004-015-C00230. Y.S.L. acknowledges the fellowship of the BK21 Program. References and Notes (1) Tanner, C.; Manca, C.; Leutwyler, S. Science 2003, 302, 1736. (2) Rini, M.; Magnes, B.-Z.; Pines, E.; Nibbering, E. T. Science 2003, 301, 349. (3) Park, S.; Kwon, O.-H.; Kim, S.; Choi, M.-G.; Cha, M.; Park, S. Y.; Jang, D.-J. J. Am. Chem. Soc. 2005, 127, 10070. (4) Kwon, O.-H.; Lee, Y.-S.; Yoo, B. K.; Jang, D.-J. Angew. Chem., Int. Ed. 2006, 45, 415. (5) Kwon, O.-H.; Lee, Y.-S.; Park, H. J.; Kim, Y.; Jang, D.-J. Angew. Chem., Int. Ed. 2004, 43, 5792. (6) Agmon, N. J. Phys. Chem. A 2005, 109, 13. (7) Geissler, P. L.; Dellago, C.; Chandler, D.; Hutter, J.; Parrinello, M. Science 2001, 291, 2121. (8) Inoue, J.; Tomioka, N.; Itai, A.; Harayama, S. Biochemistry 1998, 37, 3305. (9) le Coutre, J.; Tittor, J.; Oesterhelt, D.; Gerwert, K. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 4962. (10) (a) Tanner, C.; Thut, M.; Manca, C.; Leutwyler, S. J. Phys. Chem. A 2006, 110, 1758. (b) Bach, A.; Coussan, S.; Mu¨ller, A.; Leutwyler, S. J. Chem. Phys. 2000, 112, 1192. (11) Tolbert, L. M.; Solntsev, K. M. Acc. Chem. Res. 2002, 35, 19. (12) Solntsev K. M.; Tolbert, L. M.; Cohen, B.; Huppert, D.; Hayashi, Y.; Feldman, Y. J. Am. Chem. Soc. 2002, 124, 9046. (13) (a) Solntsev, K. M.; Huppert, D.; Agmon, N. Phys. ReV. Lett. 2001, 86, 3427. (b) Solntsev, K. M.; Huppert, D.; Agmon, N. J. Phys. Chem. A 2001, 105, 5868. (c) Solntsev, K. M.; Agmon, N. Chem. Phys. Lett. 2000, 320, 262. (14) Cohen, B.; Huppert, D.; Agmon, N. J. Phys. Chem. A 2001, 105, 7165. (15) Solntsev, K. M.; Huppert, D.; Tolbert, L. M.; Agmon, N. J. Am. Chem. Soc. 1998, 120, 7981. (16) Tolbert, L. M.; Haubrich, J. E. J. Am. Chem. Soc. 1994, 116, 10593. (17) Mason, S. F.; Philp, J.; Smith, B. E. J. Chem. Soc. A 1968, 3051. (18) Konijnenberg, J.; Ekelmans, G. B.; Huizer, A. H.; Varma, C. A. G. O. J. Chem. Soc., Faraday Trans. 2 1989, 85, 39. (19) Itoh, M.; Adachi, T.; Tokumura, K. J. Am. Chem. Soc. 1984, 106, 850. (20) Nakagawa, T.; Kohtani, S.; Itoh, M. J. Am. Chem. Soc. 1995, 117, 7952.

