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J. Phys. Chem. B 2006, 110, 3910-3916
Local Environments of Coumarin Dyes within Mesostructured Silica-Surfactant Nanocomposites Akira Yamaguchi,†,‡ Yosuke Amino,† Kentaro Shima,† Shintaro Suzuki,† Tomohisa Yamashita,† and Norio Teramae*,† Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Aoba-ku, Sendai 980-8578, Japan, and PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan ReceiVed: NoVember 6, 2005; In Final Form: January 18, 2006
The local environments surrounding dye molecules were studied with use of coumarin dyes in a mesostructured silica-surfactant nanocomposite, which was formed in a porous alumina membrane by a surfactant-templated method and has an average pore diameter of 3.4 nm. Coumarin dyes, such as coumarin 480 (C480), coumarin 343 (C343), and propylamide coumarin 343 (PAC343), were extracted into the silica-surfactant nanocomposite and time-resolved fluorescence spectra of these dyes were examined. C480 and C343 show slow dynamic Stokes shifts and the decay curve can be fitted by a biexponential function. The decay-time constants obtained from the fitting are almost identical for C480 and C343: 0.87 and 7.5 ns for C480, and 0.86 and 7.6 ns for C343. In contrast to these two coumarin dyes, short decay-time constants (0.50 and 4.8 ns) were obtained for PAC343 in the silica-surfactant nanocomposite. These results indicate that the local environments of C480 and C343 are almost identical but different from that of PAC343. By considering the origin of the dynamic Stokes shift and the mesostructure of the silica-surfactant nanocomposite, the location and microenvironment of coumarin dyes within the silica-surfactant nanocomposite are discussed.
1. Introduction Mesostructured silica thin films and membranes synthesized by a surfactant-templated method are potentially useful in many fields because of their inherent properties, such as uniform pore diameter of molecular dimensions, high surface area, and high adsorption capacity.1-7 These mesoporous materials are generally synthesized via spontaneous self-organization of silicasurfactant nanocomposites at solid/liquid interfaces. Selforganization is often caused by an evaporation-induced selfassembly process in which solvent evaporation induces formation of surfactant micelles and further organization of the silicasurfactant nanocomposite.4,8 The possibility of controlled placement of functional molecules within the silica-surfactant nanocomposite has attracted much interest from the point of view of developing functional mesostructured silica thin films and of separating molecules. Zink and co-workers9,10 demonstrated the placement of luminescent molecules in three distinct regions of the silica-surfactant nanocomposite: the silica framework, the hydrophobic interior of the surfactant micelles, and the ionic interface containing the charged headgroups of the ionic surfactant and their counterions. By using a one-step synthesis method, the desired molecule may be placed in a specific region of the mesostructure. In this case, a metal complex with condensable trialkoxysilane groups was incorporated into the silica framework, and luminescent organic dye molecules with no trialkoxysilane groups were placed into the hydrophobic interior of the micelles and/or the ionic interface. It has been reported that the hydrophobic interior of the micelles and the ionic interfacial region play different roles in the extraction of molecules into the silica-surfactant nanocom* To whom corresponding should be addressed. Fax: +81-22-795-6552. E-mail:
[email protected]. † Tohoku University. ‡ Japan Science and Technology Agency.
posite.11-15 When a mesostructured silica membrane was prepared by using an acidic precursor solution containing cetyltrimethylammonium bromide (CTAB) as a surfactant, and the membrane was used as a solid extraction material, cationic organic dyes were extracted into the micellar phase via formation of an ion pair, whereas extraction of anionic organic dyes was predominantly based on an anion-exchange process at the ionic interface between the anionic dye and the bromide that is a counteranion of the cationic cetyltrimethylammonium (CTA) headgroup.11 For extraction of neutral organic molecules, it has been reported that the hydrophobic interior of the micellar phase plays an important role by solubilizing the molecules.12-15 The properties of mesostructured silica-surfactant nanocomposites have been often studied with use of fluorescent probe molecules. Hydrophobic 2-p-toluidinyl naphthalene-6-sulfonate was used to monitor the formation of mesostructured silicasurfactant nanocomposites at a solid/liquid interface.3 A rigidochromic Re complex was used to study molecular motion and environmental rigidity within a silica-surfactant nanocomposite formed by a dip-coating procedure. From the results of steady-state fluorescence and fluorescence depolarization measurements, it was proposed that the environment of a Re complex, located at the ionic interface, acted like a fluid, allowing the complex to rotate freely.16 Another investigation with a similar rigidochromic Re complex showed that the surfactant molecules within the silica framework retained some fluidity up to a pressure of about 10 GPa.17 The purpose of the present work is to study the local environment of fluorescent probe dyes extracted into the silicasurfactant nanocomposite by measuring the time-dependent fluorescence spectral shift (dynamic Stokes shift) of probe dyes. The dynamic Stokes shift is caused by the solvent relaxation process, which is related to an instantaneous change in the charge distribution of a probe dye upon excitation. As the solvent
