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Jan 28, 2011 - The effective polarity of the air/water interface, which was determined ... (17-26) Among them, the electronic sum frequency generation...
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Molecules at the Air/Water Interface Experience a More Inhomogeneous Solvation Environment than in Bulk Solvents: A Quantitative Band Shape Analysis of Interfacial Electronic Spectra Obtained by HD-ESFG Sudip Kumar Mondal, Shoichi Yamaguchi, and Tahei Tahara* Molecular Spectroscopy Laboratory, Advanced Science Institute (ASI), RIKEN 2-1 Hirosawa, Wako 351-0198, Japan

bS Supporting Information ABSTRACT: To evaluate the inhomogeneity of the solvation environment at the air/water interface, we quantitatively analyzed the shape of the electronic spectra at the air/water interface for the first time. We measured the interface-selective electronic χ(2) (second-order nonlinear susceptibility) spectra of solvatochromic coumarin molecules at the air/water interface by heterodyne-detected electronic sum frequency generation (HD-ESFG) spectroscopy. The observed imaginary χ(2) (Im[χ(2)]) spectra were well reproduced by the convolution of a line shape function with a Gaussian distribution of a frequency shift, which enabled us to quantitatively determine the peak position and bandwidth of the Im[χ(2)] spectra. The effective polarity of the air/water interface, which was determined by the peak position, was found to be dependent on each coumarin, which agreed with our previous homodyne-detected ESFG study. Interestingly, the spectra at the air/water interface showed substantially broader bandwidths than those in equally polar bulk solvents or even bulk water, indicating that the solvation environment at the air/water interface is more inhomogeneous than that in bulk solvents. At the air/water interface, the stabilization energy of the solvation not only changes with the change of the position and orientation of the surrounding solvents but also varies with the change of the position and orientation of the solute at the interface. We consider that this unique situation arising from the anisotropy along interface normal brings about a broader distribution of the stabilization energy of solvation at the air/water interface. The present work showed that the air/water interface provides a more inhomogeneous solvation environment than equally polar bulk solvents because of this broader distribution of the local solvation structure at the interface.

1. INTRODUCTION The arrangement of solvent molecules around a solute molecule, which is termed the solvation environment, strongly affects the physical and chemical properties of a solute. The solvation environment at a liquid interface is obviously different from that in a bulk solvent because, at an interface, a part of the solute is solvated by one medium and the rest by the other. Due to this unique situation at the interface, many biological and chemical processes occur at interfaces differently from those in bulk solvents.1-5 Knowledge about the solvation environment at interfaces is essentially important to understand the role of the interfacial solvation for such processes. The measurement of the electronic spectra of solvatochromic molecules is one of the simplest ways to obtain information about the solvation environment.6,7 The dipole moments of the electronically ground and excited states of solvatochromic molecules are substantially different (usually the excited state has a larger dipole), and they gain different stabilization energy, which causes the shift (usually red shift) of the transition energy depending on the polarity of the solvation environment.8 Therefore, the peak position of the electronic spectrum of the solvatochromic solute can be used as a measure of the polarity of the environment. r 2011 American Chemical Society

Moreover, the bandwidth of the electronic spectrum indicates the distribution of the local solvation environment. The solute molecules are distributed among a range of different local solvation environments that provides different solute-solvent interactions, which gives rise to a distribution of the stabilization energy and hence a distribution of the transition energy (i.e., inhomogeneous broadening of the electronic spectrum). Consequently, we can obtain information about the solvation environment and its distribution from the peak position and bandwidth of the electronic spectra of solvatochromic molecules. Using this well-established idea, we can study the solvation environment at liquid interfaces by measuring electronic spectra of the solvatochromic molecules at the interface. Nevertheless, this is not an easy task because the spectral information of solvatochromic molecules at the interface is hidden by the signal arising from a large number of the same molecules in the bulk solvent in ordinary spectroscopic measurements.9 In the past, electronic spectra of the solute at liquid interfaces have been measured and Received: November 2, 2010 Revised: January 1, 2011 Published: January 28, 2011 3083

