Layer-by-Layer Assemblies Composed of Polycationic Electrolyte

Article pubs.acs.org/JPCC

Layer-by-Layer Assemblies Composed of Polycationic Electrolyte, Organic Dyes, and Layered Silicates Juraj Bujdák* Department of Physical and Theoretical Chemistry, Faculty of Natural Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovakia Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 845 36 Bratislava, Slovakia S Supporting Information *

ABSTRACT: The formation of layer-by-layer (LBL) assemblies is a simple procedure, suitable for the construction of thin films with well-defined structures and compositions. LBL assemblies based on layered silicates, cationic organic dyes, and polycationic electrolyte were investigated by absorption and fluorescence spectroscopies. The formations of two simple films were investigated: one film bearing a laser dye, oxazine 1, and the second based on J-aggregates of pseudoisocyanine. A detailed study based on chemometric methods (principal component analysis, multivariate curve resolution-alternating least squares) confirmed the very complex nature of the LBL assembly formation. During the assembly deposition, the outer surface with the adsorbed dye often exhibited significant changes upon the formation of a new layer. These changes mainly included a partial desorption of the dye and structural rearrangement of the adsorbed dye species, including molecular aggregation. A complex film composed of alternating layers of the two dyes was made mainly for the purpose of investigating photophysical phenomena, such as fluorescence quenching and intermolecular resonance energy transfer. The results obtained can be useful for further studies leading to the development of functional materials based on photoactive dyes, inorganic layered carriers, and polyelectrolytes. used as sensors,19,20 systems for light harvesting and excitation energy transfer,21−23 or models for drug delivery systems.24 One strategy for building dye-embedded LBL assemblies is based on the incorporation of photoactive moieties covalently attached to polyelectrolyte macromolecules.25 Another approach is based on the intercalation of small dye molecules bound to polyelectrolyte layers through electrostatic attraction forces. LBL assemblies with uncharged hydrophobic molecules are rather rare and require the use of surfactant micelles as carriers.26 The deposition process of ionic dyes in LBL films is regular and highly reproducible, as confirmed by spectroscopy methods.24,27 Dye loading and release from LBL films is significantly influenced by pH and ionic strength. 28,29 Futhermore, pH of the polyeletrolyte solutions can influence the surface morphology and roughness of the layers.30 Some films exhibit highly reversible dye loading. Under specific conditions, it is possible to design LBL films to prevent the diffusion of embedded dye ions through the polyelectrolyte layers.21 Spectroscopy methods have confirmed the formation of dye molecular aggregates in LBL films.31 The formed aggregates can be significantly different from those commonly occurring in aqueous solutions.32

1. INTRODUCTION Nanomaterials and hybrid materials represent a new era of science and technology. One of the simplest methods used for the formation of hybrid materials with layered structures is layer-by-layer (LBL) deposition. It is a simple procedure, suitable for the construction of thin hybrid films at the molecular level with well-defined structures and compositions.1 LBL assemblies are prepared by step-by-step molecular deposition, often using a strategy of alternating the charge of single components.2 A typical example of LBL assemblies is alternating layers of polyelectrolytes of opposite charge. However, other types of chemical bonds aside from ionic forces (e.g., hydrogen bonds, van der Waals forces) can also be applied.3,4 A broad spectrum of the components potentially used in LBL assemblies include micelle-like molecular assemblies,5 simple or more complex molecules, polymer chains, nanoparticles of any shape,6 and layered particles.7,8 The novel properties and functionalities of these materials depend on the precise nanoscale organization of the components. Already proven or potential applications include materials for special or multifunctional coatings;9 biomedical applications and drug delivery systems;5,10−13 magnetic assemblies;14 and photonic, photocatalytic,15 and electronic16−18 materials. LBL assemblies containing organic dyes exhibit interesting optical and photophysical properties. Such materials can be © 2014 American Chemical Society

Received: November 13, 2013 Revised: March 6, 2014 Published: March 7, 2014 7152

dx.doi.org/10.1021/jp411155x | J. Phys. Chem. C 2014, 118, 7152−7162

The Journal of Physical Chemistry C

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cascade RET, and fluorescence quenching. Some phenomena occurred in dependence on the properties of the components or were specific to only certain deposition steps. Understanding all of these phenomena can be useful for future strategies to develop functional hybrid materials based on LBL assemblies. Such materials might include light-harvesting and energytransfer systems, luminescent materials, photosensors, photocatalysts, and photosensitization materials, among others.