J. Phys. Chem. B, Vol. 110, No. 24, 2006 12003 (21) Kohtani, S.; Tagami, A.; Nakagaki, R. Chem. Phys. Lett. 2000, 316, 88. (22) (a) Fang, W. J. Am. Chem. Soc. 1998, 120, 7568. (b) Fang, W. J. Phys. Chem. A 1999, 103, 5567. (23) (a) Chou, P.-T.; Wei, C.-Y.; Wang, C.-R. C.; Hung, F.-T.; Chang, C.-P. J. Phys. Chem. A 1999, 103, 1939. (b) Chou, P.-T.; Wei, C.-Y. J. Phys. Chem. B 1998, 102, 3305. (24) Garcı´a-Ochoa, I.; Dı´ez Lo´pez, M.-A.; Vio`as, M. H.; Santos, L.; Martı´nez Ataz, E.; Sa´nchez, F.; Douhal, A. Chem. Phys. Lett. 1998, 296, 335. (25) Garcı´a-Ochoa, I.; Bisht, P. B.; Sa´nchez, F.; Martinez Ata´z, E.; Santos, L.; Tripathi, H. B.; Douhal, A. J. Phys. Chem. A 1998, 102, 8871. (26) Lahmani, F.; Douhal, A.; Breheret, E.; Zehnacker-Rentien, A. Chem. Phys. Lett. 1994, 220, 235. (27) (a) Bach, A.; Tanner, C.; Manca, C.; Frey, H.-M.; Leutwyler, S. J. Chem. Phys. 2003, 119, 5933. (b) Meuwly, M.; Bach, A.; Leutwyler, S. J. Am. Chem. Soc. 2001, 123, 11446. (28) Matsumoto, Y.; Ebata, T.; Mikami, N.; J. Phys. Chem. A. 2002, 106, 5591. (29) Kwon, O.-H.; Doo, H.; Lee, Y.-S.; Jang, D.-J. ChemPhysChem 2003, 4, 1079. (30) Park, H. J.; Kwon, O.-H.; Ah, C. S.; Jang, D.-J. J. Phys. Chem. B 2005, 109, 3938. (31) Yu, H.; Kwon, O.-H.; Jang, D.-J. J. Phys. Chem. A 2004, 108, 5932. (32) Kim, T. G.; Kim, Y.; Jang, D.-J. J. Phys. Chem. A 2001, 105, 4328. (33) Lee, S.-I.; Jang, D.-J. J. Phys. Chem. 1995, 99, 7537. (34) Poizat, O.; Bardez, E.; Buntinx, G.; Alain, V. J. Phys. Chem. A 2004, 108, 1873. (35) Bardez, E.; Fedorov, A.; Berberan-Santos, M. N.; Martinho, J. M. G. J. Phys. Chem. A 1999, 103, 4131. (36) Bardez, E.; Devol, I.; Larrey, B.; Valeur, B. J. Phys. Chem. B 1997, 101, 7786. (37) Bardez, E.; Chaˆtelain, A.; Larrey, B.; Valeur, B. J. Phys. Chem. 1994, 98, 2357. (38) Bardez, E.; Devol, I.; Chaˆtelain, A. J. Colloid Interface Sci. 1998, 205, 178. (39) Mehata, M. S.; Joshi, H. C.; Tripathi, H. B. Chem. Phys. Lett. 2002, 359, 314. (40) Bhattacharyya, K. Acc. Chem. Res. 2003, 36, 95. (41) Nandi, N.; Bhattacharyya, K.; Bagchi, B. Chem. ReV. 2000, 100, 2013. (42) Bhattacharyya, K.; Bagchi, B. J. Phys. Chem. A 2000, 104, 10603. (43) Cohen, B.; Huppert, D.; Solntsev, K. M.; Tsfadia, Y.; Nachliel, E.; Gutman, M. J. Am. Chem. Soc. 2002, 124, 7539. (44) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: Orlando, FL, 1987; Chapter 5. (45) Maitra, A. J. Phys. Chem. 1984, 88, 5122. (46) Hauser, H.; Haering, G.; Pande, A.; Luisi, P. L.; J. Phys. Chem. 1989, 93, 7869. (47) Venables, D. S.; Huang, K.; Schmuttenmaer, C. A. J. Phys. Chem. B 2001, 105, 9132. (48) Moran, P. D.; Bowmaker, G. A.; Cooney, R. P.; Bartlett, J. R.; Woolfrey, J. L. Langmuir 1995, 11, 738. (49) Onori, G.; Santucci, A. J. Phys. Chem. 1993, 97, 5430. (50) Dutta, P.; Sen, P.; Mukherjee, S.; Halder, A.; Bhattacharyya, K. J. Phys. Chem. B 2003, 107, 10815. (51) Sen, S.; Dutta, P.; Sukul, D.; Bhattacharyya, K. J. Phys. Chem. A 2002, 106, 6017. (52) Pal, S. K.; Mandal, D.; Sukul, D.; Bhattacharyya, K. Chem. Phys. Lett. 1999, 312, 178. (53) Sarkar, N.; Das, K.; Datta, A.; Das, S.; Bhattacharyya, K. J. Phys. Chem. 1996, 100, 10523. (54) (a) Hazra, P.; Chakrabarty, D.; Sarkar, N. Chem. Phys. Lett. 2003, 371, 553. (b) Hazra, P.; Chakrabarty, D.; Sarkar, N. Langmuir 2002, 18, 7872. (55) (a) Riter, R. E.; Willard, D. M.; Levinger, N. E. J. Phys. Chem. B 1998, 102, 2705. (b) Mittleman, D. M.; Nuss, M. C.; Colvin, V. L. Chem. Phys. Lett. 1997, 275, 332. (56) Abel, S.; Sterpone, F.; Bandyopadhyay, S.; Marchi, M. J. Phys. Chem. B 2004, 108, 19458. (57) Balasubramanian, S.; Pal, S.; Bagchi, B. Phys. ReV. Lett. 2002, 89, 115505. (58) (a) Faeder, J.; Ladanyi, B. M. J. Phys. Chem. B 2005, 109, 6732. (b) Faeder, J.; Ladanyi, B. M. J. Phys. Chem. B 2001, 105, 11148. (c) Faeder, J.; Ladanyi, B. M. J. Phys. Chem. B 2000, 104, 1033. (59) Escabi-Perez, J. R.; Fendler, J. H. J. Am. Chem. Soc. 1978, 100, 2234. (60) Politi, M. J.; Brandt, O.; Fendler, J. H. J. Phys. Chem. 1985, 89, 2345. (61) Politi, M. J.; Chaimovich, H. J. Phys. Chem. 1986, 90, 282. (62) Bardez, E.; Monnier, E.; Valeur, B. J. Phys. Chem. 1985, 89, 5031.

