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Spectral properties and structure of the J-aggregates of pseudoisocyanine dye in layered silicate films
Journal of Colloid and Interface Science 326 (2008) 426–432
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Spectral properties and structure of the J -aggregates of pseudoisocyanine dye in layered silicate films Juraj Bujdák a,b,∗ , Nobuo Iyi b a b
Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, Bratislava, SK-845 36, Slovak Republic Nanoscale Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 23 April 2008 Accepted 14 June 2008 Available online 18 July 2008
Hybrid films composed of pseudoisocyanine (PIC) and layered silicates were prepared by direct adsorption from dye solution. Properties of the films were characterized by absorption and fluorescence spectroscopy, which indicated the formation of two types of J -molecular aggregates. Molecular arrangement and structure of the J -aggregates were investigated by means of linearly polarized absorption spectroscopy and X-ray diffraction (XRD). Structural models of the two main types of J -aggregates with oblique and those of parallel molecular alignments were designed. The spectral properties of these species were studied in detail. All the films contained a mixture of both types of the J -molecular assemblies. Predominance of either species likely depends on the layer charge of the silicate template. © 2008 Elsevier Inc. All rights reserved.
Keywords: Pseudoisocyanine Layered silicates UV/vis absorption spectroscopy Fluorescence J -aggregates
1. Introduction Pseudoisocyanine (PIC) dye (see supplementary information (SI) No. 1) attracts much attention due to its unique properties as well as industrial applications [1]. One of the most studied phenomena related to this dye is the formation of supramolecular assemblies called J -aggregates. The specific structures of the J -aggregates are typical of a head-to-tail intermolecular association between the molecular units [2]. Formation of the J -aggregates of PIC has been reported for various types of reaction systems, e.g., solutions [2,3] and interfaces [4]. Preference for the PIC cations to form J -aggregates rather than the generally more common H -aggregates is mainly due to the molecular structure of PIC (Fig. 1). The cations bear two hydrophobic ethyl groups, which are close to the center of the molecule, thus preventing formation of sandwich-type assemblies called H -aggregates [5]. The J -aggregates are true organic nanoassemblies [6]. Under specific conditions, they can be built from tens to thousands of assembled molecules and exhibit variable macrostructural features [7,8]. One of the main properties of the J -aggregates is the absorption spectrum: Light absorption by chromophores, which form J -aggregates, is characterized by a sharp spectral band of relatively high molar absorption coefficients and at significantly lower energies than the absorption associated with the transitions of individual (nonaggregated) dye
cations [1,9]. Other properties include those having been observed for J -aggregates in general, such as fluorescence [10], spectral-hole burning phenomena [11], nonlinear optical properties [12,13] and energy transfer [14,15]. There have been numerous attempts to prepare various hybrid materials based on inorganic substrates with embedded organic dyes. Such materials could be applicable in modern industrial fields [16]. The inorganic substrate would provide an inert and optically inactive matrix. Hence, an advantageous property of such materials would be the isolation of dye molecules from the chemical environment. Inorganic carrier would neither directly contribute to optical properties of the material nor actively take part in photochemical processes. Potential stabilization of the chromophores against decomposition has already been observed for some cyanine dyes in layered silicate dispersions based on natural and synthetic clay minerals [5,17]. The preferential orientation of dye molecules on the inorganic substrate is another important property, which is expected for this kind of materials [18]. It could be very important for some applications in optics, where materials exhibiting optical anisotropy are required. Well-organized
*
Corresponding author. Permanent address: Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, Bratislava, SK-845 36, Slovak Republic. E-mail addresses: [email protected] (J. Bujdák), [email protected] (N. Iyi). 0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.06.036
© 2008
Elsevier Inc. All rights reserved.
Fig. 1. Molecular structure of pseudoisocyanine cation.
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Table 1 Basic parameters of used layered silicates Specimen
Symbol
CEC (mmol g−1 ) with references
S ion (nm2 )
Fluorohectorite Laponite RDS Kunipia montmorillonite
FH L K
1.50 [5] 0.87 [31] 1.15 [32]
0.44 0.75 0.54
Note. CEC, cation exchange capacity. S ion , surface area occupied by one monovalent cation, i.e., to the average surface area per one electron charge.
J -type assemblies are formed on planar surfaces or can be stabilized as intercalated species on various substrates of layered, two-dimensional structures [19]. Layered inorganic compounds include phosphates [20], niobates [21], and silicates. The choice of an inorganic component is often a key factor in successfully preparing materials of optimal properties. One could note the swelling of the inorganic material, the property necessary for homogeneous and controlled dye intercalation [22,23], charge of the layered templates, controlling the distribution of the dye cations [16], and structural properties of the surface [24]. There have been several attempts to prepare various hybrid materials based on PIC or other cyanine dyes and inorganic layered compounds [25–28]. Detailed studies have been aimed at the interaction with layered silicates (mostly represented by clay minerals) in aqueous dispersions [5,29]. Partial decomposition of PIC was observed in diluted layered silicate dispersions. The decomposition was specifically related to the layer charge density of the inorganic component [5]. The effects of other properties and parameters on the formation of J -aggregates were claimed in another study [30]. Intercalation of the PIC was achieved also in a poorly swelling sodium titanate with preadsorbed propylammonium cations [18,22]. Inorganic silicate magadite grafted with hydrophobic alkyl chains exhibited significantly different properties for the intercalation of PIC cations [31]. More knowledge on the formation and structural aspects of the PIC J -aggregates has been published for the systems of mica [24,32] and glass [33] surfaces. The objective of this work was to shed more light on the formation, structure, and properties of thin films of layered silicates with intercalated PIC J -aggregates. The preparation of the hybrid materials was achieved via a direct PIC intercalation from solution into swellable silicate films. The influence of the properties of the inorganic component on the structure and optical properties of the intercalated phases of PIC cations was investigated in detail. 2. Experimental methods Two synthetic layered silicates, fluorohectorite (Corning, Inc., USA) [5] and Laponite RDS (Laporte, Ltd.) [34], and commercially available Kunipia F montmorillonite of natural origin (Kunimine Industries, Japan) [35] were used in the form as received without any purification and pretreatment. Cation exchange capacities (CEC) of the layered silicate specimens are listed in Table 1. Average surface areas available to one monovalent cation (S ion ) and the mean distances between the exchangeable cations (dion ) were calculated. Molecular mass (in unified atomic mass units) equivalent to the unit O20 (OH)4 (or O20 F4 ) was calculated from structural formulas [36,37] as a sum of all atomic masses multiplied by stoichiometric coefficients. This value is equal to the mass of 1 mol of structural units (6.022 × 1023 ) expressed in g mol−1 (M unit ). The surface area S was calculated from S=
NA · a · b M unit
,
(1)
where N A and a and b represent the Avogadro constant (6.022 × 1023 mol−1 ) and the parameters of unit cell dimensions 0.51 and
427
0.90 nm (0.53 and 0.93 nm for trioctahedral smectites), respectively. The theoretical surface area was used for the calculation of area per monovalent cation according to S ion =
S CEC · N A
,
(2)
