Preparation and Characterization of Rhodamine 6G

9 Dec 2008 - Preparation and Characterization of Rhodamine 6G/Alkyltrimethylammonium/Laponite Hybrid Solid Materials with Higher Emission Quantum ...
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J. Phys. Chem. C 2009, 113, 415–421

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Preparation and Characterization of Rhodamine 6G/Alkyltrimethylammonium/Laponite Hybrid Solid Materials with Higher Emission Quantum Yield Ryo Sasai,*,†,‡ Takanori Itoh,† Wataru Ohmori,† Hideaki Itoh,‡ and Michiko Kusunoki‡ Department of Applied Chemistry, Graduate School of Engineering, and DiVision of EnVironmental Research, EcoTopia Science Institute, Nagoya UniVersity, F3-3(250), Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

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ReceiVed: June 13, 2008; ReVised Manuscript ReceiVed: NoVember 1, 2008

To realize the solid materials with higher emission quantum yield from the rhodamine 6G (R6G) monomers, typical laser-dye, we hybridized both alkyltrimethylammonium (CnTMA+) and R6G cations into the laponite (Lap) interlayer nanospace and investigated their photoemission and nanostructure. The hybrid solid materials were prepared by dissolving an appropriate amount of CnTMA+Br- powder with Lap ethanol/aqueous (1/1 v/v) suspension with 0.1% or 0.5% cation-exchange capacity (CEC) of R6G cations. From X-ray diffraction (XRD) and 13C cross-polarization magic-angle spinning (CP MAS) NMR of the present solid materials, it was found that CnTMA+ cations were incorporated as the trans-gauche form into the Lap interlayer nanospace and R6G cations were intercalated parallel to the surface of Lap nanosheet. Emission quantum yield, φ, of the R6G/CnTMA+/Lap solid materials became higher than that of R6G/Lap and increased with increasing carbon number in alkyl chain, n, and was a constant as ca. 80% beyond n > 10. The R6G/C16TMA+/Lap solid material prepared under optimal conditions exhibited ca. 80% φ, even when the molar concentration of R6G cation was 10-2 mol/dm3. Introduction Luminous dye is an essential compounds for display, lighting, laser, and paint, utilizing organic compounds as base functional materials. Now, luminous dyes/compounds are attracting much attention since Kido et.al.1 reported on novel organic electroluminescence materials and system. It is well-known that most luminous dyes lose fluorescence ability in the high concentration range due to intermolecular interactions and/or readsorption phenomena, even when dyes are dissolved in solvent. Thus, it is very difficult to prepare luminous materials with high performance in the solid state, generally. In dye-laser systems, a high-concentration dye solution was usually required but was undesirable for gaining the high quantum yield. Most current dye-laser systems are of the circulation type, in which dilute dye solution is dissolved in some toxic organic solvents, and thus, there are some problems: difficulties of downsizing the module, large amount of toxic waste solvent, and so on. Development of luminous solid materials with high emission intensity and/or quantum yield will be a better way to solve these problems. However, a simple solid of the dye compounds cannot be achieved because of strong intermolecular interaction among dye molecules. Thus, many researchers have investigated solidification of dye by fixing dye molecules in various host materials such as polymers,2-10 silicas,11-18 clay,19-31 and so on.32 Some researchers have succeeded in preparing luminous solid materials with lasing ability, but their efficiency was still lower than that of dilute dye solution because the formation of dye aggregates cannot be fully inhibited. Clay is a natural product and has been applied in various industries such as civil engineering, cosmetics, pharmaceuticals, ceramics, and plastics.33-37 It is well-known that clays such as * To whom correspondence should be addressed: phone/fax +81-52789-5859; e-mail [email protected]. † Department of Applied Chemistry, Graduate School of Engineering. ‡ Division of Environmental Research, EcoTopia Science Institute.

