Determination of Absolute Orientations of Hemicyanine Dyes

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J. Phys. Chem. C 2007, 111, 18159-18163

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Determination of Absolute Orientations of Hemicyanine Dyes Incorporated into the Channels of Silicalite-1 Films Haesik Min, Yoonnam Jeon, Jaeho Sung, Sangjun Seok, Doseok Kim,* Hyunsung Kim,† and Kyung Byung Yoon† Department of Physics and Interdisciplinary Program of Integrated Biotechnology, Sogang UniVersity, Seoul 121-742, Korea, and Center for Microcrystal Assembly, Department of Chemistry, and Interdisciplinary Program of Integrated Biotechnology, Sogang UniVersity, Seoul 121-742, Korea ReceiVed: April 25, 2007; In Final Form: August 26, 2007

Hemicyanine dyes can be incorporated into silicalite-1 channels in uniform orientations.1 To find out the underlying mechanism, two different hemicyanine dyes having a long hydrophobic C18-alkyl chain on opposite ends of the dipolar nonlinear optical chromophore were adsorbed into the vertically oriented channels of silicalite-1 films supported on a glass plate. Second-harmonic phase measurement revealed that the above two molecules in the silicalite-1 channels were indeed oppositely oriented. A Langmuir monolayer of a hemicyanine was used as a reference sample to determine the absolute orientation of the dyes in the above samples, which allowed us to confirm that the hemicyanine dyes prefer to enter the zeolite channels with the hydrophobic tail part first.

I. Introduction Aligned inclusion of nonlinear optical (NLO) dipolar organic dye molecules into zeolites and the related nanoporous materials has received great attention as a possible means to prepare highly sensitive nonlinear optical materials. Various dye molecules including para-nitroaniline (PNA), 2-methyl-4-nitroaniline, and 2-amino-4-nitropyridine have been found to enter the zeolite (AlPO4-5) channels unidirectionally.1-4 By preparing the oriented silicate-1 film on a glass plate using the self-assembly technique, alignment of adsorbed hemicyanine dye molecules over macroscopic volume was achieved.1 The underlying mechanism of this unidirectional inclusion is believed to be the affinity of the certain moiety of the dipolar molecule with the wall of the zeolite used. However, definite experimental confirmation of the proposed mechanism was not trivial. Measurement of UV-visible absorption by polarized light cannot determine the absolute orientation of the included dipolar molecules. Nonlinear optical techniques like secondharmonic generation (SHG) were essential to determine the degree of unidirectional order of included NLO molecules.5-8 However, the square of the second-harmonic field is commonly measured, from which the needed phase information is lost and the absolute polar orientation of the molecules cannot be determined. Only a few techniques could determine the absolute orientation of the polar molecules. Maxwell displacement current measurement has been proposed to determine the polar orientation of the molecules.9 Second-harmonic phase measurement also has been successful to determine the absolute polar orientation of adsorbed molecular dipoles by measuring the interference between the SHG signal from the sample and another source (reference) while varying the optical phase between them.10-13 It has been used mainly to compare the * Corresponding author. E-mail: [email protected]. † Center for Microcrystal Assembly, Department of Chemistry, and Interdisciplinary Program of Integrated Biotechnology.

molecular orientation of the molecular thin films or monolayers under different conditions such as slight alteration of the adsorbing molecules, change of the substrate properties, and different deposition processes. In this paper, we prepared two hemicyanine molecules with hydrophobic C18-alkyl chains on opposite ends of the dipolar nonlinear optical chromophore and incorporated them into the vertical channels of a continuous silicalite-1 film grown on a glass plate. The second-harmonic phase was indeed opposite between the silicalite-1 films with the above incorporated molecules, showing that these two molecules enter the silicalite-1 channels in an opposite direction. A Langmuir monolayer of the hemicyanine molecules was used as a reference sample to determine the absolute orientation of the dyes in these samples, which allows us to find out that the hydrophobic tail part of the molecule prefers to enter the silicalite-1 pore first. II. Experiment Preparation of Samples. Figure 1 shows the two hemicyanine dyes (4-[4-(dimethylamino)styryl]-1-octadecylpyridinium bromide (HC-18) and 1-ethyl-4-[4-(methyloctadecylamino)styryl]pyridinium bromide (R-HC-18) with the hydrophobic C18alkyl chain on the opposite end of the dipolar nonlinear optical chromophore. The HC-18 was prepared by refluxing the acetonitrile solution (25 mL) of 4-[4-(dimethylamino)styryl]pyridin (224.3 mg, 1.0 mmol) and 1-bromooctadecane (333.4 mg, 1.0 mmol) under vigorous stirring for 6 days. After cooling to room temperature, acetonitrile was removed in vacuo. Ethyl acetate (100 mL) was introduced into the reaction flask containing the residue. The precipitated product was separated from the reaction mixture by adsorbing it onto silica gel, and the gel was washed thoroughly with ethyl acetate. The product on the silica gel was dissolved with MeOH. After evaporation of the MeOH, the red 4-[4-(dimethylamino)styryl]-1-octadecylpyridinium bromide was obtained. The R-HC-18 was prepared by refluxing the aceto-

