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J. Phys. Chem. B 2002, 106, 92-100
Adsorption of Sulforhodamine Dyes in Cationic Langmuir-Blodgett Films: Spectroscopic and Structural Studies Krishanu Ray† and Hiroo Nakahara*,† Department of Chemistry, Faculty of Science, Saitama UniVersity, Saitama 338-8570, Japan ReceiVed: May 21, 2001; In Final Form: September 25, 2001
This paper reports the incorporation of sulforhodamine B (SRB) in the octadecylamine Langmuir-Blodgett (LB) films and the adsorbability of the cationic LB films have been investigated. It was observed that the adsorption process mainly occurred due to the ionic interaction between the cationic amino group of the long-chain amine and the sulfonic group of the SRB dye. Surface pressure-area isotherm studies confirmed the strong interaction between the long-chain amine and the SRB dye and the change of orientation of the molecules were observed with variation of pH of the subphase. Absorption, steady-state and ultrafast timeresolved fluorescence spectroscopic studies suggested the presence of the monomer and dimer of the SRB molecule in the restricted geometry of LB films. Furthermore, the contents of the monomer and dimer depend on the concentration of the dye and the adsorption process. The orientation of the SRB dye molecule in the LB films determined by polarized absorption spectra indicated a lying-flat orientation of the SRB molecule. Structural characteristics of the LB films such as orientation, layer-layer distance, and topography of the film surface have been investigated by FTIR spectroscopy, low-angle X-ray diffraction, and atomic force microscopy (AFM). Our preliminary results imply that the functional molecules in LB films are incorporated with a desired orientation by means of designing the molecular structure of the adsorbates.
Introduction The potential applications of the ultrathin films by the Langmuir-Blodgett (LB) and self-assembled methods in fundamental sciences as well as in technologies have stimulated research with a wide variety of materials, especially the immobilization with the functional molecules in LB films are suitable for use in sensors and molecular electronics.1 The adsorption LB method for fabrication of films is one of the unique techniques toward realization of molecular electronic devices, as the molecular arrangement and orientation of molecules can be tailored by choosing proper parameters, namely molecular structure, deposition surface pressure, pH, and temperature of the aqueous subphase. Functional molecules without long-chain substitution can be incorporated as LB films by adsorption of a floating monolayer from the subphase or immersing the functionalized LB films in solution.2 Water soluble molecules were incorporated in LB films with phospholipid dimyristol phosphatidic acid as the latter one was negatively charged in neutral pH and interacted with the positively charged molecule in the aqueous subphase due to electrostatic interaction and facilitated the retention of watersoluble molecule in the monolayer.3 Ionic interaction4 is a strong attractive force between adsorbent and adsorbate and in the process has high adsorbability for oppositely charged ionic substances. The adsorption of electrolytes at the solid/liquid interface is an important step in many technological processes where selective adsorption can be made between a functionalized surface and molecules.5 Cationic film materials interact strongly with a variety of materials such as surfactants, polymers, dispersed materials, proteins, biological colloids, and lipids at the interface, and their † Fax: + 81-48-858-3700. E-mail:
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mechanisms are very important in practical applications.6 The LB films fabricated with cationic film materials are expected to have specific adsorbabilities, which are different from those of the LB films formed with anionic materials such as fatty acids. Octadecylamine is analogous to stearic acid but the deposition of cationic material to solid substrate is somewhat difficult7 and may occur due to the ionization of the amino group to the ammonium ion. By choosing suitable parameters such as pH and deposition conditions, a highly ordered LB film can be obtained by retaining their cationic properties.8 Site-selective binding of the anionic molecules can be achieved by ionic adsorption process on these cationic films. However, little information is available about the specific adsorbability of the cationic film materials. An important parameter affecting the fluorescent behavior of organic dyes is the relationship that molecules can establish with neighboring molecules to prevent forming dimers or aggregates. When fluorescent dyes are incorporated in the restricted geometries, they usually show deviations from the characteristic fluorescent behavior observed in solution, which is typically considered as