where CEC represents the cation-exchange capacity in moles per gram considering the presence of monovalent cations (Table 1). The films were prepared on fused silica slides transparent in an ultraviolet–visible (UV–vis) spectral range. The slides had been cleaned in a NaOH water/ethanol (10/90 v/v) solution, followed by washing in water and dilute H2 SO4 aqueous solution. Clean slides were then washed in highly pure deionized water. Aqueous suspensions of layered silicates (0.5% w/w), prepared using ultrasonic bath treatment (20 min), were deposited on the slides using a spin-coating method. The slides with thin films of the silicates were dried in air and directly used for the dye intercalation. PIC was dissolved in high-purity deionized water to obtain the final dye concentration of 1 mmol dm−3 . Slides with silicate films were immersed in the PIC solution for 3 h and heated at 60 ◦ C. These conditions were suitable for effective intercalation of the dye cations. The dye amount in the solution was in high excess with respect to the adsorption capacities of the silicates and had not been significantly changed during the intercalation. The excess dye solution was removed by multiple washings with water until no free or reversibly adsorbed dye was released. The films were then dried in air. UV–vis spectra were measured by a V-550 UV–vis spectrophotometer (Jasco Co., Ltd.) at room temperature. Linearly polarized UV–vis spectroscopy was applied to characterize the molecular orientation of the dye cations. A Jasco polarizer was used for recording the spectra using both horizontally (x-axis direction) and vertically polarized light ( y-axis direction). The slide orientation was changed around the x-axis at angles α with respect to the y-axis only. More details on the theoretical background and measurement setup have been published elsewhere [19,38,39]. The basic scheme is described in SI (No. 2). The fluorescence spectra of identical PIC/silicate films were measured using a Shimazu RF5300PC spectrofluorometer. Both the excitation and the emission spectra were measured. The measurement conditions such as wavelengths of light used for the excitation or wavelengths whose intensities were measured are introduced in the text below. The XRD measurements of the identical films were recorded at room temperature for the range of low angles of 2θ to identify the basal spacing values. A RINT-1200 powder X-ray diffractometer (Rigaku Co., Ltd.) with Ni-filtered CuK α radiation was used. 3. Results and discussion 3.1. Absorption spectroscopy Fig. 2 shows absorption spectra of PIC intercalated in the films of layered silicates. The film of K montmorillonite exhibits the largest absorbance values, especially at the highest wavelengths. The strong absorption band is assigned to the transitions associated with J -type molecular assemblies [40,41]. The lower wavelengths’ bands are assigned to H -type molecular assemblies and isolated PIC cations. The FH film exhibited relatively the lowest absorption in a J -band region. The J -band position was only slightly dependent on the silicate template used. Analysis of second derivative spectra (see SI, No. 3) showed slightly decreasing energy of maximal absorption in the order of L (570 nm), FH (571 nm), and K (572 nm). These values are a few nanometers lower, if taken directly from the absorption spectra (Fig. 2) (L, 568 nm; FH, 565 nm; K, 571 nm), which
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Fig. 2. Absorption spectra of pseudoisocyanine cations intercalated in the films of layered silicates.
is due to an overlap with the bands at lower energies. Interpretation of the spectra could be based on considering variable amounts of the dye present in the films, as well as a different degree and type of molecular aggregation. Ordered and large J -aggregates typically have narrow absorption bands and large molar absorption coefficients. On the other hand, disordered and smaller size J aggregates could exhibit relatively broader absorption bands. Molar absorption coefficients of the J -aggregates are usually significantly higher than those of corresponding isolated molecules [42]. The narrow shape of the J -band is also due to the absence of vibrational component broadening. Two types of the J -aggregates have been observed for other systems [43,44] and are characterized in detail in the paper, dealing with the PIC aggregation at an aqueous/borosilicate interface [45]: J -aggregates absorbing light at lower wavelengths (maximal absorption at 570 nm) were formed in the bulk solution phase; species absorbing at 579 nm interacted directly with the glass surface [45]. J -aggregates absorbing at 578 nm were observed in hydrophobic Langmuir–Blodgett films [46]. Energy of absorbed light is influenced by the effects of the electrostatic interaction of PIC cations with counterions [47], which can be different in hydrated environments or at the interface. If these interpretations are applied for this study, PIC aggregates absorbing at lower wavelengths (about 570 nm) are probably those in a relatively more hydrophilic phase in the film, coexisting with water
molecules still present between the silicate layers. Band tailing at higher wavelengths was observed for the PIC/K film representing a hydrophobic type of J -aggregates (Fig. 2, see also SI, No. 4). On the other hand, L and FH films contain relatively larger amounts of other species, absorbing at lower wavelengths (435–550 nm). Spectral properties and structural characterization of the types of J -aggregates are summarized in Table 2. The absorption spectra and the trends described above can be interpreted in terms of the effects of inorganic template properties. Specific interaction between the aggregate and inorganic template may lead to significant changes in optical properties, as has been observed for the systems based on PIC and polyelectrolytes [48]. Sensitive stereospecific interaction between PIC cations on the surface of mica was revealed using atomic force microscopy [32]. Dye cations orient specifically with respect to the shape and structure of ditrigonal cavities on the mica surface. The structures of the basal surfaces of mica and smectites are very similar [49]. Variable spectral properties of the dye in the films could be assigned to the structural aspects of the silicates used in this study. Slight structural differences between dioctahedral K montmorillonite and trioctahedral minerals, FH and L, might be taken into account [5]. Miyamoto et al. [18] assumed that the particle size of clay minerals could play a significant role in the formation of the PIC J -aggregates. The bigger the diameter of the silicate platelets, the larger the size of the J -aggregates that can theoretically be achieved. The particle size of K montmorillonite is nonuniform, with diameters of a broad range of about 0.1–1.0 μm [50]. Synthetic L is composed of much smaller particles with a diameter of about 30 nm [51]. FH is composed of relatively large particles of 2 μm or more [52]. This fact is in contradiction to Miyamoto’s interpretations [22], due to a low tendency of FH to accommodate PIC cations in the form of J -aggregates (Fig. 2). We assume that layer charge distribution in silicate layers (Table 1) may specifically affect the formation of PIC J -aggregates, as has been shown for this dye in clay mineral dispersions [5] or for the systems with other chromophores [16]. Medium-charge K forms the largest amount of the J -aggregates. A similar trend was observed in spectra of this dye in colloidal dispersions of layered silicates [5]. 3.2. Structure of the dye molecular assemblies A combination of X-ray diffraction and linearly polarized absorption spectroscopy provided basic information on the structure of intercalated dye cations. The results of these methods were used for designing possible arrangements of intercalated dye cations. XRD was used mainly for determination of the basal spacing of the PIC/layered silicate films. The thickness of the interlayer spaces occupied with organic cations was calculated as the difference between the measured basal spacing and the thickness of the silicate layer (0.96 nm) [49]. Fig. 3 shows XRD patterns of nonmodified FH and K films compared with those modified with PIC. The structure of the silicates turned out to be less ordered on the dye adsorption. FH film composed of relatively large layered particles exhibited a strong reflection at around 6◦ (2θ ), which relates to a basal spacing
Table 2 Characterization of two types of PIC J -aggregates coexisting in the films of layered silicates Type of J -aggregates
Oblique
Head-to-tail
Chemical environment Presence in silicate films Absorption band (nm) Emission bands Excitation bands Structure
Hydrophilic In all the films ∼570 nm ∼610–615, 665 nm ∼570 nm Oblique, nonparallel orientation of assembled molecules (Fig. 5a) 2.0 nm/1.0 nm
Hydrophobic [45] Preferentially in K film >570 nm, ∼579 [45], Fig. 2 ∼627 nm, Figs. 6–8 ∼585 nm, Figs. 6–8 Head-to-tail, parallel orientation of assembled molecules (Fig. 5b) <2.0 nm/1.0 nm ∼1.6 nm/0.6 nm (for K film), Fig. 3
Basal spacing/interlamellar distance
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429
Fig. 4. Angles between transition moments of PIC cations and layered silicates surfaces calculated based on the data from linearly polarized absorption spectroscopy.
