J. Phys. Chem. C 2007, 111, 18595-18604
18595
Dynamics of Rhodamine 6G Sorption into a Porous Tri-block Copolymer Thin Film Nebojsˇa Pantelic´ and Carl J. Seliskar* Department of Chemistry, 301 Clifton Court, UniVersity of Cincinnati, Cincinnati, Ohio 45221-0172 ReceiVed: August 5, 2007; In Final Form: September 22, 2007
The dynamics of sorption of the laser dye rhodamine 6G (R6G+) into a thin ion-exchange polymer film was measured by spectroscopic ellipsometry. The system under study consisted of a nanometer-sized polymer film in direct contact with a liquid phase. A thin film of a partially sulfonated tri-block copolymer preconcentrated the dye from dilute solutions (10-5 M) driven by ion-exchange to a film concentration of about 0.1 M. Spectroscopic ellipsometry was used to simultaneously measure the dynamics of the film physical (thickness) and optical changes (refractive index n(λ) and extinction coefficient k(λ)). The sorption is ultimately accompanied by dye aggregation in the film, which results in shifts and splitting of features in the monomer absorbance spectrum. The formation of nonluminescent aggregates was also confirmed by emission spectroscopy. Desorption of the dye from the film resulted in a reversal of film physical changes. Spectroscopic results were quantitatively accounted for by constructing an optical model that assumed a dye monomer/ dimer (R6G+/(R6G+)2) equilibrium in the film. The relatively low magnitude of the dimerization constant in the polymer (KD ∼27 M-1) suggests an approach to minimize molecular aggregation. Concurrent quartz crystal microgravimetry of the sorption process showed generally similar results, confirming the quantitative results from ellipsometry measurements.
Introduction The tendency of planar dyes to form dimers or higher order aggregates in solution is a well-documented phenomenon. Examples include rhodamine,1-12 phenosafranine,13 and fluorescein14 dyes. Studies of dye electronic and optical properties have been recently revisited because of their potential uses in solid-state lasers, photonic devices, sensors, and various photoinduced reactions. Rhodamine 6G (R6G+, also rhodamine 590) is one of the most extensively studied and often used laser dyes due to its high lasing efficiency and photostability. The optical properties (absorbance and luminescence) of highly concentrated dye solutions are often different from those of dilute solutions. For example, several types of dye aggregates that form at higher concentrations have low or zero fluorescence efficiency. The aggregation process depends on the dye concentration and the physical properties of the dye itself and the surrounding medium (polarity, charge, etc.). For example, whereas the R6G+ aggregates in water were detected at relatively low concentrations (∼10-5 M), aggregation was not observed in several solid matrices even at concentrations approaching 10-2 M.2,3,6 Similarly, R6G+ aggregates in ethanol were observed only at concentrations greater than 10-2 M, which is much higher than the same critical concentration in water.3 The effect of charge in silicate layers on sorbed R6G+ aggregation has been extensively studied.4,5 As a result, doping of solid materials such as clays (layered silicates)2,3,12,15,16 and polymers11 has been proposed to minimize dye aggregation. Coincidently, incorporation of a solution-phase molecule into a thin porous polymer film on a transducer is an essential process in optical chemical sensing. Such polymer films can preconcentrate the target molecule to nearly 1 M concentration.17 Recently, we have developed sensing methods based on * Corresponding author. Tel.: (513) 556-9213; fax: (513) 556-9239; e-mail:
[email protected].
luminescence measurements, and much lower detection limits were readily achieved with respect to optical absorbance measurements.18 The formation of nonluminescent aggregates at higher concentrations of a target molecule, thus, could have an important influence on sensor response and the associated quantitative analysis. A model system composed of a planar laser dye and a thin polymer film represents a good model system for studying such phenomena as they relate both to chemical sensing via luminescence and to potential dye use in photonic devices. The partially sulfonated poly(styrene)-blockpoly(ethylene-ran-butylene)-block-poly(styrene) (henceforth, SSEBS) material is one for which we have developed19 a detailed physical optics model, and the R6G+ molecule has a high tendency for aggregation. As a result, we have chosen this combination as a model system to study the dynamics of dye sorption, desorption, and aggregation in a thin polymer film exposed to ambient solution. Absorbance and fluorescence spectroscopies have been methods of choice for studies of dye aggregation. Polarized light spectroscopies were utilized to estimate the orientation of the adsorbed molecules on solid supports or intercalated within solid matrices.10,12 Nearly all studies focused on aggregation in inorganic materials. We are aware of only one report that treats aggregation of R6G+ in bulk polymer material11 and one in relatively thin polymer films.12 We have developed an in situ optical method based on spectroscopic ellipsometry for thin film dynamic studies that can also be used for studies of dye aggregation in thin films.20-22 Spectroscopic ellipsometry has important advantages over other optical methods because both film physical changes (thickness) and optical properties of the dye and the film itself are measured in time in a single experiment. The method is sensitive enough to identify aggregation in very thin films and of molecules characterized by weak extinction. To our knowledge, spectroscopic ellipsometry has not been used to date for such studies. This is one more
10.1021/jp076274r CCC: $37.00 © 2007 American Chemical Society Published on Web 11/21/2007
18596 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Pantelic´ and Seliskar
SCHEME 1: Structural Formula of SSEBS Polymer