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Figure 1. (A) Ray diagram of HD-ESFG setup. λ/2 stands for a half-wave plate, and P stands for a polarizer. (B) Energy diagram of two-photon (ω1 þ ω2) resonant and one-photon (ω1 or ω2) nonresonant ESFG processes.

discussed with wavelength-scanning second harmonic generation (SHG)10-16 because the second-order nonlinear spectroscopy (or even-order nonlinear spectroscopy, in general) can selectively give spectral information at the interface where the inversion symmetry is broken.9 However, SHG can provide only interfacial electronic spectra consisting of a small number of data points with a very limited quality. In addition, the homodyne detection of SHG provides electronically resonant |χ(2)|2 spectra that cannot directly be compared with absorption spectra in the bulk solution. Because of these difficulties, any quantitative analysis of the interfacial electronic spectra and their band shapes has not been reported so far. Recently, our group developed a series of new interfaceselective nonlinear spectroscopy methods to study liquid interfaces.17-26 Among them, the electronic sum frequency generation (ESFG) technique gives |χ(2)|2 spectra with a very high signal-tonoise ratio that is comparable to the absorption spectra in solution. The high-quality |χ(2)|2 spectra obtainable with ESFG enabled us to study the effective polarity, viscosity, protein folding, and ultrafast dynamics at interfaces.21-24 Moreover, we realized heterodyne detection of ESFG (HD-ESFG),17 which provides very high quality electronic χ(2) spectra with imaginary and real parts separately. In particular, when the spectra are measured under two-photon (ω1 þ ω2) resonant and one-photon (ω1 or ω2) nonresonant conditions, Im[χ(2)] spectra plotted against ω1 þ ω2 represent interfacial electronic spectra that can directly be compared with absorption spectra (Im[χ(1)]) in solution. High-quality Im[χ(2)] spectra at liquid interfaces obtainable with HD-ESFG enable us to perform a quantitative analysis of interfacial electronic spectra to obtain information about the solvation environment and its distribution at interfaces. In the present work, we applied the HD-ESFG technique to three coumarins at the air/water interface. We carried out band shape analysis of the Im[χ(2)] spectra at the air/water interface for the first time to obtain quantitative information about the solvation environment and its distribution at the interfaces. Comparing with the results for the Im[χ(1)] spectra of the same

coumarins in several bulk polar solvents, it was found that the solvation environment at the air/water interface is substantially more inhomogeneous than that of bulk solvents showing equivalent polarity.

2. EXPERIMENTAL SECTION 2.1. HD-ESFG Setup. A schematic of the HD-ESFG setup is given in Figure 1. Details of the setup have been described elsewhere.17 Briefly, a Ti:sapphire regenerative amplifier (Spitfire Pro XP, Spectra Physics) seeded by a mode-locked oscillator (Tsunami, Spectra Physics) generated 795 nm, 3.3 W, 120 fs output at a repetition rate of 1 kHz. A part of this output was used as the ω1 pulse, and another part was focused into water to generate a white light continuum which was used as the ω2 pulse. The spectrum of the white light continuum was extended from 540 nm to 1.2 μm. The ω1 and ω2 pulses were noncollinearly focused at the sample interface. The linear polarization of the ω1 and ω2 pulses were p and s polarized, respectively. When the ω1 and ω2 pulses were temporally overlapped, the sum frequency (ω1 þ ω2) was generated at the interface. The ω1, ω2, and ω1 þ ω2 pulses were again focused by a spherical concave mirror onto a GaAs (110) surface to generate the sum frequency once more. A fused silica glass plate of 1 mm thickness was placed between the sample and the concave mirror to delay the ω1 þ ω2 pulse from the sample relative to the reflected ω1 and ω2 pulses. The ω1 þ ω2 pulses from the sample and GaAs propagated collinearly and sequentially through an analyzer selecting the s polarization and entered into a polychromator. The optical (or spectral) slit width was set at 1 nm. After being spectrally dispersed, the ω1 þ ω2 light was detected by a multichannel detector. The detected raw spectrum contains fine fringes. The fringes are due to the interferences between the sum frequency light from the sample and GaAs. The complex χ(2) spectra were calculated from this fringe spectrum by Fourier analyses. 2.2. Sample Preparation. Coumarin 110 (C110) and coumarin 314T (C314T) were purchased from Exciton and used as 3084

