Clay Hybrids in the Solid State

Jan 8, 2017 - In the present study, we have attempted to investigate, for the first time, the photophysical behavior of 1,1′-diethyl-2,4′-cyanine ...
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Photophysical Behavior of Isocyanine/Clay Hybrids in the Solid State† Silvano R. Valandro, Alessandra L. Poli, Thaís F. A. Correia, Patricia C. Lombardo, and Carla C. Schmitt* Instituto de Química de São Carlos, Universidade de São Paulo, Caixa Postal 780,13560-970 São Carlos SP, Brasil ABSTRACT: In the present study, we have attempted to investigate, for the first time, the photophysical behavior of 1,1′diethyl-2,4′-cyanine (ICY)/clay mineral hybrids in the solid state. The effects promoted by ICY loading and clay type on the spectroscopic properties were studied by UV−vis diffuse reflectance spectroscopy (DR) and different fluorescence techniques. The hybrids were characterized by X-ray diffraction (XRD) and thermogravimetric analysis (TGA). UV−vis−DR revealed the formation of ICY H-aggregates in Wyoming montmorillonite (SWy-1) and Laponite (Lap); however, Jaggregates were predominant for ICY on Arizona (SAz-1) and Barasym (SYn-1) montmorillonites. The formation of Jaggregates was favored on clays with a high layer charge density (SAz-1 and SYn-1). Increasing ICY loading leads to an increase in H-aggregates, which become predominant in all of the samples. The fluorescence spectra of ICY-Lap and ICY-SYn-1 hybrids showed two emissive bands, and they were assigned to the monomeric and J-aggregate species. The fluorescence lifetime showed consistent and distinct values for the two species. The longer fluorescence lifetime can be assigned to the ICY monomers, while the second component has a short lifetime value and may be attributed to J-aggregate emission species. Moreover, confocal fluorescence micrographs showed two different fluorescent domains; monomers (greenish domain) and J-aggregates (orange domain) can be clearly distinguished. For ICY adsorbed on SWy-1 and SAz-1, the intensities of the fluorescence spectra were very low, and it was not possible to measure the fluorescence lifetimes due to high iron content in these clays, which acts as an efficient quencher of the excited singlet state of the dye molecules. XRD and TGA curves showed that the intercalation of ICY into the interlayer regions of SWy-1, SAz-1, and SYn-1 occurred for high dye concentration only. In the case of Laponite, ICY adsorbs on the external surface of the layer. Our studies indicate that the ICY-clays, in particular, ICY-SYn-1 and ICY-Lap, are promising hybrid materials with interesting optical and photophysical properties.

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INTRODUCTION Dyes incorporated on clay minerals (organic−inorganic hybrid materials) have historically been used for the design of photofunctional devices, such as solid lasers, optical switches, and sensors.1−4 It is well known that dye−clay hybrids are chemically,5,6 photo,7 and thermally8,9 more stable than pure dyes. However, a challenge for the development of these new hybrid materials is to control the properties of the dyes molecules because the adsorption of many dyes into clay can induce the formation of aggregates.10−14 For instance, the dye loading, media, surface area, and clay charge density are some of the factors that can affect the final properties of the materials.10,15 Several groups including us have been investigating the photophysical behavior of organic−inorganic hybrids based on organic dyes and clay minerals.10,15−23 For example, the formation of the triphenylbenzene derivative/clay hybrid greatly increased the fluorescence intensity compared to that of a bulk aqueous solution without clay due to the restriction of molecular motion of the molecules on the clay surface.19 © XXXX American Chemical Society