Fig. 3. X-ray diffraction patterns of the PIC/layered silicate films.
of about 1.5 nm. On the adsorption of PIC cations, the diffraction band shifted to lower angles due to an increase in the spacing to about 2.0 nm. The expansion of the interlayer spaces was accompanied by the formation of a less ordered structure, which is indicated by broadening of the reflection band. For the case of PIC adsorption on K film, the shift seems to be much lower. A small reflection was observed at 1.6 nm, but with low tailing to larger angles 2θ . There is a significant decrease in the band intensity on PIC adsorption on K. The reflection band tailing to lower angles 2θ indicates the coexistence of various spacings of 1.6 nm and higher. The reflection band was undetectable for the case of the PIC/L film (not shown), probably due to two contributing factors: the disordered structure of the intercalated dye and very small diameter of the L particles. Linearly polarized absorption spectroscopy provided information on the orientation of PIC cations. Molecular orientation was determined by measuring the directions of transitional moments with respect to the silicate surface. The orientation of the transition moment in a PIC cation is parallel to the longest axis of the cation, i.e., to a line connecting the nitrogen atoms [18]. The angles of molecular orientation calculated from linearly polarized absorption spectra are shown in Fig. 4 as a function of the wavelength of absorbed light. No perpendicular orientation is observed in any case. Most of the dye cations’ transition moments exhibit angles in the range of 25–30◦ with respect to the plane representing the silicate surface. Interestingly, PIC cations adsorbed on an FH surface exhibit a molecular orientation characterized of relatively lower angles. This seems to be in contradiction to the fact of the highest spacing in the PIC/FH film (2 nm) found using XRD (Fig. 3). With increasing wavelength of the absorbed light, the angle decreases dramatically. This trend starts at the wavelengths of maximal light absorption (570 to 580 nm). Significant variation in
orientation angles in the range 570–590 nm supports the assumption of the existence of structurally different J -aggregates, which was indicated by absorption spectra (Table 2). For the phases detected by XRD and related to the spacings of 1.6 and 2.0 nm, respectively, and considering dimensions of PIC cation (SI, No. 5), two basic arrangements of the J -aggregates were designed. The orientation of the longest axis of the cation at the angles 30◦ with respect to the silicate surface was considered (Fig. 4). As discussed above, a 2.0 nm spacing denotes about 1 nm thickness of the interlayer phase. This relatively large expansion of the interlamellar space occurs for the film of FH (see scheme in Fig. 5a). The planes of quinoline heterocyclic skeletons retain a nearly perpendicular orientation in order to form a relatively densely packed phase in the interlayer spaces. Empty spaces between the PIC cation and the silicate surface (Fig. 5a) could be filled with residual water molecules. Thus water molecules could play the role of bridges interconnecting the dye cations and oxygen atoms at the silicate surface via ion/dipole, dipole/dipole interactions. This type of structure has been considered for methylene blue in a water/clay mineral interface [53]. Indeed, the PIC/FH film absorbs light with a tail to relatively low wavelengths (<570 nm) (Fig. 2), which is typical for the J -aggregates formed in hydrophilic environments [43] (Table 2). K montmorillonite exhibits a lower charge density compared to that of FH, and therefore, it can accommodate fewer cations per surface unit bound via ionic interactions (Table 1). Consequently, PIC cations are less concentrated on K compared to the adsorption on FH. Lower PIC concentration would allow molecular arrangement, where a single PIC cation occupies a larger surface and the molecular alignment of quinoline heteroaromatic rings is more parallel to the silicate surface (Fig. 5b). Such a structure would be characterized by a spacing as low as 1.6 nm in agreement with the XRD measurement. There would be no empty space for water molecules in the interlayer spaces (in contrast to a 2 nm phase, shown in Fig. 5a). This is in agreement with the results of absorption spectroscopy, which indicated the presence of a more hydrophobic phase in PIC/K film (see discussion above).
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(a)
Fig. 6. Fluorescence spectra of PIC/K film. Emission spectra were measured for excitations at 571 (em571, solid line) and 585 nm (em585, dashed line). Excitation spectra were taken for emissions at 610 (ex610, dotted line), 627 (ex627, dashed line), and 665 nm (ex665, solid line).
3.3. Fluorescence of pseudoisocyanine J-aggregates
(b) Fig. 5. Scheme showing the models of molecular orientation and arrangement of pseudoisocyanine molecules intercalated between the layers of silicates. Arrangements for the phases characterized by 2.0 (a) and 1.6 nm (b) basal spacings.