10.1021/jp073178h CCC: $37.00 © 2007 American Chemical Society Published on Web 11/13/2007

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Figure 2. Schematic describing the reflection-geometry experiment for SHG phase measurement.

Figure 1. Structures of (a) HC-18 and (b) R-HC-18.

nitrile solution (25 mL) of 4-[4-(methyloctadecylamino)styryl]pyridine (0.64 g, 1.38 mmol) and the corresponding bromoethane (0.15 g, 1.38 mmol) for varying periods of time until the initially colorless solution turned dark red. The purification procedure was similar to that of HC-18 dye. Continuous silicalite-1 films supported on glass (denoted as SL/g) plates were prepared according to the method reported previously.1,14-16 Having a high ratio of silica to alumina, pores of silicalite-1 are very hydrophobic because of the electrical neutral microporous framework structure and are stable up to high temperatures. The structural porosity of silicalite-1 consists of straight, circular vertical channels (5.4-5.6 Å) interconnected with sinusoidal, elliptical pores (5.4-5.6 Å).17 Hemicyanine dyes were incorporated into the silicalite-1 channels by placing SL/g plates for 12 h in a vial (25 mL capacity) containing a methanol solution of HC-18 or R-HC-18 (10 mL, 1 mM). After equilibration, the HC-18- or R-HC-18-incorporating SL/g plates were removed from the solution, washed with copious amounts of fresh methanol, and dried in the air. The amounts of HC-18 or R-HC-18 molecules incorporated in SL/g films were analyzed quantitatively according to the following procedures. A dilute aqueous solution of hydrofluoric acid (1 mL, a mixture of 0.2 mL of 49% HF and 0.8 mL of distilled deionized water) was introduced into a 50-mL plastic flask containing 10 HC-18-incorporating SL/g plates. After gentle swirling of the mixture for 5 minutes at room temperature, the solution was neutralized by adding aqueous NaOH solution (1 mL, 5 M). Methanol (8 mL) was subsequently added into the neutral solution to ensure dissolution of all of the dye into the solution. After removal of the clean glass substrates from the solution, the solution was centrifuged to precipitate silica particles. The clean supernatant solution was decanted into one of the matched pair of quartz cells for a spectroscopic measurement. To increase the accuracy of the quantitative analysis, we independently determined the molar extinction coefficient of each HC-18 at a concentration similar to that of the extracted solution. For this, a mixed solvent with a composition similar to that of the extracted solution was prepared. Freshly calcined silicalite (20 µg), 10 pieces of clean glass plates (18 × 25 mm2), and hydrofluoric acid (10 mL consisting of 2 mL 49% HF and 8 mL of distilled deionized water) were sequentially introduced into a 100-mL plastic beaker. After swirling of the beaker for 5 min, the solution was neutralized by adding aqueous NaOH solution (10 mL, 5.0 M). Methanol (80 mL) was subsequently added to the solution, and the clean supernatant solution was collected by centrifugation. Independently, a stock solution of HC-18 dye in methanol was prepared by dissolving a known amount of HC-18 dye (20-100 µg) in methanol (10 mL). An

