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Langmuir 1998, 14, 7456-7462
Solvatochromism of a Merocyanine Derivative in a Self-Assembled Monolayer on Gold Substrates Katsuhiko Fujita,*,†,‡ Masahiko Hara,† Hiroyuki Sasabe,† and Wolfgang Knoll†,§ Frontier Research Program, The Institute of Physical and Chemical Research (Riken), Hirosawa 2-1, Wako, Saitama 351-0198, Japan, and Max-Planck-Institute for Polymer Science, Ackermannweg 10, 55021 Mainz, Germany
Kazuma Tsuboi, Kotaro Kajikawa,* Kazuhiko Seki, and Yukio Ouchi Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan Received May 21, 1998. In Final Form: September 15, 1998 The merocyanine, 1-alkyl-4-(4-hydroxystyryl)pyridinium halide, shows solvatochromism depending on environmental condition such as pH and polarity of the solvent. We synthesized the merocyanine derivative with a disulfide group tethering it on metal surfaces to form a self-assembled monolayer. The adsorption process of the merocyanine derivative was monitored by in situ surface plasmon spectroscopy (SPS). The SPS and X-ray photoelectron spectroscopy independently indicated that the monolayer was densely packed on a gold substrate. To investigate the solvatochromism following the exposures to organic solutions we applied reflection spectroscopy in the UV-vis region. For more detailed discussions on the reflection spectra, theoretical calculations were carried out on the basis of Fresnel formulas. The results indicated that the merocyanine group shows solvatochromism even in the densely packed layer.
Introduction Organosulfur compounds exemplified by thiol or disulfide derivatives spontaneously form self-assembled monolayers (SAMs) on metal substrates such as gold and silver by exposure of the substrates to solutions.1,2 It has been well accepted that these SAMs can exhibit a highly ordered and densely packed structure.3,4 Although most of the studies have been accomplished for SAMs without functional groups, such as alkanethiolate SAMs, increasing interest has been directed toward SAMs containing chromophores for optical applications.5 Several efforts have been reported for SAMs containing azobenzene derivatives6-8 which show photochromism on the basis of a conformational change from the cis to the trans isomer. It is difficult to observe the photochromism in such densely packed monolayers. Among various chromophores, the merocyanines, 1-alkyl-4-(4-hydroxystyryl) pyridinium halides, are popu†
The Institute of Physical and Chemical Research (Riken). Max-Planck-Institute for Polymer Science. Present address: Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan. E-mail: katsuf@ asem.kyushu-u.ac.jp. § ‡
(1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (2) Reviewed in: Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991; Part 3. (3) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (4) Widrig, C.; Alves, C. A.; Porter, M. J. J. Am. Chem. Soc. 1991, 113, 2805. (5) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (6) Wolf, H.; Ringsdorf, H.; Delamarche, E.; Takami, T.; Kang, H.; Michael, B.; Gerber, C.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Phys. Chem. 1995, 99, 7102. (7) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Durbin, K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071. (8) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Ishida, T.; Hara, M.; Knoll, W. Langmuir, submitted for publication.