One specific group of LBL assemblies consists of films with alternating layers of charged particles of inorganic compounds and polyelectrolyte chains bearing an opposite electric charge. Studies of such materials have focused on the spectral properties of LBL assemblies composed of the cations of polyelectrolytes and negatively charged layered silicate particles with preadsorbed cationic dyes. The adsorption of cationic dyes onto the particles of layered silicates is very strong.33 This is important for the successful formation of assemblies bearing dye-modified silicate particles. The effect of the surface charge of the silicates on dye molecular aggregation is well-known34,35 and can be used for the fabrication of films with required properties.36,37 There have been only a few reports dealing with LBL assemblies based on cationic polymer and clay minerals or related synthetic layered silicates.7,38−41 The successful preparation of such LBL assemblies has been repeatedly demonstrated. However, not much is known about the changes in photoactivity and spectral properties of dye components incorporated into this type of LBL assembly. Some photophysical phenomena occurring in LBL assemblies have already been observed in other systems. Resonance energy transfer (RET) between dye molecules has been reported in a few cases.8,21,42 Little is known about the phenomena of dye molecular aggregation occurring in LBL assemblies based on layered silicates and organic dyes. Only in a few cases have detailed analyses of the changes in optical data occurring during the deposition of LBL films been performed.43,44 No study has applied chemometric methods to detect slight, barely observable changes of dye spectral properties during the LBL assembly formation process. The objective of this work was to perform a detailed investigation of the spectral changes taking place during the formation of LBL assemblies. For this purpose, we have fabricated LBL films by alternating layers of polyelectrolyte (polydiallyldimethylammonium, Pdda) and two specimens of layered silicates (LSs) with the preadsorbed dye cations oxazine 1 (Ox) or pseudoisocyanine (Pic). The structural formulas of the dye cations are shown in Figure 1. Three films were

2. EXPERIMENTAL SECTION Two smectites (expandable LSs) were used for the construction of the films based on LBL assemblies: the montmorillonite (Mmt) Kunipia F and the synthetic saponite (Sap) Sumecton. Both of these silicates were commercially available pure standards obtained from Kunimine Industries, Tokyo, Japan, and were used as received without any further purification. According to product characterization data from the producer, the average particle diameter of Sap was about 20 nm, whereas the diameter of the Mmt particles was larger and fell in the range of 100−500 nm. Salts of Ox and Pic (Figure 1) were purchased from Exciton (Dayton, OH) and Nippon Kanko Shikiso Co. (Okayama, Japan), respectively. In previous studies, Mmt Kunipia F exhibited an extraordinary tendency to induce the formation of J-aggregates of Pic cations.45,46 Therefore, a Pic/Mmt colloid was used in this work to create layers containing J-type assemblies. Sap is a typical smectite with a low-density negative layer charge. This property is important to suppress the extensive formation of nonluminescent Haggregates.34 Therefore, Ox was combined with Sap to create hybrid particles with photoactive dye monomers. The Ox concentration in the Ox/Sap colloid was 10−5 mol L−1, and the ratio of the components was 0.05 mmol g−1. The Pic concentration and its ratio to silicate were 4 times larger (i.e., 4 × 10−5 mol L−1, 0.20 mmol g−1) to promote the formation of J-aggregates. Pdda was purchased from Sigma-Aldrich. The approximate average molar mass of the polymer used was 106 g mol−1. An aqueous solution of Pdda (0.01 mol L−1, calculated for a monomeric unit) was used for the formation of cationic polyelectrolyte layers. Dye/LS aqueous colloids and Pdda solution were used as the precursors for the construction of the films. Both colloids were based on dispersed LS particles with preadsorbed dye cations. The ionic bond between the dye cations and the negatively charged surface of the LSs guaranteed a strong association between the components.33 In both cases, the LS surface was undersaturated with dye cations to maintain an overall negative charge on the particles. LBL films were built by alternating the negatively charged silicate layers and cationic Pdda molecules. The formation of the films started on the surface of UV−vistransparent quartz slides. After treatment with a basic ammonia solution (pH 10) and washing with deionized water, the surface of the slides had a slightly negative charge. Such a surface exhibited the irreversible adsorption of a monomolecular layer of Pdda from solution. An extensive washing of the slides with deionized water led to the release of excess polymer, leaving a single layer of adsorbed Pdda chains on the surface. The Pdda layer made the surface charge of the slide positive. After the slide had been dried in air, it was measured as a blank sample using both absorption and fluorescence spectroscopies. At this point, the sample is designated as a film with zero layers, and the layer of Pdda is designated as the “zeroth” layer. The positively charged film bearing a Pdda monolayer alone was placed into a silicate colloid containing preadsorbed dye

