Microviscosity inside a Nanocavity: A Femtosecond Fluorescence Up

Oct 15, 2010 - The experimental results reveal a strong dependence of S1 state relaxation ... As we increase the w0 from 2 to 40, the microviscosity d...
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J. Phys. Chem. B 2010, 114, 13988–13994

Microviscosity inside a Nanocavity: A Femtosecond Fluorescence Up-Conversion Study of Malachite Green Shahnawaz Rafiq, Rajeev Yadav, and Pratik Sen* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, 208 016, UP, India ReceiVed: April 26, 2010; ReVised Manuscript ReceiVed: August 29, 2010

Femtosecond fluorescence up-conversion measurements of malachite green (MG) have been carried out to confirm the relaxation pathway and subsequently to probe the microviscosity of water trapped in a nanoconfined environment using an AOT (sodium dioctylsulfosuccinate, aerosol-OT) reverse micelle as a model system. The experimental results reveal a strong dependence of S1 state relaxation dynamics of MG on solvent viscosity while a very weak dependence has been observed for the S2 state relaxation. The time-dependent density functional theory (TD-DFT) calculations have been used to construct potential energy surfaces of MG by pursuing an intramolecular rotation along the torsional coordinate of the phenyl rings. On synchronization with the experimental observations, the computational results comprehend the existence of a conical intersection along the S1 and S0 potential energy surfaces, which leads to mixed vibrational levels of S1 and S0 characteristics. The results suggest that the conical intersection is along the torsional coordinate of N,N-dimethyl substituted phenyl ring. Correlating the observed dynamics of MG in a confined system with the relaxation time of MG in different glycerol-water mixtures, we assert the determination of the microviscosity of water inside the AOT reverse micelle. The data confer that the microviscosity of water in an AOT water pool of w0 ) 2 (9 cP) is almost 9 times higher than the bulk water. As we increase the w0 from 2 to 40, the microviscosity decreases monotonically to 5.68 cP, and the decrease is observed to be exponential in nature. 1. Introduction The ground and excited state dynamics of triphenylmethane (TPM) dyes in solution is a sensitive probe of the influence of the environment on intramolecular motion along a reactive potential energy surface.1–5 A number of studies have been carried out to understand the molecular structure, electronic states, and relaxation dynamics of TPM dyes.1–5 The dynamics of the first singlet excited state (S1) of TPM dyes is observed to show a strong viscosity dependence, and it has been ascribed to strong coupling between electronic states and the torsional degrees of freedom.1–5 The nature of molecular electronic states of TPM dyes bears a strong correlation with the angle between the central sp2 hybridized carbon and the plane of phenyl rings. The fluorescence lifetime, quantum yield, and the ground state recovery time of TPM dyes have undergone a great deal of study. In general, these quantities have been found to be proportional to the viscosity (η) raised to a power ranging roughly between 0.33 and 0.66.2,4,5 The model for the radiationless decay of the excited singlet state to the ground state is believed to involve a rotation of the phenyl rings toward an equilibrium geometry displaced from that in the ground state. It is proposed that the internal conversion rate is much higher near the equilibrium (through a conical intersection) geometry of the excited state and the viscosity dependence of the relaxation dymanics arises simply from the solvent frictional effect in resisting the phenyl ring rotation. The initial ground state is subsequently repopulated by reverse torsional motion from high up on the ground state surface.1–5 Because of the friction dependence of the excited state lifetime, TPM dyes can be used as a local viscosity probe. The effect of solvent polarity * Corresponding author. Pratik Sen, Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, 208 016, UP, India. E-mail: [email protected]. Fax: +91-512-259-7436.