example of the usefulness of dynamic spectroscopic ellipsometry in studying the physical chemistry of thin porous films at the molecular level. Experimental Procedures Chemicals and Materials. All chemical reagents were used without further purification. SSEBS (average MW 80 000, 5 wt % solution in 1-propanol and dichloroethane, 29 wt % styrene, 55-65% sulfonation) (Scheme 1) and 3-aminopropyltriethoxysilane (APTS) were purchased from Aldrich. o-(6Ethylamino-3-ethylimino-2,7-dimethyl-3H-xanthen-9-yl)benzoic acid ethyl ester (R6G+) was purchased from Exciton. All solutions were prepared using deionized water (D2798 Nanopure water purification system, Barnstead) in a 0.1 M KNO3 electrolyte solution. Fine annealed SF11 bulk glass and a 1 mm thick 1737F glass sheet were obtained from Schott and Corning, respectively. SF11 substrates were prepared by cutting and fine polishing 8 mm thick pieces. Preparation of SSEBS Films on Substrates. Substrates were thoroughly cleaned with soap, rinsed with ethanol and deionized water, and argon plasma cleaned for about 30 min prior to spincoating films. Generally, a 5% stock SSEBS solution was used for film preparation. When making the films with varied thicknesses, the stock solution was further diluted to the desired concentration (2, 1, and 0.5%) using 2-propanol. A 50 µL aliquot of solution was pipetted onto the substrates and spun at different speeds (between 1000 and 6000 rpm) for 30 s. For dynamic in situ studies, the films were prepared by spinning a 5% SSEBS polymer solution at 2500 rpm on gold-coated quartz and SF11 substrates. The variation of film thickness at constant concentration of SSEBS (1% solution) versus spin speed, and variation of air-dried film thickness at constant spin speed (3000 rpm) versus SSEBS concentration, followed exponential relationships. It is to be noted that on exposure to aqueous 0.1 M KNO3, air-dried SSEBS films expand in thickness by ca. 100%. In certain cases, the substrates were functionalized with the bifunctional linker APTS to improve film adhesion.22,23 For this specific procedure, clean glass substrates were soaked overnight in 2 M NaOH to activate the surface. Quartz crystals for QCM measurements were activated for a shorter period of time by dipping to prevent the damage of the crystal. After rinsing with water, the substrates were soaked in a 5% APTS in acetate buffer, pH 5.5, at 90 °C for 5 h. The substrates were then rinsed with deionized water, spun dry for 30 s, and immediately used for film-coating. Film-coated substrates were left overnight to further cure under ambient conditions. Instrumentation. Normal incidence absorbance-transmission measurements were recorded using a Varian Analytical Instruments Cary model 50. Ellipsometry measurements were made using a J.A. Woollam, Inc. variable angle spectroscopic ellipsometer (vertical configuration). This instrument was equipped with an adjustable retarder (Auto Retarder) that enabled measurements of Ψ and ∆ over the full angular range
Figure 1. Normalized absorption spectra at normal incidence of 10-5 M R6G+ in 0.1 M KNO3 solution (solid line) and equilibrated in a SSEBS film cast onto 1737F glass (dashed line). Arrows show directions of changes of the film spectrum relative to that of R6G+ in dilute solution.
(0-90 and 0-360°, respectively). The instrument also permitted depolarization of the light to be measured. Woollam WVASE32 software was used for optical modeling. The details of the construction of the ellipsometer flow-cell for back-side measurements can be found in our previous publication.20 A peristaltic pump (Cole-Parmer) was used to circulate solutions (1.0 L volume, 10 mL/min) through the ellipsometer flow-cell. A plasma cleaner (Harrick Scientific) was used for the final cleaning of substrates. A spin-coater (model 1-PM101DT-R485 Photo-Resist-Spinner, Headway Research, Inc.) was used for the preparation of films of different thicknesses. A quartz crystal microbalance controller (QCM100), crystal oscillator (QCM25), universal time interval counter (model SR620), QCM flow-cell, and gold-coated 1 in. diameter ATcut quartz crystals were purchased from Stanford Research Systems. A peristaltic pump (model Mini-pump, Fisher Scientific) was used to circulate solutions (100 mL volume, 10 mL/ min) through the QCM flow-cell. R6G+ luminescence was excited with a 532 nm laser (model 85-GCB-020, Melles Griot). Laser power was attenuated to a maximum of 0.1 mW at the launching prism face of a multiple internal reflectance (MIR) optics apparatus18 using a variable neutral density filter (Newport) to prevent overload of the detector (R6G+ quantum yield 0.95 in ethanol).24 The intensity of the emitted light was monitored at the wavelength of the maximum emission. Fluorescence spectra presented have not been corrected for instrument response. All experiments were carried out with liquids in thermal equilibrium with the surrounding laboratory temperature (∼25 °C) and under flow conditions (syringe pump, model 341B, Sage Instruments, flow speed 0.1 mL/min). Results and Discussion Exposure of a film of SSEBS to the R6G+ solution results in dye sorption into the porous film driven by ion-exchange. The R6G+ concentration in such a film typically reaches 0.10.5 M at equilibrium when a film is exposed to a dye solution of 10-5 M. Comparison of the normalized absorbance spectra of a 10-5 M R6G+ exchanged thick (approximately 2 µm) SSEBS film and a 0.1 M KNO3 R6G+ solution is shown in Figure 1. The arrows in the figure designate the directions of changes of the film-based spectrum with respect to that of the dye free in solution. The solution maximum at 526 nm is redshifted to 534 nm in SSEBS (∆λ ) 8 nm). Whereas, the solution spectrum is characterized by a shoulder at shorter wavelengths
Rhodamine 6G Sorption into Copolymer Thin Film with respect to the maximum, this shoulder is not obvious in the R6G+ spectrum in SSEBS. Rather, a new absorbance band appears at nearly the same position near 500 nm. The new maximum has here a lower intensity than the one at 534 nm. Changes in the film spectrum are also visible at the extremes of the wavelength region shown. Absorbances in solution have low values just below 425 nm and are near zero above 550 nm. In the film, the absorbances are still significant in these regions and extend above 600 nm. The R6G+ spectrum in solution also has several bands in the near-UV region (not shown). These same bands in the film were also significantly distorted and shifted relative to the solution spectrum. Other authors have observed similar changes in the absorbance spectra of R6G+ in other solid matrices.8-10 The general explanation for such spectral behavior was the aggregation of R6G+ into dimers and higher order aggregates. Deconvolution of Film R6G+ Spectrum. At any point in time during R6G+ sorption into a SSEBS film there exists a distribution among monomeric and aggregated dye structures. These structures are predominately dimers at lower concentrations but also can be higher order aggregates. If one makes the assumption that only dimers are formed, the spectrum of R6G+ in the film can be deconvoluted into the contributions from the monomer and dimer.7-10,12 (From a practical standpoint, the basic problem of deconvolution is that if one uses a fixed monomer spectrum shape, the residual intensity as a function of wavelength could be described by one or more contributions from aggregates. If one makes the simplest assumption, as we and others have, that only one contributor (the dimer) is significant, the residuals can be uniquely deconvoluted. Lacking independently determined spectra for higher order aggregates leaves little choice but to assume a two-state (monomer-dimer) model. Thus, while we suspect that higher order aggregates are present to some probably small extent, we have no meaningful way to account for them. The net result is that when one assumes a two-state model, the deconvoluted dimer spectrum is very consistent with that predicted theoretically, and it is determined within experimental error (vide infra).) When the mole fraction of monomers in equilibrium with dimers is not known, the dimer spectrum obtained from spectral subtraction is not uniquely determined but rather depends on absorbance magnitudes used in calculation. On the other hand, the result of subtraction can be used to estimate the position of the maximum (or maxima) in the absorbance spectrum of the dimer. Because the fractions of monomers and dimers in the film were initially unknown, the following systematic procedure was used to deconvolute the dimer spectrum. As a first step, the difference between the film spectrum and the spectrum measured in solution was taken, and the maxima in the dimer spectrum were estimated. We found for the dimer absorbance spectrum two bands positioned at 502.5 and 546 nm (the film absorption maximum of the monomer was at about 525 nm and slightly shifted from that in solution). Therefore, dimerization resulted in the spectrum splitting into two components. Similar positions for the R6G+ dimer bands in clay materials have been reported.8,9 The positions of maxima in the dimer spectrum also can be estimated by taking the second derivative of the film spectrum, d2k/dλ2. Several authors have used this procedure to estimate the position of the maxima.16,25 Figure 2 shows the results of this calculation. The wavelength positions are coded in pairs such that the first number at each feature represents the position obtained by differentiation and the second by spectral subtraction. The agreement between the pairs of values (which differ
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18597
Figure 2. Extinction spectrum for a SSEBS film equilibrated in 10-5 M R6G+ solution (solid line) and second derivative of the curve (dotted line). Pairs of numbers associated with each spectral feature denote wavelength positions calculated from the derivative curve (first number) and subtraction of the film curve from the solution (monomer) spectrum (second number).
by only a few nanometers) is very good. The central peak at 520 nm is the position of the monomer maximum. The peaks at 546 and 502.5 nm are the maxima in the dimer spectrum. The 479 nm wavelength was used as an important indicator of an additional maximum in the dimer spectrum (vide infra). Having identified the positions of the maxima, we then deconvoluted the film spectrum into individual monomer and dimer spectra. It was assumed that the isolated monomer absorption spectra in the polymer film and in aqueous solution were identical, and this is consistent with both the available literature on this point3,5,7-10 and the very small shifts measured for R6G in water and alcohol.9 The task was accomplished using WVASE32 software and oscillator functions described previously.20,21 The strategy was to minimize the number of partial bands needed to reproduce the observed film spectrum. The monomer extinction profile was independently modeled using six Tauc-Lorentz oscillators.26 The oscillator parameters for the monomer are given in Table 1S of the Supporting Information. Six oscillators appeared to be the minimum number for adequate modeling. Having determined the oscillators for monomer extinction, the R6G+ equilibrated film spectrum was then uploaded into the special optical layer (genosc) in WVASE32 that allowed for modeling with oscillator functions. The portions unaccounted for by the monomer spectrum in the film spectrum were assumed to be uniquely due to the dimer. The dimer spectrum was modeled with four Gaussian oscillators as has been similarly done previously.7-10,15,16,27 Three oscillators were positioned at the maxima determined by peak identification (502.5, 546, and 474 nm). A relatively broad oscillator was positioned at 420 nm to fit the remainder of the spectrum. These four Gaussian oscillators were the minimum number necessary for adequate description of the dimer spectrum. The parameters for the Gaussian oscillators are also given in Table 1S of the Supporting Information. The choice of two different functions for the description of the monomer and dimer spectra was driven by two points. First, Tauc-Lorentz functions were characterized with long tailing toward higher energies and were determined by four parameters. As a result, this choice is good for the description of many optical absorptions.20,21 Second, Gaussian functions have been exclusively used by others in simulating dimer absorptions.7-10,12,15 Most importantly, both types of functions are Kramers-Kroning consistent, which is an essential requirement for a physically meaningful optical model. As a final step in the deconvolution, a total of 10 oscillator functions (six Tauc-Lorentz (monomer) and four Gaussian
18598 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Pantelic´ and Seliskar parameter was allowed to vary during the experimental data fitting procedure. The analytical expression for extinction coefficient in the monomer optical model had the form 6
k(E) ) pole + noffset + kmon
Figure 3. Extinction spectrum of the film (solid line) deconvoluted into oscillator components (dashed lines). R6G+ monomer band (M) and dimer bands (D1, D2, and unlabeled band at 474 nm) are shown.