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Figure 2. Complex χ(2) spectra (measured in SPS polarization configuration) of (A) C110, (B) C6H, and (C) C314T at the air/water interface. The red lines represent imaginary χ(2) (Im[χ(2)]), and the black lines shaded with gray stand for real χ(2) (Re[χ(2)]).

received. Coumarin 6H (C6H) was purchased from SigmaAldrich and used as received. Purified water (Millipore, 18.2 MΩ cm resistivity) was used. Special-grade cyclohexane and methanol were purchased from Nacalai Tesque; special-grade ethanol, butanol, hexanol, and isopropanol and HPLC-grade tetrahydrofuran (THF) were purchased from Wako; dioxane (99.8%), ethyl ether (99%), and butyl ether (99.3%) were purchased from Sigma-Aldrich and used as received. Saturated solutions of the dyes were prepared for HD-ESFG measurements by sonication for about 1 h. Room temperature was kept constant at 296 K during experiments. All UV-vis absorption spectra were measured using a standard spectrophotometer (Hitachi, U-3310), setting the optical (or spectral) slit width at 1 nm.

3. RESULTS AND DISCUSSION Figure 2A shows the complex electronic χ(2) spectrum of C110 at the air/water interface. The real and imaginary parts show the dispersive and symmetric line shape, respectively. Although this spectrum is essentially the same as the spectrum that we reported very recently,27 the present spectrum has a higher signal-to-noise ratio. Compared with the |χ(2)|2 spectrum

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Figure 3. Comparison of the electronic spectra of (A) C110, (B) C6H, and (C) C314T. The red lines represent imaginary χ(2) (Im[χ(2)]) spectra at the air/water interface. The green, cyan, violet, and blue lines represent imaginary χ(1) (Im[χ(1)]) spectra (obtained from the UV-vis absorption spectra) of the coumarins in bulk water, bulk methanol, bulk THF, and bulk cyclohexane, respectively. Dashed curves are fitted lines to the corresponding spectra.

of the same sample reported recently,21 the peak wavelength of the Im[χ(2)] spectrum is red shifted by ∼2 nm. This difference is due to the interference with the nonresonant background in the |χ(2)|2 spectrum that distorts electronic spectra (vide infra). Because the Im[χ(2)] spectrum of the coumarin is measured under two-photon (ω1 þ ω2) resonant and one-photon (ω1 or ω2) nonresonant condition, we can directly compare with Im[χ(1)] spectra measured in bulk solvents (see Supporting Information for a detailed discussion). In Figure 3A, the Im[χ(2)] spectrum of C110 at the air/water interface is compared with the Im[χ(1)] spectra of C110 in bulk water, methanol, and cyclohexane. These Im[χ(1)] spectra were calculated from the UV-vis absorption spectra by the following equation28 Im½χð1Þ  ¼ ln 10ncω-1 l-1 A

ð1Þ

where A is the absorbance, ω is the angular frequency of the absorbed light, l is the thickness of the sample, and n is the refractive index of water which is almost constant for the present spectral wavelength range. The peak wavelength of the Im[χ(2)] 3085

