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Fluorescence properties of the dye-intercalated smectite
J. Phys. Chem. Solids Vol. 47. No. 8, pp. 799-804. Printed in Great Britain.
1986
0022.3697186 $3.00 + 0.00 Pergamon Journals Ltd.
FLUORESCENCE PROPERTIES OF THE DYE-INTERCALATED SMECTITE T. ENDO, T. SATO and M. SHIMADA Department of Applied Chemistry, Faculty of Engineering, Tohoku University, Sendai, Miyagi 980, Japan (Received 18 November 1985; accepted 13 February 1986)
Abstract-The fluorescence properties of the dye Rhodamine 590 and Rhodamine 640 intercalated into montmorillonite were studied. All of the intercalation reactions were accomplished in an ethanol solution which contained the corresponding quality of Rhodamine. The TG-DTA and the X-ray diffraction data showed that the immobilization of Rhodamine depended on the amount of ethanol incorporated in the interlayer space of montmorillonite. The 14.8 A basal spacing of products indicated a conformation of Rhodamine with the xanthene nucleus and the phenyl group positioned parallel and perpendicular to the silicate layer, respectively. Also, the infrared pleochromism, which supported the proposed orientation of the phenyl group, was observed at 1250 and 712 cm-‘. The main fluorescence bands of both products were observed near 600 nm when excited by the 514.5 nm line of an argon ion laser. The results indicated that the mesomeric structure determined the fluorescence behavior, which was little different for the two products owing to the environment of the interlayer. These data are briefly discussed relative to the reaction conditions utilized for the synthesis of products. Also, the results support the idea that the products have a high potential for use as a host for a tunable laser. Keywords: Intercalation,
smectite, montmorillonite,
dye, rhodamine, luminescence, fluorescence, tunable
1. INTRODUCTION Studies concerned with the reactions in the interlayer of inorganic materials, that is intercalation reactions, are attractive for obtaining information on the interfacial profile between the organic and inorganic phases. Smectite and compounds with related layer structures often show swelling after absorbing organic species of many different kinds in the form of molecules or ions. Such behavior was compared to an absorption in which the “host” material was treating the “guest” material hospitably [l]. The intercalation reactions of smectites are influenced by the negatively charged aluminosilicate layers. In this context, previous work focused on the coordination chemistry of molecules or ions governed by the charge density and its distribution in the interlayer space. Occasionally, useful materials were developed as catalysts, for instance, petroleum cracking catalysts with a high yield of gasoline owing to the geometrical selectivity and high acidity of the interlayer surface [2, 31. Also, applications as ion exchangers, an electrode and electrolite for batteries have been widely developed. These “intercalation” chemistry studies seemed to be motivated by the unusual reaction environment provided by the geometrical space and surfaces in the interlayer. On the other hand, the interlayer environment may alter the character of the “guest” material and/or the complex itself. Here, we report on the synthesis of smectite intercalates containing xanthene dyes. These dyes, well-known examples being Rhodamine 590 and Rhodamine 640, are useful as laser P.C.S47,&-E
materials quencies.
being
tunable
over a wide range
2. EXPERIMENTAL
of fre-
PROCEDURE
2.1. Material and preparation Natural Na-montmorillonite with a cation exchange capacity (CEC) of ca 115 meq/lOO g was obtained from Kunimine Industry Co. After suspending in a 10 vol.% ethanol-90 vol.% water solution, the largest fraction of starting material was freeze-dried, or pipetted on to an aluminum plate and dried overnight in air to form a thin film sample. In typical experiments, Rhodamine 590 and Rhodamine 640 were used without further purification because the purity and quality of each dye were dye-laser grade. The dye-clay complexes were prepared by dispersing the montmorillonite in the ethanol solution containing the dye, at a concentration corresponding to 2.5 and 1.5 times the CEC, respectively. The present experimental procedure leads to an almost complete intercalation at room temperature in 2-7 days, or at 75°C for 2-3 days when the concentration of the corresponding dye is conserved by use of a reflex condenser. The complex was recovered by filtration and washed several times with ethanol to eliminate excess dye, and then dried in air.