oscillators (dimer)) was fitted to experimental spectral data. The oscillator functions for the monomer were added and scaled by a single computer-fitting parameter. In this manner, the absolute profile of the monomer absorption spectrum was preserved as required in a physically meaningful optical model. The positions of the Gaussian oscillators were also fixed, but their width and amplitude terms were variables in the fit. The oscillators were fitted to the experimental spectrum acquired for the film equilibrated in R6G+ solution under the conditions of a static spectroscopic ellipsometric scan21,22 to determine the film extinction coefficient, k(λ). It was found that only one combination of the monomer extinction and dimer oscillator functions was able to adequately reproduce the experimental extinction spectrum. The results of modeling are shown in Figure 3. The deconvoluted spectra are shown by dashed lines and the original film spectrum by a solid line in Figure 3. The two dimer bands are designated D1 (546 nm) and D2 (502.5 nm) and the monomer band with symbol M. A broad Gaussian oscillator at shorter wavelengths (about 420 nm, not shown in Figure 3) was used to model the remainder of the dimer spectrum. Both dimer J-bands (identical to D1) and H-bands (identical to D2) are present in the absorption spectrum. In accord with theoretical considerations and the proposed method of classification of such dimers, the observed type is either a twisted sandwich or an oblique head-to-tail dimer.8,9 Band D1 is more intense than band D2. The corresponding areas under the curves are AD1 ) 3.859 and AD2 ) 3.524, and the associated ratio is AD2/AD1 ) 0.910. According to the proposed classification method, the dimer appears to be probably of mixed character. On the basis of the calculated ratio of 0.910, the fraction of head-to-tail dimers is larger in the mixture. According to Martinez et al.,8 the two low intensity Gaussian functions placed at higher energies (475 and 420 nm and 2.62 and 2.96 eV respectively) can be attributed to corresponding vibronic transitions associated with the two main Gaussians. Formulation of an Appropriate Optical Model for Dynamic Data. Having deconvoluted the film spectrum into individual monomer and dimer components, we then modeled the dynamics of R6G+ sorption into a SSEBS film as recorded by in situ spectroscopic ellipsometry scans. Two different optical models were used. The first one (monomer optical model) accounted only for monomer absorbance, and the contribution of dye aggregates to the overall spectrum was ignored. As has been already described, six Tauc-Lorentz oscillators were linked together to describe the R6G+ monomer spectrum (Supporting Information). The oscillators were added together and then multiplied by a single computer-scaling parameter (kmon, eq 1). Only the absolute magnitude of this scaling
TLi ∑ i)1
(1)
A second optical model (monomer/dimer optical model) accounted for an equilibrium between R6G+ monomers and dimers. The model was similar to the first with one important difference. An additional term was added to eq 1 that accounted for the absorbance of R6G+ dimers. The term was composed of four Gaussian oscillators added together in the form of an equation and then multiplied by a single computer-scaling parameter (kdim, eq 2). Only the absolute magnitude of this fitting parameter was allowed to vary during the experimental data fitting procedure in addition to kmon. The analytical equation for the extinction coefficient in the monomer/dimer optical model thus had the form 6
k(E) ) pole + noffset + kmon
∑ i)1
4
TLi + kdim
Gaussi ∑ i)1
(2)
Visual Data Interpretation of Ellipsometric Data. A film was initially equilibrated in 0.1 M KNO3 solution (exposure to potassium cation ion-exchanged film protons giving the potassium form of the polymer). Spectroscopic ellipsometry data were then acquired in time as the film was exposed to 10-5 M R6G+. Dye preconcentration in the film was driven by ion-exchange between R6G+ (solution) and K+ (film). However, given the relative hydrophobic character of R6G+, it is also possible that R6G+ ions diffused into some of the available neutral polymer domains (nonsulfonated styrene or ethylene/butylene hydrophobic domains). We are currently unable to differentiate between these two detailed sorption mechanisms. The experimental data are presented as a contour plot in Figure 4a. Two dimensions in the contour are the wavelength of light and the time. The third dimension is the ellipsometric angle Ψ, whose magnitude is represented by continuous gray scale. As the film changes in time during R6G+ sorption, its thickness, refractive index, extinction coefficient, and ellipsometric angles Ψ and ∆ change. (The analogous contour plot for ∆ gives essentially the same information and is not shown here.) The extremes of angle Ψ are at 24 and 42° (designated by white and black scale extremes). These extremes are at the same time the optical interference maxima and minima. Only the first 850 min of sorption is shown. During that period, the changes were almost complete; in full scale, the sorption spanned 940 min to equilibrium. The largest portion of the contour is white or gray. This is because most of the Ψ angles have small values between 24 and 32°. Two interference minima and three maxima are seen. The minimum at short wavelengths is narrow, whereas the one at long wavelengths is much wider and extends from 650 -to 875 nm (broadening of the minima and maxima toward the red is a general feature of the ellipsometric data). The two maxima centered around 600 and 975 nm are relatively narrow and black. A new incomplete maximum appears near 400 nm. The number of maxima in the contour plot is directly proportional to the film thickness. The shift in their positions, in general, is a direct consequence of film thickness variations. The maxima shift toward longer wavelengths when the film expands and toward shorter wavelengths when the film contracts. The biggest changes in the contour occurred during the first 300 min. The
Rhodamine 6G Sorption into Copolymer Thin Film
Figure 4. Contour plots (Ψ-λ-t) representing sorption of 10-5 M R6G+ into a SSEBS film recorded at 60° incidence angle. (a) Experimental data, (b) monomer optical model-generated data, and (c) monomer/dimer optical model-generated data. SSEBS film was previously equilibrated in 0.1 M KNO3 solution. Ψ values are shown in continuous scale. Contour lines at 29° are represented with dashed lines. Individual contours are designated with numbers (1-4) and discussed in the text.
interference maxima first shifted toward longer wavelengths and then starting from the 120 min mark shifted toward shorter wavelengths. The contours at 29° are designated with a dashed line and marked with numerals (1-4) for easier visual tracking of such changes. Therefore, based on the analysis of experimental data, it follows that the film thickness first increased, then was constant for a short period of time, and then decreased. The film thickness continued to decrease until full equilibration in the R6G+ solution. From the intercepts of contour line 4 with the wavelength axis at times t ) 0 and t ) 850 min, it follows that the film is slightly thinner after reaching equilibrium than the film in the dye-free solution (KNO3 electrolyte). Each interference maximum has its own gray region that surrounds it. The gray regions broaden as the film thickness increases and narrow as the film contracts. This can be easily observed using contour lines 3 and 4. The changes in the gray regions are a consequence of the film thickness change. A new broad gray region appears centered around 530 nm (the wide gray surface touching contour line 2 and marked with an arrow).