Figure 1. Structural formulas of oxazine 1 and pseudoisocyanine.

prepared: Two were built from a single dye component, and one complex film was a combination of the two dye components. Each deposition step was monitored by absorption and fluorescence spectroscopies to record the changes in spectral properties. Chemometric methods were used to obtain the most significant information from the spectral data recorded after each deposition step. We observed various phenomena that have never before been described for this type of LBL assembly: dye desorption from an outer surface upon Pdda layer formation, J-aggregate formation, 7153



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cations for 30 s. The excess colloid was removed by washing with deionized water. The step that deposited the first layer, which followed the zeroth layer of Pdda, is formally designated as the first deposition step. The first step could start with either the Pic/Mmt or Ox/Sap colloid. The dried slide formed after the first step contained dye cations that were already detectable by both spectroscopy methods. Both the absorption and fluorescence spectra were measured after each preparation step. The layers based on Pic/Mmt and Ox/Sap are referred to below as Pic and Ox layers. After the first step, the surface was negatively charged due to the presence of LS particles. The second step was based on the adsorption of a second Pdda layer, reverting the surface charge back to positive. We observed significant changes in the spectra during the Pdda layer formation steps that have never been reported before. The third step was in principle identical to the first and continued with dye/LS adsorption from the colloid. In the end, films were formed with a series of multilayers with alternating charges. They contained two types of layers (Pdda and dye/LS). The layers of each type were almost identical in terms of their structure and properties. Three types of slides were prepared. The first two types contained a single type of dye, either Ox or Pic. These films were built from either Pic/ Mmt or Ox/Sap colloids. Each film was built by the deposition of the “zeroth” layer followed by eight steps for the alternating deposition of four layers of Pdda and four layers of LS with preadsorbed dye cations. These films are designated as either an Ox or Pic films. One complex film was prepared that contained the two dyes in alternating order, namely, a Pic/Pdda/Ox/Pdda sequence of layers in the film, containing a total of 16 layers, four of each Pic and Ox and eight of Pdda. The final compositions of the films are described in the Supporting Information (page S1). A scheme illustrating the composition of the complex film is shown in Figure 2. Absorption spectra were recorded using a V-550 UV−vis absorption spectrophotometer (Jasco Co., Ltd.). Fluorescence spectra were measured with a Shimadzu spectrophotometer (RF5300PC) at 90° with the film orientation at 35° with respect to the excitation beam. Both measurements were done at room temperature. Blank substrates based on a quartz slide with a Pdda layer were also measured and represented spectral baselines. Chemometric methods including principal component analysis (PCA), multivariate curve resolution-alternating least squares (MCR), and partial least-squares analysis (PLS) were performed using Unscrambler (CAMO Software AS, Oslo, Norway) and the R Project Software Environment for Statistical Computing. The same fitting conditions were used as in a previous study.47 PCA was calculated using a singular value decomposition (SVD) algorithm. The baseline was first subtracted from the spectral data. The wavelength range was minimized, individually for each MCR fitting procedure, to reduce the influence of the low-signal region.

Figure 2. Composition and arrangement of the layers in the complex film and changes taking place at the outer surface during the deposition of the eighth (Pdda) layer.

Figure 3. Absorption spectra of a Pic film recorded during LBL assembly deposition: odd deposition steps of Pic layers (solid lines) and even deposition steps of Pdda layer (dashed lines).