optimum.9 Xanthene dyes are one of the most important classes of pigments used in dye lasers and in various photosensitized reactions. These dyes tend to form aggregates and dimers in solution and different matrixes that result in spectral shift, hypsochromism, and changes in vibronic structure.10 Both molecular structure and solvent play considerable roles in nonradiative pathways of deactivation of the excited state of these dyes.11 It is known that the formation at the adsorbed state of nonfluorescent and fluorescent dimers depends on the geometry adopted by the monomer constituents on the surface.12 The photophysical behaviors of xanthene dyes adsorbed on the silica surface revealed that the size of the pore plays a predominant role on the formation of nonplanar conformers, deeply affecting the luminescent properties of the
10.1021/jp011946d CCC: $22.00 © 2002 American Chemical Society Published on Web 12/06/2001
Adsorption of Sulforhodamine Dyes in LB Films CHART 1
dyes.13 Recent studies by two-photon photoemission spectroscopy showed that the electronic properties of dye-adsorbed semiconductor surfaces varied significantly due to the strong interaction of the anionic sulfo group with the silicon oxide substrate.14 Incorporation of amphiphilic rhodamine dyes in mixed LB films by time-resolved spectroscopy revealed formation of different dimers upon the matrix component, lateral arrangement of dyes, energy migration among different sites with a fractal-like distribution due to an energetic or spatial disorder leading finally to energy trapping by higher aggregates.15 However, to our knowledge water-soluble sulforhodamine molecules without long chains have not been incorporated in LB films. In this paper, we investigate the interaction of the sulforhodamine (SRB) dye adsorbed on the cationic film. This study has been undertaken in order to reveal the relationship between the molecular structure and its adsorption behavior and the changes of the electronic properties of the adsorbate-cationic molecular organized film systems. The isotherms of octadecylamine monolayers on the aqueous SRB subphase were found to be markedly expanded relative to those without SRB at acidic and neutral pH’s. Spectral and dynamic fluorescence properties of SRB-adsorbed octadecylamine films have been studied. Structural characterization of the LB films was carried out by small-angle X-ray diffraction. Morphology of the dye-adsorbed LB film surface has been characterized by using atomic force microscopy (AFM). Experimental Section Octadecylamine and sulforhodamine B (SRB) were purchased from Tokyo Chemical Industry ltd. The molecular structures are given in Chart 1. The monolayers were spread from the chloroform solution on the distilled water and on the aqueous SRB solution (1 × 10-4 M). After allowing 15 min for the chloroform to evaporate, the monolayer at the air-water interface was compressed slowly. Surface pressure-area isotherms were measured by a Langmuir-type film balance (Lauda) at 20 °C with different pH’s. The monolayers formed at the air-water interface were compressed to a surface pressure of 45 mN/m at a pH of 10.3, and subsequently the monolayer was transferred by the LB technique onto the solid substrate. The dipping speed was 7 mm/min for both the up and down stroke. Sufficient time for drying was given in transferring the monolayers onto the solid substrate. A quartz plate was used for UVvisible absorption and emission spectroscopy. Calcium fluoride plates, coated by five layers of cadmium arachidate to make the surface hydrophobic were used for transmission FTIR measurements. Glass plates coated with ferric stearate were used for X-ray diffraction. The absorption spectra of the LB films were recorded on a Hitachi U-3210 spectrophotometer. Fluorescence emission spectra of the LB films were recorded on a
J. Phys. Chem. B, Vol. 106, No. 1, 2002 93 Hitachi MPF-3 fluorescence spectrophotometer. A streak camera was used to measure the fluorescence lifetime of the LB films. The fundamental output (wavelength 775 nm, pulse width ≈130 fs, repetition rate 1 kHz) from a femtosecond Ti:Sapphire regenerative amplifier seeded by the second harmonic of a mode-locked Er-doped fiber laser (Clark-MXR, CPA-2000) was used to excite an optical parametric amplifier (Clark-MXR, IROPA). The second harmonic (wavelength 550 nm) of the signal wave from the optical parametric amplifier was used to excite the sample. The laser beam was focused on the sample in a quartz cell under a nitrogen atmosphere. Scattered light at 90° to the laser beam was collected and focused onto the entrance slit of a single spectrograph (Acton Research, SpectraPro 150). Filters were placed between the collimate lens and the entrance slit. A streak camera (Hamamatsu, C4334-01) was attached on the output port of the spectrograph. The streak camera was operated with a synchronous delay generator (Hamamatsu, C4792-01) to