799
2.2. Physical
measurements
DTA-TG measurements were carried out with a Rigaku Denki Thermal Analyzer, which was oper-
800
T. ENDO et al.
Fig. 1. SEM photographs
of clay (a) before and (b) after intercalating of Rhodamine
ated at heating rates of 10 and 2O”C/min from room temperature to 700°C in air. Fmely ground samples were used with a-A@, as a reference throughout. Before and after the intercalation reaction, the products were examined by X-ray diffraction, scanning electron microscope (SEM) and infrared spectra. X-ray diffraction patterns were obtained by a Rigaku Denki X-ray diffractometer with Ni filtered CuKa radiation. The SEM photographs were taken with a JSM-TZO of Jeol Ltd. Infrared absorption spectra were obtained with a Perkin-Elmer Model- 237B grating infrared spectrophotometer. The Pleochrorsm effect was measured by use of the thin film sample. Electronic absorption spectra and emission spectra of the dye-clay complexes were measured at
590.
room temperature by using a Hitachi 850 Fluorescence Spectrophotometer, which was operated over a wave length of 300-750 rim. Luminescence spectra excited by the 514.5 nm line of an argon ion laser were measured at room temperature and liquid double helium using a temperature by Czerney-Turner scanning monochromator Model 1402 Fluorolog Spex Spectrofluorometer.
3. RESULTS AND DISCUSSION On addition of Na-montmorillonite to the Rhodamineethanol solution, the color of the solution gradually faded within a few hours, even when
Fluorescence properties of the dye-intercalated smectite at room temperature. The products showed intense colors, namely, bright red for Rhodamine 590 and purple for Rhodamine 640. The results can be interpreted in two ways; one is due to the chemical or physical absorption on the surface of clay grains, and/or the other is a result of the interlayer absorption. The results of the powder X-ray diffraction measurements showed that the (001) reflections from the products changed in a stepwise manner in the range from 14.8 and 16.7 A. Since the thickness of the silicate layer is estimated ideally to be cu 9.6 A, the increase in basal spacing was attributed to the swelling, which resulted from the intercalation of the dye and/or the ethanol used as solvent. Figure 1 shows the SEM photographs of clays before and after the intercalation of Rhodamine 590. As the distinctive profile, some slits were observed along the silicate sheets in the intercalated sample. This fact seemed to result from the mechanical stress induced by the swelling of the layers. A large number of slits might be arranged with a separation corresponding to more than 100 layers of basal spacings. In the TG-DTA data for the Rhodamine 590 product, endothermic and exothermic changes were observed in the regions from 7&l 15°C and 39%42O”C, respectively. No thermal changes were observed relative to the melting of Rhodamine 590, whose temperature was reported to be ca 258°C [5]. The exothermic peak was observed with little or no change of weight, whereas the endothermic peak was correlated with a fairly large (5-6%) loss of weight. The color of the product remained a bright red up to 35O”C, and then it turned immediately to black as the resuIt of burning above 400°C. These DTA features were attributed to the evaporation of ethanol, and the burning and graphitization of Rhodamine 590 in air, respectively. The thermal stability of the products were examined by X-ray diffraction. Figures 2(a) and (b) show the X-ray patterns of the product before and after heating treatments at 125°C. The clay complex before heating shows an intense and broad peak given as the mean value, which results from the overlap of several sharp peaks. On the other hand, the (001) reflection of the clay complex after heating was satisfactorily separated into two peaks at 14.8 and 9.6& the latter corresponding to an unintercalated clay. The results indicated that ethanol also contributed to the swelling of the interlamellar clay. Therefore, we consider that the quantity of ethanol in the intercalation reaction was controlled by the reaction temperature conditions. When the reaction temperature is increased, the amount of ethanol in the interlayer tended to exchange readily with the dye in the ethanol solution. This result indicated that intercalation at higher temperature conditions was more profitable for packing the dye tightly in the interlayer. The basal spacing of all the intercalates formed by