3. RESULTS AND DISCUSSION 3.1. Absorption Spectroscopy of Films Containing One Dye Component. Spectra were measured after each step of LBL assembly deposition (Figures 3 and 4). The numbers in the figures represent the step numbers of the LBL assembly deposition process. Absorption spectra measured during Pic film formation are shown in Figure 3. Solid lines show the absorption spectra obtained after a layer of Pic/Mmt was formed (deposition steps 1, 3, 5, and 7). Dashed lines represent the spectra obtained after a Pdda layer was formed (deposition

steps 2, 4, 6, and 8). A dotted line shows the baseline recorded after the deposition of the zeroth layer. The spectra exhibit qualitatively similar properties. They are dominated by a band at 563 nm, which represents the J-aggregates of the dye. The steps during which Pdda layer was added (steps 2, 4, 6, and 8) did not contribute to any very significant spectral changes. Increasing the number of Pic layers contributed to an increase in the absorbance values. 7154



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Figure 4. Absorption spectra of an Ox film built by LBL assembly deposition. The numbering of the deposition steps is similar to that in Figure 3. The positions of the peaks are shown by vertical lines.

Very different trends were observed for the absorption spectra recorded for the Ox film (Figure 4). The spectra, recorded after odd deposition steps (1, 3, 5, and 7; solid lines), exhibited maximal absorbance at 643 nm (Figure 4, solid vertical line). The addition of each Pdda layer (deposition steps 2, 4, 6, and 8; dashed lines) significantly contributed to the reduction in overall Ox absorbance, and the absorption peak shifted strongly to longer wavelengths (660 nm, dashed vertical line). The changes induced by Pdda layer formation can most likely be explained by the phenomenon of competitive adsorption of Pdda and Ox at the surface of Sap. A portion of the Ox cations preadsorbed on Sap particles might have been replaced by cationic groups of Pdda chains and released from the film. Ox remaining in the film after Pdda addition represents J-type aggregates of this dye (660 nm). The absorption band near 643 nm, assigned to Ox monomers, almost disappeared when the Pdda layer was deposited. This phenomenon was repeatedly observed after each odd step of the deposition. J-aggregates probably exhibited a stronger resistance against replacement by Pdda cationic chains from the film. Another interpretation would be based on the transformation of the Ox monomers upon the adsorption of Pdda. One can conclude that the behaviors of Ox and Pic dyes are completely different when building films based on LBL assemblies. The highly complex spectral properties of solid hybrid materials, such as LBL films, should be considered. The limited number of spectral species that contribute most significantly to spectral variation are called the spectral components. Chemometry provides excellent tools for deconstructing the spectra of complex systems and simplifying their analysis. Therefore, chemometric methods were applied to identify the main features of the spectral properties of these films during their formation, to help reveal processes related to dye cations during LBL assembly formation (desorption, aggregation), and to characterize some photophysical phenomena. PCA was calculated from the spectral data series obtained for the Pic film and shown in Figure 3. Figure 5a shows the loadings of the first two principal components (PCs) carrying the most spectral information. The third component was highly influenced by spectral noise (not shown). The loading values of each component represent the relationships between variables. These relationships are specific and unique for each of the components. PC1 represented the largest variation from the mean representation of the spectra. It was characterized by a sharp negative band at 567 nm with a shoulder at about 500

Figure 5. Chemometry results for a series of absorption spectra recorded during the formation of a Pic film: (a) loadings obtained by the PCA method, (b) spectra obtained by the MCR method.