There is another main difference between the aggregates represented by the arrangements shown in Fig. 5. The arrangement of the PIC cations in Fig. 5a is characterized by interacting molecules in a nonparallel orientation. This type of assemblies is often called “intermediate” or “oblique” J -aggregates. On the other hand, an assembly in Fig. 5b exhibits head-to-tail association with the molecules oriented roughly parallel to each other. For a head-totail arrangement with molecules of parallel transition moments, the lower energy state has a larger transition moment, which results in a strong red shift of the absorption [10]. Indeed, PIC intercalated in a K film with parallel alignment of the cations exhibits relatively the strongest absorption at low energies (Fig. 2, SI, No. 4). On the other hand, oblique-type assemblies result in an energetic split of the excited states [1]. As the result of the splitting, two bands usually appear at both the low and the high energies of the absorption spectrum. In the case of the PIC/FH film, less intense absorption of the J -band is observed, which might be due to coexistent allowed transitions to a higher energy state, reflected as the presence of an H -band appearing at lower wavelengths (Fig. 2, dotted line). Structures and characterization of J -aggregate types are listed in Table 2.
PIC cations in a monomeric state do not exhibit significant fluorescence. They thermally deactivate due to molecular motions based on flip–flop movements of the two heteroaromatic rings [54]. However, the J -aggregates of PIC are fluorescently active. They emit at energies near maximal light absorption due to a negligible Stokes shift. Detailed analysis of the PIC/K film revealed two main fluorescent species (Fig. 6). The first species exhibited maximal emission if excited at 571 nm (solid line, em571). The second species was efficiently excited at larger wavelengths, 585 nm (dashed line, em585). Probably, both species are PIC J -aggregates with the properties summarized in Table 2. The aggregates absorbing at about 570 nm assigned to the molecular assemblies of oblique structure emitted with a negligible Stokes shift. At the low energy part of the main band of declining emission, two hardly resolvable shoulders were found at 610–615 and 665 nm (Fig. 6, solid line, em571). Excitation spectra of both the 610 nm (dotted line) and the 665 nm (solid line) emissions clearly identified an excitation band near 572 nm. The J -aggregates built from molecules with a paralleltype alignment and being excited at 585 nm exhibit a single and relatively strong emission at 627 nm (dashed line, em585). A close relation of the emission at 627 nm to the specific species has been confirmed with the excitation spectrum for this wavelength (Fig. 6, dashed line, ex627). Slight differences in the fluorescence spectra have been observed dependent on the silicate substrate. The largest emission at about 627 nm on excitation at its most efficient excitation wavelength (582–585 nm) was observed from a PIC/K film (Fig. 7, line 1). Strong emission related to the head-to-tail-type J -aggregates from the PIC/K film is in accordance with a relatively larger amount of the J -aggregates absorbing at higher wavelengths (>570 nm) (Fig. 2). For the excitation spectrum of a PIC/L film, the 628-nm emission identifies two excitation bands: one at 583 nm, common also
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Fig. 7. Emission and excitation spectra of PIC/layered silicate films identifying the J aggregates absorbing at higher wavelengths. Excitation wavelengths: 582–585 nm. Wavelength of emitted light: 627, 628 nm. These conditions were chosen based on the maxima of the bands in excitation (Ex, solid line) and emission spectra (Em, dashed line). Layered silicates: Kunipia montmorillonite (1), Laponite RDS (2), and fluorohectorite (3).
431
Fig. 8. Emission and excitation spectra of PIC/layered silicate films identifying the J aggregates absorbing at lower wavelengths. Excitation wavelength: 567 nm. Wavelength of emitted light: 610 nm. Layered silicates: Kunipia montmorillonite (1), Laponite RDS (2), and fluorohectorite (3).
could be due to quenching processes but does not need to reflect the low amount of these species. 4. Conclusions
in other reaction systems, and the other at 565 nm. The band at 583 nm is of relatively low intensity with respect to the other systems. The band at 565 nm could be assigned to monomers, which transfer part of the energy to the J -aggregates absorbing at higher wavelengths. In order to characterize the fluorescence of the J -aggregates, which absorb light at lower wavelengths (around 570 nm), the emission spectra on excitation at 567 nm and the excitation spectra for the 610-nm emission were measured (Fig. 8). No wellresolved bands were recognized in the emission spectra (dashed lines), which indicates energy transfer processes to head-to-tailtype J -aggregates coexisting in the films. The excitation spectrum of a PIC/K film for the 610-nm emission exhibits a strong band at about 570 nm. The intensity of this band for the PIC/Lap film is one-half to one-third. Interestingly, the smallest intensity of emitted light is observed from a PIC/FH film. In order to interpret these facts, one has to consider the results obtained from other methods (absorption spectra and XRD). Large basal spacing allows the formation of oblique-type J -aggregates. These are intermediate and structurally very similar to the H -aggregates. Absorption spectra indicate that large amounts of the H -aggregates (absorbing at lower wavelengths) coexist with these J -type assemblies. Indeed, a large density of the layer charge could lead to sandwichtype intermolecular association at the expense of the J -aggregates. A similar trend has been observed for the case of PIC assemblies formed in the systems of negatively adsorbed polyelectrolytes [54]. The highest yields of the J -aggregates required a specific ratio of the dye per polyelectrolyte amount. Undersaturation or exceeding the number of the PIC cations led to a dramatic decrease in the J -aggregates in the system. In summary, the low emission of the J -type assemblies excited at lower wavelengths in the FH film
Formation of pseudoisocyanine J -aggregates in the films of layered silicates was experimentally proven by absorption and fluorescence spectroscopies. Linearly polarized absorption spectroscopy did not find a significant variation in the molecular orientation. The dye cations were partially tilted with longest molecular axis with respect to the silicate surface (25–30◦ ). Molecular orientation is ruled by electrostatic interactions between the dye cations and the silicate surface but is likely significantly influenced by the formation of molecular assemblies. The results of spectroscopy methods and X-ray diffraction indicated the presence of at least two main types of the J -aggregates (Table 2): (1) an oblique type with quinoline heteroaromatic ring planes more perpendicular to the silicate surface but retaining the longest axis of the molecule almost parallel; and (2) the J -aggregates with a parallel alignment of the quinoline groups partially tilted with respect to the silicate surface. The layer charge of the silicate surface probably affects the structure and properties of the molecular assemblies. Montmorillonite with a moderate layer charge density exhibited optimal properties to form relatively high yields of J -aggregates, which is in accordance with observations on PIC/layered silicate dispersions [5]. Acknowledgments This work was supported by the Slovak Research and Development Agency under contract No. APVV-51-027405 and grant agency VEGA (project 2/6180/27). Supplementary material Supplementary data for this article may be found in the online version at DOI: 10.1016/j.jcis.2008.06.036. The data include
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the following. (1) Information about pseudoisocyanine. (2) Scheme showing the setup for polarized spectroscopy measurements. (3) Second derivative spectra of PIC/silicate films at a J -band region. (4) Normalized absorption spectra of PIC/silicate films. (5) Structural characterization of pseudoisocyanine cation. This material is available free of charge via Internet. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Spectral properties and structure of the J -aggregates of pseudoisocyanine dye in layered silicate films Juraj Bujdák a,b,∗ , Nobuo Iyi b a b
Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, Bratislava, SK-845 36, Slovak Republic Nanoscale Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 23 April 2008 Accepted 14 June 2008 Available online 18 July 2008
Hybrid films composed of pseudoisocyanine (PIC) and layered silicates were prepared by direct adsorption from dye solution. Properties of the films were characterized by absorption and fluorescence spectroscopy, which indicated the formation of two types of J -molecular aggregates. Molecular arrangement and structure of the J -aggregates were investigated by means of linearly polarized absorption spectroscopy and X-ray diffraction (XRD). Structural models of the two main types of J -aggregates with oblique and those of parallel molecular alignments were designed. The spectral properties of these species were studied in detail. All the films contained a mixture of both types of the J -molecular assemblies. Predominance of either species likely depends on the layer charge of the silicate template. © 2008 Elsevier Inc. All rights reserved.