is not found to be significant on the excited state dynamics in solvents of similar viscosities but with different polarities.6 The weak solvent polarity dependence suggests that the potential energy surfaces are to be similar in the different environments. One of such TPM dye is malachite green (MG) and has been well-studied to a great extent. Contrary to Kasha’s rule, MG has been found to show fluorescence emission from S2 state, imparting its significance in excited state processes.4a,5b Various efforts have been made to characterize the relaxation dynamics of S1 and S2 states of MG. Using femtosecond fluorescence spectroscopy, Mokhtari et al. suggested the presence of a hot ground state in the relaxation pathway of the S1 state of MG.5a Several other transient absorption studies also reveal the same conclusion for the excited state relaxation pathway of MG.1–4 All studies emphasized the presence of a conical intersection between the S1 and the S0 states of MG. Yoshizawa et al. studied the S2 fluorescence of MG by fluorescence up-conversion spectroscopy in different solvents.5b According to them, the S2 state relaxation follows a single exponential decay in low viscous solvents. The origin of the decay component has been explained by both the torsional configuration change and the internal conversion from the S2 state to the S1 state.5b In another work, Bhasikuttan et al.5c on the basis of kinetic data along with the time-resolved fluorescence anisotropy measurements proposed a relaxation pathway along an interaction of S2 and S1 potential energy surfaces forming a conical intersection. It has also been proposed that the conical intersection is along the torsional coordinate of unsubstituted phenyl ring of the MG dye.5c The ground state recovery dynamics of MG have already been measured in several systems to understand the microviscosity of the medium. Canva et al. studied the viscosity of xerogel matrix by probing the ground state recovery dynamics of MG and found that the viscosity is about 25 cP in the matrix.7a

10.1021/jp1037238  2010 American Chemical Society Published on Web 10/15/2010

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Nakatsuka et al. measured the fluorescent decay time of MG doped in an onion cell and tried to correlate with the microscopic dynamics in different parts of the cell.7b Recently, Nagasawa et al. utilize MG to probe microscopic viscosity of the water-alcohol binary mixture.7c There has also been a lot of work regarding the relaxation dynamics of MG at the different interfaces.8 Until now, a lot of work has been devoted to understand the dynamics of MG in bulk solvents and at the interface. However, only a little concern has been paid to explore its dynamics in a biological environment. As a first choice, one can study the dynamics of MG in a reverse micelle which can be an elegant model to mimic the biological systems.9 Among all of the reverse-micellar systems, AOT (sodium dioctylsulfosuccinate, Aerosol-OT) has been studied extensively using many techniques, and it is reported that water trapped in the nano cavity behaves quite differently than the bulk water.10 One of the important questions is: what is the nature of water in the water pool of the AOT reverse micelle? In the simplest model, many authors have postulated the presence of two types of water in the reverse micelles: one is the interfacial (shell) water, which exhibits distinct properties than bulk water, and the other is interior (core) water, which behaves similarly to bulk water.11–16 Several groups have employed this concept to elucidate the structure and dynamics of solvents inside the reverse micelles. Levinger et al. have used a combination of steady state, vibrational lifetime, orientational relaxation and vibrational echo spectroscopies to study the OD stretch of diluted HOD in H2O in AOT reverse micelles, and their results strongly support the core-shell model.15,17,18 They concluded that confinement by an interface to form an nanoscopic water pool is a primary factor governing the slow dynamics of nanoscopic water rather than the presence of charged groups at the interface. Correa’s group has studied the new insights of AOT water pool by using the photophysics of fluorescent molecules like coumarin 343 and prodan.19–21 Recently they have used a cationic reverse micelle to create water with a super hydrogen-bond-donor capacity for enzymatic catalysis.22 Amararene et al. reported that, even in a large water pool (w0 ) 27), the compressibility is 2 times larger than that in bulk water.24a Venables et al. reported a 25% shift in liberation frequency at 670 cm-1 in an AOT reverse micelle compared to that in bulk water.16 The terahertz (THz) absorption spectrum of confined water molecules is in the region of 0.1-1.3 THz which is very different from that of bulk water; as in a water pool size of 1.5 nm, the terahertz frequency shifts from 25 cm-1 to about less than 10 cm-1 in a 2.9 nm water pool.24b The size of the water pool in reverse micelles is expressed in terms of a ratio of number of water molecules to the number of surfactant molecules and is denoted by w0. With an increase in the magnitude of w0, the radius of the water pool (rw) increases. On the basis of NMR data Maitra formulated an expression for the radius of water core (rw) inside the reverse micelle10a as

rw )

(

90w0nag 4π

)

1/3

(1)

where nag is the mean average aggregation number of surfactant per droplet. Kinugasa et al. estimated the size of the AOT reverse micelle based on the following expression.23

rw )