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The Journal of Physical Chemistry C spectrum of C110 at the interface is very close to that of the Im[χ(1)] spectrum in bulk methanol. This suggests that the effective polarity sensed by C110 at the air/water interface is close to the polarity of bulk methanol. (In our previous homodyne ESFG work, we claimed that the effective polarity sensed by C110 at the air/water interface is close to the polarity of ethanol.21 This difference is due to the apparent peak shift of |χ(2)|2 spectra, which arises from the interference with the nonresonant background.) However, Figure 3A clearly showed that the bandwidth of the Im[χ(2)] spectrum at the interface is noticeably broader than that of the Im[χ(1)] spectra in bulk methanol. This indicates that the solvation environment at the air/water interface is more inhomogeneous than that in bulk methanol. Figure 2B shows the complex χ(2) spectrum of C6H at the air/ water interface. Compared with the |χ(2)|2 spectrum reported recently,21 the peak wavelength of the present Im[χ(2)] spectrum is red shifted by 6 nm (vide infra). In Figure 3B, the Im[χ(2)] spectrum of C6H at the air/water interface is compared with the Im[χ(1)] spectra of C6H in bulk water, methanol, and cyclohexane. The peak position of the Im[χ(2)] spectrum at the interface is close to that of the Im[χ(1)] spectrum in methanol, which shows that the effective polarity sensed by C6H at the air/water interface is also close to the polarity of bulk methanol. The bandwidth of the Im[χ(2)] spectrum at the air/water interface is broader than the Im[χ(1)] spectra in methanol, which is again an indication of more inhomogeneous environment at the air/water interface than in the equally polar bulk solvent. Figure 2C shows the complex χ(2) spectrum of C314T at the air/water interface. In Figure 3C, the Im[χ(1)] spectra in bulk water, THF, and cyclohexane are shown with the Im[χ(2)] spectrum at the air/water interface for comparison. The peak position of the Im[χ(2)] spectrum at the air/water interface is close to that of the Im[χ(1)] spectrum in bulk THF, indicating that C314T at the air/water interface experiences an effective polarity nearly equal to the polarity of bulk THF. The bandwidth of the Im[χ(2)] spectrum is broader than that of the Im[χ(1)] spectrum in bulk THF, which also indicates that the air/water interface provides a more inhomogeneous environment than the equally polar bulk solvent. As already mentioned, the peak positions of the Im[χ(2)] spectra measured with HD-ESFG are slightly deviated from those of the |χ(2)|2 spectra obtained by homodyne-detected ESFG.21 This is because the |χ(2)|2 spectra are determined not only by the electronic resonant term from coumarins but also by the nonresonant term arising from water. Because the nonresonant χ(2) of water under the present experimental condition is real and negative,29 addition of the nonresonant χ(2) component to the resonant χ(2) of the coumarins makes the apparent peak of the |χ(2)|2 spectra appear at a shorter wavelength than the peak of the Im[χ(2)] spectra. The peak wavelength deference between the Im[χ(2)] and the |χ(2)|2 spectra depends on the relative amplitudes between the nonresonant background and the electronic resonant term. Actually, the difference in the peak wavelength of the |χ(2)|2 and Im[χ(2)] spectra is larger for C6H (6 nm shift), which gives a smaller electronic resonant signal, compared with C110 (2 nm shift). The peak shift due to the interference with nonresonant background is small but noticeable in |χ(2)|2 spectra. Therefore, it is crucial to use the Im[χ(2)] spectra for quantitative discussion of the spectral feature of the electronic spectra at the interface. The Im[χ(2)] spectra of the three coumarins measured by HD-ESFG clearly show two facts. First, different coumarins