10.1021/jp0564086 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/11/2006
Local Environments Surrounding Dye Molecules CHART 1: Chemical Structures of Coumarin Dyes
relaxation process is related to the composition and motion of solvent molecules surrounding the probe dye,18-30 the measurement of dynamic Stokes shift provides valuable information on the local environments of probe dye molecules within the silicasurfactant nanocomposite. In addition, it is possible to find out the location of a probe dye, because the ionic interface and the hydrophobic micelle interiors are expected to have different environments. In this study, three coumarin dyes were used as fluorescent probes. Coumarin dyes are well-known to exhibit dynamic Stokes shift in time-resolved fluorescence spectra depending on the local environment.18-26 The chemical structures of the dyes used in this study are shown in Chart 1: coumarin 480 (C480) and propylamide coumarin 343 (PAC343) are neutral dyes and coumarin 343 (C343) is an anionic dye. The mesostructured silica membrane was prepared by formation of silicasurfactant nanocomposites inside the columnar pores of an anodic alumina membrane.7 The silica-surfactant nanocomposite inside the alumina pore is an assembly of surfactanttemplated silica-nanochannels with a channel diameter of 3.4 nm and the channel direction is predominantly oriented along the wall of the columnar alumina pore, as illustrated in Figure 1. Since the nanocomposite is formed by using an acidic precursor solution containing CTAB and tetraethoxysilane (TEOS), there are bromide ions inside the nanocomposite as counteranions of the cationic headgroup of CTA.11 Hereafter, we refer to the mesostructured silica membrane as a nanochannel-incorporated alumina membrane (NAM). Coumarin dyes were extracted into the silica-surfactant nanocomposite by immersing the NAM in an aqueous solution containing a coumarin dye, and time-resolved fluorescence spectra of the dye within the silica-surfactant nanocomposite were measured. The local environment and the location of coumarin dyes with different functional groups are discussed on the basis of the analysis of the dynamic Stokes shifts. 2. Experimental Section C480 and C343 were purchased from Sigma-Aldrich Japan (Tokyo). PAC343 was synthesized as described in the Supporting Information, using C343, N-hydroxysuccinimide, and 3-(3dimethylaminopropyl)-1-ethylcarbodiimide hydrochloride. Dioxane and other chemicals were purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan), and were used as received. Milli-Q water was used for all experiments. The details of the preparation procedures of NAM are described in a previous report.7 An anodic alumina membrane with an average pore diameter of 200 nm (Whatman International Ltd., England) was placed in an ordinary membrane filtration apparatus. A precursor solution, composed of CTAB, TEOS, HCl, ethanol, and water, was dropped onto the anodic alumina membrane while moderate aspiration was applied, so that the precursor solution penetrated into the columnar alumina pores. The alumina membrane incorporating the precursor solution was then dried in air at room temperature, allowing
J. Phys. Chem. B, Vol. 110, No. 9, 2006 3911 formation of an assembly of silica-surfactant nanocomposites inside the columnar alumina pore. The average diameter of the mesopores of the silica framework was estimated by nitrogen adsorption/desorption isotherms and transmittance electron microscopy to be 3.4 ( 0.2 nm. As-synthesized NAM was rinsed carefully with water and used for further experiments. To estimate the number of CTA molecules within the NAM, measurements of thermal gravimetry and differential thermal analysis (TG-DTA; Rigaku, TG-8120) were performed. The assynthesized NAM was ground in an agate mortar prior to the TG-DTA measurement. The ground sample was placed in a platinum pan and heated from room temperature to 1000 K at a heating rate of 10 deg/min in air. To examine the extraction behavior of coumarin dyes into the silica-surfactant nanocomposite, the rinsed NAM was immersed in a water-dioxane mixture (99.8/0.2 mol/mol, 10 mL) that contained 1 µM coumarin dye (C-480, C-343, or PAC343). After immersing the