Keywords: Pseudoisocyanine Layered silicates UV/vis absorption spectroscopy Fluorescence J -aggregates
1. Introduction Pseudoisocyanine (PIC) dye (see supplementary information (SI) No. 1) attracts much attention due to its unique properties as well as industrial applications [1]. One of the most studied phenomena related to this dye is the formation of supramolecular assemblies called J -aggregates. The specific structures of the J -aggregates are typical of a head-to-tail intermolecular association between the molecular units [2]. Formation of the J -aggregates of PIC has been reported for various types of reaction systems, e.g., solutions [2,3] and interfaces [4]. Preference for the PIC cations to form J -aggregates rather than the generally more common H -aggregates is mainly due to the molecular structure of PIC (Fig. 1). The cations bear two hydrophobic ethyl groups, which are close to the center of the molecule, thus preventing formation of sandwich-type assemblies called H -aggregates [5]. The J -aggregates are true organic nanoassemblies [6]. Under specific conditions, they can be built from tens to thousands of assembled molecules and exhibit variable macrostructural features [7,8]. One of the main properties of the J -aggregates is the absorption spectrum: Light absorption by chromophores, which form J -aggregates, is characterized by a sharp spectral band of relatively high molar absorption coefficients and at significantly lower energies than the absorption associated with the transitions of individual (nonaggregated) dye
cations [1,9]. Other properties include those having been observed for J -aggregates in general, such as fluorescence [10], spectral-hole burning phenomena [11], nonlinear optical properties [12,13] and energy transfer [14,15]. There have been numerous attempts to prepare various hybrid materials based on inorganic substrates with embedded organic dyes. Such materials could be applicable in modern industrial fields [16]. The inorganic substrate would provide an inert and optically inactive matrix. Hence, an advantageous property of such materials would be the isolation of dye molecules from the chemical environment. Inorganic carrier would neither directly contribute to optical properties of the material nor actively take part in photochemical processes. Potential stabilization of the chromophores against decomposition has already been observed for some cyanine dyes in layered silicate dispersions based on natural and synthetic clay minerals [5,17]. The preferential orientation of dye molecules on the inorganic substrate is another important property, which is expected for this kind of materials [18]. It could be very important for some applications in optics, where materials exhibiting optical anisotropy are required. Well-organized
*
Corresponding author. Permanent address: Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, Bratislava, SK-845 36, Slovak Republic. E-mail addresses: [email protected] (J. Bujdák), [email protected] (N. Iyi). 0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.06.036
© 2008
Elsevier Inc. All rights reserved.
Fig. 1. Molecular structure of pseudoisocyanine cation.
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Table 1 Basic parameters of used layered silicates Specimen
Symbol
CEC (mmol g−1 ) with references
S ion (nm2 )
Fluorohectorite Laponite RDS Kunipia montmorillonite
FH L K
1.50 [5] 0.87 [31] 1.15 [32]
0.44 0.75 0.54
Note. CEC, cation exchange capacity. S ion , surface area occupied by one monovalent cation, i.e., to the average surface area per one electron charge.
J -type assemblies are formed on planar surfaces or can be stabilized as intercalated species on various substrates of layered, two-dimensional structures [19]. Layered inorganic compounds include phosphates [20], niobates [21], and silicates. The choice of an inorganic component is often a key factor in successfully preparing materials of optimal properties. One could note the swelling of the inorganic material, the property necessary for homogeneous and controlled dye intercalation [22,23], charge of the layered templates, controlling the distribution of the dye cations [16], and structural properties of the surface [24]. There have been several attempts to prepare various hybrid materials based on PIC or other cyanine dyes and inorganic layered compounds [25–28]. Detailed studies have been aimed at the interaction with layered silicates (mostly represented by clay minerals) in aqueous dispersions [5,29]. Partial decomposition of PIC was observed in diluted layered silicate dispersions. The decomposition was specifically related to the layer charge density of the inorganic component [5]. The effects of other properties and parameters on the formation of J -aggregates were claimed in another study [30]. Intercalation of the PIC was achieved also in a poorly swelling sodium titanate with preadsorbed propylammonium cations [18,22]. Inorganic silicate magadite grafted with hydrophobic alkyl chains exhibited significantly different properties for the intercalation of PIC cations [31]. More knowledge on the formation and structural aspects of the PIC J -aggregates has been published for the systems of mica [24,32] and glass [33] surfaces. The objective of this work was to shed more light on the formation, structure, and properties of thin films of layered silicates with intercalated PIC J -aggregates. The preparation of the hybrid materials was achieved via a direct PIC intercalation from solution into swellable silicate films. The influence of the properties of the inorganic component on the structure and optical properties of the intercalated phases of PIC cations was investigated in detail. 2. Experimental methods Two synthetic layered silicates, fluorohectorite (Corning, Inc., USA) [5] and Laponite RDS (Laporte, Ltd.) [34], and commercially available Kunipia F montmorillonite of natural origin (Kunimine Industries, Japan) [35] were used in the form as received without any purification and pretreatment. Cation exchange capacities (CEC) of the layered silicate specimens are listed in Table 1. Average surface areas available to one monovalent cation (S ion ) and the mean distances between the exchangeable cations (dion ) were calculated. Molecular mass (in unified atomic mass units) equivalent to the unit O20 (OH)4 (or O20 F4 ) was calculated from structural formulas [36,37] as a sum of all atomic masses multiplied by stoichiometric coefficients. This value is equal to the mass of 1 mol of structural units (6.022 × 1023 ) expressed in g mol−1 (M unit ). The surface area S was calculated from S=
NA · a · b M unit
,
(1)
where N A and a and b represent the Avogadro constant (6.022 × 1023 mol−1 ) and the parameters of unit cell dimensions 0.51 and
427
0.90 nm (0.53 and 0.93 nm for trioctahedral smectites), respectively. The theoretical surface area was used for the calculation of area per monovalent cation according to S ion =
S CEC · N A
,
(2)