(

1 6VwCwnag 2 πCs

)

1/3

(2)

where Vw, Cw, Cs, and nag represent the volume of water, the concentration of water, the concentration of surfactant, and the aggregation number, respectively. For w0 between 2 and 20, one can derive the following simplified equation for the diameter of water pool of AOT reverse micelle

dAOT ) (0.29w0 + 1.1) nm

(3)

In the present study, we suggest the nature of potential energy profile of S0, S1, and S2 electronic states of MG and also the probable involvement of the three different phenyl groups on the relaxation dynamics by using the femtosecond fluorescence up-conversion measurements in different glycerol-water mixed solvents and quantum mechanical calculations in vacuum. As discussed by different authors,1–5 the rotation of phenyl rings in the excited state is the major deactivation process. However, there was no such attempt to investigate which of the phenyl ring(s) is responsible for such rapid deactivation of the excited state. From the quantum mechanical calculations we also estimate the contribution of different phenyl rings in the deactivation process. The observed viscosity-dependent relaxation dynamics has been further exploited to determine the microviscosity of nanoconfined environment with different dimensions using an AOT reverse micelle as a model system. 2. Experimental Section 2.1. Materials. MG was purchased from Spectrochem, India as MG oxalate and used without further purification. Dioctyl sulfosuccinate sodium salt (AOT) was purchased from Sigma Chemical Company and vacuum-dried before being used. Highperformance liquid chromatography (HPLC) grade distilled water (Merck, India) and glycerol (Aldrich) and n-heptane (Spectrochem, India) were used for the sample preparation. Solvents with different viscosity used in this study are water and mixtures of glycerol in water ranging from 10% to 70% glycerol. All AOT samples were prepared under nitrogen atmosphere. 2.2. Steady State Measurements. The UV-visible absorption and fluorescence spectra of the sample solutions have been measured by a commercial UV-visible spectrophotometer (UV2401, Shimadzu) and fluorimeter (Spex, fluoroMax-3, JobinYvon), respectively. 2.3. Femtosecond Fluorescence Up-Conversion Study. The fluorescence transients have been measured by a femtosecond fluorescence up-conversion setup (FOG-100, CDP Corp.). The details of the setup are discussed elsewhere.25 Briefly, the sample has been excited at 410 nm using the second harmonic of a mode-locked Ti-sapphire laser (Tsunami, Spectra Physics), pumped by a 5 W Millennia (Spectra Physics). To generate a second harmonic, we used a nonlinear crystal (1 mm BBO, θ ) 25°, φ ) 90°). The fluorescence emitted from the sample was up-converted in another nonlinear crystal (0.5 mm BBO, θ ) 38°, φ ) 90°) by using the fundamental beam as a gate pulse. The up-converted light is dispersed in a monochromator and detected by photon counting electronics. A cross-correlation function obtained with use of the Raman scattering from water displayed a full width at half-maximum (fwhm) of 350 fs. The femtosecond fluorescence decays were deconvoluted using a Gaussian shape for the exciting pulse using commercial software (IGOR Pro, WaveMetrics). The S2 and S1 state fluorescence transients of MG were monitored at 500 and 670 nm, respectively. All measurements were done at 20 °C.

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Figure 1. Molecular structure of malachite green (MG).