aliquot (1 mL) of the stock solution of HC-18 was added into 100 mL of the above simulated solution, and the absorbance of the mixed solution was recorded. More aliquots were added into the simulated solution until the absorbance of the final solution became close to the one obtained from the HC-18 solution extracted from HC-18-incroporating SL/g plates. The quantitative analysis of the amount of R-HC-18 in each SL/g plate was similarly carried out. A Langmuir monolayer of HC-18 was prepared by spreading 1.2 mM solution of HC-18 in chloroform on a water (Milli-Q, 18 MΩ) surface with a microsyringe to make the surface area/ molecule 35 Å2/molecule.18,19 It has been known from previous results that at this surface area HC-18 molecules form a neat monolayer with the alkyl chain upright and the dimethylamine group into the water.20-22 The surface pressure decreased slowly, and the dyes dissolved into the subphase water over a time scale of ∼1 h; presumably, the C18-alkyl chain is not hydrophobic enough to maintain the monolayer for a long period. So, SHG phase measurement was conducted as soon as the monolayer was prepared. Second-Harmonic Phase Measurement. The laser system for the SHG experiment was a mode-locked picosecond Nd: YAG laser (45-ps pulsewidth, 10-Hz repetition rate). The typical input energy was ∼1.3 mJ/pulse. The fundamental input beam was usually p-polarized, and p-polarized second-harmonic output (532 nm) was sent to the detector (p-in p-out for short). The incident beam was directed onto the sample at a 70° incidence angle for reflection mode and ∼60° incidence angle (when the p-in p-out SHG signal was maximum) for transmission mode. The outgoing SHG beam was sent to an analyzer, filtered by 532-nm pass filters and a monochromator, and detected by a photomultiplier tube. Figure 2 shows the experimental setup for SHG phase measurement in reflection geometry.23 To prevent relative phase shift from reflection, the incident angle (70°) was made larger than the Brewster angle for both silicalite-1 and water surfaces. A 1064-nm beam reflected from the sample and the secondharmonic beam generated from the sample first passed through the fused silica plate, which was used as a phase retarder. Next, the two beams (1064 nm, 532 nm) were incident normally on a 50-µm-thick z-cut quartz plate, where the 532-nm output (reference SHG) was generated from the remaining 1064-nm input beam and interfered with the SHG from the sample. The phase measurement in transmission geometry is almost the same as that for reflection geometry.12 Because the zeolite sample surface was not smooth, dimethylsulfoxide was put onto the sample as an index matching fluid and then the surface was covered by a coverglass. When the sample was changed from a zeolite plate to a HC-18 Langmuir monolayer, the coverglass of same thickness was put on top (with a gap between the

Determination of Absolute Orientations

Figure 3. UV-vis spectra of hemicyanine dyes. Solid line: HC-18 extracted from zeolite film and in methanol, dashed line: R-HC-18 extracted from zeolite film and in methanol, dotted line: HC-18 in zeolite film (measured by the reflection method).

monolayer surface and the coverglass) to maintain the same optical path between the two cases. III. Results and Discussion The relative incorporation ability of HC-18 and R-HC-18 dyes into silicalite-1 pores was first compared using two zeolite plates immersed for 12 h in 1 mM concentration of HC-18 and R-HC18, respectively. Figure 3 shows the UV-visible absorption spectra that correspond to aqueous methanol solution of two different hemicyanine dye molecules extracted from continuous silicalite-1 films using the method in the previous section. The HC-18 were found to enter into the silicalite-1 pore more easily, with the average number of incorporated HC-18 and R-HC-18 in each 400-nm-long channel being 3.1 and 1.5, respectively. Presumably, R-HC-18 with a long alkyl chain attached in the amine group has more lateral size than the rod-like HC-18 and cannot get into the vertical pore as easily as HC-18. The vertical pores of slicalite-1 that HC-18 and R-HC-18 dyes can enter have a diameter of 5.4-5.6 Å, a tight fit as compared to the lateral size of HC-18 or R-HC-18. Thus, adsorbed HC molecules are expected to reside inside mostly as monomers. Whether the dyes formed aggregates could be checked by measuring the UV-visible absorption because the absorption peak would be distorted and the peak width would broaden for samples containing dye aggregates as compared to the dilute samples in which the dyes exist as monomers. However, UVvisible absorption measurement with our sample was not easy because the silicalite-1 film strongly scattered the light; actual spectrum from the trial measurement gave an absorption feature hidden inside the scattering noise and hardly visible. Thus, we prepared the sample with a much larger incorporation amount of HC dyes, and measured the UV-visible absorption by the reflection method. The spectrum shown in Figure 3 (dotted line) is almost similar to the UV-visible absorption spectrum from HC/methanol solution, apart from a rigid peak shift due to the change in the dielectric property the host medium (methanol vs silicalite). Because this sample with much more dyes inside is free from any spectral distortion or broadening, we can safely assume that our samples used in the manuscript are free from any aggregate at all, as expected from the physical size of silicalite-1 pore. From the second-harmonic Maker fringe experiment, SHG with p-in p-out polarization combination is more than 2000 times larger than that with s-in p-out polarization combination for both (2) samples (i.e., χzzz . χ(2) zxx), indicating that the hemicyanine molecules are mostly along the vertical channel of the silicalite-1 (2) pore.24 The nonlinear susceptibility χzzz of the HC-18 film