nm. The negative band at longer wavelengths represented Pic Jaggregates. PC1 represented the majority of the variances, reaching a value of 99.6%. PC2, although contributing to a much lower extent, provided a spectral structure still significantly above the spectral noise. Its inclusion in the model shifted the explained variances to 99.9%. Therefore, at least a small amount of another form, in addition to the Jaggregates, could have been present in Pic films. In the profile of the PC2 loadings, the peak representing the J-aggregates was negatively related to light absorption at about 494 and 534 nm. Dye monomers, which are common in dilute solutions of Pic, were identified by their absorption at 535 nm. The band at 494 nm can be assigned to H-dimers, coexisting as the third species at very low concentrations, or to vibronic transitions of the monomers. Score values are another important parameter arising from PCA calculations. They can characterize sample composition, in terms of how much each component contributes to the whole spectral pattern. The score values and their relationships to particular film formation steps are shown in Supporting Information (page S2). The values change in the same way for the equivalent film formation steps. PCA is thus able to sensitively detect the effect of Pdda addition, which was not seen as clearly directly from the spectra (Figure 3). The score values of PC2 were much lower, and their changes were irregular. The PC2 loading dropped slightly for the formation of the sixth layer. This change might be related to an error occurring during the layers’ depositions. The scores for PC3 were very low and did not exhibit any interpretable structure (Supporting Information, page S2). Evaluating the model of two independent components, residual variances were very low ( 0.994) (Supporting Information, page S6). The functions related to the deposition steps of Pdda layer addition (even steps) crossed the y axis near the origin. Those related to the odd steps of the film deposition gave a positive intercept. This difference can be interpreted in the following way: The concentrations calculated for odd steps include weakly bound Ox cations adsorbed in excess on the outer surface of the films. After the addition of a Pdda layer, the films contained only intercalated Ox inside the film, strongly bound and unaffected by the addition of further layers. Weakly bound dye molecules were desorbed from the outer surface upon the addition of Pdda polycations. 3.2. Fluorescence Spectroscopy of LBL Films Containing One Type of Dye. Fluorescence is much more complex than light absorption. It depends on light absorption itself as well as the ability of the fluorophore to emit light and is influenced by various phenomena, such as fluorescence quenching and energy transfer. The emission spectra recorded during the first four steps of Pic film formation are shown in Figure 7. They represent the emission from J-aggregates, as

Figure 7. Emission spectra recorded during the deposition of Pic and Ox films. The excitation wavelength was 550 nm. Dashed lines show the spectra after the depositions of Pdda layers. The steps of LBL assembly are indicated by numbers and arranged in order of changing emission intensity. Vertical lines (Ox film spectra) help to identify the positions of the bands.

monomers and H-type assemblies of this dye are nonluminescent. PCA calculations also confirmed the presence of a single luminescent species (not shown). The emission spectra were composed of a single band with a maximum near 574 nm. The very low Stokes shift, which is typical for J-type molecular assemblies, confirms the assignment of this band. There was a 7157



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presenting the results of MCR should, therefore, be considered with caution. To minimize the baseline effect on MCR calculations, the iteration was run with a focused range of data (620−720 nm). The resulting spectra of the components (Figure 8b) were assigned to dye monomers (660 nm) and to both types of assemblies, J- and H-aggregates. The aggregates emitted at both the higher and lower wavelengths. All of the component spectra overlapped significantly. Therefore, the MCR results should be evaluated at only a qualitative level. A small Stokes shift was found for the J-aggregates, estimated from the positions of the absorption and emission bands (Figures 6c and 8b). The concentration profiles of monomers and J-aggregates exhibited a zigzag structure, resulting from the release of the dye upon Pdda adsorption (Figure 8c). The emission from monomers decreased significantly to almost zero after the layer of Pdda had been added (second step). This indicates an extensive desorption of the monomers originally present. The fluorescence from the dye monomers decreased further with the number of layers (Figure 8c), although the amount of dye increased after the odd steps, as indicated by the absorption spectra (Figure 4). This contradiction can be explained in terms of resonance energy transfer from excited monomers to coexistent J-aggregates. Indeed, the fluorescence from J-aggregates increased with the number of equivalent layers. The ratio of the emission from monomers to that from Jaggregates decreased significantly. More efficient three-dimensional resonance energy transfer becomes more probable with increasing number of deposited layers. The emission at low wavelengths, assigned to H-aggregates, exhibited a relatively constant value that did not change with the formation of further layers and remained uniform for all deposition steps (Figure 8c). The H-aggregates were formed after the deposition of the first layer representing the interface (quartz)−Pdda−Sap. This interface might have been favorable for H-aggregate formation. The specific formation of H-aggregates in the first layer can be explained in terms of the effect of the properties of the Pdda zeroth layer. The zeroth deposition step proceeded as the adsorption of Pdda chains on the surface of the quartz slide. For further deposition steps, Pdda chains were adsorbed on a Sap monolayer. The nonequivalency between the zeroth layer and the rest of the Pdda layers could be the cause for the specific formation of Ox H-aggregates after the deposition of the first layer. 3.3. Film Composed of Layers Bearing Both Pseudoisocyanine and Oxazine 1. The complex film composed of alternating layers of the two types of LSs and dyes was prepared in 16 steps (Supporting Information, page S1). A scheme presenting the order of the layers (up to the eighth layer) in the complex film is shown in Figure 2. Selected absorption spectra are shown in Figure 9. Dyes in the complex film exhibited properties similar to those observed for single-dye component films. The appearance of Ox J-aggregates and desorption of monomers upon the adsorption of the Pdda layer were also observed for the complex film. Because of the complexity of the film, it was quite difficult to analyze the absorption spectra in detail. MCR was found to be very useful for extracting valuable information on the spectral and concentration profiles of the dye species (Figure 10). The results are considered to be only qualitative estimations. They are compared with the trends that were observed for simpler Pic and Ox films. The spectra of the components were similar to those identified by MCR analysis for the single-dye films. Pic formed two species: J-aggregates (red dotted line) were predominant, exhibiting a sharp J-band