2.4. Computational Methods. Ground state optimization of MG has been performed by using density functional theory (DFT) calculations using a B3LYP hybrid functional with a 6-31+g(d,p) basis set. A rigorous formalism (time dependentDFT or TD-DFT) has been used to calculate the transition energies within the DFT framework using the same hybrid function and the same basis set. All calculations were performed using Gaussian 03 software. We have considered the optimized structure obtained in vacuum to hold well for excited state calculations assuming not much change in the corresponding geometries. Transition energies obtained from TD-DFT calculations are the vertical excitation energies obtained without zero point corrections. MG contains two N,N-dimethyl substituted phenyl rings and another unsubstituted phenyl ring (see Figure 1). To determine the potential energy profile of ground, first, and second excited states of MG, we determine the energies of different geometries by rotating the dihedral angles of different phenyl rings. For the estimation of the effect of different phenyl rings on the relaxation process, we have selectively rotated the unsubstituted phenyl ring (C24-C23-C1-C13 dihedral angle) and substituted phenyl ring (C15-C13-C1-C23 dihedral angle) linked to the central sp2 hybridized carbon atom and compared it with the rotation of all of the three phenyl rings at a different extent from its equilibrium position. 3. Results and Discussion 3.1. Steady State Absorption and Emission Study in Glycerol-Water Mixtures. The absorption spectrum of MG in water shows two peaks, one at 425 nm corresponding to S2rS0 absorption and other at 617 nm corresponding to S1rS0 absorption. On increasing the viscosity to 70% glycerol in water, there is a little red shift in the absorption maxima from 425 to 430 nm for the S2rS0 absorption band and from 617 to 626 nm for the S1rS0 absorption band as shown in Figure 2. Upon excitation at 410 nm, the emission spectrum of MG shows two fluorescence bands. The band in the region of 430-600 nm is attributed to the S2 fluorescence. In this case, the Raman scattering from the solvent is predominant over the fluorescence as the fluorescence quantum yield is very low. The fluorescence in the 620-750 nm region clearly shows a maximum at 667 nm in bulk water and has been assigned to the S1 fluorescence. On increasing the viscosity to 70% glycerol in water, the S1 emission maximum is found to be at 665 nm which is 2 nm blue-shifted from the MG in water. The intensity of emission for both the S1 and the S2 fluorescence shows a strong viscosity dependence (Figure 3). The increase in fluorescence intensity

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Figure 2. Absorption spectra of MG in bulk water (s), 30% glycerol ( · · · · ), and 70% glycerol (- - -) in water.

Figure 3. Steady state fluorescence spectra of MG in bulk water (s), 30% glycerol ( · · · · ), and 70% glycerol (- - -) in water.

is not similar in S1 and S2; rather, it is observed that the S1 fluorescence intensity increases more rapidly compared to the S2 fluorescence intensity. At 520 nm (S2 fluorescence region) the fluorescence intensity increases 3.1 times in a 70% glycerol-water mixture compared to bulk water, whereas at 665 nm (S1 fluorescence region) there is a 5.1 times increase in the fluorescence intensity. The origin of the viscosity-dependent emission intensity can be interpreted as a consequence of change of nonradiative decay rate constant in the molecule as already suggested by several authors.1–5 From the emission spectra it is also observed that the spectral widths and peak energies do not change much with the increase in viscosity which interprets that the electronic states in MG are meagerly affected by the solvent viscosity. 3.2. Femtosecond Fluorescence Up-Conversion Study. Femtosecond resolved fluorescence lifetime measurements have been carried out in different glycerol-water mixtures of varying viscosity to understand the relaxation pathways of S1 and S2 electronic states of MG. The fluorescence lifetime of S1 and S2 states were obtained at a constant laser excitation source of 410 nm. The viscosity-dependent fluorescence transients of S1 and S2 states are shown in Figure 4. In water the fluorescence transient of the S1 state shows biexponential behavior and can be fitted with two time constants of τS1,rise ) 100 ( 50 fs and τS1,decay ) 660 ( 50 fs (Figure 4a). Transient absorption measurements of MG in water reveals the presence of three time constants, 270, 630, and 3000 fs, which have been assigned to the equilibration of high-frequency internal modes in the S1 state, the relaxation to the intermediate state Sx, and the

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Figure 4. Fluorescence transients of MG (a) measured at 670 nm (S1 state) in bulk water (+), 30% glycerol (4), and 70% glycerol (O) in water and (b) measured at 500 nm (S2 state) in bulk water (+), 30% glycerol (4), and 70% glycerol (O) in water. The sample was excited at 410 nm. The solid line represents the best fit of the data.