where CEC represents the cation-exchange capacity in moles per gram considering the presence of monovalent cations (Table 1). The films were prepared on fused silica slides transparent in an ultraviolet–visible (UV–vis) spectral range. The slides had been cleaned in a NaOH water/ethanol (10/90 v/v) solution, followed by washing in water and dilute H2 SO4 aqueous solution. Clean slides were then washed in highly pure deionized water. Aqueous suspensions of layered silicates (0.5% w/w), prepared using ultrasonic bath treatment (20 min), were deposited on the slides using a spin-coating method. The slides with thin films of the silicates were dried in air and directly used for the dye intercalation. PIC was dissolved in high-purity deionized water to obtain the final dye concentration of 1 mmol dm−3 . Slides with silicate films were immersed in the PIC solution for 3 h and heated at 60 ◦ C. These conditions were suitable for effective intercalation of the dye cations. The dye amount in the solution was in high excess with respect to the adsorption capacities of the silicates and had not been significantly changed during the intercalation. The excess dye solution was removed by multiple washings with water until no free or reversibly adsorbed dye was released. The films were then dried in air. UV–vis spectra were measured by a V-550 UV–vis spectrophotometer (Jasco Co., Ltd.) at room temperature. Linearly polarized UV–vis spectroscopy was applied to characterize the molecular orientation of the dye cations. A Jasco polarizer was used for recording the spectra using both horizontally (x-axis direction) and vertically polarized light ( y-axis direction). The slide orientation was changed around the x-axis at angles α with respect to the y-axis only. More details on the theoretical background and measurement setup have been published elsewhere [19,38,39]. The basic scheme is described in SI (No. 2). The fluorescence spectra of identical PIC/silicate films were measured using a Shimazu RF5300PC spectrofluorometer. Both the excitation and the emission spectra were measured. The measurement conditions such as wavelengths of light used for the excitation or wavelengths whose intensities were measured are introduced in the text below. The XRD measurements of the identical films were recorded at room temperature for the range of low angles of 2θ to identify the basal spacing values. A RINT-1200 powder X-ray diffractometer (Rigaku Co., Ltd.) with Ni-filtered CuK α radiation was used. 3. Results and discussion 3.1. Absorption spectroscopy Fig. 2 shows absorption spectra of PIC intercalated in the films of layered silicates. The film of K montmorillonite exhibits the largest absorbance values, especially at the highest wavelengths. The strong absorption band is assigned to the transitions associated with J -type molecular assemblies [40,41]. The lower wavelengths’ bands are assigned to H -type molecular assemblies and isolated PIC cations. The FH film exhibited relatively the lowest absorption in a J -band region. The J -band position was only slightly dependent on the silicate template used. Analysis of second derivative spectra (see SI, No. 3) showed slightly decreasing energy of maximal absorption in the order of L (570 nm), FH (571 nm), and K (572 nm). These values are a few nanometers lower, if taken directly from the absorption spectra (Fig. 2) (L, 568 nm; FH, 565 nm; K, 571 nm), which
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Fig. 2. Absorption spectra of pseudoisocyanine cations intercalated in the films of layered silicates.
is due to an overlap with the bands at lower energies. Interpretation of the spectra could be based on considering variable amounts of the dye present in the films, as well as a different degree and type of molecular aggregation. Ordered and large J -aggregates typically have narrow absorption bands and large molar absorption coefficients. On the other hand, disordered and smaller size J aggregates could exhibit relatively broader absorption bands. Molar absorption coefficients of the J -aggregates are usually significantly higher than those of corresponding isolated molecules [42]. The narrow shape of the J -band is also due to the absence of vibrational component broadening. Two types of the J -aggregates have been observed for other systems [43,44] and are characterized in detail in the paper, dealing with the PIC aggregation at an aqueous/borosilicate interface [45]: J -aggregates absorbing light at lower wavelengths (maximal absorption at 570 nm) were formed in the bulk solution phase; species absorbing at 579 nm interacted directly with the glass surface [45]. J -aggregates absorbing at 578 nm were observed in hydrophobic Langmuir–Blodgett films [46]. Energy of absorbed light is influenced by the effects of the electrostatic interaction of PIC cations with counterions [47], which can be different in hydrated environments or at the interface. If these interpretations are applied for this study, PIC aggregates absorbing at lower wavelengths (about 570 nm) are probably those in a relatively more hydrophilic phase in the film, coexisting with water
molecules still present between the silicate layers. Band tailing at higher wavelengths was observed for the PIC/K film representing a hydrophobic type of J -aggregates (Fig. 2, see also SI, No. 4). On the other hand, L and FH films contain relatively larger amounts of other species, absorbing at lower wavelengths (435–550 nm). Spectral properties and structural characterization of the types of J -aggregates are summarized in Table 2. The absorption spectra and the trends described above can be interpreted in terms of the effects of inorganic template properties. Specific interaction between the aggregate and inorganic template may lead to significant changes in optical properties, as has been observed for the systems based on PIC and polyelectrolytes [48]. Sensitive stereospecific interaction between PIC cations on the surface of mica was revealed using atomic force microscopy [32]. Dye cations orient specifically with respect to the shape and structure of ditrigonal cavities on the mica surface. The structures of the basal surfaces of mica and smectites are very similar [49]. Variable spectral properties of the dye in the films could be assigned to the structural aspects of the silicates used in this study. Slight structural differences between dioctahedral K montmorillonite and trioctahedral minerals, FH and L, might be taken into account [5]. Miyamoto et al. [18] assumed that the particle size of clay minerals could play a significant role in the formation of the PIC J -aggregates. The bigger the diameter of the silicate platelets, the larger the size of the J -aggregates that can theoretically be achieved. The particle size of K montmorillonite is nonuniform, with diameters of a broad range of about 0.1–1.0 μm [50]. Synthetic L is composed of much smaller particles with a diameter of about 30 nm [51]. FH is composed of relatively large particles of 2 μm or more [52]. This fact is in contradiction to Miyamoto’s interpretations [22], due to a low tendency of FH to accommodate PIC cations in the form of J -aggregates (Fig. 2). We assume that layer charge distribution in silicate layers (Table 1) may specifically affect the formation of PIC J -aggregates, as has been shown for this dye in clay mineral dispersions [5] or for the systems with other chromophores [16]. Medium-charge K forms the largest amount of the J -aggregates. A similar trend was observed in spectra of this dye in colloidal dispersions of layered silicates [5]. 