NAM in a cell containing a coumarin solution, a small fraction of the coumarin solution was taken from the cell and an absorption spectrum obtained. After the measurement, the sample was put back into the cell. The absorption spectrum of the dye solution was recorded on a UVVis spectrophotometer (JASCO, V-570), and the amount of dye extracted into the NAM was estimated by using the decrease in absorbance of the coumarin dye in a solution phase. The coumarin-extracted NAM (C-NAM) was rinsed twice with a water/dioxane mixture and with water, and was then immersed in water for 30 min to remove coumarin dye adsorbed at the surface of the alumina membrane. The rinsed C-NAM was set in a quartz cuvette filled with pure water and time-resolved fluorescence measurements were performed. In the following discussion, NAM containing a coumarin dye is referred to as C-NAM, and NAMs containing C480, C343, and PAC343 as C480-NAM, C343-NAM, and PAC343-NAM, respectively. The extraction and fluorescence experiments were performed at 25 °C. The experimental setup for time-resolved fluorescence spectroscopy was described in detail in a previous paper.27 The measurement system was composed of a mode-locked Ti: Sapphire laser (Spectra-Physics, Tsunami 3960; ca. 100 fs at 780 nm, 82 Hz) pumped by a CW visible laser (Spectra-Physics, Millennia-Pro), a polychromator (Hamamatsu, C5094), and a streak scope (Hamamatsu, C4334). The repetition rate of the Ti:Sapphire laser was reduced to 4 MHz with an electrooptic modulator (Spectra-Physics, Model 3980), and the second harmonic of the Ti:Sapphire laser beam generated by an LBO crystal (Spectra-Physics, GWU) was used as an excitation source. The excitation laser beam irradiated the C-NAM in a quartz cuvette filled with water, and fluorescence from the C-NAM was focused on the slit of a polychromator. The maximum time-resolution of the present system was approximately 60 ps at 5 ns full scale and 200 ps at 20 ns full scale. The positions of the maxima of the time-resolved fluorescence spectra are estimated by fitting to a log-normal function.18,32 3. Results 3.1. Extraction of Coumarin Dyes into the Silica-Surfactant Nanocomposite. Figure 2 shows the time dependence of the absorbance ratio of coumarin dye solutions after immersion of the NAM. In Figure 2, At/A0 is plotted against the immersion time t, where A0 is the initial absorbance before immersion and At is the absorbance at time t after immersion. For every dye system, a decrease of more than 50% in peak
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Yamaguchi et al.
Figure 1. Schematic illustration of (a) the assemblies of silica-surfactant nanocomposites (channel diameter ) 3.4 nm) formed inside the columnar alumina pore (pore diameter ) 200 nm) and (b) the mesostructure of the silica-surfactant nanocomposite.
Figure 2. Time dependence of the relative peak absorbance (At/A0) of solutions containing coumarin dyes after immersion of NAM: C480 (O), C343 (b), and PAC343 (0).
absorbance was observed within 6 h after immersing the NAM. When an alumina membrane without the silica-surfactant nanocomposite was immersed into the coumarin dye solutions, decreases in the peak absorbance were below 1% at 6 h after immersion. These results indicate that at least 49% of the dye is extracted into the silica-surfactant nanocomposite. Since the decrease in the peak absorbance became slower 6 h after immersion of the NAM, the C-NAMs were prepared with an immersion period of 6 h and used for time-resolved fluorescence measurements. The C-NAM was immersed in water during the measurement of time-resolved fluorescence, but little elution of coumarin dye from the C-NAM was observed. By measuring absorption spectra of 1.0 µM C343 in aqueous solutions at various pH values, adjusted using 0.1 M HCl and 0.1 M NaOH, the pKa value of C343 in a bulk aqueous solution was estimated as 4.5. In the extraction experiment, the pH value of the C343 solution was 5.5, under which condition 90% of the molecules exist in a dissociated form, based on the estimated pKa value. In our previous study, it was confirmed that the predominant method of extraction of an anionic dye molecule into the silica-surfactant nanocomposite was via replacement of a bromide ion inside the nanocomposite.11 Thus, it is probable that most of the C343 molecules within the silica-surfactant nanocomposite are in the dissociated anionic form. In the TG-DTA measurements of the NAM, exothermic weight loss of ca. 3% was observed in a temperature range between 150 and 430 °C. Since this weight loss corresponds to the amount of combustion and decomposition of organic surfactant molecules (CTA), the amount of the surfactant within the silica framework was estimated as ca. 30 mg (1 × 10-4 mol) per 1 g of NAM. It was reported that a silica-surfactant nanocomposite contained an equal amount of chloride against CTA when the nanocomposite was formed by using an acidic precursor solution containing cetyltrimethylammonium chloride