TABLE 1: Kinetic Parameters of Relaxation Dynamics of Malachite Green in Different Glycerol-Water Mixtures % of glycerol

η (cP)

τS1,risea (fs)

τS1,decayb (fs)

τS2a (fs)

0 10 30 50 60 67 70

1.005 1.31 2.5 6.0 10.8 17.7 22.5

100 110 120 140 150 160 190

660a 840a 1420 2900 4300 6060 7620

260 280 350 410 480 500 520

a

( 50 fs. b ( 100 fs.

relaxation from Sx state to the ground state, respectively.3d Similar to the present work, Yoshizawa et al. got a biexponential fluorescence transient for MG in water with a rise component of 430 fs followed by a decay component of 540 fs.5b Mokhtari et al. also got a biexponential fluorescence transient for MG in water (λex ) 625 nm) with two time constants of 150 and 600 fs, respectively.5a The observation of only two time constants in the fluorescence transient data for S1 state relaxation is because the intermediate state Sx is nonfluorescent. The longer component has been interpreted in terms of the intramolecular diffusion toward the optimum configuration,5a which essentially corresponds to the relaxation from the S1 state to the intermediate state Sx. With the increase in viscosity of the solvent to 70% glycerol in water, the rise component of the S1 fluorescence changes from 100 to 190 fs and is thus almost independent of viscosity, while the decay component shows a strong dependence on viscosity. In the 70% glycerol-water mixture the τS1,decay increases from 660 ( 50 fs (in bulk water) to 7300 ( 100 fs. The respective relaxation kinetic parameters of MG in different glycerol-water mixtures are tabulated in Table 1. The increase in decay time of the S1 state (τS1,decay) is attributed to the larger dependence of torsional configuration change on viscosity, that is, the restricted rotation of phenyl rings of MG.4,5 The fluorescence transients of the S2 state (monitored at 500 nm) were found to be a single exponential (τS2) in nature (Figure 4b). In pure water the observed decay time is 260 ( 50 fs. On increasing the solvent viscosity to 70% glycerol in water, there has been a little increase in the lifetime of S2 state from 260 ( 50 to 510 ( 50 fs. Yoshizawa et al. also observed similar decay behavior for MG in water (270 fs).5b This may be assigned to both the torsional configuration change and the internal conversion from the S2 to the S1 state as suggested by Yoshizawa et al. Figure 5 shows the dependence of the S1 rise, the S1 decay, and the S2 decay time of MG on solvent viscosity using different

Figure 5. Dependence of the time constants τS1,decay (O), τS2 (4), and τS1,rise (]) of MG as a function of viscosity (η) of different glycerol-water mixtures.

glycerol-water mixtures. It can be readily seen that the rise time of the S1 state (τS1, rise) and decay time of the S2 state (τS2) have a very meager dependence on the solvent viscosity. Both τS1,rise and τS2 components show a very weak power law dependence on viscosity as η0.19 and η0.21, respectively, while S1 decay (τS1,decay) shows a much larger dependence on viscosity with the decay time increasing with the increase in viscosity and follows an η0.75 dependence. From these lifetime measurements it is observed that only the S1 excited state decay time is affected by the viscosity of solvent, while the S2 decay time constant seems to be inert with respect to changing viscosity. For another TPM dye with similarly substituted phenyl rings (crystal violet) Ben-Amotz et al. found that the barrierless torsional motion of the phenyl rings are responsible for the rapid relaxation of excited state. They reported the effect of temperature on the relaxation dynamics of crystal violet and found that, for iso-viscous solvents, at different temperatures there is no change in the relaxation time which rejects the presence of any activation energy barrier in the S1 potential energy surface.2b As MG is as well a similar kind of dye, one can assume that the excited state relaxation of MG is also barrierless in nature. Since MG has two different kinds of phenyl rings, it is important to have a deeper understanding of the effect of different types of phenyl rings on the relaxation process of MG. Bhasikuttan et al. proposed a relaxation pathway along an interaction of S2 and S1 potential energy surfaces forming a conical intersection and also proposed that the conical intersection is along the torsional coordinate of unsubstituted phenyl ring of MG.5c Here, we tried to investigate the role of different phenyl rings on the

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Figure 6. Potential energy surfaces of MG in vacuum for the ground and first two singlet excited states along the torsional coordinate of (a) all three phenyl rings, (b) the unsubstituted phenyl ring, and (c) the N,N-dimethyl substituted phenyl ring.