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Figure 4. SHG interference fringes in transmittance mode observed by rotating the fused quartz plate between two SHG sources. Solid line is a fit to the data. (a) HC-18 monolayer in zeolite and (b) R-HC18 monolayer in zeolite.

from the SHG measurement was found to be ∼3 times larger than that of the R-HC-18 film, and more than 10 times larger 1,12 than that of crystalline quartz, for which χ(2) xxx ∼ 0.5 pm/V. Figure 4 shows the SHG phase measurement data taken from these two silicalite-1 films before they were dissolved in HF solution for UV-visible absorption measurement. The experiment was done in transmission geometry with a 60° incidence angle at the silicalite-1 films. The relative phase found from the fitted result (shown as a solid line in each graph) was ∼179°, which clearly shows that the relative phase of the secondharmonic signal from the samples with the NLO molecules HC18 and R-HC-18 is opposite. So, HC-18 and R-HC-18 molecules were found to enter into the vertical channel of the zeolite pore in the opposite direction. Although the above result shows that the two hemicyanine molecules are oppositely oriented, the absolute orientation of each molecule in the sample is still undetermined. In a previous study,1 we compared the incorporated amounts of HC-3 (hemicyanine dye with an n-propyl chain attached at the pyridinium moiety) and n-octadecane, respectively, into silicalite-1 powder. HC-3 and n-octadecane were chosen to represent the hydrophilic head- and hydrophobic alkyl tail-part of HC-18 (or R-HC-18), respectively. The included number of n-octadecane from the experiment was as much as 480 times larger than that of HC-3, demonstrating that the hydrophobic silicate-1 channels have a much higher affinity toward the hydrophobic tail part. Although it was anticipated that both molecules enter the silicalite-1 channels with the hydrophobic tail part first, this proposition was put to an explicit test. To check which part of the molecule enters into the pore first, we used a Langmuir monolayer of HC-18 hemicyanine molecules on a water surface as a reference to determine the absolute polar orientation. For HC-18/silicalite film, the p-in p-out SHG signal is dominated by χ(2)zzz because the contribution from χ(2)zxx or χ(2)xzz is found to be insignificant. This is because in silicalite film HC-18 molecules are constrained to have vertical alignment following the pore direction, which also allowed us to find out that the hyperpolarizability component β(2) ζζζ of the hemicyanine molecule is much larger than the off(2) diagonal component βζηη (ζ is along the long axis, and η is perpendicular to the molecular long axis of hemicyanine).24 For the HC-18 Langmuir monolayer, the situation is more complicated because the hemicyanine molecules on water would not have near-vertical alignment in general, although they are known to have net polar order with the dimethylamine part of the molecules in the water side and the alkyl chain pointing to the

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(2) Figure 5. (a) χ(2) PP vs molecular tilt angle (dashed line is for χPP ) 0). (b) I(2ω)PP/I(2ω)PS vs molecular tilt angle (dashed horizontal line indicates the experimental value). The incidence angle was set to δ ) 70° for each calculation.

air side. Thus, several terms would contribute to the nonlinear susceptibility χ(2) PP in the latter case, and the overall sign including its change with molecular conformation should be considered carefully. The second-harmonic nonlinear susceptibilities χ(2) PS (for s-in (2) p-out polarization combinations) and χPP (for p-in p-out polarization combinations) are as follows25 2 (2) χ(2) PS ) χzyyLzz(2ω)[Lyy(ω)] sin δ 2 3 2 (2) (2) χ(2) PP ) χzzzLzz(2ω)[Lzz(ω)] sin δ + χzxx Lzz(2ω)[Lxx(ω)] 2 sin δ cos2 δ - 2χ(2) xxzLxx(2ω) Lxx(ω) Lzz(ω) sin δ cos δ (1)

where δ is the incidence angle of the light and Lii(Ω) is the local field factor in the i direction with frequency Ω. In eq 1, each susceptibility component can be estimated if the tilt angle of the molecule is known. Because the molecular hyperpolarizability of hemicyanine is dominated by βζζζ,24 the susceptibility components in eq 1 are26

Figure 6. Ratio of the second-harmonic signal I(2ω)PP/I(2ω)PS and surface pressure of the HC-18 molecular monolayer with the change in the molecular area. Dashed vertical line at 35 Å2 indicates the condition in which SHG phase measurement was performed.