Hybrid material composed of clay minerals and cationic subporphyrins also exhibited strong luminescence shifts when the molecules were flattening on the clay sheets.20 We recently investigated the photophysical properties of hybrids based on a diphenylmethane dye (Auramine O) and montmorillonites. These hybrids exhibit an intense fluorescence emission and interesting aggregation properties.10,15 Along these lines, some studies have investigated the molecular aggregation properties of dyes in different environments.21,24−26 Organic dye aggregation was described as supramolecular assemblies bound by noncovalent interactions,27,28 which exhibit significantly different spectral properties compared to the monomeric form.29 For this reason, molecular aggregation plays an important role in materials science, chemistry, and biology. Cyanine dyes in past decades received intense scientific attention because of their ability to form molecular aggregates. Received: October 26, 2016 Revised: January 6, 2017 Published: January 8, 2017 A

DOI: 10.1021/acs.langmuir.6b03898 Langmuir XXXX, XXX, XXX−XXX

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UV-2550 spectrophotometer (Shimadzu, Kyoto, Japan) using an integrating sphere attachment (ISR-240A) with the light incident angle at 0°. The cell holder for the transmission sample consists of a plate for the BaSO4 standard white board and a plate for the powder sample. Fluorescence spectra were recorded on a Hitachi F-4500 spectrofluorimeter (Hitachi, Tokyo, Japan). For the emission studies, isocyanine/clay samples were excited at 490 nm from the front surface. The absolute quantum yield (Φ) for the hybrids was recorded on a Quanta-φ-F-3029 integrating sphere (Horiba, Kyoto, Japan). Fluorescence lifetimes were measured on an Easy Life V lifetime fluorometer by using the stroboscopic technique (Optical Building Blocks Corporation, Edison, NJ). The fluorescence decay was obtained by using a pulsed light of diode LED of 445 nm as the excitation source and a long-pass filter of 550 nm to isolate the ICY emission from the scattered light. The instrument response function (IRF) was measured by using a Ludox scattering solution. Four scans were averaged for the fluorescence decay experiments. For every scan, the number of channels was 600, and the time of integration, over which the signal was averaged for every point of each scan, was 4 s. The accuracy of the analysis was determined by χ2, and the randomness of the residual pattern, the Durbin−Watson (DW) parameter. The Durbin−-Watson values must be near 2 for a satisfactory fit.39 For 0.01−0.1 wt % ICY/Lap and 0.01 wt % SYn-1, the curves were fitted using monoexponential analysis. For the other samples, the fluorescence decay curves were fitted using a biexponential analysis. Confocal fluorescence micrographs of ICY/clay hybrids were recorded using a Zeiss LSM 780 confocal microscope (Carl Zeiss, Jena, Germany) with a numerical aperture of NA = 0.8, 20×. A 488 nm argon laser was used as the excitation source. The X-ray diffraction (XRD) data of the samples were collected at 2θ = 3−30° by using a Rigaku Rotaflex-RU 200B diffractometer (Rigaku, Tokyo, Japan) with Cu radiation (λ = 0.154 nm) at 50 kV, 100 mA. The basal spacing of the clays was calculated by using Bragg’s equation.40 Thermogravimetric analysis for ICY, clays, and the ICY/clay hybrids was performed in an SDT-Q 50 (TA Instruments, New Castle, DE). The TGA curves were obtained under a dynamic nitrogen atmosphere flowing at 60 mL min−1. Samples were placed in open α-alumina crucibles and heated to 800 °C at a rate of 10 °C min−1.

J-type cyanine aggregates can provide new properties for materials, such as a strong nonlinear optical response, giving rise to a wide range of applications.30−33 Cyanine dyes are classified by the different links between quinoline rings. In pseudoisocyanines (PIC), both quinoline rings are linked in position 2. On the other hand, an isocyanine (ICY) is formed when a 2-quinoline ring is bonded in position 4 (Chart 1).34 Chart 1. Molecular Structure of 1,1′-Diethyl-2,4′-cyanine (Isocyanine, ICY)

In the literature we found several studies focused on the aggregation and decomposition of pseudoisocyanine (PIC) in suspensions and films of clays.13,16,35−37 On the other hand, hybrids based on isocyanine and clay minerals have not been studied. Moreover, despite broad exploitation of the aggregates’ photophysical properties in different fields of research and application, dyes adsorbed on solid matrixes have been insufficiently characterized. Dyes adsorbed onto a solid clay system can exhibit optical and chemical properties different from those obtained in solution.15,16,18,22 In this article, we investigate the effect of ICY loading and clay type on the photophysical properties of 1,1′-diethyl-2,4′cyanine/clay hybrids. The interaction between ICY and clay minerals was evaluated by spectroscopic and thermal techniques.