relaxation process and the nature of potential of different energy states by quantum mechanical calculations. 3.3. Quantum Mechanical Calculations. We tried to explore the relaxation pathway and the role of different phenyl rings of MG through TD-DFT calculations to reveal the mechanistic details of the viscosity dependence of relaxation time. The ground state optimization of MG in vacuum has been performed through DFT calculations using a B3LYP hybrid function with the 6-31+g(d,p) basis set. In the ground state optimized structure of MG, the dihedral angle between the unsubstituted phenyl ring and the central carbon atom (C24C23-C1-C13) is ∼43° (see Figure 1), and the angle between two N,N-dimethyl substituted phenyl rings with the central carbon atom (i.e., C15-C13-C1-C23 and C3-C2-C1-C23) is ∼28.5°. The ground state dipole moment of MG in vacuum is found to be 3.28 D. A rigorous formalism (TD-DFT) is employed to calculate the transition energies within the DFT framework using the same hybrid function and the same basis set. The calculated transition energies of the S1rS0 and S2rS0 transition are found to be ∼489 nm and ∼403 nm with very high oscillator strengths of 0.86 and 0.34, respectively. These calculated values of transition energies are overestimated as compared to the experimental results. However, the calculated ratio of S1rS0 and S2rS0 oscillator strength is 2.53 which is in quite good agreement with the experimental value of ∼2.7. To monitor the shape of the potential energy surfaces of MG, we first rotate all of the three phenyl rings in clockwise and anticlockwise directions from the geometrically optimized position. The data obtained from these calculations when plotted in the form of energy versus the torsional coordinate (dihedral angle) depict the similar nature of potential for both the S1 and the S2 states with a little potential barrier around 45° as shown in Figure 6a, which narrates the similar dependence of excited state decay behavior of both S1 and S2 states on the viscosity of the solvent. However, the present experimental observations completely reject this type of viscosity-dependent relaxation. A similar kind of potential energy plots for S0, S1, and S2 states are obtained on rotating only the unsubstituted phenyl ring as shown in Figure 6b. Upon the rotation of one among the two N,N-dimethyl substituted phenyl rings, the potential energy curves we obtained are shown in Figure 6c. The plot clearly shows the barrierless potential energy variation of S1 with the rotational diffusion of the N,N-dimethyl substituted phenyl ring which supports the previous understanding and our experimental data. This barrierless potential energy curve clearly shows the dependence of S1 relaxation dynamics on viscosity. The S2 curve has a potential well, that is, the S2 relaxation dynamics do not change much with the rotation of substituted phenyl ring, which

is also observed in the present femtosecond fluorescence spectroscopic measurements. This leads to the meager dependence of S2 relaxation dynamics on solvent viscosity and thus also supports our experimental observations. The theoretical calculations are also an indicative of the presence of a conical intersection or an avoided conical intersection between the S0 and the S1 electronic states. This may be regarded as an evidence of the presence of some intermediate state (Sx) in the deactivation pathway of S1 state. The present calculations thus suggest that the rotation of the substituted phenyl ring in MG may be responsible for the excited state relaxation dynamics of both S1 and S2 electronic states. 3.4. Microviscosity of Water in the Nanopool of the AOT Reverse Micelle. The observed viscosity dependence of the S1 lifetime of MG is exploited to determine the microviscosity of a nanoconfined environment. Previously there was an attempt to measure the viscosity of micellar interface by Bhattacharyya and co-workers.26 They measured the photoisomerization of 3,3′diethyloxadicarbocyanine iodide in different micelles and found that the viscosity of micellar interface is very high compared to bulk water. Hasegawa and co-workers used Auromine O, whose fluorescence quantum yield increases with increasing solvent viscosity, as a probe to determine the microviscosity of AOT reverse micelle. They have observed that for the AOT reverse micelle of w0 ) 4 the microviscosity is about 80 times higher compared to bulk water.27 Levinger’s group and Fayer’s group have contributed a lot to understand the rigidity of water inside the reverse micelles.28–30 In present study, we choose the AOT reverse micelle in n-heptane with varying water pool sizes, w0 ) 2, 5, 10, 20, 30, and 40, as a model nanoconfined environment. Being an ionic species, MG is completely insoluble in n-heptane and also in the reverse micellar side chain, which enables it to be found only inside the water pool. We are relying on the core-shell model of water inside the AOT reverse micelle.31 Since MG is carrying a positive charge and the AOT surface is negative, MG will prefer to stay in the interfacial region rather than in bulk. The work of Crans et al. has concluded that a molecule always prefers to reside in the interfacial region of reverse micelle using many techniques.29–32 Thus, the viscosity which we are reporting is actually the microviscosity at the interface. The absorption maxima of MG in the water pool of the AOT reverse micelle are found to be at 425 nm (S2rS0) and 630 nm (S1rS0), while emission maxima of the S2 and S1 fluorescence are observed at approximately 470 and 690 nm, respectively, for all of the AOT reverse micelles of w0 ) 2, 5, 10, 20, 30, and 40. The absorption and emission spectrum of MG in the AOT reverse micelle of w0 ) 20 is shown in Figure 7. To