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18599 The appearance of the new maximum is not caused by the film thickness change but is due to an increase in the film extinction coefficient during R6G+ sorption. In the beginning, the film has a very low extinction (k(λ) ≈ 0), and this gray region is not visible in the contour. With time, the region becomes uniformly darker, indicating steadily increasing dye sorption. For the last 200 min, small changes of intensity in this region are seen. This is an indication that sorption is almost complete. Interestingly enough, contour lines 1 and 2 blue-shifted (time range of 300-850 min), which was not the case for contours 3 and 4 that are essentially parallel with the time axis. This is due to the broadening of the R6G+ absorbance spectrum in the film caused by the formation of R6G+ aggregates (see also Figure 1). Figure 4b,c shows model-generated data for 10-5 M R6G+ sorption. The results for the monomer optical model (Figure 4b) and the monomer/dimer optical model (Figure 4c) are shown. From inspection, it follows that the general agreement between experimental (Figure 4a) and model-generated data is relatively good. Indeed, the overall profiles among three different contours are very similar. In turn, this means that modelgenerated thicknesses are correctly determined. However, on closer inspection, several important differences between each theoretical model and experimental contour are evident. The biggest differences are in the wavelength region where R6G+ absorbs light. The arrows in Figure 4 designate this region. First, the gray surface was narrower (smaller area) for the monomer optical model than for the monomer/dimer model. Second, the positions of the maxima in the gray regions (represented by vertical dashed lines in Figure 4) are considerably different. For the monomer/dimer model, the maximum is significantly shifted toward longer wavelengths, which is to be attributed to R6G+ dimerization with a corresponding shift in the absorbance maximum (see also Figure 1). The monomer model does not take into account such a possibility, and no shift is observed. Third, the contour line far to the left is blue-shifting in time in the monomer/dimer model. A similar shift is seen in the experimental data. No such shift is observed in the contour for the monomer optical model. The final important difference is in the range of Ψ angles. For experimental and model-generated data based on the monomer/dimer model, the Ψ angles range between 24 and 42°. For the monomer model, the Ψ values extend to 44°. Taken as a whole, the obvious conclusion is that the theoretical model that accounts for dimer formation is to be chosen as the better theoretical model. Dynamics of R6G+ Sorption. The film thickness changes concomitant with R6G+ sorption are shown in Figure 5. The film thickness increased rapidly on initial exposure to R6G+ solution, then leveled off near the 100 min mark for a short period of time and, finally, from there very slowly decreased. A decrease of film thickness continued for a long period, and the film was fully equilibrated only after about 1000 min. The equilibrated film thickness in the R6G+ solution was very slightly (ca. 20 nm) smaller than before exposure to the dye. The initial increase in film thickness was certainly caused by ion-exchange of the larger R6G+ ion (solution) for the K+ ion (film). As the concentration of the dye in the film increased, aggregation began (assumed to be dimerization only), and a turning point was reached in the thickness. At some point in time, the fraction of dimers (overall 2+ charge) became higher than that of the monomers. One might speculate that to screen the divalent charge, the polymer negatively charged sulfonate groups move closer together. This electrostatic cross-linking might then cause the film to contract as would be indicated by
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Figure 5. Spectroscopic ellipsometry determined film thickness vs time for a SSEBS film exposed to 10-5 M R6G+ solution in 0.1 M KNO3 solution (left panel) and then exposed to 0.1 M KNO3 solution (right panel).
SCHEME 2: Schematic Representation of Stages of R6G+ Sorption into SSEBS
measured film thickness reduction. As we have seen previously in ion-exchange films,21 the contraction of the film probably also induced partial dehydration of the film, and the desorption of water would contribute in the same manner to the overall film thickness change. Overall, the detailed experimental data and results of modeling point to film changes in time induced by R6G+ sorption as depicted in Scheme 2. The model-generated extinction scaling factors for the sorption (kmon and kdim, eqs 1 and 2) are shown in Figure 6 in normalized form. It is important to re-emphasize that their absolute values shown here are not extinction coefficients but numbers that scale the oscillator amplitudes to model the extinction coefficient. Additionally, the model-generated data (solid data points in Figure 6) were fitted (lines) with an exponential function (monomer) and a sigmoidal function (dimer). (An exponential function fit was also tried for the dimer, but the fit was worse than with the sigmoidal function.) The time-based changes of these functions reflect the kinetics of parallel monomer sorption and attendant dimer formation within the film. As might be expected, the timing of the two events was very different. The sorption kinetics for the monomer has a steeper initial slope than the curve for dimer formation, indicating fast initial sorption. This curve begins to level off after 300 min, and the process was nearly complete after 600 min. The first several points in the kinetics for the dimer have small but definite values (the reason that this curve was fitted with a sigmoidal function). We speculate that this might be due to a tendency of the monomers to adsorb at the solution/film interface. Because the monomer concentration at the film surface might initially be quite high, it would be likely for dimers to first form at the interface. Alternatively, the small amount of dimer might be attributed to those that might be first formed in the bathing solution; however, this seems unlikely given the low dye solution concentration. The kinetic curve for the dimer continued to increase steadily but slowly. Small changes were still observed after even the 700 min mark where the curve for