801
Fig. 2. The (001) reflections of the products, (a) before and (b) after heating treatment at 125°C.The intercalation of the product was carried out at room temperature. The (001) reflection of the product after intercalating at 75°C (c). reaction in ethanol at 75°C exhibited a (001) spacing of ca 14.8 A as shown in Fig. 2 (c). As described by Serratosa [6], the observation of IR pleochroic effects can be an effective method for determining the orientation of phenyl groups intercalated between the siIicate sheets. In infrared absorption data, all bands arising from Rhodamine 590 were faithfully reproduced with slight shifts or splittings. In particular, the two bands at 1250 and 712 cm-’ could be assigned to in-plane and out-ofplane vibrations due to the dipole moment characteristic of the phenyl group, and they exhibited significant pleochtoism when the plane of an oriented film sample was inclined relative to the path of the infrared beam. As seen in Fig. 3, the band at 1250cm-’ decreased in intensity while the band at 712 cm-’ increased in intensity. These observations suggest that the carboxy-phenyl groups are held in a position almost perpendicular to both the silicate layer and the xanthene nucleus, owing to the steric hindrance of the carboxyl group. When the ethanol was expelled from the interlayer, the Rhodamine molecule, which became rigid, underwent somewhat of a tilting of the phenyl ring. This is suggested by the difference between the observed (ca 5.2 A) and calculated (ca 5.6 A) values for the size of the carboxy-phenyl group. The conformation of the Rhodamine 590 molecule intercalated is schematically illustrated in Fig. 4. The X-ray diffraction data of products after heating above 400°C indicated the collapse of spaces in
T. ENDS et al.
802
Wove
number (cm-‘)
Fig. 3. Selected infrared absorption spectrum of the Rhodamine SO-clay complex for two angles of (90”) and (-- 45”) to incident beam.
the interlayer space, so that the (001) reflection was abruptly shifted to 12.612.3 A. In addition, no clay complex with a basal spacing of 14.8 A could readsorb water molecules on the surface of the interlayer, and swell further. In the visible and ultra-violet spectra, the main absorption band was observed at 520 nm for the Rhodamine 590-clay and at 575 nm for the Rhodamine 640-clay. On comparing the data of Rhodamine in ethanol, we observed bathochromic shifts of 3-7 nm due to weak electric binding of the dyes to the silicate layers. The small amount of ethanol which remained in the interlayer had only a minor influence on the absorption spectrum. Figure 5 shows the fluorescence spectra of the Rhodamine 590-clay complex at room temperature and 10 K, which was excited by the 514.5 nm line of an argon ion laser. It can be seen that the peak profile becomes sharp and looks correctly like that of a Rhodamine-ethanol solution at 10 K. The maximum quantum peak was also given at ca 645 nm for the Rhodamine 640-clay. As for the results of fluorescence spectra of both products with ethanol in the interlayer, it was reasonable to assume that the position of the fluorescence maximum had shifted, because changes are known to occur as a function of concentration, temperature and viscosity of the solvent.
Fig. 4. Schematic illustration of Rhodamine 590 oriented in the interlayer of montmorillonite. are omitted for clarity.
Protons
Fluorescence properties of the dye-intercalated smectite
803
(bl
I
550
I
L
600
650 Wovelength
I
I
650
600 (m-n1
Fig. 5. The quantum spectrum of fluorescence for Rhodamine 590-montmorillonite
(arbitrary units) (a)
at room temperature, (b) at 10K.