smectite have a layered structure, have exchangeable cations such as sodium, potassium, and calcium ions in the interlayer space for compensating excess layer charge, and have complete lamination in aqueous media.33-37 As a result of these properties, various ionic/polar organic compounds can be easily intercalated in the two-dimensional (2D) interlayer nanospace by ionexchange reaction and enlargement of interlayer clearance. Moreover, it is also well-known that the intercalated molecules form an ordered structure in the 2D interlayer nanospace spontaneously; that is, this indicates that solid materials with unique functions, such as anisotropic, nonlinear, and nonequilibrium properties, can be easily prepared. Considering these useful characteristics, many researchers have studied the hybridization of various functional organic compounds with ionexchangeable layered clay and those various characterizations.19-32,35-63 Additionally, clay has no absorption band in the visible range, and many researchers have also studied the incorporation of various photofuntional organic compounds in the interlayer nanospace. Thus, the hybridization of laser-dye molecules with various clays such as laponite, montmorillonite, saponite, and so on, has been investigated by many researchers.19-32,40-56,59-63 Such hybridization systems of laser-dye and clay are very effective for clarifying the basic principles for function/structural control of the organic/clay hybrid materials such as the adsorption state/behavior of laser-dye, the steric/ electronic interaction between laser-dye and clay, and so on. From the point of view of the optical function of laser-dye/ clay nanohybrid solid materials, these hybrid materials can make it possible to convert the lasing medium of dye-laser oscillator from liquid to solid. However, it is difficult to observe monomeric emission from laser-dye/clay hybrid solid materials, even when the intercalation amount of laser-dye against the cation-exchange capacity (CEC) of clay is extremely low. The reason is that dye aggregates without luminescent ability, for example, head-to-head aggregates, are easily formed. Recently, we succeeded in preparing solid materials with highly mono-

10.1021/jp805201n CCC: $40.75  2009 American Chemical Society Published on Web 12/09/2008

416 J. Phys. Chem. C, Vol. 113, No. 1, 2009 meric luminescence ability from rhodamine 6G by using montmorillonite, a clay belonging to the smectite group, as inorganic host.24,26,29 Alkyltrimethylammonium salts used in this hybrid system play the role of aggregation inhibitor of the intercalated rhodamine 6G molecules in the montmorillonite interlayer nanospace. Moreover, it was found that an alkyltrimethylammonium salt with a longer alkyl chain exerted a higher effect. The montmorillonite we used was a natural product, with ca. 2 mass % Fe species as impurity, which is known to work as a quencher. Thus, the emission of this hybrid solid material was quenched and did not exhibit high emission quantum yield, although monomeric emission from the intercalated rhodamine 6G molecules could be observed. To develop hybrid solid materials with higher emission quantum yield from rhodamine 6G, synthetic smectite clays with high purity are required. Thus, we used laponite XLG, which is synthesized and sold by Rockwood Additives, as the smectite clay host. In this study, to prepare the luminescent solid materials with as high emission quantum yield as rhodamine 6G dilute solution, we attempted to hybridize rhodamine 6G and/or alkyltrimethylammonium salts with laponite by ion-exchange method. Then, the structural and optical properties of the prepared solid materials were investigated in detail. Finally, we could find out the optimal conditions for preparing the rhodamine 6G/alkyltrimethylammonium/ laponite hybrid solid materials with higher emission quantum yield, such as carbon number in alkyl chain and intercalation amount of alkyltrimethylammonium salts. Moreover, we compared the prepared solid materials with dilute solution, and could present the superiority of our materials as photoemitting solid materials. Experimental Section Materials. Laponite XLG {abbreviation Lap; structural formula Na+0.7[(Si8Mg5.5Li0.3)O20(OH)4]-0.7; cation-exchange capacity (CEC) 0.77 mequiv/g; purchased from Rockwood Additives Ltd.; mean particle diameter ca. 25 nm; thickness of particle 0.92 nm} was used as layered clay host without further purification. Rhodamine 6G (abbreviation R6G, purchased from Exciton) was used as laser dye without further purification. Alkyltrimethylammonium salts, CnH2n+1N+(CH3)3Br-, with n ) 6, 10, 12, and 16 (abbreviation CnTMA+, purchased from Tokyo Chemical Industries Co. Ltd.) was used as surfactant, which served as dye-aggregation suppressor, without further purification. The highest grade reagents were used for all other chemicals without further purification. Distilled deionized water was used for all experiments. Sample Preparation. Lap powder (0.5 g) was suspended in 0.5 dm3 of water for exfoliation of Lap nanosheet. After 0.5 dm3 of R6G ethanol solution was added to the Lap aqueous suspension, where [R6G] ) 0.1% or 0.5% CEC, this mixed suspension were stirred for 30 min at room temperature; we called this suspension SUS1. (1) R6G/Lap powders were prepared by completely evaporating water from SUS1. (2) To prepare R6G/CnTMA+/Lap powders, CnTMA+Brpowder was added to SUS1 and dissolved ([CnTMA+] ) 300% CEC). After this suspension was stirred for 5 min at room temperature, R6G/CnTMA+/Lap powder was collected by filtration under reduced pressure. After being washing with water several times, the collected powder was dried at 60 °C overnight and at 30 °C for 2 h under reduced pressure. From CHN elemental analyses of the prepared R6G/CnTMA+/Lap, the intercalation degree of CnTMA+ cations into the Lap was ca. 70% CEC, independent of the n value. Moreover, all of the