the monomer had nearly saturated, indicating that the parallel process of aggregation continued somewhat beyond initial monomer sorption. It appears that R6G+ monomers are relatively mobile inside the film. The curve for dimers leveled off at the 900 min mark, indicating a steady-state in the dye sorption and aggregation processes. Model-generated refractive index and extinction coefficient growth curves for R6G+ sorption are shown in Figure 7. The refractive index for the film equilibrated in 0.1 M KNO3 solution is also shown for comparison. The direction of changes of these functions is clarified using arrows. Sorption and aggregation of R6G+ molecules induced enormous changes in the film refractive index. The anomalous dispersion region was found at and to shorter wavelengths of the R6G+ absorbance maximum; a Cauchy dispersion region was found at longer wavelengths. A red-shift in the maximum of the n(λ) function near 560 nm is indicated by the vertical line in Figure 7. The extinction coefficient spectrum of the film increased uniformly at all wavelengths as a consequence of R6G+ sorption. As the fraction of dimers increased in time, the spectrum became more distorted. The shift in the absorbance maximum to longer wavelengths can also be seen (vertical line indication in Figure 7). The shoulder at lower wavelengths with respect to the maximum in the monomer absorbance spectrum vanished, and a new maximum appeared at nearly the same position. At equilibrium, the measured film extinction coefficients were kmonomer,525 ) 0.048, kdimer,502.5 ) 0.107, and kdimer,546 ) 0.115. From the corresponding literature molar extinction coefficients (monomer,525 ) 1.06 × 105 L mol-1 cm-1 and dimer,502.5 ) 1.92 × 105 L mol-1 cm-1),28 the estimated concentrations22 of the monomer and dimer at equilibrium are Cmonomer ) 0.047 M and Cdimer ) 0.060 M. The calculated association constant, KD ) [D]/[M]2, is ∼27 M-1, and this is much smaller than reported for solutions (KD ) ∼2020 in water and KD ) ∼1010 in glycerol)28 and more like low molecular weight alcohol solutions.11 It thus appears that sorption of R6G+ from an aqueous salt solution into the SSEBS film serves to tip the balance of monomer to dimer far toward the monomer, and this is consistent with results for the sol-gel processed materials.2,3 Desorption of Film R6G+ into KNO3 Solution. R6G+ loaded SSEBS films were studied further. A dye loaded film monitored by dynamic spectroscopic ellipsometry was exposed to a fresh circulating 0.1 M KNO3 solution. Ellipsometric data were analyzed, and the desorption kinetics of film-bound monomers and dimers was determined. The results of this reequilibration are shown in the form of film thickness versus time (Figure 5) and extinction factors versus time (Figure 6) desorption type curves. The desorption process was also slow
Rhodamine 6G Sorption into Copolymer Thin Film
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18601
Figure 6. Monomer/dimer model-generated extinction scaling factors vs time for 10-5 M R6G+ sorption into SSEBS film (top panels, dots) and then after exposure to 0.1 M KNO3 solution (bottom panels, dots). Time-varying extinction scaling factor data points for the sorption were fitted to exponential (monomer) and sigmoidal (dimer) functions (solid lines). Time-varying data points for desorption (lower panels) were fitted to exponential curves.
and lasted for 1000 min, similar to the time needed to completely load the film with dye. The film thickness increased in time with the curve that resembled a sigmoidal function. The shape of the curve is analogous with the second part of the curve when the sorption of R6G+ was measured. The film thickness increased from an initial value of 610 to 680 nm in electrolyte solution. A very similar overall change in film thickness was measured during the R6G+ sorption but in the reverse direction. Film swelling for this re-equilibration is thus in accord with the previous explanation (vide supra). A small majority of the R6G+ molecules in the fully loaded film was in the aggregated (assumed dimer) form. As these aggregates are desorbed from the film, the reverse process of electrostatic cross-linking took place, and the film swelled. The effect of the desorbing monomers on film thickness was screened by the dimer desorption. Concurrent Spectroscopic Ellipsometry and Quartz Crystal Microgravimetry. A method for modeling mass transport in thin polymer films has been previously described.21,22 It is based on the assumption that the proportionality between ellipsometry determined film thickness and film mass has the form
dt - d∞ Mt ≈ M ∞ d∞ - d 0
(3)
In this strategy, film thicknesses measured under dynamic and static experimental conditions were directly input for modeling film mass transport. The measurement of film thickness in ellipsometry is through indirect determination based on changes of the light polarization upon reflection from the surface expressed as angles Ψ and ∆. When these two functions
Figure 7. Refractive index and extinction coefficient vs time for 10-5 M R6G+ sorption into a SSEBS film. Refractive index for the film equilibrated in 0.1 M KNO3 solution is designated by a dotted line for comparison.