lar as solvatochromic dyes without a drastic conformational change.9-12 The color change is induced by the change of protonation and the bond reorganization illustrated in Figure 1a. The merocyanines take the protonated form in an acidic solution, while two kinds of structures are possible in basic solutions: one is a zwitterionic form and the other is a quinoid form. The former appears in a polar solvent such as alcohol, while the latter appears in a nonpolar environment such as chloroform or dichloromethane.13 In the present study we intend to form SAMs of a merocyanine derivative and to investigate their attractive optical characteristics. For this purpose we synthesized a merocyanine derivative with a disulfide group, ω,ω′dithiodi[1-undecyl-4-(4-hydroxystyryl)pyridinium bromide] (HSPC11SS), illustrated in Figure 1b, and prepared SAMs by immersion of gold substrates into the solution. To characterize the structure of the SAM, in situ surface plasmon spectroscopy (SPS) and X-ray photoelectron spectroscopy (XPS) were employed. Reflection spectroscopy was also used to monitor the solvatochromism of the SAM on Au substrates. The observed reflection spectra for the gold substrate were too complicated to assign the molecular forms. For overcoming this problem, we adopted a Lorentz oscillator model in order to simulate the optical density of the organic monolayer with absorption in the UV-vis region. Then the solvatochromism of the merocyanine SAM was evaluated by comparing the reflection spectra with the simulated ones. Our results showed that (9) Giriling, I. R.; Kolinsky, P. V.; Montgomery, C. M.; Cade, N. A.; Earls, J. D.; Peterson, I. R. Opt. Commun. 1985, 55, 289. (10) Giriling, I. R.; Cade, N. A.; Montgomery, C. M. Electron. Lett. 1985, 21, 169. (11) Hayden, L. M.; Kowel, S. T.; Srinivasan, M. P. Opt. Commun. 1987, 61, 289. (12) Itoh, K.; Hayashi, K.; Hamanaka, Y.; Yamamoto, M.; Araki, T.; Iriyama, K. Langmuir 1992, 8, 140. (13) Gains, G. L., Jr. Anal. Chem. 1976, 48, 450.
10.1021/la9806000 CCC: $15.00 © 1998 American Chemical Society Published on Web 12/03/1998
Solvatochromism of a Merocyanine SAM
Langmuir, Vol. 14, No. 26, 1998 7457 was collected by filtration and dried in vacuo. The product was purified by recrystallization from 10 µM HBr/methanol and from methanol twice. 1H NMR (CD3OD): δ 8.67 (d, 4H), 8.05 (d, 4H), 7.83 (d, 2H), 7.58 (d, 4H), 7.14 (d, 2H), 6.5 (d, 4H), 4.46 (t, 4H), 2.64 (t, 4H), 1.9 (m, 4H), 1.6 (m, 4H), 1.5-1.2 (m, 28H). Anal. Calcd for C48H66O2N2S2Br2 H2O: C, 61.59; H, 7.21; N, 2.99; S, 6.85; Br, 17.07. Found: C, 61.68; H, 7.25; N, 2.94; S, 6.93; Br, 16.99.
Methods
Figure 1. Schematic representation of (a) three forms the merocyanine showing solvatochromism and (b) molecular structure of HSPC11SS.
the merocyanine group shows solvatochromism based on protonation and deprotonation even in a densely packed monolayer. Experimental Section Materials. Octadecanethiol, 11-bromoundecanol-1, diisopropyl azodicarboxylate, and thioacetic acid were purchased from Tokyo Chemical Industry (Tokyo). Triphenylphosphine, γ-picoline, p-hydroxybenzaldehyde, and the other organic solvents were from Kanto Chemicals (Tokyo). HSPC11SS Synthesis. Triphenylphosphine (23.0 g) was dissolved in dry tetrahydrofuran (THF) and cooled in an ice bath under nitrogen atmosphere. A THF solution of diisopropyl azodicarboxylate (17.7 g) was added dropwise with vigorous stirring. After 30 min stirring at 0 °C, 11-bromoundecanol (20.0 g) and thioacetic acid (6.7 g) dissolved in dry THF were added to the reaction mixture. The solution was stirred at 0 °C for 30 min and warmed to room temperature. One milliliter of methanol was added to terminate the reaction, and the solvent was removed with a rotary evaporator. To the residual oil was added hexane/ethyl acetate (2:1, v/v), and the formed white precipitate was removed by filtration and discarded. The filtrate was purified by flash chromatography using an eluent of hexane and ethyl acetate (2:1, v/v) to obtain a colorless syrup. The syrup was dissolved in ice-cold methanol saturated with dry HCl. After being stirred under nitrogen atmosphere for 4 h until transesterification was completed, the solution was poured into 3 L of degassed water. The organic layer was extracted with hexane twice, and the hexane solution