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experience different effective polarities at the same air/water interface. This result essentially agrees with the conclusion of our previous homodyne ESFG work.21 Second, the electronic spectra of the coumarins at the air/water interface have broader bandwidths than the electronic spectra in equally polar bulk solvents. This result is contrary to the argument of an earlier scanning SHG study, which concluded that the electronic spectrum at the interface is narrower than that in the bulk solvent.12 This discrepancy is highly likely due to the low quality of the SHG spectra that consist of only a few data points. The broader bandwidth of Im[χ(2)] spectra observed in the present study disclosed higher inhomogeneity of the solvation environments at the air/water interface. To make our discussion on peak shifts and bandwidth quantitative, we analyzed the electronic spectra measured at the interface and in the bulk solvents (shown in Figure 3) using the method proposed by Fee and Maroncelli.6,7 In this analysis, one assumes a single line shape function for the electronic spectrum of each solute molecule, and this line shape function accounts for the underlying vibronic structure as well as the homogeneous broadening of the spectrum. In the present study, we adopted the Im[χ(1)] spectrum of each coumarin in a nonpolar solvent (cyclohexane) as the line shape function. Then, taking account of the effect of the polarity on the electronic spectrum, it is assumed that an electronic spectrum in a polar environment is represented as a superposition of line shape functions shifted by a different amount of δ. δ represents the solvation stabilization energy, and its distribution p(δ) is brought about by the distribution of the stabilization energy arising from different local environments. First, we carried out the analysis on the electronic spectra measured in the bulk solvents. In this case, Im[χp(1)] of coumarins in polar solvents is expressed as follows Z Im½χð1Þ ðνÞ µ Im½χð1Þ ð2Þ p np ðν - δÞpðδÞdδ where ν is the frequency and Im[χ(1)p(ν)] is the imaginary part of the linear susceptibility of the coumarin in a polar solvent which is directly obtained from the absorption spectrum using eq 1. Im[χ(1)np(ν)] is the line shape function obtained from the absorption spectrum in cyclohexane. The site distribution p(δ) is assumed to be a Gaussian function " # 1 -ðδ-δ0 Þ2 pðδÞ ¼ pffiffiffiffiffiffi exp ð3Þ 2σ2 2πσ where δ0 is the center frequency of the shift and σ is the standard deviation of the distribution. The δ0 value represents the average shift from the spectra in nonpolar cyclohexane, and therefore, it represents the polarity sensed by the solute. The σ value is the parameter that represents the bandwidth due to the inhomogeneous broadening; so, it quantifies the inhomogeneity of the solvation environment. By using eqs 2 and 3, fitting analyses were performed for all the Im[χ(1)] spectra in the polar bulk solvents with δ0 and σ treated as fitting parameters. The best fits are shown by the dashed curves in Figure 3, which well reproduced the experimental Im[χ(1)] spectra. The σ and δ0 values obtained for different solvents are plotted in Figure 4 and listed in Table 1. As clearly seen in Figure 4, there is a clear correlation between δ0 and σ for the three coumarins. The solvent inducing a larger frequency shift 3086

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larger for C314T. It is not the scope of the present work to make a detailed discussion on the correlation between δ0 and σ in the electronic spectra in solution. Nevertheless, it is clear that, with theoretical backgrounds, the shifts (δ0) and broadening (σ) of the electronic spectra in various polar solvents have a clear relationship. Accordingly, in bulk solvents, σ can be predicted if δ0 is specified for each coumarin. In other words, the inhomogeneity (i.e., σ) can be predicted from the polarity (i.e., δ0) of the solvation environment for the bulk solvents. To quantitatively compare the effective polarity and inhomogeneity at the air/water interface with those in bulk solvents, we then carried out the same analysis for the Im[χ(2)(ν)] spectra of the coumarins at the air/water interface. For the analysis at the interface, we simply replaced Im[χ(1)p(ν)] with Im[χ(2)(ν)] in eq 2 and the fitting was performed based on the following formula Z ð5Þ Im½χð2Þ ðνÞ µ Im½χð1Þ np ðν - δÞpðδÞdδ

Figure 4. Plots of σ vs δ0 in several bulk solvents (open circles). The σ and δ0 values represent the inhomogeneity and polarity of the solvation environment, respectively. Dashed lines are the fitted lines of the experimental points to eq 4. The solid black lines are the eye guides to show the definite relationship between σ and δ0. The red points with error bars are the points corresponding to the air/water interface. The solvents used are (1) butyl ether, (2) ethyl ether, (3) dioxane, (4) THF, (5) acetone, (6) acetonitrile, (7) isopropanol, (8) hexanol, (9) butanol, (10) ethanol, (11) methanol, and (12) water. Molecular structures of the coumarins are given in the respective panels.