12004 J. Phys. Chem. B, Vol. 110, No. 24, 2006 (63) Bardez, E.; Goguillon, B.-T.; Keh, E.; Valeur, B. J. Phys. Chem. 1984, 88, 1909. (64) Das, S.; Datta, A.; Bhattacharyya, K. J. Phys. Chem. A 1997, 101, 3299. (65) (a) Kwon, O.-H,; Jang, D.-J. J. Phys. Chem. B 2005, 109, 8049. (b) Kwon, O.-H,; Jang, D.-J. J. Phys. Chem. B 2005, 109, 20479. (66) Kim, T. G.; Topp, M. R. J. Phys. Chem. A 2004, 108, 10060. (67) Maroncelli, M.; Fleming, G. R. J. Chem. Phys. 1987, 86, 6221. (68) Cho, C. H.; Chung, M.; Lee, J.; Nguyen, T.; Singh, S.; Vedamuthu, M.; Yao, S.; Zhu, J.-B.; Robinson, G. W. J. Phys. Chem. 1995, 99, 7806. (69) Kaatze, U. Chem. Phys. Lett. 1993, 203, 1. (70) Berden, G.; Meerts, W. L.; Schmitt, M.; Kleinermanns, K. J. Chem. Phys. 1996, 104, 972. (71) Basran, J.; Sutcliffe, M. J.; Scrutton, N. S. Biochemistry 1999, 38, 3218. (72) Hasegawa, M.; Sugimura, T.; Suzuki, Y.; Shindo, Y.; Kitahara, A. J. Phys. Chem. 1994, 98, 2120. (73) Cho, C. H.; Urquidi, J.; Singh, S.; Robinson, G. W. J. Phys. Chem. B 1999, 103, 1991.

Kwon et al. (74) Pines, E.; Pines, D.; Barak, T.; Magnes, B.-Z.; Tolbert, L. M.; Haubrich, J. E. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 511. (75) Pines, E.; Magnes, B.-Z.; Lang, M. J.; Fleming, G. R. Chem. Phys. Lett. 1997, 281, 413. (76) Solntsev, K. M.; Huppert, D.; Agmon, N. J. Phys. Chem. A 1999, 103, 6984. (77) Robinson, G. W. J. Phys. Chem. 1991, 95, 10386. (78) Luecke, H.; Richter, H.-T.; Lanyi, J. K. Science 1998, 280, 1934. (79) Tanimoto, T.; Shibata, M.; Belenky, M.; Herzfeld, J.; Kandori, H. Biochemistry 2004, 43, 9439. (80) Qian, J.; Mills, D. A.; Geren, L.; Wang, K.; Hoganson, C. W.; Schmidt, B.; Hiser, C.; Babcock, G. T.; Durham, B.; Millett, F.; FergusonMiller, S. Biochemistry 2004, 43, 5748. (81) Folmer, D. E.; Wisniewski, E. S.; Stairs, J. R.; Castleman, A. W., Jr. J. Phys. Chem. A 2000, 104, 10545. (82) Ferna´ndez-Ramos, A.; Smedarchina, Z.; Siebrand, W.; Zgierski, M. Z.; Rios, M. A. J. Am. Chem. Soc. 1999, 121, 6280. (83) Chou, P.-T.; Martinez, M. L.; Cooper, W. C.; McMorrow, D.; Collins, S. T.; Kasha, M. J. Phys. Chem. 1992, 96, 5203.