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EXPERIMENTAL SECTION

Materials. The dye, 1,1′-diethyl-2,4′-Cyanine (Sigma-Aldrich, St. Louis, MO), was used as received. The montmorillonites (Na+-SWy-1, Na+-SAz-1, and Na+-SYn-1) were provided by the Source Clays Repository of the Clay Minerals Society, University of Missouri, Columbia, MO. The cation-exchange capacities (CECs) for SWy-1, SAz-1, and SYn-1 are 76.4, 120, and 140 mequiv/100 g, respectively. The structural formulas of these clays are described by the Source Clays Repository as follows: M+0.61[Al3.01Fe(III)0.41Mn0.01Mg0.54Ti0.02][Si 7 . 9 8 Al 0 . 0 2 ]O 2 0 (OH) 4 for SWy-1, M + 1 . 1 6 [Al 2 . 7 1 Mg 1 . 1 1 Fe(III)0.12Mn0.01Ti0.03][Si8]O20(OH)4 for SAz-1, and M+0.32[Al3.99Fe(III)trMntrTitr][Si6.50Al1.50]O20(OH)4 with an unbalanced charge of −1.17 for SYn-1. Synthetic Laponite RD was supplied from Laporte Industries (Luton, U.K.), and the structural formula are described as Na+0.7[(Si8Mg5.5Li0.3)O20(OH)2.5F1.5] and CEC 73.3 mequiv/100 g. All of the clays were used after removing impurities as described earlier.38 Isocyanine/Clay Hybrid Preparation. The isocyanine/clay hybrids were obtained by the addition of 1,1′-diethyl-2,4′-cyanine solution to the clay suspensions. The amount of clay used was 0.25 g, and the cyanine proportion was in the range from 0.01 to 10 wt %. For SWy-1 and Laponite, [ICY] = 0.01% or ∼0.03% CEC, [ICY] = 0.1% or ∼0.3% CEC, [ICY] = 1% or ∼3% CEC, [ICY] = 5% or ∼14% CEC, and [ICY] = 10% or ∼30% CEC. For SAz-1 and SYn-1, [ICY] = 0.01% or ∼0.02% CEC, [ICY] = 0.1% or ∼0.2% CEC, [ICY] = 1% or ∼2% CEC, [ICY] = 5% or ∼9% CEC, and [ICY] = 10% or ∼20% CEC. The mixtures were stirred for 24 h at room temperature, and the samples were freeze-dried to obtain dry solid samples of isocyanine/ clay hybrids. Characterization of Isocyanine/Clay Hybrids. The UV−vis diffuse reflectance spectra (DR) were recorded on Shimadzu model

RESULTS AND DISCUSSION UV−Vis and Fluorescence Spectroscopy of ICY/Clay Hybrids. ICY is a cationic dye that exhibits two absorption bands in the visible spectral range with maxima at 520 and 556 nm in aqueous solution (Figure 1). The ICY molecules in

Figure 1. Absorption and fluorescence spectra of ICY (3.5 × 10−6 mol L−1) in aqueous solution. ICY solution was excited at λexc = 520 nm, and excitation and emission slit widths were set at 10 and 20 nm, respectively. B

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Figure 2. UV−vis−DR spectra of ICY in the presence of (a) SWy-1, (b) Lap, (c) SAz-1, and (d) SYn-1 clays.