Figure 8. Chemometry results obtained for fluorescence spectra recorded during the deposition of an Ox film: (a) loadings calculated by PCA and (b) spectra and (c) concentrations of components determined by MCR.

analysis of absorption spectra (Figure 6a). Fluorescence exhibits a higher sensitivity and also better selectivity by exciting the species preferentially at their lowest wavelengths. The residuals of the PCA calculations are slightly improved when the third component is included. The presence of the species emitting at the lowest wavelengths seemed to have been dominant after the second step of LBL assembly formation (see Figure 7). The score values calculated by PCA of the first three components are shown in Supporting Information (page S9), but their analysis was too complicated for useful information to be obtained. MCR was applied to the same data to estimate the real structure of the spectra (Figure 8b,c). An SVD algorithm and non-negativity constraints for both the spectra and concentrations were applied. The three principal components indicated by the results of PCA were also confirmed by MCR. The concentration values do not reflect the amounts of the specific species. They are rather the fractions of the species emitting light of a specific energy. Part of the species present might be inactive, exhibit poor luminescent properties, or transfer their excitation energy through a resonance-energytransfer mechanism. The term “concentration” used for 7158



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aggregates would confirm a partial penetration of dye molecules through the Pdda layers, which might have taken place during the release of weakly bound Ox molecules. The concentrations of Ox species apparently increased when an Ox layer was added (Figure 10). After additions of Pic layers (steps 1, 5, 9, and 13), no significant changes occurred in the concentrations of Ox species. Only small changes in the concentrations of Pic species were observed with the deposition of Ox layers. A small fraction of Pic monomers converted to J-aggregates upon the adsorption of Ox layers. For example, small increases in the J-aggregates of Pic were observed in deposition steps 3, 7, 11, and 15, when the Ox/Sap layers were added. This change could be interpreted as the influence of the Ox layer on the charge distribution in an upper Pdda layer. When an additional Pdda layer was deposited onto an Ox layer, the concentrations of Pic species returned to roughly their previous values. These changes would indicate a longer range of the electrostatic effect on the formation of dye species. Mixed-type aggregates might also play a role in these trends. A conclusive interpretation would require further studies. The MCR method was also used for the analysis of fluorescence spectra (Figure 11). The Pic J-aggregates exhibited

Figure 9. Absorption spectra of a film built by LBL assembly and containing both dyes. The spectra are identified according to the particular step of LBL assembly by their numbers. The maxima of the peaks are shown by the vertical solid and dashed lines (644 and 661 nm, respectively). Solid lines represent the spectra obtained after an Ox/Sap layer was formed (steps 3, 7, 11, and 15), whereas dashed lines represent the spectra obtained after a Pdda layer was formed (steps 4, 8, 12, and 16).

Figure 10. Results of MCR analysis applied to absorption spectra obtained during complex film formation. Top: Spectra assigned to Pic monomers, Pic J-aggregates, Ox monomers, and Ox J-aggregates. Bottom: Concentrations of species.