was dried with MgSO4 and purified by flash chromatography using an eluent of hexane/ethyl acetate (2:1, v/v). The resulting colorless oil was dissolved in degassed ethanol and was titrated with a 100 mM Solution of iodine in ethanol to form a disulfide. The reaction mixture was concentrated by a rotary evaporator and was extracted with ethyl acetate. The ether phase was washed twice with water, dried with MgSO4, and concentrated. The resulting disulfide (14.3 g) and γ-picoline (4.5 g) were dissolved in 500 mL of toluene and refluxed with vigorous stirring. A viscous oil appeared as the reaction proceeded. After a 12-h reflux, the solvent was removed by evaporation, and the residual oil was dissolved in ethanol. The ethanol solution was poured into ethyl acetate to obtain a slightly pink precipitate. The precipitate was collected by filtration and dried in vacuo. Five grams of the precipitate was dissolved in methanol. p-Hydroxybenzaldehyde (2.1 g) and piperidine (0.5 mL) were added to the methanol solution followed by reflux for 4 h. After being cooled to room temperature, the solution was condensed by evaporation and was poured into ethyl acetate. The resulting orange precipitate
SAM Preparation. Glass slides (LaSFN9 for SPS and fused silica for XPS, reflection spectroscopy and ellipsometry) were cleaned with 5% Extran in Milli-Q ultrapure water and were rinsed with Milli-Q water carefully. The slides were then dried in a nitrogen stream and placed in an Edwards Auto 360 evaporator. A gold film of 50-nm thickness was vacuum-deposited directly on the LaSFN9 glass, and the substrate was used for SPS measurements immediately after the deposition. In the case of fusedsilica glass, 1 nm of chromium was evaporated first to promote the gold adhesion, following which a gold film of 300-nm thickness was deposited. The substrates were exposed either to a 0.5 mM ethanol solution of HSPC11SS for 2-24 h or to a 1 mM ethanol solution of octadecanethiol for 3 h. SPS. The measurements were performed with a homemade Kretschmann configuration apparatus14 with a HeNe laser (λ ) 633 nm) as the light source. A Teflon cell with an inlet and outlet ports was used for immersing the substrate to measure the adsorption kinetics on the gold surface. The reflectivity as a function of the incident angle was recorded before and after the SAM formation on the substrate. These spectra are then used to determine the optical thickness of the SAM layers by comparison of them with a simulation based on Fresnel formulas. For the kinetic measurement, the incident angle was fixed at an angle 0.5° smaller than the angle for the surface plasmon resonance observed as a dip in the spectra, and the reflection intensity was monitored as a function of time. XPS. The XPS measurements were performed with an ESCALAB 220i (Vacuum Generators) instrument using monochromatic Mg KR radiation. All spectra were measured for photoelectrons emitted normal to the substrate. The absolute energy was calibrated by comparison with the Au(4f7/2) binding energy of 83.2 eV. The composition and the area of each peak were obtained by a curve-fitting procedure using both Gaussian and Lorentzian functions. Ellipsometry. Ellipsometry was carried out with a VASE 32 ellipsometer (J. A. Woollam Co., Inc) using a Xe light source. The measurements were performed at a 10nm interval in the range of 300-800 nm. The untreated metal substrates were first examined with incident angles of 45, 55, 60, 65, 70, 75, and 80° relative to the surface normal. In the present study, we used the refractive index of gold obtained by ellipsometry for analyzing the other data, because it was slightly different from that of the bulk value reported in the literature.15 UV-vis Spectra. The absorption spectra of solutions were measured with a U-2000 spectrophotometer (Hitachi) using an optical cell with a 1-cm path length. HSPC11SS was dissolved at a concentration of 2.5 × 10-5 M in ethanol/ water (95/5, v/v) or dichloromethane. For pH adjustment, the ethanol solution was titrated with 0.01N NaOH and 0.01N HCl aqueous solutions. Triethylamine was also used (14) Knoll, W. MRS Bull. 1991, XVI, 29. (15) Lynch, W. L.; Hunter, W. R. In Handbook of Optical Constants of Solids; Palik, E. D., Ed.; Academic Press Inc.: San Diego, CA, 1985; Vol. 1, pp. 275-367.