Figure 3. 1,1′-Diethyl-2,4′-cyanine incorporated in Lap (a) fluorescence emission spectra λexc= 490 nm with slit widths were set to 2.5 nm. (b) Fluorescence decays and Ludox scattering solution. (c) Confocal fluorescence images excited at 488 nm for 0.01% ICY. (d) 0.1% ICY and (e) 1% ICY.

C

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Langmuir Table 1. Fluorescence Lifetime Parameters Obtained for ICY in the Presence of Clays sample 0.01% ICY/Lap 0.1% ICY/Lap 0.5% ICY/Lap 1% ICY/Lap 0.01% ICY/SYn-1 0.1% ICY/SYn-1 0.5% ICY/SYn-1 1% ICY/SYn-1

τ1

τ2

a1

0.50 ± 0.02 0.46 ± 0.02

0.70 ± 0.01 (61%) 1.48 ± 0.02 (84%)

0.63 ± 0.04 0.40 ± 0.03 0.37 ± 0.02

0.52 ± 0.02 (54%) 0.95 ± 0.06 (67%) 1.73 ± 0.05 (86%)

4.63 4.42 2.3 1.53 4.16 3.8 2.30 2.3

± ± ± ± ± ± ± ±

a2 0.01 0.05 0.1 0.07 0.04 0.1 0.05 0.1

0.644 0.653 0.45 0.29 0.42 0.45 0.47 0.27

± ± ± ± ± ± ± ±

0.004 (100%) 0.002(100%) 0.03 (39%) 0.02 (16%) 0.01 (100%) 0.01 (46%) 0.01 (33%) 0.01 (14%)

χ2

DW

1.10 1.12 0.93 0.92 1.14 1.07 1.10 0.98

1.96 1.76 1.68 1.85 1.74 1.95 1.71 1.73

Figure 4. 1,1′-Diethyl-2,4′-cyanine incorporated into SYn-1. (a) Fluorescence emission spectra λexc = 490 nm with slit widths were set to 5 nm. (b) Fluorescence decays and Ludox scattering solution. (c) Confocal fluorescence images excited at 488 nm for 0.01% ICY. (d) 0.1% ICY and (e) 1% ICY.

aqueous solution exhibited a low fluorescence emission (Φ < 0.5%) with maxima at 565 and 605 nm (Figure 1) due to the rotational relaxation of the molecules, which competes with the radiant relaxation.23,41 UV−vis DR spectra of ICY incorporated onto SWy-1 and Lap show three absorption bands at 556, 518, and 485 nm (Figure 2a,b) for all dye loading. The absorptions at 556 and 518 nm were attributed to the monomeric species of 1,1′diethyl-2,4′-cyanine. The third band in the higher-energy region (485 nm) suggests that the dye adsorption on the clay particles results in the formation of H-type aggregates of the ICY molecules. The H-type aggregation represents a sandwich-type stacking of the ICY molecules adsorbed on SWy-1 and Lap particles. A similar H-type aggregation has been observed in the case of cationic dyes (auramine O,15 pseudoisocyanine,29 and pinacyanol34) adsorbed on host surfaces. Figure 2c,d shows the UV−vis DR spectra of ICY in the presence of SAz-1 and SYn-1 montmorillonites, respectively. It is possible to observe, for lower contents of ICY (0.01−0.5 wt %), the band assigned to monomer in the range from 546 to

556 nm. In the SAz-1 samples, the H-aggregate band was observed at 470 nm, and a new band at 573 nm appears, which can be assigned to ICY J-aggregate formation on clays.42 For SYn-1 clay, the results showed that at lower ICY loading (0.01−0.5 wt %), the formation of J-aggregates (580 nm) was favored. ́ According to Czimerová et al.,43 the CEC is proportional to the layer charge density of the clays. It is worth noticing that the formation of J-aggregates of the ICY was favored in the presence of SYn-1 and SAz-1 clays, which shows a higher CEC and consequently a higher layer charge density.43 It has been reported6,14,35 that the layer charge density of clays affects the Jaggregation of dyes such as cyanine and its derivatives. A lower charge density may induce greater distances between neighboring dye molecules adsorbed at the clay particles, which promotes the suppression of ICY aggregation.28 For SYn-1 and SAz-1 clays, the spectra show that the Haggregates band (482 nm) increases with the loading of ICY (5 and 10 wt %). In these cases, the J-band at 573 nm becomes D