Figure 6 shows the fluorescence spectra of the products without ethanol in the interlayer. There was a maximum quantum peak at 600 nm for both samples. On comparing Rhodamine 590 with Rhodamine 640, it can be seen that little structural difference is observed in relation to the fluorescence. The present results indicate that substitution of an amino group and carboxy-phenyl group at the 3- and 6-positions of the xanthene nucleus are not part of the chromophore of these dyes, so that they have only a minor influence on the fluorescence spectra of both products. Consequently, the dominant emission of fluorescence realized in the xanthene chromophore was primarily due to the n-n* transition related to the mesomeric structures [7], which are not substantially distinguishable between the two Rhodamines. 4. CONCLUSIONS
Previous work has focused on the fact that most intercalated species retain their solution-like mobility, even when they are electrically and geometrically restricted in the interlayer [8]. Therefore, present
550
600
results indicate that Rhodamine, one of the xanthene dyes, could be immobilized and thermally stabilized in the interlayer of montmorillonite. In addition, the results seem to give some new aspects on intercalation studies in expectation of the materials having a solid function. On the basis of the fact that the xanthene nucleus was not out of planarity, the mesomeric structure realized the n-electron without any interruptions. Moreover, the reaction conditions were examined for controlling the content of dye to govern the efficiency of fluorescence. It is expected that the present results of the dye-clay complex suggest possibilities for tunable laser host and fluorescent substrates. Acknowledgement-The authors gratefully acknowledge Dr. F. Minami of the National Institute for Research in Inorganic Materials, and Professor G. Adachi of Osaka University for measurements of fluorescence. REFERENCES 1. Pinnavaia T. J., Science, Wash. 4595, 365 (1983). 2. Fripiat T. J. and Crez-Cumplido M. I., A. Rev. Earth Planet Sci. 2, 239 (1974).
650
600
650
Wavelengthlnmi
Fig. 6. The quantum spectrum of fluorescence for Rhodamine 590 (a) and Rhodamine 640 (b) intercalated in montmorillonite.
804
T. ENDO et al.
3. Shimoyama A. and Johns D., Nature, Land. Phys. Sci. 232, 140 (1971). 4. Ghosh P. K. and Bard A. J., J. Am. them. Sot. 105, 5691 (1983). 5. Beilsteins Handbuch der Organischen Chemie 19, 344.
6. Serratosa J. M., Clays and Clay Minerals 14,385 (1966). 7. Drexhage K. H., Dye Lasers (Edited by F. P. Schafer), Vol. 1, p. 144. Springer, Berlin, Heidelberg (1977). 8. Pinnavaia T. J. and Welty P. K., J. Am. them. Sot. 97, 3819 (1975).
1986
0022.3697186 $3.00 + 0.00 Pergamon Journals Ltd.
FLUORESCENCE PROPERTIES OF THE DYE-INTERCALATED SMECTITE T. ENDO, T. SATO and M. SHIMADA Department of Applied Chemistry, Faculty of Engineering, Tohoku University, Sendai, Miyagi 980, Japan (Received 18 November 1985; accepted 13 February 1986)
Abstract-The fluorescence properties of the dye Rhodamine 590 and Rhodamine 640 intercalated into montmorillonite were studied. All of the intercalation reactions were accomplished in an ethanol solution which contained the corresponding quality of Rhodamine. The TG-DTA and the X-ray diffraction data showed that the immobilization of Rhodamine depended on the amount of ethanol incorporated in the interlayer space of montmorillonite. The 14.8 A basal spacing of products indicated a conformation of Rhodamine with the xanthene nucleus and the phenyl group positioned parallel and perpendicular to the silicate layer, respectively. Also, the infrared pleochromism, which supported the proposed orientation of the phenyl group, was observed at 1250 and 712 cm-‘. The main fluorescence bands of both products were observed near 600 nm when excited by the 514.5 nm line of an argon ion laser. The results indicated that the mesomeric structure determined the fluorescence behavior, which was little different for the two products owing to the environment of the interlayer. These data are briefly discussed relative to the reaction conditions utilized for the synthesis of products. Also, the results support the idea that the products have a high potential for use as a host for a tunable laser. Keywords: Intercalation,
smectite, montmorillonite,
dye, rhodamine, luminescence, fluorescence, tunable
1. INTRODUCTION Studies concerned with the reactions in the interlayer of inorganic materials, that is intercalation reactions, are attractive for obtaining information on the interfacial profile between the organic and inorganic phases. Smectite and compounds with related layer structures often show swelling after absorbing organic species of many different kinds in the form of molecules or ions. Such behavior was compared to an absorption in which the “host” material was treating the “guest” material hospitably [l]. The intercalation reactions of smectites are influenced by the negatively charged aluminosilicate layers. In this context, previous work focused on the coordination chemistry of molecules or ions governed by the charge density and its distribution in the interlayer space. Occasionally, useful materials were developed as catalysts, for instance, petroleum cracking catalysts with a high yield of gasoline owing to the geometrical selectivity and high acidity of the interlayer surface [2, 31. Also, applications as ion exchangers, an electrode and electrolite for batteries have been widely developed. These “intercalation” chemistry studies seemed to be motivated by the unusual reaction environment provided by the geometrical space and surfaces in the interlayer. On the other hand, the interlayer environment may alter the character of the “guest” material and/or the complex itself. Here, we report on the synthesis of smectite intercalates containing xanthene dyes. These dyes, well-known examples being Rhodamine 590 and Rhodamine 640, are useful as laser P.C.S47,&-E
materials quencies.