18602 J. Phys. Chem. C, Vol. 111, No. 50, 2007 are measured across some spectral region, the film thickness can be determined through modeling followed by determination of optical data. Mass accumulation in polymer films generally involves sorption of molecules into a porous structure and diffusion into all accessible volumes. As a result of sorption, the film swells or contracts depending on the specific conditions. Uptake of mass and response of the polymer to penetrant mass transport are mutually dependent processes but do not necessarily coincide in time. In a hypothetical experiment, film mass and film thickness in a dynamic sorption experiment could be measured to reveal some processes that are not clearly visible from thickness measurements alone. For example, Mukherjee and Singh29 measured film mass and thickness change of high molecular weight poly(acrylamide) in a single experiment. The results suggested that the rate of mass sorption is faster than film swelling. In addition, the poly(acrylamide) film thickness continued to change after the mass change was completed.29 Given that in our experiments the calculated film thickness might be conditioned by the uncertainties intrinsic to optical modeling, and that the polymer response to the penetrant R6G+ and the corresponding mass kinetics might not necessarily be the same in time, we sought out an independent mass-dependent experiment for R6G+ sorption into a thin SSEBS film. Quartz crystal microgravimetry (QCM) is a method capable of measurement of in situ thin film mass. We have directly compared the results of the two complementary methods of spectroscopic ellipsometry and quartz crystal microgravimetry for the sorption of R6G+ into a thin SSEBS film. No attempt was made this time to theoretically model thickness changes on penetrant mass transport because of inadequate diffusion models for this sorption. QCM and spectroscopic ellipsometry were performed equivalently but independently to measure SSEBS film mass, thickness, and refractive index change during exposure to R6G+ solution. A carefully applied stepwise procedure involved separate experiments with the following film exposures: water, 0.1 M KNO3, and R6G+ solution prepared in a 0.1 M KNO3 solution. The SSEBS films for dynamic studies were prepared by spinning 5% polymer solution on substrates at 2500 rpm for 30 s. The resulting air-dried film thicknesses measured with ellipsometry had the following values: at SF11 substrate, 356 nm and at quartz crystal, 325 nm at the geometric center of the spun film mass. The thickness of the air-dried film was also measured with QCM based on frequency changes. Following the resonant frequency measurement of the unaltered crystal in air, the film was spin-coated on the crystal surface and left to dry overnight under ambient conditions. The next day, the decrease in resonant frequency was measured. From the magnitude of this change and based on the known film density (0.966 g/cm3), we calculated the SSEBS film thickness using the Sauerbrey equation30 to be 530 nm. This value was higher than that obtained by spectroscopic ellipsometry. The probable reason for this difference is that the QCM measures the average thickness over the entire crystal surface, whereas ellipsometry measures thickness at the center of the crystal (sampling surface is determined by the diameter of the probe beam), where the films are generally thinner in spin-cast materials. Films equilibrated in 0.1 M KNO3 solution were further exposed to R6G+ solution. The plot of ellipsometry measured film thickness versus time for this equilibration is repeated as an inset in Figure 8 for direct comparison with the mass curve determined by QCM. The initial sorption of R6G+ was followed by film swelling. After 2 h, the curve leveled off and then
Pantelic´ and Seliskar
Figure 8. QCM determined film mass vs time for a SSEBS film exposed to 10-5 M R6G+ solution in 0.1 M KNO3. Inset: concurrently determined film thickness (from ellipsometry) vs time.
continued to change again but in the opposite direction. The corresponding shrinking segment was slower with respect to the swelling segment, and only after about 14 h was the film completely equilibrated. The equilibrated film thickness was lower than the thickness equilibrated in the supporting electrolyte. The corresponding QCM mass changes for the same equilibration are also shown. Remarkably, the curves for thickness and mass changes have the same overall shapes. Film mass initially increased, then leveled off for a short time, and then decreased until the equilibrium state was reached. The timing of the changes between mass and thickness curve was, however, different. While the thickness and the mass reached the maximum values at the same time mark (2 h), different times were required for the films to equilibrate in the solution based on mass (8 h) and thickness (14 h) data. Apparently, the film continued to shrink even after most of the mass was desorbed. The small difference in the air-dried film thicknesses for QCM and ellipsometric experiments is not able to explain this discrepancy. This result was similar to the finding of Mukherjee and Singh29 when measuring sorption into poly(acrylamide). Additional experiments would be necessary to further probe and understand this interesting behavior. Dynamics of R6G+ Sorption Using MIR Optics. The sorption of R6G+ was also studied using multiple internal reflection (MIR) optics as described previously.18 The sorption was monitored in both absorbance and fluorescence modes in separate experiments. The measurements were performed on a very thin, 24 nm (air-dried thickness), SSEBS film. In this case, one expects the thinner film to sorb dye faster than that used in ellipsometry and QCM studies. The results are shown in Figure 9. Approximately 2.5 min after the introduction of the dye solution into the flow-cell, a measurable signal was detected. The results of the absorbance and fluorescence measurements were very different overall. Whereas the absorbance of the film increased and then leveled off at about the 90 min mark, the fluorescence signal initially increased rapidly, peaked at about 30 min, and then decreased. The decrease in signal cannot be due to the dye leaching from the film for three reasons. First, the dye solution was continually replenished in the flow-cell under the constant flow conditions used. As a result, any dye that might desorb from the film would be replaced by a new dye entering from the flow-cell. Second, the sorption of this dye was also measured with spectroscopic ellipsometry (vide supra), and the results confirmed that dye desorption did not occur under the conditions used and within the time frame of the experiment. Finally, absorbance measurements on the film showed a net sorption over time.
Rhodamine 6G Sorption into Copolymer Thin Film
Figure 9. Absorbance and fluorescence of film sorbed R6G+ vs time measured during R6G+ sorption into a 24 nm SSEBS film under MIR optics. Ambient solution concentration was 10-5 M for absorbance and 10-6 M for luminescence measurements.
In accordance with previous literature studies,9,10 this interesting behavior can be explained by the formation of nonfluorescent R6G+ aggregates in the film. The initial increase of fluorescence intensity is due the majority of sorbed R6G+ molecules being monomeric. As the parallel processes of monomer sorption and dimer formation in the film occurred, the fraction of nonfluorescent dimers8 became higher, and the total fluorescence intensity began to decrease. Coincident with this, the measured absorbance is the sum of the absorbances of the R6G+ monomers and dimers. As expected, such dramatic changes were not measured in the absorbance mode. Conclusion We have previously developed a method for dynamic in situ studies of thin polymer films based on spectroscopic ellipsometry. The focus of the studies reported here was on the aggregation (assumed dimerization) of the dye R6G+ in a thin polymer film. We have demonstrated for the first time how the method can be used to monitor both film physical changes and optical changes that together provide new insights into the process of film-based dye sorption and molecular aggregation. The partially sulfonated polymer film sorbs R6G+ from a dilute solution of the dye to a film concentration of about 0.1 M, giving a distribution coefficient of about 10 000. Film absorbance spectra of the R6G+ monomer in equilibrium with the dimer were deconvoluted using Kramers-Kroning consistent oscillator functions.31,32 It was found that the dimer spectrum was split into two bands positioned on either side of the monomer absorbance band. The ratio of intensities of the dimer bands indicated probably a mixture of dimers (twisted sandwich and oblique head-to-tail configuration) in equilibrium with the film bound monomer. Two film optical models were constructed and tested against experiment: one that accounted for monomer only and one that treated monomer and dimer in equilibrium. Our analysis, like many others’, ignored the possibility of the higher order R6G+ aggregates. It was found that within the error of the experimental measurements, the model that accounted directly for dimer formation was superior and reproduced the experimental data with small errors. Film thickness changes during R6G+ sorption were relatively complex. A possible cause for the parabolic thickness kinetics observed was the formation of the doubly charged dimers and resulting electrostatic cross-linking of the polymer chains. The effect of dye aggregation on film luminescence measurements was also observed using MIR optics.