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Figure 3. Schematic geometry of the arrangement used for the analysis of reflection spectra in this study. Medium A with refractive index nA: air or solution. Medium B with refractive index nB: metal substrate. SAM with refractive index nsam: SAM with thickness d.
Figure 2. Experimental setup for reflection spectroscopy at the incident angle of (a) 0° and (b) 45°.
for controlling the solvent basicity by adding to the dichloromethane solution in various concentrations. Reflection Spectra. The reflection spectra were recorded with a multichannel photodetector, MCPD-1000 (Photal Otsuka Electronics), with both I2 and D2 lamps. The experimental setup is shown in Figure 2. The sample plates were placed in the appropriate solutions or in air. The light was guided to the sample plate at a certain incident angle from the lamps by an optical fiber equipped with a polarizer on the tip. The reflected light was brought to the photodetector also by a fiber. The optical density of the SAM, Asam, was determined using a reference signal recorded with an untreated substrate in the same condition and is denoted as Asam ) -log(R/R0), where R and R0 are the reflectance with and without the SAM on the surface, respectively. In the analysis, we used a model to simulate the reflectivity16 as depicted in Figure 3. Medium A corresponds to the air or solution used in the experiment. The substrate is introduced as Medium B, which is a semiinfinitely thick medium. The general expression with the Fresnel formulas delivers the reflectivity in the present system, under the supposition that the SAM is an isotropic slab with a finite thickness d, while it is actually much thinner than the wavelength of light. In the analysis, the refractive index of the SAM, nsam(ω), at a frequency ω is estimated on the basis of the Lorentz oscillator model, which is described as
nsam(ω) ) nopt(ω) + inr nopt(ω) )
x
0 +
ω0
2
F - ω2 + iωΓ
where ω0 is the resonance frequency, Γ is the damping factor, and F is an intensity factor including the oscillator strength. 0 is introduced as a dielectric constant independent of the frequency ω. The first term, nopt, originates from the optical property of the merocyanine SAM itself, (16) Bethune, D. S. J. Opt. Soc. Am. B 1989, 6, 910.
and the second term, inr, comes from the physical structure of the monolayer. It is likely that the SAM on the evaporated-gold surface is undulated because it will reflect the roughness of the gold surface. Since it is difficult to treat the roughness factor exactly, we always consider that it appears as an imaginary part of the refractive index. Actually, the reflection spectrum of an alkanethiolate SAM on gold was perfectly simulated by introducing this imaginary part of the refractive index as the roughness factor.17 Results and Discussion Solvatochromism of the Merocyanine Dissolved in Solution. Absorption spectra of HSPC11SS dissolved in ethanol and dichloromethane are shown in Figure 4. An absorption peak is observed at 400 nm in the acidic solution, which is attributed to the protonated cationic species. As the basic solution was added, this absorption peak became weaker, and another absorption peak appeared at longer wavelength. When a 6-fold equivalent of NaOH was added to the ethanol solution, the absorption spectrum showed a single peak at 520 nm. The absorbance at this peak was unchanged by further addition of NaOH. Since the zwitterionic form is known to predominate over the quinoid form in a polar environment,13 the absorption peak at 520 nm in the ethanol solution can be attributed to the deprotonated zwitterionic form. Absorption spectra of HSPC11SS in the dichloromethane solution showed a significant change by addition of triethylamine to increase the basicity of the solution. The intensity of the peak at 400 nm was decreased drastically, and another absorbance maximum appeared at 570 nm in the 0.1 M triethylamine in dichloromethane. The peak at 570 nm in dichloromethane solution can be attributed to the quinoid form. Thus, the three forms shown in Figure 1 could actually be prepared in solutions under controlled conditions. Adsorption. The adsorption isotherm of HSPC11SS on gold substrates from the ethanol solution was measured by SPS (Figure 5a). The vertical axis in Figure 5a was converted from the reflectivity at the incident angle of 58.3°, which is 0.5° smaller than the angle for the surface plasmon resonance, to the adsorbed layer thickness. The adsorption curve showed a spontaneous increase of the (17) Tsuboi, K.; Kajikawa, K.; Seki, K.; Ouchi, Y.; Fujita, K.; Hara, M.; Sasabe, H.; Knoll, W. Manuscript in preparation.