being
tunable
over a wide range
2. EXPERIMENTAL
of fre-
PROCEDURE
2.1. Material and preparation Natural Na-montmorillonite with a cation exchange capacity (CEC) of ca 115 meq/lOO g was obtained from Kunimine Industry Co. After suspending in a 10 vol.% ethanol-90 vol.% water solution, the largest fraction of starting material was freeze-dried, or pipetted on to an aluminum plate and dried overnight in air to form a thin film sample. In typical experiments, Rhodamine 590 and Rhodamine 640 were used without further purification because the purity and quality of each dye were dye-laser grade. The dye-clay complexes were prepared by dispersing the montmorillonite in the ethanol solution containing the dye, at a concentration corresponding to 2.5 and 1.5 times the CEC, respectively. The present experimental procedure leads to an almost complete intercalation at room temperature in 2-7 days, or at 75°C for 2-3 days when the concentration of the corresponding dye is conserved by use of a reflex condenser. The complex was recovered by filtration and washed several times with ethanol to eliminate excess dye, and then dried in air.
799
2.2. Physical
measurements
DTA-TG measurements were carried out with a Rigaku Denki Thermal Analyzer, which was oper-
800
T. ENDO et al.
Fig. 1. SEM photographs
of clay (a) before and (b) after intercalating of Rhodamine
ated at heating rates of 10 and 2O”C/min from room temperature to 700°C in air. Fmely ground samples were used with a-A@, as a reference throughout. Before and after the intercalation reaction, the products were examined by X-ray diffraction, scanning electron microscope (SEM) and infrared spectra. X-ray diffraction patterns were obtained by a Rigaku Denki X-ray diffractometer with Ni filtered CuKa radiation. The SEM photographs were taken with a JSM-TZO of Jeol Ltd. Infrared absorption spectra were obtained with a Perkin-Elmer Model- 237B grating infrared spectrophotometer. The Pleochrorsm effect was measured by use of the thin film sample. Electronic absorption spectra and emission spectra of the dye-clay complexes were measured at
590.
room temperature by using a Hitachi 850 Fluorescence Spectrophotometer, which was operated over a wave length of 300-750 rim. Luminescence spectra excited by the 514.5 nm line of an argon ion laser were measured at room temperature and liquid double helium using a temperature by Czerney-Turner scanning monochromator Model 1402 Fluorolog Spex Spectrofluorometer.