and TEOS.33 In the NAM, bromide is the counteranion of CTA.11 Hence, it is considered that the NAM contains approximately equal amounts (1 × 10-4 mol) of CTA molecules and bromide ions within the silica framework. However, the amount of coumarin dye extracted by the NAM is ca. 5 × 10-9 mol after 6 h of immersion in the coumarin dye solution (50% extraction), and this value is 20 000 times smaller than the amount of CTA molecules and bromide ions. Accordingly, we suggest that the extracted coumarin dye scarcely rearranges the structure of the ionic interface and the micellar phase inside the silica framework. 3.2. Fluorescence Properties of Coumarin Dyes in Various Media. As the maximum wavelength of the fluorescence spectrum of a coumarin dye reflects the polarity of the surrounding medium, spectra of coumarin dyes within the silica-surfactant nanocomposite were compared with those in bulk solution (Figure 3). In bulk solutions such as a waterdioxane mixture (99.8 mol % of water), methanol, ethanol, and butanol, the maximum wavelength shifts to the shorter wavelength region as the polarity of the solution decreases (Figure 3a,b). The maximum wavelengths observed for C480-NAM and PAC343-NAM are much shorter than those observed in a bulk water-dioxane mixture and close to those observed in a bulk methanol solution. For C343-NAM, the maximum wavelength of the fluorescence spectrum is also shorter than that found in a bulk water-dioxane mixture (Figure 3c). Thus, we conclude that the local environment inside the silicasurfactant nanocomposite is less polar than a bulk aqueous phase and its polarity resembles that of a bulk methanol solution. 3.3. Dynamic Stokes Shift of Coumarin Dyes within the Silica-Surfactant Nanocomposite. In the C-NAMs, all coumarin dyes exhibit wavelength-dependent fluorescence decay profiles. Figure 4 shows a typical example of the fluorescence decay curves of C480-NAM measured in the three different wavelength regions. The time response of the fluorescence shows a fast decay in the short-wavelength region (435 to 440 nm, Figure 4a), which becomes slower in the middle-wavelength region (470 to 475 nm, Figure 4b). Both rise and decay components appear in the long-wavelength region (550 to 565 nm, Figure 4c). Accordingly, it can be concluded that C480 within the silica-surfactant nanocomposite exhibits wavelengthdependent fluorescence decay profiles which show rise and decay in the long-wavelength region and decay in the shortwavelength region. Similar wavelength-dependent fluorescence decay profiles were observed for C343-NAM and PAC343NAM. Figure 5 (upper row of figures) shows time-resolved fluorescence spectra of coumarin dyes in the silica-surfactant
Local Environments Surrounding Dye Molecules
J. Phys. Chem. B, Vol. 110, No. 9, 2006 3913
Figure 3. Fluorescence spectra of (a) C480, (b) PAC343, and (c) C343 in various bulk solutions and in NAM: (i) water-dioxane mixture (O), (ii) NAM (b), (iii) methanol (0), (iv) ethanol (4), and (v) butanol (]).
Figure 4. Fluorescence decay profiles of C480 in the silica-surfactant nanocomposite at different wavelength regions: (a) 435-440, (b) 470475, and (c) 550-565 nm. All decay profiles were measured by a streak scope at 5 ns full scale.
nanocomposite. The time-dependent Stokes shift can be easily recognized, although no shifts in fluorescence maximum could be recognized in bulk solutions containing coumarin dyes. The time-dependent fluorescence spectra were quantitatively analyzed by using a time-correlation function, C(t), which is defined as follows:
C(t) )
ν(t) - ν(∞) ν(0) - ν(∞)
(1)
where ν(0), ν(t), and ν(∞) are the maximum fluorescence frequencies at times 0, t, and ∞, respectively.18 The decay of C(t) is shown in Figure 5 (bottom row of figures). The observed decay profiles, ν(t), can be fitted by a biexponential function as follows:
C(t) ) a1 exp(-t/τ1) + a2 exp(-t/τ2)
(2)
where τ1 and τ2 are solvent relaxation times, and a1 and a2 are their amplitudes. The average relaxation time 〈τs〉 is given by the following equation:
〈τs〉 ) a1τ1 + a2τ2
(3)
The analyzed decay parameters are summarized in Table 1. The biexponential fitting of C(t) for C480-NAM gives decaytime constants of 0.87 (a1 ) 0.54) and 7.5 ns (a2 ) 0.46) and an average decay-time constant of 〈τs〉 ) 4.0 ns. The decaytime constants obtained for C343-NAM are consistent with those obtained for C480-NAM within the limits of fitting error. The ratios of the fast and slow components (a1/a2) are 0.85 and 1.2 for C480-NAM and C343-NAM, respectively. The ratios are somewhat different, but the net decay-time constants are consistent with each other. In contrast, the decay of C(t) for PAC343-NAM is different from those observed for C480NAM and C343-NAM. For PAC343-NAM, the decay-time constants are described by two components of 0.5 and 4.8 ns
(Table 1). These decay-time constants are much larger than those observed for C480 and C343 in a bulk aqueous phase (