Solvatochromism of a Merocyanine SAM
Figure 4. Absorption spectra of 2.5 × 10-5 M HSPC11SS (a) in ethanol/water (95/5, v/v) containing HCl at the concentration of 3.0 × 10-5 M(-) and containing NaOH at the concentration of 5.0 × 10-5 M(- - -) and 1.5 × 10-4 M(- -) (b) in dichloromethane containing triethylamine at the concentration of 0 (s), 6.8 × 10-2 M(- - -) and 1.0 × 10-1 M(- -).
layer thickness and reached a plateau after a 3-h exposure to the solution. After flushing pure ethanol to rinse off the physisorbed overlayer, we observed a slight decrease of the film thickness. The SPS spectra were recorded as a function of an angle of incidence before the injection of HSPC11SS solution and after the rinse with ethanol (Figure 5b). The adsorbed-layer thickness was calculated to be 2.0 nm from the best fit of the SPS spectra in Figure 5b under the assumption that the refractive index of the layer at the wavelength of the incident light (633 nm) is equal to 1.6 + 0.8i. The justification for this choice of the refractive index of the merocyanine will be discussed later. The length along the longer axis of HSPC11SS was calculated to be approximately 2.4 nm from a molecular model with a fully extended (all-trans) configuration. The layer thickness is consistent with the calculated molecular length when the tilt angle from the surface normal is assumed to be 30°.18 The HSPC11SS SAM prepared by 24-h exposure to the ethanol solution was examined by XPS (Figure 6). The peaks at 401.3 and 162.6 eV are attributed to N 1s (18) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
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Figure 5. (a) Adsorption isotherm of HSPC11SS at the concentration of 25 µM in ethanol, measured by SPS at the incident angle of 58.3°. An arrow indicates the time at which ethanol was flushed to rinse the cell, (b) SPS obtained before (O) and after (+) the SAM adsorption from 25 µM ethanol solution of HSPC11SS. Solid line and dashed line represent the fit to the theoretical calculation before and after the adsorption, respectively.
and S 2p, respectively,19 indicating the adsorption of HSPC11SSon the gold substrate. To estimate the molecular density of the SAM, we performed the XPS measurements of an SAM consisting of octadecanethiol in the same measuring condition. It was reported that the thickness of an octadecanethiol SAM prepared by 3-h exposure to a solution is 2.2 ( 0.1 nm.20 Since the thicknesses of the HSPC11SS and the octadecanethiol SAMs are close, the difference in the escape depth of the photoelectron for these SAMs can be neglected. It is thus possible to regard the ratio of the signals of the S 2p region for these SAMs as the ratio of the surface densities of the sulfur atoms. The intensity of the S 2p signals in both spectra and their ratio are summarized in Table 1. The octadecanethiol SAM is known to have a densely packed structure with an area per molecule of 0.214 nm2 on an Au(111) surface.21 This molecular area is consistent with the limiting area observed for a monolayer of fatty acid at the air/water interface. An amphiphilic merocyanine monolayer at the air/water interface shows a molecular area of 0.40 nm2, which is in good agreement with the cross section across (19) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (20) Nishida, N.; Hara, M.; Sasabe, H.; Hahn, C.; Knoll, W. Jpn. J. Appl. Phys. Part 1 1997, 36, 2379. (21) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546.
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Figure 7. Reflection spectra of HSPC11SS SAM prepared on a gold substrate by 3-h exposure to the ethanol solution, measured in the air with the incident angle of 0°.