3.2. Structure of the dye molecular assemblies A combination of X-ray diffraction and linearly polarized absorption spectroscopy provided basic information on the structure of intercalated dye cations. The results of these methods were used for designing possible arrangements of intercalated dye cations. XRD was used mainly for determination of the basal spacing of the PIC/layered silicate films. The thickness of the interlayer spaces occupied with organic cations was calculated as the difference between the measured basal spacing and the thickness of the silicate layer (0.96 nm) [49]. Fig. 3 shows XRD patterns of nonmodified FH and K films compared with those modified with PIC. The structure of the silicates turned out to be less ordered on the dye adsorption. FH film composed of relatively large layered particles exhibited a strong reflection at around 6◦ (2θ ), which relates to a basal spacing
Table 2 Characterization of two types of PIC J -aggregates coexisting in the films of layered silicates Type of J -aggregates
Oblique
Head-to-tail
Chemical environment Presence in silicate films Absorption band (nm) Emission bands Excitation bands Structure
Hydrophilic In all the films ∼570 nm ∼610–615, 665 nm ∼570 nm Oblique, nonparallel orientation of assembled molecules (Fig. 5a) 2.0 nm/1.0 nm
Hydrophobic [45] Preferentially in K film >570 nm, ∼579 [45], Fig. 2 ∼627 nm, Figs. 6–8 ∼585 nm, Figs. 6–8 Head-to-tail, parallel orientation of assembled molecules (Fig. 5b) <2.0 nm/1.0 nm ∼1.6 nm/0.6 nm (for K film), Fig. 3
Basal spacing/interlamellar distance
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429
Fig. 4. Angles between transition moments of PIC cations and layered silicates surfaces calculated based on the data from linearly polarized absorption spectroscopy.
Fig. 3. X-ray diffraction patterns of the PIC/layered silicate films.
of about 1.5 nm. On the adsorption of PIC cations, the diffraction band shifted to lower angles due to an increase in the spacing to about 2.0 nm. The expansion of the interlayer spaces was accompanied by the formation of a less ordered structure, which is indicated by broadening of the reflection band. For the case of PIC adsorption on K film, the shift seems to be much lower. A small reflection was observed at 1.6 nm, but with low tailing to larger angles 2θ . There is a significant decrease in the band intensity on PIC adsorption on K. The reflection band tailing to lower angles 2θ indicates the coexistence of various spacings of 1.6 nm and higher. The reflection band was undetectable for the case of the PIC/L film (not shown), probably due to two contributing factors: the disordered structure of the intercalated dye and very small diameter of the L particles. Linearly polarized absorption spectroscopy provided information on the orientation of PIC cations. Molecular orientation was determined by measuring the directions of transitional moments with respect to the silicate surface. The orientation of the transition moment in a PIC cation is parallel to the longest axis of the cation, i.e., to a line connecting the nitrogen atoms [18]. The angles of molecular orientation calculated from linearly polarized absorption spectra are shown in Fig. 4 as a function of the wavelength of absorbed light. No perpendicular orientation is observed in any case. Most of the dye cations’ transition moments exhibit angles in the range of 25–30◦ with respect to the plane representing the silicate surface. Interestingly, PIC cations adsorbed on an FH surface exhibit a molecular orientation characterized of relatively lower angles. This seems to be in contradiction to the fact of the highest spacing in the PIC/FH film (2 nm) found using XRD (Fig. 3). With increasing wavelength of the absorbed light, the angle decreases dramatically. This trend starts at the wavelengths of maximal light absorption (570 to 580 nm). Significant variation in
orientation angles in the range 570–590 nm supports the assumption of the existence of structurally different J -aggregates, which was indicated by absorption spectra (Table 2). For the phases detected by XRD and related to the spacings of 1.6 and 2.0 nm, respectively, and considering dimensions of PIC cation (SI, No. 5), two basic arrangements of the J -aggregates were designed. The orientation of the longest axis of the cation at the angles 30◦ with respect to the silicate surface was considered (Fig. 4). As discussed above, a 2.0 nm spacing denotes about 1 nm thickness of the interlayer phase. This relatively large expansion of the interlamellar space occurs for the film of FH (see scheme in Fig. 5a). The planes of quinoline heterocyclic skeletons retain a nearly perpendicular orientation in order to form a relatively densely packed phase in the interlayer spaces. Empty spaces between the PIC cation and the silicate surface (Fig. 5a) could be filled with residual water molecules. Thus water molecules could play the role of bridges interconnecting the dye cations and oxygen atoms at the silicate surface via ion/dipole, dipole/dipole interactions. This type of structure has been considered for methylene blue in a water/clay mineral interface [53]. Indeed, the PIC/FH film absorbs light with a tail to relatively low wavelengths (<570 nm) (Fig. 2), which is typical for the J -aggregates formed in hydrophilic environments [43] (Table 2). K montmorillonite exhibits a lower charge density compared to that of FH, and therefore, it can accommodate fewer cations per surface unit bound via ionic interactions (Table 1). Consequently, PIC cations are less concentrated on K compared to the adsorption on FH. Lower PIC concentration would allow molecular arrangement, where a single PIC cation occupies a larger surface and the molecular alignment of quinoline heteroaromatic rings is more parallel to the silicate surface (Fig. 5b). Such a structure would be characterized by a spacing as low as 1.6 nm in agreement with the XRD measurement. There would be no empty space for water molecules in the interlayer spaces (in contrast to a 2 nm phase, shown in Fig. 5a). This is in agreement with the results of absorption spectroscopy, which indicated the presence of a more hydrophobic phase in PIC/K film (see discussion above).
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(a)
Fig. 6. Fluorescence spectra of PIC/K film. Emission spectra were measured for excitations at 571 (em571, solid line) and 585 nm (em585, dashed line). Excitation spectra were taken for emissions at 610 (ex610, dotted line), 627 (ex627, dashed line), and 665 nm (ex665, solid line).
3.3. Fluorescence of pseudoisocyanine J-aggregates
(b) Fig. 5. Scheme showing the models of molecular orientation and arrangement of pseudoisocyanine molecules intercalated between the layers of silicates. Arrangements for the phases characterized by 2.0 (a) and 1.6 nm (b) basal spacings.