Figure 11. Results of MCR analysis applied to fluorescence spectra obtained during complex film formation. Top: Spectra assigned to Pic J-aggregates and Ox monomers and J-aggregates. Bottom: Concentrations of species.

(568 nm). A lower fraction of the dye exhibited properties similar to Pic monomers, possibly including some H-aggregates. The spectrum of this form was relatively noisy, with barely distinguishable broad bands near 495 and 530 nm. Ox monomers had a main absorption band at 640 nm and a vibronic component near 590 nm. A more complex spectrum was found for the Ox aggregated form (brown line with cross symbols). A sharp band near 670 nm exhibited the typical features of J-type assemblies. It was probably in equilibrium with other species represented by the broad bands at 495, 545, and 605 nm. These bands can be assigned to mixed-type assemblies composed of both dyes, Ox and Pic. Interestingly, the amounts of Ox J-aggregates and mixed-type assemblies seemed to be linearly correlated. The existence of mixed-type

a very small Stokes shift. Therefore, there was a problem in resolving the scattered light originating from the excitation source and the fluorescence from Pic species. Only a very small impurity in the signal is thought to have a large influence on the MCR calculations. Therefore, the range of wavelengths had to be reduced to consider only a fraction of the Pic emission band. MCR identified three uncorrelated components in the spectra: PIC J-aggregates, with a band near 550 nm, and two species of Ox, monomers and J-aggregates, absorbing near 660 and 675 nm, respectively. The spectra were similar to those identified for the simpler films (Figures 7 and 8b). On the other hand, the concentration profiles from MCR analysis (Figure 11) were 7159



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probably penetrate through a less efficient barrier of a polyelectrolyte layer. Further investigation is needed to optimize the assembly parameters. The questions of how both the structure and properties of a polyelectrolyte and inorganic layered template affect the parameters of embedded chromophores and the whole process of layer-by-layer deposition still remain to be answered. Chemometric methods, especially multivariate curve resolution, are very useful for the identification of even small changes in spectral properties taking place during deposition. Only a detailed study of these properties will potentially help to find hints toward modifying and optimizing the parameters of prepared multicomponent films of this type. Nevertheless, layer-by-layer assemblies including layered silicate particles with preadsorbed dyes are highly promising hybrid materials with controllable structures and properties. Assemblies based on photoactive components can be used as prototype materials for optical and optoelectronic devices or sensors.

significantly different from those obtained from the analysis of absorption spectra (Figure 10). Both spectroscopy methods were able to identify the strong desorption of Ox upon the addition of Pdda layers. However, some unexpected trends were observed in the fluorescence spectra that can be assigned to resonance energy transfer from Pic J-aggregates to Ox species: (1) The addition of an Ox layer led to significant quenching of the fluorescence from Pic J-aggregates (steps 3, 7, 11, and 15). (2) Deposition of Pdda on an Ox layer led to a decrease in Ox fluorescence in favor of emission from Pic Jaggregates (steps 4, 8, 12, and 16). A lower amount of Ox species playing the role of energy acceptor led to a lower fluorescence quenching of Pic J-aggregates. (3) The addition of a Pic layer, when the first Ox layer was already present in the film (steps 5, 9, and 13), led to an increase in the fluorescence from Ox species. The distance being in the range of a few nanometers was likely favorable for resonance energy transfer. The thickness of LS particles is 0.96 nm. Similar values are expected for Pdda layers. There was small but sufficient overlap between the absorption spectrum of Ox monomers (Figure 6c) and the emission spectrum of Pic J-aggregates (Figure 7). Resonance energy transfer was probably not limited to merely neighboring layers. It proceeded to some extent to more distant layers. This is in agreement with the fluorescence from Ox monomers being the highest after the third step of the LBL deposition. With further deposition steps it continually decreased in favor of the emission of Ox J-aggregates.