Sasai et al. SCHEME 1: Flow Chart for Sample Preparation Procedure

added R6G cations were incorporated into the Lap interlayer space in the absence and presence of CnTMA+ cations. (3) R6G/C12 or 16TMA+/Lap powders with different intercalated degrees of CnTMA+ cations were prepared as follows: (3-1) For R6G/C12 or 16TMA+/Lap powders with 25% and 50% CEC of C12 or 16TMA+, 25% or 50% CEC of the C12 or 16TMA+Br- powder was added to SUS1 and dissolved. After this suspension was stirred for 5 min at room temperature, it was evaporated until red precipitates could be observed. R6G/ C12 or 16TMA+/Lap powders with 25% and 50% CEC of C12 or 16TMA+ was collected by filtration under reduced pressure, and then the collected precipitate was dried at 60 °C overnight and at 30 °C for 2 h under reduced pressure after being washed with water several times. (3-2) For R6G/C12 or 16TMA+/Lap powders with more than 70% CEC of C12 or 16TMA+, 300% CEC of the C12 or 16TMA+Br- powder was added to SUS1 and dissolved. After this suspension was stirred for 5 min at room temperature, it was evaporated until the residual solvent decreased down to an appropriate volume. R6G/C12 or 16TMA+/ Lap powder with more than 70% CEC of C12 or 16TMA+ was collected by filtration under reduced pressure, and then the collected precipitate was dried at 60 °C overnight and at 30 °C for 2 h under reduced pressure after being washed with water several times. In Scheme 1, the procedure for preparing R6G/ CnTMA+/Lap powder is shown. In Table 1, the amount of intercalated R6G and C12 or 16TMA+ cations compared with the CEC of Lap, as well as gallery clearance L, are shown. Here, it was found that excess C12 or 16TMA+ was intercalated as salt from elemental analysis by energy-dispersive X-ray spectroscopy (data not shown). However, the amount of intercalated CnTMA+ was calculated as cation in all cases in Table 1. Characterization. The intercalation degree of CnTMA+ cations was estimated from CHN elemental analysis (MTA-

R6G/CnTMA/Lap Hybrids with Higher Quantum Yield

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TABLE 1: Amount of Intercalated R6G and C12 or 16TMA+ and Gallery Clearance of Prepared R6G/C12 or 16TMA+/Lap Samples C16TMA+ [CnTMA+] (% CEC) [R6G] (% CEC) gallery clearance, L (nm)