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Figure 7. Steady state absorption (s) and emission spectrum (- - -) of MG in the AOT reverse micelle of w0 ) 20.

Figure 8. Fluorescence transients of MG at 670 nm (S1 state) in AOT reverse micelle of different water pool sizes; w0 ) 2 (O), w0 ) 10 (4), and w0 ) 40 (/). The sample was excited at 410 nm. The solid line represents the best fit with the multiexponential function.

TABLE 2: Observed Value of τS1,decay of MG and Estimated Viscosity of Water Inside the Water Pool of Different AOT Reverse Micelles with Varying Water Content

a

w0

τS1,decaya (fs)

ηb (cP)

2 5 10 20 30 40

3750 3500 3200 2800 2750 2650

9.0 8.2 7.3 6.1 6.0 5.7

( 100 fs. b ( 0.3 cP.

determine the viscosity of water inside the reverse-micellar system, we measured the S1 state relaxation time of MG in the water pool. The MG was excited with an excitation source of 410 nm, and the fluorescence transients were collected at a magic angle polarization at 670 nm. Figure 8 shows the fluorescence transients of MG in AOT reverse micelle with different amounts of water loading (e.g., w0 ) 2, 10, and 40). It is observed that the S1 decay of MG inside the water pool exhibits a biexponential behavior with a fast rise component and slower decay component similar to that in bulk water and in different glycerol-water mixtures (Table 2). In all of the samples (w0 ) 2, 5, 10, 20, 30, and 40), we observed that the rise time (τS1,rise) is almost constant with a value of 250 ( 50 fs. The decay time of the S1 state (τS1,decay), however, keeps on decreasing from 3750 fs in w0 ) 2 to 2650 fs in w0 ) 40. Thus, with an increase in the size of water pool the decay time of S1 state keeps on decreasing. On correlating the decay time of the S1 state of MG in the AOT reverse micelle interface with the viscosity-dependent S1 decay time of MG in glycerol-water mixtures, we estimate the microviscosity of water trapped inside the nanocavity as shown in Figure 9a. The measured S1 decay time readily reveals that the water trapped inside the reverse micelle is more viscous than bulk water. In w0 ) 2 the measured microviscosity is 9.0 cP which is 9 times higher compared to the bulk water. With an increase in the size of the water pool from w0 ) 2 to w0 ) 40 the microviscosity kept on decreasing from 9.0 to 5.68 cP as shown in Figure 9b. Without a proper mathematical formulation we propose the decrease of microviscosity with the increasing size of the nanocavity to follow an exponential behavior. These experimental results thus confirm that the behavior of a solvent is quite different when it is trapped in the interfacial region of nano dimensional size cavity compared to that in bulk as already discussed by many authors.10–15,23,37 As observed from the experimental results, the microviscosity increases with the decrease in the water pool size. It is well documented that in smaller water pool sizes there is a higher water structure.11,13,17,29,30,32 Bakker and co-workers proposed that there are almost six to seven hydrogen bonds per surfactant molecule, closely matching the three sulfonate oxygen atoms per AOT molecule.33 Levinger and others also observed a slow solvation dynamics in the interior of reverse micelle compared to that in bulk water.34–36 All of these were interpreted as a consequence of stronger hydrogen bonding of water molecules in the interfacial region than in the bulk of reverse micelle.15,37

Figure 9. (a) Viscosity of water inside the AOT reverse micelle of water pool sizes w0 ) 2, 5, 10, 20, 30, and 40 directly obtained by comparing the τS1,decay of MG with the viscosity-dependent dynamics of MG shown in Figure 5. (b) Exponential decrease of viscosity of water inside the AOT reverse micelle as a function of water loading (w0).