(δ0) gives rise to a larger inhomogeneous broadening (σ). The correlation between shifts and broadening of the electronic spectra in polar solvents has been already discussed in linearized solvation theories.30-32 Actually, in the linearized solvation theories, the radial distribution function of solvent molecules around the solute is assumed to be linear to the permanent dipole moment of the solute, and it was shown that the shift and width of the electronic spectra have a general relationship, being independent of solvents σ2 ¼ akTδ0

ð4Þ

Here, a is a constant determined by the dipole moment of the ground and excited states of the solute, k is the Boltzmann constant, and T is the temperature. The dashed curves shown in Figure 4 are the best fits based on eq 4 using the constant a as a fitting parameter. For C110 and C6H, the theoretical curves fit fairly well with the experimental data whereas the deviation is

The Im[χ(2)] spectra of all three coumarins at the air/water interface were well reproduced by the curves based on the eq 5. The best fitted curves are also plotted with dashed curves in Figure 3, and the obtained δ0 and σ values are given in Table 1 and plotted in Figure 4 (red filled circles). The errors were determined from the values obtained from several independent experiments. The data points for the air/water interface are substantially deviated from the correlation line found for bulk solutions. This demonstrates that the δ0 and σ values at the interface do not obey the relation seen in the bulk solvents. This result is more or less understandable, because the solvation structure at the interface is essentially different from that in the bulk solvent. In fact, a recent MD simulation carried out in our group clearly showed that C110 is only “half” hydrated at the air/water interface.27 In the half hydration, the hydrophilic part of C110 at the interface is hydrated in a nearly identical way with that in the bulk but the hydrophobic part at the interface is virtually exposed into the air and is not hydrated at all. Because of the absence of the water molecules surrounding the hydrophobic part, it is obvious that the radial distribution function of water about C110 at the interface is totally different from that in the bulk solvent where all parts of the solute molecule are surrounded by the solvent and is fully solvated. This essential difference in the solvation structure makes δ0 and σ of the air/water interface largely deviated from the correlation lines obtained from the values in the bulk solvents. The present study not only shows deviation of the σ and δ0 values from the correlation seen in bulk solvents but also revealed that the σ values (spectral broadenings) at the interface are much larger than that in the bulk solvent showing an equivalent δ0 value (spectral shift). This demonstrates that the air/water interface provides a more inhomogeneous solvation environment than equally polar bulk solvents for the solvatochromic molecules studied in this work. This is the most important finding of the present study. Although we cannot make quantitative arguments for the observed larger inhomogeneity at the air/water interface here, we can rationalize qualitatively (and microscopically) why the air/ water interface provides a more inhomogeneous solvation environment than equally polar bulk solvents. In bulk solvents, the stabilization energy is a function of the position and orientation of the solvent molecules surrounding the solute but the position and orientation of the solute do not affect the stabilization 3087

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Table 1. Result of Fitting Analysis of the Electronic Spectra of Three Coumarins in Several Bulk Polar Solvents and at the Air/ Water Interfacea C110 solvents

-1

C6H -1

-1

δ0 (10 cm )

σ (10 cm )

δ0 (10 cm )

3

3

3

C314T -1

σ (10 cm ) 3

-1

δ0 (10 cm ) 3

σ (103 cm-1)