25 0.5 0.48

48 0.5 0.52

78 0.5 0.55

87 0.5 0.54

620, Yanaco). The intercalation degree of R6G cations was estimated from absorbance at 530 nm of filtrate solution measured on a Jasco V-550 UV-vis spectrophotometer. X-ray diffraction analysis of the sample powders was carried out with a RINT 2500 diffractometer (Rigaku) by use of Ni-filtered Cu KR radiation (50 kV and 100 mA). 13C cross-polarization magicangle spinning (CP MAS) NMR measurement of the sample powder was carried out with an Avance 300 (Bruker) with a 7 mm MASBB/1H probe. Photoemission spectra of the sample powders were recorded in the region of 500-700 nm on an F-4500 fluorospectrometer (Hitachi). Emission quantum yield of the sample powder excited at 500 nm monochromic light was evaluated from C9920-02 emission quantum yield measurement apparatus (Hamamatu Photonics). Results and Discussion Nanostructure of R6G/CnTMA+/Lap Solid Materials. In Figure 1, gallery clearance L [calculated by subtracting the thickness of Lap nanosheet, 0.96 nm, from the basal spacing estimated from the (001) diffraction peak] of prepared R6G/ CnTMA+/Lap powders with 0.1% or 0.5% CEC of [R6G] and ca. 70% CEC of [CnTMA+] was plotted against the carbon number, n, in the alkyl chain of CnTMA+ cation. The L values of all prepared powders were larger than that of the Lap powder. This implies that R6G and/or CnTMA+ cations were incorporated into the Lap interlayer nanospace by our preparation procedures. The L value of the R6G/Lap materials with 0.1% and 0.5% CEC of R6G was ca. 0.43 nm. The thickness of the xanthene ring in R6G cations is 0.4-0.5 nm,21 because slight flattening of the R6G molecule will occur due to the intercalation [it is evidence of this flattening that the fluorescence peaks of dye monomers show a large red shift (cf. Figure 4)]. This value was almost same as the L value. Thus, the R6G molecules are intercalated almost parallel to the surface of the Lap layer as its xanthene ring. Moreover, incorporation of CnTMA+ increased the L values to ca. 0.53 nm, even when the length of the alkyl chain of CnTMA+ was varied. This value is larger

Figure 1. Dependence of gallery clearance on n (carbon number in alkyl chain) of CnTMA+Br-. (b) 0.1 and (9) 0.5 mol % R6G molecules vs CEC of Lap. n ) 0 means R6G/Lap powder. [CnTMA+] ) ca. 70% CEC.

C12TMA+ 123 0.5 0.61

143 0.5 0.81

168 0.5 1.12

25 0.5 0.51

50 0.5 0.49

99 0.5 0.51

than the thickness of CnTMA+ cation (0.47 nm). These results indicate that the CnTMA+ cations are intercalated almost parallel to the surface of the Lap layer as its alkyl chain, too. Figure 2 shows the 13C CP MAS NMR spectra of prepared R6G/CnTMA+/Lap with 0.1% or 0.5% CEC of [R6G] and ca. 70% CEC of [CnTMA+] and C16TMA+Br- powders. For C16TMA+Br- powder, a sharp peak could be observed at ca. 35 ppm. It is well-known that this peak originates from the alltrans structure of alkyl chain. A double peak could be observed in the range from 30 to 40 ppm in spectra of the prepared R6G/ CnTMA+/Lap powder. This newly appeared peak observed at ca. 33 ppm is well-known as the signal from the gauche form of the alkyl chain. From analysis of the NMR spectra, the intensity of peaks from alkyl chain of the trans and gauche forms was almost the same; that is, the abundance of trans and gauche forms in the alkyl chain would be approximately equal in R6G/ CnTMA+/Lap. Schematic structure models of R6G/CnTMA+/Lap with 0.1% or 0.5% CEC of [R6G] and ca. 70% CEC of [CnTMA+] and R6G/Lap nanohybrid materials are shown in Figure 3. In the interlayer nanospace of Lap, the xanthene ring of the intercalated R6G cation would be parallel to the layer surface of the Lap regardless of the absence or presence of CnTMA+ cations. The CnTMA+ cation could be intercalated as the trans-gauche mixed form, and its alkyl chain would be parallel to the layer surface of the Lap, even when the n value was varied. The layer surface area required by these intercalated molecules was ca. 1.6 nm2. This value is slightly larger than the surface area per cation-exchange site. However, it is assumed that both cationexchangeable sites on upper and lower layer surfaces face each other, but these sites actually slip off. The conformation of the incorporated C16TMA+ is not all-trans but trans-gauche, and then the exclusive area becomes less than 1.2 nm2. Moreover, both R6G and CnTMA+ cations will be incorporated in the same interlayer nanospace but not segregated, because only a single series of basal diffraction could be observed in XRD patterns

Figure 2. 13C CP MAS NMR spectra of (a-d) R6G/CnTMA+/Lap powders, where n ) 16, 12, 10, and 6, respectively, and of (e) C16TMA+Br- powder. [CnTMA+] ) ca. 70% CEC.