There is another main difference between the aggregates represented by the arrangements shown in Fig. 5. The arrangement of the PIC cations in Fig. 5a is characterized by interacting molecules in a nonparallel orientation. This type of assemblies is often called “intermediate” or “oblique” J -aggregates. On the other hand, an assembly in Fig. 5b exhibits head-to-tail association with the molecules oriented roughly parallel to each other. For a head-totail arrangement with molecules of parallel transition moments, the lower energy state has a larger transition moment, which results in a strong red shift of the absorption [10]. Indeed, PIC intercalated in a K film with parallel alignment of the cations exhibits relatively the strongest absorption at low energies (Fig. 2, SI, No. 4). On the other hand, oblique-type assemblies result in an energetic split of the excited states [1]. As the result of the splitting, two bands usually appear at both the low and the high energies of the absorption spectrum. In the case of the PIC/FH film, less intense absorption of the J -band is observed, which might be due to coexistent allowed transitions to a higher energy state, reflected as the presence of an H -band appearing at lower wavelengths (Fig. 2, dotted line). Structures and characterization of J -aggregate types are listed in Table 2.
PIC cations in a monomeric state do not exhibit significant fluorescence. They thermally deactivate due to molecular motions based on flip–flop movements of the two heteroaromatic rings [54]. However, the J -aggregates of PIC are fluorescently active. They emit at energies near maximal light absorption due to a negligible Stokes shift. Detailed analysis of the PIC/K film revealed two main fluorescent species (Fig. 6). The first species exhibited maximal emission if excited at 571 nm (solid line, em571). The second species was efficiently excited at larger wavelengths, 585 nm (dashed line, em585). Probably, both species are PIC J -aggregates with the properties summarized in Table 2. The aggregates absorbing at about 570 nm assigned to the molecular assemblies of oblique structure emitted with a negligible Stokes shift. At the low energy part of the main band of declining emission, two hardly resolvable shoulders were found at 610–615 and 665 nm (Fig. 6, solid line, em571). Excitation spectra of both the 610 nm (dotted line) and the 665 nm (solid line) emissions clearly identified an excitation band near 572 nm. The J -aggregates built from molecules with a paralleltype alignment and being excited at 585 nm exhibit a single and relatively strong emission at 627 nm (dashed line, em585). A close relation of the emission at 627 nm to the specific species has been confirmed with the excitation spectrum for this wavelength (Fig. 6, dashed line, ex627). Slight differences in the fluorescence spectra have been observed dependent on the silicate substrate. The largest emission at about 627 nm on excitation at its most efficient excitation wavelength (582–585 nm) was observed from a PIC/K film (Fig. 7, line 1). Strong emission related to the head-to-tail-type J -aggregates from the PIC/K film is in accordance with a relatively larger amount of the J -aggregates absorbing at higher wavelengths (>570 nm) (Fig. 2). For the excitation spectrum of a PIC/L film, the 628-nm emission identifies two excitation bands: one at 583 nm, common also
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Fig. 7. Emission and excitation spectra of PIC/layered silicate films identifying the J aggregates absorbing at higher wavelengths. Excitation wavelengths: 582–585 nm. Wavelength of emitted light: 627, 628 nm. These conditions were chosen based on the maxima of the bands in excitation (Ex, solid line) and emission spectra (Em, dashed line). Layered silicates: Kunipia montmorillonite (1), Laponite RDS (2), and fluorohectorite (3).
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Fig. 8. Emission and excitation spectra of PIC/layered silicate films identifying the J aggregates absorbing at lower wavelengths. Excitation wavelength: 567 nm. Wavelength of emitted light: 610 nm. Layered silicates: Kunipia montmorillonite (1), Laponite RDS (2), and fluorohectorite (3).
could be due to quenching processes but does not need to reflect the low amount of these species. 4. Conclusions
in other reaction systems, and the other at 565 nm. The band at 583 nm is of relatively low intensity with respect to the other systems. The band at 565 nm could be assigned to monomers, which transfer part of the energy to the J -aggregates absorbing at higher wavelengths. In order to characterize the fluorescence of the J -aggregates, which absorb light at lower wavelengths (around 570 nm), the emission spectra on excitation at 567 nm and the excitation spectra for the 610-nm emission were measured (Fig. 8). No wellresolved bands were recognized in the emission spectra (dashed lines), which indicates energy transfer processes to head-to-tailtype J -aggregates coexisting in the films. The excitation spectrum of a PIC/K film for the 610-nm emission exhibits a strong band at about 570 nm. The intensity of this band for the PIC/Lap film is one-half to one-third. Interestingly, the smallest intensity of emitted light is observed from a PIC/FH film. In order to interpret these facts, one has to consider the results obtained from other methods (absorption spectra and XRD). Large basal spacing allows the formation of oblique-type J -aggregates. These are intermediate and structurally very similar to the H -aggregates. Absorption spectra indicate that large amounts of the H -aggregates (absorbing at lower wavelengths) coexist with these J -type assemblies. Indeed, a large density of the layer charge could lead to sandwichtype intermolecular association at the expense of the J -aggregates. A similar trend has been observed for the case of PIC assemblies formed in the systems of negatively adsorbed polyelectrolytes [54]. The highest yields of the J -aggregates required a specific ratio of the dye per polyelectrolyte amount. Undersaturation or exceeding the number of the PIC cations led to a dramatic decrease in the J -aggregates in the system. In summary, the low emission of the J -type assemblies excited at lower wavelengths in the FH film
Formation of pseudoisocyanine J -aggregates in the films of layered silicates was experimentally proven by absorption and fluorescence spectroscopies. Linearly polarized absorption spectroscopy did not find a significant variation in the molecular orientation. The dye cations were partially tilted with longest molecular axis with respect to the silicate surface (25–30◦ ). Molecular orientation is ruled by electrostatic interactions between the dye cations and the silicate surface but is likely significantly influenced by the formation of molecular assemblies. The results of spectroscopy methods and X-ray diffraction indicated the presence of at least two main types of the J -aggregates (Table 2): (1) an oblique type with quinoline heteroaromatic ring planes more perpendicular to the silicate surface but retaining the longest axis of the molecule almost parallel; and (2) the J -aggregates with a parallel alignment of the quinoline groups partially tilted with respect to the silicate surface. The layer charge of the silicate surface probably affects the structure and properties of the molecular assemblies. Montmorillonite with a moderate layer charge density exhibited optimal properties to form relatively high yields of J -aggregates, which is in accordance with observations on PIC/layered silicate dispersions [5]. Acknowledgments This work was supported by the Slovak Research and Development Agency under contract No. APVV-51-027405 and grant agency VEGA (project 2/6180/27). Supplementary material Supplementary data for this article may be found in the online version at DOI: 10.1016/j.jcis.2008.06.036. The data include
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