1

butyl ether

-0.27

0.30

2

ethylether

-0.35

0.40

-0.34

0.40

-0.35

0.29

3 4

dioxane THF

-0.70 -0.80

0.45 0.45

-0.68 -0.78

0.46 0.48

-0.60 -0.80

0.32 0.34

5

acetone

-1.00

0.50

-1.05

0.55

-1.05

0.35

6

acetonitrile

-1.15

0.60

-1.20

0.62

-1.20

0.35

7

isopropanol

-1.35

0.60

-1.45

0.60

-1.25

0.32

8

hexanol

-1.50

0.61

9

butanol

-1.45

0.60

-1.55

0.60

-1.35

0.32

10

ethanol

-1.45

0.65

-1.60

0.65

-1.35

0.35

11 12

methanol water

-1.55 -2.20

0.65 0.85

-1.70 -2.25

0.70 1.00

-1.50 -2.00

0.35 0.40

air/water interface

-1.60

0.90

-1.90

1.20

-0.90

0.50

The δ0 value represents the average shift from the spectra in nonpolar cyclohexane. The σ value is the parameter that represents the bandwidth due to the inhomogeneous broadening.

a

Figure 5. Sketch of solvation environments around coumarin at the air/water interface and in bulk solvent showing equal polarity. The arrangements of solvent dipoles encircled by the red dashed line are the solvation environment of the respective coumarins. At the interface, with the change of position along the interface normal and the change in orientation of the coumarins, the number of solvent dipoles in the solvation environment changes, whereas in the bulk all solvation environments have the same number of solvent dipoles. As a result, the distribution function p(δ) at the air/water interface (red curve) is broader than that in equally polar (same δ0) bulk solvent (blue curve).

energy because of the isotropic nature. At the air/water interface, on the other hand, the stabilization energy is a function of not only the position and orientation of solvent molecules surrounding the solute but also those of the solute itself because of the anisotropy along the interface normal. Figure 5 sketches this microscopic situation that gives rise to the interface-specific broader distribution of the stabilization energy of the solvation. As sketched, the number of solvent molecules surrounding the solute is significantly changed with the change of the vertical position of the solute along the normal, which alters the stabilization energy efficiently. The orientation of the solute at the air/water interface regulates the accessibility of solvent molecules to the solute, which should also substantially influence the stabilization energy. These origins of the inhomogeneity do not exist in bulk solutions. As a result, the distribution of the

stabilization energy at the air/water interface becomes broader than in bulk solvents. Consequently, we conclude that the air/ water interface provides a more inhomogeneous solvation environment than equally polar bulk solvents because of the intrinsically anisotropic nature of the interface.

4. CONCLUSION We quantitatively analyzed the spectral shape of the electronic spectra of the solvatochromic molecules at the air/water interface for the first time using the precise Im[χ(2)] spectra measured by HD-ESFG spectroscopy. In the analysis, the Im[χ(2)] spectra were reproduced by the convolution of the line shape function with the Gaussian distribution of the frequency shift, and the average and standard deviation of the frequency shift were 3088

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The Journal of Physical Chemistry C obtained which represent the effective polarity and inhomogeneity at the interface, respectively. The effective polarity of the air/ water interface sensed by the solute was found to be dependent on each coumarin and is located in between the polarities of a bulk nonpolar solvent and bulk water, which is in agreement with our previous homodyne-detected ESFG study.21 More importantly, the analysis disclosed that the broadening of the electronic spectra at the interface is substantially larger than that in equally polar bulk solvents (or even bulk water) contrary to the previous argument based on the SHG spectra.12 This finding demonstrates that the solvation environment at the air/water interface is more inhomogeneous than that in bulk solvents having equivalent polarity. At the air/water interface, the stabilization energy of the solvation changes not only with a change of the position and orientation of the surrounding solvent but also those of the solute itself because of the anisotropy along the interface normal. We proposed that this brings about larger distribution of the stabilization energy of the solvation at the interface. The air/water interface provides a more inhomogeneous solvation environment than equally polar bulk solvents because of this characteristic feature of interfacial solvation.