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Figure 3. Schematic nanostructural model of (a) R6G/C16TMA+/Lap and (b) R6G/Lap. [CnTMA+] ) ca. 70% CEC and [R6G] ) 0.1% CEC.

of the R6G/CnTMA+/Lap powders. Thus, it is expected that the R6G cations are highly dispersed as monomer species in the Lap interlayer space; that is, the present nanohybrid powder exhibits light emission from R6G monomers with higher emission quantum yield. Spectroscopic Properties of R6G/CnTMA+/Lap Solid Materials. Figure 4 shows typical normalized emission spectra of R6G cations in aqueous solution ( · · · ) and in CnTMA+/Lap interlayer nanospace (s). The emission spectrum of R6G cations dissolved in aqueous solution originated from R6G monomer state, because the concentration of R6G cations is extremely low at 1.0 µmol/dm3. The emission spectrum of R6G cations incorporated in the CnTMA+/Lap interlayer nanospace exhibited a bathochromic effect, but its shape was similar to the spectrum of extremely dilute R6G aqueous solution. Moreover, such tendency could be observed in the emission spectra of all R6G/ CnTMA+/Lap samples, regardless of the length of alkyl chain and R6G content. Thus, it can be concluded that the R6G cations exist as approximately monomers in the interlayer nanospace of all CnTMA+/Lap samples. The observed bathochromic effect

would be caused by electrostatic interaction between R6G cations and cation-exchange sites or by complexation of R6G molecules, because such bathochromic behavior has already been reported in other research papers.26,29 Emission quantum yield, φ, was plotted against n (carbon number in alkyl chain of CnTMA+) in Figure 5. For [R6G] ) 0.1% CEC, the φ values increased with cointercalation C6TMA+ up to ca. 80% and were almost constant, even when the n values increased. These results indicate that the cointercalation of CnTMA+cations can inhibit the emission quenching due to intermolecular interaction of R6G cations, such as aggregation and excimer formation, and then can generate light emission from R6G monomers in the solid state. Thus, it was found that the R6G/CnTMA+/Lap with 0.1% CEC of R6G exhibits high emission quantum yield, the same as the R6G dilute aqueous solution. Previously, we have studied the hybridization of R6G and C16TMA+ cations with montmorillonite (natural product, CEC ) 1.19 mequiv/g, 2 mass % Fe).26,29 In this previous case, it was found that fluorescence originating from the intercalated

R6G/CnTMA/Lap Hybrids with Higher Quantum Yield

Figure 4. Typical normalized photoluminescence spectra of ( · · · ) R6G molecules in aqueous solution ([R6G] ) 1.0 µmol/dm3, excitation λ ) 532 nm) and (s) CnTMA+/Lap ([R6G] ) 0.1% CEC, [CnTMA+] ) ca. 70% CEC, excitation λ ) 532 nm).

Figure 5. Dependence of emission quantum yield, φ, on n (carbon number in alkyl chain) of CnTMA+Br-. (b) 0.1 and (9) 0.5 mol % R6G molecules vs CEC of Lap. n ) 0 means R6G/Lap powder. [CnTMA+] ) ca. 70% CEC.

monomeric R6G molecules could be observed. Then, the diffusion reflectance spectrum of this hybrid indicated that the most of intercalated R6G could exist as monomers due to the C16TMA+ cations (data not shown), but its emission quantum yield was very low, ca. 20%. On the other hand, in the laponite case, the diffusion reflectance and fluorescence spectra also indicated that most of the intercalated R6G could exist as monomers in the interlayer space (cf. Figure 4), and its emission quantum yield was very high, ca. 80%. Such a large difference in the φ value could be seen by the difference of host. Although the charge density, orientation structure of the intercalants, and so on is different in comparing the montmorillonite and laponite cases, this large difference of the φ value could be caused by the Fe species included in the natural montmorillonite as impurity. Therefore, it is better to use the synthetic layered clay as host. For [R6G] ) 0.5% CEC, φ gradually increased with increasing n and then became constant at ca. 70% beyond n ) 12. These φ values of R6G/CnTMA+/Lap with 0.5% CEC of R6G were smaller than those for 0.1% CEC of R6G. This result indicates that ca. 70% CEC of the CnTMA+ cations cannot demonstrate enough effect on the inhibition of emission quenching due to the intermolecular interaction of R6G, such as aggregation and excimer formation, for [R6G] ) 0.5% CEC. Therefore, optimization of [CnTMA+] would be required to