3 (2) χ(2) zzz ) N〈cos θ〉βζζζ

χ(2) zxx )

N 〈sin2 θ cos θ〉β(2) ζζζ 2

χ(2) xxz )

N 〈sin2 θ cos θ〉β(2) ζζζ 2

(2)

where N is the surface number density of the molecules and θ is the angle between the molecular ζ axis and the surface normal (z axis). By assuming that the surface molecules have a δ-function-like distribution of polar tilt angle θ, the χ(2) PP is indeed shown to change sign as in Figure 5a. Figure 5b shows that the ratio of the second-harmonic signal I(2ω)PP/I(2ω)PS is sensitive to the change in θ and can be used to find out the tilt angle of the hemicyanine molecules at the surface.27 Figure 6 shows the ratio of the second-harmonic signal I(2ω)PP/I(2ω)PS from the experiment as the surface area/ molecule (A) is reduced from initial spread by using a barrier, shown along with a surface pressure (π) of the monolayer. I(2ω)PP/I(2ω)PS is shown to start at about 0.5 for very large area/molecule and remains at this value up to ∼30 Å.2 This value corresponds to θ ∼ 44° from Figure 5b. At this angle, Figure 5a indicates that χ(2) PP has a positive sign and has the (2) same sign as the term proportional to χzzz among the terms in eq 1. One would normally expect that the molecule would change the tilt angle to stand up as it is compressed. For the case of a Langmuir monolayer consisting of octyl-cyanobiphenyl (8CB) molecules, the angle was shown to change from 53° ∼ 66° (changes depending on the monolayer refractive index value assumed) when the surface pressure was 20 dyn/cm, to 43 ∼

Figure 7. SHG interference fringes (in reflection mode) observed by rotating the fused quartz plate between two SHG sources. Solid line is a fit to the data. (a) HC-18 monolayer on the water surface and (b) HC-18 monolayer in zeolite.

49° when it is compressed to 40 dyn/cm.28 In contrast, the result in Figure 6 indicates that the molecular tilt angle of hemicyanine does not change appreciably upon compression.29 Because I(2ω)PP/I(2ω)PS is never close to zero as the monolayer is compressed, it also indicates that θ is always smaller than ∼53° (the angle where χ(2) PP is shown to vanish from Figure 5a) even at very large area/molecule of ∼80 Å.2 The fact that the molecules are not lying flat at this large area may seem unphysical at first but can be understood if the interaction between the hemicyanine headgroup and the subphase water molecules is larger than that between neighboring hemicyanine molecules such that compression would not change the molecular conformation appreciably from that of the isolated hemicyanine molecule on water.

Determination of Absolute Orientations

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18163 Acknowledgment. This work was supported by Korea Research Foundation Project KRF-2006-312-C00554 and Seoul Research and Business Development Program (10816). References and Notes

Figure 8. Schematic picture of hemicyanine molecules in zeolite and on the water surface. The dipole direction is shown with arrows in each figure. (a) HC-18 molecule in zeolite, (b) R-HC-18 molecule in zeolite, and (c) HC-18 monolayer on the water surface.

Because the sign of χ(2) PP is now unambiguous (the same as (2) the sign of χzzz ) for a hemicyanine monolayer, it can be used to determine the absolute polar orientation of the HC-18/silicalite film using the SHG phase measurement in reflection geometry by substituting the Langmuir monolayer in Figure 2 by HC18/silicalite film. Figure 7 shows the SHG phase measurement result, which shows that the relative phase between SHG from the Langmuir monolayer and that from the HC-18 sample in zeolite is ∼178°. Thus, we came to know that the HC-18 molecules indeed prefer to enter into the zeolite pore with the hydrophobic, alkyl chain part first. This combined with the phase measurement with HC-18 and R-HC-18 samples (Figure 4) shows that the R-HC-18 molecule also enters with the hydrophobic part first, as shown in Figure 8 IV. Conclusions Second-harmonic phase measurement was used to find out the absolute orientation of the hemicyanine dye molecules adsorbed into silicalite-1 films. The second-harmonic phase was opposite between the molecules with hydrophobic part of the molecule attached on opposite sides of the NLO chromophore. Comparison of the second-harmonic phase of one of the samples with the Langmuir monolayer of hemicyanine allowed the determination of the absolute orientation of the two hemicyanine molecules. It was found that the hemicyanine molecules enter into the pore of the silicalite-1 with the hydrophobic part first.