compensate for the time delay in the electronic circuit of the streak camera. The two-dimensional data from the streak camera were accumulated and analyzed with a personal computer. All measurements were performed with the photon counting mode. The FWHM of the instrument response function was about 30 ps. To examine the excited-state dynamics of LB films, the fluorescence decay curves were measured at 620 nm. A fluorescence decay curve was fitted to the multiexponential function deconvoluated with the instrumental response function. The weighted mean fluorescence lifetime was calculated from the fitted parameters. All the measurements were performed at room temperature and putting the LB films in nitrogen. FTIR spectra were measured with a Perkin-Elmer FTIR spectrophotometer with a TGS detector. Values of the long spacing for the layered structures of the built-up films on glass plates were measured by an X-ray diffractometer (Rigaku, RadB, Cu KR radiation, 40 kV, 30 mA) equipped with a graphite monochromator. AFM images were recorded on a Seiko SPA300 in air and under ambient conditions. We used microfabricated rectangular Si3N4 cantilevers with integrated pyramidal tips with a given force constant of 0.09N/m. Results and Discussion Monolayer Characteristics at the Air-Water Interface. Figure 1A presents the surface pressure versus area per molecule (π-A) isotherms of the octadecylamine monolayer on the aqueous subphase at various pH’s. When the long-chain amine was spread on the just distilled water (pH ) 5.8) without any ions, the condensed monolayer with a relatively small molecular area was formed, which can be considered to be due to a little hydrophilicity of the amino group, being hydorated only in part. It is evident that protonation of octadecylamine occurred in the acidic region, leading to formation of an expanded monolayer with larger compressibility at pH 3 with HClaq, whereas a typical condensed phase behavior was observed at pH 10.3 with NaOHaq. These isotherms are consistent with those in the early works.8,16,17 The value of pK ) 9.9 for a long-chain amine was estimated by Betts and Pethica.18 At basic pH the protonation of the amino group in the octadecylamine molecule is practically forbidden. These results were confirmed by IR spectra of the deposited films obtained at different pH’s of the aqueous subphase. Figure 1B presents the π-A isotherms of the octadecylamine molecule on the aqueous subphase of SRB molecule (1 × 10-4 M). As shown in Figure 1A,B, the area occupied by the octadecylamine molecule increased markedly on the SRB aqueous subphase at the lower pressure region. These results suggest that the octadecylamine molecule is
94 J. Phys. Chem. B, Vol. 106, No. 1, 2002
Ray and Nakahara
Figure 2. Adsorption isotherm of SRB in nine-layer octadecylamine amine LB film: absorbance at 579 nm versus immersion time. Inset presents the UV-vis absorption spectra of the dye-adsorbed octadecylamine LB film for various immersion times: (a) 5, (b) 12, (c) 20, (d) 30, (e) 60, and (f) 120 min.
Figure 1. (A) Surface pressure-area isotherms (20 °C) of octadecylamine monolayers on distilled water subphase at pH 3.1 (with HClaq), 5.8, and 10.3 (with NaOHaq). (B) Surface pressure-area isotherms of octadecylamine monolayer on SRB aqueous subphase (1 × 10-4 M) at different pH: 3.0, 6.2, 9.2, 10.3, and 11.5. Insets show the molecular models of octadecylamine and SRB in panels A and B, respectively.
interactive with the SRB molecule. The expanded monolayer obtained at the pH below 10.3 is due to the bulky SRB chromophore. It is worth mentioning that the molecular area of SRB is very large, about 218 Å2 (calculated with a ChemDraw 3D software, as shown in the inset of Figure 1B), where the molecular lengths along the short and long axis of SRB molecule are 11.5 and 19 Å, respectively. This molecular area of SRB roughly coincides with the obtained area by extrapolating the isotherm of octadecylamine on the SRB aqueous subphase. The π-A isotherms of the octadecylamine monolayer on the SRB aqueous subphase at different pH’s show the flat plateaulike regions. Very likely, this plateau indicates a phase transition corresponding to the reorientation of the long-chain amine molecules from lying flat at the air-water interface at low surface pressures to an almost upright vertical alignment of the amine molecules closely packed with its long chain oriented nearly perpendicular to the air-water interface at higher surface pressures. Thus, the orientation and packing of the molecules are changed at different pH’s and surface pressures. The octadecylamine monolayer on the aqueous SRB subphase reveals a considerable decrease in the molecular area above pH 10.3 while resulting in the similar condensed phase at higher surface pressures as that obtained on the subphase without SRB at a pH of 10.3. The fact that the isotherms of octadecylamine
at lower pH (