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Figure 5. Confocal fluorescence micrographs (λexc= 488 nm) for (a) 0.1 wt % ICY/SAz-1 and (b) 0.1 wt % ICY/SWy-1.

less clear, indicating that the H-aggregate type was predominant. Further information about the adsorption of ICY on the clays was obtained from fluorescence measurements (Figure 3a−e). The ICY molecules in aqueous solution exhibited low fluorescence emission; however, ICY monomer in the presence of Lap (from 0.01 up to 0.1 wt % dye concentration) exhibits a higher emission band (Φ = 10.1%) with a maximum at 560 nm (Figure 3a). In these concentrations of ICY, the fluorescence emission in the micrograph is attributed to the monomers. The monomer spectra obtained under the microscope (insets in Figure 3c,d) were quite similar to fluorescence spectra in Figure 3a. Figure 3b shows the fluorescence decay of 0.01 wt % ICY on Laponite and Ludox scattering solution, from which the lifetime was determined to be 4.63 ns (Table 1). The fluorescence micrographs indicate that the adsorption of ICY molecules into the clay surfaces generates a highly luminescent material (Figure 3c,d). This phenomenon is due to the interaction of ICY molecules with the clay surface, which restricts the degree of freedom of the dye. Therefore, this restriction decreases the degree of the radiationless process, providing high fluorescence emission.10,15,41 With increasing ICY concentration (at 1% ICY), J-aggregates were detected. It is possible to see that ICY monomers coexist homogeneously with J-aggregates (Figure 3e). It has been proposed that clay minerals with smaller particle size such as Laponite (particle size estimated to be about 25 nm)44 cannot offer enough spaces for the formation of J-aggregates.45 However, fluorescence emission was observed for these hybrids, suggesting that the smaller J-aggregate domains are present on the Laponite particles.46 Moreover, the fluorescence lifetime of 1% ICY/Lap shows two components, and it can be related to two luminescent species (Table 1). The longer fluorescence lifetime can be assigned to the ICY monomers, and the second component has a short lifetime value and may be attributed to J-aggregate emission species.47 The fluorescence intensity and lifetime of ICY monomers decrease with increasing dye content due to the formation of nonfluorescent structures (Haggregates), which quenched the fluorescence of the monomers. A lower fluorescence lifetime can be promoted by a faster deactivation process from the fluorescence excited state; the presence of aggregates generates extra deactivation pathways.9 A similar effect was observed for Auramine O adsorbed on montmorillonites15 and films of rhodamine 6G adsorbed on Laponite.17