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Figure 6. Dependence of emission quantum yield, φ, on the intercalated amount of CnTMA+ of R6G/CnTMA+/Lap powder. [R6G] ) 0.5% CEC. (0) n ) 12; (O) n ) 16.

prepare R6G/CnTMA+/Lap with 0.5% CEC of R6G with an emission quantum yield as high as the R6G dilute aqueous solution. Effect of Intercalation Amount of CnTMA+. In Figure 6, φ values of R6G/CnTMA+/Lap with n ) 12 or 16 were plotted against the intercalation amount of CnTMA+ cations, [CnTMA+]. The φ values increased up to [CnTMA+] ) ca. 50% CEC. This improvement of the φ value would be caused by disaggregation of R6G, because R6G and CnTMA+ cations could be fully compatible in the interlayer nanospace; that is, the incorporated CnTMA+ cation will play a role as inhibitor of R6G molecular aggregation. And then, whole R6G molecules would exist as monomers in the interlayer space at [CnTMA+] ) ca. 50% CEC, and thus, the highest φ value could be observed. The φ value decreased at more than 50% CEC. With an increase in [CnTMA+], it is predicted that the intercalated CnTMA+ cations form a self-assembled or self-organizing structure to reduce the total energy of this hybrid system, like a micelle. This self-assembly behavior of CnTMA+ cations can hinder full compatibility between R6G and CnTMA+ cations in the interlayer space. As a result, the intermolecular interaction among intercalated R6G cations was accelerated with an increase in [CnTMA+]. Thus, the φ value decreased with increasing [CnTMA+]. In conclusion, it was found that the optimal [CnTMA+] value was ca. 50% CEC for R6G/CnTMA+/Lap with n ) 12 or 16 and [R6G] ) 0.5% CEC. On the other hand, it was also found that the optimal [CnTMA+] value depended on the [R6G] value. This difference may be caused by the balance of interaction and/or affinity among R6G and CnTMA+ cations, but the correct reason is not still clear. Anyway, we succeeded in preparing R6G/ CnTMA+/Lap solid materials with higher CnTMA+ and [R6G] ) 0.1% or 0.5% CEC that have ca. 80% emission quantum yield. Comparison of Emission Quantum Yield of R6G/C16TMA+/ Lap Solid Material with R6G in Solution. Figure 7 compares the molar concentration dependence of the φ value of R6G in solid materials and in solution. In aqueous solution, the φ value of R6G rapidly decreased with increasing molar concentration of R6G and then became almost 0% at more than 10-3 mol/ dm3. In EtOH solution, more than 80% of the φ value was maintained up to 10-3 mol/dm3, but the φ value decreased down to ca. 40% at 10-2 mol/dm3. This lowering of the φ value in higher concentration range is well-known as “concentration quenching” and “quenching due to aggregation”. In contrast,

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Figure 8. Emission quantum yield, φ, of R6G/C16TMA+/Lap hybrid material with [R6G] ) 0.5% CEC and [C16TMA+] ) 70% CEC against dry/wet repetition procedures. Figure 7. Dependence of emission quantum yield, φ, on the molar concentration of R6G in (O) aqueous solution, (0) ethanol, and (b) R6G/C16TMA+/Lap powder. Inset photographs are photoluminated R6G/C16TMA+/Lap powders under black light irradiation: (a) [R6G] ) ca. 10-3 mol/dm3; (b) [R6G] ) ca. 10-2 mol/dm3. [CnTMA+] ) ca. 70% CEC.