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As MG is a positively charged molecule, it always prefers to stay in the negatively charged interface of AOT reverse micelle. We strongly believe that the observed high microviscosity of the interface in the small water pool compared to the larger one is due to the stronger hydrogen bonding in the interfacial region. Previous results also show that the dynamics is getting faster as the size of the water pool increases and were explained as an outcome of weakening of the water structure. Consequently, the friction posed by the water molecules will also reduce with increase in the size of the water pool. We believe this as the origin of the water pool size-dependent microviscosity of the AOT reverse micelle. 4. Conclusion The ultrafast relaxation mechanism of MG has been revisited by using femtosecond fluorescence up-conversion spectroscopy combined with quantum mechanical calculations. The femtosecond measurements and theoretical calculations completely oppose the view proposed by Bhasikuttan et al.5c In this paper we propose that teh conical intersection does not exist between S2 and S1 states; rather, it is between S1 and S0 electronic states, as suggested by several other authors.1–4,5a,b This work also proposes that the conical intersection is not along the torsional coordinate of unsubstituted ring; rather, it is along the torsional coordinate of N,N-dimethyl substituted phenyl ring. The femtosecond fluorescence up-conversion measurements and TDDFT calculations support the viscosity-dependent barrierless relaxation of the S1 state, and they also justify the viscosityindependent behavior of the S2 state relaxation due to the presence of a potential well and an activation energy barrier. Finally, the microviscosity of interfacial water inside the nano water pool of the AOT reverse micelle has been measured by correlating the relaxation time of MG in a reverse micelle with the relaxation time of MG in different glycerol-water mixtures. The data reveal that the decay time of the S1 excited state of MG in the AOT water pool with w0 ) 2 is almost 5.6 times greater than in bulk water, which leads us to conclude that the microviscosity of water in the nano water pool of the AOT reverse micelle (w0 ) 2) is almost 9 times higher than the viscosity of bulk water. It is also observed that, as the size of the water pool increases from w0 ) 2 to w0 ) 40, the microviscosity of water goes on decreasing, and the decrease is assumed to follow an exponential nature. Acknowledgment. S.R. and R.Y. thank CSIR (Counsel of Scientific and Industrial Research, India) and UGC (University Grants Commission, India), respectively, for awarding a fellowship. We thank Professor Kankan Bhattacharyya for providing the femtosecond fluorescence up-conversion facility required for the present study under Department of Science and Technology, India Project No. IR/I1/CF-01/2002. This work is financially supported by Project No. IITK/CHM/20080339 from Indian Institute of Technology Kanpur. References and Notes (1) Duxbury, D. F. Chem. ReV. 1993, 93, 381. (2) (a) Ippen, E. P.; Shank, C. V.; Bergman, A. Chem. Phys. Lett. 1976, 38, 611. (b) Ben-Amotz, D.; Harris, C. B. J. Chem. Phys. 1987, 86, 4856. (c) Mokhtari, A.; Fini, L.; Chenoy, J. J. Chem. Phys. 1987, 86, 3429. (3) (a) Sundstorm, V.; Gillbro, T.; Bergstorm, H. Chem. Phys. 1982, 73, 439. (b) Sundstorm, V.; Gillbro, T. J. Chem. Phys. 1984, 81, 3463. (c) Saikan, S.; Sei, J. J. Chem. Phys. 1983, 79, 4154. (d) Mokhtari, A.; Fini, L.; Chesnoy, J. J. Chem. Phys. 1987, 87, 3429. (4) (a) Janowski, A.; Rzeszotarska, J. J. Lumin. 1980, 21, 409. (b) Nagasawa, Y.; Ando, Y.; Kataoka, D.; Matsuda, H.; Miyasaka, H. J. Phys.

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