It can be seen in Figure 4a, for 0.01% ICY/SYn-1 hybrids, that the fluorescence spectrum with two emission bands at 560 and 600 nm correspond to monomers of ICY. Only the emission from monomeric species can be observed in the micrograph, indicating no J-aggregates (Figure 4c). The lifetime (Figure 4b) and Φ determined for the ICY monomer in SYn-1 were 4.16 ns and 6.9%, respectively. Upon increasing the ICY concentration to 0.1%, the band at 560 nm decreases and the band at 600 nm increases because of J-aggregate formation (Figure 4a). The nature of this additional band at 600 nm has been ascribed to the J-aggregate fluorescence emission.16 This behavior can be confirmed by fluorescence micrographs, which revealed two different fluorescent domains. Monomers (greenish domain) and Jaggregates (orange domain) can be clearly distinguished by their own color in the image (Figure 4d). The inset in Figure 4d shows the fluorescence spectra for different regions obtained under the microscope. The increase in ICY loading also leads to a significant change in the fluorescence lifetime (Table 1). The lifetime of the short component becomes faster (0.63 down to 0.37 ns) when the loading of ICY was increased from 0.1 to 1 wt %. This reduction in the short component is indicative of a superradiant state, which is evidence for the formation of J-aggregates.42 As observed for ICY/Lap hybrids, the presence of H-aggregates in higher ICY concentrations adsorbed on SYn-1 promoted shorter lifetimes for monomers. At 1 wt % of ICY on SYn-1, the orange J-aggregates emission is predominant in Figure 4e, and a new reddish domain appears. The fluorescence spectrum of these reddish domains showed an emission band at 640 nm. According to Demir et al.,42 the nature of this emission band arises from different types of dye stacking faults or defects in the molecular arrangement at a high concentration of dye. For ICY adsorbed on SWy-1 and SAz-1, it was not possible to measure the fluorescence lifetimes due to the very low fluorescence intensity. This behavior is due to the fact that SAz1 and SWy-1 montmorillonites are naturally produced, containing an iron component in the form of Fe2O3. The Fe3+ cations are known to act as an efficient quencher of the excited singlet state of the dye molecules.48 For these clays, the fluorescence emission is weak and can be detected only by fluorescence microscopy. Figure 5a,b presents the fluorescence images of 0.1 wt % ICY/SAz-1 and 0.1 wt % ICY/SWy-1, respectively. For ICY adsorbed on SAz-1, the slight emission of fluorescence is E

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Figure 6. X-ray pattern of clays and ICY/clays.

enhance the photophysical properties of the dye but improve the thermostability. The thermal properties of ICY/clays hybrids were verified by thermogravimetic analysis. The TGA curves were obtained for pure clays, ICY, and 10 wt % ICY/clay hybrid materials. The TGA/DTG data obtained for the pure ICY under a nitrogen atmosphere (Figure 7) showed that there was no weight loss below 200 °C and there was a drastic weight loss at 276 °C, which was associated with the molecule decomposition.

attributed to the presence of monomers and J-aggregates. In the case of ICY on SWy-1, the weak emission was related to monomers only. Structural Characterization and Thermal Properties of ICY/Clay Hybrids. Figure 6a,b shows the XRD pattern of SAz1, SWy-1, and their ICY hybrids. The basal spacing values for the clays and the respective hybrids are shown in Table 2. Pure Table 2. Interlamellar Spacing for Clays and ICY/Clays d (Å) clay

0 wt %

0.01 wt %

0.1 wt %

1 wt %

5 wt %

10 wt %

SAz-1 SWy-1 SYn-1 Laponite

12.9 11.6 11.3 14.9

13.2 11.4 11.4 14.2

12.8 11.5 11.4 14.7

13.1 12.3 11.4 15.7

13.7 12.4 11.5 15.3

14.8 13.5 11.9 15.1

SAz-1and SWy-1 exhibit reflection peaks at 2θ = 6.9 and 7.6°, respectively. These reflection peaks correspond to a basal spacing (d) of 12.9 Å for SAz-1 and 11.6 Å for SWy-1. For the ICY/SAz-1 and ICY/SWy-1 hybrids, an increase in the basal spacing was observed with increased loading, showing that the adsorption of the dye occurs not only in the external surface but also in the interlayer region of these clays. The basal spacing for ICY/SYn-1 and ICY/Lap remains the same as for the pure clays. Nevertheless, a decrease in the intensity of the d(001) signal with the increase in ICY loading was observed (Figure 6c,d), suggesting that ICY adsorption on clays promotes a partial delamination of layers, which increases the surface area available for ICY adsorption. For applications such as photonics, the incorporation of a compound in a solid host material in general should not only

Figure 7. TGA/DTG curves for pure ICY.

Figure 8a−d show the TGA/DTG curves for SWy-1, SYn-1, SAz-1, and Laponite clays and their respective ICY/clay hybrids. For all of the clays, two weight losses were observed: (i) at low temperatures (