over 60% of the φ value was maintained for the R6G/CnTMA+/ laponite hybrids, even when the molar concentration of R6G was as high as 0.015 mol/dm3, which is calculated from dividing the mole number of R6G molecules per cation-exchange site by the volume occupied by a cation-exchange site, V ) L (gallery clearance from XRD) × SA (surface area of cationexchange site, )1.5 nm2). The emission could be clearly observed by the naked eye under UV light irradiation (cf. insets a and b in Figure 7). Such clear and strongly incandescent emission of the present optimal solid materials is far ahead of other solid materials such as sol-gel silica glass and polymer film containing R6G molecules. Thus, we successfully developed novel solid materials with higher luminescent ability from R6G monomers intercalated in the Lap interlayer nanospace, even when the R6G concentration is much higher than in the solution system. Stabilization of Photoemission Ability under Humid Conditions. In applying the present luminous hybrid solid materials to practical luminous devices such as solid laser oscillator and luminous painting, stability to environmental changes is one of the important factors. In particular, the stability with respect to water adsorption (that is, relative humidity change of the surrounding environment) should be as high as possible. Because smectite clays and their hybrids generally exhibit high water adsorption ability, the effect of adsorbed water molecules on photoemission function was investigated. Thus, we investigated the photoemission of R6G/C16TMA+/Lap with [R6G] ) 0.5% CEC and [C16TMA+] ) 70% CEC under humid conditions. The preparation of dried and wet powder samples was carried out as follows: (1) Dried samples were prepared by vacuum drying at 30 °C for 2 h. (2) Wet samples were prepared by leaving the powder samples in humid atmosphere (relative humidity 50-60%). In Figure 8, the φ values were plotted against the repetition for both dry and wet procedures. This result indicates that no photoemission change from the R6G/C16TMA+/Lap could be observed and the photoemission is very stable for the dry/wet repetition. From the quantitative analysis of water content in both dry and wet samples by thermogravimetry, the water contents were 3.5 and 9.6 mass %, respectively. The hydrophobic field formed by the intercalated C16TMA+ molecules could be responsible for this small

water content, and thus, the state of the intercalated R6G molecules did not change, even when humidity changed. It was found that the present sample, R6G/C16TMA+/Lap with [R6G] ) 0.5% CEC and [C16TMA+] ) 70% CEC, was not affected by the humidity of the surrounding environment and was very stable material to humidity changes. Conclusions In the present study, it was found that solid materials with higher emission quantum yield, ca. 80%, could be prepared by incorporating both R6G and CnTMA+ cations in the 2D interlayer nanospace of Lap. The optimal amount of CnTMA+ cations depended on the intercalated amount of R6G cations, and thus it was found that R6G/CnTMA+/Lap with high luminescent ability could be prepared by control of the balance between the intercalated amount of R6G and CnTMA+ cation, even when the intercalated amount of R6G was relatively high. Moreover, we succeeded in preparing solid materials with high emission quantum yield in a higher concentration range than the solution case by the present hybridization. It was found that the present hybrid materials with C16TMA+ were very stable to humidity changes in the surrounding environment. Although the present materials could be utilized as stable fluorescent materials, they are difficult to utilize as raw materials for optical and luminescent devices such as dye lasers because all solid materials obtained in the present study were powders. As a future work, we will need to establish the technique to make a bulk material and/or film with high optical transparency. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Areas (417) and Grant-in-Aid for Young Scientist (B) (16750172) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT). We express our thanks for fruitful discussion and significant advices for the emission behavior by Professor F. L. Arbeloa and his laboratory members (University of the Basque Country UPV/EHU). References and Notes (1) Kido, J.; Kimura, M.; Nagai, K. Science 1995, 267, 1332. (2) Taylor, J. R.; Sibbett, W.; Corminer, A. J. Appl. Phys. Lett. 1977, 31, 732. (3) Dyumaev, K. M.; Manenkov, A. A.; Maslyukov, A. P.; Matyushin, G. A.; Nechitailo, V. S.; Prokhorov, A. M. J. Opt. Soc. Am. B 1992, 9, 143. (4) Rahn, M. D.; King, T. A. Appl. Opt. 1995, 34, 8260. (5) Deshpande, A. V.; Namdas, E. B. Chem. Phys. Lett. 1996, 263, 449.

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