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ASSOCIATED CONTENT

S Supporting Information *

Composition of films prepared by LBL deposition (page S1), score values calculated by PCA for the spectra recorded during Pic film deposition (page S2), concentration profiles calculated by MCR for the spectra recorded during Pic film deposition (page S3), spectral profiles calculated by MCR for Ox LBL film considering three components (page S4), concentration profiles calculated by MCR for the spectra recorded during Ox film deposition (page S5), linear fits of the concentrations of Ox forms as a function of the number of layers in the film (page S6), PLS for the correlation of fluorescence and absorption spectra for Pic film (page S7), relationship of Pic emission at 573 nm to component concentration obtained by multivariate curve resolution from absorption spectra (page S8), score values calculated by PCA of fluorescence spectra recorded during the formation of Ox film (page S9), and variable residuals of MCR fits (pages S10−12). This information is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSIONS The method of the deposition of layer-by-layer assemblies was found to be effective for preparing films based on cationic polyelectrolyte and layered negatively charged particles of silicates with preadsorbed cationic dyes. Several phenomena were identified with the help of chemometric analysis of the spectra recorded during the layer deposition process. The dye amount depended on the type of upper layer in the film. Significant desorption of organic dye cations took place from the outer surface during deposition of the polyelectrolyte layer. Oxazine 1 monomers were desorbed in larger amounts. Pseudoisocyanine forming J-aggregates was adsorbed more strongly and was not desorbed much with the polyelectrolyte molecules. The formation of dye aggregates induced significant changes in the photophysical properties of the dyes, affecting their absorption and fluorescence spectra. These changes affected photophysical phenomena taking place in the films such as energy transfer and fluorescence quenching. The prepared multidye films exhibited the phenomenon of energy transfer from pseudoisocyanine J-aggregates to monomers and J-aggregates of oxazine 1. The efficiency of energy transfer increased with the number of deposited layers. The effect of longer-range electrostatic forces on dye aggregation was observed for the complex film: The deposition of the oxazine 1/saponite layer could slightly affect pseudoisocyanine aggregation through the polyelectrolyte layer between them. Using layered silicate particles as sorbents for cationic dyes was advantageous for efficiently preventing diffusion of the dye molecules into adjacent or remote layers in the film. Only a very small fraction of mixed aggregates was detected by chemometric methods. A small fraction of dye molecules could

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +241 2 59410 459. Fax: +241 2 59410 444. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Slovak Research and Development Agency under Contract APVV-0291-11. Support from Grant Agency VEGA (2/0107/13, 1/0943/13) is also acknowledged.

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ABBREVIATIONS AND EXPLANATIONS OF TERMS zeroth layer = first layer deposited on the quartz slide substrate, a Pdda layer first through Nth layer = either Pdda or dye/silicate monolayer formed in the Nth deposition step first through Nth (deposition) step = deposition step for building the Nth layer, of either Pdda or dye/layered silicate equivalent layer or deposition step = term can reflect the equivalency of the layers in their composition, arrangement, dx.doi.org/10.1021/jp411155x | J. Phys. Chem. C 2014, 118, 7152−7162


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or order in a multilayer film. There could be two types of layers based on layered silicates with preadsorbed dyes or Pdda in simple films. In a complex film based on the two dyes and in some cases in simple films, equivalent layers are those that have the same position relative to their neighboring layers. Equivalent layers are formed by equivalent deposition steps. For example, with simple films, layered silicates with preadsorbed dyes are equivalent and represent the odd layer formed by equivalent (odd) deposition steps. For the complex film, Pdda layers with the same neighbors are equivalent. The same or at least similar structure and properties are expected for equivalent layers LBL = layer-by-layer LS = layered silicate MCR = multivariate curve resolution-alternating leastsquares Mmt = montmorillonite, used as a carrier of pseudoisocyanine Ox = oxazine 1 PC = principal component, principal factor PCA = principal component analysis Pdda = polydiallyldimethylammonium Pic = pseudoisocyanine Pic or Ox layer = silicate monolayer with adsorbed dye (either saponite with oxazine 1 or montmorillonite with pseudoisocyanine Pic or Ox film = film formed by LBL assembly deposition based on a single dye, either oxazine or pseudoisocyanine; in contrast, the complex film was built from alternating layers bearing either dye PLS = partial least-squares analysis RET = resonance energy transfer Sap = saponite, used as a carrier of oxazine 1 SVD = singular value decomposition

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Layer-by-Layer Assemblies Composed of Polycationic Electrolyte

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