Figure 6. XPS profiles of HSPC11SS SAM prepared by 24-h exposure to 0.5 mM ethanol solution in the range corresponding to (a) N1s and (b) S2p. Table 1. Peak Areas of S2p in XPS of the SAM in Arbitrary Unit and the Area Ratio batch
HSPC11SS
C18H37SH
ratio
1 2 3
4178.2 3570.0 2985.4
9153.1 7836.1 9421.7
0.456 0.456 0.317
the longer axis of the chromophore.22 Supposing that the area occupied by an HSPC11SS molecule on the gold surface is similar to the limiting area of the Langmuir monolayer, the ratio of the molecular density of the octadecanethiol and HSPC11SS on the gold surface is calculated to be 0.54, which is close to the surface-density ratio of sulfur atoms determined by XPS (Table 1). This indicates that the monolayer of HSPC11SS is a densely packed monomolecular film. Reflection Spectra of HSPC11SS SAM. Figure 7 displays the reflection spectra of an HSPC11SS SAM prepared on a gold substrate by 3-h exposure to the ethanol solution, measured in the air at an incident angle of 0°. It seems that there exist two bands: one at 450 nm and the other at 520 nm. However, detailed examination (22) Hall, R. A.; Thistlethwaite, P. J.; Grieser, F.; Kimizuka, N.; Kunitake, T. Langmuir 1994, 10, 3743.
Figure 8. Reflection spectra of HSPC11SS SAM prepared on a gold substrate by 3-h exposure to the ethanol solution. They were measured at the incident angles of (a) 0° and (b) 45° in ethanol containing 0.1 N HCl aqueous solution at the concentration of 4.8 × 10-3 M (s) and in ethanol containing 1 N NaOH aqueous solution at the concentration of 4.8 × 10-2 M (- -).
revealed that the peaks do not originate from the absorption of the merocyanine SAM, but from the characteristics of the refractive index of gold in this region. Actually, a similar apparent absorption is observed even in an alkanethiol SAM which possesses no absorption in the corresponding region.17 Figure 8 displays the spectra measured in ethanol at the incident angles of (a) 0° and (b) 45° with a p-polarized light. The peak observed at around 420 nm in the spectra
Solvatochromism of a Merocyanine SAM
Langmuir, Vol. 14, No. 26, 1998 7461
Figure 10. Simulated reflection spectrum of the SAM consisting of HSPC11SS in the protonated form in air, calculated with nA ) 1.0, d ) 2.1 nm, the incident angle ) 0°, and nB determined by ellipsometry.
Figure 9. Wavelength-dependence simulation of the real part (s) and the imaginary part (- -) of nsam for (a) the protonated form and (b) the zwitterionic form, where 0 ) 2.25, nr ) 0.8, (a) ω0 ) 4.7 × 1015 s-1, Γ ) 5.3 × 1014 s-1, F ) 4.9 × 1030 and (b) ω0 ) 3.6 × 1015 s-1, Γ ) 3.8 × 1014 s-1, F ) 4.2 × 1030.
under acidic conditions was diminished upon addition of an aqueous NaOH solution and that at 520 nm was enhanced by addition of the same. Although the solvatochromic behavior is very similar to that of the HSPC11SS ethanol solution (Figure 4a), it is necessary to simulate the spectra in order to confirm that the chromism is based on the deprotonation. The simulation requires the refractive index for the merocyanine layer, nsam. The damping constant Γ is obtained from the half width of absorption spectra of the merocyanine: Γ ) 5.3 × 1014 s-1 for the peak at 400 nm and Γ) 3.8 × 1014 s-1 for the peak at 520 nm. The intensity factor F for the peak at 400 nm was estimated to be 4.9 × 1030 by the absorption spectrum of a similar chromopholic monolayer of hemicyanine.23 The F value for the peak at 520 nm was determined to be 4.2 × 1030 by the absorbance ratio of the HSPC11SS solution at the acidic and basic conditions. The reflection spectroscopy of an alkanethiol SAM on a gold substrate used in the present study and the simulation of the spectrum result in nr ) 0.8.17 With the parameters discussed above, the nsam used in the present study was obtained as shown in Figure 9. It was found that nsam for the protonated form at 633 nm is 1.6 + 0.8i, and this was used for the thickness determination by SPS. Figures 10 and 11 show the simulations of the reflection spectra of the merocyanine SAM for the protonated form in the air (nA ) 1.0) and the protonated and the zwitterionic forms in ethanol (nA ) 1.36), respectively. The absorption (23) Kajikawa, K.; Shirota, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. Part 1 1991, 30, 1050.