3. RESULTS AND DISCUSSION On addition of Na-montmorillonite to the Rhodamineethanol solution, the color of the solution gradually faded within a few hours, even when
Fluorescence properties of the dye-intercalated smectite at room temperature. The products showed intense colors, namely, bright red for Rhodamine 590 and purple for Rhodamine 640. The results can be interpreted in two ways; one is due to the chemical or physical absorption on the surface of clay grains, and/or the other is a result of the interlayer absorption. The results of the powder X-ray diffraction measurements showed that the (001) reflections from the products changed in a stepwise manner in the range from 14.8 and 16.7 A. Since the thickness of the silicate layer is estimated ideally to be cu 9.6 A, the increase in basal spacing was attributed to the swelling, which resulted from the intercalation of the dye and/or the ethanol used as solvent. Figure 1 shows the SEM photographs of clays before and after the intercalation of Rhodamine 590. As the distinctive profile, some slits were observed along the silicate sheets in the intercalated sample. This fact seemed to result from the mechanical stress induced by the swelling of the layers. A large number of slits might be arranged with a separation corresponding to more than 100 layers of basal spacings. In the TG-DTA data for the Rhodamine 590 product, endothermic and exothermic changes were observed in the regions from 7&l 15°C and 39%42O”C, respectively. No thermal changes were observed relative to the melting of Rhodamine 590, whose temperature was reported to be ca 258°C [5]. The exothermic peak was observed with little or no change of weight, whereas the endothermic peak was correlated with a fairly large (5-6%) loss of weight. The color of the product remained a bright red up to 35O”C, and then it turned immediately to black as the resuIt of burning above 400°C. These DTA features were attributed to the evaporation of ethanol, and the burning and graphitization of Rhodamine 590 in air, respectively. The thermal stability of the products were examined by X-ray diffraction. Figures 2(a) and (b) show the X-ray patterns of the product before and after heating treatments at 125°C. The clay complex before heating shows an intense and broad peak given as the mean value, which results from the overlap of several sharp peaks. On the other hand, the (001) reflection of the clay complex after heating was satisfactorily separated into two peaks at 14.8 and 9.6& the latter corresponding to an unintercalated clay. The results indicated that ethanol also contributed to the swelling of the interlamellar clay. Therefore, we consider that the quantity of ethanol in the intercalation reaction was controlled by the reaction temperature conditions. When the reaction temperature is increased, the amount of ethanol in the interlayer tended to exchange readily with the dye in the ethanol solution. This result indicated that intercalation at higher temperature conditions was more profitable for packing the dye tightly in the interlayer. The basal spacing of all the intercalates formed by
801
Fig. 2. The (001) reflections of the products, (a) before and (b) after heating treatment at 125°C.The intercalation of the product was carried out at room temperature. The (001) reflection of the product after intercalating at 75°C (c). reaction in ethanol at 75°C exhibited a (001) spacing of ca 14.8 A as shown in Fig. 2 (c). As described by Serratosa [6], the observation of IR pleochroic effects can be an effective method for determining the orientation of phenyl groups intercalated between the siIicate sheets. In infrared absorption data, all bands arising from Rhodamine 590 were faithfully reproduced with slight shifts or splittings. In particular, the two bands at 1250 and 712 cm-’ could be assigned to in-plane and out-ofplane vibrations due to the dipole moment characteristic of the phenyl group, and they exhibited significant pleochtoism when the plane of an oriented film sample was inclined relative to the path of the infrared beam. As seen in Fig. 3, the band at 1250cm-’ decreased in intensity while the band at 712 cm-’ increased in intensity. These observations suggest that the carboxy-phenyl groups are held in a position almost perpendicular to both the silicate layer and the xanthene nucleus, owing to the steric hindrance of the carboxyl group. When the ethanol was expelled from the interlayer, the Rhodamine molecule, which became rigid, underwent somewhat of a tilting of the phenyl ring. This is suggested by the difference between the observed (ca 5.2 A) and calculated (ca 5.6 A) values for the size of the carboxy-phenyl group. The conformation of the Rhodamine 590 molecule intercalated is schematically illustrated in Fig. 4. The X-ray diffraction data of products after heating above 400°C indicated the collapse of spaces in
T. ENDS et al.
802
Wove
number (cm-‘)
Fig. 3. Selected infrared absorption spectrum of the Rhodamine SO-clay complex for two angles of (90”) and (-- 45”) to incident beam.