(1) Kim, H. S.; Lee, S. M.; Ha, K.; Jung, C.; Lee, Y.-J.; Chun, Y. S.; Kim, D.; Rhee, B. K.; Yoon, K. B. J. Am. Chem. Soc. 2004, 126, 673. (2) Cox, S. D.; Gier, T. E.; Stucky, G. D.; Bierled, J. J. J. Am. Chem. Soc. 1988, 110, 2987. (3) Cox, S. D.; Gier, T. E.; Stucky, G. D. Chem. Mater. 1990, 2, 609. (4) Werner, L.; Caro, J.; Finger, G.; Kornatowski, J. Zeolites 1992, 12, 658. (5) Simpson, G. J.; Rowlen, K. L. J. Phys. Chem. B 1999, 103, 3800. (6) Twieg, R. J.; Dirk, C. W. Chem. Phys. 1986, 5, 3537. (7) Feller, M. B.; Chen, W.; Shen, Y. R. Phys. ReV. A 1991, 43, 6778. (8) Oh-e, M.; Hong, S. C.; Shen, Y. R. J. Phys. Chem. B 2000, 104, 7455. (9) Tojima, A.; Matsuo, Y.; Hiyoshi, R.; Manaka, T.; Majima, Y.; Iwamoto, M. Thin Solid Films 2001, 393, 86. (10) Smiley, B. L.; Vogel, V. J. Chem. Phys. 1995, 103, 3140. (11) Sato, O.; Baba, R.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. 1991, 95, 9636. (12) Kajikawa, K.; Yoshida, I.; Seki, K.; Ouchi, Y. Chem. Phys. Lett. 1999, 308, 310. (13) Kimura-Suda, H.; Sassa, T.; Wada, T.; Sasabe, H. J. Phys. Chem. B 2001, 105, 1763. (14) Wang, Z.; Hedlund, J.; Sterte, J. Microporous Mesoporous Mater. 2002, 52, 191. (15) Finger, G.; Richter-Mendau, J.; Bulow, M.; Kornatowski, J. Zeolites 1991, 11, 443. (16) Ha, K.; Lee, Y.; Chun, Y. S.; Park, Y. S.; Lee, G. S.; Yoon, K. B. AdV. Mater. 2001, 13, 594. (17) Flanigen, E. M.; Bennett, J. M.; Grose, R. W.; Cohen, J. P.; Patton, R. L.; Kirchner, R. M.; Smith, J. V. Nature 1978, 271, 512. (18) Lam, J. Y. S.; Stroeve, P. Thin Solid Films 1994, 243, 361. (19) Sharath Chandra, M.; Ogata, Y.; Kawamata, J.; Radhakrishnan, T. P. Langmuir 2003, 19, 10124. (20) Niidome, Y.; Ayukawa, H.; Yamada, S. J. Photochem. Photobiol., A 2000, 132, 75. (21) Sato, O.; Baba, R.; Hashimoto, K.; Fujishima, A. Jpn. J. Appl. Phys. 1993, 32, 1201. (22) Kajikawa, K.; Yamada, T.; Yokoyama, S.; Okada, S.; Matsuda, H.; Nakanishi, H.; Kakimoto, M.; Imai, Y.; Takezoe, H.; Fukuda, A. Langmuir 1996, 12, 580. (23) Kim, H.; Sohn, K.; Jeon, Y.; Min, H.; Kim, D.; Yoon, K. AdV. Mater. 2007, 19, 260. (24) Shim, T. K.; Kim, D.; Lee, M. H.; Rhee, B. K.; Cheong, H. M.; Kim, H. S.; Yoon, K. B. J. Phys. Chem. B 2006, 110, 16874. (25) Zhuang, X.; Miranda, P. B.; Kim, D.; Shen, Y. R. Phys. ReV. B 1999, 59, 12632. (26) Mitchell, S. A. J. Phys. Chem. B 2006, 110, 883. (27) In Figure 5a and b, the monolayer refractive index was set to n′ ) 1.33. (28) Zhang, T. G.; Zhang, C. H.; Wong, G. K. J. Opt. Soc. Am. B 1990, 7, 902. (29) Shirota, K.; Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. 1990, 29, 750.