Figure 11. Simulated reflection spectra of the HSPC11SS SAM in ethanol at the incident angles of (a) 0° and (b) 45°, where nA ) 1.36, d ) 2.1 nm, and nB determined by ellipsometry. (O) and (+) denote that nsam(ω) for the protonated form (Figure 9a) and nsam(ω) for the zwitterionic form (Figure 9b) were used for the simulation, respectively.
band of the protonated form at 420 nm appears as a shoulder at around 420 nm in the simulated spectra, indicated by an arrow in Figures 10 and 11. Since the real part of nsam results in the offset in the region shorter than 500 nm of the reflection spectrum, the dip of the real part of nsam at 360 nm resulting from the resonance (Figure 9a) appears as the dip in the simulated spectrum (Figures 10 and 11 (O)). Similarly, the dip at 470 nm in Figure 11 (+)
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indicates the resonance of the zwitterionic SAM at 520 nm, although the absorption at 520 nm is hardly distinguished from the peak due to the anomaly of the refractive index of gold. These simulated results are in good agreement with the observed spectra (Figures 7 and 8). It is concluded that the merocyanine shows solvatochromism based on the transformation between the protonated and the deprotonated zwitterionic form even in the densely packed SAM. The SAM prepared in a basic, nonpolar solution (0.5 mL of triethylamine in 10 mL of dichloromethane) was also examined by reflection spectroscopy. The spectrum measured at the incident angle of 0° in the air was identical with that shown in Figure 7 for the SAM prepared in a polar solution. This observation indicates that the merocyanine was protonated in the SAM after the plate was taken out of the solution into the air, although the dissolved merocyanine adopted the deprotonated quinoid form in the basic, nonpolar solution. The protonation in the air, which was observed in an LB membrane of a merocyanine derivative,24 should be caused by the acidic nature of carbonic acid resulting from carbon dioxide in the atmosphere and water adsorbed on the surface. Reflection spectra of the SAM were also measured in dichloromethane with and without addition of triethylamine (0.5 mL of triethylamine in 10 mL of dichloromethane). They were almost identical with those in ethanol before and after addition of aqueous NaOH. The apparent absorption attributed to the anomaly of the gold substrate is observed near the absorbance peaks of both the zwitterionic form at 520 nm and the quinoid form at (24) Daniel, M. F.; Smith, G. W. Mol. Cryst. Liq. Cryst. 1984, 102, 193.
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570 nm. Thus, it is difficult to distinguish the two deprotonated forms in the reflection spectra. HSPC11SS spontaneously formed a densely packed monomolecular film on a gold substrate. The observed reflection spectra of the SAMs were in good agreement with the spectra simulated by use of the Lorentz oscillator model. We found that careful examination of the apparent absorption ascribed to the refractive index dispersion of the substrates is important for considering the opticaldensity profile of the adsorbed organic thin films. The merocyanine moiety showed solvatochromism based on the protonation and the deprotonation in the SAM, which, in turn, depended on the acid-base condition of the environment. The moieties are closely packed and located in a polar local dielectric situation even in the nonpolar solvent. It is likely that the zwitterionic form is more favorable than the quinoid form, though the two forms are hardly distinguished in the reflection spectra due to the apparent absorption. Since the zwitterionic form of merocyanine has the second largest nonlinear polarizability among the three forms, the merocyanine SAM should be an effective example for nonlinear optical studies. Acknowledgment. We thank Professor Hideo Takezoe and Dr. Toshiki Yamada of Tokyo Institute of Technology for kindly allowing us to use the spectrometer. Part of this work was supported by Grant Aid for Scientific Research (Nos. 07CE2004, 09241214 and 09750014) and the Venture Business Laboratory Project “Advanced Nanoprocess Technologies” at Nagoya University. K.F. expresses his sincere thanks to the support from the Special Postdoctoral Researchers Program in Riken. LA9806000