the interlayer space, so that the (001) reflection was abruptly shifted to 12.612.3 A. In addition, no clay complex with a basal spacing of 14.8 A could readsorb water molecules on the surface of the interlayer, and swell further. In the visible and ultra-violet spectra, the main absorption band was observed at 520 nm for the Rhodamine 590-clay and at 575 nm for the Rhodamine 640-clay. On comparing the data of Rhodamine in ethanol, we observed bathochromic shifts of 3-7 nm due to weak electric binding of the dyes to the silicate layers. The small amount of ethanol which remained in the interlayer had only a minor influence on the absorption spectrum. Figure 5 shows the fluorescence spectra of the Rhodamine 590-clay complex at room temperature and 10 K, which was excited by the 514.5 nm line of an argon ion laser. It can be seen that the peak profile becomes sharp and looks correctly like that of a Rhodamine-ethanol solution at 10 K. The maximum quantum peak was also given at ca 645 nm for the Rhodamine 640-clay. As for the results of fluorescence spectra of both products with ethanol in the interlayer, it was reasonable to assume that the position of the fluorescence maximum had shifted, because changes are known to occur as a function of concentration, temperature and viscosity of the solvent.
Fig. 4. Schematic illustration of Rhodamine 590 oriented in the interlayer of montmorillonite. are omitted for clarity.
Protons
Fluorescence properties of the dye-intercalated smectite
803
(bl
I
550
I
L
600
650 Wovelength
I
I
650
600 (m-n1
Fig. 5. The quantum spectrum of fluorescence for Rhodamine 590-montmorillonite
(arbitrary units) (a)
at room temperature, (b) at 10K.
Figure 6 shows the fluorescence spectra of the products without ethanol in the interlayer. There was a maximum quantum peak at 600 nm for both samples. On comparing Rhodamine 590 with Rhodamine 640, it can be seen that little structural difference is observed in relation to the fluorescence. The present results indicate that substitution of an amino group and carboxy-phenyl group at the 3- and 6-positions of the xanthene nucleus are not part of the chromophore of these dyes, so that they have only a minor influence on the fluorescence spectra of both products. Consequently, the dominant emission of fluorescence realized in the xanthene chromophore was primarily due to the n-n* transition related to the mesomeric structures [7], which are not substantially distinguishable between the two Rhodamines. 4. CONCLUSIONS
Previous work has focused on the fact that most intercalated species retain their solution-like mobility, even when they are electrically and geometrically restricted in the interlayer [8]. Therefore, present
550
600
results indicate that Rhodamine, one of the xanthene dyes, could be immobilized and thermally stabilized in the interlayer of montmorillonite. In addition, the results seem to give some new aspects on intercalation studies in expectation of the materials having a solid function. On the basis of the fact that the xanthene nucleus was not out of planarity, the mesomeric structure realized the n-electron without any interruptions. Moreover, the reaction conditions were examined for controlling the content of dye to govern the efficiency of fluorescence. It is expected that the present results of the dye-clay complex suggest possibilities for tunable laser host and fluorescent substrates. Acknowledgement-The authors gratefully acknowledge Dr. F. Minami of the National Institute for Research in Inorganic Materials, and Professor G. Adachi of Osaka University for measurements of fluorescence. REFERENCES 1. Pinnavaia T. J., Science, Wash. 4595, 365 (1983). 2. Fripiat T. J. and Crez-Cumplido M. I., A. Rev. Earth Planet Sci. 2, 239 (1974).
650
600
650
Wavelengthlnmi
Fig. 6. The quantum spectrum of fluorescence for Rhodamine 590 (a) and Rhodamine 640 (b) intercalated in montmorillonite.
804
T. ENDO et al.
3. Shimoyama A. and Johns D., Nature, Land. Phys. Sci. 232, 140 (1971). 4. Ghosh P. K. and Bard A. J., J. Am. them. Sot. 105, 5691 (1983). 5. Beilsteins Handbuch der Organischen Chemie 19, 344.
6. Serratosa J. M., Clays and Clay Minerals 14,385 (1966). 7. Drexhage K. H., Dye Lasers (Edited by F. P. Schafer), Vol. 1, p. 144. Springer, Berlin, Heidelberg (1977). 8. Pinnavaia T. J. and Welty P. K., J. Am. them. Sot. 97, 3819 (1975).