’ ASSOCIATED CONTENT Information. bS (1)Supporting (2)

Discussion about the band shape of χ and χ spectra (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION

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(15) Steel, W. H.; Damkaci, F.; Nolan, R.; Walker, R. A. J. Am. Chem. Soc. 2002, 124, 4824–4831. (16) Steel, W. H.; Walker, R. A. Nature 2003, 424, 296–299. (17) Yamaguchi, S.; Tahara, T. J. Chem. Phys. 2008, 129, 101102. (18) Yamaguchi, S.; Tahara, T. Angew. Chem., Int. Ed. 2007, 46, 7609–7612. (19) Yamaguchi, S.; Tahara, T. J. Chem. Phys. 2006, 125, 194711. (20) Yamaguchi, S.; Tahara, T. J. Phys. Chem. B 2004, 108, 19079– 19082. (21) Sen, S.; Yamaguchi, S.; Tahara, T. Angew. Chem., Int. Ed. 2009, 48, 6439–6442. (22) Sen, P.; Yamaguchi, S.; Tahara, T. J. Phys. Chem. B 2008, 112, 13473–13475. (23) Sen, P.; Yamaguchi, S.; Tahara, T. Faraday Discuss. 2010, 145, 411–428. (24) Sekiguchi, K.; Yamaguchi, S.; Tahara, T. J. Chem. Phys. 2008, 128, 114715. (25) Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. J. Chem. Phys. 2009, 130, 204704. (26) Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. J. Am. Chem. Soc. 2010, 132, 6867–6869. (27) Watanabe, H.; Yamaguchi, S.; Sen, S.; Morita, A.; Tahara, T. J. Chem. Phys. 2010, 132, 144701. (28) Mukamel, S. Principles of Nonlinear Optical Spectroscopy; Oxford University Press: New York, 1999. (29) Yamaguchi, S.; Shiratori, K.; Morita, A.; Tahara, T. Manuscript in preparation. (30) Shemetulskis, N. E.; Loring, R. F. J. Chem. Phys. 1991, 95, 4756– 4764. (31) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M. J. Phys. Chem. 1995, 99, 17311–17337. (32) Marcus, R. A. J. Chem. Phys. 1965, 43, 1261–1274.

Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research on Priority Area (No. 19056009) from MEXT and Grantin-Aid for Scientific Research (B) (No. 22350014) from JSPS. S. K.M. thanks JSPS for a postdoctoral fellowship. ’ REFERENCES (1) Gennis, R. B. Biomembranes; Springer: New York, 2003. (2) Volkov, A. G. Interfacial Catalysis; Marcel Dekker: New York, 2003. (3) Finlayson-Pitts, B. J.; Pitts, J. N. J. Chemistry of Upper and Lower Atmosphere; Academic Press: San Diego, 2000. (4) Benjamin, I. Chem. Rev. 2006, 106, 1212–1233. (5) Eisenthal, K. B. Chem. Rev. 1996, 96, 1343–1360. (6) Fee, R. S.; Maroncelli, M. Chem. Phys. 1994, 183, 235–247. (7) Fee, R. S.; Milsom, J. A.; Maroncelli, M. J. Phys. Chem. 1991, 95, 5170–5181. (8) Reichardt, C. Chem. Rev. 1994, 94, 2319–2358. (9) Shen, Y. R. The Principles of Nonlinear Optics; Wiley: New York, 1984. (10) Wang, H. F.; Borguet, E.; Eisenthal, K. B. J. Phys. Chem. A 1997, 101, 713–718. (11) Wang, H. F.; Borguet, E.; Eisenthal, K. B. J. Phys. Chem. B 1998, 102, 4927–4932. (12) Zimdars, D.; Eisenthal, K. B. J. Phys. Chem. B 2001, 105, 3993– 4002. (13) Benderskii, A. V.; Eisenthal, K. B. J. Phys. Chem. B 2001, 105, 6698–6703. (14) Benderskii, A. V.; Henzie, J.; Basu, S.; Shang, X. M.; Eisenthal, K. B. J. Phys. Chem. B 2004, 108, 14017–14024. 3089

dx.doi.org/10.1021/jp110456t |J. Phys. Chem. C 2011, 115, 3083–3089