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18603 Concurrent spectroscopic ellipsometry and quartz crystal microgravimetry studies were performed. It was shown that the two methods for dynamic in situ measurements were in good overall agreement. The differences between results of these two methods might be attributed to film viscoeleastic changes and/ or polymer relaxations. Although parallel QCM measurements of resonant frequency and resonant resistance were performed, a detailed analysis of viscoelastic changes in the polymer was not attempted. A major advantage of the spectroscopic method over the gravimetric method is the number of physical parameters obtained. Only the film mass could be obtained by QCM as opposed to film thickness and optical constants by spectroscopic ellipsometry. However, dynamic spectroscopic ellipsometry, as we currently apply it, is slower, and fast film changes that can be monitored by QCM are not able to be measured by ellipsometry. On the other hand, QCM is complicated by, for example, polymer viscoelastic changes that are hard to identify.33 The results presented are in good overall agreement with the large body of work on rhodamine 6G sorption into solid materials published using other techniques. We have demonstrated the dynamics of the sorption process with a detailed account of the aggregation process. It appears that a porous polymer thin film can reversibly exchange this dye from very dilute solution and at the same time achieve a high film concentration. This preconcentrating ability may have uses in making and renewing thin films of photoactive dyes. One might be able to manipulate the film dye concentration in favor of monomeric species, thus maximizing the photoluminescence, and by extension, possible laser action. Our future work will examine this possibility. Acknowledgment. Support from the Office of Environmental Management Sciences Program of the U.S. Department of Energy (Grant DE-FG0799ER62331) is greatly acknowledged. The purchase of the spectroscopic ellipsometer was made possible by a grant from the Hayes Fund of the State of Ohio. We gratefully acknowledge numerous helpful discussions with Dr. William R. Heineman. We also thank Dr. Aigars Piruska for automation of the QCM instrument and Sara E. Andria for her expert help in MIR experiments. Supporting Information Available: Overview of optical modeling with oscillators when refractive index (dielectric function) is a complex value (k(λ) * 0), table containing parameters for Tauc-Lorentz and Gaussian oscillator functions used to mimic monomer and dimer extinction coefficient spectra, and rationale for mass measurement when polymer-coated quartz crystal is immersed in liquid. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gavrilenko, V. I.; Noginov, M. A. J. Chem. Phys. 2006, 124, 44301. (2) Avnir, D.; Kaufman, V. R.; Reisfeld, R. J. Non-Cryst. Solids 1985, 74, 395. (3) Blonski, S. Chem. Phys. Lett. 1991, 184, 229. (4) Bujdak, J.; Iyi, N. J. Phys. Chem. B 2006, 110, 2180. (5) Bujdak, J.; Iyi, N.; Kaneko, Y.; Czimerova, A.; Sasai, R. Phys. Chem. Chem. Phys. 2003, 5, 4680. (6) Chaudhuri, R.; Arbeloa, F. L.; Arbeloa, I. L. Langmuir 2000, 16, 1285. (7) Arbeloa, F. L.; Martinez, V. M.; Prieto, J. B.; Arbeloa, I. L. Langmuir 2002, 18, 2658. (8) Martinez, V. M.; Arbeloa, F. L.; Prieto, J. B.; Lopez, T. A.; Arbeloa, I. L. J. Phys. Chem. B 2004, 108, 20030. (9) Martinez, V. M.; Arbeloa, F. L.; Prieto, J. B.; Arbeloa, I. L. J. Phys. Chem. B 2005, 109, 7443. (10) Arbeloa, F. L.; Martinez, V. M. Chem. Mater. 2006, 18, 1407.
18604 J. Phys. Chem. C, Vol. 111, No. 50, 2007 (11) Vogel, R.; Harvey, M.; Edwards, G.; Meredith, P.; Heckenberg, N.; Trau, M.; Rubinsztein-Dunlop, H. Macromolecules 2002, 35, 2063. (12) Sasai, R.; Fujita, T.; Iyi, N.; Itoh, H.; Takagi, K. Langmuir 2002, 18, 6578. (13) Arbeloa, I. L.; Rohatgi-Mukherjee, K. K. Spectrochim. Acta, Part A 1988, 44, 423. (14) Arbeloa, I. L. J. Chem. Soc., Faraday Trans. 2 1981, 1725. (15) Sasai, R.; Shichi, T.; Gekko, K.; Takagi, K. Bull. Chem. Soc. Jpn. 2000, 73, 1925. (16) Sasai, R.; Shin’ya, N.; Shichi, T.; Takagi, K.; Gekko, K. Langmuir 1999, 15, 413. (17) Shi, Y.; Slaterbeck, A. F.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 3679. (18) Kaval, N.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 2003, 75, 6334. (19) Pantelic´, N.; Andria, S. E.; Heineman, W. R.; Seliskar, C. J., unpublished work. (20) Zudans, I.; Heineman, W. R.; Seliskar, C. J. J. Phys. Chem. B 2004, 108, 11521.
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